Wafer-Size and Single-Crystal MoSe2 Atomically Thin Films Grown on

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article 2

Wafer-size and single-crystal MoSe atomically thin films grown on GaN substrate for light emission and harvesting Zuxin Chen, Huiqiang Liu, Xuechen Chen, Guang Chu, Sheng Chu, and Hang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04768 • Publication Date (Web): 13 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Wafer-size and single-crystal MoSe2 atomically thin films grown on GaN substrate for light emission and harvesting Zuxin Chen† , Huiqiang Liu† , Xuechen Chen† , Guang Chu‡, Sheng Chu*†, and Hang Zhang§ □

†

□

□

State key Laboratory for Optoelectronics Materials and Technology Sun Yat-sen University,

Guangzhou, 510275, China. ‡

School of Metallurgy and Environment, Central South University, Changsha, 410083, China.

§

Division of Engineering and Applied Science, California Institute of Technology, Pasadena,

CA, USA, 91125. Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing, China, 10090. Z. Chen, H. Liu and X. Chen contributed equally.

□

*

corresponding author: S. Chu ([email protected])

KEYWORDS: transition metal dichalcogenides, MoSe2, GaN, heterojunction diode, epitaxy.

ABSTRACT: Two-dimensional (2D) atomic-layered semiconductors are important for nextgeneration electronics and optoelectronics. Here, we designed the growth of MoSe2 atomic layer

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

on a lattice-matched GaN semiconductor substrate. The results demonstrated that the MoSe2 films were less than three atomic layers thick and were single crystalline of MoSe2 over the entire GaN substrate. The ultra-thin MoSe2/GaN heterojunction diode demonstrated ~850-nm light emission and could also be used in photovoltaic applications.

INTRODUCTION The growth and fabrication of thin-film devices on semiconducting wafers constitute the bulk of the semiconductor industry.1,2 Therefore, to be able to grow high-quality two-dimensional (2D) materials on semiconducting substrates will be technologically advantageous and having broad applications. Regarding 2D material, much progress has been made in growing, for example, MoS2,3,4 MoSe2,5,6 WSe2,7 WS2,8 and black phosphorene,9 etc. on insulating wafers, i.e., SiO2 and sapphire. A major progress is that Dumcenco et al recently reported the growth of singledomain, very large MoS2 monolayer films on c-sapphire substrates.10 Although these growth techniques are well established, the resulting 2D materials still need to be transferred to semiconducting substrates to be integrated into the mainstream microelectronics or optoelectronics technologies. In fact, transfer processes11-13 consume additional time and cause serious interfacial defects. Hence, the direct growth of 2D materials on semiconducting wafers or films can bypass these problems and result in ideal-quality films. Many attempts have been made to grow 2D materials directly on semiconducting substrates, including Si,14,15 SiC,16 GaAs,17 or others,18,19 and preliminary heterojunctional devices have been fabricated. However, because of unfavourable lattice mismatches, the domain orientation and size uniformity are not yet satisfactory. In contrast, 2D material growth on more suitable semiconductor substrates may potentially offer excellent crystal quality as well as large size. For example, in the growth of MoSe2 on the Ga-face c-GaN substrate, the hexagonal symmetric GaN surface matches with


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoSe2. The c-plane lattice constants of GaN and MoSe2 are a = 3.19 Å20 and 3.28 Å, respectively, corresponding to a small mismatch of 3%. Moreover, the Ga-polar substrate is uniformly terminated with Ga2+ cations, which facilitate the absorption of Se1- anions and the growth of smooth Se-Mo-Se layers. As a comparison, the SiO2 surface terminates with randomly Si or O, and as a result, the MoSe2 on top becomes more distorted. Eventually it will decrease the carrier mobility due to more scattering. In addition, the 2D material/GaN itself is a semiconducting hetero-structure. Therefore, it can be readily integrated into mature GaN technology-based electronics and optoelectronics. Very recently, Ruzmetov et al successfully fabricated MoS2 parallel nanosheets on GaN substrate.21 However, single crystal layer and large scale devices have not been realized. In this work, we report the direct chemical vapour deposition (CVD) growth of wafer-scale, atomic MoSe2 thin films onto GaN single-crystal substrates and p-type GaN thin films. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) studies clearly indicate that sharp, few-layered (fewer than three layers) MoSe2 films in single-crystal configurations were achieved. Furthermore, the growth mechanisms of MoSe2 layers is comprehensively investigated. To confirm the application potential of the resulting material, diodes based on the MoSe2/GaN heterojunction demonstrated UV-infrared dual-wavelength light emission and a significant photovoltaic response. RESULTS AND DISCUSSION The growth of MoSe2 on GaN was done by a conventional CVD process. Details of the growth process are described in the experimental section. The successful synthesis of a few layers MoSe2 film with controlled lattice orientation through MoO3 selenization is shown in Figure 1a. The growth of MoSe2 on GaN can be categorized into three major phases, as displayed


