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Thermoelectric materials have the ability to transform thermal energy into electrical energy and vice versa. For an extended period of time, only inorganic materials have been considered for thermoelectric research and application, but recent advances in the field of organic semiconductors established the basis for organic thermoelectrics research. Pristine organic semiconductors often face the problem of undesirable suboptimal transport properties and, therefore, recent approaches have focused on combining different materials, where both organic–organic and organic–inorganic hybrid systems have led to improved efficiency in organic thermoelectrics. This review aims to provide a general overview on the recent advances for organic-based thermoelectrics with an emphasis on the most thoroughly investigated material classes and the approaches employed to control their thermoelectric transport properties. [ABSTRACT FROM AUTHOR]

We report on a systematic study of electronic transport in tetracene single crystals by means of space charge limited current spectroscopy and time of flight measurements. Both I–V and time of flight measurements show that the room-temperature effective hole mobility reaches values close to μ≃1 cm[sup 2]/V s and that, within a range of temperatures, the mobility increases with decreasing temperature. The experimental results further allow the characterization of different aspects of the tetracene crystals. In particular, the effects of both deep and shallow traps are clearly visible and can be used to estimate their densities and characteristic energies. The results presented in this article show that the combination of I–V measurements and time of flight spectroscopy is very effective in characterizing several different aspects of electronic transport through organic crystals. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nanometer sized Au clusters deposited on a silicon substrate forming Au–SiO[sub 2]–Si structure are important for the development of contacts in nanotechnology. Systematic x-ray reflectivity, scanning probe microscopy, and scanning tunneling spectroscopy measurements were done to understand the relationship between morphology and electrical transport properties of this nanostructural metal–insulator–semiconductor system. The presence of an interfacial layer at the metal–insulator interface dictates the tunneling current through this structure and exhibits a gap leading to a suppression of current. Local density of states and electron density/thickness of the interfacial layer have been extracted from the measurements to understand the evolution of metallicity of this Au–SiO[sub 2]–Si structure. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We analyze the impact of the first order structural phase transition between the α- and β-phases in crystalline zinc-phthalocyanine thin films on their luminescent behavior. Upon optical excitation, two intensity maxima governing the dual fluorescence can be attributed to thin film excimer emission of the α-phase at 930 nm and bulk Frenkel exciton emission of the corresponding β-phase at 780 nm. Comprehensive temperature and time dependent studies on the structural phase transition reveal a controllable shift of spectral weight between these two maxima. Applying this approach to thin film devices, we demonstrate the preparation of dual luminescent organic light emitting diodes with tunable emission characteristics in the near infrared, which are of interest as electro-optical units in future hybrid photonic devices. [ABSTRACT FROM AUTHOR]

Presents a study that examined ferromagnetic resonance measurements on radio frequency sputtered cobalt-manganese multilayers. Use of ferromagnetic resonance and x-ray scattering; Background on thin and ultrathin magnetic layers.

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces. [ABSTRACT FROM AUTHOR]

In this work, we address the microscopic effects related to the implementation of a bathophenanthroline (BPhen) exciton blocking layer (EBL) sandwiched between Ag cathode and molecular diindenoperylene (DIP)/C60 bilayer of a photovoltaic cell. Complementary studies of current density, external quantum efficiency, and photoluminescence quenching for EBL thicknesses up to 50 nm indicate that Ag atoms are able to penetrate through the whole 35 nm thick C60 film into the polycrystalline DIP layer underneath, thereby enhancing exciton quenching if no blocking layer is applied. In contrast, an optimal trade-off between exciton blocking, suppression of metal penetration, and electron transport is achieved for a 5 nm thick BPhen layer yielding an improvement of power conversion efficiency by more than a factor of 2. [ABSTRACT FROM AUTHOR]

Ferromagnetic resonance (FMR) is performed together with superconducting quantum interference device and Faraday magnetometry to investigate the magnetic anisotropy of epitaxial bcc Fe(001) films on Ag(001) and Cr(001). The Fe/Ag(001) and Fe/Cr(001) layers as well as corresponding wedge structures were grown by molecular-beam epitaxy. On these samples the following investigations were done. Firstly the temperature dependence of the surface anisotropy of the Fe/Ag system was determined. To achieve high precise values of the surface anisotropy a locally resolved FMR technique was applied to an Fe wedge film coated in Ag layers. Secondly the influence of strong deformation of the Fe layer on the magnetic properties was investigated by highdose Fe-ion implantation in thermally immiscible Fe/Ag(001) layer structures. FMR measurements have shown a pronounced tetragonal symmetry of the anisotropy, which can be precisely controlled by implantation conditions. Thirdly in Fe/Cr single-crystal layers the magnetic parameters of the Fe films show an anomalous temperature behaviour for a distinct Cr thickness range that can be related to the spin-flip transition. The transition temperature decreases continuously with decreasing Cr thickness, ceasing in a suppression of the spin flip for Cr thicknesses below 60A. Below the transition temperature an increase in the dynamic contribution to the FMR linewidth occurs which depends on the Cr thickness. The temperature variation in the dynamic line broadening is explained by a band theoretical description which agrees qualitatively with the temperature variation in the magnetic Fe moment. [ABSTRACT FROM AUTHOR]

We report on crystalline pyramidal structures grown via self-organization on the rubrene (001) surface. The analysis of their spectral response by means of photoluminescence with micrometer lateral resolution reveals an intensity enhancement on-top of the surface structures. As we demonstrate this intensity increase can be related to the excitation processes at the molecular level in combination with exciton confinement within the pyramids. [ABSTRACT FROM AUTHOR]

The photovoltaic behavior of Schottky barrier devices consisting of a single diindenoperylene (DIP) layer sandwiched between an indium tin oxide and Ag electrode has been investigated. Correlating the spectral dependence of the photocurrent and the absorption coefficient, we estimated the exciton diffusion length in DIP to ∼100 nm along the c′ direction. X-ray structural analysis yielded this length to be in agreement with the average crystallite size, thereby, revealing domain boundaries to be the limiting effect on the exciton transport. The corresponding exciton diffusion constant of 5×10-3 cm2/s resembles that of highly ordered single crystals of polyaromatic hydrocarbons. [ABSTRACT FROM AUTHOR]

Diindenoperylene (DIP) single crystals were grown by vapor phase sublimation growth technique, yielding crystals with preferential growth in the ab crystallographic plane. Time-of-flight (TOF) and x-ray measurements were performed over the temperature range between 300 and 405 K. By TOF the mobility for electrons is found to be higher by one order of magnitude than for holes over the studied temperature range. The temperature dependent mobility behavior indicates the effect of structural phase transition in accordance with temperature dependent x-ray measurements. Observation of electron and hole transport in DIP single crystals makes this compound interesting for ambipolar organic electronics. [ABSTRACT FROM AUTHOR]

A universal approach is described for preparing single crystals of organic materials that can only be sublimed, in this case tetracene, with extensions of several millimeters along all three dimensions. It is observed that crystals grown by this technique are more brittle than crystals grown by conventional sublimation methods. The mechanical properties are found to be governed by cracks oriented along (001) basal planes. By Laue diffraction it is revealed that the crystal is composed of subgrains. These grains are separated by small angle tilt boundaries that cause visible traces along the [100] direction on (001) fracture planes. The crack formation during growth is explained to result from tilt boundaries terminating within the crystal. [ABSTRACT FROM AUTHOR]

The hole mobilities in a vapor-Bridgman-grown crystal and in sublimation-grown crystals of tetracene were measured along the c*-direction in a temperature range from 300 to 450 K by the time-of-flight method. The mobility was found to increase as a function of temperature in the vapor-Bridgman-grown crystal, indicating that the transport was thermally activated. After reaching its maximum of about 1 cm2/V s at 320 K, the mobility in the sublimation-grown crystals decreased with temperature in accordance with band transport models. Gas chromatography measurements indicated that the difference in the electronic transport properties could be related to a higher concentration of chemical impurities in the vapor-Bridgman crystal. [ABSTRACT FROM AUTHOR]

The metallization of organic thin films is a crucial point in the development of molecular electronics. However, there is no method established yet to detect trace amounts of metal atoms in those thin films. Radiotracer measurements can quantify even very small amounts of material penetrating into the bulk, in our case less than 0.01% of a monolayer. Here, the application of this technique on two different well-characterized organic thin film systems (diindenoperylene and pentacene) is demonstrated. The results show that Ag is mainly adsorbed on the surface, but indicate that already at moderate deposition temperatures Ag can penetrate into the organic thin films and agglomerate at the film/substrate interface. [ABSTRACT FROM AUTHOR]

We have developed coherent two-dimensional (2D) fluorescence micro-spectroscopy which probes the nonlinear optical response at surfaces via fluorescence detection with sub-micron spatial resolution. This enables the investigation of microscopic variations in laterally heterogeneous film samples which are of interests for sub-wavelength opto-electronic devices.

Studies the change of the thickness dependent anisotropies of high dose ion-implanted Ag/Fe/Ag-(001)-layers by ferromagnetic resonance (FMR). Significance of ion implantation of thin multilayered films; Determination of magnetic anisotropies; Discussion on FMR-spectra and the angle dependence of the resonance fields.

Studies the aspects of the intrinsic spin structure of chromium (Cr) single layers via the magnetic parameters of thin epitaxial iron (Fe) layers grown on top of the Cr films. Information on the intrinsic structure of Cr; Details on Ferromagnetic Resonance (FMR) experiments and superconducting quantum interference devices (SQUID) magnetometry; Angular dependence of the FMR resonance line position.

We present an investigation of the microscopic interplay between excitons and charge carriers by means of combined photoluminescence (PL) and charge carrier transport measurements on organic thin film transistors (OTFTs). For this purpose, the prototypical organic semiconductor diindenoperylene was utilized as the active material. The OTFT accumulation layer provides a spatially defined interaction zone for charges and photogenerated excitons leading to a PL intensity reduction of up to 4.5%. This effect correlates with the accumulated hole carrier density and provides a lower estimate of about 1.1 × 10-10 cm³/s for the recombination rate of nonradiative exciton-hole processes. It is rationalized that these processes are preferentially mediated by trapped holes. [ABSTRACT FROM AUTHOR]

Abstract: We report on a novel type of photoemission detector, the Angle Resolved Time Of Flight electron energy analyzer (ARTOF 10k), which enables electronic band structure determination under measurement conditions that are ideal for radiation-sensitive samples. This is facilitated through the combination of very high electron transmission and wide accessible angular range in one geometry. These properties make the ARTOF 10k predestined to investigate specimens that strongly suffer from radiation damage during photoemission experiments under “standard” conditions, such as organic single crystals, as extremely low fluxes can be used while not compromising spectra accumulation times and signal-to-noise ratio. Even though organic single crystals are of increasing fundamental and applied scientific interest, knowledge of their electronic properties is still largely based on theoretical calculations due to major experimental challenges in measuring photoemission. In this work we show that the band structures of rubrene and tetracene single crystals can be obtained with unprecedented quality using the ARTOF 10k detector. The dispersion of the highest occupied band in rubrene is confirmed in accordance with an earlier report and we disclose the absence of notable dispersion for the highest occupied energy level on the surface of tetracene single crystals. [Copyright &y& Elsevier]