Goldene: Free-standing Single-atom-thick Sheets of Gold

Goldene: Free-standing Single-atom-thick Sheets of Gold | 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 Research Article Goldene : Free-standing Single-atom-thick Sheets of Gold Shun Kashiwaya, Yuchen Shi, Jun Lu, Davide Sangiovanni, Mike Andersson, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2807259/v2 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 15 Apr, 2024 Read the published version in Nature Synthesis → Version 2 posted You are reading this latest preprint version Show more versions Abstract The quest to make free-standing monolayer gold has hitherto been limited to free-standing several-atoms-thick layers, or monolayers but confined on or inside another template. Examples are monolayers diffused into double hydroxides 1 , membranes framed in alloys under electron irradiation 2 , nano-ribbons suspended in graphene 3 , quantum dots on hexagonal BN 4 , monolayers in between SiC wafers and monolayer graphene 5 , and fragments produced via thermal dewetting on sapphire 6 . Here, we report the synthesis of free-standing single-atom-thick 2D gold (named goldene ) by wet-chemically etching away Ti 3 C 2 from Ti 3 AuC 2 , a nano-laminated MAX-phase ceramic initially formed by substituting Si in Ti 3 SiC 2 with Au 7 . The free-standing goldene layers are revealed by scanning transmission electron microscopy. While ab initio molecular dynamics simulations show that 2D goldene is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges. Prospects for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented. Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials Physical sciences/Materials science/Nanoscale materials/Synthesis and processing Free-standing elemental 2D materials Figures Figure 1 Figure 2 Introduction The discovery of graphene commenced the era of 2D materials featuring thus far mostly non-metallic elements and composites consisting of multiple covalently bonding elements 8,9 . Not much so yet for metals, however. Gold calls for attention with its various low-symmetry allotrope structures with superior and unique properties compared to its bulk counterparts 10 . Reducing size, cluster Au 5-13 - anions 11 form planar molecules due to the strong relativistic effects 12 that stabilize the outer 6s shell and destabilize the 5d shell, where the planarity is attributed to the unique hybridization of the half-filled 6s orbital with the fully occupied orbital 13,14 . Boron (a metalloid) has also been predicted to form planar or quasi-planar clusters and favors the formation of 2D allotropes, borophenes, due to its short covalent radius and attainable sp 2 hybridization 15,16 . Inspired by the experimental realization of free-standing monolayer iron suspended in graphene pores, theoretical studies based on density-functional theory (DFT) and ab initio molecular dynamics (AIMD) predicted stable 2D Au membranes with or without confinement by graphene pores 17-21 : specifically, stable Au membranes suspended in graphene pores of diameter up to 20 nm. Notably, the DFT structural optimization indicated that the favored 2D crystal would be of densely packed hexagonal structure without buckling that corresponds to the bulk monolayer of Au(111) fcc lattice. Epitaxial growth of single-crystal metal films has been demonstrated by numerous types of vapor deposition on lattice-matched substrates 22 . During deposition, however, gold thus far unavoidably forms islands due to a strong thermodynamic tendency for coalescence. This results in, at best, nm-thick continuous Au layers 23 . Meanwhile, unique synthetic routes of atomically-thin gold sheets have been developed. Wang et al. reported synthesis of (001)-oriented gold nanosheets with a thickness of 0.2-0.4 nm, corresponding to 1-2 atomic-thick layers diffused into a layered double hydroxides template 1 . The gold sheets were attempted to be isolated, but removal of the hydroxide matrix transformed the 2D-Au structures into nanoparticles. Wang et al. fabricated free-standing single-atom-thick Au framed in bulk Au-Ag alloy by electron beam irradiation 2 . Similarly, Zhao et al. observed free-standing single-atom-thick Au nano-ribbons suspended in graphene 3 . Bhandari et al. prepared single-atom-thick gold quantum dots (QDs) stabilized on hexagonal boron nitride surfaces 4 . Surprisingly, the QDs show tunable bandgaps – from 2.8 to 0.95 eV depending on their size and shape, consistent with the DFT prediction. Forti et al. synthesized single-atom-thick Au layers, stabilized between SiC(0001) and the buffer zero-layer graphene via intercalation at elevated temperature 5 . Although both the large 2D and 3D clusters are expected to show more metallic properties as their size increases 24 and infinite free-standing Au is predicted to be metallic 19 , the intercalated Au monolayer showed semiconducting properties with the valence band maximum 50 meV below the Fermi level. Recently, Sharma et al. claimed to have realized goldene by thermal dewetting of thin Au films on sapphire substrates, however, with likely multilayers from the appearance of high-resolution scanning transmission electron microscopy (HR-STEM) images and the nature of Au islands 6 . State-of-the-art of the thinnest free-standing ‘2D’ gold without physical confinements is two-atoms-thick gold nanosheets, fabricated via a wet-chemical route employing methyl orange which plays a critical role in suppressing the nanosheet thickness 25 . Yet, truly free-standing single-atom-thick 2D Au structures at the large scale remain unrealized. Results And Discussion Figure 1 shows HR-STEM images of a free-standing single-atom-thick 2D Au membrane, namely goldene, realized in this study (supplementary section 1). The 2D Au was achieved by etching away Ti 3 C 2 slabs from nano-laminated Ti 3 AuC 2 MAX-phase films by alkaline potassium ferricyanide solution (Murakami’s reagent) together with cetrimonium bromide (CTAB) and cysteine as stabilizers. M n+1 AX n phases (n = 1, 2, 3, or 4) are a set of intrinsically laminated ternary materials 26 . M is a transition metal, A is an element of groups 13–16, and X is carbon or nitrogen. In perspective, new 2D materials of MXenes are produced by etching away A layers from MAX-phases, leaving M n+1 X n layers: most A elements are weakly bonded to the neighboring M elements while the M-X bondings are relatively strong 27 . HF is, for instance, employed to selectively remove A elements from most MAX-phases; meanwhile, MX layers are inert to HF. We invert this concept by using Murakami’s reagent to etch away MX layers and leave free-standing single-atom-thick A layers. The lateral