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

schematically in Figure 1a. Phase 1 is the process of the formation of small, triangular MoSe2 domains with parallel side facets. This process is dominated by surface defect-assisted nucleation; Phase 2 consists of the formation of triangular voids. In this phase, the MoSe2 triangles in Phase 1 continue to expand because of more incoming source molecules. Eventually, the boundaries of the triangles merge together, and voids form between them; Phase 3 consists of the further filling of MoSe2 inside the voids to form a continuous single-crystalline film. The three phases can be controlled by the samples grown at different distances from the source, as also noted in Figure 1a. Nearer to the MoO3 source the coverage of the GaN surface is larger because of the higher source vapour pressure. The surface coverage of GaN by MoSe2 (in %) vs. the substrate distance is plotted in Figure 1b. The regions corresponding to Phases 1-3 are approximately marked in the figure. Figure 1c is a photograph of a few-layered MoSe2 film grown on a 1 cm × 1 cm GaN single crystal in Phase 3. Figure 1d shows the optical microscopy image of a Phase 2 sample. Although the contrast on GaN is much lower than that on SiO2, the MoSe2 voids are still observable. Figure 1e presents the scanning electron microscopy (SEM) images from Phase 1 to Phase 3. The SEM images for Phases 1 and 2 indicate that the most of the single-crystal domains are well aligned regarding the relative orientations of their edges. The SEM image for Phase 3 shows that the entire GaN surface is covered with a fairly smooth and uniform MoSe2 layer. Furthermore, the SEM images at different distances from the source, showing the increasing coverage of GaN surface, are given in Figure S1. In addition, the X-ray photoelectron spectroscopy (XPS) analysis in Figure S2 demonstrates that the obtained material is MoSe2 without other impurities. To check the thickness of the obtained MoSe2 films, AFM is performed and the results are shown in Figure 2a, b, and c for Phases 1, 2 and 3, respectively. These images re-demonstrate


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fine structures of the controllable crystal orientations on GaN substrates. The top part of Figure 2a unambiguously confirms that the majority of the MoSe2 domains have parallel side facets, where the orientations are pre-determined by the GaN lattice. The line scan AFM height profile in the bottom part of Figure 2a indicates that the layer thickness is ~0.8 nm, which is consistent with the values reported from other substrates.22,23 Similarly, the top part of Figure 2b clearly shows well-defined equilateral triangle voids with parallel sides. The height analysis yields a void depth of ~1.6 nm, corresponding to a double-layered MoSe2. Figure 2c of Phase 3 manifests that a continuous MoSe2 with a fairly smooth surface is formed. In general, the evolution shown by the AFM study is in good agreement with the SEM trend shown in Figure 1e, verifying that uniform and smooth MoSe2 atomic films can be controllably grown in the wafer scale on semiconducting substrates through simple CVD growth. Next, we examine the quality of the MoSe2 sample by performing optical characterizations. Raman spectrum is firstly gathered from the Phase 2 sample, as shown in Figure 3b. Three main peaks appear in the spectral range of 100-400 cm-1: The peak at ~143 cm-1 reflects the E 2L line from the wurtzite GaN substrate, which is derived from the zone folding of the transverse acoustic (TA) mode in GaN;24 the out-of-plane vibrations of Se atoms (A1g, sharp) with a Raman shift of ~241 cm-1 and the in-plane vibrations of MoSe2 (E2g) at 287 cm-1 are evident and similar to those reported in the literature.21,22,25 Furthermore, peak around ~352 cm-1 is known to be related to interlayer interactions,24 is not observed in our MoSe2 film. To investigate the spatial variation of the peak intensity, Raman intensity mapping of the thin MoSe2 from the Phase 2 sample is performed. Figure 3a is the Raman intensity map of the A1g mode (~241 cm-1), which shows a homogeneous intensity distribution over the surface of the MoSe2 film. The lowintensity regions in the map correspond to the bare GaN surface that is exposed to air. Figure 3b


