Epitaxial Growth of Two-Dimensional Metal-Semiconductor Transition

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Epitaxial Growth of Two-Dimensional Metal-Semiconductor TransitionMetal Dichalcogenide Vertical Stacks (VSe/MX) and their Band-Alignments 2

2

Zhepeng Zhang, Yue Gong, Xiaolong Zou, Porun Liu, Pengfei Yang, Jianping Shi, Liyun Zhao, Qing Zhang, Lin Gu, and Yanfeng Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08677 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Epitaxial Growth of Two-Dimensional Metal-Semiconductor Transition-Metal Dichalcogenide Vertical Stacks (VSe2/MX2) and their Band-Alignments

Zhepeng Zhang1,2, Yue Gong3,4, Xiaolong Zou5, Porun Liu6, Pengfei Yang1,2, Jianping Shi1,2, Liyun Zhao2, Qing Zhang2, Lin Gu3,4, Yanfeng Zhang1,2*

1

Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of

Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China 2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing

100871, P. R. China 3

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of

Sciences, Beijing 100190, P. R. China 4

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China

5

Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, P. R. China

6

Centre for Clean Environment and Energy, Griffith University, Gold Coast 4222, Australia

*

Address correspondence to Yanfeng Zhang ([email protected]).

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Abstract Two-dimensional (2D) metal-semiconductor transition-metal dichalcogenides (TMDCs) vertical heterostructures play a crucial role in device engineering and contact tuning fields, while their direct integration still being challenging. Herein, a robust epitaxial growth method is designed to construct multiple lattice-matched 2D metal-semiconductor TMDCs vertical stacks (VSe2/MX2, M: Mo, W; X: S, Se) by a two-step chemical vapor deposition method. Intriguingly, the metallic VSe2 preferred to nucleate and extend from the energy-favorable edge site of the semiconducting MX2 underlayer to form VSe2/MX2 vertical heterostructures. This growth behavior was also confirmed by density functional theory calculations of the initial adsorption of VSe2 adatoms. Particularly, the formation of Schottkydiode or Ohmic contact-type band-alignments were detected for the stacks between VSe2 and p-type WSe2 or n-type MoSe2, respectively. This work hereby provides insights into the direct integration, bandalignment engineering, and potential applications of such 2D metal-semiconductor stacks in nextgeneration electronics, optoelectronic devices, and energy-related fields.

Keywords transition-metal dichalcogenides, chemical vapor deposition, metal-semiconductor vertical stacks, VSe2, WSe2

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Metal-semiconductor heterostructures are considered as intriguing building blocks in two-dimensional (2D) electronics and optoelectronics.1–3 Fermi-level pinning, lattice strain, polymer residues, and metalinduced gap states at the interface of metal-semiconductor heterostructures can greatly suppress the charge carrier transport and contact tunability, resulting in performance degradations especially in atomically-thin devices.1,2,4 2D transition-metal dichalcogenides (TMDCs) are a class of layered materials consisting of many compositional and electronic variants,5–8 including 2D metallic TMDCs (m-TMDCs) such as VX2,9,10 NbX2,11–13 and TaX2 (where X= S, Se, etc.),14–16 and their semiconducting counterparts (s-TMDCs) such as MoX217,18 and WX2.19,20 High-quality, sharp-interface, vertically stacked metal-semiconductor heterostructures based on 2D TMDCs are highly desired to resolve the above issues.21–24 Although various monolayer p–n junctions,19,25,26 superlattices,27–29 and alloys,30,31 as well as vertically stacked type-II van der Waals heterostructures based on s-TMDCs,32–34 have been fabricated using chemical vapor deposition (CVD), the direct integration of high-quality metallicsemiconducting heterostructures with a clean interface is still challenging.24,35–37 As reported by Kappera et al.38 and Sung et al.,35 coplanar 1T-2H MoS2 and 1T′-2H MoTe2 polymorph metal-semiconductor homostructures were fabricated by lithium intercalation and temperature-triggered CVD method for high-performance field-effect transistors, respectively. However, the instability of 1TMoS2 and the phase mixtures for 1T′- and 2H-MoTe2 in such homostructures are inevitable in their fabrication

processes.

