Thermal transport in individual thermoelectric nanowires: a review

Abstract

Extensive research efforts have been devoted to nanowires because of their novel electronic, optical and thermoelectric properties due to spatial confinement in two dimensions. Among various fields, nanowires have been of interest in the thermoelectric community not only for their novel thermoelectric properties but also for their ease of use in fundamental scientific studies as the physics learned using nanowires can be applied in bulk thermoelectric nanocomposites. In this paper, we limit our discussion to experimental thermal transport in thermoelectric nanowires such as Bi–Te, Pb–Te and Si–Ge nanowires. After reviewing the reasons why nanowires are of interest in the thermoelectric community, we discuss various synthesis methods and thermal transport measurements. Next, we evaluated how thermal transport in nanowires is affected by various scattering mechanisms such as phonon boundary scattering, alloy scattering, etc. We also discuss a recent study concerning how the surface roughness affects phonon transport. This article is useful to gain insight into how to manage thermal transport in various applications.     

EQUILIBRIUM MOLECULAR DYNAMICS STUDY OF PHONON THERMAL TRANSPORT IN NANOMATERIALS

In this work, the thermal conductivity of nanofilms, nanowires, and nanoparticles are studied using molecular dynamics simulation. It is found that their thermal conductivity depends significantly on the characteristic size until it reaches a large value. Comparison with results of the lattice Boltzmann method reflects strong effects of surface structure, especially when the film thickness is comparable to the mean free path of phonons. Study of the phonon thermal transport in nanowires and nanoparticles reveals much stronger boundary-scattering effect on thermal transport than in nanofilms, which is attributed to the more confined phonon movements in these two- and one-dimensional nanomaterials.     

Structure and optical properties of ZnO nanowires coated with silica

Abstract

Abstract

It is essential to passivate one-dimensional nanostructures with insulating materials to protect them from contamination and oxidation as well as to avoid cross-talking between the building blocks of complex nanoscale circuits. The ZnO nanowires synthesised by the thermal evaporation of ZnO powders were coated with SiO₂ by the sputtering technique. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the ZnO core–SiO₂ shell nanowires were single crystal wurtzite type ZnO and amorphous SiO₂ respectively. Photoluminescence measurements at room temperature showed that the passivation of the ZnO nanowires was successfully achieved with SiO₂ without nearly degrading the near band edge emission from the wires. However, subsequent thermal annealing treatment was found to be undesirable owing to the degradation of the near band edge emission in intensity.     

Prediction of thermal conductivity of nanostructures: Influence of phonon dispersion approximation

In this study, the influence of phonon dispersion approximation on the prediction of in-plane and out-of-plane thermal conductivity of thin films and nanowires is shown. Results obtained using the famous Holland dispersion approximation and the Brillouin zone boundary condition (BZBC) dispersion curves are compared. For (in-plane and out-of-plane) thermal conductivity predictions based on BZBC dispersion curves, new relaxation time parameters fitted from experimental data of bulk silicon thermal conductivity are reported. The in-plane thermal conductivity of nanostructures (films of thicknesses 20 nm, 100 nm, and 420 nm and nanowires of widths 22 nm, 37 nm, and 100 nm) in the temperature range 20–1000 K is calculated from the modified bulk thermal conductivity model by scaling the bulk phonon mean free path (MFP) by the Fuch–Sondheimer factor of boundary scattering developed for nanostructures with rectangular cross-section. The pseudo out-of-plane thermal conductivity of films of thicknesses 20 nm, 100 nm, and 420 nm and in the temperature range 150–1000 K is calculated from the solution of the Boltzmann transport equation (BTE) for phonons by using the Discrete ordinate method (DOM), and the Monte Carlo (MC) simulation. In order to confirm the current results, the calculated in-plane thermal conductivity of silicon thin films and silicon nanowires are compared with existing experimental data. Moreover, due to lack of experimental and theoretical data of out-of-plane thermal conductivity of thin films, comparison of the DOM and MC simulation is performed. The current work shows that a drastic simplification of dispersion curves can lead to wrong prediction of both in-plane and out-of-plane thermal conductivities of nanostructures, especially for ultra thin nanostructures and/or at high temperatures. Comparison with experimental data of in-plane thermal conductivity of silicon thin films and silicon nanowires proves that more refined dispersion approximation such as the BZBC is well adequate for phonon transport calculations when confinement has negligible effect. Moreover, the comparison between the thermal conductivity in the out-of-plane direction and that in the in-plane direction enables one to quantify the anisotropy of thermal conductivity of the film.     

