Thermal Conductivity Of Nanowires Research Articles

Electrical and Thermal Conductivities of Single CuxO Nanowires.

Copper oxide nanowires (NWs) are promising elements for the realization of a wide range of devices for low-power electronics, gas sensors, and energy storage applications, due to their high aspect ratio, low environmental impact, and cost-effective manufacturing. Here, we report on the electrical and thermal properties of copper oxide NWs synthetized through thermal growth directly on copper foil. Structural characterization revealed that the growth process resulted in the formation of vertically aligned NWs on the Cu growth substrate, while the investigation of chemical composition revealed that the NWs were composed of CuO rather than Cu2O. The electrical characterization of single-NW-based devices, in which single NWs were contacted by Cu electrodes, revealed that the NWs were characterized by a conductivity of 7.6 × 10-2 S∙cm-1. The effect of the metal-insulator interface at the NW-electrode contact was analyzed by comparing characterizations in two-terminal and four-terminal configurations. The effective thermal conductivity of single CuO NWs placed on a substrate was measured using Scanning Thermal Microscopy (SThM), providing a value of 2.6 W∙m-1∙K-1, and using a simple Finite Difference model, an estimate for the thermal conductivity of the nanowire itself was obtained as 3.1 W∙m-1∙K-1. By shedding new light on the electrical and thermal properties of single CuO NWs, these results can be exploited for the rational design of a wide range of optoelectronic devices based on NWs.

Thermal transport in kinked nanowires through simulation.

The thermal conductance of nanowires is an oft-explored quantity, but its dependence on the nanowire shape is not completely understood. The behaviour of the conductance is examined as kinks of varying angular intensity are included into nanowires. The effects on thermal transport are evaluated through molecular dynamics simulations, phonon Monte Carlo simulations and classical solutions of the Fourier equation. A detailed look is taken at the nature of heat flux within said systems. The effects of the kink angle are found to be complex, influenced by multiple factors including crystal orientation, details of transport modelling, and the ratio of mean free path to characteristic system lengths. The effect of varying phonon reflection specularity on the heat flux is also examined. It is found that, in general, the flow of heat through systems simulated through phonon Monte Carlo methods is concentrated into a channel smaller than the wire dimensions, while this is not the case in the classical solutions of the Fourier model.

Phonon localization and resonance in thermal transport of pillar-based GaAs nanowires

Exploring the possibility of nanostructures to modulate thermal conductivity (TC) contributes to promote a deeper comprehension of phonon diffusion and transport processes with the design of thermally insulated devices with high ZT values, and the GaAs nanowires (NWs) widely used in optoelectronic and microelectronic devices exhibit nondiffusive phonon thermal transport phenomena attributed to size effects, while ignoring the wave effects of phonons. Here, we simulate the TC of pillar-based GaAs NWs using non-equilibrium molecular dynamics and Monte Carlo simulations. The spatial distribution of density of states, temperature and heat flow distribution clouds, phonon participation rate, dispersion curves and phonon transmittance of atoms were calculated to investigate the phonon thermal transport processes in pillar-based NWs. The calculation results show that the pillar-based surface reduce the TC by 16%, the TC of pristine NW increases with axial and equivalent diameter, and the TC of pillar-based NW increases nonlinearly with axial length and increases with radial length. The phonon-surface scattering intensity is enhanced by the perturbation introduced by the pillared surface with a substantial decrease in phonon transmission capacity and a break in long-wavelength phonon transport even annihilated, which leads to surface phonon localization. Nanopillars not only enhance the phonon-surface scattering intensity at low frequencies, but also reconfigure the dispersion curve to reduce the group velocity. A series of flat resonance phonon modes are generated throughout the whole spectrum due to the hybridization between the local resonance phonon modes of the nanopillar and the phonon modes of the substrate NWs, resulting in the phonon modes shifting to lower frequencies. The pillar-based surface induced surface phonon localization and local resonance phenomenon contributes to the modulation of phonon thermal transport in GaAs-based field-effect transistors.

