Organic−Organic Epitaxy of Incommensurate Systems:  Quaterthiophene on Potassium Hydrogen Phthalate Single Crystals

Abstract

Hot-wall epitaxy and molecular-beam epitaxy have been employed for growing quaterthiophene thin films on the (010) cleavage face of potassium hydrogen phthalate, and the results are compared in terms of film properties and growth mode. Even if there is no geometrical match between substrate and overlayer lattices, these films are epitaxially oriented. To investigate the physical rationale for this strong orientation effect, optical microscopy, atomic force microscopy, and X-ray diffraction are employed. A clear correlation between the morphology of the thin films and the crystallographic orientation is found. The results are also validated by surface potential calculations, which demonstrate the primary role played by the corrugation of the substrate surface.

Pseudomorphic growth of organic semiconductor thin films driven by incommensurate epitaxy

“…Such a unit cell distortion determines the loss of geometric lattice match between ␣-4T / LT͑001͒ and TEN͑001͒ so that its role in driving the overlayer orientation is ruled out, as in incommensurate organic epitaxy. 5,12 Rather, if one assumes this variation in the unit cell parameters, the spacing of the ␣-4T͑110͒ planes increases from 4.8 to about 5.0 Å, therefore matching the spacing of the TEN͑110͒ planes ͑4.96 Å͒ almost perfectly. These families of planes ͓sketched in Fig.…”

“…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][6][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.…”

“…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.…”

“…The band line shape reflects the anisotropy of the known optical absorption of ͑001͒-oriented single crystalline thin films of ␣-4T with the structure of the low temperature ͑␣-4T / LT͒ polymorph having a monoclinic unit cell with parameters a = 6.09 Å, b = 7.86 Å, c = 30.48 Å, and ␤ = 91.8°. 5,[23][24][25] Indeed, the difference A ʈa ␣-4T − A Ќa ␣-4T between the absorbance for polarized light with the electric field parallel ͑ ʈ a ␣-4T ͒ and perpendicular ͑Ќa ␣-4T ͒ to the a ␣-4T axis, reported for such a single crystalline ␣-4T film in Fig. 2͑b͒, shows a positive band due to the strong absorption feature under ʈ a ␣-4T polarized light, negligible under Ќa ␣-4T polarized light; 24,25 hence, by the comparison between Figs.…”

Reflectance anisotropy spectroscopy: A probe to explore organic epitaxial growth

“…13 The substrates have been maintained at room temperature ͑␣-4T/LT single crystals͒ or at 146 K ͑KAP͒ during the deposition process. The structure of the ␣-4T films grown at room temperature has been identified 14,15 as that of ␣-4T/LT single crystals. The RAS experiments reported here were carried out in situ at near-normal incidence with a homemade apparatus with two polarizers ͑one polarizer plus one analyzer͒ in the spectral range of 1.5-5.0 eV.…”