MoS2

Growth of Single-Crystalline Cadmium Iodide Nanoplates, CdI2/MoS2 (WS2, WSe2) van der Waals Heterostructures, and Patterned Arrays Ruoqi Ai,† Xun Guan,† Jia Li,† Kangkang Yao,† Peng Chen,† Zhengwei Zhang,† Xidong Duan,*,† and Xiangfeng Duan‡ †

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China ‡ Department of Chemistry and Biochemistry, California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Two-dimensional layered materials (2DLMs) have attracted considerable recent interest for their layer-number-dependent physical and chemical properties, as well as potential technological opportunities. Here we report the synthesis of two-dimensional layered cadmium iodide (CdI2) nanoplates using a vapor transport and deposition approach. Optical microscopy and scanning electron microscopy studies show that the resulting CdI2 nanoplates predominantly adopt hexagonal and triangular morphologies with a lateral dimension of ∼2−10 μm. Atomic force microscopy studies show that the resulting nanoplates exhibit a thickness in the range of 5−220 nm with a relatively smooth surface. X-ray diffraction studies reveal highly crystalline CdI2 in hexagonal phase, which is also confirmed by the characteristic Raman Ag mode at 110 cm−1. High-resolution transmission electron microscopy and selected area electron diffraction reveal that the resulting CdI2 nanoplates are single crystals. Taking a step further, we show the CdI2 nanoplates were readily grown on other 2DLMs (e.g., WS2, WSe2, MoS2), forming diverse van der Waals heterostructures. Using prepatterned WS2 monolayer square arrays as the nucleation and growth templates, we also show that regular arrays of CdI2/WS2 vertical heterostructures can be prepared. The synthesis of the CdI2 nanoplates, heterostructures, and heterostructure arrays offers a valuable material system for 2D materials science and technology. KEYWORDS: 2D materials, CdI2 nanoplates, transition metal dichalcogenides, van der Waals heterostructures, patterned array he discovery of graphene1−6 has ignited intense interest in two-dimensional layered materials (2DLMs),7−9 including graphene, hexagonal boron nitride (BN), and two-dimensional transition metal dichalcogenides (2DTMDs) (e.g., MoS2,10−13 MoSe2,14 WSe2,15,16 and WS216). With a strong covalent bond in each atomic layer and weak van der Waals interactions between the layers, these 2DLMs can be readily isolated or synthesized as single- or few-atom layers and flexibly combined to create artificially stacked van der Waals heterostructures (vdWHs) with atomically sharp modulation of chemical composition and electronic structure. They can thus offer a rich playground for exploring fundamental physics and chemistry at single- or few-atom thicknesses and developing exciting technologies beyond the reach of the existing materials. Beyond graphene and BN, the studies on 2DLMs have been largely focused on chalcogenide materials, including 2DTMDs9−23 and IIIA−VIA group (e.g., GaS,24−26 GaSe,26,27 InSe,28,29) and IVA−VIA group (e.g., SnSe2,30 SnS231)

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© 2017 American Chemical Society

materials. These chalcogenide materials often exhibit layernumber-dependent electronic and optical properties and have been the focus of considerable recent interest, with many exciting devices already demonstrated. Beyond chalcogenide materials, IVA−VIIA group metal halides (e.g., PbI232) have also attracted some recent attention mostly in the context of methylammonium lead halide perovskite materials. In general, there is a large family of metal halide materials with layered crystal structures. To expand the 2DLM library and enable more diverse vdWHs, here we report the synthesis of metal halide nanoplates and their van der Waals heterostructures. In particular, we focus on cadmium iodide as an example. Cadmium iodide (CdI2) has a layered structure with a hexagonal unit cell and crystallizes in Received: March 2, 2017 Accepted: March 15, 2017 Published: March 17, 2017 3413

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Figure 1. Crystal structure of CdI2. (a) Top view of the CdI2 layered structure. Cd atoms are pale yellow, and I atoms are brown. (b) Side view of the CdI2 layered structure.