Meanwhile,

NbSe2/WxNb1–xSe2/WSe2 heterostructured

Ohmic

contact

devices21,23,39,40 have been fabricated via high-temperature selenization followed by layer-by-layer stacking techniques, and NbS2/MoS2 heterostructured Schottky diodes22 have been constructed through chemical vapor transport followed by mechanical exfoliation processes. Nevertheless, such fabrication processes are tedious and incompatible with scalable production, which significantly prohibits their wider application. Therefore, a direct integration route through a sequential CVD growth of the m-TMDC/sTMDC stacks should be very promising.

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Herein, we have accomplished a direct construction of 2D metal-semiconductor vertical stacks by sequentially growing single-layer s-TMDCs (WSe2, MoSe2, WS2, and MoS2) flakes and then multilayer metallic 1T-VSe2 on the sapphire (Al2O3) substrates via a facile “two-step” CVD route. This specific vertical heterostructure was selected with following considerations: 1) the CVD growth temperature of VSe2 is lower than the chemical-degradation temperature of MX2, which can effectively prevent atommixing during the second growth step; 2) the VSe2 layers possess a robust stability and electrical conductivity at ambient conditions, which makes them perfect electrodes for s-TMDCs; 3) a small lattice mismatch between VSe2 and MX2, which may induce an epitaxial growth mode and thus a commensurate interlayer stacking geometry. Intriguingly, multilayer 1T-VSe2 was found to selectively nucleate and grow from the energy-preferred edge sites of s-TMDCs monolayers and extend inward to form complete 2D vertical stacks, in a van der Waals epitaxial growth mode. More significantly, metal-semiconductor heterostructures with Schottky or Ohmic band-alignments could be readily constructed with either p-type or n-type s-TMDCs (e.g., WSe2, MoSe2) as growth substrate, respectively. Our work is hereby expected to promote the direct synthesis of large-scale, interface-sharp 2D metal-semiconductor heterostructures, as well as propel their versatile applications.

Results and discussion As reported previously, a two-step CVD method has been widely used to controllably synthesize various semiconducting 2D TMDC heterostructures,19,25,41–44 which has presented distinct advantages over the one-pot CVD method, such as a clean interface, sharp grain boundary, and absence of alloy formations. Meanwhile, metallic 1T-phase VSe2 (a stable phase with small mismatches with s-TMDCs, 2.2% for WSe2, 1.9% for MoSe2, as depicted in Figure S1 and Table S1) has become popular in recent years because of its intriguing charge density wave order45 and 2D magnetism,10 as well as intrinsic metallicity with an extra-high conductivity (up to 106 S m–1).9 In addition, as previously reported,9 the growth temperature of high-quality 1T-VSe2 (500–600 °C) is comparable to or even lower than the growth and

