Initial Stage of para-Hexaphenyl Thin-Film Growth Controlled by the Step Structure of the Ion-Beam-Modified TiO₂(110) Surface

Abstract

Organic electronics require a precise control over properties of a molecule–substrate interface as well as film growth morphology, from both fundamental points of view, when a clean vacuum environment is needed and also under ambient air conditions. In this paper, we present submonolayer molecular films of para-hexaphenyl (6P) formation on the rutile TiO2(110) substrates and ways of affecting the growth and morphology via ion-beam nanopatterning. Ultrahigh vacuum deposition and measurements are followed by the film evolution study upon air exposure. Strongly anisotropic TiO2(110) surfaces, in the form of terraced ripples with a preserved (1 × 1) structure, were controllably fabricated utilizing ion-beam bombardment and characterized by means of high-resolution scanning tunneling microscopy and low-energy electron diffraction. 6P thin films were prepared using organic molecular beam epitaxy and characterized in situ by noncontact atomic force microscopy. Ex situ characterization was performed by tapping-mode atomic force microscopy, scanning electron microscopy, and noncontact atomic force microscopy with molecular resolution. We have demonstrated that by changing the size of locally preserved (1 × 1) surface areas, determined by the ripple parameters, different 6P assemblies can be promoted. With the shrinking size of the uninterrupted (1 × 1) terminated areas, 6P changes its growth morphology from needlelike to islandlike accompanied by a reorientation of the molecules from flat-lying to upright-standing. The resulting morphology depends on the structure of a two-dimensional phase of lying molecules formed at the initial stage of deposition, which can be either a well-ordered wetting layer or a two-dimensional mobile lattice gas. The postgrowth remainders of these two-dimensional phases participate in additional nucleation processes forming small islands or clusters.

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

“…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. , …”

“…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.…”

“…20 Also, the formation of a 6P molecular wetting layer (WL) on TiO 2 (110) was reported. [21][22][23]44 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.…”