A New Approach to Unveiling Individual Atomic Layers of 2D Materials

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A New Approach to Unveiling Individual Atomic Layers of 2D Materials and their Heterostructures Irfan Haider Abidi, Lu-Tao Weng, Chi Pui Jeremy Wong, Abhishek Tyagi, Lin Gan, Yao Ding, Man Li, Zhaoli Gao, Ruiwen Xue, Md Delowar Hossain, Minghao Zhuang, Xuewu Ou, and Zhengtang Luo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05371 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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Chemistry of Materials

A New Approach to Unveiling Individual Atomic Layers of 2D Materials and their Heterostructures Irfan Haider Abidi,‡ Lu-Tao Weng,‡,†,* Chi Pui Jeremy Wong,‡ Abhishek Tyagi,‡ Lin Gan,§ Yao Ding,‡ Man Li,§ Zhaoli Gao,# Ruiwen Xue,‡ Md Delowar Hossain,‡ Minghao Zhuang,‡ Xuewu Ou,‡ and Zhengtang Luo*, ‡ ‡

Department of Chemical and Biological Engineering, †Materials Characterization and Preparation Facility, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, §

State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China #

Department of Physics and Astronomy, University of Pennsylvania, 209S 33rd Street, Philadelphia,

Pennsylvania 19104 6396, USA ABSTRACT: Visualization of the chemical structures of two-dimensional (2D) materials and their interfaces at virtually atomic scale, is an imperative step towards devising the highly efficient ultrathin optoelectronic devices. Herein, we demonstrate a universal method featuring time-of-flight secondary ion mass spectrometry (ToF-SIMS), coupled with the structure simplicity of 2D materials, as a versatile tool to reveal the vertical atomic layers of various two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). We demonstrated that the vertical atomic layers of those 2D materials can be unveiled layer-by-layer using a strategy of ToF-SIMS three-dimensional (3D) analysis developed in this work. Moreover, we found that the extreme surface sensitivity and chemical specificity of ToF-SIMS also enable the examination of the lateral uniformity of 2D materials. During this process, we firstly removed interference of adsorbed organic contamination by annealing, which allows the high quality signals specific to the 2D materials which were accumulated along the sputtering depth. The accumulated signal was found to be linearly proportional to the number of atomic layers in the z-direction, thus providing a chemical intensity contrast that can be directly used to reveal the number of atomic layers in vertical direction. For the case of CVD-grown graphene, up to 6 individual adlayers have been resolved. The technique developed in this work is substrate-independent and can be directly applied to the grown substrate, which circumvents the time-consuming transfer process and avoids any potential hazard to the delicate 2D materials. Our approach provides an efficient characterization tool for analyzing the 2D materials and their heterostructures, with extensive implications towards preparation of high-quality 2D-crystals for atomically thin optoelectronic devices.

INTRODUCTION The present decade is an era of two-dimensional (2D) layered materials, starting from the birth of graphene it evolved into the addition of other members to the family of 2D crystals such as hexagonal boron nitride (h-BN) and a variety of transition metal dichalcogenides (TMDs, including MoS2, WS2 and so on).1-4 These atomically thin materials offer exceptional optical and electronic

properties owing to their structural built, which boost their potential for a wide spectrum of applications.5-9 Recently, scientists came up with another exciting approach of stacking multiple atomic layers of different 2D crystals, either in-plane or vertical configuration to build heterostructures at atomic level precisions, which exhibit inimitable and synergistic properties.10-12 This interesting concept brings an aspiration of building an artificial “lego” of high quality 2D crystals with desired sequences, to

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accomplish the ultimate goal of large area high-speed ultrathin electronic, photonic and optoelectronic devices.13-16 Nevertheless, ensuring the quality of 2D crystals in terms of layer uniformity and surface contaminations is the key step to fabricate such high performance nanodevices based on single or multiple layers of 2D materials.17 Among all the synthesis methods, chemical vapor deposition (CVD) technique is a widely adopted and scalable method to grow a variety of 2D materials and their heterostructures.18-21 Although enormous research has already been done on CVD growth of graphene,22,23 h-BN24,25 and most of the TMDs materials,26-28 yet more efforts are required to efficiently characterize the grown 2D crystals and obtain deep understanding about their growth mechanism. Thus, it is desirable to have an efficient tool to probe the quality of 2D materials and their heterostructures especially in terms of chemical information at virtually atomic-level, preferably directly on the growth substrate. Conventional characterization techniques however possess certain difficulties to provide simultaneously morphological and chemical information. For instance, it is very challenging to reveal adlayers of graphene or h-BN films directly on the Cu substrate, by traditional characterization techniques such as atomic force microscopy (AFM) and Raman spectroscopy due to the surface roughness and strong fluorescence effect of the metal substrate.29,30 Although SEM offers limited success, but yet it provides a poor contrast for the in-plane heterostructures and lack of information for the stacking order of the vertical heterostructures of the 2D materials. Very often, the 2D materials have very poor contrast in SEM because the secondary electrons coming from substrates dominate the total signal. Apparently all conventional techniques have a common limitation – the measurements are “substrate-dependent”. This is also the reason why the 2D materials often need to be transferred to another substrate (300 nmSiO2/Si), which adversely complicates the characterization results. 29 Therefore, it is imperative to develop a new technique that is able to unveil the homo- or heterostructures built by atomic layers of 2D crystals grown by CVD method or even stacked manually. It is even more desirable if the technique can be directly applied to the grown substrates. Recently, time-of-flight secondary ion mass spectrometry (ToF-SIMS) emerges as an alternative tool for characterizing atomically thin 2D materials.30-34 For instance, a few studies employed ToF-SIMS to analyze polymer35,36 and metallic contaminations37 on the transferred graphene