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degradation temperature of high-quality s-TMDCs,46,47 thus potentially minimizing their thermal damage or intermixing in the second growth step. In this regard, metallic 1T-VSe2 may serve as a perfect metal electrode for s-TMDCs (e.g., MoSe2 and WSe2). This can be achieved by a facile two-step CVD route through sequentially growing the two composite materials. Taking the VSe2/WSe2 heterojunction as an example (as shown in the schematic in Figure 1a), singlelayer semiconducting single-crystalline 1H-WSe2 was firstly grown on an Al2O3 substrate by a typical CVD method (in step I). In step II, multilayer metallic 1T-VSe2 was further synthesized on the predeposited triangular WSe2 flake in a separate furnace. Intriguingly, multilayer 1T-VSe2 was observed to firstly nucleate and grow on the edges of the WSe2 flake, and then extend inward to form a completely stacked multilayer-VSe2/single-layer-WSe2 (ML-VSe2/SL-WSe2) structure. The overall optical microscope (OM) morphology evolutions regarding different growth steps of VSe2/WSe2 are presented in Figure 1b–d, in line with that of Figure 1a. Notably, such edge-induced growth behavior robustly takes place in large area ML-VSe2/SL-WSe2 stacks (Figure 1e). In such edge-induced growth process, the metallic VSe2 can be precisely deposited on the edges of semiconducting WSe2, which is distinct from the random nucleation of 1T’-2H MoTe2 polymorph heterostructures through the temperature-triggered one-pot CVD route.35 In addition, the vertically stacked ML-VSe2/SL-WSe2 structures possess considerably larger contact areas at the interfaces compared to that of the previously reported lateral contact structures,35–37 which should expand their applications in various fields. More significantly, the ML-VSe2/SL-WSe2 heterostructures can be perfectly transferred onto 300 nm SiO2/Si substrates for further characterizations or applications. The edge of the monolayer WSe2 domain (inner purple region) is clearly observed to be covered by multilayer VSe2 featuring a bright yellow region, as shown by the OM image in Figure 1f. Raman spectroscopy was further used to confirm the chemical composition and the stacking geometry. Figure 1g shows the color-coded Raman line mapping image along the dashed arrow in Figure 1f. The co-existence of peaks at about 206 and 250 cm–1, which are typical for A1g of 1T-VSe2 and E1 2g of 1H-WSe2, respectively, tentatively validates their pure phases ACS Paragon Plus Environment

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and vertical stacking geometry.9,48 Nevertheless, the decrease in WSe2 Raman signal acquired from the stacked area should arise from the optical absorption of the over-stacked multilayer VSe2. The atomic force microscopy (AFM) image of transferred ML-VSe2/SL-WSe2 heterostructures (Figure 1h) shows that the ML-VSe2 with thickness about 50 nm is grown on the edges of triangular SL-WSe2 with the formation of a “void island” shape. As reported previously, the exfoliated and CVD-grown thicker VSe2 samples both presented better metallic property and stability than the thinner counterparts.14 Therefore, stacking thicker metallic VSe2 layers on 2D s-TMDCs should be promising for achieving highperformance electronic and optoelectronic devices. Intriguingly, this edge-induced growth mode was robustly confirmed in the growth of ML-VSe2/SLMoSe2, ML-VSe2/SL-MoS2, and ML-VSe2/SL-WS2 metal-semiconductor vertical stacks. Typical OM images are depicted in Figure 2a–c and Figure S2. The corresponding AFM image on the sub-covered ML-VSe2/SL-MoSe2 stack is also captured in Figure 2d, and shows the same “void island” shape. Moreover, from the Raman spectra, the VSe2/MoSe2 stacking area and the underlying MoSe2 layer can be clearly identified (Figure 2e) by the different signals from VSe2 and MoSe2. Notably, no additional peak emerges in the ML-VSe2/SL-MoSe2 area, possibly indicating the chemical bonding-free feature between the two composite layers. Meanwhile, the MoSe2 A1g peak position in the as-grown VSe2/MoSe2 heterojunction shows a negligible shift of -0.2 cm–1, compared to that of as-grown single-layer MoSe2 on Al2O3 (Figure 2f). It is thus proposed that the 2D heterointerface is free of lattice strain, as mainly originates from the weak van der Waals interaction and lattice mismatch at the interface. This deduction is also confirmed by the negligible Raman shift from the ML-VSe2/SL-WSe2 stack, as exhibited in Figure S3. Briefly, benefiting from the chemical bonding-free van der Waals interface, and the small lattice mismatch, multiple heterostructures of VSe2 direct stacking on the MX2 can be robustly obtained by our two-step CVD strategy.

