We have examined the nucleation, growth, and dynamics of the adsorption of N,N 0 -ditridecylperylene-3,4,9, 10-tetracarboxylic diimide (PTCDI-C 13 ) on SiO 2 surfaces modified by self-assembled monolayers (SAMs) and on a predeposited monolayer of pentacene using in situ synchrotron X-ray scattering and ex situ atomic force microscopy. From realtime X-ray scattering, we find that PTCDI-C 13 exhibits prolonged layer-by-layer growth for approximately the first 10 monolayers of deposition on all three SAMs examined. Concerning adsorption on the pristine SAM-terminated surfaces, in all cases, we observe a smooth decrease in the probability of adsorption of PTCDI-C 13 with increasing incident kinetic energy, indicative of trapping-mediated adsorption. Once these surfaces are covered by PTCDI-C 13 , the probability of adsorption no longer depends on the identity of the SAM, but still exhibits a significant decrease with increasing incident kinetic energy. The adsorption probability of PTCDI-C 13 on itself is similar to that observed on two SAMs that possess aromatic end groups, but it differs significantly from that observed on a relatively short, methyl-terminated SAM. These differences could reflect mechanisms, such as direct molecular insertion of PTCDI-C 13 into either the existing PTCDI-C 13 film or the longer chain SAMs with aromatic end groups, or possibly the effects of greater internal degrees of freedom of the near surface layers in these cases. Concerning growth in the submonolayer regime, we find that nucleation is homogeneous and that the absolute density of islands depends on the nature of the surface, while the relative change of the island density with increasing growth rate is essentially independent of the underlying SAM. From the latter, we find that a critical island size of a single molecule of PTCDI-C 13 can describe all the data.
We have examined the growth of bilayers and superlattices of pentacene and perylene derivatives (PTCDI-C n ) using in situ real time X-ray synchrotron radiation techniques and ex situ atomic force microscopy. We find that the growth of PTCDI-C n layers on 1 monolayer (ML) of pentacene is initially 2D layer-by-layer (LbL), eventually transitioning to a mode of growth that is more 3D after several monolayers have been deposited. We find that the extent of 2D LbL growth depends on the length of the alkyl end chains, C n : the smoothest films are formed with PTCDI-C 5 , while the roughest are formed with PTCDI-C 13 . These observations reflect a difference in the Ehrlich−Schwoebel barrier for step-edge crossing with alkyl end-chain length. When the sequence of deposition is reversed, we observe spectacular changes in the evolution of surface roughness for the growth of pentacene thin films on 1 ML of PTCDI-C n . The growth is immediately 3D, while still remaining crystalline. The morphology of these thin films indicates significant reorganization of the deposited pentacene during and/or subsequent to growth, producing 3D islands that can only form if "uphill" transport is operative. Surface energy is driving this process, where the growth of high surface energy layers (pentacene) on low surface energy materials (PTCDI-C n ) is not favored, resulting in significant reorganization. Well-ordered superlattices of pentacene and PTCDI-C n cannot be grown, consistent with our results on the bilayers. We find that thin film roughness increases abruptly with the deposition of the second ML of pentacene or the formation of the first pentacene-on-PTCDI-C n interface. Our results indicate that the successful growth of superlattices of small molecular organic thin films will require matching of surface energies to minimize the driving forces for reorganization.
The authors have examined the nucleation of diindenoperylene (DIP) on SiO2 employing primarily atomic force microscopy and focusing on the effect of incident kinetic energy employing both thermal and supersonic sources. For all incident kinetic energies examined (Ei = 0.09–11.3 eV), the nucleation of DIP is homogeneous and the dependence of the maximum island density on the growth rate is described by a power law. A critical nucleus of approximately two molecules is implicated by our data. A re-examination of the nucleation of pentacene on SiO2 gives the same major result that the maximum island density is determined by the growth rate, and it is independent of the incident kinetic energy. These observations are readily understood by factoring in the size of the critical nucleus in each case, and the island density, which indicates that diffusive transport of molecules to the growing islands dominate the dynamics of growth in the submonolayer regime.
Four members of a family with spinocerebellar degeneration and slow saccadic eye movements are described. Detailed electrophysiological studies revealed abnormalities of neurological pathways not apparent clinically. The patients had slow saccades as measured electrophysiologically, as well as absence of rapid eye movements (REM) despite REM stages of sleep. These studies suggest that although saccadic eye movement and REM are mediated through the pontine paramedian reticular formation, other characteristics of REM sleep are not necessarily mediated through the same neurons.
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