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surface, while the other work investigated adlayers of graphene assisted with isotope-labeling technique.38 Nonetheless, limited success has been obtained for the use of ToF-SIMS for resolving atomic layers of 2D heterostructures.30 From an analytical point of view, ToF-SIMS bears the following unique capabilities for 2D materials characterization: (1) when operated in static mode, its extreme surface sensitivity ( < 1 nm) allows investigation of the surface distribution of atomically thin 2D materials with high lateral resolution; (2) when combined with a low energy sputter source, the in-depth distribution of 2D materials could be resolved at nanometer resolution; (3) the combination of lateral imaging and depth profiling constitutes a powerful three dimensional (3D) chemical analysis; which could be used to reveal hetero-structures at 3D space, and (4) with its down to ppb level detection limit, the impurities (organic or inorganic) introduced during growth or transfer process could be identified and spatially located. Furthermore, as ToF-SIMS provides direct chemical identification of 2D materials, the interference of the substrates should be minimal, making this technique “substrate-independent”. In this work, we performed a systematic study to explore the potential of ToF-SIMS as a tool to investigate the identical or non-identical individual atomic layers of a variety of 2D materials both laterally and vertically using purpose-built samples grown by CVD technique. Our study was first focused on CVD-grown graphene with a particular emphasis on the evaluation of monolayer uniformity and the determination of layer number in multilayer graphene (MLG) domains. As graphene signal in ToF-SIMS was seriously affected by the organic contaminations adsorbed on graphene surface, we developed an in-situ thermal annealing protocol to remove these contaminations. Our results showed that the uniformity of monolayer graphene could be easily evaluated with ToF-SIMS surface mapping because of its extreme surface sensitivity while the individual atomic layers of MLG could be precisely revealed from a new approach proposed in this study. Although the depth resolution of ToF-SIMS is not enough to resolve the atomic layers in MLG, the graphene signal (C2-) accumulated from the depth profile was instead found to be proportional to the number of graphene layers in MLG, and therefore allows us to interrogate atomic layers of other 2D materials such as h-BN and WS2 or even the buried interfaces of vertical heterostructures of monolayer graphene and MoS2 single-crystals. Our approach is independent of the substrate and thus avoids the need for transferring to other substrate for


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characterization. Integrating our approach with the fundamental research of the 2D materials can boost the ongoing progress towards achieving the superior quality 2D crystals for the fabrication of next generation high-performance nanodevices.

RESULTS AND DISCUSSION Single and Multilayer Graphene. Figure 1(a) depicts an optical micrograph of as grown singlecrystal graphene domains on the Cu foil (graphene/Cu) using Chemical Vapor deposition (CVD) method reported in our previous work.33 After the CVD synthesis, the Cu foil was slightly oxidized to visualize the hexagonal graphene domains. The Raman spectrum acquired from the marked spot shows the typical features of single layer graphene (SLG) with 2D/G band ratio >2,39 as shown in the inset of Figure 1(a). Almost negligible D peak at 1350 cm-1 is seen, evidencing low density of structural defects of the samples.39 Signal from organic contaminants adsorbed on the graphene surface is weak attesting the effectiveness of the transfer process. Figure 1(b) shows the ToF-SIMS chemical map of the area shown in Figure 1(a), constructed by overlaying the map of C2- and that of the ions representing Cu (including Cu-, O2-, CuO- etc.). The right panel (i) and (ii) illustrates the distribution of C2- and the Cu- ions, individually. The chemical map of C2- clearly replicates the graphene domains present in the optical image, indicating C2- arising exclusively from the graphene domains in this case. Therefore probing the C2- complemented the uniformity of the graphene film grown on the Cu foil.37 Here, the main advantage of ToF-SIMS is its effectiveness of obtaining an overall image of the surface with high lateral resolution (~200 nm), as opposed to Raman mapping, which take tens of hours to obtain similar images, along with relatively low spatial resolutions. It must be pointed out that the chemical map shown in Figure 1(b) was acquired after performing in-situ thermal annealing of graphene/Cu samples at 450 °C under ultra-high vacuum conditions for one hour, to get rid of all the organic contaminations adsorbed on the graphene surface from the environment.36 Without in-situ thermal annealing step, organic compounds also contribute to the C2- ions with a much higher sensitivity, adversely reduce the resolution, which smears edges of the graphene domains, as shown in the Figure S1 (supporting information I). We found the pre-thermal annealing is even more crucial to investigate the surface of the transferred graphene.

Figure 1. ToF-SIMS chemical imaging of single crystal graphene on Cu and SiO2. (a) Optical image of as-grown single crystal graphene domains on the Cu foil. Inset is a Raman spectrum acquired from the marked area, indicating single layer graphene (SLG). (b) Surface high lateral resolution ToF-SIMS image acquired from the same area shown in (a), constructed by chemical map overlayers of C2- and the Cu characteristic ions (accumulation of Cu-, O2-, CuO2-). Panel (i) and (ii) depict the individual chemical map of C2- and the Cu characteristic ions, respectively. (c) Optical image of single crystal graphene domains transferred on 300 nm SiO2/Si substrate. Raman spectrum obtained from the indicated area is shown in the inset. (d) ToF-SIMS image of the area (c), using chemical map overlayers of C2- and SiO2-, with individual panels as (iii) and (iv). The ToFSIMS images shown in (b) and (d) are obtained after one hour of in-situ thermal annealing at 450 °C, to remove all the organic contaminations adsorbed on the graphene surface. The scale bar is 100 µm in panel (i,ii) and 200 µm in panel (iii, iv) and y-axis scale: counts/pixel.

