Qinghu You

Enhanced photoelectrochemical activity of vertically aligned ZnO-coated TiO2 nanotubes

Vertically aligned ZnO-TiO2 hetero-nanostructures constructed of anatase TiO2 nanotubes (NTs) and wurtzite ZnO coatings are fabricated by atomic layer deposition of ZnO coatings on electrochemical anodization formed TiO2 NTs, and their photoelectrochemical activities are studied through photoelectrochemical and electrochemical characterization. Compared with bare TiO2 NTs, the transient photocurrent increases to over 1.5-fold for the annealed ZnO-coated TiO2 NTs under visible illumination. The ZnO-coated TiO2 NTs also show a longer electron lifetime, a lower charge-transfer resistance and a more negative flat-band potential than the bare TiO2 NTs, confirming the improved photoelectrochemical activity due to the enhanced charge separation.

Manipulations from oxygen partial pressure on the higher energy electronic transition and dielectric function of VO2 films during a metal–insulator transition process

Optical properties and metal–insulator transition (MIT) of vanadium dioxide (VO2) films grown by pulsed laser deposition with different oxygen pressures (5 to 50 mTorr) have been investigated by temperature dependent transmittance spectra. Three interband critical points (E1, E2 and E3) can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E1 and E2 is presented. The VO2 film grown at optimized oxygen pressure exhibits the well-defined resistivity drop (∼103 Ω cm) across the MIT process. It is found that the metal–insulator transition temperature (TMIT) increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure. It is believed that the oxygen pressure can lead to lattice defects, which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies, respectively. The donor level provides electrons for higher empty π* bands, which can make the energy barrier lower and decrease critical temperature. On the contrary, electrons jumping from the d∥ band can be recombined by holes on the acceptor, impeding the MIT occurrence. It is claimed that the electronic orbital occupancy is closely related to oxygen pressure, which changes the energy barrier and manipulates the phase transition temperature. The present results are helpful to understand the fundamental mechanism of VO2 films and practical applications for VO2-based optoelectronic devices.

Formation of diatomic molecular radicals in reactive nitrogen-carbon plasma generated by electron cyclotron resonance discharge and pulsed laser ablation

The reactive nitrogen-carbon plasma generated by electron cyclotron resonance (ECR) microwave discharge of N2 gas and pulsed laser ablation of a graphite target was characterized spectroscopically by time-integrated and time-resolved optical emission spectroscopy with space resolution for a study of gas-phase reactions and molecular radical formation in the plasma. The plasma exhibits very high reactivity compared with the plasma generated solely by ECR discharge or by pulsed laser ablation and contains highly excited species originally present in the ambient gaseous environment and directly ablated from the target as well as formed as the products of gas-phase reactions occurring in the plasma. The space distribution and the time evolution of the plasma emission give an access to the gas-phase reactions for the formation of C2 and CN radicals, revealing that C2 radicals are formed mainly in the region near the target while CN radicals can be formed in a much larger region not only in the vicinity of the ...

Effects of crystal orientation on electronic band structure and anomalous shift of higher critical point in VO2 thin films during the phase transition process

The phase transition behaviour of vanadium dioxide (VO2) with different thicknesses has been investigated by temperature-dependent optical transmittance and Raman spectra. It is found that the crystal orientation has a great effect on the metal-insulator transition (MIT) of VO2 films. The x-ray diffraction (XRD) analysis shows that the films are polycrystalline and exhibit the characteristics of the monoclinic phase. The preferential growth crystal orientation (0 2 0) is converted to the ( 1 1) plane with the film thickness increasing. It is believed that the ( 1 1) plane is the reflection of a twinned structure with (0 1 1) crystal orientation, which will lead to the arrangements of oxygen atoms and vanadium atoms deviating from the pure monoclinic structure. It is found that the highest order transition (E 3) is highly susceptible to the crystal orientation, whereas the lowest order transition (E 1) is nearly unaffected by it. The E 3 exhibits an anomalous temperature dependence with an abrupt blue-shift (~0.5 eV) in the vicinity of the metal-insulator transition (MIT) for VO2 film with a thickness of 84 nm. The findings show that the empty band can be driven close to the Fermi level when the (0 2 0) orientation is converted to the ( 1 1) orientation. Compared to the VO2 films with thicknesses of 39 and 57 nm, the E 3 decreases by 0.8 eV and the E 2 increases by about 0.1 eV at the insulator state for the VO2 film with a thickness of 84 nm. The abnormal electronic transition and the variation of energy band is likely caused by the lattice distortion and V–V dimerisation deviation from the monoclinic axis.

Spectroscopic characterization of the plasma generated during the deposition of AlxGa1−xN films by pulsed laser co-ablation of Al and GaAs targets in electron cyclotron resonance nitrogen plasma

A nitrogen–aluminum–gallium–arsenic plasma is formed by pulsed laser co-ablation of an Al target and a GaAs target in electron cyclotron resonance discharge-generated nitrogen plasma for AlxGa1−xN film deposition. The formed plasma was characterized by time-integrated and time-resolved optical emission spectroscopy measurements and the process of AlxGa1−xN deposition was discussed. The plasma contains excited species originally present in the working N2 gas and energetic species ablated from the targets, and its emission is abundant in the emission bands of diatomic nitrogen molecules and molecular ions and the emission lines of monoatomic aluminum, gallium, and arsenic atoms and atomic ions. The temporal and spatial features of the plasma emission reveal that the nitrogen species in the electron cyclotron resonance nitrogen plasma experience additional excitations due to the expanding ablation plumes, and the ablated species are excited frequently when traveling with the expanding plumes in the nitrogen plasma, making the formed plasma very reactive, which is very important in the process of AlxGa1−xN film deposition. The deposited film was evaluated for composition analysis by energy-dispersive x-ray spectroscopy and structure characterization by x-ray diffraction. The AlxGa1−xN film is slightly nitrogen rich with an aluminum content x of about 0.6 and featured with hexagonal wurtzite crystal structure with preferred c-axis orientation.