Haiping Chen

Solvothermal Synthesis of Ternary Cu2MoS4 Nanosheets: Structural Characterization at the Atomic Level

Cu2 MoS4 nanosheets are synthesized by a solvothermal method in which the Cu2 O starting material acts as a sacrificial template. The microstructure of the Cu2 MoS4 nanosheets is characterized at the atomic level, and the growth mechanism is monitored at the nanoscale through systematic time-dependent experiments. As a result, the unprecedented observation of the allotropic phase change in Cu2 MoS4 that occurs during the solvothermal process is possible.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Raman scattering of single crystal Cu2MoS4 nanosheet

Polarized Raman spectra were recorded under parallel and perpendicular configurations in back scattering geometry for a single crystal Cu2MoS4 nanosheet of c-plane (001) faced orientation. The angular dependence of Raman peaks intensity for different phonon modes were studied by rotating from 0 to 180 degree around the c-axis of the ternary layered crystal. In combination with the theoretical calculations, the experimentally observed Raman peaks were assigned to four active Raman modes accordingly.

Cube-like Cu2MoS4 photocatalysts for visible light-driven degradation of methyl orange

Cube-like Cu2MoS4 nanoparticles with low-index facets and high crystallinity were fabricated via a hydrothermal method. The as-obtained nanocubes with an average size of 40-60 nm are composed of stacking-Cu2MoS4 layers separated by a weak Van der Waals gap of 0.5 nm. A strong absorption at visible light region is observed in the nanocube aqueous solution, indicating its optical-band gap of 1.78 eV. The photocatalytic measurements reveal that the nanocubes can thoroughly induce the degradation of methyl orange under visible light irradiation with good structural stability. Our finding may provide a way in design and fabrication of transition metal dichalcogenide nanostructures for practical applications.

High-metallic-phase-concentration Mo 1– x W x S 2 nanosheets with expanded interlayers as efficient electrocatalysts

In most cases, layered transition metal dichalcogenides (LTMDs), containing metallic phases, show electrochemical behavior different from their semiconductor counterparts. Typically, two-dimensional layered metallic 1T-MoS2 demonstrates better electrocatalytic performance for water splitting compared to its 2H counterpart. However, the characteristics of low metallic phase concentration and poor stability limit its applications in some cases. Herein, we demonstrate a simple and efficient bottom-up wet-chemistry strategy for the large-scale synthesis of nanoscopic ultrathin Mo1–xWxS2 nanosheets with enlarged interlayer spacing and high metallic phase concentration. Our characterizations, including X-ray absorption fine structure spectroscopy (XAFS), high-angle annular dark-fieldscanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS) revealed that the metallic ultrathin ternary Mo1–xWxS2 nanosheets exhibited distorted metal–metal bonds and a tunable metallic phase concentration. As a proof of concept, this optimized catalyst, with the highest metallic phase concentration (greater than 90%), achieved a low overpotential of about–155 mV at a current density of –10 mA/cm2, a small Tafel slope of 67 mV/dec, and an increased turnover frequency (TOF) of 1.3 H2 per second at an overpotential of –300 mV (vs. reversible hydrogen electrode (RHE)), highlighting the importance of the metallic phase. More importantly, this study can lead to a facile solvothermal route to prepare stable and high-metallicphase-concentration transition-metal-based two-dimensional materials for future applications.

Tuning the composition of ternary Bi2Se3xTe3(1-x) nanoplates and their Raman scattering investigations

We present the composition engineering and Raman scattering study of Bi2Se3xTe3(1-x) nanoplates that were synthesized by chemical vapor deposition method using different substrates, including fluorophlogopite mica, SiO2/Si. The characterizations revealed high crystallinity and layered-structure in the ternary Bi2Se3xTe3(1-x) products. Raman spectra of Bi2Se3xTe3(1-x) ranging from 80-200 cm-1 as a function of different Se-doping levels shows that intrinsic Raman peaks of Bi2Se3xTe3(1-x) nanoplates shift to higher frequency as the ratio of doped-Se increasing. The discontinuity of Raman peaks was found and discussed.

Electronic structures of layered Ta2NiS5 single crystals revealed by high-resolution angle-resolved photoemission spectroscopy

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically studied the electronic structures of quasi-one-dimensional (1D) ternary material Ta2NiS5 single crystals. Contrary to its sister compound Ta2NiSe5, which is a candidate material for excitonic insulators, we found that Ta2NiS5 cannot realize the ground state of an excitonic insulator according to temperature-dependent valence band dispersions along the Γ–X direction. In particular, the experimental ARPES data reveal that the electronic structures show strong two-dimensional characteristics along with a considerable in-plane anisotropy indicating evident coupling among the one-dimensional chain structures. Moreover, theoretical band calculations have to be compressed by a factor of about 15% in energy in order to match the experimental band structures, implying that some weak electron correlations are neglected in the band calculations.

Probing lattice vibration and surface electronic state in a layered (NH4)2V3O8 single crystal

Two-dimensional layered structure of a single crystal is regarded as an ideal feature for physical and chemical fundamental studies. Herein, we demonstrated a high-quality (NH4)2V3O8 single crystal with a layered tetragonal structure prepared via a hydrothermal method. The lattice vibrational behavior and surface electronic state of (NH4)2V3O8 layers were systematically investigated via polarized Raman scattering spectroscopy and ultraviolet photoelectron spectroscopy (UPS), respectively. It was found that all Raman peaks of (NH4)2V3O8 could be clearly identified as four active Raman modes through parallel and perpendicular polarization configurations in the backscattering geometry for (001) crystal surface. The UPS results indicated that the valence band maximum of (NH4)2V3O8 was mainly composed of localized vanadium 3d states, which was further confirmed by the density functional theory calculations.

Angle-/temperature-dependence of Raman scattering in layered NbSe3 crystal

Distinguishing the lattice vibrational modes of different Raman peaks and further studying the temperature-dependent behaviour of the different Raman modes are fundamental subjects for Raman spectroscopy of single crystal. Herein, we conducted a comprehensive study of angle- and temperature-dependant Raman scattering behaviour in high-crystalline NbSe3 layers prepared by chemical vapour transport method. The polarization experiments performed on NbSe3’s (100) crystal surface revealed that the Raman peaks could be clearly identified as Ag and Bg vibrational modes, indicating significantly anisotropic structure. Meanwhile, the effect of charge density wave (CDW) transition on the lattice vibrational behaviour was studied by investigating temperature-dependent Raman scattering spectra above and below the CDW transition temperature. It was observed that Raman shifts versus temperature for the four prominent Raman peaks of NbSe3 which belonged to Ag vibrational mode exhibited linear softening behaviour as the temperature increased from 83k to 293k. The temperature coefficient of Raman peaks fitted by Gruneisen model further confirmed the anharmonicity of the lattice in NbSe3 crystal, suggesting that the changes of ionic positions across the CDW transition has small effect on its Raman scattering behaviour.