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The crystal structures of the synthesized metal-semiconductor heterostructures were characterized using transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-angle annular dark-field aberration-corrected scanning transmission electron microscopy (HAADF-STEM). As demonstrated in Figure 1a, multilayer VSe2 prefers to nucleate and grow from the edge of the singlelayer WSe2 to form a ML-VSe2/SL-WSe2 structure. This edge-induced growth mode is also confirmed by the typical TEM and SAED images (Figure 3a–e). The ML-VSe2 region presents a hollow triangular shape, which is somewhat larger than that of the underlying WSe2 flake (labeled with a red dashed triangle). This implies that VSe2 tends to grow both outward and inward along the edge of the singlelayer WSe2 domain. However, the inward growth of VSe2 is more favorable for extending to the center of the triangular WSe2 domain, and the outward growth terminates quickly or continues with a relatively low growth rate. As displayed in Figure 3a, the width of the inward grown VSe2 is longer than that of the outward growth, corresponding to growth rates of 4 versus 1 µm/min. The OM image captured from the face-down transferred sample also confirms the different growth rates (Figure S4a–d). The origin of the interesting behavior will be discussed in the next section. Moreover, the SAED pattern of the central area (red dot in Figure 3a) shows only one set of six-fold points that is typical for single-crystalline 1H-WSe2 (Figure 3b). The SAED patterns of the edge areas (stacked region in purple, blue, and yellow dots in Figure 3a) show two sets of six-fold diffraction points with almost no misorientation (Figure 3c–e). Clearly, the inner set of the SAED pattern should be assigned to VSe2, while the outer part is attributed to WSe2, as VSe2 and WSe2 have different lattice constants aVSe2 = 3.35 nm and aWSe2 = 3.28 nm with a very small lattice mismatch of –2.2%. Additionally, as previously reported in twisted bilayer MoS2,49 MoS2/Graphene,50 and WSe2/WS2 van der Waals heterostructures,51 the twist angle between adjacent layers plays a great role in the evaluation of growth model, interlayer coupling, and electrical contact. Therefore, it is critical to extract the relative rotation angle between 1T-VSe2 and 1H-WSe2 from the SAED patterns. The relative rotation angle histogram reveals a very small value close to 0° for all the samples (Figure 3f, from 35 data points), strongly suggesting a van der Waals epitaxial growth feature.52 ACS Paragon Plus Environment

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The top and side views of the proposed stacking schematics between 1T-VSe2 (top) and 1H-WSe2 (bottom) with a well-aligned orientation are also presented as insets in Figure 3f. Such kind of commensurate TMDCs vertical stacks (herein VSe2/MX2) should present better electrical contacts comparing with that of the randomly twisted stacks according to the published references,33,49,50 thus possessing great application potentials in high-performance electronic devices. Moreover, the wellaligned orientation in the CVD-synthesized heterostructures affirms the single-crystalline features of the high-quality bottom WSe2 and top VSe2 layers, which is in sharp contrast to the recently reported lateral heterostructures based on polycrystalline MoS2 and VS2.37 To further determine the structure of stacked VSe2/WSe2, a high-resolution HAADF-STEM image across the boundary between the VSe2/WSe2 stack and single-layer WSe2 region is also collected in Figure 3g. A clear contrast variation (dark and gray) arises from the thickness difference between the two types of regions. As labeled with dashed lines, the VSe2 layer exhibits a similar crystal orientation to that of the neighboring WSe2 layer. These data again address the van der Waals epitaxial growth feature of MLVSe2 on SL-WSe2. Additionally, the TEM image of the folded edge of WSe2 confirms its single-layer nature (Figure 3h). More interestingly, the folded edge of the VSe2/WSe2 stack shows a clear layer-bylayer stacking interface, thus suggesting the epitaxial growth mode in the vertical stacks (Figure 3i). Moreover, atomic-resolution HAADF-STEM images on the different surface regions reveal well-defined 1H-phase WSe2 and 1T-phase VSe2 structures, as well as their high crystal quality (Figure 3j, k), respectively. It should be noted that the growth temperature of VSe2 (500–600 °C) is much lower than that of WSe2 (800–900 °C), which should avoid the introduction of damage or phase-mixing in the vertical stacks. Although such edge-induced growth behavior has also been reported in the two-step CVD growth of semiconducting WSe2/MoSe2 vertical heterostructures,25 its underlying mechanism has not yet been appropriately clarified. To uncover the growth mechanism of our ML-VSe2/SL-WSe2 vertical stack, density functional theory calculations were carried out to simulate the absorption energies of V, VSe, ACS Paragon Plus Environment