length of the free-standing 2D Au is about 5 nm, in agreement with the theoretically predicted size 18 (Fig. 1). With correction made using well-defined values for Ti 3 AuC 2 lattice parameters, we obtain the Au-Au spacing of the 2D Au of 2.62 Å, close to a calculated Au-Au bond length of 2.755 Å for an optimized closely packed 2D Au 19 (supplementary section 1). Au-Au spacing values reported for sub-nanometer 2D Au are shorter than 2.80 Å, the shortest Au-Au distance typically found for high density and closely packed crystal structures (supplementary section 2), because the reduced 3D→2D dimensionality strengthens in-plane bonding. Next, we confirm the structural and dynamic stability of goldene, a monolayer of fcc Au(111), using AIMD (supplementary section S3). The dynamics of 2D defect-free goldene is followed for 5 ps at 300 K. The AIMD simulations attest the dynamic stability of the stoichiometric planar motif of goldene in the hexagonal triangular structure, consistent with previous ab initio calculations and simulations (Fig. 2) 19 . However, goldene layers produced by etching show curling up and blob formation near the etched free edges (Fig. 1 and supplementary section S1). Thus, we further investigated the dynamic stability of goldene considering various structures and effects of point defects: (i) infinite goldene with Au adatoms, (ii) unstrained and initially strained (tension and compression) goldene nano-ribbons with differently oriented edges, (iii) goldene nano-ribbons with Au vacancies and Si impurities, and (iv) goldene bilayers (with and without defects) with various interlayer distances (supplementary section S3.2-3.6). The simulations indicate stability and planarity of goldene nano-ribbons cut along and edges, for which the calculated out-of-plane vibrational amplitudes are similar to that of an infinite defect-free goldene. On the other hand, the simulations suggest that Si impurities would distort the bond network to generate ripples in the layer if located near the edges of goldene. Conversely, Au vacancies have an indirect stabilization effect by acting as trapping sites for Si impurities. Small rippling is observed in the AIMD simulations for an isolated goldene monolayer. However, the amplitude of the ripples increases as the interlayer distance becomes smaller: manifestation of mutual attraction between Au layers. Geim et al. recently discovered that nano-rippling is unavoidable for graphene due to thermal fluctuations and local mechanical strain; essentially, such nano-rippling is inherent to atomically thin free-standing 2D materials in general and beneficial to chemical reactions 28 . Accordingly, the rippling nature of 2D Au layers observed in our STEM analysis substantiates that they are free-standing. Thus, goldene is intrinsically stable regardless of its lineaments, and the curling and blob formation observed in our experiments would be attributed to external factors related to initial Au interaction into Ti 3 SiC 2 to produce Ti 3 AuC 2 and subsequent etching processes, such as electron radiation damage during 300 kV STEM imaging. Experimentally, we realized various forms of 2D Au layers by using different etching conditions (supplementary section S4). The alkaline solution of potassium ferricyanide consisting of 1 g of KOH, 1 g of K 3 [Fe(CN) 6 ], and 10 mL H 2 O is conventionally called the Murakami’s reagent, and this concentration is referred to as 100 %. Using this concentration resulted in complete decomposition of Ti 3 AuC 2 into amorphous TiC and spherical gold nanoparticles; however, a 2D Au monolayer at the interface of 4H-SiC and Ti 3 AuC 2 survived the aggressive etching. Besides the etchant concentration, surfactants are essential to mitigate the coalescence of goldene layers and blob formation mediated by Au adatoms or Si impurities, as predicted by the AIMD simulation (supplementary section S3). Combining the lower concentration of 1-3 % Murakami’s reagent with the usage of CTAB and cysteamine as surfactants, freestanding rippling membranes of two- and three-atom-thick 2D Au were fabricated from Ti 3 Au n C 2 (n = 2 and 3); here, blob formation at the edges of these free-standing membranes was observed (supplementary section S4). The free-standing 2D single-atom-thick Au, goldene (Fig.1 and supplementary section S1), was achieved by employing further lowered etchant concentrations of 0.2 and 1 % Murakami’s reagent together with CTAB and cysteine. CTAB is a long molecule with a length of up to 20 Å; meanwhile, the distance between Au layers within Ti 3 Au n C 2 is 9.28 Å. Thus, CTAB can infiltrate into between Au layers exposed after etching, only in parallel to Au layers, and would block the pathway for other CTAB to enter deeper. Cysteine and cysteamine have respective lengths below 9.28 Å, smaller than CTAB, and would more easily diffuse in-between Au layers, although they would eventually block the pathway too (supplementary section S5). Therefore, the free-standing 2D Au appears only at the edge of several nm near the etching frontline. The mild etching is crucial for goldene derivatives from Ti 3 Au n C 2 compounds: the lower the concentration is, the better the yield and quality of resulting Au layers are. However, this trend ends with around concentrations of 0.2 and 0.5 % Murakami’s reagent. 0.1 % resulted in complete decomposition of 2D Au layers into 3D spherical nanoparticles, due to the lower concentration needing much longer etching duration. The etchants would gradually attack the free-standing Au layers initially stabilized by surfactants as the radical nascent oxygen produced from the etchants eventually attack the surfactants. Therefore, considering the balance of mildness and duration of etching, the concentration between 0.2 and 1 % would be optimal for the Au layers to survive. 