Cailei Yuan

Enhanced Gas-Sensing Properties of the Hierarchical TiO2 Hollow Microspheres with Exposed High-Energy {001} Crystal Facets

Anatase hierarchical TiO2 with innovative designs (hollow microspheres with exposed high-energy {001} crystal facets, hollow microspheres without {001} crystal facets, and solid microspheres without {001} crystal facets) were synthesized via a one-pot hydrothermal method and characterized. Based on these materials, gas sensors were fabricated and used for gas-sensing tests. It was found that the sensor based on hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets exhibited enhanced acetone sensing properties compared to the sensors based on the other two materials due to the exposing of high-energy {001} crystal facets and special hierarchical hollow structure. First-principle calculations were performed to illustrate the sensing mechanism, which suggested that the adsorption process of acetone molecule on TiO2 surface was spontaneous, and the adsorption on high-energy {001} crystal facets would be more stable than that on the normally exposed {101} crystal facets. Further characterization indicated that the {001} surface was highly reactive for the adsorption of active oxygen species, which was also responsible for the enhanced sensing performance. The present studies revealed the crystal-facets-dependent gas-sensing properties of TiO2 and provided a new insight into improving the gas sensing performance by designing hierarchical hollow structure with special-crystal-facets exposure.

Strain induced tetragonal SrTiO3 nanoparticles at room temperature

SrTiO3 nanoparticles with tetragonal phase at room temperature were fabricated using pulsed laser deposition method and rapid thermal annealing technique. High-resolution transmission electron microscope images demonstrate that SrTiO3 nanoparticle experiences a structural phase transition from cubic with indirect band gap to tetragonal with direct band gap with growing size. It has been found that the net deviatoric strain from Lu2O3 matrix can cause the rotation of the TiO6 octahedra and the change of the bond length, which is suggested to explain the structural phase transition. Strain engineering is an effective tool for tailoring the properties of SrTiO3 nanoparticles.

Monolayer-by-monolayer stacked pyramid-like MoS2 nanodots on monolayered MoS2 flakes with enhanced photoluminescence

The precise control of the morphology and crystal shape of MoS2 nanostructures is of particular importance for their application in nanoelectronic and optoelectronic devices. Here, we describe a single step route for the synthesis of monolayer-by-monolayer stacked pyramid-like MoS2 nanodots on monolayered MoS2 flakes using a chemical vapor deposition method. First-principles calculations demonstrated that the bandgap of the pyramid-like MoS2 nanodot is a direct bandgap. Enhanced local photoluminescence emission was observed in the pyramid-like MoS2 nanodot, in comparison with monolayered MoS2 flakes. The findings presented here provide new opportunities to tailor the physical properties of MoS2via morphology-controlled synthesis.

Novel p-type thermoelectric materials Cu3MCh4 (M = V, Nb, Ta; Ch = Se, Te): high band-degeneracy

The good thermoelectric performance of some half-Heusler (HH) alloys has been stimulating substantial efforts in searching for more materials with similar crystal structures but better properties. In this work, we predict a new class of thermoelectric materials Cu3MCh4 (M = V, Nb, Ta; Ch = Se, Te) with similar lattice structures to HH alloys. The electronic and thermal transport properties of these materials are quantitatively evaluated using first-principles calculations in combination with the semi-classical transport theory. The largest ZT values at 1000 K for p-type Cu3MTe4 and Cu3MSe4 (M = V, Nb, Ta) can reach up to 2.06, 2.22, 2.36 and 1.91, 2.35 and 2.03, respectively. It is suggested that high band-degeneracy near the valence band edge is the main physical source for the predicted excellent thermoelectric performance. Simultaneously, a smaller effective mass of holes than that of electrons also benefits the enhanced thermoelectric properties of these p-type materials.

