M. Biswas

Growth of ZnO nanostructures on Au-coated Si: Influence of growth temperature on growth mechanism and morphology

ZnO nanostructures were grown on Au-catalyzed Si silicon substrates using vapor phase transport at growth temperatures from 800 to 1150 °C. The sample location ensured a low Zn vapor supersaturation during growth. Nanostructures grown at 800 and 850 °C showed a faceted rodlike morphology with mainly one-dimensional (1D) growth along the nanorod axis. Samples grown at intermediate temperatures (900, 950, and 1050 °C) in all cases showed significant three dimensional (3D) growth at the base of 1D nanostructures. At higher growth temperatures (1100 and 1150 °C) 3D growth tended to dominate resulting in the formation of a porous, nanostructured morphology. In all cases growth was seen only on the Au-coated region. Our results show that the majority of the nanostructures grow via a vapor-solid mechanism at low growth temperatures with no evidence of Au nanoparticles at their tip, in sharp contrast to the morphology expected for the vapor-liquid-solid (VLS) process often reported as the growth mechanism on Au-c...

Carbothermal reduction vapor phase transport growth of ZnO nanostructures: Effects of various carbon sources

ZnO nanostructures were grown via carbothermal reduction vapor phase transport with carbon black, activated carbon, and graphite powders. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, although with different morphologies compared to graphite. The surface areas of the carbon black and activated carbon are higher than those of graphite; this has been used previously to explain the origin of such growth and morphology differences. We use different ZnO∕graphite ratios to equalize surface areas compared to carbon black and eliminate this effect, but differences in nanostructure growth and morphology remain. We discuss the effects of thermodynamics and carbon purity and conclude that the high surface activities of the carbon black and activated carbon are the reason for our results.

Multiphoton-absorption induced ultraviolet luminescence of ZnO nanorods using low-energy femtosecond pulses

Multiphoton-absorption (MPA) induced ultraviolet (UV) luminescence of ZnO nanorods grown by vapor phase transport was demonstrated using ultrafast excitation at pulse energies in the few nanojoules range, directly generated by a Ti:sapphire laser oscillator at wavelengths around 800 nm. The dependence of the UV luminescence on the excitation density reveals a two-photon absorption process as the responsible excitation mechanism. The broad spectral bandwidth of the excitation pulses obviously promotes the feasibility of the observed two-photon channel. Theoretical estimates concerning the contribution of nonlinear absorbance strongly support the experimental findings. The essential conditions for proper utilization of this process are discussed.

Carbothermal reduction growth of ZnO nanostructures on sapphire-comparisons between graphite and activated charcoal powders

Zinc oxide (ZnO) nanostructures were grown by the vapour phase transport (VPT) method on a-plane sapphire substrates via carbothermal reduction of ZnO powders with various carbon powders. Specifically, graphite powder and activated charcoal powder (of larger total surface area but similar mesh size) were used. ZnO nanostructures can be grown at lower temperatures (~800^oC) using activated charcoal than those required using graphite powder. Furthermore, the morphologies of ZnO nanostructures obtained using activated charcoal were different to those obtained using graphite. At higher temperatures (~950^oC), where well-aligned nanorods were obtained using graphite powder, no nanostructures were found using activated charcoal. In contrast to previous results on Si substrates we find that the effects on ZnO nanostructure growth on a-sapphire cannot be explained solely in terms of increased Zn vapour pressure due to the enhancement of the carbothermal reduction reaction rate by the high surface area activated charcoal.

Spatial inhomogeneity of donor bound exciton emission from ZnO nanostructures grown on Si

We report low temperature cathodoluminescence spectroscopy measurements of the band edge emission from ZnO nanostructures grown by vapour phase transport on Si. A range of donor bound exciton emission lines are found and the Al-related emission at 3.3605 eV in particular shows a marked inhomogeneity in its distribution throughout the sample. Increased 3.3605 eV emission is seen at a range of locations in nanorods and nanosheets where different nanostructures cross or coalesce, suggesting aggregation of Al donors in ZnO in regions of crystal structure disruption. However, localized crystal structure disruption appears to be a necessary rather than a sufficient condition for Al aggregation, since increased 3.3605 eV emission is seen only in such regions, but not all such regions show increased emission, implying that the microscopic nature of such regions is important in determining Al aggregation. Supporting data are presented from well-aligned, non-crossing, nanorods on a-sapphire.

Microscopic origins of the surface exciton photoluminescence in ZnO nanostructures

Photoluminescence (PL) studies of the surface exciton peak in ZnO nanostructures at ~3.367 eV are reported to elucidate the nature and origin of the emission and its relationship to nanostructure morphology. Localised voltage application in high vacuum and different gas atmospheres show a consistent PL variation (and recovery), allowing an association of the PL to a bound excitonic transition at the ZnO surface modified by an adsorbate. Studies of samples treated by plasma and of samples exposed to UV light under high vacuum conditions show no consistent effects on the surface exciton peak indicating no involvement of oxygen species. X-ray photoelectron spectroscopy data indicate involvement of adsorbed OH species. The relationship of the surface exciton peak to the nanostructure morphology is discussed in light of x-ray diffraction, scanning and transmission electron microscopy data.

Strong multiphoton-absorption-induced UV luminescence from ZnO nanorod arrays grown by vapour-liquid-solid mechanism

ZnO is one of the most promising materials for optoelectronic devices for UV spectral range because of its wide and direct bandgap (3.37 eV) and high exciton binding energy (60 meV) [1]. The generation of multiphoton absorption (MPA) induced luminescence in ZnO by highly intense fs-lasers in near infrared offers one effective route for optical excitation. Therefore, much attention has been paid recently to investigate this specific material behaviour [2]. Here we report on the observation of strong bandedge emission with little or no deep level emission in c-axis oriented ZnO nanorod arrays grown by vapour-liquid-solid (VLS) mechanism (Fig. 1) after illuminating with focused fs-pulses. UV-VIS Luminescence Spectra were measured with an Ocean Optics spectrometer and compared with data from a reference single crystal and nanorods fabricated by low temperature chemical procedure [3] (Fig. 2). VLS-type ZnO nanorods were grown via vapour phase transport on a-plane (11–20) sapphire substrates with Au as catalyst. The SEM image in Fig 1 shows hexagonal nanorods which are relatively well aligned perpendicular to the substrate and have an average (diagonal) diameter of about 90 nm.