Figure 1(c) shows an optical image of single-crystal graphene domains transferred to SiO2/Si substrate using polymer assisted wet transfer technique. Apparently, optical micrograph along with the Raman spectrum reflect high quality SLG, but does not provide information about physisorbed or chemisorbed contaminations on the graphene surface during the transfer process. However, ToFSIMS chemical mapping of transferred graphene samples reveals the species like C2H3O2-, C2O- and CH3O- ions, originated from the residues of polymer (PMMA) and acetone (solvent used for PMMA removal),36,40 respectively. While in-situ ToF-SIMS investigation reveals the gradual removal of these residues as indicated by the decline in the intensity of C2H3O2- C2O- and CH3O- signals during the



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Figure 2. Determining the layers number of multilayer graphene (MLG) stack. (a) C2- map reconstructed by accumulating the signals for (a) 20 seconds (b) 100 seconds (c) 300 seconds of Cs+ sputtering, showing the distribution of 1st , 2nd and 3rd layer of as-grown trilayer graphene on the Cu substrate, respectively. (d) Plot of the C2- intensity integrated from the area line scan shown in (c). A linear increment in the C2- signals is observed with the increasing number of graphene layers. (e) ToF-SIMS image (C2- map) of as-grown MLG/Cu, revealing more than 6 adlayers of graphene based on the C2- intensity difference. (f) Optical image of MLG transferred to SiO2/Si substrate to identify the number of layers by optical contrast. (g) Corresponding ToF-SIMS (C2- map) image of MLG/SiO2, demonstrating equal number of distinguishable graphene adlayers (≥ 6 layers) analogous to the optical image (f). For (a), (b), (c), (e) and (g): y-axis scale is counts/pixel.

thermal annealing treatment at 450 °C, as shown in Figure S1. Thermal annealing for about one hour resulted in almost negligible amount of adsorbed organic residues, consistent with the recent report.35 Figure 1(d) shows a high lateral resolution ToF-SIMS surface image obtained by overlaying the chemical maps of C2- and SiO2- acquired from the area shown in Figure 1(c). Chemical map shown in panel (iii) reflects C2- as a perfect marker for the hexagonal graphene domains, after the removal of organic contaminations via aforementioned thermal annealing. Hence, the uniformity of graphene domains on Cu or SiO2/Si substrate can easily be probed by ToF-SIMS surface C2- mapping, assisted with in-situ thermal annealing. Besides providing information about the graphene uniformity, we also attempted to unveil the buried adlayers of MLG stack by ToF-SIMS threedimensional (3D) analysis. The best depth resolution of ToF-SIMS, even using a very low energy sputter source (e.g. 500 V), is around 1 nanometer, which is obviously bigger than the atomic layer thickness (0.34 nm) of graphene. Therefore, it is difficult to deduce the layer number of graphene in MLG

directly from the depth profile. In practice, the number of layers and their thickness are indirectly estimated from the simulation of depth profile intensity using models (see below and ref 34). In this work, we propose a new approach to tackle this problem. Our approach was based on the ToF-SIMS 3D analysis with a rationale that the graphene signal accumulated from the 3D volume should be proportional to the number of graphene layer. This approach is illustrated in Figure 2 for MLG grown on the Cu substrate. The MLG samples were thermally annealed at 450 °C for one hour under high vacuum (10–9 Torr), before the 3D analysis. This annealing step enhances the lateral resolution of the surface mapping for graphene domains as shown in Figure S2. Thereafter, the MLG film on Cu was subjected to ToF-SIMS 3D analysis, which consisted of a series of cycles of high lateral resolution imaging followed by very slow sputtering to remove the graphene materials. The imaging was realized using 25-keV Bi3+ primary ion beam while the sputtering was using 3 keV Cs+ sputter ion beam. As an illustration, we reconstructed the chemical map of C2- after accumulating the signals for 20, 100 and 300 seconds


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of sputtering as shown in Figure 2(a-c), which correspond to the signal accumulated from different depths. With 20 seconds of sputtering time (Figure 2a), we barely observed a uniform graphene layer, while for prolonging sputtering for 100 s, another “david-star” shape start to emerge underneath as shown in the Figure 2(b). Further we found, it takes approximately 100 seconds of Cs+ sputtering for the removal of first layer of graphene at our present conditions. We further confirmed this by acquiring C2- map from the same area after 100 seconds of sputtering, which shows minimal signals in the specified region, as illustrated in Figure S3. Prolonging the Cs+ sputtering for longer time exposes all the buried adlayers underneath the top layer, such that we found a total three layers of graphene after accumulating C2- signals for 300 seconds of sputtering, as shown in Figure 2(c). A three-layer structure can be easily seen from the accumulated C2intensity. If an area line scan is drawn across the three layers (red dotted line in Figure 2(c)), a stepwise increase of C2- intensity with the number of graphene layers is observed. More importantly, the intensity at each step is linearly proportional to the number of graphene layers (Figure 2(d)), confirming that the rationale of our approach is correct. This result is remarkable because it shows that the C2signal depends only on the quantity of graphene but has nothing to do with the substrate. Implementing this approach enables us to successfully distinguish as many as six layers of as-grown graphene directly on the Cu substrate, as shown in Figure 2(e). It is important to point out that our approach does not require any prior surface treatment of Cu foil 41,42 or isotope labelling technique38 to differentiate and visualize the individual adlayers of graphene, as reported previously. As a proof of concept, we also implemented our approach to the transferred MLG film on SiO2/Si substrate. Figure 2(f) displays an optical image of the MLG domain transferred to SiO2/Si substrate. The optical contrast between the substrate and graphene layers enables us to identify the number of layers. Likewise, we also observed six individual layers of graphene in the C2- map presented in Figure 2(g), which was acquired through depth profiling the same MLG region (shown in Figure 2(f)). Obviously, ToF-SIMS chemical image perceived as an analogous to its optical image, which vindicates our strategy of identifying the layer number of graphene on Si/SiO2 as well as on Cu substrate. The layer number identification was also verified by acquiring Raman spectra of the individual adlayers of MLG, as shown in Figure S4. Consequently, as ToF-SIMS chemical

