Defect structures are generally difficult to control and characterize yet offer unprecedented ability to control material properties in advanced technological applications such as photonics, catalysis, and energy conversion. Experimental determination of the mechanisms responsible for the observed behavior is a continuing challenge, as no tools or methods exist with which to probe dynamic response – e.g. strain-coupled thermal and electronic transport – at the atomic level. In this talk, I will discuss our recent results on three areas where atomic-scale non-equilibrium can bring about profound enhancements in both physical and chemical properties, and preview the new methodologies that we are developing to establish atomic-level structure-transport property relationships in individual nanomaterials using custom multi-property measurement platforms and in situ transmission electron microscopy.
- Strain is predicted to offer orders of magnitude changes in many important properties across many different scientific fields from catalysis to optics. Using mechanical force alone, we have recently demonstrated a two orders of magnitude increase in the excitonic recombination rate of an indirect semiconductor, WSe2. This is the first time that extrinsic control over an indirect-to-direct electron band gap transition has been conclusively reported.
- Single photon generation is a requirement for quantum key distribution needed for advanced encrypted communication technologies. We have developed a method to create highly-spatially localized and well-separated emission sites in a continuous film of nominally bilayer WSe2 using an ultra-sharp dielectric tip array. The small tip radii are postulated to give rise to electronic localization effects through morphology alone, as the dielectric environment surrounding the emission site is constant. Importantly, we have measured the second order photon correlation parameter for our localized defect emission to be below 0.3.
- Reducing metal oxides thermally is a scalable and industrially promising route to defect-engineered materials, but has been perceived as problematic due to their high thermal stability. For example, the Mo─O binding energy in MoO3 is one of the highest among the metal oxides making it historically difficult to efficiently create oxygen vacancy sites on its surface. We have discovered a new mechanism to thermally reduce metal oxides at temperatures as low as 400-600 oC in MoO3, where dipole interactions between a polyelectrolyte and surface oxygen site facilitate bond breaking at temperatures up to half as low as would be required thermodynamically.
These discoveries have general implications to defect engineering, achieved thermally or otherwise, and hence impact varied and promising technological areas from catalysis and thermochemical energy storage to quantum communication. My methodology of collaboration with theorists provides general insight for predictive efforts to design materials with enhanced functionalities, and the new experimental methodologies we are developing will have the ability to influence investigations beyond the materials and applications I present as case studies.
Michael Pettes leads the Nanoscale Transport Laboratory at UConn where he has been awarded more than $1M in funding. He earned his B.S. in Mechanical Engineering from Duke University in 2001, and his M.S. and Ph.D. in Mechanical Engineering from the University of Texas at Austin in 2007 and 2011, respectively. Among his honors, he received the National Science Foundation Early Faculty Career Development (CAREER) Award in 2016, the National Science Foundation Graduate Research Fellowship in 2006, and the Donald D. and Sybil B. Harrington Doctoral Fellowship at the University of Texas at Austin in 2005. His educational innovation was recognized by invitation to the National Academy of Engineering’s Frontiers of Engineering Education symposium in 2016. His research contributions focus on experimental structure-processing-property relationships at the extremes of size and conductivity for application in advanced manufacturing processes and sustainable energy technologies.