Molecular-Scale Structure of Pentacene Interfaces with Si (111)
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Molecular-Scale Structure of Pentacene Interfaces with Si (111) Soonjoo Seo1 and Paul G. Evans2 1 Materials Science, University of Wisconsin-Madison, 1509 University Ave., Madison, WI, 53706 2 Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave., Madison, WI, 53706
ABSTRACT The morphology and crystal structure of the first few molecular layers of organic semiconductor thin films at organic-inorganic interfaces are important from both electronic and structural perspectives. The first upright layer of pentacene on Si (111) forms on top of a disordered layer of strongly bonded pentacene molecules in a structure similar to the pentacene monolayers formed on insulators. We describe a high-resolution structural study of this crystalline phase of pentacene using low-temperature scanning tunneling microscopy (STM). The arrangement of molecules in these layers observed with STM agrees the results of with structural studies using scattering techniques. The imaging conditions and sample preparation techniques necessary to achieve molecular resolution can be adapted to subsequent STM and scanning tunneling spectroscopy experiments probing individual structural defects including vacancies, dislocations and grain boundaries within and between islands.
INTRODUCTION Investigating the structural and electronic properties of organic-inorganic interfaces is important because the electrical characteristics of organic thin films are influenced by the crystal structure of the first few layers between organic molecules and inorganic materials. Charge transport through grain boundaries in polycrystalline thin films can be depends on the arrangement of molecules near the junctions of grains [1]. Structural defects such as vacancies, dislocations and grain boundaries influence the electrical properties of organic thin films. These defects are traps for charge carriers and can degrade charge transport within and between organic islands [1]. Tsiaousis and Munn showed in electronic structure calculations that molecular vacancies in anthracene can trap charges [2]. The electrical effects of grain boundaries on charge transport have been studied both experimentally [3] and theoretically [4]. High angle grain boundaries, for instance, cause reduction in the magnitude of photocurrent in bicrystals [3]. The structures of these defects can be probed using both microscopy and diffraction techniques. Attempts to relate the electrical properties to the microscopic structure of defects have previously used atomic force microscopy (AFM) [5]. Xray scattering studies have attributed a broadening of Bragg peaks to the presence of dislocations pentacene thin films [6].
STM complements other microscopy techniques in molecular-scale structural studies because it has spatial resolution sufficient to resolve individual molecules. In addition, both morphological and spectroscopic information can be obtained from STM experiments. The structural aspect of STM experiments the orientations (for example, lying-down o
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