Arthur R. Woll

Oriented 2D Covalent Organic Framework Thin Films on Single-Layer Graphene

Microporous covalent organic frameworks, which usually form as insoluble powders, grow as crystalline films on graphene. Covalent organic frameworks (COFs), in which molecular building blocks form robust microporous networks, are usually synthesized as insoluble and unprocessable powders. We have grown two-dimensional (2D) COF films on single-layer graphene (SLG) under operationally simple solvothermal conditions. The layered films stack normal to the SLG surface and show improved crystallinity compared with COF powders. We used SLG surfaces supported on copper, silicon carbide, and transparent fused silica (SiO2) substrates, enabling optical spectroscopy of COFs in transmission mode. Three chemically distinct COF films grown on SLG exhibit similar vertical alignment and long-range order, and two of these are of interest for organic electronic devices for which thin-film formation is a prerequisite for characterizing their optoelectronic properties.

Lattice Expansion of Highly Oriented 2D Phthalocyanine Covalent Organic Framework Films

Expanding into application: covalent organic framework (COF) films are ideally suited for vertical charge transport and serve as precursors of ordered heterojunctions. Their pores, however, were previously too small to accommodate continuous networks of complementary electron acceptors. Four phthalocyanine COFs with increased pore size well into the mesoporous regime are now described.

van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.

van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 μm(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation.

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.

Guiding Crystallization around Bends and Sharp Corners

Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

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.

Tuning Polymorphism and Orientation in Organic Semiconductor Thin Films via Post-deposition Processing

Though both the crystal structure and molecular orientation of organic semiconductors are known to impact charge transport in thin-film devices, separately accessing different polymorphs and varying the out-of-plane molecular orientation is challenging, typically requiring stringent control over film deposition conditions, film thickness, and substrate chemistry. Here we demonstrate independent tuning of the crystalline polymorph and molecular orientation in thin films of contorted hexabenzocoronene, c-HBC, during post-deposition processing without the need to adjust deposition conditions. Three polymorphs are observed, two of which have not been previously reported. Using our ability to independently tune the crystal structure and out-of-plane molecular orientation in thin films of c-HBC, we have decoupled and evaluated the effects that molecular packing and orientation have on device performance in thin-film transistors (TFTs). In the case of TFTs comprising c-HBC, polymorphism and molecular orientation are equally important; independently changing either one affects the field-effect mobility by an order of magnitude.

Post-deposition processing methods to induce preferential orientation in contorted hexabenzocoronene thin films.

The structuring in organic electrically active thin films critically influences the performance of devices comprising them. Controlling film structure, however, remains challenging and generally requires stringent deposition conditions or modification of the substrate. To this end, we have developed post-deposition processing methods that are decoupled from the initial deposition conditions to induce different out-of-plane molecular orientations in contorted hexabenzocoronene (HBC) thin films. As-deposited HBC thin films lack any long-range order; subjecting them to post-deposition processing, such as hexanes-vapor annealing, thermal annealing, and physical contact with elastomeric poly(dimethyl siloxane), induces crystallization with increasing extents of preferential edge-on orientation, corresponding to greater degrees of in-plane π-stacking. Accordingly, transistors comprising HBC thin films that have been processed under these conditions exhibit field-effect mobilities that increase by as much as 2 orders of magnitude with increasing extents of molecular orientation. The ability to decouple HBC deposition from its subsequent structuring through post-deposition processing affords us the unique opportunity to tune competing molecule-molecule and molecule-solvent interactions, which ultimately leads to control over the structure and electrical function of HBC films.

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,

Quantitative modeling ofin situx-ray reflectivity during organic molecule thin film growth

Synchrotron-based x-ray reflectivity is increasingly employed as an \textit{in situ} probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO