Since 1988, the Merrill Presidential Scholars Program has honored Cornell University’s most outstanding graduating seniors, while also recognizing the teachers who have played a significant role in ensuring their success. This unique program was created by the late Philip Merrill ’55 and is made possible through annual support from the Merrill Family Foundation. Cornell University is grateful to the Merrill family—Eleanor Merrill, Douglas Merrill ’89, MBA ’91, Catherine Merrill Williams ’91, and Nancy Merrill ’96—for their continued commitment to the Merrill Presidential Scholars program.
Whitney excelled in scientific research, she demonstrated exemplary leadership in the Cornell University Sustainable Design project and, she has mastered the chemical engineering curriculum. Based on her extensive accomplishments, Whitney stands out as one of Cornell’s most accomplished seniors.
Dr. Will Baumgardner defends his PhD!
Abstract of Will’s thesis:
Quantum confined semiconductor nanocrystals, or quantum dots (QD), are a material class with tremendous technological potential. As research intensifies, there is a growing awareness that many of the fundamental assumptions of QD production are not well understood. A higher level of control over the processing methods that affect QD crystallinity, shape, and inter-QD coupling is necessary for the production of reliable, reproducible, and efficient devices. I present original research focused on the optimization of QD structures through the detailed investigation of chemical and physical processing procedures. The first synthesis of size tunable, monodisperse, quantum confined SnSe QDs is described. I discuss the implications of an unusual precursor injection sequence on the nucleation and growth process, and QD crystal structure. I continue my investigations of the synthesis method in a report describing shape control of PbSe QDs. I show that the shape of PbSe QDs can be tuned from pseudo-spherical to cubic by changing only the post-injection temperature ramp rate, a parameter not generally included in literature synthesis descriptions, exposing critical insights into the reproducibility and growth mechanisms of PbSe QDs. Device applications
often demand strict control over the inter-QD coupling while still maintaining quantum confinement. I present two studies on the fabrication of confined-but-connected (CBC) QD films. First, I introduce the pulsed laser annealing (PLA) processing strategy to the QD platform. After defining the process and its effect on QD films, I report the production of CBC structures by the PLA of amorphous silicon encapsulated PbSe QD films. The next report describes the fabrication of CBC PbSe films by facet specific displacement of surface ligands, resulting in epitaxial fusion of proximate QDs. Optical investigations demonstrate that the films retain quantum confinement while improving electrical conductivity by more than 3 orders of magnitude. Finally, the complex nanosized phase behavior of Au and Fe2O3 is investigated using in-situ temperature dependent scanning transmission electron microscopy (STEM). I demonstrate that heating the nanocrystals together results in a new bulk-forbidden composite phase. Using the STEM data, I present the first ever single-particle nanophase diagram with composition, temperature, and size dependent phase behavior.
Cornell chemical engineering student Kaifu Bian was recently awarded the 2012 Austin Hooey Award for Outstanding Thesis Research. The Hooey Graduate Research Excellence Recognition Award is the highest award given to a graduate student by Cornell’s School of Chemical and Biomolecular Engineering. This award recognizes excellence in graduate research and service to the research group, department and community. Kaifu’s research focused on fundamental processing-structure-property relationships of nanocrystal assemblies as designer solids.
During Cornell’s 2012 graduation ceremony Josh is hooded as a Doctor of Philosophy in recognition of his accomplishments on “NANOCRYSTAL QUANTUM DOTS AS BUILDING BLOCKS FOR ARTIFICIAL SOLIDS AND THEIR APPLICATIONS IN OPTOELECTRONIC DEVICES”
Nanocrystal quantum dots exhibit size-dependent optoelectronic properties and provide intriguing scientific and technological opportunities. Most proposed technologies based on nanocrystals depend on macroscopic functional assemblies of nanocrystals in which the nanocrystals interact with each other to give rise to new collective properties – also called as artificial solids. As in the analogous atomic crystals, the optoelectronic properties of artificial solids are governed by (1) the energy levels of nanocrystals, (2) electronic coupling between nanocrystals, and (3) the symmetry of the nanocrystal superlattice. These issues add many levels of complexity to the design of artificial solids and, for the successful development of nanocrystal based technologies, it is crucial to gain deep understanding on the structure-property relationship of nanocrystals on multiple length scales.
In this dissertation, we will present studies that show insights into the three governing factors of the optoelectronic properties of artificial solids mentioned above. (1) Nanocrystal energy levels: we show a direct correlation between interfacial energy level offsets between lead chalcogenide nanocrystals and ZnO layers with photovoltaic device performance. Based on obtained insights on the size dependent photovoltaic properties of lead chalcogenide nanocrystals, first demonstration of solution processed nanocrystal tandem solar cells was achieved. (2) Inter-nanocrystal electronic coupling: we probed rate of photogenerated exciton dissociation in
nanocrystal assemblies as a function of inter-nanocrystal spacing. We show that excitons dissociate via tunneling induced delocalization among neighboring nanocrystals. Based on insights obtained from this work, we demonstrate drastically improved performance of solution processed nanocrystal infrared light emitting diodes. (3) Nanocrystal superlattice symmetry: interaction between ligand molecules on the surface of nanocrystals play critical roles in self-assembly process. Differences in the coverage of surface ligands bound to nanocrystals can be exploited to tune the shape of nanocrystal interaction potential during the self-assembly. Denser ligand coverage causes nanocrystals to interact as spheres and face-centered cubic structure is formed. In contrast, sparse ligand coverage amplifies the aspherical shape of the core crystallite and can cause non-close packed structures such as body-centered cubic.
Nanomaterials provide a fertile opportunity space for fundamental scientific discoveries and technological advances, but one aspect that has been underestimated by the community is whether the process of becoming a nanoscientist working with these materials influences hairstyles. The cases presented below provide an initial answer to what question. If you have additional data points from your own personal experience, please let us know.