Role Of Geminate Recombination In Photovoltaic Spectral Responses Of Polyacetylene Junctions

Photoresponses of Schottky-type polyacetylene junctions are measured and compared with a theoretical model. Evidences for the importance of geminate recombination of photogenerated pairs are presented.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

Epr Study of Polarons In a Conducting Polymer With Nondegen-Erate Ground States: asF5 Complexes of Poly(p-phenylene)

EPR measurements of temperature-dependent susceptibility and linewidth are reported for poly(p-phenylene) as a function of exposure time to dopant. Most of the charge is in bipolarons, rather than in the spin-associated polarons. Observed decreases in high temperature spin concentration as a function of doping time and post-doping anneal likely arise from both polaron-polaron reaction and polaron ionization to form bipolarons. The susceptibility of all the investigated AsF₅-doped samples strongly deviate from Curie-Weiss behavior at low temperatures. The observed temperature dependence can be explained by the low-temperature condensation of isolated polarons to form intermolecular polaron pairs in singlet ground states. The interaction between spins is antiferro-magnetic and the energy of the singlet-state polaron pair relative to two isolated polarons (-2.4 to -4.8 meV) is similar to that for the n-doped polymer (-2.4 to -2.7 meV).     

Double Injection and Electroluminescence in p-Terphenyl Single Crystals

Abstract

Double injection currents in p-terphenyl are followed by light emission from decaying singlet excitons produced by the recombining electron-hole pairs. Only one singlet exciton is produced out of ～ 100 recombinations, the other recombinations are non-radiative. The current densities with gold and Na-Hg amalgam as hole and electron injecting contacts, respectively, approached the limit of volume-controlled currents, but were limited to well defined spots covering only 10 ³-10 ⁴ of the contacts area.

This work is a study of double injection currents and recombination radiation in p-terphenyl. Gold and Na-Hg amalgam contacts were used as hole and electron injectors, but efficient injection took place in only 10^?3-10^?4 of the contacts area. In that area the current densities approached the limit of volume-controlled currents. The recombination radiation has the same spectral distribution as the ordinary crystal fluorescence, both result from the radiative decay of singlet excitons. The major proportion of electron-hole recombinations is non-radiative. Only about one singlet exciton is produced per about 100 recombination events.     

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20 K transits to the power law decay due to the electron-hopping at localized band tail states near 60 K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2 K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~ 1 ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~ 1 μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.     

Ultrafast Optical Response in Quasi-One Dimensional Halogen-Bridged Mixed-Valence Complexes [Pt(en)2][PtBr2(en)2](ClO4)4 and [Pd(en)2][PdCl2(en)2](ClO4)4

Abstract

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t ^β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the reflection spectrum by the Kramers-Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 eV to 2.5 eV were calculated to be described with three components. They correspond to free excitons with lifetime of about 800 fs, self-trapped excitons with lifetime of about 3 ps, and polaron pairs which hardly relax within 100 ps.     

EPR Studies of Electron Spin Polarization of Triplet Exciton and its Transfer into Triplet Traps of Naphthalene-TCNQ and Naphthalene-pChloranil Guest Complexes in the Naphthalene-TCNB Host

The EPR spectra of triplet excitons in single crystals of Naphthalene-TCNB (N-TCNB) charge-transfer (CT) complexes show optical electron spin polarization (OEP) which is accounted for by a spin selective populating process from the excited CT singlet into an excited triplet state localized on the acceptor. OEP is also observed for triplet traps in N-TCNB crystals doped with TCNQ and pChloranil. The analysis of the orientation dependence of spin polarization allows one to establish that also selective decay from the trap spin sublevels is important for the onset of the observed OEP features.     

Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Electron-paramagnetic resonance and photoluminescence study of Si nanocrystals-photosensitizers of singlet oxygen molecules

Si nanocrystals formed by electrochemical porosifying of c-Si wafers are investigated by means of the electron-paramagnetic resonance (EPR) and photoluminescence (PL) techniques. The PL spectra and transients give evidence of the photosensitization of singlet oxygen molecules by the energy transfer from excitons confined in Si nanocrystals to oxygen molecules adsorbed on the nanocrystal surfaces. The EPR experiments show that the singlet oxygen generation is accompanied by a slowing down of the spin-spin relaxation time of Si dangling bonds on the nanocrystal surfaces. This effect is explained by the transition of a large part of the adsorbed O₂ molecules in their ground (triplet) states to the excited (singlet) ones. The EPR data allow us to estimate the concentration of the photosensitized singlet oxygen molecules to be on the order of 10¹⁸ cm⁻³.     

Current-induced magnetic switching and dynamics in spin valves

Spin torque and current-induced magnetic switching (dynamics) in spin valve nanopillars is considered theoretically in the diffusive transport regime. Basic characteristics of the precessional states are discussed. Current-induced switching in other spin valves is analyzed, particularly in ferromagnetic single-electron transistors based on magnetic molecules.     

