Faculty Publications
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Item Electronic structure engineering of tin telluride through co-doping of bismuth and indium for high performance thermoelectrics: A synergistic effect leading to a record high room temperature ZT in tin telluride(Royal Society of Chemistry, 2019) Shenoy, U.S.; Bhat, D.K.The ever increasing demand for alternative clean energy sources has led to intense research towards the optimization of thermoelectric performance of known systems. In this work, we engineer the electronic structure of SnTe by co-doping it with Bi and In. The co-doping not only results in the formation of two different resonance states and a reduced valence band offset, as in the case of previously reported co-doped SnTe, but also leads to opening of the band gap, which otherwise was closed in the case of Bi and In doped SnTe configurations, leading to suppression of bipolar diffusion. The synergistic action of all these effects leads to an increased Seebeck co-efficient throughout the temperature range and a ZTmax of ?1.32 at 840 K. This strategy of co-doping two different resonant dopants resulted in a record high room temperature ZT of ?0.25 at 300 K for SnTe based materials. This work suggests that appropriate combination of dopants to engineer the electronic structure of a material can lead to unpredictable results. © 2019 The Royal Society of Chemistry.Item SnTe thermoelectrics: Dual step approach for enhanced performance(Elsevier Ltd, 2020) Bhat, D.K.; Shenoy, U.S.Doping of SnTe to achieve desirable properties has been a wide spread approach in the recent past to enhance its thermoelectric performance. Herein, we apply a dual approach: Pb doping for reduction of thermal conductivity and Zn doping for improving the power factor. The theoretical prediction of enhanced Seebeck due to increase in the band gap, introduction of the resonance levels by Zn and dominance of the heavy hole valence band, is realized experimentally as improved power factor throughout the temperature range. The accompanying reduction in the thermal conductivity by co-doping Pb and Zn leads to a record high room temperature figure of merit, ZT of 0.35 (@ 300K) and ZT of 1.66 at 840 K. The ZTaverage of ?0.9 with 300 K as cold end and 840 K as hot end sets a new record for SnTe based materials. © 2020 Elsevier B.V.Item Resonance levels in GeTe thermoelectrics: Zinc as a new multifaceted dopant(Royal Society of Chemistry, 2020) Bhat, D.K.; Shenoy, U.S.Recently doping has been widely used in enhancing the thermoelectric properties of lead-free GeTe. But much of the work has been concentrated on carrier concentration tuning or phonon scattering. Until now, only indium has been reported to be the best resonant dopant in cubic GeTe. Herein, for the first time we introduce zinc as a resonant dopant to the cubic GeTe family. We show that zinc in GeTe not only introduces resonance states but also increases the band gap and raises the heavy hole valence band above the light hole valence band leading to enhanced Seebeck values. This multifunctional dopant incorporation in GeTe leads to enhanced transport properties as predicted by Boltzmann transport properties calculations based on first principles density functional theory electronic structure calculations. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.Item Mg/Ca doping ameliorates the thermoelectric properties of GeTe: Influence of electronic structure engineering(Elsevier Ltd, 2020) Bhat, D.K.; Shenoy, U.S.GeTe, though originally believed to be a poor thermoelectric material due to its inherent Ge vacancies has recently attracted the attention of the scientific community due to its tunable electronic structure. Herein, we study the electronic structure modifications of GeTe by means of doping it with Mg and Ca. Both Mg and Ca increases the band gap of GeTe and brings about valence band convergence decreasing the energy offset. The enhanced Seebeck co-efficient due to tuning of the electronic structure results in improved thermoelectric properties as predicted by the Boltzmann transport calculations. This strategy of doping could be very well extended to other dopants for improving the thermoelectric properties of GeTe. © 2020 Elsevier B.V.Item Complementary effect of co-doping aliovalent elements Bi and Sb in self-compensated SnTe-based thermoelectric materials(Royal