Introduction to Nanotechnology

260 SELF-ASSEMBLY AND CATALYSIS
the strain builds up as several additional layers are added. Eventually, beyond a
transition region the strain subsides, and thick films are only strained in the transition
region near the substrate. This mismatch complicates the free-energy discussion
following Eq. (10.2), and favors the growth of three-dimensional islands to
compensate for the strain and thereby minimize the free energy. This is called the
Stranski-Krastanov growth mode. A common occurrence in this mode is the initial
formation of a monolayer that accommodates the strain, and acquires a critical
thickness. The next stage is the aggregation of three-dimensional islands on the twodimensional
monolayer. Another eventuality is the coverage of the surface with
monolayer islands of a preferred size to better accommodate the lattice mismatch
strain. This can be followed by adding further layers to these islands. A
typical size of such a monolayer island is perhaps 5 nm, and it might contain 12 unit
cells.
In addition to the three free energies included in Eq. (10.2), the formation and
growth of monolayer islands involves the free energies of the strain due to the lattice
mismatch, the free energy associated with the edges of a monolayer island and the
free energy for the growth of a three-dimensional island on a monolayer, A great
deal of experimental work has been done studying the adsorption of atoms on
substrates for gradually increasing coverage from zero to several monolayers thick.
The scanning tunneling microscope images presented in Fig. 10.1 show the
successive growth of islands of InAs on a GaAs (001) substrate for several fractional
monolayer coverages. We see from the data in Table B.l that the lattice constant
a = 0.606~1 for InAs and a = 0.565nm for GaAs, corresponding to a lattice
mismatchf = 7.0% from Eq. (10.3), which is quite large.
10.1.3. Monolayers
A model system that well illustrates the principles and advantages of the self-
assembly process is a self-assembled monolayer (Wilber and Whitesides 1999). The
Langmuir-Blodgett technique, which historically preceded the self-assembled
approach, had been widely used in the past for the preparation and study of optical
coatings, biosensors, ligand-stabilized AuSS clusters, antibodies, and enzymes. It
involves starting with clusters, forming them into a monolayer at an air-water
interface, and then transferring the monolayer to a substrate in the form of what is
called a Langmuir-Blodgettjlm. These films are difficult to prepare, however, and
are not sufficiently rugged for most purposes. Self-assembled monolayers, on the
other hand are stronger, are easier to make, and make use of a wider variety of
available starting materials.
Self-assembled monolayers and multilayers have been prepared on various
metallic and inorganic substrates such as Ag, Au, Cu, Ge, Pt, Si, GaAs, SiOz,
and other materials. This has been done with the aid of bonding molecules or ligands
such as alkanethiols RSH, sulfides RSR’, disulfides RSSR’, acids RCOOH, and
siloxanes RSiOR3, where the symbols R and R’ designate organic molecule groups
that bond to, for example, a thiol radical -SH or an acid radical -COOH. The
binding to the surface for the thiols, sulfides, and disulfides is via the sulfur atom;