Ultrathin Magnetic 2D Single‐Crystal CrSe

2D magnetic materials have generated an enormous amount of attention due to their unique 2D‐limited magnetism and their potential applications in spintronic devices. Recently, most of this research has focused on 2D van der Waals layered magnetic materials exfoliated from the bulk with random size and thicknesses. Controllable growth of these materials is still a great challenge. In contrast, 2D nonlayered magnetic materials have rarely been investigated, not especially regarding their preparation. CrnX (X = S, Se and Te; 0 < n < 1), a class of nonlayered transition metal dichalcogenides, has rapidly attracted extensive attention due to its abundance of structural compounds and unique magnetic properties. Herein, the controlled synthesis of ultrathin CrSe crystals, with grain size reaching the sub‐millimeter scale, on mica substrates via an ambient pressure chemical vapor deposition (CVD) method is demonstrated. A continuous CrSe film can also be achieved via precise control of the key growth parameters. Importantly, the CVD‐grown 2D CrSe crystals possess obvious ferromagnetic properties at temperatures below 280 K, which has not been observed experimentally before. This work broadens the scope of the CVD growth of 2D magnetic materials and highlights their significant application possibilities in spintronics.


DOI: 10.1002/adma.201900056
The ability to control spin orientations makes 2D magnets an excellent candidate for the development of spintronic logic and memory devices. These magnetic materials provide extraordinary opportunity to study intrinsic magnetic and physical phenomena at the nanometer scale without introducing any additional magnetic dopants. [17] In particular, the very recent discovery of magnetic long-range order in 2D van der Waals (vdW) materials [18][19][20][21] opens the door for fundamental physics research and device applications, which is a substantial breakthrough and has attracted much attention.
Very recently, several experiments focusing on the intrinsic and extrinsic magnetism of 2D materials have been conducted. Ferromagnetic behavior has occurred at exfoliated 2D van der Waals magnetic semiconductors, such as Fe 3 GeTe 2 , [22] CrGeTe 3 , [14] and CrI 3 . [23,24] For example, Fe 3 GeTe 2 [21] was shown to exhibit robust 2D ferromagnetism with strong perpendicular anisotropy when reduced down to monolayer thickness. Its Curie temperature (T c ) showed various layer-number-dependent magnetic phenomena. Interestingly, introducing an ionic gate raised the T c to room temperature, much higher than the T c of the bulk material, which was a great breakthrough. Afterward, van der Waals heterostructures formed by an ultrathin ferromagnetic semiconducting CrI 3 and monolayered WSe 2 have been created and exhibited unprecedented control of the spin and valley pseudospin, which indicated their important potential in spin and valleytronics. [23] However, most of these magnetic 2D materials are unstable in the air, which impedes research into their intrinsic magnetism and practical applications. Hence, exploiting new magnetic materials is a major challenge. Nonlayered 2D materials are good candidates due to their abundance of material systems and structures, which exhibit outstanding performance. [25,26] However, among these materials, 2D nonlayered magnetic materials have yet to be explored. In addition, recently reported 2D magnetic materials have mostly been layered structures stacked by weak vdW interactions, which leads to their being easily exfoliated into few-layered or monolayered structures from the bulk. However, the exfoliation method is time-consuming, affords little control over the thickness and domain size, and is incompatible with batch production. The exfoliation method is not suitable for the synthesis of 2D nonlayered materials due to its strong chemical bonds. Recently, the chemical vapor deposition (CVD) method has 2D magnetic materials have generated an enormous amount of attention due to their unique 2D-limited magnetism and their potential applications in spintronic devices. Recently, most of this research has focused on 2D van der Waals layered magnetic materials exfoliated from the bulk with random size and thicknesses. Controllable growth of these materials is still a great challenge. In contrast, 2D nonlayered magnetic materials have rarely been investigated, not especially regarding their preparation. Cr n X (X = S, Se and Te; 0 < n < 1), a class of nonlayered transition metal dichalcogenides, has rapidly attracted extensive attention due to its abundance of structural compounds and unique magnetic properties. Herein, the controlled synthesis of ultrathin CrSe crystals, with grain size reaching the sub-millimeter scale, on mica substrates via an ambient pressure chemical vapor deposition (CVD) method is demonstrated. A continuous CrSe film can also be achieved via precise control of the key growth parameters. Importantly, the CVD-grown 2D CrSe crystals possess obvious ferromagnetic properties at temperatures below 280 K, which has not been observed experimentally before. This work broadens the scope of the CVD growth of 2D magnetic materials and highlights their significant application possibilities in spintronics.

