Spectroscopic Visualization of Grain Boundaries of Monolayer

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Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers Seki Park, Min Su Kim, Hyun Kim, Jubok Lee, Gang Hee Han, Jeil Jung, and Jeongyong Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04977 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 11, 2015

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Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers Seki Park†, #, ‡, Min Su Kim#, ‡, Hyun Kim†, #, Jubok Lee†, #, Gang Hee Han#, Jeil Jung§, *, Jeongyong Kim†, #, * #

Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea

†

Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea §

Department of Physics, University of Seoul, Seoul 130-743, Republic of Korea ‡

These authors contributed equally to this work. *(J.K.) Email: [email protected] *(J.J.) Email: [email protected]

Keywords: monolayer molybdenum disulfide, grain boundary, indirect band gap photoluminescence, stacked bilayer, interlayer coupling

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Abstract

Polycrystalline growth of molybdenum disulfide (MoS2) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS2-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS2 has traditionally required atomic resolution microscopy or non-linear optical imaging techniques. Here we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS2 using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS2 grains with tilt angles as small as 6º in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS2 in the vicinity of the 0º and 60º commensuration angles which were explained by elastic deformation model of the MoS2 membranes.

Graphene-like two dimensional (2D) materials offer ample possibilities of superb electronic and photonic device functions realized in the atomic thickness scale.1-4 Although the initial discoveries of most 2D materials were made by mechanical cleaving, the large area synthesis of monolayer 2D materials is a crucial prerequisite to achieve reliable performance of photonic and electronic applications using 2D materials. The chemical vapor deposition (CVD) method has been applied successfully in the growth of 2D materials such as graphene, MoS2 and others, preparing large-area continuous films.5,

6

These films are usually 2

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polycrystalline consisting of multiple microscopic grains with different atomic orientations resulting in grain boundaries (GBs). Recent studies suggest that for the semiconductor 2D materials, such as MoS2, WS2 and MoSe2, the GBs have a large influence in the electric and optical performance.7-11 Identification of GBs of CVD grown MoS2 is a crucial aspect in assessing the film quality and also in studying the mechanism of GB formation during CVD growth. While direct examination of the atomic arrangement of monolayer MoS2 by using transmission electron microscopy, scanning tunneling microscope and electron diffraction would provide a clear identification of the grains and GBs of CVD grown MoS2,9, 10, 12-14 optical visualization methods are desirable for their convenient inspection capability of large size grains. In this context, the second-harmonic generation (SHG) using ultrafast optics can show the contrast on the edge and the GBs,15, 16 and UV exposure for selective defect site oxidation, mostly GBs, were effective too.17, 18 Optical spectroscopic methods are especially useful for MoS2 because it shows a dramatic variation of electronic band structures and thus of optical properties as the layer thickness changes from monolayer to bilayer to bulk, related with the transition from a direct to an indirect band gap.1,

4, 19

Engineering of stacked 2D layers, either by direct growth or

mechanical transfer of 2D monolayers on one another, is an active area of study.20-26 While spontaneously exfoliated MoS2 bilayers display only the so-called 2H space group phase, it is possible to achieve bilayers with almost any stacking twist angle between the two monolayers either through mechanical or CVD techniques, which has allowed systematic studies of interlayer coupling in stacked MoS2 bilayers.26-29 The interlayer coupling strength between two MoS2 monolayers in stacked bilayer MoS2 was shown to depend on the twist angle, and to influence the PL emission intensity and the center wavelength of the indirect band transition.26-29 Here we used the spectroscopic map technique to visualize the local twist angle of stacked bilayers, which enabled the clear contrast of the GB between monolayer 3 ACS Paragon Plus Environment

