1 Introduction
Recently, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (h-BN) have demonstrated excellent mobility, chemical stability, suitable bandgap, and thermal conductivity [1-9]. These properties make them highly advantageous in field-effect transistors, photodetectors, and sensors [10-14]. In particularly, TMDs such as MoS2 exhibit layer-dependent energy band structures with tunable bandgap and span a wide range of material systems from metallic, semi-metallic to semiconducting. This wide range of tunable bandgap, presents promising applications in photodetectors for detecting light across ultraviolet, visible and even infrared wavebands [15-19]. Additionally, the high mobility, surface flatness and atomic layer thickness of TMDs make them ideal candidates for electronic devices in the post-Moore era [20-22]. Furthermore, the unique electronic structures including strong electronic interactions and spin‒orbit coupling effects of TMDs, provide opportunities for exploring 2D spintronics [23-27]. However, there are still certain limitations in the regulation of properties pertaining to monolayer TMDs materials. Currently, through the realization of 2D heterojunction electronic device fabrication, the application range of 2D materials can be significantly expanded, leading to the emergence of novel properties such as distinctive interlayer excitons and ultrafast charge transfer [28-30].
Since 2017, the discovery of 2D CrI3, Cr2Ge2Te6, and other 2D materials has initiated extensive research on single-layer 2D magnetic materials [31-33]. The presence of magnetic anisotropy enables the possibility of 2D magnetism possible, and while the search for 2D ferromagnetic materials with room-temperature Curie temperatures (TC) has become a major focus of research objectives in the field [34-38]. The Cr-X (X = S, Se, Te) chalcogenides exhibiting NiAs-type structures have predominantly been studied in bulk form, and the diverse crystal phases of such materials contribute to their rich magnetic, thermoelectric, and optoelectronic properties [39-43]. The Cr-X system has a CrX2 skeleton structure, in which the diverse phases are formed through the intercalation of Cr atoms between the different layers [44]. Among these compounds, semiconducting properties are observed in CrS and Cr2S3, while most other compounds exhibit semi-metallic or metallic behavior [45]. Most phase structures display ferromagnetic or antiferromagnetic properties, even exhibiting Curie temperatures at room temperature. The current research on 2D Cr-X materials focuses on the electrical and optoelectronic properties of Cr-S, the 2D magnetic and thermoelectric properties of Cr-Se, the magnetic behavior of Cr-Te, and the construction of various types of heterojunctions [46-51]. Furthermore, unlike conventional layered 2D materials, most of the CrXn systems exhibit a non-layered three-dimensional structure. Achieving controlled synthesis of such non-layered materials in 2D limited thickness remains a crucial area for further investigation [52-55]. Therefore, a comprehensive overview of the current synthesis methods and intrinsic material properties of non-layered 2D Cr-based chalcogenides is significant for guiding their application in future spintronic and optoelectronic devices.
In this review, we present a comprehensive summary of the crystal structure, synthetic methods, and physical properties of 2D CrXn, as shown in Fig.1. Firstly, a variety of CrXn phase structures resulting from different Cr atoms intercalation in layered CrX2 materials are discussed. Secondly, we introduce the preparation of CrXn for these 2D layered and non-layered structures, and highlight the challenge of achieving precise control over the CrXn phases through CVD. We also summarize the preparation and theoretical study of heterojunctions based on 2D nanosheets of CrXn. Additionally, we emphasize on room-temperature ferromagnetism of 2D CrXn including magnetic anisotropy properties, magnetic phase transition behavior and strain-induced magnetic modulation effects. Finally, we introduce electrical and optoelectronic properties associated with semiconductor-like behavior observed in Cr2S3 as well as device applications for 2D CrXn.
2 Development and structure of 2D Cr-based chalcogenides
2.1 Two-dimensional magnetic materials
Previously, according to the Mermin-Wagner theorem [56], 2D spin-ordered terms cannot be formed due to thermal fluctuation at non-zero temperature. And thus, no two-dimensional long-range magnetic order exists. However, the existence of magnetic anisotropy could break this limitation. In 2017, Zhang et al. [32] and Xu et al. [31] demonstrated the existence of intrinsic ferromagnetism in two-dimensional conditions for Cr2Ge2Te6 (Heisenberg magnetic anisotropy) and CrI3 (2D Ising model), respectively, which has raised the research frenzy of 2D magnetic materials since then. As shown in Fig.2(a), Xu et al. [31] used the mechanical exfoliation method to exfoliate the van der Waals layered CrI3 single-crystal bulk to monolayer, and found that even the monolayers of CrI3 still have obvious hysteresis with the measurement of magneto-optical Kerr measurements (MOKE), which exhibit out-of-plane spin polarization properties. Moreover, the ferromagnetic behavior of CrI3 has obvious layer-dependent properties, showing that monolayer CrI3 is ferromagnetic, bilayer CrI3 is antiferromagnetic, and trilayer and bulk CrI3 transform to ferromagnetic again. Meanwhile, as shown in Fig.2(b), Zhang et al. [32] investigated the temperature-dependent magnetism of mechanically exfoliated bilayers of Cr2Ge2Te6 under a 0.075 T vertical field. As the temperature decreases, bilayer Cr2Ge2Te6 nanoflakes exhibited significant ferromagnetism at non-zero temperature, that is, the existence of 2D ferromagnetic ordering. After that, a series of typical intrinsic ferromagnetic 2D materials have been found due to the presence of magnetic anisotropy.
Fig.2 Structure and magnetic properties of 2D magnetic materials. (a) Crystal structure and MOKE measurement of CrI3 single layer [31]. (b) Optical morphology and temperature-dependent Kerr rotation of bilayer Cr2Ge2Te6 [32]. (c) Atomic structure and layer-dependent variation of ferromagnetic phase diagram of monolayer Fe3GeTe2 [33]. (d) The electronic band structures of Cr2S3, Cr2Se3 and Cr2Te3 [57]. |
For example, the more extensively studied van der Waals 2D Fe3GeTe2 (FGT) material, whose structure is shown in Fig.2(c), was obtained as a monolayer FGT sample from a bulk single crystal using the Al2O3-assisted exfoliation method [33]. Obviously, the TC of FGT samples significantly decreased from 180 K in the bulk to 20 K in the monolayer. They also found that the TC of the FGT materials could be improved by doping. The doping mentioned here is electron doping, through the positive gate voltage to insert lithium ions into the FGT nanosheets, resulting in a certain degree of electron doping. This kind of ion gate induced electron doping can significantly affect the electronic band of FGT, resulting in the change of DOS at the Fermi level and the obvious regulation of ferromagnetism. The interesting intrinsic ferromagnetism and spin properties of these 2D magnetic materials have attracted widespread attention of physicists and further enriched the development of the family of 2D materials. In recent years, the 2D magnetic materials represented by Cr-based chalcogenides (CrXn, X = S, Se, Te) have made great development, and this large class of materials contains both layered and nonlayered, with unique characteristics of narrow band gap, ferromagnetism, and thermoelectric properties [40, 45, 58]. The calculated electronic band structure of Cr2X3 (X = S, Se, Te) is shown in Fig.2(d). It is evident that Cr2S3 is a typical narrow band gap semiconductor structure (Eg ~ 0.45 eV), Cr2Se3 has semi-metallic and semiconductor properties at different symmetry points (Eg ~ 0.034 eV), and Cr2Te3 exhibits obvious metallicity [57]. Furthermore, the spin-selective tuning of the band gap and the induced magnetic phase transition can be achieved under strain modulation.
2.2 Atomic structures of the stoichiometric ratio of CrXn
The atomic structures of CrXn are generally formed by alternating stacks of Cr-rich and Cr-poor atomic layers filled in the X6 octahedron, in which the different filling ability of Cr atoms results in the phase diversity of CrXn including layered CrX2 and other non-layered structures. By utilizing the particle swarm optimization algorithm in conjunction with first-principles calculations, Liu et al. [59] identified several possible crystal structures of CrTe, as illustrated in Fig.3(a). These crystal configurations include the unstable Cmca and Rm structures, as well as the experimentally prepared ground-state P63/mmc structure [60]. Under high pressures of 34 and 42 GPa, CrTe undergoes phase transformations to Rm and Fmm phases from its ground state. In addition to CrTe, a stable phase of CrSe has also been successfully prepared in the experiment among the Cr-based chalcogenides. This non-layered material exhibits typical 2D ferromagnetic properties and shares the same space group P63/mmc as the ground state of CrTe [61].
