Fabrication of ion bombardment induced rippled TiO₂ surfaces to influence subsequent organic thin film growth

Abstract

Control over organic thin film growth is a central issue in the development of organic electronics. The anisotropy and extended size of the molecular building blocks introduce a high degree of complexity within the formation of thin films. This complexity can be even increased for substrates with induced, sophisticated morphology and anisotropy. Thus, targeted structuring like ion beam mediated modification of substrates in order to create ripples, pyramids, or pit structures provides a further degree of freedom in manipulating the growth morphology of organic thin films. We provide a comprehensive review of recent work on para-hexaphenyl (CH, 6P) as a typical representative of the class of small, rod-like conjugated molecules and rutile TiO(1 1 0) as an example for a transparent oxide electrode to demonstrate the effect of ion beam induced nanostructuring on organic thin film growth. Starting from molecular growth on smooth, atomically flat TiO(1 1 0) (1 × 1) surfaces, we investigate the influence of the ripple size on the resulting 6P thin films. The achieved 6P morphologies are either crystalline nano-needles composed of flat lying molecules or islands consisting of upright standing 6P, which are elongated in ripple direction. The islands' length-to-width ratio can be controlled by tuning the ripples' shape.

Molecular Structure and Electronic Properties ofpara-Hexaphenyl Monolayer on Atomically Flat Rutile TiO2(110)

“…We assign the α-peak to the 6P multilayer desorption, which arises from the nanoneedles formed when the coverage exceeds 2 MLs. The same behavior has also been reported for a number of different substrates (also for the case of TiO 2 (110)). ,, Following Redhead’s approach, the desorption energy of the monolayer was calculated from the measured TPD spectra, assuming first-order desorption of the β-peak as follows where E des is the desorption energy, R is the gas constant, T max is the maximum of desorption peak, ν 1 is the pre-exponential factor for Polanyi–Wigner first-order desorption, and β is the heating rate. Although we were not able to determine the ν 1 factor directly from the β-peak because of the very low signal-to-noise ratios, we used ν 1 ≈ 2 × 10 25(2) s –1 deduced by the “leading edge” method from the α-peak for several 6P coverages.…”

“…The same behavior has also been reported for a number of different substrates (also for the case of TiO 2 (110)). 21,22,70 Following Redhead's approach, 71 the desorption energy of the monolayer was calculated from the measured TPD spectra, assuming firstorder desorption of the β-peak as follows…”

“…For 6P, grown on reconstructed Cu(110) (2 × 1)–O substrates, it was proven that the formation of the needle nuclei results from the spontaneous dewetting of a second 6P layer on top of the compact first molecular layer, which is stabilized by the interaction with the substrate . Growth processes can be controlled not only by substrate materials but also by functionalization of their surfaces, which might be accomplished either by ion-beam irradiation ,,,− or passivation . Furthermore, growth can be controlled via the deposition parameters, such as the substrate temperature ,,− or deposition rate. , …”

“…It was already shown that 6P forms molecular nanoneedles on atomically flat TiO 2 (110) surfaces when deposited under ultra-high vacuum (UHV) conditions . On the contrary, 6P deposited on the ion-beam-modified TiO 2 (110) surfaces (with ripple-like morphology ,, ) assembles into elongated islands formed of upright standing molecules, whose length-to-width ratio is strongly correlated to the substrate’s topographical anisotropy . Also, the formation of a 6P molecular wetting layer (WL) on TiO 2 (110) was reported. − , However, the existence of a WL was only indirectly proven, but so far not directly observed.…”

“…On the contrary, 6P deposited on the ion-beam-modified TiO 2 (110) surfaces (with ripple-like morphology ,, ) assembles into elongated islands formed of upright standing molecules, whose length-to-width ratio is strongly correlated to the substrate’s topographical anisotropy . Also, the formation of a 6P molecular wetting layer (WL) on TiO 2 (110) was reported. − , However, the existence of a WL was only indirectly proven, but so far not directly observed. The 6P WL, as the initial regular structure formed during 6P deposition, is very decisive for the successive growth of 6P nanoneedles.…”