Y. Gong

Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification

We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution.

Diameter Control and Photoluminescence of ZnO Nanorods from Trialkylamines

A novel solution method to control the diameter of ZnO nanorods is reported. Small diameter (2-3 nm) nanorods were synthesized from trihexylamine, and large diameter (50–80 nm) nanorods were synthesized by increasing the alkyl chain length to tridodecylamine. The defect (green) emission of the photoluminescence (PL) spectra of the nanorods varies with diameter, and can thus be controlled by the diameter control. The small ZnO nanorods have strong green emission, while the large diameter nanorods exhibit a remarkably suppressed green band. We show that this observation supports surface oxygen vacancies as the defect that gives rise to the green emission.

Structure of Zn–Se–Te system with submonolayer insertion of ZnTe grown by migration enhanced epitaxy

We here report results of high resolution x-ray diffraction, x-ray reflectivity (XRR), as well as optical absorption and reflection measurements on ZnSe samples grown by molecular beam epitaxy, with insertion of planar (δ-) regions of both N as an acceptor dopant and Te as a “co-dopant” to facilitate a p-type doping. We note that to enhance the surface diffusion of Te, migration enhanced epitaxy was adopted in the growth of the “δ-layers;” i.e., Te is deposited in the absence of Zn flux. Structural parameters were extracted by simulating the experimental x-ray diffraction curves using a dynamical model. The results show that only the “δ-layers” (with submonolayer thickness) are rich in ZnTe, while the nominally undoped “spacers” have only a low Te concentration. Moreover, the morphology of the surface and interfaces are studied by XRR. Furthermore, the optical absorption and reflection results show that our samples largely preserve the optical properties of the host material (ZnSe). We note that our resul...

Non-Equilibrium Acceptor Concentration in GaN:Mg Grown by Metalorganic Chemical Vapor Deposition

It is shown that the high p-type conductivity in GaN:Mg, grown by metal-organic chemical vapor deposition followed by post-growth annealing, is due to non-equilibrium acceptor concentrations. A series of samples cut from a single GaN:Mg wafer, which initially had undergone rapid thermal annealing (RTA) after growth, has been investigated. The samples were annealed at various temperatures in nitrogen ambient for over 12 hours, and temperaturedependent Hall effect measurements were performed. For samples annealed at temperatures higher than 850 °C, the hole concentrations decrease by at least an order of magnitude, compared with the original sample. This behavior is explained by an Mg acceptor concentration in excess of its equilibrium solubility limit in the original sample; thus, at high enough temperatures, in the absence of hydrogen, Mg acceptors diffuse either to form electrically inactive precipitates or are eliminated. It is worth noting that the acceptor activation energy remains the same for all samples.