High-resolution TEM characterization of ZnO core-shell nanowires for dye-sensitized solar cells

Recently ZnO nanowire films have been used in very promising and inexpensive dye-sensitized solar cells (DSSC). It was found that the performance of the devices can be enhanced by functionalising the nanowires with a thin metal oxide coating. This nm-scale shell is believed to tailor the electronic structure of the nanowire, and help the absorption of the dye. Core-shell ZnO nanowire structures are synthesised at low temperature (below 120°C) by consecutive hydrothermal growth steps. Different materials are investigated for the coating, including Mg, Al, Cs and Zr oxides. High resolution TEM is used to characterise the quality of both the nanowire core and the shell, and to monitor the thickness and the degree of crystallisation of the oxide coating. The interface between the nanowire core and the outer shell is investigated in order to understand the adhesion of the coating, and give valuable feedback for the synthesis process. Nanowire films are packaged into dye-sensitised solar cell prototypes; samples coated with ZrO2 and MgO show the largest enhancement in the photocurrent and open-circuit voltage and look very promising for further improvement. © 2010 IOP Publishing Ltd.

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness. © 2007 IOP Publishing Ltd.

Ultra-high density carbon nanotubes on Al-Cu for advanced vias

An integration scheme for carbon nanotube via interconnects is described to produce nanotube densities of 2.5 1012 tubes/cm2 or 8 1012 walls/cm2 on metallic Al-Cu lines, an order of magnitude beyond the previous state of art, and, for first time, close to that needed for implementation. ©2010 Crown.

Nonvolatile memory functionality of ZnO nanowire transistors controlled by mobile protons.

We demonstrated the nonvolatile memory functionality of ZnO nanowire field effect transistors (FETs) using mobile protons that are generated by high-pressure hydrogen annealing (HPHA) at relatively low temperature (400 °C). These ZnO nanowire devices exhibited reproducible hysteresis, reversible switching, and nonvolatile memory behaviors in comparison with those of the conventional FET devices. We show that the memory characteristics are attributed to the movement of protons between the Si/SiO(2) interface and the SiO(2)/ZnO nanowire interface by the applied gate electric field. The memory mechanism is explained in terms of the tuning of interface properties, such as effective electric field, surface charge density, and surface barrier potential due to the movement of protons in the SiO(2) layer, consistent with the UV photoresponse characteristics of nanowire memory devices. Our study will further provide a useful route of creating memory functionality and incorporating proton-based storage elements onto a modified CMOS platform for FET memory devices using nanomaterials.

Development of the microstructure of uranium-doped Nd-Ba-Cu-O

Melt grown Nd-Ba-Cu-O (NdBCO) has been reported to exhibit higher values of critical current density, Jc and irreversibility field, Hirr, than other (RE)BCO superconductors, such as YBCO. The microstructure of NdBCO typically contains 5-10 μm sized inclusions of the Nd4Ba2Cu2O10 phase (Nd-422) in a superconducting NdBa2Cu3O7-δ phase (Nd-123) matrix. The average size of these inclusions is characteristically larger than that of the Y2BaCuO5 (Y-211) inclusions in YBCO. As a result, there is scope to further refine the Nd-422 size to enhance Jc in NdBCO. Large grain samples of NdBCO superconductor doped with various amounts of depleted UO2 and containing excess Nd-422 have been fabricated by top seeded melt growth under reduced oxygen partial pressure. The effect of the addition of depleted UO2 on the NdBCO microstructure has been studied systematically in samples with and without added CeO2. It is observed that the addition of UO2 refines the NdBCO microstructure via the formation of uranium-containing phase particles in the superconducting matrix. These particles are of approximately spherical geometry with dimensions of around 1 μm. The average size of the nonsuperconducting phase particles in the uranium-doped microstructure is an order of magnitude less than their size in un-doped Nd-123 prepared with excess Nd-422. The critical current density of uranium-doped NdBCO is observed to increase significantly compared to the undoped material.

Large single grain (RE)-Ba-Cu-O superconductors with nano-phase inclusions

Nano-phase (5-20 nm) particles of YBa2(Cu0.5M 0.6)O6 [where M = Nb, Ta, Mo, W, Zr and Hf] have been introduced successfully into RE-Ba-Cu-O single grain superconductors. A study to enlarge the size of a single grain containing these particles has been carried out involving measurement of the growth rate as a function of YBa 2(Cu0.5M0.6)O6 phase concentration and degree of un-dercooling. The influence of the change in YBa2 (Cu0.8M0.5)O6 concentration on microstructural features is also investigated and the superconducting properties of these large grain superconductors are presented. © 2005 IEEE.

CNTs and CNT/ZnO nanostructures for use as electron sources

The aim of this paper is to describe the growth and optimization of carbon nanotube (CNT) and CNT/Zinc Oxide nanostructures to produce novel electron sources. The emitters studied in this project are based on regular array of vertically aligned 5 μm height and 50 nm diameter CNTs with a pitch of 10 μm as described previously (1). Such a cathode design allows us to minimize electric field shielding effects and thus to help in optimizing the emitted current density. We have previously obtained a current density of 1 A/cm 2 from such arrays in DC mode, and over 12 A/cm2 in pulsed mode at RF frequencies. © 2010 IEEE.