Publications by authors named "Umesh V Waghmare"

84 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.
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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

Predicting the DNA Conductance Using a Deep Feedforward Neural Network Model.

J Chem Inf Model 2021 Jan 15;61(1):106-114. Epub 2020 Dec 15.

Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India.

Double-stranded DNA (dsDNA) has been established as an efficient medium for charge migration, bringing it to the forefront of the field of molecular electronics and biological research. The charge migration rate is controlled by the electronic couplings between the two nucleobases of DNA/RNA. These electronic couplings strongly depend on the intermolecular geometry and orientation. Estimating these electronic couplings for all the possible relative geometries of molecules using the computationally demanding first-principles calculations requires a lot of time and computational resources. In this article, we present a machine learning (ML)-based model to calculate the electronic coupling between any two bases of dsDNA/dsRNA and bypass the computationally expensive first-principles calculations. Using the Coulomb matrix representation which encodes the atomic identities and coordinates of the DNA base pairs to prepare the input dataset, we train a feedforward neural network model. Our neural network (NN) model can predict the electronic couplings between dsDNA base pairs with any structural orientation with a mean absolute error (MAE) of less than 0.014 eV. We further use the NN-predicted electronic coupling values to compute the dsDNA/dsRNA conductance.
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http://dx.doi.org/10.1021/acs.jcim.0c01072DOI Listing
January 2021

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

Chemical Route to Twisted Graphene, Graphene Oxide and Boron Nitride.

Chemistry 2020 May 31;26(29):6499-6503. Epub 2020 Mar 31.

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

The recently discovered twisted graphene has attracted considerable interest. A simple chemical route was found to prepare twisted graphene by covalently linking layers of exfoliated graphene containing surface carboxyl groups with an amine-containing linker (trans-1,4-diaminocyclohexane). The twisted graphene shows the expected selected area electron diffraction pattern with sets of diffraction spots out with different angular spacings, unlike graphene, which shows a hexagonal pattern. Twisted multilayer graphene oxide could be prepared by the above procedure. Twisted boron nitride, prepared by cross-linking layers of boron nitride (BN) containing surface amino groups with oxalic acid linker, exhibited a diffraction pattern comparable to that of twisted graphene. First-principles DFT calculations threw light on the structures and the nature of interactions associated with twisted graphene/BN obtained by covalent linking of layers.
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http://dx.doi.org/10.1002/chem.202000277DOI Listing
May 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.
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http://dx.doi.org/10.1002/anie.202000343DOI Listing
March 2020

Role of van der Waals interaction in enhancing the photon absorption capability of the MoS/2D heterostructure.

Phys Chem Chem Phys 2020 Feb 17;22(5):2775-2782. Epub 2020 Jan 17.

SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur 603203, Tamil Nadu, India and Department of Physics, SRM University-AP, Amaravati 522502, Andhra Pradesh, India.

van der Waals (vdW) interaction-based heterostructures are known for enhanced photon absorption. However, the origin of these phenomena is not yet completely understood. In this work, using first-principles calculations, we provide a comprehensive study to show the effect of vdW interactions on the optical and electrical characteristics of the device and its origin. Herein, MoS/2D (where 2D varies as graphene, black and blue phosphorene, and InSe) vdW heterojunctions are considered as model structures. The change in the band gap of the heterostructures is because of hybridisation and the non-linearity of the exchange-correlation functional. Hybridisation is correlated with strain and the difference in interstitial potential between layers of the heterostructure and the vacuum level. Significantly, the estimated values of energy conversion efficiency are high in the case of MoS/InSe and MoS/BlackP vdW heterostructures as compared to MoS/GR and MoS/BlueP, suggesting their potential application in efficient and atomically thick excitonic solar cell devices.
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http://dx.doi.org/10.1039/c9cp05782jDOI Listing
February 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.
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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.
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http://dx.doi.org/10.1021/jacs.9b11405DOI Listing
December 2019

Prediction of Coupled Electronic and Phononic Ferroelectricity in Strained 2D h-NbN: First-Principles Theoretical Analysis.