Figure 2. (a) Typical OM image of CdI2 nanoplates grown on a SiO2/Si substrate showing that the nanoplates adopt hexagonal and triangular geometry, with variable color corresponding to different thickness. Scale bar: 10 μm. The inset image is a typical SEM image of the CdI2 nanoplates. Scale bar: 2 μm. (b−d) Optical microscope images and (e−g) the corresponding atomic force microscope images of individual CdI2 nanoplates with different thicknesses (8, 25, 30, 50 nm). Scale bars in (b)−(g): 2 μm.

space group P63mc, which is similar to many halides and transition metal dichalcogenides. The side view of the CdI2 crystal structure displays each sheet of Cd atoms sandwiched between two sheets of I atoms through strong ionic bonding,33 and the atomic layers are combined through weak van der Waals forces (Figure 1a,b). Crystalline CdI2 thin films have been previously grown on glass substrates via chemical bath deposition33 or a thermal evaporation approach.34 However, the controlled synthesis of single-crystal and ultrathin CdI2 nanoplates has not been reported. Herein we report the synthesis of single-crystalline CdI2 nanoplates on SiO2/Si substrates, CdI2/TMD van der Waals heterostructures, and patterned arrays of CdI2/TMD van der Waals heterostructures via a physical vapor transport and deposition method. Optical microscopy (OM) and scanning electron microscopy (SEM) studies show hexagonal triangular nanoplates can be readily grown with lateral dimensions of ∼2−10 μm. Atomic force microscopy (AFM) studies show the nanoplates have a thickness in the range of 5−220 nm. X-ray diffraction (XRD) studies show the highly crystalline nature of the resulting nanoplates, with CdI2 showing the hexagonal phase. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies reveal that the CdI2 nanoplates are single crystals. Raman characterizations reveal the characteristic out-of-plane Ag vibration mode at ∼110 cm−1.35 Taking a step further, we show the CdI2 nanoplates can be grown on other 2DLMs (e.g., WS 2 , WSe 2 , MoS 2 ) to form diverse van der Waals heterojunctions.36−41 Using patterned WS2 monolayer square arrays as the nucleation and growth templates, we also show that regular arrays of CdI2/WS2 vertical heterostructures can be readily grown. The synthesis of the CdI2 nanoplates and

heterostructures expands the library of 2D materials and 2D heterostructures and may find broad opportunities in diverse areas.

RESULTS AND DISCUSSION The CdI2 nanoplates were synthesized using a home-built physical vapor transport and deposition system (Supplementary Figure S1). The precursor powder (CdI2, ∼0.1 g) was placed in a ceramic boat at the center of a quartz tube furnace (1 in.). A piece of Si/SiO2 (300 nm) substrate was placed at the downstream end of the tube furnace. Before heating, Ar flow was introduced into the system and pumped and flushed for several minutes to ensure complete removal of O2 and H2O. The furnace was then ramped up to 320 °C at a rate of 20 °C/ min, and Ar was used as the carrier gas at a flow rate of 125 sccm to transport the CdI2 vapor downstream to nucleate and grow on the substrate. After 40 min of growth, the furnace was naturally cooled to room temperature. A typical OM image shows the resulting CdI2 nanoplates mostly exhibit a hexagonal or triangular shape, with the lateral dimension ranging from 2 to 10 μm (Figure 2a), which is also confirmed by SEM studies (Figure 2a, inset). The nanoplates show highly distinct colors under a bright-field optical microscope, which can be attributed to different optical interference resulting from the variable thickness. The thickness of the resulting nanoplates was further determined with AFM. AFM studies show that the thickness of the nanoplates varies between 5 nm (Figure S2a) and 220 nm (Figure S2r). Figure 2b−d further show the OM images of the obtained CdI2 nanoplates with different geometries and thicknesses. We have achieved 5, 8, 10, 15, 25, 30, 50, 75, 160, and 220 nm 3414

