Seed-Particle-Independent In-situ Synthesis of Surface-Templated Shape-Selected Palladium Nanoparticle Arrays

Seed-Particle-Independent In-situ Synthesis of Surface-Templated Shape-Selected Palladium Nanoparticle Arrays | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Seed-Particle-Independent In-situ Synthesis of Surface-Templated Shape-Selected Palladium Nanoparticle Arrays Christoph Langhammer, Jordi Piella, Carl Andersson, Joachim Fritzsche This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6715528/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Surface-templated metal nanoparticle synthesis combines the best of lithography-based nanofabrication on surfaces and colloidal nanocrystal growth in solution, and it is well accepted that the final structure and crystallinity of nanoparticles grown using the seed-mediated method is strongly linked to the structure, morphology and crystallinity of the initial seed. Here, we challenge this paradigm by introducing regioselective particle growth enabled by a sacrificial polymer layer used to tailor the fraction of seed particle surface exposed to a growth solution and thereby defining the growth mode of the particle. Mechanistically, this confines metal deposition to a small, selected region of the seed surface and decouples the growth mode from size, morphology, crystallinity and composition of the seed. As we show on the example of Pd, this enables the in situ surface-templated growth of regular arrays of polycrystalline nanoflowers, spiky nanostars and single crystalline nanocubes with over 90% yield, using identical growth conditions and nanolithography-fabricated regular arrays of morphologically poorly defined polycrystalline seeds, as well as crafting heterodimeric single crystalline Pd nanocubes at the tip of a large polycrystalline Au nanocone. This widens the practical applicability of surface-templated nanofabrication with rationally arranged metal particles with well-defined morphologies, crystallinity and composition. Physical sciences/Chemistry/Chemical synthesis/Nanoparticle synthesis Physical sciences/Materials science/Nanoscale materials/Nanoparticles Physical sciences/Nanoscience and technology/Nanoscale materials/Nanoparticles Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Combining bottom-up colloidal synthesis with top-down nanolithography methods represents the best means, if not the only, to rationally arrange complex noble metal nanoparticles with well-defined morphologies, crystallinity and composition on substrate surfaces. Traditionally, these two radically different, but highly complementary, approaches have been combined by first synthesizing the nanoparticles in solution, where colloidal chemistry in general, and the impressive-collection of seed-mediated growth protocols for the direct noble metal nanoparticle formation along orderly and well-controlled pathways in particular, have been the key concepts. 1 , 2 , 3 Subsequently, these colloidal nanoparticles are assembled on a templated surface that utilizes micro- or nanolithography-defined features in combination with a tailored external force to drive the colloidal nanoparticles to specific positions and have them anchor there. 4 , 5 , 6 Typically, this assembly process involves multiple, case-specific and lengthy steps of colloidal nanoparticle purification/centrifugation from the reaction mixture, ligand exchange/surface functionalization, and self- or directed-assembly to the surface positions. These steps are of limited scalability and reproducibility and hinder the crafting of large-area nanostructure arrays with high fidelity and true long-range order, free of registration errors. Contrary to this conventional synthesis-then-positioning paradigm, surface-templated in-situ particle growth has recently emerged as a promising alternative where nanoparticles are formed directly at predefined positions on a templated surface that is immersed into a growth solution. 7 , 8 To implement size and shape control, identical or slightly adapted seeded-mediated growth protocols from colloidal synthesis in solution are used and applied to supported seed particles that either are nucleated or nanofabricated by lithography methods on the substrate surface in well-ordered arrays. 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 In this way, the rich and exciting chemistry of colloidal synthesis, and its associated functionalities, are brought to the substrate surface, while circumventing the aforementioned lengthy post-synthesis steps associated with the anchoring of the nanoparticles once synthesized. Additionally, numerous other interesting new synergies unique to surface-templated in-situ particle growth have started to just being explored. For example, since particles grow from supported seeds, colloidal stability does not need to be considered and reaction conditions that are ‘forbidden’ in colloidal synthesis in solution (e.g., reactions in high ionic strength media in the absence of stabilizing surfactants) become accessible. 22 , 23 Another advantage stems from the possibility to quickly separate the growing nanoparticles from the reaction solution to stop the reaction at any time, or to redirect it by immersing the surface-attached nanoparticles into a new solution without the need for centrifugation or precipitation steps. 14 , 15 , 24 , 25 Similarly, the nanoparticles can be easily purified from the growth solution. 23 Inherent to the growth of surface-attached seeds is also the truncation of the particle geometry at the substrate interface. 26 , 27 This, in turn, improves adhesion of the nanostructures, particularly of those with geometries like spheres or branched structures that otherwise will have small contact areas. 28 Similarly, truncation reinforces the electronic coupling between the substrate and the nanoparticles. 28 Additionally, substrate-imposed epitaxial growth has been used as a means to control the crystallographic orientation of the nanoparticles. 29 , 30 In summary, it is clear that surface-templated in-situ particle growth offers a myriad of opportunities that holds the potential to deliver complex nanostructures with morphologies and compositions inaccessible to standard colloidal synthesis, and with a control of the position of the nanoparticles on the surface unattainable by any other means. The key to all of this is the preparation of seed nanoparticles in arrays of choice on a specific substrate of interest, where both components are compatible with seeded-growth modes. For this purpose several methods exist, such as in-situ reduction on chemically patterned surfaces 16 , 17 , 31 , block copolymer lithography 9 , 11 , 32 , templated solid-state dewetting of metal thin films 33 or nanolithography approaches where the seed material of choice is evaporated through a lithographically-defined mask (e.g., electron-beam lithography (EBL), nanoimprint lithography, colloidal lithography or photolithography). Out of these methods, EBL-based nanolithography is unrivalled in terms of enabling accurate control of seed particle position inside an array with well-defined array parameters. Furthermore, since no solvents, surfactants and other chemicals are used in the coalescence and growth process of the seed particles by means of physical vapor deposition (PVD), the obtained seeds also have clean/pristine surfaces. As the main drawback, however, nanolithography, as well all the other mentioned methods, typically produce rather large particles in the tens to hundred(s) of nanometer range that are highly polycrystalline and ill-defined in terms of crystallinity, morphology and faceting. This is problematic because it is widely accepted that the growth of shape-selected nanoparticles requires the initial nucleation of seeds with specific and well-defined crystalline structures. 34 , 35 , 36 To overcome this limitation, thermal annealing steps at temperatures close to the used metals’ melting points (often > 1000°C) have proven necessary to induce recrystallization of the surface-attached seeds into Wulff/Winterbottom-shaped nanocrystals prior to growth in solution. 8 , 14 , 19 However, in most cases, such high temperatures trigger chemical reactions between the metal seeds and the substrate surface and are therefore highly problematic. Furthermore, withstanding such high temperatures demands special and expensive substrates, such as sapphire, SrTiO 3 or MgO, that are of limited practical use. To this end, Neretina et al. produced ̴ 50 nm Au, Ag, Cu, Pt and Pd single crystalline nanoparticle arrays by thermal processing of PVD nanofilms and nanodisks and used them as seeds for the surface-templated growth of shape selected nanostructures. They employed a dynamic templating process where sacrificial Sb and/or Bi layers were deposited between the substrate surface and the seed particles to facilitate metal dewetting and restructuring at lower temperatures. 1 3 , 3 3 However, even with these sacrificial layers, annealing temperatures above 750°C were required, with Pd requiring 1100°C due to its high melting temperature of 1555°C. In this work, we introduce an alternative route to overcome the polycrystalline seed problem in surface-templated colloidal particle synthesis, by introducing a methodology that allows the selective exposure of only a fraction of the seed particle surface to the growth solution. Thereby, it provides access to a single-crystal growth mode, despite the distinct polycrystallinity – and if desired large size – of the seed. This is enabled by a thin poly(methyl methacrylate) (PMMA) layer spin-coated on the substrate surface decorated with nanolithography-fabricated seed particle array. The thickness of this PMMA layer is precisely adjusted by plasma etching to expose only the desired fraction of the seed surface. Using this approach, we show on the example of Pd how the growth mode of identical surface-templated polycrystalline seeds can be progressively unlinked from their size and crystalline structure as more of the seeds’ surface, together with associated grain boundaries and crystal defects, is covered by the PMMA layer. Thereby, it enables formation of structures that range from polycrystalline nanoflowers to single crystal nanocubes with high yield simply by controlling the fraction of the seed surface that is exposed to the growth solution. Furthermore, we extend this methodology to the growth of Pd nanostars, where the PMMA layer thickness determines the number of spikes, and to multimetallic hybrid structures with seed particles made from a different metal than the grown crystal, such as the selective growth of a Pd nanocube on the tip of a large Au nanocone. Results Seed particle nanofabrication by Electron Beam Lithography (EBL) To establish the surface-templated in situ particle growth protocol with a PMMA layer, we first EBL-fabricated regular arrays of disk-shaped Pd seed particles of 125 diameter and 30 nm thickness onto an oxidized silicon wafer substrate “chip” using a well-established protocol of PVD of Pd trough a EBL-fabricated mask (Fig. 1 a and Figure S1 ). 37 , 38 As anticipated, this methodology yielded disks with highly polycrystalline structures composed of randomly oriented small crystallites or “grains” of 10–15 nm, as clearly revealed in transmission electron microscopy (TEM) images (Fig. 1 b and Figure S2 ) and selected-area electron diffraction (SAED - Fig. 1 c) of the Pd seeds from an array fabricated onto a SiN x Transmission Electron Microscopy (TEM) membrane, 39 as well as in top view (Fig. 1 d) and tilted (Fig. 1 e) high resolution Scanning Electron Microscopy (SEM) images of the corresponding seeds on the standard oxidized Si substrate. To, in the next step, investigate the extent to which a transformation of the crystallinity of the Pd seeds is possible by thermal treatment, we annealed them for 2 h in an atmospheric pressure tube furnace flushed with a mixture of 98% Ar and 2% H 2 to prevent Pd oxidation. 40 Under these conditions, the maximum temperature that we could reach without observing evidence of sample deterioration caused by thermally induced chemical interactions of the Pd seeds with the SiO 2 /Si substrate was 600°C ( Table S1 and Figure S3 and corresponding discussion). 40 , 41 , 42 As expected, even this highest annealing temperature barely transformed a few of the seed particles into single crystals, and their majority remained polycrystalline, typically with a few large crystalline grains and the presence of defects, including high- and low- angle grain boundaries, twining and stacking faults, that varied from particle to particle (Fig. 2 a and extended in Figure S4 ). As for the morphology of the annealed seeds (Fig. 2 b-d), the slight contraction in diameter from 125 ± 2 nm to 110 ± 3 nm and concomitant adoption of a geometry best described as a somewhat faceted (truncated) oblate spheroid reflects their tendency to restructure towards more isotropic Winterbottom shapes at elevated temperatures. At the same time, the fact that many of them had non-equiaxed top-view cross-sections with slightly broader diameter and circularity distributions compared to the pre-annealed seeds (Fig. 2 c, d) evidence that this restructuring process is not thermodynamically completed. Exposed seed surface control by the PMMA layer It is reasonable to assume that the above described Pd seeds are poor candidates for the seeded-growth synthesis of shape-selected nanostructures in high yield, as this generally requires (nearly) single crystal seeds with specific and perfectly defined structures. 34 Here, however, we challenge this paradigm by postulating that if (unwanted) parts of an ‘imperfect’ seed particle could be ‘hidden’ from the growth solution, for instance crystalline defects and grain boundaries, their impact on the growth process could potentially be eliminated, such that it would then be possible for structurally different seeds to sustain the same grow mode. To demonstrate this concept, we have developed a methodology that in a controlled fashion covers a tailored fraction of the seed particle surface with a PMMA layer. This polymeric layer is first deposited by spin-coating on the substrate surface templated with the seeds at a thickness significantly higher than the seed height, to ensure a uniform flat covering of the surface. For the seeds tested here (up to 50 nm in height, vide infra ), we have used a PMMA layer thickness of 140 nm. Subsequently, this PMMA layer is etched down by O 2 plasma to a specific thickness below the seed height at which it exposes the desired fraction of the seed’s surface (Fig. 3 a ) . As key aspects of this methodology, we first note the high uniformity of the PMMA layer thickness and its lineal dependence on the plasma etching time that allows excellent control of this parameter (Fig. 3 b). We also highlight (i) the high monodispersity in terms of height of the annealed Pd seeds which together with (ii) the homogeneous etching and flatness of the PMMA layer surface unlock exposing the top of the seed particles with high accuracy, as revealed by AFM profiles showing peaks with an average height of 46 ± 3 nm for the uncoated annealed Pd seeds (Fig. 3 c), and a flat profile and peaks with average heights of 7 ± 3 and 24 ± 3 nm for equivalent seeds coated with PMMA layers with thickness adjusted to 60, 35 and 20 nm respectively ( Fig. 3 d-f ) . Note here that the average height of the annealed Pd seeds after annealing (46 ± 3 nm) is larger than the nominal height of the as-fabricated Pd seeds (30 nm), and that this well-correlates with the dewetting of Pd seeds after annealing discussed above (cf. Figure 2 c). As a final remark, we noticed systematic discrepancies of 2–4 nm between the seed height measured by AFM (Fig. 3 c) and the thickness of the deposited PMMA layer measured by ellipsometry plus the height of the uncoated seed fraction measured by AFM (Fig. 3 d). This, we attribute to the softness of the PMMA layer potentially yielding an apparent lower film thickness in the AFM measurements. Templated Pd nanocrystal growth on annealed seed particles To overgrow the supported seeds partially covered by PMMA, we immersed them in an aqueous solution containing ascorbic acid (AA) as the reducing agent and cetyltrimethylammonium bromide (CTAB) as a complexing and capping agent at 40°C. Then, we fast-injected a PdCl 2 solution as the precursor to start the growth process and let the seeds rest in this growth solution for 10 min (see experiment setup in Figure S5 ). To this end, the reduction of Pd ions by AA in the presence of CTAB has been widely used in the colloidal synthesis of single-crystal Pd nanocubes, where the cube-forming growth process is driven by the preferential adsorption of the Br − from CTAB onto the (100) facets of Pd. 43 , 44 , 45 , 46 As a key point here, we highlight that in these processes, solution-nucleated sub-20 nm seeds with a well-defined crystallinity (typically single-crystal truncated octahedron) were critical for a sizable yield of Pd nanocubes. 