Scalable Synthesis of Highly Crystalline MoSe2 and Its Ambipolar

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Scalable synthesis of highly crystalline MoSe2 and its ambipolar behavior Yue Li, Kailiang Zhang, Fang Wang, Yulin Feng, Yi Li, Yemei Han, Dengxuan Tang, Baojun Zhang, and H.-S. Philip Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10693 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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ACS Applied Materials & Interfaces

Scalable synthesis of highly crystalline MoSe2 and its ambipolar behavior Yue Li1, Kailiang Zhang*,1, Fang Wang1, Yulin Feng1, Yi Li1, Yemei Han1, Dengxuan Tang1, Baojun Zhang1, H. S. Philip Wong*,2 1

School of Electrical & Electronic Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, Tianjin, 300384, China

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Department of Electrical Engineering and Center for Integrated Systems, Stanford University, Stanford, California 94305, United States.

ACS Paragon Plus Environment

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ABSTRACT

Atomically thin, two-dimensional material molybdenum diselenide (MoSe2), has been shown to exhibit significant potential for diverse applications. The intrinsic bandgap of MoSe2 allows it to overcome the shortcomings of the zero-bandgap graphene, while its higher electron mobilities when compared to molybdenum disulfide (MoS2) make it more appropriate for practical devices in electronics and optoelectronics. However, its controlled growth has been an ongoing challenging for investigations and practical applications of the material. Here, we present an atmospheric pressure chemical vapor deposition (CVD) method to achieve highly crystalline, single- and few-layered MoSe2 using a SiO2/Si substrate. Our findings suggested that careful optimization of the flow rate can result in the controlled growth of large area MoSe2 with desired layer numbers due to the adjustment of gaseous MoSe2 partial pressure and nucleation density. The FETs fabricated on such as-synthesized MoSe2 displayed different transport behavior depending on the layer numbers, and can be attributed to the formation of Se vacancies generated during low flow rates. Monolayer MoSe2 showed n-type characteristics with an Ion/Ioff ratio of ~106 and a carrier mobility of ~19 cm2V-1s-1, while bilayer MoSe2 showed n-type-dominant ambipolar behavior with an Ion/Ioff ratio of ~105 and a higher mobility of ~65 cm2V-1s-1 for electron as well as ~9 cm2V-1s-1 for hole. Our results provide a foundation for propertycontrolled synthesis of MoSe2 and offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

KEYWORDS: two-dimensional materials, transition metal dichalcogenides, molybdenum diselenide, layer numbers, flow rates, chemical vapor deposition, ambipolar transport behavior


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INTRODUCTION The discovery of graphene in 2004,1 has since triggered world-wide research interest concerning atomically thin two-dimensional (2D) materials. As a complement of graphene, transition metal dichalcogenides (TMDCs) have recently attracted tremendous interest due to their sizable bandgap and unique optical and electronic properties.2,3 Both theoretical and experimental studies have revealed that mono- and multi-layered TMDCs have exclusive properties with potential for wide ranging applications.4-14 Although various numbers of layers can exist, monolayer TMDCs represent the ultimate material dimension control in the vertical direction and have unprecedented properties compared to their bulk counterparts. With decreasing thickness of TMDCs down to the monolayer, the bandgap changes from bulk indirect structure to direct structure.4 This transition provides excellent electrostatic control and much stronger luminescence quantum efficiency for monolayer TMDCs compared to bulk TMDCs, offering a more attractive alternative for low power applications,6 and optoelectronics.8 Bilayer TMDCs are also unique in that they are the first multilayer system involving van der Waals (vdW) interactions that also exhibit useful properties that are different from monolayer TMDCs. In comparison to monolayer TMDCs, bilayer TMDCs have a higher mobility,7 and the bandgaps of bilayer TMDCs have been demonstrated can be continuously reduced to zero by applying a vertical electric field,15 making them versatile materials that are appealing for various applications. Hence, investigations regarding mono- and bilayered TMDCs have been the focus of extensive research. A prerequisite for studies and practical applications of mono- and bilayered TMDCs concerns the controlled synthesis of large-area, high quality TMDCs crystals with precise thickness. In order to obtain these high quality TMDCs crystals, numerous approaches have been explored,