Lattice Thermal Conductivity in Nanowires: Coupling the Bechmann–Kirchhoff Boundary Scattering Model With a Monte Carlo Framework

In this article, the effect of rough surface on phonon transport has been studied for silicon nanowire structures. The diffusive boundary scattering has been treated using the Beckmann–Kirchhoff (B-K) surface roughness scattering (SRS) model, considering the effect of roughness height, correlation length, phonon wavelength, and incident/reflected angles. The model is more comprehensive and accurate than the conventional approaches, where surface roughness is usually modeled based on experimental fitting parameters or only phonon wavelength. The B-K SRS model has been integrated within a particle-based Monte Carlo phonon transport (MCPT) simulator to study the thermal conductivity of different nanowire structures. The simulator is benchmarked against the experimental data for both bulk and nanowire devices. The reduction of thermal conductivity in nanowires as a function of the degree of roughness has been discussed. It is found that for a 70-nm-width, 6- $\mu \text{m}$ -long silicon structure, using 2.3-nm roughness height and 8.9-nm correlation length, 89% reduction in thermal conductivity occurs at 300 K. The sensitivity of the developed simulator with varying degree of roughness and the effect of SRS on different phonon spectral branches have been studied. These observations can be useful in designing materials with low thermal conductivity for thermoelectric cooling applications.

Thermal conductivity in nanoscale polysilicon structures with applications in sensors

Thermal conductivity of polysilicon nanowires and thin films has been experimentally studied. Thermal conductivity has been measured with a microstructure fabricated alongside the polysilicon nanowires and thin films and a robust fabrication process has been developed. Results suggest that the in-plane and out-of-plane thermal conductivity are limited by phonon grain boundary and film boundary scattering respectively. The nanowires are fabricated by patterning a thin layer of low-pressure chemical vapor deposited polysilicon using e-beam lithography, consequently circumventing the prevalent process of pick-and-place process, in which metallic contacts are deposited to insure significant heat flow to the nanowire. By avoiding the need for metallic contacts a significant source of error in the calculation of the thermal conductivity is eliminated. Thermal conductivity of nanowires with a cross section of ˜60 nm × 100 nm and thin film with a cross-section of 3μm × 108 nm were measured to be both ˜3.5 W/m.K, suggeting that the thermal conductiviy is dominated by the out-of-plane phonon scattering in both cases. This is an almost 8× reduction from the thermal conductivity of bulk polysilicon at ˜30 W/m.K. The effect of grain size and doping concentration on the thermal conductivity of ˜100 nm polysilicon films is also experimentally studied.

Thermoelectrics of Nanowires

The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to enhance it, including earlier theoretical and experimental work on quantum confinement effects and semimetal-to-semiconductor transition, surface engineering and complex heterostructures to enhance the carrier mobility and power factor, and the recent emergence of topological insulator NWs. For phonon transport, we broadly categorize the work on thermal conductivity of NWs into five different effects: classic size effect, acoustic softening, surface roughness, complex NW morphology, and dimensional crossover. Finally, we discuss the integration of NWs for device applications for thermoelectric power generation and cooling. We conclude our review with some outlooks for future research.

Impact of topological line defects on wall roughness and thermal conductivity of carbon nanotubes: A molecular dynamics study

Understanding the influence of defects on thermal conductivity of nanowires and nanomaterials is important due to its application for heat management in the nanodevices. In the present study, we investigate the influence of topological line defects on thermal conductivity of single-walled carbon nanotube (SWCNT) through molecular dynamics simulations. To model interaction between carbon atoms in the carbon nanotube, we employed the three-body Tersoff potential. Thermal conductivity was obtained in situations, which the 5-8-5 defects have been distributed with several patterns on the surface of carbon nanotube (CNT). We examined the impact of defect concentration and found that thermal conductivity decreases with increasing defect concentration. We also investigated the effects of length, temperature and the temperature difference between two ends of carbon nanotube on its thermal conductivity. The increase of length leads to an increment in thermal conductivity, while the increase of temperature causes thermal conductivity decreases. The cross-section of the nanotubes changes with the pattern of defect. Our results can be applicable in the heat management of carbon nanotube-based nanodevices.