44 , 46 Here, instead, our supported seeds are significantly larger and distinctly polycrystalline ( cf. Figure 2 ), but with the key feature that only a portion of the seed surface, controlled by the PMMA layer thickness ( cf. Figure 3 ), is accessible to the growth solution. Specifically, to investigate the critical role of the exposed surface area of the seeds in the nanoparticle’s growth mechanism, we exposed nominally identical arrays of the Pd seeds coated by PMMA layers with thickness systematically reduced from 60 nm down to 12 nm to the same growth solution and analyzed the structure of the resulting Pd nanoparticles after 10 min. Because the average seed height is 46 ± 3 nm ( cf. Figure 3 c), this PMMA thickness range stretches from fully covering the seeds to exposing most of their surface area (Fig. 4 a). The use of PMMA layers thinner than 10 nm was hampered by the arrays becoming highly contaminated by secondary nanoparticles nucleated in solution that adsorbed on the substrate surface due to their affinity for the SiO 2 surface, thereby indicating that the PMMA no longer prevented it. The use of PMMA layers thinner than 10 nm was hampered by the arrays becoming increasingly contaminated by adsorbed secondary Pd nanoparticles nucleated in solutions. We attribute this to a favored interaction of these solution-nucleated nanoparticles for the SiO 2 surface, 47 which is no longer prevented by the thin PMMA layer. Furthermore, to minimize potential oxidation of the Pd seeds by O 2 plasma used to adjust the PMMA thickness, we use a very low power (25 W) during the etching the PMMA layer ( Figure S6 and corresponding discussion about the impact of O 2 plasma on the Pd seed surface). Remarkably, some of the seed arrays produced Pd nanocubes in high yield, with the yield being strongly correlated with the thickness of the PMMA layer. Plotting the yields of grown particles and cubes in the arrays derived from SEM characterization as function of PMMA thickness indeed reveals clearly discernible growth regimes (Fig. 4 b, see Figure S7 for corresponding SEM images). Specifically, on one hand, almost no seeds sustained a growth process for PMMA layers above 50 nm (Fig. 4 b, case 0 ), which indicates that even a PMMA coating only slightly thicker than the average seed height (46 ± 3 nm) forms an impermeable barrier that prevents the monomers in solution from reaching the seed surface ( Figure S7a, b ). Notably, the number of cubes formed in these samples was consistent with the number of nanoparticles grown. When the PMMA layer was adjusted to 40 nm, which is only 6 nm below the average seed height (Fig. 4 b, case I ), the number of seeds that sustained a growth process and formed a cube on top increased dramatically and the cube yield in the array reached 90%, while only a residual fraction presented other (irregular) shapes (Fig. 4 c and Figures S7c, d and S8 ). Notably, this yield is in par with the state-of-the-art Pd cube synthesis fully in solution. 45 , 48 , 49 Interestingly, further decreasing the PMMA thickness down to 20 nm (Fig. 4 b, case II ), and hence exposing a large fraction of the seed surface to the growth solution, exponentially dropped the cube yield, while the overall yield of particles growing remained close to 100% ( Figure S7e-h ). Specifically looking at the morphology of these particles (Fig. 4 d) reveals irregular anisotropic structures with configurations that vary from particle to particle and thereby likely reflect the inherent diversity in the seed particle structures ( cf. Figure 2 ) that now is exposed to the growth solution. To further study the growth mechanism in the different regimes identified above and investigate how the growing particles evolve at the early stage in detail, we exposed equivalent Pd seed arrays partially covered with PMMA layer thickness of 40 and 20 nm (assuming that the Pd seeds have the shape of a spherical cap, this represents covering roughly 90% and 40% of their surface area, respectively) to the growth solution for 30 s, 1 min and 2 min, in addition to the 10 min exposed samples discussed above, and compared the respective structure of representative particles imaged by SEM (Fig. 4 e, f). Starting with Pd nanoparticles grown on seeds partially covered by the 40 nm PMMA layer (Fig. 4 e, extended in Figures S9 ), we find that single cubes emerge locally from the small, exposed surface at the very top of the seeds, and that, in consequence, these cubes are initially much smaller than the seed size. Remarkably, after 30 s, the cubes on top of the seeds are only 10–15 nm (leftmost image in the bottom row of Fig. 4 e), that is roughly an order of magnitude smaller than the seed diameter. These single cubes grow upon longer reaction times, at which it becomes apparent that the shape is truncated at a distance from the substrate that coincides with the thickness of the PMMA layer (tilted SEM images in Figs. 4 e and S9). Representative particles corresponding to the fraction of Pd nanoparticles in the array that contain structures different from a single cube (multiple misoriented cubes, twinned cubes, irregular shapes, etc.) are shown in Figure S10 . We attribute this residual fraction to the specific case where seed crystal defects are located specifically at the small top area exposed by the PMMA. Notably, since the cube structure is preserved throughout the growth process, iterative immersion into fresh growth solution(s) in 10 min steps enabled the synthesis of cubes as large as 300 nm ( Figure S11 ). A second interesting aspect is the orientation and truncation of these cubes, that are distinct for the individual particles. To this end, it has been reported that deposition of Pd atoms from solution on Pd seed is epitaxial. 34 , 44 Hence, one possible reason for the different cube orientations is that the (100) surface of the seed on which the cube grows orient differently with respect to the substrate surface on different seeds, as expected for the amorphous substrate used here. Alternatively, cubes do not only nucleate and grow on (100) surfaces but also on other low index facets abundant on the annealed seeds that may be exposed by the seed surface as well. 13 Similar analysis of the Pd nanoparticles produced with the 20 nm PMMA layer (Fig. 4 f, extended in Figure S12 ) reveals that metal deposits initially on a large area on the seed surface, as expected for a larger exposed seed area, and that this deposition is not conformal layer-by-layer but starts with the nucleation of multiple small cubes. For a small fraction of the seeds, these early cubes are all oriented in the same direction and eventually merge at longer reaction time into a single larger cube. However, in most cases, cubes in different regions of the same seed orient in different directions and at the boundary of these regions fast-growing highly anisotropic semi-planar structures (petal-like) arise. Particles characterized by these irregular structures, often spreading from one side of the particle to the other, with regions with more cube-like structures (top row of Fig. 4 f), clearly indicate growth pathways that override the thermodynamic tendency to form highly symmetric faceted structures by locally activating the rapid growth of branched structures along certain crystallographic directions. This requires the formation of nucleation points from which the accelerated highly directional growth occurs. Crystal defects, such as grain boundaries, twinning and stacking faults have been identified as such nucleation points in numerous protocols for the synthesis of highly anisotropic nanoparticles, such as nanostars and nanoplates, whose synthesis relies on the use of defective seed particles 50 , 51 . Hence, we argue that the presence of similar defects in our seeds ( cf. Figure 2 ), when exposed to the solution by a thinner PMMA layer, are likely the main reason for the observed irregular growth mode. Finally, seed arrays exposed to iterative immersions into fresh growth solution(s) in 10 min steps presented mainly particles with systematically increasing petal size and only a residual fraction of single large cubes ( Figure S13 ); which contrasts with the large cubes seen in high yields in equivalent iterative growth samples coated by the thicker 40 nm PMMA layer ( Figure S11 ). Templated Pd nanocrystals growth on unannealed seed particles To further evaluate the influence of crystalline defects on the particle growth modes and corroborate the general hypothesis developed above, we exposed the non-annealed, and thus highly polycrystalline and defect-rich, 125 x 30 nm Pd seeds ( cf. Figure 1 ) to exactly the same growth conditions used above for the annealed Pd seeds, and again we focused on two different PMMA layer thicknesses: (I) 20 nm tailored to expose a sizable fraction of the seed surface, and (II) 30 nm tailored to only just expose a very small surface area at the very top of the seed. (Fig. 5 a). Focusing first on scenario (I), SEM imaging of the nanoparticle arrays obtained after 10 min immersion in the growth solution reveals a distinctly flower-like morphology that is composed of many crystalline petals (Fig. 5 b). As predicted by our hypothesis, these petals are much more abundant than those seen in the corresponding Pd nanoparticles produced from the annealed seeds ( cf. Figure 4 d). Furthermore, characterization of the “nanoflowers” at different stages reveals a heterogeneous deposition of Pd on the highly polycrystalline Pd seeds and the formation of small crystals (petal-like) at many sites that are oriented in different directions and grow over time (Fig. 5 c). Hence, the reason for this flower-like morphology must be the higher abundance of crystalline grains and defects in as-fabricated seeds compared to annealed seeds, as they likely define the sites and extent of petal nucleation. Notably, here also, we were able to increase the size of the nanoflowers up to several hundred nanometers by consecutive growth steps ( Figure S14 ). Focusing on scenario (II), SEM analysis of the Pd nanoparticles grown using a PMMA layer with the same thickness as the nominal height of the seeds indeed reveals that for the unannealed seeds that exhibit any growth at all (20%), a sizable fraction has formed a single cube on top (66% of the growing seeds - Fig. 5 d). Since the only difference compared to the nanoflowers grown along scenario (I) is the thickness of the PMMA layer, and thus the extent of the seed surface exposed to the growth solution, this is the critical factor. Hence, by decreasing the exposed surface area of the seed, it is possible to progressively decouple the structural/morphological properties of the seed from its growth mode and, as a consequence, from the structure of the crystal grown on top. This is here demonstrated by the formed dimeric polycrystalline disk – single crystalline cube structure (Fig. 5 d). Notably, even the chemical surface state of the two particles is different since the grown Pd cube is covered by the shape directing agent, here CTAB, whereas the surface of the underlying Pd disk is clean after removing the protecting PMMA layer. As a final point, we note that in this scenario (II) the overall yield of grown particles was low when the yield of cubes is significant (leftmost image in Fig. 5 d). On one hand, we attribute this to the high polycrystallinity of the unannealed Pd seeds, and hence the need to only expose a very small fraction of the surface to be compatible with the single cube growth. On the other hand, we identify the flat disk shape of the unannealed seeds as a key reason for the overall low yield since it requires very accurate tuning of the PMMA layer thickness to expose the “just right” small fraction of surface. In this situation most of the seeds became fully covered by the PMMA layer. These two factors thus render the window of transition between cube to nanoflower growth modes in these highly polycrystalline Pd seeds very narrow. Seed particle size independence of templated growth In the same way as regioselective growth on Pd seeds enables the seed-particle- structure -independent synthesis of Pd cubes, we postulate that it enables the synthesis on seed particles of any size , if the PMMA thickness is adjusted accordingly to expose the “right” amount of seed surface area for each case. To demonstrate this, in addition to the 125 ± 2 nm Pd disk seeds investigated above ( c.f. Figure 4 ) , we had nanofabricated on the same substrate arrays of Pd disks with systematically smaller diameters of 110 ± 1, 95 ± 2, 70 ± 1 and 40 ± 2 nm at a constant height of 30 nm. Hence, they were also annealed at 600°C to induce recrystallization and the slight Winterbottom-reshaping necessary to obtain high cube yield arrays. The average seed size measured by SEM (diameter) and AFM (height) of the Pd seeds after annealing were: 110 ± 3 nm, 103 ± 2 nm, 88 ± 2 nm, 67 ± 1 nm and 42 ± 3 nm, and 46 ± 3 nm, 47 ± 2 nm, 41 ± 2 nm, 39 ± 1 nm and 34 ± 1 nm, respectively (Fig. 6 a-e ) . Here again it is important to note the high degree of monodispersity of all seed sizes. Subsequent exposure of the Pd seeds covered by systematically reduced PMMA layer thicknesses ranging from 60 to 12 nm to the growth solution for 10 min revealed an optimal exposed surface area for all seed sizes that enabled growth of Pd cubes with very high yields (up to 90% for the largest seeds and up to 97% for the rest) (Fig. 6 f-j, and Figures S15 and S16 for large-size SEM images of selected Pd cube arrays). Hence, globally, all tested seed sizes showed a similar trend. At the same time, interesting more subtle differences can be noted in the dependence of the cube yield on the PMMA layer thickness for the different seed sizes. For example, the optimal PMMA thickness range that enables a high cube yield broadens as the size of the Pd seeds decreases (Fig. 6 f-j). In other words, the growth pathway depends less on changes in the PMMA layer thickness as the seeds are smaller. The reason for this is likely to be related to an increase in the degree of (mono)crystallinity and a decrease in the overall defect density induced by thermal annealing as the seed size decreases. It is well known that particle size plays an important role in recrystallization upon annealing, and that small particles recrystallize more easily than their larger counterpars. 