CrSe Single Crystals
Recently, the extensive amount of investigations into 2D materials, such as MoS 2 , [1][2][3][4] WS 2 , [5,6] MoTe 2 , [7] and NbSe 2 , [8] has revealed a wide range of intriguing electronic phenomena and performance, which is attributed to their unique structures and excellent physical properties resulting from their reduced dimensionality. [9][10][11] Among these compounds, 2D magnetic materials have rarely been reported in recent years. [12][13][14][15][16] www.advmat.de www.advancedsciencenews.com been considered a swift and effective approach for the synthesis of various 2D atomic crystals, such as metallic graphene, [27] insulating h-BN, [28] and semiconducting MoS 2 . [29] However, obtaining 2D magnetic materials via the CVD method remains a great challenge. Recent research has mainly focused on 2D vdW magnetic materials, the variety for which is limited. In contrast, 2D nonlayered materials, which have an abundance of structures, have hardly been studied, providing a huge challenge for expanding the available magnetic materials systems and studying 2D-limited magnetism.
Cr n X (X = S, Se and Te; 0 < n < 1), a new class of nonlayered Cr-based chalcogenides, has been explored and has received tremendous attention due to its magnetic properties. For example, Cr 2 S 3 is a ferrimagnetic semiconductor with Néel temperature of T N ≈ 120 K. [30] Cr 2 Te 3 crystallizes in a NiAs-type structure and has been proven to be a ferromagnet, for which the Curie temperature is ≈200 K. [31] Other compounds, such as CrTe, Cr 0.62 Te, and Cr 5 Te 8 , also exhibit obvious ferromagnetic properties. [32][33][34] Additionally, Cr n X (X = S, Se and Te; 0 < n < 1) possesses a number of binary compounds with different component ratios, which leads to a number of structural phases and physical properties. [35][36][37][38][39] The Cr n Se obtained thus far has mainly been in bulk state and few properties have been reported for its 2D structure. Besides, it is suspected that CrSe crystals have anomalous magnetic properties, but few works have been reported on CrSe, not to mention the synthesis of 2D CrSe crystals. Herein, we have exploited vdW epitaxy to controllably synthesize ultrathin nonlayered CrSe crystals on mica substrates via an ambient pressure chemical vapor deposition (APCVD) approach. Optical microscopy (OM) and atomic force microscopy (AFM) studies revealed that the synthesized CrSe flakes exhibit triangular shapes with a lateral size of up to 150 µm and thicknesses as thin as 2.5 nm. By controlling the growth time and source-substrate distance, the key factors in the CVD growth process, high-quality and large-size CrSe flakes can be synthesized. A continuous CrSe film was obtained via precise control of the growth parameters. Interestingly, we have characterized the ferromagnetic properties of CVD-grown CrSe flakes for the first time, which paves the way for further studies on these magnetic materials and magnetism-related applications in spintronics.
The ground state of CrSe crystallizes into space group P63/mmc (194) with the lattice parameters a = 3.61 Å, b = 3.61 Å, and c = 5.78 Å, which are described by the X-ray diffraction (XRD) standard card (PDF#02-0883). Figure 1a depicts the crystal structure of nonlayered CrSe, which shows that the unit cell of CrSe has a Se-Cr-Se-Cr-Se quintuple layered structure. This structure can be considered to be two CrSe 2 layers stacked together by sharing a layer of Se atoms. The top view and side view of the CrSe crystal structure are provided in Figure S1 (Supporting Information). The thickness of the unit cell between the top and bottom CrSe layer is ≈0.6 nm. Figure 1b schematically illustrates the surface reaction that occurs during the epitaxial growth of CrSe crystals on freshly cleaved mica substrates via an APCVD system. More details on the growth process and experimental setup are provided in the Experimental Section and Figure S2 (Supporting Information). Briefly, chromium trichloride (CrCl 3 ) and selenium (Se) powders were used as reactants. Se powder, CrCl 3 mixed with a small amount of NaCl powder, and growth substrates (mica) were sequentially placed from the upstream to downstream, in a quartz tube. Notably, the use of NaCl powder, which has a low melting point, decreased the energy barrier and increase the growth rate, which has been discussed in previously reported  works. [6,40] During the APCVD process, an adequate amount of selenium vapor is conveyed downstream by a flow of 100 sccm Ar and 10 sccm H 2 and reacts with evaporated CrCl 3 at the rather high temperature of 735 °C. As a result, triangular CrSe flakes are successfully produced on the mica substrates, as seen in the OM in Figure 1c. Figure 1c shows the typical OM-observed morphologies of the as-grown CrSe triangles; the brightly contrasting flakes on the surface represent CrSe and the surrounding regions correspond to the mica substrates. All the CrSe crystals were triangularly shaped, which is in agreement with the crystal structure of CrSe. The maximum edge length of the CrSe domains was 150 µm, as confirmed by the inset in Figure 1c. Figure 1d shows a representative AFM image and demonstrates