Synthesis of biodiesel from waste cooking oil using immobilized lipase in fixed bed reactor

Waste cooking oil (WCO) is the residue from the kitchen, restaurants, food factories and even human and animal waste which not only harm people's health but also causes environmental pollution. The production of biodiesel from waste cooking oil to partially substitute petroleum diesel is one of the measures for solving the twin problems of environment pollution and energy shortage. In this project, synthesis of biodiesel was catalyzed by immobilized Candida lipase in a three-step fixed bed reactor. The reaction solution was a mixture of WCO, water, methanol and solvent (hexane). The main product was biodiesel consisted of fatty acid methyl ester (FAME), of which methyl oleate was the main component. Effects of lipase, solvent, water, and temperature and flow of the reaction mixture on the synthesis of biodiesel were analyzed. The results indicate that a 91.08% of FAME can be achieved in the end product under optimal conditions. Most of the chemical and physical characters of the biodiesel were superior to the standards for 0(#)diesel (GB/T 19147) and biodiesel (DIN V51606 and ASTM D-6751).

Real-time counting of single electron tunneling through a T-shaped double quantum dot system

Real-time detection of single electron tunneling through a T-shaped double quantum dot is simulated, based on a Monte Carlo scheme. The double dot is embedded in a dissipative environment and the presence of electrons on the double dot is detected with a nearby quantum point contact. We demonstrate directly the bunching behavior in electron transport, which leads eventually to a super-Poissonian noise. Particularly, in the context of full counting statistics, we investigate the essential difference between the dephasing mechanisms induced by the quantum point contact detection and the coupling to the external phonon bath. A number of intriguing noise features associated with various transport mechanisms are revealed.

Ultrafast Kerr rotations and zero-field dephasing time of electron spins in InAs/GaAs quantum disks

Time-resolved Kerr rotation (TRKR) measurements based on pump-probe arrangement were carried out at 5 K on the monolayer fluctuation induced InAs/GaAs quantum disks grown on GaAs substrate without external magnetic field. The lineshape of TRKR signals shows an unusual dependence on the excitation wavelength, especially antisymmetric step-shaped structures appearing when the excitation wavelength was resonantly scanned over the heavy- and light-hole subbands. Moreover, these step structures possess an almost identical decay time of similar to 40 Ps which is believed to be the characteristic spin dephasing time of electrons in the extremely narrow InAs/GaAs quantum disks.

Optical study of heterointerface configuration in narrow GaAs/AlGaAs single quantum wells prepared with growth interruption

Photoluminescence and time-resolved photoluminescence were used to study the heterointerface configuration in GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with growth interruption. Photoluminescence spectra of the growth-interrupted sample are characterized by multiplet structures, with energy separation corresponding to a 0.8 monolayer difference in well width, rather than 1 monolayer as expected from the ''atomically smooth island'' picture. By analyzing the thermal transfer process of the photogenerated carriers and luminescence decay process, we further exploit the exciton localization at the interface microroughness superimposed on the extended growth islands. The lateral size of the microroughness in our sample was estimated to be 5 nm, less than the exciton diameter of 15 nm. Our results strongly support the bimodal roughness model proposed by Warwick et al. [Appl. Phys. Lett. 56, 2666 (1990)]. (C) 1996 American Institute of Physics.

Resonant Raman Scattering and Photoluminescence Emissions from Above Bandgap Levels in Dilute GaAsN Alloy

The transitions of E0 ,E0 ＋A0, and E＋ in dilute GaAs（1-x） Nx alloys with x = 0.10% ,0.22% ,0.36% ,and 0.62% are observed by micro-photoluminescence. Resonant Raman scattering results further confirm that they are from the intrinsic emissions in the studied dilute GaAsN alloys rather than some localized exciton emissions in the GaAsN alloys. The results show that the nitrogen-induced E E＋ and E0 ＋ A0 transitions in GaAsN alloys intersect at a nitrogen content of about 0.16%. It is demonstrated that a small amount of isoelectronic doping combined with micro-photoluminescence allows direct observation of above band gap transitions that are not usually accessible in photoluminescence.

