Direct Chemical Vapor Deposition Growth and Band-Gap

Mar 23, 2017 - †Center for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sc...
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Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS2/h‑BN van der Waals Heterostructures on Au Foils Zhepeng Zhang,† Xujing Ji,§ Jianping Shi,†,‡ Xiebo Zhou,†,‡ Shuai Zhang,∥ Yue Hou,⊥ Yue Qi,† Qiyi Fang,†,‡ Qingqing Ji,† Yu Zhang,†,‡ Min Hong,†,‡ Pengfei Yang,†,‡ Xinfeng Liu,∥ Qing Zhang,‡ Lei Liao,⊥ Chuanhong Jin,*,§ Zhongfan Liu,† and Yanfeng Zhang*,†,‡ †

Center for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, and ‡ Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China § State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ∥ CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ⊥ Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China S Supporting Information *

ABSTRACT: Stacked transition-metal dichalcogenides on hexagonal boron nitride (h-BN) are platforms for high-performance electronic devices. However, such vertical stacks are usually constructed by the layer-by-layer polymer-assisted transfer of mechanically exfoliated layers. This inevitably causes interfacial contamination and device performance degradation. Herein, we develop a two-step, low-pressure chemical vapor deposition synthetic strategy incorporating the direct growth of monolayer h-BN on Au foil with the subsequent growth of MoS2. In such vertical stacks, the interactions between MoS2 and Au are diminished by the intervening h-BN layer, as evidenced by the appearance of photoluminescence in MoS2. The weakened interfacial interactions facilitate the transfer of the MoS2/h-BN stacks from Au to arbitrary substrates by an electrochemical bubbling method. Scanning tunneling microscope/spectroscopy characterization shows that the central h-BN layer partially blocks the metal-induced gap states in MoS2/h-BN/Au foils. The work offers insight into the synthesis, transfer, and device performance optimization of such vertically stacked heterostructures. KEYWORDS: chemical vapor deposition growth, MoS2/h-BN, Au foil, van der Waals heterostructure, transfer

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unascertainable relative orientation, and incompatibility with scalable production. In contrast, chemical vapor deposition (CVD) was used to synthesize high-quality van der Waals heterostructures with a clean interface, including graphene/hBN,8 MX2/graphene,9,10 and MX2/MX2.11 Specifically, highquality MoS2/h-BN stacks are thus suitable for investigating the intrinsic transport properties of MoS2 because of the lack of dangling bonds and charged impurities as well as the wide band gap (∼5.9 eV) of h-BN.12

wo-dimensional (2D) layered materials, such as graphene,1,2 hexagonal boron nitride (h-BN),3 and transition-metal dichalcogenides MX2, where M = Mo or W and X = S, Se, or Te, have recently attracted worldwide attention for their great potential in clarifying fundamental physical issues and for application in new electronic and optoelectronic devices.4,5 van der Waals heterostructures of multiple stacked 2D materials allow the exploration of basic physical phenomena, like the Hofstadter butterfly effect in graphene/h-BN, the construction of high-performance optoelectronic devices (e.g., with MoS2/WS2)6 and the interlayer electron−phonon coupling in WSe2/h-BN heterostructures.7 However, layer-by-layer assembly of heterostructures often entails interfacial contamination, uncontrollable layer thickness, © 2017 American Chemical Society

Received: March 3, 2017 Accepted: March 23, 2017 Published: March 23, 2017 4328

DOI: 10.1021/acsnano.7b01537 ACS Nano 2017, 11, 4328−4336

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Figure 1. (a) Schematic of h-BN growth with H3NBH3 complex precursor; (b) XPS and (c) SEM image of h-BN on Au; (d) OM and (e) AFM images of transferred h-BN on 300 nm SiO2/Si showing uniform monolayer h-BN with height of ∼0.75 nm relative to SiO2; (f) large-scale STM image (VT = −0.08 V, IT = 4.16 nA, T = 298 K) of h-BN on hex-Au(001) showing periodic striped superstructure; (g) magnified STM image (−0.28 V, 5.71 nA, 298 K) of pattern with average period of ∼1.43 nm; (h) simplified sphere model of orientation of h-BN parallel to hex-Au(001); (i) atomic-resolution STM image (−0.01 V, 14.81 nA, 298 K) of h-BN with lattice constant of ∼0.25 nm; corresponding FFT pattern inset, upper right.

coverage monolayer h-BN was successfully synthesized on the Au foil. Submonolayer or fully monolayer MoS2 films were deposited on the full-coverage h-BN/Au by adjusting either the precursor−substrate distance or the growth time in the second CVD growth step. The vertical stacking geometry and intrinsic electronic properties of the MoS2/h-BN/Au foils were characterized by atomic-resolution scanning transmission electron microscopy (STEM), Raman spectroscopy, photoluminescence (PL) spectroscopy, and scanning tunneling microscope/spectroscopy (STM/STS). An electrochemical bubbling transfer method was developed to transfer the vertical MoS2/h-BN stacks from the Au foils onto arbitrary substrates. This nondestructive transfer route permits the low-cost batch production of MoS2/h-BN stacks, facilitating their practical application. STM/STS was also employed to investigate the blocking effect of monolayer h-BN on the metal-induced gap states in the as-grown MoS2/h-BN/Au system.