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and VSe2 radicals on the surface and edge of WSe2. The V, VSe, and VSe2 were found to exhibit much lower absorption energies of approximately 0.4, 0.8, and -0.06 eV on the Se-terminated edge than on the surface, 4.5, 3.0, and 2.4 eV (Figure 4a), respectively. Moreover, the absorption of such radicals on the W-terminated edges show the same tendency (Figure S5). In this regard, the VSe2 radicals prefer to adsorb on the edges of WSe2 to form seed points rather than on the surface. Such edge-induced growth mechanism can be further confirmed by the fact that the edge nucleation rate is distinctly higher than that on the surface (Figure S6), which is in good agreement with the theoretical calculations. In addition, as displayed in the AFM image of WSe2 (Figure S7), no distinct particles appeared in the edges of our synthesized WSe2 flake, thus the particle-induced mechanism which was reported in the growth of WS2/MoS2 vertical stacks can be excluded in our system.53 Interestingly, with the further increase in the growth time, VSe2 prefers to continuously grow inward until fully covering the underlying WSe2 flake. Moreover, the outward growth rate along the WSe2 edge (towards the bare Al2O3 surface) (Figure 4b and 1a) is much slower than that of the inward growth. This different growth behavior is proposed to be mediated by the disparate dangling-bond densities of the WSe2 and Al2O3 surfaces. The Al2O3 surface possessing rich dangling bonds is less suitable for the lateral growth of VSe2, compared to the danglingbond-free WSe2 surface, as reported previously.9,54 More significantly, this edge-induced growth model applies to multiple types of edges, including randomly terminated edges of fractal domains (Figure S8), and mechanically exposed new edges (Figure 4c). Notably, the edge-induced growth behavior in our two-step CVD growth strategy should be compatible with future applications in 2D integrated circuits, as such highly conductive VSe2 layers can be selectively deposited and serve as edge electrodes for s-TMDC channels. The band profiles of metal-semiconductor heterostructures are key parameters for evaluating their application potentials in electronic and optoelectronic devices.55–57The synthesized TMDCs metalsemiconductor vertical stacks were transferred onto the conductive Si substrates to perform Kelvin probe force microscope (KPFM) characterizations. Typical AFM morphology images of ML-VSe2/SL-WSe2 ACS Paragon Plus Environment

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and ML-VSe2/SL-MoSe2 heterostructures are presented in Figure 5a, e. The single-layer WSe2 triangle is superimposed with multilayer VSe2 at the edges, forming “void island” shape structures. The thicknesses of VSe2 are measured as about 50 and 20 nm by AFM height profiles, respectively. Interestingly, the areas of the stacked ML-VSe2/s-TMDCs and single-layer s-TMDCs exhibit distinctly different surface potentials in the KPFM surface potential images (Figure 5b, f), while both exhibit darker contrasts regarding that of the Si substrate. Accordingly, the Fermi-level difference between s-TMDCs and s-TMDCs and VSe2 (Δϕ) can be estimated from the KPFM characterization by the following formula:55 Δ𝜙 = 𝜙sTMDCs ― 𝜙VSe2 = 𝜙tip ―𝑒𝑉CPD ― sTMDCs ― (𝜙tip ― 𝑒𝑉CPD ― VSe2) = 𝑒(𝑉CPD ― VSe2 ― 𝑉CPD ― sTMDCs)

