Bismuth-induced effects on optical,lattice vibrational,and structural properties of bulk GaAsBi alloys

Bulk GaAs_{1 - x}Bi_x/GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm^-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm^-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.

PACS

78.55Cr 78.55-m 78.20-e 78.30-j     

Molecular beam epitaxy and properties of GaAsBi/GaAs quantum wells grown by molecular beam epitaxy: effect of thermal annealing

We have grown GaAsBi quantum wells by molecular beam epitaxy. We have studied the properties of a 7% Bi GaAsBi quantum well and their variation with thermal annealing. High-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy have been employed to get some insight into its structural properties. Stationary and time-resolved photoluminescence shows that the quantum well emission, peaking at 1.23 μm at room temperature, can be improved by a rapid annealing at 650°C, while the use of a higher annealing temperature leads to emission degradation and blue-shifting due to the activation of non-radiative centers and bismuth diffusion from the quantum well.     

Dynamics of time-resolved photoluminescence in GaInNAs and GaNAsSb solar cells

We report a time-resolved photoluminescence study for GaInNAs and GaNAsSb p-i-n bulk solar cells grown on GaAs(100). In particular, we studied the extent to which the carrier lifetime decreases with the increase of N content. Rapid thermal annealing proved to significantly increase the decay times by a factor of 10 to 12 times, for both GaInNAs and GaNAsSb heterostructures, while for the 1-eV bandgap GaNAsSb structure, grown at the same growth conditions as the GaInNAs, the photoluminescence decay time remained slightly below 100 ps after annealing; the approximately 1.15-eV GaInNAs p-i-n solar cell exhibited a lifetime as long as 900 ps.

PACS

78.47.D; 78.55.Cr; 88.40.hj     

Bismuth incorporation and the role of ordering in GaAsBi/GaAs structures

The structure and composition of single GaAsBi/GaAs epilayers grown by molecular beam epitaxy were investigated by optical and transmission electron microscopy techniques. Firstly, the GaAsBi layers exhibit two distinct regions and a varying Bi composition profile in the growth direction. In the lower (25 nm) region, the Bi content decays exponentially from an initial maximum value, while the upper region comprises an almost constant Bi content until the end of the layer. Secondly, despite the relatively low Bi content, CuPt_B-type ordering was observed both in electron diffraction patterns and in fast Fourier transform reconstructions from high-resolution transmission electron microscopy images. The estimation of the long-range ordering parameter and the development of ordering maps by using geometrical phase algorithms indicate a direct connection between the solubility of Bi and the amount of ordering. The occurrence of both phase separation and atomic ordering has a significant effect on the optical properties of these layers.

PACS

78.55.Cr III-V semiconductors; 68.55.Nq composition and phase identification; 68.55.Ln defects and impurities: doping, implantation, distribution, concentration, etc; 64.75.St phase separation and segregation in     

Steady-state behavior of ring polymers in dilute flowing solutions via Brownian dynamics

A bead and spring model is considered for the Brownian dynamics simulation of the behavior of cyclic polymer chains (rings) in a dilute solution under shear or elongational flow. Finite extensibility, excluded volume, and hydrodynamic interaction are taken into account to make the polymer model as realistic as possible. In shear flow, the deformation of the chain and the shear rate viscosity dependence (the flow curve), are studied and characterized. In elongational flow, the coil-stretch phenomenon is described and the relationship between the critical elongational rate and the molecular weight is given. The qualitative behavior obtained for ring polymers is analogous to that of linear polymers.     

Steady-state behavior of star polymers in dilute flowing solutions via Brownian dynamics

A bead and spring model is considered for the Brownian dynamics simulation of the behavior of regular star polymer chains in a dilute solution under both shear flow and extensional (or elongational) flow. Finite extensibility, excluded volume, and hydrodynamic interaction are taken into account to make the polymer model as realistic as possible. The behavior of star-like chains in flow is similar to that of linear and ring polymers. Thus, dependence of a given property with the arm molecular weight is analogous to that found for linear polymers when using the total molecular weight. In shear flow, the deformation of the chain and the shear rate viscosity dependence (the flow curve), are studied. We find a slope for the shear-thinning region of the flow curve close to −2/3. In elongational flow the coil-stretch transition is characterized by giving the relationship between the critical elongational rate and the arm molecular weight, which turns out to be similar to the power law found in linear chains.     

