importance of surface energy considerations for explaining polymorphism at small dimensions. [19] The arguments presented here have demonstrated that the length scale at which a solid-state phase change occurs in pentacene films, given by h c , is related to the ratio of the differences between the (001) surface energies and the bulk energies for the two distinct phases. ExperimentalPentacene powder as received from Aldrich was thermally evaporated from a tungsten wire basket onto room-temperature amorphous carbon coated mica substrates under a vacuum of~10 ±5 torr. Films approximately 20 nm in thickness were evaporated at a rate of approximately 1 nm s ±1 . The pentacene films on amorphous carbon were then floated off the mica onto deionized water for collection with copper grids. Low voltage electron microscopy [20] was done on a LVEM5 from Delong Instruments operating in transmission mode at 4.7 kV. HREM was performed on a 400 kV JEOL 4000 EX operating under low dose conditions [21]. The end-point electron dose for destruction of crystallinity in pentacene was determined to be 0.25 ± 0.05 C cm ±2 at 400 kV [22]. Electron diffraction patterns were taken with a dose of 10 ±4 to 10 ±3 C cm ±2 . HREM images were taken with a dose of less than 0.09 C cm ±2 . Typical HREM images were taken at a magnification of 60 000 , a screen current density of 15 pA cm ±2 and an exposure time of 1.4 s. Negatives were digitized using an Imacon Flextight scanner with 5760 dpi resolution. The Cerius 2 v. 4.6 software package from Accelrys was used for modeling of the pentacene structure and simulation of ED patterns. The Dreiding 2.21 force field [17] was used to describe inter-and intramolecular interactions with a Buckingham exponential-6 potential and a simple harmonic oscillator, respectively. The triclinic pentacene lattice parameters and atomic coordinates were directly entered from the experimentally determined crystal structure [11].

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