Single silicon nanowires as inherent heaters and thermometers for thermal conductivity measurements

It is traditionally challenging to measure the thermal conductivity of nanoscale devices. In this Letter, we demonstrate a simple method for the thermal conductivity measurements of silicon nanowires by using the silicon nanowire under test as the thermometer and heater. The silicon nanowire (SiNW) arrays are patterned out of a silicon-on-insulator (SOI) wafer by standard microfabrication processes. The thermal conductivity of SiNWs with a width from 150nm to 400nm (while the thickness is fixed at 220nm) are measured in the temperature range of 20-200K. At low temperature range, the nanowire thermal conductivity exhibits a strong size dependency since the effective phonon mean-free path is dominated by the nanowire radial size. At high temperature range, the phonon mean-free path is significantly smaller than the nanowire radial size. The nanowire thermal conductivity is strongly temperature correlated and nearly independent of the nanowire size. Density function theory calculations are also performed on the SiNWs and the calculated thermal conductivity of SiNWs are largely consistent with the experimental data, showing that our method is valid for the thermal conductivity measurements of nanoscale devices.

Critical temperature in feedback-controlled electromigration of gold nanostructures

This paper presents several experiments demonstrating the need for a more nuanced picture of electromigration (EM) than that of a fixed critical junction temperature at which EM onset occurs. Our data suggests that even for a fixed cross-sectional geometry the critical junction temperature for EM, Tc, varies with environmental temperature, thermal resistance of adjacent regions, and even the direction of the current flow in asymmetric structures. We have performed feedback-controlled EM on nanowires at environmental temperatures between 75 and 260 K and fit the EM onset points with a constant junction power model. We find that average fit critical power is monotonically increasing with decreasing temperature, but is decidedly nonlinear at lower temperatures. We extract and compare the corresponding Tc values using several different thermal models which utilize measured values of nanowire thermal conductivity for our devices: these models all agree on a moderately increasing Tc with decreasing environmental temperature. This is tentatively explained by enhanced current-driven annealing on the voltage ramp prior to EM onset which decreases structural scattering, thereby increasing the critical temperature at which wind-force-driven hopping events will achieve a critical atomic flux. We also obtain fit critical power for a series of bowtie structures of identical constriction but varying adjacent thermal resistance (Rth), and estimate that Tc in the constriction varies with Rth for higher resistance structures. Critical power measurements on a second series of asymmetric bowties further suggests that Tc also depends on the alignment of the electron flow with the temperature gradient at the constriction.

Reduction of Thermal Conductivity in Nanowires by Combined Engineering of Crystal Phase and Isotope Disorder.

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed 28Si x30Si1- x nanowires with composition close to the highest mass disorder ( x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices.

Thermal conductivity reduction in silicon fishbone nanowires

Semiconductor nanowires are potential building blocks for future thermoelectrics because of their low thermal conductivity. Recent theoretical works suggest that thermal conductivity of nanowires can be further reduced by additional constrictions, pillars or wings. Here, we experimentally study heat conduction in silicon nanowires with periodic wings, called fishbone nanowires. We find that like in pristine nanowires, the nanowire cross-section controls thermal conductivity of fishbone nanowires. However, the periodic wings further reduce the thermal conductivity. Whereas an increase in the wing width only slightly affects the thermal conductivity, an increase in the wing depth clearly reduces thermal conductivity, and this reduction is stronger in the structures with narrower nanowires. Our experimental data is supported by the Callaway-Holland model, finite element modelling and phonon transport simulations.

A comparative study of molecular dynamics simulation methods for evaluation of the thermal conductivity and phonon transport in Si nanowires

In the present study, various MD methods including Equilibrium Molecular Dynamics (EMD) and two different Non- Equilibrium Molecular Dynamics (NEMD) methods are studied and compared for evaluation of the thermal conductivity and total and partial phonon density of states spectra in silicon nanowires, as a case study. The thermal conductivity of nanowires was determined as a function of length, cross section width and temperature. According to the results obtained via various MD methods, the thermal conductivity increases by increasing the length and cross section or decreasing the MD temperature. However, it was observed that despite the same initial conditions, different MD methods could predict considerably different values for the thermal conductivity which was found to be due to the different equilibrium temperature achieved in different methods. The total phonon density of states spectra was then employed to analyze the phonon transport properties of a 45 nm SiNW simulated using various MD techniques. Two major peaks were observed at around 16.5 and 5 THz which are attributed to Si-Si bond modes. Finally partial phonon density of states was calculated to differentiate the contributions to the phonon DOS from surface atoms compared to central atoms.