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

shows selected Raman spectral data along the line in Figure 3a. Figure S4 presents the whole Raman spectrum for point A (exposed GaN region in the voids) in Figure 3a. The GaN Raman signals at 567 cm-1 and 738 cm-1 are clearly observed and can be assigned to GaN E2H and A1 (LO),26 respectively. In addition to Raman spectroscopy, the emission features of MoSe2 are investigated using micro photoluminescence (PL) spectra. Figure 3c is the PL spectra for the Phase 1 MoSe2 layer. A 820-nm peak is observed and can be attributed to the near band edge (NBE) emission of MoSe2.22,27,28 The shorter wavelength PL signals (685 nm and 750 nm) are assigned to the GaN substrate, as proved by the black-line spectrum of bare GaN in Figure 3c. The PL characterizations of MoSe2 films with different phase are given in Figure S5. The PL map (for 820-nm light) of a certain region and the corresponding SEM image are given in Figure 3d. The bright region in the mapping profile is well linked to the two MoSe2 triangles in the SEM image, indicating that the synthesized MoSe2 film is a direct bandgap semiconductor. To further assess the thickness and quality of the MoSe2, the large-area crystal structure and its relative orientation are analysed using cross-sectional TEM imaging and selected-area electron diffraction (SAED) for Phase 3 sample. A thin slice of a cross-sectional sample was prepared by a focused ion beam (FIB) lift-out method (Figure S6). Figure 4a presents the HRTEM image of the MoSe2/GaN specimen, which obviously shows two layers of the MoSe2 sheet. The growth of MoSe2 exhibits preferred alignment direction, i.e., MoSe2 parallel to the GaN direction. A periodic arrays of MoSe2 (100) planes with a spacing of 0.28 nm can be observed and matched with the MoSe2 data. In addition, interlayer spacings of 0.52 nm and 0.26 nm for the GaN (001) and (100) planes are also observed. Multiple cross-sectional TEM characterizations are performed throughout the entire specimen, and a nonhomogeneous distribution of the sample thickness is observed. Figure 4b shows a region with 3 layers of


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoSe2. Four or more MoSe2 layers are not found, indicating it is the thin nature of the 2D MoSe2 atomic sample. Top-view TEM characterization of the MoSe2 nanosheet is performed by fragmenting of the specimen (Phase 3) and extracting the MoSe2 film for characterization. Figures 4c-d display the top-view TEM and HRTEM images of the MoSe2 layer. This layer exhibits clear-cut crystal lattice fringes with an interplanar spacing of ~0.28 nm, and the SAED (inset of Figure 4d) pattern corresponding to the (100) planes of hexagonal MoSe2 obviously presents six diffraction spots. Finally, the relative lattice configuration between MoSe2 and GaN is summarized in Figure 4e-f by TEM analysis. From this schematic, it can be further seen that MoSe2 and GaN have close lattice similarity, which results in single crystallinity films. The method in this paper provides a general scheme to grow 2D materials on semiconducting substrate. If the lattice mismatch between certain substrate and 2D layer is too large, one can consider to form an intermediate layer between them to buffer the mismatch. A heterojunctional device can be readily fabricated with this MoSe2/GaN structure. Previous studies have shown that atomically thin 2D semiconductors in contact with 3D conventional semiconductors can still form diodes.29,30 Because MoSe2 is an n-type semiconductor (We measured the electrical properties of the MoSe2 film by fabricating field effect transistors (FETs) on transferred freestanding MoSe2 film onto SiO2/Si substrate. The carrier concentration and mobility are ∼6×1012 cm-2, 60.1-100.6 cm2/V·S, respectively. The details of the FETs and corresponding results will be published elsewhere), the MoSe2/GaN substrate is an n-n junction. To produce a more versatile device, MoSe2 layers are grown on p-type GaN epi-layers (GaN layers are grown on sapphire using metal-organic chemical vapor deposition (MOCVD). The ptype region thickness, carrier concentration and mobility are 1.5 µm, 3×1017 cm-3, 20 cm2/V·S, respectively. Pure p-type GaN wafers are not currently available), resulting in a similar growth


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

phase pattern Figure S7. Schematics of the MoSe2/GaN n-n and p-n junctions and electrodes arrangement are shown in Figures 5a and b, respectively. First, because the MoSe2 layer is extremely thin, the interface electronic band alignment must be understood thoroughly. Here, ultraviolet photoelectron spectroscopy (UPS)31,32 is used to determine the work function and the energy difference between the valence band maximum and the Fermi level. The UPS spectra for the bare GaN wafer and MoSe2 are given in Figure S8, where the valence band positions (relative to the Fermi level) extracted from the low-energy tail of the spectra are 1.4 eV and 3.1 eV for MoSe2 and GaN, respectively. Thus the band diagram for the n-n junction is summarized in Figure 5c. The band diagram for the p-n junction is analysed in a similar manner (Figure S8) and is shown in Figure 5d. The p-n band diagram contains a minor barrier of ~0.2 eV at the valence band, which is toleratable for current injection in optoelectronic device operation. Both devices demonstrate non-linear I-V curves, as shown in Figure 5e, whereas the p-n junction shows a slightly stronger rectifying feature. The electroluminescence (EL) properties of the p-n junction under forward injection current are shown in Figure 5f. The left-hand and right-hand spectra correspond to emissions near the GaN and MoSe2 bandgaps, respectively. Evidently, under an injection current of > 40 mA, EL peaks at approximately 850 nm are observed and can be attributed to electron-hole recombination in MoSe2 side. However, because of the much lower material quantity, this emission is substantially weaker than the recombination at the GaN side. As a result, the operating device display strong blueish colour in Figure 5g. It should be noted that the emission profile is fairly uniform throughout the sample. Here we attributed this phenomena to the waveguide effect that made the light appears with fairly uniformity throughout the sample. The light generation should happen in the area just beneath the top elctrodes area rather than spreading through the whole sample, given the generally low conductivity from the


