M.K. Gupta

Supramolecular Interaction of Coumarin 1 Dye with Cucurbit[7]uril as Host: Combined Experimental and Theoretical Study

Molecules of the coumarin family have fluorescence characteristics that are highly sensitive to their environment, and thus, they have been used as fluorescent sensors in chemical and biological systems. However, the very poor fluorescence yield of most coumarin dyes in aqueous media limits their applications. We have adopted a supramolecular strategy to improve the fluorescence intensity of coumarin dye through its interaction with the relatively new host cucurbit[7]uril (CB[7]). The virtually nonfluorescent coumarin 1 (Φ(f) = 0.04) was converted into a highly fluorescent (Φ(f) = 0.52) entity in water upon addition of the nonfluorescent host CB[7]. Various spectroscopy techniques, namely, UV-vis absorption and steady-state and time-resolved fluorescence spectroscopies, established the formation of a strong 1:1 dye-CB[7] inclusion complex with a high binding constant of (1.2 ± 0.1) × 10(5) M(-1) for the dye. The stable inclusion complex of the neutral molecule was supported by density-functional-theory- (DFT-) based quantum chemical calculations. Energy decomposition analysis of various interaction factors in the host-guest complex revealed that key components providing stability to the complex were electrostatic, polarization, and charge-transfer energies. These new results on the formation of a strong inclusion complex of the versatile fluorophore coumarin 1 with the nontoxic host CB[7] could lead to the design of efficient molecular-scale biological probes, sensors, and photostable aqueous UV dye lasers.

Rational Design of Boradiazaindacene (BODIPY)‐Based Functional Molecules

Three boradiazaindacene (BODIPY) dyes with different-coloured (greenish-yellow, orange and red) fluorescence and good Stokes shifts were synthesised starting from the greenish-yellow BODIPY dye PM546. The high Stokes shifts of the dyes are due to the release of the steric strain in their excited states relative to that in the highly twisted ground states. One of these compounds might be a useful water-soluble fluorophore, whereas the other two are promising H(+) sensors.

Growth of single crystals in Bi2(Sr, Ca)n + 1CunOx system

Abstract Flux growth of crystals in the general system Bi 2 (Sr, Ca) n + 1 Cu n O x with n = 1 and n = 2 is described. The solute concentration in solution has been found to govern the phase, morphology and quality of the grown crystals. A controlled excess of CuO and CaO when added to the solute of stoichiometry 2212 yielded crystals of superconducting phase free from secondary growth of CuO needles.

Growth of yttria and dysprosium thin films by molecular beam epitaxy and their characterization

Abstract The thin film deposition of yttria (Y 2 O 3 ) and dysprosium (Dy) on single crystal substrates of YSZ and SrTiO 3 has been studied under MBE conditions. Yttria is found to be very stable under electron beam evaporation. Depending upon the growth temperatures, two growth regimes which give rise to epitaxial growth of Y 2 O 3 on 〈100〉 YSZ substrates and predominantly oriented films along 〈222〉 are identified. In the case of Dy films on 〈100〉SrTiO 3 substrates, for growth temperatures above 600°C the growth orientation is found to be 〈101〉, whereas below this temperature the films are polycrystalline in nature. ESCA measurements made on Dy films reveal their special affinity to oxygen.

Characterization of YBa2Cu3O7- delta single crystals grown from charges containing silver nitrate

Single-crystal growth of YBa2Cu3O7- delta has been studied using starting charges containing known amounts of Ag in the form of AgNO3. The as-grown crystals are found to undergo a sharp transition to the superconducting state at 90 K, showing that the addition of AgNO3 facilitates their oxygenation. The measured room-temperature resistivities, both in plane ( rho ab) and out of plane ( rho c), for the grown crystals are found to be the lowest among the values reported so far. The anisotropy in resistivity ( rho c/ rho ab) at 300 K is found to be roughly one-half of the value measured for the crystals grown from charges free from AgNO3. The angular dependence of transport critical current for the doped crystals has been measured at 82 K and for an applied magnetic field of 0.9 T. The results have been analysed using the existing theoretical models.

STM studies of growth spirals in bulk single crystals of superconducting Y-123 and Bi-2122

Abstract The STM observations of growth spirals in bulk single crystals of Y-123 and Bi-2122 high Tc cuprates are reported for the first time. The density of screw dislocations present is of the same order as recently reported for sputtered epitaxial thin films of Y-123. Growth features are briefly discussed and it is argued that the observed dislocation density is still too low for realizing optimum flux pinning in these HTSC systems.

Thin film deposition of BaO by molecular beam epitaxy

Thin film deposition of BaO using BaO2 as the source material is studied by molecular beam epitaxy. The crystalline nature of the films grown at different substrate temperatures is studied by RHEED. Chemical composition of the films has been ascertained from ESCA measurements carried out without breaking the vacuum. The substrate temperature has been found to govern the composition of the films. Whilst at low growth temperature metallic Ba films are obtained, those grown at high temperatures (∼ 600°C) are of BaO.

Preparation of adherent Y-Ba-Cu-O thick films and the effect of silver doping

Thick films of YBa2Cu3Ox have been prepared on (100)SrTiO3 substrates by the paint-on method. The effect of silver doping on the adhesion, void density and superconducting properties of the film has been investigated. The results show that the addition of silver promotes the growth of grains as well as their alignment. It also affects the nature of intergrain connectivity from superconductor-insulator-superconductor (S-I-S) type in undoped samples to superconductor-metal-superconductor (S-N-S).

Effect of the Lorentz force on dissipation in YBa2Cu3Ox thin films

Abstract The current-voltage characteristics of YBa 2 Cu 3 O x , (YBCO) thin films have been measured, with both current and magnetic field applied parallel to the ab -plane, to study the effect of the Lorentz force on dissipation. The angle (θ) between the field and the current is varied to obtain the angular dependence of the I–V curve. A power law behavior for the I–V curves is observed for different fields and θ values studied. The results cannot be explained on the basis of the existing flux flow and flux creep models. A model for the vortex motion has been developed to explain the results.

Bulk synthesis of (BiPb)2Sr2Ca2Cu3Ox superconductor

Abstract A simple and effective method for the large scale production of high quality single phase high-T c (T c = 108K) Bi-2223 material is described.