Fluorescence of laser created electron-hole plasma in graphene

We present an experimental observation of non-linear up- and down-converted optical luminescence of graphene and thin graphite subject to picosecond infrared laser pulses. We show that the excitation yields to a high density electron-hole plasma in graphene. It is further shown that the excited charge carries can efficiently exchange energy due to scattering in momentum space. The recombination of the resulting non-equilibrium electron-hole pairs yields to the observed white light luminescence. Due to the scattering mechanism the power dependence of the luminescence is quadratic until it saturates for higher laser power. Studying the luminescence intensity as a function of layer thickness gives further insight into its nature and provides a new tool for substrate independent thickness determination of multilayer flakes

Comparative study of the growth of sputtered aluminum oxide films on organic and inorganic substrates

We present a comparative study of the growth of the technologically highly relevant gate dielectric and encapsulation material aluminum oxide in inorganic and also organic heterostructures. Atomic force microscopy studies indicate strong similarities in the surface morphology of aluminum oxide films grown on these chemically different substrates. In addition, from X-ray reflectivity measurements we extract the roughness exponent \beta of aluminum oxide growth on both substrates. By renormalising the aluminum oxide roughness by the roughness of the underlying organic film we find good agreement with \beta as obtained from the aluminum oxide on silicon oxide (\beta = 0.38 \pm 0.02), suggesting a remarkable similarity of the aluminum oxide growth on the two substrates under the conditions employed

Strongly Enhanced Thermal Stability of Crystalline Organic Thin Films Induced by Aluminum Oxide Capping Layers

We show that the thermal stability of thin films of the organic semiconductor diindenoperylene (DIP) can be strongly enhanced by aluminum oxide capping layers. By thermal desorption spectroscopy and in-situ X-ray diffraction we demonstrate that organic films do not only stay on the substrate, but even remain crystalline up to 460C, i.e. 270 deg. above their desorption point for uncapped films (190C). We argue that this strong enhancement of the thermal stability compared to uncapped and also metal-capped organic layers is related to the very weak diffusion of aluminum oxide and the structurally well-defined as-grown interfaces. We discuss possible mechanisms for the eventual breakdown at high temperatures.Comment: 5 pages, 4 figures, submitted to Adv. Mat., for further information see http://www.physchem.ox.ac.uk/~f

Thermoelectric properties of nanostructured tetrathiotetracene iodide crystals in a two-dimensional model

Previously, it has been predicted that nanostructured crystals of tetrathiotetracene iodide are very promising candidates for thermoelectric applications. However, these predictions are based on a strictly one-dimensional (1D) model. In order to verify these conclusions, a two-dimensional (2D) model is elaborated which explicitly takes into account the weak interaction of carriers with the nearest conductive chains. It is shown that for crystals with a rather low degree of purity this interaction does not affect significantly the results obtained by the 1D approximation, but for ultrapure crystals this interaction can no longer be neglected