Second+Semester+Final+Project


 * __Second Semester Final Project Draft__**

This semester, I have continued using Gold(Au(111)) surfaces coated with Octanethiol molecules by submersing the sample in a solution of Octanethiol for usually around 24 hours. I have, for the majority of the semester, spent my time researching the effects that blue-violet laser radiation (405 nm wavelength) has on the Octanethiol molecules on the surface of the gold. With the functionality of the Scanning Tunneling Microscope (STM), I am able to take scans of the samples. I am looking for changes on the surface between the before and after images, specifically how the Octanethiol molecules rearrange themselves after being heated.
 * Overview of Research**


 * Setup and Changes**

media type="custom" key="18929188" The photo above shows the Scanning Tunneling Microscope and the sample holder. The STM takes scans of surfaces by using a metal tip that can conduct electricity. I utilize a Platinum-Iridium (Pt-Ir) tip in my research. The microscope sends an electrical current that passes from the tip, through a small gap of air, to the surface. More advanced Scanning Tunneling Microscopy is done in vacuum to minimize noise and other factors that could negatively affect the surface or tip. The changes in surface height and density result in changes of current. These changes in current are the resulting images which we see. The sample holder that I use is hollow, as is the sample puck. This allows for the light of the laser to pass through the sample holder without moving it, so as to be able to scan the same region before and after the firing of the laser. The green box on top of the sample holder in the image above is a piezoelectric stack. The piezoelectric stack is connected by the wires extending out of it to a system which can charge the stack with up 135 V at maximum output. The piezoelectric stack extends a very small distance when charged with electricity. This can be useful for my research because I can scan a sample while having the piezo stack charged with electricity. Then, I reduce it to shoot the laser so the sample can avoid crashing in to the tip and I can take a scan of the sample afterwards by raising the voltage back to its original voltage and scanning. Beginning this semester, during April, I stopped using this functionality and simply scanned without any volts charged to the stack. I did this for a couple reasons. I noticed, when this special sample holder broke one day and I had to use a standard one, that the quality of the scan greatly improved when I was not using the system or charging it with any amount of voltage. Also, when I would try to retract the sample holder so I could fire the laser at it, the sample holder repeatedly crashed into the tip and I was unable to get an after scan. I began trying to take before and after images without utilizing the withdrawal feature of the piezoelectric stack and had much more success. media type="custom" key="18929670" Above: The laser that I use that emits light at a wavelength of 405 nm. The majority of my finished scans do not offer any real insight into the molecules on the surface. I took very few images that I thought were worthwhile enough on which to use the laser and most of these ended with the sample crashing when I would decrease the voltage on the piezoelectric stack. After an incredibly long period of unsuccessful attempts, I stopped using the piezoelectric functionality of the system and began shooting the laser without moving the sample. I found a decent bit of success doing this. I completed several different before and after image sets that greatly resembled each other with slight difference. One of the first times I did this with a fair bit of success came in April. The set below, 4-25-12AD, 4-25-12AE, and 4-25-12AF was a set of three images that resemble each other yet have very significant changes between them. media type="custom" key="18943602"
 * Results**

The far left image is the original image, "AD".I shot the laser at it for 30 seconds, let it cool for 45 seconds, and then scanned. In the purple, I have outlined the regions and features inside of them that do not change significantly from this before image to the after image in the middle, "AE". The region outlined in black in the lower left corner houses clumps of Octanethiol molecules that look to be not included in the after image as a result of the scanning region moving upward combined with the overall expansion of distance between every feature. This is most evident in the region enclosed in purple because it has greatly increased in size from "AD" to "AE". The clump of Octanethiol molecules in the blue region has disappeared between "AD" and "AE" while the two valleys seem to have become one or the area between them has stretched and the lower of the two is the valley beginning at the bottom left of "AE". The latter is more likely based on how the rest of the features stretched along with the distance between them. The area in the teal region contains two valleys that I think may have stretched,became larger, and transformed into the region in teal in the second image. I think this because of the two clumps of Octanethiol molecules above and to the left of them that are in a similar position between the two images. The region in red directly above the region in teal houses a clump of Octanethiol molecules that could have either disappeared or combined with the clump slightly lower and to the left of it. The red region below the teal region does not seem to be part of the after image, along with most of the region enclosed in silver. The region in black on the right side looks to have stretched and transformed into the region in red in "AE". Once more, I shot the laser for 30 seconds at "AE", let it cool for 45 seconds, and then scanned. Not nearly as much changed between "AE" and "AF" as did between "AD" and "AE". Except for slight differences in size, the features inside the lime green region remained the same between "AE" and "AF". The two clumps of Octanethiol molecules inside the black regions have disappeared, and the lower valley in the teal region looks like it has decreased greatly in size, turning into the very think streak that occupies a similar location. Also two of the valleys in the bottom left look like they have combined to form a much longer valley. The rest of the image is fairly unexciting though. Professor Kandel suggested that, because of a lack of success with the laser system, we should move in a different direction come summer time. He still wants me to look at the effects that heat have on the surface molecules but he wants me to do it in a different way. I have drawn a diagram of what the new setup should look like. media type="custom" key="18929890" The idea is to send a current through a wire connected to the sample. Here is a sample of the Gold on micah. As the current passes through the gold sample, the electrons that will pass through to the other wire will cause friction and create heat. A low voltage will be able to generate a fairly high amount of heat because the mass of the sample is so small. There will be another wire on the other end to ground the circuit. However, this will only work if the gold is a strong enough resistor. Guido and I began testing gold samples for resistance to see if the project would be plausible. media type="custom" key="18929876" We checked the resistances of old gold samples and then created patterns like the one in the above image to increase the resistance. With a pattern like the one above, we were able to reach a high of 3.2 Ohms of resistance.
 * New Project**

The next step was to see if I could attach wires to the sample of the gold and still be able to have a current flow through the system with enough resistance to be able to generate the necessary heat. media type="custom" key="18943252" I attached wires to the surface of the gold using Silver conductive paint. Then I used a voltmeter and checked the resistance at the two ends of the wires and consistently registered a resistance between 0.7 V and 1.1 V I am not sure exactly what I will need to do next to further the progress on this project. I assume I will need to develop an efficient process for connecting the wires to the gold while maintaining the integrity of the surface. I will continue working on this project and turning it into a reality this summer. If successful, I will use this system rather than the laser mechanism if it is possible. This semester I have learned, through a large enough amount of trials, that the use of the Piezoelectric capabilities of my system is not exactly necessary. Also, its use usually resulted in a lower quality scan, probably because there was a higher amount of energy in the system when the piezoelectric stack was charged. I have had fewer accidents this semester that resulted in lost opportunities. Perhaps this was because I had fewer opportunities to do so but I have improved my technique to minimize such accidents. Most of my failed laser attempts were caused by the sample crashing after I reduced the voltage. I solved this by not using the piezoelectric capabilities, which also resulted in higher quality scans. I look forward to my new project and hope that it brings more success than my current one.I think that it will have a potential for bringing more frequent opportunities than my current project because I will not have to rely on the piezoelectric stack and I will be able to take higher quality scans before and after heating up the sample. I believe that assembling the project will be harder but most likely more rewarding than my current laser based project.
 * Conclusion**