Thermally driven smoothening of molecular thin films: Structural transitions in n-alkane layers studied in real-time

Abstract

We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.

Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets

“…It is known that the crystallinity and packing of thiol molecules on a gold surface depend on the chain length. With longer alkyl chains, the assembled thiol molecules are more densely packed and better organized because of the increased intermolecular interactions. ,, The self-assembled alkanethiols with a longer chain show a higher degree of surface coverage with fewer surface defects. − Such details in the molecular packing may account for a lower number density of the nanodroplets with the chain extension because there are fewer nucleation sites available for droplet formation. The same reason may explain the largest derivation in the droplet size observed on the tetradecanethiol surface in Figure a.…”

2,4,5,7,9,10-Hexaethynylpyrenes: Synthesis, Properties, and Self-Assembly

“…This finding strongly supports the belief that the diffraction peak at ca. 24° at 40 °C is due to the crystal lattice from the packing of the decyloxy chains; thus, the endothermic peak at 81.2 °C corresponds to the melting of the decyloxy segments. The WAXD profile in the LC state at 100 °C showed distances of 29.4 nm (200), 23.2 nm (100), 20.1 nm (300), 17.7 nm (220), 14.7 nm (400), and 1.19 nm (500), which correspond to a rectangular columnar (Col r ) lattice ( P 2/ a ) with lattice parameters of a = 5.88 nm, b = 2.52 nm, and c = 0.77 nm (see Figure c).…”

Enhanced photoelectrical response of thermodynamically epitaxial organic crystals at the two-dimensional limit

“…A thermodynamical equilibrium shape with a surface energy minimum is expected, showing a 2D growth mode (Fig. 3k) 31,32 . Thus, the residues and thick crystals decrease with increasing temperature.…”