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bandgap relation was present for further application of the layered GaSe in bendable flexible lightemission devices.

Single crystals of oxygen-incorporated ZnS (i.e., ZnS (1– x ) O x series) are environment-friendly wide-band-gap semiconductors available for light-emitting devices and solar cell use. The series of materials has considerable potential for use in visible ultraviolet areas with flexibility for palette emissions. In this study, we grow oxygen-incorporated ZnS series crystals by chemical vapor transport method with iodine (I 2 ) as the transport agent. Three different oxygen-incorporated crystals of undoped ZnS, ZnS 0.94 O 0.06 , and ZnS 0.88 O 0.12 are studied. Through structural studies, ZnS doped with oxygen crystallizes in the main sphalerite phase and a little wurtzite structure. The lattice constants of the major cubic phase are determined to be a = 5.43 Å (ZnS), 5.41 Å (ZnS 0.94 O 0.06 ), and 5.39 Å (ZnS 0.88 O 0.12 ). Three band-edge excitonic transitions are simultaneously detected by thermoreflectance measurement for the ZnS, ZnS 0.94 O 0.06 , and ZnS 0.88 O 0.12 series samples. The energy positions of the band-edge transitions decrease as the oxygen content increases in the ZnS (1– x ) O x series. Defect-state and surface-state emissions, including sulfur vacancy, oxygen vacancy, zinc interstitial, and so forth, can emit approximately full-color spectra from the near band edge of the ZnS (1– x ) O x series crystals. With adjusting the oxygen content, the ZnS (1– x ) O x can be a series of color-palette luminescence matters that applied for fluorescent display or light-emitting device.

ReS and ReSe have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS and WSe) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS Se (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS Se is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E, E and E-are detected. For the thick (bulk) ReS Se , more stacking of the ReX monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX. The transmittance absorption edge of each bulk ReX (a few microns thick) usually has a lower energy than those of the direct E and E excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.

Two-dimensional (2D) semiconductors play a crucial role in high-efficiency photocatalysts because of their high surface-to-volume ratio. The surface property is a key part of photocatalysis. In this work, the enhanced photocatalytic behavior of the layered ReS2 with optical polarization along the Re4 nano-diamond-chain (DC) direction (b axis) has been demonstrated. The unpolarized photoconductivity (PC) response of ReS2 with an applied bias along the b axis is approximately 1 order higher than that of the applied bias perpendicular to the b axis. The polarization-dependent PC spectra of E ∥ b also reveal a higher photoresponsivity with respect to those measured along the E ⊥ b polarization for the layered ReS2. This result indicates that stronger polarization dipoles as well as a larger amount of photogenerated carriers and surface states can contribute to the C-plane ReS2 under the illumination of E ∥ b polarized photons. With the special axial effect, the layered ReS2 2D photocatalyst shows much faster degradation rates of 5.6 and 12.3 than the other transition-metal dichalcogenides of TaS2 and MoS2 for the degradation of methylene blue (MNB) solution. For the polarization-dependent photodegradation test, the degradation rate of illuminated E ∥ b polarized photons is also approximately 12 times faster than that of the illuminated E ⊥ b polarized light in a 25 μM MNB solution. The enhanced photocatalytic behavior of ReS2 along the DC also shows a peak photoreponse of ∼25 μV detected in the polarized photovoltaic spectrum of the 0.5 μM MNB dye-sensitized solar cell positioned at ∼1.99 eV. The formation of a nano-DC and a one-layer trigonal crystalline phase is beneficial for the versatile energy applications of ReS2.