Qing Lan Ma

Development of Porous Silicon Based Visible Light Photodetectors

Porous silicon based visible light photodetectors with the characteristic structures of Al/porous silicon/Si were developed by evaporating aluminum contact onto the top surface of porous silicon films to form metal-semiconductor-metal Schottky junctions. The spongy nanostructures of the porous silicon film were characterized with the scanning electron microscopy. The current-voltage characteristics, the biased voltage dependent photocurrents and the illumination intensity dependent photocurrents were measured for the Al/porous silicon/Si visible light photodetectors. It is found that the photocurrents as large as 4 mA/cm2 can be achieved for the porous silicon based visible light photodetectors under the normal illumination of one 500 W tungsten lamp

Preparation and Performance of ZnO Based Gas Sensors Working at Room Temperature

In this presentation, we present the preparation and performance of ZnO gas sensors on glass substrates. The phase and microstructures of the prepared ZnO nanocrystals were characterized with the X-ray diffractometer and transmission electron microscope. The transient response characteristics of the ZnO based sensors were recorded at room temperatures in saturated ethanol gas. It has been observed that the ZnO sensors can be operated at room temperature.

Morphology and Photoluminescence of Zn/ZnO Core-Shell Structures by Water-Boiling Method

Zn/ZnO core-shell structured composites were firstly synthesized by water-boiling method using induction cooker and electric cooker. The synthesized Zn/ZnO core-shell structures were characterized with X-ray diffraction (XRD), scanning electronic microscopy (SEM) and photoluminescence (PL) spectrophotometer, respectively. The XRD pattern confirms that the shells of the Zn/ZnO core-shell composites are composed of wurtzite ZnO crystals. Based on SEM analysis, the Zn/ZnO core-shell structures formed by intermittent boiling under induction cooker show a tendency for spherical morphology with stacked and bending ZnO shells while the ones formed by continuous boiling under electric cooker exhibit a spherical morphology with the irregular ZnO nanorods on the surface of Zn spheres, and the continuous boiling under electric cooker can promote the peeling and regeneration of ZnO shells on the surface of Zn cores. The PL spectra of the Zn/ZnO core-shell structures have been recorded at room temperature and observed two peaks around 379 nm and 538 nm. However, the defect emission is much stronger than the UV emission in the Zn/ZnO core-shell structures synthesized under electric cooker.

Ultrasonication Induced Synthesis of Zn/ZnO Core-Shell Structures

Metal-semiconductor Zn/ZnO core-shell structures are fabricated by a simple ultrasonic process of raw Zn particles in water. The crystal structure, morphologies and fluorescence of the Zn/ZnO core-shell particles are characterized with X-ray diffraction (XRD), scanning electronic microscopy (SEM) and fluorescence spectrophotometer. The XRD pattern confirms that the shells of the Zn/ZnO core-shell particles are composed of Wurtzite ZnO crystals. The SEM study clearly exhibits the formation of ZnO on the surface of Zn spheres. The room-temperature fluorescence spectra have been observed a strong ultraviolet (UV)-visible emission at 381 nm and a yellow-green deep-level emission at about 534 nm. It is noted that the very broad green emission at 510 nm appears in the ZnO crystals with increasing the content of ZnO shell on the surface of Zn spheres. Furthermore, a possible formed mechanism of ZnO shells on the Zn core is discussed.

Blue Fluorescent Liquid Crystal N-(4-methoxybenzylidene)-4-n-alkoxybenzenamine

Rod-like liquid crystals N-(4-methoxybenzylidene)-4-n-alkoxybenzenamine (MBnBA, n = 1, 2, 10) were synthesized and then were investigated by differential scanning calorimetry, polarizing optical microscopy, ultraviolet-visible spectrophotometry and fluorescence spectroscopy. Our results demonstrated that the dilute tetrahydrofuran solutions of each member in the family of MBnBA could give off blue fluorescence under the 325 nm excitation. The calculated electronic structures revealed that the fluorescence recorded at 469 nm (2.65 eV) was the result of π* → n transitions in the molecules of the synthesized liquid crystals MBnBA.

