60th Annual Report on Research 2015

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 continued in the same three directions as last year: 1. Optical pump-probe measurements of non-diffusive thermal transport in nanostructures; 2. Alignment of a multidimensional spectrometer and analysis of multidimensional spectroscopy lineshapes; and 3. Phonon mode analysis 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. Recent improvements in laser modulation stability and electronics impedance matching have enabled a 100x sensitivity increase, which we are exploiting to measure phonon dynamics in thermally-isolated platforms for the first time. We are excited to compare these experimental results to electrical calorimetry to determine the true connection between thermoreflectance signals and thermal dynamics, which has been questioned in recent nanoscale thermoreflectance experimental work.

Second, are working to implement our passively phase-stable multidimensional spectroscopy setup for novel phonon spectroscopy measurements. We have also developed an analytical model capable of calculating a 2D spectrum lineshape from fundamental properties, which will be essential for recovering physical parameters from multidimensional spectra. In particular, we will be able to directly connect the 2D lineshape with temperature in experimental data. This work was published in Optics Letters earlier this year. We are now working to extend this work to allow for phonon spectra to be obtained from a multidimensional exciton spectrum of quantum dots.

Finally, we have successfully synthesized PbS quantum dots using a standard technique, and have modified a new method for getting much smaller PbS QDs than was previously possible. 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 multidimensional spectrosocpy setup. We have also started theoretical work studying the quantized phonon modes in these nanocrystals; it turns out that the modes for PbS are not well-understood, and knowing the phonon modes will enable a thorough analysis of the experimental data. This phonon model is nearing completion and we will submit our results to Physical Review B.

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. Preliminary results obtained as a result of this funding were used to obtain NSF funding for using 2D spectroscopy for phonon spectroscopy of nanomaterials.