image is independent of the substrate, this technique provides an efficient and precise way to probe the number of layers of CVD-graphene directly on the Cu substrate, without requiring a subsequent tedious transfer process. In order to further validate our approach, a more detailed analysis on the above ToF-SIMS 3D data was performed. Figure 3(a) presents ToF-SIMS lateral map of accumulated C2- signals acquired from MLG/Cu, also discussed above as Figure 2(e). To show the distribution of C2- ions in perpendicular direction (z-axis), we took slices such as xz and yz in horizontal and vertical directions, respectively, as shown with the marked lines in Figure 3(a). The cross-section map of C2- distribution within xz and yz slice is shown in Figure 3(b). The shallow distribution of C2- for xz slice, represents the single-layer of graphene, while for the yz slice in-depth distribution of C2- were observed particularly in the central region indicating the existence of the multilayer graphene domains in center. These cross-section maps of C2also corresponds well to the lateral intensity map of C2- (Figure 3a), reflecting the exceptional capability of ToF-SIMS for in-depth analysis of the atomic layers. In addition, Figure 3(c) exhibits 3D overlay distribution of the secondary ions C2- and Curepresenting MLG layers stacked on the Cu substrate, respectively. Figure 3(d) illustrates the depth profile (intensity vs sputtering time) of the C2and Cu- secondary ions reconstructed from the micro-areas of one, two and three layers of graphene on Cu. The intensity profiles clearly reflect the C2ions distributed deeper for the 3-layers graphene region as compared to that for 1-layer graphene, as it requires longer sputtering time to expose the Cu surface (Cu-). However, as pointed out above, the number of graphene layers cannot be resolved from the depth profiles as the depth resolution of ToFSIMS is not enough. In order to estimate the thickness of individual graphene layers from C2intensity, we implemented a sputtering rate model proposed by Hoffmann,43 to convert the sputtering time into depth (nm) (see supporting information II for details). A typical converted profile for tri-layer graphene is shown in Figure 3(e), indicating a good agreement between the C2- profiles simulated by model with the experimental data points. Implementing this calculation method, the thickness of the utmost four layers of MLG is calculated and plotted against the number of layers (Figure 3(f)), together with the theoretical thickness of graphene. The results clearly demonstrate a nice agreement between the calculated thickness from the depth profiling and the theoretical thickness of graphene



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Figure 3. Depth profiling and 3D imaging of multilayer graphene on Cu substrate. (a) ToF-SIMS image of as-grown MLG stack on the Cu substrate, constructed by sequential depth profiling of C 2- ions. The horizontal and vertical lines indicating xz and yz slices for cross-section analysis. (b) The cross-section views of C2- (graphene) distribution in z-axis obtained from mapping xz and yz slices. The yz cross-section reveals relatively much deeper distribution of C2- ions, which corresponds to the multilayer graphene stacks. (c) A 3D representation of C2- ions distribution on Cu substrate denoted by Cu- ions. (d) Depth profiles reconstructed from the micro-area of one, two and three layers of graphene on Cu, represented by intensity of C2- and Cu- secondary ions as a function of sputtering time. The deeper interface of C2/Cu- indicates thick layer of graphene. (e) Normalized intensity C2- simulated profile against the thickness (nm) for a trilayer graphene, converted using sputtering rate model. (f) A plot illustrating experimentally calculated thickness vs the number of graphene layers, which shows a great agreement with the theoretical thickness of graphene adlayers.

(see supporting information II for details). Such an indirect thickness determination of graphene adlayers also vindicated our approach for MLG identification through lateral high-resolution C2mapping above. Therefore, following few simple steps of in-situ thermal annealing and ToF-SIMS depth profiling enable us to visualize and distinguish individual layers of as-grown MLG film directly on the Cu foil. The main advantage of the proposed strategy is that no prior surface treatment of the Cu foil or isotope labelling is required for layer determination, which otherwise makes the visualization process complicated and limited. Consequently, the presented approach is a milestone as an effective quality evaluation technique for CVDdriven graphene which can boost up the research in this field. 2D Materials beyond Graphene. We stretched our approach to other members of 2D materials

family, such as h-BN and transition metal dichalcogenides (TMDCs, MoS2, WS2 etc.). Figure 4(a) illustrates an optical micrograph of as grown single-crystal h-BN on the Cu foil. Likewise graphene visualization method, h-BN domains were visualized under optical microscope by slightly oxidizing Cu surface after the CVD growth. However, for the chemical identification of these domains, some other characterization tools are required. Unfortunately, Raman signals of h-BN on the growth substrate are extremely weak44 or sometimes invisible (Figure S5) and hence it is impractical to map a large area of asgrown h-BN domains using Raman spectroscopy. Therefore, it necessitates polymer-assisted transfer of h-BN flakes to SiO2/Si substrates for further probing the quality of domains. Nevertheless, to-date revealing the multi-layer h-BN domains directly on the Cu substrate is yet challenging due to the low contrast in SEM and extremely weak Raman signals.