Fig.3 Typical CrXn crystal structure for different atomic ratios. (a) Calculation of the theoretically predicted atomic structures of different space groups of CrTe: Cmca, P63/mmc, Rm and Fmm [59]. (b) Top and side views of the atomic structure of CrS2 in the 2H, 1T and 1T' phase [62]. (c) Atomic structures of air-stable α-Cr2S3, β-Cr2S3 and theoretically predicted metastable phase γ-Cr2S3 [63]. (d) Crystal structure of Cr1+xTe2 formed by different Cr atom intercalation filling rates [51]. |
Currently, the experimentally synthesized 2D Cr-S nanomaterials mainly consist of van der Waals layered CrS2 [64] and non-layered Cr2S3 [53, 65]. These two compounds also exhibit multiple phase states. Fig.3(b) presents the monolayer atomic structures of layered CrS2 in different phases. The generalized gradient approximation (GGA) function was utilized to calculate the relative formation energies of these three structures with 2H-CrS2 as a reference. The calculated values for the relative formation energy per CrS2 unit are 0.521 and 0.37 eV for 1T and 1T'-CrS2, respectively. Meanwhile, the Perdew−Burke and Ernzerhof (PBE) function theoretically predicts the band gap, revealing that 2H-CrS2 possesses a band gap of approximately 0.95 eV, while 1T-CrS2 and 1T'-CrS2 exhibit metallic and semi-metallic properties with a band gap size of around 10 meV, respectively [62]. For Cr2S3, there exist two primary stable phases, the trigonal and rhombohedral phases [66, 67]. Both of them are semiconductive properties, with a theoretically predicted band gap of 0.45 eV for rhombohedral Cr2S3 [45]. The key to distinguish two structures is the stacking arrangement of the Cr1/3S layer and the CrS layer. The crystal structures of Cr2S3 in various phases and the newly predicted relatively unstable γ-Cr2S3 are illustrated in Fig.3(c) [63]. Firstly, it is obvious that α-Cr2S3 is a rhombohedral R structure with an “ABCABC...” stacking sequence, while β-Cr2S3 exhibits a trigonal P1c structure with an “ABAB...” arrangement. Both two phases are typical non-van der Waals crystal structures, wherein each atom forms covalent bonds in three-dimensional space. In contrast, the novel predicted structure of γ-Cr2S3 is linked along the [111] direction by van der Waals forces in an “ABCAB...” stacking mode. The indirect band gap increases from 1.23 eV in bulk material to 1.97 eV in a single layer of γ-Cr2S3 according to HSE06 calculation. So far, the crystal structure of Cr-Se system is relatively simple, mainly including P63/mmc CrSe [61], 1T-CrSe2, 2H-CrSe2, Pm1 Cr3Se4, trigonal Cr2Se3 and rhombohedral Cr2Se3. Similarly, only CrSe2 is a vdW layered material, while the rest remain unlayered and have a crystal structure similar to that of the Cr-S system. Sawa et al. [68] synthesized 1T-CrSe2 with a Pm1 space group structure. Tankeshwar et al. [42] calculated the band structure of 2H-CrSe2, whose space group is Pm2, from the bulk material to single-layer CrSe2, whose band gap changes from an indirect band gap of 0.56 eV to a direct band gap of 0.75 eV. Using first-principles calculations, Ma et al. [27] concluded that the single-layer Cr2Se3 space group is Pm2 and also demonstrated that the single-layer Cr2Se3 is a valleytronic semiconductor with an indirect band gap of 0.57 eV. The Cr2Se3 nanosheets synthesized by Zhou and Wang et al. [69, 70] all have a rhombohedral structure with R.
In terms of the diversity of phases, the Cr-Te system among Cr-based chalcogenides exhibits greater complexity than the Cr-S and Cr-Se compounds. Fig.3(d) displays the top and side views of several typical structures for stoichiometric Cr1+xTe crystals [51]. As previously discussed, the wide range of atomic structure ratios in Cr-based chalcogenides can be attributed to the filling densities of the Cr-filled layers. Only CrTe2 is a van der Waals layered material, while different levels of Cr atom filling between the two Te-Cr-Te atomic layers can lead to stable non-layered Cr-Te compounds with various ratios. At present, the synthesized CrTe2 includes 1T phase with a space group of Pm1 and 2H phase with P3m1 [23, 71]. The monolayer 1T-CrTe2 is composed of Cr atoms containing Te octahedral vacancies, and the interlayers are held together by van der Waals forces. The insertion of a specific number of Cr atoms into the van der Waals gap between two layers of 1T-CrTe2 can result in the formation of various phases [44, 51, 72], such as the trigonal and monoclinic Cr5Te8 structures formed by Cr1/4Te and CrTe [73], Cr1/3Te and CrTe overlapping Cr2Te3 with a space group of P1c [74], C2/m space group structured Cr3Te4 formed by Cr1/2Te and CrTe [75], and complete filling of the gap with Cr resulting in pure phase of CrTe. In addition, there are also various structures such as CrTe3, Cr4Te5, and all of them present unique magnetic or electrical properties [76-78].
2.3 The atomic structure of non-stoichiometric ratios of Cr1+xTe2
As previously mentioned, this unique Cr intercalation behavior of CrTe2 enables not only a variety of atomic ratios of CrTen, but also often a variety of non-stoichiometric ratios due to the experimentally controlled nature of the compounds. Fig.4(a) depicts the typical atomic structure of P63/mmc CrTe. While wang et al. [79] successfully prepared a single crystal of Cr0.87Te with Cr vacancies through thermal annealing, demonstrating not only the ability to obtain specific ratios of intercalated CrTen materials but also the possibility of synthesizing non-stoichiometric Cr1+xTe2 by introducing Cr defects. Fig.4(b) shows the crystal structure of Cr1.53Te2, which is composed of Cr atom intercalation and belongs to the monoclinic C2/m space group, consistent with the structure of monoclinic Cr3Te4 [37]. Further decreasing the filling rate of Cr atoms in the van der Waals gap, a Cr1.2Te2 single crystal was prepared by Xiang et al. [80]. As depicted in Fig.4(c), Cr1.2Te2 exhibits the same Pm1 space structure similar as CrTe2. Sun et al. [81] reported the discovery of a hexagonal Cr4.14Te8 single crystal with a Pm1 structure as shown in Fig.4(d), in which embedded Cr atoms are located within the van der Waals gap between the CrTe2 layers, resulting in lattice distortion and alteration of the lattice constant. Based on these reports, it is obvious that all Cr1+xTe2 compounds with non-stoichiometric ratios have a crystal structure similar to that of the stoichiometric compounds. The observed different properties are mainly attributed to specific lattice distortions induced by the intercalation of Cr atoms or vacancies. From the aforementioned studies, it is evident that the key to the wide variety of phases in Cr-based chalcogenides can be identified, while it is particularly crucial to change the number of intercalated Cr vacancies through different synthesis methods to achieve the phase modulation of such multiphase materials. Moreover, the special properties of Cr-X compounds with various atomic ratios are also significant for their application in optoelectronic and spintronic devices.
Fig.4 Crystal structure of the non-stoichiometric ratio Cr1+xTe2. (a) Top and side views of the crystal structures of CrTe [79]. (b) Schematic diagram of Cr intercalated CrTe2 structure and side view of Cr1.53Te2 [37]. (c) Crystal structure of Cr1.2Te2 composed of Cr intercalated CrTe2 van der Waals gap [80]. (d) Side view of the intercalated atomic structure of Cr4.14Te8 [81]. |
3 Synthesis of 2D CrXn nanomaterials
Cr-based chalcogenides systems comprise both layered materials, such as MoS2 [21], WS2 [82], and non-layered compounds. Achieving precise control over the thickness, phase, and size of 2D CrXn nanosheets, whether they are layered or non-layered, has been a major research focus for investigating their physical properties and device applications. Various methods such as mechanical exfoliation, solution method, hydrothermal synthesis, molecular beam epitaxy and chemical or physical vapor deposition have been used to synthesize CrXn nanomaterial systems. Among them, the chemical vapor deposition method has been extensively studied up to now.
3.1 Diverse preparation methods for two-dimensional CrXn
3.1.1 Liquid phase exfoliation
Liquid-phase exfoliation is a widely used method for the preparation of 2D materials, in which chemical ion intercalation has been developed as an assisted liquid-phase exfoliation to obtain 2D MX2 nanosheets. The most common method involves using compounds such as LixMX2 or NaxMX2 to form dispersions in solvents, resulting in ultrathin dispersed MX2 nanosheets after sonication [83-85]. As illustrated in Fig.5(a), Coraux et al. [73] synthesized KCrTe2 single crystal using a specific molar mixture of Cr, K, and Te at 1170 K for 8 days. Subsequently, the deintercalation of K ions was completed by reacting KCrTe2 with an iodine solution in acetonitrile. Finally, the iodide was removed using acetonitrile followed by filtration and drying to obtain millimeter-sized flakes of CrTe2. In a similar way, Xiang et al. [80] subjected the K, Cr, and Te mixture to a 900 °C heating process for 24 h and maintained it for 7 days to obtain KCr1.2Te2 single crystal. The grown crystal was then immersed in the solution of iodine in acetonitrile to achieve the deintercalation of K ions. Finally, Cr1.2Te2 was obtained by repeated washing in acetonitrile solution, filtering and drying. By this method, nanoflakes with a lowest thickness of 5.61 nm were obtained as shown in Fig.5(a) (bottom). Obviously, the intercalation liquid phase exfoliation method is particularly common in the synthesis of 2D CrTe2 flakes and can be easily prepared in large quantities [86-88]. But it is almost exclusively applicable to this class of layered structures, so other synthetic methods are needed for other Cr-based chalcogenides with non-layered phase structures.
