Formation of intra-island grain boundaries in pentacene monolayers

Abstract

To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O 3 treatment). We investigated the influence of the kinetic energy of the impinging molecules on the sub-monolayer growth by comparing organic molecular beam deposition (OMBD) and supersonic molecular beam deposition (SuMBD). For pentacene films fabricated by OMBD, higher pentacene island-density and higher polycrystalline island density were observed on UV/O 3 -treated silicon oxide as compared to pristine silicon oxide. Pentacene films deposited by SuMBD exhibited about one order of magnitude lower island-and polycrystalline island densities compared to OMBD, on both types of substrates. Our results suggest that polycrystalline growth of single islands on amorphous silicon oxide is facilitated by structural/chemical surface pinning centres, which act as nucleation centres for multiple grain formation in a single island. Furthermore, the overall lower intra-island grain boundary density in pentacene films fabricated by SuMBD reduces the number of charge carrier trapping sites specific to grain boundaries and should thus help achieving higher charge carrier mobilities, which are advantageous for their use in organic thin-film transistors.

Toward a comprehensive understanding of molecular doping organic semiconductors (review)

“…This "standard" mode of OFET operation is referred to as accumulation mode, as the transistor is "off" unless a gate voltage is applied, which is in contrast to the depletion mode, where it is "on" without bias voltage and the source-drain current can be reduced by the applied gate voltage. Importantly, charge transport in a working OFET is confined to percolation pathways in the vicinity to the gate dielectric, e.g., in the first two monolayers for PEN based OFETs [70], where both structure, molecular orientation, and the overall structural quality of the film (e.g., the grain-boundary density [71,72]) are crucial for the charge carrier mobility and, consequently, the overall OFET performance [73]. In particular, OSCs tend to adopt molecular arrangements in the vicinity of a substrate that can significantly differ from their bulk crystal structures, that is, substrate-mediated polymorphs can occur in thin-films of application relevant OSCs [74][75][76][77][78][79][80][81].…”

Real Space Demonstration of Induced Crystalline 3D Nanostructuration of Organic Layers

“…The second layer growth before the monolayer completion, i.e., growth of pentacene on pentacene, already indicates that during OMBD the initial layer-by-layer growth converts to a much faster, ascribed to a defect assisted growth . Intralayer grain boundaries and new nucleation sites may lead to the formation of intralayer domains and stacking faults (or even dislocations) influencing the thin film crystallinity. ,, The investigation of the local frictional properties by means of FFM has shown to be a valuable tool for revealing structural details of molecular films, ,− which are difficult to visualize with other techniques and ultimately due to the stick–slip mechanism at atomic scale . This technique has been exploited during the controlled manipulation described here to evidence structural domains and layer stacking; therefore, surface morphology and frictional response were verified at every stage of the process.…”

Quasi-Layer-by-Layer Growth of Pentacene on HOPG and Au Surfaces

“…Over the past 20 years, organic molecular electronics, one of the most important topics in the scientific and industrial fields, has significantly progressed in terms of performance and industrial product development. − The unique properties of organic materials hold great promise for applications such as portable, flexible electronics with reduced manufacturing costs. Previous investigations regarding material design, film control, and device engineering have promoted both performance improvements and a fundamental understanding of the solid materials. − However, relative to inorganic material (mainly silicon) electronics, fundamental knowledge of organic materials, thin films, and devices is limited. ,− Basic issues include the organic thin film growth mechanism and intrinsic charge transport behavior, specifically with resolution at the molecular level. Most of the organic thin films used in devices are characterized at a large scale using techniques such as atomic force microscopy (AFM) and X-ray diffraction (XRD).…”

“…Successful high-resolution characterization of organic semiconductors is usually performed on conductive single-crystal substrates using techniques such as scanning tunneling microscopy (STM). However, the molecular films assembled through the weak van der Waals forces present significant substrate-sensitive aggregation behavior. ,− …”