CAREER: Phononic Properties of Colloidal Nanocrystal Superlattices

Arizona State University, Scottsdale AZ

Investigators

Abstract

Non-Technical Abstract Electricity, light, sound, and heat are common phenomenon encountered in everyday life. Materials that enable advanced control over the transmission of electricity and light are known as electronic and photonic materials, respectively. These materials have made possible numerous modern technologies such as laptops, cellular phones, fiber optics, lasers, and microscopes. In contrast, technological control over sound and heat has lagged far behind that of light and electricity. This project focuses on the creation of phononic materials that could enable advanced control over the transmission of sound and heat. These phononic materials will consist of organized nanoparticle-molecule assemblies that possess vibrational characteristics that do not arise in naturally occurring materials. These assemblies will then be used to create filters, mirrors, and one-way valves that manipulate the transmission of sound and heat. This project is also integrated with a variety of educational activities. It will engage K-12 students and help train future STEM educators through the Science is Fun program at the LeRoy Eyring Center for Solid State Science. In addition, a laboratory module on phononic crystals will be developed for undergraduate students. Research results will be incorporated into a graduate student course on nanoscale heat transfer. Technical Abstract Phononic crystals are artificially structured materials with periodic variations in acoustic impedance (i.e., alternating hard and soft materials). This periodicity results in a phononic band gap that enables the creation of many phononic devices such as phonon filters, waveguides, diodes, and mirrors. The objectives of this project are to: (1) Demonstrate the potential of nanocrystal superlattices for phononic applications, (2) Achieve phononic band gaps in the > 100 GHz regime for 3-dimensional phononic crystals, (3) Control transmission and reflection of heat transporting phonons in nanocrystal superlattices, and (4) Create high frequency phononic devices such as phonon filters, mirrors, and diodes. The phononic band gap center and phononic band gap width will be tuned via nanocrystal size and composition as well as ligand composition. DNA will also be used to direct nanocrystal assembly and enable diverse structural possibilities. To ensure mechanical, chemical, and thermal stability of the superlattices, the DNA linkers will be converted into an inorganic matrix via sol-gel chemistries. Phonon transmission and reflection will be experimentally studied using phonon spectroscopy, which uses monochromatic phonon generators and detectors to directly measure frequency-resolved phonon transport. These experiments will be complemented with computational modeling that calculates the phononic band diagram and simulates phonon transport using plane wave expansion methods and finite-difference time-domain methods, respectively. The nanocrystal superlattices will also be integrated with substrates by growing them from templates fabricated via electron beam lithography; thereby opening the door for future chip-integrated applications.