Page 163 - Science at the nanoscale

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7.3. The Self-Assembly Processes
This topic has attracted much attention as it allows us to engi-
neer special surface architectures required in new technological
applications.
Well-defined surfaces, particularly crystalline planes of metallic
solids, are found to provide versatile platforms for the assembly
of molecules into clusters, chains, 2D arrays, and even 3D super-
8
lattice architectures. This is often carried out by chemical vapour
deposition or molecular beam epitaxy inside a high vacuum chamber.
The visualisation of these structures is often aided with scanning
tunneling microscopy (STM), a nano-tool discussed in Chapter 8.
In general, the self-assembly is driven by interactions between the
assembled molecules and the substrate surface, as well as between
the molecules in adjacent layers. This is the driving force towards
the reduction of the overall Gibbs energy.
Researchers have tried to construct complex surface architec-
tures using non-covalent interactions such as hydrogen bonding,
π − π stacking, van der Waals interaction, etc. between neigh-
bouring molecules. In some cases, the first few layers of adsorbed
molecules define the architecture capable of trapping other enti-
ties in the subsequent layer. An example is illustrated in Fig. 7.7
whereby monolayers and bilayers of α-sexithiophene (6T) adsorb
on the Ag(111) surface to form stripe-like patterns (Fig. 7.7(a)).
In subsequent adsorption experiments, preferential adsorption of
C 60 molecules in linear molecular chains is observed on the bilayer
6T nanostripes (Fig. 7.7(b)).It is proposed that this arises from the
donor-acceptor interaction between 6T and C 60 .
The adsorption of some molecules can also be performed on
specific surfaces in solutions to form self-assembled monolayers
(SAMs). These are monolayers of amphiphilic molecules that
remain intact after the substrates are removed from the solution. 153 ch07
These SAMs are stable in air and ordinary temperatures, offer-
ing a convenient route to tailor the properties of an entire sur-
face. SAMs can be prepared using different sets of molecules
and substrates, examples include alkyl silanes such as octade-
cyltrichlorosilane (OTS, CH 3 (CH 2 ) 17 SiCl 3 ) on various oxide sur-
faces; alkyl carboxylates such as fatty acid on aluminium or mica
8 J. V. Barth, Annu. Rev. Phys. Chem. 58, 375–407 (2007).
9 H. L. Zhang, W. Chen, L. Chen, H. Huang, X. S. Wang, J. Yuhara and A. T. S. Wee,
“C 60 molecular chains on α-sexithiophene nanostripes”, Small 3, 2015–2018
(2007).