Thermoluminescence and defect study of MgSiO3 ceramics

Enstatite (MgSiO₃) ceramic powders were synthesised by a low-temperature initiated self-propagating, gas-producing solution combustion process. The prepared powders were characterised by powder X-ray diffraction, scanning electron microscopy and Brunauer–Emmer–Teller specific surface area measurements. Defect centres induced by radiation were studied using the techniques of thermoluminescence (TL) and electron spin resonance (ESR). A well-resolved glow with peak at 178°C and a shouldered peak at 120°C were observed. Two defect centres were identified by ESR measurements, which were carried out at room temperature, and these were assigned to an O^? ion and F⁺ centre. The O^? ion (hole centre) appears to correlate with the main TL peak at 178°C.     

Quantum well engineering of InAlAs/InGaAs HEMTs for low impact ionization applications

The performance of InAlAs/InGaAs quantum well field effect transistors are subject to high impact ionization and band-to-band tunneling (BTBT) due to its narrow bandgap feature. In this work, the energy gap is engineered using strain and quantization techniques to increase the effective energy gap leading to low impact ionization and BTBT leakage current. It is shown that the impact ionization is reduced in 5 nm channel device as compared to 13 nm device with onset at approximately E_geff/q. Also the band-to-band-tunneling current is reduced due to the increase in effective energy gap. We have also investigated the effects of quantum well engineering on the dc performance of InGaAs HEMTs.     

Electronically Rich N-Substituted Tetrahydroisoquinoline 3-Carboxylic Acid Esters: Concise Synthesis and Conformational Studies

Recent work in our laboratory has shown that the highly substituted, electronically rich 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (THIQ3CA) scaffold is a key building block for a novel class of promising anticoagulants (Al-Horani et al. J. Med. Chem.2011, 54, 6125-6138). The synthesis of THIQ3CA analogs, especially containing specific, electronically rich substituents, has been a challenge and essentially no efficient methods have been reported in the literature. We describe three complementary, glycine donor-based strategies for high yielding synthesis of highly substituted, electronically rich THIQ3CA esters. Three glycine donors studied herein include hydantoin 1, (±)-Boc-α-phosphonoglycine trimethyl ester 2 and (±)-Z-α-phosphonoglycine trimethyl ester 3. Although the synthesis of THIQ3CA analogs could be achieved using either of the three, an optimal, high yielding approach for the desired THIQ3CA esters was best achieved using 3 in three mild, efficient steps. Using this approach, a focused library of advanced N-arylacyl, N-arylalkyl, and bis-THIQ3CA analogs was synthesized. Variable temperature and solvent-dependent NMR chemical shift studies indicated the presence of two major conformational rotamers in 3:1 proportion for N-arylacyl-THIQ3CA analogs, which were separated by a high kinetic barrier of ~17 kcal/mol. In contrast, N-arylalkyl and bis-THIQ3CA variants displayed no rotamerism, which implicates restricted rotation around the amide bond as the origin for high-barrier conformational interconversion. This phenomenon is of major significance because structure-based drug design typically utilizes only one conformation. Overall, the work presents fundamental studies on the synthesis and conformational properties of highly substituted, electronically rich THIQ3CA analogs.     

The stereochemical diversity of a new SNONS binucleating ligand towards 3d metal ions

A new binucleating ligand containing phenoxide as an endogenous bridging group, 2,6-diformyl-p-cresol bis(2-furanthiocarboxyhydrazone) and its binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes with chloride ion as an exogenous bridge, have been obtained. The complexes were characterized by elemental analysis, molar conductivities, magnetic moment measurements at room temperature, electronic, IR, 1H-NMR, EPR, FAB spectral studies and thermal data. The copper complex assumes a tetranuclear structure composed of two binuclear units related by a center of symmetry. The dimeric nature of copper(II) complex is supported by FAB. This complex is EPR silent. Room temperature magnetic moment reveals the operation of a significant antiferromagnetic spin exchange between the metal centers. Ligand and its copper and zinc complexes exhibit fluorescence at room temperature in DMF. All the compounds show an appreciable antimicrobial activity.     