Fig.5 The main preparation methods of Cr-based sulfur nanomaterials. (a) OM and AFM images of a few atomic layers of Cr1.2Te2 and CrTe2 prepared by liquid-phase exfoliation method [73, 80]. (b) SEM image of Cr2S3 nanorods prepared by solution method [89]. (c) SEM image of hydrothermally synthesized h-Cr2Se3 nanosheets [90]. (d) Schematic diagram and STM topography image of CrTe2 films synthesized on graphene by MBE (top) [91]; atomic-resolution STM images of 1T-CrSe2 and 1T'-CrSe2 and STM image of Cr2Se3 grown by MBE (bottom) [92]. (e) Schematic diagram of interface connection of dangling bond (top), van der Waals gap (middle) and quasi van der Waals gap (bottom) [93]. (f) Schematic diagram of improved CVD synthesis of 2D Cr5Te8 crystals and OM images of the grown Cr5Te8 nanosheets at different temperatures (left) [94]; schematic illustration of 2D Cr2S3 crystals by confined-space CVD, accompanied by OM and AFM images of Cr2S3 nanoflakes (right) [65]. |
3.1.2 Solution and hydrothermal synthesis methods
Solution based chemical synthesis methods are commonly used for preparing 2D thin films and nanostructures, offering the advantages such as large-scale production, and controlled low-temperature reactions. Ali et al. [89] successfully synthesized low-dimensional Cr2S3 nanomaterials with different morphologies via chemical solution methods utilizing a single precursor source or a multi-source precursor, respectively. Firstly, a single precursor source of thiourea-chromium complex was synthesized by adding CrCl·6H2O, thiourea and acetone in a round bottom flask. Using thiourea-chromium complex as the reaction precursor source, it was dissolved in ethylene glycol and heated to 180 °C and kept overnight, and finally other by-products such as ethylene glycol were removed with methanol to obtain short rods of Cr2S3 as shown in Fig.5(b) [89]. However, compared to the single source precursor, the multi-source precursor approach involves direct addition of thiourea and CrCl2·6H2O into an autoclave containing ethylene glycol. The resulting mixture is then subjected to overnight heating at 180 °C within a furnace before being washed with methanol to yield Cr2S3 nanoparticle. Obviously, different synthesis paths can significantly affect the morphology of Cr2S3. Jie et al. [95] have successfully synthesized sub-micron CrSe2 flakes on Si (110) and glass substrates using a solvothermal method. 2 mmol of Se and Cr powder were introduced into 30 mL of ethylenediamine, followed by stirring for 10 min. The glass, Si (110) substrates and the mixture were put into an autoclave. The autoclave was maintained at 200 °C for two weeks before removing the substrate. After cleaning and drying with ethanol, the substrates were heated to 300 °C for 2 hours under an argon atmosphere to obtain CrSe2 nanosheets. The synthesized low-dimensional CrSe2 exhibited the sizes ranging from 5 to 15 μm and thicknesses around several hundred nanometers. Kariper et al. [96, 97] utilized chemical bath deposition to synthesize CrSe film. Hydrochloric acid, chromium nitrate, Se source solution and ethanolamine were employed as precursors. The substrate was immersed in the mixed bath at a certain pH and held at 50 °C for 3 hours. As shown in Fig.5(c), Velusamy et al. [90] used chromium acetate and selenium as precursors to synthesize h-Cr2Se3 hexagonal sheets for the first time using a hydrothermal method. The high crystallinity of as-synthesized h-Cr2Se3 indicates that it is suitable for the preparation of high-quality metal-sulfide compounds. We anticipate that chemical solution-based synthesis and hydrothermal synthesis of 2D materials will have a significant influence on the future large-scale production and improved device application of two-dimensional Cr-based chalcogenides.
3.1.3 Molecular beam epitaxy
Molecular beam epitaxy (MBE) is the predominant technique utilized for the preparation of crystalline thin films, which has been extensively employed in the fabrication of condensed physics and diverse optoelectronic devices [98-100]. In recent years, the application of MBE in the preparation of CrXn thin films has gained widespread attention due to the rapid development of ferromagnetic 2D Cr-based materials [43, 100, 101]. Fig.5(d) (top) illustrates the schematic and scanning tunneling microscope (STM) images of 2D CrTe2 thin films [91]. The 1T-CrTe2 films grown by MBE on bilayer graphene/SiC substrates at a deposition rate of 0.73 Å/min, using high-purity Cr and Te as the electron-beam evaporation source while maintaining the substrate temperature of 375 °C, exhibit the transverse dimension up to 5 mm. The STM topology image reveals that the grown 1T-CrTe2 films possess a typical hexagonal lattice structure and demonstrate layer-by-layer growth patterns with atomic-level flatness. Moreover, Wang et al. [102] used the MBE method to epitaxially grow hexagonal Cr2Te3 films on Al2O3 substrates. The X-ray diffraction (XRD) analysis of this film showed that it has a single [001] orientation and its c-axis lattice constant showed a slight increasing with the increased sample thickness. As shown in Fig.5(d) (bottom), Wee et al. [92] synthesized monolayer CrSe2 with the coexistence of 1T and 1T'' phases on HOPG substrates using MBE under ultra-high vacuum (10−9 mbar). From the atomic-resolution STM images, 1T-CrSe2 exhibits hexagonal symmetry with a lattice spacing of (3.3 ± 0.1) Å and a 1 × 1 surface periodicity. While the distorted 1T''-CrSe2 shows six asymmetric points around the brighter spots with a lattice spacing of (6.5 ± 0.1) Å and a 2 × 2 surface periodicity. Furthermore, upon annealing the monolayer of CrSe2 at a temperature of 300 °C, the sample underwent a transformation into monolayer Cr2Se3 and displayed a thickness increasement from 0.8 nm to 1.1 nm. To overcome the defects in the Cr2Se3 samples, monolayer Cr2Se3 was synthesized directly on the HOPG substrate at 300 °C, in which the high-quality film with specific shapes shows a lattice spacing of (3.6 ± 1) Å under lower defect density. In short, the MBE growth is commonly used for high-quality 2D materials synthesis method for layered and nonlayered materials, which requires an extremely demanding growth environment, equipment and very clean substrates. The high-quality films via the MBE method are of utmost important for investigating the physical properties of 2D Cr-based chalcogenides and also plays a crucial role in the improved device performance of electronic and optoelectronic devices.
3.1.4 Chemical vapor deposition
Chemical vapor deposition (CVD), the most prevalent method for synthesizing 2D materials, involves the adsorption and diffusion of active precursors on a substrate surface, followed by a chemical reaction to produce solid products, and has enabled the preparation of a variety of 2D layered and nonlayered materials [52, 103, 104]. Among the Cr-based chalcogenides, only CrS2, CrSe2 and CrTe2 exhibit typical layered materials in this materials family, while the rest phases are non-layered materials with a significant number of unsaturated dangling bonds on the surface, which make it difficult to obtain 2D nanosheets or thin films with controllable thickness by conventional epitaxy methods. Considering the difficulty of epitaxial growth on substrates, many researchers have resorted to using epitaxial substrates in order to overcome lattice mismatch at the interface of materials. Fig.5(e) depicts a schematic diagram of interfacial bonding under three different kinds of epitaxial conditions [93]. In conventional epitaxial growth, the epitaxial material is chemically bonded to the substrate, and a strict lattice match between them is necessary to eliminate the problem of excessive stress caused by interfacial bonds. However, due to the diversity of materials, it is difficult to find a universal epitaxial substrate for all of materials. The proposed van der Waals epitaxial growth offers a practical solution to address this issue. As shown in Fig.5(e) (middle) for the van der Waals epitaxial growth, the interface between the layered material and substrate is connected by weak van der Waals forces, in which the lattice mismatch could be relaxed up to 10% that the interface shows the negligible interfacial stress. This greatly enhances lattice mismatch tolerance and reduces material selectivity to the substrate. Subsequently, the van der Waals epitaxial growth technique was further developed to the quasi-van der Waals epitaxy, as shown in Fig.5(e) (bottom). The quasi-van der Waals epitaxy refers to the process of growing layered materials on non-layered substrates or non-layered materials on layered substrates. This epitaxy also does not involve the formation of chemical bonds at the interface, but instead relies on weak van der Waals forces, which can greatly improve the tolerance of lattice mismatch. At present, most CVD growth of Cr-based chalcogenides is based on quasi-van der Waals epitaxial growth method and can achieve precise control over size, morphology, and thickness [55, 61, 105].
The schematic diagram and morphology of Cr5Te8 (left) and Cr2S3 (right) nanosheets prepared by CVD are illustrated in Fig.5(f). Xu et al. [94] used a tube-in-tube CVD growth method to epitaxially synthesize Cr5Te8 nanosheets with high quality and controlled thickness on mica substrates, using CrCl2 and Te as precursors. This method differs from conventional CVD, in which a small quartz tube (1.5 cm in diameter) is incorporated within the tube furnace quartz tube and is fully open at one end and only features a small 3 mm aperture at the other end. The utilization of such a growth strategy ensures a continuous and stable supply of Cr and Te precursor sources, thereby driving the reaction equilibrium towards the synthesis of high-quality Cr5Te8 nanosheets. In addition, a detailed investigation was conducted on the impact of growth temperature on the synthesis of Cr5Te8 nanosheets. The thickness and lateral size of the nanosheets increased from 1.2 to 26 nm and from 5 to 30 μm, respectively, as the temperature rose from 600 °C to 800 °C. Meanwhile, the morphology transformed from trigonal to hexagonal shapes. Moreover, this tube-in-tube CVD method is particularly effective in regulating the CrxTey components, and by adjusting the supplying quality of the precursors. This report showed the structures such as Cr3Te4 and CrTe2 can be also obtained. This preparation technique produces 2D chromium chalcogenides nanosheets with high crystal quality and excellent air stability. The Cr2S3 nanosheets were epitaxially grown on the mica surface using a space-confined CVD method, as shown in Fig.5(f) (right) [65]. Similarly, the use of CrCl2 and powder S as precursors for Cr and S, respectively, in conjunction with two stacked mica sheets as growth substrates separated by a gap (less than 100 μm), serves to limit precursor supply and facilitates epitaxial growth of non-layered materials on mica surfaces. The thickness of Cr2S3 nanosheets was successfully reduced to 2.5 nm for the first time, while achieving a lateral size of up to 40 μm. Additionally, during the investigation of Cr2S3 growth on SiO2/Si substrates, clear evidence of vertical nanosheet growth was observed, further confirming the previously mentioned inhibition of lateral growth in non-layered materials by surface dangling bonds. As an important preparation method for 2D materials, CVD growth stands as the most promising future industrial approach due to its exceptional crystal quality, precise controllability, large-scale wafer production capability, and low cost. Meanwhile, van der Waals epitaxy-based CVD growth also shows the great potentials in achieving precise control of non-layered 2D Cr-based chalcogenides materials, thereby creating favorable samples quality for exploring their physical properties.
