The discovery of graphene in 2004^[1] has opened up intense research of various two-dimensional (2D) materials, such as transition-metal dichalcogenides,^[2] black phosphorus,^[3] hexagonal boron nitride,^[4] InSe,^[5] etc. The studies on these materials as well as stacked heterostructures^{[6, 7]} based on them have revealed interesting properties and potential applications. Despite the vast family of 2D crystals, only a few exhibit intrinsic magnetic orders. The discovery of suitable 2D magnetic materials holds promise for future nanoelectronics and spintronic applications. Therefore, recently some investigations on ferromagnetic CrI₃,^[8] Cr₂Ge₂Te₆,^[9] and antiferromagnetic transition-metal trichalcogenides with the chemical formula of MPX ₃^{[10, 11]} have been reported.

Among all the predicted and experimentally observed antiferromagnetic transition-metal trichalcogenides, a very promising material of particular interest is FePS₃. There have been many studies of FePS₃ since its discovery,^[12–16] not only in the field of spintronics,^{[17, 18]} but also in the areas of catalyst chemistry^[19] and electronic devices. For example, Raman spectroscopy measurements show the Ising-type antiferromagnetic order can even survive down to the monolayer limit.^[20] FePS₃ quantum sheets demonstrate a substantially accelerated photocatalytic H₂ generation rate, which is up to three times higher than the bulk counterpart.^[21] Both negative and positive photoconductivity exist in FePS₃-based photodetectors and can be controllably switched with bias voltage.^[22] Existing studies make FePS₃ an ideal candidate for 2D magnetic as well as optoelectronic systems. However, the corresponding research is still in its infancy, which motivates us to deeply investigate the crystallographic and magnetic properties of FePS₃ using various experimental characterization methods.

In this work, the chemical vapor transport (CVT) route, which has been considered as a promising way to synthesize layered materials, is utilized to grow FePS₃ with high quality in a large scale. The as-grown crystals show highly anisotropic magnetic properties. The frequency-independent behavior of alternating current (ac) magnetic susceptibility rules out the spin freezing. Instead, they represent the clear antiferromagnetic phase transition and the magnetic ordering is of Ising-type.

High quality single crystals of FePS₃ were grown by a typical CVT method. The stoichiometric amount high purity elements (99.99% Fe, 99.999% P, and 99.999% S from Alfa Aesar) and iodine as the transport agent were placed in a quartz ampoule and sealed under vacuum. The ampoule was further placed in a horizontal 2-zone furnace. The temperatures were set at 750 °C (hot zone) and 650 °C (cold zone) and kept at that condition for 21 days. The quality of crystals was characterized by a Rigaku Dmax-rB diffractometer with Cu radiation. A scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) was used to check the surface morphology and composition ratio of the synthesized crystals. The microstructure of crystals was also observed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) using JEM-200CX microscope. For the study of Raman spectra, a He–Ne laser (λ =488 nm) was used as the excitation beam. The x-ray photoelectron spectroscopy (XPS) spectra were obtained using PHI 5000 VersaProbe instrument. The direct current (dc) magnetizations from 300 K to 10 K were measured with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design) under a magnetic field of 500 Oe, and the low temperature ac magnetic susceptibility was measured by physical property measurement system (PPMS, Quantum Design).

In FePS₃ crystal, the magnetic Fe atoms form a honeycomb arrangement and each of them is octahedrally coordinated by six S atoms with a small trigonal distortion. The P atom is coordinated with three S atoms and one P atom to form a [P₂S₆]⁴⁻ unit. The S atomic layers are arranged along the c direction in an ABCABC stacking sequence, forming in a monoclinic structure with the space group of C2/m. The layered nature is apparent from the crystal structure in Fig. 1(b), where the topography of van der Waals gap separating layers is visible. Hence, it is possible to make FePS₃ easily exfoliated by scotch tape.

Fig. 1.

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	Fig. 1. (a) In-plane and (b) out-of-plane crystal structures of FePS3.