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MoS2 grains. Unlike other optical visualization techniques that use oxidation to locate the defect sites,17, 18 or requires ultra-fast pulse lasers,15, 16, 30 the proposed method uses standard photoluminescence (PL) spectroscopic tools and specifically determines the atomic orientation of the MoS2 grains, leading to an unambiguous identification of grains and GBs of monolayer MoS2. Results and Discussions The strategy that we designed to optically identify grains and GBs using stacked bilayers is schematically illustrated in Figure 1a. A single crystalline monolayer MoS2 was mechanically stacked on top of polycrystalline MoS2 flakes to make the stacked bilayer. The presence of top and bottom MoS2 monolayers and the stacked bilayers was confirmed by Raman spectroscopy, where the monolayer MoS2 region and stacked bilayer region showed respectively 18.0 cm-1 and 20.6 cm-1 for the frequency difference between the E12g Raman peak and A1g Raman peak (See Figure S1). No noticeable traces of strain or external doping effect due to transfer process were found, as peak positions and intensities of PL A exciton and Raman ( E12g and A1g modes) were the same between top and bottom monolayers (See Figure S2 and S3). PL spectral mapping was then carried out on stacked bilayers by taking full PL spectra at each pixel position and the map of indirect band PL peak position displayed the discontinuous optical contrast across the GB due to the different twist angle and interlayer coupling in the stacked bilayer. We estimated the twist angles in the stacked bilayers by directly measuring the relative angle between the triangular edges of the top and bottom MoS2 monolayers because of their known correlation with crystal orientation.10, 16 Therefore the twist angle was defined only in the range 0º -60º and the type of the zigzag edge (Mo- or S- terminated) was not distinquished here. The position of the peak of the indirect transition ranged from 760 to 840 nm, depending on the twist angle, as shown in Figure 1b, which displays representative PL spectra obtained 4 ACS Paragon Plus Environment

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from stacked bilayers with twist angles of 0º, 4º, 21º and 60º. Each PL spectrum was deconvoluted into 4 known PL peaks: the A neutral exciton (A) peak, A trion (A-) peak, B exciton (B) peak and the indirect band (I) transition peak. The A and B peaks result from optical transitions at the K point in the 1st Brillouin zone, while the I peak is the indirect band optical transition occurring between K and Γ points.4 In our experiments, and consistent with previous reports, the wavelength of the I peak showed the most variation as the twist angle was changed.27-29 This marked dependence of the I transition peak position can be used to optically visualize the distribution of local twist angles in stacked bilayers, as shown in the representative optical images of Figure 1c; here, the brightness was set according to the peak position of the I peak, which ranged from 760 to 840 nm. Each stacked bilayer displayed uniform contrast over the MoS2 flakes (except for across the GB as described below) indicating the high reliability of spectral mapping in I peak position determination. We were able to observe a large change in contrast between twist angles of 0º and 4º, which indicates the high sensitivity of the peak wavelength of the I peak to small differences in twist angles. We inspected 45 stacked bilayers and plotted the peak wavelengths of the I peak vs. the twist angle, as displayed in Figure 1d. The peak wavelengths were observed to be the largest, i.e., at about 840 nm, for twist angles of about 0º (3R phase) and 60º (2H phase); for intermediate twist angles of 15º -45º, the peak stayed within the range of 765 - 785 nm, which is consistent with the previous reports.27, 28 In addition, the I peak wavelength was observed to gradually increase as the twist angle decreased from ~15º to 0º or as it increased from ~45º to 60º. The peak position of the I transition of an exfoliated MoS2 bilayer is shown for comparison (red-dashed line in Figure 1d). Traces of monotonic variations of peak wavelength near the commensurate angles of 0º and 60º, corresponding respectively to 3R and 2H space group symmetries, were also observed27 although the experimental uncertainties together with the difficulty of studying theoretically extremely large supercell 5 ACS Paragon Plus Environment