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoSe2 film obtained by electrical characterization. However, the GaN/sapphire structure can form a waveguide since the refractive index for GaN (n=2.2) is larger than top material (air n=1) and bottom material (sapphire n=1.7). Such waveguide makes the light propagate easily on the horizontal direction. As a result, the whole device appears with more even light distribution. In contrast to light emission, the bandgap of MoSe2 makes it to be an excellent solar radiation absorber. Previous studies have shown that ultrathin monolayer films have strong optical absorptions up to 5-10% of incident light.33,34 Hence, the p-n junction device can be utilized as a solar cell if a GaN thin film is taken as the window layer. The photovoltaic response under AM 1.5 simulated sun light is shown in Figure 5h. The power conversion efficiency of the MoSe2/GaN p-n junctions based photovoltaic (PV) devices reached 1.29%, which was lower than that of the reported MoS2/Si35-37 and MoS2/GaAs17 solar cells, but our device shows a higher open-circuit voltage of 0.62 V. Considering the thickness of the MoSe2 material, the performance of the heterojunction in light-emitting diodes (LEDs) and solar cells is promising, and thus, this device has many potential applications. CONCLUSIONS In summary, we successfully synthesized MoSe2 layered films on GaN single-crystal substrates and thin films via a CVD furnace. The MoSe2 grown on GaN single crystals exhibited fairly good optical properties. We observed a distinct crystallographic orientation between the grown MoSe2 and GaN, because of interlayer coulombic interactions. Moreover, the MoSe2/GaN heterojunction demonstrated good optoelectronic device performance. The present method is suitable for exploring the optical and electronic properties of many other atomic-layer materials on other semiconductor substrates, thereby opening up new applications. METHODS, MATERIALS AND INSTRUMENTATION


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

MoSe2 layer growth procedure: We used a two-temperature zone furnace for the CVD growth of MoSe2. Specifically, Se powders (870 mg, 99.999%, Sigma-Aldrich, USA) were placed in the first upstream zone, and MoO3 powders (450 mg, 99.99%, Sigma-Aldrich, USA) were placed in the second zone. The distance between the two sources was tuned and optimized at 14 cm. A 6-in. quartz reaction tube was first flushed with 500 sccm of Ar for 10 min, and then, the furnace was ramped to the designated temperature at a ramp rate of 30゜C/min for growth. The temperatures of Se and MoO3 can be controlled over a wide range by their locations in the furnace. The materials presented in Figure 1 were grown at conditions where the MoO3 zone temperature was 850 ℃, and the Se zone temperature was 540℃. The GaN single crystal (1 cm×1 cm) substrates were placed on top of the MoO3 powders facing down. During the growth process, the flow rate of Ar/H2 was tuned and optimized at 320/20 sccm, and the growth occurred at ambient pressure. After the reaction, the furnace was naturally cooled to room temperature under 320/20 sccm of Ar/H2. Morphology characterization and component analysis: The SEM images were taken with a Zeiss Auriga-39-34 (Oberkochen, Germany) microscope operating at 15 kV. The samples were also imaged using AFM (Dimension Fastscan) operating in AC mode. Component analysis was conducted with an XPS instrument (ESCALab250, Thermo Fisher Scientific, USA). Optical characterization: Raman and PL spectroscopies were performed on a commercial system (inVia microRaman, Renishaw, UK) using a 532-nm (2.33-eV) laser for excitation. The laser power was kept at 5-15 mW, and the Raman emission was collected by a 100× objective via backscattering. A spectral grating with 2400 lines/mm was used for all Raman spectra, whereas one with 1200 lines/mm was used for the PL measurements. Regions of interest were


