Molecular beam epitaxial step-edge growth of Bi2Te3/multi-stepped Sb2Te3 nanoplate hetero-structures

Molecular beam epitaxial step-edge growth of Bi2Te3/multi-stepped Sb2Te3 nanoplate hetero-structures | 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 Molecular beam epitaxial step-edge growth of Bi 2 Te 3 /multi-stepped Sb 2 Te 3 nanoplate hetero-structures Gyu-Chul Yi, Yoonkang Kim, Sangmin Lee, Eunsu Lee, Seongbeom Kim, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4586406/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 We report the synthesis of multiple Bi 2 Te 3 shells on multi-stepped Sb 2 Te 3 nanoplates using molecular beam epitaxial (MBE) step-edge growth. For the growth of Bi 2 Te 3 /Sb 2 Te 3 hetero-structures, multi-stepped Sb 2 Te 3 nanoplates with stair-like morphology following layer-by-layer (LBL) growth mode were obtained by optimizing the growth temperature, and the growth of Bi 2 Te 3 on the step-edges of the Sb 2 Te 3 nanoplates was followed. Width of Bi 2 Te 3 on the Sb 2 Te 3 nanoplates was controlled by changing the growth time. Structural properties of the hetero-structures were investigated using aberration-corrected (C s -corrected) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), revealing the interface between Sb 2 Te 3 and Bi 2 Te 3 . In-plane epitaxial relation at the interface was confirmed using fast Fourier transforms (FFTs). Compositional analysis of Bi 2 Te 3 and Sb 2 Te 3 was verified through energy-dispersive X-ray spectroscopy. Furthermore, we performed density functional theory (DFT) calculations to confirm the preferential growth of Bi 2 Te 3 on the step-edges of Sb 2 Te 3 . By forming multi-stepped core structure, it would be feasible to create various integrated hetero-structures. multi-stepped nanoplate integrated hetero-structure step-edge growth molecular beam epitaxy transmission electron microscopy energy-dispersive X-ray spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction There have been tremendous interests in two dimensional (2D) van der Waals (vdW) nano materials hetero-structures for diverse electronic and optical devices. This led to investigations on fabricating various vertical hetero-structures by layer-by-layer stacking of 2D materials 1 , 2 . Concurrently, there has been an emerging interest in lateral hetero-structures 3 – 6 . Unlike vertical hetero-structures, these are formed through mechanisms involving interactions such as dangling bonds and chemical covalent bonding, typically achieved by integrating different materials using growth techniques like chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Despite significant advancements, the field still faces some challenges, particularly in integrating diverse structural configurations within a single material system. Generally known configurations of vertical and lateral hetero-structures are (A/B)/C and (A-B)/C, respectively, where A and B are as-grown materials while C is the substrate. In this respect, previous hetero-structure fabrications had limited configurations, leaving the coexistence of vertical and lateral hetero-structures within the same sample—which we refer to as integrated hetero-structures—an unexplored domain of research. Integrated hetero-structures have the potential to form multiple junctions, similar to those created by sequential epitaxy in multi-lateral hetero-structures 7 , and even more complex properties can be expected, regarding the mix of in-plane bonding and vdW interactions. Previous research has documented the growth of integrated (vertical and lateral) hetero-structures using transition metal dichalcogenides (TMDs) 8 . However, the lateral overgrowth of secondary materials (WSe 2 ) occurred, making it challenging to maintain them separately due to the rapid growth rates associated with the CVD method. Here, we address this issue by employing MBE, which allows for controlled and slow growth rates 9 , 10 , to develop integrated hetero-structures of Bi 2 Te 3 /multi-stepped Sb 2 Te 3 , avoiding Bi 2 Te 3 to merge with each other. Specifically, we utilize multi-stepped Sb 2 Te 3 nanoplates with stair-like configurations as the core material, enabling the growth of multiple Bi 2 Te 3 shells on each Sb 2 Te 3 step through step-edge growth. Since Sb 2 Te 3 and Bi 2 Te 3 are of the most intriguing topological insulator (TI) materials 11 – 13 , our hetero-structures can provide a promising platform for exploring more complex quantum phenomena. The multiple Bi 2 Te 3 shells formed on Sb 2 Te 3 steps would potentially enable the formation of multiple loops of quantum wires. Notably, as the loop decreases in size towards the core, and with graphene as the base material for the outermost shell, unlike the inner shells’ Sb 2 Te 3 base, distinct properties can be investigated through this platform. Materials and methods A. MBE growth of Sb 2 Te 3 and Bi 2 Te 3 on graphene Multi-stepped Sb 2 Te 3 nanoplates and Bi 2 Te 3 /Sb 2 Te 3 hetero-structures were grown on graphene layers using a custom-designed ultrahigh vacuum (UHV) MBE system. The base pressure of the growth chamber was in the range of mid 10 − 9 Torr. Prior to the growth, thermal cleaning was carried out at 820 \(\text{℃}\) for 10 minutes in an UHV. Then, the temperature decreased to 285 \(\text{℃}\) with sufficient stabilizing (> 20 min). For multi-stepped Sb 2 Te 3 growth, high-purity Sb (99.9999%) and Te (99.9999%) were supplied, with Sb and Te fluxes maintained at approximately 0.02 \(\dot{A}\) /s and 0.35 \(\dot{A}\) /s, respectively 14 , 15 , as measured by a quartz crystal microbalance (QCM). Following approximately 10 minutes of growth, the shutter of the Sb Knudsen cell was closed, and without altering the growth temperature, Bi (99.9999%) was supplied at a flux of approximately 0.02 \(\dot{A}\) /s, with the Te Knudsen cell shutter remaining open. After the completion of Bi 2 Te 3 growth, a natural cooling process to 250 \(\text{℃}\) was proceeded, with continued Te supply. B. Growth substrate preparation For substrate preparation, graphene layers were transferred either onto TEM-compatible membrane or Si wafer covered with a 300 nm thick SiO 2 layer. For transfer onto membrane, dry pick-up method using polypropylene carbonate (PPC) on polydimethylsiloxane (PDMS) was used while mechanical exfoliation technique was employed for transfer onto SiO 2 /Si wafers. Especially, membrane preparation involved several steps. Initially, a ∼200 nm SiN x film was deposited on both sides of a ∼200 µm (100) Si wafer using low-pressure chemical vapor deposition (LPCVD). Subsequently, the upper and lower SiN x layers were selectively removed through conventional lithography and reactive ion etching (RIE). After RIE, wet etching was employed to eliminate the Si wafer, thereby creating a window-like freestanding region for transferring the graphene layers 16 , 17 . The width of this freestanding region was between 2∼5 µm. C. Morphological and microstructural characterizations The structural analysis of the as-grown materials was conducted using electron microscopy and atomic force microscopy techniques. Specifically, a field emission scanning electron microscope (FE-SEM) (MERLIN Compact, ZEISS) was utilized, operating at an electron acceleration voltage of 3kV, allowing the acquisition of detailed surface morphology. For more in-depth plan-view structural analysis at atomic resolution, an aberration-corrected (C s -corrected) high-angle annular dark field scanning TEM (HAADF-STEM) (JEM-ARM200F, JEOL) was employed, with an acceleration voltage set at 200kV. In addition, energy-dispersive X-ray spectroscopy (EDS), integrated into the HAADF-STEM system, was utilized to analyze the elemental properties of the samples. Furthermore, to examine the multiple steps characteristic of Sb 2 Te 3 nanoplates, an atomic force microscope (NX10, Park Systems) was used, alongside the accompanying XEI software for data analysis. D. Computational details Ab initio calculations were performed using the Vienna ab initio simulation package (VASP) 18 , employing density functional theory within the generalized gradient approximation. The optimization of lattice parameters and atomic positions proceeded until the forces on each atom reached a threshold of < 0.01 eV \({\dot{A}}^{-1}\) . Electronic wave functions were expanded using a plane wave basis with a kinetic energy cutoff set at 500 eV. To account for van der Waals interactions, Grimme’s DFT-3 method was incorporated 19 . A vacuum layer exceeding 20 \(\dot{A}\) was introduced to prevent spurious interactions between adjacent slabs. Brillouin zone sampling utilized a 1 × 13 × 1 k-point mesh based on the Monkhorst–Pack scheme 20 . To calculate the interfacial energy, we conducted calculations for the interfacial energy of Bi 2 Te 3 /Graphene using a \({(5\times 5)}_{{Bi}_{2}{Te}_{3}}\) - \({(9\times 9)}_{Graphene}\) epitaxial supercell, taking into account the lattice constants of different structures. For the Bi 2 Te 3 /Sb 2 Te 3 interface, calculations were carried out using a coherent Sb 2 Te 3 -Bi 2 Te 3 hetero-structure, given the insignificant lattice mismatch. Results and Discussion