Rapid and Large-Area Visualization of Grain Boundaries in MoS2 on SiO2 Using Vapor Hydrofluoric Acid

Grain boundaries in two-dimensional (2D) material layers have an impact on their electrical, optoelectronic, and mechanical properties. Therefore, the availability of simple large-area characterization approaches that can directly visualize grains and grain boundaries in 2D materials such as molybdenum disulfide (MoS2) is critical. Previous approaches for visualizing grains and grain boundaries in MoS2 are typically based on atomic resolution microscopy or optical imaging techniques (i.e., Raman spectroscopy or photoluminescence), which are complex or limited to the characterization of small, micrometer-sized areas. Here, we show a simple approach for an efficient large-area visualization of the grain boundaries in continuous chemical vapor-deposited films and domains of MoS2 that are grown on a silicon dioxide (SiO2) substrate. In our approach, the MoS2 layer on a SiO2/Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO2 at the MoS2 grain boundaries and SiO2 underneath the MoS2 grains as a result of VHF diffusing through the defects in the MoS2 layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO2 pattern using optical microscopy, scanning electron microscopy, or Raman spectroscopy. This method allows for a simple and rapid evaluation of grain sizes in 2D material films over large areas, thereby potentially facilitating the optimization of synthesis processes and advancing applications of 2D materials in science and technology.


INTRODUCTION
Two-dimensional (2D) materials such as molybdenum disulfide (MoS 2 ) have been studied for use in potential applications 1−3 such as transistors, 4 light emitters, 5 photodetectors, 6 modulators, 7 pressure sensors, 8 resonators, 9 biosensors, 10 gas sensing, 11 photocatalysis, 12 and electrochemical applications. 13 For using these 2D materials in device applications, it is critically important to realize the industrialscale and reliable synthesis of high-quality monolayers of the 2D materials at low cost. Chemical vapor deposition (CVD) is a potential large-scale 2D material synthesis approach suitable for industrial applications and one of the most developed approaches for the preparation of large-area MoS 2 of good quality. 14−17 For the development and optimization of CVDgrown single-layer MoS 2 , it is important to characterize the properties of the grown MoS 2 layers that typically consist of irregularly shaped individual grains that connect the adjacent grains 18−21 through grain boundaries. As a result, the grain sizes and grain boundaries of the domains or continuous films of MoS 2 have an important impact on its electrical, 18,22−24 optical, 18 optoelectronic, 25,26 mechanical, 27,28 and chemical properties 29 as well as on the characteristics of devices made of MoS 2 . Furthermore, the properties of grain boundaries in MoS 2 might be beneficially used in specific applications by controlled defects engineering. 30−32 For the above reasons, fast and simple methods to directly observe the large-area distribution of grains and grain boundaries in MoS 2 are of increasing importance.
Grains and grain boundaries in MoS 2 films can be characterized by atomic resolution using transmittance electron microscopy (TEM) 18−20 and scanning tunneling microscopy (STM), 32,33 providing detailed information about the crystal structure of the grains and the grain boundaries. The MoS 2 grain boundaries can also be identified using atomic force microscopy (AFM) by decorating a self-assembled octadecylphosphonic acid monolayer on the MoS 2 surface. 34 However, these techniques are time-consuming, require complex sample preparation procedures, and/or are limited to the characterization of very small areas. Another approach to observe grain boundaries in MoS 2 is the use of nonlinear optics. 35−38 For instance, the grain boundaries between the adjacent MoS 2 grains can be distinguished by stacking MoS 2 bilayers and using photoluminescence imaging based on second harmonic generation. 37 However, photoluminescence imaging is generally limited to MoS 2 domains that feature a large rotation of the crystal axis as compared to the neighboring grains, requires sophisticated optical systems, and is typically slow. 39 Compared to photoluminescence imaging, a faster approach for visualizing grain boundaries in CVD-grown MoS 2 is multiphoton microscopy based on thirdharmonic generation, which is also independent of the degree of crystal axis rotation. 39 Yet, this approach still requires a relatively sophisticated optical system and therefore is not easily accessible. Grain boundaries in CVD-grown MoS 2 layers can also be visualized by oxidizing MoS 2 using UV irradiation in a moisture-rich environment and subsequently imaging the layer with scanning electron microscopy (SEM) or AFM. 40 However, with this approach, additional visible oxidized line defects within the MoS 2 grains were produced alongside the grain boundaries because of easy oxidation of MoS 2 . 40 The grains and grain boundaries of large MoS 2 layers were also visualized by depositing nematic liquid crystals on the MoS 2 layers in combination with polarized optical microscopy 41 or by visualizing the differential diffusion of gold on the surface of MoS 2 between the grain boundaries and the inner grain areas after depositing gold on the MoS 2 surface using optical microscopy. 42 However, these techniques require elaborate and controlled preparation of the MoS 2 samples.