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including mechanical exfoliation,1 hydrothermal synthesis,16 chemical vapor deposition (CVD),17,18 atomic layer deposition,19 and molecular beam epitaxy.20 Among them, the CVD method has shown to be the most promising approach to obtain high-quality, wafer-scaled 2D materials,17,18,21-25 that are compatible with the standard fabrication process in the semiconductor industry. Historically, the majority of studies have concentrated on molybdenum disulfide (MoS2) as the TMDC of choice in 2D research. However, a recent shift in interest has brought attention to molybdenum diselenide (MoSe2) due to its diverse potential applications in energy conversions involving solar spectrum,26 room temperature exciton-polaritonic devices,27 and heterostructures integrating multiple TMDCs to extend their joined properties.28 The CVD synthesis of MoSe2 is more challenging than MoS2 synthesis due to the lower chemical reactivity of Se compared to S. This limitation may introduce barriers in the investigation and application of this versatile material. We believe that solving the challenge of controlled MoSe2 synthesis is priority for advancing the utilization of this material. By carefully optimizing the processing parameters and studying the underlying mechanisms of MoSe2 synthesis we intend to establish a standard fabrication process. Previous works have demonstrated that processing parameters such as temperature,29 pressure,30 and precursor amounts,31 have influence on TMDC thickness. According to the principles of CVD,32 flow rate is an essential factor that can affect the reactants mass transfer and precipitation, and ultimately may further influence the quality of grown-film. To this end, we explored the influence of flow rate on CVD-MoSe2 properties. We found that the MoSe2 layer numbers increase as the flow rate decreases and the underlying mechanism of this phenomena was discussed. As a result of our synthesized mono- and bi-layered MoSe2 samples, we can systematically characterize the as-synthesized MoSe2 and evaluate its electrical performance.


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The field effect transistors (FETs) fabricated on our MoSe2 samples showed that as-synthesized MoSe2 crystals can exhibit different transport behavior depending on the layer numbers, e.g. monolayer MoSe2 shows n-type characteristic while bilayer MoSe2 shows n-type-dominant ambipolar behavior. Our results not only pave the way to property-controlled synthesis of MoSe2 but also offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

RESULTS AND DISCUSSION The MoSe2 crystals were grown on SiO2/Si substrates with 50 mg Se and 15 mg MoO3 powder as precursors, using a two-temperature-zone tube furnace (Figure 1a). A mixture of Ar and H2 (10% H2, which is essential for the growth of MoSe233,34) was used as the carrier gas and reducing atmosphere during the deposition period. The photograph of a bare SiO2/Si substrate and the as grown mono- and bi-layered MoSe2 samples are shown in Figure S1. Flow rates of carrier gas during MoSe2 growth ranged between 5 and 40 standard cubic centimeters per minute (sccm) and microscopy images revealed different growth patterns at these different rates (Figure 1b-i). Interestingly, the MoSe2 crystal layer numbers increased as the flow rate decreased. The Raman spectrum of as-grown MoSe2 with different thickness is shown in Figure 1j. A blue shift in the A1g mode and a red shift in the E2g1 mode was observed, confirming the thickness transformations.33,35 When the flow rate was cut to 5 sccm, no precipitation of MoSe2 occurred and only some oxide was observed (Figure 1b). Particles and nanocrystals were obtained when the flow rate was set to 10 sccm (Figure 1c). As the flow rate was gradually increased, thicker MoSe2 crystals were obtained (Figure 1d-e). When flow rate was increased to 25 sccm, a significant amount of bi- and tri-layered MoSe2 crystals were obtained (Figure 1f). By further increasing the flow rate, bilayer MoSe2 crystals where the second layer fully covered the