0.2 % Murakami’s reagent with CTAB allowed 5x5 nm free-standing 2D Au to survive, but the blob formation at the edge of the 2D Au progresses simultaneously. Further etching would advance the blob formation at the edges and Au atoms of the blob diffuse laterally through the layer. Therefore, we observed gold layers to become thicker after etching for long duration (supplementary section S4). A previous report demonstrated the free-standing two-atomic layer thick ‘2D’ Au membranes where the main structure is fcc, but hcp reconstruction was observed at the edges 25 . This non-fcc feature has been found to stabilize planar Au structures 29 . Here, the blob formed at the edges next to the free-standing 2D Au in our observation would function as a stabilizer to maintain free-standing goldene. Appropriate selection of etchant concentrations and smaller, more suitable surfactants would trigger such reconstruction of the free-standing 2D Au edges and enable higher yield of goldene. Besides etchant concentrations and surfactants, a decisive factor for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases is the interlayer distance between Au layers within the MAX-phases. Our AIMD simulations demonstrate that free-standing 2D Au layers coalesce with the presence of Si impurities and if brought within 7 Å interlayer distance between them; however, as their separation distance increases from 7 to 10, 12, and 14 Å, the goldene-goldene interlayer interaction decreases (supplementary section S3). The distance between Au layers within Ti 3 AuC 2 is 9.28 Å. Thus, instant interlayer interaction is expected without surfactants as soon as goldene layers are isolated from Ti 3 AuC 2 during etching. Here, we produced the first Au-intercalated 413 MAX-phases of Ti 4 AuC 3 from Ti 4 SiC 3 (supplementary section S6). The distance between Au layers within Ti 4 AuC 3 is about 12 Å. Therefore, a delay of goldene interactions after etching is expected, giving rise to a longer grace period of surfactants to stabilize the goldene sheets before they coalesce and a greater likelihood of their survival. Conclusion In conclusion, analytical electron microscopy corroborated by AIMD simulations shows the existence of goldene , monolayer sheets of Au , as prepared by etching away Ti 3 C 2 slabs from Ti 3 AuC 2 . Essential are the precise dilution of the Murakami’s reagent as an etchant and the appropriate usage of passivating surfactants of CTAB and cysteine. Our developed processing scheme (supplementary section S5-7) has potential for expansion of the goldene sheet area by means of etching and surfactant optimization. The template SiC(0001) wafer size that sets the area limit of growing epitaxial Ti 3 SiC 2 for subsequent Au intercalation is 300 mm 30 , which would give an ample allowance for practical goldene application. References 1 Wang, L. et al. Two-dimensional gold nanostructures with high activity for selective oxidation of carbon–hydrogen bonds. Nature communications 6 , 6957 (2015). 2 Wang, X., Wang, C., Chen, C., Duan, H. & Du, K. Free-standing monatomic thick two-dimensional gold. Nano Letters 19 , 4560-4566 (2019). 3 Zhao, L., Ta, H. Q., Mendes, R. G., Bachmatiuk, A. & Rummeli, M. H. In situ observations of freestanding single‐atom‐thick gold nanoribbons suspended in graphene. Advanced Materials Interfaces 7 , 2000436 (2020). 4 Bhandari, S. et al. Two-dimensional gold quantum dots with tunable bandgaps. ACS nano 13 , 4347-4353 (2019). 5 Forti, S. et al. Semiconductor to metal transition in two-dimensional gold and its van der Waals heterostack with graphene. Nature communications 11 , 1-7 (2020). 6 Sharma, S. K., Pasricha, R., Weston, J., Blanton, T. & Jagannathan, R. Synthesis of Self-Assembled Single Atomic Layer Gold Crystals-Goldene. ACS Applied Materials & Interfaces 14 , 54992-55003 (2022). 7 Fashandi, H. et al. Synthesis of Ti 3 AuC 2 , Ti 3 Au 2 C 2 and Ti 3 IrC 2 by noble metal substitution reaction in Ti 3 SiC 2 for high-temperature-stable Ohmic contacts to SiC. Nature materials 16 , 814 (2017). 8 Glavin, N. R. et al. Emerging applications of elemental 2D materials. Advanced Materials 32 , 1904302 (2020). 9 Shanmugam, V. et al. A Review of the Synthesis, Properties, and Applications of 2D Materials. Particle & Particle Systems Characterization , 2200031 (2022). 10 Hong, X., Tan, C., Chen, J., Xu, Z. & Zhang, H. Synthesis, properties and applications of one-and two-dimensional gold nanostructures. Nano Research 8 , 40-55 (2015). 11 Goldsmith, B. R. et al. Two-to-three dimensional transition in neutral gold clusters: The crucial role of van der Waals interactions and temperature. Physical Review Materials 3 , 016002 (2019). 12 Pyykko, P. Relativistic effects in structural chemistry. Chemical Reviews 88 , 563-594 (1988). 13 Fernández, E. M., Soler, J. M., Garzón, I. L. & Balbás, L. C. Trends in the structure and bonding of noble metal clusters. Physical Review B 70 , 165403 (2004). 14 Häkkinen, H., Moseler, M. & Landman, U. Bonding in Cu, Ag, and Au clusters: relativistic effects, trends, and surprises. Physical review letters 89 , 033401 (2002). 15 Feng, B. et al. Experimental realization of two-dimensional boron sheets. Nature chemistry 8 , 563 (2016). 16 Mannix, A. J. et al. Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs. Science 350 , 1513-1516 (2015). 17 Ono, S. Dynamical stability of two-dimensional metals in the periodic table. Physical Review B 102 , 165424 (2020). 18 Koskinen, P. & Korhonen, T. Plenty of motion at the bottom: Atomically thin liquid gold membrane. Nanoscale 7 , 10140-10145 (2015). 19 Yang, L.-M., Dornfeld, M., Frauenheim, T. & Ganz, E. Glitter in a 2D monolayer. Physical Chemistry Chemical Physics 17 , 26036-26042 (2015). 20 Yang, L.-M., Ganz, A. B., Dornfeld, M. & Ganz, E. Computational Study of Quasi-2D Liquid State in Free Standing Platinum, Silver, Gold, and Copper Monolayers. Condensed matter 1 (2016). 21 Nevalaita, J. & Koskinen, P. Stability limits of elemental 2D metals in graphene pores. Nanoscale 11 , 22019-22024 (2019). 22 Campbell, C. T. Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties. Surface Science Reports 27 , 1-111 (1997). 23 Norrman, S., Andersson, T., Granqvist, C. & Hunderi, O. Optical properties of discontinuous gold films. Physical Review B 18 , 674 (1978). 24 Natarajan, G., Mathew, A., Negishi, Y., Whetten, R. L. & Pradeep, T. A unified framework for understanding the structure and modifications of atomically precise monolayer protected gold clusters. The Journal of Physical Chemistry C 119 , 27768-27785 (2015). 25 Ye, S. et al. Sub‐nanometer thick gold nanosheets as highly efficient catalysts. Advanced Science 6 , 1900911 (2019). 26 Lei, X. & Lin, N. Structure and synthesis of MAX phase materials: a brief review. Critical Reviews in Solid State and Materials Sciences 47 , 736-771 (2022). 27 Lim, K. R. G. et al. Fundamentals of MXene synthesis. Nature Synthesis 1 , 601-614 (2022). 28 Sun, P. et al. Unexpected catalytic activity of nanorippled graphene. Proceedings of the National Academy of Sciences 120 , e2300481120 (2023). 29 Kondo, Y. & Takayanagi, K. Gold nanobridge stabilized by surface structure. Physical Review Letters 79 , 3455 (1997). 