Over the past two summers, Materials Processing Center-Center for Materials Science and Engineering Summer Scholars Megan Beck (2013) and Sarah Arveson (2014) worked in the Tisdale Lab as interns, drawing praise for their contributions from William A. Tisdale, the Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT.
Beck was co-author of a paper with fourth-year MIT chemical engineering graduate student Mark C. Weidman on controlled synthesis of ordered lead sulfide quantum dots. Arveson worked with former postdoctoral associate Pooja Tyagi on excitonic properties of organometal halide perovskites. A paper is in preparation.
Controlled quantum-dot synthesis
Beck assisted Weidman’s work on controlling synthesis of lead sulfide quantum dots to produce highly ordered, air-stable nanocrystals of consistent size. For applications, making quantum dots of all the same size, or monodisperse, is important. Nanocrystal lead sulfide is an important material for solar cells, LEDs, near infrared, and photodetectors.
“Mark and Megan were able to make extremely monodisperse, unprecedented monodispersity, in this particular type of nanocrystal, lead sulfide,” Tisdale says.
Beck varied the ratio of precursor concentration, with results showing that adding excess lead, even up to 24 times as much lead as sulfur, produced monodisperse nanocrystals. “She picked up the synthesis really fast so was able to get a lot of genuinely valuable work done for us that summer. …Mark followed along and did some very good work to unravel the mechanism of why that is,” Tisdale says.
“What we stumbled upon is that the ratio of precursors in the reaction had a huge effect on monodispersity of the final ensemble,” Tisdale explains. Beck is pursuing a PhD in Materials Science and Engineering at Northwestern University, in the Hersam Research Group.
High-efficiency solar
Arveson studied thin films of methyl ammonium lead bromide, an organometal halide perovskite with recently reported record solar-cell efficiencies. She experimented with different synthesis techniques, varying thickness and temperature and comparing spin coating with drop casting. She also made scanning electron microscope images of the perovskite films, which form into cubic crystals at room temperature and pressure.
“We’d like to deconstruct these materials, find out why they work so great, and if we can find out why they work so great and how they’re operating, then that can give us essential information to make further advances. So we were unraveling some of the fundamental questions about the nature of the exciton in these materials, which is essential to optimizing the solar cell device, having that understanding,” Tisdale explains.
“We found that they continue to behave like bulk crystals down to very, very small length scales, very, very small size crystals, which was surprising and counter to some previously published claims,” Tisdale says.
“Sarah’s work was really important in that project. She did a lot of the materials characterization work. She did a lot of X-ray diffraction work to prove that the crystal structure of these small nanocrystallites was the same crystal structure as the bulk crystals that we could make, as well as perfecting the synthesis of these materials to make them reproducibly and reliably. And then she also did a lot of electron microscopy work to study their overall shape and morphology. So those were really essential characterization strategies that we need to draw meaningful conclusions from the spectroscopy work that Pooja was doing,” Tisdale says.
Arveson is now a senior at University of California at Berkeley, where she is majoring in geophysics and applied mathematics. "I was very fortunate to have Will as my summer mentor," Arveson says. "Discussions with Will were always helpful and thought-provoking. He plays a very active role in the group, fostering a great environment for communication and learning."
Nine-week program
The Materials Processing Center and the Center for Materials Science and Engineering jointly run the nine-week Summer Scholars research program for undergraduate students at MIT with funding from the National Science Foundation. This next internship program will run from June 8 to August 8, 2015.
The application deadline is Feb. 13, 2015.
