Session V4: Electronic Excitations in Organic Molecular Crystals/Fluctuating Fronts: Beyond a Popular Mean-Field Theory

Photoemission study on the charge transport mechanism in pentacene thin film

Organic molecules are attracting much interest to use for a variety of electronic applications. Pentacene (Pn), which is one of such molecules, has a high application potential due to its high hole mobility. The hole mobility of Pn is almost comparable to that of amorphous silicon. At low temperature, the hole transport in Pn crystal has been reported to have a band-like nature, and the band-like charge transport is expected to play a major part at room temperature as well. One of the origins of the band-like transport is the adequate overlap of the pi-orbitals of adjacent molecules, which produces orbital-derived electronic bands. It is therefore essential to have a proper understanding on the electronic band structures in order to fully comprehend the charge transport mechanism of a Pn crystal. By using angle-resolved photoelectron spectroscopy (ARPES), we have measured the dispersions of the highest occupied molecular orbital (HOMO)-derived bands of single crystal Pn monolayer films grown on substrates. Two HOMO-derived, whose band dispersion widths are larger than the values predicted by theoretical calculations, were clearly observed in the ARPES spectra. Our result indicates that the overlap of the pi-orbitals of adjacent Pn molecules is larger than what was expected from theoretical calculations, and the observed dispersions suggest that the higher binding energy HOMO-derived band mainly contributes to the band-like charge transport mechanism of a Pn crystal. By analyzing the dispersions within a simple tight-binding approximation, the obtained results lead to a hole mobility of $mu_h>$34.1 cm$^2$/Vs at 140 K.   

Bandlike transport in organic molecular crystals revealed by subpicosecond transient photoconductivity

The nature of charge carrier photogeneration and transport in organic molecular crystals is not completely understood. In particular, the mechanism responsible for the observed bandlike behavior of charge carriers in these materials, where the carrier mobility increases as the temperature decreases, remains unresolved and is the focus of much research. Using typical device structures to explore intrinsic properties of charge transport in organic semiconductors is complicated by the presence of defects and the necessity to make contacts to the sample. Recently, however, ultrafast techniques that use terahertz (THz) pulses for assessing the electronic properties of materials have been developed. In particular, time-resolved THz spectroscopy allows transient photoconductivity in materials to be probed with subpicosecond time resolution, providing a sensitive non-contact tool for studying the transport of charge carriers before they are trapped at defect sites. This talk will provide an overview of how THz pulses can be used to probe the nature of conductivity and bandlike behavior in organic molecular crystals and thin films.   

Polarons and Coulomb interactions in organic transistors

In organic Field Effet Transistors (FETs), charge carriers accumulate in a two-dimensional layer at the interface between an organic crystal and a gate dielectric. The possibility of tuning several microscopic parameters such as the carrier density, the electron-electron and electron-phonon interactions makes these devices an interesting playground for fundamental physics. Recent experiments have demonstrated that depending on the gate insulator used, the electric mobility in organic FETs can be tuned from metallic-like to insulating-like. This phenomenon can be explained in terms of the formation of small polarons, due to the remote interaction of the charge carriers with the phonons of the gate material [1]. In the devices with the highest polarizabilities, experiments performed at large gate voltages (corresponding to $\sim 0.1$ carriers/molecule) have revealed a further reduction of the mobility, suggesting the onset of electron-electron interactions [2]. The physics of this novel regime involving both strong electron-phonon and long-range electron-electron interactions will be discussed. If time allows, I shall briefly present how the above picture is modified when the narrow-band organic crystal is replaced by graphene ---a two-dimensional sheet of carbon atoms. Although the effect is less striking in that case, the remote scattering with the substrate phonons still constitutes an important limiting factor of the mobility at room temperature, that should be addressed for the design of future graphene devices [3]. \\ References:\\ $[1]$ I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iosad, G. Rastelli, S. Ciuchi and A. F. Morpurgo, Nature Materials 5, 982-986 (2006)\\ $[2]$ S. Fratini, H. Xie, I. N. Hulea, S. Ciuchi, and A. F. Morpurgo, arXiv:0710.2845 preprint (2007)\\ $[3]$ S. Fratini, F. Guinea, arXiv:0711.1303 preprint (2007)   

Particle versus density models in spark formation: X-rays from pulled fronts?

Streamer discharges govern the early stages of sparks and lightning, of spark-like phenomena in water, oil, and semiconductors, in industrial corona reactors, or in gigantic sprite discharges above thunderclouds [1,2]. Thunderstorms recently have been found to emit terrestrial gamma-ray flashes or X-rays towards satellites and towards the ground. These emissions might be explained by particle models of ``pulled'' streamer ionization fronts. In general, the growing discharge channel has an inner structure with multiple scales [1-3]. While the largest part of this channel can be treated in a density approximation for the electrons and ions, the dynamics of the ionization front is that of a pulled front; it is determined in the leading edge where the density approach eventually breaks down. We therefore investigate a realistic MC particle model for the motion of single electrons in a discharge in pure nitrogen. The particle model not only incorporates particle fluctuations, but also shows that the electron energies are systematically larger in the leading edge of the front than in the corresponding density model, and that the ionization level behind the front is higher as well, while the front velocity hardly changes [3]. These effects increase with increasing applied electric field and might actually cause the recently observed X-ray emission from lightning through rare very energetic runaway electrons in the tail of the distribution. Comparing the leading edge of the particle front with a linear particle avalanche, the avalanche shows the same mean density gradient and energy overshoot in its leading edge as the nonlinear front; hence the pulled front concept in this sense applies to discrete particle models as well [3]. This gives a key to understanding the above effects through analytical approximations and to develop efficient numerical methods coupling particle and density models in space.\\ {[1]} U. Ebert {\it et al.}, Plasma Sources Sci. Techn. {\bf 15}, S118 (2006) (arXiv:physics/0604023).\\ {[2]} {\it Streamers, sprites, leaders, lightning: From micro- to macroscales}, workshop in Oct. 2007: \\ {\tt http://www.lorentzcenter.nl/lc/web/2007/265/info.php3?wsid=265}; and cluster issue in J. Phys. D in fall 2008; organizers/editors: U. Ebert and D.D. Sentman.\\ {[3]} C. Li {\it et al.}, J. Appl. Phys. {\bf 101}, 123305 (2007) (arXiv:physics/0702129).   

Fluctuation Effects on Propagating Waves of Self-Assembly in Organosilane Monolayers.

Wavefronts associated with reaction--diffusion and self-assembly processes are ubiquitous in the natural world. For example, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes, in polymerization fronts involved in cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. Although it is often claimed that self-sustaining or autocatalytic front propagation is well described by mean-field ``reaction-- diffusion'' or ``phase field'' ordering models, it has recently become appreciated from simulations and theoretical arguments that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean-field theory (MFT) of this type of front propagation. The present work explores these fluctuation effects in a real physical system. In particular, we consider a high-resolution near-edge x-ray absorption fine structure spectroscopy (NEXAFS) study of the spontaneous frontal self-assembly of organosilane (OS) molecules into self-assembled monolayer (SAM) surface-energy gradients on oxidized silicon wafers. We find that these layers organize from the wafer edge as propagating wavefronts having well defined velocities. In accordance with two-dimensional simulations of this type of front propagation that take fluctuation effects into account, we find that the interfacial widths w(t) of these SAM self-assembly fronts exhibit a power-law broadening of in time w(t) $\sim $ t$^{\beta }$, rather than the constant width predicted by MFT. Moreover, the observed exponent values accord rather well with previous simulation and theoretical estimates. These observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials-processing contexts.