Browsing by Author "Nayak S.K."

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Absence of ferromagnetic interaction in Co-Co nearest neighbor impurity pairs in ZnO: An analysis from GGA+U studies
(2011) Nayak S.K.; Ney A.; Gruner M.E.; Tripathi G.S.; Behera S.N.; Entel P.
We study the magnetic interactions of Co doped in ZnO, with the Co atoms occupying the nearest neighbor cation sites. We perform electronic structure calculations using the local density approximation (LDA), generalized gradient approximation (GGA), and GGA+U. The Hubbard U is treated separately on d-orbitals of Zn and Co, and simultaneously on the d-orbitals of both Zn and Co. Results of GGA+U studies confirm that the nearest neighbor Co-Co pair favor antiferromagnetic interaction, where the Co spins align oppositely. This is different from the LDA and GGA predictions. A general comparison of our results with experiments shows fairly good agreement. � 2012 American Institute of Physics.
Anisotropic ferromagnetism in carbon-doped zinc oxide from first-principles studies
(2012) Nayak S.K.; Gruner M.E.; Sakong S.; Sil S.; Kratzer P.; Behera S.N.; Entel P.
A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the nonspinpolarized C Zn impurity is under almost all conditions thermodynamically more stable than the C O impurity which has a magnetic moment of 2? B, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize d0 ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C O-C O impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal ab plane of wurtzite ZnO than along the c axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C O impurities in ZnO. From the calculated results, we derive that a C O concentration between 2% and 6% should be optimal to achieve d0-ferromagnetism in C-doped ZnO. � 2012 American Physical Society.
Atomically thin layers of B�N�C�O with tunable composition
(2015) Ozturk B.; De-Luna-Bugallo A.; Panaitescu E.; Chiaramonti A.N.; Liu F.; Vargas A.; Jiang X.; Kharche N.; Yavuzcetin O.; Alnaji M.; Ford M.J.; Lok J.; Zhao Y.; King N.; Dhar N.K.; Dubey M.; Nayak S.K.; Sridhar S.; Kar S.
In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp2-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm2 V?1 s?1, and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < Eg < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform. 2015 � The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.
Band gap modulation of graphene by metal substrate: A first principles study
(2018) Sahoo M.R.; Sahu S.; Kushwaha A.K.; Nayak S.K.
Due to high in-plane charge carrier mobility with high electron velocity and long spin diffusion length, graphene guarantees as a completely unique material for devices with various applications. Unaffected 2pz orbitals of carbon atoms in graphene can be highly influenced by substrates and leads to tuning in electronic properties. We report here a density functional calculation of graphene monolayer based on metallic substrate like nickel surfaces. Band-gap of graphene near K points opens due to interactions between 2pz and d-orbitals of nickel atoms and the gap modulation can be done with the increasing number of layers of substrates. � 2018 Author(s).
Band structure modulation in MoS2 multilayers and heterostructures through electric field and strain
(2016) Lanzillo N.A.; O'Regan T.P.; Nayak S.K.
We use density functional theory to investigate the effect of finite external electric fields along with strain on the electronic structure of MoS2 multilayers and heterostructures composed of MoS2/MoSe2/MoS2 and MoS2/WS2/MoS2. We find that the presence of an electric field decreases the values of both direct and indirect band gaps, in agreement with earlier works. However, when either compressive or tensile strain is applied to the system the strength of the coupling between the electric field and direct band gap is increased while the coupling strength between the electric field and the indirect band gap is decreased. In contrast, in the heterostructures the coupling with the direct band gap remains unchanged while the coupling with indirect band gap is decreased. Taken together, these results show rich behavior of the electric field effects in MoS2 multilayers and heterostructures. � 2015 Elsevier B.V. All rights reserved.
Carbon doped ZnO: Synthesis, characterization and interpretation
(2013) Mishra D.K.; Mohapatra J.; Sharma M.K.; Chattarjee R.; Singh S.K.; Varma S.; Behera S.N.; Nayak S.K.; Entel P.
A novel thermal plasma in-flight technique has been adopted to synthesize nanocrystalline ZnO and carbon doped nanocrystalline ZnO matrix. Transmission electron microscopy (TEM) studies on these samples show the average particle sizes to be around 32 nm for ZnO and for carbon doped ZnO. An enhancement of saturation magnetization in nanosized carbon doped ZnO matrix by a factor of 3.8 has been found in comparison to ZnO nanoparticles at room temperature. Raman measurement clearly indicates the presence of Zn-C complexes surrounded by ZnO matrix in carbon doped ZnO. This indicates that the ferromagnetic signature in carbon doped ZnO arises from the creation of defects or the development of oxy-carbon clusters, in the carbon doped ZnO system. Theoretical studies based on density functional theory also support the experimental analyses. � 2012 Elsevier B.V.
Charge transfer and hybridization effect at graphene-nickel interface: A tight binding model study
(2019) Sahu S.; Sahoo M.R.; Kushwaha A.K.; Rout G.C.; Nayak S.K.