30 Fashandi, H. et al. Single-step synthesis process of Ti 3 SiC 2 ohmic contacts on 4H-SiC by sputter-deposition of Ti. Scripta Materialia 99 , 53-56 (2015). Methods Ti 3 Au n C 2 films were prepared by Au-intercalation into Ti 3 SiC 2 . After polishing away the residual Au, the films were etched by the mixture of KOH and K 3 [Fe(CN) 6 ] with various concentrations together with CTAB, cysteine, and cysteamine in the aqueous solution, to produce free-standing single- and multiple-atom-thick 2D Au. Structural analysis was carried out by high-resolution scanning transmission electron microscopy high angle annular dark field (HRSTEM-HAADF) imaging within Linköping’s double CS corrected FEI Titan 3 60−300 microscope operated at 300 kV. Detailed information can be found in the supplementary information section S7. AIMD simulations were carried out using the VASP code implemented with the projector augmented wave method 31,32 and the generalized-gradient electronic exchange and correlation approximation 33 . An average temperature of 300 K is controlled via the Nosé-Hoover thermostat (NVT ensemble). The classical equations of motion are integrated on timesteps of 1 fs. Spin-orbit coupling and relativistic effects are neglected. Van der Waals interactions are modeled using Grimme’s approximation 34 . At each ionic step, the energy is calculated with accuracy of 10 –5 eV/supercell, using 300 eV cut-off for the planewave basis set and Γ-point sampling of the reciprocal space. Detailed information can be found in the supplementary information section S3. Data availability The data supporting the findings of this work is available within the article and the corresponding supplementary information. References 31 Kresse, G. & Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B 54 , 11169-11186 (1996). 32 Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Physical Review B 59 , 1758-1775, doi:10.1103/PhysRevB.59.1758 (1999). 33 Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Physical Review Letters 77 , 3865-3868, doi:10.1103/PhysRevLett.77.3865 (1996). 34 Grimme, S. Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction. Journal of computational chemistry 27 , 1787-1799 (2006). Declarations Acknowledgments This work was supported by the Swedish Research Council project grant 2017-03909 (to L.H.) and 2021-04426 (to D.G.S.), Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University SFO-Mat-LiU 2009 00971, and the Wallenberg Scholar Program KAW 2019.0290 (to L.H.). All simulations were carried out using the resources provided by the Swedish National Infrastructure for Computing (SNIC) – partially funded by the Swedish Research Council through Grant Agreement Nº VR-2018-05973. J. R. acknowledges funding from the Göran Gustafsson foundation for Research in Natural Sciences and Medicines. Author contributions L.H. conceived and supervised the study. J.R, S.K., and L.H. designed the study. Implementation was made by S.K. (MAX phase film growth, intercalation experiments, etching/surfactant protocols, and STEM sample preparation), Y.S (MAX phase film growth, intercalation experiments, and STEM sample preparation), M.A (Au film growth). J.L. (STEM analysis), and D.G.S. (AIMD simulations). S.K., Y.S., J.L., and D.G.S. performed the data analysis. S.K., D.G.S., and L.H. wrote the manuscript. All authors contributed to the final version of the manuscript. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary materials available at … (information added when published online) Additional Declarations There is NO Competing Interest. Supplementary Files 20230416goldeneSI.docx Supplementary information Cite Share Download PDF Status: Published Journal Publication published 15 Apr, 2024 Read the published version in Nature Synthesis → Version 2 posted You are reading this latest preprint version Show more versions 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-2807259","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":191656455,"identity":"09a662cc-554d-4e87-b19d-bbbbb259a591","order_by":0,"name":"Shun Kashiwaya","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYDACCRBxgIHBgBnMtZFhI1VLGg8JWiDcwzwEdcjP7jF7wHDGzt6cncfwM0/NeR4+iQTmDx/waDG4c8bcgOFGcuLOZh5jaZ5jt3nYJBLYJGfg0yKRYybB8IE5weAw7wZpHjaIFmZ8zpOfAdZSbw/Usvk3z79zIC3Mn//g88wNkJYbhxk3HObdJs3bdgCkhUEanw6DG2llEglnjiduOMz/zXJuXzIPG8/DNskevA5L3ibx4Vi1vcH5Y8k33nyzk5NvTz784Qc+a0AgAUozQXzN2EBIAwIwEjR8FIyCUTAKRiQAAB3WRzG55eGkAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-0578-8218","institution":"Linköping University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Shun","middleName":"","lastName":"Kashiwaya","suffix":""},{"id":191656456,"identity":"ed047539-b82f-4f9f-ae8b-9e27dcd52457","order_by":1,"name":"Yuchen Shi","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yuchen","middleName":"","lastName":"Shi","suffix":""},{"id":191656457,"identity":"31852d41-08b1-467a-956e-7f96bd8bdfd0","order_by":2,"name":"Jun Lu","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jun","middleName":"","lastName":"Lu","suffix":""},{"id":191656458,"identity":"c26cf378-34d3-4b1e-8525-51df5b5a7fc8","order_by":3,"name":"Davide Sangiovanni","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Davide","middleName":"","lastName":"Sangiovanni","suffix":""},{"id":191656459,"identity":"2307fbae-f287-4e6b-8b9b-56bb778e3be8","order_by":4,"name":"Mike Andersson","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Mike","middleName":"","lastName":"Andersson","suffix":""},{"id":191656460,"identity":"ea6dafa0-6537-4ba2-b7e2-f582c28aa7b5","order_by":5,"name":"Johanna Rosen","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Johanna","middleName":"","lastName":"Rosen","suffix":""},{"id":191656461,"identity":"168d6498-5a46-43bf-b378-276baebf0628","order_by":6,"name":"Lars Hultman","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Lars","middleName":"","lastName":"Hultman","suffix":""}],"badges":[],"createdAt":"2023-04-12 14:46:29","currentVersionCode":2,"declarations":"","doi":"10.21203/rs.3.rs-2807259/v2","doiUrl":"https://doi.org/10.21203/rs.3.rs-2807259/v2","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s44160-024-00518-4","type":"published","date":"2024-04-16T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":35957639,"identity":"01e41f84-4ba3-4829-81ad-63420b555817","added_by":"auto","created_at":"2023-04-18 20:05:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":283628,"visible":true,"origin":"","legend":"\u003cp\u003eSee above image for figure legend.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-2807259/v2/17f42251ae7920d3006373cf.png"},{"id":35957640,"identity":"0fa911c1-581a-4c32-bd70-31a8771f17f7","added_by":"auto","created_at":"2023-04-18 20:05:16","extension":"tiff","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":842701,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePlan and side orthographic views of a defect-free goldene monolayer.