Efficient charge transfer and field-induced tunneling transport in hybrid composite device of organic semiconductor and cadmium telluride quantum dots

Temperature and photo-dependent current-voltage characteristics are investigated in thin film devices of a hybrid-composite comprising of organic semiconductor poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT: PSS) and cadmium telluride quantum dots (CdTe QDs). A detailed study of the charge injection mechanism in ITO/PEDOT: PSS-CdTe QDs/Al device exhibits a transition from direct tunneling to Fowler-Nordheim tunneling with increasing electric field due to formation of high barrier at the QD interface. In addition, the hybrid-composite exhibits a huge photoluminescence quenching compared to aboriginal CdTe QDs and high increment in photoconductivity (similar to 400%), which is attributed to the charge transfer phenomena. The effective barrier height (Phi(B) approximate to 0.68 eV) is estimated from the transition voltage and the possible origin of its variation with temperature and photo-illumination is discussed. (C) 2015 AIP Publishing LLC.

NH3 adsorption on PtM (Fe, Co, Ni) surfaces: Cooperating effects of charge transfer, magnetic ordering and lattice strain

Adsorption of a molecule or group with an atom which is less electronegative than oxygen (0) and directly interacting with the surface is very relevant to development of PtM (M = 3d-transition metal) catalysts with high activity. Here, we present theoretical analysis of the adsorption of NH3 molecule (N being less electronegative than 0) on (111) surfaces of PtM (Fe, Co, Ni) alloys using the first principles density functional approach. We find that, while NH3-Pt interaction is stronger than that of NH3 with the elemental M-surfaces, it is weaker than the strength of interaction of NH3 with M-site on the surface of PtM alloy. (C) 2016 Published by Elsevier B.V.

Visualisation of charge-transfer excitations in donor-acceptor molecules using the particle-hole map: a case study

Charge-transfer (CT) excitations are essential for photovoltaic phenomena in organic solar cells. Owing to the complexity of molecular geometries and orbital coupling, a detailed analysis and spatial visualisation of CT processes can be challenging. In this paper, a new detail-oriented visualisation scheme, the particle-hole map (PHM), is applied and explained for the purpose of spatial analysis of excitations in organic molecules. The PHM can be obtained from the output of a time-dependent density-functional theory calculation with negligible additional computational cost, and provides a useful physical picture for understanding the origins and destinations of electrons and holes during an excitation process. As an example, we consider intramolecular CT excitations in Diketopyrrolopyrrole-based molecules, and relate our findings to experimental results.

Metal Auger intensity ratios as a probe for change-transfer effects in aluminum-covered transition metal

Marked changes in the LVV/LMV and LVV/LMM Auger intensity ratios of Co, Ni and Cu are observed on depositing Al on their surfaces. These changes, ascribed to charge-transfer or hybridization effects, are accompanied by changes in the intensity of the satellites next to the core levels of the transition metals.

Synthesis,characterization and photoluminescence properties of CaSiO3 :Dy3+ nanophosphors

CaSiO3 : Dy3+ (1-5 mol. %) nanophosphors were synthesized by a simple low-temperature solution combustion method. Powder X-ray diffraction patterns revealed that the phosphors are crystalline and can be indexed to a monoclinic phase. Scanning electron micrographs exhibited faceted plates and angular crystals of different sizes with a porous nature. Photoluminescence properties of the Dy3+-doped CaSiO3 phosphors were observed and analyzed. Emission peaks at 483, 573 and 610 nm corresponding to Dy3+ were assigned as F-4(9/2)-> H-6(15/2), F-4(9/2) -> H-6(13/2) and F-4(9/2) -> H-6(11/2) transitions, respectively, and dominated by the Dy3+ F-4(9/2) -> H-6(13/2) hyperfine transition. Experimental results revealed that the luminescence intensity was affected by both heat treatment and the concentration of Dy3+ (1-5 mol. %) in the CaSiO3 host. Optimal luminescence conditions were achieved when the concentration of Dy3+ was 2 mol. %. UV-visible absorption features an intense band at 240 nm, which corresponds to an O-Si ligand-to-metal charge transfer band in the SiO32- group. The optical energy band gap for the undoped sample was found to be 5.45 eV, whereas in Dy3+-doped phosphors it varies in the range 5.49-5.65 eV. The optical energy gap widens with increase of Dy3+ ion dopant.

Impedance parameters and the state-of-charge. III. Zinc-manganese dioxide and magnesium-manganese dioxide dry batteries

The problem of non-destructive determination of the state-of-charge of zinc- and magnesium-manganese dioxide dry batteries is examined experimentally from the viewpoint of internal impedance and open-circuit voltage at equilibrium. It is shown that the impedance is mainly charge-transfer controlled at relatively high states-of-charge and progressively changes over to diffusion control as the state-of-charge decreases in the case of zinc-manganese dioxide dry batteries. On the other hand, the impedance is mainly diffusion controlled for undischarged batteries but becomes charge-transfer controlled as soon as there is some discharge in the case of magnesium-manganese dioxide batteries. It is concluded that the determination of state-of-charge is not possible for both types of batteries by the measurement of impedance parameters due to film-induced fluctuations of these parameters. The measurement of open-circuit voltage at equilibrium can be used as a state-of-charge indicator for Zn-MnO2 batteries but not for Mg-MnO2 batteries.

Impedance parameters and the state-of-charge. II. Lead-acid battery

The determination of the state-of-charge of the lead-acid battery has been examined from the viewpoint of internal impedance. It is shown that the impedance is controlled by charge transfer and to a smaller extent by diffusion processes in the frequency range 15–100 Hz. The equivalent series/parallel capacitance as well as the a.c. phase-shift show a parabolic dependence upon the state-of-charge, with a maximum or minimum at 50% charge. These results are explained on the basis of a uniform transmission-line analog equivalent circuit for the battery electrodes.

Quantum and non-Markovian effects in the electron transfer reaction dynamics in the Marcus inverted region

Electron transfer reactions in large molecules may often be coupled to both the polar solvent modes and the intramolecular vibrational modes of the molecule. This can give rise to a complex dynamics which may in some systems, like betaine, be controlled more by vibrational rather than by solvent effects. Additionally, a significant contribution from an ultrafast relaxation component in the solvation dynamics may enhance the complexity. To explain the wide range of behavior that has been observed experimentally, Barbara et al. recently proposed that a model of an electron transfer reaction should minimally consist of a low-frequency classical solvent mode (X), a low-frequency vibrational mode (Q), and a high-frequency quantum mode (q) (J. Phys. Chem. 1991, 96, 3728). In the present work, a theoretical study of this model is described. This study generalizes earlier work by including the biphasic solvent response and the dynamics of the low-frequency vibrational mode in the presence of a delocalized, extended reaction zone. A novel Green's function technique has been developed which allowed us to study the non-Markovian dynamics on a multidimensional surface. The contributions from the high-frequency vibrational mode and the ultrafast component in the non-Markovian solvent dynamics are found to be primarily responsible for the dramatic increase in charge transfer rate over the prediction of the classical theories that neglect both these factors. These, along with a large coupling between the reactant and the product states, may combine to render the electron transfer rate both very large and constant over a wide range of solvent relaxation rates. A study on the free energy gap dependence of the electron transfer rate reveals that the rates are sensitive to changes in the quantum frequency particularly when the free energy gap is very large.

Theory of electron transfer processes via chemisorbed intermediates: Part II. Current-potential characteristics

Current-potential characteristics are obtained numerically for a lone-adsorbate-mediated anodic charge transfer at the electrode-solution interface. An increase in the overpotential leads to the appearance of maxima in the anodic current-potential plots instead of the extended activationless region (i.e. a saturation current at large positive overpotentials) predicted by the direct heterogeneous outer-sphere anodic charge transfer process. A detailed analysis of the dependence of current-potential profiles and other kinetic parameters on various system parameters is also presented.