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Figure 4. ToF-SIMS analysis of other 2D materials beyond graphene. (a) Optical image of CVD grown single crystal h-BN domains on the Cu substrate. The Cu surface was slightly oxidized to visualize the h-BN flakes. (b) ToF-SIMS high lateral resolution surface map of B+ secondary ions representing isolated h-BN domains distribution, consistent with the optical image. Inset is the 50 µm x 50 µm B+ map of the marked region to show enlarged image of h-BN domains. (c) B+ map accumulated from depth profiles, revealing the multilayers of h-BN domains. (d) Optical image of single-crystal WS2 domains stacked on the SiO2/Si substrate. Inset is the Raman spectrum acquired from the marked spot reflecting monolayer of WS2. (e) An overlayer image constructed by chemical maps of S- and O- secondary ions, presenting the distribution of WS2 crystals on SiO2 substrate. (f) S- ion map accumulated from depth profiling revealing multilayer domains of WS2, inset is an optical image which corresponds to the SIMS analysis area.

Next, we will demonstrate that ToF-SIMS high lateral resolution mapping and depth profiling enable us not only to reveal the large-area h-BN domains but also to distinguish the number of adlayers in multilayer h-BN domains. Figure 4(b) shows a large area surface high lateral resolution ToF-SIMS image acquired from the same area as in Figure 4a. We map the B+ secondary ions as the representative of the h-BN domains, showing consistent domain shape and size with that in the optical image. From the inset of Figure 4(b) showing enlarged image of B+ mapping of the four domains from the marked region, we can observe the uniform distribution of B+ signals within the h-BN domains indicating isolated islands of single-crystal, predominately monolayer h-BN. The B+ map accumulated from depth profiling, however reveals that some h-BN domains are in fact multilayer, which are invisible under optical microscope, as shown in Figure 4(c). We believe this approach of direct visualization of multilayer h-BN stacks on the Cu foil is remarkable as it provides an efficient route to

probe large area CVD-grown h-BN, which can guide us for optimizing growth conditions for uniform high quality h-BN films. We have also employed TMDCs materials to evaluate their quality in terms of uniformity using our approach. Figure 4(d) displays an optical image revealing single-crystal CVD-grown WS2 domains stacked on SiO2/Si substrate. The Raman spectrum acquired from the marked area showed characteristics Raman bands of monolayer WS2 at 352 cm-1 (overlapping of second order mode 2LA(M) and in-plane E12g vibrational mode) and out-of-plane A1g mode at 417 cm-1,45 as shown in the inset of Figure 4(d). The surface high lateral resolution chemical mapping was performed on the same area, specifying S- and O- secondary ions as representative of WS2 and SiO2 surfaces, respectively. Figure 4(e) depicts an overlay image of chemical maps of S- and Osecondary ions, clearly presenting the isolated WS2 domains on SiO2 substrate, consistent with the optical image (Figure 4d). Interestingly, some of the



non-uniform region inside the WS2 domain can also be observed by ToF-SIMS mapping, which vindicates the subatomic surface sensitivity of this technique. Likewise, we revealed the isolated single-crystal domains of MoS2 on SiO2/Si substrate by mapping the S- secondary ions, as shown in Figure S6. Further, we also investigated the layer uniformity of WS2 stacks using ToF-SIMS 3D analysis. Figure 4(f) depicts an S- map accumulated from depth profiling of WS2 flakes shown in the inset optical image. The multilayers of WS2 stacks can easily be distinguished on the basis of S- signal intensity, as marked in the Figure 4(f). Indeed comprehensive analysis enable us to differentiate between one, two or more individual adlayers of WS2 based on the gradual increase in the signal intensity, which is also consistent with the optical image shown in the inset of Figure 4(f). We verified our assumptions about layer number of WS2 by Raman spectroscopy shown in Figure S7. Therefore, we suggest our presented approach is applicable as a universal methodology to analyze a variety of 2D materials. Lateral and Vertical Heterostructures of 2D Materials. Owing to its ultra-high chemical selectivity, ToF-SIMS emerges as a powerful tool to study the interfaces and buried layers of atomic heterostructures. Figure 5 (a) shows an optical image of CVD-grown in-plane heterostructure of monolayer graphene and h-BN, after one month of

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aging under ambient atmosphere. The dark contrast of graphene region indicates that the underneath Cu surface was oxidized in ambient environment,46 whereas the surface covered with h-BN film was protected from the oxidation hence showing a lighter contrast.47 Generally, as-grown heterostructure on Cu substrate is required to transfer onto another substrate for further investigating the quality of such heterostructures. We also transferred graphene/hBN heterostructure on SiO2/Si substrate and the Raman spectra were acquired to validate the existence of graphene domains and h-BN film in grown heterostructure, as shown in Figure S8. In contrast, ToF-SIMS enables us to directly reveal the as-grown graphene/h-BN heterostructures on the Cu substrate. Figure 5 (b) illustrates an overlay SIMS image of surface high lateral resolution chemical maps of C2- and BN- secondary ions representing graphene and h-BN regions, respectively. As for graphene grown on Cu, the sample was thermally annealed at 450 °C for one hour in order to remove the interference of adsorbed organic compounds. The distribution of C2- and BN- ions clearly elucidates the isolated graphene domains stitched through a sharp interface with continuous h-BN layer, which also corresponds well with the optical image shown in Figure 5(a). The right panels (i,ii) display the individual maps of C2- and BN- ions, presenting the sharp edges of graphene and h-BN, respectively.