\u003c/strong\u003e Supercell model used in AIMD simulations at 300 K. The crystallographic axes are relative to an Au(111) monolayer. The length of visible Au–Au bonds is ≤3.6 Å.\u003c/p\u003e","description":"","filename":"Figure2Planandsideorthographicviewsofadefectfreegoldenemonolayer.tiff","url":"https://assets-eu.researchsquare.com/files/rs-2807259/v2/e7f620c5ea5f292d0e52653a.tiff"},{"id":55107286,"identity":"56e95eb7-43a9-428d-b0b1-de664d0269d1","added_by":"auto","created_at":"2024-04-22 17:21:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1228785,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2807259/v2/831855ec-da81-4efa-9230-77f32f21b10f.pdf"},{"id":35957641,"identity":"a036860c-2f18-4c58-a764-b813b6ada75f","added_by":"auto","created_at":"2023-04-18 20:05:17","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":57639967,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary information\u003c/p\u003e","description":"","filename":"20230416goldeneSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-2807259/v2/16e523c789ac1b119e4fd6c5.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003ci\u003eGoldene\u003c/i\u003e: Free-standing Single-atom-thick Sheets of Gold","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe discovery of graphene commenced the era of 2D materials featuring thus far mostly non-metallic elements and composites consisting of multiple covalently bonding elements\u003csup\u003e8,9\u003c/sup\u003e. Not much so yet for metals, however. Gold calls for attention with its various low-symmetry allotrope structures with superior and unique properties compared to its bulk counterparts\u003csup\u003e10\u003c/sup\u003e. Reducing size, cluster Au\u003csub\u003e5-13\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e anions\u003csup\u003e11\u003c/sup\u003e form planar molecules due to the strong relativistic effects\u003csup\u003e12\u003c/sup\u003e that stabilize the outer 6s shell and destabilize the 5d shell, where the \u0026nbsp;planarity is attributed to the unique hybridization of the half-filled 6s orbital with the fully occupied\u0026nbsp;\u0026nbsp;orbital\u003csup\u003e13,14\u003c/sup\u003e. Boron (a metalloid) has also been predicted to form planar or quasi-planar clusters and favors the formation of 2D allotropes, borophenes, due to its short covalent radius and attainable sp\u003csup\u003e2\u003c/sup\u003e hybridization\u003csup\u003e15,16\u003c/sup\u003e. Inspired by the experimental realization of free-standing monolayer iron suspended in graphene pores, theoretical studies based on density-functional theory (DFT) and \u003cem\u003eab initio\u003c/em\u003e molecular dynamics (AIMD) predicted stable 2D Au membranes with or without confinement by graphene pores\u003csup\u003e17-21\u003c/sup\u003e: specifically, stable Au membranes suspended in graphene pores of diameter up to 20 nm. Notably, the DFT structural optimization indicated that the favored 2D crystal would be of densely packed hexagonal structure without buckling that corresponds to the bulk monolayer of Au(111) fcc lattice.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eEpitaxial growth of single-crystal metal films has been demonstrated by numerous types of vapor deposition on lattice-matched substrates\u003csup\u003e22\u003c/sup\u003e. During deposition, however, gold thus far unavoidably forms islands due to a strong thermodynamic tendency for coalescence. This results in, at best, nm-thick continuous Au layers\u003csup\u003e23\u003c/sup\u003e. Meanwhile, unique synthetic routes of atomically-thin gold sheets have been developed. Wang \u003cem\u003eet al.\u003c/em\u003e reported synthesis of (001)-oriented gold nanosheets with a thickness of 0.2-0.4 nm, corresponding to 1-2 atomic-thick layers diffused into a layered double hydroxides template\u003csup\u003e1\u003c/sup\u003e. \u0026nbsp;The gold sheets were attempted to be isolated, but removal of the hydroxide matrix transformed the 2D-Au structures into nanoparticles. Wang \u003cem\u003eet al.\u003c/em\u003e fabricated free-standing single-atom-thick Au framed in bulk Au-Ag alloy by electron beam irradiation\u003csup\u003e2\u003c/sup\u003e. Similarly, Zhao \u003cem\u003eet al.\u003c/em\u003e observed free-standing single-atom-thick Au nano-ribbons suspended in graphene\u003csup\u003e3\u003c/sup\u003e. Bhandari \u003cem\u003eet al.\u003c/em\u003e prepared single-atom-thick gold quantum dots (QDs) stabilized on hexagonal boron nitride surfaces\u003csup\u003e4\u003c/sup\u003e. Surprisingly, the QDs show tunable bandgaps \u0026ndash; from 2.8 to 0.95 eV depending on their size and shape, consistent with the DFT prediction. Forti\u003cem\u003e\u0026nbsp;et al.\u0026nbsp;\u003c/em\u003esynthesized single-atom-thick Au layers, stabilized between SiC(0001) and the buffer zero-layer graphene via intercalation at elevated temperature\u003csup\u003e5\u003c/sup\u003e. Although both the large 2D and 3D clusters are expected to show more metallic properties as their size increases\u003csup\u003e24\u003c/sup\u003e and infinite free-standing Au is predicted to be metallic\u003csup\u003e19\u003c/sup\u003e, the intercalated Au monolayer showed semiconducting properties with the valence band maximum 50 meV below the Fermi level. Recently, Sharma\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e claimed to have realized goldene by thermal dewetting of thin Au films on sapphire substrates, however, with likely multilayers from the appearance of high-resolution scanning transmission electron microscopy (HR-STEM) images and the nature of Au islands\u003csup\u003e6\u003c/sup\u003e. State-of-the-art of the thinnest free-standing \u0026lsquo;2D\u0026rsquo; gold without physical confinements is two-atoms-thick gold nanosheets, fabricated via a wet-chemical route employing methyl orange which plays a critical role in suppressing the nanosheet thickness\u003csup\u003e25\u003c/sup\u003e. Yet, truly free-standing single-atom-thick 2D Au structures at the large scale remain unrealized.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003eFigure 1 shows HR-STEM images of a\u0026nbsp;free-standing single-atom-thick 2D Au membrane, namely goldene, realized in this study (supplementary section 1). The 2D Au was achieved by etching away Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e slabs from nano-laminated Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e MAX-phase films by alkaline potassium ferricyanide solution (Murakami\u0026rsquo;s reagent) together with cetrimonium bromide (CTAB) and cysteine as stabilizers. \u0026nbsp;M\u003csub\u003en+1\u003c/sub\u003eAX\u003csub\u003en\u003c/sub\u003e phases (n = 1, 2, 3, or 4) are a set of intrinsically laminated ternary materials\u003csup\u003e26\u003c/sup\u003e. M is a transition metal, A is an element of groups 13\u0026ndash;16, and X is carbon or nitrogen. In perspective, new 2D materials of MXenes are produced by etching away A layers from MAX-phases, leaving M\u003csub\u003en+1\u003c/sub\u003eX\u003csub\u003en\u003c/sub\u003e layers: most A elements are weakly bonded to the neighboring M elements while the M-X bondings are relatively strong\u003csup\u003e27\u003c/sup\u003e. HF is, for instance, employed to selectively remove A elements from most MAX-phases; meanwhile, MX layers are inert to HF. \u003cem\u003eWe invert this concept by using Murakami\u0026rsquo;s reagent to etch away MX layers and leave free-standing single-atom-thick A layers.