Research

Nanostructuring of thermoelectric (TE) materials leads to improved energy conversion performance; however, it requires a perfect fit between the nanoprecipitates’ chemistry and crystal structure and those of the matrix. We synthesize bulk Bi₂Te₃ from molecular precursors and characterize their structure and chemistry using electron microscopy and analyze their TE transport properties in the range of 300–500 K. Te-nanoprecipitates decorating the Bi₂Te₃ grain boundaries (GBs), which yield enhanced TE performance with a power factor (PF) of ∼19 μW cm⁻¹ K⁻² at 300 K. First-principles calculations validate the role of Te/Bi₂Te₃ interfaces in increasing the charge carrier concentration, density of states, and electrical conductivity. These optimized TE coefficients yield a promising TE figure of merit (zT) peak value of 1.30 at 450 K and an average zT of 1.14 from 300 to 500 K. This is one of the cutting-edge zT values recorded for n-type Bi₂Te₃ produced by chemical routes. We believe that this chemical synthesis strategy will be beneficial for future development of scalable n-type Bi₂Te₃-based devices.

An order of magnitude rise in the thermoelectric (TE) performance of the PbSe, a scalable and easy-to-manufacture TE material, has been achieved by incorporating reduced graphene oxide (Gr) nanoplatelets in a PbSe/PbSeO₃ heterostructure formed by acoustic cavitation-assisted oxidation. The fabricated Gr/PbSe/PbSeO₃ nanocomposites exhibit high TE performance with an exceptionally high Seebeck coefficient coupled with low thermal conductivity. The variation in the Seebeck coefficient has been attributed to a reduction in charge carrier mobility due to the ferroelectric polarization effect. Furthermore, the increase in electrical resistivity is minimized by adding graphene. This study shows substantial changes in the TE properties of PbSe through the incorporation of graphene and PbSeO₃. Gr:PbSe/PbSeO₃ exhibits lower thermal conductivity of ~ 0.2 W/mK between 450 and 600 K. The understanding and methodology developed in this study can be exploited for the scalable manufacturing of high-performance TE materials.

ZnO is a promising thermoelectric (TE) material for high-temperature applications; however, its TE performance is limited by strong coupling between the electrical and thermal transport properties. In this study, NaCl and CdO are co-doped with Al into ZnO samples synthesized by spark plasma sintering and establish a strong correlation with TE properties. NaCl's alloy with ZnAlO is doped simultaneously at cationic and anionic sites, while the embedded CdO contributes to doping and surface modification. The methodology produced an ultrahigh σ ≥ 3600 S/cm at 300 K with significantly lower lattice thermal conductivity ~3.1 W/mK at high temperature. The structural, electrical, and thermal properties are analyzed to elucidate CdO's influence, and a correlation between the power factor and the energy barrier height discussed. This yields the best power factor of ~10.9 µW/cm K² at 1173 K in the ZnAlCdO matrix, resulting because of carrier energy filtering effect, which is 1.5 times above the reported data. 

Herein, we investigate the correlation between grain size, misorientation, and lattice strain in Bi₂Te₃ and its TI signature, aiming to improve its TE performance. We reveal an unusual behavior showing that electron mobility increases upon the increase of grain size, reaching at a maximum value of 495 cm²/V·s for an optimum grain size of 600 nm and most-frequent GB misorientation angle of 60° and then decreases with increasing grain size. It is also indicated that the combined effects of grain size reduction and point defects induce lattice strain in the Bi₂Te₃ matrix that is essential to trigger the TI contribution to TE transport. This trend is corroborated by first-principles calculations showing that compressive strains form multiple valleys in the valence band and opens the TI band gap. Such a combination of physical phenomena in a well-known TE material is unique and can promote our understanding of the nature of TE transport with implications for TE energy conversion.

Novel compound Ca₃La₂W₂O₁₂:xEu³⁺ synthesized and characterized for luminescence application. Correlation of experimental results and theoretical calculation validates the abilities of Ca₃La₂W₂O₁₂:xEu³⁺ for advanced display technology. Powder X-ray diffraction (PXRD) analysis confirmed that the phosphors crystallize in a hexagonal crystal system with the space group R3̅m, and Raman shift from 324 to 336 cm⁻¹ could be correlated with the structural and electronic changes after Eu-doping. The chromaticity coordinates, color purity, and correlated color temperature (CCT) for this optimal concentration were (0.60, 0.40), 79.63%, and 1712.79 K, respectively. Additionally, theoretical calculation is used to describe and quantify the intensity of electric-dipole transitions using Judd-Ofelt (J-O) theory. The comprehensive characterization and favorable photoluminescent properties of Ca₃La₂W₂O₁₂:Eu³⁺ phosphors underscore their potential as superior red-emitting materials for integration into white light-emitting diodes (w-LEDs). These findings could pave the way for advancements in solid-state lighting technologies, highlighting the significant impact of optimized Eu³⁺ doping concentrations on the performance of phosphor materials. 

With the help of Raman spectroscopy, we demonstrate how ferromagnetism arises in diamagnetic materials. Polycrystalline lead selenide (PbSe) doped with copper (Cu) and nickel (Ni) was prepared to understand its magnetic behavior and Raman activity. The processing conditions and influence of dopants (magnetically active and non-active) and their respective compositions on the magnetic properties and Raman-active mode were studied. A surprising/anomalous room-temperature ferromagnetism (hysteresis loop) is noticed in bulk diamagnetic PbSe, which is found to be a natural or inherent characteristic of material, and depends on the crystallite size, dopant, and developed strain due to dopant/defects. In these doped PbSe, the shifting of longitudinal (LO) phonon mode was also studied by the Raman spectroscopy. The shifting of LO mode is found to be dopant dependent, and the frequency shift of LO mode is associated with the induced strain that created by the dopants and vacancies. This asymmetry in LO phonon mode (peak shift and shape) may be due to the intraband electronic transition of dopants.