Due to the recent expanding interest in two-dimensional layered materials, molybdenum disulfide (MoS2) has been receiving much research attention. Having an ultrathin layered structure and an appreciable direct band gap of 1.9 eV in the monolayer regime, few-layer MoS2 has good potential applications in nanoelectronics, optoelectronics, and flexible devices. In addition, the capability of controlling spin and valley degrees of freedom makes it a promising material for spintronic and valleytronic devices. In this review, we attempt to provide an overview of the research relevant to the structural and physical properties, fabrication methods, and electronic devices of few-layer MoS2. Recent developments and advances in studying the material are highlighted.

Few-Layer MoS2: A Promising Layered Semiconductor

Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

/pdf/room-temperature-ferromagnetism-in-graphite-driven-by-two-36ae7e3kaz.pdf

Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

The development of an active, earth-abundant, and inexpensive catalyst for the oxygen evolution reaction (OER) is highly desirable but remains a great challenge. Here, by combining experiments and first-principles calculations, we demonstrate that MoS2 quantum dots (MSQDs) are efficient materials for the OER. We use a simple route for the synthesis of MSQDs from a single precursor in aqueous medium, avoiding the formation of unwanted carbon quantum dots (CQDs). The as-synthesized MSQDs exhibit higher OER activity with a lower Tafel slope in comparison to that for the state of the art catalyst IrO2/C. The potential cycling of the MSQDs activates the surface and improves the OER catalytic properties. Density functional theory calculations reveal that MSQD vertices are reactive and the vacancies at the edges also promote the reaction, which indicates that the small flakes with defects at the edges are efficient for the OER. The presence of CQDs affects the adsorption of reaction intermediates and dramaticall...

MoS2 Quantum Dots as Efficient Catalyst Materials for the Oxygen Evolution Reaction

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.

/pdf/ferromagnetism-in-freestanding-mos2-nanosheets-1awbz9ebrt.pdf

Ferromagnetism in freestanding MoS2 nanosheets

Room-temperature ferromagnetism is observed in undoped sphalerite and wurtzite CdS nanostructures which are synthesized by hydrothermal methods. Scanning electron microscopy and transmission electron microscopy results indicate that the sphalerite CdS samples show a spherical-like shape and the wurtzite CdS ones show a flower-like shape, both of which are aggregated by lots of smaller particles. The impurity of the samples has been ruled out by the results of X-ray diffraction, selected-area electron diffraction, and X-ray photoelectron spectroscopy. Magnetization measurements indicate that all the samples exhibit room-temperature ferromagnetism and the saturation magnetization decreases with the increased crystal sizes, revealing that the observed ferromagnetism is defect-related, which is also confirmed by the post-annealing processes. This finding in CdS should be the focus of future electronic and spintronic devices.

/pdf/ferromagnetism-in-sphalerite-and-wurtzite-cds-nanostructures-4p3j8jgicy.pdf

Ferromagnetism in sphalerite and wurtzite CdS nanostructures

The room temperature ferromagnetism at the interface between CuO and Cu2O microspheres is reported in this letter. Magnetic measurement results indicate that the interface-based ferromagnetism can be tuned by changing the phase rations of the CuO/Cu2O compounds synthesized by one-step hydrothermal method at different reaction times. The indirect double-exchange model was employed to explain the observed ferromagnetism. This finding suggests that tailoring the interfaces is an effective route to engineer architecture with enhanced magnetic properties and establish a striking example of emergent phenomena at oxide interfaces.

Room temperature ferromagnetism in CuO/Cu2O microspheres: Towards interface effect

In this letter, we report the interface-related room temperature ferromagnetism in bulk CuO/Cu2O composites which were synthesized by heat-decomposing or oxidation methods with the CuO and Cu2O as the precursors, respectively. Magnetic measurement results indicate that all the CuO/Cu2O composites show obvious room temperature ferromagnetism, while the samples with the pure CuO and Cu2O phases are paramagnetic. What is more, the ferromagnetism can be tuned by changing the phase rations of the CuO and Cu2O in the composites. The observed ferromagnetism in CuO/Cu2O composites is considered to be related to the interface between CuO and Cu2O phases.

Interface mediated ferromagnetism in bulk CuO/Cu2O composites

In this paper, we report the observation of intrinsic room temperature ferromagnetism in pure La2O3 nanoparticles. Magnetism measurement indicates that all of the samples exhibit room temperature ferromagnetism and the saturation magnetization for the samples decreases with the increase in annealing temperature from 700 to 1,000 °C. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in the La2O3 nanoparticles. The fitting results of the O 1s spectrum indicate that the variation of the oxygen vacancy concentration is in complete agreement with the change of the saturation magnetization. It is also found that the saturation magnetization of the La2O3 nanoparticles can be tuned by post-annealing in argon or oxygen atmosphere. These results suggest that the oxygen vacancies are largely responsible for the room temperature ferromagnetism in pure La2O3 nanoparticles.

Observation of room temperature ferromagnetism in pure La2O3 nanoparticles

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