Post-Deposition Wetting and Instabilities in Organic Thin Films by Supersonic Molecular Beam Deposition

Abstract

We discuss the formation and post-deposition instability of nanodrop-like structures in thin films of PDIF-CN2 (a perylene derivative) deposited via supersonic molecular beam deposition technique on highly hydrophobic substrates at room temperature. The role of the deposition rate on the characteristic lengths of the organic nanodrops has been investigated by a systematic analysis of atomic force microscope images of the thin films and through the use of the height-height correlation function. The nanodrops appear to be a metastable configuration for the freshly-deposited films. For this reason, post-deposition wetting effect has been examined with unprecedented accuracy throughout a year of experimental observations. The observed time scales, from few hours to months, are related to the growth rate, and characterize the thin films morphological reordering from three-dimensional nanodrops to a well-connected terraced film. While the interplay between adhesion and cohesion energies favors the formation of 3D-mounted structures during the growth, wetting phenomenon following the switching off of the molecular flux is found to be driven by an instability. A slow rate downhill process survives at the molecular flux shutdown and it is accompanied and maybe favored by the formation of a precursor layer composed of more lying molecules. These results are supported by simulations based on a non-linear stochastic model. The instability has been simulated, for both the growth and the post-growth evolution. To better reproduce the experimental data it is needed to introduce a surface equalizer term characterized by a relaxation time taking into account the presence of a local mechanism of molecular correlation.

Post-Growth Dynamics and Growth Modeling of Organic Semiconductor Thin Films

“…Actually, in most cases, the post-growth evolution is not contemplated possibly because of the high stability of the films. When observed, it is not considered for the extraction of the scaling exponents from the HHCF analysis …”

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors

“…They are all listed in Table . Here, the difference λ – ξ can be interpreted as a measure of the mean distance between the picene islands (in a certain way it is the measure of the degree of connection between the islands) . Our attention is focused on this parameter because, generally, the charge carrier transport properties are remarkably correlated to the island contiguity as well as the quality of the grain boundaries .…”

“…Here, the difference λ – ξ can be interpreted as a measure of the mean distance between the picene islands (in a certain way it is the measure of the degree of connection between the islands). 44 Our attention is focused on this parameter because, generally, the charge carrier transport properties are remarkably correlated to the island contiguity as well as the quality of the grain boundaries. 45 The minimum value of the λ – ξ difference is observed in the case of the heterostructure 60/15 (60 nm of picene grown on 15 nm of PDIF-CN 2 ) which, in terms of morphological properties, represents the most promising choice for the ambipolar device realization.…”

Growth, morphology and molecular orientation of controlled Indigo thin films on silica surfaces

“…The average diameter of grains on such large faced structures is equal, within the experimental error, to the average diameter obtained by TEM analysis (see Figure S3 and S5 in the SI). It is not possible with topographic AFM images to discern if grains, possibly formed by instabilities of molecular films (like post-deposition wetting 61 ) are only on the surface, or also within the structure. However, the average step width (see inset of Figure 5a) is similar to the average diameter of grains, suggesting possible re-organization phenomena effects.…”