Nonmonotonic Diameter Dependence of Thermal Conductivity of Extremely Thin Si Nanowires: Competition between Hydrodynamic Phonon Flow and Boundary Scattering

Abstract

By carefully and systematically performing Green-Kubo equilibrium molecular dynamics simulations, we report that the thermal conductivity (κ) of Si nanowires (NWs) does not diverge but converges and increases steeply when NW diameter (D) becomes extremely small (dκ/dD < 0), a long debate of one-dimensional heat conduction in history. The κ of the thinnest possible Si NWs reaches a superhigh level that is as large as more than 1 order of magnitude higher than its bulk counterpart. The abnormality is explained in terms of the dominant normal (N) process (energy and momentum conservation) of low frequency acoustic phonons that induces hydrodynamic phonon flow in the Si NWs without being scattered. With D increasing, the downward shift of optical phonons triggers strong Umklapp (U) scattering with acoustic phonons and attenuates the N process, leading to the regime of phonon boundary scattering (dκ/dD < 0). The two competing mechanisms result in nonmonotonic diameter dependence of κ with minima at critical diameter of 2-3 nm. Our results unambiguously demonstrate the converged κ and the clear trend of κ ∼ D for extremely thin Si NWs by fully elucidating the competition between the hydrodynamic phonon flow and phonon boundary scattering.

A minimal Tersoff potential for diamond silicon with improved descriptions of elastic and phonon transport properties

“…Thermal transport in silicon based materials has been extensively studied by classical molecular dynamics (MD) simulations [1][2][3][4][5][6][7][8][9][10][11][12]. The results strongly depend on the empirical interatomic potential used.…”

Decouple electronic and phononic transport in nanotwinned structures: a new strategy for enhancing the figure-of-merit of thermoelectrics

“…The charge distribution on the TB (circle A in Figure 1b) has been strongly altered with respect to a pristine diamond structure (circle B in Figure 1b A quite dense K points mesh, 15 , but it does not affect directly the comparison and conclusion since all the calculations are performed with the same method, which has been proved by previous studies 26,28,29 . All the results of ph  are calculated by Green-Kubo equilibrium molecular dynamics (GK-EMD) simulations [30][31][32] (details can be found in Sec. 4), in which the interatomic interaction is described by .…”

Phonon coherence and its effect on thermal conductivity of nanostructures

“…Semiconductor NWs have attracted considerable attention due to their potential applications in many areas, such as electronic devices [117,118], biosensors [119,120], thermoelectric devices [121][122][123][124] and optoelectronic devices [125][126][127][128]. The interest of investigating phonon transport in SiNWs has been greatly stimulated [129][130][131][132][133][134][135][136][137][138][139][140][141][142][143][144][145] since experiments [12,29] revealed that an approximately 100-fold reduction in thermal conductivity over bulk Si can be achieved in SiNWs, while the electrical conductivity and electron contribution to Seebeck coefficient are still similar to those of bulk silicon, which indicated that SiNWs could be applied as high-performance nanoscale thermoelectric materials. Further reducing thermal conductivity of SiNWs is critically important for achieving higher thermoelectric performance.…”