Suppression of exciton recombination in symmetric GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum wells induced by an in-plane magnetic field

Suppression of the exciton recombination in GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum well (CQW) induced by an external magnetic field is investigated theoretically. Unlike the usual electro-Stark effect, the exciton energy dispersion of an exciton is modified by an external in-plane magnetic field, the ground state of the magnetoexciton shifts from a zero in-plane center of mass (CM) momentum to a finite CM momentum, and the Lorentz force induces the spatial separation of electron and hole. Consequently, this effect renders the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. This effect depends sensitively on the thickness and height of GaAs0.7Sb0.3 layer, therefore it could provide us useful infometion about the band alignment of CQW. (C) 2004 American Institute of Physics.

Structural and optical properties of strain-compensated GaAsSb/GaAs quantum wells with high Sb composition

The structural and optical properties of GaAsSb/GaAs quantum wells (QWs) and strain-compensated GaAsP/GaAs/GaAsSb/GaAs/GaAsP QWs grown on a GaAs substrate by molecular beam epitaxy are investigated using high-resolution x-ray diffraction and photoluminescence (PL) measurements. We demonstrated that the insertion of tensile GaAsP layers into the active region of GaAsSb/GaAs QWs effectively improves the structural and optical quality. Even the Sb composition is as high as 0.39. The PL spectra at 11 K and room temperature indicate that the PL peak of strain-compensated QWs has a narrower linewidth and higher intensity in comparison to the sample without strain compensation. The results of PL peak blueshift with increasing excitation show the strain-compensated GaAsSb/GaAs interface characteristic of type-I band alignment. (C) 2003 American Institute of Physics.

Interband luminescence and absorption of GaNAs/GaAs single-quantum-well structures

We have investigated the interband electron transitions in a GaNAs/GaAs single quantum well (QW) by photoluminescence and absorption spectra. The experimental results show that the dominant photoluminescence at low temperature and high excitation intensity originates from transitions within the GaNAs layer. The interband transition energy for QWs with different well widths can be well fitted if a type-II band line up of GaNAs/GaAs QWs is assumed. (C) 2000 American Institute of Physics. [S0003-6951(00)03220-4].

Optical transitions in GaNAs/GaAs single quantum well

We have investigated GaNAs/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL), time-resolved PL (TRPL) and photovoltaic (PV) techniques. The low temperature PL is dominated by spatially direct transitions involving electrons confined in GaNAs well and holes localized in the same GaNAs layer. This assignment was supported by PL decay time measurements and absorption line-shape analysis derived from the PV measurements. By fitting the experimental data with a simple calculation, the band offset of the GaN0.015As0.985/GaAS heterostructure was estimated, and a type II band lineup in GaN0.015As0.985/GaAs QWs was suggested. Moreover, DeltaE(C), the discontinuity of conductor band, is found to be a nonlinear function of the nitrogen (N) composition (x), and the average variation of DeltaE(C) is about 0.110eV per %N, The measured band bowing coefficient shows a strong function of x, giving an experimental support to the theoretic calculation of Wei et al [Ref.2].

Elucidating the optical properties of novel heterolayered materials based on MoTe2-InN for photovoltaic applications

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Studies of degradation of organic solar cell materials using electrical scanning probe microscopy