Society of Chemistry, 2021) Kihoi, S.K.; Shenoy, U.S.; Bhat, D.K.; Lee, H.S.Research on Pb-free thermoelectric materials as a potential eco-friendly and solid-state source of energy has continuously advanced over time, with SnTe-based materials having shown utmost promising properties owing to their tunable electronic structure and scalable thermal conductivity. In this study, we self-compensate Sn to reduce inherent Sn vacancies, and further tune the carrier concentration by doping with Bi. Sb is further alloyed to incorporate nanostructures that significantly reduce the thermal conductivity. Multiple aliovalent dopants result in a continually decreased carrier concentration and subsequent significantly decreased electrical conductivity. The Seebeck values are seen to increase with temperature, where a maximum value of ?171 ?V K?1is reported with a maximum power factor of ?22.7 ?W cm?1K?2. We show through first principles DFT calculations the synergistic effect of Bi and Sb to introduce resonance states and an additional valence band convergence effect with increasing Sb that contribute to improved electronic properties. A decreased phonon frequency with co-doping is also reported. A maximumZTof ?0.8 at 823 K is reported in the Sn0.90Bi0.03Sb0.10Te composition, showing good potential in Sb co-doped SnTe-based materials. © The Royal Society of Chemistry 2021.Item Vanadium: A Protean Dopant in SnTe for Augmenting Its Thermoelectric Performance(American Chemical Society, 2021) Shenoy, U.S.; Bhat, D.K.In this work, a second n-type resonant dopant in the form of vanadium is introduced in the SnTe thermoelectrics family. The electronic structure simulated using density functional theory calculations revealed that V not only opens the band gap but also causes convergence of both valence and conduction sub-bands. Apart from introducing the resonance levels at the Fermi energy, the unique trait exhibited is the Rashba splitting of the conduction band to introduce multiple valleys. The advantage is the proximity of these features to the Fermi level, which eliminates the need for a co-dopant to harness the benefits. The Boltzmann transport calculations predicted promising transport properties, which showed the dual nature of vanadium, being capable of acting as a p-type as well as an n-type dopant in SnTe with corresponding maximum ZT values of 1.66 and 1.31, respectively, at 800 K, thus making it a highly potential high-performance thermoelectric candidate for future experimental studies. © 2021 American Chemical SocietyItem Optimized Mn and Bi co-doping in SnTe based thermoelectric material: A case of band engineering and density of states tuning(Chinese Society of Metals, 2021) Kihoi, S.K.; Kahiu, J.N.; Kim, H.; Shenoy, U.S.; Bhat, D.K.; Yi, S.; Lee, H.S.Tin telluride (SnTe) overwhelmingly continues to be studied owing to its promising thermoelectric properties, tunable electronic structure, and its potential as an alternate to toxic lead telluride (PbTe) based materials. In this research, we engineer the electronic properties of SnTe by co-doping Mn and Bi below their individual solubility limit. The First principles density functional theory studies reveal that both Bi and Mn introduce resonance states, thereby increasing the density of states near the Fermi level leading to enhanced Seebeck coefficient. This unique combination of using two resonant dopants to introduce flatter bands is effective in achieving higher performance at lower temperatures manifesting into a large Seebeck value of ?91 ?V/K at room temperature in the present case. Both elements optimally co-doped results in a very high power factor value of ?24.3 ?W/cmK2 at 773 K when compared to other high performance SnTe based materials. A zT of ?0.93 at 773 K is achieved by tuning the proportion of the co-dopants Mn and Bi in SnTe. The hardness value of pristine SnTe was also seen to increase after doping. As a result, synergistic optimized doping proves to be a suitable means for obtaining thermoelectric materials of superior characteristics without the need for heavy doping. © 2021Item Improving the: ZT of SnTe using electronic structure engineering: Unusual behavior of Bi dopant in the presence of Pb as a co-dopant(Royal Society of Chemistry, 2021) Shenoy, U.S.; Bhat, D.K.Electronic structure engineering of SnTe by doping various elements to improve its figure of merit has been the