52 , 53 , 54 As a second aspect, we note that the unit change in exposed seed surface area per unit change in thickness of the PMMA layer depends on the size and shape of the seed. Accordingly, the correlation between the thickness of the PMMA and the exposed seed area is stronger for particles with large aspect ratio (like our largest seeds, Fig. 6 a) than for more spherical seeds (like our smaller seeds, Fig. 6 e). Hence, this is likely the second reason for the negative correlation between cube yield and PMMA layer thickness as the size of the seeds decreases. To summarize, it becomes apparent that the cube growth mode is determined by structural factors of the exposed seed fraction rather than the overall seed size and structure. Furthermore, for the same reason, large differences in seed size should not be reflected in the final size of the cubes for a given growth time if the area exposed by the PMMA layer is of similar size. Indeed, this is confirmed when comparing two representative cubes grown on our largest and smallest seeds, respectively, as they barely show differences in size (Fig. 6 k, l). Discussion In traditional colloidal synthesis in solution, nanostructures with radically different morphologies, such as polycrystalline flowers and single-crystalline cubes, require different seed particles and/or synthesis conditions (directing agents, reaction kinetics, etc.). 2 , 36 It is thus indeed remarkable that we have produced this variety of morphologies employing the same initial seed and growth conditions, solely by tuning the fraction of seed surface exposed to the growth solution, in this case, by adjusting the thickness of the PMMA layer. As the mechanism, we postulate that the smaller the exposed area of a ‘defective’ seed, the less likely it is to expose (a sizable number of) crystal defects to the growth solution that could contribute to an irregular growth process. In essence, mechanistically this is similar to physically shrinking the seed to the typical sizes and crystallinity used in colloidal synthesis. While our results indeed nicely corroborate this proposed mechanism so far, they do not fully exclude alternative scenarios. For example, Mirkin et al. demonstrated that Pd cubes can grow from supported 12 nm multiple twinned Au seeds under synthesis conditions very similar to ours. 9 While, in principle, twinned seeds are not compatible with the cube growth mode 34 , 44 , 55 , they described the growth process as being characterized by the initial deposition of Pd at low-coordination high-surface-energy sites, leading to the multiple nucleation of Pd around facet edges and corners. As more Pd subsequently deposits on the seed, these discrete nucleation islands initially formed merge into a primitive shell that develops (100) facets upon growth, as determined by the face-selected adsorption of the Br- ions of the used CTAB capping agent, and it does not necessarily form an epitaxial crystalline relation with the seed. The main implication of this growth mechanism for our work at hand here is that the formation of a cube may not necessarily require an epitaxial relation with the seed. Applying this reasoning to our case, one or even multiple defects in a small enough surface area exposed to a growth solution (as well as, hypothetically, a different seed material with significantly different lattice constant) may not prevent a cube to still nucleate and grow since the external conditions imposed by the growth solution and the used capping agents may compensate the lattice mismatch induced by the defects, provided they are limited in number – as ensured by a small exposed seed surface area. For this particular case, we propose a final mechanism where the cube yield is strongly dependent on the absolute exposed seed surface area and only weakly dependent on said area’s defectuousness, provided the exposed defectuous area is small enough. To further verify this mechanism with particular focus on the proposed only weak dependence on the crystallinity of the small exposed seed surface, and a first step towards using a different seed material (for which, however, it has still been demonstrated that epitaxial growth of Pd is possible) 56 , we exchanged the polycrystalline disk-shaped unannealed Pd seed particles (cf. Figure 5 ) to corresponding unannealed Au seeds, nanofabricated into arrays using the same EBL procedure, and subsequently coated them with 20 nm and 30 nm PMMA layers, respectively (Fig. 7 ). As anticipated, 10 min exposure to the same Pd growth solution used above yielded Pd nanoflowers on the polycrystalline Au seeds for 20 nm PMMA exposing a large fraction of the seed surfaces area (Fig. 7 a), and a single Pd cube on the polycrystalline Au seeds for 30 nm PMMA covering nearly all the seed surface (Fig. 7 b), thereby corroborating our overall proposed mechanism and also showcasing the ability of our approach to generate surface templated arrays of hybrid nanostructures comprised of two different elements, and particles with different sizes and shapes. Notably, such hybrid structures have found interest in a variety of applications that range from nanophotonics and sensing 57 , 58 to (photo)catalysis 59 , 60 . As the final step, to hint at the new possibilities in surface-templated synthesis enabled by our PMMA-layer approach and the concept of controlling the exposed seed surface area, we demonstrate as a first example the selective growth of a single Pd nanocube at the tip of a polycrystalline Au nanocone seed with nominally 100 nm base diameter and 80 nm height (Fig. 8 a). In these experiments, the Au seed cones were nanofabricated using Shrinking-Hole Colloidal Lithography, 61 which, in short, exploits the deposition of material at the rim of the hole of the used nanolithography mask, and the corresponding closing of the hole, as a means to grow cone-like structures using PVD. By exposing just a few nanometers of the tip cone, enabled by a PMMA layer of 75 nm thickness, Pd cube growth on top of the Au cone was indeed obtained as proof of concept. As the second example, we demonstrate the applicability of the PMMA-layer strategy on growth conditions that otherwise would be unattainable in colloidal nanoparticle synthesis by using a surfactant-free growth solution where CTAB was substituted by the ionic salt KBr. Under these conditions, the first observation was much faster reaction kinetics, indicated by the growth solution turning black (due to spontaneous formation of solution-nucleated secondary Pd nanoparticles) after just 1 min from the start of the synthesis, rather than the 5-to-10 min exposures to the growth solution used above with CTAB. We attribute this to a stronger complexing effect of CTAB with Pd ions in solution compared to just Br − , that slows down the reduction kinetics. 62 Interestingly, when we immersed arrays of the unannealed Pd seeds (cf. Figure 5 ) partially coated with 20 nm PMMA (and hence exposing a large surface area of the seeds) to this growth solution for 10 min, we observed the formation of large Pd nanostars with many narrow and well defined spikes (Fig. 8 b and S17). These highly spiky nanostars turned into nanostars with only just a few spikes when arrays of annealed Pd seeds (cf. Figure 4 ) also covered by a PMMA layer of 20 nm were used instead (Fig. 8 c). Alternatively, they turned into concave-like nanocubes when the annealed Pd seeds were covered by a PMMA layer of 40 nm thickness to expose a smaller fraction of the seed surface (Fig. 8 d). Thus, the overall range of morphologies from nanoflowers to nanocubes previously described and discussed using the CTAB growth solution (cf Figs. 4 and 5 ) is here similarly observed with the KBr solution but with kinetically favored products like nanostars and concave nanocubes. Overall, these results corroborate that the surface-templated in-situ synthesis methodology we have developed here, can be extended to seeds of different sizes and compositions, as well as to different synthesis conditions, and therefore it has the potential to produce arrays of nanoparticles with morphologies and structures different from those explored here. Conclusions In this work, we have introduced regioselective seed growth enabled by a finely tuned PMMA sacrificial layer in surface-templated metal nanoparticle synthesis as a new paradigm for controlling the morphology and crystalline structure of in-situ grown nanoparticles. This PMMA layer is used to tailor the fraction of seed particle surface exposed to a growth solution and thereby define the growth mode of the particle independent from the seed. As we have shown in the first part of this work, this enables the in situ surface-templated growth of regular arrays of polycrystalline Pd nanoflowers, spiky nanostars and single crystalline nanocubes with over 90% yield, using identical growth conditions and tailored PMMA layers, with CTAB as the surfactant and nanolithography-fabricated regular arrays of morphologically poorly defined polycrystalline seeds. Furthermore, it made it possible to synthesize heterodimeric single crystalline Pd nanocubes at the tip of a large polycrystalline Au nanocone, as a first step towards seed-particle-material-independent surface templated nanoparticle synthesis. Finally, we have demonstrated the applicability of the PMMA-layer strategy to growth conditions inaccessible in colloidal synthesis in solution by generating arrays of surface-templated spiky Pd nanostars and concave nanocubes using a surfactant-free growth solution where CTAB was substituted by KBr. Mechanistically, we propose that this all can be understood as the PMMA layer confining the metal deposition to a small, selected region of the seed surface, whose size can be finely tuned by adjusting the PMMA layer thickness. In this way, we effectively decouple the growth mode of the nanoparticle from the size, morphology, crystallinity and composition of the seed since we reduce the number of defect sites exposed to the growth solution (or the area with a lattice mismatch in case of a different seed material) to below the critical level where irregular growth would occur. In essence, this approach shrinks the effective seed size, i.e., the exposed seed surface area, down to the typical sizes and crystallinity levels used in colloidal synthesis in solution. Looking forward, we predict that our presented concept will widen the practical applicability of surface-templated nanofabrication with rationally arranged metal particles with well-defined morphologies, crystallinity and composition, and pave the way for mechanistic studies of the factors that govern shape evolution in nanoparticle synthesis. Furthermore, combining our approach with other, more advanced templated surfaces, such as epitaxially oriented seed particles with tailored crystallinity, will certainly unlock new possibilities in in-situ nanoparticle growth. Methods Chemicals and Materials A N-type (100)-Si wafer of 500 μm thickness (SiMat) with 80 nm dry oxide layer thermally grown (Centrotherm) and diced in 1 x 1 cm chips was used as a substrate. For samples compatible with TEM characterization 25-nm-thick SiNx grids were used. 39 Palladium(II) chloride (≥99.9%, PdCl2, Sigma), L-ascorbic acid (AA) (99%, Sigma), cetyltrimethylammonium bromide (CTAB) (≥99%, Acros), and MilliQ water (18.2 MΩ) were used without further purification in the preparation of the growth solution. An aqueous stock solution of H 2 PdCl 4 (20 mM) was prepared by dissolving 35.5 mg of PdCl 2 in 2 mL of HCl (200 mM), stirring overnight at room temperature and finally adding 8 mL of MiliQ water. Pd seed particle nanofabrication Periodic arrays of highly polycrystalline disk-like Pd seeds of 125 nm in diameter and 30 nm in height were fabricated on oxidized silicon substrate following a standard procedure described previously. 38 The procedure consists of thermally evaporating (Kurt J. Lesker PVD 225) a 30 nm Pd layer on a mask previously defined on the substrate surface by electron beam lithography (EBL, RAITH EBPG 5200). The EBL-mask contains openings through which the metal atoms reach the substrate surface. This is followed by a lift-off procedure in which the sample is immersed overnight in acetone then in IPA for 1 min and finally blow-dry in air. The EBL-mask dissolves in the acetone and the Pd that raises on its surface peels away into the solution leaving a pristine substrate surface with only Pd disk-like seeds at the predefined positions. Arrays of Pd seeds with smaller diameters from 110 to 40 nm at a constant height of 30 nm were fabricated on the substrate surface by varying the size of the openings in the EBL-mask. Annealing of the Pd seed arrays was carried out in a quartz tube furnace (Nabertherm R50/250/12) fluxed with a gas mixture of Ar with 2% of H 2 (300 mL min –1 ) and where the sample was placed in an alumina crucible, heated for 2 h at 600 °C and then allowed to cool to room temperature. PMMA layer deposition and etching To cover the Pd seed arrays with a PMMA layer of the desired thickness, we casted a few drops of a resist PMMA 950k A3 on the substrate surface template with the seed arrays. The sample was then spun at 3000 rpm for 60 s to obtain a uniform PMMA coating with a thickness of 140 nm and subsequently baked on a hot plate at 130 °C for 5 min in air. While the thickness of the PMMA layer initially deposited is not critical, it is important to cover the substrate surface with a PMMA layer thicker than the height of the seeds to form a homogeneous flat coating layer that completely covers the seeds. Etching of the PMMA layer down to the desired thickness was performed in successive steps, where the seed templated surface with the PMMA layer was exposed to oxygen plasma (25 W) in a RIE (Plasma-Therm) system for a specific time and then the PMMA layer thickness was measured by ellipsometry. By etching the PMMA layer further than the height of the Pd seeds, the seed top surface was progressively uncovered. Arrays of Pd nanocubes Pd nanocube arrays were produced from different Pd seed arrays in terms of seed-type (size and crystallinity) that were partially coated with a PMMA layer with a thickness experimentally optimized for each seed-type to maximize cube yield (Table S2) and incubated in a growth solution for 10 min. To growth the seeds in solution, we placed the substrate surface templated with the Pd seed arrays and the PMMA layer at the bottom of a 20 ml glass vial containing 2.75 mL of miliQ water and 0.25 mL of CTAB (200 mM) and heated to 40 °C in an oil bath as depicted in Figure S5 . The temperature was allowed to equilibrate for 5 min, after which we added 1 mL of freshly prepared ascorbic acid (75 mM). After allowing the temperature to equilibrate for another 2 min, we rapidly injected 1 mL of H 2 PdCl 4 (2 mM) preheated to 40 °C to initiate the reaction and then we let the particles to rest undisturbed in the growth solution for 10 min. As the reaction progressed, the colour of the growth solution changed from orange to dark brown in about 6 to 8 min due to the spontaneous nucleation in solution of secondary Pd nanoparticles. To terminate the reaction, the substrate was removed from the growth solution, rinsed with IPA, immersed in acetone for 1 min to dissolve the PMMA layer, immersed in IPA again for 1 min and finally blow-dry with N 2 . Alternatively, to extend the growth process, the substrate was removed from the growth solution after 10 min and immediately introduced in a new growth solution. Repeating this step 5 times resulted in Pd nanocubes up to 300 nm in size. Despite the significant nucleation of Pd nanoparticles in the growth solution, as indicated by the solution’s black colour at the end of the 10 min reaction time, their presence on the substrate surface was relatively low. On one hand, we attribute this to the low-energy and low-charged PMMA layer surface that limited the adsorption of these ‘unwanted’ by-products, and on the other hands, to the posterior dissolution of the PMMA layer that peeled away those nanoparticles eventually adsorbed, leaving a pristine substrate surface with only the nanoparticle arrays. Pd nanocubes on Au seeds were produces by replacing the nanofabricated Pd seeds with analogously fabricated Au seeds. Pd cube on Au cone tip . The protocol is the same as described for the Pd cubes but replacing the Pd seeds for 100 nm in diameter and 80 nm in height Au cones fabricated according to Syrenova et al. 61 and coated with a PMMA layer with a thickness adjusted accordingly to expose approximately the top 5 nm of the Au cone tip to the growth solution. Nanoflowers, nanostars and concave nanocubes Pd nanoflowers . Arrays of Pd nanoflowers were produced by subjecting arrays of unannealed and thus highly polycrystalline disk-shaped Pd seeds of 125 nm in diameter and 30 nm in height partially coated with a PMMA layer of 20 nm to the exact same growth conditions of the Pd nanocubes. Pd nanoflowers of sizes up to 500 nm were produced by consecutive growth steps in the exact same manner described for the Pd nanocubes. Pd nanoflowers on Au seeds were produces by replacing the nanofabricated Pd seeds by analogously fabricated Au seeds. Pd nanostars and concave Pd nanocubes. These structures were produced as described for the Pd nanoflowers and Pd nanocubes, respectibely, using a growth solution containing KBr (40 mM) instead of CTAB (10 mM) with no other changes in the protocols. Characterization and data analysis SEM imaging was carried out using a Zeiss Supra 55 operated at 15 kV. SEM images of grown Pd nanoparticle arrays were taken after peeling off the PMMA layer without further treatment of the samples. SEM images of Pd seed coated with PMMA have a 5 nm Cr layer deposited by PVD to facilitate imaging. Tilted SEM images were taken at 60°. Mean particle diameter and roundness were calculated by averaging the diameter and roundness of a random population of at least 100 particles using ImageJ. 