ascent project.

I've been meaning to write a post about my research. Now that I've had some success in that area, I've decided it's time. Also, I'm going to use proper English this time around (sorry Sarah). I found some good links for some of the terminology, so if you are curious you can follow the links and find out more.

Computers as we know them have been built on the science of microelectronics. The word “micro” means million, so a micrometer is one millionth of a meter. The Ascent Project is pioneering what is starting to be called nanoelectronics. The word “nano” means billion, so a nanometer is one billionth of a meter.

In order to have nanoelectronics, we have to have nanowires (wires that are so small they can be measured on a nanometer scale). To achieve this, we are using a variety of techniques including e-beam lithography and metal-coated DNA.

Wires are important, but a complex electrical device, such as a computer, needs more than wires – it needs transistors. Single-wall carbon nanotubes (SWCNTs) are semiconductors that are only a few atoms wide (less than two nanometers), so they are perfect candidates for transistors in nanoelectronics. Click on the picture of the nanotube to see an animation.

This is where my work comes in. Not all SWCNTs are made equal. Some are too fat, others don’t have the electrical properties we want, etc. Basically, my job is to separate the good from the bad. This is called chromatography.

First, I have to dissolve the nanotubes in water. Unfortunately, this isn’t like mixing up a batch of Koolaid. The nanotubes don’t dissolve naturally, so it takes about 5 hours of sonication. Ultrasonic sound waves break the nanotubes apart and disperse them throughout the water, much like a Sonicare toothbrush breaks up plaque on your teeth (highly recommended).

Then, I have to mix in a few chemicals and head over to the ultracentrifuge. Basically, the ultracentrifuge spins really fast (41,000 RPMs to be exact - the engine of a Formula One racecar tops out at around 20,000 RPMs). The heavier (less-desirable) SWCNTs sink to the bottom leaving the lightweight (desirable) ones on the top. It seems like at those speeds it wouldn’t take long, but it takes a total of 20 hours to achieve good results.

When all is said and done, we have visible SWCNT bands. Each band represents a layer of SWCNTs with unique electrical properties. The last problem is getting the layers out of the tube without messing everything up.

In order to get the fractions out, we use a technique called upwards displacement fractionation. Basically, a needle is inserted into the bottom of the tube and high-density liquid is injected. The SWCNT fractions are lifted upwards and are slowly extracted through the top needle. This step is very important – if any mixing whatsoever occurs, the density gradient is ruined. Even an air bubble that sneaks in through the lower needle could destroy the gradient (this has happened to me several times now). I’m trying to think up a better way to extract the fractions, but so far I haven’t come up with anything.

The last separation attempt was a huge success and as a result, Dr. Davis wants to hire me for the Spring and Summer! My sweet wife was kind enough to pull an all-nighter with me to finish the fractionation (hence my drooping eyes in the picture with the ultracentrifuge). I'll keep you posted on future progress.