Photoluminescence study of multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot at various temperature

The photoluminescence of self-assembled multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot (QD) was measured at various temperatures. Strong photoluminescence of wetting layer (WL) and quantum dots were observed at the same time. Furthermore, direct excitons thermal transfer process between the wetting layer and quantum dots was observed. In the study of temperature dependence of PL intensity it was found that the PL peak of wetting layer contains two quenching processes: at low temperature, excitons are thermally activated from localized states to extended two-dimensional states and then trapped by QDs; at high temperature excitons quench through the X valley of barriers. Using rate equation excitons thermal transfer and quenching processes were analyzed quantitatively.

Room-temperature microwave oscillation in AlAs/GaAs superlattices

Room-temperature microwave (MW) oscillations are observed in GaAs/AlAs (10 nm/2 nm) doped weakly coupled superlattices (SLs) in the first plateau of the I-V curve. Oscillations induced by sequential resonant tunneling are detected in a temperature range from 15 to 300 K by applying DC bias on the Si, diodes. The temperature dependence of current at small fixed bias voltage is also measured. Through analysis, it is found that the dominant transport mechanisms are sequential resonant tunneling and phonon-assisted tunneling when the temperature is below 300 K. The low bias voltage at which oscillations are realized is helpful to restrain thermionic emission through the X valley of AlAs barriers in the room-temperature transport. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

High Characteristic Temperature 1.3 mu m InAs/GaAs Quantum-Dot Lasers Grown by Molecular Beam Epitaxy

We report the molecular beam epitaxy growth of 1.3 mu m InAs/GaAs quantum-dot (QD) lasers with high characteristic temperature T-0. The active region of the lasers consists of five-layer InAs QDs with p-type modulation doping. Devices with a stripe width of 4 mu m and a cavity length of 1200 mu m are fabricated and tested in the pulsed regime under different temperatures. It is found that T-0 of the QD lasers is as high as 532K in the temperature range from 10 degrees C to 60 degrees C. In addition, the aging test for the lasers under continuous wave operation at 100 degrees C for 72 h shows almost no degradation, indicating the high crystal quality of the devices.

Electron concentration dependence of exciton localization and freeze-out at local potential fluctuations in InN films

InN films with electron concentration ranging from n similar to 10(17) to 10(20) cm(-3) grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 10(18) cm(-3) show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy sigma (loc) is found to follow the n (5/12) power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.

CALCULATION OF SPIN-REORIENTATION TEMPERATURE IN ND2FE14B

The spin-reorientation phenomenon in Nd2Fe14B has been investigated using an angular dependent free energy approach. A magnetic Hamiltonian which includes the crystal electric field term and the exchange term has been established using realistic band structure results. The temperature dependence of the molecular field is accounted for by introducing the Brillouin function and the magnetic Hamiltonian is diagonalized within the ground state multiplet of the Nd ion. The eigenstates are then used to form the partition function for the free energy. At each temperature, the direction of the molecular field is obtained by searching for the minimum in the angular parameter space of the free energy. Our calculations show that for Nd2Fe14B, the net magnetic anisotropy direction is canted away from the c axis at a temperature close to the experimentally reported spin-reorientation temperature of 150 K. The temperature dependence of the magnetic structure is found to be very sensitive to the size of the second order crystal field parameter B20.

Interplay effects of temperature and injection power on photoluminescence of InAs/GaAs quantum dot with high and low areal density

In this report, we have investigated the temperature and injection power dependent photoluminescence in self-assembled InAs/GaAs quantum dots (QDs) systems with low and high areal density, respectively. It was found that, for the high-density samples, state filling effect and abnormal temperature dependence were interacting. In particular, the injection power-induced variations were most obvious at the temperature interval where carriers transfer from small quantum dots (SQDs) to large quantum dots (LQDs). Such interplay effects could be explained by carrier population of SQDs relative to LQDs, which could be fitted well using a thermal carrier rate equation model. On the other hand, for the low density sample, an abnormal broadening of full width at half maximum (FWHM) was observed at the 15-100 K interval. In addition, the FWHM also broadened with increasing injection power at the whole measured temperature interval. Such peculiarities of low density QDs could be attributed to the exciton dephasing processes, which is similar to the characteristic of a single quantum dot. The compared interplay effects of high-and low-density QDs reflect the difference between an interacting and isolated QDs system.