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


identified from maps for which single spectra were obtained with an acquisition time of 20 s and by integrating for twice. All spectra were obtained with an acquisition time of 20 s and by integrating for twice. TEM: Cross-sectional samples were prepared by FIB (Helios nanolab 600, FEI, USA) to directly observe the vertical morphology. Platinum deposition was conducted to protect the sample surface during FIB processing. A thick cross-sectional sample was obtained (~65 nm thick) through a FIB-assisted method. TEM was performed using a Tecnai G2 F20 S-Twin instrument operated at an energy of 200 kV. The top-view TEM sample was prepared by dropping the MoSe2/GaN sample into liquid nitrogen and collecting sample fragments for TEM characterization. UPS measurement: UPS was performed in a vacuum chamber using a 21.2-eV helium lamp as the excitation source. Device fabrication and characterization: Metal contacts were deposited on the heterojunction device using a pre-designed mask. The I-V characteristics were measured using Agilent 4156C on a probe station. The EL properties were measured with a photomultiplier tube and an Oriel monochromator with a chopper. The photovoltaic response was achieved using solar simulator (Type S, Zolix, China). ASSOCIATED CONTENT Supporting Information. SEM images of the MoSe2/GaN samples from Phase 1-3; XPS spectra on the MoSe2 grown on GaN; Additional Raman spectra on MoSe2 as well as bare GaN surface; PL characterizations of MoSe2 films with different phase; Additional TEM image of the FIB cut


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

slide; SEM images of the MoSe2 grown on p-type GaN surface; UPS spectra on MoSe2.This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel.: +86-20-84110402, Present address: 1, Sun Yat-sen University, Guangzhou, China, 510275 Author Contributions S. C. and H. Z. conceived and designed the experiments. Z. C., H. L. and X. C did the MoSe2 film growth. Z. C., H. L. and G. C. carried out the characterization experiments. S. C. and G. C. co-wrote the paper. Z. C., H. L., and X. C. contributed equally to this work. All authors contributed to analyzing and reviewing the data in this manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank Dr. Y. Chen for assistance in TEM imaging. The work was supported by National Science Foundation of China (grant no. 11204097 and U1530120). REFERENCES (1) Pierret, R. F. Semiconductor Device Fundamentals. Ch. 4 (Addison Wesley, New Jersey, 1996). (2) Van Ruyven, L. J. Double Heterojunction Lasers and Quantum Well Lasers. J. Lumin. 1984, 29, 123-161.


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman J. N.; Strano, M. S. Electronics and Optoelectronics of Two-dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (4) Zhang, L.; Liu, K.; Wong, A. B.; Kim, J.; Hong. X.; Liu, C.; Cao, T.; Louie, S. G.; Wang, F.; Yang, P. D. Three-Dimensional Spirals of Atomic Layered MoS2. Nano Lett. 2014, 14, 6418-6423. (5) Heo, H.; Sung, J. H.; Jin, G.; Ahn, J. H.; Kim, K.; Lee, M. J.; Cha, S.; Choi, H.; Jo, M. Rotation-misfit-free Heteroepitaxial Stacking and Stitching Growth of Hexagonal Transition-metal Dichalcogenide Monolayers by Nucleation Kinetics Controls. Adv. Mater. 2015, 27, 3803-3810. (6) Xia, J.; Huang, X.; Liu, L.; Wang, M.; Wang, L.; Huang, B.; Zhu, D.; Li, J.; Meng, X. CVD Synthesis of Large-area, Highly Crystalline MoSe2 Atomic Layers on Diverse Substrates and Application to Photodetectors. Nanoscale 2014, 6, 8949-8955. (7) Liu, B.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y.; Zhou, C. Chemical Vapor Deposition Growth of Monolayer WSe2 with Tunable Device Characteristics and Growth Mechanism Study. ACS Nano 2015, 9, 6119-6127. (8) Gutiérrez, H. R.; Lopez, N. P.; Elias, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; Lopez-Urias, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447-3454. (9) Li, L. K.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Hui, X.; Zhang, Y. Black Phosphorus Field Effect Transistors. Nat. Nanotechnol 2014, 9, 372-377. (10) Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazic, P.; Gilbertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y. C.; Krasnozhon, D.; Kis, A. Large-area Epitaxial Monolayer


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

MoS2. ACS Nano 2015, 9, 4611-4620 . (11) Gao, L.; Ni, G.; Liu, Y.; Liu, B.; Neto, A. H. C.; Loh, K. P. Face to Face Transfer of Wafer Scale Grapheme Films. Nature 2013, 505, 190-194. (12) Lee, E. W.; Lee, C. H.; Paul, P. K.; Ma, L.; McCulloch, W. D.; Krishnamoorthy, S.; Wu, Y.; Arehart, A. R.; Rajan, S. Layer Transferred MoS2/GaN PN Diodes. Appl. Phys. Lett. 2015, 107, 103305. (13) Lin, Z.; Zhao, Y.; Zhou, C.; Zhong, R.; Wang, X.; Tsang, Y. H.; Chai, Y. Controllable Growth of Large Size Crystalline MoS2 and Resist Free Transfer Assisted with a Cu Thin Film. Sci. Rep. 2015, 5, 18596. (14) 14. Lopez-Sanchez, O.; Alarcon, L. E.; Koman, V.; Fontcuberta, M. A,; Radenovic, A.; Kis, A. Light Generation and Harvesting in a Van Der Waals Heterostructure. ACS Nano 2014, 6, 3042-3048. (15) Liu, Y.; Hao, L.; Gao, W.; Wu, Z.; Lin, Y.; Li, G.; Guo, W.; Y, L.; Zhang, H.; Zhu, J.; Zhang, W. Hydrogen Gas Sensing Properties of MoS2/Si Heterojunction. Sensor. Acurator B: Chem. 2015, 211, 537-543. (16) Lee, E. W.; Ma, L.; Nath, D. N.; Lee, C. H.; Arehart, A.; Wu, Y.; Rajan, S. Growth and Electrical Characterization of Two Dimensional Layered MoS2/SiC Heterojunctions. Appl. Phys. Lett. 2014, 105, 203504. (17) Lin, S.; Li, X.; Wang, P.; Xu, Z.; Zhang, S.; Zhong, H.; Wu, Z.; Xu, W.; Chen, H. Interface Designed MoS2/GaAs Heterostructure Solar Cell with Sandwich Stacked Hexagonal Boron Nitride. Sci. Rep. 2015, 5, 15103. (18) Vishwanath, S.; Liu, X.; Rouvimov, S.; Mende, P. C.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Feenstra, R. M.; Furdyna, J. K.; Jena, D.; Xing, H. G. Comprehensive Structural and


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Optical Characterization of MBE Grown MoSe2 on Graphite, CaF2 and Grapheme. 2D Mater. 2015, 2, 024007. (19) Kim, J.; Baryram, C.; Park, H.; Cheng, C.; Dimitrakopoulos, C.; Ott, J. A.; Reuter, K. B.; Bedell, S. W.; Sadana, D. K. Principle of Direct Van Der Waals Epitaxy of Singlecrystalline Films on Epitaxial Grapheme. Nat. Commun. 2014, 5, 4836 (2014). (20) Strite, S.; Morkoc, H. GaN, AlN, and InN: a Review. J. Vac. Sci. Technol. B 2002, 10, 12371267. (21) Ruzmetov, D.; Zhang, K. H.; Stan, G.; Kalanyan, B.; Bhimanapati, G. R.; Eichfeld, S. M.; Robert. A.; Burke, P. B.; Shah, T. P.; O’Regan, F. J.; Crowne, A.G.; Birdwell, J. A.; Robinson, A.; Davydov, V.; Ivanov, T. G. Vertical 2D/3D Semiconductor Heterostructures Based on Epitaxial Molybdenum Disulfide and Gallium Nitride. ACS Nano 2016, 10, 3580. (22) Wang, X.; Gong, Y.; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Liu, Z.; Ringe, E.; Tay, B. K.; Ajayan, P. M. Chemical Vapor Deposition Ggrowth of Crystalline Monolayer MoSe2. ACS Nano 2014, 8, 5125-5131. (23) Shaw, J. C.; Zhou, H.; Chen, Y.; Weiss, N. O.; Liu, Y.; Huang, Y.; Duan, X. Chemical Vapor Deposition Growth of Monolayer MoSe2 Nanosheets. Nano Res. 2014, 7, 511-517. (24) Harima, H. Properties of GaN and Related Compounds Studied by Means of Raman Scattering. J. Phys.: Condens. Matter 2002, 14, R967-R993. (25) Rhyee J. S.; Kwon J.; Dak P.; Kim J. H.; Kim S. M.; Park J.; Hong Y. K; Song W. G.; Omkaram I.; Alam M. A.; Kim S. High-mobility Transistors Based on Large-area and Highly Crystalline CVD-grown MoSe2 films on insulating substrates. Adv. Mater. 2016, 28, 2316-2321. (26) Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.; Kloc,


