Gengmin Zhang

Field emission from zinc oxide nanotowers: the role of the top morphology

Arrays of novel nanometer-scale tower-shaped structures of zinc oxide (ZnO nanotowers) were synthesized by a simple thermal evaporation method. Due to the difference in fabrication conditions, ZnO nanotowers with similar body structure but different top morphologies were obtained. These ZnO nanotowers with different top morphologies showed obvious disparity in field emission, despite their overall field enhancement factor and density being the same. The nanotowers with the sharpest top had the lowest turn-on and threshold electric field. This disparity is attributed to the different local field enhancement factors at the nanotower tops, which were calculated both from the field emission data and by simulation. The above results have demonstrated the essential importance of the top morphology of a ZnO nanostructure in field emission.

TiO2 Nanotip Arrays: Anodic Fabrication and Field-Emission Properties

In contrast to the main-stream strategy of growing convex nanostructures upward from the substrates and using them as cold electron sources, it is illustrated in this article that growing concave nanostructures downward into substrates also results in configurations suitable for field emission. Well-ordered TiO₂ nanotube arrays were developed on the titanium foils in two-step anodizations. Simultaneously, arrays of sharp nanotips, which resembled the Spindt emitter arrays in appearance, also manifested themselves on the outmost surface of the foils. These nanotips were actually the remainder of the titanium foil surfaces that survived dissolution during anodization. Annealing transformed the amorphous TiO₂ nanotips into anatase crystals and further to rutile. Despite the lack of an overall large aspect ratio, the sharpness of these nanotips guaranteed sufficiently strong electric fields for electron extraction. As a result, field emission was readily obtained from the TiO₂ nanotip arrays, either before or after annealing. Photoelectron spectroscopy of the samples demonstrated that the majority of the emitted electrons came from local states in the band gap. Annealing at an appropriate temperature increased these local states and improved the field-emission capability of the samples.

Field emission from carbon nanotube arrays fabricated by pyrolysis of iron phthalocyanine

Arrays of multi-walled carbon nanotubes (MWCNTs) were fabricated by pyrolysis of iron phthalocyanine (FePc). A silicon wafer and a stainless steel plate were used as the substrates. MWCNTs grown on the silicon wafer were packed closely to each other and were thus well aligned, while those grown on the stainless steel plate had a low density and were oriented randomly. Field emission was achieved from the MWCNT arrays on both substrates. The turn-on electric fields of the silicon-based and stainless steel-based arrays were measured to be 1.9?V??m?1 and 3.4?V??m?1, respectively. The emission site distribution was also studied using a transparent anode. The field emission from the MWCNTs on the silicon substrate occurred mainly at the edge regions, while that from the MWCNTs on the stainless steel substrate exhibited a much better uniformity. We attribute this disparity in the emission site distribution to the screening effect of the electric field.

Highly ordered hierarchical TiO2 nanotube arrays for flexible fiber-type dye-sensitized solar cells

Highly ordered hierarchical TiO2 nanotube (HTNT) arrays are obtained with a facile and low-cost approach, which can be applied in flexible fiber-type dye-sensitized solar cells (DSSCs). To achieve the HTNT arrays, smooth TiO2 nanotube (STNT) arrays are fabricated by a two-step anodization process followed by hydrothermal treatment in the presence of the aqueous solution of NH4F. Both STNT and HTNT arrays can serve as photoanodes in the flexible fiber-type DSSCs. We demonstrate that the conversion efficiency of the HTNT array-based flexible fiber-type DSSC (8.6%), which has increased by more than 30% compared with that of the STNT array-based flexible fiber-type DSSC. Moreover, the conversion efficiency is highly stable, even when the flexible fiber-type DSSC is subjected to extreme mechanical deformation, such as 180° bending. This promising conversion efficiency may originate from the significant dye loading capability of the photoanode, which has an enormous surface area benefited from the hierarchical nanostructures.

Preparation, characterization and catalytic property of CuO nano/microspheres via thermal decomposition of cathode-plasma generating Cu2(OH)3NO3 nano/microspheres.