The most widely used substrates for h-BN growth (Cu, Ni, and Pt foils, similar to graphene growth) react with S vapor during MoS2 growth, hindering the direct CVD synthesis of MoS2/h-BN stacks. Two routes avoiding this issue have been proposed. One entails molecular beam epitaxy growth of MoS2 under greatly reduced S concentrations and ultrahigh vacuum on graphene/Pt,13 while the other uses thermal decomposition of ammonium ethiomolybdate ((NH4)2MoS4) directly onto graphene/Cu.9 However, the high cost of vacuum systems and the uncontrollable thickness of the MoS2 films render these methods unsuitable for large-area production. Some S-resistant substrates, i.e., Al2O3,14,15 SiO2,16 transferred h-BN/SiO217 and exfoliated h-BN/SiO2,18−20 have been previously employed for the growth of h-BN films, or WS2/hBN or MoS2/h-BN heterostructures. However, their catalytic inertness to h-BN synthesis, transfer-induced impurities, and limited size of exfoliated h-BN flakes caused unsatisfactory heterostructure quality and degraded device performances. Recently, Fu et al.21 reported the direct growth of high-quality MoS2/h-BN heterostructures on sulfide-resistant nickel-based alloys. Nevertheless, exploring the direct synthesis of high quality MoS2/h-BN heterostructures on a pure metal substrate and investigating the bandgap characters remain unaddressed. Liao et al. reported that placing mono- or bilayer h-BN between monolayer MoS2 and metal electrodes greatly reduced the Schottky barrier height with a small tunneling resistance at the contact, thus dramatically improving the carrier mobility of MoS2.22 The direct synthesis of such vertical stacks on electrode materials should permit investigations of the internal mechanism for this tunneling resistance reduction. S-resistant Au demonstrated good characteristics for the direct synthesis of large-domain monolayer MoS2 or WS2.10,23 Considering of its catalytic ability23,24 to the growth of MoS2 and h-BN, Au foil should be a perfect conducting platform for directly synthesizing MoS2/h-BN van der Waals heterostructures. Herein, we demonstrate the direct synthesis of high-quality h-BN and subsequent MoS2 on h-BN/Au foils via a facile lowpressure CVD (LPCVD) method toward constructing MoS2/hBN van der Waals heterostructures. A high-quality, full-

RESULTS AND DISCUSSION LPCVD was used previously to grow high-quality 2D materials including h-BN on metal substrates,25 MX2 on insulating substrates,26 and graphene/h-BN vertical and in-plane heterostructures on Cu foils.8,27 Very recently, MoS2/graphene vertical stacks were synthesized by CVD on Au foils.10 A direct two-step LPCVD strategy for synthesizing MoS2/h-BN van der Waals heterostructures on Au foils is designed based on these examples, as shown in Figure S1. The growth of h-BN on Au foil is displayed in Figure 1a, wherein a solid H3NBH3 complex is the h-BN precursor and the catalytic properties of Au promote growth, as reported previously.28 Notably, a relatively high growth temperature is needed for the growth of h-BN on Au foils relative to that on Cu foils because of the chemical inertness of Au foils and the relatively weak interfacial interactions of h-BN/Au foils (see Figure S2), as supported by previous theoretical calculations.29 After growth, X-ray photoelectron spectroscopy (XPS) characterizations were used to confirm the formation of h-BN; B 1s and N 1s peaks appear at ∼190.0 eV and ∼397.5 eV (Figure 1b; survey spectra shown in Figure S3), respectively, as previously seen.25 The uniform film 4329

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Figure 2. (a) Schematic of MoS2/h-BN stack growth via two-step LPCVD method; (b−d) SEM images of triangular MoS2 flakes on h-BN/Au; coverages of ∼96.35%, ∼42.18%, and ∼9.25% achieved with precursor-to-substrate distances (D) of ∼2.5 cm, ∼3.0 cm, and ∼3.5 cm, respectively; (e) Raman spectra from three samples each of MoS2/h-BN/Au and MoS2/Au; (f) PL spectra of MoS2/h-BN/Au and MoS2/Au recorded at room temperature (rt, 298 K) and 100 K. Spectra are normalized to the Raman A1g peak intensity. The B excitonic transition, the neutral excitonic transition of A, and the charged excitonic transition of are labeled with B, A, and A−, respectively. Temperature dependence of Raman spectra of (g) MoS2/Au and (h) MoS2/h-BN/Au, respectively; positions of (i) E2g and (j) A1g Raman modes of MoS2/Au (blue) and MoS2/h-BN/Au (red) plotted as functions of testing temperature, showing fit lines and linear temperature coefficients X.

