Ziyu Hu

Hydrogenated arsenenes as planar magnet and Dirac material

Arsenene and antimonene are predicted to have 2.49 and 2.28 eV band gaps, which have aroused intense interest in the two-dimensional (2D) semiconductors for nanoelectronic and optoelectronic devices. Here, the hydrogenated arsenenes are reported to be planar magnet and 2D Dirac materials based on comprehensive first-principles calculations. The semi-hydrogenated (SH) arsenene is found to be a quasi-planar magnet, while the fully hydrogenated (FH) arsenene is a planar Dirac material. The buckling height of pristine arsenene is greatly decreased by the hydrogenation, resulting in a planar and relatively low-mass-density sheet. The electronic structures of arsenene are also evidently altered after hydrogenating from wide-band-gap semiconductor to metallic material for SH arsenene, and then to Dirac material for FH arsenene. The SH arsenene has an obvious magnetism, mainly contributed by the p orbital of the unsaturated As atom. Such magnetic and Dirac materials modified by hydrogenation of arsenene may have potential applications in future optoelectronic and spintronic devices.

Temperature dependent phonon Raman scattering of highly a-axis oriented CoFe2O4 inverse spinel ferromagnetic films grown by pulsed laser deposition

Lattice vibrations of highly a-axis oriented CoFe2O4 (CFO) films have been investigated by Raman scattering in the temperature range of 80-873 K. The five phonon modes T1 g (2), T1 g (3), E g , A1 g (1), A1 g (2), and their evolutions can be uniquely distinguished. It was found that an electron transfer between Co2+ and Fe3+ cations occurs in octahedral sites at about 173 K. The structure disorder in the CFO films appears with increasing the temperature, which indicates the cation migration between tetrahedral and octahedral sites. The phenomena suggest the structural transformation trend from inverse spinel to normal spinel at the elevated temperatures.

Temperature dependent phonon evolutions and optical properties of highly c-axis oriented CuGaO2 semiconductor films grown by the sol-gel method

Transparent conductive CuGaO2 oxide films were prepared on sapphire substrates by the sol-gel method. The highly c-axis orientation and optical transparency (60%-80%) in the visible region were obtained. It indicates that the A 1 g phonon mode shifts about 20 cm−1 with the temperature due to the thermal expansion of the lattice and anharmonic phonon coupling. Moreover, temperature-dependent dielectric function has been investigated and three electronic transitions located at about 1.05, 2.67, and 3.99 eV can be uniquely assigned. It was found that the optical band gap of the CuGaO2film decreases with the temperature, which mainly originated from the electron-phonon interactions.

Inherent optical behavior and structural variation in Na0.5Bi0.5TiO3-6%BaTiO3 revealed by temperature dependent Raman scattering and ultraviolet-visible transmittance

Optical properties of Na 0.5Bi0.5TiO3-6%BaTiO3 (NBT-6%BT) single crystal have been studied by temperature dependent Raman and ultraviolet-visible spectra from 25 to 180 °C. With increasing the temperature, the absorption edge approximately decreases from 3.13 to 3.04 eV. Moreover, abnormal changes of phonon mode and spectral transmission are observed at 83, 106, and 150 °C, which can be unambiguously correlated with thermal evolutions of polar nano-regions and phase transition. It indicates that there is an inherent relationship between optical behavior and structural variation of NBT-6%BT, which provides a valid methodology to explore the phase transition of relaxor ferroelectric oxides.

Structural and electronic properties of atomically thin germanium selenide polymorphs

Using comprehensive density functional theory calculations, we systematically investigate the structure, stability, and electronic properties of five polymorphs of GeSe monolayer, and highlight the differences in their structural and electronic properties. Our calculations show that the five free-standing polymorphs of GeSe are stable semiconductors. β-GeSe, γ-GeSe, δ-GeSe, and ε-GeSe are indirect gap semiconductors, whereas α-GeSe is a direct gap semiconductor. We calculated Raman spectra and scanning tunneling microscopy images for the five polymorphs. Our results show that the β-GeSe monolaye r is a candidate for water splitting.中文摘要本文利用密度泛函理论, 系统研究了五种单层GeSe晶型的结构、稳定性和电子结构特性, 并着重分析了其结构和电子性质差异. 研究结果表明, 五种单层GeSe晶型均表现出稳定的半导体特性. 不同的是β-GeSe、γ-GeSe、δ-GeSe和ε-GeSe晶型结构是间接带隙半导体材料, 而α-GeSe是直接带隙半导体. 计算进一步提供了五种晶型结构的拉曼光谱和扫描隧道显微镜图像. 带边排布分析表明β-GeSe单层材料适用于光催化分解水.

