title: Sub-amorphous thermal conductivity in ultrathin crystalline silicon nanotubes.
abstract: Thermal transport behavior in nanostructures has become increasingly important for understanding and designing next generation electronic and energy devices. This has fueled vibrant research targeting both the causes and ability to induce extraordinary reductions of thermal conductivity in crystalline materials, which has predominantly been achieved by understanding that the phonon mean free path (MFP) is limited by the characteristic size of crystalline nanostructures, known as the boundary scattering or Casimir limit. Herein, by using a highly sensitive measurement system, we show that crystalline Si (c-Si) nanotubes (NTs) with shell thickness as thin as ∼5 nm exhibit a low thermal conductivity of ∼1.1 W m(-1) K(-1). Importantly, this value is lower than the apparent boundary scattering limit and is even about 30% lower than the measured value for amorphous Si (a-Si) NTs with similar geometries. This finding diverges from the prevailing general notion that amorphous materials represent the lower limit of thermal transport but can be explained by the strong elastic softening effect observed in the c-Si NTs, measured as a 6-fold reduction in Young's modulus compared to bulk Si and nearly half that of the a-Si NTs. These results illustrate the potent prospect of employing the elastic softening effect to engineer lower than amorphous, or subamorphous, thermal conductivity in ultrathin crystalline nanostructures.
authors.affiliation: †Department of Mechanical and Aerospace Engineering, University of California-San Diego, La Jolla, California 92093, United States.
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Wingert
authors.affiliation: ‡Materials Science and Engineering Program, University of California-San Diego, La Jolla, California 92093, United States.
authors.forename: Soonshin
authors.initials: S
authors.lastname: Kwon
authors.affiliation: §Institute of Mineral Engineering, Division of Materials Science and Engineering, RWTH Aachen University, 52064 Aachen, Germany.|∥Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, Department of Mechanical and Process Engineering, ETH Zürich, CH-8092 Zürich, Switzerland.
authors.forename: Ming
authors.initials: M
authors.lastname: Hu
authors.affiliation: ∥Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, Department of Mechanical and Process Engineering, ETH Zürich, CH-8092 Zürich, Switzerland.
authors.forename: Dimos
authors.initials: D
authors.lastname: Poulikakos
authors.affiliation: ‡Materials Science and Engineering Program, University of California-San Diego, La Jolla, California 92093, United States.|⊥Department of Electrical and Computer Engineering, University of California-San Diego, La Jolla, California 92093, United States.
authors.forename: Jie
authors.initials: J
authors.lastname: Xiang
authors.affiliation: †Department of Mechanical and Aerospace Engineering, University of California-San Diego, La Jolla, California 92093, United States.|‡Materials Science and Engineering Program, University of California-San Diego, La Jolla, California 92093, United States.
authors.forename: Renkun
authors.initials: R
authors.lastname: Chen
keywords: Thermal conductivity; elastic modulus; nanotubes; nanowires; phonon; phonon softening