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sizes31 precluded any conclusion regarding their behavior in the long moiré pattern period regimes near these two commensurate angles. The matter remained somewhat controversial since subsequent experimental28 and theoretical studies28, 32 proposed that there are only two distinct interlayer coupling phases, namely those for the two discrete 3R and 2H phases at 0º and 60º, and all the other intermediate rotated configurations that share a similar uniform behavior. While these discrepancies could be due to differences in sample preparation techniques leading to different equilibrium geometries, our experimental results confirm that the peak positions related with the interlayer coupling vary continuously between commensurate and large twist angle phases. All previous analyses28, 32 agree upon the fact that the observed I peak positions measure the steric effects due to the repulsion between the S atoms at the interface and are coupled with the interlayer separations between the two MoS2 layers. From this viewpoint only two distinct interlayer coupling phases are expected28, 32, corresponding to commensurate and twisted configurations. This is because any finite twist angle introduced in two rigid lattices leads to approximately equal distribution of every possible local stacking configuration irrespective of the specific angle33 and preserves the average interlayer coupling strength. For this reason the average value of the interlayer separation would remain approximately constant for every twist angle. However, recent studies indicate that in the limit of long moiré pattern periods substantial in and out-plane strains can lead to finite values of average quantities due to an unequal distribution of local stacking configurations. An expansion and compression of local stacking configurations will take place to minimize the overall area in the higher energy configuration and is strongest near the commensuration angles because the elastic energy has an inverse square dependence with respect to the moiré period34. This kind of expansion and compression of the graphene lattice within the moiré unit cell was observed for graphene on hexagonal boron and the corresponding theoretical explanation for the moiré 6 ACS Paragon Plus Environment

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strains of graphene subject to periodic honeycomb potentials explained the role of strains in the generation of an average band gap35. One plausible explanation for the continuous variation of the I peak position can be given in terms of elastic deformations of the MoS2 membranes in the small twist angle limit around the commensurate stacking configurations. Considering that each stacking configuration is closely related with the local interlayer separations34, 36 we expect that the compression and expansion of different local stacking regions will dictate the average interlayer distance between the layers and therefore the position of the I peak. To explain the observed dependence of I peak position on the twist angle, we used the following fitting function between 0o and 60o for the observed peak positions

λI(θ) = λmin + ( λ0o - λmin ) exp( - θ C0o)

+ ( λ60o - λmin

) exp( - (π/3 - θ) C60o)

where the coefficients C0o and C60o are written in terms of the stacking energy differences, the twist angle, and the bulk modulus, as indicated in the SI. The wavelengths λ0o = 850 nm, λ60o = 840 nm for the two commensurate geometries, and the minimum λmin = 760 nm where all three stacking geometries are expected in equal share, are read from the experimental data. In result, an excellent fit to the observed I peak position was found. Fitting result and details on the elastostatic problem of the MoS2 membrane subject to moiré commensuration potentials are outlined in the SI. The systematic and reproducible dependence of the I peak wavelength on the twist angle of the stacked bilayer can be used to distinguish the GBs. Figure 2 displays a series of GBs of monolayer polycrystalline MoS2, revealed by stacking bilayers. The “tilt angle”, defined as the relative angle difference between the two underlying monolayer MoS2 grains neighboring each other is given across the GBs in each image. A tilt angle of 0º, which would not form a 7 ACS Paragon Plus Environment

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GB,9, 10 did not show any contrast. A tilt angle between the two grains as small as 6º provided a very clear contrast, which indicates the superb performance of this method in distinguishing different grains and GBs. The identification of GBs was also possible without de-convoluting each PL spectrum at each pixel of a PL mapping image. We found that a simple "center of mass" procedure (See the SI for the algorithm of center of mass process) performed for the PL spectral map image yielded a similar trend as did the actual peak position of the deconvoluted I peaks and still provided, though with somewhat less contrast, clear identifications of grains and GBs. (The images corresponding to Figure 2a but processed with the center-of-mass algorithm are shown in Figure S4.) Here we point out that our scheme is not applicable for all GBs formed with arbitrary twist angles and tilt angles. For example, the 60º tilt angle does not provide the contrast between neighboring grains, even though a GB is expected to be present at their interface. This lack of contrast is due to the symmetry around 30º of the I peak dependence on the twist angle, as the plot in Figure 1d is shown. Also if the twist angles of both grains are in the range 15º–45º, which could happen with 25% probability for the “random” stacking, they would not produce the high enough contrast due to the almost flat dependence of I peak position within this twist angle range. (See the examples in Fig. S5). However the large change of I band peak positions between near 0º and other angles implies if overlaid monolayer is intentionally aligned with either of two adjacent grains, any tilt angle GB will be distinguished within the uncertainty of twist angle determination. The requirement of stacking bilayers for GB identification that inevitably modifies monolayer characteristics of MoS2 is another drawback of our spectroscopic GB visualization. Nevertheless our method is advantageous in terms of the unambiguous GB identification and the convenience of routine spectroscopic instrumentation, suitable for investigation of the formation mechanism of specific GBs found in merging MoS2 grains. 8 ACS Paragon Plus Environment

