Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 26

RINSE

RINSE DRY

DRY

water water water

anionic
solution

cationic
solution

Fig. 1.22 The principle of the electrostatic self-assembly method

alkanethiol
molecules

Au
nanoparticle

Au substrate

SAM

MPC

Fig. 1.23 Schematic representations of the SAM and MPC methods

properties of the final nanostructure are a mixture of the properties of the constituent
layers, which is a typical characteristic for self-assembled nanostructures.
Chemical self-assembly methods are very frequently used to fabricate nanostruc-
tures, among them the SAM (self-assembled monolayers) and MPC (monolayer-
protected clusters) techniques being prevalent. In the SAM method, monolayers
form spontaneously by immersion of an appropriately chosen substrate into a
solution, whereas MPC uses chemisorption to obtain nanoclusters with surfaces
derivatized by ligand molecules, as displayed in Fig. 1.23 .
Finally, proteins and DNA are the basic constituents of biomolecular self-
assembly, which is used to fabricate (1) self-assembly of semiconducting nanopar-
ticles, for example, ensembles of CdSe quantum dots; (2) functionalized metal-
lic nanoparticles; or (3) biomolecular-metal complexes, for instance, DNA-Au
complexes.
Many other self-assembly methods for nanostructures exist, the self-assembly
strategy being one of the major techniques for fabricating new devices or materials
( Shenhar et al. 2004 ). As an example, metal nanowires with lengths in the 20-50
m range and widths between 2 and 50 nm can be fabricated by self-assembly
techniques in a defined 3D geometry, without lithography ( Saif et al. 2003 ). These
long and ultrathin metal nanowires are obtained by PECVD coating a Si substrate
with SiO 2 incorporating OH impurities, which produce compressive stress in the
Si=SiO 2 film. This film is subsequently annealed at 600 ı C for a long time to