Avik Ghosh, Patrick E. Hopkins & Lloyd R. Harriott
Engineering the conduction of heat in solid materials is essential for a wide array of applications, ranging from thermal management of electronics, to power generation, to information technology. Many of these modern systems include materials and structures with characteristic length scales on the order of tens to hundreds of nanometers. At these length scales, the major thermal resistances arise at the interfaces between two materials, leading to thermal properties of nanosystems that are strongly dictated by these solid interfaces. Therefore, it is of utmost importance to develop mechanisms of controlling and tuning the thermal transport across interfaces to be able to accurately manage the thermal properties of nanosystems. To achieve this, this project will explore thermal transport between two solid layers, controlled by properties of organic molecules grown at their interface. Through the interplay between the wide-band, spatially localized molecular vibrons and the narrow-band, spatially delocalized substrate phonons, the project will aim for the systematic and targeted engineering of the composition, morphology and phonon bandstructures of material interfaces at the molecular scale in order to achieve a high degree of tunability of the interfacial thermal conductance. A combined and closely coupled theoretical and experimental study will be launched exploring various classes of self-assembled monolayers (SAMs) of organic molecules as thermal interfacial materials, as a function of variables such as the SAM and substrate quality and material, utilizing self-assembly and most importantly a rich variety of functional chemistry. The thermal transport across the wide array of SAM-based interfaces will be measured with time domain thermoreflectance. The experimental results will be strongly coupled with ab initio modeling efforts utilizing Nonequilibrium Green's Functions formalisms. The science discovered in this project will also be widely applicable to thermal engineering of generic metal and semiconductor interfacial systems.
The intellectual merit of this proposed work is in the development of a new understanding of nanoscale interfacial thermal properties. This will prove critical for the atomistic control of heat flow. Through the close synergy between theory and experiment, the study will enhance the understanding of heat flow and dissipation at their most fundamental, microscopic limits, combining atomistic concepts behind molecular heat flow with solid state concepts of bulk heat flow. It will bridge disciplines, materials, and ultimately the boundary between fundamental science and technological applications. Accurate, well benchmarked simulations will address the critical role of band-alignment and chemistry at the solid-molecular interface. Experiments will focus on fabrication of well-defined SAM-based interfacial structures, modifying them through molecular chemistry, characterization and measurement of thermal boundary resistances with a goal towards creating high quality tunable thermal boundaries.
The broader impacts of this project include both engineering relevance and education/outreach components. The knowledge gained will introduce a new concept in nanoscale science: thermal interface control and engineering via molecular chemistry. Nanoscale thermal management and control will bring about disruptive changes to science, technology and economics, ranging from superior quality and tailor-made thermal coatings and thermoelectric refrigerators, to better heat management in the multi-billion dollar semiconductor industry. Educational and outreach activities will involve creation of a nano-curriculum at UVa, tutorial creation on the thermalHUB, integration of research and education through thermal transport labs in courses taught by the PIs, conference organization of session on molecular heat transfer with undergraduate involvement, engaging minority undergraduate students for summer projects through the REU program and training middle school teachers for curriculum development utilizing UVA's Center for Diversity in Engineering.