150 consecutive particles were visualized by SEM and evaluated by eyes to determine the cube yield in the arrays. TEM imaging and selected area electron diffraction analysis were carried out in a FEI Tecnai T20 LaB6 operated at 200 kV. Samples were prepared on a compatible 25-nm-thick SiNx membrane instead of the standard oxidized silicon substrate. 39 The height of the Pd seeds before annealing is assumed to be the same as the thickness of the PVD-deposited Pd layer during the nanofabrication process (30 ± 0.5 nm). The height of the Pd seeds after annealing was determined by AFM images using a Bruker Dimension 3100 for measurements and Nasoscope v6.12. program for analysis. No less than 100 particles were counted to determine mean particle height. PMMA layers thickness was measured by ellipsometry with a A J.A. Woollam RC2 and represents the average of three measurements taken at substrate surface positions separated by 50 µm. Declarations Data Availability The underlying data for this publication are available at Zenodo, 10.5281/zenodo.15479860 Corresponding Authors Jordi Bagaria − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0002-5788-3145; Email: [email protected] Christoph Langhammer − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0003-2180-1379; Email: [email protected] Authors Jordi Piella − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0002-5788-3145 Joachim Fritzsche − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0001-8660-2624 Carl Anderson − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/ 0009-0007-7216-4433 Christoph Langhammer − Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0003-2180-1379; Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements This research has received funding from the Wenner-Gren Foundation via contract UPD2020-0287, the Marie-Sklodowska-Curie Actions Fellowship (MSCA) under the European Union’s Horizon Europe research and innovation program (DOI 10.3030/101032880/FASINA) and the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation program (101043480/NACAREI). Part of this work was carried out at the Chalmers MC2 cleanroom facility and at the Chalmers Materials Analysis Laboratory (CMAL). We also thank Prof. Kasper Moth-Poulsen for fruitful discussions and support in the early phases of this project References He M-Q, Ai Y, Hu W, Guan L, Ding M, Liang Q. Recent Advances of Seed-Mediated Growth of Metal Nanoparticles: from Growth to Applications. Advanced Materials 35 , 2211915 (2023). Xia Y, Gilroy KD, Peng H-C, Xia X. Seed-Mediated Growth of Colloidal Metal Nanocrystals. 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Janus Nanoparticles: From Fabrication to (Bio)Applications. ACS Nano 15 , 6147-6191 (2021). Additional Declarations There is NO Competing Interest. Supplementary Files Piellaetal.SI.docx Supplementary info for publication Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6715528","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":460911298,"identity":"2e28544a-a292-44bf-b430-26ea0cce5cfc","order_by":0,"name":"Christoph Langhammer","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7klEQVRIiWNgGAWjYDACdiBmbGBg4GfgYWB4QJQWZqgWyQaglgSStBgcIFYLfzPzs4dfd9hFGx8/e/BDYo5NvnwD+0O8DpQ4zGZuLHsmOXfbmbxkicRtaZYbDvAYG+DTYsDMYCYt2cacu+0GjwFQy2EDAwYeNgn8Wti/AbXU526ewWP8I3HbfwOgw57/wK+Fx0zyY9vh3A0SPGZAWw4YMBxgMMOnA+gXnjJpxrbjuTPO5JhZJG5LNjA4zGOM12H87e3bJH+2Vef2t58xvvFxm52BfHv7ww94rQECZh5ULiH1QMCI17ejYBSMglEwCgBqukb9CUKLrgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-2180-1379","institution":"Chalmers University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Christoph","middleName":"","lastName":"Langhammer","suffix":""},{"id":460911299,"identity":"a3ed82b2-fc0a-4815-81ea-b2878b2c360e","order_by":1,"name":"Jordi Piella","email":"","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Jordi","middleName":"","lastName":"Piella","suffix":""},{"id":460911300,"identity":"da755edf-48ad-4cea-ad2a-4693f03265b9","order_by":2,"name":"Carl Andersson","email":"","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Carl","middleName":"","lastName":"Andersson","suffix":""},{"id":460911301,"identity":"a71f0e15-0561-4677-843a-fcaba4ec3e9c","order_by":3,"name":"Joachim Fritzsche","email":"","orcid":"https://orcid.org/0000-0001-8660-2624","institution":"Chalmers Univerity of Technology","correspondingAuthor":false,"prefix":"","firstName":"Joachim","middleName":"","lastName":"Fritzsche","suffix":""}],"badges":[],"createdAt":"2025-05-21 10:25:18","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6715528/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6715528/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83424739,"identity":"ab8a8776-eedb-4a4b-8ed6-f3be484d43dd","added_by":"auto","created_at":"2025-05-26 03:58:50","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":81746,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCharacterization of as-fabricated Pd seed arrays.\u003c/strong\u003e (a) SEM images taken at different magnifications of a square-lattice array of disk-shaped Pd seeds fabricated onto a standard oxidized Si substrate. (b) TEM image and (c) corresponding SAED pattern showing the highly polycrystalline structure of a representative Pd seed from an equivalent array fabricated onto a TEM-compatible SiNx membrane. (d) Top- and (e) tilted-view high magnification SEM images of a single Pd seed on the standard oxidized Si substrate. For SEM and TEM characterization of additional representative particles see \u003cstrong\u003eFigures S1 and S2 \u003c/strong\u003erespectively.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/8b848ee6fa1229f22813ebd8.jpg"},{"id":83424740,"identity":"8b7d07bb-5b21-4bf2-aaa8-59b821420cb5","added_by":"auto","created_at":"2025-05-26 03:58:50","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":101332,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCharacterization of 600 °C annealed Pd seed arrays.\u003c/strong\u003e (a) TEM characterization of two representative annealed Pd seeds from an array fabricated onto a TEM-compatible SiNx membrane. The SAED pattern of the particle on the left reveals a nearly single crystal with a high-angle grain boundary as indicated by the extra diffraction spots in the pattern and clearly visible in the corresponding TEM image (dotted yellow line). The presence of multiple crystalline defects in the annealed particle on the right is revealed by the corresponding high magnification TEM image of a selected area (dotted orange square). (b) SEM images of a Pd seed particle array on the standard oxidized silicon substrate after annealing and high magnification SEM images of a representative particle in top and tilted views. Histograms compare the top-view diameter (c) and circularity (d) distributions of the Pd seeds in the array before and after annealing. For TEM characterization of additional representative annealed Pd seeds see \u003cstrong\u003eFigure S3\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/bbfeccb5f1c81fa95ef379c4.jpg"},{"id":83425050,"identity":"933dceb6-d081-403a-859f-b7964f921cf0","added_by":"auto","created_at":"2025-05-26 04:14:50","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":107781,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExposed seed particle surface controlled by a PMMA layer.\u003c/strong\u003e (a) Tilted SEM images of an array of annealed Pd seeds embedded in a spin-coated 140 ± 1 nm thick PMMA layer (top) and etched down to 20 nm by O\u003csub\u003e2\u003c/sub\u003e plasma (bottom). The insets depict the process steps. b) Linear correlation of the PMMA layer thickness measured by ellipsometry with the etching time. Error bars reveal a homogeneous deposition and etching of the PMMA layer on the substrate surface by depicting the standard deviation of three thickness measurements taken at different positions on the chip’s surface. (c, d) AFM analysis of an uncoated Pd seed array after annealing (c) and equivalent seed arrays coated with PMMA layers of 60, 35 and 20 nm (d). AFM profiles show a flat surface for a PMMA layer thicker than the seed height and peaks corresponding to the seed’s non-coated fraction when the PMMA layer thickness is below the seed height. (e, f) Tilted SEM images taken at different magnifications of the Pd seeds fully coated (e) and only partially coated (f) with the PMMA layer.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/8c312f9dbd6bed516cd5ce8e.jpg"},{"id":83424748,"identity":"d67ec5ce-211b-4cf3-a5d6-d78d03d9d42a","added_by":"auto","created_at":"2025-05-26 03:58:50","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":155807,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of PMMA layer thickness on the morphology of Pd nanoparticles grown from annealed Pd seeds.\u003c/strong\u003e a) Schematic representation of the experiment design. b) Plot of grown particle yield (left axis) and cube yield (right axis) derived from SEM characterization (cf. \u003cstrong\u003eFigure S7\u003c/strong\u003e) as a function of PMMA layer thickness. The yellow-colored region denotes the mean seed height with standard deviation limits determined by AFM (cf. \u003cstrong\u003eFigure 3c)\u003c/strong\u003e. c) SEM images of an array of Pd nanocubes grown from Pd seeds partially covered with a 40 nm PMMA layer (case I in (a, b)). d) SEM images of an array of defective Pd nanocubes with diverse morphologies produced using a 21 nm PMMA layer (case II in (a, b)). In both cases, equivalent annealed Pd seeds arrays were subjected to the exact same growth solution for 10 min with the only difference being the PMMA layer thickness. e, f) High-magnification SEM images comparing selected representative particles from equivalent arrays immersed in the growth solution for 10 min (top row) and 30 s, 1 min and 2 min (bottom row) for case I (e) and case II (f). For high-magnification SEM characterization of additional selected particles see Figures S9 and S10 (case I) and S12 (case II). All SEM images are of samples after removing the PMMA layer. Pd seed equivalent to those shown in Figure 2 were used in all tests.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/d54198c9e393920aec13468d.jpg"},{"id":83424814,"identity":"c9fc0404-f9e5-475e-a222-43649b2be86b","added_by":"auto","created_at":"2025-05-26 04:06:50","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":100017,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of PMMA layer thickness on the morphology of Pd nanoparticles grown from non-annealed highly polycrystalline Pd seeds\u003c/strong\u003e. a) Schematic of the experiment design. b) SEM images taken at different magnifications of an array of Pd nanoflowers produced from a non-annealed disk-shaped Pd seed array (\u003cstrong\u003ecf. Figure 1\u003c/strong\u003e) with a 20 nm PMMA layer and 10 min immersion in the growth solution (scenario I in (a)). The inset depicts a zoom-in SEM image of a nanoflower partially detached from the substrate surface where the underlying fraction of the Pd seed that was coated by the PMMA layer and hence not exposed to the growth solution is clearly visible. c) High-magnification SEM images of a representative Pd seed and representative Pd nanoflowers grown in solution for 30 s, 1 min, 2 min and 10 min, respectively. d) SEM images taken at different magnifications of a sample equivalent to that shown in (b) but produced using a PMMA layer of the same thickness as the nominal height of the seeds (30 nm, case II in (a)). Inset green squares on the image on the left indicate positions with structures consisting of a Pd cube grown on top of the seed as shown in the high-magnification images on the right. All SEM images were taken after dissolving the PMMA layer except for the tilted images in (b) where PMMA layer is present.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/d84d2377bba7f4c84efbe7cb.jpg"},{"id":83424817,"identity":"4db09427-61a8-41e8-a331-9c04aae7fc59","added_by":"auto","created_at":"2025-05-26 04:06:50","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":153457,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of PMMA layer thickness on Pd cube yield for different seed sizes\u003c/strong\u003e. Arrays of Pd disks with decreasing diameter from 125 to 40 nm at a constant height of 30 nm were fabricated on the same chip, annealed at 600 °C, coated with a PMMA layer of the desired thickness and immersed in the growth solution for 10 min. (a-e) SEM images depict a representative seed particle of each array type after annealing, together with the average diameter, \u003cem\u003ed\u003c/em\u003e, and height, \u003cem\u003eh\u003c/em\u003e of the seeds derived from SEM and AFM analysis, respectively. Scale bars represent 100 nm. f-j) Plots show grown particle yield (left axis) and cube yield (right axis) in the arrays as a function of PMMA layer thickness. Trendlines are guides to the eye. Yellow shaded areas depict average seed height within standard deviation limits as indicated in (a-e). Note that (f) is the same plot as Figure 4b since this sample was located on the same chip and processed simultaneously. The panel on the right shows representative SEM images of the arrays for two PMMA layer thicknesses (40 and 20 nm) with the insets depicting the SEM images with particles color-coded according to their structure. (k, l) Tilted SEM images comparing two similarly oriented Pd cubes grown on Pd seeds of different sizes and with the PMMA layer adjusted accordingly for each size. The truncation of the Pd cube is closer to the SiO₂ surface and the underlying seed is not visible in the tilted image in (l) because of the smaller seed size and thinner PMMA layer used compared to (k).\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/d5707663bcbf43cdcb6c4990.jpg"},{"id":83424744,"identity":"e38029bf-af45-45af-83a6-12837ccc650a","added_by":"auto","created_at":"2025-05-26 03:58:50","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":42177,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIn situ templated growth of hybrid Pd/Au nanocrystals\u003c/strong\u003e. a) Array of Pd\u003cstrong\u003e/\u003c/strong\u003eAu nanoflowers prepared by growing Pd on as-fabricated unannealed Au nanodisks (nominally 100 nm diameter and 30 nm height) partially covered by a 20 nm thick PMMA layer. On the left a STEM image of a representative particle on a sample fabricated on a TEM-compatible SiNx substrate is depicted, together with a corresponding EDX elemental map revealing the Au disk (red) and Pd flower grown on top (green). The SEM image on the right shows an array of hybrid Pd/Au nanoflowers on the standard oxidized SiO\u003csub\u003e2\u003c/sub\u003e wafler substrate. The inset depicts a zoom-in SEM image of a single particle in the array b) SEM image of an array of Pd nanocubes grown on Au nanodisk seeds using the exact same methodology as in (a) except for covering the Au disks with a PMMA layer of the same thickness as the nominal seed height (30 nm) to expose eventually only a very small fraction of the seed surface. The insets depict zoom-in SEM images of single particle in the array. Scale bar in all insets represents 100 nm.