纳米硅锗热电材料和纳米器件的研究进展

近年来,纳米技术逐渐被用来设计和制备硅锗（Si−Ge）热电材料和新型器件。为了提高Si−Ge热电材料的热电性能,研究学者利用各种纳米结构对Si−Ge热电材料进行了理论研究。其中,利用纳米线、超晶格和量子点等结构中的能带机理与散射机理,从理论上设计了降低Si−Ge纳米结构热导率和提高其功率因子的途径。同时,高效的Si−Ge纳米热电材料被制备出来,包括纳米块体材料的热电性能得到大幅度提高,室温下薄膜和纳米线的热电性能实现了重大突破。在高性能材料的基础上,新型Si−Ge纳米热电器件的研发除了关注于制备工艺优化外,还包括传热结构和原型器件的设计。     

Formation,rapid thermal oxidation and passivation of solar grade silicon nanowires for advanced photovoltaic applications

Uniform and regular silicon nanowires (SiNWs) arrays are fabricated on both sides of solar grade silicons (SiGS) by silver assist-electrochemical etching. SiNWs arrays exhibit an excellent antireflection character with an overall reflectance of 2% in the range from 300 to 1000 nm. More importantly, the effective lifetimes of the symmetric SiNWs/Si structures decreased due to the high densities of dangling bonds and surface defects. Surface passivation to overcome lifetime degradation is realized by means of rapid thermal oxidation (RTO). Following rapid oxidation, Fourier Transform Infrared spectroscopy reveals that oxygen diffusion is enhanced inside silicon nanowires where the morphological structure is preserved during RTO. Moreover, it is shown that even the rapid thermal oxidation process is not effective to recover initial τ_eff due to the high density of imperfections involved during nanowires formation and the contamination level induced by silver. The interdiffusion between residual silver and metal contaminants in the core of the nanowire can probably limit the passivation effect due to the segregation of metal atoms at SiO₂ and to the redistribution of both impurities across the wire.     

Nonlocal behavior in phonon transport

Nonlocal behavior is accommodated in the framework of thermal lagging to describe heat transport in phonon systems. In steady state the emphasis is on capturing the linear dependence of the effective thermal conductivity of thin nanowires on their radius. In ultrafast transient validation of the nonlocal/lagging model is furnished by its precise correlations with the thermomass model. A new type of thermal wave results from the first order nonlocality. Entering the second-order effects of nonlocality, special features include diminution of the sharp wavefront, a localized zone with a finite width, and much wider affected regions due to phonon scattering.     

Fast and highly-sensitive hydrogen sensing of Nb2O5 nanowires at room temperature

The single-crystalline Nb₂O₅ nanowires with tetragonal phase structures were synthesized through thermal oxidation process. The Nb₂O₅ nanowires were grown along [001] orientation and formed a layer of free-standing nanowire membrane. A pair of platinum electrodes was deposited on the surface of the nanowire layer to fabricate a Pt/Nb₂O₅ nanowire hydrogen sensor. The Pt/Nb₂O₅ nanowire hydrogen sensor exhibited fast, highly-sensitive and selective hydrogen response at room temperature, which may be attributed to the hydrogen induced interface and surface effects together with the high specific surface area of the Nb₂O₅ nanowires.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

CLAMPED NANOWIRE THERMAL CONDUCTIVITY BASED ON PHONON TRANSPORT EQUATION

The phonon Boltzmann transport equation is used to calculate the thermal conductivity in clamped silicon nanowires and study both boundary and confining effects of phonon scattering. The solution method includes partly diffuse and specular phonon reflections at the boundaries and introduces a spectral extinction coefficient. This parameter is derived from the Umklapp relaxation time accounting for thermal resistive processes. A numerical scheme based on the discrete ordinates method has been developed. Results are presented for several wire lengths and extrapolated to infinite wires. Fourier's law is retrieved both theoretically and numerically for acoustically thick media when only specular reflections occur.     