Multi-stepped Sb 2 Te 3 nanoplates and hetero-structures (Bi 2 Te 3 /Sb 2 Te 3 ) were grown on graphene layers using an UHV MBE system. The graphene substrates were prepared onto either SiO 2 /Si wafers or SiN x /Si TEM-compatible membranes. Initially, Sb 2 Te 3 was grown on graphene layers atop SiO 2 /Si to study its morphology. Once the optimal conditions were identified, hetero-structures were then grown on graphene layers supported by membranes, allowing for TEM observations to be conducted without the need for additional sample processing. High-purity sources of Sb (99.9999%), Bi (99.9999%), and Te (99.9999%) were used to facilitate material growth under Te-rich conditions, while maintaining the background pressure of the growth chamber at a low 10 − 9 Torr. For more details, refer to the Materials and methods sections A & B. We investigated the surface morphology of Sb 2 Te 3 grown at various temperatures using field-emission scanning electron microscopy (FE-SEM). Figure 1 (a) illustrates the morphology of Sb 2 Te 3 grown at 250, 270, 285, and 320 \(\text{℃}\) . At lower temperatures of 250 and 270 \(\text{℃}\) , high-density, multi-spiral structures covered the surface of the sample, preventing the formation of isolated islands. At 320 \(\text{℃}\) , the islands drastically shrank in size with minimal material remaining on the graphene surface, indicating that the coverage by as-grown materials decreases as temperature increases. To delve deeper into the structural changes, we analyzed the contrasts in the FE-SEM images, which are indicative of topographic contrasts due to variations in the height of the as-grown materials affecting the emission of secondary electrons and thus the image brightness 21 . Notably, the material height increases towards the center for all Sb 2 Te 3 grown at various temperatures. However, a change in the morphology from spiral to triangular stacking was observed as temperature increased to 285 \(\text{℃}\) , indicating a growth mode transition. According to the Burton-Cabrera-Frank (BCF) theory 22 , 23 , crystal growth can proceed via dislocation-driven or layer-by-layer (LBL) modes. Dislocation-driven growth is characterized by the addition of atoms at defect sites within the crystal, often resulting in spiral steps around these dislocations 24 . Conversely, the LBL mode involves sequential atom deposition, building new layers on the existing surface 25 , 26 . Previous studies have reported spiral morphology in Sb 2 Te 3 nanoplates due to screw dislocation-driven (SDD) growth 27 , 28 . In this respect, a transition in the growth mode occurred at temperatures between 270 and 285 \(\text{℃}\) , shifting from SDD to LBL mode 29 . Consequently, the multi-stepped Sb 2 Te 3 nanoplates grown at 285 \(\text{℃}\) appear to follow the LBL growth mode, exhibiting multiple layers with a decreasing lateral size towards the center. Surface morphology of a multi-stepped Sb 2 Te 3 nanoplate, grown at 285 \(\text{℃}\) , was investigated using atomic force microscopy (AFM), providing a nanometer scale information of the surface. Figure 1 (b) presents the 2D surface morphology and corresponding line profile of the Sb 2 Te 3 nanoplate. We revealed that the height of the outermost step was 2∼3 nm and the heights of inner steps were specified to be less than 1 nm. In addition, each terrace has width of ∼100 nm, which offers sufficient space for shell growth with width of several tens of nanometers, avoiding lateral overgrowth. So the presence of multiple steps with sufficient terrace width suggests the feasibility of step-edge growth, depicted in Fig. 2 (a). The surface morphologies of multi-stepped Sb 2 Te 3 nanoplates can be classified into two types; rotational and parallel hierarchical, as depicted in the left sides of Figs. 2 (b), (c). For the first case where the below step is triangular (or hexagonal) and the upper step is also triangular (or hexagonal) with all pairs of edges parallel to each other, the parallelism of the edges across the hierarchical steps, maintains a consistent orientation from one level to the next. For the second case, where the below step is triangular (or hexagonal) and the upper step is also triangular (or hexagonal) but is rotated 180 degrees from the below step, the rotational transformation applied to the subsequent hierarchical step, indicates a significant orientation change while preserving the shape’s symmetry. As previously mentioned, our Sb 2 Te 3 follows LBL growth mode where vdW interaction exists between the layers. In this respect, parallel hierarchical ones would be straightforward and energetically favorable configuration, considering R \(\stackrel{-}{3}\) m group’s symmetry (from Fig. 3 (c)). However, due to the triangular (or hexagonal) shape’s inherent symmetry, rotating by 180 degrees does not change the relative positions of the lattice points. Thus, rotational hierarchical configuration is not abnormal. The point is that despite their distinct geometric features, they commonly possess multiple step-edges which allows the subsequent Bi 2 Te 3 growth. Following Fig. 2 (a), preceding hetero-structure formation was done on graphene layers on TEM-compatible membranes. For the growth, Bi 2 Te 3 was grown at same temperature with time of 2 minutes right after Sb 2 Te 3 was grown. Right sides of Figs. 2 (b), (c) show the FE-SEM images of Bi 2 Te 3 /Sb 2 Te 3 hetero-structures. For a single Sb 2 Te 3 growth, the contrasts on the Sb 2 Te 3 nanoplate become brighter as they approach the core due to their multiple steps. However, for Bi 2 Te 3 /Sb 2 Te 3 , bright and dark contrasts alternately repeated from the outer to the core of the nanoplate. This change implies Bi 2 Te 3 was grown on the multi-stepped Sb 2 Te 3 nanoplate. In addition, we controlled the width of Bi 2 Te 3 shell by changing the growth time. Figure 2 (d) depicts the FE-SEM images of the three cases-growth time of Bi 2 Te 3 being 1, 2, and 3 minutes. The widths of outermost shells were ∼20 nm and ∼40 nm for 1 and 2 minutes, respectively. However, for 3 minutes of growth, it was difficult to mark the outermost shell, and the inner steps were not clearly specified due to lateral overgrowth, previously mentioned in Introduction section. The result made it possible to tune the degree of horizontal growth next to the step of the Sb 2 Te 3 suggesting the possibility to control the shell width to some extent, by varying growth parameters beyond just growth time, such as source fluxes. If such refined width control becomes feasible, it opens up the possibility for in-depth research on various structures. A prime example of this would be nanowires made of topological insulators. In the case of hetero-structures of topological insulators, the topological properties are preserved 14 . Under these circumstances, by considering each shell as a wire structure and adjusting the width, one can examine the variations in properties such as quantum confinement of surface states 30 . Given that each step’s height is same or less than 2 nm, and the shell’s width does not surpass the terrace’s width avoiding lateral overgrowth, this platform offers the potential for significant dimensional confinement. In addition, as the horizontal length of the shells decreases towards the interior of the island, it becomes possible to investigate the impact of this length on transport characteristics 31 , in one specimen. To characterize the crystal structure and the epitaxial relationship between Sb 2 Te 3 and Bi 2 Te 3 , plan-view TEM observations were proceeded. Since the membrane is TEM- compatible, no additional sampling processes were required. Figure 3 (a) shows the low- magnification plan-view image obtained using high-angle annular dark field scanning TEM (HAADF-STEM). The inset shows a FE-SEM image of the target nanoplate. First, the outermost region boxed with the red line (the first step of the multi-stepped Sb 2 Te 3 nanoplate and the lateral Bi 2 Te 3 shell) was investigated. Aberration-corrected (C s -corrected) atomic resolution images are exploited as shown in Fig. 3 (b). The overlaid ball-stick model is obtained through the calculation of lattice parameters from FFT patterns. The ratio of d-spacing of {10 − 10} ST, BT to {11 − 20} ST, BT is \(\sqrt{1/3}\) which can be converted to ratio of \(\sqrt{3}\) in real space. In addition, {11 − 20} ST, BT peaks make an angle of \({30}^{^\circ }\) to {10 − 10} ST, BT peaks. Thus, Sb 2 Te 3 and Bi 2 Te 3 both are classified as rhombohedral phase with the trigonal crystal system (space group of R \(\stackrel{-}{3}\) m, a = b ≈ 4.25 \(\dot{A}\) for Sb 2 Te 3 and a = b ≈ 4.4 \(\dot{A}\) for Bi 2 Te 3 ) 32 . Since the contrast shown in the STEM image is dependent on Z values, the interface of Sb 2 Te 3 and Bi 2 Te 3 is apparently shown. Then, to understand the relation between two materials, fast Fourier transforms (FFTs) from three regions-Sb 2 Te 3 , Bi 2 Te 3 , and their interface were obtained. Figure 3 (c) depicts the FFT patterns. From the peaks’ orientation; {11 − 20} BT ∥ {11 − 20} ST and {10 − 10} BT ∥ {10 − 10} ST , their lateral relation is confirmed. The pattern from the interface