Sexithiophene thin films epitaxially oriented on polytetrafluoroethylene substrates: Structure and morphology

Spectroscopic Evidence for a Substrate Dependent Orientation of Sexithiophene Thin Films Deposited onto Oriented PTFE

“…The large difference in optical absorption as a function of light polarization reveals a strong molecular orientation. 6 All the observed bands within the range 1.55-4.1 eV appear L-polarized whereas band E located at 4.5 eV is M-polarized in agreement with the literature data. 2b, 7 The dichroic ratio at normal incidence in the range 1.55-3.64 eV is defined by eq 1:…”

“…The frequencies of the split bands are very close to those of the single crystal (∆ν < 2 cm -1 ), i.e., the film and the single crystal appear to have the same crystalline structure 8b ), in agreement with electron diffraction data. 6 When the electric field E is in the substrate plane and normal to the PTFE chains, the b-polarized components of the doublets are weak, especially the one at 690 cm -1 , which has half the intensity of the c-polarized component at 705 cm -1 . The ratio b/c of ∼1/2 is small compared to that (about 2) found in the spectrum of 6T in KBr pellets in which the molecules have no preferential orientation.…”

“…In a parallel paper, 6 we show by electron microscopy and electron diffraction on the same samples that a good epitaxial relationship exists at the 6T-oriented PTFE film interface, the ac or (010) plane of 6T crystals lying parallel to the (100) plane of the substrate (a and c are the long and short crystallographic axes of 6T crystal respectively 8 ): the distance between the PTFE chains (0.56 nm) is very close to an intermolecular distance of 6T molecules (0.55 nm) (see Scheme 2a,b). The molecules have their long axis L parallel to the PTFE fibers and consequently the a* axis (or, within less than 1°, the a axis) is tilted by (23.5°w ith respect to the PTFE chain axis.…”

Weak Epitaxy Growth Affording High‐Mobility Thin Films of Disk‐Like Organic Semiconductors

“…[3][4][5][15][16][17] A known epitaxial growth technology [6][7][8]18] was used to fabricate phthalocyanine thin films, as used in the inorganic semiconductor industry. [19] But the organic molecule usually showed the epitaxy growth on a single-crystal substrate [20,21] or oriented substrate [22][23][24] with molecules tending to adhere flat to their substrate due to the strong interaction between the molecules and the substrate. As a consequence, the higher conductive direction between the molecules cannot be parallel to the film plane, leading to low in-plane mobility.…”

“…We call this method a weak epitaxy growth (WEG) to distinguish it from other oriented epitaxy growth on single-crystal substrates [20,21] or oriented substrates. [22][23][24]27] The WEG approach produces highly oriented and continuous organic thin films of disk-like phthalocyanine compounds with the molecule p-p interaction direction parallel to the substrate, leading to a significant improvement of the carrier transportation in organic electronic devices. Exceptional electronic performance is especially expected in an organic field-effect transistor (OFET), where carriers are accumulated at the interface of the organic semiconductor near the gate insulator, with the transport direction parallel to the substrate.…”