Mixed-metal organic framework-coated ZnO nanowires array for efficient photoelectrochemical water oxidation

Designing of high-performance photoanodes is essential for efficient solar energy conversion in photoelectrochemical (PEC) water splitting. Herein, we report an effective approach to synthesize three dimensional (3D) mixed-metal organic framework-coated ZnO nanowires array (ZnNi MOF@ZnO) for the effective PEC performance. The ZnO nanowires act as photon absorber as well as rapid charge transporter; whilst the ZnNi MOF provides the active sites for PEC process by lowering the energy barrier of water oxidation and suppressing electron-hole recombination. The 3D nanostructure of ZnNi MOF@ZnO nanowires array provides intimate interfacial contact through covalent interactions between the ZnNi MOF and ZnO nanowires which facilitates the rapid charge transfer during photocatalytic oxygen evolution reactions. As a result, the ZnNi MOF@ZnO nanowires array exhibited excellent photoelectrochemical water oxidation with very low onset potential (0.31 V vs. RHE) and high photocurrent density (1.40 mA/cm²) as compared to the Zn MOF @ZnO and ZnO nanowires array. This facile strategy provides a promising direction towards high performance photoanode design for adequate solar energy conversion.     

Theoretical study of the hydrogen adsorption on AlB nanowire

We studied AlB nanowires as hydrogen storage materials based on density functional theory and Rigged QED theory. In this paper, we focused on the adsorption energy and the electronic structure of models. AlB nanowire models are compared with an Al nanowire model and AlB₂ crystal structure in terms of density of states, electron density, kinetic energy density, tension density and stress tensor density. These results revealed AlB nanowires do not have the conductivity, while the Al nanowire and AlB₂ bulk have it. It was also shown that the stabilization energies of AlB nanowires for the hydrogen adsorption are larger than that of Al nanowire. Adsorped hydrogens are more stable in the AlB nanowires than the Al nanowire.     

Lattice Thermal Conductivity of Pristine Si Nanowires: Classical Nonequilibrium Molecular Dynamics Study

We investigate lattice thermal conductivities of Si nanowires (SiNWs) based on classical nonequilibrium molecular dynamics (NEMD) simulations. The SiNWs are supposed to have a crystalline diamond structure and extend along the [001] axial direction with the {100} sidewall facets on which the surface Si atoms are all dimerized and fully passivated by hydrogen. Various sizes in the square cross section are considered, and the lengths are varied to find the diffusive limits of the lattice thermal conductivities. The upper limits of the diffusive lattice thermal conductivities of SiNWs are 11.2, 16.0, 22.8, and 28.0 W/mK for SiNWs with widths of 2.7, 4.9, 8.1, and 11.3 nm, respectively. The mode-averaged mean free paths of phonons are 50, 52, 63, and 80 nm for the SiNWs, respectively, which are 7–19 times longer than the SiNW widths.     