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bottom layer were formed (Figure 1g). The color contrast homogeneity in the optical images demonstrated the high uniformity within the bilayer MoSe2 crystals. When the flow rates continued to increase, both monolayer MoSe2 crystals and bilayer MoSe2 crystals with bottom layer edges exposed were observed (Figure 1h). The substrate was covered with a majority of monolayer MoSe2 crystals when the flow rate was set to 40 sccm (Figure 1i). In summary, we found that a decrease in flow rate can improve the vertical growth of MoSe2 while an increase in flow rate can facilitate the lateral growth of MoSe2.


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Figure 1. Synthesis process and flow rate effect on the layer numbers of CVD-MoSe2. (a) Synthesis schematic of MoSe2 crystals on a SiO2/Si substrate grown in a quartz tube with Ar/H2 flow used as carrier gas. (b-i) Optical microscope images of MoSe2 grown at flow rates between 5-40 sccm. Mono- (1L), bi- (2L) and trilayer (3L) crystals are marked in (f-h) denoting their thickness-dependent contrasts. (j) The Raman spectra of the as-grown MoSe2 for different thicknesses with an inset showing the schematics of the A1g and E2g1 modes. To elucidate the role of flow rate on crystal formation, we examined our synthesis process by dividing it into four major steps according to the principles of CVD,32 and previous works.36 These steps include (1) the sublimation of the precursors (MoO3 and Se), (2) the mass transport


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of the precursors to the reacting region close to growth substrate in Zone I (as described in Experimental Methods), (3) the reaction between MoO3, Se and H2 to form gaseous MoSe2, and (4) the diffusion of gaseous MoSe2 across the boundary layer towards the growth substrate and precipitation of MoSe2 on it. In order to better explain the reaction processes during step (3), we further divided the reaction between MoO3, Se and H2 into two parts based on the reaction between MoO3 and S.37,38 The first step being the MoO3 reduction by Se and H2 to form the volatile intermediate MoO3-x species, and the second step being the MoSe2 species resulting from the further selenization of the intermediate MoO3-x. We hypothesize that the reactions occur as follows: x/2 Se + x H2+ 2 MoO3 = 2MoO3-x + x/2 SeO2 + x H2O (1) MoO3-x + (7-x)/2 Se = MoSe2 + (3-x)/2 SeO2 (2) We correlated the influence of flow rate on the layer numbers of MoSe2 to its effect on the partial pressure of gaseous MoSe2 (PMoSe2), and we postulate that the PMoSe2 can be equated by the following equation: PMoSe2 = (nMoSe2/nMoSe2 + nSe + ncarrier) * Ptotal (3) where nMoSe2, nSe and ncarrier are the moles of gaseous MoSe2, Se and carrier gas (Ar/H2), respectively. Ptotal is the total pressure of synthetic process. When the flow rate was set at a low level (≤10 sccm), only small amounts of Se were brought to the reacting region. At these low flow rates, eq. (1) preferentially occurred, and only a few MoSe2 crystals were formed. It was also important to consider that at these low flow rates large amounts of vapored MoO3 cannot be removed, resulting in gaseous MoO3 that easily diffused across the “plug flow” regime and boundary layer and ultimately precipitated on the growth substrate. This resulted in the oxides and nanocrystals observed in Figure 1b-c. Increased flow