Ultrafast dynamics of dye molecules in solution as a function of temperature

Femtosecond time-resolved studies using fluorescence depletion spectroscopy were performed on Rhodamine 700 in acetone solution and on Oxazine 750 in acetone and formamide solutions at different temperatures. The experimental curves that include both fast and slow processes have been fitted using a biexponential function. Time constants of the fast process, which corresponds to the intramolecular vibrational redistribution (IVR) of solute molecules, range from 300 to 420 fs and increase linearly as the temperature of the environment decreases. The difference of the average vibrational energy of solute molecules in the ground state at different temperatures is a possible reason that induces this IVR time-constant temperature dependence. However, the time constants of the slow process, which corresponds to the energy transfer from vibrational hot solute molecules to the surroundings occurred on a time scale of 1-50 ps, changed dramatically at lower temperature, nonlinearly increasing with the decrease of temperature. Because of the C-H...O hydrogen-bond between acetone molecules, it is more reasonable that acetone molecules start to be associated, which can influence the energy transfer between dye molecules and acetone molecules efficiently, even at temperatures far over the freezing point.

Effects of temperature, glucose and inorganic nitrogen inputs on carbon mineralization in a Tibetan alpine meadow soil

High levels of available nitrogen (N) and carbon (C) have the potential to increase soil N and C mineralization We hypothesized that with an external labile C or N supply alpine meadow soil will have a significantly higher C mineralization potential and that temperature sensitivity of C mineralization will increase To test the hypotheses an incubation experiment was conducted with two doses of N or C supply at temperature of 5 15 and 25 C Results showed external N supply had no significant effect on CO2 emission However external C supply increased CO2 emission Temperature coefficient (Q(10)) ranged from 113 to 1 29 Significantly higher values were measured with C than with N addition and control treatment Temperature dependence of C mineralization was well-represented by exponential functions Under the control CO2 efflux rate was 425 g CO2-Cm-2 year(-1) comparable to the in situ measurement of 422 g CO2-Cm-2 year(-1) We demonstrated if N is disregarded microbial decomposition is primarily limited by lack of labile C It is predicted that labile C supply would further increase CO2 efflux from the alpine meadow soil (C) 2010 Elsevier Masson SAS All rights reserved

Excimer-laser irradiated polyimide films as temperature-sensitive materials

The temperature dependence of the resistivity of KrF laser irradiated polyimide films was studied. In all cases, the resistivity decreased with increasing temperature. The irradiated polyimide film exhibited a typical semiconducting property. This result indicated that the irradiated polyimide films can be used as temperature-sensitive materials. We demonstrated that both the sensitivity and the sensitive temperature range of the irradiated polyimide films can be altered by adjusting laser irradiation parameters. The intrinsic relationship between the temperature coefficient of the resistivity and irradiation condition was interpreted in terms of the microstructural change. The result provided a new insight into the fundamental aspects of laser irradiated polyimide film structure and a method of preparing temperature-sensitive materials. (C) 2000 Elsevier Science B.V. All rights reserved.

Transport property of polyaniline and its molecular weight dependence

The electrical conductivity of polyaniline doped with camphor sulfonic acid (PAn-CSA) was studied. The results indicate that there is a critical temperature (T-c) and the temperature dependence of PAn-CSA conductivity shows metallic and semiconductor characteristics above and below T-c, respectively. The higher the molecular weight of PAn, the lower the T-c. The conductivity was enhanced remarkably when PAn-CSA film was stretched, its room temperature conductivity is up to 750 S/cm when elogonation is 60%; however, T-c was independent of elongation.

Comblike polymer electrolyte with main chain glass transition near room temperature

A new amorphous comblike polymer (CBP) based on methylvinyl ether/maleic anhydride altering copolymer backbone and on oligooxyethylene side chain was synthesized. The dynamic mechanical properties of CBP and its Li salt complexes were investigated by means of DDV-11-EA type viscoelastic spectrometry. Results showed that there were two glass transitions (alpha-transition and beta-transition) in the temperature range from -100 to 100 degrees C. The beta-transition was assigned to oligo-PEO side chains and the temperature of beta-transition increases with increasing Li salt content. The alpha-transition was assigned to the main chain of CBP. The temperature of the alpha-transition (T-alpha) is also dependent upon the Li-salt content, but not monotonic. The value of T-alpha lies between 30-45 degrees C in the Li salt concentration range studied, near room temperature. It was found that the CBP-Li salt complexes showed an unusual dependence of ionic conductivity on Li salt content. There are two peaks in the plot of the ionic conductivity vs. Li salt concentration, which has been ascribed to the movability of the CBP main chain at ambient temperature. The temperature dependence bf the ionic conductivity indicated that the Arrhenius relationship was not obeyed, and the plot of log sigma against 1/(T - T-0) showed the unusual dual VTF behavior when using side chain glass transition temperature (T-beta) as T-0.

