Dynamics of Bimodal Growth in Pentacene Thin Films

Abstract

Previous studies have established that pentacene films deposited on silicon oxide consist of a substrate-induced "thin-film" phase, with the bulk phase of pentacene detected in thicker films only. We show that the bulk phase nucleates as early as the first monolayer, and continues to nucleate as film growth progresses, shadowing the growth of the thin-film phase. Moreover, we find that the transition between the "thin-film" and the bulk phase is not a continuous one, as observed in heteroepitaxial systems, but rather the two phases nucleate and grow independently.

One- and two-center physical space partitioning of the energy in the density functional theory

“…They are connected with the existence of bonding molecular orbitals spreading over the atoms in question, which leads to a correlation between the fluctuations of the atomic populations: 18,29 if there is a positive charge fluctuation on one atom, then on the bonded atom, a negative one may be expected-and this correlation leads to a negative energy contribution, similar to that as the correlation of fluctuating dipoles causes attractive dispersion interaction. This negative energy component appears even if the charge is completely concentrated near the nuclei: it is present 18,30,31 in the CNDO-type semiempirical theories in which the overlap populations integrate to zero ͑the orbitals are assumed orthogonal͒ and the zero differential overlap approximation is used for the integrals, and even in the case of a point-charge approximation of the two-electron integrals. 31,32 This connection between the diatomic exchange, bond orders, and negative exchange energy contributions motivated us to introduce a new quantity called "bond order density"-it will be discussed in more detail elsewhere.…”

“…This negative energy component appears even if the charge is completely concentrated near the nuclei: it is present 18,30,31 in the CNDO-type semiempirical theories in which the overlap populations integrate to zero ͑the orbitals are assumed orthogonal͒ and the zero differential overlap approximation is used for the integrals, and even in the case of a point-charge approximation of the two-electron integrals. 31,32 This connection between the diatomic exchange, bond orders, and negative exchange energy contributions motivated us to introduce a new quantity called "bond order density"-it will be discussed in more detail elsewhere. The bond order density ␤ AB ͑r͒ for the pair of atoms A and B is obtained by integrating the diatomic exchange density AB x ͑r , rЈ͒ over the coordinate rЈ; for the closed-shell case considered here, one gets…”

Two complementary molecular energy decomposition schemes: The Mayer and Ziegler–Rauk methods in comparison

“…A simple alternative of these methods are various bondorder schemes. [10][11][12][13][14][15] Their advantage is that a characteristic value ͑bond order or bond index͒ is attributed to each pair of atom in a molecule. Thus, the bond order is a continuous measure of bonding, available at any molecular configuration.…”

“…Thus, the bond order is a continuous measure of bonding, available at any molecular configuration. The most widely used bond-order scheme is due to Mayer,13,14 which is essentially a generalization of Wiberg bond indices 12 for the case of nonorthogonal basis functions. Since the Mayer bond orders are easy to evaluate and to interpret, they are frequently used to elucidate molecular structures.…”

“…They are related to the one-atom "operator of the number of electrons" used by Mayer within the secondquantization formalism. 14 The one-atom density matrix A ͑x ͉ xЈ͒ is also closely related to Mayer bond orders B AB as follows:…”