shows that peaks from Sb 2 Te 3 and Bi 2 Te 3 almost overlap indicating that the orientations of the two materials are lateral to each other. Furthermore, energy-dispersive X-ray spectroscopy (EDS) was performed to validate the growth of Bi 2 Te 3 at each step of the multi-stepped Sb 2 Te 3 . First, the outermost region is investigated. The HAADF-STEM image of the target region is shown in Fig. 4 (a). The image includes three regions-I: the outermost Bi 2 Te 3 shell (lateral to the first-step of Sb 2 Te 3 ), II: the first-step of Sb 2 Te 3 and III: the inner Bi 2 Te 3 shell (vertical/lateral to the first/second-step of Sb 2 Te 3 ). As depicted in 2D EDS spectra map, in region I, the Bi signal is dominant, whereas a negligible Sb signal is observed, while it is vice versa in region II. The interfaces of regions I and II in the 2D EDS image match well with the interface shown in the HAADF-STEM image. For region III, since it is not the outermost (the first) shell of the nanoplate, it is the second Bi 2 Te 3 shell with the first-step of Sb 2 Te 3 beneath it. Therefore, the Sb signal (from the first-step) emerges in region III together with the Bi signal. The EDS data also represent Te signals emerging regardless of the region, which is trivial. The inner parts of the nanoplate are shown in Fig. 4 (b). Similar to the previous investigation, four regions were identified. I: the first-step of Sb 2 Te 3 , II: the second Bi 2 Te 3 shell (vertical/lateral to the first/second-step of Sb 2 Te 3 ), III: the second-step of Sb 2 Te 3 , and IV: the third Bi 2 Te 3 shell (vertical/lateral to the second/third-step of Sb 2 Te 3 ). Since regions I and III are suspected to be the regions where only Sb 2 Te 3 is grown, weak Bi signals are observed. Furthermore, considering the 2D EDS spectra for regions II and IV, which are suspected to be the shell regions, the sections where Bi signals appear well match the contrasts shown in HAADF-STEM image. Since the investigated area contains the first-(region I and beneath II), and the second-(region III and beneath IV) step of Sb 2 Te 3 , the Sb signal comes out from the entire area just like Te signal. Unlike the atomic resolution HAADF-STEM image shown in Fig. 3 (b), the 2D EDS data reveal more than just the presence of the outermost Bi 2 Te 3 shell; they also indicate the formation of inner shells, each vertically/laterally grown above/to, the respective steps of the Sb 2 Te 3 nanoplate. This is further corroborated by the alignment between the alternating Bi signals and the contrasts observed in 2D EDS and HAADF- STEM images, respectively. This makes it possible to claim that multiple Bi 2 Te 3 shells are separately grown at each step of the multi-stepped Sb 2 Te 3 nanoplate. Computational analyses were conducted to investigate the formation of multi-stepped Sb 2 Te 3 nanoplate-based hetero-structures through density functional theory (DFT) calculations. At first, this study aimed to ascertain the preference of Bi 2 Te 3 towards step-edge growth by evaluating its total energy. As illustrated in Fig. 5 (a), four distinct scenarios were established in accordance with the methodologies outlined in the ‘D. Computational Details’ section. Each scenario involved a multi-stepped Sb 2 Te 3 nanoplate configuration, characterized by a reduced domain width at the second step, which served as a constant parameter across all models. The top row of Fig. 5 (a) describes instances of vertical Bi 2 Te 3 growth, which occurs via van der Waals (vdW) interactions with the base layer of Sb 2 Te 3 at the second step. Conversely, the bottom row presents scenarios that incorporate a combination of two distinct interactions: the formation of dangling bonds with the Sb 2 Te 3 at the second step, and vdW interactions with the Sb 2 Te 3 at the first step. A comparative analysis of the total energies for each pair (column) of scenarios reveals that the configurations in the bottom row exhibit lower total energies; 4.6852 and 4.7415 eV lower, respectively. This suggests a thermodynamic preference for Bi2Te3 growth at the step edges of Sb2Te3 nanoplates. Additionally, the findings generally align well with the growth behavior that favors a mix of in-plane bonding and vdW interactions rather than isolated interactions 33 , 34 . We have also calculated the interfacial energies for Bi 2 Te 3 /Graphene and Bi 2 Te 3 /Sb 2 Te 3 with supercells depicted in Fig. 5 (b). As-grown Bi 2 Te 3 shells can be classified into two; the outermost and the others. They are commonly grown via step-edge growth, assisted by dangling bonds from the side of Sb 2 Te 3 . So the only difference is induced from the base materials. Since Bi 2 Te 3 and Sb 2 Te 3 have negligible lattice mismatch compared to graphene, the interfacial energy was expected to be lower. The calculation reveals that Bi 2 Te 3 /Graphene has approximately 0.0058 eV/ \({\dot{A}}^{2}\) higher interfacial energy which might be induced from the existence of the strains. As previously mentioned, the hetero-structure we have grown can act as a platform offering multiple Bi 2 Te 3 nanowires. In this context, the calculation highlights the hetero-structure’s potential for investigating distinct strain-induced transport behaviors 35 – 37 . Conclusions In conclusion, we created Bi 2 Te 3 /multi-stepped Sb 2 Te 3 nanoplate hetero-structures. For the growth of the hetero-structures, multi-stepped Sb 2 Te 3 nanoplates with stair-like morphology were prepared at the growth temperature of 285 \(\text{℃}\) , and the growth of Bi 2 Te 3 shell layers on the step-edges of the Sb 2 Te 3 nanoplates was subsequently performed at the same temperature. Through FE-SEM and AFM, the steps of Sb 2 Te 3 and morphologies of hetero-structures were verified. Moreover, we were able to control the width of Bi 2 Te 3 on the Sb 2 Te 3 nanoplates by changing the growth time. Through C s -corrected HAADF-STEM, the in-plane epitaxy between two materials at the outermost, was investigated; {11 − 20} BT ∥ {11 − 20} ST and {10 − 10} BT ∥ {10 − 10} ST . For compositional analysis, 2D EDS spectra were mapped showing the Bi signals well matching with the alternating contrasts shown in HAADF-STEM images. Furthermore, DFT calculations indicated a preference for Bi 2 Te 3 to grow along the step-edges of Sb 2 Te 3 nanoplates, verifying the step-edge growth. Additionally, the calculated interfacial energy of Bi 2 Te 3 /Graphene was higher than that of Bi 2 Te 3 /Sb 2 Te 3 , which can be attributed to lattice mismatches causing strain. We believe that multi-stepped 2D materials have the potential to act as effective cores for integrated hetero-structure synthesis, possessing the potential to create multiple quantum structures through step-edge growth. Declarations Acknowledgements This work was financially supported by National Research Foundation (NRF) of Korea (NRF-2021R1A5A1032996) and the Science Research Center (SRC) for Novel Epitaxial Quantum Architectures. This research was also supported by grants NRF-2022R1A2C3007807, and NRF-2019M3D1A1079215 from the NRF of Korea. Additionally, we also acknowledge the Brain Korea 21-Plus Program, the Institute of Applied Physics (IAP). Research Institute of Advanced Materials (RIAM) at Seoul National University. Conflict of interest The authors declare no competing interests. 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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-4586406","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":318678039,"identity":"1970d3d4-c3ff-4546-9146-75bf2443bf96","order_by":0,"name":"Gyu-Chul Yi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAArUlEQVRIiWNgGAWjYJCCAx+gDMYGYrUcnEGyFmYekrTIu58xPGzz505iA/vhB4wz9xChxfBMjsHh3LZniQ08aQaMG54Ro2UGD1BLw+HEBoYcBsYHB4jVYvEHqIX/DZFa5CWAWhjYgFokgLZsIEaLAU9awcHetsPGbRLPDA7OIMqW9sObP/z4c1i2nz/54cMeomw5wGEAZrABMTEagLY0sD8gSuEoGAWjYBSMYAAAcQA7iSytxCQAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-7703-3455","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":true,"prefix":"","firstName":"Gyu-Chul","middleName":"","lastName":"Yi","suffix":""},{"id":318678040,"identity":"fe21963d-813c-4776-a715-4d2eeb1deb2c","order_by":1,"name":"Yoonkang Kim","email":"","orcid":"https://orcid.org/0009-0004-7641-7491","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Yoonkang","middleName":"","lastName":"Kim","suffix":""},{"id":318678041,"identity":"d5f5f916-e7d7-4472-9ac6-fb32d4656a7b","order_by":2,"name":"Sangmin Lee","email":"","orcid":"","institution":"DEPARTMENT OF MATERIAL SCIENCE ENGINEERING and Research Institute of Advanced Materials, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Sangmin","middleName":"","lastName":"Lee","suffix":""},{"id":318678042,"identity":"239a34c8-95cb-4355-a0a9-feaecbb3f09e","order_by":3,"name":"Eunsu Lee","email":"","orcid":"","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Eunsu","middleName":"","lastName":"Lee","suffix":""},{"id":318678043,"identity":"7b125736-5760-491c-b2a0-e6a1a6aa27e9","order_by":4,"name":"Seongbeom Kim","email":"","orcid":"","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Seongbeom","middleName":"","lastName":"Kim","suffix":""},{"id":318678044,"identity":"71634c38-6556-4b59-98fc-45df5051b50f","order_by":5,"name":"Wonwoo