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nm can be indexed to the (110) and (100) planes of the CdI2 hexagonal structure (Figure 3d). The SAED pattern of the nanoplate demonstrates a single set of diffraction spots in a hexagonal symmetry, suggesting single-crystalline structure (Figure 3d, inset). With a relatively low growth temperature, the CdI2 nanoplates can be readily grown on two-dimensional transition metal dichalcogenides (such as MoS2, WSe2, WS2) to form van der Waals heterostructures. To this end, three 2D-TMDs (MoS2, WS2, WSe2) nanosheets were first synthesized on a SiO2/Si substrate using a previously reported approach42−45 and then used as the substrate for the subsequent growth of CdI2 nanoplates (see Methods for more details). OM images (Figure 4a, Figure 5a,d, and Figure S3) reveal that CdI2 nanoplates selectively nucleate and grow on the top surface of MoS2 (or WSe2, WS2) nanoplates, forming the CdI2/MoS2 (CdI2/WSe2, CdI2/WS2) van der Waals heterostructures. For CdI2/MoS2 vertical heterostructures, the OM images show two concentric triangles. Both CdI2 (110 cm−1) and MoS2 (384, 400 cm−1) Raman features were observed in the center area (marked with “2” in Figure 4b), while only the CdI2 Raman signal was observed in the edge area (marked with “1” in Figure 4b) (Figure 4c).35 We can deduce the inner triangle is the underlying MoS2 nanosheet, and the bigger triangle is the CdI2 nanoplate grown on the MoS2 nanoplate (Figure 4a,b). Since the CdI2 nanoplate is almost transparent, the MoS2 substrate underneath the CdI2 nanoplate is visible under optical microscope. The Raman mapping image at 110 cm−1 clearly shows the CdI2 signal across the entire triangle (Figure 4d), while the mapping image at 384 cm−1 demonstrates that the MoS2 signal is present only in the center triangle (Figure 4e). Both OM and Raman mapping studies indicate that there is obvious overlap between the MoS2 nanosheet and CdI2 nanoplate, confirming the formation of CdI2/MoS2 vertical heterostructures (Figure 4d−f). As for the CdI2/WSe2 vertical heterostructure, optical microscopy and Raman characterizations show similar results to those of the CdI2/MoS2 heterostructures. OM images show almost the same growth characteristics as CdI2/MoS2. The inner hexagon is the underlying WSe2 nanosheet, and the bigger one is the CdI2 nanoplate grown on top of the WSe2 nanosheet (Figure 5a). The Raman spectrum from the center region features both the WSe2 Raman peak at 251 cm−1 and the CdI2 Raman peak at 110 cm−1 (marked with “2” in Figure 5b), while the Raman spectrum from the outer hexagon area shows only a single peak at 110 cm−1 (mark with “1” in Figure 5b), in agreement of the Raman signal of CdI2. The Raman mapping studies can further reveal the formation of the CdI2/WSe2, with the central part consisting of a smaller hexagon domain of WSe2 and the entire big hexagon showing a CdI2 signal (Figure 5c). For CdI2/WS2 vertical heterostructures (Figure 5d), Raman spectra show that both the CdI2 (110 cm−1) and the WS2 (348 and 417 cm−1) Raman peaks are observed in the overlapping area marked with “2” (Figure 5e), indicating the formation of CdI2/WS2 vertical heterostructures. The Raman mapping studies can further reveal the formation of the CdI2/WS2 vertical heterostructures (Figure 5f). These studies unambiguously confirm the generality of our approach for the growth of the CdI2/MX2 vertical heterojunctions. It is particularly interesting to note that the triangular CdI2 nanoplates almost exclusively nucleate and grow on the MoS2 templates and essentially all the triangular domains of MoS2 are covered by CdI2 nanoplates (Figure 4a), suggesting a highly

nanoplates, as confirmed by the AFM studies (Figure S2a−r). The color of the CdI2 nanoplates under the optical microscope evolves from blue to red as the thickness of the CdI2 nanoplates increases. XRD can be used to determine the crystalline phases of the resulting CdI2 nanoplates grown on the Si/SiO2 substrate. The XRD pattern shows strong diffraction peaks that can be indexed to the hexagonal P63mc (186) space group with lattice parameters of a = b = 4.248 Å, c = 13.726 Å (Figure 3a).

Figure 3. (a) XRD patterns of the CdI2 nanoplates grown on a Si/ SiO2 substrate. (b) Raman spectra of CdI2 nanoplates. The Raman experiment was performed in a confocal Raman microscope using a 532 nm excitation laser; the inset images show a typical OM image of a CdI2 nanoplate and the corresponding Raman intensity mapping at 110 cm−1. Scale bar: 2 μm. (c) Low-magnification TEM image of a CdI2 nanoplate. Scale bar: 50 nm. (e) High-resolution TEM image of a CdI2 nanoplate. Scale bar: 2 nm. The inset image is the SAED pattern of the nanoplate. Scale bar: 2 1/nm.

