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Anodes derived from SnO2 offer a greater specific capacity comparative to graphitic carbon in lithium-ion batteries (LIBs); hence, it is imperative to find a simple but effective approach for the fabrication of SnO2 . The intelligent surfacing of transition metal oxides is one of the favorite strategies to dramatically boost cycling efficiency, and currently most work is primarily aimed at coating and/or compositing with carbon-based materials. Such coating materials, however, face major challenges, including tedious processing and low capacity. This study successfully reports a new and simple WO3 coating to produce a core-shell structure on the surface of SnO2 . The empty space permitted natural expansion for the SnO2 nanostructures, retaining a higher specific capacity for over 100 cycles that did not appear in the pristine SnO2 without WO3 shell. Using WO3 -protected SnO2 nanoparticles as anode, a coin half-cell battery was designed with Li-foil as counter-electrode. Furthermore, the anode was paired with commercial LiFePO4 as cathode for a coin-type full cell and tested for lithium storage performance. The WO3 shell proved to be an effective and strong enhancer for both current rate and specific capacity of SnO2 nanoarchitectures; additionally, an enhancement of cyclic stability was achieved. The findings demonstrate that the WO3 can be used for the improvement of cyclic characteristics of other metal oxide materials as a new coating material.

A new method for electropolymerization of polythiophenes as anode buffer layer in combination with a PEDOT:PSS buffer in P3HT–PCBM solar cells is introduced. This approach uses a stack of PEDOT:PSS and electropolymerized polythiophene (ePT) as an anode buffer layer. A significant improvement in the short circuit current is seen in solar cells with additional thin buffer layers of ePT grown on top of PEDOT:PSS. The cells with ePT buffer layer grown with constant potential mode compared to solar cells without additional ePT buffer layer have an 18.7% increase in power conversion efficiency. However, the cells with ePT grown with cyclic potential mode result in an additional enhancement in short circuit current with a further increase of 11.7% in power conversion efficiency. This improvement is attributed to better quality of the buffer layer obtained by cyclic potential electropolymerization technique with respect to film quality and area coverage.

A model is presented that describes the gate-voltage-dependent contact resistance and channel-length-dependent charge carrier mobility of small-molecule-based organic thin-film transistors in top and bottom drain/source contact configuration.

To achieve high performance organic field effect transistors (OFETs) with enhanced currents, high switching speeds and improved integration density the channel length L is downscaled to the sub-micrometer regime. The influence of important scaling parameters on the electrical performance of bottom contact devices is analyzed. High-mobility oligo- and polythiophenes were used as semiconductor, which combine the advantages of highly ordered growth with processibility from solution. Devices with optimized parameters exhibit high switching frequencies beyond 2 MHz. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface structures of two model systems in the field of II-VI semiconductors – the ZnSe(001)-c(2×2) and the BeTe(001)-2×1 surface – are derived from spot-profile analysis low-energy electron diffraction and surface X-ray diffraction (SXRD). For the first, a Zn-vacancy model could be unambiguously confirmed by the comparison of our SXRD data with density functional theory calculations. The BeTe surface consists of [10]-oriented Te dimers inducing a large amount of surface strain. This causes pronounced vertical and lateral atomic displacements in the underlying BeTe bilayers which effectively relieve the surface strain. The experimentally derived atomic geometry is in very good agreement with the results of density functional theory calculations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the lattice dynamics of abrupt and intermixed interfaces of II-VI-layers and quantum dots, studied by Raman spectroscopy from embedded CdSe monolayers in BeTe, and from CdSe quantum dots in ZnSe. It is demonstrated that the best CdSe/BeTe interface quality is achieved, when CdTe interface bonds are applied. The corresponding vibration modes are described very well by density functional calculations, and they also reveal the onset of CdSe quantum dot formation. For these interfaces, the reduced symmetry (i.e. non-equivalence of [011] and [01-1] in-plane axes) is analyzed from the polarization dependent Raman scattering intensity. Similar results are presented for Raman scattering from Te surface dimers on BeTe(100). Furthermore, the (s,p)-d exchange energy of diluted magnetic II-VI semiconductors is studied by spin flip Raman spectroscopy. For n-doped (Zn,Mn)Se, a doping-induced reduction of the exchange energy and enhancement of the spin relaxation time T is demonstrated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate the potential of resonant Raman spectroscopy from vibration modes for the analysis of embedded monolayers and quantum dots. As representative systems, we present results for CdSe, embedded either in BeTe or in ZnSe. The vibration spectra reflect a distinctly different behaviour. For CdSe in BeTe, they reveal atomically sharp interfaces, especially when the interface bonds are chosen to be Cd-Te. In addition, the vibration modes reflect the onset of quantum dot formation. They are described by ab initio calculations. In contrast, in case of CdSe quantum dots in ZnSe, the vibration spectra are explained in terms of ternary (Cd,Zn)Se modes with increasing Cd content for increasing nominal CdSe coverage. Thus, they reveal a noticeable interfacial intermixing on the scale of a few monolayers.

Resonante Raman-Spektroskopie analysiert anorganische und organische Schichten bis in kleinste Bedeckungsbereiche und empfiehlt sich als Untersuchungsmethode in der technisch immer wichtigeren Funktionalisierung von Oberflachen.

