Sub-millimeter-Scale Growth of One-Unit-Cell-Thick Ferrimagnetic

Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Sub-millimeter-Scale Growth of One-Unit-Cell-Thick Ferrimagnetic Cr2S3 Nanosheets Junwei Chu,†,‡,# Yu Zhang,†,# Yao Wen,† Ruixi Qiao,∥ Chunchun Wu,‡ Peng He,† Lei Yin,† Ruiqing Cheng,† Feng Wang,† Zhenxing Wang,† Jie Xiong,*,‡ Yanrong Li,‡ and Jun He*,†,§,⊥

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CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China ‡ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China § Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ∥ School of Physics, Peking University, Beijing 100871, China ⊥ School of Physics and Technology, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: Two-dimensional (2D) magnetic materials provide an ideal platform for the application in spintronic devices due to their unique spin states in nanometer scale. However, recent research on the exfoliated monolayer magnetic materials suffers from the instability in ambient atmosphere, which needs extraordinary protection. Hence the controllable synthesis of 2D magnetic materials with good quality and stability should be addressed. Here we report for the first time the van der Waals (vdW) epitaxial growth of oneunit-cell-thick air-stable ferrimagnet Cr2S3 semiconductor via a facile chemical vapor deposition method. Single crystal Cr2S3 with the domain size reaching to 200 μm is achieved. Most importantly, we observe the as grown Cr2S3 with a Néel temperature (TN) of up to 120 K and a maximum saturation magnetic momentum of up to 65 μemu. As the temperature decreases, the samples show a transition from soft magnet to hard magnet with the highest coercivity of 1000 Oe. The one-unit-cell-thick Cr2S3 devices show a p-type transfer behavior with an on/off ratio over 103. Our work highlights Cr2S3 monolayer as an ideal magnetic semiconductor for 2D spintronic devices. The vdW epitaxy of nonlayered magnets introduces a new route for realizing magnetism in 2D limit and provides more application potential in the 2D spintronics. KEYWORDS: Cr2S3, ferrimagnetic, nonlayered, van der Waals epitaxial growth

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necessary. Recently, few works about monolayer magnets including MnSe and VSe2 have been reported, which were obtained via a molecular beam epitaxy (MBE) method. But restricted by the rigorous condition, MBE method cannot be widely used.12−14 Differently, chemical vapor deposition (CVD) method has been considered as a facile and effective route for the synthesis of various 2D layered materials, such as graphene, h-BN, and a series of transition-metal dichalcogenides (TMDs), which all possess high crystal quality and demonstrate outstanding performance.15−17 Consequently, the controllable synthesis of ultrathin magnetic materials in a convenient CVD method is still a great challenge. Moreover, most of recent disquisitive magnetic materials, such as CrI3, Fe3GeTe2, and CrGeTe3, suffer from instability in ambient atmosphere, which hinders their practical applications and the research on intrinsic magnetism.5−7 Hence, exploring new kinds of magnetic materials is a grand significance. 2D

wo-dimensional (2D) materials have attracted great consideration in recent years due to their high potential in new generation electronics for their unique electronic and mechanical behaviors.1−3Recently, as motivated by the research in spintronics and valleytronics, 2D magnetic materials with especially ferromagnetism and ferrimagnetism are under the spotlight.4 These magnets have fascinating behaviors at the 2D limit, different from the magnetism in their bulk state. For example, layered dependent ferromagnetism is observed in CrI3 nanosheets, which even exhibits a record magnetoresistance ratio up to 19,000%.5,6 Further, Fe3GeTe2 flakes demonstrate room-temperature ferromagnetism by introducing an ionic gate into the system.7 Similar behaviors have also been observed in CrI3 systems.8,9 However, most of the recently reported 2D magnetic materials, such as FePS3, CrI3, Fe3GeTe2, and Cr2Ge2Te6, are obtained by an exfoliation method, which always exhibits random domain size and thickness.5−7,10,11 Actually, the exfoliation method provides little control over the thickness and size of obtained samples and goes against batch production. Hence, exploring an available method for the synthesis of 2D magnetic materials is © XXXX American Chemical Society

Received: January 27, 2019 Revised: February 18, 2019 Published: February 21, 2019 A