Au foils mediated by the hex-reconstruction of Au(001).24 The high-resolution STM image in Figure 1g shows that the zigzag direction of the h-BN layer is always aligned with the strip direction, thus indicating the near-epitaxial growth of h-BN on the hex-Au(001) surface. The atomic-resolution STM image in Figure 1i shows the representative honeycomb structure of hBN with a lattice constant of ∼0.25 nm, again suggesting the formation of a high-quality h-BN layer on Au foil. A simplified sphere model is shown in Figure 1h, addressing the obtained stacking geometry of h-BN on hex-Au(001). In brief, we reported the facile CVD synthesis of high-quality full-coverage monolayer h-BN on Au foil. As illustrated in Figure 2a, monolayer MoS2 is synthesized on the full-coverage monolayer h-BN/Au using MoO3 and S as precursors. After growth, triangular MoS2 domains are clearly identified from dark contrasts in SEM imaging (Figure 2b), suggesting the formation of MoS2 on the h-BN/Au. The coverage and domain size of MoS2 were precisely tuned by altering the precursor−substrate distance (Figure 2b−d). The coverage of MoS2 nanosheets decreases with increasing distances between the MoO3 precursor and substrate, possibly because of the decreased MoO3 precursor concentration. After MoS2 growth, h-BN wrinkles remain noticeable in SEM images with almost unchanged shapes (Figure 2c,d). This indicates the near-perfect preservation of the h-BN layer even after growing MoS2. In order to confirm this, the chemical composition of the MoS2/h-BN stacks was obtained by XPS measurement (Figure S5). The Mo 3d5/2 (at ∼230.0 eV) and 3d3/2 (at ∼233.2 eV) peaks align with those of Mo4+, while the S 2p3/2 (at ∼162.9 eV) and 2p1/2 (at ∼164.1 eV) peaks can be assigned to S2−, which agree well with standard XPS data for

morphology of h-BN on the Au foils is verified by the uniform color contrast of the scanning-electron microscope (SEM) image shown in Figure 1c. The randomly distributed white lines may be wrinkles in the h-BN films, generated by the different thermal expansion coefficients of Au (∼14 × 10−6 K−1 at 300 K) and h-BN (∼−2.72 × 10−6 K−1 at 300 K).30,31 The uniform color contrast and wrinkle formation also indicate the evolution of full-coverage h-BN layers on the Au foils. To characterize the structural properties, the as-grown h-BN films were transferred from the Au foils onto 300 nm SiO2/Si substrates by an electrochemical bubbling method. This method avoids etching the Au foils and thus guarantees the recyclability of the foils.23 An optical image of the transferred hBN film is shown in Figure 1d, where the uniform color contrast indicates large-area homogeneity in the transferred film thickness. Figure 1e shows a typical atomic force microscopy (AFM) image of the edge of a transferred film. The corresponding height profile analysis yields an average height of ∼0.75 nm relative to the substrate, suggesting monolayer hBN. High-resolution TEM imaging (see Figure S4) shows uniform contrast in the sample, again indicating homogeneous layer thickness. This homogeneous monolayer growth of h-BN was attributed to the rather low solubility and migration barrier of boron and nitrogen atoms in/on the Au substrate.32 STM is also used to characterize the atomic-scale structures of the as-grown h-BN layer on Au foil. Striped patterns with the periodicity of ∼1.43 nm are distributed uniformly on different Au terraces (mainly comprising the Au(001) facet), as depicted in Figure 1f. Otherwise, two types of rotational domains at angles of ∼90° universally occur on the sample surface. This quasi-1D striped morphology was also reported for graphene/ 4330

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Figure 3. (a) Schematic of electrochemical bubbling transfer of sample. MoS2/h-BN/Au is covered by PMMA. (b) Top panel: PMMA/MoS2/ h-BN gradually separated from Au foil by cathode-generated H2 bubbles under constant voltage (2.8 V). Bottom panel: (left) PMMA/MoS2/ h-BN on SiO2/Si and (right) Au foil after removing vertical stack by 20 s bubbling. (c) SEM image of full-coverage MoS2/h-BN film on Au. (d) Optical image and (e) Raman map of MoS2/h-BN transferred to SiO2/Si. (f) Corresponding PL and (inset) Raman spectra of transferred MoS2/h-BN.