\u003c/em\u003e The lateral length of the free-standing 2D Au is about 5 nm, in agreement with the theoretically predicted size\u003csup\u003e18\u003c/sup\u003e (Fig. 1). With correction made using well-defined values for Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u0026nbsp;\u003c/sub\u003elattice parameters, we obtain the Au-Au spacing of the 2D Au of 2.62 \u0026Aring;, close to a calculated Au-Au bond length of 2.755 \u0026Aring; for an optimized closely packed 2D Au\u003csup\u003e19\u003c/sup\u003e (supplementary section 1). Au-Au spacing values reported for sub-nanometer 2D Au are shorter than 2.80 \u0026Aring;, the shortest Au-Au distance typically found for high density and closely packed crystal structures (supplementary section 2), because the reduced 3D\u0026rarr;2D dimensionality strengthens in-plane bonding.\u003c/p\u003e\n\u003cp\u003eNext, we confirm the structural and dynamic stability of goldene, a monolayer of fcc Au(111), using AIMD (supplementary section S3). The dynamics of 2D defect-free goldene is followed for 5 ps at 300 K. The AIMD simulations attest the dynamic stability of the stoichiometric planar motif of goldene in the hexagonal triangular structure, consistent with previous \u003cem\u003eab initio\u003c/em\u003e calculations and simulations (Fig. 2)\u003csup\u003e19\u003c/sup\u003e. However, goldene layers produced by etching show curling up and blob formation near the etched free edges (Fig. 1 and supplementary section S1). Thus, we further investigated the dynamic stability of goldene considering various structures and effects of point defects: (i) infinite goldene with Au adatoms, (ii) unstrained and initially strained (tension and compression) goldene nano-ribbons with differently oriented edges, (iii) goldene nano-ribbons with Au vacancies and Si impurities, and (iv) goldene bilayers (with and without defects) with various interlayer distances (supplementary section S3.2-3.6). The simulations indicate stability and planarity of goldene nano-ribbons cut along \u0026lt;110\u0026gt; and \u0026lt;112\u0026gt; edges, for which the calculated out-of-plane vibrational amplitudes are similar to that of an infinite defect-free goldene. On the other hand, the simulations suggest that Si impurities would distort the bond network to generate ripples in the layer if located near the edges of goldene. Conversely, Au vacancies have an indirect stabilization effect by acting as trapping sites for Si impurities. Small rippling is observed in the AIMD simulations for an isolated goldene monolayer. However, the amplitude of the ripples increases as the interlayer distance becomes smaller: manifestation of mutual attraction between Au layers. Geim \u003cem\u003eet al.\u003c/em\u003e recently discovered that nano-rippling is unavoidable for graphene due to thermal fluctuations and local mechanical strain; essentially, such nano-rippling is inherent to atomically thin free-standing 2D materials in general and beneficial to chemical reactions\u003csup\u003e28\u003c/sup\u003e. Accordingly, the rippling nature of 2D Au layers observed in our STEM analysis substantiates that they are free-standing. Thus, goldene is intrinsically stable regardless of its lineaments, and the curling and blob formation observed in our experiments would be attributed to external factors related to initial Au interaction into Ti\u003csub\u003e3\u003c/sub\u003eSiC\u003csub\u003e2\u003c/sub\u003e to produce Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e and subsequent etching processes, such as electron radiation damage during 300 kV STEM imaging.\u003c/p\u003e\n\u003cp\u003eExperimentally, we realized various forms of 2D Au layers by using different etching conditions (supplementary section S4). The alkaline solution of potassium ferricyanide consisting of 1 g of KOH, 1 g of K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e], and 10 mL H\u003csub\u003e2\u003c/sub\u003eO is conventionally called the Murakami\u0026rsquo;s reagent, and this concentration is referred to as 100 %. Using this concentration resulted in complete decomposition of Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e into amorphous TiC and spherical gold nanoparticles; however, a 2D Au monolayer at the interface of 4H-SiC and Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e survived the aggressive etching. Besides the etchant concentration, surfactants are essential to mitigate the coalescence of goldene layers and blob formation mediated by Au adatoms or Si impurities, as predicted by the AIMD simulation (supplementary section S3). Combining the lower concentration of 1-3 % Murakami\u0026rsquo;s reagent with the usage of CTAB and cysteamine as surfactants, freestanding rippling membranes of two- and three-atom-thick 2D Au were fabricated from Ti\u003csub\u003e3\u003c/sub\u003eAu\u003csub\u003en\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e (n = 2 and 3); here, blob formation at the edges of these free-standing membranes was observed (supplementary section S4). The free-standing 2D single-atom-thick Au, goldene (Fig.1 and supplementary section S1), was achieved by employing further lowered etchant concentrations of 0.2 and 1 % Murakami\u0026rsquo;s reagent together with CTAB and cysteine. CTAB is a long molecule with a length of up to 20 \u0026Aring;; meanwhile, the distance between Au layers within Ti\u003csub\u003e3\u003c/sub\u003eAu\u003csub\u003en\u003c/sub\u003eC\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eis 9.28 \u0026Aring;. Thus, CTAB can infiltrate into between Au layers exposed after etching, only in parallel to Au layers, and would block the pathway for other CTAB to enter deeper. Cysteine and cysteamine have respective lengths below 9.28 \u0026Aring;, smaller than CTAB, and would more easily diffuse in-between Au layers, although they would eventually block the pathway too (supplementary section S5). Therefore, the free-standing 2D Au appears only at the edge of several nm near the etching frontline.