We have investigated here, the electronic and magnetic properties of graphene�nickel system by tight-binding mean-field approach. Strong hybridization between the 2pz orbital of graphene and 3dz2 orbital of nickel occurs when monolayer graphene is placed over a single layer of ferromagnetically ordered Ni (111) metal due to the excellent lattice matching between the two layers. This hybridization greatly affects the electronic and magnetic properties of the bilayer system, resulting in a significantly reduced local magnetic moment of the nickel layer and an enhanced induced spin polarization on the graphene layer. The calculated Hamiltonian revealed critical information regarding the first-, second-and third-nearest-neighbour hopping integrals of ?? electrons of graphene besides the Coulomb correlation of electrons in nickel (111). The Hubbard type Coulomb interactions present in nickel lattices were treated within the mean-field approximation. Zubarev's technique was employed to calculate electronic Green's functions and subsequent investigation of the temperature dependent ferromagnetic magnetization of nickel (111)was carried out through self-consistent calculation. Further calculations regarding the induced magnetization in the graphene, total magnetization in bilayer layer system, electronic band dispersion, spin resolved density of states (DOS) and spin polarization efficiency have been carried out. The results were corroborated by experimental observations. � 2018 Elsevier Ltd
A conserved threonine spring-loads precursor for intein splicing
(2013) Dearden A.K.; Callahan B.; Van Roey P.; Li Z.; Kumar U.; Belfort M.; Nayak S.K.
Protein splicing is an autocatalytic process where an "intein" self-cleaves from a precursor and ligates the flanking N- and C-"extein" polypeptides. Inteins occur in all domains of life and have myriad uses in biotechnology. Although the reaction steps of protein splicing are known, mechanistic details remain incomplete, particularly the initial peptide rearrangement at the N-terminal extein/intein junction. Recently, we proposed that this transformation, an N-S acyl shift, is accelerated by a localized conformational strain, between the intein's catalytic cysteine (Cys1) and the neighboring glycine (Gly-1) in the N-extein. That proposal was based on the crystal structure of a catalytically competent trapped precursor. Here, we define the structural origins and mechanistic relevance of the conformational strain using a combination of quantum mechanical simulations, mutational analysis, and X-ray crystallography. Our results implicate a conserved, but largely unstudied, threonine residue of the Ssp DnaE intein (Thr69) as the mediator of conformational strain through hydrogen bonding. Further, the strain imposed by this residue is shown to position the splice junction in a manner that enhances the rate of the N-S acyl shift substantially. Taken together, our results not only provide fundamental understanding of the control of the first step of protein splicing but also have important implications in various biotechnological applications that require precursor manipulation. � 2013 The Protein Society.
Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering
(2017) Pal S.; Sahoo M.; Veettil V.T.; Tadi K.K.; Ghosh A.; Satyam P.; Biroju R.K.; Ajayan P.M.; Nayak S.K.; Narayanan T.N.
Controlled assembly of mesoscopic structures can bring interesting phenomena because of their interfaces. Here, carbon nanotubes (CNTs) are cross-coupled via a C-C bonding through Suzuki reaction resulting in three-dimensional (3D) CNT sponges, and these 3D CNTs are studied for their efficacy toward the electrocatalytic hydrogen evolution reaction (HER) in acidic medium - one of the promising methods for the production of a renewable energy source, hydrogen. Both single and multiwall CNTs (SWCNTs and MWCNTs) are studied for the development of 3DSWCNTs and 3DMWCNTs, and these 3D CNTs are found to be HER active with small reaction onset potentials and low charge-transfer resistances unlike their uncoupled counterparts. First-principle density functional calculations show that the combination of electron acceptor and donor bonded to the CNT network can provide a unique band structure modulation in the system facilitating the HER reaction. This study can provide possibilities for band engineering of CNTs via functionalization and cross-coupling reactions. (Graph Presented). � 2017 American Chemical Society.
Electronic conduction through quantum dots undergoing Jahn-Teller transition
(2014) Bose S.M.; Ndengeyintwali D.; Behera S.N.; Nayak S.K.; Entel P.
Electronic conduction through quantum dots undergoing Jahn-Teller distortion is studied utilizing a model presented recently in connection with investigation of possible magnetovoltaic effect in this system. The quantum dot connected to two metallic leads is described by the single impurity Anderson model (SIAM) Hamiltonian along with two additional terms describing the Jahn-Teller distortion and an applied magnetic field. The self-consistent calculation shows that the Jahn-Teller (J-T) order parameter which is a measure of the splitting of the degenerate dot level is maximum at zero temperature and smoothly goes to zero at the structural transition temperature, Ts. The conductance is greatly suppressed by the J-T distortion at low temperatures, slowly increases and attains a maximum at Ts, above which it shows a slow decrease. When plotted as a function of the energy of the dot level, the conductance shows two peaks corresponding to the two split J-T levels at temperatures below Ts, which further develops into a four peak structure in the presence of a magnetic field. � 2013 Elsevier B.V.