Impact of torsion and disorder on the thermal conductivity of Si nanowires: A nonequilibrium molecular dynamics study

In this paper, we investigate the thermal transport in Si nanowires by using nonequilibrium molecular dynamics simulations (NEMD). We focus on the effects of axial torsion and impurity on the thermal conductivity of the Si nanowires. Stillinger–Weber interatomic potential is employed to describe the interaction between silicon atoms. Also, Tersoff potential is used to simulate the thermal conductivity of Si nanowires with carbon impurity. Furthermore, the dependence of the thermal conductivity on tensile strain and mean temperature is evaluated in different crystallography directions such as directions [100], [110] and [111]. We find that by increasing the torsional angle and the impurity concentration, the thermal conductivity of the nanowires at all three crystallography directions decreases. As well as, we realized that the thermal conductivity of the strained Si nanowires decreases at [100] direction but continuously increases for [110] and [111] directions. Our simulation results exhibit that increasing the mean temperature of the system leads to reduction in the thermal conductivity. Our findings provide insights into the thermal control of silicon-based nanodevices for different applications such as thermoelectric ones.

Thermal conductivity of Bi2Te3 nanowires: how size affects phonon scattering.

This work provides an in-depth study of how the thermal conductivity of stoichiometric [110] Bi2Te3 nanowires becomes affected when reducing its diameter from an experimental and theoretical point of view. The thermal conductivity was observed to decrease more than 70% (from 1.78 ± 0.46 W K-1 m-1 to 0.52 ± 0.35 W K-1 m-1) when the diameter of the nanowire was reduced one order of magnitude (from 300 nm to 25 nm). The Kinetic-Collective model was used to understand such a reduction, which can be explained by the impact that surface scattering has in acoustic phonons. The smaller the diameter of the nanowires is, the larger the alteration in the mean free path of the low-frequency phonons is. The model agrees well with the experimental data, and the reduction in the thermal conductivity of the nanowires can be explained in terms of an increment of phonon scattering.

Thermal and Thermoelectric Transport in Highly Resistive Single Sb2Se3 Nanowires and Nanowire Bundles.

In this study, we measured the thermal conductivity and Seebeck coefficient of single Sb2Se3 nanowires and nanowire bundles with a high resistivity (σ ~ 4.37 × 10−4 S/m). Microdevices consisting of two adjacent suspended silicon nitride membranes were fabricated to measure the thermal transport properties of the nanowires in vacuum. Single Sb2Se3 nanowires with different diameters and nanowire bundles were carefully placed on the device to bridge the two membranes. The relationship of temperature difference on each heating/sensing suspension membranes with joule heating was accurately determined. A single Sb2Se3 nanowire with a diameter of ~ 680 nm was found to have a thermal conductivity (kNW) of 0.037 ± 0.002 W/m·K. The thermal conductivity of the nanowires is more than an order of magnitude lower than that of bulk materials (k ~ 0.36–1.9 W/m·K) and highly conductive (σ ~ 3 × 104 S/m) Sb2Se3 single nanowires (k ~ 1 W/m·K). The measured Seebeck coefficient with a positive value of ~ 661 μV/K is comparable to that of highly conductive Sb2Se3 single nanowires (~ 750 μV/K). The thermal transport between wires with different diameters and nanowire bundles was compared and discussed.

Reduction of phonon mean free path: From low-temperature physics to room temperature applications in thermoelectricity

Abstract It has been proposed for a long time now that the reduction of the thermal conductivity by reducing the phonon mean free path is one of the best way to improve the current performance of thermoelectrics. By measuring the thermal conductance and thermal conductivity of nanowires and thin films, we show different ways of increasing the phonon scattering from low-temperature up to room-temperature experiments. It is shown that playing with the geometry (constriction, periodic structures, nano-inclusions), from the ballistic to the diffusive limit, the phonon thermal transport can be severely altered in single crystalline semiconducting structures; the phonon mean free path is in consequence reduced. The diverse implications on thermoelectric properties will be eventually discussed.