(1)

where 𝜙sTMDCs, 𝜙VSe2and 𝜙tip are the work functions of the s-TMDCs, VSe2/WSe2, and AFM tip, respectively. 𝑉CPD ― VSe2 and 𝑉CPD ― sTMDCs are the contact potential differences (CPDs) between the AFM tip and local areas of the VSe2/WSe2 and s-TMDC, respectively. 𝑒 is the charge of an electron. In our discussion, the CPD of the VSe2/s-TMDC stacks is treated as the CPD of VSe2, as the surface potential contribution of the bottom single-layer s-TMDC to the metallic multilayer VSe2 is negligible. Figure 5c, g presents the line profiles of the 𝑉CPD of VSe2-sTMDC heterostructures. The Fermi-level difference between the s-TMDC and metallic VSe2 can be calculated using Eq. (1) as approximately 163.0 meV for VSe2/WSe2 and approximately 123.0 meV for VSe2/MoSe2. This implies that the work function of metallic VSe2 is distinctly smaller than that of s-TMDCs relative to the vacuum level. Therefore, when metallic VSe2 is connected with s-TMDCs, electrons should diffuse from VSe2 to s-TMDCs to form a depletion region. According to the physics of metal-semiconductor junctions,58 a Schottky or an Ohmic contact can be formed in the junction of metal with p-type or n-type semiconductor, respectively, when the work function of metal is smaller than that of semiconductor. Notably, the CVD-synthesized single-layer WSe2 or MoSe2 was widely defined as a p-type48,59 or ntype60 semiconductor, respectively. The schematics of the band profiles of VSe2/WSe2 and VSe2/MoSe2

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heterostructures are illustrated in Figure 5d, h, respectively.58 Interestingly, the heterostructures of VSe2 with p-type WSe2 or n-type MoSe2 show totally different contact characteristics. In the VSe2/WSe2 heterostructure, a typical Schottky barrier diode is formed because of the formation of high resistance layer in the depletion region at WSe2 side. Such diodes should find great application potentials in optoelectronics as the photo-induced electron–hole pairs can be well separated and located at the two sides of the interface.32 In the VSe2/MoSe2 heterostructure, an Ohmic contact is formed. Such vertical stacks should find great application potentials in high-performance electronic devices accompanied with relatively low-contact resistances.61 Further device fabrication based on our synthesized stacks will be demonstrated in our next study. Our results therefore offer a controllable method to obtain vertically stacked metal-semiconductor TMDC heterostructures with tunable band alignment, and is a critical step for building all-2D-based electronics by the synthetic route.

Conclusion In summary, we have demonstrated the direct growth of lattice-matched VSe2/MX2 heterostructures with clean van der Waals interfaces, via a scalable two-step CVD method. We find that the secondly grown metallic 1T-VSe2 prefers to grow on the energy-favorable edge sites of monolayer semiconducting 1HMX2 to form an edge-elongated stack at the initial growth stage, and finally form a complete vertical stack, following an epitaxial growth mechanism. The interface-sharp vertical stacks offer us perfect platforms for the further exploration of the carrier transport and charge transfer on the 2D metalsemiconductor heterostructures that present either 2D Schottky barrier diodes or Ohmic contact-type junctions depending on different band alignments (e.g., VSe2 with WSe2 or MoSe2). All these efforts should shed light on the direct integration and band-alignment engineering of 2D layered materials into various vertical metal-semiconductor stacks, as well as promote their future applications in electronic and optoelectronic devices, and energy-related fields.

