Publications by authors named "Kanishka Biswas"

67 Publications

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
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.
View Article and Find Full Text PDF

Download full-text PDF

Source
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).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c11015DOI Listing
November 2020

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator BiSe.

ACS Appl Mater Interfaces 2020 Nov 16;12(47):53409-53415. Epub 2020 Nov 16.

Laboratory for Nanomagnetism and Magnetic Materials (LNMM), School of Physical Sciences, National Institute of Science Education and Research (NISER), HBNI, Jatni, Khurda 752050, India.

BiSe is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λ). We report an λ of ∼0.36 nm, which is the highest ever observed in BiSe. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated BiSe/CoFeB bilayer. The BiSe thickness dependence of λ, perpendicular surface anisotropy (), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of . The SCCE is found to be high when the value of is small.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.0c13540DOI Listing
November 2020

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler.

Angew Chem Int Ed Engl 2021 Feb 16;60(8):4259-4265. Epub 2020 Dec 16.

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

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe , which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46-0.31 W m  K ) in the 300-673 K. We established an intrinsic rattling nature in TlInTe by studying the local structure and phonon vibrations using synchrotron X-ray pair distribution function (PDF) (100-503 K) and inelastic neutron scattering (INS) (5-450 K), respectively. We showed that while 1D chain of covalently bonded transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm inside distorted Thompson cage formed by . This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe .
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202013923DOI Listing
February 2021

Virtual Special Issue: Materials for Thermoelectric Energy Conversion.

ACS Appl Mater Interfaces 2020 Oct;12(42):47113-47114

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.0c17369DOI Listing
October 2020

2D layered all-inorganic halide perovskites: recent trends in their structure, synthesis and properties.

Nanoscale 2020 Oct;12(41):21094-21117

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

Recently, halide perovskites have appeared as a superior class of materials for diverse applications, mainly in optoelectronics and photovoltaics. Perovskite halides are broadly classified as hybrid organic-inorganic and all-inorganic analogues depending on the chemical nature of the A cation in the ABX3-type structure. Immense progress has already been achieved in halide perovskites focusing mainly on the hybrid equivalents and all-inorganic three-dimensional (3D) structures, however all-inorganic two-dimensional (2D) layered halide perovskites are relatively new and their nanostructures have gained significant attention in the last few years. In this minireview, we presented a discussion on the recently developed all-inorganic 2D layered halide perovskites highlighting their crystal structure, synthetic methodologies, chemical transformations, and optical properties. We have demonstrated a significant number of examples of Pb-free 2D halide perovskite nanostructures. Strategies for the shape-controlled synthesis of nanostructures and their excitonic properties are discussed in detail. Thermal conductivity and thermoelectric properties are emphasized along with the magnetic properties of layered transition-metal based perovskites. We have also mentioned the recent examples of all-inorganic 2D halide perovskites as photocatalysts for solar-driven CO2 reduction. Finally, we have concluded the article with an outlook for the further progress in 2D all-inorganic halide perovskites toward the structural diversity and prospective new applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0nr06138gDOI Listing
October 2020

Charge Transfer in the Heterostructure of CsPbBr Nanocrystals with Nitrogen-Doped Carbon Dots.

J Phys Chem Lett 2020 Oct 10;11(19):8002-8007. Epub 2020 Sep 10.

Heterostructures of inorganic halide perovskites with mixed-dimensional inorganic nanomaterials have shown great potential not only in the field of optoelectronic energy devices and photocatalysis but also for improving our fundamental understanding of the charge transfer across the heterostructure interface. Herein, we present for the first time the heterostructure integration of the CsPbBr nanocrystal with an N-doped carbon dot. We explore the photoluminescence (PL) and photoconductivity of the heterostructure of CsPbBr nanocrystals and N-doped carbon dots. PL quenching of CsPbBr nanocrystals with the addition of N-doped carbon dots was observed. The photoexcited electrons from the conduction band of CsPbBr are trapped in the N-acceptor state of N-doped carbon dots, and the charge transfer occurs via quasi type II-like electronic band alignment. The charge transfer in the halide perovskite-based heterostructure should motivate further research into the new heterostructure synthesis with perovskites and the fundamental understanding of the mechanism of charge/energy transfer across the heterostructure interface.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.0c02139DOI Listing
October 2020

