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.
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.
- Aharon E, Albo A, Kalina M, Frey GL: Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Adv Funct Mater 2006, 16: 980. 10.1002/adfm.200500458View ArticleGoogle Scholar
- Lee HS, Min SW, Chang YG, Park MK, Nam T, Kim H, Kim JH, Ryu S, Im S: MoS2 nanosheet phototransistors with thickness-modulated optical energy gap. Nano Lett 2012, 12: 3695. 10.1021/nl301485qView ArticleGoogle Scholar
- Seayad AM, Antonelli DM: Recent advances in hydrogen storage in metal-containing inorganic nanostructures and related materials. Adv Mater 2004, 16: 765. 10.1002/adma.200306557View ArticleGoogle Scholar
- Mosleh M, Atnafu ND, Belk JH, Nobles OM: Modification of sheet metal forming fluids with dispersed nanoparticles for improved lubrication. Wear 2009, 267: 1220. 10.1016/j.wear.2008.12.074View ArticleGoogle Scholar
- Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A: Single-layer MoS2 transistors. Nat Nanotech 2011, 6: 147.View ArticleGoogle Scholar
- Mak KF, Lee C, Hone J, Shan J, Heinz TF: Atomically thin MoS2: a new direct-gap semiconductor. Phys Rev Lett 2010, 105: 136805.View ArticleGoogle Scholar
- Matte HSSR, Gomathi A, Manna AK, Late DJ, Datta R, Pati SK, Rao CNR: MoS2 and WS2 analogues of graphene. Angew Chem Int Edit 2010, 49: 4059. 10.1002/anie.201000009View ArticleGoogle Scholar
- Lauritsen JV, Kibsgaard J, Helveg S, Topsoe H, Clausen BS, Laegsgaard E, Besenbacher F: Size-dependent structure of MoS2 nanocrystals. Nat Nanotech 2007, 2: 53. 10.1038/nnano.2006.171View ArticleGoogle Scholar
- Zhan Y, Liu Z, Najmaei S, Ajayan PM: Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate. Small 2012, 8: 966. 10.1002/smll.201102654View ArticleGoogle Scholar
- Eda G, Yamaguchi H, Voiry D, Fujita T, Chen MW, Chhowalla M: Photoluminescence from chemically exfoliated MoS2. Nano Lett 2011, 11: 5111. 10.1021/nl201874wView ArticleGoogle Scholar
- Mathew S, Gopinadhan K, Chan TK, Yu XJ, Zhan D, Cao L, Rusydi A, Breese MBH, Dhar S, Shen ZX, Venkatesan T, Thong JTL: Magnetism in MoS2 induced by proton irradiation. Appl Phys Lett 2012, 101: 102103. 10.1063/1.4750237View ArticleGoogle Scholar
- Li H, Yin Z, He Q, Li H, Huang X, Lu G, Fam DWH, Tok AIY, Zhang Q, Zhang H: Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing NO at room temperature. Small 2012, 8: 63. 10.1002/smll.201101016View ArticleGoogle Scholar
- Furimsky E: Role of MoS.sub.2 and WS.sub.2 in hydrodesulfurization. Catal Rev Sci Eng 1980, 22: 371. 10.1080/03602458008067538View ArticleGoogle Scholar
- Braga D, Gutiérrez Lezama I, Berger H, Morpurgo AF: Quantitative determination of the band gap of WS2 with ambipolar ionic liquid-gated transistors. Nano Lett 2012, 12: 5218. 10.1021/nl302389dView ArticleGoogle Scholar
- Fang H, Chuang S, Chang TC, Takei K, Takahashi T, Javey A: High-performance single layered WSe2 p-FETs with chemically doped contacts. Nano Lett 2012, 12: 3788. 10.1021/nl301702rView ArticleGoogle Scholar
- Zhao WJ, Ghorannevis Z, Chu LQ, Toh ML, Kloc C, Tan PH, Eda G: Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. ACS Nano 2013, 7: 791. 10.1021/nn305275hView ArticleGoogle Scholar
