Thermal Transport

Thermoelectric materials, which can both generate electricity from waste heat and use electricity for solid-state Peltier cooling, are to date inefficient, compared to conventional generators and refrigerators. One way to obtain systems with improved efficiency is to engineer nanostructured semiconductors, so as to reduce the thermal conductivity of the crystalline materials while preserving their electronic properties. Such a strategy has been recently applied to several semiconductors with promising results.

We are developing simulation frameworks to investigate thermal transport in nanostructured materials, that are based on molecular dynamics simulations and the Boltzman transport equation. We are particularly interested in solid-state thermoelectric materials, which can both generate electricity from waste heat and use electricity for solid-state Peltier cooling.

Molecular Dynamics and Boltzmann Transport approaches

Two major approaches, namely molecular dynamics (MD) simulations and calculations solving approximately the Boltzmann transport equation (BTE), have been developed to compute the lattice thermal conductivity. We performed a detailed direct comparison of these two approaches, using as prototypical cases MgO and PbTe. The comparison, carried out using empirical potentials, takes into account the effects of fourth order phonon scattering, temperature-dependent phonon frequencies (phonon renormalization), and investigates the effects of quantum vs classical statistics. We clarified that equipartition, as opposed to Maxwell-Boltzmann, govern the statistics of phonons in MD simulations. We found that lattice thermal conductivity values from MD and BTE show an apparent, satisfactory agreement; however such an agreement is the result of error cancellations. We also showed that the primary effect of statistics on thermal conductivity is via the scattering rate dependence on phonon populations.

First principles molecular dynamics of thermal transport

Advances in understanding heat transport in solids were recently reported by both experiment and theory. However an efficient and predictive quantum simulation framework to investigate thermal properties of solids, with the same complexity as classical simulations, has not yet been developed. Here we present a method to compute the thermal conductivity of solids by performing ab initio molecular dynamics at close to equilibrium conditions, which only requires calculations of first-principles trajectories and atomic forces, thus avoiding direct computation of heat currents and energy densities. In addition the method requires much shorter sequential simulation times than ordinary molecular dynamics techniques, making it applicable within density functional theory. We discuss results for a representative oxide, MgO, at different temperatures and for ordered and nanostructured morphologies, showing the performance of the method in different conditions.

Perovskites for solar-thermal applications

Using ab initio calculations, we showed that hybrid organic/inorganic CH3NH3AI3 (A = Pb and Sn) perovskites may be promising materials for solar thermoelectric applications. We found that their large carrier mobilities mainly originate from a combination of the small effective masses of electrons and holes and a relatively weak carrier–phonon interaction. We propose that by tuning the carrier concentration to values of the order of ∼1018 cm-3, the thermoelectric figure of merit of Sn and Pb based perovskites may reach values ranging from 1 to 2.

Nanostructured clathrates

Using first-principles calculations, we investigated the thermoelectric properties of a newly synthesized Si-based ternary clathrate K8Al8Si38, composed of ∼1 nm hollow cages with a metal atom inside. We found that, similar to other nanostructured type I clathrates, this system is a semiconductor and has a low thermal conductivity (∼1 W/mK). It was long believed that the mere presence of rattling centers was responsible for the low lattice thermal conductivity of type I clathrates. We found instead that the cage structural disorder induced by atomic substitution plays a crucial role in determining the conductivity of these materials, in addition to the dynamics of the guest atoms. Our calculations showed that the latter is substantially affected by the charge transfer between the metal and the cages.

SiGe at the nanoscale

We computed the thermal conductivity of planar superlattices, arrays of Ge nanowires and nanodots embedded in Si, by using a fully atomistic Monte Carlo solution of the Boltzmann transport equation, and we investigated how dimensionality affects heat transport in Si-Ge superlattices.

We also investigated thermal transport in thin SiGe wires and in nanoporous SiGe, using molecular dynamics and lattice dynamics calculations, and we investigated how morphology affects changes in the thermal conductivity of SiGe at the nanoscale.

Nanoporous and Amorphous Si

We showed that the thermal conductivity of nanoporous silicon may attain values 10−20 times smaller than in bulk Si for porosities and surface-to-volume ratios similar to those obtained in recently fabricated nanomeshes. Further reduction of almost an order of magnitude is obtained in thin films with thickness of 20 nm, in agreement with experiment. Our study was conducted using molecular dynamics simulations and lattice dynamics calculations and results were compared to those for amorphous Si, studied with the same methodologies.

Si Nanowires

We carried out a series of molecular dynamics, lattice dynamics, and Boltzmann transport equation calculations aimed at understanding heat transport in Silicon nanowires. We found that the computed thermal conductivity strongly depends on the surface structure and defects present in the bulk of the wires. Our results were used to rationalize several experiments showing strong reduction of the conductivity in Si nanowires.