DOI: 10.1021/acs.nanolett.9b00386 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. The structure and morphology of Cr2S3 nanosheets. (a, b) Structural portrait for Cr2S3 structures in R3̅ space group. (a) Top view showing the edge shared CrS6 octahedra network. Inset: Perspective view of the Cr2S3 crystal with one-unit-cell thickness. (b) Side view containing CrS2 layers and intercalated Cr atoms, showing the stacking sequence of the partially filled CrS6 octahedral layer. The indexes a, b, and c indicate the positions occupied by Cr of the partially filled layer. This a-b-c stacking in c direction leads to the crystal form with R3̅ space group. (c) The schematic view for the Cr2S3 growth process, yellow: S powder, green: CrCl3 powder. (d) Typical OM image of as-grown Cr2S3 crystals on mica, showing an average size of 40 μm; scale bar: 20 μm. (e) AFM image and its corresponding height profile of a one-unit-cell-thick Cr2S3 triangle with typical thickness is ∼1.78 nm. The height image of AFM demonstrates the atomically sharp surface with no nucleation center and secondary nucleation. Scale bar: 4 μm. (f) A typical OM image of single Cr2S3 crystal with the domain size reaching up to 200 μm; inset: The corresponding AFM graph; scale bar: 5 μm.

for the vdW epitaxy of various 2D magnetic materials and provide the significant application potentials in spintronics. Result and Discussion. Figure 1a shows the crystal structure of Cr2S3 single crystal, which can be viewed as alternating stacked edge shared and independent CrS 6 octahedra layers. This material can also be relabeled as Cr1/3CrS2, which accounts for a fraction of the chromium atoms intercalated within the gaps between CrS2 layers.24,25 The quasi-layered structure provides great possibility for the epitaxy on vdW substrates like mica. CrS2 layers are face shared with the independent CrS6 octahedra layers, and two structures with different magnetic behaviors are consequently resulted from the different stacking sequences of independent layers. The rhombohedral and trigonal structure of Cr2S3 lattice correspond to a-b-c sequence in R3̅ space group and a-b-a-b sequence in P31c space group, with magnetic transition temperatures of 120 and 100 K, respectively.24 The thickness of one unit cell between the top and bottom CrS2 layer in R3̅ space group is 16.6 Å (Figure 1b). As depicted in the Methods section and Figure 1c, Cr2S3 nanosheets are grown on freshly cleaved mica substrate at 750 °C by an APCVD method using CrCl3 and S powder as precursors. By introducing hydrogen, the more reductive hydrogen sulfide can be generated and reacted with CrCl3 to eventually yield Cr2S3. Under optimized growth parameters, uniform nanosheets with an average size of around 40 μm are obtained and exhibited by the OM image in Figure 1d. A one-unit-cell thickness feature of the obtained material is apparent from the atomic force microscope (AFM) image illustrated in Figure 1e. It can be clearly observed that the obtained material owns smooth surface with thickness of 1.78 nm. Moreover, a maximum edge length of as-grown Cr2S3 crystals can be as large as 200 μm, whose thickness is still only one-unit-cell, as depicted in Figure 1f. These results confirm that we realized the epitaxial growth of Cr2S3 crystals with one-

nonlayered materials maybe good candidates due to their abundant structures and significant performance.18,19 But 2D nonlayered materials are hard to obtain by exfoliating from the bulk ones due to their strong chemical bonds. Recently, van der Waals (vdW) epitaxy techniques have fulfilled the growth of nonlayered materials on vdW substrates.20 A CdTe nanosheet with thickness down to 4.8 nm has been fulfilled on mica substrate by chemical vapor deposition (CVD) method, which forecasts the possibility to the synthesis of 2D nonlayered magnetic material via a facile CVD method.21 However, 2D nonlayered magnetic materials have been sparsely explored and prepared now, which may provide an important possibility to widen the magnetic materials systems and study the magnetism in the 2D limit. Cr2X3 (X = S, Se, Te) is a new magnetic family with nonlayered crystal structure. Cr2X3 behaves as ferrimagnetic, antiferromagnetic, and ferromagnetic with X ranging from S, Se, and Te, respectively.22,23 Cr2S3 is a ferrimagnetic semiconductor with monoclinic NiAs-type crystal structure and TN ∼ 120 K. It possesses significant value for the investigation of magnetism order and other physical properties in rhombohedral phase.24 However, the synthesis of 2D Cr2S3 crystals is still an enormous challenge. In this paper, ferrimagnetic Cr2S3 nanoflakes are synthesized and studied in depth. Utilizing vdW epitaxy techniques, nonlayered Cr2S3 with one-unit-cell thickness down to 1.78 nm is successfully synthesized via a facile ambient pressure chemical vapor deposition (APCVD) method. And single crystal Cr2S3 with a grain size reaching to 200 μm is demonstrated, suggestive of the potential for large-scale integrated circuits. Furthermore, the ferrimagnetic behavior is exhibited with a Néel temperature of 120 K. A transition from soft-magnet state to hard-magnet state is also observed with the reduction of temperature. Our Cr2S3-based devices show a field-effect on/off ratio over 103. We believe that the findings in our work pave a promising way B


