Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

Band Engineering of Thermoelectric Materials

High Seebeck coefficient by creating large density-of-states effective mass through either electronic structure modification or manipulating nanostructures is commonly considered as a route to advanced thermoelectrics However, large density-of-state due to flat bands leads to large transport effective mass, which results in a simultaneous decrease of mobility In fact, the net effect of such a high effective mass is a lower thermoelectric figure of merit, zT, when the carriers are predominantly scattered by phonons according to the deformation potential theory of Bardeen–Shockley We demonstrate that the beneficial effect of light effective mass contributes to high zT in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction

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Low effective mass leading to high thermoelectric performance

High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.

Low Effective Mass Leading to High Thermoelectric Performance

Among the various material growth techniques, physical vapor deposition of thin films encompasses many experimental forms. They can be categorized into equilibrium and none-quilibrium processes according to the ways in which the source materials are vaporized. Equilibrium process is typified by thermal evaporation including the conventional resistive heating, the more elaborate e-beam evaporation. as well as molecular beam epitaxy (MBE). Nonequilibrium processes include sputtering and many of its derivative forms such as ion beam deposition. In comparison with these techniques, thin-film deposition by laser evaporation is less well known and does not fall cleanly into either category. Depending on the laserconditions, the evaporation process can be either thermal, nonthermal, or a mixture of the two.

Growth of thin films by laser-induced evaporation

The Seebeck coefficient of p-type PbTe can be enhanced at 300 K, either due to the addition of Tl-resonant states or by manipulation of the multiple valence bands by alloying with isovalent compounds, such as MgTe. PbTe alloyed with MnTe shows a similar thermopower enhancement that could be due to either mechanism. Here we investigate the characteristics that distinguish the resonant state mechanism from that due to multiple valence bands and their effect on the thermoelectric figure of merit, zT. Ultimately, we find that the transport properties of PbTe alloyed with MnTe can be explained by alloy scattering and multiple band model that result in a zT as high as 1.6 at 700 K, and additionally a ~30% enhancement of the average zT.

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Thermopower enhancement in Pb1-xMnxTe alloys and its effect on thermoelectric efficiency

Resonant Raman scattering by ion beam synthesized in silicon matrix ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase is studied. The samples are prepared with the implantation of $^{24}\mathrm{Mg}^{+}$ ions with dose $4\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and with two different energies 40 and $60\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ into (100)Si substrates. The far infrared spectra are used as criteria for the formation of the ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase. The Raman spectra are excited with different lines of ${\mathrm{Ar}}^{+}$ laser, with energies of the lines lying in the interval from $2.40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The resonant scattering can be investigated using these laser lines, as far as according to the ${\mathrm{Mg}}_{2}\mathrm{Si}$ band structure, there are direct gaps with energies in the same region. The energy dependences of the scattered intensities in the case of the scattering by the allowed ${F}_{2g}$ and the forbidden LO-type modes are experimentally obtained and theoretically interpreted. On the base of the investigation energies of the interband transitions in the ${\mathrm{Mg}}_{2}\mathrm{Si}$ are determined. It is found also that the resonant Raman scattering appears to be a powerful tool for characterization of a material with inclusions in it. In the particular case it is concluded that the ${\mathrm{Mg}}_{2}\mathrm{Si}$ phase is present in the form of a surface layer in the sample, prepared with implantation energy $40\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and as low-dimensional precipitates, embedded in the silicon matrix, in the sample, prepared with the higher implantation energy.

Resonant Raman scattering in ion-beam-synthesized Mg 2 Si in a silicon matrix

A detailed temperature dependence of the energy gap E0 in the PbSe and PbTe was obtained from the transmittance measurements. A non-linear change of E0 in the vicinity of the Debye temperature was noticed. An attempt to find an empirical formula describing the experimental dependence E0(T) led to the conclusion that the non-linear change can be attributed to the electron interaction with the optical phonons.

https://iopscience.iop.org/article/10.1088/0953-8984/2/13/003/pdf

M I Baleva

Papers

Resonant Raman scattering in ion-beam-synthesized Mg 2 Si in a silicon matrix

On the temperature dependence of the energy gap in PbSe and PbTe

Laser-assisted sputtering of Pb1−xCdxSe films

Pressure coefficient of the PbTe metastable CsCl-type-phase energy gap

Optical Characterization of Lead Monoxide Films Grown by Laser-Assisted Deposition