Phys Rev Lett 2019 Jul;123(3):037601

Theoretical Sciences Unit, School of Advanced Materials and Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India.

Using first-principles density functional theoretical analysis, we predict coexisting ferroelectric and semimetallic states in a two-dimensional monolayer of h-NbN subjected to an electric field and in-plane strain (ε). At strains close to ε=4.85%, where its out-of-plane spontaneous polarization changes sign without inverting the structure, we demonstrate a hysteretic response of its structure and polarization to an electric field, and uncover a three-state (P=±P_{o}, 0) switching during which h-NbN passes through Dirac semimetallic states. With first-principles evidence for a combination of electronic and phononic ferroelectricity, we present a simple model that captures the energetics of coupled electronic and structural polarization, and show that electronic ferroelectricity arises in a material which is highly polarizable (small band gap) and exhibits a large electron-phonon coupling leading to anomalous dynamical charges. These insights will guide the search for electronic ferroelectrics, and our results on 2D h-NbN will stimulate development of piezofield effect transistors and devices based on the multilevel logic.
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http://dx.doi.org/10.1103/PhysRevLett.123.037601DOI Listing
July 2019

Defect-enriched tunability of electronic and charge-carrier transport characteristics of 2D borocarbonitride (BCN) monolayers from ab initio calculations.

Nanoscale 2019 Nov 5;11(41):19398-19407. Epub 2019 Aug 5.

International Center for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, P.O Jakkur, Bangalore 560064, India.

Development of inexpensive and efficient photo- and electro-catalysts is vital for clean energy applications. Electronic and structural properties can be tuned by the introduction of defects to achieve the desirable electrocatalytic activity. Using first-principles molecular dynamics simulations, the structural, dynamical, and electronic properties of 2D borocarbonitride (h-BCN) sheets have been investigated, highlighting how anti-site defects in B and N doped graphene significantly influence the bandgap, and thereby open up new avenues to tune the chemical behavior of the 2D sheets. In the present work, all of the monolayers investigated display direct bandgaps, which reduce from 0.99 eV to 0.24 eV with increasing number of anti-site defects. The present results for the electronic structure and findings for bandgap engineering open up applications of BCN monolayers in optoelectronic devices and solar cells. The influence of the anti-site distribution of B and N atoms on the ultra-high hole/electron mobility and conductivity is discussed based on density functional theory coupled with the Boltzmann transport equation. The BCN defect monolayer is predicted to have carrier mobilities three times higher than that of the pristine sheet. The present results demonstrate that BN doped graphene monolayers are likely to be useful in the next-generation 2D field-effect transistors.
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http://dx.doi.org/10.1039/c9nr04096jDOI Listing
November 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.
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http://dx.doi.org/10.1039/c9sc00485hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6521233PMC
May 2019

Dependence of the Properties of 2D Nanocomposites Generated by Covalent Crosslinking of Nanosheets on the Interlayer Separation: A Combined Experimental and Theoretical Study.

Chemphyschem 2019 07 12;20(13):1728-1737. Epub 2019 Jun 12.

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

Covalently cross-linked heterostructures of 2D materials are a new class of materials which possess electrochemical and photochemical hydrogen evolution properties. It was of considerable interest to investigate the role of interlayer spacing in the nanocomposites involving MoS and graphene sheets and its control over electronic structures and catalytic properties. We have investigated this problem with emphasis on the hydrogen evolution properties of these structures by a combined experimental and theoretical study. We have linked MoS based nanocomposites with other 2D materials with varying interlayer spacing by changing the linker and studied their hydrogen evolution properties. The hydrogen evolution activity for these composites decreases with increasing linker length, which we can link to a decrease in magnitude of charge transfer across the layers with increasing interlayer spacing. Factors such as the nature of the sheets, interlayer distance as well as the nature of the linker provide pathways to tune the properties of covalently cross-linked 2D material rendering this new class of materials highly interesting.
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http://dx.doi.org/10.1002/cphc.201900292DOI Listing
July 2019

Structural Features and HER activity of Cadmium Phosphohalides.

Angew Chem Int Ed Engl 2019 May 10;58(21):6926-6931. Epub 2019 Apr 10.

