[MUSIC] Hello, I'm Carrie Donley. And I'm the Director of the Chapel Hill, analytical and nanofabrication laboratory, or CHANL, at UNC. With me today is Catherine McKenas who will be showing you how to take x-ray, photo electron spectroscopy or XPS Data. >> Thank you Carrie, as a reminder, this is a schematic of the XPS chamber. In XPS, we will hit the sample with x-rays that have enough energy to knock electrons out. Those electrons will be collected by the electron energy analyzer and and detected. The spectrum that is produced will tell us what elements are present on the surface of the sample. I'm trying to bind chlorine and sulfur to the surface of a carbon film with a chemical reaction shown here. Let's go to the lab to see if I was successful. This is the XPS chamber. It's a large vacuum chamber that's held at pressures in the 10 to the negative 9 to our range. Let me point out the important parts of the chamber to you before we get started. This is an aluminium x-ray gun that produces x-rays that then travel up through a monochromator. The x-ray gun produces x-rays at a couple of different energies and the monochromator selects just one energy of x-rays and allows them to travel to the sample. The samples sit in the center of this chamber and the electrons produced travel up through these electron objects, through an analyzer which separates them as a function of their energy, and then onto a detector which is on the other side of the chamber. Now let's load the samples I prepared earlier. First I'll vent part of the chamber to atmospheric pressure. After a few minutes, I can open the door to the chamber and load my samples onto a transfer road. Then, I can close the chamber door, and re-pump the system. Depending on the size of the chamber that needs to pump down. This can take anywhere from 20 minutes to 2 hours. Once the chamber has pumped down, I can move the samples into the analysis chamber, turn on the x-rays and find the spot on my sample that I want to analyse. Once this is done I can start collecting data. It will take anywhere between 15 minutes to an hour per sample, depending on how many elements I'm interested in looking at. And how concentrated they are in the sample. The scans for carbon only take a minute or two because there is a lot of carbon present in my samples. The scans for chlorine and sulfur will take longer. About ten minutes each because there are fewer of these atoms on the surface. Here are the schematic diagrams and spectra for the first two samples I analyzed today. The first sample is composed primarily of carbon and oxygen, as shown by the C1S and O1S peaks. After the surface has been reacted with the phosphorus pentachloride, two new peaks appear. If we take a closer look at this region of the spectrum, we can see that the two new peaks are our energy's consistent with the addition of chlorine to the surface. The chlorine peak is much weaker than the carbon or oxygen peaks because there is much less chlorine on the surface. The next step in this experiment is to react the chlorine terminated surface with sodium hydrosulfide in order to replace the chlorine atoms with sulfur atoms. Again, we see some changes in the spectra following the reaction. And if we zoom in to the lower binding energy region, we can see that the two new peaks are at energy's consistent with the addition of sulfur to the surface. But there still small chlorine peaks present. The picture at the top is what we hoped the surface would look like after this reaction, but it looks like the reaction was not complete. A more accurate picture would look something like this, where most but not all of the chlorine has been replaced with sulphur. We can also take high resolution scans for that elements of interests, in order to understand the surface chemistry even better. If we take a closer look at the sulphur 2p peak, we can see that it is actually composed of two components. The larger peak at lower binding energies is due to an unoxidized sulfur atoms while the smaller peak at a higher binding energy is due to sulfur atoms that have been oxidized. In order to determine how much of each form of sulfur is present, we will need to do some peak fitting within the analysis software. The first step is to fit a base line to the data. Then we can start adding peaks to fit the data. The sulfur 2p line is composed of a doublet, and the spacing between the peaks is published in reference books. The software will allow me to add a new peak with the correct position and relative height to the original peak. Once I've added enough peaks, I can use the Auto-Fit feature to try to obtain the best fit to the data. Finally, I can use this fitted data to determine that 14.6% of my sulfur is in the oxidized form. This oxidized sulphur is an unwanted by-product and I'm working on ways to reduce its concentration on the surface of the sample. Without a surface sensitive technique like XPS, it would be impossible to determine if I had been successful in changing the surface chemistry of my carbon surfaces. I hope you enjoyed this demonstration on taking XPS data. Thank you for joining me.