In short, there are various methods to achieve the growth of 2D Cr-based chalcogenides, and all the atomic ratio structures of Cr-based chalcogenides and the corresponding space groups and synthesis methods are given in Tab.1. These abundant synthetic methods facilitate the application of 2D Cr-based chalcogenides in different fields such as optoelectronic devices, magnetic measurements and photocatalysis.
Tab.1 Structures and synthesis of 2D Cr-based chalcogenides compounds. |
| CrXn (X=S/Se/Te) | Structure | Space group | Synthesis | Refs. |
|---|---|---|---|---|
| S | CrS2 | P63/mmc, P31 | CVD | [62] |
| Cr2S3 | R, P1c | CVD | [106] | |
| Cr3S4 | Pm1 | − | [107] | |
| Cr5S6 | P1c | − | [108] | |
| Se | CrSe | P63/mmc | CVD, MBE, CBD | [61, 97, 101] |
| CrSe2 | Pm1, Rm | Solvothermal, CVD, MBE | [92, 95, 109] | |
| Cr2Se3 | R, Pm1 | CVD, Hydrothermal | [69, 90] | |
| Cr3Se4 | Pm1 | − | [110] | |
| Te | CrTe | Cmca, P63/mmc, Rm, Fmm | MBE, CVD | [111, 112] |
| CrTe2 | Pm1, P3m1 | CVT, CVD, MBE | [87, 113, 114] | |
| Cr2Te3 | P1c | MBE, CVD | [74, 100] | |
| Cr3Te4 | C2/m | MBE, CVD | [75, 115] | |
| Cr5Te8 | Pm1 | CVD, CVT | [25, 55] | |
| CrTe3 | P2/m | MBE | [116] | |
3.2 Phase control of 2D Cr-based chalcogenides by chemical vapor deposition
3.2.1 Synthesis of 2D Cr-S nanoflakes based on CVD
Currently, for the Cr-X system, the synthesis of 2D nanosheets with different phases has been successfully achieved by CVD. Recently, Wu et al. [117] provided a comprehensive analysis of the in-situ transmission electron microscopy (TEM) study on the phase transformation behavior of Cr2S3 at various temperatures [Fig.6(a)]. This study elucidates the intricate mechanism underlying the phase transition process in this multiphase compound from an atomic structural perspective and offers the theoretical guidance for controlling the synthesis of multiphase materials. The annular dark-field aberration-corrected scanning transmission electron microscopy (ADF-STEM) and selected area electron diffraction (SAED) images of Cr2S3 nanosheets at 25, 400, and 600 °C are depicted in Fig.6(a) (left). By analyzing the atomic structure of Cr-S compounds at [001], [110], and [010] zone axis, it can be observed that a phase transition occurs at this temperature. Specifically, R-Cr2S3 transforms to T-Cr2S3, then to M-Cr3S4, and finally to the coexistence of M-Cr3S4 and T-Cr5S6. In addition, Fig.6(a) (right) presents a schematic diagram illustrating the movement of the Cr atoms during the phase transition in the Cr-S compounds. The diagram reveals that the migration of Cr atoms in the CrS2 layer to the Cr-poor layer is primarily responsible for this transition, while the horizontal movement and rearrangement of these atoms result in formation of a new phase. He et al. [53] explored the epitaxial growth of Cr2S3 nanosheets on mica substrates by atmospheric pressure CVD (APCVD) at 750 °C, showing with lateral size up to 200 μm and the single unit cell thicknesses of 1.78 nm. Moreover, the size of the samples exhibited a significant increase with growth temperature from 680 °C to 750 °C and an extension of growth time from 5 °C to 60 min. Yuan et al. [118] achieved in situ nitrogen doping of Cr2S3 nanosheets, inducing a transition from the R phase to the P1c phase and obtaining N-Cr2S3 nanosheets. The structures of Cr2S3 nanosheets before and after N doping are R rhombohedral (P-Cr2S3) and P1c trigonal (N-Cr2S3), respectively. 2D pure phase Cr2S3 (P-Cr2S3) and N-Cr2S3 were grown via a molten salt-assisted plasma-enhanced chemical vapor deposition (PECVD) method under N2 atmosphere at 760 °C, with the SiO2/Si substrate and quartz boat maintained at a 45° angle as shown in Fig.6(b). The doping level of N element in N-Cr2S3 was adjusted by the input power of the plasma reactor. Fig.6(b) (bottom right) displays the hexagonal-shaped P-Cr2S3 and N-Cr2S3 nanosheets grown on SiO2/Si substrate, with thicknesses of 1.79 and 1.68 nm, respectively, and lateral dimensions of approximately 10 μm. Liu et al. [64] used space-confined CVD with the assistance of molten salt to synthesize 1T-CrS2 nanosheets on SiO2/Si substrates at the temperatures of 670−680 °C. The resulting nanosheets have a semi-hexagonal morphology and possess a thickness as low as 2 nm.
Fig.6 Schematic illustration and characterization of the synthesis of 2D Cr-based chalcogenides in different phases based on chemical vapor deposition. (a) ADF-STEM and SAED images of Cr2S3 at 25, 400, and 600 °C, respectively (left); schematic representation of the atomic movement process of Cr in Cr2S3 during the phase transition (right) [117]. (b) PECVD synthetic process for N-doping in Cr2S3 nanosheets, and the OM images of P-Cr2S3 and N-Cr2S3 synthesized on SiO2/Si substrates [118]. (c) Schematic illustration of CVD synthesis of Cr2Se3 (top left) and XRD spectra of two-dimensional Cr2Se3 nanosheets (top right); OM images of the synthesized Cr2Se3 nanosheets at different temperatures (bottom) [70]. (d) SEM image of Cr3Te4 nanoflakes and corresponding EDS mapping images with Cr and Te (left); the cross-sectional ADF-STEM image of Cr3Te4 nanoflakes and corresponding atomic structures (right) [119]. (e) A CVD synthesis route of 1T-CrTe2 and atomic-resolution STEM-HADDF images of top-view and cross-section [113]. |
3.2.2 Synthesis of 2D Cr-Se nanoflakes based on CVD
Similarly, He et al. [61] successfully used APCVD to epitaxially grow CrSe nanosheets on mica substrates, all of which were trigonal shapes with lateral dimensions up to 150 μm and thicknesses down to 2.5 nm. By controlling the growth temperature and source-substrate distance, high quality and large size CrSe nanosheets could be prepared, and further control of growth parameters could lead to continuous CrSe films. While the CrSe2 nanosheets with controlled thickness can be successfully prepared by CVD using an epitaxial growth substrate of WSe2 with a dangling bond-free surface [109]. Compared with SiO2/Si substrate, it is easier to grow regular trigonal shapes with relatively lower thickness on the WSe2 substrate. In particular, the thickness of CrSe2 strongly depends on the growth temperature. The average thickness increases from 4 to 7 nm as the temperature raises from 700 °C to 710 °C, and further reaches 10 nm when the temperature is elevated to 720 °C. Moreover, Wang et al. [70] explored the thermodynamic effects on the growth of 2D Cr2Se3 nanosheets and successfully synthesized rhombohedral Cr2Se3 nanosheets with adjustable thickness by CVD on various substrates such as mica, SiO2/Si, and glassy silica. As shown in Fig.6(c), the XRD pattern indicates that the Cr2Se3 nanosheets grown on SiO2/Si substrate exhibit the (001) orientation, excellent single crystallinity, and high crystal quality. As can be observed from the optical images in Fig.6(c) (bottom), the thickness of Cr2Se3 nanosheets on mica substrates increases from 1.8 nm to tens of nm as the growth temperature from 820 to 900 °C, and the lateral size can increase to 120 μm with the changed shapes from triangular to hexagonal. In contrast, the optical microscope (OM) of Cr2Se3 on SiO2/Si and glass substrates are predominantly triangular nanosheets. This phenomenon may be attributed to the factor of the different nucleation potential of Cr2Se3 on different substrate surfaces.
