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Fall 2015 Hooey Graduate Research Award Winners: Eugene Choi and Jonathan Saathoff

Friday, December 4, 2015

The winners of this distinguished prize will present a seminar on their work at 9:00 am on Monday, December 7, 2015 in 165 Olin Hall. A reception will be held at 8:45am in the Rhodes Lounge, 128 Olin Hall. 

Eugene Choi of the Clancy Research Group presents, Exploring the phase diagram of metastable water, no-man’s land, and clouds.

Water, the seemingly ordinary liquid is anything but ordinary: it’s peculiar properties, including the density maxima at 4 °C, have fascinated scientists to pursue the origin to water’s unique properties for a long time. Recent advancements in experimental and computational studies in metastable liquid water (i.e., liquids that equilibrate away from their thermodynamically stable domain due to kinetic barrier for phase transition) have shined some clues to what this “origin” might be, but the important parts of the phase diagram of metastable liquid water remain largely unexplored.

In this seminar, I will present my experimental quests to answer some of interesting questions about metastable liquid water and their applications, particularly in cloud-aerosol interactions. In the first part of my talk, I will briefly discuss my efforts to study superheated-stretched water (i.e., water at negative pressure), moving towards the doubly-metastable regime where liquid water is metastable with respect to both vapor and ice, and show how these studies have led me into discovering the continuity of vapor-liquid curve at supercooled temperatures (i.e., below melting point of water), even into the No-Man’s land.  I will discuss the implications of this discovery for the current perspectives on thermodynamics of liquid water in No-Man’s land.

In the second part of my talk, I will present on applying the results from the first part to understand kinetics and thermodynamics of heterogeneous nucleation from vapor and on the importance of this process in understanding cloud-aerosol interactions that involve supercooled water. I will show that by taking a reductionist’s approach to the complicated physical processes in clouds, I can provide useful insights to elucidating the underlying mechanisms for the formation of supercooled water-ice clouds. I will conclude with discussion on the impacts of this work in the context of global climate modeling, where the current lack of understanding in cloud-aerosol interactions imposes one of the biggest uncertainties. 

Jonathan Saathoff of the Clancy Group presents, Molecular Modeling of Solution-Processed Graphene Nanoribbons.

There has been great interest in graphene due to its single atom thickness and high electron and hole mobilities. It has the intriguing property that if formed into thin enough ribbons it develops a band gap, making it semiconducting rather than metallic. Graphene nanoribbons (GNRs) that are 2 nm across or less combine the low dimensionality of graphene with band gaps comparable to silicon, making them attractive for electronic device applications. My work is part of a multi-PI effort to fabricate GNRs through novel “bottom up” synthetic methods that produce atomically precise ribbon widths and pre-specified edge geometries. In practice, side-chains need to be appended to the ribbon edges to tune properties such as the dispersibility of the GNRs in solution, their ability to self-assemble on a substrate or in solution, and their electronic characteristics.  Of these, the dispersibility issue has been the most critical to help mitigate.

Experimentally, it is difficult to determine what role these side-chains play to alter macroscopic properties like dispersibility. Using computational approaches, we have uncovered the complexity and interdependence of interactions between side-chains, GNR, and solution. We have discovered that all these entities play a significant role in the material’s overall behavior in solution. In addition, we used free energy calculations to investigate possible molecular-scale mechanisms by which GNRs aggregate.  Together these studies have produced the first fundamentals-based understanding of GNRs in solution.

Closing the loop from processing to structure to function, we have probed the effect of these final aggregate morphologies on the GNRs’ electronic properties. To date, most studies of GNR have been confined to ab initio calculations of ideal situation of an isolated GNR; in this work, we show how electronic performance is degraded as the realism of the system is increased. To do so, we study the effect of GNR aggregation, misalignment and side-chain intervention on the electronic properties of the system. We show that our results for these more realistic systems compare well to experimental measurements from devices fabricated by our Princeton collaborators (Gao and Loo). 


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