Violation of Fourier's law and anomalous heat diffusion in silicon nanowires (original) (raw)
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Heat Conductance Is Strongly Anisotropic for Pristine Silicon Nanowires
Nano Letters, 2008
We compute atomistically the heat conductance for ultra-thin pristine silicon nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature thermal conductance is found to be highly anisotropic: wires oriented along the 110 direction have 50-75% larger conductance than wires oriented along the 100 and 111 directions. We show that the anisotropies can be qualitatively understood and reproduced from the bulk phonon band structure. Ab initio density functional theory (DFT) is used to study the thinnest wires, but becomes computationally prohibitive for larger diameters, where we instead use the Tersoff empirical potential model (TEP). For the smallest wires, the thermal conductances obtained from DFT-and TEP calculations agree within 10%. The presented results could be relevant for future phononengineering of nanowire devices.
Journal of Electronic Materials, 2013
We study the thermal properties of ultra-narrow silicon nanowires (NW) with diameters from 3 nm to 12 nm. We use the modified valence-force-field method for computation of phononic dispersion and the Boltzmann transport equation for calculation of phonon transport. Phonon dispersion in ultra-narrow 1D structures differs from dispersion in the bulk and dispersion in thicker NWs, which leads to different thermal properties. We show that as the diameter of the NW is reduced the density of long-wavelength phonons per cross section area increases, which increases their relative importance in carrying heat compared with the rest of the phonon spectrum. This effect, together with the fact that low-frequency, low-wavevector phonons are affected less by scattering and have longer mean-free-paths than phonons in the rest of the spectrum, leads to a counter-intuitive increase in thermal conductivity as the diameter is reduced to the sub-ten-nanometers range. This behavior is retained in the presence of moderate boundary scattering.
Anomalous diameter dependence of thermal transport in ultra-narrow Si nanowires
Journal of Applied Physics, 2014
We present atomistic valence force field calculations of thermal transport in Si nanowires of diameters from 12nm down to 1nm. We show that as the diameter is reduced, the phonon density-of-states and transmission function acquire a finite value at low frequency, in contrast to approaching zero as in the bulk material. It turns out that this effect results in what Ziman described as the "problem of long longitudinal waves" , which states that the thermal conductivity of a material increases as its length is increased due to the vanishing scattering for long-wavelength phonons. We show that this thermal transport improvement also appears in nanowires as their diameter is decreased below D=5nm (not only as the length increases) originating from the increase in the density of the long wavevector modes. The observation is present under ballistic transport conditions, and further enhanced with the introduction of phonon-phonon scattering. Because of this, in such ultra-narrow nanowires, as the diameter is reduced, phonon transport is dominated more and more by lower energy phonons with longer mean-free paths. We show that ~80% of the heat is carried by phonons with energies less than 5meV, most with mean-free paths of several hundreds of nanometers.
Thermal Conductivity in Thin Silicon Nanowires: Phonon Confinement Effect
Nano Letters, 2007
Thermal conductivity of thin silicon nanowires (1.4−8.3 nm) including the realistic crystalline structures and surface reconstruction effects is investigated using direct molecular dynamics simulations with Stillinger−Weber potential for Si−Si interactions. Thermal conductivity as a function of decreasing nanowire diameter shows an expected decrease due to increased surface scattering effects. However, at very small diameter (<1.5 nm), an increase in the thermal conductivity is observed, which is explained by the phonon confinement effect.
2008
The thermoelectric efficiency of a material depends on the ratio of its electrical and thermal conductivity. In this work, the cross-sectional dependence of electron mobility and lattice thermal conductivity in silicon nanowires has been investigated by solving the electron and phonon Boltzmann transport equations. The effects of confinement on acoustic phonon scattering (both electron-phonon and phonon-phonon) are accounted for in this study. With decreasing wire crosssection, the electron mobility shows a non-monotonic variation, whereas the lattice thermal conductivity exhibits a linear decrease. The former is a result of the decrease in intervalley and intersubband scattering due to a redistribution of electrons among the twofold-degenerate ∆ 2 and fourfold-degenerate ∆ 4 valley subbands when the cross-section is below 5 × 5 nm 2 , while the latter is because of the monotonic increase of three phonon umklapp and boundary scattering with decreasing wire cross-section. Among the wires considered, those with a cross-section between 3 × 3 nm 2 and 4 × 4 nm 2 have the maximal ratio of the electron mobility to lattice thermal conductivity, and are expected to provide the maximal thermoelectric figure of merit.
Kink effects on thermal transport in silicon nanowires
International Journal of Heat and Mass Transfer, 2019
Kinks in nanowires have recently been shown to be able to effectively tune the nanowire thermal conductivities; however, the underlying mechanisms have not been fully understood yet. To further disclose the details of phonon transport in kinked nanowires, here we report on non-equilibrium molecular dynamics studies of thermal transport through kinked and straight silicon nanowires. Results show that kinks can induce additional resistance and lead to lower thermal conductivity for kinked nanowires than that of corresponding straight wires. Detailed analysis indicates that kinks produce additional resistance through reflecting phonons back into their incoming arms. Moreover, through introducing heavier isotope atoms in the kink region, the simulation replicates the experimental observation that defects in the kink regime, instead of posing additional resistance, can actually facilitate thermal transport through deflecting phonons into the opposite arm.
Thermal Transport in Silicon Nanowires at High Temperature up to 700 K
Nano letters, 2016
Thermal transport in silicon nanowires has captured the attention of scientists for understanding phonon transport at the nanoscale, and the thermoelectric figure-of-merit (ZT) reported in rough nanowires has inspired engineers to develop cost-effective waste heat recovery systems. Thermoelectric generators composed of silicon target high-temperature applications due to improved efficiency beyond 550 K. However, there have been no studies of thermal transport in silicon nanowires beyond room temperature. High-temperature measurements also enable studies of unanswered questions regarding the impact of surface boundaries and varying mode contributions as the highest vibrational modes are activated (Debye temperature of silicon is 645 K). Here, we develop a technique to investigate thermal transport in nanowires up to 700 K. Our thermal conductivity measurements on smooth silicon nanowires show the classical diameter dependence from 40 to 120 nm. In conjunction with Boltzmann transport...
Atomistic simulations of heat transport in real-scale silicon nanowire devices
Applied Physics Letters, 2012
Utilizing atomistic lattice dynamics and scattering theory, we study thermal transport in nanodevices made of 10 nm thick silicon nanowires, from 10 to 100 nm long, sandwiched between two bulk reservoirs. We find that thermal transport in devices differs significantly from that of suspended extended nanowires, due to phonon scattering at the contact interfaces. We show that thermal conductance and the phonon transport regime can be tuned from ballistic to diffusive by varying the surface roughness of the nanowires and their length. In devices containing short crystalline wires phonon tunneling occurs and enhances the conductance beyond that of single contacts.