New Chemistry Unit, Sheikh Saqr Laboratory, School of Advanced Materials and Theoritical Science Unit, Jawharlal Nehru Centre for Advanced Scientific Research, Jakkur, P.O., 560064, Bangalore, India.

We have carried out a combined experimental and theoretical investigation of the structures and properties of a family of cadmium phosphochlorides with varying Cl/Cd and P/Cd ratios, Cd P Cl, Cd P Cl , Cd PCl and Cd P Cl . Their optical band gaps are in the visible region and the values are sensitive to the Cl/Cd and P/Cd ratios, leading to an increase and decrease, respectively. First-principles calculations were used to understand the bonding and electronic structures. All phosphochlorides except Cd P Cl possess direct band gaps. The calculated dielectric constants and Born effective charges illustrate the bonding, hybridization, and ionic character in these compounds. The band positions indicate the thermodynamic feasibility to perform water splitting. All systems can be used in the hydrogen evolution reaction (HER), where Cd P Cl has the highest activity and Cd PCl the lowest. The apparent quantum yield is highest in Cd P Cl (20.1 %) even without the assistance of a co-catalyst. The HER activity can be understood on the basis of photoelectrochemical measurements.
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http://dx.doi.org/10.1002/anie.201900936DOI Listing
May 2019

TiNF and Related Analogues of TiO : A Combined Experimental and Theoretical Study.

Chemphyschem 2018 12 20;19(24):3410-3417. Epub 2018 Nov 20.

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

Aliovalent anion substitution in inorganic materials brings about marked changes in properties, as exemplified by N,F-codoped metal oxides. Recently, complete substitution of oxygen in ZnO by N and F was carried out to generate Zn NF. In view of the important properties of TiO , we have attempted to prepare TiNF by employing an entirely new procedure involving the reaction of TiN with TiF . While the reaction at low temperature (450 °C) yields TiNF in the anatase phase, reaction at a higher temperature (600 °C) yields TiNF in the rutile phase. This is interesting since the anatase phase of TiO also transforms to the rutile phase on heating. The lattice parameters of TiNF are close to those of the parent oxide. Partial substitution of oxygen in TiO by N and F reduces the band gap, but complete substitution increases the value comparable to that of the oxide. We have examined properties of N,F-codoped TiO , and more interestingly N,F-codoped Ti O , both with lower band gaps than the parent oxides. A detailed first-principles calculations has been carried out on structural and electronic properties of N,F-TiO and the TiNF phases. This has enabled us to understand the effects of N,F substitution in TiO in terms of the crystal structure, electronic structure and optical properties.
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http://dx.doi.org/10.1002/cphc.201800778DOI Listing
December 2018

Phonons and thermal conducting properties of borocarbonitride (BCN) nanosheets.

Nanoscale 2018 Dec 24;10(47):22148-22154. Epub 2018 Oct 24.

Institute for Computational Molecular Science (ICMS), Temple University, Philadelphia, PA 19122, USA.

Hexagonal borocarbonitrides (BCN) are a class of 2D materials, which display excellent catalytic activity for water splitting. Here, we report the analysis of thermal stability, phonons and thermal conductivity of BCN monolayers over a wide range of temperatures using classical molecular dynamics simulations. Our results show that in contrast to the case of graphene and boron nitride monolayers, the out-of-plane phonons in BCN monolayers induce an asymmetry in the phonon density of states at all temperatures. Despite possessing lower thermal conducting properties compared to graphene and BN monolayers, the BCN nanosheets do not lose thermal conductivity as much as graphene and BN in the studied temperature range of 200-1000 K, and thus, BCN nanosheets are suitable for thermal interface device applications over a wide range of temperatures. Besides their promising role in water splitting, the above-mentioned results highlight the possibility of expanding the use of BCN 2D materials in thermal management applications and thermoelectrics.
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http://dx.doi.org/10.1039/c8nr07373bDOI 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.
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http://dx.doi.org/10.1002/anie.201809841DOI Listing
November 2018

CdNF, an analogue of CdO.