The structural and electronic properties of semiconductor interfaces crucially influence the behaviour of modern nanoscale heterostructures. We apply optical spectroscopy (Raman spectroscopy and electroreflectance (ER)) with special reference to the interface-induced vibronic signature and electronic interband transitions in wide-band gap II–VI materials. This material system allows the combination of materials with extremely different vibronic and electronic properties, which have to be matched at the interfaces. As examples we present the analysis of ZnSe/BeTe superlattices (SL) and CdSe monolayers, embedded between BeTe barriers, grown on GaAs(100) substrates. Both systems exhibit type II electronic band alignment with a rather high valence band offset of about 1 eV and an electron confinement potential in the range of 2 eV. Since these II–VI compounds contain no common elements, two types of interface bond configurations are possible: either Be–Se or Zn–Te (Cd–Te). We show that the properties of the type-II spatially indirect transition are completely governed by this interface layer. The normal and inverted electronic interface signature can be addressed individually in ER by applying an external dc electric field or by using polarised light with polarisation either along the [110] or [10] direction. In addition the chemical nature of the interface is analysed, using its phonon properties in Raman spectroscopy: They are completely altered for different interface bonding conditions in the case of embedded monolayers. Moreover, the ER results from the epilayers are also used for the deduction of the band offset at the interface substrate–epilayer and Schottky barrier of the outer interfaces (epilayer–metal).

The lattice dynamical properties of cubic CdS/ZnS(e) and hexagonal CdSe/CdS superlattices are investigated by means of Raman and FTIR spectroscopy. Due to the good lateral and in-depth homogeneity of the layer thicknesses folded longitudinal acoustic phonons are observed for all three superlattice systems. The determined frequencies agree quite well with calculations based on the Rytov model. For the optical phonons strain-induced energy shifts become important. Besides, we observe higher order confined longitudinal optical phonons in CdS/ZnSe superlattices. Our experimental results are compared with calculations based on a linear-chain model showing that in this AB/CD type superlattice real interface modes can be observed.

We have determined the dielectric function (DF) of beryllium chalcogenides between 2.5 and 25 eV using an ellipsometer operating with synchrotron radiation in the VUV range. The VUV ellipsometer is, in particular, useful for the characterization of wide band gap semiconductors since the spectral range of spectrometers equipped with conventional light sources is limited to below 6 eV. Beryllium chalcogenides are a relatively new class of material with still unknown optical and electronic properties. Here we determine the dielectric function of BeTe and BexZn1—xSe for the full composition range (0 ≤ x ≤ 1). We verify that they are wide band gap semiconductors with their direct band gaps at 4.2 eV (BeTe) and 5.55 eV (BeSe).

The dispersion curves for phonons propagating along the trigonal axis of the layer-structure emiconductor Bi2Te3 at 77 K have been determined by coherent inelastic neutron scattering techniques. Analysis of these results in terms of a simple interplanar force model shows a) that the forces between adjacent planes of Te atoms are comparable to those between nearest neighbour planes of Bi and Te and b) that foreces between next-nearest-neighbour planes have to be considered. In addition, a limited number of phonons were measured at 300 K and some low-frequency modes propagating along the bisectrix within the planes were determind. The results are compared, where possible, with previous Raman scattering, infrared reflection, and elastic constant measurements.

The antiferromagnetic resonance (AFMR) in NiO doped with either Fe2+ or Co2+ was studied at liquid helium temperatures in the far infrared. A strong frequency shift of the AFMR line was observed in Co2+-doped crystals. In the case of the Fe2+-doped crystals, this shift was much smaller. In addition, a second absorption line was found at larger Co2+ concentrations (5% to 7%), the nature of which has not yet definitely been established. A simple model calculation which primarily employs the single ion anisotropies of the Co2+ and Fe2+ ions was performed for a qualitative interpretation of the experimental results. Die antiferromagnetische Resonanz (AFMR) wurde untersucht bei Temperaturen des flussigen Heliums an NiO, dotiert mit Fe2+ oder Co2+, im fernen Infrarot. Fur die mit Co2+ dotierten Kristalle wurde eine starke Verschiebung der AFMR-Linie beobachtet. Im Falle der mit Fe2+ dotierten Kristalle war die Verschiebung deutlich kleiner. Bei grosen Co2+-Konzentrationen (5% bis 7%) wurde eine zusatzliche Absorptionslinie gefunden, deren Ursache bislang noch nicht restlos geklart wurde. Einfache Modellbetrachtungen, die wesentlich den Einflus der Einzelionenanisotropien der Co2+- oder Fe2+-Ionen beruck-sichtigen, dienen zu einer mehr qualitativen Deutung der Mesergebnisse.

The lattice absorption of I2 single crystals was measured with polarized radiation from 20 to 300 cm−1 at 110 and 300 K. Besides the absorption of the ir-active phonons there is considerable absorption situated near the inactive internal vibration of the molecule. This absorption is due to two-phonon processes involving an internal vibrational mode. The corresponding selection rules were deduced. The lattice absorption persists in the liquid phase near the melting point. However, it is altered by strong absorption of the internal vibration. An Jodeinkristallen wurde mit polarisierter Strahlung die Absorption zwischen 20 und 300 cm−1 bei 110 und 300 K gemessen. Auser der Absorption durch die beiden ir-aktiven Phononen des Kristalls treten im Bereich der inaktiven inneren Schwingung des Molekiils starke Absorptionsbanden auf. Es handelt sich dabei urn Zwei-Phonon-Prozesse zwischen der inneren Schwingung und auseren Gitterschwingungen. Dazu werden die entsprechen-den Auswahlregeln angegeben. Die Gitterabsorption wird auch in flussigem Jod nahe am Schmelzpunkt beobachtet. Sie wird jedoch im Gebiet der inneren Schwingung durch deren starke Absorption abgeandert.