Thermoelectric properties of nanostructured materials

Nanostructures may display transport properties very different from their bulk counterparts. This makes them appealing for several applications ranging from thermoelectric power generation, where low thermal conductivity is desirable, to heat dissipation, requiring high thermal conductivity. We are using both molecular dynamics simulations and electronic structure calculations to study thermoelectric properties of SWCNT, silicon nanowires and nanoporous silicon.

  • Silicon nanowires
  • Nanoporous silicon
  • Carbon Nanotubes interacting with external media

Thermal Transport of Si nanowires
Crystalline silicon nanowires (upper figure of left panel) and wires with amorphous like surfaces (lower figure of left panel and right panel).

We have studied thermal transport in Silicon nanowires by molecular dynamics and lattice dynamics methods. Our calculations elucidate the reasons why lattice thermal conductivity may change by up to two order of magnitudes, depending on the surface structure of the wires. Consistent with experiment, wires with rough (amorphous-like) surfaces are found to have much lower thermal conductivity (up to 2 orders of magnitude) than bulk Si and they may exhibit values of the thermoelectric figure of merit ZT close to 1 [1,2].


Structure of nanoporous Si (orange) passivated with H (purple) atoms along with structural variables [pore size dp and pore distance ds] used in the calculations: circular (left) and square (right) pores along the [001] direction.
Structure of nanoporous Si

In collaboration with J.C.Grossman ’s group at UC Berkeley we have investigated room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores. We have used a combination of classical molecular dynamics for lattice thermal conductivity [3] and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity [4]. We predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk Si. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. While alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required.


SWCNT on a Si Surface
We have investigated the thermal conductivity of single-wall carbon nanotubes (SWCNT) either isolated or in contact with external media by using equilibrium molecular dynamics and the Boltzmann transport equation. Both methods yield a finite value of the thermal conductivity for infinitely long tubes, as opposed to the case of 1D, momentum-conserving systems. Acoustic and flexure modes with mean free paths of the order of a few microns are major contributors to the high value of SWCNT conductivity. Remarkably, the interaction of the tubes with an external medium may substantially decrease the lifetime of the low-frequency vibrations, reducing the thermal conductivity of by up to 2 orders of magnitude [5].
SWCNT on a Si Surface     

References

  1. “Atomistic simulations of heat transport in silicon nanowires” D. Donadio and .G. Galli, Phys. Rev. Lett. 102, 1195901 (2009)
  2. "Atomistic design of thermoelectric properties of silicon nanowires", T. Vo, A. Williamson, V. Lordi and G. Galli, Nano Lett., 8, 1111 (2008).
  3. "Lattice thermal conductivity of nano-porous Si: molecular dynamics study", J.-H. Lee, J. C. Grossman, J. Reed, and G. Galli, Appl. Phys. Lett., 91, 223110 (2007).
  4. "Nanoporous Si as an efficient thermoelectric material", J-H. Lee, G. Galli and J. Grossman, Nanolett. 11, 3750 (2008)—featured in Nature Materials.
  5. "Thermal conductivity of isolated and interacting carbon nanotubes: Comparing results from molecular dynamics and the Boltzmann transport equation", D. Donadio and G. Galli, Phys. Rev. Lett., 99, 255502 (2007) and Virtual Journal of Nanotechnology, Jan, 7th 2008..