Experimental verification of minima in excited long-range Rydberg states of Rb_2

Recent theoretical studies with alkali atoms A$^{\ast}$ excited to high Rydberg states predicted the existence of ultra long-range molecular bound states. Such excited dimers have large electric dipole moments which, in combination with their long radiative lifetimes, make them excellent candidates for manipulation in applications. This letter reports on experimental investigations of the self-broadening of Rb principal series lines, which revealed multiple satellites in the line wings. The positions of the satellites agree quantitatively with theoretically-predicted minima in the excited long-range Rydberg states of Rb$_2$.Comment: 3 figures, 5 pages in two-column forma

Space Charge Limited Transport and Time of Flight Measurements in Tetracene Single Crystals: a Comparative Study

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 $\mu \simeq 1$ cm$^2$/Vs and show 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 paper 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.Comment: Accepted by J. Appl. Phys.; tentatively scheduled for publication in the January 15, 2004 issue; minor revisions compared to previous cond-mat versio

SPR DETECTION OF SINGLE NANO PARTICLES AND VIRUSES

Abstract Here we report a novel method for detection of single individual particles and viruses by means of surface plasmon assisted optical microscopy. The size of the studied objects may be at least one order of magnitude less than the wavelength of the light used for the imaging. This allows studying of nanoparticles and viruses in natural surrounding (enviroment) by means of cheap and well-developed visible-light sources. The signal reflected from the nanoparticle is enhanced due to excitation of the surface plasmon polariton waves. Combination with modern image-processing procedure allows automatic detection of nano-sized objects

Effect of Impurities on Pentacene Thin Film Growth for Field-Effect Transistors

Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material at number densities from 0.001 to 0.474 to quantify the relative effects of impurity content and grain boundary structure on transport in pentacene thin-film transistors. Atomic force microscopy (AFM) and electrical measurements of top-contact pentacene thin-film transistors have been employed to directly correlate initial structure and final film structures, with the device mobility as a function of added impurity content. The results reveal a factor four decrease in mobility without significant changes in film morphology for source PnQ number fractions below ~0.008. For these low concentrations, the impurity thus directly influences transport, either as homogeneously distributed defects or by concentration at the otherwise-unchanged grain boundaries. For larger impurity concentrations, the continuing strong decrease in mobility is correlated with decreasing grain size, indicating an impurity-induced increase in the nucleation of grains during early stages of film growth.Comment: 18 pages, 4 Figures, 1 Tabl

Tetracene - crystal growth and electronic transport

In dieser Arbeit werden die elektronischen Materialeigenschaften des organischen Halbleiters Tetracen untersucht. Für die Untersuchungen wurden Tetracen-Einkristalle durch zwei unterschiedliche Techniken hergestellt. Plattenförmige Kristalle mit Flächen bis 1 cm^2 wurden unter konstantem Wasserstoffstrom in einem Rohrofen gezogen. Um Kristalle mit größeren Ausdehnungen in allen drei Raumrichtungen zu erhalten, wurde erstmals die Gasphasen-Bridgman-Methode erfolgreich auf Tetracen angewandt. Die Beweglichkeit der Löcher in der c'-Richtung wurde mit der Flugzeitmethode als Funktion der Temperatur gemessen. Die Beweglichkeit folgt dem Hoesterey-Letson-Modell und hatte ein Maximum von 0.98 cm^2/Vs bei 320 K. Der Temperaturverlauf deutete an, dass die intrinsische Löcher-Beweglichkeit in c'-Richtung eine T^(-n)-Abhängigkeit hat, dem Bändermodell des Transports entsprechend. In der ab-Ebene wurde die Löcher-Beweglichkeit in Abhängigkeit von der Temperatur durch die Feldeffekt-Transistor-Technik gemessen. Hierfür wurde das Polymer PPX als Gate-Isolator mit einer Durchbruchfeldstärke von 3 MV/cm eingesetzt. Mit 0.79 cm^2/Vs bei 320 K wurde eine maximale Löcher-Beweglichkeit in Tetracen erzielt, die doppelt so hoch ist wie die bisher publizierte. Um das beobachtete Temperaturverhalten der Beweglichkeit zu erklären, wurden die Kristalle strukturell und chemisch charakterisiert. Durch Anwendung von Ätztechniken wurden Versetzungen sichtbar gemacht, die allerdings keinen signifikanten Einfluss auf den Ladungsträgertransport haben. Um die Art und die Konzentration der chemischen Verunreinigungen zu bestimmen, wurde die kombinierte Gaschromatographie-Massenspektrometrie eingesetzt. Die gefundene Gesamtkonzentration an chemischen Verunreinigungen stimmt mit der aus dem Temperaturverlauf der Beweglichkeit abgeschätzten Fallendichte überein. Die Hauptverunreinigung stellt ein Photoreaktionsprodukt des Tetracens, 5,12-Tetracenchinon, dar. In einer oberflächenselektiven Untersuchung wurde gezeigt, dass die Konzentration des Chinons an der Oberfläche der Einkristalle stark erhöht ist. Konsequenzen für den Einsatz von Polyacenen in der organischen Elektronik werden aufgezeigt.This work focuses on the electronic properties of the organic semiconductor tetracene. For the transport studies tetracene single crystals were grown by two different techniques. Plate-like crystals with surface areas up to 1 cm^2 were grown in a stream of hydrogen in a horizontal tube furnace. In order to grow crystals with larger extensions in all three crystallographic directions, the vapor-phase Bridgman method was successfully applied to tetracene for the first time. The mobility of holes as a function of temperature in the c'-direction was measured using the time-of-flight method. Following a Hoesterey-Letson-behavior, the maximum mobility resulted in 0.98 cm^2/Vs at 320 K. From the temperature dependence it is concluded that the intrinsic mobility in the c'-direction follows a T^(-n) law consistent with the band model. In the ab-plane the hole mobility was measured as a function of temperature by the field-effect transistor technique. For this purpose the polymer PPX was used as gate insulator with a maximum electric breakdown field of 3 MV/cm. With 0.79 cm^2/Vs at 320 K a maximum mobility was obtained, which is twice as high as the field effect mobility for tetracene published so far. In order to explain the observed temperature dependence of the mobility, the crystals were characterized structurally and chemically. Dislocations were visualized by the use of etching techniques. However, these defects have no significant influence on the charge carrier transport. In order to determine the type and the concentration of chemical impurities, the combined gas chromatography-mass spectrometry was used. The total concentration of chemical impurities matches the trap density that was estimated from the temperature dependence of the mobility. The main contaminant present in the crystals is a photoreaction product of tetracene, 5,12-tetracenquinone. In a surface selective investigation it was shown that the concentration of the quinone is strongly increased at the surface of a single crystals. Consequences for the use of polyacenes in organic thin film transistors are discussed