Dalton Trans 2018 Jul;47(28):9303-9309

New Chemistry Unit, International Centre for Materials Science, Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore-560064, India.

Cd2NF, isoelectronic with CdO, has been prepared by ammonolysis of CdF2. Cd2NF has the rock salt structure of CdO and shows electronic properties similar to CdO. First principles calculations shed light on the electronic structure and properties.
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http://dx.doi.org/10.1039/c8dt01137kDOI Listing
July 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.
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http://dx.doi.org/10.1021/jacs.8b02691DOI Listing
May 2018

Origin of the monolayer Raman signature in hexagonal boron nitride: a first-principles analysis.

J Phys Condens Matter 2018 May 21;30(18):185701. Epub 2018 Mar 21.

Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, United Kingdom.

Monolayers of hexagonal boron nitride (h-BN) can in principle be identified by a Raman signature, consisting of an upshift in the frequency of the E vibrational mode with respect to the bulk value, but the origin of this shift (intrinsic or support-induced) is still debated. Herein we use density functional theory calculations to investigate whether there is an intrinsic Raman shift in the h-BN monolayer in comparison with the bulk. There is universal agreement among all tested functionals in predicting the magnitude of the frequency shift upon a variation in the in-plane cell parameter. It is clear that a small in-plane contraction can explain the Raman peak upshift from bulk to monolayer. However, we show that the larger in-plane parameter in the bulk (compared to the monolayer) results from non-local correlation effects, which cannot be accounted for by local functionals or those with empirical dispersion corrections. Using a non-local-correlation functional, we then investigate the effect of finite temperatures on the Raman signature. We demonstrate that bulk h-BN thermally expands in the direction perpendicular to the layers, while the intralayer distances slightly contract, in agreement with observed experimental behavior. Interestingly, the difference in in-plane cell parameter between bulk and monolayer decreases with temperature, and becomes very small at room temperature. We conclude that the different thermal expansion of bulk and monolayer partially 'erases' the intrinsic Raman signature, accounting for its small magnitude in recent experiments on suspended samples.
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http://dx.doi.org/10.1088/1361-648X/aab883DOI 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.
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http://dx.doi.org/10.1002/anie.201801491DOI Listing
April 2018

Structural and Electronic Descriptors of Catalytic Activity of Graphene-Based Materials: First-Principles Theoretical Analysis.

Small 2018 03 28;14(10). Epub 2017 Dec 28.

SRM Research Institute and Department of Physics and Nanotechnology, SRM University, Kattankulathur, 603203, Tamil Nadu, India.

Characteristic features of the d-band in electronic structure of transition metals are quite effective as descriptors of their catalytic activity toward oxygen reduction reaction (ORR). With the promise of graphene-based materials to replace precious metal catalysts, descriptors of their chemical activity are much needed. Here, a site-specific electronic descriptor is proposed based on the p (π) orbital occupancy and its contribution to electronic states at the Fermi level. Simple structural descriptors are identified, and a linear predictive model is developed to precisely estimate adsorption free energies of OH (ΔG ) at various sites of doped graphene, and it is demonstrated through prediction of the most optimal site for catalysis of ORR. These structural descriptors, essentially the number of ortho, meta, and para sites of N/B-doped graphene sheet, can be extended to other doped sp hybridized systems, and greatly reduce the computational effort in estimating ΔG and site-specific catalytic activity.
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http://dx.doi.org/10.1002/smll.201703609DOI Listing
March 2018

Unique Features of the Photocatalytic Reduction of HO and CO by New Catalysts Based on the Analogues of CdS, CdPX (X = Cl, Br, I).

ACS Appl Mater Interfaces 2018 Jan 10;10(3):2526-2536. Epub 2018 Jan 10.

New Chemistry Unit, International Centre for Material Science (ICMS), CSIR Centre for Excellence in Chemistry, Sheikh Saqr Laboratory, Jawharlal Nehru Centre for Advanced Scientific Research , Jakkur, Bangalore 560064, India.