MoS2.33 Notably, the Mo 3d and S 2s peaks are blue-shifted relative to those from MoS2/Au. This indicates that the h-BN layer weakens the electron-doping effect from Au to MoS2 by the same mechanism as that of graphene in MoS2/graphene/ SiO2.34 Additionally, the B 1s (at 190.5 eV) and N 1s (at 398.2 eV) peaks are identical to those described elsewhere.25 Raman and PL spectra determined the lattice strains, adlayer−substrate interactions, doping levels, and layer numbers of the 2D materials. Raman spectra from the asgrown MoS2/h-BN/Au and MoS2/Au were also compared to verify the stacking geometry and crystal quality of the vertical stacks (Figure 2e). It is known that the E2g mode represents the in-plane vibration of Mo and S, which is sensitive to the built-in strain in 2D MoS2, while the A1g mode represents the out-ofplane vibration of S associated with the doping level and adlayer−substrate interactions.35 In Figure 2e, the E2g and A1g peaks of the as-grown MoS2 on h-BN/Au are centered at ∼383.9 and ∼404.7 cm−1, respectively, while those of the asgrown MoS2/Au are at ∼382.8 and ∼404.2 cm−1. The blueshifting of the E2g peak for MoS2/h-BN/Au indicates decreased interfacial interactions between the MoS2 and h-BN/Au relative to those of the MoS2 and Au. The stiffening of the A1g peak at ∼0.5 cm−1 and high intensity ratio of A1g to E2g for MoS2 on hBN/Au (compared to that on Au) are attributed to the reduced electron density of MoS2 (see Figure S6), as demonstrated previously.17 Moreover, the frequency difference in the two typical Raman peaks of MoS2 on h-BN/Au is ∼20.8 cm−1, suggesting the monolayer nature of MoS2. Surprisingly, a strong PL signal from the as-grown MoS2 on h-BN/Au appears at ∼668 nm at room temperature, which corresponds to a direct band gap of ∼1.86 eV as shown in Figure 2f. This differs from the obvious PL quenching effect observed in MoS2/Au, again confirming the vertical stacking geometry of the MoS2/h-BN/Au.36 For characteristic monolayer MoS2, the two pronounced PL peaks A and B occur at ∼668 and ∼618 nm, arising from neutral exciton emission from the direct transition at the K-point and exciton emission from another direct transition between the conduction band and a lower lying valence band, respectively.37 The third peak, located at lower energy (∼719 nm) than that of the A peak, is associated with the recombination of negatively charged excitons of A (trion emission, A−). Generally, the integrated PL intensity ratio from A and A− (A/A−) is variable with the

doping level of MoS2 . A higher doping level usually corresponds to a lower A/A− ratio.17 The A/A− ratios for our MoS2/h-BN/Au samples are approximately at 8.0 at a room-temperature testing condition and 18.0 at 100 K. This value is obviously higher than that of MoS2/SiO2 samples (A/ A− ∼ 3.1) shown in the Figure S7, thus indicating the lower doping level of our synthesized heterostructures. The weak interaction of MoS2 with h-BN/Au, relative to that with Au, is further confirmed by the temperature-dependent Raman spectra from 100 to 425 K. As shown in Figure 2g−j, the E2g and A1g modes of both systems are red-shifted with increasing temperature, presenting linear temperature dependences. This linear dependence is attributed to the anharmonic vibrations of the MoS2 lattice.38 Parts i and j of Figure 2 show the linear temperature coefficients X extracted from the slopes of the E2g and A1g modes in MoS2/Au and MoS2/h-BN/Au, respectively. The X difference between E2g and A1g modes in MoS2/h-BN/Au (∼0.002 cm−1 K−1) is much smaller than that of MoS2/Au (−0.006 cm−1 K−1). This indicates the weak strain in the upper MoS2 layer in the MoS2/h-BN/Au system. MoS2 monolayers with little lattice strain and no doping were successfully synthesized on the h-BN/Au surface by LPCVD. Considering the chemical inertness of the noble-metal Au foil, the electrochemical bubbling transfer method was developed to separate MoS2/h-BN stacks from the Au foil. This transfer method guarantees reusability of the Au substrate and avoids chemical etchants, as illustrated in Figure 3a. First, the MoS2/h-BN/Au foil sample is covered by a poly(methyl methacrylate) (PMMA) layer. Second, the PMMA/MoS2/hBN stack is gradually separated from the foil by H 2 microbubbles produced in a water-splitting reaction at the cathode when a constant voltage of 2.8 V is applied between cathode and anode (W foils).23 The separated PMMA/MoS2/ h-BN film and intact Au foil are shown in the lower panel of Figure 3b. Finally, the PMMA layer is removed by a typical acetone bath process. The transfer method is compatible with full-coverage MoS2/h-BN on Au foil (SEM image shown in Figure 3c). Figure 3d shows an optical image of the MoS2/hBN film transferred to a SiO2/Si substrate. The homogeneous color contrast indicates the uniform film thickness of MoS2/hBN. The black dashed lines in Figure 3d correspond to wrinkles in the continuous MoS2/h-BN film. 4331