3.2.3 Synthesis of 2D Cr-Te nanoflakes based on CVD
In the Cr-Te system, addressing the issue of structural diversity is crucial during material growth, and CVD method serves as a vital tool for achieving phase modulation. Duan et al. [75] employed the APCVD method to synthesize Cr3Te4 nanosheets on SiO2/Si substrates, adding 15 sccm H2 to the transport carrier gas Ar in order to achieve an adequate supply of Te. Similarly, the nanosheets exhibited a precise temperature-dependent thickness variation down to 1 nm at 670 °C. Recently, Zou et al. [119] achieved epitaxial growth of Cr3Te4 nanosheets on mica substrates with a thickness ranging from 10 to 150 nm, as illustrated in Fig.6(d). The scanning electron microscope (SEM) image reveals a typical hexagonal nanosheet with an exceptionally smooth surface and a highly uniform distribution of Cr and Te elements. A cross-sectional STEM analysis of this sample, as depicted in Fig.6(d) (right), presents a clear the atomic arrangement that is consistent with the Cr3Te4 structure. Gong et al. synthesized 2D 1T-CrTe2 nanosheets of controlled thickness on SiO2/Si substrates using CrCl2 and Te precursors by CVD, as shown in Fig.6(e) [113]. The effect of reaction temperature ranging from 973 to 993 K on the synthesis of samples was investigated in details. As a result, the thickness of single crystalline 1T-CrTe2 nanosheets gradually increased from 1.2 nm to about 50 nm as the growth temperature increased, while all nanosheets showed hexagonal structures. Further STEM analysis was conducted on 1T-CrTe2, revealing an atomic-resolved STEM-HAADF image from a top view that clearly depicts the well-organized arrangement of atoms in a hexatomic ring. This observation, as shown in Fig.6(e) (middle), reflects the six-fold symmetry of 1T-CrTe2, which corresponds to its hexagonal morphology. The cross-sectional STEM in Fig.6(e) (right) shows that the atomic arrangement is consistent with the atomic structure of 1T-CrTe2, providing the evidence for successful preparation of layered 1T-CrTe2 nanosheets. In addition, Zhou et al. [55] achieved the preparation of Cr5Te8 nanosheets using pure Ar with a larger flow rate (200 sccm) as the carrier gas. Furthermore, Liu et al. [60] provided a method for synthesizing very large size CrTe nanosheets. Using a mixture of Cr and CrCl3·6H2O as the metal precursor source and Te powder as the Te precursor, 5 nm single crystal of CrTe nanosheets with lateral dimensions up to 250 μm were synthesized on SiO2/Si substrates at 700 °C. The control of the reaction rate is primarily achieved by regulating the ratio of Cr and CrCl3·6H2O in the mixed Cr precursor, while Te is added to reduce the melting point of the Cr metal. Using this method, the growth of 250 μm nanosheets can be accomplished within a mere 2 min, which greatly improves the sample preparation efficiency. The above synthesis for Cr-Te compounds involves modifying the precursor type, carrier gas and growth temperature to achieve the phase structure modulation of CrxTey. However, the thickness and size of the nanosheets are strongly influenced by the growth temperature and time. In conclusion, the phase control of Cr-based chalcogenides based on CVD growth method is a feasible and high-quality approach for preparing single crystals with special phases, which can be applied in 2D optoelectronic and magnetoelectronic devices.
4 Properties and applications of 2D CrXn nanoflakes
4.1 Two-dimensional Cr-based heterojunctions
In recent years, the family of 2D Cr-based chalcogenides has been continuously expanded, and 2D heterojunctions based on CrXn (X = S, Se and Te) have likewise received widespread attention. Zheng et al. [120] used first principle calculations to understand the electronic and optical properties of InSe/CrS2 heterojunction. Fig.7(a) (left) illustrates the positions of the band edges of CrS2 and InSe, respectively, which are relative to the vacuum energy level. It is evident that electron-hole transfer occurs from InSe to CrS2 across the interface of their heterojunction. Further calculation of the energy bands and density of states of the InSe/CrS2 vdWH, as shown in Fig.7(a) (right), indicates that the conduction band of this heterojunction is mainly contributed by the CrS2 layer. However, the valence band is contributed by both CrS2 and InSe. The monolayer CrS2 and InSe undergo the coupling of the valence band, which is somewhat different from Fig.7(a) (left). This van der Waals heterojunction exhibits an intrinsic mixed band alignment with a band gap of 1.2 eV. PDOS calculations reveal that by manipulating the electric field, a finely tuned mixed-band alignment can be achieved between type-I and type-II. Consequently, this heterojunction holds significant potential for applications in optical and optoelectronic devices. Cao et al. [121] conducted a theoretical investigation on the electronic structure of the CrS2/BP heterojunction, as depicted in Fig.7(b). The energy band dispersion of the heterojunction indicates that both the CBM and VBM are located at the K point, resulting in an extremely narrow direct band gap with a value of Eg = 0.221 eV. From the P-DOS calculations, the VBM and CBM of the CrS2/BP heterojunction are mainly attributed to the P-p and Cr-d orbitals, respectively. These properties endow the heterojunction with a broader optical absorption and a more favorable energy band structure for carrier transport. The nonequivalent energy valleys present in the first Brillouin zone of TMD materials render them a promising candidate for valley electronics. Therefore, Xiong et al. [122] investigated the effect of axial strain on the valley splitting of WSe2 based on the WSe2/CrSe2 heterojunction. As shown in Fig.7(c), the red bubbles represent the contribution of the W element, and the spin splitting between K(α) and K'(α') varies under different strains, thereby resulting in magnetically induced valley splitting. A notable feature is that the bottom of energy valley shifts away from the high symmetry point under uniaxial strain, which can be attributed to the reduced crystal symmetry of TMDs. When the uniaxial strain is increased, the reduction of the structural symmetry leads to the decrease or even disappearance of the valley splitting phenomenon, which has significant implications for the field of valley electronics.
Fig.7 Heterojunction properties of 2D Cr-based sulfides. (a) Energy band distribution of the relative vacuum energy levels of monolayer CrS2 and InSe and DFT calculation of the electronic energy band structure and density of states of CrS2/InSe heterojunction [120]. (b) Energy band structure and PDOS of CrS2/BP heterojunction, where blue and pink lines are CrS2 and BP, respectively [121]. (c) Projected energy band structures of W atoms in WSe2/CrSe2 heterojunctions with no strain, 10% biaxial strain and 10% uniaxial strain [122]. (d) Low-resolution TEM image of the CrSe2/WSe2 heterojunction and EDS elemental mapping of Cr, Se, and W, and the SAED pattern of this heterojunction [109]. (e) Cross-sectional HADDF-STEM images of ZrTe2/CrTe2 heterojunction and SOT-assisted magnetization switching schematic of ZrTe2/CrTe2 heterojunction device [123]. (f) Schematic diagram of the CrTe2/Bi2Te3 bilayer heterojunction structure with a van der Waals gap [124]. (g) The dI/dV conductance mapping at the CrTe2/CrTe3 metal-semiconductor lateral heterojunction interface and the STM image of this monolayer lateral heterojunction [125]. |
Duan et al. [109] synthesized a CrSe2/WSe2 heterojunction, as shown in Fig.7(d) (left). The distribution of individual elements in the low-resolution TEM-EDS mapping clearly reveals that typical trigonal CrSe2 nanosheets grow on the surface of the WSe2 film. Fig.7(d) (right) demonstrates that the heterojunction possesses two sets of six-fold-symmetric diffraction spots in the SAED image, which show the lattice spacings of 0.289 and 0.314 nm, corresponding to the (100) plane of WSe2 and CrSe2, respectively. Additionally, its high-resolution TEM images also reveal the well-defined moiré superlattice. The ferromagnetic properties of the heterojunction were further investigated. It was found that there were obvious thickness-dependent ferromagnetic properties of the CrSe2 nanosheets, which were primarily attributed to the charge transfer of WSe2 and the interlayer coupling within CrSe2. Samarth et al. [123] synthesized monolayer 1T-CrTe2 and ZrTe2/CrTe2 heterojunction on a (001) sapphire substrate using MBE, and the cross-sectional HAADF-STEM view of this heterojunction is shown in Fig.7(e) (left). In addition, a single layer of 1T-CrTe2 grown on the ZrTe2 surface was found to still have long-range ferromagnetic ordering. The CrTe2/ZrTe2 heterojunction Hall bar device was constructed, as shown in Fig.7(e) (right). And the relationship between the magnetization to the current for the CrTe2 was probed by anomalous Hall effect (AHE), which proved that the current flows in parallel in both ZrTe2 and CrTe2 layers. Herein, the current-induced magnetization switching of this ferromagnetic CrTe2/ZrTe2 heterojunction device was realized. The successful fabrication of wafer-scale vdW 2D ferromagnets and topological semi-metallic heterojunctions can catalyze the exploration of integrated quantum devices for advanced research. Bian et al. [124] employed molecular beam epitaxy to create a 2D vdW ferromagnet/topological insulator heterojunction consisting of CrTe2/Bi2Te3 with several atomic layer thicknesses, separated by a van der Waals gap as depicted in Fig.7(f). In this heterojunction, a significant topological Hall effect (THE) signal is observed with a resistivity of up to 1.39 μΩ·cm at 10 K (much larger than that of bilayer heterojunctions [126-128]), which is mainly due to the combination of the 2D ferromagnetism of CrTe2, the strong spin-orbit coupling of Bi2Te3, and the atomic-level sharp interface. Zhao et al. [125] synthesized monolayer and bilayer antiferromagnetic CrTe3 on HOPG with semiconducting properties, exhibiting band gaps of 0.92 eV and 0.71 eV, respectively. By annealing the monolayer of CrTe3, a portion of the monolayer can transform into a monolayer CrTe2, resulting in the formation of lateral magnetic metal-semiconductor heterojunctions (CrTe2/CrTe3) with strong chemical bonding at the interface. In Fig.7(g) (right), this in-plane heterojunction clearly observed by the atomic resolution STM images. Additionally, the dI/dV conductivity map presented in Fig.7(g) (left) clearly illustrates the energy band bending state at the heterojunction interface, which is a typical characteristic of Schottky barriers observed at metal-semiconductor interface. The height of the Schottky barriers were measured to be approximately 0.5 V. Simultaneously, they predicted that the antiferromagnetic and ferromagnetic behaviors of this heterojunction or the metal and semiconductor properties of this heterojunction could switch over time under the in-plane stresses. Moreover, Lin et al. [51] also prepared a Cr2Te3/Cr5Te8tr lateral heterojunction. Unlike the CrTe2/CrTe3 heterojunction, this one has a mixed-phase transition region of about 1 μm between Cr2Te3 and Cr5Te8tr, in which the state of the CrxTey phase is random and the arrangement of Cr atoms is disordered. This transition region has a great influence on the magnetic decoupling effect of the Cr2Te3/Cr5Te8tr lateral heterojunction, enabling the maintenance of magnetism up to 210 K. Based on the above theoretical and experimental study, we can see that Cr-based chalcogenides heterojunction devices exhibit rich electrical and magnetic properties, which can play an important role in the future applications of magnetism and spintronics.
