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

doi:10.21203/rs.3.rs-4586406/v1

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

https://doi.org/10.21203/rs.3.rs-4586406/v1

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We report the synthesis of multiple Bi₂Te₃ shells on multi-stepped Sb₂Te₃ nanoplates using molecular beam epitaxial (MBE) step-edge growth. For the growth of Bi₂Te₃/Sb₂Te₃ hetero-structures, multi-stepped Sb₂Te₃ nanoplates with stair-like morphology following layer-by-layer (LBL) growth mode were obtained by optimizing the growth temperature, and the growth of Bi₂Te₃ on the step-edges of the Sb₂Te₃ nanoplates was followed. Width of Bi₂Te₃ on the Sb₂Te₃ 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 Sb₂Te₃ and Bi₂Te₃. In-plane epitaxial relation at the interface was confirmed using fast Fourier transforms (FFTs). Compositional analysis of Bi₂Te₃ and Sb₂Te₃ was verified through energy-dispersive X-ray spectroscopy. Furthermore, we performed density functional theory (DFT) calculations to confirm the preferential growth of Bi₂Te₃ on the step-edges of Sb₂Te₃. 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

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-structures⁷, 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 (WSe₂) 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 Bi₂Te₃/multi-stepped Sb₂Te₃, avoiding Bi₂Te₃ to merge with each other. Specifically, we utilize multi-stepped Sb₂Te₃ nanoplates with stair-like configurations as the core material, enabling the growth of multiple Bi₂Te₃ shells on each Sb₂Te₃ step through step-edge growth. Since Sb₂Te₃ and Bi₂Te₃ 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 Bi₂Te₃ shells formed on Sb₂Te₃ 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’ Sb₂Te₃ base, distinct properties can be investigated through this platform.

A. MBE growth of Sb₂Te₃ and Bi₂Te₃ on graphene

Multi-stepped Sb₂Te₃ nanoplates and Bi₂Te₃/Sb₂Te₃ 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 Sb₂Te₃ 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 Bi₂Te₃ 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 SiO₂ 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 SiO₂/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 Sb₂Te₃ 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 incorporated¹⁹. 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 scheme²⁰. To calculate the interfacial energy, we conducted calculations for the interfacial energy of Bi₂Te₃/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 Bi₂Te₃/Sb₂Te₃ interface, calculations were carried out using a coherent Sb₂Te₃-Bi₂Te₃ hetero-structure, given the insignificant lattice mismatch.

Multi-stepped Sb₂Te₃ nanoplates and hetero-structures (Bi₂Te₃/Sb₂Te₃) were grown on graphene layers using an UHV MBE system. The graphene substrates were prepared onto either SiO₂/Si wafers or SiN_x/Si TEM-compatible membranes. Initially, Sb₂Te₃ was grown on graphene layers atop SiO₂/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 Sb₂Te₃ grown at various temperatures using field-emission scanning electron microscopy (FE-SEM). Figure 1(a) illustrates the morphology of Sb₂Te₃ 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 brightness²¹. Notably, the material height increases towards the center for all Sb₂Te₃ 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 dislocations²⁴. Conversely, the LBL mode involves sequential atom deposition, building new layers on the existing surface^25,26. Previous studies have reported spiral morphology in Sb₂Te₃ 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 mode²⁹. Consequently, the multi-stepped Sb₂Te₃ 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 Sb₂Te₃ 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 Sb₂Te₃ 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 Sb₂Te₃ 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 Sb₂Te₃ 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 Bi₂Te₃ growth.

Following Fig. 2(a), preceding hetero-structure formation was done on graphene layers on TEM-compatible membranes. For the growth, Bi₂Te₃ was grown at same temperature with time of 2 minutes right after Sb₂Te₃was grown. Right sides of Figs. 2(b), (c) show the FE-SEM images of Bi₂Te₃/Sb₂Te₃ hetero-structures. For a single Sb₂Te₃ growth, the contrasts on the Sb₂Te₃ nanoplate become brighter as they approach the core due to their multiple steps. However, for Bi₂Te₃/Sb₂Te₃, bright and dark contrasts alternately repeated from the outer to the core of the nanoplate. This change implies Bi₂Te₃ was grown on the multi-stepped Sb₂Te₃ nanoplate.