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Figure 4. (a) Low magnification ADF-STEM image of transferred MoS2/h-BN showing MoS2 flakes over the complete h-BN layer. (b) Atomicresolution ADF-STEM image resolving the lattice atoms and edges of a MoS2 domain. (c) Corresponding FFT pattern of the area in (b) showing two sets of 6-fold symmetric spots labeled with blue and red circulars corresponding to MoS2 and h-BN, respectively. (d) Histogram of the distribution of the relative rotation angles between MoS2 and h-BN. (e, f) ADF-STEM images showing two representative image focused on the overlapped regions of two adjacent MoS2 domains. Inset: corresponding FFT patterns with the relative rotation angles indicated.

Figure 5. (a) Optical image of the as-grown single-crystal MoS2 on h-BN/Au. Inset: MoS2 triangular domain crystal with the edge length of ∼20 μm (temperature of S powders is 65 °C). (b) SEM image of the as-grown single-crystal MoS2 on h-BN/Au. (c) SEM image of the MoS2 triangular domain with the edge length of ∼20 μm. (d) Growth rates plotted as a function of the single-crystalline domain sizes of the CVDsynthesized MoS2 on h-BN obtained in this work and those reported in the literature. The data points are labeled with the reference number. Red: using S and MoO3 as precursor. Blue: using H2S and Mo foils as precursor. (e) The Ids−Vds characteristics for the FET device of transferred MoS2/h-BN heterostructures. Inset: optical image of a FET device of transferred MoS2/h-BN heterostructures. (f) Transfer characteristics (Ids−Vg) of the device at different Vds.

Figure 3e shows Raman mapping from the edge of the film, which confirms the existence and continuity of the MoS2/h-BN stack. The PL spectrum (PL peak at ∼666 nm) and Raman spectrum (with a frequency difference of ∼21 cm−1), as shown in Figure 3f, reveal the high-quality monolayer nature of the top MoS2 layer. An electrochemical bubbling transfer was also employed for sub-monolayer MoS2 on h-BN/Au foil. AFM was

employed to characterize the transferred films, as depicted in Figure S8. The corresponding height histogram profile confirms a monolayer MoS2 domain on h-BN. The obvious wrinkle across both MoS2 domains and their gaps reveals that the underlying h-BN was transferred simultaneously onto the SiO2 substrate with MoS2. In brief, MoS2/h-BN stacks are transferred intact from Au onto SiO2 substrates by this electrochemical 4332

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Figure 6. (a) Atomic-scale STM morphology (inset, − 0.28 V, 5.71 nA, 298 K) and STS spectrum of monolayer h-BN on hex-Au(001) (1.20 V, 200 pA, 78 K; Vrms = 10 mV, f = 932 Hz). (b, c) High-magnification STM images (3.00 V, 200 pA, 78 K; 0.91 V, 0.89 nA, 78 K) of monolayer MoS2 on h-BN/Au(001) showing striped patterns similar to those of MoS2/Au(001) with a period of ∼4.6 nm and 60° rotation of MoS2 lattice relative to stripe direction. (d) Large-area STM image (1.30 V, 300 pA, 78 K) of monolayer MoS2 on Au(001). (e) Magnified STM image (0.75 V, 300 pA, 78 K) showing MoS2 lattice (∼0.32 nm lattice constant) aligned with striping direction (∼7.5 nm in period). (f) Superstructure period and height profile analyses of systems. (g) STS analyses of MoS2 on bare Au(001) (black) and h-BN/Au(001) (red) surfaces showing different band gaps. Inset: lattice spacing of MoS2.