4.2 Magnetic properties of two-dimensional Cr-based materials
Since the experimental demonstration of 2D magnetism, extensive research has been conducted in the field of 2D magnetic materials. Meanwhile, the emergence of 2D transition metal chalcogenides, particularly Cr-based chalcogenides, which exhibit excellent ferromagnetism, anomalous Hall effect and magnetic anisotropy, has also garnered significant attention from researchers. These properties are key to future applications in spintronic and magnetoelectronic, and magneto-optoelectronic devices.
4.2.1 Ferromagnetic properties of 2D CrXn materials
Cr-based halides, such as CrI3 and CrBr3, exhibit ferromagnetic behavior in the monolayer limit with a pronounced layer dependence [31, 129]. Extensive studies have also been conducted on the ferromagnetism of other Cr-based chalcogenides, especially Cr-S, Cr-Se and Cr-Te compounds [67, 72, 79, 130]. Notably, some of these compounds demonstrate Curie temperatures (TC) above room temperature including but not limited to Cr2Te3, Cr3Te4, and CrSe [61, 74, 115]. The magnetic phase diagram of CrSe2, as shown in Fig.8(a) (left), indicates that the TC remains at 110 K until the number of layers is reduced to 16 [109]. However, as the number of layers decreases, there is a significant reduction in the Curie temperature and ultimately, for a single layer, it drops to 65 K. A Hall bar device of 7-layer CrSe2 was constructed to investigate its magnetic transport properties. The anomalous Hall effect is shown in Fig.8(a) (right), and both the residual anomalous Hall resistance and coercivity exhibit a decreasing trend with increasing temperature. Furthermore, the magnetic transport properties confirm that the CVD-synthesized CrSe2 nanosheets possess typical ferromagnetic characteristics. Wang et al. [107] utilized first-principles calculations to predict a series of Cr3X4 monolayers, among which Cr3S4 exhibits ferrimagnetism while both Cr3Se4 and Cr3Te4 display ferromagnetism. The Heisenberg model suggests that strong ferromagnetic ordering is enabled by significant inter- and intra-layer coupled exchange interactions in the monolayers of Cr3Se4 and Cr3Te4. The Monte Carlo method was employed to simulate the temperature-dependent magnetization and specific heat capacity of monolayer Cr3Se4 and Cr3Te4, as depicted in Fig.8(b). It was discovered that the TC of monolayer Cr3Se4 and Cr3Te4 are 370 and 460 K, respectively, which surpass those of other 2D van der Waals materials. These findings suggest that monolayer Cr3Se4 and Cr3Te4 hold great potential as prime candidates for spintronic devices.
Fig.8 Magnetic properties of Cr-based sulfides. (a) Layer-dependent properties of the Curie temperature (TC) of CrSe2 and anomalous Hall resistance (RAHE) variation of seven layers of CrSe2 at given temperatures [109]. (b) Magnetic moments (μB), magnetization (χ) and specific heat (CV) of monolayers of Cr3Se4 and Cr3Te4 as a function of temperature, simulated using the Monte Carlo method based on the 2D Heisenberg model [107]. (c) Magnetic temperature curves of CrTe2 films with different thicknesses in field-cooling mode (left) and vertical magnetization curves of 7 ML CrTe2 sample at different temperatures (right) [91]. (d) Schematic diagram of electrical transport measurements of Cr5Te8 devices under magnetic field and magnetic field-temperature phase diagram of Cr5Te8 [25]. (e) Hysteresis curves of Cr2Te3 and Cr2Te3/Cr2Se3 heterojunctions at 10 K and their temperature dependent magnetization intensity after field cooling and zero field cooling [131]. |
Xu et al. [91] conducted superconducting quantum interference device (SQUID) measurements to investigate the room temperature ferromagnetism of CrTe2 thin films, as illustrated in Fig.8(c). The magnetization of the CrTe2 films decreases significantly with increasing temperature under field cooling and a perpendicular external magnetic field of 1000 Oe. Even when reduced to three layers, the CrTe2 film maintains a TC near room temperature, and at seven layers, it exhibits a significant magnetization at 300 K, indicating its maintained ferromagnetic ordering at this temperature. Fig.8(c) (right) gives the out-of-plane hysteresis of the seven-layer CrTe2 film at different temperatures, with its coercive field increasing to 1000 Oe as the temperature drops from 300 to 20 K, indicating typical hard magnet behavior. Furthermore, through X-ray magnetic circular dichroism (XMCD) characterization, it has been demonstrated that even a single layer of CrTe2 can still exhibit high Curie temperatures (TC ~ 200 K). Later, they explored the intercalation of Cr atoms to achieve antiferromagnetic interlayer exchange coupling in Cr5Te8, presenting a novel approach for designing two-dimensional magnetic structures [25]. Previous research has established that Cr5Te8 is comprised of van der Waals CrTe2 materials with interlayer Cr atoms, in which the CrTe2 interlayers exhibit robust ferromagnetism. However, the insertion of interlayer Cr atoms greatly affects the magnetic transport of Cr5Te8, resulting in an antiparallel spin configuration. The TC of Cr5Te8 is determined to be approximately 150 K based on the variation curves of magnetization intensity with temperature under different magnetic fields. With the increase of temperature, this Cr5Te8 sample exhibits typical antiferromagnetic behavior with a Néel temperature (TN) of 180 K. As the magnetic field increases, the TN decreases until it disappears when the magnetic field exceeds 0.5 T. A schematic diagram depicting the electrical transport device of Cr5Te8 is presented in Fig.8(d) (left). As the temperature approaches TC, a sharp peak emerges in the resistance of the device, and with further increase in temperature, the peak broadens significantly, signifying a magnetic phase transition from ferromagnetic to antiferromagnetic phase. The magnetic phase transition diagram of Cr5Te8 is presented in Fig.8(d) (right), where the joint modulation of temperature and magnetic field enables the realization of ferromagnetic, antiferromagnetic, and paramagnetic phase transitions in Cr5Te8. Chun et al. [131] investigated the THE resulting from interlayer exchange coupling in ferromagnetic Cr2Te3/noncoplanar antiferromagnetic Cr2Se3 (CT/CS) heterojunctions. Firstly, the magnetization curves of CT and CT/CS are compared in Fig.8(e). The fabricated heterojunction significantly reduces the saturated magnetization of CT but does not substantially alter the magnitude of the coercive field and causes a slight decrease for the magnetic phase transition temperature. The presence of noncoplanar ordering in the antiferromagnet plays a crucial role in the generation of the topological Hall effect. Measurement of the topological Hall resistivity of CT/CS heterojunctions with different Cr2Se3 thicknesses indicates that the THE intensity remains unaffected by sample thickness, and disappears beyond the Néel temperature. This topological Hall effect observed in CT/CS heterojunctions arises from the interfaces between the ferromagnetic (FM)/noncoplanar antiferromagnetic (AFM) materials with antisymmetric exchange coupling, providing a promising avenue for investigating the interplay between interfacial exchange and spin chirality.
In ferromagnetic materials, the Curie temperature normally decreases significantly as the material size is reduced, such as CrTe and Cr5Te8 [94, 112]. However, He et al. [74] investigated the ferromagnetism of 2D Cr2Te3 and observed an opposite trend: when the thickness of Cr2Te3 nanosheets was decreased from 40.3 to 7.1 nm, the TC increased from 160 K to 280 K due to atomic reconfiguration in low dimensions. In addition, Zhang et al. [44] obtained symmetry-broken layered Cr1.5Te2 nanosheets by precisely modulating the Cr atom intercalation and investigated the ferromagnetic behavior for self-intercalated Cr1+xTe2 nanosheets using MOKE measurements. The Curie temperature observed in Cr1.5Te2 nanosheets was (285 ± 5) K, which is similar with that of bulk materials and does not vary with thickness. This interesting variation of Curie temperature due to Cr atom intercalation is a unique property of Cr-based chalcogenides, which help obtain ferromagnetic 2D materials with high Curie temperature [132, 133]. In conclusion, the abundant ferromagnetism exhibited by 2D Cr-based chalcogenides positions them as a crucial member of the 2D magnetic material family. The construction of 2D Cr-based chalcogenides heterojunctions and the alteration of Cr atom intercalation can provide more possibilities for their applications in spintronic devices.