Boundary scattering effect on the thermal conductivity of nanowires

Tuning the surface boundary structures of crystal nanowires (NWs) is one of the most popular ways to control the thermal conductivity of NWs. In this paper, non-equilibrium molecular dynamic simulations (NEMD) and time domain normal mode analysis (TDNMA) are performed to investigate the thermal transport properties of Ar (length of 52.9 nm, width of 4.23 nm) and Ge (56.5 nm long and 4.52 nm wide) NWs with different surface boundary structures. Results show that both the total and modal thermal conductivity have a dramatic difference among the NWs with the same diameter (the maximal and minimal thermal conductivity of Ar (Ge) NWs are 1.25 (15.3) and 0.60 (5.64) W/mK, respectively), which suggests the efficiency of nanoscale thermoelectric devices and heat dissipation devices can be largely tuned by only changing the boundary orientation. The fundamental mechanism responsible for such a discrepancy is found to mainly originate from different phonon group velocity (combined with phonon relaxation time for Ar NWs). The results are quite helpful for understanding the boundary scattering effect on thermal transport and also facilitating the design of nanoscale devices by tailoring the boundary structures.

Predictive Model for the Thermal Conductivity of Rough and Smooth Silicon Nanowires

The thermal conductivity of semiconductor nanowires (NWs) is significantly reduced with respect to their bulk counterparts. It decreases when the NW diameter is reduced and, in addition, it is dependent on the NW surface morphology, resulting in very low thermal conductivities for rough NWs. The thermal conductivity is a crucial physical magnitude for the thermal management of the nanodevices based on NWs; therefore, a great research effort has been focused in constructing theories that account for the thermal conductivity of semiconductor NWs. However, among all of these approaches, the possibility of predicting the value of the thermal conductivity of the NWs on the basis of the sole knowledge of the nature and morphology of the NWs is still lacking. We present herein a predictive approach for determining the thermal conductivity of both smooth and rough silicon NWs, based on a modified Callaway-Holland formalism. It correlates well with the existing experimental data, and greatly simplifies the complex evaluation of thermal conductivity at the nanoscale by means of simple mathematical expressions. These expressions allow for the estimation of the Si NW thermal conductivity for any combination of diameter and surface morphology parameters.

Influence of magnetic domain walls and magnetic field on the thermal conductivity of magnetic nanowires.

We investigated the influence of magnetic domain walls and magnetic fields on the thermal conductivity of suspended magnetic nanowires. The thermal conductivity of the nanowires was obtained using steady-state Joule heating to measure the change in resistance caused by spontaneous heating. The results showed that the thermal conductivity coefficients of straight and wavy magnetic nanowires decreased with an increase in the magnetic domain wall number, implying that the scattering between magnons and domain walls hindered the heat transport process. In addition, we proved that the magnetic field considerably reduced the thermal conductivity of a magnetic nanowire. The influence of magnetic domain walls and magnetic fields on the thermal conductivity of polycrystalline magnetic nanowires can be attributed to the scattering of long-wavelength spin waves mediated by intergrain exchange coupling.

Thermal conductivity prediction of copper hollow nanowire

Because its superficial area is larger than that of a nanowire, hollow nanowires have drawn more and more attention in solar cells, lithium ion batteries, and other potential applications. The thermal property of a hollow nanowire is useful, but there is little study on the matter. This paper marks the first careful study of the metallic hollow nanowire's thermal conductivity. The electron mean free path was simulated, and a fitting model for easy calculation was established based on that simulation. With an electron mean free path already obtained, the electronic thermal conductivity can be easily determined. At the same time, a model based on the ballistic-diffusive effective medium model was established to calculate the lattice thermal conductivity of a hollow nanowire. In the study of copper hollow nanowires, it turns out that: model results of the lattice thermal conductivity agree with results obtained by molecular dynamics. The size effect of the electronic or lattice thermal conductivity of a hollow nanowire is characterized well by the equivalent diameter. With a similar diameter, the electronic or lattice thermal conductivity of a hollow nanowire approximately equals with that of a nanowire. From the point of thermal conductivity, the hollow nanowire will be a good substitute for the nanowire.