Suman Kalyan Pal

Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3

The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the s...

Geminate Charge Recombination in Polymer/Fullerene Bulk Heterojunction Films and Implications for Solar Cell Function

We have studied the influence of three different fullerene derivatives on the charge generation and recombination dynamics of polymer/fullerene bulk heterojunction (BHJ) solar cell blends. Charge generation in APFO3/[70]PCBM and APFO3/[60]PCBM is very similar and somewhat slower than charge generation in APFO3/[70]BTPF. This difference qualitatively matches the trend in free energy change of electron transfer estimated from the LUMO energies of the polymer and fullerene derivatives. The first order (geminate) charge recombination rate is significantly different for the three fullerene derivatives studied and increases in the order APFO3/[70]PCBM < APFO3/[60]PCBM < APFO3/[70]BTPF. The variation in electron transfer rate cannot be explained from the LUMO energies of the fullerene derivatives and single-step electron transfer in the Marcus inverted region and simple considerations of expected trends for the reorganization energy and free energy change. Instead we suggest that geminate charge recombination occurs from a state where electrons and holes have separated to different distances in the various materials because of an initially high charge mobility, different for different materials. In a BHJ thin film this charge separation distance is not sufficient to overcome the electrostatic attraction between electrons and holes and geminate recombination occurs on the nanosecond to hundreds of nanoseconds time scale. In a BHJ solar cell, we suggest that the internal electric field in combination with polarization effects and the dynamic nature of polarons are key features to overcome electron-hole interactions to form free extractable charges.

Calculations of interfacial interactions in pyrene-Ipa rod sensitized nanostructured TiO2.

Pyrene chromophores carrying different rigid rod spacer groups (ethynylene, ethynylene-phenylene-ethynylene, and ethynylene-bicyclo[2.2.2]octylene-ethynylene) and bound to TiO2 nanostructured materials via an isophthalic acid (Ipa) anchor group have been investigated using quantum chemical calculations in order to elucidate structural and electronic properties of dye-sensitized semiconductor structures capable of long-range photoinduced interfacial electron transfer. The calculations are used to study firstly the effect of the anchor and spacer groups on the electronic properties of the pyrene-dyes, secondly the binding of isophthalic acid to nanostructured TiO2, and thirdly the interfacial electronic interactions for dye-sensitized nanostructured TiO2 relevant to dye-sensitized solar cell applications. Together, these calculations provide theoretical insights into the effect of incorporating rigid rod anchor-cum-spacer group motifs in sensitizers for e.g. solar cell applications. In particular, the calculations help to rationalize the strong influence of the rods on the photophysical properties of the sensitizers in terms of electronic interactions between the individual chromophore, spacer, and anchor segments, as well as to provide information about interfacial electronic interactions of interest for the capabilities of the rods to act as efficient mediators of photoinduced interfacial charge separation.

Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase

Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric ...

Global Sampling of the Photochemical Reaction Paths of Bromoform by Ultrafast Deep-UV Through Near-IR Transient Absorption and ab initio Multiconfigurational Calculations

Ultrafast deep-ultraviolet through near infrared (210-950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr3 leads to the ground-state iso-CHBr3 product in a large quantum yield (∼35%), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr3 and the ground electronic state of iso-CHBr3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C-Br-Br bending and Br-Br stretching modes of ground-state iso-CHBr3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water.

Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study

The quenching of the fluorescence of 1-aminopyrene (1-Ap) by reduced graphene oxide (rGO) has been investigated using spectroscopic techniques. In spite of the upward curvature in the Stern-Volmer plot, the unchanged spectral signature of the absorption of 1-Ap in the presence of rGO and the decrease in fluorescence lifetime with increasing rGO concentration point toward the dynamic nature of the quenching. Detailed analysis of steady state and time-resolved spectroscopic data has shown that the quenching arises due to the photoinduced electron transfer from 1-Ap to rGO. This is again supported by estimating the Gibbs free energy change for the ground as well as excited state electron transfer. Ab initio calculations under the density functional theory (DFT) formalism reveal that the possibility of π-π stacking is very slim in the 1-Ap-rGO system and the electron density resides completely on 1-Ap in the highest occupied molecular orbital (HOMO) and on graphene in the lowest unoccupied molecular orbital (LUMO), supporting the experimental findings of the intermolecular electron transfer between 1-Ap and rGO in the excited state.

Photochemistry of monochloro complexes of copper(II) in methanol probed by ultrafast transient absorption spectroscopy.

Ultrafast transient absorption spectra in the deep to near UV range (212-384 nm) were measured for the [Cu(II)(MeOH)(5)Cl](+) complexes in methanol following 255-nm excitation of the complex into the ligand-to-metal charge-transfer excited state. The electronically excited complex undergoes sub-200 fs radiationless decay, predominantly via back electron transfer, to the hot electronic ground state followed by fast vibrational relaxation on a 0.4-4 ps time scale. A minor photochemical channel is Cu-Cl bond dissociation, leading to the reduction of copper(II) to copper(I) and the formation of MeOH·Cl charge-transfer complexes. The depletion of ground-state [Cu(II)(MeOH)(5)Cl](+) perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between [Cu(II)(MeOH)(5)Cl](+) and [Cu(II)(MeOH)(4)Cl(2)] is established on a 10-500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative.

A radiation sensitive hybrid polymer based on an Mn-Anderson polyoxometalate cluster and a UV active organic monomer: synergistic effects lead to improved photocurrent in a photoresponse device

A new radiation sensitive polyoxometalate/polymer hybrid, POM–MAPDST, has been synthesized from an Mn-Anderson cluster based hybrid and a UV active organic monomer and tested for its photo-response properties by fabricating an ITO/POM–MAPDST/Al device. This device showed improved I–V characteristics under illumination possibly due to the synergistic interactions between the polyoxometalate cluster and the organic components of the hybrid polymer.

Exponentially distributed trap-controlled space charge limited conduction in graphene oxide films

Temperature dependent electrical conduction in as prepared and annealed graphene oxide (GO) thin films was investigated. The electrical measurements on a sandwich structure of GO reveal space charge limited conduction in the presence of an exponential trap distribution within the temperature range of 83–288 K. However, for the annealed GO, at all temperatures and low bias voltage, charge transport is governed by a bulk limited process with a bias dependent crossover from Ohmic to trap free space charge limited conduction. At the high bias voltage and low temperature, the conduction becomes space charge limited with exponential distribution of traps. Moreover, we estimated trap densities in both pristine and annealed GO.

Ab Initio Assessment of the Structural and Optoelectronic Properties of Organic–ZnO Nanoclusters

Structural, electronic, and optical properties of a new coumarin dye, zinc oxide (ZnO) nanoclusters of varying sizes, and their complexes have been investigated using density functional theory (DFT). The band gap of oxide nanoclusters varies with size validating quantum confinement effect in small particles. Energy level diagrams of dye, ZnO nanoclusters, and redox electrolyte are in favor of efficient electron injection from dye to nanocluster and regeneration of the ionized dye. The adsorption of the organic dye to nanocluster is tested for anchoring through three different functional groups (cyano, carbonyl, and hydroxyl) of the dye. We have compared simulated absorption spectra of the dye, nanoclusters, and dye functionalized nanoclusters and discussed the matching with the solar irradiance spectrum. A strong new band appeared in the low energy side of the absorption spectra for dye adsorbed nanoclusters. Frontier molecular orbital calculations reveal that the first absorption band of dye-ZnO complexes is charge transfer (CT) in character. Excitation of this band leads to direct electron transfer to the conduction band (CB) of the nanocluster, making dye-ZnO complexes suitable for type II DSSCs as well.