that the thickness can be as low as ≈2.5 nm, which is extremely thin for a nonlayered material. In addition, the AFM image in Figure S3 (Supporting Information) gives a thickness of 0.6 nm for an occasional CrSe step edge, which is in good accordance with the thickness of the unit cell of the CrSe crystal. Further, XRD was employed to identify the crystal structure of as-grown samples. Notably, the XRD results were collected after transferring the sample on onto SiO 2 /Si to eliminate the interference from mica substrates. As shown in Figure 1e, the XRD pattern of our CVD-grown sample is well matched to the (001) crystal planes of the standard hexagonal CrSe pattern (PDF#02-0883), which confirms the formation of CrSe crystals and their single-crystalline nature. Furthermore, the chemical states and elemental composition of the as-synthesized samples were determined by X-ray photoelectron spectroscopy (XPS; Figure 1f,g). It can be seen that the sample consists of Cr and Se atoms. The chemical states of Cr 2p 1/2 and 2p 3/2 can be identified from the peaks at binding energies of 584 and 574 eV (Figure 1f), whereas the peaks at 55 and 53 eV (Figure 1g) correspond to the 2p 1/2 and 2p 3/2 states of Se, respectively. Raman spectroscopy measurements were also performed on the CrSe sample that had been transferred onto SiO 2 /Si to explore the intrinsic optical properties of a newly synthesized material. Figure 1h shows the two characteristic Raman peaks at 251 and 284 cm −1 (under 514 nm laser excitation), and layer-thickness-dependent Raman spectra are provided in Figure S4 (Supporting Information). Additionally, the time-dependent Raman spectra also emphasize the stability of our CVD-grown CrSe samples in air, as shown in Figure S5 (Supporting Information). In brief, these results reveal that large-scale ultrathin CrSe crystals were epitaxially grown on mica through an APCVD method.
To optimize the growth results, detailed systematic studies were conducted to investigate the effect of growth time and source-substrate distance (D ss ) on the growth results (Figure 2). With the other growth parameters (growth temperature, carrier gas, and D ss ) remaining constant, the CrSe nanosheets  proper growth conditions, the domain size of the CrSe triangles increases from 5 to 100 µm with increasing growth time from 5 to 30 min. Briefly, when there is a suitable nucleation density on growth substrates, the domain size of the CrSe flakes can be efficiently increased, until joining together to form a film, by providing a continuous and sufficient amount of the reactants. These results clearly show that the growth time critically affects the crystal size. In addition, a change from very small domain size to large domain size with increasing growth time is observed (Figure 2i). Furthermore, in the same growth batch, the distance between the CrCl 3 source and the mica substrates (D ss ) was also found to be an essential parameter for determining the growth results. It is expected that the concentration of available CrCl 3 will decrease with increasing D ss , leading to different coverage of the CrSe flakes on the mica substrates. As a consequence, the coverage of CrSe on the mica substrates can be effectively tuned from 95% to 15% for the different D ss (1, 3, 4, 6, and 7 cm), as shown in the representative OM images of the as-grown CrSe flakes on mica (Figure 2e-h). In summary, CrCl 3 powder is placed upstream, as a growth precursor, and transported downstream by carrier gas. Then the evaporated CrCl 3 has a distinct concentration gradient with increasing distance, which ultimately leads to different nucleation densities between upstream and downstream locations. Obviously, the nucleation density and domain size are very sensitive to the precursor concentration. Figure 2e shows a nearly continuous CrSe film that formed when D ss ≈ 1 cm due to the high nucleation density, which is in contrast with the discrete triangles that formed when D ss ≈ 7 cm (Figure 2h) due to the low nucleation density. At the same time, another trend for increasing D ss can be observed. As shown in Figure 2e-h, the domain sizes of the CrSe crystals become larger with increasing D ss . The reason is that a low nucleation density is beneficial for achieving a large domain size at the proper growth time and growth temperature. A similar change from a high degree of substrate coverage to a small degree of substrate coverage is observed with increasing growth D ss , as shown in Figure 2j. These growth trends are in good accordance with those seen in the synthesis of layered WS 2 via the CVD method. [41,42] In this regard, through appropriate control of the growth time and D ss , near-complete high-quality and large-scale CrSe films can be obtained directly on mica via a facile APCVD method. As shown in Figure 2k, a typical OM image of the sample surface shows a highly uniform color contrast, indicating the formation of an in-plane CrSe film. This atomic thin film layer can also be transferred onto flexible poly(ethylene terephthalate) (PET) substrates (inset photograph in Figure 2k), which shows its potential application in the field of flexible electronic/optoelectronic devices.