A study of the dependence of ferrocene diffusion on chain length in polymer electrolytes

The diffusion rates of ferrocene have been estimated in five kinds of poly(ethylene glycol) solution, containing the electrolyte LiClO4, by using non-steady-state chronoamperometry. The D-app of ferrocene increases with increasing temperature, and the dependency of D-app on temperature obeys the Williams-Landel-Ferry equation. The D-app of ferrocene decreases with increasing polymer chain length. Both the chain length and temperature dependence conform to a simple free volume model. A relation between current and polymer chain length is suggested at room and high temperatures.

STUDIES ON RADIATION EFFECTS OF PTFE .2. RADIATION-INDUCED STRUCTURAL-CHANGES AND THE TEMPERATURE EFFECT

In this work PTFE sheets irradiated with gamma-rays at 150-degrees-C and 200-degrees-C were studied using x-ray photoelectron spectroscopy (XPS). The main structural changes in PTFE due to radiation are the formation of CF3 and CF groups. An irradiation temperature dependence of the relative content of the three kinds of groups in irradiated PTFE was observed. The CF3 groups, especially when irradiation is carried out a lower temperatures, can defluorinate in the same manner as previosly reported for CF2 groups. The CF groups, on the other hand, are observed to increase with increasing irradiation dose and irradiation temperature; the latter was explained as due to an increase in branching structures.

Temperature and biomass influences on interannual changes in CO2 exchange in an alpine meadow on the Qinghai-Tibetan Plateau

Three years of eddy covariance measurements were used to characterize the seasonal and interannual variability of the CO2 fluxes above an alpine meadow (3250 m a.s.l.) on the Qinghai-Tibetan Plateau, China. This alpine meadow was a weak sink for atmospheric CO2, with a net ecosystem production (NEP) of 78.5, 91.7, and 192.5 g C m(-2) yr(-1) in 2002, 2003, and 2004, respectively. The prominent, high NEP in 2004 resulted from the combination of high gross primary production (GPP) and low ecosystem respiration (R-e) during the growing season. The period of net absorption of CO2 in 2004, 179 days, was 10 days longer than that in 2002 and 5 days longer than that in 2003. Moreover, the date on which the mean air temperature first exceeded 5.0 degrees C was 10 days earlier in 2004 (DOY110) than in 2002 or 2003. This date agrees well with that on which the green aboveground biomass (Green AGB) started to increase. The relationship between light-use efficiency and Green AGB was similar among the three years. In 2002, however, earlier senescence possibly caused low autumn GPP, and thus the annual NEP, to be lower. The low summertime R-e in 2004 was apparently caused by lower soil temperatures and the relatively lower temperature dependence of R-e in comparison with the other years. These results suggest that (1) the Qinghai-Tibetan Plateau plays a potentially significant role in global carbon sequestration, because alpine meadow covers about one-third of this vast plateau, and (2) the annual NEP in the alpine meadow was comprehensively controlled by the temperature environment, including its effect on biomass growth.

Structural, Electrical, And Optical Properties Of Inasxsb1-X Epitaxial Films Grown By Liquid-Phase Epitaxy

The InAsxSb1-x films were grown on (100) GaSb substrates by liquid-phase epitaxy, and their structural, electrical, and optical properties were investigated. The high-resolution x-ray diffraction results reveal that the single crystalline InAsxSb1-x films with a midrange composition are epitaxially grown on the GaSb substrates. Temperature dependence of the Hall mobility was theoretically modeled by considering several predominant scattering mechanisms. The results indicate that ionized impurity and dislocation scatterings dominate at low temperatures, while polar optical phonon scattering is important at room temperature (RT). Furthermore, the InAsxSb1-x films with the higher As composition exhibit the better crystalline quality and the higher mobility. The InAs0.35Sb0.65 film exhibits a Hall mobility of 4.62x10(4) cm(2) V-1 s(-1). The cutoff wavelength of photoresponse is extended to about 12 mu m with a maximum responsivity of 0.21 V/W at RT, showing great potential for RT long-wavelength infrared detection. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2989116]