Photochemical reduction of HO and CO has been investigated with a new family of catalysts of the formula CdPX (X= Cl, Br, I), obtained by the complete aliovalent substitution of the sulfide ions in CdS by P and X (Cl, Br, I). Unlike CdS, the CdPX compounds exhibit hydrogen evolution and CO reduction from water even in the absence of a sacrificial agent or a cocatalyst. Use of NiP as the cocatalyst, enhances hydrogen evolution, reaching 3870 (apparent quantum yield (AQY) = 4.11) and 9258 (AQY = 9.83) μmol h g, respectively, under artificial and natural (sunlight) irradiation, in the case of CdPBr/NiP. Electrochemical and spectroscopic studies have been employed to understand the photocatalytic activity of this family of compounds. Unlike most of the semiconductor-based photocatalysts, CdPX catalysts reduce CO to CO and CH in the absence of sacrificial-agent or cocatalyst using water as the electron source. CO, CH, and H have been obtained with these catalysts under artificial as well as sun-light irradiation. First-principles, calculations have been carried out to understand the electronic structure and catalytic features of these new catalysts.
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http://dx.doi.org/10.1021/acsami.7b15992DOI Listing
January 2018

Photochemical Water Splitting by Bismuth Chalcogenide Topological Insulators.

Chemphyschem 2017 Sep 8;18(17):2322-2327. Epub 2017 Aug 8.

Chemistry and Physics Materials Unit, Theoretical Sciences Unit, International Centre for Materials Science and Sheik Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., Bangalore, 560064, India.

As one of the major areas of interest in catalysis revolves around 2D materials based on molybdenum sulfide, we have examined the catalytic properties of bismuth selenides and tellurides, which are among the first chalcogenides to be proven as topological insulators (TIs). We find significant photochemical H evolution activity with these TIs as catalysts. H evolution increases drastically in nanosheets of Bi Te compared to single crystals. First-principles calculations show that due to the topology, surface states participate and promote the hydrogen evolution.
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http://dx.doi.org/10.1002/cphc.201700344DOI Listing
September 2017

Intrinsic Rattler-Induced Low Thermal Conductivity in Zintl Type TlInTe.

J Am Chem Soc 2017 03 14;139(12):4350-4353. Epub 2017 Mar 14.

Department of Chemistry, Indian Institute of Science Education and Research (IISER) , Pune 411008, India.

Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal conductivity for various applications including thermoelectrics. TlInTe, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe.
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http://dx.doi.org/10.1021/jacs.7b01434DOI Listing
March 2017

Covalent Functionalization of Nanosheets of MoS and MoSe by Substituted Benzenes and Other Organic Molecules.

Chemistry 2017 Jan 19;23(4):886-895. Epub 2016 Dec 19.

New Chemistry Unit, International Centre for Materials Science, CSIR Centre of Excellence in Chemistry, Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore, 560 064, India.

Covalent functionalization has been effectively employed to attach benzene functionalities to MoS and MoSe nanosheets by the reaction with para-substituted iodobenzenes bearing -OCH , -H, and -NO as the substituents, where the electron-donating and electron-withdrawing power of the para substituent varies significantly. The functionalization is based on the formation of a C-S or C-Se linkage at the expense of the C-I bond on reaction of the iodobenzene with electron-rich 1T-MoS or 1T-MoSe . The degree of functionalization is in the range 4-24 % range, the value increases with the electron-withdrawing power of the para substituent. Semiconducting 2H-MoS and 2H-MoSe nanosheets can also be functionalized with iodobenzene by carrying out the reaction in the presence of a Pd catalyst. We have also carried out functionalization of 1T-MoS with pyrene, coumarin, and porphyrin derivatives. By using first-principles density functional calculations, we show that the bonding of the functional groups with the 1T phase is stronger than with the 2H phase. This is reflected in notable changes in the electronic structure of the former upon functionalization; a gap opens up in the electronic spectrum of the 1T phase. Functionalization with para-substituted benzenes leads to a change in the work function.
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http://dx.doi.org/10.1002/chem.201604176DOI Listing
January 2017

An improved d-band model of the catalytic activity of magnetic transition metal surfaces.

