Temperature-dependent optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]: spectroscopic ellipsometry and first-principles calculations.

The temperature-dependent ([Formula: see text]) optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] were investigated through spectroscopic ellipsometry over the spectral range of 0.73-6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5-4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from [Formula: see text] to [Formula: see text] for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron-phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.

Gate tunable giant anisotropic resistance in ultra-thin GaTe.

Anisotropy in crystals arises from different lattice periodicity along different crystallographic directions, and is usually more pronounced in two dimensional (2D) materials. Indeed, in the emerging 2D materials, electrical anisotropy has been one of the recent research focuses. However, key understandings of the in-plane anisotropic resistance in low-symmetry 2D materials, as well as demonstrations of model devices taking advantage of it, have proven difficult. Here, we show that, in few-layered semiconducting GaTe, electrical conductivity anisotropy between x and y directions of the 2D crystal can be gate tuned from several fold to over 103. This effect is further demonstrated to yield an anisotropic non-volatile memory behavior in ultra-thin GaTe, when equipped with an architecture of van der Waals floating gate. Our findings of gate-tunable giant anisotropic resistance effect pave the way for potential applications in nanoelectronics such as multifunctional directional memories in the 2D limit.

Deep-ultraviolet Raman scattering spectroscopy of monolayer WS2.

Raman scattering measurements of monolayer WS2 are reported as a function of the laser excitation energies from the near-infrared (1.58 eV) to the deep-ultraviolet (4.82 eV). In particular, we observed several strong Raman peaks in the range of 700∼850 cm-1 with the deep-ultraviolet laser lights (4.66 eV and 4.82 eV). Using the first-principles calculations, these peaks and other weak peaks were appropriately assigned by the double resonance Raman scattering spectra of phonons around the M and K points in the hexagonal Brillouin zone. The relative intensity of the first-order [Formula: see text] to A1g peak changes dramatically with the 1.58 eV and 2.33 eV laser excitations, while the comparable relative intensity was observed for other laser energies. The disappearance of the [Formula: see text] peak with the 1.58 eV laser light comes from the fact that valley polarization of the laser light surpasses the [Formula: see text] mode since the [Formula: see text] mode is the helicity-exchange Raman mode. On the other hand, the disappearance of the A1g peak with the 2.33 eV laser light might be due to the strain effect on the electron-phonon matrix element.

Electric-field control of magnetism in a few-layered van der Waals ferromagnetic semiconductor.

Manipulating a quantum state via electrostatic gating has been of great importance for many model systems in nanoelectronics. Until now, however, controlling the electron spins or, more specifically, the magnetism of a system by electric-field tuning has proven challenging1-4. Recently, atomically thin magnetic semiconductors have attracted significant attention due to their emerging new physical phenomena5-13. However, many issues are yet to be resolved to convincingly demonstrate gate-controllable magnetism in these two-dimensional materials. Here, we show that, via electrostatic gating, a strong field effect can be observed in devices based on few-layered ferromagnetic semiconducting Cr2Ge2Te6. At different gate doping, micro-area Kerr measurements in the studied devices demonstrate bipolar tunable magnetization loops below the Curie temperature, which is tentatively attributed to the moment rebalance in the spin-polarized band structure. Our findings of electric-field-controlled magnetism in van der Waals magnets show possibilities for potential applications in new-generation magnetic memory storage, sensors and spintronics.

Sensitive Phonon-Based Probe for Structure Identification of 1T' MoTe2.

In this work, by combining transmission electron microscopy and polarized Raman spectroscopy for the 1T' MoTe2 flakes with different thicknesses, we found that the polarization dependence of Raman intensity is given as a function of excitation laser wavelength, phonon symmetry, and phonon frequency, but has weak dependence on the flake thickness from few-layer to multilayer. In addition, the frequency of Raman peaks and the relative Raman intensity are sensitive to flake thickness, which manifests Raman spectroscopy as an effective probe for thickness of 1T' MoTe2. Our work demonstrates that polarized Raman spectroscopy is a powerful and nondestructive method to quickly identify the crystal structure and thickness of 1T' MoTe2 simultaneously, which opens up opportunities for the in situ probe of anisotropic properties and broad applications of this novel material.

Control of Surface and Edge Oxidation on Phosphorene.