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Figure 2. Characterization of obtained Cr2S3 crystals. (a) XRD peaks for Cr2S3 nanosheets on sapphire substrate. Three characteristics peaks corresponding well with the [00l] series of Cr2S3 are distinguished from sapphire substrate. (b) XPS spectrum for obtained Cr2S3 nanosheets. Left: Cr 2p; right: S 2p. (c) SHG signal for obtained monolayer Cr2S3 nanosheet. (d) Raman spectrum for Cr2S3 flakes with different thickness.

2c, the three-fold symmetric structure of our lattice is elucidated, confirming the single crystalline nature of Cr2S3 nanosheets. The hexagonal structure of our nanosheets is in consistent with the result predicted by the R3̅ space group. The thickness-dependent Raman spectrum provides a convenient way for thickness identification through the facile optical method. The Raman tests were performed on the Cr2S3 flakes with different thickness transferred on sapphire substrate using 514 nm excitation laser. It is evident in Figure 2d (more information is presented in Figure S3) that two obvious Raman peaks are observed at 253.3 and 284.6 cm−1 for Cr2S3 sheets with a thickness around 200 nm (blue spectrum), which is in good accordance with the previously reported experimental and theoretical results.27,28 Considering the nonlayered structure, the two peaks cannot be simply divided into interor intra- layer signals and may correlate with the longitudinal optical (LO) and transverse optical (TO) vibration modes from the vibration of covalent Cr−S bonds.27 The reduction of thickness down to 10 nm is accompanied by the red shift of the above-mentioned modes, resulting in the advent of a new mode at around 335 cm−1 (green spectrum). For one-unit-cellthick nanosheets, the intensity of the LO and TO modes reduces and fuses together with the mode at 335 cm−1 (red spectrum).

unit-cell thickness by a convenient APCVD approach for the first time. As mentioned above, Cr2S3 possesses two possible phases with distinct magnetic transition temperatures.24 Thus, the Xray diffraction (XRD) is employed to clarify the crystal structure of the obtained nanosheets. As shown in Figure 2a, the XRD signals of transferred samples on sapphire indicate the single-crystallized pattern of Cr2S3 nanosheets.24 The obtained XRD signal corresponds well with the [00l] crystal planes of rhombohedral structured Cr2S3 (PDF# 10-0340). Further, X-ray photoelectron spectroscopy (XPS) characterization was utilized to analyze the chemical composition of the as-grown nanosheets. The XPS results in Figure 2b and Figure S1 demonstrate the pure phase of Cr2S3 without oxidation. Considering the strong observed C−O XPS peak, which may cover up the oxidation signal form Cr−O bond, a timedependent Raman test was further conducted. As shown in Figure S2, the Raman spectrum obtained from the sample after a month in ambient atmosphere exhibits no new peaks or peak shift. This result further confirms the good stability in air of Cr2S3 nanosheets. The 2p1/2 and 2p3/2 states of Cr are corroborated from the peaks at binding energies ≈583.9 and 574.4 eV, whereas those at binding energies ≈161.1 and 160.3 eV are meant for 2p3/2 and 2p1/2 states of S, respectively.26 Utilizing the second-harmonic generation spectrum in Figure C


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Figure 3. Modulation of Cr2S3 nanosheets in size and morphology. (a−c) Obtained Cr2S3 nanosheets under growth conditions of 15 min at 680 °C, 720 °C, and 750 °C; scale bar: 20 μm. (d) The dependence of growth temperature on edge length of Cr2S3 triangular nanosheets. (e−g) Typical OM images of Cr2S3 nanosheets obtained at different growth times of 5, 20, and 40 min under growth temperature of 750 °C; scale bar: 20 μm. The nanosheets are all with regular shape in triangular or hexagonal morphology. (h) The dependence of growth time on edge length of Cr2S3 nanosheets.

Figure 4. TEM characterization of the Cr2S3 nanosheets. (a) Low-magnification image of a single-crystal Cr2S3 domain transferred on a copper grid; scale bar: 2 μm. (b, c) Corresponding EDS mapping of Cr and S atoms. (d) HAADF-STEM image of the inner area of the Cr2S3 nanosheet. Inset: SAED of Cr2S3 nanosheet, corresponding lattice indexes are identified; scale bar: 2 nm. (e−h) Typical SAED patterns acquired from different regions of the sample in (a).