CuO nano/microspheres with a wide diametric distribution were prepared by thermal decomposition of Cu(2)(OH)(3)NO(3) nano/microspheres formed in a simple asymmetric-electrode based cathodic-plasma electrolysis. The morphological, componential, and structural information about the two kinds of spheres were characterized in detail by SEM, TEM, EDX, XPS and XRD, and the results revealed that the morphology of the spheres were well kept after the componential and structural transformation from Cu(2)(OH)(3)NO(3) into CuO. The TGA/DSC study showed that the CuO nano/microspheres could be explored to be a promising additive for accelerating the thermal decomposition of ammonium perchlorate (AP). Combining with the current curve and emission spectrum measured in the plasma electrolysis, formation mechanism of the Cu(2)(OH)(3)NO(3) spheres was also discussed.

TiO2 hierarchical nanostructures: Hydrothermal fabrication and application in dye-sensitized solar cells

Arrays of TiO2 hierarchical nanostructures that consisted of rutile nanorods and anatase branches were hydrothermally fabricated and employed as photoanodes in dye-sensitized solar cells (DSSCs). Each hierarchical nanostructure array was attained in two steps. First, a primary nanorod array was synthesized in aqueous solutions of hydrochloric acid (HCl) and tetrabutyl titanate (C16H36O4Ti); subsequently, secondary branches were grown on the nanorods in aqueous solutions of ammonium hexafluorotitanate ((NH4)2TiF6) and boric acid (H3BO3). The secondary anatase branches filled part of the space among the primary rutile nanorods and gave rise to a larger surface area. Light-harvesting capability of the DSSCs with TiO2 hierarchical nanostructures as photoanodes was appreciably improved because more dye molecules could be loaded on the photoanodes and more light could be scattered inside the DSSCs. Therefore, the conversion efficiencies of the DSSCs were doubled by replacing the photoanode of primary TiO2 nanor...

Field emission of individual carbon nanotubes on tungsten tips

Individual multiwalled carbon nanotubes (MWCNTs) were assembled onto tungsten tips in a transmission-electron microscope. Then they were transferred into a field-emission microscope for the measurement of field-emission properties. Stable field emission was established after repeated heat treatment and extraction of field-emission current, which are believed to have cleaned and blunted the MWCNT ends. Even under high voltages and large currents, most of the emitted electrons that hit the screen were found to be still restricted within an ∼10−2 solid angle, indicating the possible availability of a high brightness.

Room-temperature fabrication of dual-functional hierarchical TiO2 spheres for dye-sensitized solar cells

At room temperature, hierarchical TiO2 spheres with dual functions of high dye loading capacity and light scattering ability were synthesized by a facile and low-cost method. The dye-sensitized solar cells using the hierarchical TiO2 spheres as the scattering layer displayed improved cell performance when compared with the commercial scattering layer.

Amazing ageing property and in situ comparative study of field emission from tungsten oxide nanowires

In this work, needle-shaping of tungsten oxide nanowires occurred during field emission characterization. Compared with nanowires with a flat apex, needle-shaped emitters showed a lower threshold field of 11.9 V µm(-1) for 1 mA cm(-2) and a higher emission current of 1120 µA at 16.2 V µm(-1). Most notably, the measured ageing current dramatically increased by more than four times until it slightly decreased, tending towards stability. In addition, the samples showed striking difference in their nonlinear Fowler-Nordheim plot before and after ageing tests. Selected area diffraction and transmission electron microscope characterizations were used to further study these amazing results.

Hydrothermally formed functional niobium oxide doped tungsten nanorods.

Nanorod forms of metal oxides are recognized as one of the most remarkable morphologies. Their structure and functionality have driven important advancements in a vast range of electronic devices and applications. In this work, we postulate a novel concept to explain how numerous localized surface states can be engineered into the bandgap of niobium oxide nanorods using tungsten. We discuss their contributions as local state surface charges for the modulation of a Schottky barrier height, the relative dielectric constant and their respective conduction mechanisms. Their effects on hydrogen gas molecule interaction mechanisms are also examined herein. We synthesized niobium tungsten oxide (Nb17W2O25) nanorods via a hydrothermal growth method and evaluated the Schottky barrier height, ideality factor, dielectric constant and trap energy level from the measured I-V versus temperature characteristics in the presence of air and hydrogen to show the validity of our postulations.