Biomedical Engineering Reference
In-Depth Information
FIGURE 1.6
Positioning single atoms with a scanning tunneling microscope
[5]
.
goes on in the corral by solving the classic eigenvalue problem in quantum mechanics—a particle in a
hard-wall box.
Probably the most publicized material in recent years has been carbon nanotubes. Carbon nano-
tubes, long, thin cylinders of carbon, were discovered in 1991 by Iijima. These are large macromol-
ecules that are unique for their size, shape, and remarkable physical properties. They can be thought
of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. These intriguing struc-
tures have sparked much excitement in the recent years and a large amount of research has been dedi-
cated to their understanding. Currently, the physical properties are still being discovered and disputed.
What makes it so difficult is that nanotubes have a very broad range of electronic, thermal, and struc-
tural properties that change depending on the different kinds of nanotube (defined by its diameter,
length, and chirality, or twist). To make things more interesting, besides having a single cylindrical
wall (SWNTs), nanotubes can have multiple wall (MWNTs) cylinders inside the other cylinders.
Bower et al.
[7]
have grown vertically aligned carbon nanotubes using microwave plasma-
enhanced CVD system using a thin film cobalt catalyst at 825°C (
Figure 1.8
). The chamber pressure
used was 20 Torr. The plasma was generated using hydrogen which was replaced completely with
ammonia and acetylene at a total flow rate of 200 sccm.
Lithographic methods are important for micro- and nanofabrication. Lithography (in Greek
'lithos' means stone; 'graphein' means to write) is a planographic printing technique using a plate or
stone with a smooth surface. In micro- and nanofabrication we mean pattern transfer. Due to limita-
tions in current (and future) photolithographic processes there is a challenge to develop novel litho-
graphic processes with better resolution for smaller features. One such development is that of
dip-pen
nanolithography
(DPN). Dip-pen technology in which ink on a pointed object is transported to a
surface via capillary forces is approximately 4,000 years old. The difference with DPN is that the
pointed object has a tip which has been sharpened to a few atoms across in some cases. DPN is a
scanning probe nanopatterning technique in which an atomic force microscope (AFM) tip is used to
deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmo-
sphere. It is a direct-write technique and is reported to give high-resolution patterning capabilities for
a number of molecular and biomolecular “inks” on a variety of substrates, such as metals, semicon-
ductors, and monolayer functionalized surfaces.
DPN allows one to precisely pattern multiple patterns with good registration. It is both a fabrication
and an imaging tool, as the patterned areas can be imaged with clean or ink-coated tips. The ability to
achieve precise alignment of multiple patterns is an additional advantage earned by using an AFM tip to
