M. P. Joshi

THREE-DIMENSIONAL OPTICAL CIRCUITRY USING TWO-PHOTON-ASSISTED POLYMERIZATION

An approach using an as-formed solid bulk sample containing a blend of photocurable and thermally curable epoxies together with a highly efficient two-photon excitable fluorophore is demonstrated for in situ fabrication of three-dimensional optical circuitry with a precise control of dimension and spatial locations. Confocal volume images of a 1×2 splitter and a grating coupler are shown. End-fire coupling of a He–Ne laser beam into the 1×2 splitter has been achieved.

Investigation of optical limiting in C60 solution

Results of experimental and theoretical investigation of optical limiting for 30 ns pulses at 527 nm in C60 are presented which suggest that apart from higher excited state absorption, other nonlinear processes may have an important role in the experimentally observed limiting action.

On the contribution of nonlinear scattering to optical limiting in C60 solution

We present experimental results which bring out the contribution of nonlinear scattering to the energy limiting of 527 nm, 30 ns pulses in C60 solution. To perform these measurements we used a specific experimental arrangement to separate the effects of nonlinear refraction and scattering. Our results show that scattering reduces the output significantly and contributes to limiting in C60 solution.

Negative electric field dependence of mobility in TPD doped Polystyrene

Abstract A total negative field dependence of hole mobility down to low temperature was observed in N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site energies. The simulation supports our experimental observations and justification on the basis of low values of disorder parameters.

Phase-coherent electron transport in (Zn, Al)Ox thin films grown by atomic layer deposition

A clear signature of disorder induced quantum-interference phenomena leading to phase-coherent electron transport was observed in (Zn, Al)Ox thin films grown by atomic layer deposition. The degree of static-disorder was tuned by varying the Al concentration through periodic incorporation of Al2O3 sub-monolayer in ZnO. All the films showed small negative magnetoresistance due to magnetic field suppressed weak-localization effect. The temperature dependence of phase-coherence length ( lφ∝T−3/4), as extracted from the magnetoresistance measurements, indicated electron-electron scattering as the dominant dephasing mechanism. The persistence of quantum-interference at relatively higher temperatures up to 200 K is promising for the realization of ZnO based phase-coherent electron transport devices.

Observation of dopant-profile independent electron transport in sub-monolayer TiOx stacked ZnO thin films grown by atomic layer deposition

Dopant-profile independent electron transport has been observed through a combined study of temperature dependent electrical resistivity and magnetoresistance measurements on a series of Ti incorporated ZnO thin films with varying degree of static-disorder. These films were grown by atomic layer deposition through in-situ vertical stacking of multiple sub-monolayers of TiOx in ZnO. Upon decreasing ZnO spacer layer thickness, electron transport smoothly evolved from a good metallic to an incipient non-metallic regime due to the intricate interplay of screening of spatial potential fluctuations and strength of static-disorder in the films. Temperature dependent phase-coherence length as extracted from the magnetotransport measurement revealed insignificant role of inter sub-monolayer scattering as an additional channel for electron dephasing, indicating that films were homogeneously disordered three-dimensional electronic systems irrespective of their dopant-profiles. Results of this study are worthy enough...

Electron irradiation effects on TGA-capped CdTe quantum dots

8MeV electron irradiation effects on thioglycolic acid (TGA)-capped CdTe quantum dots (QD) are discussed in this study. CdTe QDs were characterized using x-ray diffraction (XRD), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS). Steady-state and time-resolved emission spectroscopy and UV-visible absorption spectroscopy were performed before and after irradiation with 8MeV electrons. XRD and TEM confirm the growth of TGA-capped CdTe QDs. The photoemission wavelength, intensity and lifetimes were found to vary with electron dose. At lower doses, they were found to be increasing (red-shift of photoluminescence (PL) peak and intensity) while the intensity decreased at higher electron doses. The observed changes in PL property, XPS and XRD analysis suggest possible epitaxial growth of the CdS shell on the CdTe core. This work demonstrates electron beam induced formation of the CdS layer on the CdTe core, which is a key step towards growth of the water soluble CdTe/CdS core-shell structure for biomedical labelling applications.

Electrical and optical characterization of photooxidized TPD

Changes in current–voltage characteristics of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′diamine (TPD) dispersed in polymer films are studied when solutions of TPD are exposed to UV light. Higher current density is observed in films cast with solutions exposed under UV for an optimum time. Observations are rationalized on the basis of the photoinduced oxidation of TPD when chlorinated solvents are used. Photoinduced generation of the TPD cation (TPD+) is confirmed using linear absorption spectroscopy when compared with salt-based generation (using CuCl2) of TPD+. Results suggest another possible route for generating TPD+ and the possibility of unintentional oxidation of TPD when chlorinated solvents like chloroform or dichloromethane are used for making devices.

Dimensional crossover of electron weak localization in ZnO/TiOx stacked layers grown by atomic layer deposition

We report on the dimensional crossover of electron weak localization in ZnO/TiOx stacked layers having well-defined and spatially-localized Ti dopant profiles along film thickness. These films were grown by in situ incorporation of sub-monolayer TiOx on the growing ZnO film surface and subsequent overgrowth of thin conducting ZnO spacer layer using atomic layer deposition. Film thickness was varied in the range of ∼6–65 nm by vertically stacking different numbers (n = 1–7) of ZnO/TiOx layers of nearly identical dopant-profiles. The evolution of zero-field sheet resistance (R◻) versus temperature with decreasing film thickness showed a metal to insulator transition. On the metallic side of the metal-insulator transition, R◻(T) and magnetoresistance data were found to be well corroborated with the theoretical framework of electron weak localization in the diffusive transport regime. The temperature dependence of both R◻ and inelastic scattering length provided strong evidence for a smooth crossover from 2D ...

Band offset at TiO2/MDMO PPV and TiO2/PEDOT PSS interfaces studied using photoelectron spectroscopy

We report band alignment and band offset studies across the interfaces of hetero-structures of TiO2 with poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylene vinylene] (MDMO PPV) and poly(styrenesulfonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT PSS) using photoelectron spectroscopy. In both the cases the band alignment was found to be type II with significant band bending in the range 0.2 to 0.3 eV. In the case of the TiO2/MDMO PPV hetero-structure, the valance band offset (VBO)/conduction band offset (CBO) were found to be 2.68 ± 0.1 eV/1.68 ± 0.1 eV, while the TiO2/PEDOT PSS hetero-structure exhibited VBO/CBO as 2.3 ± 0.1 eV/0.7 ± 0.1 respectively. Based on these results, schematic band diagrams for these organic/inorganic hetero-structures were proposed. Our studies are important for the optimum design of various photovoltaic devices based on these organic/inorganic hetero-junctions.