Publications by authors named "Subhajit Roychowdhury"

11 Publications

  • Page 1 of 1

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance.

Angew Chem Int Ed Engl 2021 Feb 22. Epub 2021 Feb 22.

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560064, India.

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe) (AgBiTe )  , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe) (AgBiTe )  . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe) (AgBiTe )  . (GeSe) (AgBiTe ) has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 μW cm  K at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm  K at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe) (AgBiTe ) . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.
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http://dx.doi.org/10.1002/anie.202101283DOI Listing
February 2021

Enhanced atomic ordering leads to high thermoelectric performance in AgSbTe.

Science 2021 02;371(6530):722-727

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India.

High thermoelectric performance is generally achieved through either electronic structure modulations or phonon scattering enhancements, which often counteract each other. A leap in performance requires innovative strategies that simultaneously optimize electronic and phonon transports. We demonstrate high thermoelectric performance with a near room-temperature figure of merit, ~ 1.5, and a maximum ~ 2.6 at 573 kelvin, by optimizing atomic disorder in cadmium-doped polycrystalline silver antimony telluride (AgSbTe). Cadmium doping in AgSbTe enhances cationic ordering, which simultaneously improves electronic properties by tuning disorder-induced localization of electronic states and reduces lattice thermal conductivity through spontaneous formation of nanoscale (~2 to 4 nanometers) superstructures and coupling of soft vibrations localized within ~1 nanometer around cadmium sites with local strain modulation. The strategy is applicable to most other thermoelectric materials that exhibit inherent atomic disorder.
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http://dx.doi.org/10.1126/science.abb3517DOI Listing
February 2021

Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)(SnSe)(SnS).

J Am Chem Soc 2020 Nov 20. Epub 2020 Nov 20.

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India.

Thermoelectric (TE) energy conversion demands high performance crystalline inorganic solids that exhibit ultralow thermal conductivity, high mechanical stability, and good TE device properties. Pb-free germanium telluride (GeTe)-based material has recently attracted significant attention in TE power generation in mid temperatures, but pristine GeTe possesses significantly higher lattice thermal conductivity (κ) compared to that of its theoretical minimum (κ) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κ of (GeTe)(SnSe)(SnS) very near to its κ. The (GeTe)(SnSe)(SnS) system behaves as a coexistence of point-defect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κ by effective phonon scattering because of the excess point defects and rich microstructures. In the second step, introduction of Sb-doping leads to additional phonon scattering centers and optimizes the -type carrier concentration. Notably, 10 mol % Sb-doped (GeTe)(SnSe)(SnS) exhibits ultralow κ of ∼0.30 W/mK at 300 K. Subsequently, 10 mol % Sb-doped (GeTe)(SnSe)(SnS) exhibits a high TE figure of merit (zT) of ∼1.9 at 710 K. The high-performance sample exhibits a Vickers microhardness (mechanical stability) value of ∼194 that is significantly higher compared to the pristine GeTe and other state-of-the-art thermoelectric materials. Further, we have achieved a high output power, ∼150 mW for the temperature difference of 462 K, in single leg TE device based on 10 mol % Sb-doped (GeTe)(SnSe)(SnS).
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http://dx.doi.org/10.1021/jacs.0c11015DOI Listing
November 2020

Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral -Type GeSe Crystal.

J Am Chem Soc 2020 Jul 30;142(28):12237-12244. Epub 2020 Jun 30.

Department of Physical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, S. A. S. Nagar, Manauli 140306, India.

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: ), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: 3), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe. We show ultralow lattice thermal conductivity (κ) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the -type rhombohedral (GeSe)(AgBiSe) crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)(AgBiSe) are directly evident in the low temperature heat capacity () and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κ and enhances the thermoelectric performance in rhombohedral (GeSe)(AgBiSe) crystals.
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http://dx.doi.org/10.1021/jacs.0c03696DOI Listing
July 2020

Highly Converged Valence Bands and Ultralow Lattice Thermal Conductivity for High-Performance SnTe Thermoelectrics.

Angew Chem Int Ed Engl 2020 Jun 28;59(27):11115-11122. Epub 2020 Apr 28.

New Chemistry Unit, International Centre for Materials Science and School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560064, India.

A two-step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self-compensated SnTe (that is, Sn Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn Ag Te resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔE decreases to 0.10 eV in Sn Ag Mn Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 μW cm  K at 816 K in Sn Ag Mn Te was attained. The lattice thermal conductivity of Sn Ag Mn Te reached to an ultralow value (ca. 0.3 W m  K ) at 865 K, owing to the formation of Ag Te nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn Ag Mn Te.
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http://dx.doi.org/10.1002/anie.202003946DOI Listing
June 2020

Stabilizing n-Type Cubic GeSe by Entropy-Driven Alloying of AgBiSe : Ultralow Thermal Conductivity and Promising Thermoelectric Performance.

Angew Chem Int Ed Engl 2018 Nov 15;57(46):15167-15171. Epub 2018 Oct 15.

New Chemistry Unit and School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, India.

The realization of n-type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high-symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n-type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe (30-50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe concentration. The band gap of n-type cubic (GeSe) (AgBiSe ) (0.30≤x≤0.50) has a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe) (AgBiSe ) exhibits an ultralow lattice thermal conductivity (κ ≈0.43 W m  K ) in the 300-723 K range. The low κ is attributed to significant phonon scattering by entropy-driven enhanced solid-solution point defects.
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http://dx.doi.org/10.1002/anie.201809841DOI Listing
November 2018

The journey of tin chalcogenides towards high-performance thermoelectrics and topological materials.