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe mild etching is crucial for goldene derivatives from Ti\u003csub\u003e3\u003c/sub\u003eAu\u003csub\u003en\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e compounds: the lower the concentration is, the better the yield and quality of resulting Au layers are. However, this trend ends with around concentrations of 0.2 and 0.5 % Murakami\u0026rsquo;s reagent. 0.1 % resulted in complete decomposition of 2D Au layers into 3D spherical nanoparticles, due to the lower concentration needing much longer etching duration. The etchants would gradually attack the free-standing Au layers initially stabilized by surfactants as the radical nascent oxygen produced from the etchants eventually attack the surfactants. Therefore, considering the balance of mildness and duration of etching, the concentration between 0.2 and 1 % would be optimal for the Au layers to survive. 0.2 % Murakami\u0026rsquo;s reagent with CTAB allowed 5x5 nm free-standing 2D Au to survive, but the blob formation at the edge of the 2D Au progresses simultaneously. Further etching would advance the blob formation at the edges and Au atoms of the blob diffuse laterally through the layer. Therefore, we observed gold layers to become thicker after etching for long duration (supplementary section S4). A previous report demonstrated the free-standing two-atomic layer thick \u0026lsquo;2D\u0026rsquo; Au membranes where the main structure is fcc, but hcp reconstruction was observed at the edges\u003csup\u003e25\u003c/sup\u003e. This non-fcc feature has been found to stabilize planar Au structures\u003csup\u003e29\u003c/sup\u003e. Here, the blob formed at the edges next to the free-standing 2D Au in our observation would function as a stabilizer to maintain free-standing goldene. Appropriate selection of etchant concentrations and smaller, more suitable surfactants would trigger such reconstruction of the free-standing 2D Au edges and enable higher yield of goldene.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBesides etchant concentrations and surfactants, a decisive factor for preparing goldene from a series of non-van der Waals Au-intercalated MAX-phases is the interlayer distance between Au layers within the MAX-phases. Our AIMD simulations demonstrate that free-standing 2D Au layers coalesce with the presence of Si impurities and if brought within 7 \u0026Aring; interlayer distance between them; however, as their separation distance increases from 7 to 10, 12, and 14 \u0026Aring;, the goldene-goldene interlayer interaction decreases (supplementary section S3). The distance between Au layers within Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e is 9.28 \u0026Aring;. Thus, instant interlayer interaction is expected without surfactants as soon as goldene layers are isolated from Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e during etching. Here, we produced the first Au-intercalated 413 MAX-phases of Ti\u003csub\u003e4\u003c/sub\u003eAuC\u003csub\u003e3\u003c/sub\u003e from Ti\u003csub\u003e4\u003c/sub\u003eSiC\u003csub\u003e3\u003c/sub\u003e (supplementary section S6). The distance between Au layers within Ti\u003csub\u003e4\u003c/sub\u003eAuC\u003csub\u003e3\u003c/sub\u003e is about 12 \u0026Aring;. Therefore, a delay of goldene interactions after etching is expected, giving rise to a longer grace period of surfactants to stabilize the goldene sheets before they coalesce and a greater likelihood of their survival.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn conclusion, analytical electron microscopy corroborated by AIMD simulations shows the existence of \u003cem\u003egoldene\u003c/em\u003e,\u003cem\u003e\u0026nbsp;monolayer sheets of Au\u003c/em\u003e, as prepared by etching away Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e slabs from Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e. Essential are the precise dilution of the Murakami\u0026rsquo;s reagent as an etchant and the appropriate usage of passivating surfactants of CTAB and cysteine. Our developed processing scheme (supplementary section S5-7) has potential for expansion of the goldene sheet area by means of etching and surfactant optimization. The template SiC(0001) wafer size that sets the area limit of growing epitaxial Ti\u003csub\u003e3\u003c/sub\u003eSiC\u003csub\u003e2\u003c/sub\u003e for subsequent Au intercalation is 300 mm\u003csup\u003e30\u003c/sup\u003e, which would give an ample allowance for practical \u003cem\u003egoldene\u003c/em\u003e application. \u003c/p\u003e"},{"header":"References","content":"\u003cp\u003e1\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Wang, L.\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e Two-dimensional gold nanostructures with high activity for selective oxidation of carbon\u0026ndash;hydrogen bonds. \u003cem\u003eNature communications\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 6957 (2015).\u003c/p\u003e\n\u003cp\u003e2\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Wang, X., Wang, C., Chen, C., Duan, H. \u0026amp; Du, K. 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Gold nanobridge stabilized by surface structure. \u003cem\u003ePhysical Review Letters\u003c/em\u003e \u003cstrong\u003e79\u003c/strong\u003e, 3455 (1997).\u003c/p\u003e\n\u003cp\u003e30\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Fashandi, H.\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e Single-step synthesis process of Ti\u003csub\u003e3\u003c/sub\u003eSiC\u003csub\u003e2\u003c/sub\u003e ohmic contacts on 4H-SiC by sputter-deposition of Ti. \u003cem\u003eScripta Materialia\u003c/em\u003e \u003cstrong\u003e99\u003c/strong\u003e, 53-56 (2015).\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eTi\u003csub\u003e3\u003c/sub\u003eAu\u003csub\u003en\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e films were prepared by Au-intercalation into Ti\u003csub\u003e3\u003c/sub\u003eSiC\u003csub\u003e2\u003c/sub\u003e. After polishing away the residual Au, the films were etched by the mixture of KOH and K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] with various concentrations together with CTAB, cysteine, and cysteamine in the aqueous solution, to produce free-standing single- and multiple-atom-thick 2D Au. Structural analysis was carried out by high-resolution scanning transmission electron microscopy high angle annular dark field (HRSTEM-HAADF) imaging within Link\u0026ouml;ping\u0026rsquo;s double CS corrected FEI Titan\u003csup\u003e3\u003c/sup\u003e 60\u0026minus;300 microscope operated at 300 kV. Detailed information can be found in the supplementary information section S7.