Effect of thermal annealing on dual photoluminescence emission characteristics of chemically synthesized uncapped Mn2+ doped ZnS quantum dots

Dual photoluminescence (PL) emission characteristics of Mn2+ doped ZnS (ZnS:Mn) quantum dots (QDs) have drawn a lot of attention recently. However, here we report the effect of thermal annealing on the PL emission characteristics of uncapped ZnS:Mn QDs of average sizes similar to 2-3 nm, synthesized by simple chemical precipitation method by using de-ionized (DI) water at room temperature. As-synthesized samples show dual PL emissions, having one UV PL band centred at similar to 400 nm and the other in the visible region similar to 610 nm. But when the samples are isochronally annealed for 2 h at 100-600 degrees C temperature range in air, similar to 90% quenching of Mn2+ related visible PL emission intensity takes place at the annealing temperature of 600 degrees C. X-ray diffraction data show that the as-synthesized cubic ZnS has been converted to wurtzite ZnO at 600 degrees C annealing temperature. The nanostructural properties of the samples are also determined by transmission electron micrograph, electron probe micro-analyser and UV-vis spectrophotometry. The photocatalytic property of the annealed ZnS:Mn sample has been demonstrated and photo-degradation efficiency of the as-synthesized and 600 degrees C annealed ZnS:Mn sample has been found out to be similar to 35% and similar to 61%, respectively, for the degradation of methylene blue dye under visible light irradiation. The synthesized QDs may find significant applications in future optoelectronic devices. (C) 2014 Elsevier B.V. All rights reserved.

Electron transfer statistics and thermal fluctuations in molecular junctions

We derive analytical expressions for probability distribution function (PDF) for electron transport in a simple model of quantum junction in presence of thermal fluctuations. Our approach is based on the large deviation theory combined with the generating function method. For large number of electrons transferred, the PDF is found to decay exponentially in the tails with different rates due to applied bias. This asymmetry in the PDF is related to the fluctuation theorem. Statistics of fluctuations are analyzed in terms of the Fano factor. Thermal fluctuations play a quantitative role in determining the statistics of electron transfer; they tend to suppress the average current while enhancing the fluctuations in particle transfer. This gives rise to both bunching and antibunching phenomena as determined by the Fano factor. The thermal fluctuations and shot noise compete with each other and determine the net (effective) statistics of particle transfer. Exact analytical expression is obtained for delay time distribution. The optimal values of the delay time between successive electron transfers can be lowered below the corresponding shot noise values by tuning the thermal effects. (C) 2015 AIP Publishing LLC.

Fully developed thermal transport in combined pressure and electroosmotically driven flow of nanofluid in a microchannel under the effect of a magnetic field

This paper critically analyzes, for the first time, the effect of nanofluid on thermally fully developed magnetohydrodynamic flows through microchannel, by considering combined effects of externally applied pressure gradient and electroosmosis. The classical boundary condition of uniform wall heat flux is considered, and the effects of viscous dissipation as well as Joule heating have been taken into account. Closed-form analytical expressions for the pertinent velocity and temperature distributions and the Nusselt number variations are obtained, in order to examine the role of nanofluids in influencing the fully developed thermal transport in electroosmotic microflows under the effect of magnetic field. Fundamental considerations are invoked to ascertain the consequences of particle agglomeration on the thermophysical properties of the nanofluid. The present theoretical formalism addresses the details of the interparticle interaction kinetics in tune with the pertinent variations in the effective particulate dimensions, volume fractions of the nanoparticles, as well as the aggregate structure of the particulate system. It is revealed that the inclusion of nanofluid changes the transport characteristics and system irreversibility to a considerable extent and can have significant consequences in the design of electroosmotically actuated microfluidic systems.

A novel interferometric technique to estimate thermal diffusivity of `optically transparent solid' using `Isothermal surface velocimetry'

One-dimensional transient heat flow is interpreted as a procession of `macro-scale translatory motion of indexed isothermal surfaces'. A new analytical model is proposed by introducing velocity of isothermal surface in Fourier heat diffusion equation. The velocity dependent function is extracted by revisiting `the concept of thermal layer of heat conduction in solid' and `exact solution' to estimate thermal diffusivity. The experimental approach involves establishment of 1 D unsteady heat flow inside the sample through Step-temperature excitation. A novel self-reference interferometer is utilized to separate a `unique isothermal surface' in time-varying temperature field. The translatory motion of the said isothermal surface is recorded using digital camera to estimate its velocity. From the knowledge of thermo-optic coefficient, temperature of the said isothermal surface is predicted. The performance of proposed method is evaluated for Quartz sample and compared with literature.