Chem Commun (Camb) 2018 Jun;54(50):6573-6590

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India.

Lead chalcogenides and their alloys belong at the heart of thermoelectrics due to their large thermoelectric figure of merit (zT). However, recent research has shown a limitation in the use of lead (Pb)-based materials due to their toxicity and efforts have been made to produce non-toxic analogues of lead chalcogenides. Tin chalcogenides have been predicted to be promising for this purpose due to their unique electronic structure and phonon dispersion properties. Here, we discuss the journey of tin chalcogenides in the field of thermoelectrics and topological materials with the main emphasis on the bonding, crystal structures, electronic band structures, phonon dispersion and thermoelectric properties. The thermal transport properties of tin chalcogenides are explained based on lattice dynamics, where resonant bonding and local structural distortion play an important role in creating lattice anharmonicity, thereby lowering the lattice thermal conductivity. Since thermoelectric and topological materials, especially topological insulators and topological crystalline insulators, share similar material features, such as a narrow band gap, heavy constituent elements and significant spin-orbit coupling, we have discussed the thermoelectric properties of several topological tin chalcogenides from a chemistry perspective. This feature article serves as a useful reference for researchers who strive to improve the properties of tin chalcogenides and advance the field of thermoelectric and topological materials.
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http://dx.doi.org/10.1039/c8cc02230eDOI Listing
June 2018

Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe.

Angew Chem Int Ed Engl 2018 04 12;57(15):4043-4047. Epub 2018 Mar 12.

New Chemistry Unit and Theoretical Science Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, India.

Crystalline solids with intrinsically low lattice thermal conductivity (κ ) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we show an ultralow κ of 0.35 Wm  K in AgCuTe, which has a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.
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http://dx.doi.org/10.1002/anie.201801491DOI Listing
April 2018

Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe) (AgSbSe ).

Chemistry 2017 Jun 15;23(31):7438-7443. Epub 2017 May 15.

New Chemistry Unit & International Center for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur, Bangalore, India.

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZT ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZT ). Herein an ultrahigh ZT of 1.4 for (GeTe) (AgSbSe ) [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZT of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (κ ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe in GeTe results in κ of ≈0.4 W mK in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κ ) of GeTe. Additionally, (GeTe) (AgSbSe ) exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.
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http://dx.doi.org/10.1002/chem.201701480DOI Listing
June 2017

Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping.

Angew Chem Int Ed Engl 2015 Dec 28;54(50):15241-5. Epub 2015 Oct 28.

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore (India) http://www.jncasr.ac.in/kanishka/.

Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb0.6 Sn0.4 Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb0.6 Sn0.4 Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb0.6 Sn0.4 Te increases p-type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb0.58 Sn0.40 Na0.02 Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications.
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http://dx.doi.org/10.1002/anie.201508492DOI Listing
December 2015

Unique influence of cholesterol on modifying the aggregation behavior of surfactant assemblies: investigation of photophysical and dynamical properties of 2,2'-bipyridine-3,3'-diol, BP(OH)2 in surfactant micelles, and surfactant/cholesterol forming vesicles.

J Phys Chem B 2014 Aug 30;118(31):9329-40. Epub 2014 Jul 30.

Department of Chemistry, Indian Institute of Technology , Kharagpur 721302, West Bengal, India.

The binding and rotational properties of an excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,2'-bipyridine-3,3'-diol, BP(OH)2 has been investigated in alkyltrimethylammonium bromide containing (CnTAB, n = 12, 14, and 16) micelles and alkyltrimethylammonium bromide/cholesterol (CnTAB (n = 14 and 16)/cholesterol) forming vesicles using fluorescence-based spectroscopy techniques. The formation of thermodynamically stable unilamellar self-assemblies of alkyltrimethylammonium bromide/cholesterol are characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. Individually, aqueous solutions of all these alkyltrimethylammonium bromide form micelles after certain surfactant concentration (critical micelle concentration, cmc) of surfactant, whereas cholesterol molecules are insoluble in water. But with the variation of the cholesterol-to-surfactant molar ratio (Q = [cholesterol]/[surfactant]), uniform distribution of vesicular aggregates in aqueous solution can be obtained. The micelle-to-vesicle transition of surfactant solution upon addition of cholesterol also influences the steady state emission profile, fluorescence lifetime, and rotational dynamics of BP(OH)2 molecule. The diketo tautomer of BP(OH)2 molecule gets stabilized as the concentration of surfactant increases in aqueous solution. Fluorescence lifetime and rotational time constant of the BP(OH)2 molecule are also influenced by the variation of alkyl chain length of surfactant molecule. The emission quantum yield (Φ) is also found to be sensitive with surfactant concentration, variation in chain length of surfactants, and it saturates after the cmc of surfactants. The rigid and restricted microenvironment of vesicle bilayer enhance the lifetime and also rotational relaxation of BP(OH)2 significantly. The rotational behavior of BP(OH)2 in surfactant/cholesterol self-assemblies is also explained by using analytical parameters related to time-resolved anisotropy following two-step process and wobbling in a cone models.
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http://dx.doi.org/10.1021/jp503938bDOI Listing
August 2014