4.2.2 Magnetic modulation and magnetic anisotropy of 2D CrXn chalcogenides
The magnetic properties of Cr-based chalcogenides are of great significant importance in spintronics, and the high Curie temperature of 2D magnetic materials has been a focal point in research on magnetic materials. As previously mentioned, Cr-based chalcogenides can exhibit high ferromagnetic phase transition temperature by adjusting the concentration of Cr intercalation layer. Furthermore, in 2D materials, the ferromagnetic properties such as Curie temperature and coercive field can be easily manipulated through external adjustments, which can greatly enhance their magnetic characteristics and expanding their potential applications in spintronic devices. The strain-modulated reflective magnetic circular dichroism (RMCD) sweeps of Cr2Te3 nanosheets at different temperatures are given in Fig.9(a) [134]. The coercive field of Cr2Te3 nanosheet decreases significantly (only 4% of that of the unstrained nanosheet) at tensile strain (+0.37%), while the TC increases from 140 to 180 K. However, when a compressive strain (−0.37%) is applied, the TC drops from 140 to 110 K and simultaneously, the coercive field also experiences a significant reduction (about 33% of that of unstrained Cr2Te3). Moreover, the reduction of TC caused by the compressive strain becomes more significant with increasing thickness of the nanosheets, resulting in an enhanced negative modulation that can lower TC to 90 K. This stress-induced alteration of magnetism is mainly attributed to the changes in super-exchange interaction between the ions due to variations in Cr−Te−Cr angle under uniaxial strain, with larger angles contributing more significantly to magnetism. The angle increases from 131.8° to 132.3° when the strain increases from −1.0% to +1.0%, which is also reflected in the increase of TC. In addition, Moodera et al. [135] have investigated the impact of strain in the magnetic modulation of Cr2Te3. It was discovered that AHE exhibits a unique sign reversal with a hump-shaped Hall feature under the varying temperature and strain, which is mainly related to the presence of multiple magnetic layers/domains under interface strain conditions. Duan et al. [136] conducted a comprehensive analysis using DFT calculations to investigate the impact of strain and Cr atom intercalation on the magnetic properties of CrTe2. The transition of CrTe2 from intra-layer antiferromagnetic coupling to ferromagnetic coupling, and ultimately to inter-layer antiferromagnetic coupling, occurs as the applied strain changes from −6% to 4%. This phenomenon is attributed to the alteration in super-exchange interaction resulting from increased distances between the Cr atoms in the facets and decreased inter-layer spacing. In the meantime, the Cr intercalation within the CrTe2 layers induces a transition of CrTe2 to ferromagnetism. These tunable magnetic transitions hold great significance for further exploration of CrTe2-based magnetic strain sensors and spin transistors.
Fig.9 Novel magnetic properties of 2D Cr-based materials. (a) RMCD sweeps of Cr2Te3 nanosheets under +0.37% tensile strain (15.4 nm) and −0.37%compressive strain (13.6 nm) at different temperatures [134]. (b) Temperature-dependent characteristics of the out-of-plane phase of magnetization at different frequencies (f = 10, 50, 100 and 500 Hz) [137]. (c) Magnetoresistance (MR) properties of Cr2Te3 at parallel (θ = 90°) and perpendicular (θ = 0°) magnetic fields and variation of MR with magnetic field direction at 2 K [100]. (d) Illustration of the measured magnetoresistance of CrTe under an out-of-plane magnetic field and the temperature-dependent characteristics of the resistance at different θ under a 4 T magnetic field [138]. (e) Schematic and optical diagrams of a few-layer CrTe2 device, and schematic diagrams of anisotropic magnetoresistance measurements [87]. |
The tunable magnetic properties of such 2D materials effectively expand their investigation fields in magnetism and spintronics. Furthermore, the complex magnetic properties of Cr-based sulfides that are yet to be discovered are gradually being explored. Banerjee et al. [100] were the first to discover the possible spin-glass behavior in MBE-grown Cr2Te3, where a spin-glass phase emerged in Cr2Te3 nanosheet below 35 K, resulting in the randomization of magnetic moments and a rapid decline in magnetization strength. Later, Zhou et al. [137] reported the observation of intriguing spin-glass states in two-dimensional Cr2Se3 nanosheets with a spin-freezing temperature of 28 K. Notably, an increase in frequency led to a significant enhancement in the spin-freezing temperature of Cr2Se3 nanosheets, as depicted in Fig.9(b). The inherent spin glass behavior in 2D Cr-based materials arises from the geometrical frustration of their crystalline symmetry, providing a foundation for investigating the disordered spin in 2D systems.
Anisotropic magnetoresistance (AMR) is an inherent property exhibited by ferromagnetic materials. Since the discovery of the ferromagnetism of 2D Cr-based materials, their magnetic anisotropy has been successively reported. Banerjee et al. [100] conducted magneto-transport measurements on a 4 nm Cr2Te3 nanosheet, which displayed typical metallic material-like AMR, as illustrated in Fig.9(c). The magnetic resistance (MR) of a 4 nm Cr2Te3 nanosheet varies with the direction of the magnetic field, indicating the presence of perpendicular magnetic anisotropy (PMA) in the film. Additionally, Zheng et al. [139] observed a semiconducting-to-metallic transition at 175 K in MBE-grown epitaxial films of Cr2Te3. Negative MR effects were observed in all Cr2Te3 films, with the strongest effect occurring at TC. The PMA of the films is indicated by their AHE and out-of-plane hysteresis, with the susceptibility magnetization axis along the c-axis direction. Furthermore, the magnetic transport measurements for different magnetic field directions show that Cr2Te3 has a typical anisotropic magnetoresistance, in which the out-of-plane magnetoresistance is significantly larger than that of in-plane magnetoresistance. The AMR of ferromagnetic CrTe nanosheets was investigated by Zhai et al. [138], and the device schematic along with the AMR results are depicted in Fig.9(d). These hexagonal CrTe nanosheets exhibit the magnetization along their normal direction, and the AMR ratio exhibits a linear increase as the room temperature decreases, reaching 5.2% at 5 K, which is comparable to that of numerous magnetic metals and alloys. To investigate the spin transport properties of 1T-CrTe2, Zhang et al. [87] fabricated h-BN/CrTe2/h-BN devices on SiO2/Si substrates, as illustrated in Fig.9(e). Remarkably, CrTe2 exhibits a negative/positive AMR at ambient/low temperature, demonstrating an entirely contrasting AMR behavior compared to other vdW 2D ferromagnetic materials. Zhang et al. [80] analyzed the THE of Cr1.2Te2 and observed a significant magnetic field angle dependence of the Hall resistivity in the nanosheets, while the magnetic field angle dependence measurements of the magnetoresistance also proved the existence of AMR.
In summary, the 2D Cr-based chalcogenide materials exhibit exceptional physical properties, such as high Curie temperature, tunable ferromagnetism, anomalous Hall effect and anisotropic magnetoresistance, render them highly promising candidates for magnetic storage and spintronic devices. Besides, it is crucial to explore additional avenues for optimizing the ferromagnetic properties of 2D Cr-based chalcogenides materials and uncovering novel physical phenomena in order to enhance the application fields for these 2D CrXn compounds.
4.3 Novel optoelectronic properties of 2D CrXn materials
4.3.1 Nonlinear optical characteristics of 2D CrXn
The diverse phases of Cr-based chalcogenides have led to the development of a family of materials with great potential for various applications, such as nonlinear optics, electronic and optoelectronic devices. The schematic of second harmonic generation (SHG) is shown in Fig.10(a), showing the process where two photons with a frequency of ω are incident on a nonlinear optical medium and interact to emit a photon with doubled-frequency of 2ω [94]. Xu et al. [94] investigated the nonlinear optical properties of Cr5Te8 hexagonal nanosheets and observed a strong SHG signal when exposing the samples to a 900 nm incident laser with varying powers. The fitting data of Cr5Te8 nanosheets shows a second-order correlation between the SHG signals and laser power [Fig.10(b)], in which the strong SHG intensity confirms that the 2D Cr5Te8 material has the broken symmetry. Furthermore, the angle-resolved polarization-dependent SHG () of this nanosheet is shown in Fig.10(c), which reveals the six-fold symmetry of Cr5Te8 is consistent with the crystal structure. He et al. [53] measured the SHG spectra of monolayer Cr2S3 nanosheets, as shown in Fig.7(d). The SHG polarization with threefold symmetry is consistent with the Cr2S3 structure, and it is believed that its SHG signal originates from the interaction between the Cr2S3/air interface. At the Cr2S3/air interface, the surface potential field is composed of boundary conditions due to the structure of the material, and the interaction between the nonlinear electric dipole at the interface and the incident light contributes to the second harmonic generation. This interface interaction is localized at the boundary and decreases with the increase of the thickness of the 2D material. The SHG measurements are of extraordinary significance in the study of crystal structure symmetry, lattice orientation, and characterization of heterojunction. Additionally, it also enables the characterization of magnetically ordered structures, magnetic domains, and plays a crucial role in many new 2D magnetic materials [140, 141].