Exciton Localization and Delocalization in GaNAs/GaAs Quantum Wells

We have studied exciton localization and delocalization effect in GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) and timeresolved PL measurements. Studied results suggest that, at low temperature and under a conventional CW excitation, measured PL spectra were dominated by localized exciton (LE) emission caused by potential fluctuations in GaNAs layer. However, under short pulse laser excitation, it is different. An extra high-energy PL peak comes out from GaNAs/GaAs QWs and dominates the PL spectra under high excitation and/or at high temperature. By investigation, we have attributed the new PL peak to the recombination of delocalized excitons in QWs. This recombination process competes with the localized exciton emission, which, we believe, constitutes the "S-shaped" temperature-dependent emission shift often reported in ternary nitrides of InGaN and AlGaN in the literature.

Investigation of Residual Donor Defects in Undoped and Fe-Doped LEC InP

The free electron concentration of as-grown liquid encapsulated Czochralski (LEC) InP measured by Hall effect is much higher than the concentration of net donor impurity determined by glow discharge mass spectroscopy. Evidence of the existence of a native donor hydrogen-indium vacancy complex in LEC undoped and Fe-doped InP materials can be observed with infrared absorption spectra. The concentration increase of the donor complex correlates with the increase of ionized deep acceptor iron impurity Fe~(2+) concentration in Fe-doped semi-insulating (SI) InP. These results indicate that the hydrogen-indium vacancy complex is an important donor defect in as-grown LEC InP, and that it has significant influence on the compensation in Fe-doped SI InP.

Long-wavelength SiGe/Si MQW resonant-cavity-enhanced photodetectors (RCE-PD) for application in optical fiber communication network

　

Quantum dynamics of coupled quantum-dot qubits and dephasing effects induced by detections

　

MOCVD growth of high quality crack-free GaN on Si(III) substrates

High quality crack free GaN epilayers were grown on Si(111) substrates. Low temperature AlN interlayer grown under low V/III ratio was used to effectively eliminate the formation of micro-cracks. It is found that tensile stress in the GaN epilayer decreases as the N/Al ratio decreases used for AlN interlayer growth. The high optical and structural qualities of the GaN/Si samples were characterized by RBS, PL and XRD measurements. The RT-PL FWHM of the band edge emission is only 39.5meV The XRD FWHM of the GaN/Si sample is 8.2arcmin, which is among the best values ever reported.

Fabrication of semiconductor optical amplifiers and a novel gain measuring technique

The design and fabrication of 1550 nm semiconductor optical amplifiers (SOAs) and the characteristics of the fabricated SOA are reported. A novel gain measurement technique based on the integrations of the product of emission spectrum and a phase function over one mode interval is proposed for Fabry-Perot semiconductor lasers.

MMI optical couplers and switches with SOI technology

MMI (multimode interference) coupler, modulator and switch based on SOI (silicon- on-insulator) have been become more and more attractive in optical systems since they show important performances. SiO2 thin cladding layers (<1.0mum) can be used in SOI waveguide due to the large index step between Si and SiO2, making them compatible with the VLSI technology. The design and fabrication of multimode interference (MMI) optical coupler, modulator and switche in SOI technology are presented in the paper. The results demonstrated that the modulator has an extinction ratio of -11.0dB and excess loss of -2.5dB, while the optical switch has a crosstalk of -12.5dB and responding time of less than 20 mus.

2 x 2 electrooptical switch in silicon-on-insulator waveguide

A 2 x 2 Mach-Zehnder interferometer electrooptical switch integrated in silicon-on-insulator using multimode interference 3-dB couplers as splitter and combiner has been proposed and fabricated. Free carriers plasma dispersion effect was utilized to realize light modulation in silicon. Switching operation was achieved at an injection current of 358mA and which can be much reduced by optimizing the PIN structure and improving fabrication process. Extinction ratio of 7.7dB and crosstalk of 4.8dB has been observed.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.