Patterned Nanofibers: The making of "NanoBucky"
Nanobucky is a fun example of the ability to control the synthesis of nanoscale materials such as carbon nanofibers. Nanobucky is made entirely from tiny "hairs" of carbon nanofibers. These carbon nanofibers are about 50-75 nanometers in diameter, each about 1,000 times thinner than a human hair. The entire image of Bucky is about 15 microns (15,000 nm) in size. That means that we could fit approximately 9,000 complete NanoBuckys onto the head of a pin. NanoBucky was created by graduate students Sarah Baker, Kiu-Yuen Tse and Jeremy Streifer, postdoc Matthew Marcus, and Prof. Robert Hamers, at UW-Madison.
The carbon nanofibers that make up Bucky are of great interest for practical applications such as chemical and biological sensing and as high surface-area materials for use in a applications such as energy storage. So, while NanoBucky is fun, there is some serious science behind making structures such as this. Below is a description of how Bucky was made.
The process starts by defining a pattern or shape in a computer program. A special computer-aided design (CAD) program is used. This represents various shapes as a series of regions that are bright and dark. The first stept in making NanoBucky was to import a picture of Bucky Badger into a computer, and then to use a computer mouse to trace over this using a computer program called AutoCad Lite. This produced a set of polygons that, taken together, described the shape of Bucky. Any shape can be approximated as a set of polygons, so that this is a general method for creating different shapes in the computer. Below is a screen shot shoing how Bucky looks on the computer screen.
The sample itself starts off as a standard silicon wafer (represented in blue below). The wafer is coated with a thin layer of a polymer "resist" (represented in pink). The resist is a polymer (plastic) that will change its chemical composition when it is exposed to light or, in our case, electrons. The silicon sample is coated with liquid resist and then heated to turn the resist into a solid film. The resist-covered sample is introduced into a scanning electron microscope (SEM). Here is a photo of the SEM we used.
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While most SEMs are used to image things, we also use the SEM as tool for making objects. We do this using a technique known as electron-beam lithography (EBL). In this technique, we control the SEM with a computer. The computer steers the focused beam of electrons to specific locations on the sample and can turn the beam of electrons on or off very quickly. So, the computer moves the focused electron beam across the sample, and turns the beam on or off to correspond to the pattern that we drew earlier. The beam of electrons can be focused down to a spot about 30 nanometers in diameter, making it possible to create very fine patterns.
When the electrons strike the resist, they change its chemical structure in way that makes the exposed regions (darker red) more easily able to be dissolved in certain solvents. The figure below shows a simple stripe, but the technique can be used to make very simple patterns and very complex patterns (such as NanoBucky).
The exposed regions of resist are now dissolved away. The regions of the sample that we not exposed to the electron beam are still coated with resist.
Now we want to use the resist to control the spatial position of a metal catalyst that control the nanofiber growth. We do this by placing the sample into a metal "evaporator", a specialized vacuum system that allows us to deposit metals onto the sample by heating them to high temperatures where they "evaporate", and then they condense on the sample to produce a thin metal coating of nickel. The metal coats the entire sample.
Even though nickel metal coats the entire sample, some of it is in direct contact with the original substrate (blue), and some is on the resists (pink). If we now place the sample into a solvent that will dissolve all the photoresist, the metal that is on top of the photoresist is removed along with the resist. This process is called "lift-off", and leaves us with metal only in the desired regions of the sample.
We are now ready to start growing nanofibers!
There are two important roles of the nickel: First, the nickel metal acts as a catalyst, meaning that it accelerates the rate of chemical reaction. In growing nanofibers, the nickel acts as a catalyst that accelerates the rate at which acetylene (C2H2) decomposes into carbon and hydrogen, via C2H2 ® C + H2 . We also have some ammonia (NH3) present. The role of the ammonia is not not completely understood, but it seems to keep the catalyst surface clean and probably reduces any oxides that form. Secondly, the nickel controls the nanometer-scale structure of the nanofibers. In the growth chamber, the nickel becomes hot and "balls up" into small nanoparticles that are roughly 50 nanometers in diameter.
When acetylene gas hits the catalytic nickel nanoparticles, it decomposes into carbon and hydrogen. The hydrogen goes off as a gas and is pumped away. The carbon dissolves into the nickel, forming a molten alloy of nickel and carbon. As the reaction goes on, the amount of carbon in the nanoparticles increases, until finally it exceeds the solubility limit. When this happens, pure carbon begins to deposit. Because the nanoparticles are very small, the carbon only deposits into a small-diameter fiber. The catalyst rides on top of the fiber.
In the end, this grows nanofibers wherever the catalyst was. The nanofibers we grow have another very special property, which is that they are vertically aligned. This means that the fibers grow nearly straight up, instead of wandering in all different directions. This happens because during the growth, we have a large voltage of ~500 Volts applied to the sample with respect to the top electrode. This produces a big electric field that tugs on the nanofibers as they are growing, in exactly the same way that your hair stands on end if you touch a van de Graaf machine!
Note that in order for this to work it is very important for the decomposition of acetylene to take place only on the metal nanoparticles! If the decomposition took place everywhere, then the fibers would just get very big in diameter and turn into one big blob of carbon.