Four primary XRD peaks can all be indexed to {001} family planes ((002), (004), (006), (008)) of hexagonal phase CdI2, suggesting the nanoplates are all well aligned along the [001] direction on the growth substrate. The Raman spectrum shows a prominent peak around 110 cm−1 (Figure 3b), which is in agreement with the characteristic peak of the Ag mode of bulk CdI2.35 Due to the limit of our Raman spectroscopy, we were not able to obtain the Eg mode expected at 45.1 cm−1.35 The upper inset of Figure 3b shows a typical optical microscopy image of a CdI2 nanoplate used for the Raman mapping test. The spatially resolved mapping of the Raman signal shows highly uniform contrast throughout the entire nanoplate (Figure 3b, lower inset), suggesting a highly uniform crystal structure. We have next employed transmission electron microscopy (TEM) to investigate the microstructure of the CdI2 nanoplates in detail (Figure 3c,d). A top-view low-magnification TEM image of a nanoplate confirms the well-faceted triangular geometry (Figure 3c). The HRTEM image shows clearly resolved lattice fringe, further confirming the single-crystalline structure (Figure 3d). The lattice spacing of 0.212 and 0.367 3415

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Figure 4. Growth of CdI2/MoS2 vertical heterostructures. (a) OM image of the CdI2/MoS2 heterostructures on the Si/SiO2 substrate. Scale bar: 10 μm. (b) OM image of a CdI2/MoS2 heterostructure used for Raman characterization. Scale bar: 5 μm. (c) Raman spectra taken from the two areas marked in (b). (d, e) Raman intensity mappings at 110 and 384 cm−1, respectively. (f) Composite Raman intensity mapping at 110 and 384 cm−1. Scale bars in (d)−(f): 5 μm.

Figure 5. Growth of CdI2/WSe2 (WS2) vertical heterostructures. (a) OM image of a CdI2/WSe2 heterostructure. (b) Raman spectra taken from two areas marked in (a). (c) Raman intensity mapping of the CdI2/WSe2 vertical heterostructure. Scale bars in (a) and (c) are 5 μm. (d) OM image of a CdI2/WS2 heterostructure. (e) Raman spectra taken from two areas marked in (a). (f) Composite Raman intensity mapping of the CdI2/WS2 vertical heterostructure. Scale bars are 2 μm in (d) and (f).

In the case of WS2/CdI2 heterostructures (Figure S3c,d), since the underlying WS2 substrates are considerably larger than MoS2 or WSe2 domains (∼100 μm vs 10 μm), the CdI2 nanoplates do not fully covert the underlying WS2 substrates within our growth duration. Nonetheless, it is apparent that CdI2 nanoplates selectively nucleate and grow on the WS2 substrate and particularly at the edge of WS2 domains. By studying the relative crystal orientation between 2D-TMDs and the CdI2 nanoplates, we find that MoS2/CdI2 vertical heterostructures and WSe2/CdI2 vertical heterostructures show a parallel orientation between the underlying TMD and the top CdI2 (see Figure 4a, Figure S3a−c), and for the CdI2/ WS2 vertical heterostructures, most of the CdI2 nanoplates show a relative angle of ∼30° to the WS2 nanosheets, with few CdI2 nanoplates being parallel to the WS2 substrate (see Figure

selective nucleation and growth process. Similarly, the WSe2/ CdI2 heterostructures also showed highly selective and exclusive growth of CdI2 on WSe2 (Figure S3a,b). Interestingly, the resulting CdI2 nanoplates typically adopt a hexagonal geometry, following the guide of the underlying WSe2 template. These studies suggest that the underlying 2D substrate can not only guide the nucleation location but also guide the growth and dictate the final morphology of the resulting CdI2 nanoplates. We believe this may be attributed to the much faster growth rate of CdI2 on TMDs than on the silicon oxide substrate. Because of the much faster growth rate on TMD, CdI2 could quickly grow on TMD to adopt the geometry of the underlying TMD before it starts to overgrow beyond the TMD template at a much slower rate. 3416

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whisper gallery mode optical cavity to effectively confine the photons within the nanoplates, limiting photon escape from the in-plane area and allowing photons to preferentially leak out from the edge.