Experimental Section

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First-step CVD Growth of s-TMDCs. 1H-phase single-layer WSe2 and MoSe2 flakes were firstly grown by atmospheric-pressure CVD (APCVD). Commercial Se (alfa) and WO3 (alfa) were used as precursors and heated to 200 and 1000 °C in the growth process of WSe2, respectively. A mixed flow of H2 (25 sccm) and Ar (80 sccm) was used as the carrier gas. The Al2O3 substrate was placed downstream and heated to 775 °C for a growth time of 30 min. Se and MoO3 (alfa) were used as precursors and heated to 150 and 850 °C in the growth process of MoSe2, respectively. A mixed flow of H2 (15 sccm) and Ar (20 sccm) was used as the carrier gas. The Al2O3 substrate was placed downstream and heated to 850 °C for a growth time of 15 min. Second-step CVD Growth of VSe2. 1T-VSe2 was grown via a similar APCVD route. Commercial VCl3 and Se were used as precursors, and were heated to 400 and 360 °C in the growth process, respectively. The s-TMDCs/Al2O3 sample was then used as a substrate and placed downstream (12 cm from the VCl3) for the second growth of VSe2, at the growth temperature of 500–600 °C for 2 to 10 min of growth. A mixed flow of H2 (3 sccm) and Ar (100 sccm) was used as the carrier gas. Transfer of VSe2/MX2 stacks. A Poly(methyl methacrylate) (PMMA)-assisted transfer method was employed to transfer the vertical stacks. Firstly, the as-grown VSe2/MX2/Al2O3 samples were spin-coated with commercial PMMA (950 K, ALLRESIST, AR-P 679.04) at a speed of 800–1000 rpm (1 min). The spin-coated samples were heated to 70–80 °C for 10 min on a hotplate to solidify the PMMA layers. Secondly, the PMMA/VSe2/MX2/Al2O3 stacks were soaked in deionized water for 30 min to separate the PMMA/VSe2/MX2 films from the Al2O3 substrates. Finally, the PMMA/VSe2/MX2 films were transferred onto the target substrates (e.g., SiO2/Si with 300 nm SiO2 layer, conductive Si). The PMMA layers were then removed by soaking PMMA/VSe2/MX2/target-substrate in acetone for 20 min. For the inversional transfer process, the Scotch tapes were used to separate the PMMA/VSe2/MX2 films from the Al2O3 substrates in the second step. Characterizations. Optical microscopy (Eclipse LV100ND, Nikon), Raman spectroscopy (LabRAM HR800, HORIBA, excitation wavelength of 514 nm), AFM (Dimension Icon, Bruker), TEM (JEMACS Paragon Plus Environment

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2100F, JEOL), and STEM (JEM-ARM200F, JEOL) were used to characterize the as-grown or transferred samples on SiO2, Si, or Cu TEM grids. AFM tips (RTESP-300, Bruker) with a high force constant of 40 N/m were used to scratch the single-layer MX2 under a contact mode. The scratching force was controlled by the factor of “Amplitude Setpoint”. Conductive AFM tips (SCM-PIT, Bruker) were used for the KPFM characterizations under the electrical lift mode with a 100 nm lift scan height. The conductive Si substrates for the transferred samples were connected with the sample chuck of the AFM equipment by using the conductive adhesive tapes. Theoretical Calculation. All density functional theory calculations were performed by Vienna ab-initio simulation package (VASP) using Perdew-Burke-Ernzerhof functional and projector-augmented wave potentials. The growth fronts of WSe2 were modeled by zigzag nanoribbons (NR) with stable W and Se terminated edges. The NRs consisted of 6 zigzag chains, and their periodic length was set as 13.18 Å, corresponding to 4 primary units. For the adsorption on surface, a 4×4 unit cell was used. A vacuum layer larger than 10 Å was selected to ensure the interactions from the nearest images negligible. The van der Waals interaction was taken into account using the method developed by Tkatchenko and Scheffler. All structures were fully relaxed until the force and energy were converged to 0.01 eV/Å and 10-5 eV/Å, respectively. The chemical potentials of V and Se in bulk states were chosen as references for adsorption energies.

Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51861135201, 51472008, 51290272, 61774003, 61521004), and the National Program on Key Basic research Project (Grant Nos. 2016YFA0200103, 2017YFA0304600, 2017YFA0205700), the Open Research Fund Program of the State Key Laboratory of Low Dimensional Quantum Physics (Nos. KF201601, KF201604).

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Supporting Information Available. Figures S1–S8 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org Conflict of Interest. The authors declare no competing financial interest.

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Figure 1. Schematic of the synthetic strategy, morphology evolutions, and Raman spectra of the CVDderived metal-semiconductor vertical stacks, e.g., ML-VSe2/SL-WSe2 on Al2O3. (a) Schematic of the twostep CVD synthetic route for the vertical stack. A single-layer s-TMDCs flake is firstly grown on the Al2O3 substrate (Step I (left)), and metallic VSe2 is then deposited, which selectively nucleates and grows from the edges of the s-TMDC (middle, Step II). With further increase in growth time, a fully stacked ML-VSe2/SL-TMDC stack is obtained (right). (b, c, d) Optical images for ML-VSe2/SL-WSe2 synthesized at different stages corresponding to (a). (e) Large-area OM image of the vertical stack from the step in (c). (f) Optical image of the transferred ML-VSe2/SL-WSe2/SiO2 from the similar sample shown in (c). (g) Raman mapping and single-point spectra from the arrow and superimposed points labeled in (f). (h) AFM image for ML-VSe2 growth on SL-WSe2 with the formation of a “void island”.