Intrinsically Ultralow Thermal Conductivity in Ruddlesden-Popper 2D Perovskite CsPbICl: Localized Anharmonic Vibrations and Dynamic Octahedral Distortions.

J Am Chem Soc 2020 Sep 28;142(36):15595-15603. Epub 2020 Aug 28.

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

Fundamental understanding of the correlation between chemical bonding and lattice dynamics in intrinsically low thermal conductive crystalline solids is important to thermoelectrics, thermal barrier coating, and more recently to photovoltaics. Two-dimensional (2D) layered halide perovskites have recently attracted widespread attention in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κ) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, CsPbICl, synthesized by the Bridgman method. We have measured the anisotropic κ value of the CsPbICl single crystal and observed an ultralow κ value of ∼0.37-0.28 W/mK in the temperature range of 295-523 K when measured along the crystallographic -axis. First-principles density functional theory (DFT) analysis of the phonon spectrum uncovers the presence of soft (frequency ∼18-55 cm) optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s lone pair. We provide experimental evidence for such low energy optical phonon modes with low-temperature heat capacity and temperature-dependent Raman spectroscopic measurements. The strong anharmonic coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons to ultrasoft frequency (maximum ∼37 cm). The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic-optical phonon coupling results in an intrinsically ultralow κ value in the all-inorganic layered RP perovskite CsPbICl.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c08044DOI Listing
September 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c03696DOI Listing
July 2020

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs Bi I Cl Perovskite.

Angew Chem Int Ed Engl 2020 Jul 7;59(31):13093-13100. Epub 2020 Jul 7.

New Chemistry Unit and School of Advanced Materials, Bangalore, 560064, India.

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs Bi I Cl , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202005966DOI 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202003946DOI Listing
June 2020

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n-Type BiTe.

Angew Chem Int Ed Engl 2020 Mar 12;59(12):4822-4829. Epub 2020 Feb 12.

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, India.

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero-structural nature which has a low lattice thermal conductivity (κ ). BiTe, a unique member of the (Bi ) (Bi Te ) homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi Te (where m:n=0:1). Herein, we report intrinsically low κ of 0.47-0.8 W m  K in the 300-650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm  V  s and 707 cm  V  s along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202000343DOI Listing
March 2020

Ultralow Thermal Conductivity in Chain-like TlSe Due to Inherent Tl Rattling.

J Am Chem Soc 2019 Dec 13;141(51):20293-20299. Epub 2019 Dec 13.

School of Basic Sciences , Indian Institute of Technology Mandi , Mandi , Himachal Pradesh 175005 , India.

Understanding the mechanism that correlates phonon transport with chemical bonding and solid-state structure is the key to envisage and develop materials with ultralow thermal conductivity, which are essential for efficient thermoelectrics and thermal barrier coatings. We synthesized thallium selenide (TlSe), which is comprised of intertwined stiff and weakly bonded substructures and exhibits intrinsically ultralow lattice thermal conductivity (κ) of 0.62-0.4 W/mK in the range 295-525 K. Ultralow κ of TlSe is a result of its low energy optical phonon modes which strongly interact with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic rattler-like vibration of Tl cations in the cage constructed by the chains of (TlSe), as evident in low temperature heat capacity, terahertz time-domain spectroscopy, and temperature dependent Raman spectroscopy. Density functional theoretical analysis reveals the bonding hierarchy in TlSe which involves ionic interaction in Tl-Se while Tl-Se bonds are covalent, which causes significant lattice anharmonicity and intrinsic rattler-like low energy vibrations of Tl, resulting in ultralow κ.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b10551DOI Listing
December 2019

Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe Alloying.