- Gutierrez HR, Perea-Lopez N, Elias AL, Berkdemir A, Wang B, Lv R, Lopez-Urias F, Crespi VH, Terrones H, Terrones M: Extraordinary room-temperature photoluminescence in WS2 triangular monolayers. Nano Lett 2013, 13: 3447. 10.1021/nl3026357View ArticleGoogle Scholar
- Murugan P, Kumar V, Kawazoe Y, Ota N: Atomic structures and magnetism in small MoS2 and WS2 clusters. Phys Rev A 2005, 71: 063203.View ArticleGoogle Scholar
- Ma YD, Dai Y, Guo M, Niu CW, Lu JB, Huang BB: Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers. Chem Chem Phys 2011, 13: 15546. 10.1039/c1cp21159eView ArticleGoogle Scholar
- Ramakrishna Matte HSS, Maitra U, Kumar P, Rao BG, Pramoda K, Rao CNR, Anorg Z: Synthesis, characterization, and properties of few-layer metal dichalcogenides and their nanocomposites with noble metal particles, polyaniline, and reduced graphene oxide. Allg Chem 2012, 638: 2617. 10.1002/zaac.201200283View ArticleGoogle Scholar
- Coleman JN, Lotya M, O'Neill A, Bergin SD, King PJ, Khan U, Young K, Gaucher A, De S, Smith RJ, Shvets IV, Arora SK, Stanton G, Kim HY, Lee K, Kim GT, Duesberg GS, Hallam T, Boland JJ, Wang JJ, Donegan JF, Grunlan JC, Moriarty G, Shmeliov A, Nicholls RJ, Perkins JM, Grieveson EM, Theuwissen K, Mccomb DW, Nellist PD, Nicolosi V: Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science 2011, 331: 568. 10.1126/science.1194975View ArticleGoogle Scholar
- Gao DQ, Si MS, Li JY, Zhang J, Zhang ZP, Yang ZL, Xue DS: Ferromagnetism in freestanding MoS2 nanosheets. Nanoscale Res Lett 2013, 8: 129. 10.1186/1556-276X-8-129View ArticleGoogle Scholar
- Mayer JC, Chuvilin A, Algara-Siller G, Biskupek J, Kaiser U: Selective sputtering and atomic resolution imaging of atomically thin boron nitride membranes. Nano Lett 2009, 9: 2683. 10.1021/nl9011497View ArticleGoogle Scholar
- Yen PC, Huang YS, Tiong KK: The growth and characterization of rhenium-doped WS2 single crystals. J Phys Condens Matter 2004, 16: 2171. 10.1088/0953-8984/16/12/025View ArticleGoogle Scholar
- Rao CNR, Matte HSSR, Subrahmanyam KS, Maitra U: Unusual magnetic properties of graphene and related materials. Chem Sci 2012, 3: 45. 10.1039/c1sc00726bView ArticleGoogle Scholar
- Enoki T, Takai K: Unconventional electronic and magnetic functions of nanographene-based host–guest systems. Dalton Trans 2008, 8: 3773.View ArticleGoogle Scholar
- Zhang J, Soon JM, Loh KP, Yin J, Ding J, Sullivian MB, Wu P: Magnetic molybdenum disulfide nanosheet films. Nano Lett 2007, 7: 2370. 10.1021/nl071016rView ArticleGoogle Scholar
- Vojvodic A, Hinnemann B, Nørskov JK: Magnetic edge states in MoS2 characterized using density-functional theory. Phys Rev B 2009, 80: 125416.View ArticleGoogle Scholar
- Ataca C, Sahin H, Akturk E, Ciraci S: Mechanical and electronic properties of MoS2 nanoribbons and their defects. J Phys Chem C 2011, 115: 3934. 10.1021/jp1115146View ArticleGoogle Scholar
- Shidpoura R, Manteghian M: A density functional study of strong local magnetism creation on MoS2 nanoribbon by sulfur vacancy. Nanoscale 2010, 2: 1429. 10.1039/b9nr00368aView ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.