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Figure 2. Overall morphologies and Raman spectra of the representative ML-VSe2/SL-MoSe2 vertical stacks on Al2O3. (a, b, c) Typical OM images of the single-layer MoSe2 triangular flake, VSe2 nucleation at the edges of MoSe2, and complete ML-VSe2/SL-MoSe2 vertical structures, respectively. (d) AFM image of the sub-covered ML-VSe2/SL-MoSe2 stack. (e) Raman spectra collected from the lower MoSe2 layer and the stacked VSe2/MoSe2, as marked with blue and red triangular dots in (b), respectively. (f) Histograms of the A1g peak positions of MoSe2 collected from either the ML-VSe2/SL-MoSe2 stack (red) or single-layer MoSe2 areas (blue), respectively (collected from 80 randomly selected points). The average values are located at 240.1 and 240.3 cm–1, respectively.

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Figure 3. Structural characterizations of the ML-VSe2/SL-WSe2 metal-semiconductor heterostructure. (a) Low-magnification TEM image of the ML-VSe2/SL-WSe2 stack. The typical area of the underlying single-layer WSe2 is labeled with a red dot, and the stacked regions are labeled with purple, blue, and yellow dots, respectively. The inward and outward growth directions of ML-VSe2 starting from the triangular edge are labeled with red and white arrows, respectively. (b) SAED pattern for monolayer WSe2 recorded from the indicated dot in (a). (c, d, e) Top: SAED patterns of the areas labeled with purple, blue, and yellow dots in (a), respectively. Bottom: corresponding zoom-in images of the square regions. (f) Histogram of the rotation angles between VSe2 and WSe2. Inset: proposed top and side views of the stacked VSe2/WSe2. (g) High-resolution HAADF-STEM image across the boundary between the lower WSe2 and the stacked VSe2/WSe2. The crystal orientations of VSe2 and WSe2 are labeled with green and red dashed lines, respectively. (h) TEM images of the folded edge covering the pure WSe2 and stacked VSe2/WSe2, respectively. In (i), the layered structures of single-layer WSe2 and multilayer VSe2 are labeled with red and green, respectively. (j, k) Atomic-resolution HAADF-STEM images of WSe2 and the stacked VSe2 on WSe2, respectively.

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Figure 4. Growth mechanism exploration of the two-step CVD-grown VSe2/WSe2 stack. (a) Theoretical calculation of the absorption energies of V, VSe, and VSe2 radicals on the surface and the edge of a monolayer WSe2 flake, respectively. (b) Side-view of the initial growth behavior of VSe2 along/above the edge of a monolayer WSe2 flake. (c) Left: schematic of the cutting of a triangular WSe2 domain. Right: OM image of multilayer VSe2 grown on the patterned edge of monolayer WSe2.

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Figure 5. Band profiles of the metal-semiconductor (ML-VSe2/SL-WSe2; ML-VSe2/SL-MoSe2) vertical stacks. (a, e) and (b, f) AFM images and KPFM surface potential maps of transferred ML-VSe2/SL-WSe2 and ML-VSe2/SL-MoSe2 on conductive Si substrates, respectively. c, g Line profiles of the surface potential differences of the two types of vertical stacks. (d, h) Corresponding schematics of the band profiles for ML-VSe2/SL-WSe2 and ML-VSe2/SL-MoSe2 according to KPFM characterizations. Insets of (d, h): Charge distribution in the depletion region of metal-semiconductor (SC) junctions. A high (low) resistance (R) layer is formed in the depletion region of metal-p (n)-type-SC interface.

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