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rates not only delivered more Se to the reacting region, but also removed excess vapored MoO3. During increased flow rates, the amount of Se was proportional to the rest of MoO3, and eq. (2) readily occurred. For simplicity, we assumed that the amount of Se was enough to fully transform the MoO3 into gaseous MoSe2. With this assumption, the nMoSe2 depended on the amount of the remaining MoO3 (nMoO3). In our synthetic process, the nSe was much larger than the nMoSe2 and ncarrier. Therefore, eq. (3) can be further described as: PMoSe2 = (nMoO3/nSe) * Ptotal (4) In eq. (4) the PMoSe2 decreases with the decrease of nMoO3 and the increase of nSe. In other words, the PMoSe2 decreases as the flow rate increases, resulting in the gradual decrease in layer number, as shown in Figure 1d-i. We also synthesized MoSe2 at different Se source temperature to confirm the influence of PMoSe2 (see the detail in Figure S2). The influence of partial pressure on film thickness has also been demonstrated previously for MoS2.31 Although here we mainly attributed the change in layer number to the flow rate, there may be other factors. For example, the interaction with the substrate can play an important role in the control of layer number as described in a previous report.31 Therefore, flow rate regulation may provide an appropriate partial pressure of gaseous MoSe2 for optimizing the layer number in as-synthesized MoSe2 crystals. In order to comprehensively investigate the influence of flow rate on the grown-MoSe2, we further increased the flow rates to range between 10 sccm and 110 sccm. We observed that the MoSe2 grain sizes enhanced with increasing flow rates but decreased at high levels (≥90 sccm) (Figure S3). We attributed the experimental results to the adjustment of nucleation density by elevated flow rate. This was in agreement with conclusions from previous reports.29,39 This further demonstrated that large grain size MoSe2 can be obtained by optimizing flow rates.


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Based on our experimental results, we focused on the mono- and bilayer MoSe2 crystals and systematically investigated the properties of as-synthesized MoSe2 via broad characterization methods. FETs were fabricated to study and compare the electrical performance of the monoand bilayer MoSe2 crystals. To analyze the thickness and uniformity as well as the crystal quality, atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) were performed. The AFM images and height profiles of monolayer and bilayer MoSe2 crystals were surveyed (Figure 2a-d). The MoSe2 triangular crystal showed homogeneous color contrast and a step height of ~0.71 nm (Figure 2a-b), confirming its monolayer characteristics. The second layer height was measured at ~0.69nm (Figure 2c-d), and is in agreement with previous reports regarding bilayer characteristics.33 The Raman and PL measurements are effective methods for the characterization of layer numbers and crystal quality in TMDCs.40,41 Two characteristic peaks appeared in the Raman spectra of the monolayer MoSe2 crystals (Figure 2e): a sharp one at a low wavenumber (240.5 cm-1) attributed to the A1g mode of MoSe2 (out of plane vibration) and a broad one at higher wavelength (286 cm-1) attributed to the E2g1 mode (in-plane vibration). The A1g and E2g1 modes of bilayer MoSe2 were located at 241 cm-1 and 285.6 cm-1, respectively. The uniformity of the Raman intensity maps of the A1g mode (Figure 2g-h) further confirmed the high homogeneity of the as-synthesized mono- and bilayer MoSe2. The PL spectra of mono- and bilayer MoSe2 were also measured (Figure 2f). It was noticed that the monolayer MoSe2 exhibited a prominent emission peak at ~1.55 eV with a strong intensity, while the bilayer MoSe2 showed a shifted peak at ~1.52 eV with a sharp decline in intensity. The change in bandgap and intensity noticeably indicated the transition from direct to indirect bandgap and is in agreement with previous reports.26,42


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Figure 2. Morphology, Raman spectrum and PL spectrum of as-grown MoSe2. (a-d) AFM images of the as-grown mono-(a) and bilayer (c) MoSe2. Panels (b) and (d) show the height profile at the marked red dotted lines from (a) and (c), respectively. The monolayer MoSe2 was 0.71 nm in thickness and the second layer height in bilayers was measured at 0.69 nm. Panel (e) shows the Raman spectrum of the as-grown monolayer and bilayer MoSe2. The inset shows a schematic of the A1g and E2g1 modes. Panel (f) denotes room-temperature PL spectrum of the asgrown monolayer and bilayer MoSe2. (g-h) Raman intensity maps of A1g mode for monolayer (g) and bilayer (h) MoSe2. The elemental composition and bonding of the monolayer (Figure 3a-b) and bilayer (Figure S4) MoSe2 crystals were examined by X-ray photoelectron spectroscopy (XPS). In the monolayer, the peaks at 232.5 and 229.4 eV corresponded to Mo 3d3/2 and 3d5/2 core levels while the Se 3d5/2 and Se 3d3/2 core level peaks were located at 54.8 and 55.6 eV, respectively (Figure 3a-b). This is in agreement with the results obtained in previous reports.34,43 The ~1:2 Mo/Se ratio obtained from the integrated peak areas suggested that the MoSe2 crystals have the