most promising approach recently sought after. Pb doped in SnTe is well known to decrease the thermal conductivity but fails to beneficially tune its electronic properties. Herein, we co-dope Bi in SnTe doped with Pb, to improve the power factor of the material. Bi in the presence of Pb exhibits unusual features not shown in the case of Bi doped SnTe. The synergistic action leads to an increase in the band gap and valence band convergence. Bi also introduces resonance states just below the conduction band edge and causes conduction band convergence. An enhanced power factor due to modification of the electronic structure combined with reduced thermal conductivity translates into an enhanced figure of merit of up to ?1.58 at 800 K as predicted using Boltzmann transport calculations, making it a potential thermoelectric material worthy of further study. This journal is © The Royal Society of Chemistry.Item Synergistic manifestation of band and scattering engineering in the single aliovalent Sb alloyed anharmonic SnTe alloy in concurrence with rule of parsimony(Royal Society of Chemistry, 2021) Basu, R.; Mandava, S.; Shenoy, U.S.; Bhat, D.K.; Khasimsaheb, B.; Debnath, A.K.; Singh, A.; Neeleshwar, S.Several endeavors were adapted to improve the thermoelectric performance of SnTe as a substitute of toxic PbTe and the booming approaches comprise introduction of nanostructuring, resonance states, valence band convergence and interstitial or substitutional defects. In this study, a stratagem was designed to incorporate single aliovalent Sb in SnTe by a one-step approach which successfully modulates the electronic and thermal transport properties by integration of several approaches, viz. energy-filtering effect, valence band convergence and phonon scattering at all length scales synergistically. Here, the alteration of the band structure of SnTe incorporated with Sb leads to substantial improvement of the Seebeck coefficient, essentially beneficial for the performance of thermoelectric alloys, beyond the designated critical temperature at 473 K which shows the onset of strong contribution of the heavy (?) valence band. The experimental finding of band convergence by Sb was for the first time corroborated by theoretical validation by Density Functional Theory (DFT) calculations. In addition, the presence of mass fluctuation, secondary precipitates, interfaces and the long-range interactions due to resonant bonding leading to optical phonon softening, large phase space available for three-phonon scattering and strong anharmonicity enables an ultralow lattice thermal conductivity of ?0.5 W m-1 K-1. Thus, a zT value of ?0.72 at 775 K is recorded for the SnTeSb0.05 composition, which is 154% enhancement compared to our pristine SnTe and is strongly competing with numerous reported zT values using considerably less abundant Ag, Ge, In and highly toxic Pb, Hg, Bi, Cd multiple elements as either a dopant or an additive. Thus, the law of parsimony is maintained with reduction in the cost of the thermoelectric module. © The Royal Society of Chemistry.Item Ultralow Lattice Thermal Conductivity and Enhanced Mechanical Properties of Cu and Sb Co-Doped SnTe Thermoelectric Material with a Complex Microstructure Evolution(American Chemical Society, 2022) Kihoi, S.K.; Shenoy, U.S.; Kahiu, J.N.; Kim, H.; Bhat, D.K.; Lee, H.S.SnTe is an exceptionally promising eco-friendly thermoelectric material that continues to draw immense interest as a source of alternative energy recovered from waste heat energy. Here, we investigate the effect of introducing Cu as a single doping element rather than phase separated in SnTe followed by Sb co-doping to tune the lattice thermal conductivity. A microstructure evolution was observed which influences the thermoelectric performance of these SnTe-based materials. An overall power factor of ∼22 μW/cmK2 and an ultralow lattice thermal conductivity of 0.39 W/mK are reported. A maximum ZT of 0.86 is also reported with an all-time record high hardness value of 165 Hv among SnTe-based thermoelectric materials. Through DFT calculations, we show that Cu opens the band gap of SnTe, whereas Sb in the presence of Cu introduces resonance levels and causes band convergence. This kind of enhanced thermoelectric performance is paramount for the application of SnTe in recovery of heat into useful electrical energy. © 2022 American Chemical Society