Anomalous temperature dependence of Fermi-edge singularity in modulation-doped AlGaAs/InGaAs/GaAs hetero-structures

The temperature and power dependence of Fermi-edge singularity (FES) in high-density two-dimensional electron gas, specific to pseudomorphic AlxGa1-xAs/InGa1-yAs/GaAs heterostructures is studied by photoluminescence (PL). In all these structures, there are two prominent transitions E-11 and E-21 considered to be the result of electron-hole recombination from first and second electron sub-bands with that of first heavy-hole sub-band. FES is observed approximately 5-10 meV below the E-21 transition. At 4.2 K, FES appears as a lower energy shoulder to the E-21 transition. The PL intensity of all the three transitions E-11, FES and E-21 grows linearly with excitation power. However, we observe anomalous behavior of FES with temperature. While PL intensity of E-11 and E-21 decrease with increasing temperature, FES transition becomes stronger initially and then quenches-off slowly (till 40K). Though it appears as a distinct peak at about 20 K, its maximum is around 7 - 13 K.

Reverse bias capacitance measurements on copper-doped germanium p-n junctions in the breakdown region

Results of measurements at a high frequency on reverse bias capacitance of copper-doped germanium junctions are reported. Phenomenal increase in capacitance is found in the breakdown region, particularly at low temperatures.

A novel sheet 4f-3d mixed-metal pyridine dicarboxylate: synthesis, structure, photophysical properties and its transformation to a perovskite oxide

The synthesis, characterization and photophysical properties of a 4f-3d mixed metal compound, Gd(H2O)(3)Co[C5N1H3-(COO)(2)](3), are described; the structure is unique, consisting of sheets with large pores ( ca. 7 angstrom diameter) in the sheets and transforms to a perovskite oxide at moderate temperatures.

Dimethyl sulphoxide complexes of lanthanide and yttrium nitrates

Dimethyl sulphoxide complexes of lanthanide and yttrium nitrates of the general formula M(DMSO)n(NO3)3 where M = La, Ce, Pr, Nd, Sm or Gd; n = 4 and M = Y, Ho or Yb; n = 3 have been isolated and characterized. The i.r. data besides excluding the presence of D3h nitrate, reveal co-ordination through the oxygen atom of the dimethyl sulphoxide. The complexes are monomeric in acetonitrile. Molecular conductance data in acetone, acetonitrile, dimethyl formamide and dimethyl sulphoxide suggest a co-ordination number of eight for the lighter lanthanides and seven for yttrium and the heavier lanthanides.

Highly Luminescent Mn-Doped ZnS Nanocrystals: Gram-Scale Synthesis

Following growth doping technique highly luminescent (quantum yield >50%) Mn-doped ZnS nanocrystals are synthesized via colloidal synthetictechnique. The dopant emission has been optimized with varying reaction parameters and found the ratio of Zn and S as well as the percentage of introduced dopant in the reaction mixture are key factors for controlling the intensity. The method is simple, hassle free, and can be scalable to gram level without hindering the quality of nanocrystals. These nanocrystals retain their emission during various ligand exchange processes and aqueous dispersion.

Nanostructured Copper(II) oxide thin film for alcohol sensing

Nanostructured copper(II) oxide film was deposited using reactive DC magnetron sputtering. It has been characterized using XRD, EDAX, XPS, and FESEM. The grain size of copper oxide film was found to be 40-65 nm with size distribution. The entire study was divided into two parts. In the first part, the film has been studied for its response to alcohol at different temperatures to find the optimum sensing temperature, whereas in the second part, the film sensitivity to different alcohol concentrations were studied at fixed optimum operating temperature. The optimum temperature for the response of ethanol was observed to be 400 C,and the response for different concentrations was found to be almost linear.

Reactions with disulphur monoxide solutions obtained by the reduction of cupric oxide by elemental sulphur

When the products of reaction between elemental sulphur and copper oxide at elevated temperature in vacuum are bubbled through chilled inert organic solvents like carbontetrachloride, orange-yellow solutions were obtained indicating the presence of lower oxide of sulphur. This lower oxide has been found to be disulphur monoxide as shown by three different types of reactions; (1) Mercury decomposition, (2) Reaction with hydrogen iodide and hydrolytic reaction in an alkaline homogeneous medium.

Recombination and trapping in n-type cobalt-doped germanium

The role of cobalt centers in promoting the recombination and trapping processes in n-type germanium has been investigated. Data on lifetime measurements carried out by the steadystate photoconductivity and photo-magneto-electric methods in the temperature range 145 to 300°K on n-type germanium samples containing cobalt in the concentration range 1·1013 to 5.·014/cm3 are presented. The results are analysed on the basis of Sah-Shockley's multi-level formula to yield the capture cross-sections Sp= (hole capture cross-section at doubly negatively charged center) and Sn-(electron capture cross-section at singly negatively charged center) and temperature dependence. It is found that Sp= is (22 ± 6). 10-16 cm2 and Sn- is ∼ 0·1. 10-16 cm2 at 145°K. Sp= varies (n = 3·5 to 4·5) in the range 145-220°K; above 225°K the index 'n' tends to a smaller value. Sn- is practically temperature independent below 180°K and increases with increase of temperature above 180°K. The value of Sp= and its temperature variation lead one to the conclusion that during capture at attractive centers, the phonon cascade mechanism is responsible for the dissipation of the recombination energy.