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appropriate stoichiometry. The XPS analysis of the bilayer MoSe2 showed that Mo 3d3/2 and 3d5/2 core levels were located at 232.1 and 229 eV while the Se 3d5/2 and Se 3d3/2 core level peaks were located at 54.7 and 55.6 eV, respectively (Figure S4). The 1:1.96 Mo/Se ratio confirmed by the XPS result indicated that there was some Se vacancy in the bilayer MoSe2 samples. The crystalline structure of the synthesized mono- and bilayer MoSe2 was further elucidated by using transmission electron microscopy (TEM). A poly (methyl methacrylate) (PMMA) assisted method,23 was used to transfer the MoSe2 samples to a TEM grid. The continuity of the transferred film (Figure 3c) suggested the high quality of the synthesized MoSe2. Some wrinkles and small particles observed on the TEM sample were caused by the transfer process. The high-magnification TEM image (Figure 3d) showed the interface between mono- and bilayer, as marked in Figure 3c. The HRTEM images and corresponding selected area electron diffraction (SAED) patterns of monolayer area and bilayer area were displayed in Figure 3e-f, respectively, confirming the single crystalline nature of the sample with a hexagonal structure. The lattice spacing measured from the HRTEM image (Figure 3e) are ~0.28nm and ~0.16nm, corresponding to ｛1010｝ and ｛1120｝ planes, in agreement with previous reports.33,35,44


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Figure 3. Elemental composition and structural characterizations of mono- and bilayer MoSe2. (a-b) XPS spectrum of monolayer MoSe2 showing the (a) Se 3d core level region and (b) Mo 3d core level region, respectively. (c) Low-magnification TEM image of a bilayer MoSe2 crystal transferred onto a TEM grid. (d) High-magnification TEM image of the mono/bilayer interface


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as marked in (c). HRTEM images and corresponding SAED patterns (inset) of the (e) monolayer and (f) bilayer regions as shown in (d). To evaluate the electrical performance of the synthesized MoSe2 crystals, back-gate FETs were fabricated based on the mono- and bilayer MoSe2 samples using electron beam lithography, as schematically illustrated in Figure 4a. Typically, 10 nm Ti and 100 nm Au were deposited as the electrodes using electron beam evaporation and magnetron sputtering, respectively. Prior to electrical measurements, the devices were annealed at 200 °C for 2 h under atmospheric conditions (100 sccm Ar and 10 sccm H2) to improve contact and remove residues. The electrical performance of typical devices with a channel length (L) of 2 µm and channel width (W) of 1 µm, as shown in inset of Figure 4c and Figure 5a, were measured at room temperature under a vacuum down to 10-3 Pa. The typical transfer and output characteristics of the device fabricated on monolayer MoSe2 are shown in Figure 4b-c. The transfer characteristics (Ids−Vbg) on a semilog scale can be seen in the inset of Figure 4b. The results suggested that the fabricated devices displayed n-type behavior, consistent with previous results.34,45 The linear and symmetric curves shown in Figure 4c indicated that ohmic contacts were formed between the electrodes and MoSe2. The field-effect mobilities can be estimated according to the equation µ = [dIds/dVbg] × [L/(WCgVds)], where L, W, and Cg are the channel length, width, and the gate capacitance per unit area, respectively.6 The gate capacitance can be estimated by the equation Cg = εrε0/d, where εr and ε0 are the relative static permittivity of silica (≈ 3.9) and vacuum permittivity (≈8.9 × 10−12 F/m), and d is the thickness of dielectric layer (300 nm). The Cg is estimated to be ≈ 1.15×10-8 Fµm−2. A statistical distribution of the carrier mobility and the Ion/Ioff ratio measured from 20 FETs based on the monolayer MoSe2 were shown in Figure 4d. The average carrier mobility and Ion/Ioff ratio were ~19 cm2V-1s-1 and 4×106, respectively, and were comparable to the exfoliated