In addition, we controlled the width of Bi₂Te₃ shell by changing the growth time. Figure 2(d) depicts the FE-SEM images of the three cases-growth time of Bi₂Te₃ 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 Sb₂Te₃ 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 preserved¹⁴. 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 states³⁰. 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 characteristics³¹, in one specimen.

To characterize the crystal structure and the epitaxial relationship between Sb₂Te₃ and Bi₂Te₃, 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 Sb₂Te₃ nanoplate and the lateral Bi₂Te₃ 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, Sb₂Te₃ and Bi₂Te₃ 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 Sb₂Te₃ and a = b ≈ 4.4 \(\dot{A}\) for Bi₂Te₃)³². Since the contrast shown in the STEM image is dependent on Z values, the interface of Sb₂Te₃ and Bi₂Te₃ is apparently shown. Then, to understand the relation between two materials, fast Fourier transforms (FFTs) from three regions-Sb₂Te₃, Bi₂Te₃, 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 Sb₂Te₃ and Bi₂Te₃ 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 Bi₂Te₃ at each step of the multi-stepped Sb₂Te₃. 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 Bi₂Te₃ shell (lateral to the first-step of Sb₂Te₃), II: the first-step of Sb₂Te₃ and III: the inner Bi₂Te₃ shell (vertical/lateral to the first/second-step of Sb₂Te₃). 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 Bi₂Te₃ shell with the first-step of Sb₂Te₃ 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 Sb₂Te₃, II: the second Bi₂Te₃ shell (vertical/lateral to the first/second-step of Sb₂Te₃), III: the second-step of Sb₂Te₃, and IV: the third Bi₂Te₃ shell (vertical/lateral to the second/third-step of Sb₂Te₃). Since regions I and III are suspected to be the regions where only Sb₂Te₃ 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 Sb₂Te₃, 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 Bi₂Te₃ shell; they also indicate the formation of inner shells, each vertically/laterally grown above/to, the respective steps of the Sb₂Te₃ 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 Bi₂Te₃ shells are separately grown at each step of the multi-stepped Sb₂Te₃ nanoplate.

Computational analyses were conducted to investigate the formation of multi-stepped Sb₂Te₃ nanoplate-based hetero-structures through density functional theory (DFT) calculations. At first, this study aimed to ascertain the preference of Bi₂Te₃ 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 Sb₂Te₃ 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 Bi₂Te₃ growth, which occurs via van der Waals (vdW) interactions with the base layer of Sb₂Te₃ 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 Sb₂Te₃ at the second step, and vdW interactions with the Sb₂Te₃ 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 Bi₂Te₃/Graphene and Bi₂Te₃/Sb₂Te₃ with supercells depicted in Fig. 5(b). As-grown Bi₂Te₃ 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 Sb₂Te₃. So the only difference is induced from the base materials. Since Bi₂Te₃ and Sb₂Te₃ have negligible lattice mismatch compared to graphene, the interfacial energy was expected to be lower. The calculation reveals that Bi₂Te₃/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 Bi₂Te₃ nanowires. In this context, the calculation highlights the hetero-structure’s potential for investigating distinct strain-induced transport behaviors^35–37.

In conclusion, we created Bi₂Te₃/multi-stepped Sb₂Te₃ nanoplate hetero-structures. For the growth of the hetero-structures, multi-stepped Sb₂Te₃ nanoplates with stair-like morphology were prepared at the growth temperature of 285 \(\text{℃}\), and the growth of Bi₂Te₃ shell layers on the step-edges of the Sb₂Te₃ nanoplates was subsequently performed at the same temperature. Through FE-SEM and AFM, the steps of Sb₂Te₃ and morphologies of hetero-structures were verified. Moreover, we were able to control the width of Bi₂Te₃ on the Sb₂Te₃ 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 Bi₂Te₃ to grow along the step-edges of Sb₂Te₃ nanoplates, verifying the step-edge growth. Additionally, the calculated interfacial energy of Bi₂Te₃/Graphene was higher than that of Bi₂Te₃/Sb₂Te₃, 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.

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

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Abstract

Figures

Introduction

Materials and methods

Results and Discussion

Conclusions

Declarations

References

Additional Declarations

Status:

Version 1