Here, we present a simple and rapid method for visualizing grain boundaries in large areas of CVD-grown single-layer MoS 2 on a SiO 2 surface using SEM, optical microscopy, or Raman spectroscopy. Similar to the previously reported method for observing grain boundaries in CVD-grown graphene placed on a SiO 2 surface using vapor hydrofluoric acid (VHF) exposure, 43 in the proposed method, we first expose the MoS 2 layer on the SiO 2 surface to VHF, which causes VHF molecules to diffuse through the defects in the lattice structure of the MoS 2 grain boundaries. The diffusion of the VHF through these defects results in etching of SiO 2 underneath MoS 2 , with a difference between the etching speed of SiO 2 directly at the grain boundaries and the etching speed of SiO 2 in the areas below the grains away from the grain boundaries. The resulting etch pattern in the SiO 2 layer along the MoS 2 grain boundaries is then visible and can be imaged using optical microscopy, SEM, or Raman spectroscopy. Because the MoS 2 and the underlying SiO 2 layers are exposed to VHF and etched to some extent in our method, this is an invasive approach. SiO 2 is one of the most commonly used growth substrates for large-area CVD-grown MoS 2 , and thus, our method will be useful in the development, characterization, and optimization of large-area MoS 2 synthesis processes.

EXPERIMENTAL SECTION
In all experiments, we used CVD-grown single-layer MoS 2 on a SiO 2 / Si substrate that was bought from 2D Semiconductors (USA), in which the SiO 2 layer was 285 nm thick. MoS 2 was composed of domains with continuous films and individual grains with visible grain boundaries that were not fully stitched to the adjacent MoS 2 grains. For VHF etching, we exposed the SiO 2 /Si chips with the MoS 2 films to VHF that was evaporated from a liquid HF solution with a HF concentration of 25%. The VHF reacts with SiO 2 underlying the grain boundaries of the MoS 2 layer by penetrating the defects of the grain boundaries. To control the amount of H 2 O that is present at the substrate surfaces, the substrate temperature was kept at 40°C. 44,45 For details on the experimental setup of the VHF chamber, our previous publication on the visualization of grain boundaries in graphene could be referred. 43 The VHF etching of SiO 2 involves reactions 1 and 2. (1) Before and after exposing MoS 2 on the SiO 2 /Si substrate to VHF for different times, we used optical microscopy (OLYMPUS, BX60, Japan) and SEM (Gemini, Zeiss, Ultra 55, Germany) to image the morphologies of the MoS 2 /SiO 2 surfaces on the Si substrates with different magnifications, respectively. We used a Raman spectrometer (alpha300 R Microscope, WITec, Germany) to evaluate the quality of the MoS 2 films on the SiO 2 surface. For the Raman scans with areas of 15 μm × 15 μm (100 × 100 points per map), we used a laser with a wavelength of 532 nm, a laser power of 1.5 mW, and an integration time of 0.3 s. For the Raman scans with the areas of 5 μm × 5 μm (25 × 25 points per map), we used a laser with a wavelength of 532 nm, a laser power of 1 mW, and an integration time of 3 s. To analyze the topography of the MoS 2 /SiO 2 surfaces before and after exposing MoS 2 on the SiO 2 /Si substrates to VHF for different times, we used an AFM tool (Dimension Icon, Bruker) with a cantilever (Olympus AC240TM) and an AFM tip (tip radius = 15 nm) in tapping mode.

