Abstract
Pentacene is one of the most studied organic semiconducting materials. While many aspects of the film formation have already been identified in very thin films, this study provides new insight into the transition from the metastable thin-film phase to bulk phase polymorphs. This study focuses on the growth behavior of pentacene within thin films as a function of film thickness ranging from 20 to 300 nm. By employing various X-ray diffraction methods, combined with supporting atomic force microscopy investigations, one crystalline orientation for the thin-film phase is observed, while three differently tilted bulk phase orientations are found. First, bulk phase crystallites grow with their 00L planes parallel to the substrate surface; second, however, crystallites tilted by 0.75° with respect to the substrate are found, which clearly dominate the former in ratio; third, a different bulk phase polymorph with crystallites tilted by 21° is found. The transition from the thin-film phase to the bulk phase is rationalized by the nucleation of the latter at crystal facets of the thin-film-phase crystallites. This leads to a self-limiting growth of the thin-film phase and explains the thickness-dependent phase behavior observed in pentacene thin films, showing that a large amount of material is present in the bulk phase much earlier during the film growth than previously thought.
Highlights
Pentacene is a prototypical organic semiconductor material for use in organic thin-film transistors.[1−4] Well-defined deposition conditions underhigh-vacuum conditions allow for a high reproducibility in thin-film fabrication, which makes it an ideal model molecule for studying nucleation and crystal growth of conjugated organic molecules on surfaces in general.[5−7] Pentacene is reported to grow in various different polymorphs, among which three distinct crystallographic phases have been observed on silicon oxide.[8−10] During initial film growth, a monolayer of vertically upright-standing molecules forms directly at the organic−substrate interface.[11]
The metastable thin-film phase can be transformed into the Campbell phase by either heat treatment, solvent treatment, or aging, which is evident from experimental studies and molecular dynamics calculations.[20−23] some recent results show that Campbell phase crystallites grow with their 001 contact plane parallel to the thin film polymorph, but with their 001 contact plane inclined by well-defined angles of 5°, 10°, or 13°.24−26 While homoepitaxy between the 001TF and 001C (Campbell) planes might appear obvious, the appearance of Received: December 12, 2016
By combining the results presented here with similar examples from literature, two possibilities have been suggested for the formation of the Campbell phase domains: (i) the majority of Campbell phase crystals either adopt to characteristic thin-film phase island surface slopes, or (ii) the Campbell phase nucleates at stacking fault sites between adjacent dendritic branches in thin-film phase islands, as discussed in relation to the literature
Summary
Pentacene is a prototypical organic semiconductor material for use in organic thin-film transistors.[1−4] Well-defined deposition conditions under (ultra)high-vacuum conditions allow for a high reproducibility in thin-film fabrication, which makes it an ideal model molecule for studying nucleation and crystal growth of conjugated organic molecules on surfaces in general.[5−7] Pentacene is reported to grow in various different polymorphs, among which three distinct crystallographic phases have been observed on silicon oxide.[8−10] During initial film growth, a monolayer of vertically upright-standing molecules forms directly at the organic−substrate interface.[11]. An approximate maximum of 50 nm (∼30 monolayers)[10] is typically found to arrange in the thin-film phase, after which the film growth is dominated by the Campbell phase instead The reason for this transition in growth behavior is still not fully understood, many studies demonstrate how the extension of the thin-film phase can be manipulated by employing different substrate treatments, deposition conditions (e.g., deposition rate, glancing-angle deposition, temperature, pressure, etc.),[17] or by codeposition with chemically modified pentacene species, where phase separation promotes growth in the thin-film phase.[18,19] The metastable thin-film phase can be transformed into the Campbell phase by either heat treatment, solvent treatment, or aging, which is evident from experimental studies and molecular dynamics calculations.[20−23] some recent results show that Campbell phase crystallites grow with their 001 contact plane parallel to the thin film polymorph, but with their 001 contact plane inclined by well-defined angles of 5°, 10°, or 13°.24−26 While homoepitaxy between the 001TF (thin film) and 001C (Campbell) planes might appear obvious, the appearance of Received: December 12, 2016 Accepted: March 13, 2017 Published: March 13, 2017
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