J Am Chem Soc 2019 Dec 2;141(49):19505-19512. Epub 2019 Dec 2.

Successful applications of a thermoelectric material require simultaneous development of compatible n- and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe)(AgBiSe) for ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe)(AgBiSe). This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe)(AgBiSe) has promising thermoelectric performance in the mid-temperature range (400-500 K) with maximum thermoelectric figure of merit, , reaching ∼1.3 in p-type (GeTe)(AgBiSe) at 467 K and ∼0.6 in n-type (GeTe)(AgBiSe) at 500 K.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b11405DOI Listing
December 2019

Ultrathin Free-Standing Nanosheets of BiOSe: Room Temperature Ferroelectricity in Self-Assembled Charged Layered Heterostructure.

Nano Lett 2019 Aug 1;19(8):5703-5709. Epub 2019 Aug 1.

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

Ultrathin ferroelectric semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ferroelectric insulators, the ferroelectric transition temperature decreases drastically with decreasing material thickness and ceases to exist below certain critical thickness owing to depolarizing fields. Herein, we show the emergence of an ordered ferroelectric ground state in ultrathin (∼2 nm) single crystalline nanosheets of BiOSe at room temperature. Free-standing ferroelectric nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid, and scalable wet chemical procedure at room temperature. The existence of ferroelectricity in BiOSe nanosheets is confirmed by dielectric measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby resulting in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by atomic resolution scanning transmission electron microscopy and density functional theory calculations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.9b02312DOI Listing
August 2019

Mechanochemical Synthesis and Temperature-Dependent Optical Properties of Thermochromic (Ag Cu ) HgI.

Chem Asian J 2019 Dec 25;14(24):4641-4644. Epub 2019 Jul 25.

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

Thermochromic materials are generally synthesized via high-temperature melting reaction or solution-based synthesis. Herein, all-inorganic thermochromic compounds of (Ag Cu ) HgI were synthesized by solvent-free simple and scalable mechanochemical grinding at room temperature. Temperature-dependent electronic absorption spectroscopy along with DSC analysis confirmed the thermochromic events within these materials, and the phase transition temperature varied with solid solution compositions. The photoluminescence (PL) spectra is red-shifted with the increase in the Cu content in (Ag Cu ) HgI (x=0-1).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/asia.201900716DOI Listing
December 2019

Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe.

Inorg Chem 2019 Jul 25;58(14):9236-9245. Epub 2019 Jun 25.

Institute of Physical Chemistry , Justus-Liebig-University Giessen , Heinrich-Buff-Ring 17 , D-35392 Giessen , Germany.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI family, and especially AgBiSe, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe, to evaluate possible stereochemical activity of the 6s lone pair of Bi, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBiSbSe (0 ⩽ ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.9b00874DOI Listing
July 2019

Bonding heterogeneity and lone pair induced anharmonicity resulted in ultralow thermal conductivity and promising thermoelectric properties in n-type AgPbBiSe.

Chem Sci 2019 May 3;10(18):4905-4913. Epub 2019 Apr 3.

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

Efficiency in generation and utilization of energy is highly dependent on materials that have the ability to amplify or hinder thermal conduction processes. A comprehensive understanding of the relationship between chemical bonding and structure impacting lattice waves (phonons) is essential to furnish compounds with ultralow lattice thermal conductivity () for important applications such as thermoelectrics. Here, we demonstrate that the n-type rock-salt AgPbBiSe exhibits an ultra-low of 0.5-0.4 W m K in the 290-820 K temperature range. We present detailed analysis to uncover the fundamental origin of such a low . First-principles calculations augmented with low temperature heat capacity measurements and the experimentally determined synchrotron X-ray pair distribution function (PDF) reveal bonding heterogeneity within the lattice and lone pair induced lattice anharmonicity. Both of these factors enhance the phonon-phonon scattering, and are thereby responsible for the suppressed . Further optimization of the thermoelectric properties was performed by aliovalent halide doping, and a thermoelectric figure of merit () of 0.8 at 814 K was achieved for AgPbBiSeI which is remarkable among n-type Te free thermoelectrics.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c9sc00485hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6521233PMC
May 2019

Realization of High Thermoelectric Figure of Merit in Solution Synthesized 2D SnSe Nanoplates via Ge Alloying.

