I work primarily with Au(III) and occasionally graphite surfaces. Au(III) refers to gold (Au) that is cut in a certain way, in order to have a flatter surface. This is then put in a vial with a certain amount of octanethiol. Octanethiol (abbreviated 8thiol), is a self-assembling monolayer consisting of eight carbon atoms with a sulfur on the end. This sulfur bonds to the gold surface and the molecules assemble themselves in a signature fashion
*image belongs to Christopher Agostino*
This is an example of octanethiol arrangement on Au(III). The faint lines, particularly in the middle, are the 8thiol.
After scanning the gold or graphite, it is exposed to radioactive bombardment.
The idea behind this is quite simple- a radioactive ‘source’, or an element which emits radioactive particles, in this case Strontium-90, a beta particle emitter. We used to use Polonium-210, an alpha emitter. The chief difference between the two is the particle emitted during decay- Strontium-90′s beta particle is significantly larger and heavier than Polonium-210′s alpha particle.
What we expect to see is what we would expect from a human-sized impact on a surface- a crater and surrounding debris. The graphite, being a ‘softer’ element than gold, is more easily damaged by particles and is easier to scan- if a scope is ever failing to scan or scanning badly, we use graphite, which we are quite familiar with, to figure out what is wrong with scope and correct it.
Being that beta particles are larger and heavier than alpha particles, they would be expected to have a greater effect on the surface, even if craters are not actually the product. Logic dictates that a crater would be expected, but it’s tricky to know for sure
-This is the apparatus used to expose the gold and graphite samples. The tray on screws (bottom black box) is where the sample is placed with the radioactive sample for exposure. The tube on the left carries a constant stream of nitrogen and the metal enclosure itself is only filled with nitrogen, a ‘nitrogen purge’. The glove on the right is inflated when the nitrogen is flowing, alerting the lab students when the flow has stopped.
The pure nitrogen environment ensures that no ‘air’ particles will effect the surface of the sample.
Scanning Tunneling Microscope:
This is the Scanning Tunneling Microscope that I use. It is a portable scope in the sense that it is relatively lightweight and easy to move. This is especially helpful when bringing it to area high schools for demonstrations.
The gold part itself is the scope with the grey-brown cord on the left connecting to the black cylindrical box behind, which contains the computer element of the scope. This box is then connected to a laptop, which i use to control the scope and take images.
The silver cylinder in the scope is the sample piezo. It has a magnet on the end which is used to attach the sample holder which clamps down on the sample.
The picture above is of a sample. The gold is attached to the clear mica underneath. The dark stone is the base of the scope and assists in isolating vibrations.
Because of the extremely small scale on which we work, any and all vibrations can wreak havoc on the image and potentially damage a sample if it crashes into the tip or holder.
The scope is a very delicate piece of equipment and, because of this, can be easily disturbed and difficult to use effectively. This has led to long stretches of time with few, if any results.
However, there are other times when the scope performs exceptionally well, and it usually is some in between these two extremes- not excellent images, but they’re still useful.
This is an example of when the scope is not scanning well. The cause behind this failure of an image was traced to the sample holder, but it serves to illustrate what happens occasionally.
This is an example of a very good, though not exceptional image.
The black boxes were added by me to highlight the interesting features of the gold surface. The holes present throughout are typical of gold surfaces. The ‘islands’ however, are not due to the exposure. I have been told they are products of a process that smoothes the surface of the gold called annealing, in which the sample is heated with a hydrogen torch, then allowed to cooled, then heated again. This expands and contracts the gold, making it even flatter.
These islands are seen before the exposure and can be used as reference points in judging the effect of the radioactive particles. If the same area is actually observed before and after exposure, which is very difficult, nigh impossible due to the scale and technique, they would easily relate how the surface had been effected by the exposure.
Exposed to beta source for an hour and fifty minutes.
Au(III) exposed to beta source for one hour
This image provides an excellent baseline for comparison in a search for change in the surface. The ‘flaking’, or layers of gold, are distinct and varied in both height and shape.
The data we have gathered thus far is difficult to judge. We don’t exactly know what we’re looking for, though we have an idea of what it could be. Because of this, our results are inconclusive until looked at in a larger context. Several small similarities, months apart, could reveal a trend. This is merely an example to illustrate how grand an operation this is.
Since starting in the lab in June of 2010, I have learned a great deal from both the grad students in the lab and professors. They have helped me get much better at scanning, as well as teaching me how to prep samples and various other odd jobs related to scanning. The lab is a dynamic environment and always has something new for a researcher to learn.
This semester provides a climax for all that I hav learned the past year and a half or so. I hope to narrow my errors and entice the scope into behaving and churning out some good images.
For this semester, my lab liaison has in mind to increase the exposure time with the beta particles. The longer the exposure, the more likely impact on the surface is, which means a higher chance of encountering such an impact. This being my last semester of high school and, by extension working in the lab, I hope to have better results and spend extra time in the lab if need be.
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