One of the cool things about the nanowire growth process is how easy it is to change variables like pressure and temperature. With just the turn of a knob you can completely alter the environment that the reaction is taking place in. After giving up on my studies of Steel as a growth substrate, I decided to do a quick study to see how varying the Pressure would affect the growth of the nanowires.
I kept the temperature constant at 685 oC and ran three tests, with pressure values of 75 torr, 150 torr, and 250 torr. For reference sake, atmospheric pressure is 760 torr. In order to grow any wires, the pressure must be lowered because the Cadmium Sulfide needs to be able to vaporize. You can think of increasing the pressure as extra weight pushing down on the CdS and the substrate. So the lower pressures actually allow more CdS to evaporate and thus have a chance to form wires. However, if the pressure becomes too low you run the risk of evaporating the gold as well.
Below are some comparisons of the results using the SEM.
75 torr vs. 250 torr (20 µm scale)
The sample that was run at a lower pressure (left) is much denser, but this is not necessarily a good thing. There is a good amount of growth in both cases, but clearly the yield is higher in the lower pressure environment. This makes sense because lower pressure leads to a greater evaporation rate for the CdS. However, my research group is only interested in wires of a certain diameter, and even in this first image you can see that the low pressure sample has very inconsistent diameters, many of which would actually be considered "belts" and many that would just be too thick to be of interest.
250 torr (500 nm scale)
Here is one of the smaller nanowires that we found on the sample run at 250 torr, measuring just 70 nm in diameter. Notice that right next to this thin wire is a wire that is almost 4 times as wide. These inconsistencies prevailed throughout all the samples, but were most noticeable in the lower pressure samples.
The results of the Pressure study are somewhat inconclusive and obviously could benefit from many repeated trials, but the clear trends that I noticed were that lower pressure environments will lead to greater deposition, but generally larger and lower quality wires. It would be interesting to see what happens if the pressure is increased further beyond 250 torr.
Finally, there was an additional unexpected takeaway when we compared the front end to the back end of the samples. In the furnace, the front end is more directly on the coil of the furnace so there is a temperature gradient across the sample.
Even though the furnace was set to 685 C, it can be assumed that the back end of the sample was significantly cooler. Here we compare two images from the same sample. The sample on the right is from the back end of the substrate and is thus cooler.
Front End vs. Back End (250 torr)
Both images show significant growth, but the section that was in cooler temperatures has higher quality wires while the hotter section has more variety and a greater percentage of thick wires. This would make for an interesting study to see if this trend is consistent, but it seems that cooling down the temperature may help improve the consistency and the quality of the nanowires. In order to grow ideal nanometers with a small diameter, the growth should happen in only one direction off of the gold particles. One possible explanation for the trend shown above is that in hotter environments the CdS may be using the additional energy to attach itself to growing wires and multiple directions, causing the growth of thicker wires and belts.