Nucleation of organic semiconductors on inert substrates

Abstract

We have adapted the microscopic theory of nucleation for the epitaxial growth of inorganic materials to the nucleation of organic small molecules on an inert substrate like the gate dielectric of an organic thin-film transistor. The parameters required to explore the model were calculated with the standard MM3 force field and also include experimentally determined vapor pressure data, as well as film growth data. Sufficient agreement is found between the experimentally determined equilibrium crystal shape and heats of sublimation on the one hand and the calculated parameters on the other hand. The growth of pentacene, tetracene, and perylene on inert substrates has been studied in terms of this theory, especially focusing on the two-dimensional ͑2D͒ to 3D nucleation transition. It is demonstrated that 3D nucleation leads to ill-connected grains, while 2D nucleated grains form continuous films suitable for charge transport. The analysis of this transition allows for the experimental determination of the molecule-substrate interactions for a given molecule on a given surface. It was found that the deposition conditions for 2D growth shift to less favorable substrate temperatures and deposition rates as the difference between interlayer interactions and molecule-substrate interactions increase and the intralayer interactions decrease. Moreover, those interactions affect the nucleation rate and therefore the ultimate 2D grain size that can be obtained.

A benchmark theoretical study of the electronic ground state and of the singlet-triplet split of benzene and linear acenes

“…8,9 Naphthacene ͑also referred to as tetracene͒, pentacene, and derivative compounds have been used recently to prepare highly ordered conducting organic materials or thin films with particularly large charge carrier mobilities. 10 In contrast with the prevailing view 11 that large acenes have a closed-shell singlet or open-shell triplet electronic ground states, a few groups [12][13][14][15][16] recently conjectured that large acenes such as hexacene, heptacene, or octacene should rather be regarded as open-shell singlet biradical systems, as a result of the instability of unrestricted ͑UBLYP, UB3LYP, UPW91, UBPW91, etc.͒ wave functions, in conjunction with rather modest basis sets ͑STO-3G, 6-31G ‫ء‬ , or cc-pVDZ͒. This in-stability was diagnosed 12 from the fact that in these unrestricted calculations the two outermost singly occupied ␣ and ␤ spin orbitals do not smoothly follow the D 2h symmetry point group imposed by the nuclear frame, but rather reflect an overwhelmingly strong symmetry breaking of the electronic wave function, 12 in the form of a localization of the two frontier electrons on opposite polyacetylenic strands.…”

Nucleation and growth of thin films of rod-like conjugated molecules

“…Typically, one aims at a smooth film with a low number of grain boundaries and a flat surface. Grain boundaries represent inhomogeneities in the thin film and will have a negative effect on the transport properties [110][111][112][113]. On top of that they also affect the optical properties of the thin film devices such as nano fiber wave guides [104,[114][115][116].…”

Determination of crystal orientation in organic thin films using optical microscopy

“…If studied thin films form epitaxial layers such as having one crystal axis somewhat aligned with the normal to the substrate, the problem simplifies to finding the relationship between the index ellipse and the in-plane crystal orientation. In case of single crystalline epitaxial films, the index ellipse can be found experimentally using, for example, ellipsometry [19] or polarized absorption spectroscopy [21] while the crystal orientation e using the crystal shape [22] or x-ray diffraction (XRD) measurements [16]. In our work, we find optical properties using light microscopy techniques while the in-plane crystal orientation is obtained from preferential crystal cracking directions.…”