MIT Department: Chemistry
Undergraduate Institution: Vassar College
Faculty Mentor: Troy Van Voorhis
Research Supervisor: James Shepard, Nadav Geva
I am junior Chemistry major at Vassar College from Mbale, Kenya. I am primarily interested in physical and inorganic chemistry research, with a focus on materials, photocatalysis, and photovoltaics. In my free time, I enjoy reading, cooking, and hiking. I plan to pursue a PhD in Chemistry after graduating from Vassar.
2017 Research Abstract
Effects of Surface Defects on Electronic Structure in Quantum Dots
Austin Atsango, Department of Chemistry, Vassar College,
Sam Shepherd, Department of Chemistry, Massachusetts Institute of Technology,
Troy Van Voorhis, Department of Chemistry, Massachusetts Institute of Technology,
Quantum dots serve as building blocks in numerous devices, with notable applications in solar energy, biological imaging, and data storage. At the core of these applications is photon up-conversion, a process where low-energy light is converted to higher-energy light, as well as photon down-conversion, a process where the reverse occurs. Modulating the surface chemistry of quantum dots has been observed to alter their optical properties and, by extension, their applicability. Here, we seek to further investigate this effect by studying how the electronic structures for lead sulfide (PbS) and cadmium selenide(CdSe) quantum dots respond to changes in ligand shell coverage. We begin with a quantum dot fully covered in a shell of 1-propanamine, and then we construct structures for quantum dots with shells that contain different ratios of both 1-propanamine and propane. This is achieved in two ways: by either removing 1-propanamine molecules from the shell at random and replacing them with propane, or replacing 1-propanamine with propane in a more orderly fashion so that a section of the shell would contain either 1-propanamine or propane, not both. We then use density functional theory (DFT) to examine the electronic properties of all constructed structures, including one where the shell is made entirely of propane. We find that the band gap for CdSe rises with the level of 1-propanamine coverage, with a maximum value of 1.74eV that occurs when the shell is made entirely out of 1-propanamine. A plot of the band gap against the level of 1-propanamine coverage for PbS yields a concave relationship, with a maximum band gap of 1.14eV occurring when 1-propanamine makes up 40% of the shell. Overall, there is no appreciable difference between dots that have 1-propanamine replaced randomly and those where the replacement was more orderly. These data provide promising avenues for the modification of quantum dots with a view to making them better suited for specific applications.