J Am Chem Soc 2019 Apr 8;141(15):6141-6145. Epub 2019 Apr 8.

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

Recently single crystals of layered SnSe have created a paramount importance in thermoelectrics owing to their ultralow lattice thermal conductivity and high thermoelectric figure of merit ( zT). However, nanocrystalline or polycrystalline SnSe offers a wide range of thermoelectric applications for the ease of its synthesis and machinability. Here, we demonstrate high zT of ∼2.1 at 873 K in two-dimensional nanoplates of Ge-doped SnSe synthesized by a simple hydrothermal route followed by spark plasma sintering (SPS). Anisotropic measurements also show a high zT of ∼1.75 at 873 K parallel to the SPS pressing direction. Ge doping (3 mol %) in SnSe nanoplates significantly enhances the p-type carrier concentration, which results in high electrical conductivity and power factor of ∼5.10 μW/cm K at 873 K. High lattice anharmonicity, nanoscale grain boundaries, and Ge precipitates in the SnSe matrix synergistically give rise to the ultralow lattice thermal conductivity of ∼0.18 W/mK at 873 K.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b01396DOI Listing
April 2019

Single pot synthesis of indirect band gap 2D CsPbBr nanosheets from direct band gap 3D CsPbBr nanocrystals and the origin of their luminescence properties.

Nanoscale 2019 Feb;11(9):4001-4007

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

Two-dimensional (2D) perovskites recently attracted significant interest due to their unique and novel optoelectronic properties. CsPb2Br5, a 2D inorganic perovskite halide, is an indirect band gap semiconductor, and hence it is not supposed to be luminescent. However, a fundamental understanding of the origin of its luminescence properties is still lacking as there are contradictory literature reports present concerning its luminescence properties. Here, we demonstrate a single pot solution based transformation of 2D CsPb2Br5 nanosheets from the nanocrystals of 3D CsPbBr3 and investigate the origin of its luminescence properties by detailed experiments and density functional theory (DFT) calculations. The photoluminescence of CsPb2Br5 originates from the different amorphous lead bromide ammonium complexes which are present at the surface of the nanosheets. We have also highlighted the formation mechanism of 2D nanosheets from 3D CsPbBr3 nanocrystals. These combined theoretical and experimental studies offer significant insights into the optical properties and formation mechanism of 2D CsPb2Br5 perovskites.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr09349kDOI Listing
February 2019

Synthesis of Ultrathin Few-Layer 2D Nanoplates of Halide Perovskite CsBiI and Single-Nanoplate Super-Resolved Fluorescence Microscopy.

Inorg Chem 2018 Dec 26;57(24):15558-15565. Epub 2018 Nov 26.

The discovery of new two-dimensional (2D) perovskite halides has created sensation recently because of their structural diversity and intriguing optical properties. The toxicity of Pb-based perovskite halides led to the development of Pb-free halides. Herein, we have demonstrated a one-pot solution-based synthesis of 2D ultrathin (∼1.78 nm) few-layer (2-4 layers) nanoplates (300-600 nm lateral dimension), nanosheets (0.6-1.5 μm), and nanocrystals of layered CsBiI by varying the reaction temperature from 110 to 180 °C. We have established a mechanistic pathway for the variation of morphology of CsBiI with temperature in the presence of organic capping ligands. Further, we have synthesized the bulk powder of CsBiI by mechanochemical synthesis and liquid-assisted grinding and crystalline ingot by vacuum-sealed tube melting. 2D nanoplates and bulk CsBiI demonstrate optical absorption edge along with excitonic transition. Photoluminescence properties of individual nanoplates were studied by super-resolution fluorescence imaging, which indicated the blinking behavior down to the level of an individual CsBiI nanoplate along with its emission at the far-red region and high photostability.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.8b02887DOI Listing
December 2018

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201809841DOI Listing
November 2018

Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations.

