Hua Bao

Thermal conductivity of silicene from first-principles

Silicene, as a graphene-like two-dimensional material, now receives exceptional attention of a wide community of scientists and engineers beyond graphene. Despite extensive study on its electric property, little research has been done to accurately calculate the phonon transport of silicene so far. In this paper, thermal conductivity of monolayer silicene is predicted from first-principles method. At 300 K, the thermal conductivity of monolayer silicene is found to be 9.4 W/mK and much smaller than bulk silicon. The contributions from in-plane and out-of-plane vibrations to thermal conductivity are quantified, and the out-of-plane vibration contributes less than 10% of the overall thermal conductivity, which is different from the results of the similar studies on graphene. The difference is explained by the presence of small buckling, which breaks the reflectional symmetry of the structure. The flexural modes are thus not purely out-of-plane vibration and have strong scattering with other modes.

Large tunability of lattice thermal conductivity of monolayer silicene via mechanical strain

Strain engineering is one of the most promising and effective routes toward continuously tuning the electronic and optic properties of materials, while thermal properties are generally believed to be insensitive to mechanical strain. In this paper, the strain-dependent thermal conductivity of monolayer silicene under uniform biaxial tension is computed by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. Unlike the commonly believed understanding that thermal conductivity only slightly decreases with increased tensile strain for bulk materials, it is found that the thermal conductivity of silicene can increase dramatically with strain. Depending on the size, the maximum thermal conductivity of strained silicene can be a few times higher than that of the unstrained case. Such an unusual strain dependence is mainly attributed to the dramatic enhancement in the acoustic phonon lifetime. Such enhancement plausibly originates from the flattening of the buckling of the silicene structure upon stretching, which is unique for silicene as compared with other common two-dimensional materials. Our findings offer perspectives on modulating the thermal properties of low-dimensional structures for applications such as thermoelectrics, thermal circuits, and nanoelectronics.

Optical properties of ordered vertical arrays of multi-walled carbon nanotubes from FDTD simulations

A finite-difference time-domain (FDTD) method is used to model thermal radiative properties of vertical arrays of multi-walled carbon nanotubes (MWCNT). Individual CNTs are treated as solid circular cylinders with an effective dielectric tensor. Consistent with experiments, the results confirm that CNT arrays are highly absorptive. Compared with the commonly used Maxwell-Garnett theory, the FDTD calculations generally predict larger reflectance and absorbance, and smaller transmittance, which are attributed to the diffraction and scattering within the cylinder array structure. The effects of volume fraction, tube length, tube distance, and incident angle on radiative properties are investigated systematically. Low volume fraction and long tubes are more favorable to achieve low reflectance and high absorbance. For a fixed volume fraction and finite tube length, larger periodicity results in larger reflectance and absorbance. The angular dependence studies reveal an optimum incident angle at which the reflectance can be minimized. The results also suggest that an even darker material could be achieved by using CNTs with good alignment on the top surface.

An investigation of the optical properties of disordered silicon nanowire mats

Optical reflectance spectra of three disordered silicon nanowire mats with average diameters of 40, 60, and 80 nm are investigated both experimentally and theoretically. The total hemispherical reflectance spectra from 200 to 1600 nm wavelength are first measured. All three samples exhibit reflectance about 15% to 20% within the ultraviolet band. As the wavelength becomes longer, the reflectance will first increase to around 50% and then decrease to below 20%. Such reflectance spectra are attributed to the combined effect of silicon dielectric function, the nanowire geometry, and the volume fraction of the mats. An analytical method based on Mie scattering theory and two-flux model is proposed to predict the reflectance spectra of the NW mats using only the physical quantities including dielectric function and structural parameters of the nanowire mats. The experimental reflectance spectra can be well reproduced by this method.

Thermal transport in bismuth telluride quintuple layer: mode-resolved phonon properties and substrate effects.

The successful exfoliation of atomically-thin bismuth telluride (Bi2Te3) quintuple layer (QL) attracts tremendous research interest in this strongly anharmonic quasi-two-dimensional material. The thermal transport properties of this material are not well understood, especially the mode-wise properties and when it is coupled with a substrate. In this work, we have performed molecular dynamics simulations and normal mode analysis to study the mode-resolved thermal transport in freestanding and supported Bi2Te3 QL. The detailed mode-wise phonon properties are calculated and the accumulated thermal conductivities with respect to phonon mean free path (MFP) are constructed. It is shown that 60% of the thermal transport is contributed by phonons with MFP longer than 20 nm. Coupling with a-SiO2 substrate leads to about 60% reduction of thermal conductivity. Through varying the interfacial coupling strength and the atomic mass of substrate, we also find that phonon in Bi2Te3 QL is more strongly scattered by interfacial potential and its transport process is less affected by the dynamics of substrate. Our study provides an in-depth understanding of heat transport in Bi2Te3 QL and is helpful in further tailoring its thermal property through nanostructuring.

