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We have studied defect structures in the crystalline organic molecular semiconductor pentacene. Our investigations included the calculation of free surface energies on low index planes. It was found that that the (001) surface had the lowest energy ∼50 mJ/m2, roughly half that of the other low index planes, (100) and (010) ∼120 mJ/m2 and ∼140 mJ/m2 respectively. These calculations were then compared to experimental data from vapor grown crystals using optical microscopy, Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM). We also modeled dislocations dipoles of varying Burgers vector and dipole length. It was found that dislocations were accommodated by extensive molecular deformation near the defect core, as well as a well defined stacking fault that occurred down the length of the dipole. Finally, we investigated low angle tilt grain boundaries. It was seen that low angle boundaries relaxed through molecular deformation in the first layer of molecules at the boundary as well as bending of the (001) planes.
The aromatic hydrocarbon pentacene is currently under investigation for use as the active layer in electronic devices such as thin film field effect transistors. We have used X-Ray Diffraction (XRD), Electron Diffraction (ED), Low Voltage Electron Microscopy (LVEM), High Resolution Electron Microscopy (HREM) and molecular modeling to investigate the thin film phase of pentacene. We will report the orthorhombic symmetry and lattice parameters of the thin film phase measured experimentally from these techniques. The structure of extended defects such as dislocations and grain boundaries will influence the electrical and mechanical characteristics of the films. Here we show a direct image of an edge dislocation in the thin film phase and discuss the way in which the lattice accommodates the defect.