Ferromagnetism in freestanding MoS2 nanosheets
© Gao et al.; licensee Springer. 2013
Received: 21 January 2013
Accepted: 18 February 2013
Published: 16 March 2013
Freestanding MoS2 nanosheets with different sizes were prepared through a simple exfoliated method by tuning the ultrasonic time in the organic solvent. Magnetic measurement results reveal the clear room-temperature ferromagnetism for all the MoS2 nanosheets, in contrast to the pristine MoS2 in its bulk form which shows diamagnetism only. Furthermore, results indicate that the saturation magnetizations of the nanosheets increase as the size decreases. Combining the X-ray photoelectron spectroscopy, transmission electron microscopy, and electron spin resonance results, it is suggested that the observed magnetization is related to the presence of edge spins on the edges of the nanosheets. These MoS2 nanosheets may find applications in nanodevices and spintronics by controlling the edge structures.
As a kind of layered semiconducting material, molybdenum disulfide (MoS2) has attracted much research interest due its unique physical, optical, and electrical properties correlated with its two-dimensional (2D) ultrathin atomic layer structure [1–4]. Unlike graphite and layered hexagonal BN (h-BN), the monolayer of MoS2 is composed of three atom layers: a Mo layer sandwiched between two S layers. The triple layers are stacked and held together through weak van der Waals interactions [5–10]. Recently, reports demonstrate strong photoluminescence emergence and anomalous lattice vibrations in single- and few-layered MoS2 films [5, 6], which exemplify the evolution of the physical and structural properties in MoS2, due to the transition from a three-dimensional to a 2D configuration. Results also indicate that the single-layer MoS2 exhibits a high channel mobility (approximately 200 cm2 V−1 s−1) and current on/off ratio (1 × 108) when it was used as the channel material in a field-effect transistor . Most recently, it is proposed that the indirect band gap of bulk MoS2 with a magnitude of approximately 1.2 eV transforms gradually to a direct band gap of approximately 1.8 eV in single-layer samples [8, 9], which is in contrast to pristine graphene with a band gap of about 0 eV and few-layered h-BN with a band gap of about 5.5 eV [10, 11]. All these results imply that 2D MoS2 nanosheets have possible potential applications in electronics, optics, and semiconductor technologies as promising complements to graphene and h-BN [5–11].
Recently, based on first-principle calculations, lots of reports reveal the promising electronic properties of monolayer MoS2 nanosheets and nanoribbons, predicting their potential application in spintronic devices [12–15]. Calculation results indicate that MoS2-triple vacancy created in a single-layer MoS2 can give rise to a net magnetic moment, while other defects related with Mo and S atoms do not influence the nonmagnetic ground state . Shidpour et al. performed the calculation on the sulfur vacancy-related magnetic properties on the S-edge with 50% and 100% coverage of MoS2 nanoribbons, showing 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 the S-edge with 100% coverage causes this magnetic property to disappear . Most recently, for the MoS2 nanoribbons, Pan et al. and Li et al. predicted that S-terminated zigzag nanoribbons are the most stable even without hydrogen saturation. MoS2 zigzag nanoribbons are metallic and ferromagnetic, and their conductivity may be semiconducting or half metallic by controlling the edge structures saturated with H atoms. The armchair nanoribbons are semiconducting and nonmagnetic, with band gaps increased by the hydrogen saturation of their edge states [15, 16]. Inconsequently, Botello-Mendez et al. found that armchair nanoribbons could be metallic and exhibit a magnetic moment. Besides, when passivating with hydrogen, the armchair nanoribbons become semiconducting .
Though a lot of interesting magnetic properties of MoS2 nanosheets and nanoribbons had been predicted, the corresponding experimental realization on MoS2 nanosheets and nanoribbons has been at the nascent stage. The reason may be the difficulties in the synthesis methods because MoS2 tends to form zero-dimensional closed structures (fullerene-like nanoparticles) or one-dimensional nanotube structures during the experimental fabrications as well as lower crystalline structures [18–20]. What we know so far, the only experimental report on magnetism in MoS2 came from a study on MoS2 nanosheet film deposition on Si (100) and tantalum foil substrates synthesized using thermal evaporation method. A confirmatory test was also employed to rule out the samples' contaminants, where MoS2 nanotubes fabricated on an alumina template using the similar source and setup were tested to be nonmagnetic . However, the interface between the film and substrate as well as the substrate itself could influence the local structures and, subsequently, the magnetic properties of the samples . Therefore, synthesis and understanding of the edge-based magnetism in substrate-free MoS2 nanosheets or nanoribbons are very necessary, and a further sensitive experimental verification is required.
