Chongwen Zou

Strain Dynamics of Ultrathin VO2 Film Grown on TiO2 (001) and the Associated Phase Transition Modulation

Tuning the metal insulator transition (MIT) behavior of VO2 film through the interfacial strain is effective for practical applications. However, the mechanism for strain-modulated MIT is still under debate. Here we directly record the strain dynamics of ultrathin VO2 film on TiO2 substrate and reveal the intrinsic modulation process by means of synchrotron radiation and first-principles calculations. It is observed that the MIT process of the obtained VO2 films can be modulated continuously via the interfacial strain. The relationship between the phase transition temperature and the strain evolution is established from the initial film growth. From the interfacial strain dynamics and theoretical calculations, we claim that the electronic orbital occupancy is strongly affected by the interfacial strain, which changes also the electron-electron correlation and controls the phase transition temperature. These findings open the possibility of an active tuning of phase transition for the thin VO2 film through the interfacial lattice engineering.

ZnO/TiO2 core–brush nanostructure: processing, microstructure and enhanced photocatalytic activity

Heterostructure ZnO/TiO2 core–brush nanostructures were synthesized on glass substrates by a combination of aqueous solution growth and magnetron sputtering method. The microstructure and morphology were characterized using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The heterostructure core–brush shows the single crystal ZnO nanorod as the core and polycrystalline TiO2 nanowires as the brush-like outer layer. Surface electronic states and optical transmittance were measured using X-ray photoelectron spectroscopy and a UV-vis spectrometer. The growth mechanism was proposed as a slow “particle-by-particle” mechanism. The photocatalytic activity of the ZnO/TiO2 core–brush nanostructure was evaluated by the decomposition reaction of Bromo-Pyrogallol Red dye under UV (245 nm) and visible-light (450 nm) irradiation. The results revealed that the core–brush structure exhibited much higher photocatalytic activities than that of a TiO2 film and a TiO2/ZnO composite film. The photocatalytic activity enhancement of the ZnO/TiO2 core–brush could be attributed to the interaction effect, lower band gap energy and its unique core–brush feature which can lower the recombination rate of electron–hole pairs, extend the absorption range and provide a high density of active sites.

Growth and phase transition characteristics of pure M-phase VO2 epitaxial film prepared by oxide molecular beam epitaxy

VO2 epitaxial film with large size has been prepared by oxide-molecular beam epitaxy method on Al2O3 (0001) substrate. The VO2 film shows a perfect crystal orientation, uniformity, and distinct metal-insulator phase transition (MIT) characteristics. It is observed that the MIT character is closely associated with the crystal defects such as oxygen vacancies. By controlling the growth condition, the MIT temperature can be tuned through modifying the content of oxygen vacancies. The role of the oxygen vacancies on the phase transition behavior of this VO2 film is discussed in the framework of the hybridization theory and the valence state of vanadium.

Heterogeneous Lollipop-like V2O5/ZnO Array: A Promising Composite Nanostructure for Visible Light Photocatalysis

ZnO/V(2)O(5) core-shell nanostructures have been prepared by a two-step synthesis route through combined hydrothermal growth and magnetron sputtering. After annealing under oxygen ambience, a ZnO/V(2)O(5) heterogeneous lollipop-like nanoarray formed. The microstructure and crystal orientation of those nanolollipops were investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), which show single crystal structure. The optical properties were characterized by UV-vis spectroscopy and showed quite different absorption curves for the as-deposited and annealed samples. The ZnO/V(2)O(5) nanolollipops demonstrated excellent photocatalytic activity in terms of decomposing 2,6-dichlorophenol (2,6-DCP) under visible light, indicating their promising potential as catalysts for industrial wastewater and soil pollution treatments.

Microstructures and optical properties of β-V2O5 nanorods prepared by magnetron sputtering

Amorphous V2O5 films were prepared on glass substrates at room temperature by magnetron sputtering and converted to β-V2O5 nanorods by subsequent annealing in O2 flux. X-ray absorption near-edge spectroscopy and photoelectron spectroscopy with synchrotron radiation have been applied to confirm whether the nanorods are V2O5 compound, and to investigate their electronic structure. The optical properties of the V2O5 nanorods were also characterized by UV–Vis spectroscopy. It was observed that the absorption edge had an obvious red shift. The growth mechanism of the nanorods and the relationship between the microstructure and the optical properties are briefly discussed.