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/bdc2eb562ea93f365aad7f14.jpg"},{"id":83424746,"identity":"fcabd9f6-c410-46bd-954b-df4e4ebfa43d","added_by":"auto","created_at":"2025-05-26 03:58:50","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":66187,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAdditional possibilities in in-situ growth synthesis enabled by the PMMA-layer approach. \u003c/strong\u003ea) SEM images in top- (top) and tilted-view (bottom) of Pd cubes grown on the tip of Au cones enable by a PMMA layer exposing only the tip of the Au cone. b-c) Pd nanocrystals synthetized in a surfactant-free growth solution from arrays of as-fabricated (b) and annealed (c) Pd seeds with a PMMA layer of 20 nm that expose a large fraction of the seed surface, and from annealed Pd seeds with a PMMA layer of 40 nm that exposes a small fraction of the seed surface area (d). Scale bars in the tilted images in (a) and all insets in (b-d) represent 100 nm.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/06bcdec39ff01ed46af511fb.jpg"},{"id":85243543,"identity":"be26d7a9-9570-4837-8633-a7820326ab73","added_by":"auto","created_at":"2025-06-23 19:43:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2047134,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/657a5154-768e-41ad-a009-b2ff8b5f07ae.pdf"},{"id":83424749,"identity":"cea85221-ff33-451d-945d-9acca5c96eb7","added_by":"auto","created_at":"2025-05-26 03:58:53","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":167374145,"visible":true,"origin":"","legend":"Supplementary info for publication","description":"","filename":"Piellaetal.SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-6715528/v1/260eae31f6c24d2beae87ebe.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Seed-Particle-Independent In-situ Synthesis of Surface-Templated Shape-Selected Palladium Nanoparticle Arrays","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCombining bottom-up colloidal synthesis with top-down nanolithography methods represents the best means, if not the only, to rationally arrange complex noble metal nanoparticles with well-defined morphologies, crystallinity and composition on substrate surfaces. Traditionally, these two radically different, but highly complementary, approaches have been combined by first synthesizing the nanoparticles in solution, where colloidal chemistry in general, and the impressive-collection of seed-mediated growth protocols for the direct noble metal nanoparticle formation along orderly and well-controlled pathways in particular, have been the key concepts.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Subsequently, these colloidal nanoparticles are assembled on a templated surface that utilizes micro- or nanolithography-defined features in combination with a tailored external force to drive the colloidal nanoparticles to specific positions and have them anchor there.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Typically, this assembly process involves multiple, case-specific and lengthy steps of colloidal nanoparticle purification/centrifugation from the reaction mixture, ligand exchange/surface functionalization, and self- or directed-assembly to the surface positions. These steps are of limited scalability and reproducibility and hinder the crafting of large-area nanostructure arrays with high fidelity and true long-range order, free of registration errors.\u003c/p\u003e \u003cp\u003eContrary to this conventional synthesis-then-positioning paradigm, surface-templated \u003cem\u003ein-situ\u003c/em\u003e particle growth has recently emerged as a promising alternative where nanoparticles are formed directly at predefined positions on a templated surface that is immersed into a growth solution.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e To implement size and shape control, identical or slightly adapted seeded-mediated growth protocols from colloidal synthesis in solution are used and applied to supported seed particles that either are nucleated or nanofabricated by lithography methods on the substrate surface in well-ordered arrays.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e In this way, the rich and exciting chemistry of colloidal synthesis, and its associated functionalities, are brought to the substrate surface, while circumventing the aforementioned lengthy post-synthesis steps associated with the anchoring of the nanoparticles once synthesized.\u003c/p\u003e \u003cp\u003eAdditionally, numerous other interesting new synergies unique to surface-templated \u003cem\u003ein-situ\u003c/em\u003e particle growth have started to just being explored. For example, since particles grow from \u003cem\u003esupported\u003c/em\u003e seeds, colloidal stability does not need to be considered and reaction conditions that are \u0026lsquo;forbidden\u0026rsquo; in colloidal synthesis in solution (e.g., reactions in high ionic strength media in the absence of stabilizing surfactants) become accessible.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Another advantage stems from the possibility to quickly separate the growing nanoparticles from the reaction solution to stop the reaction at any time, or to redirect it by immersing the surface-attached nanoparticles into a new solution without the need for centrifugation or precipitation steps.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Similarly, the nanoparticles can be easily purified from the growth solution.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Inherent to the growth of surface-attached seeds is also the truncation of the particle geometry at the substrate interface.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e This, in turn, improves adhesion of the nanostructures, particularly of those with geometries like spheres or branched structures that otherwise will have small contact areas.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Similarly, truncation reinforces the electronic coupling between the substrate and the nanoparticles.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Additionally, substrate-imposed epitaxial growth has been used as a means to control the crystallographic orientation of the nanoparticles.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e In summary, it is clear that surface-templated in-situ particle growth offers a myriad of opportunities that holds the potential to deliver complex nanostructures with morphologies and compositions inaccessible to standard colloidal synthesis, and with a control of the position of the nanoparticles on the surface unattainable by any other means.\u003c/p\u003e \u003cp\u003eThe key to all of this is the preparation of seed nanoparticles in arrays of choice on a specific substrate of interest, where both components are compatible with seeded-growth modes. For this purpose several methods exist, such as \u003cem\u003ein-situ\u003c/em\u003e reduction on chemically patterned surfaces\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, block copolymer lithography\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, templated solid-state dewetting of metal thin films\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e or nanolithography approaches where the seed material of choice is evaporated through a lithographically-defined mask (e.g., electron-beam lithography (EBL), nanoimprint lithography, colloidal lithography or photolithography). Out of these methods, EBL-based nanolithography is unrivalled in terms of enabling accurate control of seed particle position inside an array with well-defined array parameters. Furthermore, since no solvents, surfactants and other chemicals are used in the coalescence and growth process of the seed particles by means of physical vapor deposition (PVD), the obtained seeds also have clean/pristine surfaces.\u003c/p\u003e \u003cp\u003eAs the main drawback, however, nanolithography, as well all the other mentioned methods, typically produce rather large particles in the tens to hundred(s) of nanometer range that are highly polycrystalline and ill-defined in terms of crystallinity, morphology and faceting. This is problematic because it is widely accepted that the growth of shape-selected nanoparticles requires the initial nucleation of seeds with specific and well-defined crystalline structures.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e To overcome this limitation, thermal annealing steps at temperatures close to the used metals\u0026rsquo; melting points (often\u0026thinsp;\u0026gt;\u0026thinsp;1000\u0026deg;C) have proven necessary to induce recrystallization of the surface-attached seeds into Wulff/Winterbottom-shaped nanocrystals prior to growth in solution.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e However, in most cases, such high temperatures trigger chemical reactions between the metal seeds and the substrate surface and are therefore highly problematic. Furthermore, withstanding such high temperatures demands special and expensive substrates, such as sapphire, SrTiO\u003csub\u003e3\u003c/sub\u003e or MgO, that are of limited practical use. To this end, Neretina et al. produced ̴ 50 nm Au, Ag, Cu, Pt and Pd single crystalline nanoparticle arrays by thermal processing of PVD nanofilms and nanodisks and used them as seeds for the surface-templated growth of shape selected nanostructures. They employed a dynamic templating process where sacrificial Sb and/or Bi layers were deposited between the substrate surface and the seed particles to facilitate metal dewetting and restructuring at lower temperatures.\u003csup\u003e1\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, 3\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e However, even with these sacrificial layers, annealing temperatures above 750\u0026deg;C were required, with Pd requiring 1100\u0026deg;C due to its high melting temperature of 1555\u0026deg;C.\u003c/p\u003e \u003cp\u003eIn this work, we introduce an alternative route to overcome the polycrystalline seed problem in surface-templated colloidal particle synthesis, by introducing a methodology that allows the \u003cem\u003eselective exposure\u003c/em\u003e of only a fraction of the seed particle surface to the growth solution. Thereby, it provides access to a single-crystal growth mode, despite the distinct polycrystallinity \u0026ndash; and if desired large size \u0026ndash; of the seed. This is enabled by a thin poly(methyl methacrylate) (PMMA) layer spin-coated on the substrate surface decorated with nanolithography-fabricated seed particle array. The thickness of this PMMA layer is precisely adjusted by plasma etching to expose only the desired fraction of the seed surface. Using this approach, we show on the example of Pd how the growth mode of identical surface-templated polycrystalline seeds can be progressively unlinked from their size and crystalline structure as more of the seeds\u0026rsquo; surface, together with associated grain boundaries and crystal defects, is covered by the PMMA layer. Thereby, it enables formation of structures that range from polycrystalline nanoflowers to single crystal nanocubes with high yield simply by controlling the fraction of the seed surface that is exposed to the growth solution. Furthermore, we extend this methodology to the growth of Pd nanostars, where the PMMA layer thickness determines the number of spikes, and to multimetallic hybrid structures with seed particles made from a different metal than the grown crystal, such as the selective growth of a Pd nanocube on the tip of a large Au nanocone.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSeed particle nanofabrication by Electron Beam Lithography (EBL)\u003c/h2\u003e \u003cp\u003eTo establish the surface-templated \u003cem\u003ein situ\u003c/em\u003e particle growth protocol with a PMMA layer, we first EBL-fabricated regular arrays of disk-shaped Pd seed particles of 125 diameter and 30 nm thickness onto an oxidized silicon wafer substrate \u0026ldquo;chip\u0026rdquo; using a well-established protocol of PVD of Pd trough a EBL-fabricated mask (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e).\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e As anticipated, this methodology yielded disks with highly polycrystalline structures composed of randomly oriented small crystallites or \u0026ldquo;grains\u0026rdquo; of 10\u0026ndash;15 nm, as clearly revealed in transmission electron microscopy (TEM) images (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cb\u003eFigure S2\u003c/b\u003e) and selected-area electron diffraction (SAED - Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) of the Pd seeds from an array fabricated onto a SiN\u003csub\u003ex\u003c/sub\u003e Transmission Electron Microscopy (TEM) membrane,\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e as well as in top view (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) and tilted (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee) high resolution Scanning Electron Microscopy (SEM) images of the corresponding seeds on the standard oxidized Si substrate.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo, in the next step, investigate the extent to which a transformation of the crystallinity of the Pd seeds is possible by thermal treatment, we annealed them for 2 h in an atmospheric pressure tube furnace flushed with a mixture of 98% Ar and 2% H\u003csub\u003e2\u003c/sub\u003e to prevent Pd oxidation.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e Under these conditions, the maximum temperature that we could reach without observing evidence of sample deterioration caused by thermally induced chemical interactions of the Pd seeds with the SiO\u003csub\u003e2\u003c/sub\u003e/Si substrate was 600\u0026deg;C (\u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and Figure S3\u003c/b\u003e and corresponding discussion).\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e As expected, even this highest annealing temperature barely transformed a few of the seed particles into single crystals, and their majority remained polycrystalline, typically with a few large crystalline grains and the presence of defects, including high- and low- angle grain boundaries, twining and stacking faults, that varied from particle to particle (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and extended in \u003cb\u003eFigure S4\u003c/b\u003e). As for the morphology of the annealed seeds (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb-d), the slight contraction in diameter from 125\u0026thinsp;\u0026plusmn;\u0026thinsp;2 nm to 110\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm and concomitant adoption of a geometry best described as a somewhat faceted (truncated) oblate spheroid reflects their tendency to restructure towards more isotropic Winterbottom shapes at elevated temperatures. At the same time, the fact that many of them had non-equiaxed top-view cross-sections with slightly broader diameter and circularity distributions compared to the pre-annealed seeds (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, d) evidence that this restructuring process is not thermodynamically completed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExposed seed surface control by the PMMA layer\u003c/h3\u003e\n\u003cp\u003eIt is reasonable to assume that the above described Pd seeds are poor candidates for the seeded-growth synthesis of shape-selected nanostructures in high yield, as this generally requires (nearly) single crystal seeds with specific and perfectly defined structures.