Layer Dependent Wetting in Parahexaphenyl Thin Film Growth on Graphene

Abstract

In order to exploit graphene (Gr) as a transparent electrode in organic optoelectronics, a profound understanding of molecular wetting and diffusion processes on this material is essential. Properties of Gr like band structure, work function, or elasticity depend significantly on the layer number. Here, we report on Gr layer dependent differences in the growth morphologies of hot wall epitaxy deposited sub-monolayer thin films of the one-dimensional conjugated parahexaphenyl (6P) molecule on exfoliated Gr/SiO2. At deposition temperatures around 363 K, 6P forms straight needles arranged in regular networks on single layer graphene. Within the first four Gr layers, the average needle height increases and the average needle length decreases thereby reducing the specific 6P-Gr interface area. The variations in morphology and wetting behavior with the Gr layer number arise from changes in the growth kinetics induced by differences in adhesion and diffusion properties with Gr thickness.

Effects of polymethylmethacrylate-transfer residues on the growth of organic semiconductor molecules on chemical vapor deposited graphene

“…[9][10][11][12][13][14][46][47][48][49][50][51][52][53][54][55][56][57][58] We find that 6P grows in a flat-lying needle-type morphology on CVD MLG transferred to oxidized Si wafers, irrespective of observed PMMA residue levels and MLG defects. This indicates that MLG dominates interface properties even in the case of severe contamination or damage.…”

“…Only subsequent to post-transfer H 2 anneals -that remove even minor PMMA residues -extended 6P needle growth is obtained which resembles that observed on clean exfoliated MLG. [12][13][14] We grow MLG by low-pressure CVD on Cu catalysts. 32,33 The as-grown MLG on the Cu is then covered with a PMMA layer, and wet chemical etches are used to release the PMMAcoated MLG from the Cu to subsequently transfer the stack onto SiO 2 (300 nm) covered Si-wafers.…”

“…6P is then deposited onto the transferred CVD MLG via hot wall epitaxy (HWE) with MLG substrates held at 90 C (deposition time 5 min). 12,13 For further experimental details see the supplementary material. 59 The small conjugated 6P molecule is chosen as a model system since its growth behavior is highly sensitive to surface conditions, showing two principal growth morphologies 50 Author to whom correspondence should be addressed.…”

“…For further increased annealing temperatures of 400 C and 500 C, the longer 6P needles frequently act as nucleation centers themselves yielding the formation of cross or star like needle bunches, similar to 6P deposition on exfoliated MLG. 12,13 Figure 3(a) shows a quantitative analysis of 6P needle length distributions as a function of MLG H 2 annealing temperature. For each histogram, the lengths of 250-500 needles were evaluated.…”

“…At low coverages, usually only mono-or bi-layer 6P islands are observed. 12,13 The insets show corresponding AFM images of needle and island growth on exfoliated MLG 12,13 and amorphous carbon, respectively (1.75 Â 1.75 lm 2 , z-scale 10 nm, see below).…”