Suh","email":"","orcid":"","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Wonwoo","middleName":"","lastName":"Suh","suffix":""},{"id":318678045,"identity":"8a6eeaae-2e8d-4f37-941c-c752caccbe87","order_by":6,"name":"Imhwan Kim","email":"","orcid":"","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Imhwan","middleName":"","lastName":"Kim","suffix":""},{"id":318678046,"identity":"a6ffade5-c645-4d46-af62-74a910449049","order_by":7,"name":"Junyeop Jeon","email":"","orcid":"","institution":"Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Junyeop","middleName":"","lastName":"Jeon","suffix":""},{"id":318678047,"identity":"202f60bb-78eb-4260-9c53-7705ba4e8cf5","order_by":8,"name":"Miyoung Kim","email":"","orcid":"https://orcid.org/0000-0001-8632-6711","institution":"Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Miyoung","middleName":"","lastName":"Kim","suffix":""}],"badges":[],"createdAt":"2024-06-15 11:55:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4586406/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4586406/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":60639483,"identity":"d1767c71-a359-4d4f-b3a6-e89762a8e491","added_by":"auto","created_at":"2024-07-19 03:12:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":206611,"visible":true,"origin":"","legend":"\u003cp\u003eSurface morphology of MBE-grown Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on graphene layers. (a) SEM images of one-step grown Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at different growth temperatures of 250, 270, 285, 320 respectively. (b) AFM image of an optimized multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate and corresponding line profile.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/1515d707749ce2c88ea3ca77.png"},{"id":60637940,"identity":"c2e1cce0-8692-429b-ab6d-f28e4ded5ab9","added_by":"auto","created_at":"2024-07-19 02:48:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":328052,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic of the fabrication process for step-edge growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates. (b), (c) Surface morphology of MBE-grown Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on graphene layers; SEM images of multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates showing different geometry and corresponding Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structures. (d) SEM images of as grown Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structures with different Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth times; 1, 2 and 3 minutes, respectively.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/48f1b67f0262efeb638776e2.png"},{"id":60638415,"identity":"5f4632eb-eb98-429b-8db7-9082c54c58dc","added_by":"auto","created_at":"2024-07-19 02:56:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":326726,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Low-magnification plan-view HAADF-STEM micrograph of as-grown structure, red box indicating the target region for atomic resolution imaging. (b) Atomic HAADF-STEM image (aberration corrected) of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e-Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e (ZA: [0001]). Ball-stick models overlaid [blue: Sb atom; yellow: Bi atom; pink: Te atom]. (c) FFT patterns taken from three regions-Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e (left), Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e (middle) and interface of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e (right).\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/256222019c0268ab7e7cf75e.png"},{"id":60638899,"identity":"cb8c6fe2-435e-4129-bc29-dc7636f97c94","added_by":"auto","created_at":"2024-07-19 03:04:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":523741,"visible":true,"origin":"","legend":"\u003cp\u003e(a), (b) Low-magnification plan-view HAADF-STEM micrographs and 2D EDS spectra maps of Bi, Sb and Te elements of outermost and inner regions, respectively. Left top insets of HAADF-STEM micrographs depict full morphology of the target structure with the red boxes indicating the target regions for atomic resolution imaging.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/64e0b4fd63b96c191f299b62.png"},{"id":60637944,"identity":"562f07a4-269b-47b6-a29f-7288bb8526ec","added_by":"auto","created_at":"2024-07-19 02:48:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":166277,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Four schemes (side view of ball-stick models [blue: Sb atom; yellow: Bi atom; pink: Te atom]) for one-shot total energy calculations. Top row: Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e grown via vdW interaction with Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at the second step. Bottom row: Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e grown on step-edge via dangling bond and vdW interaction. (b) Two schemes (top and side views of ball-stick models [blue: Sb atom; yellow: Bi atom; pink: Te atom, brown: C atom]) for full-relaxed interfacial energy calculations. Left and right columns indicate Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Graphene and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e super cells, respectively.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/f6fea9132e2523b7f8c7cf67.png"},{"id":61315397,"identity":"fabaa124-18ab-492a-843d-9a2a4dbac5e4","added_by":"auto","created_at":"2024-07-29 11:55:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2205687,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4586406/v1/af744095-2099-425d-8140-51489cc6ede1.pdf"}],"financialInterests":"(Not answered)","formattedTitle":"Molecular beam epitaxial step-edge growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate hetero-structures","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThere have been tremendous interests in two dimensional (2D) van der Waals (vdW) nano materials hetero-structures for diverse electronic and optical devices. This led to investigations on fabricating various vertical hetero-structures by layer-by-layer stacking of 2D materials\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Concurrently, there has been an emerging interest in lateral hetero-structures\u003csup\u003e\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Unlike vertical hetero-structures, these are formed through mechanisms involving interactions such as dangling bonds and chemical covalent bonding, typically achieved by integrating different materials using growth techniques like chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Despite significant advancements, the field still faces some challenges, particularly in integrating diverse structural configurations within a single material system. Generally known configurations of vertical and lateral hetero-structures are (A/B)/C and (A-B)/C, respectively, where A and B are as-grown materials while C is the substrate. In this respect, previous hetero-structure fabrications had limited configurations, leaving the coexistence of vertical and lateral hetero-structures within the same sample\u0026mdash;which we refer to as integrated hetero-structures\u0026mdash;an unexplored domain of research. Integrated hetero-structures have the potential to form multiple junctions, similar to those created by sequential epitaxy in multi-lateral hetero-structures\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, and even more complex properties can be expected, regarding the mix of in-plane bonding and vdW interactions.\u003c/p\u003e \u003cp\u003ePrevious research has documented the growth of integrated (vertical and lateral) hetero-structures using transition metal dichalcogenides (TMDs)\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. However, the lateral overgrowth of secondary materials (WSe\u003csub\u003e2\u003c/sub\u003e) occurred, making it challenging to maintain them separately due to the rapid growth rates associated with the CVD method. Here, we address this issue by employing MBE, which allows for controlled and slow growth rates\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, to develop integrated hetero-structures of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, avoiding Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e to merge with each other. Specifically, we utilize multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates with stair-like configurations as the core material, enabling the growth of multiple Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shells on each Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e step through step-edge growth. Since Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e are of the most intriguing topological insulator (TI) materials\u003csup\u003e\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, our hetero-structures can provide a promising platform for exploring more complex quantum phenomena. The multiple Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shells formed on Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e steps would potentially enable the formation of multiple loops of quantum wires. Notably, as the loop decreases in size towards the core, and with graphene as the base material for the outermost shell, unlike the inner shells\u0026rsquo; Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e base, distinct properties can be investigated through this platform.