Controlled vapour-phase deposition synthesis and growth mechanism of Bi2Te3 nanostructures

This work presents a study on the controlled growth and the growth mechanism of vapour-phase deposited two-dimensional Bi2Te3 nanostructures by investigating the influence of growth conditions on the morphology of Bi2Te3 nanostructures. The formation of a hexagonal plate geometry for Bi2Te3 nanostructures is a consequence of the large difference in growth rate between crystal facets along 〈0001〉 and 〈11 2¯0〉 directions. Under low Ar carrier gas flow rates (60–100 sccm), the growth of Bi2Te3 nanoplates occurs in the mass-transport limited regime, whereas under high carrier gas flow rates (130 sccm), the growth of Bi2Te3 nanoplates is in the surface-reaction limited regime. This leads to an increase in the lateral size of Bi2Te3 nanoplates with increasing the Ar carrier gas flow rate from 60 to 100 sccm, and a decrease in size for a flow rate of 130 sccm. In addition, the lateral size of Bi2Te3 nanoplates was found to increase with increasing growth time due to the kinetic characteristics of material growth...

Tuning strain and photoluminescence of confined Au nanoparticles by hydrogen passivation

Au nanoparticles confined in an Al2O3 matrix are synthesized using pulsed laser deposition method and rapid thermal annealing technique. The confined Au nanoparticles experience a compressive strain during the growth process. It is demonstrated that hydrogen passivation can be used to enhance and tailor the optical properties of confined Au nanoparticles by engineering the strain and defect states of the confined Au nanoparticles. The findings provide an insight and useful methodology to improve the emission efficiency of noble metal nanoparticle based materials for potential application in optoelectronic and photonic devices.

High Selectivity of Sulfur Doped SnO2 in NO2 Detection at Lower Operating Temperature

Resistive gas sensors based on metal oxides have aroused great interest in the sensing of NO2 gas due to their low cost, good stability, and easy fabrication. However, drawbacks such as low sensitivity and a lack of selectivity, which originate from the limited kinds of intrinsic active centers on the surface of the metal oxides that could be involved in the gas-sensing reaction, remain great challenges to overcome. To solve these problems, surface modification of SnO2 by S-doping was carried out by the sintering of flower-like SnS2. Gas-sensing tests revealed that the S-doped SnO2 showed ultra-high sensitivity to NO2 (Rg/Ra = 600 toward 5 ppm) with low optimal operating temperature (50 °C). The detection limit of the sensor was as low as 50 ppb (Rg/Ra = 11). Notably, the S-doped SnO2 showed negligible cross-responses to alcohol, acetone, HCHO, SO2, H2S, and xylene. The ultra-high sensitivity and selectivity toward NO2 were closely related to the content of the S-dopant. This phenomenon is attributed to the active role of S-dopant during the surface reactions with NO2, which was substantiated by in situ Raman characterization and DFT-based calculations. This study offers an important guide for surface modification by doping to improve the sensitivity and selectivity of metal oxides and sheds new light on material design to develop resistive gas sensors for NO2 detection.

Work function variation of monolayer MoS2 by nitrogen-doping

Monolayer MoS2 films with substantial sulfur vacancies are obtained using the laser molecular beam epitaxy (L-MBE) technique benefitted by high substrate temperature and ultrahigh vacuum growth conditions. The intrinsic sulfur vacancies present an excellent opportunity for varying the work function of monolayer MoS2 films by nitrogen doping. The in-plane doping of nitrogen atoms on L-MBE-synthesized monolayer MoS2 films is realized on the basis of rapid thermal annealing in a nitrogen environment. The as-grown and nitrogen-doped monolayer MoS2 films are evaluated by using Raman and photoluminescence spectroscopies. In accordance with the X-ray photoelectron spectroscopy results, the ultraviolet photoelectron spectroscopy investigation shows that the work function of the monolayer MoS2 films increases by 0.29 eV after covalent nitrogen doping. Nitrogen doping on monolayer MoS2 is further treated theoretically using first-principles calculations. Based on theoretical calculations and experimental validations, it is illustrated that nitrogen is a promising in-plane heteroatom dopant for work function variation of monolayer MoS2.