J. D. Ferguson

Post-deposition reorganization of pentacene films deposited on low-energy surfaces

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N2 atmosphere, beginning immediately after deposition of thin films onto SiO2 gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octyltrichlorosilane (FOTS). Films deposited on bare SiO2 remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the interfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N2 atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N2 atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than ∼5 ML remain stable under all conditions; this is attributed to the fact that the interfacial layer is buried completely for films thicker than ∼5 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N2 atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.

Measurements of surface diffusivity and coarsening during pulsed laser deposition.

Pulsed laser deposition (PLD) of homoepitaxial SrTiO(3) 001 was studied with in situ x-ray specular reflectivity and surface diffuse x-ray scattering. Unlike prior reflectivity-based studies, these measurements access both time and length scales of the evolution of the surface morphology during growth. In particular, we show that this technique allows direct measurements of the diffusivity for both inter- and intralayer transport. Our results explicitly limit the possible role of island breakup, demonstrate the key roles played by nucleation and coarsening in PLD, and place an upper bound on the Ehrlich-Schwoebel barrier for downhill interlayer diffusion.

Epitaxial Oxygen Getter for a Brownmillerite Phase Transformation in Manganite Films

Complex oxide systems are promising candidates for materials in solid oxide fuel cells, oxygen sensors, and other applications requiring oxygen anion diffusion. [ 1– 3 ] In particular, mixed mode conductors such as the manganite oxides are of interest as cathode materials for solid oxide fuel cells. [ 3– 5 ] One interesting property of some complex oxides is their ability to form distinct, oxygen-defi cient ordered phases with high ionic conductivity. [ 1 , 6– 8 ] Here, we report the discovery, using in situ synchrotron-based X-ray techniques, of a new method for creating oxygen vacancy ordered phases in epitaxial manganite thin fi lms. The method involves depositing an oxygen defi cient complex oxide fi lm on top of a stoichiometric manganite fi lm to act as an oxygen getter. Once the getter layer exceeds a critical thickness, a phase transition to an oxygen vacancy ordered superlattice occurs in the manganite fi lm. We demonstrate the use of oxygen defi cient SrTiO 3δ (STO) and LaAlO 3δ (LAO) as getter layers and superlattice formation in four manganite systems: La 0.7 Sr 0.3 MnO 3 (LSMO), Pr 0.7 Ca 0.3 MnO 3 (PCMO), La 0.7 Ca 0.3 MnO 3 (LCMO), and LaMnO 3 (LMO). The superlattices may be maintained at ambient conditions after cooling to room temperature. This growth technique constitutes a new procedure for preparing such structures, and may lead to the discovery of new, technologically diverse phases of complex oxide materials that cannot be grown by traditional deposition techniques. Refl ection high energy electron diffraction (RHEED) and X-ray scattering are commonly employed to monitor thin fi lm thickness, roughness, morphology, and structure during deposition. [ 9 – 14 ] The penetrating power of X-rays makes them uniquely suited for structural studies of the buried layers in heterostructures. To monitor fi lm thickness during deposition,

Real time monitoring of pentacene growth on SiO2 from a supersonic source

Thin film growth of pentacene on SiO2 using a supersonic source has been investigated with in situ real time synchrotron x-ray scattering and ex situ atomic force microscopy, focusing on the effects of incident kinetic energy Ei and growth rate GR on the evolution of surface roughness and the crystalline structure of the thin films. For the conditions examined here, Ei=2.5–7.2eV and GR=0.0015–0.2MLs−1, the thin film phase is always observed. We find that while the effect of Ei on interlayer transport is minimal, at high growth rates, slightly smoother films are observed.