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Indeed, we were able to examine GBs of a number of different tilt angles between two adjacent grains and found a regular pattern that helped us model the formation of the GBs (see Figure 2b). In this proposed model, the GB starts from the adjoining corners and extends linearly along the angle bisectors. These two extended lines meet at a point (S) in Figure 2bi if the vertex of only one grain contacts the side of the other. Interestingly this point is crossed by the bisecting line (

) that extends from the vertex to the center of "incoming" triangular

MoS2 grain. If the vertices of the two grains contact the sides of each other as illustrated in Figure 2bii, (Further growth of the grains in Scheme i results in this case) the extended lines from the bisecting corners stop at the lines of other bisecting diagonals (

and

) from

each triangle center, and then a GB exists between these two intermediate points (S and S'). We point out that these simple models should apply to GB formation between any two adjacent triangular grains if, the growth starts from the grain center spreading out preserving the triangular grain shape, and if the growth rates are similar for the neighboring grains. (See the SI motion picture for the intuitive understanding of GB formation pattern.) We inspected numerous GBs of our sample and the patterns described here were always observed. For example, in Fig. 2a, 6º and 14º tilt angle GBs are in good agreements with the model described in Fig. 2bi and 35º, 44º, 47º and 54º tilt angle GBs follow the exact pattern described in Fig. 2bii. (See the Fig. S6) Our model is consistent with other observation and models10, 30, 37, but is not valid in case that adjacent grains grow with different rates including when different zigzag edge types merges to form a GB.30 In our samples, most of MoS2 grains were close to the perfect triangular shape without the truncations on the edges, suggesting the edges of our CVD-grown MoS2 are single type of zigzag edge. Besides, the fact that these suggested GB formation patterns were well observed conversely suggests that the size difference of triangular monolayer MoS2 in our samples are due to the different onset time of the grain growth rather than the different rate of the growth. 9 ACS Paragon Plus Environment

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The variation of interlayer coupling in stacked bilayers causes the variation of other spectral signatures as well,27-29 and we investigated the capability of other optical contrasts to identify GBs in monolayer MoS2 films, including the spectral peak positions and intensities of the I peak, A exciton peak and Raman peak of the stacked bilayer samples prepared with various twist angles. As shown in Figure S7, the intensity map of the I peak yielded a much lower albeit discernible contrast and the intensity or peak position of A exciton peak did not lead to any discernible contrast. While other spectral features didn’t provide a practical enough contrast, interestingly we found that the intensity map of the Raman peaks of the stacked bilayer displayed discernible contrast across the GB. In Figure 3 we present the I peak position map (Figure 3a) and Raman peak intensity map (Figure 3b) of the same stacked bilayers that contains 60º and 10º tilt angles of monolayer MoS2 GBs. The local twist angles of stacked bilayers are also given on the image. As expected, the 60º tilt angle GB doesn’t appear in I band peak position map because I peak positions are the same for the stacked bilayers with 13º and 47º twist angles, while 10º tilt angle GB show a sufficient contrast. In contrast, the Raman intensity map satisfactorily showed the clear presence of GB for the 60º case, but displayed much lower contrast for the 10º tilt angles than the I peak position map. Through the inspection of many monolayer GBs, we found the Raman peak intensity contrast across the GBs was observed to be lower than that using the I peak position map, but it was still possible to distinguish the GBs in a number of grain orientations, as examples of 56º and 59º tilt angles GBs are shown in Figure 3c and 3d, respectively. We believe that this observed contrast of Raman intensity in the stacked bilayers across the GB is the result of the systematic variation of the Raman peak intensities of stacked MoS2 bilayers modified by the interlayer coupling. Indeed in Figure 3e, the E12g Raman peak intensity (normalized by E12g peak intensity obtained from the monolayer region in the same mapped images) obtained from 98 stacked bilayer regions with different twist angles within 10 ACS Paragon Plus Environment