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monolayer MoSe2 measured by the same back-gated configuration, confirming the high crystalline quality of our synthesized MoSe2 crystals. The typical transfer characteristics of the bilayer device were measured (Figure 5b). Interestingly, ambipolar behavior was exhibited, although the n-type behavior was much more prominent. A similar ambipolar behavior were reported by Lu et al..46 However, the mobility extracted from their devices is too low to be used in practical applications (0.02 cm2V-1s-1 for electron and 0.01 cm2V-1s-1 for hole). A previous study demonstrated that Se vacancies generated during their synthetic process result in the ptype-dominant ambipolar behavior.30 Similarly, we speculate that some Se vacancies may be created at the low-level flow rate condition, consistent with our XPS results. The inset of Figure 5b showed the transfer characteristics (Ids−Vbg) on a semilog scale and the I-V characteristics of the same device at a range of back-gated voltages (-60~-100) was shown in Figure 5c that further confirmed its ambipolar behavior. The output characteristics of the device at different positive back gate voltages were measured and the inset showed negative voltage results (Figure 5d). The current increased with increasing |Vbg|, which also demonstrated the ambipolarity of the bilayer device. The nonlinear data indicating a Schottky barrier formed between MoSe2 and metal contacts, which may attribute to complex Fermi level pinning. Similarly, 20 FETs based on bilayer MoSe2 were analyzed. The average mobilities calculated from these as-fabricated MoSe2 devices were ~65 cm2V-1s-1 (three-fold higher than that of monolayer MoSe2 devices) for electrons with an average Ion/Ioff ratio of 5×105 and ~9 cm2V-1s-1 for holes with an average Ion/Ioff ratio of 7×104. It should be noted that the ambipolar behavior displayed in the bilayer devices may make it ideal for complementary digital logic applications and p-n junctions,47 and the electrical performance of our devices could be further improved by optimizing the contact or using high k-dielectrics such as HfO2.


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Figure 4. Electrical performance of monolayer MoSe2. (a) Three-dimensional schematic view of back-gate FETs configuration. (b) Typical transfer characteristics (Ids-Vbg) of the monolayer MoSe2 FETs at different Vds. The inset shows transfer characteristics with semilog scale. (c) Output curves (Ids-Vds) for various Vbg. The inset shows the optical image of a monolayer MoSe2 FETs. (d) The distributions of the field-effect mobilities (left y-axis) and Ion/Ioff ratios (right yaxis) of the 20 monolayer MoSe2 FETs.


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Figure 5. Electrical performance of bilayer MoSe2. (a) Optical image of a bilayer MoSe2 FET. (b) Typical transfer characteristics (Ids-Vbg) of the bilayer MoSe2 FETs at different Vds. The inset shows the I-V characteristic on a semilog scale. (c) The transfer characteristics (Ids-Vbg) at the range of (-60~-100). (d) Typical output curves (Ids-Vds) for various positive and negative (shown


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in inset) Vbg ranges. (e) The distributions of the field-effect mobilities (left y-axis) and Ion/Ioff ratios (right y-axis) of the 20 bilayer MoSe2 FETs.

