Si/Ge Superlattice Nanowires with Ultralow Thermal Conductivity

Abstract

The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics.

Low thermal conductivity in Si/Ge hetero-twinned superlattices

“…The nonmonotonic dependence of Si/Ge hetero-twinned SLs on periodic thickness is caused by the combined action of phonon coherent and interface phonon scattering. 26 Our results also support a hypothesis that in SLs with a short periodic thickness, there are almost no interfaces and the whole SLs should be considered as a giant lattice.…”

“…The previous simulated and experimental results indicated that the thermal transport properties in SLs can be adjusted by regulating the lattice period 6,7 and the structures of interfaces. 8,9 According to the structures of the interfaces, SLs can be divided into two kinds: (1) heterostructure SLs (SLs consist of two components 10 ), and (2) homo-coherent twinned SLs (SLs consist of nanotwinned layers from one component 9 ). It has been demonstrated that the cross-plane thermal conductivity of crystalline heterostructure SLs is much lower than that of bulk materials with a single component, and, sometimes even lower than those of their corresponding alloys.…”

Impact of internal crystalline boundaries on lattice thermal conductivity: Importance of boundary structure and spacing

“…In other words, the lattice thermal conductivity reduces by increasing the boundary density. Similar observations have been recently reported for grain boundaries in 2D graphene sheet, 13 Si/Ge interface, 36 and nano-crystalline silicon. 25,37 From Figure 2, it can be seen that for the boundaries whose atomic structures are very close to the perfect crystal (coherent boundaries such as twin boundary and stacking fault), there is only a negligible effect on the overall lattice thermal conductivity.…”

Chemical Trends of Electronic Properties of Two-Dimensional Halide Perovskites and Their Potential Applications for Electronics and Optoelectronics

“…One is to enhance the Seebeck coefficient or the PF using band structure engineering methods, [5] such as modifying the electronic density of states and Fermi energy through doping heteroatoms, reducing crystal symmetry to achieve high band edge degeneracy, [6][7][8] etc. The other is to reduce thermal conductivity using methods such as nanostructuring, [9][10][11][12][13][14][15][16][17][18][19][20][21] alloying, [14,[22][23][24][25][26] etc. These strategies have achieved significant progress in the past decade.…”