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003eA. MBE growth of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on graphene\u003c/p\u003e \u003cp\u003eMulti-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structures were grown on graphene layers using a custom-designed ultrahigh vacuum (UHV) MBE system. The base pressure of the growth chamber was in the range of mid 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e Torr. Prior to the growth, thermal cleaning was carried out at 820 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e for 10 minutes in an UHV. Then, the temperature decreased to 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e with sufficient stabilizing (\u0026gt;\u0026thinsp;20 min). For multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth, high-purity Sb (99.9999%) and Te (99.9999%) were supplied, with Sb and Te fluxes maintained at approximately 0.02 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e/s and 0.35 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e/s, respectively\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, as measured by a quartz crystal microbalance (QCM). Following approximately 10 minutes of growth, the shutter of the Sb Knudsen cell was closed, and without altering the growth temperature, Bi (99.9999%) was supplied at a flux of approximately 0.02 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e/s, with the Te Knudsen cell shutter remaining open. After the completion of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth, a natural cooling process to 250 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e was proceeded, with continued Te supply.\u003c/p\u003e \u003cp\u003eB. Growth substrate preparation\u003c/p\u003e \u003cp\u003eFor substrate preparation, graphene layers were transferred either onto TEM-compatible membrane or Si wafer covered with a 300 nm thick SiO\u003csub\u003e2\u003c/sub\u003e layer. For transfer onto membrane, dry pick-up method using polypropylene carbonate (PPC) on polydimethylsiloxane (PDMS) was used while mechanical exfoliation technique was employed for transfer onto SiO\u003csub\u003e2\u003c/sub\u003e/Si wafers. Especially, membrane preparation involved several steps. Initially, a \u0026sim;200 nm SiN\u003csub\u003ex\u003c/sub\u003e film was deposited on both sides of a \u0026sim;200 \u0026micro;m (100) Si wafer using low-pressure chemical vapor deposition (LPCVD). Subsequently, the upper and lower SiN\u003csub\u003ex\u003c/sub\u003e layers were selectively removed through conventional lithography and reactive ion etching (RIE). After RIE, wet etching was employed to eliminate the Si wafer, thereby creating a window-like freestanding region for transferring the graphene layers\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. The width of this freestanding region was between 2\u0026sim;5 \u0026micro;m.\u003c/p\u003e \u003cp\u003eC. Morphological and microstructural characterizations\u003c/p\u003e \u003cp\u003eThe structural analysis of the as-grown materials was conducted using electron microscopy and atomic force microscopy techniques. Specifically, a field emission scanning electron microscope (FE-SEM) (MERLIN Compact, ZEISS) was utilized, operating at an electron acceleration voltage of 3kV, allowing the acquisition of detailed surface morphology. For more in-depth plan-view structural analysis at atomic resolution, an aberration-corrected (C\u003csub\u003es\u003c/sub\u003e-corrected) high-angle annular dark field scanning TEM (HAADF-STEM) (JEM-ARM200F, JEOL) was employed, with an acceleration voltage set at 200kV. In addition, energy-dispersive X-ray spectroscopy (EDS), integrated into the HAADF-STEM system, was utilized to analyze the elemental properties of the samples. Furthermore, to examine the multiple steps characteristic of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates, an atomic force microscope (NX10, Park Systems) was used, alongside the accompanying XEI software for data analysis.\u003c/p\u003e \u003cp\u003eD. Computational details\u003c/p\u003e \u003cp\u003eAb initio calculations were performed using the Vienna ab initio simulation package (VASP)\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, employing density functional theory within the generalized gradient approximation. The optimization of lattice parameters and atomic positions proceeded until the forces on each atom reached a threshold of \u0026lt;\u0026thinsp;0.01 eV \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\dot{A}}^{-1}\\)\u003c/span\u003e\u003c/span\u003e. Electronic wave functions were expanded using a plane wave basis with a kinetic energy cutoff set at 500 eV. To account for van der Waals interactions, Grimme\u0026rsquo;s DFT-3 method was incorporated\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. A vacuum layer exceeding 20 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e was introduced to prevent spurious interactions between adjacent slabs. Brillouin zone sampling utilized a 1 \u0026times; 13 \u0026times; 1 k-point mesh based on the Monkhorst\u0026ndash;Pack scheme\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. To calculate the interfacial energy, we conducted calculations for the interfacial energy of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Graphene using a \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({(5\\times 5)}_{{Bi}_{2}{Te}_{3}}\\)\u003c/span\u003e\u003c/span\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({(9\\times 9)}_{Graphene}\\)\u003c/span\u003e\u003c/span\u003e epitaxial supercell, taking into account the lattice constants of different structures. For the Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e interface, calculations were carried out using a coherent Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e-Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structure, given the insignificant lattice mismatch.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eMulti-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates and hetero-structures (Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e) were grown on graphene layers using an UHV MBE system. The graphene substrates were prepared onto either SiO\u003csub\u003e2\u003c/sub\u003e/Si wafers or SiN\u003csub\u003ex\u003c/sub\u003e/Si TEM-compatible membranes. Initially, Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e was grown on graphene layers atop SiO\u003csub\u003e2\u003c/sub\u003e/Si to study its morphology. Once the optimal conditions were identified, hetero-structures were then grown on graphene layers supported by membranes, allowing for TEM observations to be conducted without the need for additional sample processing. High-purity sources of Sb (99.9999%), Bi (99.9999%), and Te (99.9999%) were used to facilitate material growth under Te-rich conditions, while maintaining the background pressure of the growth chamber at a low 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e Torr. For more details, refer to the \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003eMaterials and methods\u003c/span\u003e sections A \u0026amp; B.\u003c/p\u003e \u003cp\u003eWe investigated the surface morphology of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e grown at various temperatures using field-emission scanning electron microscopy (FE-SEM). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) illustrates the morphology of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e grown at 250, 270, 285, and 320\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e. At lower temperatures of 250 and 270 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, high-density, multi-spiral structures covered the surface of the sample, preventing the formation of isolated islands. At 320 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, the islands drastically shrank in size with minimal material remaining on the graphene surface, indicating that the coverage by as-grown materials decreases as temperature increases. To delve deeper into the structural changes, we analyzed the contrasts in the FE-SEM images, which are indicative of topographic contrasts due to variations in the height of the as-grown materials affecting the emission of secondary electrons and thus the image brightness\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Notably, the material height increases towards the center for all Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e grown at various temperatures. However, a change in the morphology from spiral to triangular stacking was observed as temperature increased to 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, indicating a growth mode transition. According to the Burton-Cabrera-Frank (BCF) theory\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, crystal growth can proceed via dislocation-driven or layer-by-layer (LBL) modes. Dislocation-driven growth is characterized by the addition of atoms at defect sites within the crystal, often resulting in spiral steps around these dislocations\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Conversely, the LBL mode involves sequential atom deposition, building new layers on the existing surface\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Previous studies have reported spiral morphology in Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates due to screw dislocation-driven (SDD) growth\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. In this respect, a transition in the growth mode occurred at temperatures between 270 and 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, shifting from SDD to LBL mode\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Consequently, the multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates grown at 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e appear to follow the LBL growth mode, exhibiting multiple layers with a decreasing lateral size towards the center.