Ye Sun

Growth of ZnO thin films: Experiment and theory

Many recent studies of ZnO thin film growth have highlighted a propensity for forming c-axis aligned material, with the crystal morphology dominated by the polar {0001} surface. This is illustrated here for ZnO thin films grown by pulsed laser deposition methods, and put to advantage by using such films as templates for aligned growth of ZnO nanorods. Complementary to such experiments, we report results of periodic ab initio density functional theory calculations on thin films of ZnO which terminate with the (0001), (000), (100) and (110) surfaces. Thin (<18 layer) films which terminate with the polar (0001) and (000) surfaces are found to be higher in energy than corresponding films in which these polar surfaces flatten out forming a new ‘graphitic’-like structure in which the Zn and O atoms are coplanar and the dipole is removed. For thinner (<10 layer) slab sizes this coplanar surface is found to be lower in energy than the non-polar (100) and (110) surfaces also. The transition between the lowest energy geometries as the ZnO film thickness increases is investigated, and possible consequences for the growth mechanism discussed.

Photoluminescence from diameter-selected ZnO nanorod arrays

Arrays of ZnO nanorods with different, user-controlled, diameters were formed on Si substrates by 193xa0nm pulsed laser deposition in a low background pressure of oxygen. The effect of nanorod diameter on the photoluminescence exhibited by these arrays was investigated, as a function of incident ultraviolet (UV, 325xa0nm) laser intensity. Nanorods with large surface to volume ratios can exhibit high UV (~380xa0nm) emission efficiencies, particularly when excited at higher incident intensities. Pre-illumination of such nanorods with 325xa0nm radiation serves to enhance the UV emission measured at lower incident intensities. These observations are rationalized in terms of photo-induced desorption of surface-bound oxygen, resulting in a quenching of the depletion layer at the ZnO surface. These findings serve to reinforce the potential of ZnO nanostructures for optoelectronic device applications.

Sensitive Room Temperature Photoluminescence-Based Sensing of H2S with Novel CuO-ZnO Nanorods.

Novel CuO nanoparticle-capped ZnO nanorods have been produced using a pulsed laser deposition (PLD) method. These nanorods are shown to grow by a CuO-nanoparticle-assisted vapor-solid-solid (V-S-S) mechanism. The photoluminescence (PL) accompanying ultraviolet illumination of these capped nanorod samples shows large variations upon exposure to trace quantities of H2S gas. The present data suggest that both the Cu-doped ZnO stem and the CuO capping nanoparticle contribute to optical H2S sensing with these CuO-ZnO nanorods. This study represents the first demonstration of PL-based H2S gas sensing, at room temperature, with sub-ppm sensitivity. It also opens the way to producing CuO-ZnO nanorods by a V-S-S mechanism using gas-phase methods other than PLD.

Reduction of Threading Dislocations in Epitaxial ZnO Films Grown on Sapphire (0001)

Transmission electron microscopy was used to investigate epitaxial ZnO films on c-sapphire, produced by a two-step method. Firstly, pulsed laser deposition provided a continuous buffer ZnO, with thickness about 80 nm and a predominant alignment of (0001)ZnO//(0001)sapphire and [11–20]ZnO//[10-10]sapphire. On the top of buffer layer there was a high density of c-aligned nanorods, which revealed few, if any, threading dislocations (TDs), in contrast with the buffer layer where TD density was about 1011/cm2. Subsequent treatments by either chemical vapour deposition or hydrothermal growth caused the nanorods to grow laterally and form continuous films. Subgrain boundary dislocations were generated as nanorods coalesced, but these new TDs were also annihilated in dislocation reactions, giving to a substantial reduction in the total TD density.