Semiconductor Nanoparticles

Semiconductor nanoparticles exhibit size dependent properties, when their size is comparable to the size of Bohr diameter for exciton. This can be exploited to increase fluorescence efficiency or increase the internal magnetic field strength in doped semiconductors. Nanoparticles are usually unstable and can aggregate. It is therefore necessary to protect them. Surface passivation using capping molecules or by making core–shell particles are some useful ways. Here synthesis and results on doped and un-doped nanoparticles of ZnS, CdS and ZnO will be discussed. We shall present results on core–shell particles using some of these nanoparticles and also discuss briefly the effect of Mn doping on hyperfine interactions in case of CdS nanoparticles.     

Continuous periodic to quasiperiodic structural transformations in crystalline and superlattice structures

Attention is focussed on the fact that the β—Mn, the σ—phase and the Al₅Ti₃ and the Al₁₁Ti₇ superlattice derivatives of the δ—AlTi phase all belong to a category of structures which comprises an identical periodic arrangement of square and rhombic tiles — the shapes of the rhombic tiles being different in each of these cases. While the first and the second structures constitute periodic approximants of the octagonal and the dodecagonal quasicrystals respectively, the last two (superlattice) structures are the approximants of a hypothetical quasiperiodic superlattice structure. The strip projection method has been used here to model the intermediate structures which arise during the continuous periodic to quasiperiodic transitions between the relevant structures and was found to be quite successful in explaining many of the related experimental findings.     

Twisted aromatics, 9-anthryl and 1-pyrenyl terpyridines organize into novel multi-directional ‘ladder-like’ motifs in the solid state

9-Anthryl and 1-pyrenyl terpyridines (1 and 2, respectively), key precursors for the design of novel fluorescent sensors have been synthesized and characterized by ¹H NMR, mass spectroscopy and X-ray crystallography. Twisted molecular conformations for each 1 and 2 were observed in their single crystal structures. Energy minimization calculations for the 1 and 2 using the semi-empirical AM1 method show that the ‘twisted’ conformation is intrinsic to these systems. We observe interconnected networks of edge-to-face CHπ interactions, which appear to be cooperative in nature, in each of the crystal structures. The two twisted molecules, although having differently shaped polyaromatic hydrocarbon substituents, show similar patterns of edge-to-face CHπ interactions.The presently described systems comprise of two aromatic surfaces that are almost orthogonal to each other. This twisted or orthogonal nature of the molecules leads to the formation of interesting multi-directional ladder like supramolecular organizations. A combination of edge-to-face and face-to-face packing modes helps to stabilize these motifs. The ladder like architecture in 1 is helical in nature.     

Pulsed laser heating calculations incorporating vapourization

We present a scheme for solution of the heat flow equation in one-dimension incorporating melting and vapourization produced under pulsed laser irradiation. The method can be applied to pure materials as well as multilayered structures such as deposited films. The variation of physical properties with temperature can be easily taken into account. Results of calculation are presented for aluminium and for chromium and antimony layers deposited on aluminium. As a consequence of excessive vapourization at high energy densities, the melt depth and the melt duration do not increase beyond a certain limit. The resolidification front velocity is strongly dependent on energy density and can be controlled in an experiment by a careful choice of laser parameters. Some recent experimental data on laser treated chromium films are discussed in light of our calculations.     

Mössbauer study of the spinel system GexCu1−xFe2O4

The magnetic properties of the spinel series Ge_xCu_1?xFe₂O₄ (X = 0 to 0.8) have been investigated by means of Mössbauer spectroscopy. Mössbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to Fe³⁺ octahedral ions (B-sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A- and B-site ions and a broad central line spectrum. For 0.2 ? X ? 0.4, fast electron exchange among octahedral iron ions occurs as in Fe₃O₄. The variations of nuclear magnetic fields at the A- and B-sites are explained on the basis of AB and BB supertransferred hyperfine interactions.     

Total synthesis of (±)-Δ9(12)-capnellene

Starting from 2-cyclopentenone the title compound has been synthesized via a combination of [2+2] and [4+2] cycloaddition reactions.     

Surface oxidation study of Fe67Co18B14Si1 metallic glass using Auger electron spectroscopy

Surface composition and depth profile of native oxide on Fe₆₇Co₁₈B₁₄Si₁ metallic glass has been investigated using Auger electron spectroscopy. The native oxide is compared with chemisorbed oxygen on the cleaned surface. Results indicate that boron segregates on the surface in the presence of oxygen. The low energy L₂₃M₄₅M₄₅ peak of iron in metallic glass is compared with that in crystallized sample and pure iron foil.     

Investigations in (0-0) isotopic bands of the C2 Swann system

Employing the various isotopes of carbon, (0-0) bands of the C₂ Swann system have been investigated. Comparisons have been made between the observed rotational line positions and those obtained by computer calculations with the respective constants evaluated from ¹²C₂. There is a good agreement (within ± 0.5 cm^?1 in most of the cases) between the two sets of values. The studies include the effect of change of nuclear mass and spin on different aspects of rotational structure and intensity pattern.