Micro- and nano-phononic/photonic crystals have found application and/or hold promise in many emerging technologies such as nanophotonic devices, high ZT thermoelectric materials, phononic-photonic crystals for optical cooling, acoustic insulators, micro-acoustic components such as waveguides, cavities and filters, as well as NEMS/MEMS thermal sensors and actuators. Phonon coherence and scattering effects in these two-dimensional periodically porous (2D-PP) and one-dimensional nanoladder (1D-NL) structures may result in more than two orders of magnitude reduction in their effective thermal conductivities. Exploring the intriguing physics of phonon transport in these novel structures is the subject of this project by means of careful transport property measurements on silicon films, which offer nearly unparalleled material quality owing to the refinement of silicon on insulator fabrication technology. We explore a wide range of critical dimensions such as film thickness, hole diameter and separation ranging from 20 to 1000 nm as well as surface roughnesses ranging from 2 to 10 nm and temperatures ranging from 10 to 800 K. We implement novel pore designs to break or control the onset periodicity and induce multiple phononic bandgaps. While the primary focus of this project is to make available the largest set of accurate and well-characterized experimental data to the scientific community, we also attempt to develop a simulation tool/model that can help to guide the experimental design as well as improve upon the existing models to extend their limited range of applicability and accuracy.
This project addresses several outstanding questions on the fundamentals of phonon transport in nanostructures. For example, we would like to know the origin of extreme reduction in thermal conductivity two-dimensional and one-dimensional nanostructures and the manner in which this reduction is affected by pore geometry and temperature. This project attempts to identify the relative contributions of classical phonon scattering on pores and boundaries and phonon coherent effect (dispersion) to heat conduction. We should learn if it possible to create multiple phononic bandgaps using multiple periodicity patterns in the 2D structures and be able to understand the impact of breaking the periodicity patterns. Other areas of science and technology that can directly benefit from the proposed research include thermoelectric energy generation and cooling. Potentially, a more efficient thermoelectric two-dimensional material may result in significant improvement of these devices. Understanding of the transport in nanostructures structures will greatly benefit the performance and reliability of nanofabricated thermal sensors and actuators.
