We report ZnO nanowire/nanowall growth using a two-step vapor phase transport method on $a$-plane sapphire. X-ray diffraction and scanning electron microscopy data establish that the nanostructures are vertically well aligned with the $c$ axis normal to the substrate and have a very low rocking curve width. Photoluminescence data at low temperatures demonstrate the exceptionally high optical quality of these structures, with intense emission and narrow bound exciton linewidths. We observe a high energy excitonic emission at low temperatures close to the band-edge which we assign to the surface exciton in ZnO at $\sim 3.366\mathrm{eV}$. This assignment is consistent with the large surface to volume ratio of the nanowire systems and indicates that this large ratio has a significant effect on the luminescence even at low temperatures. The band-edge intensity decays rapidly with increasing temperature compared to bulk single crystal material, indicating a strong temperature-activated nonradiative mechanism peculiar to the nanostructures. No evidence is seen of the free exciton emission due to exciton delocalization in the nanostructures with increased temperature, unlike the behavior in bulk material. The use of such nanostructures in room temperature optoelectronic devices appears to be dependent on the control or elimination of such surface effects.

A series of Cr-doped ZnO micro-rod arrays were fabricated by a spray pyrolysis method. X-ray diffraction patterns\ud of the samples showed that the undoped and Cr-doped ZnO microrods exhibit hexagonal crystal structure. Surface\ud morphology analysis of the samples has revealed that pure ZnO sample has a hexagonal microrod morphology. From X-ray photoelectron spectroscopy studies, the Cr 2p3/2 binding energy is found to be 577.34 eV indicating that the electron binding energy of the Cr in ZnO is almost the same as the binding energy of Cr3+ states in Cr2O3. The optical band gap Eg decreases slightly from 3.26 to 3.15 eV with the increase of actual Cr content from x = 0.00 to 4.63 at % in ZnO. Photoluminescence studies at 10 K show that the incorporation of chromium leads to a relative increase of deep level band intensity. It was also observed that Cr doped samples clearly showed ferromagnetic behavior; however, 2.49 % Cr doped ZnO showed remnant magnetization higher than that of 1.07 % and 4.63 % Cr doped samples, while 4.63 % Cr doped ZnO samples had a coercive field higher than the other dopings

We report studies of multiphoton mechanisms of plasmon excitation and their influence on the femtosecond-laser induced sub-wavelength ripple generation in large-bandgap dielectric and semiconducting transparent materials. An extended Drude-Sipe formalism is applied to quantitatively estimate the real part of the dielectric function which is dependent on the carrier density. The theory is able to predict the ripple periods for selected materials in good agreement with the experimental observations. Possible limitations at very small spatial periods are also discussed.

Undoped and Ni-doped ZnO micro-rod arrays were successfully synthesized by the spray pyrolysis method on glass substrates. Analysis of the samples with x-ray diffraction and scanning electron microscopy showed that these micro-rod arrays had a polycrystalline wurtzite structure with a highly c-axis preferred orientation. Photoluminescence studies at both 300 K and 10 K show that the incorporation of nickel leads to a relative increase in the visible blue light band intensity. Magnetic measurements indicated that Ni-doped ZnO samples exhibit ferromagnetic behavior at room temperature, which is possibly related to the presence of point defects

The results of photoluminescence measurements on ZnO implanted with stable and radioactive isotopes of Zn and Ga are presented. The donor-related exciton feature $I_8$ at $3.3600\mathrm{eV}$ is suggested to be due to bound exciton recombination at Ga donors. The $I_1$ line at $3.3718\mathrm{eV}$ is also likely to be due to Ga, and is attributed to ionized Ga donor bound exciton recombination. A feature at $3.3225\mathrm{eV}$ is observed following transmutation of radioactive Ga into stable Ge, and is attributed to Ge. Finally, a damage-related band is observed in the region of $1.8\mathrm{eV}$ when the recoil energy of the decay is capable of dislodging the host atoms from their respective lattice sites.

Raman spectroscopy, X-ray diffractometry and atomic force microscopy have been used to characterise ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. The results show substantial enhancement and broadening of certain Raman features which correlate excellently with the change in width of the X-ray diffraction peaks. The 570 cm⁻¹ Raman feature showed pronounced asymmetry and enhanced intensity in the unannealed sample. An increase in grain size observed after subsequent annealing produced a substantial reduction in both the asymmetry and intensity of this peak. Our experimental data suggest that electric fields, due to charge trapping at grain boundaries, in conjunction with localised and surface phonon modes are the cause of the intensity enhancement and asymmetry of this feature.