Phase transitions and thermotropic phase boundaries in MnO2-doped (K0.5Na0.5)NbO3-0.05LiNbO3 single crystals: Raman scattering evidence at elevated temperatures

Raman scattering of (K0.5 Na 0.5)NbO3-0.05LiNbO3-yMnO2 (y = 0% and 1.0%) single crystals have been reported in the temperature range from 300 to 800 K. The spectra exhibit a competition between a soft mode and a relaxation mode upon heating across the diverse transitions. The progressive change in the conflicting displacive mechanism (soft mode) and order-disorder (relaxation mode) can explain the origin of the successive orthorhombic-tetragonal-cubic phase transitions. Moreover, the polymorphic phase transition between orthorhombic and tetragonal structures can be confirmed through the observation of thermotropic phase boundaries for MnO2-doped (K0.5 Na 0.5)NbO3-0.05LiNbO3 single crystals.

Optical bandgap and phase transition in relaxor ferroelectric Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 single crystals: An inherent relationship

We report band to band transition behaviors of relaxor ferroelectric Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 (PMN-xPT) single crystals derived from temperature-dependent spectral transmittance. A typical bandgap formula with the temperature and composition ( 8 K ≤ T exp ≤ 453 K , 0.1 ≤ x ≤ 0.4 ) has been presented. Moreover, the phase diagram of PMN-xPT crystals can be well proposed, which is based on the bandgap variations and can be explained by electronic structure evolution. It reveals an intrinsic relationship between fundamental bandgap and phase transition of PMN-xPT single crystals, which pioneers an effective methodology to explore the phase transition of ferroelectric oxides.

A first-principles study of lithium-decorated hybrid boron nitride and graphene domains for hydrogen storage

First-principles calculations are performed to investigate the adsorption of hydrogen onto Li-decorated hybrid boron nitride and graphene domains of (BN)(x)C(1-x) complexes with x = 1, 0.25, 0.5, 0.75, 0, and B0.125C0.875. The most stable adsorption sites for the nth hydrogen molecule in the lithium-decorated (BN)(x)C(1-x) complexes are systematically discussed. The most stable adsorption sites were affected by the charge localization, and the hydrogen molecules were favorably located above the C-C bonds beside the Li atom. The results show that the nitrogen atoms in the substrate planes could increase the hybridization between the 2p orbitals of Li and the orbitals of H2. The results revealed that the (BN)(x)C(1-x) complexes not only have good thermal stability but they also exhibit a high hydrogen storage of 8.7% because of their dehydrogenation ability.

Modulation of doping and biaxial strain on the transition temperature of the charge density wave transition in 1T-TiSe2

We study the effects of charge doping and biaxial strains on the transition temperature of charge density wave (CDW) transition in TiSe2. We find soft phonon modes at the M and L points in the Brillouin zone of TiSe2, and a 0.043 eV Fermi–Dirac smearing width can suppress these soft modes. In this way, the transition temperature of CDW is quantitatively represented by the smearing width. After doping with electrons or holes, we find that the critical smearing widths to eliminate the CDW transition both decrease, indicating the same suppression effect on the CDW transition of electron doping and hole doping. Comparatively, only compressive biaxial strains can lower down the transition temperature of CDW transition in TiSe2, while stretching strains enlarges the transition temperature.

Temperature dependence of terahertz optical characteristics and carrier transport dynamics in p-type transparent conductive CuCr1−xMgxO2 semiconductor films

Terahertz reflectance spectra and electrical transport of CuCr1− x Mg x O2 films have been studied in temperature range of 150–300 K. With increasing temperature, the phonon mode near 15 THz shows a redshift trend, and free carrier absorption below 6 THz becomes more prominent. Moreover, hole effective mass increases linearly from 0.028 to 0.48 m 0 with the temperature and composition. Hall coefficient shows a turning-point at about 220, 206, and 194 K for the composition of x = 0.02, 0.06, and 0.10, respectively. The phenomena can be attributed to the transition of carrier transport mechanism from a thermal activation behavior to a variable range-hopping one.