Optical properties of multi-layer type II InP/GaAs quantum dots studied by surface photovoltage spectroscopy

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details. (C) 2011 American Institute of Physics. [doi:10.1063/1.3638705]1106Bulgarian National Science Fund [D01-463/12.7.06]Alexander von Humboldt FoundationCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Bulgarian National Science Fund [D01-463/12.7.06

Average and extreme multi-atom Van der Waals interactions: Strong coupling of multi-atom Van der Waals interactions with covalent bonding

<p>Abstract</p> <p>Background</p> <p>The prediction of ligand binding or protein structure requires very accurate force field potentials – even small errors in force field potentials can make a 'wrong' structure (from the billions possible) more stable than the single, 'correct' one. However, despite huge efforts to optimize them, currently-used all-atom force fields are still not able, in a vast majority of cases, even to keep a protein molecule in its native conformation in the course of molecular dynamics simulations or to bring an approximate, homology-based model of protein structure closer to its native conformation.</p> <p>Results</p> <p>A strict analysis shows that a specific coupling of multi-atom Van der Waals interactions with covalent bonding can, in extreme cases, increase (or decrease) the interaction energy by about 20–40% at certain angles between the direction of interaction and the covalent bond. It is also shown that on average multi-body effects decrease the total Van der Waals energy in proportion to the square root of the electronic component of dielectric permittivity corresponding to dipole-dipole interactions at small distances, where Van der Waals interactions take place.</p> <p>Conclusion</p> <p>The study shows that currently-ignored multi-atom Van der Waals interactions can, in certain instances, lead to significant energy effects, comparable to those caused by the replacement of atoms (for instance, C by N) in conventional pairwise Van der Waals interactions.</p

Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (&lt;2.0 eV) and super-band-gap (&gt;2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model. © 2013 American Chemical Society

Filippov-Pliss lemma for dynamical inclusions on a time scale

Abstract In this paper we prove two variants of the well-known Filippov-Pliss lemma in the case of dynamical inclusions on a time scale. The first variant is when the right-hand side is Lipschitz continuous on the state variable. Afterward we introduce one-sided Perron conditions for multifunctions on a time scale and prove the second variant of that lemma. Some discussions on relaxed systems are presented

Optical determination of interface roughness in multilayered semiconductor structures

We have developed a new approach (the LQR method) for calculating the reflectivity and transmission spectra of a multilayer optical material with N interfaces, as an alternative to the matrix method. The approach allows the inclusion of the effects of interface roughness by introducing a ''rough'' element between adjacent layers. For this purpose we have developed an empirical model, which describes the effect of interface roughness on an optical beam passing through or being reflected from an interface. An assessment of the interface roughness of a multilayer structure was carried out by fitting the experimental reflectivity spectrum of GaAs/AlGaAs multiple quantum well samples with and without oxidation of the barrier layers. The refractive index and the thickness of the oxidized layers were also obtained from the fit

Surface photovoltage and photoluminescence spectroscopy of self-assembled InAs/InP quantum wires

The optical properties of InAs/InP multi-layer quantum wire (QWR) structures of various spacer thicknesses have been investigated by means of room temperature surface photovoltage and photoluminescence spectroscopy. Combined with empirical tight binding calculations, the spectra have revealed transitions assigned to QWR families with heights equal to integer number of 5, 6 and 7 monolayers. From the comparison of the experimental and theoretical results the atomic concentration of phosphorus in the wires has been estimated.This work was supported by the Sofia University research fund (contract 99/2009), the Bulgarian National Science Fund (contract D01-463/12.07.06), the Alexander von Humboldt Foundation, the Ministry of Education and Science of Spain (Project No. MAT2006-01825, FEDER), and the Generalitat Valenciana.Peer reviewe

Fourteen papers translated from the russian

Contains topics in such areas as representation theory, mathematical physics, Lie groups, differential equations, and random processe