Coupling effects on photoluminescence of exciton states in asymmetric quantum dot molecules

We present a theoretical study of photoluminescence from exciton states in InAs/GaAs asymmetric dot pairs, where interdot coupling is reached via magnetic field in the Faraday configuration. Electronic structure is obtained by finite element calculations, and Coulomb effects are included using a perturbative approach. According to our simulated spectra, bright excited states may become optically accessible at low temperatures in hybridization regimes where intermixing with the ground state is achieved. Our results show effective magnetic control on the energy, polarization and intensity of emitted light, and suggest these coupled nanostructures as relevant candidates for implementation of quantum optoelectronic devices.     

Dissipative particle dynamics study on the multicompartment micelles self-assembled from the mixture of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) and homopolymer poly(propylene oxide) in aqueous solution

Dissipative particle dynamics (DPD) method is applied to model the self-assembly of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) (PEE-b-PEO) and homopolymer poly(propylene oxide) (PPO) in aqueous solution. In this study, several segments are coarse-grained into a single simulation bead based on the experimental density. For the self-assembly of pure diblock copolymer PEE-b-PEO in dilute solution, the DPD simulation results are in good agreement with experimental data of micelle morphologies and sizes. The chain lengths of the block copolymers and the volume ratios between PPO and PEE-b-PEO are varied to find the conditions of forming multicompartment micelles. The micelles with core-shell-corona structure and the micelles with two compartments are both formed from the mixture of PEE-b-PEO and PPO in aqueous solution.     

Numerical Study of Dilute Polymer Solutions Using FENE Bead-Spring Chain Model

The numerical solution of viscoelastic fluid problems in complex geometries and free solutions has been an area of extensive research in polymer fluid mechanics. The governing equation for polymer chains was in terms of Brownian dynamics and finitely extensible nonlinear elastic (FENE). Explicit and semi-implicit predictor-corrector algorithms were employed to solve the Brownian dynamics problem. After all the chain displacements have been determined, the stress tensor will be calculated with the ensemble average technique. Molecular configurations, such as the maximum length of the molecular chain along the flow direction, the angle between molecular chain and the flow direction, and the mean configuration thickness of the molecule in the velocity gradient direction, are also calculated. In the procedure, the governing equation of the displacement of chain has been made dimensionless.     

Enhanced thermoelectric figure of merit in indium and ytterbium double-filled skutterudite bulk materials through simultaneously optimising power factor and reducing thermal conductivity

In this study, Yb_xCo₄Sb₁₂ and In_xYb_0.3Co₄Sb₁₂ bulk materials were fabricated via microwave synthesis combined with spark plasma sintering. Based on ytterbium single filling, indium was further filled into the voids of CoSb₃. Carrier concentration and mobility were regulated as 1.6–1.8 × 10²⁰ cm^-³ and ∼30 cm²V⁻¹S⁻¹, respectively, resulting in an improved power factor of ∼4900 μWm⁻¹K⁻². The phonon-resonant scattering, caused by indium and ytterbium double filling, was combined with other phonon scattering mechanisms such as nano-inclusion, dislocation, and grain boundary segregation, which resulted in a significantly decreased lattice thermal conductivity of 0.72–2.08 Wm⁻¹K⁻¹. Owing to the improvements in carrier concentration and phonon transport, excellent thermoelectric performance, reflected by ZT = 1.38 at 773 K, was achieved in In_0.4Yb_0.3Co₄Sb₁₂.     