Preparation of lower oxide of sulphur

RECENT work on the lower oxide of sulphur1,2 has established that disulphur monoxide (S2O) or its polymeric form is produced when sulphur is burnt in oxygen under reduced pressure. It has now been shown that it is possible to make use of an oxide of a heavy metal as a source of limited supply of oxygen to prepare the disulphur monoxide. For example, when a mixture of finely powdered cupric oxide and sulphur (1 : 5 by weight) is heated under vacuum in a glass tube gaseous products are evolved. which, on cooling in a trap surrounded by liquid air, will give an orange-red condensate (S2O)x. This condensate also gives off sulphur dioxide in stages as the temperature is raised, finally leaving a residue of elemental sulphur. Copper sulphide and excess of sulphur are left behind in the reaction tube.

Space-charge limited conduction in doped polypyrrole devices

Doping dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out on polypyrrole devices in metal-polymer-metal sandwich structure. Temperature dependent I-V measurements infer that space-charge limited conduction (SCLC) with exponential trap distribution is appropriate for the moderately doped samples, whereas trap-free SCLC is observed in lightly doped samples. Trap densities and energies are estimated, the effective mobility is calculated using the Poole-Frenkel model, and the mobility exhibits thermally activated behavior. Frequency dependent capacitance-voltage characteristics show a peak near zero bias voltage, which implies that these devices are symmetric with a negligible barrier height at the metal-polymer interface. Low frequency capacitance measurements have revealed a negative capacitance at higher voltages due to the processes associated with the injection and redistribution of space-charges. (C) 2010 American Institute of Physics.

Electron-hole recombination in cobalt-doped p-type germanium

The recombination properties of cobalt centers in p-type germanium containing cobalt in the concentration range 1014 to 1016 atoms/cm3 have been investigated. The measurement of lifetime has been carried out by steady-state photoconductivity and photo-magneto-electric methods in the temperature range 145 to 300°K. The cross-sections Sno (electron capture cross-section at neutral centers). Sn- (electron capture cross-section at singly negatively charged centers) and their temperature variations have been estimated by the analysis of the lifetime data on the basis of Sah-Shockley's multi-level formula. The value of Sno is (15±5).10-16 cm2 and is temperature independent. The value of Sn- is ≈4·10-16 cm2 around 225°K and it increases with increase of temperature. The possible mechanisms for capture at neutral and repulsive centers are discussed and a summary of the capture cross-sections for cobalt centers is given. A comparison of the cross-section values of cobalt and their temperature variations with those of the related impurities-manganese, iron and nickel-in germanium has been made.

Influence of Oxide Particle Network Morphology on Ion Solvation and Transport in ``Soggy Sand''Electrolytes

The role of oxide surface chemical composition and solvent on ion solvation and ion transport of ``soggy sand'' electrolytes are discussed here. A ``soggy sand'' electrolyte system comprising dispersions of hydrophilic/hydrophobic functionalized aerosil silica in lithium perchlorate methoxy polyethylene glycol solution was employed for the study. Static and dynamic rheology measurements show formation of an attractive particle network in the case of the composite with unmodified aerosil silica (i.e., with surface silanol groups) as well as composites with hydrophobic alkane groups. While particle network in the composite with hydrophilic aerosil silica (unmodified) were due to hydrogen bonding, hydrophobic aerosil silica particles were held together via van der Waals forces. The network strength in the latter case (i.e., for hydrophobic composites) were weaker compared with the composite with unmodified aerosil silica. Both unmodified silica as well as hydrophobic silica composites displayed solid-like mechanical strength. No enhancement in ionic conductivity compared to the liquid electrolyte was observed in the case of the unmodified silica. This was attributed to the existence of a very strong particle network, which led to the ``expulsion'' of all conducting entities from the interfacial region between adjacent particles. The ionic conductivity for composites with hydrophobic aerosil particles displayed ionic conductivity dependent on the size of the hydrophobic chemical moiety. No spanning attractive particle network was observed for aerosil particles with surfaces modified with stronger hydrophilic groups (than silanol). The composite resembled a sol, and no percolation in ionic conductivity was observed.

Investigation of activated transport in hole doped rare earth manganites in the high temperature paramagnetic regime

Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La(1-x)A(x)MnO(3), A=Ca, Sr, Ba, x similar or equal to 0.1-0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping (x), average A-site cation radius (< r(A)>), cationic disorder (sigma(2)) and strain (epsilon(zz)). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different < r(A)> reveals the importance of sigma(2) as a metric to qualify any analysis based on (r(A)). (c) 2006 Elsevier Ltd. All rights reserved.

An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals

We report here a synthetic route for high-quality Mn-doped ZnSe nanocrystals using selenourea as a selenium source, avoiding the more conventional route-using tributylphosphine (TBP) that restricts the growth of spherical ZnSe nanocrystals below 5 nm in size, besides being highly toxic and pyrophoric. Spherical ZnSe nanocrystals with unprecendented sizes (up to 12 nm) are synthesized, the large size of the host helps to keep dopant ions well inside the nanocrystal leading to intense and stable dopant emission. Mn-doped ZnSe nanocrystals with more than 50% quantum yield (QY) are synthesized in this method and found to be stable both in aqueous and nonaqueous dispersions for months.