UNS: Collaborative Research: Ultrafast Phonon Spectroscopy for Lifetime Measurements of Phonons in 2-D Transitional Metal Dichalcogenides

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1512776 / 1511199Yang, Ronggui / Siemens, MarkIn very thin materials (of a few atoms), the opto-electronic properties can be dramatically different from a macroscopic piece of the same material that has implications in conducting heat. A type of this material is transition metal dichalcogenides (TMDCs). Further progress in integrating these ultra-thin TMDCs into devices requires full understanding of their heat transfer capability, but the thinness that enables their novel physical properties also prevents the use of traditional characterization techniques. Researchers from the University of Colorado and the University of Denver propose to develop a new technique for measuring the dynamics of phonons which carry heat in TMDCs. Results from this project are expected to yield critical insight into the design of novel materials and devices with desired thermal conduction characteristics. The principal investigators will implement a hands-on program encouraging students to explore scientific questions about lasers and heat transfer. Graduate students will be involved in the research for their graduate degrees.Throughout the recent work on nanoscale thermal transport, phonon mean free path (proportional to phonon lifetime) is a central concept that governs the extraordinary phenomena observed and explored over the past two decades for ultra-low and ultra-high thermal conductivity of nanostructured materials. Although the mean free path and lifetime of carriers are simple kinetic concepts, the first-principle calculations of mean free path and lifetime of phonons are only recently being undertaken. A convincing measurement technique is yet needed to measure frequency-dependent phonon lifetime. The objective of this collaborative research project is to adapt two-dimensional coherent spectroscopy, a high-impact technique used to study electronic coherence, to the measurement of phonon lifetime. To demonstrate the efficacy of this novel technique, the frequency-dependent phonon lifetime in 2-D TMDCs will be measured and compared with the first-principles calculations. Phonon lifetime spectroscopy on monolayer and few-layer TMDCs will yield critical information about the layer-dependence of thermal conductivity and phonon in few-layer TMDCs, which can have immediate impact on the design of 2D TMDC-enabled novel devices.