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the range of 0º - 60º shows a clear increase of E12g peak intensity with the twist angle, (A1g peak showed the similar dependence), which explains the observed contrast in Raman intensity map across the GBs at 60º shown in Figure 3b. This trend is empirically certain showing the clear contrast between grains of different twist angles of stacked bilayers. This type of Raman intensity variations on the twist angle, firstly observed in transition metal dichalcogenides (TMD) stacked bilayers, were previously studied in stacked graphene bilayers theoretically and experimentally, where the intensity of 2D Raman band at ~2640 cm-1, the most sensitive Raman peak to the layer thickness of graphene, displayed the similar trend of monotonic increase with increasing twist angle.25, 38, 39 In Figure 3f, we also show the measured frequency differences between E12g and A1g peak, which are known to be affected by the twist angle of stacked MoS2 bilayers,25, 27, 28 with various twist angles. Previously this dependence of Raman peak positions on the twist angle, while not fully understood, were attributed to the variation of interlayer coupling strength between top and bottom MoS2 monolayers, similar to that of I band peak position.27, 28 Here we note that trend of Raman peak intensity vs. twist angle is quite different from that of the frequency difference. While the effect of the stacking configuration on Raman signals is a subject of active research, the monotonic increase of Raman intensity with increasing twist angle has never been observed. Our result suggests that Raman peak intensity and the PL peak position in stacked MoS2 bilayers are dictated by different physical mechanisms even though both of them show a systematic dependence with the twist angle.

Conclusion We have demonstrated a new spectroscopic method to visualize and identify grains and GBs of monolayer MoS2 grown by CVD, by stacking bilayers. Our method is based on the fine tuning of the spectral position of the I PL transition, on the stacking twist angle between the top and bottom MoS2 monolayers. In addition, we found that the Raman peak intensities 11 ACS Paragon Plus Environment

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of stacked bilayers also showed systematic twist angle dependence and enabled the visualization of 60º tilt angle GB of monolayer MoS2. The markedly different behaviors of the Raman peak intensity and the peak positions of Raman and I bands with the twist angle in stacked MoS2 bilayers strongly indicates that spectral characteristics modified by interlayer coupling in stacked MoS2 bilayers are governed by mechanisms of different physical origins. Unambiguous optical identification of GBs has allowed us to derive a simple empirical rule for determining the location of GBs between two adjacent monolayer MoS2 grains based on a model of homogenous growth of the grains from the grain center in the same microscopic area. The large number of twist angle configurations that we identified allowed us to resolve the controversy on the discrete two phase evolution of the I peak positions for twist angles close to the 2H and 3R commensurate stacking configurations. Our findings provide a convenient method to identify grains and the concepts developed herein may be extended to the study of other TMD stacked bilayers.

Methods Sample synthesis: To synthesize triangle-shaped monolayer MoS2, a promoter was prepared by dissolving perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS, 2D semiconductor) in DI water.5 The promoter solution was then dropped onto a SiO2/Si wafer and spin-cast at 2600 rpm for 1 minute. 200 mg of S (Sigma-Aldrich) and 10 mg of MoO3 were separately loaded in the middle portions of the respective zones of a two-zone furnace. The promoter-coated wafer was placed face down on an Al2O3 crucible boat that contained MoO3 powder. Zone 1 of the furnace, i.e., the upstream side, was heated to 210 ºC at a rate of 42 ºC /min, whereas zone 2, i.e., the downstream side, was ramped up to 780 ºC. This whole process was carried out under 500 sccm N2 for 15 minutes.


