title: Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.
abstract: Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.
authors.affiliation: Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Mu
authors.affiliation: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA.|School of Fundamental Studies, Shanghai University of Engineering Science, Shanghai, 201620, China.
authors.forename: Lili
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of Power Engineering, North China Electric Power University, Baoding, 071003, China.
authors.forename: Xueming
authors.initials: X
authors.lastname: Yang
authors.affiliation: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA.
authors.forename: Pu
authors.initials: P
authors.lastname: Zhang
authors.affiliation: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA.
authors.forename: Albert C
authors.initials: AC
authors.lastname: To
authors.affiliation: Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.|Center for Sustainable Energy at Notre Dame, University of Notre Dame, Notre Dame, IN 46556, USA.
authors.forename: Tengfei
authors.initials: T
authors.lastname: Luo