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSurface morphology of a multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate, grown at 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, was investigated using atomic force microscopy (AFM), providing a nanometer scale information of the surface. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) presents the 2D surface morphology and corresponding line profile of the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate. We revealed that the height of the outermost step was 2\u0026sim;3 nm and the heights of inner steps were specified to be less than 1 nm. In addition, each terrace has width of \u0026sim;100 nm, which offers sufficient space for shell growth with width of several tens of nanometers, avoiding lateral overgrowth. So the presence of multiple steps with sufficient terrace width suggests the feasibility of step-edge growth, depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe surface morphologies of multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates can be classified into two types; rotational and parallel hierarchical, as depicted in the left sides of Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), (c). For the first case where the below step is triangular (or hexagonal) and the upper step is also triangular (or hexagonal) with all pairs of edges parallel to each other, the parallelism of the edges across the hierarchical steps, maintains a consistent orientation from one level to the next. For the second case, where the below step is triangular (or hexagonal) and the upper step is also triangular (or hexagonal) but is rotated 180 degrees from the below step, the rotational transformation applied to the subsequent hierarchical step, indicates a significant orientation change while preserving the shape\u0026rsquo;s symmetry. As previously mentioned, our Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e follows LBL growth mode where vdW interaction exists between the layers. In this respect, parallel hierarchical ones would be straightforward and energetically favorable configuration, considering R\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\stackrel{-}{3}\\)\u003c/span\u003e\u003c/span\u003em group\u0026rsquo;s symmetry (from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c)). However, due to the triangular (or hexagonal) shape\u0026rsquo;s inherent symmetry, rotating by 180 degrees does not change the relative positions of the lattice points. Thus, rotational hierarchical configuration is not abnormal. The point is that despite their distinct geometric features, they commonly possess multiple step-edges which allows the subsequent Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFollowing Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a), preceding hetero-structure formation was done on graphene layers on TEM-compatible membranes. For the growth, Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e was grown at same temperature with time of 2 minutes right after Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003ewas grown. Right sides of Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), (c) show the FE-SEM images of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structures. For a single Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth, the contrasts on the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate become brighter as they approach the core due to their multiple steps. However, for Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, bright and dark contrasts alternately repeated from the outer to the core of the nanoplate. This change implies Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e was grown on the multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate.\u003c/p\u003e \u003cp\u003eIn addition, we controlled the width of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell by changing the growth time. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d) depicts the FE-SEM images of the three cases-growth time of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e being 1, 2, and 3 minutes. The widths of outermost shells were \u0026sim;20 nm and \u0026sim;40 nm for 1 and 2 minutes, respectively. However, for 3 minutes of growth, it was difficult to mark the outermost shell, and the inner steps were not clearly specified due to lateral overgrowth, previously mentioned in \u003cspan refid=\"Sec1\" class=\"InternalRef\"\u003eIntroduction\u003c/span\u003e section. The result made it possible to tune the degree of horizontal growth next to the step of the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e suggesting the possibility to control the shell width to some extent, by varying growth parameters beyond just growth time, such as source fluxes. If such refined width control becomes feasible, it opens up the possibility for in-depth research on various structures. A prime example of this would be nanowires made of topological insulators. In the case of hetero-structures of topological insulators, the topological properties are preserved\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Under these circumstances, by considering each shell as a wire structure and adjusting the width, one can examine the variations in properties such as quantum confinement of surface states\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Given that each step\u0026rsquo;s height is same or less than 2 nm, and the shell\u0026rsquo;s width does not surpass the terrace\u0026rsquo;s width avoiding lateral overgrowth, this platform offers the potential for significant dimensional confinement. In addition, as the horizontal length of the shells decreases towards the interior of the island, it becomes possible to investigate the impact of this length on transport characteristics\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, in one specimen.\u003c/p\u003e \u003cp\u003eTo characterize the crystal structure and the epitaxial relationship between Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, plan-view TEM observations were proceeded. Since the membrane is TEM- compatible, no additional sampling processes were required. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows the low- magnification plan-view image obtained using high-angle annular dark field scanning TEM (HAADF-STEM). The inset shows a FE-SEM image of the target nanoplate. First, the outermost region boxed with the red line (the first step of the multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate and the lateral Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell) was investigated. Aberration-corrected (C\u003csub\u003es\u003c/sub\u003e-corrected) atomic resolution images are exploited as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). The overlaid ball-stick model is obtained through the calculation of lattice parameters from FFT patterns. The ratio of d-spacing of {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eST, BT\u003c/sub\u003e to {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eST, BT\u003c/sub\u003e is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\sqrt{1/3}\\)\u003c/span\u003e\u003c/span\u003e which can be converted to ratio of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\sqrt{3}\\)\u003c/span\u003e\u003c/span\u003e in real space. In addition, {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eST, BT\u003c/sub\u003e peaks make an angle of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({30}^{^\\circ }\\)\u003c/span\u003e\u003c/span\u003e to {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eST, BT\u003c/sub\u003e peaks. Thus, Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e both are classified as rhombohedral phase with the trigonal crystal system (space group of R\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\stackrel{-}{3}\\)\u003c/span\u003e\u003c/span\u003em, a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;\u0026asymp;\u0026thinsp;4.25 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e for Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;\u0026asymp;\u0026thinsp;4.4 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\dot{A}\\)\u003c/span\u003e\u003c/span\u003e for Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e)\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Since the contrast shown in the STEM image is dependent on \u003cem\u003eZ\u003c/em\u003e values, the interface of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e is apparently shown. Then, to understand the relation between two materials, fast Fourier transforms (FFTs) from three regions-Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, and their interface were obtained. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c) depicts the FFT patterns. From the peaks\u0026rsquo; orientation; {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eBT\u003c/sub\u003e ∥ {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eST\u003c/sub\u003e and {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eBT\u003c/sub\u003e ∥ {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eST\u003c/sub\u003e, their lateral relation is confirmed. The pattern from the interface shows that peaks from Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e almost overlap indicating that the orientations of the two materials are lateral to each other.\u003c/p\u003e \u003cp\u003eFurthermore, energy-dispersive X-ray spectroscopy (EDS) was performed to validate the growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at each step of the multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e. First, the outermost region is investigated. The HAADF-STEM image of the target region is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). The image includes three regions-I: the outermost Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell (lateral to the first-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e), II: the first-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and III: the inner Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell (vertical/lateral to the first/second-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e). As depicted in 2D EDS spectra map, in region I, the Bi signal is dominant, whereas a negligible Sb signal is observed, while it is vice versa in region II. The interfaces of regions I and II in the 2D EDS image match well with the interface shown in the HAADF-STEM image. For region III, since it is not the outermost (the first) shell of the nanoplate, it is the second Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell with the first-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e beneath it. Therefore, the Sb signal (from the first-step) emerges in region III together with the Bi signal. The EDS data also represent Te signals emerging regardless of the region, which is trivial.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe inner parts of the nanoplate are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b). Similar to the previous investigation, four regions were identified. I: the first-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, II: the second Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell (vertical/lateral to the first/second-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e), III: the second-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, and IV: the third Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell (vertical/lateral to the second/third-step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e). Since regions I and III are suspected to be the regions where only Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e is grown, weak Bi signals are observed. Furthermore, considering the 2D EDS spectra for regions II and IV, which are suspected to be the shell regions, the sections where Bi signals appear well match the contrasts shown in HAADF-STEM image. Since the investigated area contains the first-(region I and beneath II), and the second-(region III and beneath IV) step of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, the Sb signal comes out from the entire area just like Te signal.\u003c/p\u003e \u003cp\u003eUnlike the atomic resolution HAADF-STEM image shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), the 2D EDS data reveal more than just the presence of the outermost Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell; they also indicate the formation of inner shells, each vertically/laterally grown above/to, the respective steps of the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate. This is further corroborated by the alignment between the alternating Bi signals and the contrasts observed in 2D EDS and HAADF- STEM images, respectively. This makes it possible to claim that multiple Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shells are separately grown at each step of the multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate.\u003c/p\u003e \u003cp\u003eComputational analyses were conducted to investigate the formation of multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate-based hetero-structures through density functional theory (DFT) calculations. At first, this study aimed to ascertain the preference of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e towards step-edge growth by evaluating its total energy. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), four distinct scenarios were established in accordance with the methodologies outlined in the \u0026lsquo;D. Computational Details\u0026rsquo; section. Each scenario involved a multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate configuration, characterized by a reduced domain width at the second step, which served as a constant parameter across all models. The top row of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) describes instances of vertical Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e growth, which occurs via van der Waals (vdW) interactions with the base layer of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at the second step. Conversely, the bottom row presents scenarios that incorporate a combination of two distinct interactions: the formation of dangling bonds with the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at the second step, and vdW interactions with the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e at the first step. A comparative analysis of the total energies for each pair (column) of scenarios reveals that the configurations in the bottom row exhibit lower total energies; 4.6852 and 4.7415 eV lower, respectively. This suggests a thermodynamic preference for Bi2Te3 growth at the step edges of Sb2Te3 nanoplates. Additionally, the findings generally align well with the growth behavior that favors a mix of in-plane bonding and vdW interactions rather than isolated interactions\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe have also calculated the interfacial energies for Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Graphene and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e with supercells depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). As-grown Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shells can be classified into two; the outermost and the others. They are commonly grown via step-edge growth, assisted by dangling bonds from the side of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e. So the only difference is induced from the base materials. Since Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e have negligible lattice mismatch compared to graphene, the interfacial energy was expected to be lower. The calculation reveals that Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Graphene has approximately 0.0058 eV/\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\dot{A}}^{2}\\)\u003c/span\u003e\u003c/span\u003e higher interfacial energy which might be induced from the existence of the strains. As previously mentioned, the hetero-structure we have grown can act as a platform offering multiple Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanowires. In this context, the calculation highlights the hetero-structure\u0026rsquo;s potential for investigating distinct strain-induced transport behaviors\u003csup\u003e\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn conclusion, we created Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplate hetero-structures. For the growth of the hetero-structures, multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates with stair-like morphology were prepared at the growth temperature of 285 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{℃}\\)\u003c/span\u003e\u003c/span\u003e, and the growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shell layers on the step-edges of the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates was subsequently performed at the same temperature. Through FE-SEM and AFM, the steps of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and morphologies of hetero-structures were verified. Moreover, we were able to control the width of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates by changing the growth time. Through C\u003csub\u003es\u003c/sub\u003e-corrected HAADF-STEM, the in-plane epitaxy between two materials at the outermost, was investigated; {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eBT\u003c/sub\u003e ∥ {11\u0026thinsp;\u0026minus;\u0026thinsp;20}\u003csub\u003eST\u003c/sub\u003e and {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eBT\u003c/sub\u003e ∥ {10\u0026thinsp;\u0026minus;\u0026thinsp;10}\u003csub\u003eST\u003c/sub\u003e. For compositional analysis, 2D EDS spectra were mapped showing the Bi signals well matching with the alternating contrasts shown in HAADF-STEM images. Furthermore, DFT calculations indicated a preference for Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e to grow along the step-edges of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates, verifying the step-edge growth. Additionally, the calculated interfacial energy of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Graphene was higher than that of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e, which can be attributed to lattice mismatches causing strain. We believe that multi-stepped 2D materials have the potential to act as effective cores for integrated hetero-structure synthesis, possessing the potential to create multiple quantum structures through step-edge growth.