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Here, however, we challenge this paradigm by postulating that if (unwanted) parts of an \u0026lsquo;imperfect\u0026rsquo; seed particle could be \u0026lsquo;hidden\u0026rsquo; from the growth solution, for instance crystalline defects and grain boundaries, their impact on the growth process could potentially be eliminated, such that it would then be possible for structurally different seeds to sustain the same grow mode.\u003c/p\u003e \u003cp\u003eTo demonstrate this concept, we have developed a methodology that in a controlled fashion covers a tailored fraction of the seed particle surface with a PMMA layer. This polymeric layer is first deposited by spin-coating on the substrate surface templated with the seeds at a thickness significantly higher than the seed height, to ensure a uniform flat covering of the surface. For the seeds tested here (up to 50 nm in height, \u003cem\u003evide infra\u003c/em\u003e), we have used a PMMA layer thickness of 140 nm. Subsequently, this PMMA layer is etched down by O\u003csub\u003e2\u003c/sub\u003e plasma to a specific thickness below the seed height at which it exposes the desired fraction of the seed\u0026rsquo;s surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea\u003cb\u003e)\u003c/b\u003e.\u003c/p\u003e \u003cp\u003eAs key aspects of this methodology, we first note the high uniformity of the PMMA layer thickness and its lineal dependence on the plasma etching time that allows excellent control of this parameter (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). We also highlight (i) the high monodispersity in terms of height of the annealed Pd seeds which together with (ii) the homogeneous etching and flatness of the PMMA layer surface unlock exposing the top of the seed particles with high accuracy, as revealed by AFM profiles showing peaks with an average height of 46\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm for the uncoated annealed Pd seeds (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), and a flat profile and peaks with average heights of 7\u0026thinsp;\u0026plusmn;\u0026thinsp;3 and 24\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm for equivalent seeds coated with PMMA layers with thickness adjusted to 60, 35 and 20 nm respectively \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed-f\u003cb\u003e)\u003c/b\u003e. Note here that the average height of the annealed Pd seeds after annealing (46\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm) is larger than the nominal height of the as-fabricated Pd seeds (30 nm), and that this well-correlates with the dewetting of Pd seeds after annealing discussed above (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). As a final remark, we noticed systematic discrepancies of 2\u0026ndash;4 nm between the seed height measured by AFM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) and the thickness of the deposited PMMA layer measured by ellipsometry plus the height of the uncoated seed fraction measured by AFM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). This, we attribute to the softness of the PMMA layer potentially yielding an apparent lower film thickness in the AFM measurements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eTemplated Pd nanocrystal growth on annealed seed particles\u003c/h3\u003e\n\u003cp\u003eTo overgrow the supported seeds partially covered by PMMA, we immersed them in an aqueous solution containing ascorbic acid (AA) as the reducing agent and cetyltrimethylammonium bromide (CTAB) as a complexing and capping agent at 40\u0026deg;C. Then, we fast-injected a PdCl\u003csub\u003e2\u003c/sub\u003e solution as the precursor to start the growth process and let the seeds rest in this growth solution for 10 min (see experiment setup in \u003cb\u003eFigure S5\u003c/b\u003e). To this end, the reduction of Pd ions by AA in the presence of CTAB has been widely used in the colloidal synthesis of single-crystal Pd nanocubes, where the cube-forming growth process is driven by the preferential adsorption of the Br\u003csup\u003e\u0026minus;\u003c/sup\u003e from CTAB onto the (100) facets of Pd.\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e As a key point here, we highlight that in these processes, solution-nucleated sub-20 nm seeds with a well-defined crystallinity (typically single-crystal truncated octahedron) were critical for a sizable yield of Pd nanocubes.\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e Here, instead, our supported seeds are significantly larger and distinctly polycrystalline (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), but with the key feature that only a portion of the seed surface, controlled by the PMMA layer thickness (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), is accessible to the growth solution. Specifically, to investigate the critical role of the exposed surface area of the seeds in the nanoparticle\u0026rsquo;s growth mechanism, we exposed nominally identical arrays of the Pd seeds coated by PMMA layers with thickness systematically reduced from 60 nm down to 12 nm to the same growth solution and analyzed the structure of the resulting Pd nanoparticles after 10 min. Because the average seed height is 46\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), this PMMA thickness range stretches from fully covering the seeds to exposing most of their surface area (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The use of PMMA layers thinner than 10 nm was hampered by the arrays becoming highly contaminated by secondary nanoparticles nucleated in solution that adsorbed on the substrate surface due to their affinity for the SiO\u003csub\u003e2\u003c/sub\u003e surface, thereby indicating that the PMMA no longer prevented it. The use of PMMA layers thinner than 10 nm was hampered by the arrays becoming increasingly contaminated by adsorbed secondary Pd nanoparticles nucleated in solutions. We attribute this to a favored interaction of these solution-nucleated nanoparticles for the SiO\u003csub\u003e2\u003c/sub\u003e surface,\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e which is no longer prevented by the thin PMMA layer. Furthermore, to minimize potential oxidation of the Pd seeds by O\u003csub\u003e2\u003c/sub\u003e plasma used to adjust the PMMA thickness, we use a very low power (25 W) during the etching the PMMA layer (\u003cb\u003eFigure S6\u003c/b\u003e and corresponding discussion about the impact of O\u003csub\u003e2\u003c/sub\u003e plasma on the Pd seed surface).\u003c/p\u003e \u003cp\u003eRemarkably, some of the seed arrays produced Pd nanocubes in high yield, with the yield being strongly correlated with the thickness of the PMMA layer. Plotting the yields of grown particles and cubes in the arrays derived from SEM characterization as function of PMMA thickness indeed reveals clearly discernible growth regimes (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, see \u003cb\u003eFigure S7\u003c/b\u003e for corresponding SEM images). Specifically, on one hand, almost no seeds sustained a growth process for PMMA layers above 50 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, \u003cb\u003ecase 0\u003c/b\u003e), which indicates that even a PMMA coating only slightly thicker than the average seed height (46\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm) forms an impermeable barrier that prevents the monomers in solution from reaching the seed surface (\u003cb\u003eFigure S7a, b\u003c/b\u003e). Notably, the number of cubes formed in these samples was consistent with the number of nanoparticles grown. When the PMMA layer was adjusted to 40 nm, which is only 6 nm below the average seed height (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, \u003cb\u003ecase I\u003c/b\u003e), the number of seeds that sustained a growth process and formed a cube on top increased dramatically and the cube yield in the array reached 90%, while only a residual fraction presented other (irregular) shapes (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and \u003cb\u003eFigures S7c, d\u003c/b\u003e and \u003cb\u003eS8\u003c/b\u003e). Notably, this yield is in par with the state-of-the-art Pd cube synthesis fully in solution.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e Interestingly, further decreasing the PMMA thickness down to 20 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, \u003cb\u003ecase II\u003c/b\u003e), and hence exposing a large fraction of the seed surface to the growth solution, exponentially dropped the cube yield, while the overall yield of particles growing remained close to 100% (\u003cb\u003eFigure S7e-h\u003c/b\u003e). Specifically looking at the morphology of these particles (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) reveals irregular anisotropic structures with configurations that vary from particle to particle and thereby likely reflect the inherent diversity in the seed particle structures (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) that now is exposed to the growth solution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further study the growth mechanism in the different regimes identified above and investigate how the growing particles evolve at the early stage in detail, we exposed equivalent Pd seed arrays partially covered with PMMA layer thickness of 40 and 20 nm (assuming that the Pd seeds have the shape of a spherical cap, this represents covering roughly 90% and 40% of their surface area, respectively) to the growth solution for 30 s, 1 min and 2 min, in addition to the 10 min exposed samples discussed above, and compared the respective structure of representative particles imaged by SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, f).\u003c/p\u003e \u003cp\u003eStarting with Pd nanoparticles grown on seeds partially covered by the 40 nm PMMA layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, extended in \u003cb\u003eFigures S9\u003c/b\u003e), we find that single cubes emerge locally from the small, exposed surface at the very top of the seeds, and that, in consequence, these cubes are initially much smaller than the seed size. Remarkably, after 30 s, the cubes on top of the seeds are only 10\u0026ndash;15 nm (leftmost image in the bottom row of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee), that is roughly an order of magnitude smaller than the seed diameter. These single cubes grow upon longer reaction times, at which it becomes apparent that the shape is truncated at a distance from the substrate that coincides with the thickness of the PMMA layer (tilted SEM images in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee and S9). Representative particles corresponding to the fraction of Pd nanoparticles in the array that contain structures different from a single cube (multiple misoriented cubes, twinned cubes, irregular shapes, etc.) are shown in \u003cb\u003eFigure S10\u003c/b\u003e. We attribute this residual fraction to the specific case where seed crystal defects are located specifically at the small top area exposed by the PMMA. Notably, since the cube structure is preserved throughout the growth process, iterative immersion into fresh growth solution(s) in 10 min steps enabled the synthesis of cubes as large as 300 nm (\u003cb\u003eFigure S11\u003c/b\u003e).\u003c/p\u003e \u003cp\u003eA second interesting aspect is the orientation and truncation of these cubes, that are distinct for the individual particles. To this end, it has been reported that deposition of Pd atoms from solution on Pd seed is epitaxial.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e Hence, one possible reason for the different cube orientations is that the (100) surface of the seed on which the cube grows orient differently with respect to the substrate surface on different seeds, as expected for the amorphous substrate used here. Alternatively, cubes do not only nucleate and grow on (100) surfaces but also on other low index facets abundant on the annealed seeds that may be exposed by the seed surface as well.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eSimilar analysis of the Pd nanoparticles produced with the 20 nm PMMA layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, extended in \u003cb\u003eFigure S12\u003c/b\u003e) reveals that metal deposits initially on a large area on the seed surface, as expected for a larger exposed seed area, and that this deposition is not conformal layer-by-layer but starts with the nucleation of multiple small cubes. For a small fraction of the seeds, these early cubes are all oriented in the same direction and eventually merge at longer reaction time into a single larger cube. However, in most cases, cubes in different regions of the same seed orient in different directions and at the boundary of these regions fast-growing highly anisotropic semi-planar structures (petal-like) arise. Particles characterized by these irregular structures, often spreading from one side of the particle to the other, with regions with more cube-like structures (top row of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef), clearly indicate growth pathways that override the thermodynamic tendency to form highly symmetric faceted structures by locally activating the rapid growth of branched structures along certain crystallographic directions. This requires the formation of nucleation points from which the accelerated highly directional growth occurs. Crystal defects, such as grain boundaries, twinning and stacking faults have been identified as such nucleation points in numerous protocols for the synthesis of highly anisotropic nanoparticles, such as nanostars and nanoplates, whose synthesis relies on the use of defective seed particles\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Hence, we argue that the presence of similar defects in our seeds (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), when exposed to the solution by a thinner PMMA layer, are likely the main reason for the observed irregular growth mode. Finally, seed arrays exposed to iterative immersions into fresh growth solution(s) in 10 min steps presented mainly particles with systematically increasing petal size and only a residual fraction of single large cubes (\u003cb\u003eFigure S13\u003c/b\u003e); which contrasts with the large cubes seen in high yields in equivalent iterative growth samples coated by the thicker 40 nm PMMA layer (\u003cb\u003eFigure S11\u003c/b\u003e).\u003c/p\u003e\n\u003ch3\u003eTemplated Pd nanocrystals growth on unannealed seed particles\u003c/h3\u003e\n\u003cp\u003eTo further evaluate the influence of crystalline defects on the particle growth modes and corroborate the general hypothesis developed above, we exposed the non-annealed, and thus highly polycrystalline and defect-rich, 125 x 30 nm Pd seeds (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to exactly the same growth conditions used above for the annealed Pd seeds, and again we focused on two different PMMA layer thicknesses: (I) 20 nm tailored to expose a sizable fraction of the seed surface, and (II) 30 nm tailored to only just expose a very small surface area at the very top of the seed. (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eFocusing first on scenario (I), SEM imaging of the nanoparticle arrays obtained after 10 min immersion in the growth solution reveals a distinctly flower-like morphology that is composed of many crystalline petals (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). As predicted by our hypothesis, these petals are much more abundant than those seen in the corresponding Pd nanoparticles produced from the annealed seeds (\u003cb\u003ecf.