Xijin Xu

ZnO and ZnS Nanostructures: Ultraviolet-Light Emitters, Lasers, and Sensors

ZnO and ZnS, well-known direct bandgap II–VI semiconductors, are promising materials for photonic, optical, and electronic devices. Nanostructured materials have lent a leading edge to the next generation technology due to their distinguished performance and efficiency for device fabrication. As two of the most suitable materials with size- and dimensionality-dependent functional properties, wide bandgap semiconducting ZnO and ZnS nanostructures have attracted particular attention in recent years. For example, both materials have been assembled into nanometer-scale visible-light-blind ultraviolet (UV) light sensors with high sensitivity and selectivity, in addition to other applications such as field emitters and lasers. Their high-performance characteristics are particularly due to the high surface-to-volume ratios (SVR) and rationally designed surfaces. This article provides a comprehensive review of the state-of-the-art research activities in ZnO and ZnS nanostructures, including their syntheses and potential applications, with an emphasis on one-dimensional (1D) ZnO and ZnS nanostructure-based UV light emissions, lasers, and sensors. We begin with a survey of nanostructures, fundamental properties of ZnO and ZnS, and UV radiation–based applications. This is followed by detailed discussions on the recent progress of their synthesis, UV light emissions, lasers, and sensors. Additionally, developments of ZnS/ZnO composite nanostructures, including core/shell and heterostructures, are discussed and their novel optical properties are reviewed. Finally, we conclude this review with the perspectives and outlook on the future developments in this area. This review explores the possible influences of research breakthroughs of ZnO and ZnS nanostructures on the current and future applications for UV light–based lasers and sensors.

Preparation and formation mechanism of ZnS semiconductor nanowires made by the electrochemical deposition method

ZnS nanowire arrays have been prepared for the first time by direct-current electrodeposition into the nanopores of porous anodic alumina membranes (AAMs), and the growth mechanism is that metal cations are firstly reduced and then react with elemental S to form ZnS nanowire arrays in the nanopores of AAMs. The nanopores of AAMs are of benefit to the formation of sulfide nanowires.

Vapor-phase synthesis of one-dimensional ZnS, CdS, and ZnxCd1-xS nanostructures

One-dimensional (1D) nanostructures have received prime attention due to their high potential in understanding fundamental physical concepts and constructing nanoscale electronic devices. ZnS and CdS, the well-known direct and wide bandgap semiconductors, have recently attracted significant research interest due to their special properties and applications in sensing, optoelectronics, piezoelectronics, and lasing. This article reviews the most recent activities in ZnS and CdS nanostructures, with an emphasis on the authors’ own results, and on 1D ZnS and CdS nanostructures, especially those synthesized using vapor deposition techniques. The review begins with a survey of ZnS and CdS nanostructures, and then is primarily focused on their 1D nanostructures, syntheses, characterizations, formation mechanisms, and optical and field-emission (FE) properties. Additionally, developments of ZnxCd1–xS composite nanostructures, including nanocombs and zigzag nanowires, are also discussed. Finally, we conclude this review with the perspectives and outlook on the future developments in this field.

In situ x-ray diffraction study of the thermal expansion of the ordered arrays of silver nanowires embedded in anodic alumina membranes

Thermal expansion of as-prepared and annealed ordered arrays of silver nanowires embedded in anodic alumina membranes (AAMs) was studied by in situ x-ray diffraction measurement in the temperature range from 25to800°C. The axial thermal expansion coefficient (TEC) for the as-prepared nanowires is 6.35×10−9∕°C and 6.02×10−6∕°C below and above 650°C, respectively. However, the TEC of the annealed sample turns from 2.32×10−6∕°Cto12.06×10−6∕°C when the temperature is above 350°C. The collective effects of the intrinsic expansion, surface pressure, the limit effect of AAM, and the vacancies incorporated into the silver lattice were responsible for the thermal expansion.

In situ x-ray diffraction study of the size dependent thermal expansion of silver nanowires

The thermal expansion of as-prepared and annealed silver nanowires embedded in anodic alumina membranes with different diameters was studied by in situ x-ray diffraction in the temperature range from 25to800°C. For both the as-prepared and annealed samples, the coefficients of thermal expansion have “V” shape change as the diameters increase; and the minimum values of the coefficients of thermal expansion do not correspond to the same diameters of nanowires. The collective effects of the surface tension, the limit effects of anodic alumina membrane, and the vacancies incorporated into the silver lattice were responsible for the thermal expansion.

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zni and Vo is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.

Metal oxide heterostructures for water purification

1 Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China 2 School of Physics and Technology, University of Jinan, Jinan 250022, China 3Department of Physics, University of Houston, Houston 77004, USA 4 School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

Self-assembly of semiconductor metal oxide nanostructures

1 Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China 2 School of Physics and Technology, University of Jinan, Shandong, Jinan 250022, China 3 School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 4Department of Physics, Boston College, Chestnut Hill, MA 02467, USA

Characterization, cathodoluminescence and field-emission properties of morphology-tunable CdS micro/nanostructures

High-quality, uniform CdS one-dimensional (1D) micro/nanostructures with different morphologies, e.g. microrods, sub-microwires and nanotips, are fabricated through a facile and effective thermal evaporation process. Their structural, cathodoluminescence and field-emission (FE) properties are systematically investigated. Microrods and nanotips exhibit sharp near band edge (NBE) emission and broad deep level (DL) emission, whereas sub-microwires show only the DL emission. A significant decrease in a DL/NBE intensity ratio is observed along a tapered nanotip towards a smaller diameter part. This behavior is understood under consideration of defect concentrations in the nanotips, as analyzed with high-resolution transmission electron microscopy (HRTEM). Field-emission (FE) measurements show that the nanotips possess best FE characteristics with a relatively low turn-on field of 5.28 V/µm and the highest field-enhancement factor of 4819 among all 1D CdS nanostructures reported to date. The field-enhancement factor, turn-on and threshold fields are discussed related to structure morphology and vacuum gap variations under emission.

Template-assisted electrodeposition of iron nanostructures.

A facile approach to prepare iron nanostructures (nanowires, nanotubes, branched and multi-branched nanotubes) is reported by reduction of metal sulfide salts in the pores of an anodic aluminum membranes (AAMs) template with a back side Au sheet. The crystal structures and morphologies of Fe nanostructures are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM) and transmission electron microscope. The results indicate that the Fe nanostructures can replicate the inner architectures of the templates. The thickness of the Au film deposited on the back side of the AAMs and the inner structures of AAMs are the two key factors to determine the final morphologies of Fe nanostructures. This approach can be broadened to fabricate other metal nanostructures.