Role of thermal annealing on SiGe thin films fabricated by PECVD

Amorphous Silicon Germanium (a-SiGe) thin films of 500 nm thickness are deposited on silicon substrates using Plasma Enhanced Chemical Vapour Deposition (PECVD). To obtain polycrystalline nature of films, thermal annealing is done at various temperature (450-600 degrees C) and time (1-10 h). The surface morphology of the pre- and post-annealed films is investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The crystallographic structure of the film is obtained by X-ray diffraction method. Raman spectroscopy is carried out to quantify the Ge concentration and the degree of strain relaxation in the film. Nano-indentation is performed to obtain the mechanical properties of the film. It is found that annealing reduces the surface roughness of the film and increases the Ge concentration in the film. The grain size of the film increases with increase in annealing temperature. The grain size is found to decrease with increase in annealing time up to 5 h and then increased. The results show that 550 degrees C for 5 h is the critical annealing condition for variation of structural and mechanical properties of the film. Recrystallization starts at this condition and results in finer grains. An increase in hardness value of 7-8 GPa has been observed. Grain growth occurs above this critical annealing condition and degrades the mechanical properties of the film. The strain in the film is only relaxed to about 55% even for 10 h of annealing at 600 degrees C. Transmission Electron Microscopy (TEM) observations show that the strain relaxation occurs by forming misfit dislocations and these dislocations are confined to the SiGe/Si interface. (C) 2015 Elsevier Ltd. All rights reserved.

Thermal, magnetic and electrical properties of Tb1-xDyxMnO3 multiferroics

A series of multiferroic materials with the compositional formula, Tb1 - xDyxMnO3 (where x=0, 0.1, 0.2, 0.3 and 0.4) were prepared by the sol gel method. After characterizing the samples structurally, a systematic investigation of specific heat, magnetization and dielectric properties over the temperature range, 4-300 K, was undertaken. Based on these studies, it was found that all the samples exhibit a transition at 40 K and the observed behavior may be attributed to the ordering of Mn3+ ions. Further, all the five samples are found to exhibit a ferroelectric transition in the temperature range 20-24 K. Finally, yet another transition was also exhibited by all the samples at temperatures below 10 K and is attributed to the antiferromagnetic (AF) ordering of rare-earth ionic moments. The magnetic entropy of all the samples was also computed with the help of their heat capacity data. (C) 2015 Elsevier B.V. All rights reserved.

Rapid synthesis of hybrids and hollow PdO nanostructures by controlled in situ dissolution of a ZnO nanorod template: insights into the formation mechanism and thermal stability

Hollow nanomaterials have attracted a lot of interest by virtue of their wide range of applications that arise primarily due to their unique architecture. A common strategy to synthesize hollow nanomaterials is by nucleation of the shell material over a preformed core and subsequent dissolution of the core in the second step. Herein an ultrafast, microwave route has been demonstrated, to synthesize PdO nanotubes in a single step using ZnO as a sacrificial template. The mechanism of the nanotube formation has been investigated in detail using control experiments. By tuning the starting ratio of PdCl2 : ZnO, hollow to hybrid PdO nanostructures could be obtained using the same method. Conversion of the PdO to Pd nanotubes has been shown by simple NaBH4 treatment. The thermal stability of the PdO nanotubes has been studied. The insights presented here are general and applicable for the synthesis of hybrids/hollow structures in other systems as well.

Position-dependent velocity of an effective temperature point for the estimation of the thermal diffusivity of solids

A new approach is proposed to estimate the thermal diffusivity of optically transparent solids at ambient temperature based on the velocity of an effective temperature point (ETP), and by using a two-beam interferometer the proposed concept is corroborated. 1D unsteady heat flow via step-temperature excitation is interpreted as a `micro-scale rectilinear translatory motion' of an ETP. The velocity dependent function is extracted by revisiting the Fourier heat diffusion equation. The relationship between the velocity of the ETP with thermal diffusivity is modeled using a standard solution. Under optimized thermal excitation, the product of the `velocity of the ETP' and the distance is a new constitutive equation for the thermal diffusivity of the solid. The experimental approach involves the establishment of a 1D unsteady heat flow inside the sample through step-temperature excitation. In the moving isothermal surfaces, the ETP is identified using a two-beam interferometer. The arrival-time of the ETP to reach a fixed distance away from heat source is measured, and its velocity is calculated. The velocity of the ETP and a given distance is sufficient to estimate the thermal diffusivity of a solid. The proposed method is experimentally verified for BK7 glass samples and the measured results are found to match closely with the reported value.