For the Love of Spectra

by Kara Mattioli

Almost a year ago my research advisor and I decided to start a project that I initially thought was an exciting, but difficult, task. We had been discussing a research project that I was about to start working on, and my to-do list initially looked something like this:

  1. Help grow a material called graphene
  2. Study the graphene using a technique called Raman spectroscopy
  3. Figure out a way to make hydrogen atoms bond to the graphene (this is called “hydrogenating” the graphene)

The goal of my project was to learn how to study graphene using Raman spectroscopy, and then to use what I had learned to find out whether my graphene hydrogenations would work. At first, I didn’t know how to do any of the things on my to-do list. But I also had no idea of what was in store for me: working with awesome people, discovering a new subject that would fascinate me, and realizing that I love doing research.

I didn’t accomplish step #1 by myself. Instead, I found myself working with several other undergraduate students to learn how to grow graphene by a process called chemical vapor deposition. Two important questions had to be addressed: first, what is graphene? And second, what is chemical vapor deposition?

Graphene is a single layer of carbon atoms – it is incredibly thin, so thin that scientists call it a two-dimensional material because it barely has any thickness to it! The carbon atoms are bonded together and form little hexagon shapes. Chemical vapor deposition is a process where you start with a metal, typically copper or nickel, place it in a furnace, and then flow hydrogen, methane, and nitrogen gases over the metal surface while the metal is being heated. With the right amounts of gases and with the metal in a certain temperature range, you can actually grow a layer of graphene on top of the metal! It is a very cool process.

graphene

Graphene is a single layer of carbon atoms bonded in a hexagonal arrangement.

The group of students that I was a part of was called the Graphene Factory, which is a research group formed in the Physics Department at The Ohio State University to give students an opportunity to create graphene for other researchers to study. We first made graphene on copper, but it’s hard to study graphene when it is on copper because the chemical properties of copper sometimes obscure the properties of the graphene. So we transferred the graphene onto silicon dioxide. Each student in the group had a different specialty, and mine was Raman spectroscopy. So to complete step #2 on my to-do list, I had to learn about Raman and what it actually measures.

There are many different types of spectroscopy – it is the study of light emitted from or absorbed by atoms and molecules. For Raman spectroscopy, you shine light from a laser at whatever material you want to study, and you study the light reflected from the sample. Light from a laser is at one wavelength and one frequency, but materials can reflect light at many different frequencies. Atoms in a material will absorb light at specific energies and then re-emit some of the energy. A detector collects the re-emitted light and a computer plots the intensity of light emitted at each frequency, creating a plot called a spectrum.

Raman spectroscopy measures the difference in frequency between the frequency of the laser light and the frequency of light emitted by the material. This difference is called the Raman shift.

ramaneff

Raman spectroscopy measures a “Raman shift,” which is the difference between the frequency of the laser light (red) and the frequency of light emitted from the sample (black).

I took Raman spectra on our graphene samples and learned what the spectra were telling me about our graphene. That’s right – spectra are not just a display of different peaks! You can get a lot of information from them, and that is one reason why I think spectra, and especially graphene Raman spectra, are so beautiful.

Graphene Raman spectrum

This is a Raman spectrum of graphene. The Raman shift is measured in units called wavenumbers, or cm^-1, which is proportional to frequency.

The presence of the two big peaks, and the fact that the peak on the right is larger than the peak on the left, tells me that the material is graphene. The coolest part is that the peaks appear due to the vibration of carbon atoms in graphene that are bonded together. The carbon atoms vibrate side to side as well as up and down.

The width of the peaks tells me something about how many layers of graphene I’m looking at, and the fact that I don’t see a third peak before the first one tells me that our graphene is not damaged. Sometimes if carbon atoms are missing or if other molecules are bonded to the graphene surface, the graphene can have many defects and be unusable. And those are just a few of the pieces of information I get from looking at a single spectrum!

Modern art graphene Raman spectra

Sometimes just for fun I like to make what I call “modern art” versions of my spectra!

I studied lots of graphene Raman spectra to learn as much about our graphene as I could. Then came step #3 – trying to hydrogenate graphene. To do this, I place a graphene sample in a vacuum chamber, then heat the graphene and expose it to hydrogen. I’m still fine-tuning the hydrogenation details, but I did get to build my own vacuum chamber for this project, which was really fun!

vacuum chamber picture

This is a picture of the vacuum chamber I built, which is circled in purple.

I take Raman spectra of the graphene before and after my hydrogenations to see if the “normal” Raman spectrum of graphene changed. Remember the extra peak I mentioned earlier that only shows up if the graphene is damaged? Well, when I try to hydrogenate graphene, I want that peak to appear! It means that the graphene structure would be “damaged” because it would not consist of just carbon atoms anymore – a lot of the carbon atoms would be bonded to hydrogen.

hydrogenated graphene image (Image from Science magazine)

When graphene has been hydrogenated, hydrogen atoms are bonded to one or both sides of the graphene. Source: Science

I really enjoy interpreting spectra and finding out what does and doesn’t work in an experiment. In many ways, my job is kind of like constantly solving puzzles, and I love it. I like viewing research projects as puzzles to be solved, and with that view the things on my to-do list are more like putting a few of the pieces together. I’m so glad that I have the opportunity to learn how to become a scientist, and to discover new things that I never suspected I would love so much.

———————————————————————————————————-

About Kara Mattioli

Kara Mattioli pictureWhile I have always loved science, I never imagined that I would become a physicist. I initially came to Ohio State as a pre-medicine biochemistry major and had my heart set on becoming a surgeon. I decided that I wanted to learn more physics, so I changed my major to physics and ended up loving it. My career interests changed from medicine towards pursuing a full-time career as a scientist. I’m now entering my senior year as an undergraduate at Ohio State, and I plan on applying to physics graduate school. In my free time, I enjoy reading, hiking, and visiting art museums.