Role of Spin States of Polymethine Dyes in the Generation of Electron-Hole Pairs

The spectral and luminescent properties of film composites based on photoconductive poly-N-epoxypropylcarbazole (PEPC) and electrically neutral polyvinylbutyral (PVB) with admixtures of polymethine dyes with various ionicities are studied. The external magnetic field effect on the spectral and luminescent properties of film composites based on PEPC with cationic polymethine and merocyanine dyes is studied. The magnetic field effect in the photogeneration of electron-hole pairs is explained by the participation of the singlet–triplet intersystem crossing of excited dye molecules.     

The temperature dependence f photoconductivity in a-Si

The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ?_A is situated about 0.18 eV below ge_C in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ?_V. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ?_A. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.     

Charge and spin topological insulators

The topologically nontrivial states of matter—charge and spin topological insulators, which exhibit, respectively, properties of the integer quantum Hall effect and the quantum spin Hall effect—are discussed. The topological characteristics (invariant with respect to weak adiabatic changes in the Hamiltonian parameters) which lead to such states are considered. The model of a 2D hexagonal lattice having symmetries broken with respect to time reversal and spatial inversion which was proposed by Haldane and marked the beginning of unprecedented activity in the study of topologically nontrivial states is discussed. This model relates the microscopic nature of the symmetry breaking with respect to the time reversal to the occurrence of spontaneous orbital currents which circulate within a unit cell. Such currents become zero upon summation over the unit cell, but they may form spreading current states at the surface which are similar to the edge current states under the quantum Hall effect. The first model of spontaneous currents (exciton insulator model) is considered, and the possibility of implementing new topologically nontrivial states in this model is discussed.     

Fractionally Charged Excitations in Quasi-One-Dimensional Systems

Recent theoretical studies have shown that in quasi-one-dimensional conductors having a Peierls distortion of commensurability index n (ratio of the distortion period to lattice specing), there exist excitations whose charge is Q = ω 2e/n or ω 2e/n ± e. These excitations are kinxs in the order parameter ψ describing the lattice distortion. In trans polyacetylene, n = 2 and Q = O, ± e with spin S = 1/2, O respectively. For n = 3 (e.g. TTF-TCNQ at 19Kb) Q = ± 2/3 e, ± 1/3 e, ω 4/3 e with spin S = O, 1/2, O respectively. Electronic states localized at the kink have energies in the Peierls gaps. Properties of these stable fractionally chareged objects are discussed.     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

Femtosecond transmission studies of n-type and p-type hydrogenated amorphous silicon pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on n-type and p-type hydrogenated amorphous silicon are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different materials are quite different, allowing the technique to clearly distinguish between them. It is shown that while recombination is dominant in intrinsic a-Si:H, trapping becomes important in the doped material. A model of the experimental results gives additional insight into the density of states in the band tail region.     

六方Fe2Ge合金块体及其(001)表面电子结构和磁性的第一性原理

采用基于密度泛函理论第一性原理的赝势平面波方法,计算了块体Fe₂Ge及其(001)表面的电子结构和磁性。考虑了两种类型的终端(001)表面:Ge(Ⅰ)-(001)表面和Ge(Ⅱ)-(001)表面。电子结构方面,不同类型的Fe₂Ge(001)表面都表现出金属特性,这与块体的金属性保持一致。通过计算它们的自旋极化率,得出Ge(Ⅰ)-(001)表面的自旋极化程度最高。磁性方面,在块体和Ge(Ⅱ)-(001)表面的Ge原子是铁磁自旋有序的,而在Ge(Ⅰ)-(001)表面第一层的Ge原子是亚铁磁自旋有序的。此外,Ge(Ⅱ)-(001)表面Ge原子的自旋磁矩优于块体中和Ge(Ⅰ)-(001)表面Ge原子的自旋磁矩。这些结果与Fe的d态和Ge的p态电子的杂化有关,本文中通过分析它们的态密度进行了讨论。     

MnGa合金体的电子结构与磁性质的研究

基于密度泛函理论方法系统研究了四方结构MnGa合金体的结构、形成、电子结构和磁性质。结果表明,四方MnGa合金晶胞的生成焓为-4.85 eV,高于一些不含d电子的体系。其呈现导体的能带结构,其中d电子主要形成深能级价带,定域性最强。四方MnGa合金存在着明显的自旋极化,靠近费米能级两侧的s电子和靠近费米能级下方的p电子具有较弱的自旋极化。形成浅能级价带和导带的d电子产生高强度的自旋极化,对磁性质贡献较大。Mn的s电子和Mn的p电子自旋极化作用较弱,Mn的d电子形成浅能级价带和导带,自旋极化作用最强。形成深能级价带的Ga的d电子自旋极化作用较弱,不同位置的Ga原子的自旋极化不同。四方MnGa合金体具有净有效磁矩,呈弱的亚铁磁性。