Huaijuan Zhou

Direct imaging of intrinsic molecular orbitals using two-dimensional, epitaxially-grown, nanostructured graphene for study of single molecule and interactions

Using epitaxially grown graphene on Ru(0001) as a buffer layer, the intrinsic molecular orbitals of perylene-3,4,9,10-tetracarboxylic dianhydride, pentacene, and C60 molecules were imaged by means of scanning tunneling microscope (STM). Combined with density functional theory calculations, our high resolution STM images of the molecules reveal that the graphene layer decouples the individual molecules electronically from the metallic substrate. Our results show that graphene-based moire pattern can be used as a unique way to probe the intrinsic electronic structures of molecular adsorbates and their interactions.

Self-assembly of C60 monolayer on epitaxially grown, nanostructured graphene on Ru(0001) surface

C60 molecules adsorbed on graphene/Ru(0001) substrate were investigated by scanning tunneling microscopy (STM) at 5 K. On high quality substrates, C60 molecules adopt a commensurate growth mode, leading to formation of a supramolecular structure with perfect periodicity and few defects. On under-annealed substrates with imperfections and domains, the molecules form the same closely packed hexagonal structures in spite of underlying corrugations, disorders or steps, indicating a weak molecule-substrate interaction—a conclusion that is also supported by DFT calculations. This system may be beneficial to the fabrication of carbon based devices and of other types of organic functional overlayers.

Template-free formation of various V2O5 hierarchical structures as cathode materials for lithium-ion batteries

Various V2O5 hierarchical structures were successfully synthesized via a template-free method by annealing diverse morphological VO2 sub-microspheres which can be facilely tailored by adjusting the solvothermal reaction duration. The VO2 sub-microspheres undergo a solid → yolk–shell → hollow → yolk–shell structure process with increasing time, which is believed to result from an unusual Ostwald-ripening process. After the annealing process, multi-structural VO2 sub-microspheres changed into hierarchical structures including fist-type structures consisting of nanorods, yolk–shell and hollow sub-microspheres composed of nanorods and a yolk–shell construction made up of nanoplates. As the cathode materials for lithium-ion batteries, among them, yolk–shell sub-microspheres comprised of nanoplates exhibited high reversible capacity, excellent cycling stability at high currents and good rate capacities. Without doping and compositing, the electrode delivered reversible capacities of 119.2 and 87.3 mA h g−1 at high current densities of 2400 and 3600 mA g−1, respectively, as well as a capacity retention of 78.31% after 80 cycles at 1200 mA g−1. The excellent electrochemical performance could be attributed to the purity of the phase and synergistic effect between the yolk–shell structure and hierarchical structure of the sub-microspheres, which make the yolk–shell V2O5 hierarchical structure a promising candidate for the cathode material for lithium-ion batteries.

Self-Assembled Multilayer Structure and Enhanced Thermochromic Performance of Spinodally Decomposed TiO2–VO2 Thin Film

Composite films of VO2-TiO2 were deposited on sapphire (11-20) substrate by cosputtering method. Self-assembled well-ordered multilayer structure with alternating Ti- and V-rich epitaxial thin layer was obtained by thermal annealing via a spinodal decomposition mechanism. The structured thermochromic films demonstrate superior optical modulation upon phase transition, with significantly reduced transition temperature. The results provide a facile and novel approach to fabricate smart structures with excellent performance.

Electron transfer induced thermochromism in a VO2–graphene–Ge heterostructure

As a marvelous thermochromic material, vanadium dioxide (VO2) holds great promise for applications in smart devices due to its semiconductor–metal transition (SMT), which is an active response to external temperature stimuli and near-infrared irradiation. Herein, we demonstrate for the first time that the phase transition temperature of a VO2 film can be effectively manipulated using graphene as an interlayer. A high quality graphene monolayer was deposited using chemical vapor deposition on a Ge underlayer, followed by the growth of a VO2 film onto the graphene to form a semimetal–semiconductor contact, namely, a VO2–graphene–Ge junction. The thermochromic properties of the VO2 film were demonstrated with ∼20% infrared reflectance contrast. Furthermore, the transition temperature of the VO2 film was effectively reduced from 340 K to 330 K. On the basis of the Mott–Hubbard phase transition theory, a plausible mechanism is proposed here for the first time from the perspective of charge transfer to elucidate the experimental phenomenon, which indicates that the electron transfer can increase the electron concentration in the VO2 film and destabilize the semiconductor phase of the VO2 film, thus decreasing the SMT temperature of the thermochromic VO2 film. In addition to stimulating scientific interest, this study may also contribute to thermochromic VO2-based applications in sensors, optical and electrical switches, and other nanodevices.

The optical properties of low infrared transmittance WO3−x nanocrystal thin films prepared by DC magnetron sputtering under different oxygen ratios

Low infrared transmittance WO3−x (0 < x < 1) ratios, as transparent conductors with high transmittance in the visible range and UV blocking. The infrared shielding properties could be adjusted by tuning the oxygen ratio during the sputtering process. The intrinsic defects, such as oxygen vacancy () and special atom (W5+), determine the main optical properties by localized states originated from the electron caused by interband transition and the surface dipole of the plasmon oscillation of the nanoparticle with optical scattering. The structures, morphology, defects and chemical states were investigated. With respect to oxygen ratio, it has little effect on the structure and morphology, yet the optical shielding performance, defects and chemical states can be controlled linearly by considering the oxygen ratio during the sputtering process. The WO3−x thin films have great potential for application in infrared shielding and energy conservation.

