Reports: DNI1052463-DNI10: Quantized Phonon Dynamics for Thermoelectrics

Mark Siemens, PhD, University of Denver

We are pursuing enhanced thermoelectric efficiency by measuring the non-diffusive (potentially even quantized) phonon dynamics involved in the cooling of extreme-nanoscale objects. Our work has proceeded in three directions: 1. Optical pump-probe measurements of non-diffusive thermal transport in nanostructures; 2. Construction and alignment of a multidimensional spectrometer; and 3. Synthesis of few-nanometer semiconductor quantum dots for thermal transport measurements.

First, we have measured thermal transport dynamics with sub-picosecond time resolution using our ultrafast lasers in a pump-probe (thermoreflectance) geometry. We observed non-diffusive cooling of gold nanospheres and isolated metal films. Currently we are setting up to measure the thermal transport dynamics in bulk silicon as a function of the spot size of the laser heating. This will motivate our other work by showing clearly that thermal diffusion breaks down near nanoscale heat sources, even in bulk materials and substrates.

Second, we have completed the construction and alignment of a passively phase-stable setup for performing multidimensional spectroscopy in novel thermal transport measurements. The passive phase stability of the system is a huge advantage, but also presents a number of unique alignment challenges that have been solved. We are currently finalizing the alignment by measuring a four-wave mixing signal from polymer films, and the setup will then be ready for measurements of the cooling dynamics of semiconductor nanostructures, such as quantum dots. In particular, multidimensional spectroscopy will be used to measure the cooling dynamics of an inhomogeneous distribution of QDs, providing a direct measurement of the size dependence of thermal transport in a single experiment.

Finally, in preparation for these thermal QD studies, we recently began constructing a setup and implementing a procedure for chemical synthesis of PbS semiconductor quantum dots. These QDs are semiconductor nanocrystals just a few nanometers in diameter – small enough to potentially exhibit quantized phonon transport – and we are very excited to study the cooling dynamics of these samples, both with conventional thermoreflectance and with our 2DCS setup. The new synthesis setup is being built in collaboration with colleagues at NREL; we had considered the possibility of having them do the synthesis, but it was determined that having the QD synthesis on-campus would greatly speed the iterative process of QD synthesis, optical testing, and thermal transport measurement. However, their continued collaboration and expertise has still been essential in our building up QD synthesis for testing quantized thermal transport.

Career impact

The ACS PRF DNI grant has been critical in enabling the PI to successfully establish his experimental laboratory. The majority of funds support graduate student researchers doing the work described above.