S3e,f). The observations show that the CdI2 nanoplates grow on the 2D-TMD with a certain preferred orientation. The 2DTMDs’ crystalline nature may guide the growth of CdI2 nanoplates on its top surface. The exact underlying mechanism for this distinct crystal orientation will be an interesting topic for future investigations. The above studies clearly show that the 2D-TMD nanosheets can function as excellent templates to guide the nucleation and growth of CdI2 nanoplates. Taking advantage of the selective nucleation and growth, it is possible to grow a regular array of CdI2 nanoplates if we create a patterned array of 2D-TMD templates. To this end, we have created a square array of WS2 nanosheets, by applying electron beam lithography and O2 plasma etching (see Methods for more details) on pregrown WS2. The size of the WS2 nanosheet squares is 3 μm × 3 μm, and the distance between each WS2 square is 3 μm (Figure 6a).

CONCLUSION In summary, we have shown that CdI2 nanoplates with variable thicknesses (5−220 nm) can be readily grown on SiO2/Si substrates using a one-step physical vapor deposition approach. The obtained CdI2 nanoplates exhibit a triangle and hexagonal shape with lateral dimensions up to ∼10 μm. HRTEM and SAED studies demonstrate that the achieved nanoplates are single crystals. The Raman spectra of the resulting CdI2 nanoplates are in agreement with the characteristic peaks of bulk CdI2. XRD studies show the highly crystalline nature of the resulting crystals preferentially oriented along the [001] directions. We have also shown that CdI2/MoS2, CdI2/WSe2, and CdI2/WS2 van der Waals heterostructures can be readily prepared by two-step physical vapor deposition. We further show that the CdI2/WS2 vertical heterostructure arrays can be readily prepared by using prepatterned WS2 arrays as the growth templates. The synthesis of single-crystalline CdI2 nanoplates, CdI2/MoS2 (WSe2, WS2) van der Waals heterostructures, and heterostructure arrays offers an interesting material system for exploring 2D halide materials and the related van der Waals heterojunctions, and these may find important applications in diverse areas. For example, the large band gap CdI2 nanoplate can function as a whispering gallery mode optical cavity for photoemission in the TMD materials, which may be imporant for enabling 2D-TMD lasers. METHODS Preparation of CdI2 Nanoplates. Two-dimensional CdI2 nanoplates were synthesized on SiO2/Si substrates using a home-built CVD system (Figure S1). One ceramic boat loaded with CdI2 powder (0.1 g) (99.9%, Alfa) was placed at the heating center of a quartz tube furnace. A piece of SiO2/Si substrate was placed at the downstream end of the tube furnace. The system was purged with ultra-high-purity argon (Ar) gas (∼99.999%), then ramped to 320 °C and maintained for 40 min for the growth of CdI2 nanoplates, under ambient pressure and a constant flow of 125 sccm Ar as the carrier gas. The growth was then terminated by shutting off the power of the furnace, and the sample was naturally cooled to ambient temperature under a 125 sccm Ar flow. The CdI2 nanoplates with different thicknesses can be found on different positions of the SiO2/Si substrate. By varying the growth temperature (315−325 °C) and Ar flow rate (100−150 sccm), we can also obtain CdI2 nanoplates with variable thicknesses. Preparation of WS2, WSe2, and MoS2 Nanosheets. WS2 powder (99.9%, Sigma-Aldrich) was used as the precursor and placed at the center of the tube furnace, and a piece of SiO2/Si substrate was placed at the downstream end of the furnace. The system was purged with high-purity Ar gas (∼99.999%) and then ramped to 1150 °C and maintained for 5 min for the growth of WS2 nanosheets, under ambient pressure and a constant flow of 50 sccm Ar as the carrier gas. The growth was terminated by shutting off the power of the furnace, and the sample was naturally cooled to ambient temperature under a 50 sccm Ar flow. To grow WSe2 nanosheets, WSe2 powder (99.9%, Sigma-Aldrich) was used as the source material and placed at the center of the furnace, and a piece of SiO2/Si substrate was placed at the downstream end of the furnace. Then the system was purged with high-purity Ar gas (∼99.999%) and then ramped to 1060 °C and maintained for 15 min for the growth of WSe2 nanosheets, under ambient pressure and a constant flow of 80 sccm Ar as the carrier gas. The growth was then terminated by shutting off the power of the furnace, and the sample was naturally cooled to ambient temperature under a 80 sccm Ar flow.