RESULTS AND DISCUSSION
In our experiments, we used CVD-grown MoS 2 on a 285 nm thick SiO 2 layer on a silicon (Si) substrate, which was purchased from 2D Semiconductors Inc. (USA). The CVDgrown MoS 2 was composed of domains with continuous MoS 2 films without visible grain boundaries ( Figure S1a), as well as domains in which there were presumed visible grain boundaries between the adjacent MoS 2 grains that were incompletely stitched to each other ( Figure S1b). First, we evaluated and confirmed the quality of the pristine MoS 2 samples using Raman spectroscopy ( Figure S2). The typical E 2g 1 band position (385.4 cm −1 ) and A 1g band position (406 cm −1 ) depict the single-layer MoS 2 ( Figure S2e). Furthermore, we used Raman spectroscopy to indicate the presence of grain boundaries between the adjacent MoS 2 grains that were incompletely stitched to each other ( Figure S2b−d). In these measurements, we also found E 2g 1 and A 1g bands in the Raman spectrum at the grain boundaries ( Figure S2d,e), which can be attributed to the fact that the width of a MoS 2 grain boundary is smaller than the diameter of the Raman laser spot (∼400 nm), as indicated in Figure S2d by the red and blue solid circles. Thus, the laser spot always overlaps either one or both sides of the adjacent MoS 2 grains when exciting the boundary region, resulting in a nonzero Raman signal of MoS 2 . The surface topography of a sample of pristine MoS 2 characterized by AFM further confirms that the MoS 2 grains were incompletely stitched to each other ( Figure S3).
To evaluate the proposed approach for visualizing grain boundaries in a single MoS 2 layer on a SiO 2 substrate, we exposed a sample with a large continuous MoS 2 film without visible grain boundaries (Figure 1a) to VHF at 40°C for 120 s (see Experimental Section). After VHF exposure, line patterns became visible and distinct within the continuous film in both SEM ( Figure 1b) and optical microscopy images (Figure 1c,d). The SEM (Figure 1a,b) and optical microscopy images ( Figure  1c,d) were taken at the same position of the same chip. In the images, it can be seen that the areas enclosed by a line pattern are of the order of 1 μm to several μm. We tentatively assign these line patterns to the grain boundaries in the MoS 2 film. To further explore this, we selected a chip area in which the MoS 2 film on the SiO 2 surface contained distinguishable individual grains and grain boundaries prior to VHF exposure (Figure 1e), caused by incomplete growth and stitching of the adjacent MoS 2 grains. After exposing this sample to VHF at 40°C for 120 s, we found that the same type of line pattern in the SiO 2 layer appeared at the locations of the previously visible grain boundaries (Figure 1f−h). Also, here, the SEM and optical microscopy images in Figure 1e−h were taken from the same position of the chip. Furthermore, we investigated a chip area where a continuous MoS 2 film was located directly next to a MoS 2 film, in which grain boundaries and individual grains were visible, caused by incomplete growth and stitching of the adjacent MoS 2 grains. After exposing this sample to VHF at 40°C for 120 s, we again observed that the same type of line pattern in the SiO 2 layer appeared at the locations of the previously visible grain boundaries, with a comparable pattern appearing in the area of the continuous film ( Figure S4a,b). Interestingly, there are thinner line patterns within the largearea grain boundaries of the MoS 2 sample after exposure to VHF for 120 s, indicating that the visible larger grains are composed of several small grains ( Figure S4). It should be noted that line defects other than grain boundaries, such as wrinkles in 2D materials, are easily introduced when a 2D material is transferred from its original growth substrate to a new target substrate. In contrast, the samples in our experiments consist of single-layer MoS 2 that is directly grown on the SiO 2 surface of the Si substrate by CVD, thus avoiding the transfer of the 2D material. Therefore, the MoS 2 samples used in our experiments most likely do contain only few or no wrinkles that may interfere with the visualization of the grain boundaries in MoS 2 by VHF exposure.