Further, high-resolution transmission electron microscopy (HRTEM) was employed to investigate the detailed atomic structure and crystallinity of the CVD-grown CrSe samples. The as-grown CrSe triangles on the mica substrate were first transferred onto carbon-supported Cu grids via a poly(methyl methacrylate) (PMMA) assisted transfer method [43] and then characterized by TEM. The low-magnification TEM image (Figure 3a) shows a typical triangular CrSe flake on the Cu grids, indicating the material remained intact during transfer. Further, Figure 3b shows an HRTEM image of the perfect hexagonal arrangement of the atoms. The (110) lattice plane spacing (d 110 ) measured from Figure 3b is ≈0.181 nm, which is in good accordance with that calculated from the standard XRD results (PDF#02-0883). The corresponding selective area electron diffraction (SAED) pattern (Figure 3c  www.advmat.de www.advancedsciencenews.com energy-dispersive spectroscopy (EDS) elemental mapping was utilized to analyze the elemental composition of the synthesized CrSe crystals. Figure 3d shows that our CVD-synthesized samples consist of Cr and Se atoms. The EDS maps (Figure 3e,f) present the spatial distribution of the Cr and Se atoms in the corner region of a triangle, which indicates the formation of CrSe crystals and its compositional uniformity as reflected by the uniform color distribution. Together, these TEM results confirm the achievement of ultrathin single-crystal CrSe flakes and demonstrate their highly crystalline nature, which ensures a good material foundation for further research and applications.
It is expected that these CrSe crystals, as a typical member of the Cr n X class of materials, are likely to possess distinct magnetic properties. Thus, to verify our prediction and study the magnetic properties of the synthesized CrSe crystals, a physical property measurement system (PPMS) was utilized on the CrSe samples that had been transferred onto SiO 2 /Si. Because of the very high diamagnetic background signal and potential residual signal from mica, all CrSe samples were transferred onto SiO 2 /Si substrates before the magnetic test. Subsequently, the magnetic response of the different CrSe samples to parallel and vertical magnetic field was both investigated, as shown in Figure 4 and Figure S6 (Supporting Information). In addition, the morphologies and thickness distributions of the samples are provided in Figure S7 (Supporting Information). First, a field cooling process was conducted under an external field of 1000 Oe. The temperature-dependent magnetic moments (M-T) were measured and are shown in Figure 4a,c, which indicates the obvious ferromagnetic nature of our CVD-grown CrSe samples. As shown in Figure 4a, when the temperature is just below 280 K, the spontaneous magnetization of the CrSe lattice exceeds the thermal fluctuation induced net magnetic moment. Thus, the magnetic moment reaches a saturation state with decreased temperature. Additionally, when applying a vertical magnetic field, the temperature-dependent magnetic moment shows a similar tendency. In contrast, the saturated moment under a vertical magnetic field is ≈100 µemu, which is much higher than that under a parallel magnetic field, as shown in Figure 4a. These data indicate that the out-of-plane CrSe crystals possess a much higher magnetic susceptibility than that of the in-plane crystals. Importantly, a transition from the paramagnetic state to the ferromagnetic state occurs at ≈280 K. Further, the magnet field dependent moments are also obtained and shown in Figure 4b,d. Figure 4b shows the magnetic hysteresis curves acquired at different temperatures ranging from 300 to 5 K under a parallel magnetic field, while  www.advmat.de www.advancedsciencenews.com temperature is below 280 K, regardless of whether a parallel field (Figure 4b) or a vertical field (Figure 4d) is applied. A mall magnetic hysteresis exists at 250 K, a magnified view of which is shown in Figure S8 (Supporting Information). Hence, there is a transition from the paramagnetic state to the ferromagnetic state with decreasing temperature, which is in good accordance with the M-T results. Moreover, the hysteresis in Figure 4d is much stronger than that in Figure 4b, which again confirms the higher magnetic susceptibility for the out-of-plane CrSe crystals. These results demonstrate that our CVD-grown CrSe crystals exhibit obvious ferromagnetic properties, which possess significant importance for the investigation of magnetism and magnetism-related applications.