Time-Resolved Photoluminescence and Microwave Conductivity at Semiconductor Electrodes: Depletion Layer Effects

Analytical expressions which include depletion layer effects on low-injection carrier relaxation are being presented for the first time here. Starting from the continuity equation for the minority carriers, we derive expressions for the output signal pertinent to time-resolved microwave and luminescence experiments. These are valid for the time domain that usually overlaps with the time scales of surface processes, such as charge transfer and trapping. Apart from the usual pulse form of illumination, theoretical expressions pertaining to other forms of illumination such as switch-on and switch-off transient modes, a periodic mode, and a steady state and their various inter-relationships are derived here. The expressions obtained are seen to be generalizations of existing flat-band low-injection results in the Limit of early or initial band bendings. The importance of the depletion layer as an experimental parameter is clearly seen in the limit of larger band bendings wherein it is shown, unlike the flat-band case, to exhibit pure exponential forms of carrier relaxation. Our results are consistent with the main conclusions of the numerical and experimental work published recently. Furthermore, this work provides the actual functional relationships between the applied potential and observed carrier decay. This should enable one to extract the surface kinetic parameters, after deciding on the dominant mode of carrier relaxation at the interface, whether charge transfer or trapping, by studying the potential dependence of the fate of relaxation.

A perspective of biological supramolecular electron transfer

Electron transfer is an essential activity in biological systems. The migrating electron originates from water-oxygen in photosynthesis and reverts to dioxygen in respiration. In this cycle two metal porphyrin complexes possessing circular conjugated system and macrocyclic pi-clouds, chlorophyll and hems, play a decisive role in mobilising electrons for travel over biological structures as extraneous electrons. Transport of electrons within proteins (as in cytochromes) and within DNA (during oxidative damage and repair) is known to occur. Initial evaluations did not favour formation of semiconducting pathways of delocalized electrons of the peptide bonds in proteins and of the bases in nucleic acids. Direct measurement of conductivity of bulk material and quantum chemical calculations of their polymeric structures also did not support electron transfer in both proteins and nucleic acids. New experimental approaches have revived interest in the process of charge transfer through DNA duplex. The fluorescence on photoexcitation of Ru-complex was found to be quenched by Rh-complex, when both were tethered to DNA and intercalated in the base stack. Similar experiments showed that damage to G-bases and repair of T-T dimers in DNA can occur by possible long range electron transfer through the base stack. The novelty of this phenomenon prompted the apt name, chemistry at a distance. Based on experiments with ruthenium modified proteins, intramolecular electron transfer in proteins is now proposed to use pathways that include C-C sigma-bonds and surprisingly hydrogen bonds which remained out of favour for a long time. In support of this, some experimental evidence is now available showing that hydrogen bond-bridges facilitate transfer of electrons between metal-porphyrin complexes. By molecular orbital calculations over 20 years ago. we found that "delocalization of an extraneous electron is pronounced when it enters low-lying virtual orbitals of the electronic structures of peptide units linked by hydrogen bonds". This review focuses on supramolecular electron transfer pathways that can emerge on interlinking by hydrogen bonds and metal coordination of some unnoticed structures with pi-clouds in proteins and nucleic acids, potentially useful in catalysis and energy missions.

Structural, EPR, optical and magnetic properties of alpha-Fe2O3 nanoparticles

alpha-Fe2O3 nanoparticles were synthesized by a low temperature solution combustion method. The structural, magnetic and luminescence properties were studied. Powder X-ray diffraction (PXRD) pattern of alpha-Fe2O3 exhibits pure rhombohedral structure. SEM micrographs reveal the dumbbell shaped particles. The EPR spectrum shows an intense resonance signal at g approximate to 5.61 corresponding to isolated Fe3+ ions situated in axially distorted sites, whereas the g approximate to 2.30 is due to Fe3+ ions coupled by exchange interaction. Raman studies show A(1g) (225 cm(-1)) and E-g (293 and 409 cm(-1)) phonon modes. The absorption at 300 nm results from the ligand to metal charge transfer transitions whereas the 540 nm peak is mainly due to the (6)A(1) + (6)A(1) —> T-4(1)(4G) + T-4(1)(4G) excitation of an Fe3+-Fe3+ pair. A prominent TL glow peak was observed at 140 C at heating rate of 5 degrees C s(-1). The trapping parameters namely activation energy (E), frequency factor (s) and order of kinetics (b) were evaluated and discussed. (C) 2012 Elsevier B.V. All rights reserved.