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

C.; Gordan, O.; Zahn, R. T.; Bratschitsch, R. Photoluminescence Emission and Raman Response of Monolayer MoS2, MoSe2, and WSe2. Opt. Express 2013, 21, 4908-4916. (27) Roy A.; Movva H. V. P.; Satpati B.; Kim K.; Dey R.; Rai A.; Pramanik T.; Guchhait S.; Tutuc E.; Banerjee S. K. Structural and Electrical Properties of MoTe2 and MoSe2 Grown by Molecular Beam Epitaxy. ACS Appl. Mater. Interfaces 2016, 8, 7396-7402. (28) Le C. T.; Clark D. J.; Ullah F.; Senthilkumar V.; Jang J. I.; Sim Y.; Seong M. J.; Chung K. H.; Park H.; Kim Y. S. Nonlinear Optical Characteristics of Monolayer MoSe2. Ann. Phy. 2016, 6, 1-9. (29) Davydov, V. Y.; Kitaev, Y. E.; Goncharuk, I. N.; Smirnov, A. N.; Graul, J.; Semchinova, O.; Ulfmann, D.; Smirnov, M. B., Evarestov, R. A. Phonon Dispersion and Raman Scattering in Hexagonal GaN and AlN. Phys. Rev. B 1998, 58, 12899-12907. (30) Tsai, M. L.; Su, S. H.; Chang, J. K.; Tsai, D. S.; Chen, C. H.; Wu, C. I.; Li, L. J.; Chen, L. J.; He, J. H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 27. (31) Serrano, W. Pinto, N. J.; Naylor, C. H.; Kybert, N. J.; Charlie Johnson Jr, A. T. Facile Fabrication of an Ultraviolet Tunable MoS2/p-Si Junction Diode. Appl. Phys. Lett. 2015, 106, 193504. (32) Rabalais, J. W. Principles of Ultraviolet Photoelectron Spectroscopy. (Wiley: New York, 1977). (33) Carlson, B.; Leschkies, K.; Aydil, E. S.; Zhu, X. Y. Valence Band Alignment at Cadmium Selenide Quantum Dot and Zinc Oxide (10-10) Interfaces. J. Phys. Chem. C 2008, 112, 8419-8423. (34) Bernardi, M.; Palummo, M.; Grossman, J. C. Extraodinary Sunlight Absorption and One Nanmometer Thick Photovoltaics Using Two Dimensional Monolayermaterials. Nano Lett.


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2013, 13, 3664-3670. (35) Hao, L. Z.; Liu, Y. J.; Gao, W.; Liu, Y. M.; Han, Z. D.; Xue, Q. Z.; Zhu, J. Enhanced Photovoltaic Characteristics of MoS2/Si Hybrid Solar Cells by Metal Pb Chemical Doping. RSC Advances 2016, 6, 1346. (36) Hao, L. Z.; Gao, W.; Liu, Y. J.; Han, Z. D.; Xue, Q. Z.; Xue, Q. Z.; Zhu, J.; Li, Y. R. Highperformance n-MoS2/i-SiO2/P-Si Heterojunction Solar Cells. Nanoscale 2015, 7, 8304. (37) Hao, L. Z.; Liu, Y. J.; Gao, W.; Han, Z. D.; Xue, Q. Z.; Zeng, H. Z.; Wu, Z. P.; Zhu, J.; Zhang, W. L. Electrical and Photovoltaic Characteristics of MoS2/Si p-n Junctions. Journal of applied physics 2015, 117, 114502.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

LIST OF FIGURES Figure 1. (a). Schematic of the CVD growth process. Three growth phases were identified mainly based on the substrate distance to the MoO3 source. (b). Surface MoSe2 coverage as a function of the substrate-MoO3 source distance. (c). Photos of the GaN substrate before and after MoSe2 growth. (d). Optical microscopy image of a Phase 2 sample. Scale bar: 10 µm. The image‘s contrast was manually adjusted. Regions of the MoSe2 film was manually circumscribed white dots. (e). SEM images of the MoSe2 layers on the GaN surface at Phases 1-3. Scale bar: 5 µm. Figure 2. AFM characterizations of samples at three growth phases. (a). Top: AFM image of the MoSe2 film in Phase 1; Bottom: Height profile for the blue line in the AFM image. (b). Top: AFM image of the MoSe2 film in Phase 2; Bottom: Height profile for the blue line in the AFM image. (c). Top: AFM image of the MoSe2 film in Phase 3; Bottom: Height profile for the blue line in the AFM image. Figure 3. Optical properties of the MoSe2 on GaN. (a). Mapping of the 241-cm-1 Raman peak of the Phase 2 MoSe2 film. The point A indicates a viod (exposed GaN substrate) in Phase 2. (b). Raman spectra collected at the selected points on the line in a. (c). PL spectra of the bare GaN substrate and MoSe2 film. (d). PL mapping result and corresponding SEM image (scale bar: 2 µm). Moreover, the size of the area which presents PL mapping is 3 µm × 6 µm. Figure 4. TEM characterization of the Phase 3 MoSe2 atomic film. (a). Side-view HRTEM image of double MoSe2 layers on the GaN substrate. Scale bar: 5 nm. (b). Side-view HRTEM image of triple MoSe2 layers on the GaN substrate. Scale bar: 5 nm. (c). Low-magnification topview TEM image of the lifted-up MoSe2 film. Scale bar: 20 nm. (d). Top-view HRTEM image of


