Plasticity in pentacene thin films

Abstract

We have investigated the structure, defects and plasticity of thermallyevaporated thin films of the organic molecular semiconductor pentaceneusing X-ray Diffraction (XRD), Optical microscopy (OM), TransmissionElectron Microscopy (TEM), Electron Diffraction (ED), and High ResolutionElectron Microscopy (HREM). Using XRD the degree of (001) texturingpresent in the as-grown films was characterized. The nature of pentaceneplasticity and deformation-induced molecular alignment was investigatedusing rubbing and scratching techniques, as well as nanoindentation.Rubbing of the bulk powder produced thin oriented films, and a deformationlength scale dependence was seen. Under stress pentacene crystalsinitially fail by cracking, until they reach a critical size of aboutone micron, when they tend to plastically deform into thin sheets.Alignment of thermally evaporated films was achieved under a controlledload scratch, and the degree of molecular orientation inside thescratched region was directly imaged using HREM. Finally, using nanoindentationwe measured pentacene's plastic hardness to be 0.25 GPa at a loadingrate 0.05 mN/s. A loading rate dependence of the hardness and stiffnesswas measured, with thin films behaving harder and stiffer at fasterindentation rates. (C) 2004 Kluwer Academic Publishers.