Time-resolved photoluminescence study of low-energy emission mechanisms in oligofluorene and polyfluorene films

We report time-resolved photoluminescence (PL) spectroscopy study of solutions and thin films of pure penta-fluorenes with varying alkyl side-chain structures. While no low-energy green emissions were observed in solutions, spin-cast and vacuum dried initial films exhibited low-energy emission in time-delayed PL spectra, centered at 490 nm, whose intensity varied strongly with the side-chain structure. In comparison, the films annealed at 170 °C in air showed nearly identical low-energy emission in time-delayed PL spectra, centered at 520 nm, regardless of the side-chain structures. Therefore, our experimental results obtained for pure penta-fluorenes provided clear evidence that the mechanism of the green emission of oligofluorene and polyfluorene films includes both the intermolecular aggregate/excimer formation, emitted around 490 nm, and fluorenone-type chemical defects, emitted around 520 nm.     

Numerical study on electrophoretical stretching dynamics of DNA in microcontraction

Brownian dynamics simulations are used to characterize the electrophoretical stretching process of long T4 DNA in microchannels. When DNA is forced to move through the microchannels, the pure elongational flow generated by electric field gradients in hyperbolic contraction will unravel the molecules of DNA. The effects of hydrodynamic interactions, the strain rate, the Brownian fluctuation, and the initial states of molecules on the stretching dynamics are analyzed in this paper. The computational results show us the weak dependence of polymer dynamics on hydrodynamic interactions in microcontractions. In the case of low Deborah number, the stretching process of a molecule depends on the Brownian fluctuation. However, in the case of high Deborah number, the individualistic stretching behavior can be traced to variations in the starting conformation.     

Experimental study on the solid velocity in horizontal dilute phase pneumatic conveying of fine powders

The calculation reliability of pressure drop and gas-solid drag force in horizontal dilute phase pneumatic conveying strongly depends on the accuracy of gas-solid velocity correlation. However, there are limited studies on the solid velocity in horizontal dilute phase pneumatic conveying and it is important to further validate suitability of existing correlation of gas-solid velocity, especially for fine particles (such as pulverized coal). Consequently, in this paper, a negative pressure pneumatic conveying test rig is set up and two kinds of powders with different sizes are adopted. Optical fiber probe (OFP) was used to measure the volumetric solid concentration and particle velocity. The volumetric solid concentration was also calculated by using the measured particle velocity. The results show that the solid concentrations obtained by the two methods have good agreement, and discrepancy is within ± 20%. It was found the particle velocities are different in the upper and lower part of the cross-section in the horizontal pipe. However, the difference is generally no more than 2 m/s. The velocity difference will decrease with the increasing gas velocity, and increases with the solid mass flow rate. In the experimental condition of 0.06 mm < d_s < 0.35 mm, 1400 kg/m³ < ρ_s < 2600 kg/m³, the implicit correlation based on Yang's Unified Theory gives the best prediction of particle velocity among existing studies but still with noticeable discrepancy with the comparison of the present experimental data. By modifying the solid friction factor, an improved correlation of the particle velocity was obtained, which agrees better with the experimental data given in the present and literature studies.     

Ultrafast Carrier Relaxation in InN Nanowires Grown by Reactive Vapor Transport

We have studied femtosecond carrier dynamics in InN nanowires grown by reactive vapor transport. Transient differential absorption measurements have been employed to investigate the relaxation dynamics of photogenerated carriers near and above the optical absorption edge of InN NWs where an interplay of state filling, photoinduced absorption, and band-gap renormalization have been observed. The interface between states filled by free carriers intrinsic to the InN NWs and empty states has been determined to be at 1.35 eV using CW optical transmission measurements. Transient absorption measurements determined the absorption edge at higher energy due to the additional injected photogenerated carriers following femtosecond pulse excitation. The non-degenerate white light pump-probe measurements revealed that relaxation of the photogenerated carriers occurs on a single picosecond timescale which appears to be carrier density dependent. This fast relaxation is attributed to the capture of the photogenerated carriers by defect/surface related states. Furthermore, intensity dependent measurements revealed fast energy transfer from the hot photogenerated carriers to the lattice with the onset of increased temperature occurring at approximately 2 ps after pulse excitation.     

