Si/Ge Superlattice Nanowires with Ultralow Thermal Conductivity

• Published in: Nano letters Vol. 12; no. 11; pp. 5487 - 5494
• Main Authors: Hu, Ming, Poulikakos, Dimos
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 14.11.2012
• Subjects: Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Germanium; Heat transfer; Materials science; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Nanowires; Phonons; Physics; Quantum wires; Silicon; Structure of solids and liquids; crystallography; Superlattices; Thermal conductivity; Thermal properties of condensed matter; Thermal properties of small particles, nanocrystals, nanotubes; Thermoelectricity; Transport; thermoelectrics; thermal conductivity; Si/Ge superlattice nanowire; coherent phonon; Dimensionality; Molecular dynamics method; Theoretical study; Thermoelectric devices; Interfaces; Thermoelectric materials; Monocrystals; Superlattices; Thermal conductivity; Germanium; Silicon; Nanowires; Phonon scattering; Nanostructured materials; Surface phonons
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl301971k

The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics.