Haiping Tang

Temperature-dependent photoluminescence of quasialigned Al-doped ZnO nanorods

Temperature-dependent photoluminescence (PL) properties of quasialigned Al-doped ZnO nanorods grown by thermal evaporation method were investigated. The ionization energy of the Al donor was determined to be ∼90meV. A PL peak at 3.315eV was observed at low temperature and was tentatively related to excitons bound to surface defects. The emission, along with its first longitudinal optical phonon replica, persists up to room temperature and dominates the near band edge (NBE) emission of the nanorods. The doping of Al results in a redshift of ∼0.04eV of the room-temperature NBE emission of the ZnO nanorods.

Raman scattering and photoluminescence of quasi-aligned ternary ZnCdO nanorods

Ternary ZnCdO single-crystal nanorods with a high Cd incorporation of about 16 at% were synthesized onto Au-catalyzed Si substrate using a physical vapour deposition method. Field emission scanning electron microscopy observation, x-ray diffraction analysis and Raman spectrum showed that the ZnCdO nanorods were well-aligned along the c-axis. Raman spectrum revealed that the E2 (high) mode of the ZnCdO nanorods shifted towards the higher frequency side by 5 cm−1 with respect to that of pure bulk ZnO, corresponding to biaxial compressive stress of 1.14 GPa. The near-band-edge emission of ZnCdO red-shifted to 3.04 eV due to Cd substitution. The defect-related emission exhibited anomalous phonon replicas with an energy of 102 meV, which was attributed to the disorder-activated B1 phonon mode.

Preparation and characterization of Al-doped quasi-aligned ZnO submicro-rods

Al-doped quasi-aligned ZnO submicro-rods were prepared on heavily doped n-type Si(111) substrates by the thermal evaporation method. Electrical transport measurements indicate the Al-doped ZnO submicro-rods have better conductivity than the undoped submicro-rods. The photoluminescence excitation spectrum of the annealed sample indicates an energy level ~100 meV below the conduction band minimum, which may be due to the localized state of the Al impurity. The annealed sample exhibits a strong green emission centred at 2.45 eV that can be seen by the naked eye and an infrared emission centred at 1.10 eV. The intensity ratios of the deep-level green emission to the ultraviolet emission are 0.86 and 30 for the as-grown and annealed samples, respectively. Moreover, two emissions at 2.2 and 1.96 eV were discriminated from the PL spectrum of the as-grown Al-doped ZnO submicro-rods, which were not observed in the pure ZnO submicro-rods and the annealed Al-doped sample.

Growth and properties of ZnO/hexagonal ZnMgO/cubic ZnMgO nanopagoda heterostructures

Quasialigned ZnO/hexagonal ZnMgO/cubic ZnMgO nanopagoda heterostructures have been synthesized by a simple thermal evaporation method using Zn and Mg metals as raw materials. The structural, compositional and optical properties were investigated by field emission scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction, high-resolution transmission electron microscopy and room-temperature photoluminescence. The cubic ZnMgO structure was grown on the hexagonal ZnMgO interlayer and the ZnO nucleating layer. The nanopagodas are about 600?nm at the bottom and 70?nm at the top in diameter. The molar ratio of Zn-to-Mg decreases in the nanopagodas along the direction of the tip, while the average is 0.41?:?0.59. With Mg incorporation in the hexagonal ZnMgO interlayer, the ultraviolet near-band-edge emission shows a clear blueshift of 260?meV.

Determination of the free exciton energy in ZnO nanorods from photoluminescence excitation spectroscopy

Temperature-dependent photoluminescence excitation (PLE) spectroscopy is used to study nominally undoped and indium-doped ZnO nanorods grown by thermal evaporation method. Clear differences in PLE features between the two samples are observed. We demonstrate that the first derivative of the PLE spectra can be used to determine the free exciton energy for both samples. The physics behind is understood either in terms of competing absorption and recombination to the green emission band being monitored, or based on the analogy between the first derivative of PLE and photoreflectance spectroscopy. Two residual donor levels located at about 37 and 120meV below the conduction band minimum are identified from the PLE spectra.

Synthesis and characterization of dendritic ZnMgO nanostructures

Dendritic ZnMgO nanostructures were synthesized on Si(111) substrates by a catalyst-free thermal evaporation method using Zn and Mg powder as the source materials. The structural, compositional and optical properties were investigated by field emission scanning electron microscopy, x-ray diffraction, transmission electron microscopy, Raman scattering and photoluminescence. The results indicate that the incorporated Mg in the as-grown nanostructures mainly form a separated cubic MgO phase. By annealing in oxygen, hexagonal ZnMgO can be formed with the dendritic nanostructures maintained, which exhibits a blueshift of ~0.05 eV in the near band edge emission. It is possible to tune the Mg content in the nanostructures simply by varying the distance between the substrate and the source materials.

Synthesis of ZnO/cubic (Zn, Mg)O heterostructure nanorods

ZnO/cubic (Zn, Mg)O core–shell heterostructure nanorods with wire-shaped tips have been synthesized via a catalyst-free thermal evaporation process on Si substrates. The average diameters were about 180 nm for nanorods and about 20 nm for tips. Transmission electron microscopy (TEM) investigations revealed that the tip was a single-crystalline cubic structure and grew along the [2 0 0] direction. A two-step growth process was used to synthesize the ZnO/(Zn, Mg)O heterostructure nanorods; that is, Zn and Mg powders were first evaporated at 630 °C and then the temperature was raised to 750 °C. High-resolution TEM and energy dispersive x-ray spectroscopy analysis confirmed that the product is pure ZnO at the first step. X-ray diffraction, photoluminescence and TEM results indicated that a final heterostructure nanorod consisted of a wurtzite ZnO core and a cubic (Zn, Mg)O shell. The growth temperature and process control were suggested to be responsible for the formation of heterostructure nanorods.

Effect of oxygen pressure on structural and electrical properties of pulsed laser deposition-derived Zn0.95Mg0.05O: Li thin films

Li-doped p-type Zn0.95Mg0.05O thin films have been achieved on glass substrates by pulsed laser deposition. X-ray diffraction shows that all the films exhibit good crystallinity with a highly c-axis preferential orientation. The results of the Hall measurements indicate that p-type conduction in Li-doped Zn0.95Mg0.05O films is strongly dependent on the oxygen pressure. Films deposited at 5 and 25 Pa show n-type conduction. However, at the oxygen pressure of 15 and 20 Pa, the films show definitive p-type conductivity. The maximal hole concentration was found to be 1.26 × 1018 cm−3 at a working pressure of 20 Pa with a resistivity of 17.82 Ω cm and Hall mobility of 0.285 cm2 V−1 s−1. Furthermore, all the p-type Zn0.95Mg0.05O films possess high average transmittance of ~90% in the visible region and a band gap of 3.41 eV at room temperature.