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Furthermore, characterization of the \u0026ldquo;nanoflowers\u0026rdquo; at different stages reveals a heterogeneous deposition of Pd on the highly polycrystalline Pd seeds and the formation of small crystals (petal-like) at many sites that are oriented in different directions and grow over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Hence, the reason for this flower-like morphology must be the higher abundance of crystalline grains and defects in as-fabricated seeds compared to annealed seeds, as they likely define the sites and extent of petal nucleation. Notably, here also, we were able to increase the size of the nanoflowers up to several hundred nanometers by consecutive growth steps (\u003cb\u003eFigure S14\u003c/b\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFocusing on scenario (II), SEM analysis of the Pd nanoparticles grown using a PMMA layer with the same thickness as the nominal height of the seeds indeed reveals that for the unannealed seeds that exhibit any growth at all (20%), a sizable fraction has formed a single cube on top (66% of the growing seeds - Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Since the only difference compared to the nanoflowers grown along scenario (I) is the thickness of the PMMA layer, and thus the extent of the seed surface exposed to the growth solution, this is the critical factor. Hence, by decreasing the exposed surface area of the seed, it is possible to progressively decouple the structural/morphological properties of the seed from its growth mode and, as a consequence, from the structure of the crystal grown on top. This is here demonstrated by the formed dimeric polycrystalline disk \u0026ndash; single crystalline cube structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Notably, even the chemical surface state of the two particles is different since the grown Pd cube is covered by the shape directing agent, here CTAB, whereas the surface of the underlying Pd disk is clean after removing the protecting PMMA layer. As a final point, we note that in this scenario (II) the overall yield of grown particles was low when the yield of cubes is significant (leftmost image in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). On one hand, we attribute this to the high polycrystallinity of the unannealed Pd seeds, and hence the need to only expose a very small fraction of the surface to be compatible with the single cube growth. On the other hand, we identify the flat disk shape of the unannealed seeds as a key reason for the overall low yield since it requires very accurate tuning of the PMMA layer thickness to expose the \u0026ldquo;just right\u0026rdquo; small fraction of surface. In this situation most of the seeds became fully covered by the PMMA layer. These two factors thus render the window of transition between cube to nanoflower growth modes in these highly polycrystalline Pd seeds very narrow.\u003c/p\u003e\n\u003ch3\u003eSeed particle size independence of templated growth\u003c/h3\u003e\n\u003cp\u003eIn the same way as regioselective growth on Pd seeds enables the seed-particle-\u003cem\u003estructure\u003c/em\u003e-independent synthesis of Pd cubes, we postulate that it enables the synthesis on \u003cem\u003eseed particles of any size\u003c/em\u003e, if the PMMA thickness is adjusted accordingly to expose the \u0026ldquo;right\u0026rdquo; amount of seed surface area for each case. To demonstrate this, in addition to the 125 \u0026plusmn; 2 nm Pd disk seeds investigated above (\u003cb\u003ec.f.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e, we had nanofabricated on the same substrate arrays of Pd disks with systematically smaller diameters of 110 \u0026plusmn; 1, 95 \u0026plusmn; 2, 70 \u0026plusmn; 1 and 40 \u0026plusmn; 2 nm at a constant height of 30 nm. Hence, they were also annealed at 600\u0026deg;C to induce recrystallization and the slight Winterbottom-reshaping necessary to obtain high cube yield arrays. The average seed size measured by SEM (diameter) and AFM (height) of the Pd seeds after annealing were: 110 \u0026plusmn; 3 nm, 103 \u0026plusmn; 2 nm, 88 \u0026plusmn; 2 nm, 67 \u0026plusmn; 1 nm and 42 \u0026plusmn; 3 nm, and 46 \u0026plusmn; 3 nm, 47 \u0026plusmn; 2 nm, 41 \u0026plusmn; 2 nm, 39 \u0026plusmn; 1 nm and 34 \u0026plusmn; 1 nm, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-e\u003cb\u003e)\u003c/b\u003e. Here again it is important to note the high degree of monodispersity of all seed sizes. Subsequent exposure of the Pd seeds covered by systematically reduced PMMA layer thicknesses ranging from 60 to 12 nm to the growth solution for 10 min revealed an optimal exposed surface area for all seed sizes that enabled growth of Pd cubes with very high yields (up to 90% for the largest seeds and up to 97% for the rest) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef-j, \u003cb\u003eand Figures S15 and S16\u003c/b\u003e for large-size SEM images of selected Pd cube arrays). Hence, globally, all tested seed sizes showed a similar trend.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt the same time, interesting more subtle differences can be noted in the dependence of the cube yield on the PMMA layer thickness for the different seed sizes. For example, the optimal PMMA thickness \u003cem\u003erange\u003c/em\u003e that enables a high cube yield broadens as the size of the Pd seeds decreases (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef-j). In other words, the growth pathway depends less on changes in the PMMA layer thickness as the seeds are smaller. The reason for this is likely to be related to an increase in the degree of (mono)crystallinity and a decrease in the overall defect density induced by thermal annealing as the seed size decreases. It is well known that particle size plays an important role in recrystallization upon annealing, and that small particles recrystallize more easily than their larger counterpars.\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAs a second aspect, we note that the unit change in exposed seed surface area per unit change in thickness of the PMMA layer depends on the size and shape of the seed. Accordingly, the correlation between the thickness of the PMMA and the exposed seed area is stronger for particles with large aspect ratio (like our largest seeds, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea) than for more spherical seeds (like our smaller seeds, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). Hence, this is likely the second reason for the negative correlation between cube yield and PMMA layer thickness as the size of the seeds decreases.\u003c/p\u003e \u003cp\u003eTo summarize, it becomes apparent that the cube growth mode is determined by structural factors of the exposed seed fraction rather than the overall seed size and structure. Furthermore, for the same reason, large differences in seed size should not be reflected in the final size of the cubes for a given growth time if the area exposed by the PMMA layer is of similar size. Indeed, this is confirmed when comparing two representative cubes grown on our largest and smallest seeds, respectively, as they barely show differences in size (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ek, l).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn traditional colloidal synthesis in solution, nanostructures with radically different morphologies, such as polycrystalline flowers and single-crystalline cubes, require different seed particles and/or synthesis conditions (directing agents, reaction kinetics, etc.).\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e It is thus indeed remarkable that we have produced this variety of morphologies employing the same initial seed and growth conditions, solely by tuning the fraction of seed surface exposed to the growth solution, in this case, by adjusting the thickness of the PMMA layer. As the mechanism, we postulate that the smaller the exposed area of a \u0026lsquo;defective\u0026rsquo; seed, the less likely it is to expose (a sizable number of) crystal defects to the growth solution that could contribute to an irregular growth process. In essence, mechanistically this is similar to physically shrinking the seed to the typical sizes and crystallinity used in colloidal synthesis.\u003c/p\u003e \u003cp\u003eWhile our results indeed nicely corroborate this proposed mechanism so far, they do not fully exclude alternative scenarios. For example, Mirkin et al. demonstrated that Pd cubes can grow from supported 12 nm multiple twinned Au seeds under synthesis conditions very similar to ours.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e While, in principle, twinned seeds are not compatible with the cube growth mode\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e, they described the growth process as being characterized by the initial deposition of Pd at low-coordination high-surface-energy sites, leading to the multiple nucleation of Pd around facet edges and corners. As more Pd subsequently deposits on the seed, these discrete nucleation islands initially formed merge into a primitive shell that develops (100) facets upon growth, as determined by the face-selected adsorption of the Br- ions of the used CTAB capping agent, and it does not necessarily form an epitaxial crystalline relation with the seed. The main implication of this growth mechanism for our work at hand here is that the formation of a cube may not necessarily require an epitaxial relation with the seed. Applying this reasoning to our case, one or even multiple defects in a small enough surface area exposed to a growth solution (as well as, hypothetically, a different seed material with significantly different lattice constant) may not prevent a cube to still nucleate and grow since the external conditions imposed by the growth solution and the used capping agents may compensate the lattice mismatch induced by the defects, provided they are limited in number \u0026ndash; as ensured by a small exposed seed surface area. For this particular case, we propose a final mechanism where the cube yield is strongly dependent on the absolute exposed seed surface area and only weakly dependent on said area\u0026rsquo;s defectuousness, provided the exposed defectuous area is small enough.\u003c/p\u003e \u003cp\u003eTo further verify this mechanism with particular focus on the proposed only weak dependence on the crystallinity of the small exposed seed surface, and a first step towards using a different seed material (for which, however, it has still been demonstrated that epitaxial growth of Pd is possible)\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e, we exchanged the polycrystalline disk-shaped unannealed Pd seed particles (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) to corresponding unannealed Au seeds, nanofabricated into arrays using the same EBL procedure, and subsequently coated them with 20 nm and 30 nm PMMA layers, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). As anticipated, 10 min exposure to the same Pd growth solution used above yielded Pd nanoflowers on the polycrystalline Au seeds for 20 nm PMMA exposing a large fraction of the seed surfaces area (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), and a single Pd cube on the polycrystalline Au seeds for 30 nm PMMA covering nearly all the seed surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), thereby corroborating our overall proposed mechanism and also showcasing the ability of our approach to generate surface templated arrays of hybrid nanostructures comprised of two different elements, and particles with different sizes and shapes. Notably, such hybrid structures have found interest in a variety of applications that range from nanophotonics and sensing\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e to (photo)catalysis\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs the final step, to hint at the new possibilities in surface-templated synthesis enabled by our PMMA-layer approach and the concept of controlling the exposed seed surface area, we demonstrate as a first example the selective growth of a single Pd nanocube at the tip of a polycrystalline Au nanocone seed with nominally 100 nm base diameter and 80 nm height (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). In these experiments, the Au seed cones were nanofabricated using Shrinking-Hole Colloidal Lithography,\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e which, in short, exploits the deposition of material at the rim of the hole of the used nanolithography mask, and the corresponding closing of the hole, as a means to grow cone-like structures using PVD. By exposing just a few nanometers of the tip cone, enabled by a PMMA layer of 75 nm thickness, Pd cube growth on top of the Au cone was indeed obtained as proof of concept.\u003c/p\u003e \u003cp\u003eAs the second example, we demonstrate the applicability of the PMMA-layer strategy on growth conditions that otherwise would be unattainable in colloidal nanoparticle synthesis by using a surfactant-free growth solution where CTAB was substituted by the ionic salt KBr. Under these conditions, the first observation was much faster reaction kinetics, indicated by the growth solution turning black (due to spontaneous formation of solution-nucleated secondary Pd nanoparticles) after just 1 min from the start of the synthesis, rather than the 5-to-10 min exposures to the growth solution used above with CTAB. We attribute this to a stronger complexing effect of CTAB with Pd ions in solution compared to just Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, that slows down the reduction kinetics.\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e Interestingly, when we immersed arrays of the unannealed Pd seeds (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) partially coated with 20 nm PMMA (and hence exposing a large surface area of the seeds) to this growth solution for 10 min, we observed the formation of large Pd nanostars with many narrow and well defined spikes (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb and S17). These highly spiky nanostars turned into nanostars with only just a few spikes when arrays of annealed Pd seeds (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) also covered by a PMMA layer of 20 nm were used instead (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec). Alternatively, they turned into concave-like nanocubes when the annealed Pd seeds were covered by a PMMA layer of 40 nm thickness to expose a smaller fraction of the seed surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed). Thus, the overall range of morphologies from nanoflowers to nanocubes previously described and discussed using the CTAB growth solution (cf Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) is here similarly observed with the KBr solution but with kinetically favored products like nanostars and concave nanocubes. Overall, these results corroborate that the surface-templated in-situ synthesis methodology we have developed here, can be extended to seeds of different sizes and compositions, as well as to different synthesis conditions, and therefore it has the potential to produce arrays of nanoparticles with morphologies and structures different from those explored here.