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Molecular vibration and charge transport in crystalline oligoacenes and derivatives : Raman and DFT combined study

Authors
Publisher
University of North Carolina at Chapel Hill. Library
Publication Date
Disciplines
  • Design

Abstract

All-organic devices have drawn a lot of interest over the past decades. Compared to liquid crystal display technology, organic transistors and discrete LED displays hold the potential for devices with improved characteristics, including lower power requirements, better resolution, more mechanical flexibility, and lower production costs. To find new materials for better device performance, it is necessary to understand the connection between the structural and electronic properties of molecules. The establishment of these connections will make it possible to tailor molecules for desired performance in devices. Among the organic materials, oligoacenes and their derivatives have an important position in fundamental physics research because the molecules are relatively small and simple, which facilitates understanding of the relationships among molecular structures, optical properties, and transport properties in organics. In this work, I focus on the molecular vibration and charge transport in three oligoacene and two oligoacene derivative single crystals: anthracene, tetracene, pentacene, 9,10-diphenylanthracene (DPA), and especially 5,6,11,12-tetraphenyltetracene (rubrene). By comparing the experimental Raman spectra with the Density-Functional-Theory calculation based on one isolated molecule, I am able to distinguish the intermolecular vibrations from the intramolecular vibrations in crystalline anthracene, tetracene, pentacene, and DPA. Parallel study among the oligoacenes reveals decreasing strength of intermolecular vibrations as the number of benzene ring increases. However, the intermolecular coupling is even weaker in DPA because the side phenyl groups prevent close packing, therefore several intramolecular vibrational modes predicted by the calculation can be observed in the low-frequency Raman spectrum. I report temperature-dependent Raman spectra of rubrene from 30 to 300 K. The linewidths of certain low-frequency peaks increase significantly with temperature, especially in the range 150--200 K. These peaks correspond to the vibrations of the phenyl side groups of the rubrene molecules, and their couplings to intermolecular vibrational modes. I propose a model in which the strong increase in mobility observed with increasing temperature between 30 and 150 K results from disorder as the phenyl groups exchange sides of the backbone plane and break the symmetry, and discuss on how this model explains previous experimental observations of structural and calorimetric changes near 150 K. Lastly I discuss possible temperature-dependent properties of rubrene, and the potential application of the rubrene molecule in single-molecule devices design.

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