The prominent growth parameters (such as growth time, gas flow, and growth distance) in our APCVD route were systematically optimized in such a way that it would give the desired material at ultrathin thickness. Accordingly, a selected parameter was made to vary while keeping others constant.

Figure 3 and Figure S4 depict the detail of the optimization process. It is fascinating to see that the domain sizes of Cr2S3 flakes evidently enlarge with an increasing temperature from 680 to 750 °C. At the growth temperature of 680 °C, small Cr2S3 flakes with rugged edges are obtained (Figure 3a). By D


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Figure 5. Magnetic performance of Cr2S3 nanosheets. (a) Temperature-dependent magnetization of nanosheets transferred on sapphire substrate with a parallel magnetic field at 1500 Oe. Red: the zero-field cooling process (ZFC), blue: the field cooling process (FC). (b) Magnetic hysteresis loop of Cr2S3 at different temperature in parallel magnetic field scanning within ±1 T. Inset: Temperature-dependent coercivity in parallel field. (c) Temperature-dependent magnetization of nanosheets transferred on sapphire substrate with a vertical magnetic field at 1500 Oe. (d) Magnetic hysteresis loop of Cr2S3 at different temperature in vertical field. Inset: Temperature-dependent coercivity in vertical magnetic field.

contrast, with an increase of the growth temperature to 750 °C, the synthesized Cr2S3 flakes exhibit regular triangles with a large domain size. The evolution from very small to large domain size is apparent from Figure 3d which can be associated with the increasing growth temperature along the way. Similar trend was observed in another report dealing with the synthesis of layered WSe2 via CVD method.21 In a typical CVD growth process, a higher growth temperature can promote the surface migration of precursors and the crystallization of Cr2S3 flake. However, an excessive temperature leads to desorption and destruction of the growth front and results in a smaller size (Figure S4). Thus, finding the optimum temperature guarantees the achievement of large domain size and high crystal quality. In our case, the obtained Cr2S3 flakes exhibit regular triangular shapes with large domain size at 750 °C, evidencing the fact that the optimum temperature is 750 °C. Moreover, in the CVD growth process, the growth time contributes another important factor that affects the flake morphology. Keeping the already optimized growth temperature and gas flow constant, we varied the growth time in search for the best one. Figure 3e−g and Figure S4 show the variation trend via OM images. It is clear to see that the edge length of Cr2S3 flakes increases from 30 to 90 μm with a prolonged growth time from 5 to 40 min. The dependence of domain size on growth time is illustrated in Figure 3d, wherein the growth rate of about 2.5 μm min−1 is realized, considerably with that of MoS2 and WSe2 on silica substrates.29,30 The highly linear domain size-time relationship indicates that the

diameter of nanosheets can be efficiently controlled by growth time under an optimized condition. Combined with Figure S4, an abnormal shape transition of the nanosheets from triangular to hexagonal shape is observed beyond the growth time of 30 min. The shape transition may be induced by the increased S:Cr ratio during the growth process. With a S-rich atmosphere, the Cr-terminated edges grow much faster than S-terminated edges, and truncated triangular shape is consequently introduced, which is similar to the synthesis of MoS2.31 Exposing the terminal edges remarkably influence the electronic and mechanical properties of 2D materials; this scenario is currently a hotspot in discovering novel electronic and spintronic devices.32 Our results demonstrate a controllable growth of Cr2S3 crystals by efficiently modulating the growth parameters. Atomic resolution transmission electron microscope (TEM) and high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM) were employed to investigate the detailed atomic structure and crystal quality of obtained Cr2S3 nanosheets. Figure 4a shows a lowmagnification TEM image of a Cr2S3 nanosheet on copper grid, confirming the well-assembled triangular geometry. The energy dispersive spectroscopy (EDS) mapping was also employed to identify the chemical composition. Figure 4b,c clearly exhibits the uniform distribution of Cr and S atoms throughout the triangular nanosheet. The uniform color contrasts reveal the compositional uniformity of Cr and S atoms in the synthesized Cr2S3 nanosheets. Besides, the quantified analysis of EDS results in Figure S5e indicates the E