Intense research is being done in the field of organic photovoltaics in order to synthesize low band-gap organic molecules. These molecules are electron donors which feature in combination with acceptor molecules, typically fullerene derivarntives, forming an active blend. This active blend has phase separated bicontinuous morphology on a nanometer scale. The highest recorded power conversionrnefficiencies for such cells have been 10.6%. Organic semiconductors differ from inorganic ones due to the presence of tightly bonded excitons (electron-hole pairs)resulting from their low dielectric constant (εr ≈2-4). An additional driving force is required to separate such Frenkel excitons since their binding energy (0.3-1 eV) is too large to be dissociated by an electric field alone. This additional driving force arises from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor and the acceptor materials. Moreover, the efficiency of the cells also depends on the difference between the highest occupied molecular orbital (HOMO) of the donor and LUMO of the acceptor. Therefore, a precise control and estimation of these energy levels are required. Furthermore any external influences that change the energy levels will cause a degradation of the power conversion efficiency of organic solar cell materials. In particular, the role of photo-induced degradation on the morphology and electrical performance is a major contribution to degradation and needs to be understood on a nanometer scale. Scanning Probe Microscopy (SPM) offers the resolution to image the nanometer scale bicontinuous morphology. In addition SPM can be operated to measure the local contact potential difference (CPD) of materials from which energy levels in the materials can be derived. Thus SPM is an unique method for the characterization of surface morphology, potential changes and conductivity changes under operating conditions. In the present work, I describe investigations of organic photovoltaic materials upon photo-oxidation which is one of the major causes of degradation of these solar cell materials. SPM, Nuclear Magnetic Resonance (NMR) and UV-Vis spectroscopy studies allowed me to identify the chemical reactions occurring inside the active layer upon photo-oxidation. From the measured data, it was possible to deduce the energy levels and explain the various shifts which gave a better understanding of the physics of the device. In addition, I was able to quantify the degradation by correlating the local changes in the CPD and conductivity to the device characteristics, i.e., open circuit voltage and short circuit current. Furthermore, time-resolved electrostatic force microscopy (tr-EFM) allowed us to probe dynamic processes like the charging rate of the individual donor and acceptor domains within the active blend. Upon photo-oxidation, it was observed, that the acceptor molecules got oxidized first preventing the donor polymer from degrading. Work functions of electrodes can be tailored by modifying the interface with monomolecular thin layers of molecules which are made by a chemical reaction in liquids. These modifications in the work function are particularly attractive for opto-electronic devices whose performance depends on the band alignment between the electrodes and the active material. In order to measure the shift in work function on a nanometer scale, I used KPFM in situ, which means in liquids, to follow changes in the work function of Au upon hexadecanethiol adsorption from decane. All the above investigations give us a better understanding of the photo-degradation processes of the active material at the nanoscale. Also, a method to compare various new materials used for organic solar cells for stability is proposed which eliminates the requirement to make fully functional devices saving time and additional engineering efforts.

Joseph Gusikow. Das Holz und Stroh Instrument

Signatur des Originals: S 36/G01044

Sigismund Thalberg, Brustbild

Signatur des Originals: S 36/G12378

Geschichte der völkerwanderung,

I. bd. Vorwort zur neuen auflage. Einleitung der neuen auflage. 1. buch. Die Germanen vor der völkerwanderung bis zu dem abzug der Gothen an die Donau und dem Markomannenkrieg. 2. buch. Die zeit vom auftreten der Gothen an der Donau und dem Markomannenkrieg bis zum Hunnen-einfall. Anhang zum I. band: A. Anmerkungen. B. Excurse.--II. bd. 3. buch. Von dem Hunnen-einfall bis zu den letzten bewegungen der völkerwanderung (375-568) Anhang zum II. band: A. Anmerkungen. B. Excurse. Berichtigungen und druckfehler. Alphabetisches sachregister. Quellen-und literatur-uebersicht (von F. Dahn)

Führer durch den Concertsaal

I. Abtheilung; I-II Band: Sinfonie und Suite; II. Abteilung, erster Teil: Kirchliche Werke: Passionen, Messen, Hymnen, Psalmen, Motetten, Kantaten. II. Abtheilung, zweiter Theil: Oratorien und weltliche Chorwerke. Zweite Auflage.

Sämtliche Werke /

Edition statement varies.

"Grundriss der romanischen philologie,

"Namen-, sach-, und wortverzeichnis in jedem band."

Geschichte der mosaischen institutionen und des jüdischen volks von J. Salvador.

"Anhang. Jesus vor Caiphas und Pilatus, widerlegung des kapitels: Prozess und verurtheilung Christi (II. band, buch IV.) von herrn Dupin dem aeltern."--v. 3, p. [327]-438.

Regesten der Pfalzgrafen am Rhein, 1214-1508.

1. Bd. 1214-1400, hrsg. von der Badischen historischen Commission unter Leitung von Eduard Winkelmann, bearb. von Adolf Koch und Jakob Wille.--2. Bd. Regesten König Ruprechts [1400-1410] bearb. von Graf. L. v. Obendorff. Nachträge, Ergänzungen und Berichtigungen zum I. und II. Band; Namen- und Sachregister zum II. Ban, bearb. von Manfred Krebs.

Greengenes, a chimera-checked 16S rRNA gene database and workbench compatible with ARB

A 16S rRNA gene database (http://greengenes.bl.gov) addresses limitations of public repositories by providing chimera screening, standard alignment, and taxonomic classification using multiple published taxonomies. It was found that there is incongruent taxonomic nomenclature among curators even at the phylum level. Putative chimeras were identified in 3% of environmental sequences and in 0.2% of records derived from isolates. Environmental sequences were classified into 100 phylum-level lineages in the Archaea and Bacteria.