interface interaction between MoS2 and h-BN and their random relative orientations. To investigate the electrical properties of the as-grown MoS2/h-BN heterostructures, MoS2 triangular domains with edge lengths up to ∼20 μm were deliberately obtained by a careful optimization of the growth parameters. Compared with the growth condition of the previous samples (shown in Figure 1b−d), the temperature of S powders was greatly reduced from 103 to 65 °C to weaken the poisoning of the MoO3 precursor.4 The largest domain size of our sample can be as large as ∼20 μm, as evidenced by the typical optical and SEM images shown in Figure 5a−c. Notably, the obtained MoS2 domains are all of triangular shapes and with sharp edges,40,41 indicating their rather high crystal quality and the termination type at the domain edge (Mo-terminated type). Figure 5d is a comparison of the domain size and growth rate between our work and the published results regarding the growth of MoS2/h-BN heterostructures (more details in Table S1). Apparently, the domain size (∼20 μm) and the growth rate (4 μm/min) are larger than that reported by Fu’s group (H2S precursor and Mo foil as precursor)21 and almost ∼4.4 times and ∼20 times higher than those described in the literature using the same S and MoO3,17−19 respectively. The large-domain MoS2/h-BN heterostructures were then transferred onto SiO2/Si substrates, using the method described above, to fabricate the back-gate field effect transistors (FETs). The I−V characteristics of a typical device with a channel length (L) of ∼4 μm and a channel width (W) of ∼3.5 μm were measured in vacuo (Figure 5e). The nearlinear and symmetric curves suggest the formation of ohmiclike contacts at the source and drain electrodes. The transfer curves driven at different Vds were also achieved in Figure 5f. Apparently, the Ids value increases monotonically with increasing Vg, indicative of its n-type behavior. The maximum on/off current ratio was estimated as ∼107, and the maximum mobility was determined to be ∼11.4 cm2 V−1 s−1, respectively, which is ∼6.7 times higher than the normal devices constructed from CVD-MoS2 transferred on SiO2, as shown in Figure S9.

bubbling transfer method. This facilitates the low-cost batch production of high-quality MoS2/h-BN van der Waals heterostructures. Annual dark-field (ADF)-STEM characterizations were then performed on the transferred MoS2/h-BN stacks. In Figure 4a, MoS2 flakes in brighter contrast were observed with regard to the bare h-BN area due to their stacking geometry and difference in atomic number (heavier Mo and S atoms over lighter B and N atoms). Figure 4b exhibits a high magnification ADF-STEM image of the edge of a MoS2 domain on h-BN, in which the lattice atom and edge structure at atomic scale could be ambiguously resolved. No obvious defects was observed inside the flake, suggesting the high-crystalline quality of asgrown MoS2 samples on h-BN. More specifically, the measured lattice parameters from the corresponding FFT pattern (Figure 4c) were 0.31 nm (from blue set) and 0.25 nm (from red set), which could be well assigned to be MoS2 and h-BN with a relative rotation angle of 9°. Further statistics regarding the relative rotations of MoS2 with h-BN, obtained from 12 different locations, are given in Figure 4d. It is clear that no favored rotation angle can be noticed from the synthesized samples, which is similar to the published literatures relating to both the CVD-grown MoS2 on transferred h-BN/SiO217 and the directly grown h-BN on Ni−Ga alloys21 but not fit with the reports regarding the precisely aligned or low-angle twisted MoS2 on exfoliated h-BN.18,19 For the current growth system, the deviation from strict alignment was attributed to the influences from steps and impurities on the substrates or at the interfaces as well as the different growth paramenters.9,13,19,39 Besides, some overlapping of two adjacent MoS2 flakes at the edges were also found in the samples, as determined by the formation of moiré patterns and the brighter contrasts result from the twisted MoS2 bilayer (BL) (Figure 4e,f, labeled with white dash lines). This overlapping geometry may be formed either in the transfer or in the CVD growth process as mainly mediated by the weak interaction between MoS2 and the basal plane of h-BN. Briefly, all these facts address the relative weak 4333

