Biomedical Engineering Reference
In-Depth Information
cylindrical wall (SWNTs), nanotubes can have multiple walls (MWNTs) cylinders inside the
other cylinders.
Bower et al.
[10]
have grown vertically aligned CNTs using microwave plasma enhanced CVD
system using a thin film cobalt catalyst at 825
C(
Figure 1.6
). 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: drawing or
writing on kind of yellow salty limestone so that impressions in ink can be taken and in the Oxford
Dictionary the word Lithos comes from Greek for stone. In micro- and nanofabrication we mean
pattern transfer. Due to limitations in current (and future) photolithographic processes, there is a
challenge to develop novel lithographic 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 4000 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 AFM
tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the
ambient atmosphere. 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, semiconductors, and monolayer functionalized surfaces.
DPN allows one to precisely pattern multiple patterns with good registration. It is both a fabri-
cation and 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 write as well as read nanoscopic features on a surface. These attributes make
DPN a valuable tool for studying fundamental issues in colloid chemistry, surface science, and
nanotechnology. For instance, diffusion and capillarity on a surface at the nanometer level, organi-
zation and crystallization of particles onto chemical or biomolecular templates, monolayer etching
resists for semiconductors, and nanometer-sized tethered polymer structures can be investigated
using this technique. In order to create stable nanostructures, it is beneficial to use molecules that
can anchor themselves to the substrate via chemisorption or electrostatic interactions. When alkane
thiols are patterned on a gold substrate, a monolayer is formed in which the thiol head groups form
relatively strong bonds to the gold and the alkane chains extend roughly perpendicular to surface.
Creating nanostructures using DPN is a single step process which does not require the use of
resists. Using a conventional atomic force microscope (AFM), DPN has been reported to achieve
ultrahigh-resolution features with line widths as small as 10
15 nm with approximately 5 nm spa-
tial resolution. For nanotechnological applications, it is important not only to pattern molecules in
high resolution, but also to functionalize surfaces with patterns of two or more components
(
Figure 1.7
).
Figure 1.8
shows the basic concept of nanomanufacturing. Individual atoms, which are given in
the periodic table, form the basis of nanomanufacturing. These can be assembled into molecules
and various structures using various methods including directed self-assembly and templating, and
may be positioned appropriately depending on the final requirements. Further along the devices
architecture,
integration,
in situ processing may be employed culminating in nanosystems,
molecular devices, etc.
