Pengxun Yan

Microwave absorption properties and the isotropic antenna mechanism of ZnO nanotrees

In this paper, ZnO nanowires and ZnO nanotrees have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the ZnO nanostructures and paraffin composites have been measured in a frequency of 0.1–18 GHz. Excellent microwave absorption performances have been observed in ZnO nanotree composite compared to ZnO nanowire composite, and the maximum absorption is enhanced as the concentration of the nanotrees increases in the composite. The value of minimum reflection loss for the composites with 60 vol % ZnO nanotrees is −58 dB at 4.2 GHz with a thickness of 4.0 mm. Such strong absorption is attributed to the unique isotropic antenna morphology of the ZnO nanotrees in the composite.

Morphology-controlled synthesis, growth mechanism, optical and microwave absorption properties of ZnO nanocombs

ZnO nanocombs and nanorods with different morphologies have been successfully synthesized through a simple metal vapour deposition route at 600-750 degrees C using pure zinc powder or zinc and graphite powders as source materials. The structures and morphologies of the products were characterized in detail by using x-ray diffraction, scanning electron microscopy, transmission electron microscopy and laser Raman spectrometer. The morphologies of the products can be easily controlled by tuning the following four factors: reaction temperature, the distance between the source and the substrates, the kinds of substrates and the kinds of precursors. Possible growth mechanisms for the formation of ZnO nanostructures with different morphologies are discussed. Photoluminescence studies show that there are sharp UV and broad defect-related green emissions for all products. Relative intensity of the UV to defect-related green emissions decreases from ZnO nanorods to nanocombs. Microwave absorption properties of these nanocombs are also investigated. The value of the minimum reflection loss is -12 dB at 11 GHz for the ZnO nanocomb composite with a thickness of 2.5 mm.

Red light photoluminescence emission from Mn and Cd co-doped ZnS one-dimensional nanostructures

Wurtzite ZnS one-dimensional (1D) nanostructures with Mn and Cd as dopants were synthesized rapidly with Zn and S in the vapour phase. The Au-film coated Si wafers were used as substrates. Photoluminescence measurements were carried out at room temperature on the Mn-doped, Cd-doped and Mn, Cd co-doped ZnS 1D nanostructures. A red light emission band around 636 nm was found from the Mn and Cd co-doped samples. The mechanism of energy transfer for this emission is discussed.

Structure and photoluminescence property of Eu-doped SnO2 nanocrystalline powders fabricated by sol–gel calcination process

Eu-doped SnO2 nanocrystalline powders were fabricated by the sol–gel calcination process. The effect of Eu doping concentrations on the structure and photoluminescence properties of Eu-doped SnO2 nanocrystalline powders was investigated. X-ray diffraction patterns, Fourier transformation infrared spectrum, field emission scanning electron microscope and high-resolution transmission electron microscope are employed to investigate the morphology and the structure of Eu-doped SnO2 nanocrystalline powders. The samples display reddish-orange light and red light when excited at indirect and direct excitation, respectively. Meanwhile, PL spectra indicate that the quenching concentrations are different when the excitation wavelength alters. Based on the analysis of the PL spectra, it is believed that Eu3+ ions located at different sites in the SnO2 host are selectively excited.

One-pot synthesis of ZnS hollow spheres via a low-temperature, template-free hydrothermal route

ZnS hollow nanospheres with porous shells were successfully fabricated by a simple one-pot template-free hydrothermal route with the assistance of trolamine. The hollow nanospheres with a mean diameter of about 220 nm and the pore size of the porous shell of about 3.2 nm exhibit large BET surface areas of 129.9 m2 g−1. The formation of ZnS hollow nanospheres was definitely driven by the well-known Ostwald ripening process. The morphology of the product could be controlled by adjusting the volume of trolamine in the ternary solution, which could influence the nucleation density and rate of the growing region during the initial stage of reaction. As synthesized, highly defective nanocrystalline ZnS exhibits a very strong defect-related green emission, which may pertain to the emission enhancement of the unique geometrical morphology of ZnS hollow nanospheres with a porous shell. The observed green PL emission of the wurtzite-type ZnS nanomaterials could provide an interesting application in the development of novel luminescent devices.