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Figure 6. Electrical performance of Cr2S3 nanosheets. (a) OM image of the fabricated transistor based on Cr2S3 nanosheet with one-unit-cell-thick. Inset: Corresponding AFM image of the device with thickness of 1.839 nm. (b) Transfer character with gate voltage scanning from −100 to 100 V, showing a p-type behavior. (c) Temperature-dependent resistance of Cr2S3 nanosheet. Inset: Corresponding four terminal device.

of Figure 5b,d indicates the temperature-dependent coercivity from the MH curve. Unlike the MT curve, the coercivity monotonically rises with the reduction of temperature. A maximum coercivity up to 1000 Oe is observed at 10 K with parallel field applied, that is, a transition from soft-magnet state at around 120 K to hard-magnet state at 10 K. Our result forecasts the great potential of Cr2S3 nanosheets form the highfrequency application in soft-magnet state to low-frequency application in hard-magnet state.34 The electronic behaviors of Cr2S3 crystals have also been studied. Figure 6a shows a typical optical image for constructed device of Cr2S3 with a thickness of ≈1.89 nm (shown as inset AFM image). An ohmic contact is fulfilled with Cr/Au electrodes (Figure S7). The Ids−Vgs transfer curve in Figure 6b demonstrates a typical p-type behavior of Cr2S3. The on state current of about 10 pA with on/off ratio over 103 indicates the great potential for spintronic devices. Furthermore, a fourterminated device based on Cr2S3 triangle is fabricated to study the temperature-dependent resistance, as shown in Figure 6c. It demonstrates that the resistance increases with the decreasing temperature, which indicates a pure semiconductor feature. We further investigate the c-direction magnetoresistance of obtained nanosheet, and the result is depicted in Figure S8. It can be seen that an ultrahigh relative magneto resistance up to 250% at 8 T and 7% at 2 T is revealed at 400 K. Given the very good semiconducting behavior and magnetoresistance performance, our Cr2S3 crystals can find potential applications in spintronics. Conclusion. In summary, we demonstrate the controlled vdW epitaxy of high-quality nonlayered uniform Cr2S3 down with one-unit-cell thickness via an APCVD method, for the first time. Single crystal Cr2S3 flake with the domain size reaching sub-millimeter scale (∼200 μm) is also achieved. The as-grown Cr2S3 nanosheets demonstrate excellent crystal quality with pure R3̅ phase. Moreover, our CVD-synthesized Cr2S3 nanosheets with high stability in the air reveal fascinating semiconductor behavior with an on/off ratio over 103. Most importantly, a Néel temperature of up to 120 K is observed with a maximum saturation magnetic momentum up to 65 μemu in our CVD-grown Cr2S3 crystals. A transition from softmagnet state to hard-magnet state could be realized by modifying the temperature. The findings in our work provide an ideal platform for spintronics application from high- to lowfrequency devices. Furthermore, by utilizing vdW epitaxy of nonlayered materials, many more magnetic materials could be introduced into the 2D system. Our work opens up a new route for the fabrication of 2D magnets. Methods. Cr2S3 Growth and Transfer. The growth was performed inside a dual-temperature-zone furnace (MTI) with

atomic ratio of Cr and S elements is 39.5:60.5, again confirming the formation of Cr2S3. The HAADF-STEM observations show that our Cr2S3 nanosheets have very high crystalline quality with no defects and vacancies (Figure 4d, Figure S5g,h). Extracted from Figure 3d, the (101̅0) lattice plane spacing of Cr2S3 can be revealed as 3.04 Å, thus the inplane lattice constant (a0) should be ≈0.608 nm. Selected area electron diffraction (SAED) pattern in the inset shows the six equivalent diffraction spots from the (101̅0) planes in the sixfold Cr2S3 lattice, and only one set of spots of the SAED confirms the single crystal nature. Furthermore, a series of SAED patterns obtained from different regions in the triangular nanosheet (Figure 4a) is shown in Figure 3e−h, reflecting a nearly identical orientation (deviation smaller within ±0.3°). The unitary set of diffraction spots demonstrates the single crystal feature of the Cr2S3 triangle with hexagonal structure. The TEM and STEM results demonstrate the high crystal quality of our synthesized Cr2S3 single crystals. Based on the very high crystal quality observed here, it appears to be reasonable to expect an intriguing application in magnetism and electronics. Physical property measurement systems (PPMS) was employed to further study the magnetic properties of the synthesized Cr2S3 samples. Considering the much higher diamagnetic background signal and complex components of mica, the Cr2S3 nanosheets are transferred on sapphire substrate. As shown in Figure 5, the magnetic responses to parallel and vertical magnetic fields are studied. The field cooling process is conducted under an external field of 1500 Oe, in which Cr2S3 nanosheets are magnetized to saturation (MS, saturation magnetism) as evidenced in Figure 5b,d. Our temperature-dependent moment results in Figure 5a,c confirm the ferrimagnetic behavior with TN ∼ 120 K, below which the spontaneous magnetization of Cr2S3 lattice exceeds over the thermal fluctuation-induced net magnetic moment. Meanwhile, the in plane of Cr2S3 is the easy axis, and a maximum moment of 65 μemu is observed at around 75 K. Then with the successive reduction of temperature to 5 K, the MS decreases rapidly due to the emergence of antiparallel lattice magnetization.24 Compared with the χmax ∼ 90 K in bulk Cr2S3, the reduction maybe derived from the disturbed magnetic order from the interface of the 2D Cr2S3 samples with reduced thickness down to one-unit-cell thickness.5,24,33The magnetic hysteresis in MH curve is shown in Figure 5b,d. Obvious hysteresis can be found regardless of applying parallel or vertical magnetic field with the temperature below 120 K (TN). Increasing the temperature just above TN, the hysteresis vanishes with only mixed paramagnetic signal from Cr2S3 and diamagnetic signal from the substrates (Figure S6). The inset F


