Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 54

Interchelation Polymer

mixing

C 12 H 25 NH 3 + Br -

Figure 2.14 Steps in the production of a nanocomposite based on exfoliated clays.

quaternary ammonium ions with long aliphatic chains or by melt processing with
a suitable polymer such as nylon (Figure 2.14). This then results in a powder that
can be further compounded with a polymer to produce a clay-reinforced nano-
composite that contains the platelets, which are well distributed through the
composite.

2.5.6 Top Down

Whilst some nanomaterials may be simply mined there are a whole range of materi-
als which have to be prepared in a more laborious manner. There are two main
methods for preparing nanomaterials, top down and bottom up. The top down
approach works on the basis of breaking down a large piece of material into a
smaller piece, in this case with dimensions in the nanometer range. This method
can be used on most nanomaterials and is usually related to the patterning of a
surface by either lithography and etching, or by electron or fast atom bombardment
(Mendes et al. , 2004). More recent interest has grown in the ability to use scanning
near fi eld pattering methods to chemically alter surfaces at resolutions down to
9 nm using conventional light (Leggett, 2006) or to use atomic force microscopy
tips to plough soft polymer fi lms on the nanoscale (Kunze, 2002). All of these
methods result in essentially fl at patterned surfaces; as these surfaces have some
depth they are often termed 2½D techniques. If these techniques are used to
produce a nanoscale pattern in a hard material they can then be used to prepare
moulds and be further used for nano-imprinting and other related methods for
more large scale production of devices.

2.5.7 Bottom Up

The bottom up approach relies on using small molecules to prepare the nanopar-
ticles. For example, it is well known that the addition of a solution containing sulfate
ions to a solution containing calcium ions will result in the rapid production of
calcium sulfate. Calcium sulfate, being very insoluble in water, will form a precipi-
tate, the solution will turn cloudy and the fi nal product will settle out at the bottom