Deep-Blue Photoluminescent Properties of a Liquid Crystal TB10A

A typical liquid crystal (LC) N,N-(terephthalylidene) bis (4-decylbenzenamine) (TB10A) was firstly synthesized at room temperature. The synthesized LC TB10A were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), ultraviolet-visible (UV-vis) spectrophotometry, photoluminecence (PL) spectroscopy and electronic structure calculations. Upon the 325 nm excitation, the dilute tetrahydrofuran solutions of TB10A could give off blue PL. With Hückel tight-binding method, the electronic structures of the TB10A were calculated. Our results demonstrated that the deep-blue PL observed at about 489 nm (2.54 eV) can be assigned to the π* → n transition for the TB10A.

Water Assisted Conversion of ZnO from Metallic Zinc Particles

Zinc oxide (ZnO) was synthesized by solid-liquid reaction of raw Zn particles in water. The synthesized ZnO were characterized with X-ray diffraction (XRD), scanning electronic microscopy (SEM) and photoluminescence (PL) spectrophotometer. The XRD pattern confirmed that the formation of ZnO with wurtzite structure and ZnOOH crystals. The SEM study clearly exhibited a honeycomb-shaped structure of the Zn/ZnO/ZnOOH composites. Room-temperature PL spectra have been observed a very weak UV emission at 376 nm and a very strong yellow-green emission at 538 nm. Furthermore, a possible mechanism of the water-assisted conversion of ZnO from metallic Zn particles was discussed.

Effects of Thermal Annealing in Nitrogen on Photoluminescence of Sol-Gel Derived ZnO Nanocrystals

The synthesis of ZnO nanocrystals is carried out in alkaline alcoholic solution of zinc acetate dihydrate by sol-gel method. Effects of thermal annealing in nitrogen on photoluminescence (PL) properties of the sol-gel derived ZnO nanocrystals were investigated by PL spectroscopy. With increasing annealing temperature in nitrogen, UV emissions of the ZnO nanocrystals red-shifted from 378 nm to 388 nm while deep-level luminescent bands ranging from 450 to 700 nm with three emission centers at about 502 nm, 538 nm and 602 nm are enhanced, and the green luminescent band gains its intensity at the expense of the yellow and orange luminescent bands. Our results have demonstrated that the origin yellow-green color PL was tuned towards the cyan color PL with increasing the thermal annealing temperature in nitrogen from 344 oC to 813 oC.

Anodization Current Density Independent Photoluminescence of Porous Silicon

Porous silicon (PS) is usually prepared by means of the anodization under constant current density, and fabrication of PS is a key step towards the realization of all-silicon electronic devices. It is a general belief that the photoluminescent properties of electrochemically etched PS depend on the anodization current density. In this work, we electrochemically prepared a series of PS films in the electrolyte of hydrofluoric acid by varying anodizing current density in the range of 1-70 mA/cm2. In spite of the different anodizing current density, the peak wavelength of the photoluminescence spectrum of the electrochemically anodized PS does not depend on the anodization current density. SEM has been utilized to characterize the morphology of the prepared PS films, and the mechanism is discussed for the anodization current independent photoluminescence of PS.

Transmission Spectrum of Two-Dimensional Self-Assembled SiO2 Photonic Crystals

Controlling the photon propagation in materials by photonic crystals lies at the heart of quantum optics. The motivation of our work is to investigate the direct control of the photon confinement in the self-assembled two-dimensional SiO2 photonic crystals. Here we demonstrate the realization of transmitted light controls in the self-assembled two-dimensional SiO2 PCs. Our experimental results agree well with the theoretical calculations on the photonic band gaps in the spectral region of 400-1100 nm. These results present experimental evidence on the direct control of the photon confinement in the self-assembled two-dimensional SiO2 PCs.