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAIMD simulations were carried out using the VASP code implemented with the projector augmented wave\u0026nbsp;method\u003csup\u003e31,32\u003c/sup\u003e and the generalized-gradient electronic exchange and correlation approximation\u003csup\u003e33\u003c/sup\u003e. An average temperature of 300 K is controlled via the Nos\u0026eacute;-Hoover thermostat (NVT ensemble). The classical equations of motion are integrated on timesteps of 1 fs. Spin-orbit coupling and relativistic effects are neglected. Van der Waals interactions are modeled using Grimme\u0026rsquo;s approximation\u003csup\u003e34\u003c/sup\u003e. At each ionic step, the energy is calculated with accuracy of 10\u003csup\u003e\u0026ndash;5\u003c/sup\u003e eV/supercell, using 300 eV cut-off for the planewave basis set and \u0026Gamma;-point sampling of the reciprocal space. Detailed information can be found in the supplementary information section S3.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this work is available within the article and the corresponding supplementary information.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReferences\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e31\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Kresse, G. \u0026amp; Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. \u003cem\u003ePhysical Review B\u003c/em\u003e \u003cstrong\u003e54\u003c/strong\u003e, 11169-11186 (1996).\u003c/p\u003e\n\u003cp\u003e32\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Kresse, G. \u0026amp; Joubert, D. 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Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction. \u003cem\u003eJournal of computational chemistry\u003c/em\u003e \u003cstrong\u003e27\u003c/strong\u003e, 1787-1799 (2006).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Swedish Research Council project grant 2017-03909 (to L.H.) and 2021-04426 (to D.G.S.), Swedish Government Strategic Research Area in Materials Science on Functional Materials at Link\u0026ouml;ping University SFO-Mat-LiU 2009 00971, and the Wallenberg Scholar Program KAW 2019.0290 (to L.H.). All simulations were carried out using the resources provided by the Swedish National Infrastructure for Computing (SNIC) \u0026ndash; partially funded by the Swedish Research Council through Grant Agreement N\u0026ordm; VR-2018-05973. J. R. acknowledges funding from the G\u0026ouml;ran Gustafsson foundation for Research in Natural Sciences and Medicines.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eL.H. conceived and supervised the study. J.R, S.K., and L.H. designed the study. Implementation was made by S.K. (MAX phase film growth, intercalation experiments, etching/surfactant protocols, and STEM sample preparation), Y.S (MAX phase film growth, intercalation experiments, and STEM sample preparation), M.A (Au film growth). J.L. (STEM analysis), and D.G.S. (AIMD simulations). S.K., Y.S., J.L., and D.G.S. performed the data analysis. S.K., D.G.S., and L.H. wrote the manuscript. All authors contributed to the final version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe online version contains supplementary materials available at \u0026hellip; (information added when published online)\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"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":"Free-standing, elemental 2D materials","lastPublishedDoi":"10.21203/rs.3.rs-2807259/v2","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2807259/v2","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe quest to make free-standing monolayer gold has hitherto been limited to free-standing several-atoms-thick layers, or monolayers but confined on or inside another template. Examples are monolayers diffused into double hydroxides\u003csup\u003e1\u003c/sup\u003e, membranes framed in alloys under electron irradiation\u003csup\u003e2\u003c/sup\u003e, nano-ribbons suspended in graphene\u003csup\u003e3\u003c/sup\u003e, quantum dots on hexagonal BN\u003csup\u003e4\u003c/sup\u003e, monolayers in between SiC wafers and monolayer graphene\u003csup\u003e5\u003c/sup\u003e, and fragments produced via thermal dewetting on sapphire\u003csup\u003e6\u003c/sup\u003e. Here, we report the synthesis of free-standing single-atom-thick 2D gold (named \u003cem\u003egoldene\u003c/em\u003e) by wet-chemically etching away Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e from Ti\u003csub\u003e3\u003c/sub\u003eAuC\u003csub\u003e2\u003c/sub\u003e, a nano-laminated MAX-phase ceramic initially formed by substituting Si in Ti\u003csub\u003e3\u003c/sub\u003eSiC\u003csub\u003e2\u003c/sub\u003e with Au\u003csup\u003e7\u003c/sup\u003e. The free-standing \u003cem\u003egoldene\u003c/em\u003e layers are revealed by scanning transmission electron microscopy. While \u003cem\u003eab initio\u003c/em\u003e molecular dynamics simulations show that 2D \u003cem\u003egoldene\u003c/em\u003e is inherently stable, the experiments reveal a tendency for curling and agglomeration at edges.\u0026nbsp; Prospects for preparing \u003cem\u003egoldene\u003c/em\u003e from a series of non-van der Waals Au-intercalated MAX-phases, including developing etching schemes, are also presented.\u003c/p\u003e","manuscriptTitle":"Goldene: Free-standing Single-atom-thick Sheets of Gold","msid":"","msnumber":"","nonDraftVersions":[{"code":2,"date":"2023-04-18 20:05:11","doi":"10.21203/rs.3.rs-2807259/v2","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}},{"code":1,"date":"2023-04-13 16:07:04","doi":"10.21203/rs.3.rs-2807259/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":"ad3815dc-57e9-434a-9474-1e997db3341c","owner":[],"postedDate":"April 18th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":20794259,"name":"Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials"},{"id":20794260,"name":"Physical sciences/Materials science/Nanoscale materials/Synthesis and processing"}],"tags":[],"updatedAt":"2024-04-22T17:21:30+00:00","versionOfRecord":{"articleIdentity":"rs-2807259","link":"https://doi.org/10.1038/s44160-024-00518-4","journal":{"identity":"nature-synthesis","isVorOnly":false,"title":"Nature Synthesis"},"publishedOn":"2024-04-16 00:00:00","publishedOnDateReadable":"April 16th, 2024"},"versionCreatedAt":"2023-04-18 20:05:11","video":"","vorDoi":"10.1038/s44160-024-00518-4","vorDoiUrl":"https://doi.org/10.1038/s44160-024-00518-4","workflowStages":[]},"version":"v2","identity":"rs-2807259","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2807259","identity":"rs-2807259","version":["v2"]},"buildId":"WrCJVZZCHTDjtuVLN7oU0","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}