Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Sep 21, 2007
Nanotechnology crop circles appear in Southern California!
(Nanowerk Spotlight) Just kidding - I always wanted to write a tabloid headline like that! In case you are expecting a story on the mysteries of crop circles caused by alien nanotechnology - stop reading right here; but the analogy is just too striking when you look at the amazing images coming out of the labs at the University of Southern California, where they developed a new technique to create three-dimensional carbon nanotube structures. While carbon nanotubes possess many exceptional properties which far exceed most known bulk materials, creating controlled nanotube (CNT) microstructures has always been a challenge. Overcoming this challenge is going to be key in developing useful and commercially viable CNT devices. Existing techniques for patterning three-dimensional CNT structures are based on the bottom-up growth of multiwalled CNTs (MWCNTs) from a patterned catalyst, which is limited to 2D-like geometries. Other, complex 3D microstructures have been fabricated with polymer-based resin materials but not with CNTs. The new technique developed by researchers in California uses a focused laser beam to selectively burn local regions of a dense forest of MWCNTs. This technique enables chemically sensitive fields to take advantage of nanotubes' exceptional properties and expand their possible applications into new areas.
"We created three-dimensional microstructures using a focused laser beam to selectively burn local regions of a dense forest of multiwalled carbon nanotubes" Dr. Stephen B. Cronin tells Nanowerk. "Raman spectroscopy is used to systematically quantify this process in a controlled fashion to determine the laser power threshold for burning carbon nanotubes and, also, the depth of burnout at different laser powers."
a) Concentric cylindrical structures patterned using the laser burnout
method. b) Close-up image showing a slight undercut profile. (Reprinted with permission from American Institute of Physics)
The researchers grew CNT forests by chemical vapor deposition by passing ethylene over a predeposited iron catalyst on silicon wafers. The nanotube growth takes place in a heated tube furnace at 650°C.
"We determined the minimum threshold laser power for burning carbon nanotubes in air by observing changes in the intensity of nanotube Raman
spectra before and after laser exposure" says Cronin.
CNTs were exposed at laser powers between 50 and 9000 microwatts for 1 second and the scientists found that the threshold for laser burnout occurs at 300 µW, which corresponds to a power density of 244 µW/µm2 for a 1.25 µm spot size.
"The initial burnout we observed came as quite a surprise and appeared very striking in our microscope images" says Cronin. "Typically, carbon nanotubes lying on a silicon wafer are very difficult to heat with a laser. So, in fact, this burnout effect was discovered serendipitously."
3D staircase structure fabricated in the MWNT surface. (Reprinted with permission from American Institute of Physics)
Cronin notes that they observed several interesting phenomena on the surface of the MWCNT forests after laser treatment. "SEM images revealed white spots ranging from 100 to 200 nm on top of the burned MWCNT surface. At higher magnification, these white spots can be resolved as nanotube bundles that aggregate
during the exothermic burnout process. This aggregation demonstrates the dynamic nature of the burnout process of these MWCNTs."
Potential applications that could benefit from this technique – since it doesn't involve chemicals and the resolution is limited primarily by the spot size of the objective lens – are on-chip DNA manipulation, chemical and protein identification, templates for directed stem cell growth, and gas mixture separation.
The next step for the researchers is to work closely with scientists from other fields, such as biological and micro-fluidics, to determine how this method compares to the current state-of-the-art techniques and to quantify any additional advantages provided by this method.