Dynamic tuning of optical absorbers for accelerated solar-thermal energy storage

Currently, solar-thermal energy storage within phase-change materials relies on adding high thermal-conductivity fillers to improve the thermal-diffusion-based charging rate, which often leads to limited enhancement of charging speed and sacrificed energy storage capacity. Here we report the exploration of a magnetically enhanced photon-transport-based charging approach, which enables the dynamic tuning of the distribution of optical absorbers dispersed within phase-change materials, to simultaneously achieve fast charging rates, large phase-change enthalpy, and high solar-thermal energy conversion efficiency. Compared with conventional thermal charging, the optical charging strategy improves the charging rate by more than 270% and triples the amount of overall stored thermal energy. This superior performance results from the distinct step-by-step photon-transport charging mechanism and the increased latent heat storage through magnetic manipulation of the dynamic distribution of optical absorbers.Solar-thermal energy storage based on phase-change materials suffers from slow thermal-diffusion-based charging. Here the authors alleviate this issue by introducing optical absorbers and controlling their distribution to accelerate charging process and thus improve solar-thermal energy conversion.

Molecular dynamics study of the interfacial thermal conductance of multi-walled carbon nanotubes and van der Waals force induced deformation

Thermal boundary resistance (TBR) plays an important role in the thermal conduction of carbon nanotube (CNT)-based materials and CNT networks (e.g., thin films, arrays, and aerogels). Although individual CNTs have extremely high thermal conductivity, interfacial resistances can dominate the overall resistance and largely influence their thermal performance. Using molecular dynamics simulations, we systematically study the interfacial thermal conductance (ITC, the inverse of TBR) of multi-walled carbon nanotube (MWNT)-substrate interfaces and MWNT-MWNT junctions, and compare the CNT-CNT junctions with graphene-graphene junctions. The results show that for CNTs with the diameter of a few nanometers, the total ITCs first decrease and then stabilize with the increase of the number of walls, mainly due to the changes of mechanical strength and adhesive energy. Increasing the CNT diameter leads to a larger total ITC and it is mainly due to a larger contact area. The area normalized ITC of CNT-CNT junctions incr...

Probing phonon-surface interaction by wave-packet simulation: Effect of roughness and morphology

One way to reduce the lattice thermal conductivity of solids is to induce additional phonon–surface scattering through nanostructures. However, the way in which phonons interact with surfaces, especially at the atomic level, is not well understood at present. In this work, we perform two-dimensional atomistic wave-packet simulations to investigate angular-resolved phonon reflection at a surface. Different surface morphologies, including smooth surfaces, periodically rough surfaces, and surfaces with amorphous coatings, are considered. For a smooth surface, mode conversion can occur after reflection, with the resulting wave-packet energy distribution depending on the surface condition and the polarization of the incident phonon. At a periodically rough surface, the reflected wave-packet distribution does not follow the well-known Ziman model but shows a nonmonotonic dependence on the depth of the surface roughness. When an amorphous layer is attached to a smooth surface, the incident wave packet is absorbe...

Collocation mesh-free method to solve the gray phonon Boltzmann transport equation

ABSTRACT The phonon Boltzmann transport equation (BTE) is an important governing equation for phonon transport at the subcontinuum scale. Numerically solving the BTE can help to study the heat transfer phenomena at microscale and nanoscale, for example, in electronic devices or nanocomposites. In this work, we developed a collocation mesh-free method to solve the BTE for different heat transfer regimes. The proposed numerical scheme does not require meshing the domain or performing numerical integration, and is thus advantageous for problems with complicated geometries. A few case studies show that our method can yield comparable results with semianalytical and finite volume methods.

Absorption Spectra and Electron-Vibration Coupling of Ti:Sapphire From First Principles

First-principles calculations are performed to study the absorption spectra and electronvibration coupling of titanium-doped sapphire (Ti:Al2O3). Geometry optimization shows a local structure relaxation after the doping of Ti. Electronic band structure calculation shows that five additional dopant energy bands are observed around the band gap of Al2O3, and are attributed to the five localized d orbitals of the Ti dopant. The optical absorption spectra are then predicted by averaging the oscillator strength during a 4 ps first-principles molecular dynamics (MD) trajectory, and the spectra agree well with the experimental results. Electron-vibration coupling is further investigated by studying the response of the ground and excited states to the Eg vibrational mode, for which a configuration coordinate diagram is obtained. Stokes shift effect is observed, which confirms the red shift of emission spectra of Ti:sapphire. This work offers a quantitative understanding of the optical properties and crystal-field theory of Ti-doped sapphire. The firstprinciples calculation framework developed here can also be followed to predict the optical properties and study the electron-vibration coupling in other doped materials. [DOI: 10.1115/1.4032177]