Ryan Badger
Summer 2011 REU
June 8, 2011
Carbon-Nanotube-Templated Microfabrication (CNT-M): Methods and
Filtering Applications
Research Internship Prospectus
Carbon nanotubes (CNTs) have wide range of possible applications. One
application of particular interest relates to filter/sieve creation using carbonnanotube-templated microfabrication (CNT-M). Many solutions and gases have
micro-scale impurities that are difficult to remove. The unique shape, size, and
structure of vertically aligned carbon-nanotubes (VACNT) produced using CNT-M
make this an interesting possibility.
Previous testing has shown that the CNT-M process can be divided into two
different types, labeled type-I and type-II. This testing has also shown that the
orientation and size of the CNTs, both in diameter and length, can be precisely
controlled. Type-I mircofabrication starts with a silicon wafer coated in alumina,
adhesion promoter and photo resist. The wafer is then exposed to light through a
patterned mask. Developing the wafer removes the non-exposed photo resist. Fe is
then evaporated onto the wafer and then a solvent is used to remove the remaining
photo resist coated areas. This leaves a grid-patterned layer of Fe where VACNTs
are then grown. The nanotube structure is mostly void, so infiltration with a second
material leaves a robust, patterned three-dimensional structure. Theoretically the
size of the empty spaces in the structure between the CNT walls can be as small as
500-nm. The type-I process could be used in applications requiring particles to be
filtered larger than 500-nm. The lower limits of the type-I process could be explored
by attempting to create a 500-nm filter.
The type-II CNT-M process is similar, but differs in that a solid pattern is
grown. This forest is then infiltrated and the CNTs are burned out leaving very small
tube-like voids (pores) in the solid. These pores are uniform in diameter, and can
created at 100-nm and smaller. Type-II would be used for applications requiring
filtering of particles smaller than 100-nm. The process to create sieves in this size
range as well as the sieve functionality would need to be tested.
During the fabrication process after the CNT forest had been grown, the
sample could be infiltrated with two different test elements, nickel or tungsten.
These materials and geometries seem conducive to filtering, but further testing
would be required to determine if this type of structure could be used in real world
filtering applications. The applications may include air filtering in a clean-room
environment, various gas filtering applications, and solution filtering including
organic solutions, such as blood.
To determine if the sieve is capable of filtering blood, research and testing
would be required. The actual toxins and normally regulated solutes in the blood
would need to be researched to determine the target filter size. Overall feasibility of
this possibility would also need to be determined by researching methods used in
current dialysis machines. The intent being that this method of sieve production
would reduce cost or simplify current processes.
CNT sample growth and testing outlined here would prove the use of both
the type-I and type-II process capabilities for sieve creation. The two different
infiltration elements (nickel and tungsten) could be compared to determine if either
had advantageous characteristics. Lastly, exploring the use of CNT-M sieves for
blood filtering would directly test their use in a real-world application.