Wednesday, September 21, 2016

URSP Student Iris Stone Studies Material Characterization

I have been working with Dr. Vora (Department of Physics and Astronomy) in his optics laboratory since the fall of 2014. A significant portion of our efforts focus on materials characterization, or the process of investigating the properties of various materials and the way their behaviors change under different conditions, such as temperature. Our primary methods of exploration are through absorption, photoluminescence (PL), and Raman spectroscopy, each of which provide a unique opportunity to probe the material’s underlying structure and study intermolecular interactions. As an optics lab, we rely heavily on a few essential pieces of equipment, including a spectrometer, which takes in a beam of light (typically from a laser) and uses a diffraction grating to disperse the light across a silicon CCD chip; a liquid Nitrogen-cooled camera, which contains the CCD chip and sends “images” of the dispersed light to our computer for analysis; and a cryostat, which is capable of cooling samples to temperatures as low as 4K.

Over the past two years, I have conducted experiments on a range of 2D and nanomaterials, including transition metal dichalcogenides (TMDs) and single-walled nanotubes (SWNTs). However, my primary responsibility has been the study of charge transfer (CT) crystals. Unlike atomic crystals (which are the crystals you probably learned about in chemistry class), CT crystals are formed by stacks of repeating donor and acceptor molecules. This unique arrangement gives way to emergent physics that go beyond the constituent molecules. In addition, controlling certain features of the CT crystal – for example, by modifying the choice of donor and acceptor or adjusting the molecular packing – can have a significant impact on its optoelectronic properties. Ultimately, our goal is to identify CT crystals that can act as semiconductors and would be useful for applications in organic electronic devices.

I initially received a traditional URSP grant from OSCAR to study these materials in the summer of 2015. Experiments continued over the course of the 2015-16 school year, and I was fortunate enough to be the benefactor of additional funding – this time an intensive URSP grant – to carry on with my work throughout the summer of 2016. At the outset of this project, we identified one donor-acceptor pair (PTZ-TCNQ, or phenothiazine-tetracyanoquinodimethane) as a particularly promising candidate. Most recently, our goal has been to determine the importance of stoichiometry, or the ratio of donor and acceptor molecules, on the optoelectronic properties that manifest in the CT crystals. Through a collaboration with Georgetown University, we have been able to examine five novel stoichiometries using PL and Raman spectroscopy. These measurements are crucial to understanding which of the stoichiometries support charge transfer and allow ambipolar conductivity – in other words, which combinations of molecules are preferential for use in organic electronics.

Working in a physics lab for two consecutive years has been an interesting experience, to say the least. I have been extremely fortunate to have the opportunity to contribute to so many different aspects of the research process, from constructing the lab and designing software to taking measurements and analyzing the results – and of course, learning a lot along the way!