Microstructure and optical properties of Ag-doped ZnO nanostructures prepared by a wet oxidation doping process

Silver-doped zinc oxide (Ag:ZnO) nanostructures were prepared by a facile and efficient wet oxidation method. This method included two steps: metallic Zn thin films mixed with Ag atoms were prepared by magnetron sputtering as the precursors, and then the precursors were oxidized in an O(2) atmosphere with water vapour present to form Ag:ZnO nanostructures. By controlling the oxidation conditions, pure ZnO and Ag:ZnO nanobelts/nanowires with a thickness of ∼ 20 nm and length of up to several tens of microns were synthesized. Scanning electron microscopy, transmission electron microscopy, cathodoluminescence and low temperature photoluminescence (PL) measurements were adopted to characterize the microstructure and optical properties of the prepared samples. The results indicated that Ag doping during magnetron sputtering was a feasible method to tune the optical properties of ZnO nanostructures. For the Ag:ZnO nanostructures, the intensity of ultraviolet emission was increased up to three times compared with the pure ones. The detailed PL intensity variation with the increasing temperature is also discussed based on the ionization energy of acceptor in ZnO induced by Ag dopants.

Control of the Metal-Insulator Transition in VO2 Epitaxial Film by Modifying Carrier Density

External controlling the phase transition behavior of vanadium dioxide is important to realize its practical applications as energy-efficient electronic devices. Because of its relatively high phase transition temperature of 68 °C, the central challenge for VO2-based electronics, lies in finding an energy efficient way, to modulate the phase transition in a reversible and reproducible manner. In this work, we report an experimental realization of p-n heterojunctions by growing VO2 film on p-type GaN substrate. By adding the bias voltage on the p-n junction, the metal-insulator transition behavior of VO2 film can be changed continuously. It is demonstrated that the phase transition of VO2 film is closely associated with the carrier distribution within the space charge region, which can be directly controlled by the bias voltage. Our findings offer novel opportunities for modulating the phase transition of VO2 film in a reversible way as well as extending the concept of electric-field modulation on other phase transition materials.

Study of a nitrogen-doped ZnO film with synchrotron radiation

X-ray absorption near-edge spectroscopy (XANES) and photoelectron spectroscopy (PES) with synchrotron radiation have been applied to investigate the structure and chemical states of nitrogen atoms in ZnO:N films with different annealing temperatures. The high-resolution XANES and PES spectra of N 1s reveal the chemical states of N dopants and give a direct observation of nitrogen location in the ZnO films. The results indicate that only the nitrogen atoms incorporated substitutionally at O sites act as acceptors, and contribute to the p-type characteristic of the ZnO:N film.

Temperature sensitive crystallization of V2O5: from amorphous film to β-V2O5nanorods

Amorphous V2O5 films dramatically transform to standing β-phase V2O5nanorods and flat lying nanoslices after annealing in O2 ambience within a narrow temperature range, showing a remarkable temperature-sensitive crystallization process.

Decoupling the Lattice Distortion and Charge Doping Effects on the Phase Transition Behavior of VO2 by Titanium (Ti4+) Doping

The mechanism for regulating the critical temperature (TC) of metal-insulator transition (MIT) in ions-doped VO2 systems is still a matter of debate, in particular, the unclear roles of lattice distortion and charge doping effects. To rule out the charge doping effect on the regulation of TC, we investigated Ti4+-doped VO2 (TixV1-xO2) system. It was observed that the TC of TixV1-xO2 samples first slightly decreased and then increased with increasing Ti concentration. X-ray absorption fine structure (XAFS) spectroscopy was used to explore the electronic states and local lattice structures around both Ti and V atoms in TixV1-xO2 samples. Our results revealed the local structure evolution from the initial anatase to the rutile-like structure around the Ti dopants. Furthermore, the host monoclinic VO2 lattice, specifically, the VO6 octahedra would be subtly distorted by Ti doping. The distortion of VO6 octahedra and the variation of TC showed almost the similar trend, confirming the direct effect of local structural perturbations on the phase transition behavior. By comparing other ion-doping systems, we point out that the charge doping is more effective than the lattice distortion in modulating the MIT behavior of VO2 materials.