纳米TiO_2的低温制备及晶相控制研究

以钛酸丁酯为前躯体,在低温(40℃)下制备TiO2薄膜,并评价其光催化活性。结果表明,制备的TiO2薄膜不需经过高温处理,就有较好的光催化活性;XRD分析表明,未经过热处理的样品,随着反应温度的提高,得到的TiO2粒径变大,经450℃处理2 h的样品,金红石含量随反应温度提高而减少。醋酸的加入不仅能抑制板钛矿相的生成,而且提高了锐钛矿相向金红石相转变的温度。     

Structural changes of bulk polyethylene by stress impact as studied by solid‐state NMR

Solid‐state ¹³C‐ and ¹H‐NMR spectra of bulk high‐density polyethylene samples, cylindrical in form, to which stress impacts were applied with a home‐made stress‐impact apparatus, were measured. The fraction of the noncrystalline component increases with an increase in the stress‐impact strength. In the crystalline region, the monoclinic crystalline component appear with the stress impact, in addition to the major orthorhombic crystalline component. Furthermore, dynamic characterization was carried out on the basis of the observed values of the relaxation parameters ¹H T₂ and T_CH of the ¹H and ¹³C nuclei. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2268–2272, 2001     

Tentative study on kinetics of bulk polymerization of methyl methacrylate in presence of montmorillonite

The kinetics of the bulk polymerization of methyl methacrylate (MMA) in the presence of montmorillonite (MMT) were studied. The effect of MMT on the radical polymerization of MMA was researched by determining the polymerization rate dilatometrically. It was assumed that there were both bimolecular and monomolecular termination processes involved in the termination of the radicals in the polymerization. It was found that a lower benzoyl peroxide (BPO) concentration promotes a higher fraction of monomolecular mode in chain termination. The results show that there is an optimal ratio of MMT to initiator that increases the bulk polymerization rate of MMA. The X‐ray results show that the layer structure of the formed PMMA–MMT composites was also affected by the BPO concentration. With lower initiator concentration, less pronounced layer structure will be observed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3690–3695, 2003     

A molecular dynamics study of siloxane diffusion in a polyester-melamine solution

This study examines the diffusion of small methylated and hydroxylated siloxanes through solutions of polymer and crosslinker molecules. The diffusivity of the siloxanes was calculated for bulk diffusion and for surface segregation. Our simulations demonstrate that there is significant interaction between 3,3-dimethylhexahydroxytrisiloxane (hexa-OH-TS) and the polar solvent components of the paint matrix at 298 K. The nature of these associations is largely due to hydrogen bonding between the siloxane and the polar solvent molecules. At 573 K the diffusivity of solvent molecules is sufficiently high to disrupt interactions. The smaller number of hydroxyl groups in 1,5-dihydroxyhexamethyltrisiloxane (hexa-Me-TS) results in a weaker interaction with the polar solvent. Surface segregation studies indicate that there is a slight increase in the concentration of hexa-OH-TS in the surface layer of the system as a result of curing and relaxation processes. However, for hexa-Me-TS there is in fact a decrease in the surface concentration of the siloxane relative to the bulk concentration.     

Use of bulk liquid membrane for the removal of chromium (VI) from aqueous acidic solution with tri-n-butyl phosphate as a carrier

Tri-n-butyl phosphate (TBP) was used as carrier for the transport of chromium (VI) through a hexane bulk liquid membrane. The transport efficiency of chromium (VI) by TBP was investigated under various experimental conditions such as pH of the feed phase (Cr (VI) solution), concentration of the receiving phase (NaOH solution), concentration of TBP in membrane, rate of stirring, effect of transport time, type of solvent, Cr (VI) concentration in feed phase, and effect of temperature. The transport efficiency increased with increasing carrier concentration from 7.5 × 10^− 2 to 2.25 × 10^− 1 mol/L. At high pH (donor phase) the transport rate of chromate ions decreased. At high stirring speed (300 rpm) the Cr (VI) transport from the feed phase to the strip phase was completed within 5 h at 27 °C. Under optimum conditions: donor phase 4.8 × 10^− 4 mol/L K₂Cr₂O₇ solution at pH 1.0 ± 0.1, acceptor phase 1.0 mol/L NaOH solution, membrane phase 2.25 × 10^− 1 mol/L, stirring speed 300 rpm, and temperature 27 °C, the flux rate was found to be 2.90 × 10^− 7 mol/m² s.     

First-principles study of carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.