Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd₁₃Te₁₆, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors.     

Cross sections and rate constants for OH + H2 reaction on three different potential energy surfaces for ro-vibrationally excited reagents

A systematic study of the reagent ro-vibrational excitations in H(2) + OH reaction is presented on three different potential energy surfaces using the multiconfiguration time-dependent Hartree method. An exact form of the kinetic energy operator including Coriolis coupling has been used. Coupled channel results on WDSE surface for vibrational excitation of H(2) produce very large cross sections in accordance with the previous approximate results. The rate constant obtained for H(2)(v = 1) at 300 K on the YZCL2 surface shows an excellent agreement with the most recent experimental result. Quantum dynamical results for ro-vibrational excitation of reagents obtained on the WSLFH surface show similar behavior to previous quasiclassical trajectory studies. The integral cross sections obtained for excited reagent rotations exhibit contrasting trends on the three surfaces. The effects are explained considering the different orientations of the transition state structure and the individual surface characteristics.     

Studies on the Effects of Salt and Surfactant in Wet Phase Separation of Polysulfone

This study investigates the effect of additives in the nonsolvent water in terms of cloud point during the phase inversion of Polysulfone (PS) in dimethyl formamide (DMF). The exponential pattern is observed with PS concentration (0.2 to 0.6% (w/w)). It needs a low amount of water to get the cloud point at low temperature. The cloud point varied with the nature of water matrix and depended on the amount of salt, as well as the PS amount. The presence of salts (sodium chloride and sodium sulfate) lowers the cloud point of the solution. The network distribution of the particles at the cloud point is disturbed in the presence of salt. The requirement is more for Sodium lauryl sulfate (SLS) added water to reach the cloud point in the low range of PS solution up to 0.3% PS (w/w). The morphological and distribution pattern of PS particles are very different compared to PS particles produced from pure water. XRD study of PS particles produced from the mixed water system reflects relatively more amorphous character with respect to PS particles from pure water. The presence of both surfactant and salts in water systems also influences the cloud point in synergistic manner.     

Spectrophotometric study of molecular complex formation between o-chloranil and a series of methylated pyridines

o-Chloranil has been shown to form 1:1 molecular complexes with pyridine and 2-, 3- and 4-picolines in CCl4 medium. Isosbestic points have been found but charge-transfer bands could not be detected. The formation constants of the complexes exhibit a very good linear free energy relationship from which the Hammett p parameter for the complexation reaction is found to be -3.67.     

Comparison of optical properties of periodic photonic–plasmonic and randomly textured back reflectors for nc-Si solar cells

Light trapping is essential to harvest long-wavelength red and near-infrared photons in thin film silicon solar cells. Light trapping in p-i-n thin film cells is commonly achieved with back-reflectors, and in n-i-p cells with textured front transparent conductors. We systematically compare the optical properties and performance of randomly roughened back reflectors with periodic plasmonic-photonic back-reflectors in p-i-n solar cells. The randomly textured back reflectors were prepared to have very high diffuse reflectance comparable to the state of the art. The periodic back reflectors of tapered nano-pillars/bumps show enhanced quantum efficiency and optical absorption over randomly structured back reflectors using the same solar cell architecture in nc-Si solar cells. The strong diffraction and light concentration in periodic back reflectors may be more beneficial than the light scattering offered by randomly textured back reflectors.     

Single Amino-Acid Based Self-Assembled Biomaterials with Potent Antimicrobial Activity

The design and development of soft biomaterials based on amino acid and short-peptide have gained much attention due to their potent biomedical applications. A slight alteration in the side-chain of single amino acid in a peptide or protein sequence has a huge impact on the structure and function. Phenylalanine is one of the most studied amino acids, which contains an aromatic phenyl group connected through a flexible −CH₂− unit. In this work, we have examined whether flexibility and aromatic functionality of phenylalanine (Phe) are important in gel formation of model gelator Fmoc-Phe-OH or not. To examine this hypothesis, we synthesized Fmoc-derivatives of three analogues unnatural amino acids including cyclohexylalanine, phenylglycine, and homophenylalanine; which are slightly varied from Phe. Interestingly, all these three new analogues formed hydrogels in phosphate buffer at pH 7.0 having different gelation efficacy and kinetics. This study suggests that the presence of aromatic side-chain and flexibility are not mandatory for the gelation of this model gelator. Newly synthesized unnatural amino acid derivatives have also exhibited promising antimicrobial activity towards gram-positive bacteria by inhibiting cellular oxygen consumption. We further determined the biocompatibility of these amino acid derivatives by using a hemolysis assay on human blood cells. Overall studies described the development of single amino acid-based new injectable biomaterials with improved antimicrobial activity by the slight alteration in the side-chain of amino acid.     

Pulling an adsorbed polymer chain off a solid surface

The thermally assisted detachment of a self-avoiding polymer chain from an adhesive surface by an external force applied to one of the chain-ends is investigated. We perform our study in the “fixed height” statistical ensemble where one measures the fluctuating force, exerted by the chain on the last monomer when a chain-end is kept fixed at height h over the solid plane at different adsorption strength . The phase diagram in the h - plane is calculated both analytically and by Monte Carlo simulations. We demonstrate that in the vicinity of the polymer desorption transition a number of properties like fluctuations and probability distribution of various quantities behave differently, if h rather than f is used as an independent control parameter.     

On pulsatile flow of a viscous fluid in a rotating channel

A study is made on the pulsatile flow superposed on a steady laminar flow of a viscous fluid in a parallel plate channel rotating with an angular velocity Ω about an axis perpendicular to the plates. An exact solution of the governing equations of motion is obtained. The solution in dimensionless form contain two parametersK ²=ΩL ²/v which is reciprocal of Ekmann Number and frequency parameter σ=αL ²/v. The effects of these parameters on the principal flow characters such as mean sectional velocity and shear stresses at the plates have been examined. For large σ andK ² the flow near the plates has a multiple boundary layer character.