Sci Rep 2016 11 3;6:35916. Epub 2016 Nov 3.

Indo-Korea Science and Technology Center (IKST), Bangalore, India.

The d-band center model of Hammer and Nørskov is widely used in understanding and predicting catalytic activity on transition metal (TM) surfaces. Here, we demonstrate that this model is inadequate for capturing the complete catalytic activity of the magnetically polarized TM surfaces and propose its generalization. We validate the generalized model through comparison of adsorption energies of the NH molecule on the surfaces of 3d TMs (V, Cr, Mn, Fe, Co, Ni, Cu and Zn) determined with spin-polarized density functional theory (DFT)-based methods with the predictions of our model. Compared to the conventional d-band model, where the nature of the metal-adsorbate interaction is entirely determined through the energy and the occupation of the d-band center, we emphasize that for the surfaces with high spin polarization, the metal-adsorbate system can be stabilized through a competition of the spin-dependent metal-adsorbate interactions.
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http://dx.doi.org/10.1038/srep35916DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5093898PMC
November 2016

High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe: Synergistic Effect of Resonance Level and Valence Band Convergence.

J Am Chem Soc 2016 10 22;138(39):13068-13075. Epub 2016 Sep 22.

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

Understanding the basis of electronic transport and developing ideas to improve thermoelectric power factor are essential for production of efficient thermoelectric materials. Here, we report a significantly large thermoelectric power factor of ∼31.4 μW/cm·K at 856 K in Ag and In co-doped SnTe (i.e., SnAgInTe). This is the highest power factor so far reported for SnTe-based material, which arises from the synergistic effects of Ag and In on the electronic structure and the improved electrical transport properties of SnTe. In and Ag play different but complementary roles in modifying the valence band structure of SnTe. In-doping introduces resonance levels inside the valence bands, leading to a significant improvement in the Seebeck coefficient at room temperature. On the other hand, Ag-doping reduces the energy separation between light- and heavy-hole valence bands by widening the principal band gap, which also results in an improved Seebeck coefficient. Additionally, Ag-doping in SnTe enhances the p-type carrier mobility. Co-doping of In and Ag in SnTe yields synergistically enhanced Seebeck coefficient and power factor over a broad temperature range because of the synergy of the introduction of resonance states and convergence of valence bands, which have been confirmed by first-principles density functional theory-based electronic structure calculations. As a consequence, we have achieved an improved thermoelectric figure of merit, zT ≈ 1, in SnAgInTe at 856 K.
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http://dx.doi.org/10.1021/jacs.6b08382DOI Listing
October 2016

A first-principles study of pressure-induced phase transformation in a rare-earth formate framework.

Phys Chem Chem Phys 2016 Jul 29;18(28):19032-6. Epub 2016 Jun 29.

Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India.

Among the panoply of exciting properties that metal-organic frameworks (MOFs) exhibit, fully reversible pressure-induced phase transformations (PIPTs) are particularly interesting as they intrinsically relate to the flexibility of MOFs. Recently, a number of MOFs have been reported to exhibit this feature, which is attributed to bond rearrangement with applied pressure. However, the experimental assessment of whether a given MOF exhibits PIPT or not requires sophisticated instruments as well as detailed structural investigations. Can we capture such low pressure transformations through simulations is the question we seek to answer in this paper. For this, we have performed first-principles calculations based on the density functional theory, on a MOF, [tmenH2][Y(HCOO)4]2 (tmenH2(2+) = N,N,N',N'-tetramethylethylenediammonium). The estimated lattice constants for both the parent and product phases of the PIPT agree well with the earlier experimental results available for the same MOF with erbium. Importantly, the results confirm the observed PIPT, and thus provide theoretical corroborative evidence for the experimental findings. Our calculations offer insights into the energetics involved and reveal that the less dense phase is energetically more stable than the denser phase. From detailed analyses of the two phases, we correlate the changes in bonding and electronic structure across the PIPT with elastic and electronic conduction behavior that can be verified experimentally, to develop a deeper understanding of the PIPT in MOFs.
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http://dx.doi.org/10.1039/c6cp03028aDOI Listing
July 2016