In our study, the CVT method has been shown to successfully yield high quality FePS₃ crystals. A typical SEM image of as-grown crystal is shown in Fig. 2(a), exhibiting the expected layered structure. As mentioned above, FePS₃ has a hexagonal in-plane structure. So, the straight edges with characteristic inner angles of ∼120° can be easily identified in the morphology. Most of the obtained FePS₃ flakes have an average size of about 10 mm, as shown in Fig. 2(b). The parameters of cleavage energy and cleavage strength are normally utilized to estimate the possibility of mechanical exfoliation from the bulk 2D van der Waals. Previously, it has been shown that the cleavage energy of FePS₃ is comparable to the experimentally estimated value of graphite and the cleavage strength is even lower than that of graphite,^[23] which indicates that the exfoliation of bulk FePS₃ by scotch tap methods is feasible. Here, we have mechanically exfoliated the as-grown bulk FePS₃ crystal onto freshly cleaned Si substrates with a 300-nm-thick SiO₂ top-layer (see the inset of Fig. 2(b)). The resulting FePS₃ sheets show different optical contrast due to the interference. Different colors represent different thicknesses. The preferential crack-propagation along the high-symmetry crystal axis during exfoliation results in typical 120° or 60° angles between sheet edges.^[24] The successful achievement of ultrathin FePS₃ favors for exploring the interesting low dimensional magnetism, as well as for further incorporation FePS₃ into van der Waals heterostructures. The chemical compositions of FePS₃ have been measured by EDS, which reveals that the ratio of Fe:S:P is very close to the expected 1:1:3, suggesting the formation of FePS₃ phase. The TEM morphology image is shown in Fig. 2(c), demonstrating the 2D nanosheet-like geometry of FePS₃. It seems that there are wrinkles in the sample. The SAED pattern is taken with the incident electron beam normal to the ab-plane. Well-defined diffraction spots are clearly shown, confirming the presence of characteristic planes. Our SAED pattern agrees well with that reported by Murayama et al.,^[25] suggesting that the prepared crystals are likely to have rotational twins and are pseudo-3-fold symmetrical.

Fig. 2.

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	Fig. 2. (a) SEM image, (b) photo of bulk FePS3 crystals, and the typical exfoliated FePS3 (the inset in (b)). The bright-field TEM image and electron diffraction pattern are shown in (c) and (d), respectively.

The x-ray diffraction (XRD) data are further collected at room temperature to determine the orientation of the as-grown crystals. As shown in Fig. 3(a), only (00l)- oriented peaks for monoclinic structure (PDF#30-0663) of FePS₃ appear without any other impurities peaks, exhibiting high crystalline phase purity as well as the desired layer structure. Structural characterization has also been performed by Raman spectroscopy. The Raman spectra at room temperature are collected using a 488 nm laser. On the basis of the present experiments and of previous analysis, the Raman peaks can be assigned as follows. The peak around 100 cm⁻¹ originates from the Fe atom vibrations. This peak is a spin-disorder-induced mode and could be replaced by other spin-order-induced modes below the Néel temperature of FePS₃.^[26] The peak around 155 cm⁻¹ is the strongly infrared (IR) active E_u type mode of the P₂S₆ molecule, which can be observed in all the MPS₃ compound.^[27] The other peaks come from [P₂S₆]⁴⁻ anion vibrational modes. The Raman peaks at 247 cm⁻¹ and 380 cm⁻¹ correspond to the A_1g mode, which is the out-of-plane vibration of P₂S₆ entities. The other three peaks observed at 227 cm⁻¹, 279 cm⁻¹, and 579 cm⁻¹ are related to the E_g mode. Overall, the Raman modes observed in our experiment agree well with previous studies.^{[15, 26, 27]} The careful studies on thickness dependence and temperature dependence of Raman spectra for FePS₃ are underway.

Fig. 3.

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	Fig. 3. (a) X-ray diffraction pattern and (b) Raman spectrum of the as-grown FePS3 measured at room temperature.

The surface sensitive XPS has been performed to detect the surface composition and chemical state of FePS₃. The atomic ratio of Fe:S:P is detected to be also about 1:1:3. The corresponding Fe 2p, P 2p, and S 2p XPS spectra are shown in Fig. 4. The Fe 2p spectra appear as well-resolved doublets. The Fe²⁺ state can be ascertained from the peaks of Fe 2p_1/2 and Fe 2p_3/2 at the binding energies of 722 eV and 709 eV. The two resolved S 2p peaks at 162.4 eV and 163.5 eV are assigned to 2p_1/2 and 2p_3/2, respectively, implying Fe–S and P–S entities. The P 2p peaks located at 131.9 eV and 132.5 eV correspond to P 2p_1/2 and P 2p_3/2, respectively. All the values are comparable with previous reported values.^{[19, 28]}

Fig. 4.

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	Fig. 4. XPS spectra of (a) Fe 2p, (b) P 2p, and (c) S 2p for FePS3.

In addition to structural characterization, the magnetic properties of FePS₃ crystals have been further probed. The dc magnetic susceptibilities χ, measured with the 500 Oe field along the layer stacking direction ( , out-of-plane) and with the field in the plane of the FePS₃ layers ( , in-plane), are shown in Fig. 5(a). The curves show clear magnetic phase transition from the antiferromagnetic to paramagnetic phase around Néel temperature. The first derivative of susceptibility curves indicates a transition temperature of ∼115 K, which is in agreement with previous studies.^{[20, 26]} Moreover, considerable anisotropy of χ is apparent. The change of χ out of plane is much larger than that in plane, implying that the spin is aligned along the out-of-plane direction. The anisotropy in FePS₃ is so strong that it will create an energy gap of ∼16 meV in the spin-wave spectrum.^[29] The high-temperature paramagnetic susceptibility in both directions obeys the Curie–Weiss law , where C is the Curie constant from which the effective magnetic moment μ _eff of Fe²⁺ could be deduced, and T _C is the Curie temperature. Fitting the data at high temperature gives an effective moment of , T _C=62 K for , and , T _C=−85 K for , respectively. The different sign of T _C is probably associated with the crystal-field effects arising due to the trigonal elongation of the FeS₆ octahedra.^[30] If we consider the spin of Fe ion only, the theoretical μ _eff would be 4.9 μ _B. The values after fitting are larger than the spin-only effective moment, which has also been reported in the previous studies.^{[30, 31]} This might indicate that the orbital momentum in FePS₃ is not completely quenched.^[30] However, more studies are still needed in the future to give direct evidences. The temperature dependence of the out-of-plane ac susceptibility under different frequencies is also shown in Fig. 5(b). Over the entire range of measured frequencies from 100 Hz to 2000 Hz, no frequency dispersion is observed, which rules out a spin freezing transition.

Fig. 5.

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	Fig. 5. (a) Temperature dependence of dc parallel ( ) and perpendicular ( ) magnetic susceptibility of FePS3. The fitting curve using the Curie–Weiss law is also shown. (b) Temperature dependence of real ac magnetic susceptibility of FePS3 measured at different frequencies from 100 Hz to 2000 Hz.

A comprehensive study of crystallographic and magnetic properties of high quality van der Waals layered FePS₃ single crystals has been performed using SEM, TEM, XRD, Raman, XPS, χ_dc(T), and χ_ac(T). The synthesized FePS₃ show single-crystallinity with the average size of about 10 mm. The crystals can be exfoliated down to few layers. The preferred cleavage angles are 120 degrees or 60 degrees. FePS₃ exhibits typical magnetic transition from the paramagnetic to antiferromagnetic phase. The derived effective moments are larger than the spin-only effective moment, suggesting that an orbital contribution to the total angular momentum of the Fe²⁺ could be present. The ac susceptibility lacks frequency dependence, which means that the spin freezing effect is excluded.