Page 66 - Nanotechnology an introduction

energy. Therefore, particles of almost any insoluble material synthesized in water are likely to aggregate, unless appropriate measures to ensure
their hydration are taken. A useful strategy is to synthesize the particles in the presence of a very hydrophilic material such as polyethylene glycol or
a polyion such as hexametaphosphate, which is able to adsorb on the surface of the particles and effectively hydrate them. Michael Faraday's
famous synthesis of gold nanoparticles used citrate ions to hydrate their surface [50]. Crystals of silver chloride, silver bromide and silver iodide
ranging in size from tens of nanometers to micrometers, which form the basis of conventional silver halide-based photography, are stabilized in the
emulsions used to coat glass or polymer substrates by the natural biopolymer gelatin.

Micelles and superspheres are dealt with in Section 8.2.9.
6.2. Nanofibers
“Nanofiber” is the generic term describing nano-objects with two external dimensions in the nanoscale. A nanorod is a rigid nanofiber, a nanotube
is a hollow nanofiber, and a nanowire is an electrically conducting nanofiber (Figure 6.2).
Two approaches are presently mainly used to synthesize nanofibers. For some substances, under certain conditions, the natural growth habit is
acicular. Therefore, the methods described in Section 6.1.2 can be used to generate nuclei, followed by a growth stage to elongate them.
Heterogeneous nucleation can be induced at the solid/gas interface by predepositing small catalytic clusters. Upon addition of vapor, condensation
on the clusters and growth perpendicular to the solid substrate takes place. This is used as an efficient way of synthesizing carbon nanotubes (see
Section 9.2). If uniform nanopores can be formed in a membrane (e.g., by laser drilling or by self-assembly) they can be used as templates for
nanofiber formation. The material for the fiber should be deposited as a shell on the inner surface of the pores (if the goal is to make nanotubes), or
else should completely fill the pores (for nanorods). Nanofibers, especially nanorods, formed by either of the two previous methods can also be
used as templates for making nanotubes of a different material. Lieber has reported the general synthesis of semiconductor nanowires with control
of diameter [44] and [66].

6.3. Nanoplates and Ultrathin Coatings
Many of the traditional engineering methods of fabricating thin coatings on a substrate have not produced objects in the nanoscale because
typically they have been more than 100 nm thick. Nevertheless, the trend is to develop thinner functional surfaces by coating or otherwise modifying
bulk material, and insofar as the coating or modification is engineered with atomic precision, it belongs to nanotechnology, and if it is
nanostructured either laterally or perpendicular to the substrate, it will rank as a nanomaterial even if its overall thickness exceeds 100 nm.
The surface treatment of bulk material, especially metals, is an ancient technology. In the cases where nanoscale structural features were
apparently necessary to ensure having the required attributes (e.g., in Damascus swords [146]), although the structures were created deliberately
the nanoscale aspect might be considered as essentially inadvertent since the technologist is unlikely to have explicitly envisioned the nanoscale
structuring. This is in contrast to, for example, the medicinal use of nanoparticulate gold (introduced by Paracelsus in the 16th century), when it was
realized that a metal could only be assimilated by a living human organism if it was sufficiently finely divided.
Completely in the spirit of nanotechnology are the monomolecular layers, now called Langmuir films, transferred to solid substrata using the
Langmuir–Blodgett and Langmuir–Schaefer techniques; these films might only be a few nanometers thick. Their preparation is a “top–down”
approach.

Many physical vapor deposition techniques (such as an evaporation and magnetron sputtering) create films thicker than the nanoscale and hence
are out of the scope of this book. Molecular beam epitaxy (MBE), a technique of great importance in the semiconductor processing industry, is
briefly covered in Section 6.3.1 (see also Section 8.1.1).

6.3.1. Molecular Beam Epitaxy (MBE)

MBE can be considered as a precise form of physical vapor deposition (PVD). Solid source materials are placed in evaporation cells around a
−5
centrally placed, heated, substrate. Pressures less than 10 torr ensure that the mean free path of the vapor exceeds practical vacuum chamber
dimensions (~ 1 m), the molecular beam condition. Ultrahigh vacuum (UHV) conditions are needed to ensure the absence of contamination from
residual gases (from the chamber walls, etc.). A few seconds are typically required to grow one monolayer. The technique has been developed very
successfully using a practical, empirical approach—thermodynamic analysis is difficult because the various parts of the system (sources, substrate,
chamber wall) are at different temperatures.
6.3.2. Langmuir Films

Langmuir films, first reported by Pockels at the end of the 19th century, consist of a monomolecular layer of amphiphiles (molecules consisting of an
apolar nanoblock conjoined to a polar block of roughly the same size) floating on water. The polar “heads” dip into the water and the apolar “tails”
stick up into the air. In other words, the film precursors are molecules of general formula XP, where X is (typically) an apolar chain (e.g., an alkyl
chain), called the “tail”, and P is a polar “head” group such as oligoethylene oxide, or phosphatidylcholine. When spread on water they mostly
remain at the water/air interface, where they can be compressed (e.g., using movable barriers) until the molecules are closely packed to form two-
dimensional liquid-like and solid-like arrays. Slowly withdrawing a hydrophilic plate perpendicular to and through the floating monolayer from below
will coat the plate with a packed monolayer (Figure 6.6(a)), as was extensively investigated by Blodgett; these supported layers are called
Langmuir–Blodgett (LB) films. The Langmuir–Blodgett technique refers to the transfer of the floating monomolecular films to solid substrata by
vertically dipping them into and out of the bath. In the Langmuir–Schaefer (LS) technique, the substratum is pushed horizontally through the floating
monolayer (Figure 6.7). Very stable multilayer films can be assembled by making P a chelator for multivalent metal ions, which bridge lateral
neighbors and/or successive layers (assembled head–head and tail–tail). Lateral stability can be increased by UV-irradiation of films with an
unsaturated alkyl chain (photopolymerization). The process of perpendicular traversal of the floating monolayer can be repeated many times to
build up multilayer films, provided the close packing of the monolayer is maintained, e.g., by adding fresh material pari passu in an appropriate
fashion. Exceptionally strongly laterally cohesive and rigid Langmuir films can be manipulated as free-standing objects.