Ferromagnetism in ultrathin MoS2 nanosheets: from amorphous to crystalline
© Zhang et al.; licensee Springer. 2014
Received: 10 July 2014
Accepted: 16 October 2014
Published: 22 October 2014
Two-dimensional materials have various applications in the next generation nanodevices because of their easy fabrication and particular properties. In this work, we studied the effects of crystalline order on the magnetic properties of ultrathin MoS2 nanosheets. Results indicate that all the fabricated samples show clear room temperature ferromagnetism. The amorphous sample has the larger saturation magnetization than that of the crystallized samples, where the disordered grain boundary or defects in the nanosheets are considered to be responsible for the long-range magnetic order. These MoS2 nanosheets with versatile functions may have potential applications in spintronics, nanodevices, and photodevices.
Magnetic ordering in materials that do not have partially filled d or f orbitals named defect-induced magnetism (DIM) is a topic of interest in recent years [1–3]. Until now, the DIM phenomenon has been finally observed in a broad spectrum of materials, from several oxides to carbon based materials [4, 5] and to the transition-metal dichalcogenide with the atomic thickness [6, 7].
Recently, as one of the central members in transition metal dichalcogenide compounds, MoS2 with its remarkable electronic properties such as charge density wave transitions in transition metal dichalcogenides make the material interesting from both fundamental and applied research perspectives [8–10]. For example, reports demonstrate strong photoluminescence emergence and anomalous lattice vibrations in single- and few-layered MoS2 films [11, 12]. Results also indicate that the single-layer MoS2 exhibits a high channel mobility (approximately 200 cm2V−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 [14, 15], which is in contrast to pristine graphene with a band gap of approximately 0 eV and few-layered h-BN with a band gap of approximately 5.5 eV .
Recently, theoretical studies suggested that, although bulk MoS2 is a diamagnetic material, it becomes ferromagnetic when MoS2 nanoribbons are formed with zigzag edges, defects are induced, or nonmetals such as H, B, C, N, and F are absorbed [17, 18]. In addition, formation of magnetic moments was also reported in MonS2n clusters, nanoparticles, and nanoribbons of MoS2 from first principle studies [19–21]. Experimentally, the interesting ferromagnetism phenomenon in nanosheets of MoS2 had previously been reported and the observed magnetic signal is attributed to the presence of unsaturated edge atoms . The weak ferromagnetism was also obtained in the freestanding nanosheets and in the bulk MoS2 irradiated by proton, which increases the TC up to 895 K [23, 24]. Further, the existence of magnetism in the pristine MoS2 has been reported despite that there are differences depending on the layer thickness .
Although such unexpected ferromagnetic behavior ascribed to edge states and defects in MoS2 has been reported, there are no experimental observations of ferromagnetism in MoS2 with a different degree of crystallization. In this letter, MoS2 nanosheets from amorphous to crystalline were prepared by a hydrothermal method reaction at different temperatures. The structure and magnetic properties were studied. Results indicate that all the samples show clear room ferromagnetism and the ferromagnetism weakened with the crystalline order increasing.
MoS2 nanosheets with different crystalline order were synthesized by the hydrothermal method. Sodium molybdate dihydrate, 1.5 g, and thioacetamide, 2.2 g, were dissolved together in 50 ml of deionized water and stirred for 2 h. Then, the solution was transferred into a Teflon-lined stainless steel autoclave and reacted at different temperatures (180°C, 200°C, 220°C and 240°C) for 24 h. Finally, the products were filtered and washed with water and dried in a vacuum.
X-ray diffraction (XRD, X'Pert PRO PHILIPS with Cu Kα radiation) was employed to study the crystal structure. The morphology of the samples was obtained by using the high-resolution transmission electron microscopy (HRTEM, TecnaiTM G2 F30, FEI, Hillsboro, OR, USA). X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra, Manchester, UK) was utilized to determine the bonding characteristics of the samples. The composition was confirmed by an inductively coupled plasma atomic emission spectrometer (ICP, ER/S). The measurements of magnetic properties were made using the Quantum Design MPMS magnetometer based on superconducting quantum interference device (SQUID).
Results and discussion
ICP results for the MoS 2 nanosheets
Element content (ppm)
The ultrathin MoS2 nanosheets with different crystalline order were fabricated, and their magnetic properties were studied. Results indicate that all the fabricated samples show clear room ferromagnetism and the amorphous sample has larger saturation magnetization owing to having more disordered grain boundary or defects. This unusual ferromagnetism may open perspectives for its application in spintronics devices.
This work is supported by the NSFC (Grant No. 51202101, 11474137), the Specialized Research Fund for the Doctoral Program of Higher Education (20120211120005), the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China, the Open Project of Key Laboratory for Magnetism, and the Magnetic Materials of the Ministry of Education of Lanzhou University (LZUMMM2014015).
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