Overview of Thermal Exposure

1. Overview of Thermal Exposure • Gold Samples (Au(111)) are submerged in Octanethiol Solutions and heated for around 24 hours. • The goal of the research is to study the effects of Laser heating on the surface of Gold (Au(111)) coated with Octanethiol molecules. • The samples are then scanned using a Scanning Tunneling Microscope. • The quality of the image taken depends on the sharpness of the Platinum-Iridium tip used and on the solution forming molecules on the Gold surface. If there is not enough time in solution, the molecules can be disorderly on the surface and prevent any useful imaging.

2. The sample holder has a Piezoelectric stack on top of it and is hollow to allow for the light of the laser to hit the sample without removing the sample from the STM. • The Piezoelectric stack that sits upon the sample holder is connected to a power box, which houses fifteen 9-volt batteries. When scanning, on average, 40.0-45.0 V are applied to the Piezoelectric stack – increasing its length. The reduction of the voltage results in a shortening of the Piezo stack, allowing for a detraction from the Pt-Ir tip that is small enough to minimize movement but allow for thermal expansion.

3. When I take images and find those of high definition or with obvious atomic definition, the retraction method of reducing the voltage is utilized and then the laser is shot at the sample, usually from 35-60 seconds. After this, the sample cools for a few minutes before the same voltage that was used before is once more applied and scanning resumes. This method has proven quite successful throughout the course of the research in its attempt to scan the same area before and after the heat is applied.

4. Results This image comes from June 28, 2011. It comes from the beginning days of my work in the Lab and was actually before I had begun my specific project. This image shows how Octanethiol molecules ideally arrange themselves on a surface of Gold. They arrange in groups of several chains as shown in this image. I am looking for features similar to those in this image in my own research to investigate the effects of applied laser heat to the Octanethiol molecules.

5. These images are before and after images from September 14, 2011. Although they do not demonstrate clear atomic resolution, they do show the ability of the Piezoelectric stack to retract then increase and continue scanning a very similar area. These images show similar features and regions but also show drastic changes in the surface as seen in the bottom half of the images from the before to the after image. However, the most important aspect of these images is that the quality of the image decreases drastically from the before to the after image as one can see in the vastly larger amount of streaks in the after image. This reveals the problem of the laser causing thermal expansion in the tip which can possibly decrease or increase the quality of the tip. In this case, the tip decreased in sharpness and therefore in quality.

6. Conclusions • Unfortunately, I have not been able to gain atomic resolution in my scans and this has prohibited me from learning about the effects of the Laser heating on the Octanethiol molecules. Poor samples, dull tips, and high amounts of noise all reduce the quality of the image and it is difficult to ensure the best of all of these three. In a lab with a vacuum pump nearby, it is nearly impossible to rule noise out as a factor in the low quality of the images. I shall still endeavor to cut tips as sharply as possible and do my best to obtain the best possible images. • The expansion of the tip during the laser application also creates problems. This can ruin chances of seeing how molecules change from the before to the after image because I would scan with much less certainty.

7. Pictured Here: A Blue-Violet Laser which emits light at 405 nm wavelength.

8. Pictured Here: The NanosurfEasyScan Portable Scanning Tunneling Microscope and the sample holder in operation. The Green box structure is the Piezoelectric Stack