Figure 6. Growth of CdI2/WS2 vertical heterostructure arrays. (a) Optical microscope image of a lithographically patterned square array of WS2 nanosheets. (b) Optical microscope image of a CdI2/ WS2 vertical heterostructure array grown using a WS2 array as the growth template. (c, d) Raman intensity mappings of a CdI2/WS2 vertical heterostructure array at 110 cm−1 (characteristic peak of CdI2 nanoplates) and 348 cm−1 (characteristic peak of WS2 nanosheets), respectively. Scale bars are all 5 μm.

With such a square array of WS2 as the template in the physical vapor deposition process, the CdI2 nanoplates selectively nucleate and grow on the WS2 monolayer squares, forming a regular array of CdI2/WS2 vertical heterostructures (Figure 6b). Raman mapping images clearly confirm the CdI2 nanoplates on top of the WS2 squares (Figure 6c,d). The CdI2 nanoplates typically adopt a hexagonal geometry and extend beyond the underlying square WS2 templates. It is also noted that the CdI2 hexagons typically adopt two orientations: a few (1 out of 16, highlighted in white arrows) with the edge parallel to the bottom edge of the square WS2 templates (100 direction) and most (15 out of 16) with a 30° angle with the bottom edge of the square WS2 templates, consistent with the 30° orientation offset observed in CdI2/WS2 heterostructures. It is noted that there is a bright outline around the CdI2 nanoplates (Figure 6c,d), which may be attributed to the optical cavity effect in CdI2 nanoplates. The CdI2 nanoplates can function as a 3417

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ACS Nano For the growth of MoS2 nanosheets, MoS2 powder (99.999%, Sigma-Aldrich) was placed at the center of the tube furnace, and a piece of SiO2/Si substrate was placed at the downstream end of the furnace. Then the system was purged with high-purity Ar gas (∼99.999%) and then ramped to 1200 °C and maintained for 10 min for the growth of MoS2 nanosheets, under ambient pressure and a constant flow of 120 sccm Ar as the carrier gas. The growth was terminated by shutting off the power of the furnace, and the sample was naturally cooled to ambient temperature under a 120 sccm Ar flow. Preparation of CdI2/MoS2, CdI2/WS2, and CdI2/WSe2 Vertical Heterostructures and CdI2/WS2 van der Waals Heterostructure Arrays. The as-grown MoS2 (WS2, WSe2) nanosheets were used as the growth substrate for the van der Waals epitaxial growth of CdI2 nanoplates to obtain CdI2/MoS2 (WS2, WSe2) van der Waals heterostructures, using the same growth protocols described above. To prepare CdI2/WS2 van der Waals vertical heterostructure arrays, the as-grown WS2 nanosheets were first patterned into a square array using electron beam lithography and O2 plasma etching and then used as the growth substrate for the selective nucleation and growth of CdI2 nanoplates, following the same growth protocols described above. Sample Characterization. The as-synthesized CdI2 nanoplates, CdI2/MoS2, CdI2/WS2, and CdI2/WSe2 vertical heterostructures, and CdI2/WS2 vertical heterostructure arrays were characterized by an optical microscope (DP27, Olympus), an atomic force microscope (Bioscope system, Bruker), a confocal Raman system (inVia-reflex, Renishaw) with a 532 laser, a transmission electron microscope (JEM2100F, JEOL), a scanning electron microscope (∑IGMA HD, Zeiss), and X-ray diffraction (XRD, D8-Advance, Bruker).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01507. Schematic setup for the growth, OM and AFM images of the CdI2 nanoplates, additional OM images of CdI2/ WSe2 and CdI2/WS2 van der Waals heterostructures and heterostructure array (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiangfeng Duan: 0000-0002-4321-6288 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the support from National Natural Science Foundation of China (No. 61528403). X.D. acknowledges the support by NSF DMR1508144. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Katsnelson, M.; Grigorieva, I.; Dubonos, S.; Firsov, A. Two-Dimensional Gas of Massless Dirac Fermions In Graphene. Nature 2005, 438, 197−200. (3) Schedin, F.; Geim, A.; Morozov, S.; Hill, E.; Blake, P.; Katsnelson, M.; Novoselov, K. Detection of Individual Gas Molecules Adsorbed On Graphene. Nat. Mater. 2007, 6, 652−655. (4) Geim, A. K. Graphene: Status And Prospects. Science 2009, 324, 1530−1534. 3418

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