Narrow Results

Results of research in the quantum capacitance $C_q$ of quasi-2D-crystals are presented. The detected extraordinary behavior of $C_q$ manifests itself in the existence of such energy ranges in which it is practically equal to zero. The causes for the existence of such ranges are: (a) dimensional quantization as a result of the nanoscale of the van der Waals gap, (b) a certain value of the width of the allowed zone in the plane of the layers, which is determined by the value of the effective mass. Intercalation and external electric field are effective factors capable of changing the position of ranges. Thus, in a system connected in a series of electrostatic and quantum capacitances, the resulting capacitance will significantly depend on the $C_q$ behavior. Comparative analyses of structures, chemical bonds, majority of coinciding characteristics of physical quantities in graphite, transition metal dichalcogenides (TMD), and layered crystals A₃B₆ allow us to assert that the obtained qualitative conclusions can be applied for each of them. Calculations of density of states (DOS) of quasi-2D crystals performed by authors within the framework of the improved Kohn–Sham density functional theory (DFT), namely the DFT taking into account the van der Waals forces in it, show a step-like form of DOS qualitatively similar to ours.

We report a systematic study of large-scale growth of high-quality WSe₂ atomic layers directly on SiO₂/Si substrates using a convenient method. Various parameters, especially growth temperatures, flow rate of carrier gas and tube pressure, are investigated in affecting the properties of as-grown WSe₂ flakes in terms of their sizes, shapes and thickness. The pre-annealing step is demonstrated to be a key role in achieving the large-scale growth. Under an optimized condition, the lateral size of triangular single-crystal monolayer WSe₂ is up to 30 $\mu$m and the area of the monolayer thin film can be up to 0.25 mm². And some other interesting features, such as nanoflowers, are observed, which are a promising for catalyzing research. Raman spectrum and microphotoluminescence indicate distinct layer dependent efficiency. Auger electron spectroscopy (AES) studies demonstrate the atomic concentration of the as-grown WSe₂. Electrical transport further shows that the $p$-type WSe₂ field-effect transistors exhibit excellent electrical properties with carrier mobility of $\sim$64 cm${}^2\cdot$V${}^{-1}\cdot$s${}^{-1}$ and current on/off ratio over 10⁵. These results are comparable to the exfoliated materials.

The single-, dual- and triple-wavelength passively mode-locked erbium-doped fiber lasers are demonstrated with MoS₂ and polarization-dependent isolator (PD-ISO). The saturable absorber is fabricated by wrapping an MoS₂ around a microfiber. The intracavity PD-ISO acts as a wavelength-tunable filter with a polarization controller (PC) by adjusting the linear birefringence. Single-wavelength mode-locked fiber laser can self-start with suitable pump power. With appropriate PC state, dual- and triple-wavelength operations can be observed when gains at different wavelengths reach a balance. It is noteworthy that dual-wavelength pulses exhibiting peak and dip sidebands, respectively, are demonstrated in the experiment. The proposed simple and multi-wavelength all-fiber conventional soliton lasers could possess potential applications in numerous fields, such as sensors, THz generations and optical communications.

MoS₂ films on sapphire and SiO₂/Si substrates were fabricated by sulfurization of sputtered Mo precursor films at 900^∘C in a two-zone furnace. Structural, morphological, optical and electrical properties of the films were determined. The films of MoS₂ were successfully grown on the substrates with polycrystalline structures and proposed to be used in electro-optical devices.

Few-layered MoS₂ nanostructures were successfully synthesized by a simple hydrothermal method without the addition of any catalysts or surfactants. Their morphology, structure and photocatalytic activity were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, electrochemical impedance spectra and UV-Vis absorption spectroscopy, respectively. These results show that the MoS₂ nanostructures synthesized at 180${}^{\circ }$C exhibit an optimal visible light photocatalytic activity (99%) in the degradation of Rhodamine B owing to the relatively easier adsorption of pollutants, higher visible light absorption and lower electron–hole pair recombination.

Ultrathin two-dimensional transition metal dichalcogenides represent a promising new class of materials with surface-enhanced Raman scattering (SERS) activities. Previous work mainly focused on the SERS effects of transition metal dichalcogenide crystals with sizes of micrometer scale. The enhancement effect is usually nonuniform on the SERS substrates. In this paper, we report a chemical vapor deposition method for the preparation of centimeter-scale MoSe₂ films with uniform and high SERS activity. The molybdenum precursors are supplied in a “face-to-face” manner from a silica gel plate to the growth substrate, which guarantees uniform nucleation and growth of the monolayer MoSe₂. We demonstrate that the monolayer MoSe₂ film has strong SERS effect, which can be attributed to charge transfer between MoSe₂ and the probe molecules. The detection limit of Rhodamine B on monolayer MoSe₂ is determined to be 10${}^{-7}$mol/L, and the Raman enhancement factor is estimated to be $5.58\times 10^4$. Due to its atomic uniformity and chemical stability, the MoSe₂ film can serve as an ideal two-dimensional SERS material for detection of various molecules.

The recent emergence of two-dimensional (2D) layered materials — graphene and transition metal dichalcogenides — opens a new avenue for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. Here, we provide a brief review of experimental achievements concerning electrical spin injection, spin transport, graphene nanoribbons spintronics and transition metal dichalcogenides spin and pseudospins. Future research in 2D layered materials spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including pseudospins-valley phenomena in graphene and other 2D materials.

Interface engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) has been regarded as a promising strategy for modulating their outstanding electrical and optoelectronic properties due to their inherent 2D nature and large surface-to-volume ratio. In particular, introducing organic molecules and polymers directly onto the surface of TMDs has been explored to passivate surface defects or achieve better interfacial properties with neighboring surfaces efficiently, thus leading to great opportunity for the realization of high-performance TMD-based applications. This review describes recent progress in the interface engineering of TMDs with organic molecules and polymers, which involves the modulation of their electrical and optoelectronic characteristics. Depending on the interfaces between the surfaces of the TMDs and the dielectric, conductive contacts or ambient environment, we present various strategies for introducing organic interlayers. In addition, the role of native defects on the surface of TMDs, such as adatoms or vacancies, in determining their electrical characteristics is also discussed in detail. Finally, future challenges and the outlook associated with interface engineering are highlighted.

Angle-resolved photoemission spectroscopy is a powerful technique that can directly probe the electronic structure of materials. By utilizing the synchrotron radiation, which can provide high-intense monochromatic photon beam with tunable polarization and photon energy, ARPES can resolve detailed electronic structure with ultra-high energy and momentum resolutions, detect three-dimensional momentum space, and probe electrons with different orbital characters selectively, etc. In this chapter, we will briefly review the recent progress obtained by ARPES using the synchrotron radiation. The focused materials will cover the iron-based high-T_C superconductors, the charge-density-wave materials, and Mott-insulators.