Inorg Chem 2018 Jun 31;57(12):7481-7489. Epub 2018 May 31.

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

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the AgCuS and AgCuS samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag/Cu) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The AgCuS and AgCuS samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.8b01246DOI Listing
June 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8cc02230eDOI Listing
June 2018

Localized Vibrations of Bi Bilayer Leading to Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Weak Topological Insulator n-Type BiSe.

J Am Chem Soc 2018 05 19;140(17):5866-5872. Epub 2018 Apr 19.

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth's crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi) sandwiched between two BiSe quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κ of ∼0.6 W/mK at 300 K), which is significantly lower than that of BiSe (κ of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi) (BiSe) . Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κ. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κ. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.8b02691DOI Listing
May 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201801491DOI Listing
April 2018

All-Solid-State Mechanochemical Synthesis and Post-Synthetic Transformation of Inorganic Perovskite-type Halides.

Chemistry 2018 Feb 16;24(8):1811-1815. Epub 2018 Jan 16.

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

All-inorganic and hybrid perovskite type halides are generally synthesized by solution-based methods, with the help of long chain organic capping ligands, complex organometallic precursors, and high boiling organic solvents. Herein, a room temperature, solvent-free, general, and scalable all-solid-state mechanochemical synthesis is demonstrated for different inorganic perovskite type halides, with versatile structural connectivity in three (3D), two (2D), and zero (0D) dimensions. 3D CsPbBr , 2D CsPb Br , 0D Cs PbBr , 3D CsPbCl , 2D CsPb Cl , 0D Cs PbCl , 3D CsPbI , and 3D RbPbI have all been synthesized by this method. The all-solid-state synthesis is materialized through an inorganic retrosynthetic approach, which directs the decision on the solid-state precursors (e.g., CsX and PbX (X=Cl/Br/I) with desired stoichiometric ratios. Moreover, post-synthetic structural transformations from 3D to 2D and 0D perovskite halides were performed by the same mechanochemical synthetic approach at room temperature.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/chem.201705682DOI Listing
February 2018

Synthetic Nanosheets of Natural van der Waals Heterostructures.

Angew Chem Int Ed Engl 2017 11 6;56(46):14561-14566. Epub 2017 Oct 6.

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

Creation of new van der Waals heterostructures by stacking different two dimensional (2D) crystals on top of each other in a chosen sequence is the next challenge after the discovery of graphene, mono/few layer of h-BN, and transition-metal dichalcogenides. However, chemical syntheses of van der Waals heterostructures are rarer than the physical preparation techniques. Herein, we demonstrate the kinetic stabilization of 2D ultrathin heterostructure (ca. 1.13-2.35 nm thick) nanosheets of layered intergrowth SnBi Te , SnBi Te , and SnBi Te , which belong to the Sn Bi Te homologous series, by a simple solution based synthesis. Few-layer nanosheets exhibit ultralow lattice thermal conductivity (κ ) of 0.3-0.5 W m  K and semiconducting electron-transport properties with high carrier mobility.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201708293DOI Listing
November 2017

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi.

Materials (Basel) 2017 Mar 23;10(4). Epub 2017 Mar 23.

Équipe Verres et Céramiques, ISCR CNRS UMR 6226, Université de Rennes 1, Rennes 35042, France.

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, GeTeSe₃ glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (GeTeSe₃)M (M = Cu or Bi; = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (GeTeSe₃)Bi ( = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm K at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma10040328DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5506923PMC
March 2017