Songqing Zhao

Synthesis of well-aligned ZnO nanowires by simple physical vapor deposition on c-oriented ZnO thin films without catalysts or additives

Well-aligned ZnO nanowires were synthesized by simple physical vapor deposition using c-oriented ZnO thin films as substrates without catalysts or additives. The synthesized ZnO nanowires have two typical average diameters: 60nm in majority and 120nm in minority. They are about 4μm in length and well aligned along the normal direction of the substrate. Most of the synthesized ZnO nanowires are single crystalline in a hexagonal structure and grow along the [001] direction. The c-oriented ZnO thin films control the growth direction. Photoluminescence spectrum was measured showing a single strong ultraviolet emission (380nm). Such result indicates that the ZnO nanowire arrays can be applied to excellent optoelectronic devices.

Ultraviolet fast-response photoelectric effect in tilted orientation SrTiO3 single crystals

Ultraviolet photoelectricity based on the vicinal cut as-supplied SrTiO3 single crystals has been experimentally studied in the absence of an applied bias at room temperature. An open-circuit photovoltage of 130 ps rise time and 230 ps full width at half maximum was observed under the irradiation of a 355 nm pulsed laser of 25 ps in duration. The dependence of the photoelectric effect on the tilting angles was studied, and the optimum angle is 20.9 degrees. Seebeck effect is proposed to elucidate the tilting angle dependence of laser-induced photovoltage. This work demonstrates the potential of SrTiO3 single crystals in ultraviolet detection. (c) 2006 American Institute of Physics.

Photovoltaic effect in micrometer-thick perovskite-type oxide multilayers on Si substrates

Micrometer-thick perovskite-type multilayer heterostructures with ten thousands of unit cells, [SrNb(0.05)Ti(0.95)O(3)/La(0.9)Sr(0.1)MnO(3)](3), have been fabricated on Si substrates. The structure exhibited nonlinear and rectifying current-voltage characteristics. Stable and temporal photovoltaic effects in the multilayer have been experimentally studied by using different wavelength light sources from ultraviolet to infrared, respectively. The photovoltage and photocurrent responsivities as a function of wavelength displayed that two cutoff wavelengths occurred corresponding to the band gaps of SrNb(0.05)Ti(0.95)O(3) and Si and La(0.9)Sr(0.1)MnO(3). Under the pulsed laser illumination the nanosecond response characterization of the present multilayer structure was obtained. Based on the band structure of the multilayers, a possible mechanism of photovoltaic process was proposed. c 2008 American Institute of Physics. [DOI: 10.1063/1.3010373]

Magnetical and electrical tuning of transient photovoltaic effects in manganite-based heterojunctions

Magnetically and bias current tunable transient photovoltaic (TPV) responses have been investigated in a manganite-based heterojunction composed of a La2/3Ca1/3MnO3 film and an n-type Si substrate at ambient temperature. Under irradiation of 248 nm pulsed laser with 20 ns duration the TPV peak values can be modulated in a range of -125 to 138 mV when the applied magnetic field perpendicular to the interface changes from -6.4 to + 6.4 kOe, and the relative variations (TPV(H) - TPV(0))/TPV(0) reach up to about 1000%. In addition, TPV responses can be also affected by bias current, and the photoresponse peaks change from positive to negative with the currents from -350 to 350 μA. These results indicate that the manganite-based heterojunction can be used for magnetically and electrically tunable ultraviolet photodetectors.

Ultrafast photovoltaic effects in miscut Nb-doped SrTiO3 single crystals

Picosecond photovoltaic effect in miscut Nb-doped SrTiO3 single crystal has been observed under ultraviolet pulsed laser irradiation at ambient temperature without an applied bias. The 10%–90% rise time and the full width at half maximum are 828 and 670 ps, respectively, which is faster than that of exact cut Nb-doped SrTiO3 single crystal. A model based on terrace structure is put forward to explain the observation.

Enhanced lateral photovoltaic effect in the p-n heterojunction composed of manganite and silicon by side irradiation for position sensitive detecting

Lateral photovoltaic effect has been studied in p-La0.67Ca0.33MnO3/n-Si heterojunction. Under illumination of continuous 808 nm laser beam on the film surface, a transient photovoltaic overshoot accompanied with the steady signal was observed when the laser turned off and on. The open-circuit photovoltage had a linear dependence on illuminated position, and the sensitivity reached 0.75 mV mW(-1) mm(-1) for steady value and 6.25 mV mW(-1) mm(-1) for the transient peak value. Especially, an enhancement in position detecting sensitivity was observed when the interface of this heterojunction was irradiated, which were 1.25 mV mW(-1) mm(-1) (steady value) and 26.0 mV mW(-1) mm(-1) (peak value). This work demonstrates a novel way to increase sensitivity for manganite-based position sensitive detectors.

Effect of ambient oxygen pressure on structural, optical and electrical properties of SnO2 thin films

Polycrystalline SnO2 thin films were deposited on sapphire substrates at 450 degrees C under different ambient oxygen pressures by pulsed laser deposition technique. The effect of ambient oxygen pressure on the structural, optical and electrical properties of SnO2 thin films was studied. X-ray diffraction and Hall measurements show that increasing the ambient oxygen pressure can improve crystallization of the films and decrease resistivity of the films. A violet emission peak centered at 409 nm was observed from photoluimnescence measurements for SnO2 films under deposition ambient oxygen pressure above 5 Pa, which is related to the improvement of crystalline of the films.

Enhanced photovoltage in perovskite-type artificial superlattices on Si substrates

We have fabricated a three-component perovskite-type superlattice (SL) consisting of La(0.9)Sr(0.1)MnO(3), SrTiO(3) and LaAlO(3) with atomic scale control by laser molecular beam epitaxy on Si substrates. When a He-Ne laser irradiated the superlattice by side illumination, a stable photovoltage was produced and the responsivity reached 46.7 mV mW(-1) which is six times higher than that of a similarly grown La(0.9)Sr(0.1)MnO(3) single layer on Si substrates. This work demonstrates the potential of the present SL in photo-detectors operating at room temperature.

Ultrafast and spectrally broadband photovoltaic response in quartz single crystals

Picosecond photovoltaic response has been observed in quartz single crystals under spectrally broadband laser irradiation from ultraviolet to near infrared without an applied bias at room temperature. The 10-90% rise time and full width at half maximum are about several hundred picoseconds, which are weakly dependent on the wavelength. In addition, open-circuit photovoltages under continuum solid state lasers operating in a wide wavelength range were also observed, which exhibited much higher responsivities than those under picosecond pulsed laser irradiation. The inner mechanism of the present photovoltaic response was discussed.

Detection and identification of selected alcohols using terahertz time-domain spectroscopy

The optical property and spectroscopy of selected kinds of alcohols are studied based on the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.5-2.5 THz. The B3LYP method in conjunction with the 6-31G (d) basis set was used to optimize the geometric structure of samples and simulate the alcohols spectrum. No absorption peak occurs in THz wave band based on our simulation, suggesting that the present experimental absorption peaks resulted from the intermolecular interaction.