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the lifted-up MoSe2 film. Scale bar: 5 nm. Inset: SAED pattern of this sample. (e). Top-view lattice configuration of MoSe2 and GaN. f). Side-view lattice configurations of MoSe2 and GaN. Figure 5. Device characterization of the MoSe2/GaN heterojunctiona. (a). Schematic of the device consisting of an atomic MoSe2 layer on an n-GaN substrate. (b). Schematic of the device consisting of an atomic MoSe2 layer on a p-type GaN/sapphire substrate. (c). Band diagram of the MoSe2/n-GaN substrate heterojunction. (d). Band diagram of the MoSe2/p-GaN heterojunction. (e). I-V curves of the n- MoSe2/n-GaN and n- MoSe2/p-GaN heterojunctions. (f). EL spectra of the n- MoSe2/p-GaN device in the short- and long-wavelength regions. (g). Photos of the device under different injection currents. The size of the sample is 1 cm × 1 cm. (h). Photovoltaic effect of the n- MoSe2/p-GaN device.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

FIGURE 1


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE 2


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

FIGURE 3


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE 4


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

FIGURE 5


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Use only

Synopsis Controlled epitaxial growth of wafer-scale, single-crystal MoSe2 atomically thin films is presented by a direct vapor deposition method on a lattice-matched GaN single-crystal substrates and p-type GaN thin films. Furthermore, the MoSe2/GaN heterostructures demonstrate optoelectronic device performance, which are attributed to the superior crystallinity of as-grown MoSe2.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a). Schematic of the CVD growth process. Three growth phases were identified mainly based on the substrate distance to the MoO3 source. (b). Surface MoSe2 coverage as a function of the substrate-MoO3 source distance. (c). Photos of the GaN substrate before and after MoSe2 growth. (d). Optical microscopy image of a Phase 2 sample. Scale bar: 10 µm. The image‘s contrast was manually adjusted. Regions of the MoSe2 film was manually circumscribed white dots. (e). SEM images of the MoSe2 layers on the GaN surface at Phases 1-3. Scale bar: 5 µm. 98x66mm (300 x 300 DPI)


Page 26 of 31

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. AFM characterizations of samples at three growth phases. (a). Top: AFM image of the MoSe2 film in Phase 1; Bottom: Height profile for the blue line in the AFM image. (b). Top: AFM image of the MoSe2 film in Phase 2; Bottom: Height profile for the blue line in the AFM image. (c). Top: AFM image of the MoSe2 film in Phase 3; Bottom: Height profile for the blue line in the AFM image. 95x65mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Optical properties of the MoSe2 on GaN. (a). Mapping of the 241-cm-1 Raman peak of the Phase 2 MoSe2 film. The point A indicates a viod (exposed GaN substrate) in Phase 2. (b). Raman spectra collected at the selected points on the line in a. (c). PL spectra of the bare GaN substrate and MoSe2 film. (d). PL mapping result and corresponding SEM image (scale bar: 2 µm). Moreover, the size of the area which presents PL mapping is 3 µm × 6 µm. 100x71mm (300 x 300 DPI)


Page 28 of 31

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. TEM characterization of the Phase 3 MoSe2 atomic film. (a). Side-view HRTEM image of double MoSe2 layers on the GaN substrate. Scale bar: 5 nm. (b). Side-view HRTEM image of triple MoSe2 layers on the GaN substrate. Scale bar: 5 nm. (c). Low-magnification top-view TEM image of the lifted-up MoSe2 film. Scale bar: 20 nm. (d). Top-view HRTEM image of the lifted-up MoSe2 film. Scale bar: 5 nm. Inset: SAED pattern of this sample. (e). Top-view lattice configuration of MoSe2 and GaN. f). Side-view lattice configurations of MoSe2 and GaN. 204x298mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Device characterization of the MoSe2/GaN heterojunctiona. (a). Schematic of the device consisting of an atomic MoSe2 layer on an n-GaN substrate. (b). Schematic of the device consisting of an atomic MoSe2 layer on a p-type GaN/sapphire substrate. (c). Band diagram of the MoSe2/n-GaN substrate heterojunction. (d). Band diagram of the MoSe2/p-GaN heterojunction. (e). I-V curves of the n- MoSe2/n-GaN and nMoSe2/p-GaN heterojunctions. (f). EL spectra of the n- MoSe2/p-GaN device in the short- and longwavelength regions. (g). Photos of the device under different injection currents. The size of the sample is 1 cm × 1 cm. (h). Photovoltaic effect of the n- MoSe2/p-GaN device. 98x68mm (300 x 300 DPI)


Page 30 of 31

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Controlled epitaxial growth of wafer-scale, single-crystal MoSe2 atomically thin films is presented by a direct vapor deposition method on a lattice-matched GaN single-crystal substrates and p-type GaN thin films. Furthermore, the MoSe2/GaN heterostructures demonstrate optoelectronic device performance, which are attributed to the superior crystallinity of as-grown MoSe2. 97x104mm (300 x 300 DPI)


Wafer-Size and Single-Crystal MoSe2 Atomically Thin Films Grown on

Recommend Documents