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Preparation of stacked bilayer MoS2: Polymethylmethacrylate (PMMA) was spin-coated on a CVD-grown monolayer MoS2/SiO2/Si sample and cured at 80 ºC for 30 minutes to eliminate air bubbles formed at the interface between monolayer MoS2 and PMMA and to improve the adhesion between MoS2 and PMMA. The PMMA/MoS2/SiO2/Si sample was separated from the SiO2/Si substrate by mildly etching out SiO2 in 1 mol/L KOH solution. The PMMA/MoS2 sample was transferred to DI water to reduce the amount of KOH residue, and then transferred onto a cover glass. The sample was then baked at 80 ºC for 30 minutes to eliminate air bubbles formed at the interface between PMMA/MoS2 and the cover glass and to increase the adhesion between MoS2 and the cover glass. PMMA on the MoS2/cover glass was removed by using acetone with thermal treatment at 320 ºC for 5 hours. In the same way, another monolayer MoS2 was transferred onto the MoS2/cover glass sample to make the stacked bilayer MoS2 sample.

Confocal spectral mapping measurements: A laser confocal microscope made in the laboratory with a spectrometer was used for the confocal PL and Raman spectroscopy measurements. With a 0.9 NA objective, the laser light was focused with a diameter of approximately 300 nm. Scattered light was collected using the same objective and guided to a 50 cm long monochromator equipped with a cooled CCD. A 532 nm laser line excitation of a diode-pumped solid-state laser, with a typical power of 500 µW was applied to the sample with an acquisition time of 500 ms per pixel. A more detailed description of the spectral analysis is provided in Supporting Information.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. 13 ACS Paragon Plus Environment

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Acknowledgements This work was supported by IBS-R011-D1 S. P. and M. S. K. contributed equally to this work.

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Figure 1. The peak position of the indirect band (I) PL emission depends on the twist angle. a) Schematic of stacking MoS2 bilayers and the obtained optical contrast across the grain boundary. b) De-convoluted PL spectra of the stacked bilayer MoS2 with twist angles of 0º, 4º, 21º and 60º. c) I peak position maps of stacked bilayer MoS2 with various twist angles. The insets show schematic illustrations corresponding to the bilayer stacking of each twist angle. Scale bars are 5 µm. d) I peak position of stacked bilayer MoS2 as a function of twist angle. The red-dashed line indicates the I peak position of intrinsic bilayer MoS2.


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Figure 2. Optical visualization and empirical rules of GB formation. a) Indirect band PL peak position maps of stacked bilayer MoS2. The scale was individually set for each image to obtain the optimal contrast, where the brightness was set to be proportional to the peak wavelength of the indirect band PL emission. “Tilt angles” between neighboring monolayer MoS2 grains are given on the GBs. The positions of the GBs are indicated by white arrows. The scale bars are 5 µm. b) Schematics of two ways GBs are formed in monolayer MoS2: i, the vertex of one triangular grain contacts the side of another; and ii, the vertices of two grains contact each other (see the SI motion pictures).


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Figure 3. Comparison of GB identifications using various spectral signatures. a, b) Maps of indirect band PL peak position and Raman peak intensity of stacked bilayer MoS2 that contains two GBs formed with 10º and 60º tilt angles. Local twist angles are also given (small fonts). White arrows indicate the positions of the GBs. c, d) Raman peak intensity maps of stacked bilayer MoS2 for 56º and 59º tilt angles between two monolayer MoS2 grains. White arrows indicate the starting positions of the GBs. The scale bar lengths are 5 µm. e) Distribution of Raman E12g peak intensity vs. twist angles obtained from 98 different stacked bilayer MoS2 regions having different twist angles. f) Frequency difference between E12g and A1g peaks obtained from the same stacked bilayer MoS2 regions.


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Simple optical method is presented for visualizing grain boundaries (GBs) of polycrystalline monolayer MoS2 using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities modified by the twist angle. GB between two monolayer MoS2 grains having only a 6º tilt angle in grain atomic orientations is clearly distinguished. monolayer

molybdenum

disulfide,

grain

boundary,

indirect

band

gap

photoluminescence, stacked bilayer, interlayer coupling Seki Park†, Min Su Kim†, Hyun Kim, Jubok Lee, Gang Hee Han, Jeil Jung*, Jeongyong Kim* Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers


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Spectroscopic Visualization of Grain Boundaries of Monolayer

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