To further investigate the electronic properties and magnetism-related properties of the CVD-grown CrSe crystals, four-terminal electronic devices were fabricated on the CrSe crystals after they had been transferred onto SiO 2 /Si. The inset of Figure 5a shows a typical OM image of a device constructed on a single crystal CrSe triangle with a thickness of 5.1 nm (shown in Figure S9, Supporting Information). Figure 5a demonstrates a linear I ds -V ds curve collected at room temperature, indicating ohmic contact between CrSe and the electrodes ( Figure S10, Supporting Information). Figure 5b shows the temperature-dependent resistivity of the CrSe device, which exhibits an increase in the resistance from 300 to 264 K and then an obvious decrease in the resistance from 264 to 50 K. The resistivity of the CrSe-based device is comparable to that of previously reported 2D materials. [44,45] The saturation at ≈264 K is considered to be induced by the magnetic phase transition, for which the result is well matched with the above M-T results ( Figure 4). Moreover, Figure 5c,d presents the magnet field-dependent resistance of the CrSe-based devices at 230 and 245 K. The results show an obvious negative resistance effect, which indicates the magnetic nature of CrSe crystals. The findings obtained here reveal the electronic properties and magnetic properties of single-crystal CrSe.
In summary, we have successfully synthesized high-quality ultrathin nonlayered single-crystal CrSe via a facile APCVD method, for the first time. The obtained CrSe crystals exhibit triangular shapes with a lateral size of up to ≈150 µm and a thickness as low as ≈2.5 nm and a good stability in air. And a continuous CrSe film can also be obtained via precise control of the key growth parameters. Further HRTEM and SAED characterizations reveal that the CrSe triangles are single crystals and possess high crystallinity. Importantly, the magnetic characterization reveals that the CVD-grown CrSe flakes exhibit obvious ferromagnetic properties with a transition temperature close to 280 K. Additionally, the electronic properties of the CrSe crystals are further characterized by electrical transport measurements. We believe this work paves the way for controllably growing other 2D magnetic materials via a facile CVD method and may provide a new platform for investigating 2D magnetism and versatile applications in spintronics.

Experimental Section
CVD Growth of CrSe Crystals and Their Transfer: The growth of CrSe crystal was performed inside a multitemperature-zone tube furnace (Lindberg/Blue M) equipped with a 1 in. diameter quartz tube in an ambient pressure system. The CrSe crystals were synthesized by a facile APCVD technique. Selenium powder was placed in the first hot zone and mildly sublimated at ≈300 °C. CrCl 3 powder (Alfa Aesar, purity 99.9%) and mica substrates (Nanjing MKNANO Tech. Co., Ltd., www.mukenano. com) were successively placed inside the hot zone. A mixture of Ar and H 2 gas with flow rates of 100 and 10 sccm, respectively, was used to transport the vapor species to the downstream substrates. The growth temperature was ≈735 °C, and the growth time ranged from 10 to 30 min. As a result, ultrathin CrSe crystals were obtained on the mica substrates. The as-grown CrSe samples could be transferred intact onto an arbitrary substrate with a commonly used method. In brief, the CrSe on mica was first spin-coated with PMMA (495 K, A4, Microchem Company) via a spin coater and baked for 3 min at 180 °C. Then the CrSe supported by the PMMA film was removed from the mica substrate by tweezers under deionized water. Next, it was transferred onto a target substrate (e.g., SiO 2 /Si) and dried on a heating stage. Finally, the PMMA film was removed via dissolution in acetone for ≈30 min and dried under N 2 gas. Thus, the CrSe samples were successfully transferred onto SiO 2 /Si.
Characterization: The morphologies and atomic structure of the CrSe samples were characterized by OM (Olympus BX51 M), scanning electron microscopy (Hitachi SU8220), and TEM (TecnaiF20). TEM energy-dispersive X-ray spectroscopy (TEM-EDX) elemental mapping was performed by a TecnaiF20. The thickness of the obtained samples was characterized by AFM (Bruker Icon). The Raman and photoluminescence (PL) spectra were obtained from a confocal microscope-based Raman spectrometer (Renishaw InVia, 514 nm excitation laser). X-ray photoelectron spectroscopy was performed on an ESCALAB 250 Xi.
Magnetic and Electronic Measurements: The magnetic measurements were performed in different Quantum Design PPMS with vibrational sample magnetometer utility and the high magnetic fields of up to 9 T. The as-grown CrSe flakes on mica were fabricated into devices after being transferred onto SiO 2 /Si. Standard electron-beam lithography processing was used to define the source/drain electrodes. Subsequently, a Cr/Au metal layer (Cr: 8 nm; Au: 60 nm) was deposited to form electrodes by thermal evaporation. Room-temperature measurements were performed with a semiconductor parameter analyzer (Keithley Model 4200-SCS). Temperature-dependent measurements of the resistivity and magnetic resistivity were performed using the different PPMS (Quantum Design) with temperatures ranging from 300 to 5 K.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.