Narrow Results

A silver-loaded one-dimensional (1D) vertical ZnO nanowires (NWs) array synthesized by a facile seed mediated hydrothermal-RF magnetron sputtering method has been investigated for the fabrication of a highly stable and reproducible acetylene (C₂H₂) gas sensor. Successful immobilization of silver nanoparticles (NPs) as a sensitizer on the ZnO NWs array significantly enhanced the C₂H₂ sensing properties and showed a stable sensing performance. The grown structure exhibited high response magnitude (30.8 at 1000ppm), short response time (43s) and excellent selectivity at 220${}^{\circ }$C. The enhanced performance can probably be accounted for the effect of combining the highly orientated ZnO NWs and catalytically active silver-based network. The superior sensing features toward C₂H₂ along with broad detection range (1–1000ppm), outstanding stability and excellent reproducibility indicate that the sensor is a promising candidate for practical applications.

The role of Chemical Vapor Deposition's (CVD) temperature and time duration on the mass transport properties and repercussions of these properties on the crystalline growth of ZnO nanowires inside Anodic Aluminum Oxide (AAO) nanotubes (aspect ratio = 1500) have been investigated.

CVD assisted growth of nanowires in AAO nanochannels was carried out for 27 min at deposition temperature of 813 K (A1) and for 180 min at 903 K (A2) under identical gas flow rates (Ar: 23 sccm and O₂: 0.45 sccm) under a chamber pressure of 4.45 torr.

Electron diffraction patterns (SAED) of the ZnO diffused inside A1 nanotube (exposed in Focused Ion Beam milled cross-sections) showed a ringed pattern, signifying polycrystalline morphology at the tip as well as the channel bottom. For A2, a single crystal formation at least in the tip region was confirmed by the SAED pattern while the corresponding High Resolution TEM image (HRTEM) revealed a lattice spacing of 0.24 nm corresponding to the (1 0 1) growth direction of zincite (JCPDS-36-1451).

Chemical composition of the nanowires, reflected the inherent stoichiometry of preferentially nucleated ZnO in the grooved orifices of individual nanopores. Data points of normalised values of the Zn concentration (TEM-EDS line scan) versus penetration depth (~ 0.6 μm), were empirically fitted with solutions of modified Fickian diffusion equation in 1-D. The Diffusion coefficient values of the order of 0.06 nm² s^-1 and 0.22 nm² s^-1 were obtained for ZnO flow inside nanochannels of A1 and A2, respectively. Observed variations, in diffusivity and relative density, have been explained qualitatively by considering the transport of ZnO inside the nanochannels as arising from superposition of the temperature dependent contributions of the convectional viscous creep and surface diffusion components.

We study the effect of ZnO source and growth temperatures on morphology of the ZnO nanostructures. Nanostructures grown from ZnO nanopowder show the shape evolution from nanowires to nanoribbons and then to nanorods with variation in growth temperature and zinc vapor pressure. At 750°C, only vertically aligned nanowires have been observed and with increase in growth temperature nanowires transforms to nanoribbons and then finally to nanorods at 870°C. On the other hand, in case of ZnO bulk powder as a source material, only nanowires are produced at all temperatures. Raman scattering studies on the nanostructures show distinct mode at ~ 437 cm^-1, confirming the hexagonal wurtzite phase of the as grown nanostructures. Room temperature photoluminescence spectra exhibit bound exciton related strong UV emission at ~ 379 nm indicating high quality of the as-grown nanostructures. Possible mechanism of growth and shape evolution in ZnO nanostructures are explored through systematic studies of the effect of growth temperatures and zinc vapor pressure.

The difficulty of achieving p-type conductance has severely limited the application of ZnO to electronic devices. In this work, we propose a simple and effective way to achieving p-type semiconducting ZnO nanowires (NWs) through density functional theory computations. Adsorption of tetrathiafulvalene (TTF) during synthetic procedures leads to lower formation energies, and accordingly higher concentration of p-type defects; after the formation of ZnO NWs and the desorption of TTF, p-type ZnO NWs can be achieved. Also, we present a facile synthesis route on basis of previous experiments, which provides some guidance for the realization of p-type ZnO NWs.

We report the effects of 70 keV proton (H⁺) irradiation on the structure of zinc oxide nanowires (ZnO NWs) for a wide range of irradiation doses at room temperature. It was found that at low dose 5 × 10¹⁵ ions/cm² of protons, few defects were created in ZnO NWs and the defects' density was increased with an increasing proton irradiation dose. After the irradiation dose was increased to 2 × 10¹⁷ ions/cm², the crystal structure of the ZnO NWs was almost completely damaged and the crystalline wurtzite structure of the ZnO NWs could be transformed into a disordered amorphous structure. Structural changes in the ZnO NWs upon bombardment with 70 keV protons were characterized by high resolution transmission electron microscopy (HRTEM).

In this work, we investigated the effect of organic CuPc coating on the surface of the ZnO NWs for possible improvement in the photoluminescence, photoconductivity and photoresponse. As a result of surface covering, the UV emission is enhanced by a factor of 7–8 while the green emission is reduced to half. Despite an increase in dark current after the CuPc covering, we obtained a significant improvement in the photocurrent and photoresponse rate. The photocurrent-to-dark current ratio is nearly doubled and the photoresponse process becomes faster for the ZnO/CuPc heterostructure. Improvements in the photoluminescence and photoconductivity for the ZnO/CuPc heterostructure are explained on the basis of modification of surface defects and interfacial charge transfer process.

Zinc oxide, ZnO, a popular semiconductor material with a wide band gap (3.37 eV) and high binding energy of the exciton (60 meV), has numerous applications such as in optoelectronics, chemical/biological sensors, and drug delivery. This project aims to (i) optimize the operating conditions for growth of ZnO nanostructures using the chemical vapor deposition (CVD) method, and (ii) investigate the effects of coupling radiofrequency (RF) plasma to the CVD method on the quality of ZnO nanostructures. First, ZnO nanowires were synthesized using a home-made reaction setup on gold-coated and non-coated Si (100) substrates at 950 °C. XRD, SEM, EDX, and PL measurements were used for characterizations and it was found that a deposition duration of 10 minutes produced the most well-defined ZnO nanowires. SEM analysis revealed that the nanowires had diameters ranging from 30-100 mm and lengths ranging from 1-4 µm. In addition, PL analysis showed strong UV emission at 380 nm, making it suitable for UV lasing. Next, RF plasma was introduced for 30 minutes. Both remote and in situ RF plasma produced less satisfactory ZnO nanostructures with poorer crystalline structure, surface morphology, and optical properties due to etching effect of energetic ions produced from plasma. However, a reduction in plasma discharge duration to 10 minutes produced thicker and shorter ZnO nanostructures. Based on experimentation conducted, it is insufficient to conclude that RF plasma cannot aid in producing well-defined ZnO nanostructures. It can be deduced that the etching effect of energetic ions outweighed the increased oxygen radical production in RF plasma nanofabrication.