Cadmium Sulfide is a yellow powder that is classified as a semiconductor. In order to explore its potentially interesting properties and applications, my research group must first change the structure of the CdS from its natural state into nanowires. A nanowire is basically any conductive material with a long cylindrical shape with a diameter on the nano-scale. This means the wires should have a diameter of less than 200 nm, which is very very small. Here is a quick outline of the procedure used in the lab to grow nanowires. We will then talk about potential variables in the process before we go on to explore the two projects that I undertook this summer.
Step 1: Position CdS and Subsrate into Glass Tube
Glass Tube in the Furnace Cadmium Sulfide Silicon Substrate with 40 nm Gold
The glass tube that you see above is the main vehicle for the nanowire growing process. Before it can be put in the furnace, it needs to be loaded up with our key players. A small amount of Cadmium Sulfide is loaded into a glass boat and placed in one end of the tube (see yellow dot on left side of image above). The substrate is the site of nanowire growth. This is positioned within the tube on the other side (somewhere underneath all that yellow gunk on the right side of the tube). The substrate is a thin piece of silicon that has been applied with a very small concentration of nano-sized gold colloid. The gold solution is a colloid with little gold particles that have a diameter of only 40 nm or so. When gold particles this small are suspended in a liquid, the color of the solution changes depending only on the size of the gold.
In the picture to the left, all 5 containers are suspensions of gold in a liquid. The different colors can be explained by the changing wavelength of light absorbed by different sized gold particles. So basically, not all gold is gold (colored)! Anyways, our Silicon substrate has a very small amount of 40 nm red gold colloid applied to it, but it is such a small amount that the substrate still just looks like silicon.
This set-up is utilized because the entire glass tube is going to be heated to an extremely high temperature, at which point the CdS essentially evaporates and can travel throughout the tube. There is an attraction between the CdS and the substrate, so some of the evaporated CdS will "land" on the substrate. The gold comes into play because it is the right size for nanowire growth. If everything happens according to plan, some of the CdS will attach itself to the nano-sized gold colloids and as more and more CdS makes its way to the substrate, it will continue to attach itself, causing a long but extremely thin wire to grow.
Step 2: Put the Tube Under Inert Atmosphere and Heat it Up
Before we can expect this interplay between the CdS and the Gold Colloid to happen, we need to do everything we can to assure that it is the only reaction happening. The glass tube is inserted into a setup with a vacuum line attached to it, and it is pumped down to remove as much air as possible from the environment.
There is also a tank of Argon (see image) attached to the setup. You may recall from your Chemistry class that Argon is a Noble Gas, so we don't have to worry about it getting in the way of our reaction. After the air is removed from the tube, it is placed under a steady flow of Argon until the pressure stabilizes. We generally use low pressures (240 Torr or so). Then we close the furnace and crank up the temperature. Wires have been known to grow in the 600 - 750
oC range. This means our reaction will be happening at about the same temperature as liquid hot magma. The high temperature and low pressure are necessary so that the CdS is vaporized, but you don't want to get it too hot or the Gold Colloid may evaporate as well. In order to assure that there is enough time for the wires to grow, we can now walk away for a couple hours and allow everything to run in this inert and super-hot environment.
Step 3: Check out your Wires!
Once the wires have been given enough time to grow, we can return the tube to the atmosphere and check out what happened on the substrate. As you can see, when it is heated to these extreme temperatures the CdS becomes red-hot. As it cools back down its yellow color returns. If everything went well, there should be a thin yellow film on the silicon substrate, and we can use the light microscope to see if it looks like any wires formed. Now, if you've been paying attention, you might be saying, "You can't see nano-sized wires in a light microscope!", which is partially true. Even though we won't be able to make out the diameter of any wires, the lengths are often long enough to see with a regular microscope. However, in order to truly analyze the results, we have to use a Scanning Electron Microscope (SEM).
When scientists or engineers look at a process like this, they always ask themselves "what variables are there?", in the hope that the process may be studied and improved upon. Here's a quick list of the major variables in the CdS nanowire process:
As you can see, there are many variables, and to truly optimize this procedure could be quite the laborious undertaking. Since I only had 6 weeks in the lab, I chose to focus on two variables.