Pseudomorphic growth of organic semiconductor thin films driven by incommensurate epitaxy

Abstract

A stable pseudomorphic phase of ␣-quaterthiophene, a well known organic semiconductor, is obtained by growing films with organic molecular beam epitaxy ͑OMBE͒ on a single crystal of another organic semiconductor, namely, tetracene. The structural characteristics of the new phase are investigated by monitoring in situ the OMBE process by reflectance anisotropy spectroscopy; thus assessing that incommensurate epitaxy is in this case, the driving force for tuning the molecular packing in organic molecular films and in turn, their solid state properties. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3081413͔The key factor improving the performances of thin film devices based on organic semiconductors is the achievement of high charge mobility through the control of growth mechanisms aimed at improving the structural quality and morphology of the thin films. 1 The fine tuning of the material properties by a suitable modification of the molecular orientation and packing ͑crucial for determining the electronic band structure of the solid film and its charge-transport properties͒ 2 represents an alternative experimental opportunity. The arrangement of different organic semiconductors in multilayer architectures, including semiconductor junctions, gives further beneficial effects in thin film devices. 1 In this framework, organic molecular beam epitaxy ͑OMBE͒ represents the best choice among the growth techniques, assuring high quality films and controlled interfaces in organic heterostructures in view of both fundamental studies and applications. 3,4 Among the strategies for growing samples with designed properties, one of the most promising is the use of organic substrates 5-7 whose effect in tuning the molecular orientation of the overlayer has been already demonstrated. In particular, when organic single crystals are used as substrates, highly oriented organic heterostructures are grown even in the absence of any geometric coincidence between substrate and overlayer ͑incommensurate epitaxy͒. 5,[8][9][10][11][12][13] In this letter, we investigate the properties of an organic-organic heterostructure by demonstrating the OMBE growth of an epitaxial pseudomorphic phase of the ␣-quaterthiophene ͑␣-4T͒ film on tetracene ͑TEN͒ single crystals. By means of reflectance anisotropy spectroscopy ͑RAS͒, we monitored in situ the OMBE process, showing that the ␣-4T overlayer grows with a distorted lattice structure with respect to the bulk crystal, which results in incommensurate epitaxy. In addition, the full ex situ stability of this new phase is demonstrated so that organic-organic epitaxy is definitely assessed as a powerful tool for tuning the properties of the growing films. 14 ␣-4T was synthesized and purified according to Ref. 15, while commercial TEN was purified by several sublimation steps before use. TEN single crystals were grown by physical vapor transport. 16 Its crystal structure belongs to the triclinic system, with unit cell parameters a = 6.06 Å, b = 7.84 Å, c = 13.01 Å, ␣ = 77.13°, ␤ = 72.12°, ␥ = 85.79...

Growth of pseudomorphic structures through organic epitaxy

“…The growth of 4T thin films on various organic single crystal substrates, such as KAP, sexithiophene, tetracene, rubrene, etc., has shown the presence of either the LT phase or pseudomorphic phases very close to the LT one. 18,19,37 The present study demonstrates that epitaxial 4T films can be grown also on single crystal PET, where a pseudomorphic phase very close to the HT polymorph grows as the dominant film phase. The possible epitaxial relations between the lattice of substrate and overlayer are deduced.…”

“…In the case of highly oriented crystalline thin films, polarized optical spectroscopy can give significant hints about the orientation of the film crystal structure with respect to the substrate. 17,18 In Figure 3, the absorption spectra of a 15 nm-thick 4T thin film grown on PET(001) are reported, as collected at normal incidence under polarized light. A strong anisotropy is observed, indicating a preferential orientation of 4T crystalline domains, similarly as in 4T thin films grown on KAP (where, in fact, orientation of 4T/LT is complete, with a 4T //c KAP ).…”

“…[11][12][13][14][15] In the context of organic epitaxy, the substrate surface quality and its characteristic morphology or, more precisely, its corrugation, i.e., the surface fluctuations at the atomic/molecular scale, is primarily responsible for driving the film orientation. 14,[16][17][18][19] The present a) E-mail: adele.sassella@unimib.it. work focuses on the growth of highly ordered thin films of α-quarterthiophene (4T) on organic single crystal substrates using OMBE.…”

“…We show how a proper choice of the substrate permits to select not only specific orientations, but also polymorphic and pseudomorphic phases of the overlayer. 18,20,21 In the case of 4T, the use pentaerythritol (PET) single crystal substrate allowed us to successfully grow at room temperature films composed by the high-temperature (HT) polymorph and by a preponderant new pseudomorphic phase with slight structural differences with respect to 4T/HT. Noteworthy, 4T/HT was never observed in thin films, while the usually favored polymorphic phase of 4T is the low temperature one (4T/LT).…”

Probing surface and interface structure using optics

“…RAS is in routine use for monitoring III-V and II-VI semiconductor growth and, recently, it has been shown that growth monitoring can be extended to organic molecular beam epitaxy [46,47]. RAS studies of pseudomorphic growth of α-quaterthiophene on a tetracene single crystal substrate have shown that the growth is driven by a form of incommensurate epitaxy [48].…”