Ferromagnetism in exfoliated tungsten disulfide nanosheets
© Mao et al.; licensee Springer. 2013
Received: 25 June 2013
Accepted: 18 August 2013
Published: 17 October 2013
Two-dimensional-layered transition metal dichalcogenides nanosheets have attracted tremendous attention for their promising applications in spintronics because the atomic-thick nanosheets can not only enhance the intrinsic properties of their bulk counterparts, but also give birth to new promising properties. In this paper, ultrathin tungsten disulfide (WS2) nanosheets were gotten by liquid exfoliation route from its bulk form using dimethylformamide (DMF). Compared to the antiferromagnetism bulk WS2, ultrathin WS2 nanosheets show intrinsic room-temperature ferromagnetism (FM) with the maximized saturation magnetization of 0.004 emu/g at 10 K, where the appearance of FM in the nanosheets is partly due to the presence of zigzag edges in the magnetic ground state at the grain boundaries.
KeywordsWS2 ferromagnetism nanosheet
Together with the rapidly increasing research interests on graphene and their devices in the last few years, inorganic-layered structure materials, such as tungsten disulfide (WS2) and MoS2 also attracted extensive attention because of their unique physics properties [1–5]. Similar to graphite, such layered structure materials crystallize in a van der Waals-layered structure where each layer consists of a slab of S-X-S (X = W, Mo) sandwich. MoS2 monolayers have been isolated via mechanical exfoliation, wet chemical approaches, physical vapor deposition, and sulfurization of molybdenum films [6–9]. At the same time, their electronic, optical, and magnetic properties including carrier mobilities of approximately 200 cm2V−1s−1, photoluminescence, and weak room temperature ferromagnetism have been proposed [1–5, 10, 11]. So far, MoS2 has been explored in diverse fields and integrated in transistors and sensors, and used as a solid-state lubricant and catalyst for hydrodesulfurization, hydrogen evolution, and so on [6–9, 12, 13].
Recently, mechanically exfoliated, atomically thin sheets of WS2 were also shown to exhibit high in-plane carrier mobility and electrostatic modulation of conductance similar to MoS2[14, 15]. Differential reflectance and photoluminescence spectra of mechanically exfoliated sheets of synthetic 2H-WS2 with thicknesses ranging between 1 and 5 layers were also reported, where the excitonic absorption and emission bands were found as gradually blue shifted with decreasing number of layers due to geometrical confinement of excitons . Gutiérrez et al. described the direct synthesis of WS2 monolayers via sulfurization of ultrathin WO3 films with triangular morphologies and strong room-temperature photoluminescence , which could be used in applications including the fabrication of flexible/transparent/low-energy optoelectronic devices.
Even though the electrical, mechanical, and optical properties of WS2 have been studied both theoretically and experimentally, recent studies on the magnetic response of WS2 are limited. Murugan et al. revealed by first-principles calculations that stoichiometric Mo n S2n (n = 1, 2, 5, and 6) and W6S12 clusters as well as several of the nonstoichiometric clusters are magnetic, where the magnetic moments arise due to the partially filled d states . Besides, calculation results indicate that adsorption of nonmetal elements on the surface of WS2 nanosheets can induce a local magnetic moment . In an experimental study, Matte et al. fabricated WS2 nanosheets by hydrothermal method and revealed their ferromagnetism, which was considered to be related to the edges and defects .
Developed liquid exfoliation process is considered to be an effective pathway to prepare the ultrathin two-dimensional nanosheets of intrinsically layered structural materials with high quality . In this paper, the ultrathin WS2 nanosheets were gotten by exfoliating bulk WS2 in N,N-dimethylformamide (DMF, 100 mL) solution as in our previous report , and we studied the magnetic properties of WS2 nanosheets experimentally from 300 K down to 10 K. Results indicate that the fabricated WS2 nanosheets show clear room-temperature ferromagnetism which possibly originates from the existence of zigzag edges or defects with associated magnetism at grain boundaries.
WS2 nanosheets were prepared through exfoliating of bulk WS2. In a typical synthesis progress, 0.5 g of WS2 powders was sonicated in N, N-Dimethylformamide (DMF, 100 mL) to disperse the powder. After precipitation, the black dispersion was centrifuged at 2000 rpm for about 20 minutes to remove the residual large-size WS2 powders. Then, the remainder solution was centrifuged at 10000 rpm for 1 h to obtain the black products. To remove the excess surfactant, the samples were repeatedly washed with ethanol and centrifuged. Finally, the samples were dried at 60°C in vacuum condition.
Results and discussion
Recently, similar ferromagnetic nature was also observed in other layered materials, like graphene, graphene nanoribbons, and MoS2. Matte et al. and Enoki et al. proposed that edge states as well as adsorbed species affect the magnetic properties of graphene [25, 26]. Zhang et al. prepared MoS2 samples with high density of prismatic edges and showed them to be ferromagnetic at room temperature, where the magnetism arising from nonstoichiometry of the unsaturated Mo and S atoms at the edge . Our previous results indicate that the saturation magnetizations of the exfoliated MoS2 nanosheets increase as the lateral size decreases, revealing the edge-related ferromagnetism . Density functional calculations on inorganic analog of graphite MoS2 reveal that edge states are magnetic and it appears that magnetism originates at the sulfur-terminated edges due to the splitting of metallic edge states at the Fermi level . Besides, calculation results indicate that only MoS2-triple vacancy created in a single-layer MoS2 can give rise to a net magnetic moment . Shidpour et al. indicated that a vacancy on the S-edge with 50% coverage intensifies the magnetization of the edge of the MoS2 nanoribbon, but such a vacancy on S-edge with 100% coverage causes this magnetic property to disappear . Furthermore, MoS2 and WS2 clusters (Mo6S12 and W6S12) were shown to be magnetic, where the magnetism arising from the unsaturated central metal atom is due to partially filled d orbitals . In our case, the WS2 nanosheets with 2 ~ 8 layers thick and the presence of the high density of edges can be seen from the images in Figure 2f. The bends in the layers may arise from the defects. Besides, the high-resolution TEM image of the nanosheets shown in Figure 2d reveals a hexagonal arrangement of atoms with zigzag edges. Such defective centers and edges would be associated with the W atoms, which are undercoordinated, resulting in partially filled d orbitals. A high concentration of such edges and defects in our samples could be one of the possible reasons for the observation of ferromagnetism.
In summary, even though the observed ferromagnetism in WS2 is in the bulk limit, results indicate that the ferromagnetism for exfoliated WS2 nanosheets persists from 10 K to room temperature. We attribute the existence of ferromagnetism partly to the zigzag edges and the defects in our samples. This unusual room-temperature ferromagnetism, which is an intrinsic feature similar to that observed in carbon-based materials, may open perspectives for spintronic devices in the future.
This work is supported by the National Basic Research Program of China (grant no. 2012CB933101), NSFC (grant nos. 11034004 and 51202101), the Fundamental Research Funds for the Central Universities (no. lzujbky-2012-28), and the Specialized Research Fund for the Doctoral Program of Higher Education.
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