We report on neutron transmutation doping (NTD) of isotopically (64Zn) enriched ZnO nanorods to produce material with holes as the majority mobile carrier. Nanorods of ZnO enriched with 64Zn were synthesised and the abundance of 64Zn in these samples is ∼ 71%, compared to the natural abundance of ∼ 49 %. The enriched material was irradiated with thermal neutrons which converts some 64Zn to 65Zn. The 65Zn decays to 65Cu with a half-life of 244 days and the Cu can act as an acceptor dopant. After 690 days, a hot probe technique was used to determine the majority charge carriers in non-irradiated and neutron irradiated nanorod samples. Non-irradiated samples were measured to be to have electrons as the majority mobile carrier and the irradiated samples were measured to have holes as the majority mobile carrier.

The near-ultraviolet photoluminescence of ZnO nanorods induced by multiphoton absorption of unamplified Ti: sapphire pulses is investigated. Power dependence measurements have been conducted with an adaptation of the ultrashort pulse characterization method of interferometric frequency-resolved optical gating. These measurements enable the separation of second harmonic and photoluminescence bands due to their distinct coherence properties. A detailed analysis yields fractional power dependence exponents in the range of 3-4, indicating the presence of multiple nonlinear processes. The range in measured exponents is attributed to differences in local field enhancement, which is supported by independent photoluminescence and structural measurements. Simulations based on Keldysh theory suggest contributions by three-and four-photon absorption as well as avalanche ionization in agreement with experimental findings.

We report on atomic probe microscopy (APM) of isotopically enriched ZnO nanorods that measures the spatial distribution of zinc isotopes in sections of ZnO nanorods for natural abundance natZnO and 64Zn and 66Zn enriched ZnO nanorods. The results demonstrate that APM can accurately quantify isotopic abundances within these nanoscale structures. Therefore the atom probe microscope is a useful tool for characterizing Zn isotopic heterostructures in ZnO. Isotopic heterostructures have been proposed for controlling thermal conductivity and also, combined with neutron transmutation doping, they could be key to a novel technology for producing p-n junctions in ZnO thin films and nanorods.

Zn-and O-isotopically enriched ZnO nanorods were grown with excellent optical quality allowing an identification and resolution of various bound exciton zero-phonon lines (ZPL). Furthermore, the well-known Cu-related emission at 2.86 eV could specifically be studied in order to investigate the local environment of the defect including possible involvement of native defects such as interstitials and vacancies in this deep centre. Energetic shifts of this ZPL were measured and compared to changes in the near band edge (NBE) energies as a function enrichment. No relative shift was observed in Zn-enriched samples, indicating that only O atoms lie in the immediate vicinity of the Cu atom, and that Zn interstitials and O vacancies are not involved in this defect. NBE and Cu 2.86 eV ZPL emissions in samples with O enrichment displayed a significant relative shift, and the Cu 2.86 eV ZPL line widths showed a substantial increase, which is attributed to the multiple local configurations possible for O atoms surrounding the Cu atom in these mixed isotope environments. These data provide the first direct evidence of the microscopic nature of this defect centre of a Cu atom substituting on a Zn lattice site, and is thus consistent with the conventional model of this defect.

We have developed two novel vapour phase transport methods to grow ZnO nanorod arrays isotopically enriched with 18O. Firstly, a three-step process used to grow natural and Zn-enriched ZnO nanorods has been further modified, by replacing the atmospheric O2 with enriched 18O2, in order to grow 18O-enriched ZnO nanorods using this vapour-solid method on chemical bath deposited buffer layers. In addition, 18O-enriched ZnO nanorods were successfully grown using 18O isotopically enriched ZnO source powders in a vapour-liquid-solid growth method. Scanning electron microscopy studies confirmed the success of both growth methods in terms of nanorod morphology, although in the case of the vapour-liquid-solid samples, the nanorods’ c-axes were not vertically aligned due to the use of a non-epitaxial substrate. Raman and PL studies indicated clearly that O-enrichment was successful in both cases, although the results indicate that the enrichment is at a lower level in our samples compared to previous reports with the same nominal enrichment levels. The results of our studies also allow us to comment on both levels of enrichment achieved and on novel effects of the high temperature growth environment on the nanorod growth, as well as suggesting possible mechanisms for such effects. Very narrow photoluminescence line widths, far narrower than those reported previously in the literature for isotopically enriched bulk ZnO, are seen in both the vapour-solid and vapour-liquid-solid nanorod samples demonstrating their excellent optical quality and their potential for use in detailed optical studies of defects and impurities using low temperature photoluminescence.