Synthesis, Characterization, and Microwave Absorption Property of the SnO2Nanowire/Paraffin Composites

In this article, SnO2nanowires (NWs) have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the SnO2NWs/paraffin composites have been measured in a frequency range of 0.1–18 GHz, and the measured results are compared with that calculated from effective medium theory. The value of maximum reflection loss for the composites with 20 vol.% SnO2NWs is approximately −32.5 dB at 14 GHz with a thickness of 5.0 mm.In this article, SnO2nanowires (NWs) have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the SnO2NWs/paraffin composites have been measured in a frequency range of 0.1-18 GHz, and the measured results are compared with that calculated from effective medium theory. The value of maximum reflection loss for the composites with 20 vol.% SnO2NWs is approximately -32.5 dB at 14 GHz with a thickness of 5.0 mm.

Morphology and thermal stability of Ti-doped copper nitride films

A weakly Ti-doped copper nitride (Cu3N) film was prepared by cylindrical magnetron sputtering. The XPS results indicate that Ti atoms do not substitute for the Cu atoms but probably locate at the grain boundaries. The columnar grains size is about half of that of the undoped Cu3N film and the surface is smoother. For weakly Ti-doped Cu3N films, a dense layer appears on top of the columnar crystals. The RMS of the Cu film formed by annealing of the weakly Ti-doped Cu3N film is more than twice larger than that of the film before annealing. Compared with the undoped Cu3N film, it possesses fine thermal stability both in vacuum and in atmosphere.

The effect of hydrogen on Cu3N thin films deposited by radio frequency magnetron sputtering

The Cu3N films were synthesized at room temperature by radio frequency magnetron sputtering in various H2+N2 mixture atmosphere on a glass substrate. The introduction of hydrogen promoted the crystallization of Cu3N distinctly, and the optimized growth of (100) plane was strong. Compared to the films with no hydrogen introduced, the electrical resistivity decreased by several magnitudes and the optical energy gap decreased notably too. A conspicuous improvement of electrical and optical properties was achieved, but the surface morphology did not gain any modification; on the contrary, the introduction of hydrogen engendered the protuberances on the surface of the films. The thermal stability was investigated by heating the films in vacuum chamber at different temperatures. The films decomposed at 150°C initially and at 250°C entirely; the thermal stability is not as good as Cu3N films with no hydrogen included. The films were characterized by x-ray diffraction, UV-visible spectrum, four-point probe, and f...

Effect of catalyst nanoparticle size on growth direction and morphology of InN nanowires

Au-assisted growth of InN nanowires (NWs) was accomplished by a simple chemical vapor deposition system. The as-prepared InN NWs exhibit two morphologies with different growth directions: periodic NWs (PNWs) and smooth NWs (SNWs) along and , respectively. The PNWs with crinoids morphology resulted when larger Au particles (∼40 nm in diameter) were used, while the SNWs with smooth sidewalls were obtained when smaller Au particles (∼10 nm in diameter) served as the collector. Furthermore, the mechanism of this growth behavior was discussed in terms of the effect of catalyst nanoparticle size.

Structure and optical investigation of faceted hexagonal aluminum nitride nanotube arrays

Arrays of single-crystalline aluminum nitride nanotubes (AlNNTs) were successfully synthesized at 780 °C by a facile CVD method without templates or catalysts. Faceted hexagonal AlNNTs with diameters from tens to hundreds of nanometers were observed. 264 nm deep-ultraviolet emission associated with Al vacancies was detected by photoluminescence. Raman characterization indicates that the lattice of the AlNNTs is notably defective, and disorder-activated silent B1 (low) and B1 (high) were detected at 583.0 and 730.0 cm−1 respectively, which were allowed by the breakdown of the translational crystal symmetry. The redshift of E2 (high) Raman modes indicates that biaxial tensile stress exists in the samples.