To explore the impact of the VHF exposure time on the resulting line patterns in both the continuous CVD-grown MoS 2 films and individual domains placed on a SiO 2 substrate, we exposed such samples to VHF at 40°C for different times, that is, 30, 60, and 120 s. We observed that the line patterns appear increasingly pronounced with the increasing exposure time to VHF (Figures 2 and 3). We also observed that the patterns did not change in structure for the different evaluated VHF exposure times (Figures 2 and 3). Generally, when the VHF exposure time was increased from 30 to 60 and to 120 s, thin line patterns appeared inside the large MoS 2 domains and became increasingly distinct when the VHF exposure time was   increased to 120 s (Figures 1−3 and S4). This indicates that a longer time (i.e., 120 s) of exposure to VHF reveals more information about the grain boundaries in MoS 2 than shorter VHF exposure time (i.e., 30 and 60 s). This is in agreement with the observation of grain boundaries in CVD-grown graphene on SiO 2 /Si substrates after exposure to VHF. 43 The mechanism for observing grain boundaries in CVDgrown graphene on SiO 2 /Si substrates after exposure to VHF is based on the diffusion of VHF through the defects in the graphene lattice at the grain boundaries and the different speeds of VHF etching of SiO 2 that is close to the graphene grain boundaries and that below the graphene grains. 43 In order to verify that the mechanism for the appearance of the line patterns in the SiO 2 surface covered by the MoS 2 films is the same as described above, we used AFM to image the surface topography of our samples and characterized the evolution of the MoS 2 and SiO 2 surface topography with increasing VHF exposure times (Figure 4). Before exposing our samples to VHF, we did not observe significant topographical features in the MoS 2 film on the SiO 2 surface (Figure 4a,b). The high features appearing in white in Figure  4a might be associated with the multilayered growth or the formation of by-products occurring mainly at the grain boundaries during the synthesis of CVD-grown MoS 2 , which has been reported before. 46 After exposure to VHF for 30, 60, and 120 s, the surface topography near the assumed MoS 2 grain boundaries progressively increased, with the line patterns being elevated from the other areas by about 8−10 nm ( Figure  4c (Figure  4e,f). The three-dimensional (3D) representation of the AFM data, with the surface topography of the MoS 2 samples after exposure to VHF for 30, 60, and 120 s, further illustrate that the line pattern is elevated with respect to the areas surrounded by the line pattern (Figure 5a−d). These results conclusively confirm that the VHF exposure of a MoS 2 film on a SiO 2 surface results in differential etching of SiO 2 . This can be explained by an increased etch rate of SiO 2 underneath the MoS 2 crystallites where liquid water with dissolved HF can accumulate, as compared to the SiO 2 etch rate at the MoS 2 grain boundaries where SiO 2 is directly exposed to VHF and where liquid water does not accumulate (Figure 5e). Specifically, the net reaction of etching of SiO 2 with VHF results in an excess of H 2 O molecules that get trapped underneath the MoS 2 grains and accumulate in a water reservoir along with the easily water-dissolvable HF. 43 In contrast, in the areas along the grain boundaries, the excess water can evaporate through the grain boundaries, and thus SiO 2 in these areas is directly exposed to VHF, resulting in a significantly lower SiO 2 etch rate along nanoporous grain boundaries. This mechanism results in the distinct line pattern of the surface topography along the nanoporous grain boundaries in the MoS 2 films that can be visualized by SEM or optical microscopy.
To evaluate the impact of VHF exposure on the quality of the MoS 2 films on a SiO 2 substrate, we exposed MoS 2 films with presumed visible grain boundaries between the adjacent MoS 2 grains to VHF for different exposure times, that is, 30, 60, and 120 s, and thereafter performed scanning micro-Raman spectroscopy on the samples. After exposing MoS 2 to VHF for 30 s (Figure 6a), the Raman spectroscopy maps of the intensities of the E 2g 1 and A 1g modes of MoS 2 are still relatively strong (Figure 6a2,a3) with clear E 2g 1 and A 1g band intensities (Figure 6a4,a5). This indicates that MoS 2 is of relative high quality even after exposure to VHF for 30 s. As we increased the VHF exposure time to 60 s (Figure 6b), the Raman spectroscopy maps of the intensities of the E 2g 1 and A 1g modes of MoS 2 became weaker (Figure 6b2,b3), with significantly decreased E 2g 1 and A 1g bands (Figure 6b4,b5). These results indicate that MoS 2 was damaged to a larger extent during the exposure to VHF for 60 s. As expected, when we further increased the VHF exposure time to 120 s (Figure 6c), the Raman spectroscopy intensity maps of the E 2g 1 and A 1g modes of MoS 2 substantially weakened (Figure 6c2,c3), with very weak E 2g 1 and A 1g bands (Figure 6c4,c5). This indicates that MoS 2 was substantially damaged during the exposure to VHF for 120 s. Moreover, it can be seen from Figure 6a5−c5 that the E 2g 1 and A 1g bands of MoS 2 became weaker with increasing VHF exposure times, both within the grains and at the grain boundaries. This further indicates that long VHF exposure and more etching of SiO 2 underneath the MoS 2 film degrade the quality of MoS 2 . As an additional reference, we characterized samples with continuous MoS 2 films using Raman spectroscopy before and after exposing them to VHF for 30, 60, and 120 s, respectively, and we found similar results ( Figures S5−S8). Our Raman spectroscopy characterization shows that the exposure to VHF of MoS 2 on a SiO 2 surface for short times (i.e., 30 s) only marginally affects MoS 2 , whereas the exposure to VHF of MoS 2 placed on SiO 2 for longer times (i.e. 120 s) significantly affects MoS 2 .
