Second-harmonic generation spectroscopy on?organic?nanofibers

Optical second-harmonic generation upon nanosecond laser irradiation of nanofibers grown from nonsymmetrically functionalized para-quarter-phenylene molecules on mica surfaces is investigated in the spectral region around electronic resonances. Strong multiphoton excited luminescence appears together with the second-harmonic signal in the spectra. It is demonstrated how the two components can be separated by analyzing emission spectra recorded over a broad range of wavelengths around the second-harmonic energy. Spectra of the absolute values of the second-harmonic signals are obtained for three differently functionalized molecules. Together with supplementary measurements of the structure of the nanofiber films this forms the basis for estimation of the second-harmonic susceptibility of the fiber materials. Values of the nonlinear susceptibilities are obtained, which are comparable to those of well-known inorganic nonlinear optical crystals.     

Subthreshold regime in rubrene single-crystal organic transistors

Organic field-effect transistors were fabricated with vapor-grown rubrene single crystals in a staggered top-contact configuration. The devices were electrically characterized by measuring the transfer curves at low drain voltage. In parallel to these measurements, a model is developed to account for the subthreshold regime of the transistors. The model is based on the multiple trapping and thermal release concept, which assumes that charge transport is limited by a single level of shallow traps located close to the transport band edge. It is shown that the threshold voltage no longer establishes at the transition between the depletion and accumulation regimes. Instead, the threshold corresponds to the point at which traps are filled. This results in a subthreshold current that varies linearly with gate voltage. Moreover, the subthreshold current at low drain voltages increases with drain voltage. These finding are in good agreement with the experimental data.     

Key role of molecular kinetic energy in early stages of pentacene island growth

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO _x /Si. We show that the kinetic energy of the impinging molecule (E _k ) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing E _k , the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E _k ≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.     

Polaron and ion diffusion in a poly(3-hexylthiophene) thin-film transistor gated with polymer electrolyte dielectric

Electrolytes are finding applications as dielectric materials in low-voltage organic thin-film transistors (OTFT). The presence of mobile ions in these materials (polymer electrolytes or ion gels) gives rise to very high capacitance (>10 μF/cm²) and thus low transistor turn-on voltage. In order to establish fundamental limits in switching speeds of electrolyte gated OFETs, we carry out in situ optical spectroscopy measurement of a poly(3-hexylthiophene) (P3HT) OTFT gated with a LiClO₄:poly(ethyleneoxide) (PEO) dielectric. Based on spectroscopic signatures of molecular vibrations and polaron transitions, we quantitatively determine charge carrier concentration and diffusion constants. We find two distinctively different regions: at V _G≥−1.5 V, drift-diffusion (parallel to the semiconductor/dielectric interface) of hole-polarons in P3HT controls charging of the device; at V _G<−1.5 V, electrochemical doping of the entire P3HT film occurs and charging is controlled by drift/diffusion (perpendicular to the interface) of ClO₄ ⁻ counter ions into the polymer semiconductor.     

The electronic structure of cobalt phthalocyanine

The electronic structure and morphology of organic semiconducting cobalt-phtalocyanine (CoPc) films in situ prepared on the Au(001)-5×20 superstructure have been studied by a combination of experimental and theoretical work. The CoPc molecular film was characterized by photoemission spectroscopy (PES, valence band and core-level). The experimental results were simulated and have been explained in the framework of density functional theory (DFT) calculations. The C 1s and N 1s core level spectra were analyzed by taking into account the fact that both types of atoms have different nonequivalent positions in the molecule. And finally, the experimentally obtained electronic valence band structure of CoPc is in very good agreement with ab initio density of state results, allowing a detailed site-specific insight into the system.     

Electronic properties close to Dirac cone in two-dimensional organic conductor α-(BEDT-TTF)<Subscript>2</Subscript>I<Subscript>3</Subscript>

A zero-gap state (ZGS) has been found in a bulk system of two-dimensional organic conductor, α-(BEDT-TTF)₂I₃ salt which consists of four sites of donor molecules in a unit cell. In the present paper, the characteristic of the ZGS is analyzed in detail and the electronic properties are examined in the vicinity of the Dirac point where the conduction and valence bands degenerate to form the zero-gap. The eigenvectors of the energy band have four components of respective sites, where two of them correspond to inequivalent sites and the other two correspond to equivalent sites. It is shown that the former exhibits an exotic momentum dependence around the contact point and the latter shows almost a constant dependence. The density of states of each site close to the Dirac point is calculated to demonstrate the temperature dependence of the local magnetic susceptibility and the local nuclear magnetic relaxation rate. Further, the robust property of the ZGS against the anion potential is also shown by using the second-order perturbation.     

Collaborate,compete and share

We introduce and study a model of an interacting population of agents who collaborate in groups which compete for limited resources. Groups are formed by random matching agents and their worth is determined by the sum of the efforts deployed by agents in group formation. Agents, on their side, have to share their effort between contributing to their group’s chances to outcompete other groups and resource sharing among partners, when the group is successful. A simple implementation of this strategic interaction gives rise to static and evolutionary properties with a very rich phenomenology. A robust emerging feature is the separation of the population between agents who invest mainly in the success of their group and agents who concentrate in getting the largest share of their group’s profits.     

Role of TiO2 Nanotube on Improvement of Performance of Hybrid Photovoltaic Devices

The performance of TiO₂ nanotubes in bulk heterojunction of poly (2-methoxy-5-(2'-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV)/TiO₂ nanotubes is investigated. The transport properties are studied by using the time-of-flight technique (TOF). The carrier mobilities of both holes and electrons are not improved for the MEH-PPV:TiO₂ composites compared with the pristine MEH-PPV. However, photoluminescence under the influence of the electric field indicates that the dissociation of excitons in the MEH-PPV:TiO₂ composites, which is facilitated by photoinduced charge transfer, only requires a smaller electric field.     

Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3

Blue organic light-emitting devices based on wide bandgap host material, 2-(t-butyl)-9, 10-di-(2-naphthyl) anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO₃) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO₃ (5 nm)/NPB (30 nm)/[TBADN: DSA-Ph (3 wt%)](35 nm)/BPhen (12 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the MoO₃∥BPhen-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA∥Alq₃ is based on, respectively. The J-V curves of ‘hole-only’ devices reveal that a small hole injection barrier between MoO₃∥NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Lamb wave detection of limpet mines on ship hulls

This paper describes the use of ultrasonic guided waves for identifying the mass loading due to underwater limpet mines on ship hulls. The Dynamic Wavelet Fingerprint Technique (DFWT) is used to render the guided wave mode information in two-dimensional binary images because the waveform features of interest are too subtle to identify in time domain. The use of wavelets allows both time and scale features from the original signals to be retained, and image processing can be used to automatically extract features that correspond to the arrival times of the guided wave modes. For further understanding of how the guided wave modes propagate through the real structures, a parallel processing, 3D elastic wave simulation is developed using the finite integration technique (EFIT). This full field, technique models situations that are too complex for analytical solutions, such as built up 3D structures. The simulations have produced informative visualizations of the guided wave modes in the structures as well as mimicking directly the output from sensors placed in the simulation space for direct comparison to experiments. Results from both drydock and in-water experiments with dummy mines are also shown.