In this paper, solution exfoliation method was employed to fabricate the MoS2 nanosheets with different sizes . The structure and the magnetic properties of these nanosheets were studied.
MoS2 nanosheets were prepared through exfoliation of bulk MoS2 (purchased from the J&K Chemical, Beijing, China) with different times. In a typical synthesis progress, 0.5-g MoS2 powders were sonicated in N,N-dimethylformamide (DMF, 100 mL) to disperse the powder for 2, 4, 6, 8, and 10 h, respectively. After precipitation, the black dispersion was centrifuged at 2,000 rpm for about 20 min to remove the residual large-size MoS2 powders. Then, the remainder solution was centrifuged at 10,000 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.
The morphologies of the samples were obtained by high-resolution transmission electron microscopy (HRTEM, Tecnai™ G2 F30, FEI, Hillsboro, OR, USA). X-ray diffraction (XRD, X'Pert PRO PHILIPS (PANalytical B.V., Almelo, The Netherlands) with CuKα radiation) and selected area electron diffraction (SAED) were employed to study the structure of the samples. The measurements of magnetic properties were made using the Quantum Design MPMS magnetometer (Quantum Design, Inc., San Diego, CA, USA) based on a superconducting quantum interference device (SQUID). The spectrometer at a microwave frequency of 8.984 GHz was used for electron spin resonance (ESR JEOL, JES-FA300, JEOL Ltd., Akishima, Tokyo, Japan) measurements. X-ray photoelectron spectroscopy (XPS, VG ESCALAB 210, Thermo VG Scientific, East Grinstead, UK) was utilized to determine the bonding characteristics and the composition of the samples. The vibration properties were characterized by Raman scattering spectra measurement, which was performed on a Jobin Yvon LabRam HR80 spectrometer (HORIBA Jobin Yvon Inc., Edison, NJ, USA; with a 325-nm line of Torus 50-mW diode-pumped solid-state laser (Laser Quantum, San Jose, CA, USA)) under backscattering geometry. The infrared absorption spectra of the samples were conducted with the KBr pellet method on a Fourier transform infrared spectrometer (FTIR; NEXUS 670, Thermo Nicolet Corp., Madison, WI, USA) in the range of 400 to 4,000 cm−1. Atomic force microscopy (AFM; Dimension 3100 with Nanoscope IIIa controller, Veeco, CA, USA) was used to confirm the layer number by measuring the thicknesses in tapping mode in air.
Results and discussion
First-principle calculation results reveal the nonmagnetic properties for the infinitely single-layered MoS2, and the FM in MoS2 nanoribbons is considered to be dominated by its zigzag edges [15, 16], In addition, the unit magnetic moment of MoS2 nanoribbons (magnetic moment per MoS2 molecular formula) decreases gradually with increasing ribbon width, implying that the magnetism of MoS2 nanoribbons gets weaker and weaker as the ribbon width increases and disappears finally in the infinitely single-layered MoS2 and bulk. In our case, the size of the nanosheets decreases gradually with increasing ultrasonic time in the organic solvent DMF, and the enhancement of the FM for the nanosheets was also observed as the size decreases. This is because the magnetic behavior in MoS2 nanosheets results from the unsaturated edge atoms, and the ratio of edge atoms vs. total atoms increases dramatically as the size decreases. Therefore, the observed FM in MoS2 nanosheets is considered to be related to the intrinsic morphology of the materials.
In summary, MoS2 nanosheets of different sizes were fabricated by exfoliation of bulk MoS2 in DMF solution. Magnetic measurements indicate that all the fabricated MoS2 nanosheets show obvious RT FM, and the enhanced FM was observed as the size of the nanosheets decreases. The intrinsic room-temperature FM for the samples is considered to be related to the presence of edge spins on the edges of the nanosheets.
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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