Chemistry Reference
In-Depth Information
Figure 5.1. Scheme of the different mechanisms of growth: (a) Frank-van der
Merwe, (b) Volmer-Weber and (c) Stranski-Krastanov. The substrate and over-
layers are represented by dark grey and light grey shading, respectively.
5.1 Heterogeneous growth
Three modes of growth are known and they are termed after their discoverers:
Frank-van der Merwe, Stranski-Krastanov and Volmer-Weber. A schematic of
these basic mechanisms of growth is given in Fig. 5.1, which highlights the
dimensionality of the growth: 2D (in-plane) or 3D (out-of-plane). The Frank-van
der Merwe mode, also called the layer-by-layer mode, is of the 2D type, where a
layer builds on top of the underlying one only after completion (Fig. 5.1(a)). This
is the case of complete wetting. Contrary to this mode, the Volmer-Weber is purely
3D, where part of the substrate remains uncovered, representing the non-wetting
case (Fig. 5.1(b)). In this 2D-3D competition, the Stranski-Krastanov mode occu-
pies the intermediate position, since the first layers grow 2D but the 3D mode finally
dominates (Fig. 5.1(c)). The growth is intrinsically a non-equilibrium process and
the state obtained is not necessarily the thermodynamically most stable one but
is instead kinematically determined. Herein lies the
essence
of the formation of
different polymorphs.
Organic thin films have a tendency to adopt a preferential orientation with their
molecular planes having the shortest molecule-molecule contacts (higher molecu-
lar density) parallel to the substrate surface in the case of weak molecule-substrate
interactions. This well-known experimental fact can be understood within the
framework of the heterogeneous growth model (Chernov, 1984), which is based
on the minimization of the total Gibbs free energy
G
of the system. In order to
evaluate the energy barrier
G
c
for the formation of stable crystalline aggregates or
nuclei on a foreign surface, let us consider for simplicity nuclei having the shape of
a parallelepiped of constant height
h
and sides
L
attached to a substrate single step
also of height
h
, as depicted in Fig. 5.2. A step can be regarded as a line boundary
at which the surface changes height by one or more molecular units.
In spite of its simplicity this approximation retains the necessary information and
is justified because the vast majority of MOMs are layered and thus structurally
anisotropic. Arrows in Fig. 5.2 aim to succinctly describe the basic atomistic mech-
anisms of step-flow growth, based on the BCF model (Burton
et al
., 1951).