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ACS Nano Using graphene contact and additional h-BN encapsulation may further improve the electrical performance.19 In brief, this electrical characterization confirms the high crystalline quality of our synthesized MoS2/h-BN van der Waals heterostructures. The phonon-limited mobility of MoS2 is predicted to be as high as 410 cm2 V−1 S−1;42 however, most reported experimental values are much lower than this ideal value. This is mainly caused by the existence of contact resistance, normally induced by the high Schottky barrier in the interface of MoS2/metal electrodes. Very recently, both Liao’s group and Wong’s group have developed an effective methods to avoid this issue by inserting insulating layers or tunneling layers of hBN or Ta2O5 between MoS2 and the metal electrodes.22,43 The carrier mobility of MoS2 was dramatically increased from 73.0 to 321.4 cm2 V−1 S−1 by Liao’s group, which is attributed to the blocking of metal-induced gap states.17 To achieve a deeper understanding of the internal mechanism for the dramatic increase, further explorations of the effects of the inserted layers on the electronic properties of such heterostructures are necessary. The tunneling layer in the present system is monolayer hBN. For comparison, the electronic band gap of monolayer hBN on Au was first measured by STS (Figure 6a). The atomicscale morphology of monolayer h-BN on the universally extant hex-Au(001) facet is presented in the inset (facet evidence is shown in Figure S10). The STS data show a band gap of ∼4.2 eV, which is slightly smaller than that of the h-BN/Cu system (∼5.0 eV) demonstrated in the previous work.44 For the vertical stack of MoS2/h-BN on Au, high-resolution STM analyses are provided in Figure 6b,c. As a comparison, the STM morphologies of MoS2/Au(001) are presented in Figure 6d,e. A typical stripelike superstructure with a period of ∼7.5 nm was reported for MoS2/Au(001) and attributed to the lattice symmetry and lattice constant mismatch effect,45 equivalent to the data shown in Figure 6d,e. By inserting h-BN, a similar striped superstructure occurs, but with a period of ∼4.6 nm, along with some blurring of the detailed stripe morphology. In addition to the difference in superstructure period, the relative orientation of the MoS2 lattice with the stripe orientation changes with the insertion of the h-BN layer, from 0° to 60°. A decrease in surface undulation height from ∼0.24 to ∼0.06 nm is apparent in the height profile analyses from the STM images (Figure 6f). These changes indicate that h-BN diminishes the strong interfacial interactions between MoS2 and Au. Notably, the atomically resolved image (inset in Figure 6g) clearly shows the hexagonal lattice of MoS2 with a measured lattice constant of ∼0.32 nm, equal to that of MoS2/Au(001). Further low-temperature STS measurements were performed on the vertical stacks of MoS2/h-BN on Au, as well as on MoS2/Au, as depicted in Figure 6d. For the vertical stacks, a band gap of ∼1.75 eV is found with a valence band maximum (VBM) at −1.0 eV and conduction band minimum (CBM) at 0.75 eV. This differs from that of monolayer MoS2 on Au(001), which has a band gap of ∼1.50 eV with VBM at −1.0 eV and CBM at 0.5 eV.45 Consequently, inserting the tunneling layer of h-BN increases the band gap of MoS2 on Au by ∼0.25 eV. As presented in Figure 6g, two characteristic peaks (P1 and P1′) appear for the two systems. These are both located at the same energy of ∼−1.5 eV in the STS spectra. These most likely represent the states at the Γ position of MoS2.46 The VBM of MoS2 for both systems has the same energy; thus, the 0.25 eV band gap increase is attributed to the upshift of the CBM, as similarly reported for folded bilayer MoS2.43 According to

previous studies, the metal-induced gap states should be near the conduction band of MoS2. Therefore, the band gap increase is ascribed to the suppression of metal-induced gap states by the inserted h-BN layer. Consequently, monolayer h-BN blocks most metal-induced gap states for MoS2 on bare Au(001), thereby decreasing the Schottky barrier height and increasing the mobility of MoS2. Inserting thicker h-BN layers could fully block the metal-induced gap states.