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this work, we have introduced regioselective seed growth enabled by a finely tuned PMMA sacrificial layer in surface-templated metal nanoparticle synthesis as a new paradigm for controlling the morphology and crystalline structure of \u003cem\u003ein-situ\u003c/em\u003e grown nanoparticles. This PMMA layer is used to tailor the fraction of seed particle surface exposed to a growth solution and thereby define the growth mode of the particle independent from the seed. As we have shown in the first part of this work, this enables the \u003cem\u003ein situ\u003c/em\u003e surface-templated growth of regular arrays of polycrystalline Pd nanoflowers, spiky nanostars and single crystalline nanocubes with over 90% yield, using identical growth conditions and tailored PMMA layers, with CTAB as the surfactant and nanolithography-fabricated regular arrays of morphologically poorly defined polycrystalline seeds. Furthermore, it made it possible to synthesize heterodimeric single crystalline Pd nanocubes at the tip of a large polycrystalline Au nanocone, as a first step towards seed-particle-material-independent surface templated nanoparticle synthesis. Finally, we have demonstrated the applicability of the PMMA-layer strategy to growth conditions inaccessible in colloidal synthesis in solution by generating arrays of surface-templated spiky Pd nanostars and concave nanocubes using a surfactant-free growth solution where CTAB was substituted by KBr. Mechanistically, we propose that this all can be understood as the PMMA layer confining the metal deposition to a small, selected region of the seed surface, whose size can be finely tuned by adjusting the PMMA layer thickness. In this way, we effectively decouple the growth mode of the nanoparticle from the size, morphology, crystallinity and composition of the seed since we reduce the number of defect sites exposed to the growth solution (or the area with a lattice mismatch in case of a different seed material) to below the critical level where irregular growth would occur. In essence, this approach shrinks the effective seed size, i.e., the exposed seed surface area, down to the typical sizes and crystallinity levels used in colloidal synthesis in solution.\u003c/p\u003e \u003cp\u003eLooking forward, we predict that our presented concept will widen the practical applicability of surface-templated nanofabrication with rationally arranged metal particles with well-defined morphologies, crystallinity and composition, and pave the way for mechanistic studies of the factors that govern shape evolution in nanoparticle synthesis. Furthermore, combining our approach with other, more advanced templated surfaces, such as epitaxially oriented seed particles with tailored crystallinity, will certainly unlock new possibilities in in-situ nanoparticle growth.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cem\u003eChemicals and Materials\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eA N-type (100)-Si wafer of 500 μm thickness (SiMat) with 80 nm dry oxide layer thermally grown (Centrotherm) and diced in 1 x 1 cm chips was used as a substrate. For samples compatible with TEM characterization 25-nm-thick SiNx grids were used.\u003csup\u003e39\u003c/sup\u003e Palladium(II) chloride (≥99.9%, PdCl2, Sigma), L-ascorbic acid (AA) (99%, Sigma), cetyltrimethylammonium bromide (CTAB) (≥99%, Acros), and MilliQ water (18.2 MΩ) were used without further purification in the preparation of the growth solution. An aqueous stock solution of H\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e4\u003c/sub\u003e (20 mM) was prepared by dissolving 35.5 mg of PdCl\u003csub\u003e2\u003c/sub\u003e in 2 mL of HCl (200 mM), stirring overnight at room temperature and finally adding 8 mL of MiliQ water.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003ePd seed particle nanofabrication\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003ePeriodic arrays of highly polycrystalline disk-like Pd seeds of 125 nm in diameter and 30 nm in height were fabricated on oxidized silicon substrate following a standard procedure described previously.\u003csup\u003e38\u003c/sup\u003e The procedure consists of thermally evaporating (Kurt J. Lesker PVD 225) a 30 nm Pd layer on a mask previously defined on the substrate surface by electron beam lithography (EBL, RAITH EBPG 5200). The EBL-mask contains openings through which the metal atoms reach the substrate surface. This is followed by a lift-off procedure in which the sample is immersed overnight in acetone then in IPA for 1 min and finally blow-dry in air. The EBL-mask dissolves in the acetone and the Pd that raises on its surface peels away into the solution leaving a pristine substrate surface with only Pd disk-like seeds at the predefined positions. Arrays of Pd seeds with smaller diameters from 110 to 40 nm at a constant height of 30 nm were fabricated on the substrate surface by varying the size of the openings in the EBL-mask. Annealing of the Pd seed arrays was carried out in a quartz tube furnace (Nabertherm R50/250/12) fluxed with a gas mixture of Ar with 2% of H\u003csub\u003e2\u003c/sub\u003e (300 mL min\u003csup\u003e–1\u003c/sup\u003e) and where the sample was placed in an alumina crucible, heated for 2 h at 600 °C and then allowed to cool to room temperature.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003ePMMA layer deposition and etching\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTo cover the Pd seed arrays with a PMMA layer of the desired thickness, we casted a few drops of a resist PMMA 950k A3 on the substrate surface template with the seed arrays. The sample was then spun at 3000 rpm for 60 s to obtain a uniform PMMA coating with a thickness of 140 nm and subsequently baked on a hot plate at 130 °C for 5 min in air. While the thickness of the PMMA layer initially deposited is not critical, it is important to cover the substrate surface with a PMMA layer thicker than the height of the seeds to form a homogeneous flat coating layer that completely covers the seeds. Etching of the PMMA layer down to the desired thickness was performed in successive steps, where the seed templated surface with the PMMA layer was exposed to oxygen plasma (25 W) in a RIE (Plasma-Therm) system for a specific time and then the PMMA layer thickness was measured by ellipsometry. By etching the PMMA layer further than the height of the Pd seeds, the seed top surface was progressively uncovered.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eArrays of Pd nanocubes\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003ePd nanocube arrays were produced from different Pd seed arrays in terms of seed-type (size and crystallinity) that were partially coated with a PMMA layer with a thickness experimentally optimized for each seed-type to maximize cube yield (Table S2) and incubated in a growth solution for 10 min. To growth the seeds in solution, we placed the substrate surface templated with the Pd seed arrays and the PMMA layer at the bottom of a 20 ml glass vial containing 2.75 mL of miliQ water and 0.25 mL of CTAB (200 mM) and heated to 40 °C in an oil bath as depicted in \u003cstrong\u003eFigure S5\u003c/strong\u003e. The temperature was allowed to equilibrate for 5 min, after which we added 1 mL of freshly prepared ascorbic acid (75 mM). After allowing the temperature to equilibrate for another 2 min, we rapidly injected 1 mL of H\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e4\u003c/sub\u003e (2 mM) preheated to 40 °C to initiate the reaction and then we let the particles to rest undisturbed in the growth solution for 10 min. As the reaction progressed, the colour of the growth solution changed from orange to dark brown in about 6 to 8 min due to the spontaneous nucleation in solution of secondary Pd nanoparticles. To terminate the reaction, the substrate was removed from the growth solution, rinsed with IPA, immersed in acetone for 1 min to dissolve the PMMA layer, immersed in IPA again for 1 min and finally blow-dry with N\u003csub\u003e2\u003c/sub\u003e. Alternatively, to extend the growth process, the substrate was removed from the growth solution after 10 min and immediately introduced in a new growth solution. Repeating this step 5 times resulted in Pd nanocubes up to 300 nm in size. Despite the significant nucleation of Pd nanoparticles in the growth solution, as indicated by the solution’s black colour at the end of the 10 min reaction time, their presence on the substrate surface was relatively low. On one hand, we attribute this to the low-energy and low-charged PMMA layer surface that limited the adsorption of these ‘unwanted’ by-products, and on the other hands, to the posterior dissolution of the PMMA layer that peeled away those nanoparticles eventually adsorbed, leaving a pristine substrate surface with only the nanoparticle arrays. Pd nanocubes on Au seeds were produces by replacing the nanofabricated Pd seeds with analogously fabricated Au seeds. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003ePd cube on Au cone tip\u003c/em\u003e. The protocol is the same as described for the Pd cubes but replacing the Pd seeds for 100 nm in diameter and 80 nm in height Au cones fabricated according to Syrenova et al.\u003csup\u003e61\u003c/sup\u003e and coated with a PMMA layer with a thickness adjusted accordingly to expose approximately the top 5 nm of the Au cone tip to the growth solution.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eNanoflowers, nanostars and concave nanocubes\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cu\u003ePd nanoflowers\u003c/u\u003e\u003cem\u003e.\u003c/em\u003eArrays of Pd nanoflowers were produced by subjecting arrays of unannealed and thus highly polycrystalline disk-shaped Pd seeds of 125 nm in diameter and 30 nm in height partially coated with a PMMA layer of 20 nm to the exact same growth conditions of the Pd nanocubes. Pd nanoflowers of sizes up to 500 nm were produced by consecutive growth steps in the exact same manner described for the Pd nanocubes. Pd nanoflowers on Au seeds were produces by replacing the nanofabricated Pd seeds by analogously fabricated Au seeds. \u003cu\u003ePd nanostars and concave Pd nanocubes.\u003c/u\u003eThese structures were produced as described for the Pd nanoflowers and Pd nanocubes, respectibely, using a growth solution containing KBr (40 mM) instead of CTAB (10 mM) with no other changes in the protocols.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eCharacterization and data analysis\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eSEM imaging was carried out using a Zeiss Supra 55 operated at 15 kV. SEM images of grown Pd nanoparticle arrays were taken after peeling off the PMMA layer without further treatment of the samples. SEM images of Pd seed coated with PMMA have a 5 nm Cr layer deposited by PVD to facilitate imaging. Tilted SEM images were taken at 60°. Mean particle diameter and roundness were calculated by averaging the diameter and roundness of a random population of at least 100 particles using ImageJ. 150 consecutive particles were visualized by SEM and evaluated by eyes to determine the cube yield in the arrays. TEM imaging and selected area electron diffraction analysis were carried out in a FEI Tecnai T20 LaB6 operated at 200 kV. Samples were prepared on a compatible 25-nm-thick SiNx membrane instead of the standard oxidized silicon substrate.\u003csup\u003e39\u003c/sup\u003e The height of the Pd seeds before annealing is assumed to be the same as the thickness of the PVD-deposited Pd layer during the nanofabrication process (30 ± 0.5 nm). The height of the Pd seeds after annealing was determined by AFM images using a Bruker Dimension 3100 for measurements and Nasoscope v6.12. program for analysis. No less than 100 particles were counted to determine mean particle height. PMMA layers thickness was measured by ellipsometry with a A J.A. Woollam RC2 and represents the average of three measurements taken at substrate surface positions separated by 50 µm.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe underlying data for this publication are available at Zenodo, 10.5281/zenodo.15479860\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Authors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJordi Bagaria \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0002-5788-3145; Email: [email protected]\u003c/p\u003e\n\u003cp\u003eChristoph Langhammer \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0003-2180-1379; Email: [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJordi Piella \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0002-5788-3145\u003c/p\u003e\n\u003cp\u003eJoachim Fritzsche \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0001-8660-2624\u003c/p\u003e\n\u003cp\u003eCarl Anderson \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/\u0026nbsp;0009-0007-7216-4433\u003c/p\u003e\n\u003cp\u003eChristoph Langhammer \u0026minus; Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden; orcid.org/0000-0003-2180-1379; Email: [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research has received funding from the Wenner-Gren Foundation via contract UPD2020-0287, the Marie-Sklodowska-Curie Actions Fellowship (MSCA) under the European Union\u0026rsquo;s Horizon Europe research and innovation program (DOI 10.3030/101032880/FASINA) and the European Research Council (ERC) under the European Union\u0026rsquo;s Horizon Europe research and innovation program (101043480/NACAREI). Part of this work was carried out at the Chalmers MC2 cleanroom facility and at the Chalmers Materials Analysis Laboratory (CMAL). We also thank Prof. Kasper Moth-Poulsen for fruitful discussions and support in the early phases of this project\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHe M-Q, Ai Y, Hu W, Guan L, Ding M, Liang Q. 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Janus Nanoparticles: From Fabrication to (Bio)Applications. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 6147-6191 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6715528/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6715528/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSurface-templated metal nanoparticle synthesis combines the best of lithography-based nanofabrication on surfaces and colloidal nanocrystal growth in solution, and it is well accepted that the final structure and crystallinity of nanoparticles grown using the seed-mediated method is strongly linked to the structure, morphology and crystallinity of the initial seed. Here, we challenge this paradigm by introducing regioselective particle growth enabled by a sacrificial polymer layer used to tailor the fraction of seed particle surface exposed to a growth solution and thereby defining the growth mode of the particle. Mechanistically, this confines metal deposition to a small, selected region of the seed surface and decouples the growth mode from size, morphology, crystallinity and composition of the seed. As we show on the example of Pd, this enables the \u003cem\u003ein situ\u003c/em\u003e surface-templated growth of regular arrays of polycrystalline nanoflowers, spiky nanostars and single crystalline nanocubes with over 90% yield, using identical growth conditions and nanolithography-fabricated regular arrays of morphologically poorly defined polycrystalline seeds, as well as crafting heterodimeric single crystalline Pd nanocubes at the tip of a large polycrystalline Au nanocone. This widens the practical applicability of surface-templated nanofabrication with rationally arranged metal particles with well-defined morphologies, crystallinity and composition.\u003c/p\u003e","manuscriptTitle":"Seed-Particle-Independent In-situ Synthesis of Surface-Templated Shape-Selected Palladium Nanoparticle Arrays","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-26 03:58:46","doi":"10.21203/rs.3.rs-6715528/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"254f9aea-7015-4533-a5f3-3d60ce5a4fa2","owner":[],"postedDate":"May 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":48958618,"name":"Physical sciences/Chemistry/Chemical synthesis/Nanoparticle synthesis"},{"id":48958619,"name":"Physical sciences/Materials science/Nanoscale materials/Nanoparticles"},{"id":48958620,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Nanoparticles"}],"tags":[],"updatedAt":"2025-06-23T19:35:19+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-26 03:58:46","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6715528","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6715528","identity":"rs-6715528","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}