During my time at George Mason University working for Dr. Vora, my primary focus was on studying the optoelectronic properties of charge transfer (CT) crystals. These are a class of organic materials comprised of stacked pairs of molecules. In this model, one molecule acts as an electron donor, meaning it shares an electron with another molecule (which acts as an electron “acceptor.”) How much that electron shifts from the donor to the acceptor during crystal formation determines the degree of charge transfer between the molecules.
Two other important features of CT crystals are their stoichiometry and morphology.
Stoichiometry refers to the ratio of donor to acceptor molecules (it’s not always one-to-one!). Some CT crystals only form in one ratio, while others might grow in a variety of stoichiometries.
When the ratio of the molecules changes, this often affects the shape (morphology) of the crystals. Looking through a microscope, you might see CT crystals that look like cylindrical rods, coils, rectangles, or other geometric shapes.
What Have I Done?
In the Vora Lab, I looked specifically at a type of CT crystal formed from phenothiazine (PTZ) and tetracyanoquinodimethane (TCNQ). There hasn’t been much research on PTZ:TCNQ, but theoretical work suggests that these crystals could possess desirable traits for organic electronics (more on that below). As such, my goal was to provide a complete profile of PTZ:TCNQ crystals. This meant addressing questions like:
- Do they have a preferred stoichiometry?
- What about morphology?
- How much charge transfer occurs between PTZ and TCNQ molecules?
- Does charge transfer change depending on the temperature of the crystals?
- How electrical conductive are the crystals?
Over a period of about three years while working for Dr. Vora, I used a whole host of techniques – including absorption, photoluminescence, and Raman spectroscopy and atomic force microscopy – to answer these questions.
Why Should You Care?
Because of the unique coupling process between molecules, CT crystals show a lot of promise for use in organic electronics. Using organic materials in electronics would allow us to make devices that are flexible, biocompatible, and cheaper than traditional options.
We’ve already seen one application of this in curved televisions that use OLED (organic light-emitting diode) technology. In the future, we might also have biodegradable implants, printable electronic circuits, and thinner, more efficient solar cells – all thanks to organic materials!
CT crystals are especially exciting because they exist in such great variety. By adjusting the types of molecules and their ratio, you can tune certain aspects of the crystal – like their carrier mobility and electronic band gap. These features control a whole host of different optical and electronic properties, meaning you could end up with a material that is metallic, insulating, semiconducting, or even superconducting. In summary: CT crystals are versatile, easy to make, and easy to adapt for different purposes. And really, what more could you want?!
Why I Care (A Personal Note)
This project has been important to me for a few reasons. In many ways, it was the most significant research endeavor of my undergraduate career. I got in on the ground floor, way back in 2015 when it was all just an idea. From the beginning, I took the lead on every aspect of the project – coding software, designing our optical systems, taking measurements, performing the data analysis, and writing the paper. Just as importantly, it gave me a wide-lens view of what is required to get a research project from start to finish.
Studying CT crystals were also what sparked my interest in biophysics. I was fascinated by all their potential applications in biotechnology and medicine, and it felt good that my research could one day help improve human health. Ultimately, this project ended up being the first stepping stone on a long journey to my final destination in neuroscience.