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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology of China (no. 2016YFA0200700), National Natural Science Foundation of China (nos. 61625401, 21703047, 61474033, 61574050, 11674072, 51722204), CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Key Basic Research Program of China (2014CB931702), Sichuan Provincial Fund for Distinguished Young Academic and Technology Leaders (2014JQ0011), Fundamental Research Funds for the Central Universities (ZYGX2016Z004 and ZYGX2018J036), and Sichuan Science and Technology Program (2018RZ0082). The authors also gratefully acknowledge the support of Youth Innovation Promotion Association CAS.

a 2 in. diameter quartz tube. Sulfur powder was placed in the middle of first zone at about 200 °C. CrCl3 powder mixed with a little NaCl powder (Alfa Aesar, purity 98%) was placed inside the second heat zone at 750 °C. Freshly cleaved mica substrates were placed just over the CrCl3 and NaCl powders. A mixture of 120 sccm Argon and 50 sccm Hydrogen vapor was used to convey S and react with CrCl3 vapor on the downstream substrates. The growth time was mediated to obtain nanosheets with different domain size. The as-grown Cr2S3 nanosheets can be transferred onto arbitrary substrates. A PMMA layer was spin-coated and heated at 120 °C for 10 min. Then we put the specimen in deionized water and used tweezers to lift up the Cr2S3 nanosheets supported by the PMMA layer. Arbitrary substrates including sapphire and silicon were used to catch the PPMA layer and dried at 100 °C in atmosphere. The PMMA was then dissolved in acetone. Characterization of Cr2S3 Nanosheets. Optical microscope (Olympus DX51), Raman spectroscopy (Reinshaw, 514 nm), AFM (Oxford), SEM (Hitachi S-8220; acceleration voltage, 5 kV), XRD (Bruker, D8 Focus, Cu Kα line), and TEM (FEIF20 LaB6; acceleration voltage, 200 kV) were used to characterize the nanosheets. Micro copper grids with carbon film were used for TEM characterization. In the SHG experiment, Cr2S3 nanosheets were excited by a femtosecond Optical Parametric Oscillator (OPO) laser centered at 1330 nm with the power of 18 mW (Spectra-Physics Inspire ultrafast OPO system with pulse duration of 150 fs and repetition rate of 80 MHz). The incident laser was focused on Cr2S3 by an Olympus objective (40×, NA = 0.65). The SHG signal was detected in the reflection geometry and collected by grating spectrograph (Princeton SP-2500i) with fixed integration time of 1 s. The magnetic measurements were performed in different Quantum Design PPMS with Vibrational Sample Magnetometer utility and the highest magnetic fields up to 9 T. The nanosheets were further transferred on SiO2(300 nm)/ Si substrates to fabricate field-effect devices through standard EBL method (FEI Nova Dual Beam) and characterized by Keithley 4200 Semiconductor Analyzer; 6 nm Cr and 60 nm Au are thermally evaporated as electrodes.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b00386.

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XPS, Raman, and TEM , SAED, and OM images and I− V, M−H curves (PDF)

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jie Xiong: 0000-0003-3881-6948 Jun He: 0000-0002-2355-7579 Author Contributions #

These authors contributed equally. The manuscript was discussed, written, and approved by all authors. Notes

The authors declare no competing financial interest. G


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