In this work, we report the growth of vertically aligned ZnO nanorods with excellent optical quality by both catalyst free vapour phase transport (VPT) and catalyst free pulsed laser deposition (PLD). We compare the near band edge emission of such deposits, with a focus on the identification of the origin of the 3.331eV emission feature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and low-temperature (13K) photoluminescence (PL) were used to characterise these nanorod deposits. XRD and SEM data reveal that both techniques lead to highly textured ZnO nanorod arrays with uniform c-axis orientation normal to the substrate surface. The VPT-grown nanorods are well separated and show smooth, facetted surfaces whereas the PLD-grown nanorods are more closely packed and display comparatively rougher surfaces. The optical quality of the samples obtained by both growth methods was very good and low-temperature PL spectra were dominated in both cases by a strong I6 bound exciton (BX) emission (3.36eV), and also showed emission from the surface exciton and the free exciton. A comparatively weak visible emission was also observed in samples deposited by both techniques. The main difference between the PLD- and VPT-grown nanorod samples is the presence of the 3.331eV emission in the former, and its complete absence in the latter (as well as in continuous PLD-grown seed layers) which is discussed in light of the differing surface morphologies mentioned above and which provides strong support for our previous assignment of the origin of this defect to structural defects in the inhomogeneous sub-surface region close to the rough nanorod surface.

We have studied particle acceleration in different nanostructured targets irradiated by high intensity laser pulses of high contrast. We find that the maximum energy of emitted particles and their directionality is significantly enhanced in the case of nanostructured targets with respect to plane targets. We have studied theoretically in detail the generation and propagation of fast electrons in nanowire targets. Such targets exhibit an extraordinary high conversion efficiency of laser energy into electron kinetic energy. We observe guiding of electron bunches along the wires. Results from theory and simulation compare reasonably well with the experimental data.

Multiphoton absorption induced ultraviolet luminescence in ZnO nanorods is investigated with interferometric frequency-resolved optical gating. Power dependency exponents between 3 and 4 are extracted, consistent with simulations using Keldysh theory showing three-photon and four-photon absorption.

We have produced isotopically enriched ZnO nanorods using Zn-enriched ZnO source powder by vapour phase transport on silicon substrates buffer-coated with unenriched ZnO seed layers. SEM and XRD data confirm successful growth of high quality, dense, c-axis aligned nanorods over a substantial surface area. Raman data show a shift of >1cm−1 in the peak position of the Raman scattered peaks due to the E2low and E2high phonon modes when the Zn isotope is changed from 64Zn to 68Zn, consistent with previous work, thus confirming successful isotopic enrichment. SIMS data provides additional confirmation of enrichment. The optical quality (as determined by photoluminescence feature intensity and line width) is excellent. Samples with Zn isotopic enrichment ranging from 64ZnO to 68ZnO display a shift in recombination energy of the bound excitons at the band edge (3.34–3.37eV) of ~0.6meV. This blue-shift is also consistent with previously published data, further confirming both the excellent optical quality and successful isotopic substitution of ZnO nanorods using this relatively simple growth method.

We correct Eq. (1) of our previous paper showing the pulse duration (tau) dependence on group delay dispersion (beta(2)) and initial duration of the unchirped pulse coming from the laser (tau(0)). The correct form of the equation was used in all the calculations in the previous paper however, so in all other respects our previous results and conclusions are unchanged. (C)2014 Optical Society of America

Photoluminescence emission from, and third harmonic generation by, ZnO nanorod samples grown using both low and high temperature methods are reported. Our results clearly show that PL emission at cryogenic temperatures from low temperature grown samples is both significantly weaker in intensity and spectrally broader than that from high temperature grown samples. The third harmonic generation efficiency of samples grown by both the low temperature and high temperature methods are however comparable to one another, and much larger than third harmonic generation at a bare quartz surface. Intensity dependence and interferometric frequency resolved optical gating measurements are used to study the third harmonic generation from both sample types. Laser pulse parameters are extracted for pulses subjected to both high and low chirp and our results indicate that ZnO nanostructures grown by low temperature methods allow excellent characterization of ultrafast pulses and are efficient for third harmonic generation and thus are excellent candidate materials for a variety of technological applications.

Efficient third harmonic generation was found in ZnO nanorod layers grown by phase transport and low temperature chemical bath deposition method. Interferometric frequency-resolved optical gating of few cycle fs pulses was demonstrated.