In a previous study, we have demonstrated that grains and grain boundaries in a CVD-grown single-layer graphene on a SiO 2 /Si substrate can be visualized after exposing them to VHF. 43 In the study, we used the same processing conditions for exposing and visualizing the grains in graphene as in the experiments with MoS 2 in the present study, and thus the two approaches are comparable. One advantage of the visualization of grains in MoS 2 is that single layers of MoS 2 can be directly grown on SiO 2 surfaces by CVD, which is the required substrate material for our method. In contrast, for visualizing the grains and grain boundaries in CVD-grown graphene, graphene first has to be transferred from the original growth substrate (e.g., a copper substrate) to the SiO 2 /Si substrate. A disadvantage with this is that the transfer can easily introduce additional line defects such as wrinkles, folds, and cracks as well as possible contaminations such as poly(methyl methacrylate) residues in the graphene layer, 43 which can influence the results. In addition, in our previous work, we have not been able to observe the graphene surface on exactly the same position before and after exposure to VHF for   43 In contrast to the previous work, 43 in the present investigation, we used single-layer MoS 2 that was directly synthesized on the SiO 2 /Si substrate by CVD, thereby entirely avoiding the layer transfer process with its potential disadvantages of introducing wrinkles, folds, cracks, or polymer residues. In addition, we have been able to observe MoS 2 on the SiO 2 /Si substrate at exactly the same position before and after the exposure to VHF for various times, which leads to the improved accuracy of our results. We speculate that our approach may be applicable to the large-area visualization of grains and grain boundaries in single-layer 2D materials other than MoS 2 and graphene. The prerequisites for this would be that (a) the 2D material is placed on a SiO 2 surface (either grown or transferred), (b) it is a single-layer 2D material with grains larger than approximately 0.5−1 μm, and (c) the 2D material is not permeable to VHF and does not get In summary, we demonstrate here a simple and efficient method to visualize grain boundaries over large areas in CVDgrown MoS 2 films on a SiO 2 /Si substrate. Our approach only requires VHF etching for 30−120 s and subsequent optical microscopy or SEM inspection, which are all processes and tools that are commonly available in typical cleanrooms and semiconductor labs. Although our method is invasive, that is, the sample is permanently modified in the characterization process, it has advantages such as ease of use, speed, and simple large-area analysis, which could be very useful in the development and optimization of large-scale MoS 2 synthesis processes. Our approach may also be useful for investigating and optimizing the mechanical, electrical, and chemical properties of CVD-grown MoS 2 , which are strongly influenced by grain boundaries and grain sizes, thereby ultimately promoting the utilization of MoS 2 in research and its application in future 2D material devices.

CONCLUSIONS
We demonstrated that VHF can be used to rapidly visualize the location of grains and grain boundaries over large areas in CVD-grown single-layer MoS 2 on a SiO 2 surface by using optical microscopy, SEM imaging, or Raman spectroscopy. Our approach is based on the difference in the etching behavior of SiO 2 near the MoS 2 grain boundaries and of SiO 2 below the MoS 2 grains when exposed to VHF. The resulting microscale line patterns in the SiO 2 surface are caused by higher etch rates of SiO 2 in the areas underneath the MoS 2