Seminar Series
Engineering thermal conductivity at the nanoscale by He+ ion irradiation
Abstract:
The control of thermal conductivity of materials is key to advanced heat management for novel applications such as thermal insulation, thermoelectric energy harvesting, and efficient heat dissipation, although the methods have been restricted to the bulk-/microscale without flexibility in terms of tunable thermal conductivity value in one system. Here, we introduce helium (He+) ion irradiation to control thermal conductivities of silicon membrane which be able to have arbitrary values from single-crystalline to amorphous level. The continuous suppression of its thermal conductivity as the increase of He+ ion doses is nearly two orders of magnitude, well correlating with point defect generation and the gradual amorphization process of the lattice. Tuning the spot size, energy, and dose of the irradiation, we sculpture the Si membrane into arbitrary patterns that can be designed to control heat flow along the membrane, acting as a programmable nanoscale thermal metamaterial.
Speaker: Hwan Sung Choe
Department of Materials Science and Engineering, UC Berkeley
Hwan Sung Choe received his B.S. in Physics Education department and M.S. in Physics department from Seoul National University, South Korea in 2002 and 2004, respectively. Under the supervision of professor Charles M. Lieber, he received his Ph.D. from the Physics department at Harvard University in 2013. He is currently a postdoctoral scholar employee in the Material Science and Engineering department at University of California at Berkeley, working on thermal and thermoelectric physics in low-dimensional materials with professor Junqiao Wu. His research interests include design, fabrication, and characterization of low-dimensional phase transitional materials and topological insulators for understanding their thermal and thermoelectric physics and the control of their energy transport for novel applications. He has published 12 peer-reviewed articles in major scientific journals including Nature and Physical Review Letters.
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Omnidirectional Indoor Light Harvesting using Nanocone Metastructure Arrays
Abstract:
Highly connected device networks have become omnipresent following nowadays big data trend. It is crucial to power such network systems in a reliable and sustainable way. In this talk, I will show that the power conversion efficiency limit of a single junction photovoltaic (PV) device can be greater than 80% when illuminated by monochromatic light emitting diodes (LEDs) and as high as 49.7% under white light illumination consisting of a GaN LED covered by yellow phosphor. We propose two novel device structures composed of nanocone arrays for harvesting optical energy from low intensity and angularly diffused indoor light sources. We present both experimental and simulation results to show that nanocone arrays have broadband light absorption exceeding 80%-90% for illumination angles from 0° to 40°, promising for PV powered devices for Internet of Things (IoT) applications
Speaker: Jipeng Qi
Department of Mechanical Engineering, UC Berkeley
Jipeng Qi is currently a 3rd year graduate student from Prof. Connie Chang-Hasnain’s research group at UC Berkeley. He received his B. S. degree in Materials Science and Engineering from Cornell University in 2015. His research interests are in semiconductor optoelectronics and nanophotonics. He is currently working on the design and simulation of high contrast grating (HCG), vertical-cavity surface-emitting laser (VCSEL) and metastructure for novel applications.