Figure 5. CVD grown lateral heterostructure of graphene and h-BN. (a) optical image of as-grown graphene/h-BN lateral heterostructure on Cu substrate, obtained after aging the sample for one month in ambient atmosphere. (b) High-lateral resolution SIMS image of overlayer maps of C2- and BN-, representing graphene and h-BN, respectively. Panels showing the individual chemical maps of (i) C2- and (ii) BN- secondary ions. Panel (iii) is a CuO2- map accumulated from all cycles of depth profiles, revealing localized oxidation of Cu underneath the graphene domain after one month of aging. (c) Area line scan intensity profile of C2- and BN- acquired from the marked (yellow dotted lines) area shown in (b). The sharp decline in the intensity of BN- or C2- reflect the interface between h-BN and graphene, respectively.


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The secondary ions distribution can also be examined by plotting an area line scan based on the signal intensity. Figure 5(c) plots signals intensity of C2- and BN- acquired from the area line scan (yellow dotted lines) marked in Figure 5(b). The sharp decline in the intensity of BN- at the onset of graphene domain (vice versa) identified as the interface of lateral heterostructure between h-BN and graphene. Hence, the surface selectivity of ToFSIMS enable us to reveal the sharp interfaces between lateral graphene/h-BN heterostructure directly on the growth substrate, which can stimulate the progress about CVD growth of such atomic heterostructures with significant implications for manufacturing of integrated atomically thin electronic circuits.48 In order to verify whether Cu is more oxidized under graphene in comparison with h-BN, a depth profiling was carried out on the same area. As shown in the panel (iii) of Figure 5(b), the CuO2- map accumulated from the depth profile discloses a very much similar distribution to that of C2-, which specifies that the oxide layer predominantly present underneath the graphene domains rather than h-BN film. These ToF-SIMS

depth profile analyses also indicated interesting feature of h-BN film as an oxidation protection layer. Besides lateral heterostructures, ToF-SIMS was also applied to unveil the buried atomic layers of vertical or van der Waals heterostructures. Figure 6(a) illustrates the false-color optical image revealing a vertical heterostructure of CVD-grown monolayers of single-crystal graphene and MoS2 domains, fabricated by stacking the sequential layers onto a SiO2/Si substrate by wet transfer method.49 Although the MoS2 flake (marked with green circle) is buried underneath the graphene domain, yet it is visible in the optical microscope due to the high transparency of single layer graphene. The Raman spectra shown in Figure 6(b) were acquired from the marked regions of graphene and MoS2 domains as indicated in the optical image. The Raman spectrum for graphene (orange line) exhibits a 2D/G band ratio of >2, thus reflecting a single layer of graphene. Whereas the spectrum for MoS2 (blue curve) shows resonance peaks; out-of-plane A1g mode at 403 cm-1 and the in-plane E12g mode at 384 cm-1, which implies a monolayer of MoS2 domain.50

Figure 6. Vertical heterostructure of CVD grown monolayer graphene and MoS 2 domains. (a) False-color optical image of stacked monolayer graphene/MoS2 vertical heterostructure onto SiO2/Si substrate. The MoS2 flakes are partially covered by graphene overlayer. (b) Raman spectrum acquired from the marked spots of graphene (orange) and MoS2 (blue) reflecting monolayers of each domain. (c) PL spectra of bare MoS 2 (blue) and graphene-covered MoS2 (green) demonstrating graphene/MoS2 van der Waals heterostructure. (d) SIMS surface image obtained by using overlayers of C2-, S- and SiO2- secondary ions revealing the lateral chemical map of the heterostructure, corresponds to the optical image (a). S- map accumulated from depth profiling after (e) 20 seconds (f) 50 seconds of Cs+ sputtering, unveiling the MoS2 domain by removal of atop graphene layer. The scale bar used in a, d-f images is 100 µm.



Figure 6(c) displays the photoluminescence (PL) spectra acquired from a bare MoS2 flake (blue) and graphene-covered MoS2 flake (green). The PL intensity of graphene-covered MoS2 is significantly quenched as compared to that of a bare MoS2, which is attributed to the interaction between two atomic layers and electron quenching ability of the atop graphene layer.51 However, there is a lack of direct evidence to determine the stacking order of graphene and MoS2 layers. Figure 6(d) depicts the ToF-SIMS surface image attained by overlaying the chemical maps of C2-, S- and SiO2- secondary ions, which denote graphene, MoS2 and SiO2 substrate, respectively. Again the sample had been thermally annealed at 450 °C for one hour prior to ToF-SIMS analysis to remove the interference of adsorbed organic contamination with graphene signal. The ToF-SIMS image clearly corresponds well to the optical image shown in Figure 6(a). Interestingly, the graphene-covered MoS2 flake is concealed completely in the surface map demonstrating the stacking configuration i.e, graphene atop of MoS2. The intrinsic capacity of ToF-SIMS allows us to chemically separate the atomic layers in lateral as well as vertical plane. Further, the controlled removal of graphene overlayer by slow sputtering rate can expose the underlying MoS2 layer and SiO2 substrate. Figure 6(e) shows the accumulated image of S- after 20 s of Cs+ sputtering for removal of graphene layer. The triangular marked region was designated as the graphene-covered MoS2 flake and focused for further investigation. After 20 s of sputtering, the S- signals appeared to be very weak indicating that the MoS2 is still buried underneath the graphene. The sequential Cs+ sputtering continued the graphene sublayer removal until it reaches the graphene/MoS2 interface and MoS2 surface is exposed (the serial depth profile images are shown in Figure S9). Figure 6(f) reveals that after 50 seconds of Cs+ sputtering, the MoS2 layer was unveiled after the removal of atop graphene layer as recognized by strong S- signals in the designated region. Likewise, the progressive elimination of graphene layer can also be monitored by probing C2ions, as shown in Figure S10. Consequently, prolonging Cs+ sputtering resulted in a complete removal of graphene as well as MoS2 atomic layers leading to the exposure of the SiO2 surface. It is worth mentioning here that the main focus in this study is to probe the interface between the CVDdriven atomic layers of graphene and MoS2, however the contaminations caused by the wet transfer can also be studied in-detail, as demonstrated in previous reports.30,35 We have also demonstrated revealing