Rare earth elements analysis in archaeological pottery by laser induced breakdown spectroscopy

In the present work, laser induced breakdown spectroscopy has been employed to detect rare earth elements in archaeological potteries and brick sample collected from different locations. Laser Induced breakdown spectroscopy data has been analyzed to investigate the provenance of ancient clay artifacts which is based on the geochemistry of rare earth elements in the artifacts. The qualitative study of spectra shows the presence of a set of rare earth elements such as La, Ce, Pr, Nd, Sm, Gd, Dy, and Er in archaeological potteries which specifies that these potteries were manufactured from the same clay sources.     

Multiferroic properties of nanocrystalline BaTiO3

Some of the Multiferroics [H. Schmid, Ferroelectrics 162 (1994) 317] form a rare class of materials that exhibit magneto–electric coupling arising from the coexistence of ferromagnetism and ferroelectricity, with potential for many technological applications [J.F. Scott, Nat. Mater. 6 (2007) 256; N.A. Spaldin, M. Fiebig, Science 309 (2005) 391]. Over the last decade, an active research on multiferroics has resulted in the identification of a few routes that lead to multiferroicity in bulk materials [C. Ederer, N.A. Spaldin, Nat. Mater. 3 (2004) 849; D.V. Efremov, J. van den Brink, D.I. Khomskii, Nat. Mater. 3 (2004) 853; N. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature 429 (2004) 392]. While ferroelectricity in a classic ferroelectric such as BaTiO₃ is expected to diminish with the reducing particle size, [C.H. Ahn, K.M. Rabe, J.M. Triscone, Science 303 (2004) 488; J. Junquera, P. Ghosez, Nature 422 (2003) 506] ferromagnetism cannot occur in its bulk form [N.A. Hill, J. Phys. Chem. B 104 (2000) 6694]. Here, we use a combination of experiment and first-principles simulations to demonstrate that multiferroic nature emerges in intermediate size nanocrystalline BaTiO₃, ferromagnetism arising from the oxygen vacancies at the surface and ferroelectricity from the core. A strong coupling between a surface polar phonon and spin is shown to result in a magnetocapacitance effect observed at room temperature, which can open up possibilities of new electro–magneto-mechanical devices at the nano-scale.     

Studies on third-order nonlinear optical properties and reverse saturable absorption in polythiophene/poly (methylmethacrylate) composites

We report here the studies on third-order nonlinear optical properties of two novel polythiophene composite films investigated using the Z-scan technique. The measurements were carried out using a Q-switched, frequency doubled Nd:YAG laser producing 7 nanosecond laser pulses at 532 nm. Z-scan results reveal that the composite films exhibit self-defocusing nonlinearity. The real and imaginary parts of the third-order nonlinear optical susceptibility were of the order 10⁻¹² esu. The effective excited-state absorption cross section was found to be larger than the ground state absorption cross section, indicating that the operating nonlinear mechanism is reverse saturable absorption (RSA). The polythiophene composite films also exhibit good optical power limiting of the nanosecond laser pulses. The nonlinear optical parameters are found to increase on increasing the strength of the electron-donor group, indicating the dependence of χ ⁽³⁾ on the electron-donor/acceptor units of polythiophenes.     

Bimetallic complexes of a potentially pentadentate,acyclic, symmetrical compartmental Schiff base ligand that provides suitable topology for an exogenous bridge

The binucleating ligand LH₃, 2,6-diformyl-p-cresol-bis(phenylthioacetyldrazone), a Schiff base condensation product of 2,6-diformyl-p-cresol and phenylthioacetyldrazide forms complexes of the [M₂ClL] type with Co^II, Ni^II and Cu^II ions, which were characterized by elemental analysis, magnetic susceptibility, electronic spectra, molar conductance, i.r., n.m.r., e.p.r., t.g. and FAB mass spectral measurements. Sub-normal magnetic moments indicate the operation of antiferromagnetic coupling between the metal centres. The ligand and its copper complex show a pronounced fungistatic activity.     

Symmetries, vibrational instabilities, and routes to stable structures of clusters of Al, Sn, and As

We investigate the stability of small clusters using density functional theory to compute the total energy, forces, and vibrational frequencies using linear response. We exhibit an efficient and computationally low-cost route to finding stable structures, by starting with high-symmetry structures and distorting them according to their unstable modes. We illustrate this by application to 4-, 6-, and 13-atom clusters of Al, Sn, and As. This technique also naturally elucidates the origins of stability of the lower symmetry structures, which is variously due to the linear or pseudo Jahn-Teller effect, combined with a lowering of various contributions to the total energy. We show that the situation is more complex than has generally been appreciated.