Xinhai Han

Synthesis and optical properties of well-aligned ZnO nanorod array on an undoped ZnO film

High-density well-aligned ZnO nanorod array was successfully synthesized on a PLD prepared undoped ZnO film by a catalyst-free method. X-ray diffraction and scanning electron microscopy show that the nanorods are well-oriented perpendicular to the substrate. The photoluminescence from free excitons and Raman spectra of the ZnO nanorods reflect the high purity and nearly defect free structure of nanorods. The well-aligned feature of the nanorod array is attributed to the nanorods’ epitaxial growth from the ZnO film.

Synthesis and Characterization of Aligned ZnO Nanorods on Porous Aluminum Oxide Template

Aligned high-density ZnO nanorods were successfully synthesized on porous aluminum oxide (PAO) template. The growth process involves carbonthermal reduction of ZnO as a Zn vapor source and ZnO nucleation on the PAO template without metal catalysts. Field-emission scanning electron microscope images revealed that the nanorods have uniform length distributions and hexagon end planes, and the intense c-axis growth was also confirmed by X-ray diffraction. Strong ultraviolet emission at 380 nm and weak green band emission at 520 nm at room-temperature photoluminescence clearly indicated the high quality of the ZnO nanorods. A growth mechanism is proposed that the multipore surface of the PAO template plays a critical role in the nucleation of ZnO in the initial stage of growth, and nanorods grow from the nuclei due to intense ZnO c-axis orientation growth.

Ultraviolet lasing and time-resolved photoluminescence of well-aligned ZnO nanorod arrays

Well-aligned ZnO nanorod arrays were synthesized by thermal evaporation of ZnO powder on ZnO thin films. The nanorods are almost the same length of 5.5μm and well-oriented perpendicular to the substrates. The time-integrated and time-resolved photoluminescence spectra of the nanorod arrays revealed the existence of laser action with the excitation threshold of 130μJ∕cm2 and emission lifetime of less than 30 ps. At the excitation fluence a little below threshold, the decay fits well to a biexponential function with a fast component (∼80ps) and a slower component (∼360ps).

Crystal orientation-ordered ZnO nanorod bundles on hexagonal heads of ZnO microcones: epitaxial growth and self-attraction

We demonstrate a preferential nucleation, epitaxial growth, and self-attraction of crystal orientation-ordered ZnO nanorod bundles on (0001) plane of single-crystal ZnO microcones.

Non-aqueous cathodic electrodeposition of large scale uniform ZnO and CdO nanowire arrays

We fabricated ordered ZnO and nanowire arrays in anodic alumina membrane (AAM) by cathodic electrodeposition from a non-aqueous dimethylsulfoxide (DMSO) bath containing zinc chloride and dissolved oxygen. X-ray diffraction (XRD), high-resolution transmission electron microcsopy (HRTEM), and electron diffraction (ED) show that the ZnO nanowires are well crystallized with wurtzite structure. The morphologies and structure of the ZnO nanowires have been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the ZnO nanowires with diameters of about 60nm are uniformly embedded into the hexagonally ordered nanopores of anodic alumina membrane. A sharp ultraviolet emission at 383nm and visible broad emission bands around 592nm were observed from ZnO nanowire arrays. They originated from the near-band emission due to the recombination of bound excitons and deep level emission due to defects, respectively. The cathodic electrodeposition technique in DMSO is also extended to synthesize CdO nanowire arrays.

Optical properties of ZnO cone arrays and influence of annealing on optical properties of ZnO-Zn coaxial nanocables

The novel ZnO cone arrays with controllable morphology and ZnO-Zn coaxial nanocables have been synthesized by thermal evaporating metal Zn powders at low temperature of 570°C without a metal catalyst. The ZnO cone arrays were grown on the Si (100) substrates, and clear structure evolutions were observed using scanning electron microscopy: well-aligned ZnO nanocones, double-cones with growing head cones attached by stem cones, cones with straight hexagonal pillar were achieved as the distance between the source and the substrates increased. X-ray diffraction showed that all of the cone arrays grow along c-axis. Raman and photoluminescence spectra revealed that the optical properties of the buffer layer between the ZnO cone arrays and the silicon substrates are better than those of ZnO cone arrays. ZnO-Zn nanocables were achieved in the region down stream with a temperature of 300°C. The PL measurements of the ZnO-Zn nanocables reveal a UV peak at 382nm corresponding to the free exciton emission originating from the ZnO shells, while violet luminescence centered around 424 and 431 nm are observed after annealed in Ar and air, respectively. The growth mechanisms of the ZnO cone arrays and ZnO-Zn nanocables are proposed.

Preparation and optical properties of ZnO films by cathodic electrodeposition

ZnO films on polycrystaline Zn substrates were synthesized by cathodic electrodeposition from an aqueous solution composed only of 0.05M zinc nitrate maintained at 65°C. Their microstructures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Photoluminescence (PL) spectra of ZnO thin films prepared at more positive potential (-0.6~-0.8V) show a bond exciton photoluminescence band and intensive broad emission band around 2.17 eV, and three longitudinal optical (LO) phonon replica peaks of the bound exciton around 3.34. When more negative potential (-1.0~-1.4V) was applied in electrodeposition process, ultraviolet (UV) emission bands disappeared. A weak shoulder peak at about 1.87 eV was observed for all samples. The results indicate that more positive electrodeposition potential favors the high quality ZnO film growth. Annealed ZnO films prepared at more positive electrodeposition potentials (-0.6V~-1.0V) exhibit the strong ultraviolet emission at 3.35eV and a negligibly weak deep level emission. Thermal treatment results in the enhancement and sharpening of the excitonic photoluminescence bands and decrease of the deep level emission. Thermal treatment also decreases the Eg of ZnO film prepared at -1.0V from 3.56eV to 3.29eV.