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more complex MoS2/graphene/h-BN vertical heterostructure by 3D ToF-SIMS analysis, as shown in Figure S11. Moreover, a recent report has demonstrated utilization of ToF-SIMS to deduce the stacking order of SMoSe Janus structure.52 Hence, ToF-SIMS high lateral resolution chemical mapping along with depth profiling provide an efficient platform to obtain detailed chemical analysis of the stacked 2D-atomic layers, which are critical towards optimizing their CVD synthesis to the device fabrication. However, it should be pointed out that this technique bears a drawback compared to conventional techniques: the depth profiling analysis is destructive. This may limit its applications to final devices.

CONCLUSIONS Through a series of purpose-built CVD-grown 2D materials and their heterostructures, we have systematically explored the potential of ToF-SIMS as a tool to gauge the layer uniformity and to reveal individual atomic layers in z-direction. Our results showed that the layer uniformity could be easily verified using ToF-SIMS high spatial resolution imaging owing to its extreme surface sensitivity (< 1nm) and chemical specificity. This ToF-SIMS imaging capability has been successfully applied not only to pure 2D materials such as graphene, h-BN and WS2 single crystal domains but also to the CVDdriven graphene/h-BN in-plane heterostructure. As graphene signal can be affected by the adsorbed organic contaminations, graphene-containing samples had to be subjected to an in-situ thermal annealing prior to ToF-SIMS imaging. Furthermore, we have developed a new approach based on ToFSIMS 3D analysis to unveil the atomic layer structure of 2D materials in z-direction. Given the limited depth resolution of ToF-SIMS, our approach proposed to accumulate the signal specific to 2D materials along the sputtering depth and found that this accumulated signal was linearly proportional to the number of atomic layers in z-direction. Consequently, this signal provided a chemical intensity contrast, similar to optical contrast that can be directly utilized to resolve the number of adlayers. In the case of CVD-grown MLG, for example, up to six adlayers were resolved by this method. However, contrary to optical contrast, the chemical intensity contrast does not depend on the substrate. This approach has been proven to be versatile as it has been successfully applied to various 2D materials such as graphene, h-BN and WS2. Moreover, it could unveil the multilayer structures that were otherwise invisible under optical microscope. It has also been


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used to reveal the buried interfaces of graphene/MoS2 van der Waals heterostructure. The main advantage of our approach is that the measurements are independent of substrate used and consequently it can be directly applied to grown substrate. This will avoid the time-consuming transfer process and the potential damage that may be brought in. In summary, our work presented a universal methodology to evaluate the quality of CVD grown 2D crystals and their heterostructures, which can contribute towards steering the progress of 2D materials research on the fast track.

EXPERIMENTAL SECTION CVD Growth of Single-Crystal and Multilayer Graphene. We grew graphene using our previously reported method.37 Briefly, a 25 µm thick Cu foil (Alfa Aesar, 99.8%) stacked on a Ni substrate and loaded inside the CVD furnace.33,53 The Cu foil was preoxidized at 200 °C for 5 minutes followed by ramping up the temperature to 1040 °C under the flow of 250 sccm of Ar. The Cu foil annealed at growth temperature by an Ar/H2 (10:1) mixture. The graphene growth commenced by introducing CH4 gas (5% to the total flow, 500 ppm diluted) into the system for an hour to obtain single-crystals of graphene. For multilayer graphene, the supporting substrate for Cu is replaced with quartz instead of Ni. Finally, the system cooled down rapidly to the room temperature under the flow of Ar/H2. CVD Growth of h-BN and Graphene/h-BN Heterostructure. A 25 µm thick Cu foil (Alfa Aesar, 99.8%) was cut into piece of 2 cm x 4 cm and chemically polished in acetic acid bath followed by rinsing with water. Thereafter, the Cu/quartz substrate put in the CVD furnace. The furnace ramped up to the growth temperature of 1040 °C under the stream of Ar/H2 (30:1) mixture following with annealing for 30 minutes. Thereafter h-BN precursor is introduced into the reaction chamber by sublimating the ammonia borane (AB) powder. The growth sustained for 20 minutes to attain singlecrystal of h-BN domains. Finally, the growth was terminated by fast cooling under flow of Ar. For graphene/h-BN lateral heterostructure, after obtaining the single-crystal domains of graphene the CH4 supply is terminated, and simultaneously the sublimation of ammonia borane (AB) powder started at 120 °C. The growth sustained for 30 minutes to ensure the lateral stitching of h-BN continuous sheet with graphene domains. CVD Growth of TMDs. Synthesis of WS2: Firstly, we prepared tungstic acid solution by dissolving 1.8 g Na2WO4·2H2O in 100 mL water. Then, 3 mol/L HCl