Thermochromic multilayer films of WO3/VO2/WO3 sandwich structure with enhanced luminous transmittance and durability

A novel thermochromic WO3/VO2/WO3 sandwich structure was deliberately designed and deposited by a reactive magnetron sputtering technique. The double layer of WO3 not only functions as an antireflection (AR) layer to enhance the luminous transmittance (Tlum) of VO2, but also performs as a good protective layer for thermochromic VO2. Basically, the bottom WO3 layer functions as a buffer beneficial for the formation of the intermediate VO2 layer and serves as an AR layer while the intermediate VO2 layer with primary monoclinic phase acts as an automatic solar/heat control for energy saving. The top WO3 layer acts as another AR layer, and provides protection in a complex environment. An obvious increase in Tlum by 49% (from 37.2% to 55.4%) is found for VO2 films after introducing double-layer WO3 AR coating. The VO2 deposited on glass exhibits good thermochromism with an optical transition at 54.5 °C, which decreases to 52 °C in WO3/VO2/WO3 sandwich structure, and the hysteresis loop is sharper around the transition temperature, which may be ascribed to the strain and interfacial diffusion. In comparison with single-layer VO2, the durability in automatic solar/heat control of sandwich-structure VO2 films is improved nearly 4 times in high temperature and humidity conditions. This multilayer will open a new avenue for the design and integration of advanced thermochromic heterostructures with controllable functionalities for intelligent window and sensing system applications.

Synthesis of novel ammonium vanadium bronze (NH4)0.6V2O5 and its application in Li-ion battery

A novel ammonium vanadium bronze (NH4)0.6V2O5 has been successfully synthesized via a simple hydrothermal treatment and its electrochemical performance is investigated. The as-synthesized material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrum, Raman spectrum, X-ray photoelectron spectroscopy (XPS), element analysis (EA), cyclic voltammetry (CV) and galvanostatic charge/discharge cycling test. The results revealed that a pure novel phase (NH4)0.6V2O5 was obtained with square brick-like morphology. Preparation conditions such as amount of reducing agent, temperature and reaction time have been investigated to obtain the pure phase. (NH4)0.6V2O5 square bricks are tested as a cathode material for lithium-ion batteries. It has an excellent lithium ion insertion/extraction ability with a high specific discharge capacity of 280.2 mA h g−1 and 244.3 mA h g−1 during 1.0–3.8 V at the current densities of 10 mA g−1 and 20 mA g−1, respectively.

Room temperature optical constants and band gap evolution of phase pure M 1 -VO 2 thin films deposited at different oxygen partial pressures by reactive magnetron sputtering

Spectroscopic ellipsometry study was employed for phase pure VO2(M1) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO2(M1) thin films have been determined in a photon energy range between 0.73 and 5.05 eV. The near-infrared extinction coefficient and optical conductivity of VO2(M1) thin films rapidly increase with decreasing O2-Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge (E1) at varied O2-Ar ratios are almost the same (∼2.0 eV); consequently, the absorption edge is not significantly changed. However, the optical band gap corresponding to semiconductor-to-metal phase transition (E2) decreases from 0.53 to 0.18 eV with decreasing O2-Ar ratios.

Enhanced Anti-Infective Efficacy of ZnO Nanoreservoirs through a Combination of Intrinsic Anti-Biofilm Activity and Reinforced Innate Defense

The increasing prevalence of implant-associated infections (IAIs) imposes a heavy burden on patients and medical providers. Bacterial biofilms are recalcitrant to antiseptic drugs and local immune defense and can attenuate host proinflammatory response to interfere with bacterial clearance. Zinc oxide nanoparticles (ZnO NPs) play a dual role in antibacterial and immunomodulatory activities but compromise the cytocompatibility because of their intracellular uptake. Here, ZnO NPs were immobilized on titanium to form homogeneous nanofilms (from discontinuous to continuous) through magnetron sputtering, and the possible antimicrobial activity and immunomodulatory effect of nano-ZnO films were investigated. Nano-ZnO films were found to prohibit sessile bacteria more than planktonic bacteria in vitro, and the antibacterial effect occurred in a dose-dependent manner. Using a novel mouse soft tissue IAI model, the in vivo results revealed that nano-ZnO films possessed outstanding antimicrobial efficacy, which could not be ascribed solely to the intrinsic anti-infective activity of nano-ZnO films observed in vitro. Macrophages and polymorphonuclear leukocytes (PMNs), two important factors in innate immune response, were cocultured with nano-ZnO and bacteria/lipopolysaccharide in vitro, and the nano-ZnO films could enhance the antimicrobial efficacy of macrophages and PMNs through promoting phagocytosis and secretion of inflammatory cytokines. This study provides insights into the anti-infective activity and mechanism of ZnO and consolidates the theoretical basis for future clinical applications of ZnO.