Growth of para-Hexaphenyl Thin Films on Flat, Atomically Clean versus Air-Passivated TiO₂(110) Surfaces

Abstract

We report on para-hexaphenyl (6P) ultrathin film growth on freshly prepared and air-passivated atomically flat rutile titanium dioxide single-crystal (110) surfaces. The surface morphology of the developed structures has been investigated in situ and ex situ by means of various scanning probe techniques and electron microscopy. In situ 6P deposition results in the formation of a wetting layer of lying molecules coexisting with bunches of tens of micrometers long needles oriented along the TiO 2 [11̅ 0] surface direction. The observed bunching of the 3−5 nm high needles is explained in terms of anisotropic diffusion paths along and perpendicular to the needles. Air exposure of the asprepared films induces the formation of small features at the cost of the 6P wetting layer, whereas the needles stay unchanged. In contrast, 6P deposition on already air-passivated TiO 2 (110) yields the formation of dendritic islands, composed of roughly upright-standing molecules. No 6P wetting layer forms on the air-passivated surface. In addition to air exposure, we have checked the impact of surface modification via ion beam bombardment. Growth of 6P on gradient ion-beam-modified titanium dioxide substrates kept at either room or elevated temperature reveals that a slight surface roughening is sufficient to switch the film from lying molecular orientation to upright-standing orientation. However, surface stoichiometry severely influences film properties like size, density, and shape of the 6P islands.

Thin Film Phase and Local Chirality of Surface-Bound MOP4 Nanofibers

“…Similar effects have been observed for, e.g., naphthyl end-capped thiophenes 23,49 and para-hexaphenylene 71,72 .…”

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation

“…28 It supports -sometimes after the formation of a wetting layer 29,30 -three-dimensional, epitaxial growth of organic molecules. Due to its cm surface symmetry, this growth is highly anisotropic [31][32][33] as also observed for other anisotropic surfaces such as TiO 2 (110) 34,35 and Cu(110). 36,37 On the muscovite (001) face, the mirror plane changes from along [110] to [% 110] and back at consecutive cleavage planes.…”

On the nucleation and initial film growth of rod-like organic molecules

“…5A on Au(111) [32] and Ag(111) [33], 6P on Al(111) [34], but also on non-metallic substrates, e.g. 6P on TiO 2 (110) [35] or perfluoro-pentacene on silicon dioxide [36]. In all these cases the molecules are orientated parallel to the substrate, both in the monolayer as well as within the needle-like islands.…”