We report the femtosecond time-resolved studies of room temperature exciton recombination and ultrafast stimulated emission dynamics in ZnO microcrystallite thin films. A free exciton photoluminescence lifetime of a few tens of picoseconds and a decay time of a few picoseconds for the stimulated emission were observed. The relatively slow rise time (3 ps) for the P band as the result of exciton-exciton scattering compared with the 0.8 ps rise time for the N band attributed to electron-hole plasma recombination clearly distinguished the two stimulated emission processes.
Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry–Pérot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics.
The mechanical property of polymer can be effectively changed by grafting polymer onto the surface of nano-fillers. In this paper, the crystallization behaviour of polyethylene grafted on carbon nanotubes with different grafting densities have been explored in detail by using molecular dynamics simulation. During the crystallization process, the crystallinity, the site order parameter, the mean square displacement, the radius of gyration and the bond-orientational order parameter were recorded. The results show that higher grafting density results in stronger crowding effect and weaker mobility of molecular chain. The ultimate crystallinity of the polymer increases with the increase of grafting density. The orientation of crystal bond indicates that the polymer with higher grafting density tends to be perpendicular to the surface of nano-fillers. Those findings in this work are of great values for further exploring reinforcement mechanism and also the microscopic mechanism of crystallization behaviours of polymer nanocomposites.
Mn/Y co-doped Ba 0.67 Sr 0.33 TiO 3 (Mn+Y: BST) thin films were deposited on LaNiO 3 (LNO)/SiO 2 /Si substrates by radio frequency (RF) magnetron sputtering at substrate temperature of 400 °C. Influence of the annealing temperature on microstructure and electrical properties of the Mn+Y: BST films was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations revealed that all the films have dense and crack-free surface with a perovskite structure. The Mn+Y: BST films annealed at 700 °C showed the best electrical properties, with a dielectric constant of 875, dielectric loss of 0.032, tunability of 60% at room temperature and 100 kHz, and the leakage current density of 6.7×10 -5 A/cm 2 at an electric field of 400 kV/cm, respectively.
Crystallized polycrystalline barium strontium titanate (Ba0.5Sr0.5TiO3) thin films have been synthesized on the titanium metal substrates by an environmentally conscious method of hydrothermal synthesis. The films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electronic micrographs (SEM) respectively. The XRD analyses show that the as-grown films are a cubic phase containing a little unidentified phase, which would tend to be a pure cubic phase after being annealed at 600°C for 30 minutes; the XPS analyses reveal that the composition of the as grown Ba0.5Sr0.5TiO3 films is agreement with the stoichiometry, and the valences of Ba, Sr, Ti, and O elements of the films are +2, +2, +4, and −2 respectively; the SEM photographs show that the films are condensely synthesized; and the AFM analyses show that the average surface roughness and the root-mean-square (rms) of the film measured are 0.257 μm and 0.323 μm respectively. It is concluded that an environmentally conscious method of hydrothermal synthesis can be used for preparing multi-element oxide thin films.
The effects of doping method of MgO on the characteristics of the microstructure and electric property of Ba0.3Sr0.7Zr0.18Ti0.82O3–xMgO ceramics were investigated experimentally. The MgO was introduced by solid-state processing and sol-gel processing, respectively. The results revealed that the doping method gives rise to differences in dielectric response and breakdown strength and energy storage density of Ba0.3Sr0.7Zr0.18Ti0.82O3 ceramics. Solid-state doping with MgO was beneficial to improve breakdown strength of the ceramics, and obtained higher dielectric energy stored density.
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