Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 25

Fig. 1.21 The principle of
the Langmuir-Blodgett
technique

monolayer

glass plate

surfactant

water

over a water surface. If a glass microscope slide acting as the self-assembly substrate
is introduced in the water, a monolayer attaches to it when pulling the glass slide
out of water, as shown in Fig. 1.21 . The process can be repeated, and a monolayer
is deposited each time the glass slide passes through the water surface. In practice,
the LB technique requires specialized instruments such as a Langmuir trough, a
dipping device for the lowering or raising substrate, and a movable barrier, which is
controlled by a pressure sensor and maintains a constant surface pressure by sliding
on the gas-liquid interface. The LB technique was used to fabricate polymeric films,
2D gold nanoparticle arrays, and semiconducting quantum dots. A good review
about LB films is Rietman ( 2001 ).
Electrostatic self-assembly is a widespread technique based on the electrostatic
interaction between nanoparticles or molecules. The origin of this self-assembly
technique resides in the fabrication of multilayer films, in which each layer contains
negatively and positively charged colloid particles, for example, Al and Si. The
method is used in the fabrication of nanostructured films containing semiconductors,
metals, polymers, magnetic materials, or organic molecules.
The thin films fabricated by electrostatic self-assembly are stable and uniform
because of the strong ionic bonds between positive- and negative-charged particles,
while the combination between polymers and the layers of electrically charged
nanoparticles minimizes the eventual defects. In the electrostatic self-assembly
process, a clean substrate is first immersed into a cationic solution, then rinsed,
and dried, followed by another dip of the substrate already coated with cations into
an anionic solution and subsequent rinsing and drying processes. This procedure
can be repeated several times, the resulting nanostructured film growing with one
layer/cycle (see Fig. 1.22 ). As an example, a heterostructure containing consecutive
layers of gold and silver nanoparticles can be fabricated by first self-assembling
positively charged Au nanoparticles on a negative glass substrate by dipping the
substrate into 4-ATP (aminotiophenol)-capped Au solution having pH D 4,then
depositing a monolayer of negatively charged Ag nanoparticles by dipping the
previous Au-coated glass into a 4-CTP (carboxythiophenol)-capped Ag solution
having pH D 8:5, and repeating the cycle as many times as necessary ( Huie 2003 ).
In a similar manner, it is possible to self-assemble a multilayer film consisting of
conducting polymer chains and nanosized iron oxide, which combines the strength
and flexibility of polymers with the magnetic properties of iron. In both cases, the