Fabrication of Ni nanowires for hydrogen evolution reaction in a neutral electrolyte

Hydrogen evolution reaction in 1 M Na₂SO₄ was investigated using Ni nanowires in diameter of 250 nm with exposed lengths of 20, 35, and 45 μm, respectively. The Ni nanowires were fabricated by a direct-current pulse electrodeposition technique using an anodic aluminum oxide template, followed by selective removal of the supporting pore walls. Scanning Electron Microscope images revealed structural stabilities and X-ray diffraction pattern indicated a polycrystalline fcc phase. In current–potential (i–V) polarizations, the Ni nanowires with longer exposed lengths demonstrated larger current responses. Analysis from impedance spectroscopy confirmed increasing double-layer capacitances with longer Ni nanowires. In galvanostatic lifetime experiments, the free-standing Ni nanowires exhibited a reduced overpotential over that of supported ones. Similar procedures were performed for the oxygen evolution reaction in both i–V and lifetime measurements. For the Ni nanowires of 45 μm length, we estimated the energy cost for hydrogen production was 5.24 × 10⁵ J/mole.     

MnO2 nanowires anchored on mesoporous graphitic carbon nitride (MnO2@mpg-C3N4) as a highly efficient electrocatalyst for the oxygen evolution reaction

In the present study, we report the rational design and fabrication of a novel nanocomposite, namely one-dimensional (1D) MnO₂ nanowires grew up in situ within the 2D mesoporous carbon nitride (MnO₂@mpg-C₃N₄), as a highly efficient electrocatalyst for OER. The structural, morphological and thermal properties of as-prepared MnO₂@mpg-C₃N₄ electrocatalyst were characterized by TEM, SEM, XRD, XPS, Raman, ICP-MS, and TGA. The results clearly revealed the formation of 3D-hierarchical heterostructures consisting of 1D MnO₂ nanowires anchored on mpg-C₃N₄. Next, the electrocatalytic performance of MnO₂@mpg-C₃N₄ nanocomposite was tested in OER wherein it exhibited substantially enhanced activity than pristine 1D MnO₂ nanowires. In particular, the turnover frequency (TOF) of MnO₂@mpg-C₃N₄ (0.84 s⁻¹@480 mV) was found almost three times higher than that of 1D MnO₂ nanowires (0.32 s⁻¹@480 mV). Moreover, the overpotential and Tafel slope values were successfully lowered down by using MnO₂@mpg-C₃N₄ nanocomposite compared to those of 1D MnO₂ nanowires. It was experimentally demonstrated that the superior OER performance of the MnO₂@mpg-C₃N₄ is attributed to the effective stabilization of Mn³⁺ species (Mn₂O₃) in the electrocatalyst via the help of nitrogen functional groups of mpg-C₃N₄ and the formation of 3D heterostructure that offers the following three major contributions; i) enhanced aerophobicity due to orientation modifications of growing 1D MnO₂ nanowires, ii) open structure facilitating the rapid detachment of gas bubbles from the electrode surface, iii) a large number of transport channels for the penetration of electrolyte, ions and electrons.     

Cu2O nanowires based p-n homojunction photocathode for improved current density and hydrogen generation through solar-water splitting

Hydrogen generation through solar-water splitting is expected to address the global energy crisis by providing a source for a safer and sustainable alternative fuel. Herein, we report a facile synthesis of Cu₂O nanowires and show that the magnetic field could influence the nanowires’ distribution and alignment. Orientation of nanowires was observed to become more inclined towards the magnetic field lines as the values of full-width at half maximum decreased from 140° to 46.2° with the increase in the field strength. Crystallographic, morphological, optoelectronic, and photoelectrochemical properties of the constructed p-n homojunction were analyzed by using different characterization techniques. A high built-in potential of +0.93 V vs. RHE was observed for a 50 nm layer of n-Cu₂O over p-Cu₂O nanowires that resulted in a significantly high photocurrent density of −7.42 mA/cm². The stability in the photoelectrochemical medium was maintained for 14 h, generating 20 mmol/cm² of H₂.     