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by National Research Foundation (NRF) of Korea (NRF-2021R1A5A1032996) and the Science Research Center (SRC) for Novel Epitaxial Quantum Architectures.\u0026nbsp;This research was also supported by grants NRF-2022R1A2C3007807, and NRF-2019M3D1A1079215 from the NRF of Korea. Additionally, we also acknowledge the Brain Korea 21-Plus Program, the Institute of Applied Physics (IAP). Research Institute of Advanced Materials (RIAM) at Seoul National University.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYu W J, Li Z, Zhou H, Chen Y, Wang Y, Huang Y and Duan X 2012 Vertically stacked multi-heterostructures of layered materials for logic transistors and complementary inverters Nature Mater. 12 246-252 \u003c/li\u003e\n\u003cli\u003eGong Y et al 2014 Vertical and in-plane heterostructures fromWS\u003csub\u003e2\u003c/sub\u003e/MoS\u003csub\u003e2\u003c/sub\u003e monolayers Nature Mater. 13 1135\u0026ndash;1142\u003c/li\u003e\n\u003cli\u003eWang J, Li Z, Chen H, Deng G and Niu X 2019 Recent Advances in 2D Lateral hetero- structures Nano-Micro Lett. 11 48 \u003c/li\u003e\n\u003cli\u003eZhao J, Cheng K, Han N and Zhang J 2018 Growth control, interface behavior, band alignment, and potential device applications of 2D lateral hetero-structures Wiley Interdisciplinary Reviews: Computational Molecular Science 8 e1353 \u003c/li\u003e\n\u003cli\u003eKundu B, Mohanty P, Kumar P, Nayak B, Mahato B, Ranjan P, Chakraborty S K, Sahoo S and Sahoo P K 2021 Synthesis of lateral heterostructure of 2D materials for optoelectronic devices: challenges and opportunities Emergent Materials 4 923-949 \u003c/li\u003e\n\u003cli\u003eWan X, Li H and Xu J 2020 Towards scalable fabrications and applications of 2D layered material-based vertical and lateral hetero-structures Chemical Research in Chinese Universities 36 525-550\u003c/li\u003e\n\u003cli\u003eSahoo P K, Memaran S, Xin Y, Balicas L and Guti\u0026eacute;rrez H R 2018 One-pot growth of two-dimensional lateral hetero-structures via sequential edge-epitaxy Nature 553 63-67\u003c/li\u003e\n\u003cli\u003eGong Y et al 2015 Two-Step Growth of Two-Dimensional WSe\u003csub\u003e2\u003c/sub\u003e/MoSe\u003csub\u003e2\u003c/sub\u003e Heterostructures Nano Lett. 15 6135-6141 \u003c/li\u003e\n\u003cli\u003eHuang W, Gan L, Li H, Ma Y and Zhai T 2016 2D layered group IIIA metal chalcogenides: synthesis, properties and applications in electronics and optoelectronics CrystEngComm 18-3968-84 \u003c/li\u003e\n\u003cli\u003eXu K, Yin L, Huang Y, Shifa T A, Chu J, Wang F, Cheng R, Wang Z and He J 2016 Synthesis, properties and applications of 2D layered M\u003csup\u003eIII\u003c/sup\u003e X\u003csup\u003eVI\u003c/sup\u003e (M = Ga, In; X = S, Se, Te) materials Nanoscale 8 16802-18 \u003c/li\u003e\n\u003cli\u003eEschbach M et al 2015 Realization of a vertical topological p\u0026ndash;n junction in epitaxial Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e heterostructures Nat Commun. 6 8816 \u003c/li\u003e\n\u003cli\u003eZhao Y et al 2013 Demonstration of surface transport in a hybrid Bi\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e/Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e heterostructure Sci Rep. 3 3060 \u003c/li\u003e\n\u003cli\u003eEremeev S V, Otrokov M M and ChulkovNew E V 2018 Universal Type of Interface in the Magnetic Insulator/Topological Insulator Heterostructures Nano Lett. 18 6521-6529\u003c/li\u003e\n\u003cli\u003eGuha P, Park J Y, Jo J H, Chang Y Y, Bae H H, Saroj R K, Lee H K, Kim M Y and Yi G C 2022 Molecular beam epitaxial growth of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e\u0026ndash;Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e lateral hetero-structures 2D Materials 9 025006\u003c/li\u003e\n\u003cli\u003ePark J Y, Lee G-H, Jo J, Cheng A K, Yoon H, Watanabe K, Taniguchi T, Kim M, Kim P and Yi G-C 2016 Molecular beam epitaxial growth and electronic transport properties of high quality topological insulator Bi\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e thin films on hexagonal boron nitride 2D Mater. 3 035029\u003c/li\u003e\n\u003cli\u003eShin H J, Park I S, Jang Y J, Wi S J, Lee G S and Ahn J H 2019 Fabrication of free- standing nanoscale SiN\u003csub\u003ex\u003c/sub\u003e membranes with enhanced burst pressure via improved etching process Sensors and Actuators A: Physical 297 111538 \u003c/li\u003e\n\u003cli\u003eXiong W et al 2020 SiN\u003csub\u003ex\u003c/sub\u003e films and membranes for photonic and MEMS applications Journal of Materials Science: Materials in Electronics 31 90-97\u003c/li\u003e\n\u003cli\u003eKresse G, Furthm ̈uller J 1996 Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set Phys. 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Mater. 35 2107362 \u003c/li\u003e\n\u003cli\u003eWu Y et al 2018 Quantum Wires and Waveguides Formed in Graphene by Strain Nano Lett. 18(1) 64-69\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"multi-stepped nanoplate, integrated hetero-structure, step-edge growth, molecular beam epitaxy, transmission electron microscopy, energy-dispersive X-ray spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-4586406/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4586406/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe report the synthesis of multiple Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e shells on multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates using molecular beam epitaxial (MBE) step-edge growth. For the growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e/Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e hetero-structures, multi-stepped Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates with stair-like morphology following layer-by-layer (LBL) growth mode were obtained by optimizing the growth temperature, and the growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on the step-edges of the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates was followed. Width of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on the Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e nanoplates was controlled by changing the growth time. Structural properties of the hetero-structures were investigated using aberration-corrected (C\u003csub\u003es\u003c/sub\u003e-corrected) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), revealing the interface between Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e. In-plane epitaxial relation at the interface was confirmed using fast Fourier transforms (FFTs). Compositional analysis of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e and Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e was verified through energy-dispersive X-ray spectroscopy. Furthermore, we performed density functional theory (DFT) calculations to confirm the preferential growth of Bi\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e on the step-edges of Sb\u003csub\u003e2\u003c/sub\u003eTe\u003csub\u003e3\u003c/sub\u003e. By forming multi-stepped core structure, it would be feasible to create various integrated hetero-structures.\u003c/p\u003e","manuscriptTitle":"Molecular beam epitaxial step-edge growth of Bi2Te3/multi-stepped Sb2Te3 nanoplate hetero-structures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-19 02:48:18","doi":"10.21203/rs.3.rs-4586406/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":"0618ef66-366a-45e0-864a-0e94afd7e160","owner":[],"postedDate":"July 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-29T11:46:55+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-19 02:48:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4586406","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4586406","identity":"rs-4586406","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}