X. J. Tan

Enhanced thermoelectric performance of graphene nanoribbons

The thermoelectric properties of a series of armchair and zigzag graphene nanoribbons with narrow width are examined using nonequilibrium Green function method and molecular dynamics simulations. It is found that these nanoribbons are rather stable when the edge atoms are passivated by hydrogen and those with armchair edges exhibit much better thermoelectric performance than their zigzag counterparts. Moreover, the corresponding ZT value increases with decreasing ribbon width. By optimizing the doping level, a room temperature ZT of 6.0 can be achieved for the narrowest armchair nanoribbon. The significantly enhanced ZT value makes armchair graphene nanoribbon a promising candidate for thermoelectric applications.

Thermoelectric properties of armchair and zigzag silicene nanoribbons

Using the nonequilibrium Greens function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications.

Enhanced thermoelectric performance of (Sb0.75Bi0.25)2Te3 compound from first-principles calculations

The electronic properties of (Sb0.75Bi0.25)2Te3 compound are examined by using the full-potential linearized augmented plane-wave method. The transport coefficients are then calculated within the semiclassical Boltzmann theory, and further evaluated as a function of chemical potential assuming a rigid band picture and constant relaxation time. The ZT value is thus estimated by inserting an averaged thermal conductivity. Our theoretical calculations give a valuable insight on how to enhance the thermoelectric performance of this compound, and many potential doping elements and their optimal concentrations are suggested.

The realization of a high thermoelectric figure of merit in Ge-substituted β-Zn4Sb3 through band structure modification

In this study, we demonstrate a realization of a favorable modification of band structures and an apparent increase in the density of state effective mass in β-Zn4Sb3 compound by introduction of a slight amount of Ge at the Zn site, in a manner of adding a shape peak below the valence band edge and giving rise to a significant enhancement in the power factor which is similar to the case of Tl-doped PbTe. As a consequence, the high power factor exceeding 1.4 mW m−1 K−2, coupled with the intrinsic very low thermal conductivity originated from complex crystal structures and a high degree of disorder, results in a maximum figure of merit of ∼1.35 at 680 K for the 0.25 at% Ge-substituted sample, which is ∼20% improvement as compared with that of the unsubstituted sample in this study. What is most important is the average ZT between 300 and 680 K reaches ∼1.0, which is ∼35% enhancement in comparison with the unsubstituted sample and superior to most of p-type materials in this temperature range. Furthermore, the combination of high thermoelectric performance and improvement in the thermodynamic properties makes this natural-abundant, “non-toxic” and cheap Ge-substituted β-Zn4Sb3 compound a very promising candidate for thermoelectric energy applications.

First-principles study of alkali-atom doping in a series of zigzag and armchair carbon nanotubes

First-principles calculations are performed to study the Li doping in a series of carbon nanotubes with different diameters and chiralities. It is found that the Li–Li interaction inside or outside zigzag tubes is repulsive but strongly screened. Moreover, small diameter zigzag tubes are energetically more favorable than larger ones for Li doping. In contrast, almost all the armchair tubes have the same Li binding energy, especially for the outside doping. Our theoretical results suggest that small diameter zigzag tubes could be plausible candidates for Li-ion battery application. In addition, the doping of other alkali atoms in zigzag tubes is also investigated and the optimal binding distance between them are determined.

Optimizing the thermoelectric performance of zigzag and chiral carbon nanotubes

Using nonequilibrium molecular dynamics simulations and nonequilibrium Greens function method, we investigate the thermoelectric properties of a series of zigzag and chiral carbon nanotubes which exhibit interesting diameter and chirality dependence. Our calculated results indicate that these carbon nanotubes could have higher ZT values at appropriate carrier concentration and operating temperature. Moreover, their thermoelectric performance can be significantly enhanced via isotope substitution, isoelectronic impurities, and hydrogen adsorption. It is thus reasonable to expect that carbon nanotubes may be promising candidates for high-performance thermoelectric materials.

Theoretical study of the thermoelectric properties of SiGe nanotubes

The thermoelectric properties of two typical SiGe nanotubes are investigated using a combination of density functional theory, Boltzmann transport theory, and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe nanotubes tend to have gear-like geometry, and both the (6, 6) and (10, 0) tubes are semiconducting with direct band gaps. The calculated Seebeck coefficients as well as the relaxation time of these SiGe nanotubes are significantly larger than those of bulk thermoelectric materials. Together with smaller lattice thermal conductivity caused by phonon boundary and alloy scattering, these SiGe nanotubes can exhibit very good thermoelectric performance. Moreover, there are strong chirality, temperature and diameter dependences of the ZT values, which can be optimized to 4.9 at room temperature and further enhanced to 5.4 at 400 K for the armchair (6, 6) tube.