CONCLUSION In summary, monolayer MoS2/h-BN van der Waals heterostructures were directly synthesized on Au foil using an allCVD strategy. The traits of these heterostructures, including decreased lattice strain, interfacial interactions, and doping of the monolayer MoS2, were verified by several characterization methods. The vertical stacks were successfully transferred onto SiO2 via an electrochemical bubbling transfer method, allowing reuse of the Au foils. The inserted h-BN layer also generated a blocking effect against the metal-induced gap states of MoS2 on Au. This study significantly propels investigations on the intrinsic properties of monolayer MoS2 and further improves the performance of MoS2-based electronic devices. EXPERIMENTAL SECTION Pretreatment of Au and Cu foils. The ∼25 μm thick Au foils were purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd. Before growth, the Au foils were ultrasonically cleaned in an HCl solution (∼1 mol/L) and acetone (99%) for ∼10 min to remove the oxide and organic impurities. After rinsing with distilled water, cleaned Au foils were annealed in the air conditions at ∼950 °C for ∼5 h to remove the remnants and, more importantly, to improve the crystalline properties of Au. The 25 μm Cu foil was purchased from Alfa Aesar China. Before growth, Cu foils were electropolished in a homemade plastic tank for 1 h using a mixture of phosphoric acid (85 wt %) and ethylene glycol (15 wt %) as the electrolyte. In the electro-polishing process, a 1.8 V DC voltage bias was applied between anodic Cu foil and cathodic Cu plate. h-BN Growth on Au Foils. The pretreated Au foils were loaded into a 1 in. quartz tube and heated by a Thermal split tube furnace (Lindberg/Blue M HTF55347c). A base pressure of 0.5 Pa was achieved in the tube using a rotary pump system. The Au foil was first heated from room temperature to 1030 °C in 50 min, with 200 sccm H2 and 80 sccm Ar as carrier gases. Followed by 60 min of annealing at 1030 °C, ammonia borane, a solid BN precursor, was introduced into the furnace using a heating belt at 100−110 °C for 10 min to grow hBN. MoS2 Growth and Transfer. MoS2 is grown by adopting MoO3 (Alfa Aesar, purity 99.9%) and S (Alfa Aesar, purity 99.5%) precursors through a low-pressure CVD route. The typical temperature of the hBN/Au substrate, MoO3, and S powders are 680, 530, and 65−103 °C, respectively. Ar (50 sccm) and H2 (5 sccm) were used as the carrier gases. After growth, the MoS2/h-BN vertical heterostructures were detached from Au foils and transferred onto target substrates using an electrochemical bubbling transfer method. Characterizations. The samples were characterized by OM (Olympus BX51), SEM (Hitachi S-4800, 1 kV), XPS (Kratos Analytical Ltd. AXIS-Ultra with monochromatic Al Ka X-ray), Raman and PL spectroscopy (Horiba Jobin-Yvon, LabRAM HR800, excitation light of 514 nm in wavelength), TEM (JEM 2100, acceleration voltage 200 kV), and AFM (Veeco Nanoscope ICON, peak force mode). ADF-STEM was performed with a probe-corrected TEM (FEI, Titan ChemiSTEM) which was operated at an acceleration voltage of 200 kV. The atomic-resolution ADF-STEM images were slightly processed with an improved Wiener filtering for a better display. A UNISOKU UHV-STM/STS system was used for the atomic-scale structure characterizations under a base pressure better than 10−10 mbar. The STS spectra were measured at ∼78 K by 4334

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recording the output of a lock-in system with the manually disabled feedback loop. A modulation signal of 10 mV at 932 Hz is selected under a tunneling condition of 1.20 V, 200 pA. Device Fabrication and Measurements. MoS2/h-BN films were first transferred onto 300 nm SiO2/Si substrate. Then the substrates were spin coated with MMA and PMMA, and the EBL (JEOL 6510 with NPGS) was employed to define the source and drain pattern followed by metal evaporation and lift-off processes. Here, Cr/Au (15/ 50 nm) was deposited as contact electrode. Electrical characterizations were carried out with the Lake Shore TTPX probe station and Agilent 4155C semiconductor parameter analyzer.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01537. Figures S1−S10 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lei Liao: 0000-0003-1325-2410 Zhongfan Liu: 0000-0003-0065-7988 Yanfeng Zhang: 0000-0003-1319-3270 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (Nos. 2016YFA0200103, 2016YFA0200700) and the National Natural Science Foundation of China (Nos. 51290272, 51472008, 21201012, 51121091, 51072004, and 51201069), the National Basic Research Program of China (Nos. 2013CB932603 and 2014CB932500), the Beijing Municipal Science and Technology Planning Project (No. Z151100003315013), Young Thousand Talented Program, start-up funding from Peking University (No. Y5862911ZX), 100-Talents Program of Chinese Academy of Sciences (No. Y5862912ZX), and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (No. KF201601). X.J. and C.J. are grateful for the access to microscope facilities kindly provided by the Center of Electron Microscopy of Zhejiang University. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (2) Wu, T.; Zhang, X.; Yuan, Q.; Xue, J.; Lu, G.; Liu, Z.; Wang, H.; Wang, H.; Ding, F.; Yu, Q.; Xie, X.; Jiang, M. Fast Growth of InchSized Single-Crystalline Graphene from a Controlled Single Nucleus on Cu−Ni Alloys. Nat. Mater. 2015, 15, 43−47. (3) Kim, S. M.; Hsu, A.; Park, M. H.; Chae, S. H.; Yun, S. J.; Lee, J. S.; Cho, D.-H.; Fang, W.; Lee, C.; Palacios, T.; Dresselhaus, M.; Kim, K. K.; Lee, Y. H.; Kong, J. Synthesis of Large-Area Multilayer Hexagonal Boron Nitride for High Material Performance. Nat. Commun. 2015, 6, 8662−8672. (4) Chen, W.; Zhao, J.; Zhang, J.; Gu, L.; Yang, Z.; Li, X.; Yu, H.; Zhu, X.; Yang, R.; Shi, D.; Lin, X.; Guo, J.; Bai, X.; Zhang, G. OxygenAssisted Chemical Vapor Deposition Growth of Large Single-Crystal 4335

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