Zhihua Duan

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.

Oxygen pressure manipulations on the metal-insulator transition characteristics of highly (0 1 1)-oriented vanadium dioxide films grown by magnetron sputtering

The metal–insulator transition behaviour of vanadium dioxide (VO2) films grown at different oxygen pressures is investigated. With the aid of temperature-dependent electrical and infrared transmittance experiments, it is found that the transition temperature in the heating process goes up with increasing argon–oxygen ratio, whereas the one in the cooling process shows an inverse variation trend. It is found that the hysteresis width of the phase transition is narrowed at a lower argon–oxygen ratio because the defects introduced by excess oxygen lower the energy requirement of transformation. Furthermore, the defects reduce the forbidden gap of the VO2 system due to the generation of a V5+ ion. The present results are valuable for the achievement of VO2-based optoelectronic devices.

Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods

Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.

Temperature-dependent dielectric functions and interband critical points of relaxor lead hafnate-modified PbSc1/2Ta1/2O3 ferroelectric ceramics by spectroscopic ellipsometry

The electronic band structures and dielectric functions of (1−x)PbSc1/2 Ta 1/2O3−xPbHfO3 ceramics with different composition have been investigated by variable-temperature spectroscopic ellipsometry. Using the standard critical-point (SCP) model, three typical interband transitions can be observed from the second derivative of dielectric functions. The CP transitions, which are sensitive to B-site order degree, show a redshift trend with the temperature due to the electron-phonon interactions and lattice thermal expansion. The linear temperature coefficients are varied with oxygen vacancy, B-atom (Sc, Ta, Hf) arrangement, and Pb-O bonds owing to addition of PbHfO3.

Extended photo-response of ZnO/CdS core/shell nanorods fabricated by hydrothermal reaction and pulsed laser deposition

Heterogenous nanostructures shaped with CdS covered ZnO (ZnO/CdS) core/shell nanorods (NRs) are fabricated on indium-tin-oxide by pulsed laser deposition of CdS on hydrothermally grown ZnO NRs and characterized through morphology examination, structure characterization, photoluminescence and optical absorption measurements. Both the ZnO cores and the CdS shells are hexagonal wurtzite in structure. Compared with bare ZnO NRs, the fabricated ZnO/CdS core/shell NRs present an extended photo-response and have optical properties corresponding to the two excitonic band-gaps of ZnO and CdS as well as the effective band-gap formed between the conduction band minimum of ZnO and the valence band maximum of CdS.

The A-site driven phase transition procedure of (Pb{sub 0.97}La{sub 0.02})(Zr{sub 0.42}Sn{sub 0.40}Ti{sub 0.18})O{sub 3} ceramics: An evidence from electronic structure variation

The transition of (Pb{sub 0.97}La{sub 0.02})(Zr{sub 0.42}Sn{sub 0.40}Ti{sub 0.18})O{sub 3} (PLZST) ceramic has been investigated by temperature-dependent X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). The rhombohedral and tetragonal symmetries are confirmed by XRD analysis. Two interband transitions (E{sub cp1} and E{sub cp2}) located at about 3.7 and 5.2 eV can be derived from the second derivative of the complex dielectric functions using the standard critical point (SCP) model. Except for the negative temperature coefficient parts, the transitions present additional parts corresponding to appearance of the antiferroelectric (AFE) phase. The phenomena can be attributed to variation of the electronic structure during A-site driven phase transition.

Improved electric behaviors of the Pt/Bi1−xLaxFe0.92Mn0.08O3/n+-Si heterostructure for nonvolatile ferroelectric random-access memory

Multiferroic BiFeO3 (BFO) and lanthanum-substituted Bi1−xLaxFe0.92Mn0.08O3 (BLFMx, 0 ≤ x ≤ 0.2) films have been directly deposited on heavily doped Si(100) with an electric resistivity of about 0.001 Ω cm. The La substitution effects on the microstructure and lattice dynamics of the BLFMx films have been investigated by X-ray diffraction (XRD), scanning electron microscopy, far-infrared reflectance and Raman scattering studies. XRD analysis shows that the rhombohedral structure of BFO films reduced toward the orthorhombic or tetragonal structure by Mn and La substitution. It was found that the leakage current density tended to decrease with increasing La composition in a low electric field because the La dopant can suppress the formation of oxygen vacancies by stabilizing the oxygen octahedron and controlling the volatility of Bi atoms. On the other hand, the leakage current density in the high electric field can be suppressed by Mn substitution due to compensation of the charge of Fe2+ ions. The well-saturated polarization hysteresis can be obtained in the Pt/BLFMx/n+-Si prototype devices. As an example, the electric remanent polarization (2Pr) and coercive field (2Ec) at the electric field of 1600 kV cm−1 for the BLFM0.15 film are 150 μC cm−2 and 870 kV cm−1, respectively. Moreover, its relative dielectric constant at the frequency of 3.5 × 105 Hz is about 125. These results could be crucial for future applications of silicon-based nonvolatile ferroelectric random-access memory.

Fabrication and multi-band photoluminescence of vertically aligned ZnO/ZnSe core/shell nanorod arrays

Vertically aligned ZnO/ZnSe core/shell nanorod (NR) arrays were fabricated on Si (100) using combined approach of hydrothermal growth of ZnO followed by pulsed laser deposition of ZnSe. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells with the hydrothermally grown ZnO core nanorods preferentially c-axis oriented. Upon excitation by 325 nm light at room temperature, the ZnO/ZnSe core/shell NR arrays emit multi-band photoluminescence (PL) including the near-band-edge (NBE) emissions of ZnO and ZnSe as well as the defect-related emission and the emission associated with the effective band gap formed due to the staggered band alignment of the heterojunction constructed from ZnO and ZnSe, while the NBE emission of the ZnO cores is greatly suppressed by the ZnSe shells. The PL shows a strong dependence on temperature. Post-fabrication annealing of the samples in N2 results in an improvement in crystal structure and an enhancement of PL intensity.