TiO2/ZnO core/shell nano-heterostructure arrays as photo-electrodes with enhanced visible light photoelectrochemical performance

The present article reports a facile method for preparing the vertically-aligned 1D arrays of a new type of type II n-n TiO2/ZnO core/shell nano-heterostructures by growing the nano-shell of ZnO on the electrochemically fabricated TiO2 nanotubes core for visible light driven photoelectrochemical applications. The strong interfacial interaction at the type II heterojunction leads to an effective interfacial charge separation and charge transport. The presence of various defects such as surface states, interface states and other defects in the nano-heterostructure enable it for improved visible light photoelectrochemical performance. The presence of such defects has also been confirmed by the UV-vis absorption, cathodoluminescence, and crystallographic studies. The TiO2/ZnO core/shell nano-heterostructures exhibit strong green luminescence due to the defect transitions. The TiO2/ZnO core/shell nano-heterostructures photo-electrode show significant enhancement of visible light absorption and it provides a photocurrent density of 0.7 mA cm(-2) at 1 V vs. Ag/AgCl, which is almost 2.7 times that of the TiO2/ZnO core/shell nano-heterostructures under dark conditions. The electrochemical impedance spectroscopy results demonstrate that the substantially improved photoelectrochemical and photo-switching performance of the nano-heterostructures photo-anode is because of the enhancement of interfacial charge transfer and the increase in the charge carrier density caused by the incorporation of the ZnO nano-shell on TiO2 nanotube core.

Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications

As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion-dissolution at extreme pH conditions, together with the formation of inert Zn(OH)(2) during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface-bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface-bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect-rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non-polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico-chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.

On the ultrafast charge migration and subsequent charge directed reactivity in Cl center dot center dot center dot N halogen-bonded clusters following vertical ionization

In this article, we have presented ultrafast charge transfer dynamics through halogen bonds following vertical ionization of representative halogen bonded clusters. Subsequent hole directed reactivity of the radical cations of halogen bonded clusters is also discussed. Furthermore, we have examined effect of the halogen bond strength on the electron-electron correlation-and relaxation-driven charge migration in halogen bonded complexes. For this study, we have selected A-Cl (A represents F, OH, CN, NH2, CF3, and COOH substituents) molecules paired with NH3 (referred as ACl:NH3 complex): these complexes exhibit halogen bonds. To the best of our knowledge, this is the first report on purely electron correlation-and relaxation-driven ultrafast (attosecond) charge migration dynamics through halogen bonds. Both density functional theory and complete active space self-consistent field theory with 6-31+G(d, p) basis set are employed for this work. Upon vertical ionization of NCCl center dot center dot center dot NH3 complex, the hole is predicted to migrate from the NH3-end to the ClCN-end of the NCCl center dot center dot center dot NH3 complex in approximately 0.5 fs on the D-0 cationic surface. This hole migration leads to structural rearrangement of the halogen bonded complex, yielding hydrogen bonding interaction stronger than the halogen bonding interaction on the same cationic surface. Other halogen bonded complexes, such as H2NCl:NH3, F3CCl:NH3, and HOOCCl:NH3, exhibit similar charge migration following vertical ionization. On the contrary, FCl:NH3 and HOCl:NH3 complexes do not exhibit any charge migration following vertical ionization to the D-0 cation state, pointing to interesting halogen bond strength-dependent charge migration. (C) 2015 AIP Publishing LLC.