solution was added dropwise with stirring until pH ≈ 1. After standing 24 h, the precipitate was dissolved into a concentrated solution of oxalic acid (1.4 g in 20 mL of water) by gently warming. A thin film of WO3·xH2O solution was dispersed on the Si/SiO2 substrate. For CVD growth, the Si/SiO2 substrate with the WO3 film and S powders (0.1g, 99.5%, Alfa) were placed at center and upstream of the furnace, respectively. The furnace was heated to 800 ºC within 30 minutes and kept for 30 minutes. Ar with the flow rate of 50 sccm was introduced into the quartz tube. After growth, the furnace was naturally cooled to room temperature. Synthesis of MoS2: The SiO2/Si substrate was firstly rinsed with acetone and isopropyl alcohol solution to remove the surface contaminants. The promoter, which was prepared by dissolving sodium cholate (SC) hydrate (Aldrich Sigma) into deionized (DI) water (1 wt.%), was spin-coated onto the clean substrate. Typically, 20 mg Molybdenum Trioxide powder (MoO3, Aldrich Sigma, 99.99 %) and 40 mg Sulfur powder (S, Aldrich Sigma, 99 %) were used as the solid precursor for growth. The substrate and a quartz boat with MoO3 powder was put in the center of the furnace which was heated from room temperature to 780 °C at a rate of 40 °C/min and S powder contained by a quartz boat heated to 200 °C in upstream. The dwell time for MoS2 growth at 780 °C was 10-15 min, and then the furnace was fast cooled back to room temperature. Transfer of CVD Grown Films. A typical transfer process for CVD-grown graphene and h-BN on Cu comprises of the following steps: (i) spin coating the as-grown films on Cu with 300 nm of PMMA as a mechanical support for transfer. (ii) The PMMA/graphene or h-BN stack can be detached by etching Cu into a 10% FeCl3 aqueous solution for 5 hours followed by rinsing in water bath (iii) The PMMA/graphene or h-BN stack finally scooped up on a SiO2/Si substrate, and subsequently PMMA is removed by acetone vapors. The vertical heterostructure of graphene/MoS2 was fabricated by sequential transfer method. Firstly, as grown MoS2 flakes on the SiO2/Si was spin coated with 300 nm thick layer of PMMA and baked. The MoS2/PMMA stack was separated from SiO2/Si by interfacial etching in 1M KOH solution followed by rinsing in water. Later the stack was transferred to clean SiO2/Si substrate with marker. The PMMA was removed by acetone vapors. Finally, the graphene transferred atop of MoS2 flake following the aforementioned transfer method for graphene or hBN.



Raman Spectroscopy. An instrument (InVia micro-Raman system, Renishaw) was used, equipped with Ar+ ion laser with wavelength 514.5 nm. The Raman spectra were taken by focusing the laser beam with a 50x objective lens. ToF-SIMS Analysis. ToF-SIMS measurements were carried out with a TOF-SIMS V (ION-TOF GmbH, Münster, Germany) instrument which is equipped with a bismuth liquid-metal ion source for analysis and a Cs+ ion beam for sputtering. In this work, two types of ToF-SIMS analyses were performed: (1) high spatial resolution surface chemical mapping and (2) ToF-SIMS 3D analysis. For surface chemical mapping, a 25 keV Bi3+ cluster beam was scanned over a specified raster area (varies according to the images shown in the main text) with a pixel resolution of 512x512. The primary ion beam was optimized for best spatial resolution (~ 200 nm) by scarifying mass resolution. For ToF-SIMS 3D analysis, a 3 keV-Cs+ beam scanning over typically an area of 1 x 1 cm2 with a current of 30 nA was used to sputter through the 2D materials. High spatial resolution images were taken at the center of Cs sputtering with 256 x 256 raster pixel sizes. The imaged areas varied typically from 100 x 100 m2 to 300 x 300 m2 (see main text). The whole depth profiling process was run in the interlaced mode, consisting of cycles of short pulses of Bi3+ followed by a long period of Cs+ sputtering. Data were stored in the raw data stream mode so that the ion images or intensity profiles can be reconstructed after analysis. Charging compensation was realized using a low energy flood gun. The vacuum during the analysis was about 1.5 x 10-9 mbar. A heating/cooling sample holder was used to perform in-situ thermal treatment for graphene-containing samples. We have systematically studied the in-situ annealing conditions to evaluate the efficiency of the removal of organic contaminants from the sample surfaces, such as annealing temperature range and annealing time. For graphene on Cu, thermal annealing at 450 °C for 30 min is sufficient for removing all the organic impurities while for transferred samples (graphene/SiO2-Si), it needs one hour. In order to keep the same thermal annealing conditions, all graphene-containing samples were annealed at 450 °C for one hour to make sure that all organic residues were gone.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website

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Supporting Information I: Figure S1-S11 showing ToFSIMS analysis of PMMA removal from graphene surface, analysis for multilayer graphene and Raman spectra, ToF SIMS images for h-BN, MoS2 and their vertical heterostructures. Supporting Information II: theoretical calculations for layer number determination of multilayer graphene.

AUTHOR INFORMATION Corresponding Author *(Z.L.) Email: [email protected] *(L.T.W.) Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This project is supported by the Research Grant Council of Hong Kong SAR (Project number 16204815), The Guangzhou Science & Technology Project (2016201604030023). We appreciate the support from the Center for 1D/2D Quantum Materials and the Innovation and Technology Commission (ITCCNERC14SC01 and ITS/267/15). I.H.A. appreciate financial support from Higher Education Commission (HEC) of Pakistan. Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated.

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A New Approach to Unveiling Individual Atomic Layers of 2D Materials

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