NSF/ENG/ECCS-BSF: Self-Assembled Superlattice Nanowires: A Pathway to High Efficiency Thermoelectrics

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Abstract: Non-technical: As developing countries continue to industrialize, energy demands are rapidly increasing; thus, there is an increasing need for sustainable clean energy sources driven by scientific research. Typically, the efficiency of energy-production and utilization systems is limited by heat loss; conversion of the wasted heat into usable energy may be accomplished using solid-state devices that convert heat to electricity, i.e. thermoelectrics. Thermoelectric generators are used to power satellites, probes, and rovers, but their widespread terrestrial use would require increased device efficiencies. This project explores a pathway towards high efficiency thermoelectrics using a naturally occurring phenomenon, spontaneous vertical phase separation, to achieve nanowire superlattices that extend over macroscopic lengths. The ultimate goal is to understand heat propagation across the superlattices in order to optimize the performance of several advanced nanotechnologies. The project consists of a collaboration between the University of Michigan and Ben-Gurion University, integrating the expertise of the U.S. investigators (theory and thermoelectric characterization) with that of the Israeli investigators (nanowire fabrication and characterization). The new knowledge gained will be broadly disseminated through publications and presentations, and curriculum development. Outreach activities emphasize the mentoring of women and underrepresented minorities. Technical Description: Nanometer-scale heterostructured materials have been identified as promising candidates for high efficiency thermoelectric devices. In the framework of the phonon-glass-electron crystal concept, the thermoelectric efficiency can be enhanced by reducing dimensionality, through the formation of two-dimensional thin films or superlattices, one-dimensional nanowires, or zero-dimensional quantum dots. Indeed, one-dimensional conductors, in which electrons are restricted to a narrow energy range, are predicted to enable conversion efficiencies approaching the Carnot limit. A primary goal of the project is to explore eutectic alloy catalyst-induced vertical phase separation during vapor-liquid-solid growth of nanowires. In addition to developing predictive understanding of these mechanisms, nanowire superlattices extending over macroscopic length-scales will be fabricated, and spatially-resolved Seebeck measurements will be demonstrated using scanning thermoelectric microscopy. Finally, new understanding of the mechanisms for enhanced thermoelectric figure of merit in nanowires will be developed. In particular, correlations between van Hove singularities in the density of states and enhancements of the Seebeck coefficient will be investigated using both doping and electrostatic gating to tune the Fermi level of the nanowires. The combined expertise of the team in nanowire growth, structural and thermoelectric characterization, and device simulation and fabrication will be used to develop a pathway to superlattice nanowires for thermoelectric generators.