Defects in Organic Molecular Crystals
Pentacene is an aromatic molecule which is currently under active development for use as the active layer in organic thin-film transistors (TFTs). Hole mobilites greater than 1 cm^2/V s have been measured from thermally evaporated films of this material.
We have used low-dose high resolution electron microscopy (HREM) to investigate the nature of molecular relaxations near defects such as dislocations and grain boundaries in pentacene nanocrystals. ~100 nanometer sized crystals were produced by mechanical breakup of the bulk powder between glass slides. The figure shows a HREM image of a 40 degree tilt grain boundary in a pentacene nanocrystal. The fringes visible correspond to the 1.4 nm (001) lattice spacing, and along the grain boundary a transition can be seen from continuous bending of the (001) planes across the boundary to an abrupt kink where the (001) planes are out of registry. It can also be explained as a transition from a wide to a narrow grain boundary at a constant boundary angle. The plot inset shows the angle of the (001) planes from the grain boundary plane as a function of distance from the boundary for two different regions of the crystal. The data is fit to a hyperbolic tangent function.
Larry Drummy, Paul Miska, David Martin “Electron Microscopy and Diffraction of Pentacene Thin Films” ACS Preprints (In Press)
L.F. Drummy, C. Kubel, D. Lee, A. White, D.C. Martin “Direct Imaging of Defect Structures in Pentacene Nanocrystals” Advanced Materials, 14 (1), 54-57, 2002
Paul Miska, Larry Drummy, David Martin “Molecular Modeling of Planar Defects in Pentacene” University of Michigan Macro Molecular Science Symposium, 2001
Larry Drummy, Paul Miska, David Martin “Processing and Characteristics of Pentacene Thin Films” University of Michigan Macro Molecular Science Symposium, 2001
Larry Drummy, Paul Miska, David Martin “Deformation Induced Molecular Alignment in Pentacene” ISMANAM 2001