Laser Tuning in Layered h-BN Crystals.

Integrated optics shows great potential in the current optical communication systems, sensor technology, optical computers, and other fields. Tunable laser technology within a certain range is the key to achieving on-chip optical integration; to realize which, Raman scattering is a competitive method that can effectively transfer incident laser energy to optical phonons due to the photon-phonon interaction. Here, we take hexagonal boron nitride as the energy conversion medium, and based on the angle-resolved polarized Raman spectroscopy, it is found that when laser polarization vector ei ⊥ c axis, the spectrum obtains maximal scattering across the cross section and a minimal depolarization ratio. At room temperature, h-BN obtains an output signal with a wavelength of 522.8 nm and a full-width at half-maximum of 0.24 nm under the excitation of 488 nm pump laser, and the depolarization ratio is 0.09 (theoretically, it is 0, and this difference is due to experimental errors). And then, within the temperature range of 80∼420 K, the scattered light wavelength shows a high-precision shift of 0.006 nm/25 K, indicating that continuous wavelength tuning has been successfully achieved in h-BN.

Raman Tensor of Layered SnS2.

Recently, tin disulfide (SnS2) has become a hot research focus in various fields due to its advantages of a high transistor switching ratio, an adjustable band gap in visible light range, excellent Li storage performance, sensitive gas recognition, and efficient photocatalytic capability. However, at present, studies of its basic structure mostly stay on the regulation related to the number of layers. To maximize the value of SnS2 in the application design, this paper analyzes the angle-resolved polarized Raman spectra of SnS2 crystals grown under high-temperature sealing systems. Under the parallel scattering configuration test of both the sample basal plane and the cross plane, we observed that how the Raman scattering intensity of the two test planes varies with the polarization angle is different. Combining this experimental result with theory support allows us to reach a conclusion that the differential polarizability of the phonon vibration mode along the z-axis of the cross plane of SnS2 is proven to be the strongest. This finding is expected to provide favorable support for the application of structural regulation of SnS2 and work as a reference for studying other van der Waals layered materials with greater potential.

Raman Tensor of van der Waals MoSe2.

Molybdenum selenide (MoSe2) is a van der Waals layered crystal with both the anisotropic light absorption and light scattering outside its surface. At present, the study of the Raman tensor of MoSe2, which affects and even determines the inelastic light scattering's anisotropy, is not sufficient. In this research, with the aim of studying the out-of-plane anisotropy, we performed systematic angle-resolved polarized Raman (APR) spectroscopy and abstracted complete Raman tensors both experimentally and theoretically. In addition, according to first-principles calculations, in different conditions of laser excitation, MoSe2 has various Raman tensor forms, of which the phase difference between Raman tensor elements of the A1g mode is a particular one. By studying the anisotropic optical absorption properties, we confirmed that it is the dispersion and absorption of MoSe2 under different pump light that lead to the photon-energy-dependent phase differences.

Raman tensor of layered MoS2.

Raman tensors, one of the basic physical properties of ${{\rm MoS}_2}$MoS2, are rarely reported. Here, angle-resolved polarized Raman scatterings on basal and cross planes of layered ${{\rm MoS}_2}$MoS2 were carried out using the geometry configuration of parallel polarization, and the Raman tensors of three optical vibration modes were systematically studied. As a polar vibration mode, the differential polarizability of the ${{\rm A}_{1{\rm g}}}$A1g mode corresponding to the Raman tensor along the $c$c direction is larger than that along the $a$a direction. In addition, it is also larger than that formed by ${{\rm E}_{2{\rm g}}}$E2g and ${{\rm E}_{1{\rm g}}}$E1g modes. All the experimental results above are beneficial to the understanding of inelastic light-scattering process of ${{\rm MoS}_2}$MoS2.

Hydrogen Impurities in ZnO: Shallow Donors in ZnO Semiconductors and Active Sites for Hydrogenation of Carbon Species.

ZnO, as a low-cost yet significant semiconductor, has been widely used in solar energy conversion and optoelectronic devices. In addition, Cu/ZnO-based catalysts can convert syngas (H2, CO, and CO2) into methanol. However, the main concern about the intrinsic connection between the physical and chemical properties and the structure of ZnO still remains. In this work, efforts are made to decipher the physical and chemical information encoded into the structure. Through using NMR-IR techniques, we, for the first time, report a new ZnO model with three H+ cations incorporated into one Zn vacancy. 1H magic-angle spinning NMR and IR spectra demonstrate that Ga3+ cations are introduced into the Zn vacancies of the ZnO lattice, which replace the H+ cation, and thus further confirm the feasibility of our proposed model. The exchange between the H+ cation in Zn vacancies and the D2 gas phase shows that ZnO can activate H2 because of the quantized three H+ cations in the defect site.

Critical conditions for the formation of p-type ZnO with Li doping.

The stability of Li dopants in ZnO is studied via first-principles calculations with electric dipole correction. The formation energies of substitutional Li (LiZn), interstitial Li (Lii) and the LiZn + Lii complex are calculated in large supercells and the results are extrapolated to the limit of an infinite-sized supercell. The stabilities of 2LiZn and the LiZn + Lii complex are found to depend on the temperature and absolute oxygen partial pressure. At normal experimental temperature (900 K), an extremely high absolute oxygen partial pressure (194 bar) is needed to break the coupling between LiZn and Lii and thus form p-type ZnO. The reaction barrier and the absorbance spectra are also discussed.