Synthesis of metallic copper nanowires using dielectric barrier discharge plasma and their application in hydrogen evolution reaction

In this study, metallic copper (Cu) nanowires are synthesized by reducing thermally synthesized CuO nanowires under an indigenously developed hydrogen plasma system. The X-ray diffraction (XRD) results of the plasma-synthesized nanowires indicate the presence of metallic copper [(111) and (200)] and the field emission scanning electron microscopy (FESEM) further affirms the findings by presenting a stark difference in contrast of the nanowires before and after plasma treatment with diameters of 50 and 100 nm, respectively. The nanowires are studied for hydrogen evolution reaction in a neutral medium and they show excellent performance than the previously reported studies on bulk copper, with an overpotential of 210 mV at a current density of 10 mA/cm² and an exchange current density of 60 exp-5 A/cm² which is an order of magnitude larger than the reported values on bulk copper. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicates that the surface of the nanowires is highly rich in metallic copper resulting in better electrochemical performance of the metallic Cu nanowires in a neutral environment.     

Dye-sensitized solar cells based on semiconductor morphologies with ZnO nanowires

ZnO nanowires and structures that combine nanowires and nanoparticles were used as the wide band gap semiconducting photoelectrode in dye-sensitized solar cells (DSSCs). The nanowires provide a direct path from the point of photogeneration to the conducting substrate and offer alternative semiconductor network morphologies to those possible with sintered nanoparticles. Growing nanowires with dendrite-like branched structure greatly enhances their surface area, leading to improved light harvesting and overall efficiencies. Hybrid cells based on a combination of nanowires and nanoparticles can be tailored to take advantage of both the high surface area provided by the nanoparticles and the improved electron transport along a nanowire network. Solar cells made from branched nanowires showed photocurrents of 1.6 mA/cm², internal quantum efficiencies of 70%, and overall efficiencies of 0.5%. Solar cells made from appropriate hybrid morphologies show photocurrents of 3 mA/cm² and overall efficiencies of 1.1%, while both the nanowire and hybrid cells show larger open circuit voltages than nanoparticle cells.     

Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties

Synthesis of zinc oxide (ZnO) nanowires (NWs) grown via vapor-liquid-solid (VLS) process using Gold (Au) as a catalyst metal on aluminum-doped zinc oxide (AZO) seed layer is reported in the present work. During the growth procedure, the nucleation process helps us to obtain ZnO nanowires with Au on the tip, confirming the VLS growth mechanism. Different morphologies were obtained after the variation in the growth parameters in the VLS process, and further, their role in the photocatalytic performance was studied. Changes in the structural properties of nanowires allowed us to modify the aspect ratio and surface area of the nanostructures. X-ray diffraction (XRD) showed that the principal orientation of the nanowires was (002) in the present case. Scanning electron microscopy (SEM) showed the structural properties of 1-D nanostructures (nanowires), and statistical analysis revealed that the average diameter in the present case was found to be varied from 57 to 85 nm. Scanning transmission electron microscopy (STEM) technique revealed the different elements present on the surface of ZnO NWs. Further, the compositional profile of nanostructures was cross-verified using Energy dispersive Spectroscopy (EDS). Photoluminescence (PL) and UV Visible studies were employed to study the optical properties of nanowires. UV–Vis measurements showed the role of different structural properties of nanowires on the absorption spectra, especially in the visible region. The ZnO nanowires were tested as photocatalysts for hydrogen production from water splitting reaction, and it was found in particular nanowires with random orientation with optimal diameter distribution show the stable and highest photocatalytic performance.     

A hybridized electron-selective layer using Sb-doped SnO2 nanowires for efficient inverted polymer solar cells

We developed a novel hybridized electron-selective layer comprised of Sb-doped SnO₂ nanowires for efficient inverted polymer solar cells. A device containing Sb-doped SnO₂ nanowires with 0.1 mg/ml concentration showed a significant increase in power conversion efficiency to 3.23% with an enhanced fill factor, compared to a reference device without the nanowires (2.89%). Such improvement is attributed to the high electrical conductivity of one-dimensional Sb-doped SnO₂ nanowires and to the good light transmittance through the wide band gap of tin oxide. Also the surface morphology of the hybridized electron-selective layer is made denser and improved by incorporating one-dimensional Sb-doped SnO₂ nanowires, resulting in the enhancement of the photovoltaic performance.