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

在非常薄的材料（由几个原子组成）中，光电特性可能与同一材料的宏观块体有很大不同，这对导热有影响。其中一种类型的材料是过渡金属二硫族化合物（TMDCs）。将这些超薄TMDCs集成到器件中的进一步进展需要充分了解其传热能力，但是使其具有新颖物理特性的薄性也阻碍了传统表征技术的使用。科罗拉多大学和丹佛大学的研究人员提议开发一种新技术来测量在TMDCs中携带热量的声子的动力学。该项目的结果有望对具有理想热传导特性的新型材料和器件的设计产生关键的见解。主要研究人员将实施一个实践计划，鼓励学生探索有关激光和热传递的科学问题。研究生将参与其研究生学位的研究。在最近的纳米尺度热输运研究中，声子平均自由程（与声子寿命成正比）是一个核心概念，它支配着过去二十年来观察和探索的纳米结构材料的超低和超高导热率的非凡现象。虽然载流子的平均自由程和寿命是简单的动力学概念，但声子的平均自由程和寿命的第一性原理计算直到最近才开始进行。目前还需要一种令人信服的测量技术来测量频率相关的声子寿命。该合作研究项目的目标是将二维相干光谱学（一种用于研究电子相干性的高影响技术）应用于声子寿命的测量。为了证明这种新技术的有效性，将测量二维TMDCs中频率相关的声子寿命，并与第一性原理计算进行比较。单层和几层TMDCs上的声子寿命谱将获得关于热导率和声子在几层TMDCs中的层依赖性的关键信息，这将对二维TMDCs新型器件的设计产生直接影响。

1512776 / 1511199Yang, Ronggui / Siemens, MarkIn very thin materials (of a few atoms), the opto-electronic properties can be dramatically different from a macroscopic chunk of the same material that has implications in conducting heat. One of this type of materials is the 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.