J.T. Chen

Nanoparticles and 3D sponge-like porous networks of manganese oxides and their microwave absorption properties.

Hydrohausmannite nanoparticles (approximately 10 nm) were prepared by the hydrothermal method at 100 degrees C for 72 h. Subsequent annealing was done in air at 400 degrees C and 800 degrees C for 10 h, Mn(3)O(4) nanoparticles (approximately 25 nm) and 3D Mn(2)O(3) porous networks were obtained, respectively. The products were characterized by XRD, TEM, SAED and FESEM. Time-dependent experiments were carried out to exhibit the formation process of the Mn(2)O(3) networks. Their microwave absorption properties were investigated by mixing the product and paraffin wax with 50 vol%. The Mn(3)O(4) nanoparticles possess excellent microwave absorbing properties with the minimum reflection loss of -27.1 dB at 3.1 GHz. In contrast, the Mn(2)O(3) networks show the weakest absorption of all samples. The absorption becomes weaker with the annealing time increasing at 800 degrees C. The attenuation of microwave can be attributed to dielectric loss and their absorption mechanism was discussed in detail.

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.

Photoluminescence and ZnO → Eu3+ energy transfer in Eu3+-doped ZnO nanospheres

ZnO and Eu 3+ doped ZnO nanospheres were synthesized by a wet-chemical method. Scanning electron microscopy showed that all the samples have a regular spherical shape. 5 D1 → 7 F2, 5 D0 → 7 F0 and 5 D0 → 7 F2 emissions originating from intra-atomic 4f–4f transition of Eu 3+ were observed in the photoluminescence (PL) spectra. The PL excitation spectrum showed that two strong absorptions centred around 410 and 455 nm appeared besides a weak absorption at 374 nm and three absorption peaks belonging to Eu 3+ . According to the results of the Raman scattering spectrum and x-ray photoelectron spectroscopy, the two strong absorptions were attributed to substituted Eu 3+ (EuZn) and interstitial zinc (Zni) related defect bands, respectively. Moreover, it showed that Eu 3+ ions might locate at different sites in the ZnO nanospheres and that energy transfer occurred from ZnO to Eu 3+ ions. A possible energy transfer process was described and a corresponding schematic drawing labelling the energy levels of Eu 3+ in the ZnO nanospheres was presented. In addition, a nearly white emission which was visible to the naked eye was achieved by introducing Eu 3+ into the ZnO nanospheres and this might be significant for developing white light emitters by chemical methods. (Some figures in this article are in colour only in the electronic version)

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.

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.

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...