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

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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.