Collaborative Research: Thermal Investigation of Strain-Tuned Thermal Conductivities of Thin Films

应变纳米薄膜广泛应用于电子和光电子器件、微电子机械系统、光伏、发光二极管以及固态能量转换。然而，应变薄膜的热学性质还没有得到系统的研究。该合作项目旨在研究高拉伸应变(根据薄膜厚度高达1-10%)对具有代表性的薄膜的热性能的影响，这些薄膜是当前电子工业中具有重要技术意义的材料。这项工作的结果尤其有利于它们广泛的工业应用，因为薄膜几何结构更适合于器件制造。更广泛地说，所获得的知识可能被推广到其他纳米结构材料的应变工程性质。该项目可以通过在博物馆展览中发现材料，在两所大学发展跨学科教育和研究计划，以及为K-12和高中生创建新的外联活动，使普通公众受益。这项拟议的工作将对弹性应变工程的热性质建立一个全面的理解。纳米薄膜，重点是沿拉伸应变和横向方向的热各向异性。通过将最先进的超快泵浦-探测实验技术和分子动力学(MD)模拟相结合，对具有代表性的薄膜在单轴和等双向拉伸应变下进行了一系列随温度变化的热输运研究。特别是在通常被忽视的横向声子传输方面，我们将获得新的物理见解。此外，在泵浦-探测实验中，通过调节激光加热的光斑大小和频率，也可以重建面内声子平均自由程(MFP)谱。这样的声子MFP谱将进一步用于应变薄膜的声子输运分析，与对非应变块体材料的大量研究相比。从薄膜研究中获得的知识可以潜在地扩展到其他纳米结构(例如纳米线和2D材料)和纳米结构块状材料(例如在超高压下热压的纳米复合材料)。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Strained nanofilms are widely used in electronic and optoelectronic devices, microelectromechanical systems, photovoltaics, light-emitting diodes, and for solid-state energy conversion. However, the thermal properties of a strained film have not yet been systematically investigated. This collaborative project aims to study the impact of a high tensile strain (up to 1-10% depending on the film thickness) on the thermal properties of representative thin films that are of technologically important materials in current electronic industry. Outcomes of this work can particularly benefit their broad industrial applications with the preference of thin-film geometry for device fabrication. More broadly, the obtained knowledge can be potentially generalized to other nanostructured materials for strain-engineered properties. This project can benefit a general public through materials discoveries at museum exhibitions, development of interdisciplinary education and research programs at two universities, and creation of new outreach activities to K-12 and high-school students. This proposed work will establish a comprehensive understanding on the thermal properties of ?elastic strain engineered? nanofilms, with an emphasis on the thermal anisotropy along both the tensile-strain and the transverse directions. A series of systematic temperature-dependent thermal transport studies will be carried out on representative thin films under uniaxial and equi-biaxial tensile strains, via the integration of the state-of-the-art ultrafast pump-probe experimental technique and molecular dynamics (MD) simulations. New physical insights will be gained, particularly, into the usually overlooked transverse-direction phonon transport. In addition, by tuning the spot sizes and frequencies of laser heating in the pump-probe experiments, the in-plane phonon mean-free-path (MFP) spectrum will also be reconstructed. Such a phonon MFP spectrum will be further used for the phonon transport analysis of strained films, in comparison with numerous studies on unstrained bulk materials. The knowledge gained from thin-film studies can be potentially extended to other nanostructures (e.g., nanowires and 2D materials) and nanostructured bulk materials (e.g., nanocomposites hot pressed under an ultrahigh pressure).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.