CONCLUSION In conclusion, we have demonstrated a simple chemical vapor deposition (CVD) approach that can directly grow large-area, single- and few-layered MoSe2 on a SiO2/Si substrate by finely tuning the flow rate. The influence of flow rate on layer numbers and grain size were attributed to the flow rate’s effect on the partial pressure of gaseous MoSe2 and nucleation density, respectively. FETs fabricated on such synthesized MoSe2 crystals exhibited different transport behaviors depending on the layer number. Monolayer MoSe2 showed n-type characteristics while bilayer MoSe2 showed n-type-dominant ambipolar behavior. This is attributed to the formation of Se vacancies generated during low flow rates. Additional research would be necessary to better understand the underlying mechanism. Our results provide a feasible method for creating large-area, highly crystalline, single- and few-layer MoSe2. We have also demonstrated the potential of these synthesized MoSe2 crystals for practical application such as complementary logic electronics and p-n junctions.

EXPERIMENTAL METHODS Growth of MoSe2 crystals. During the typical growth process, a ceramic boat containing MoO3 powder with a SiO2/Si substrate were placed on top of the MoO3 powder (Figure 1a). This was located downstream at the furnace Zone Ⅰ while a ceramic boat containing Se powder was located upstream at Zone Ⅱ. Prior to the growth process, the SiO2/Si substrates were sonicated in acetone and isopropyl alcohol for 10 min respectively, to remove the impurities absorbed on the surface. A quartz tube (2 in. in diameter) was first vacuum pumped to expel the air and was


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then filled with high purity Ar to atmospheric pressure. Subsequently, different flow rates of carrier gas (Ar/H2) were used for property-controlled synthesis of MoSe2 crystals. The growth process was carried out at ambient pressure. The temperature of the reacting region, i.e. Zone Ⅰ, was heated to 750oC with a heating ramp of 20oC/min and the growth process was maintained for 15 min before being naturally cooled down. The temperature of Se in Zone Ⅱ was kept at 300oC during the growth process. Characterization of MoSe2 crystals and devices. The topography of as- synthesized MoSe2 crystals were first characterized using optical microscopy (OM, Olympus) and scanning electron microscopy (SEM, Hitachi S4800). Atomic force microscopy (AFM, Agilent 5600LS), Raman spectrum (ThermoFisher DXR) and PL spectrum (Witec Alpha 300R) were employed to characterize the thickness and uniformity as well as the crystal quality of the MoSe2 crystals. For the Raman measurements, a laser power of 1 mW with 532 nm excitation wave length and 1.2 µm spot size were used while a Si peak position at 520 cm-1 was used as the standard peak. The PL measurements were performed at 532 nm wavelength with the laser power of 1 mW. The Xray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was performed with a monochromatic Al Kα X-ray source to determine the elemental composition and bonding within the MoSe2 crystals. High-resolution transmission electron microscopy (HRTEM, Talos F200X) was applied at 200 keV to characterize the crystalline structure of MoSe2 crystals transferred onto a copper TEM grid with an ultrathin carbon film. Electrical performance of mono- and bilayer MoSe2 FETs were measured using a semiconductor parameter analyzer (Agilent B1500A) under a vacuum down to levels as low as 10-3 pa using a probe station (JANIS ST500).

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Supporting Information Supporting Information Available: Additional description of evolution of MoSe2 grown at flow rates ranged between 10 sccm and 110 sccm, Optical images and SEM images of MoSe2 grown at different flow rate and XPS spectrum of the bilayer MoSe2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Author Contributions K.Z. and Y.L. conceived the idea and designed the experiments. Y.L., F.W., Y.L., D.T. and B.Z. synthesized MoSe2 crystals. Y.L., K.Z., F.W., Y.F., Y.H. and D.T. carried out the AFM, SEM, Raman, PL, XPS, and TEM characterizations. Y.L. fabricated and measured the electrical performance of the MoSe2-based field-effect transistors. Y.L., K.Z. and H.P.W. analyzed the data and cowrote the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work is supported by the National Natural Science Foundation of China (Grant Nos 61404091, 61274113, 61505144, 51502203，and 51502204），and Tianjin Natural Science Foundation (Grant Nos 17JCYBJC16100, 17JCZDJC31700).

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