Fig.10 (a) Schematic of SHG measurement of Cr5Te8 nanosheets. (b) SHG intensity dependence characteristics with laser power. (c) Angle polarization SHG of Cr5Te8 nanosheets [94]. (d) Angle-polarized SHG of monolayer Cr2S3 [53]. (e) Schematic diagram and OM image of Cr2S3 electrical device. (f) Transfer characteristic curves (Ids−Vg) of Cr2S3 nanosheets with different thicknesses (Vds = 1 V, thickness of Cr2S3 nanosheets are 2.6, 3.6, 4.8, 6 and 7.6 nm, respectively) [106]. (g) Schematic diagram of Cr2S3 homojunction transistor [105]. (h) Device schematic of Cr2S3 photodetector. (i) Detectivity dependence of Cr2S3 photodetector with effective laser power at different laser wavelengths (520, 808 and 1650 nm). (j) Time-resolved photocurrent of Cr2S3 photodetector at Vds = 1 V, a 520 nm laser and 3.28 nW laser power [142]. (k) Conductivity (black) and on/off ratio (red) of Cr2S3 transistors with Se doping concentration, respectively. (l) Time-resolved photoresponse of a Se doped Cr2S3 transistor at Vds = 1 V and 3.28 nW laser power [143]. |
4.3.2 Electronic and optoelectronic devices of 2D CrXn
In addition to their excellent magnetic properties, Cr-based chalcogenides have recently garnered attention for their electronic and optoelectronic properties. He et al. [53] built a 1.89 nm Cr2S3 field effect transistor (FET) device, which was found to exhibit typical p-type semiconductor behavior with on/off ratios up to 103. Zhang et al. [106] fabricated two-dimensional Cr2S3 transistors to study their electrical properties, and the schematic and optical images of the device are shown in Fig.10(e). Noteworthy, the thickness-dependent variation in conductivity type is exhibited by this 2D Cr2S3 device, as depicted in Fig.10(f), with a typical p-type conductivity behavior observed for 2.6 nm Cr2S3, bipolar type emerging (the thickness > 4.8 nm), and typical n-type (the thickness > 7.6 nm). The hole mobility of the 3.6 nm Cr2S3 is about 6.3 × 10−3 cm2·V−1·s−1, while the electron mobility of the 12.5 nm Cr2S3 reaches around 1.8 cm2·V−1·s−1. The large variation in conductivity type with thickness primarily arises from disparate vacancy concentrations of Cr atoms within the depleted Cr layer of Cr2S3. Subsequently, they fabricated lateral homojunctions of Cr2S3, as shown in Fig.10(g), including pm-ambipolar/n, p/ambipolar, ambipolar/n, and nm-ambipolar/n homojunctions, which were able to enhance the field-effect mobility up to a factor of six in the original direction [105]. In addition, Cr2S3 thin films were prepared by Gao et al. [54] using a two-step CVD method exhibit p-type semiconductor properties, and the conductive behavior remains nearly unaffected by variations in thickness (from 1.9 to 4.9 nm). The hole mobility of Cr2S3 exhibited an increasing trend with the thickness, as the mobility increased from 0.08 to 2.41 cm2·V−1·s−1 when the thickness increased from 1.9 nm to 4.6 nm, while the on/off ratio rose from 101 to 102. Meanwhile, they used the same approach to synthesize Cr2Se3 thin films, which showed a positive correlation between current and the thickness of Cr2Se3. The films did not have gate-regulation properties at all thicknesses. Recently, Zhou et al. [144] successfully achieved heteroatom P-element doping of Cr2S3 nanosheets, resulting in typical n-type doping and significant surface passivation, which enhanced the resistivity by 104 times compared to the intrinsic Cr2S3 samples. In addition, Liu et al. [64] measured the electrical properties of 1T-CrS2 using the four-terminal method from 2 nm to 15 nm, all of which exhibit n-type semiconductor properties. Meanwhile, owing to its 2D structure and high transparency window, as well as the capability to modulate field effect properties within the THz spectral range, it is postulated that 1T-CrS2 exhibits potential applications in the realm of THz sensing and imaging.
Zhang et al. [142] firstly investigated the photodetectivity of Cr2S3 nanosheets and found the bandgap of Cr2S3 film is about 0.15 eV, indicating a typical narrow bandgap semiconductor and has a broader wavelength photoresponse. The schematic diagram of the Cr2S3 photodetector device is depicted in Fig.10(h), in which the Cr/Au were used as the contact electrode. With the excitation light at the wavelengths of 520, 808, and 1550 nm, the device demonstrates high photoresponsivity values of 14.4, 6.0 and 2.0 A·W−1, respectively. Additionally, it exhibits high detectivity values of 4.0 × 1010 Jones, 1.7 × 1010 Jones, and 8.3 × 109 Jones at Vds = 0.1 V, respectively, as shown in Fig.10(i). Meanwhile, the Cr2S3 photodetector was measured to have a response rate tr/td of about 1.7/1.65 s at 520 nm wavelength and a 3.28 nW laser power, as illustrated in Fig.10(j). Li et al. [143] accomplished the doping of Cr2S3 with different concentrations of Se elements, where the conductivity and on/off ratio were strongly dependent on the doping concentration of Se, as shown in Fig.10(k). When the Se doping concentration reached 2.05%, the room-temperature mobility of the Cr2S3 transistor was enhanced from 5.32 × 10−3 to 1.96 × 10−1 cm2·V−1·s−1. Moreover, the 2.05% Se-doped Cr2S3 exhibited excellent optoelectronic performance, as shown in Fig.10(l). At an incident laser of 660 nm and 8.82 nW, the response rate of 2.05% Se-doped Cr2S3 device (tr/td = 28/28 ms) was approximately 200 times faster than that of undoped Cr2S3 (tr/td = 4.9/5.7 s). In addition, the maximum photoresponsivities of Cr2S3 and 2.05% Se-doped Cr2S3 devices were measured to be 3.23 and 10.41 A·W−1, respectively (an incident wavelength of 660 nm). And the corresponding detectivities were determined to be 1.05 × 1011 Jones and 2.37 × 1011 Jones.
In general, Cr-based chalcogenides transistor devices have excellent electrical properties such as mobility variation with thickness, sizable switching ratios, and thickness-dependent conductive behavior. Meanwhile, 2D Cr-based chalcogenides photodetectors are considered as the promising materials for future photodetection in the visible to near-infrared wavelength bands due to their narrow bandgap and other advantages, exhibiting a wide-band optical response, high detectivity and fast response speed. It is possible that the performance of Cr-based chalcogenides photodetectors can be significantly improved through elemental doping, heterojunction formation, and other material modifications as well as device surface and interface engineering [145‒148].
5 Conclusions and perspective
In this article, we provide a comprehensive review of the structures, synthesis strategies, and exceptional physical properties of 2D Cr-based chalcogenides materials. Firstly, from a structural perspective, Cr-X chalcogenides possesses a distinctive interlayer CrX2 intercalation structure that impacts numerous inherent novel properties. Therefore, it is of paramount importance to effectively control the ratio of Cr atom intercalation in order to achieve precise modulation of the physical properties in 2D Cr-X systems. Our primary focus lies on exploring various nanosheet synthesis methods for these systems and examining the advantages offered by liquid-phase exfoliation, solution-based approaches, MBE, and CVD techniques for synthesizing different structured CrXn nanosheets. Notably, considering that most Cr-X systems are non-layered materials, and particular emphasis is also placed on achieving controlled growth of such non-layered materials through van der Waals epitaxy method on substrates devoid of dangling bonds. The phase control of CrXn nanosheets achieved through CVD is comprehensively analyzed, with particular emphasis on the influence of temperature, precursor type, and elemental doping on their phase regulation. Furthermore, a comprehensive summary is presented on the magnetic properties, electrical characteristics, and optoelectronic behavior of CrXn nanosheets and their heterojunctions at room temperature, including their 2D ferromagnetism and antiferromagnetism, magnetic anisotropy, as well as tunable magnetic properties. Additionally, the unique electronic properties and wide-band optoelectronic detection capability arising from the narrow bandgap structure exhibited by semiconductor-characterized Cr2S3 nanosheets are discussed.
However, the growth and application of these materials are limited by several factors: (i) the van der Waals epitaxy of non-layered 2D CrXn thin films has limitations, and the synthesis of non-layered materials is more prone to interfacial stress problems due to the presence of dangling bonds, and wafer-scale ultrathin single-crystal thin film growth is currently not feasible; (ii) considering the existence of surface dangling bonds in non-layered CrXn materials, its surface defect density has a great influence on the properties of the material itself, how to realize the passivation of the surface dangling bonds in non-lamellar materials is also very important; (iii) currently, there is a paucity of studies on the heterojunctions of the Cr-X system, which should be integrated with the distinctive electrical and magnetic properties of Cr-X to fabricate novel heterojunction devices and further expand their application in transistors and spintronic devices; (iv) the utilization of ferromagnetic devices for CrXn nanosheets is still nascent, necessitating crucial device design for these 2D room-temperature ferromagnetic materials to be applied in nonvolatile storage and logic devices; (v) most Cr-X materials are semi-metallic or metallic, precluding their use as switches for electronic devices and limiting their potential applications in electronic and optoelectronic devices, requiring the search for high- carrier mobility and long carrier lifetime of the 2D Cr-X semiconductors.
In conclusion, the synthesis and device applications of 2D Cr-based sulfides still hold significant untapped potential for exploration and possess substantial research value. This review comprehensively presents the theoretical and experimental advancements in 2D Cr-based chalcogenides, while also highlighting the primary challenges and proposing future research strategies for this promising 2D material, thereby providing valuable insights into its prospective research direction.