Half the summer has passed at this point, and I can barely believe how much I have learned. Since the last blog post, I have learned a lot more. As a reminder, for my project this summer I am collecting graphene samples to measure its behavior when chlorinated using Raman spectroscopy which I discussed on my last blog post.
In addition to taking a single Raman spectrum of a sample, I have started measuring Raman spatial maps of my graphene samples. When I shine the laser on a sample to take its Raman spectrum, the spectrometer only obtains a measurement at that specific location. A map measures multiple Raman spectra at each specific location on the sample using a computer program that automatically shifts the translation stage one step at a time. Programming the stage so it moves the number of steps I want is another process I learned since the last blog post. It requires a few simple calculations and visually determining the area where the map should be taken to encompass the entire graphene sample. The spectrometer is programed to take a spectrum every 60 seconds, so if I program the stage so it moves 15 steps on the x-axis and 20 steps on the y-axis, the spectrometer will take 336 spectra (16 x 21). At 60 seconds a spectrum, it will take approximately 5.5 hours for the map to finish. For that reason, maps are usually measured overnight.
Once the map data has been collected, I upload it into the data analysis program Igor. Igor is very convenient because it analyzes all the spectra at once as opposed to me having to manually analyze them, which is especially useful when I measure a map with 336 spectra. We already have several Igor macros that were written to perform the desired functions like loading multiple files and fitting peaks. However, as part of my training, Professor Crowther taught me how to edit the macros to only display the one layer graphene sample of interest as opposed to also including other spectra of surrounding graphite or multi-layered graphene. As a result, I can now edit several mapping programs. I was delighted when the codes I wrote worked especially because learning how to code was something I always wanted to pick up and the fact that it improved the data processing was exciting.
After taking the initial map of a graphene sample, I run an experiment on it. For my project, I am conducting photochlorination experiments. I begin by exposing the silicon substrate where the graphene sample is located to chlorine gas. Chlorine gas is highly toxic, so for this part of the experiment I work in a fume hood using appropriate safety equipment. Because of the toxicity of the gas, the setup for the chlorination experiment uses a manifold to control the exposure. The sample is placed in a cuvette which is attached to the system. The whole system is evacuated and tested for leaks before the chlorination experiment can begin. Once the setup is deemed safe, the sample is exposed to chlorine gas for less than two minutes. Chlorine gas has a distinct yellow color, so once the cuvette obtains that color, it is sealed, trapping the gas with the sample. The remaining chlorine gas is pushed out of the system using nitrogen gas into a wash bottle where it is converted to harmless Cl-. Nitrogen gas continues running through the system for about ten minutes to ensure no more toxic chlorine gas is present before the cuvette can be removed and the equipment can be turned off. What makes this experiment complicated and dangerous if not done carefully is that everything must be turned on and off in the correct order.
With the sample successfully exposed to chlorine, it is time to irradiate it with a 405 nm laser which has a nice purple color. For the irradiation step, I must first find the graphene in the sample on the cuvette on an elevated stage which restricts how much the translation stage can move making it harder to find the graphene sample than if it was in a slide. Once I have found the sample and aligned both the 405 nm laser and the 532 nm laser onto the sample, I can begin irradiation. I typically irradiate the sample for prolonged time varying from 30 minutes to maybe an hour. Once the time has passed, I take a Raman spectrum of the sample. I repeat this process for a total of 5 or 6 hours.
When I take all the different Raman spectra at different time intervals, the important feature that I am observing is the change in peak intensity ratios. As I mentioned previously, a typical graphene spectrum has 2 distinguishing peaks: A G and a 2D peak. Upon photochlorinating a sample, the 2D peak lowers in intensity until it is relatively flat. Another peak also appears at around 1350 cm-1, which was not previously observed before the experiment. This peak is called the D peak. While the 2D peak decreases in intensity the longer the sample is irradiated, the D peak increases in intensity. In order to measure the effect of the irradiation on the graphene sample, the intensity of the D peak and the G peak are observed. The longer the sample is irradiated, the larger the ratio of the D peak to G peak intensity. When the photochlorination process is finished, I measure another Raman spatial map of the graphene sample to determine how the location and the intensity of the peaks changed after the experiment.
Besides working on this project, this past week I helped Professor Crowther give a talk to the students in the Pre-College Program. Professor Crowther gave a brief summary of the Barnard Chemistry Department as well as an overall explanation of the research we do in our lab. We then took the students on a department tour showing them the teaching labs as well as our own lab. Upon entering the lab, I spoke to the students about the research I am conducting this summer. I later did a Q&A explaining to them what it is like to do research and study at Barnard. It was pretty exciting because I have never really given a presentation on my research before even though this was relatively informal. Next week, I will again talk about my research during group meeting to keep my lab mates updated on what I have done this summer so far.
Overall, I feel good about the data I have gathered so far. I hope in the next few weeks to collect more data and perform more photochlorination experiments on graphene samples.