Phosphorene is emerging as an important two-dimensional semiconductor, but controlling the surface chemistry of phosphorene remains a significant challenge. Here, we show that controlled oxidation of phosphorene determines the composition and spatial distribution of the resulting oxide. We used X-ray photoemission spectroscopy to measure the binding energy shifts that accompany oxidation. We interpreted these spectra by calculating the binding energy shift for 24 likely bonding configurations, including phosphorus oxides and hydroxides located on the basal surface or edges of flakes. After brief exposure to high-purity oxygen or high-purity water vapor at room temperature, we observed phosphorus in the +1 and +2 oxidation states; longer exposures led to a large population of phosphorus in the +3 oxidation state. To provide insight into the spatial distribution of the oxide, transmission electron microscopy was performed at several stages during the oxidation. We found crucial differences between oxygen and water oxidants: while pure oxygen produced an oxide layer on the van der Waals surface, water oxidized the material at pre-existing defects such as edges or steps. We propose a mechanism based on the thermodynamics of electron transfer to interpret these observations. This work opens a route to functionalize the basal surface or edges of two-dimensional (2D) black phosphorus through site-selective chemical reactions and presents the opportunity to explore the synthesis of 2D phosphorene oxide by oxidation.

In-Plane Optical Anisotropy of Layered Gallium Telluride.

Layered gallium telluride (GaTe) has attracted much attention recently, due to its extremely high photoresponsivity, short response time, and promising thermoelectric performance. Different from most commonly studied two-dimensional (2D) materials, GaTe has in-plane anisotropy and a low symmetry with the C2h(3) space group. Investigating the in-plane optical anisotropy, including the electron-photon and electron-phonon interactions of GaTe is essential in realizing its applications in optoelectronics and thermoelectrics. In this work, the anisotropic light-matter interactions in the low-symmetry material GaTe are studied using anisotropic optical extinction and Raman spectroscopies as probes. Our polarized optical extinction spectroscopy reveals the weak anisotropy in optical extinction spectra for visible light of multilayer GaTe. Polarized Raman spectroscopy proves to be sensitive to the crystalline orientation of GaTe, and shows the intricate dependences of Raman anisotropy on flake thickness, photon and phonon energies. Such intricate dependences can be explained by theoretical analyses employing first-principles calculations and group theory. These studies are a crucial step toward the applications of GaTe especially in optoelectronics and thermoelectrics, and provide a general methodology for the study of the anisotropy of light-matter interactions in 2D layered materials with in-plane anisotropy.

In situ oxidation of carbon-encapsulated cobalt nanocapsules creates highly active cobalt oxide catalysts for hydrocarbon combustion.

Combustion catalysts have been extensively explored to reduce the emission of hydrocarbons that are capable of triggering photochemical smog and greenhouse effect. Palladium as the most active material is widely applied in exhaust catalytic converter and combustion units, but its high capital cost stimulates the tremendous research on non-noble metal candidates. Here we fabricate highly defective cobalt oxide nanocrystals via a controllable oxidation of carbon-encapsulated cobalt nanoparticles. Strain gradients induced in the nanoconfined carbon shell result in the formation of a large number of active sites featuring a considerable catalytic activity for the combustion of a variety of hydrocarbons (methane, propane and substituted benzenes). For methane combustion, the catalyst displays a unique activity being comparable or even superior to the palladium ones.

Anomalous lattice vibrations of monolayer MoS2 probed by ultraviolet Raman scattering.

We present a comprehensive Raman scattering study of monolayer MoS2 with increasing laser excitation energies ranging from the near-infrared to the deep-ultraviolet. The Raman scattering intensities from the second-order phonon modes are revealed to be enhanced anomalously by only the ultraviolet excitation wavelength 354 nm. We demonstrate theoretically that such resonant behavior arises from a strong optical absorption that forms near the Γ point and ½ΓK of the band structure and an inter-valley resonant electronic scattering by the M-point phonons. These results advance our understanding of the double resonance Raman scattering process in low-dimensional semiconducting nanomaterials and provide a foundation for the technological development of monolayer MoS2 in the ultraviolet frequency range.

Microwave absorption properties of Ni/(C, silicides) nanocapsules.

The microwave absorption properties of Ni/(C, silicides) nanocapsules prepared by an arc discharge method have been studied. The composition and the microstructure of the Ni/(C, silicides) nanocapsules were determined by means of X-ray diffraction, X-ray photoelectric spectroscopy, and transmission electron microscope observations. Silicides, in the forms of SiOx and SiC, mainly exist in the shells of the nanocapsules and result in a large amount of defects at the 'core/shell' interfaces as well as in the shells. The complex permittivity and microwave absorption properties of the Ni/(C, silicides) nanocapsules are improved by the doped silicides. Compared with those of Ni/C nanocapsules, the positions of maximum absorption peaks of the Ni/(C, silicides) nanocapsules exhibit large red shifts. An electric dipole model is proposed to explain this red shift phenomenon.