Electronic structure of oxygen-functionalized armchair graphene nanoribbons
(2013) Simbeck A.J.; Gu D.; Kharche N.; Satyam P.V.; Avouris P.; Nayak S.K.
The electronic and magnetic properties of varying width, oxygen-functionalized armchair graphene nanoribbons (AGNRs) are investigated using first-principles density functional theory (DFT). Our study shows that O-passivation results in a rich geometrical environment which in turn determines the electronic and magnetic properties of the AGNR. For planar systems, a degenerate magnetic ground state, arising from emptying of O lone-pair electrons, is reported. DFT predicts ribbons with ferromagnetic coupling to be metallic, whereas antiferromagnetically coupled ribbons present three band gap families: one metallic and two semiconducting. Unlike hydrogen-functionalized AGNRs, the oxygen-functionalized ribbons can attain a lower energy configuration by adopting a nonplanar geometry. The nonplanar structures are nonmagnetic and show three semiconducting families of band gap behavior. Quasiparticle corrections to the DFT results predict a widening of the band gaps for all planar and nonplanar semiconducting systems. This suggests that oxygen functionalization could be used to manipulate the electronic structures of AGNRs. � 2013 American Physical Society.
The energy efficiency of fractal solar grids
(2017) Kumar S.; Ramaswamy R.; Nayak S.K.
We study the energy production, usage, and stability of networks composed of solar modules connected in different topologies. In particular, we examine fractal microgrid networks of such modules with storage batteries at each node and examine the performance under typical conditions of generation, usage and storage. We also incorporate energy excess sharing on demand between the units. Experimental as well as simulation results are presented for model households in which users have a distribution of requirements for the generated energy. Our results show that under normal operational conditions, solar units connected in a network offer an advantage over isolated solar off-grid units. We find that there is a relationship between the nature of the connectivity and efficiency within a particular connection topology. Furthermore, we find that networks with loops are more efficient in energy distribution. We also consider the possibility of mixed energy generation, with nodes randomly distributed between solar and wind generators. Random fractal networks show higher efficiency: they reduce power wastage and the effects of power cuts through effective power sharing. � 2016 IEEE.
Engineering Redox Potential of Lithium Clusters for Electrode Material in Lithium-Ion Batteries
(2017) Kushwaha A.K.; Sahoo M.R.; Nanda J.; Nayak S.K.
Low negative electrode potential and high reactivity makes lithium (Li) ideal candidate for obtaining highest possible energy density among other materials. In this work we show a novel route with which the overall electrode potential could significantly be enhanced through selection of cluster size. Using first principles density functional theory and continuum dielectric model, we studied free energy and redox potential as well as investigated relative stability of Lin (n�?�8) clusters in both gas phase and solution. We found that Li3 has the lowest negative redox potential (thereby highest overall electrode potential) suggesting that cluster based approach could provide a novel way of engineering the next generation battery technology. The microscopic origin of Li3 cluster�s superior performance is related to two major factors: gas phase ionization and difference between solvation free energy for neutral and positive ion. Taken together, the present study provides insight into the engineering of redox potential in battery and could stimulate further work in this direction. � 2017, Springer Science+Business Media, LLC.
Enhanced field emission properties of doped graphene nanosheets with layered SnS2
(2014) Rout C.S.; Joshi P.D.; Kashid R.V.; Joag D.S.; More M.A.; Simbeck A.J.; Washington M.; Nayak S.K.; Late D.J.
We report here our experimental investigations on p-doped graphene using tin sulfide (SnS2), which shows enhanced field emission properties. The turn on field required to draw an emission current density of 1 ? A/cm2 is significantly low (almost half the value) for the SnS 2/reduced graphene oxide (RGO) nanocomposite (2.65 V/? m) compared to pristine SnS2 (4.8 V/? m) nanosheets. The field enhancement factor ? (? 3200 for the SnS2 and ? 3700 for SnS 2/RGO composite) was calculated from Fowler-Nordheim (F-N) plots, which indicates that the emission is from the nanometric geometry of the emitter. The field emission current versus time plot shows overall good emission stability for the SnS2/RGO emitter. The magnitude of work function of SnS2 and a SnS2/graphene composite has been calculated from first principles density functional theory (DFT) and is found to be 6.89 eV and 5.42 eV, respectively. The DFT calculations clearly reveal that the enhanced field emission properties of SnS2/RGO are due to a substantial lowering of the work function of SnS2 when supported by graphene, which is in response to p-type doping of graphene. � 2014 AIP Publishing LLC.
Enhanced Nonenzymatic Glucose-Sensing Properties of Electrodeposited NiCo2O4-Pd Nanosheets: Experimental and DFT Investigations
(2017) Naik K.K.; Gangan A.; Chakraborty B.; Nayak S.K.; Rout C.S.
Here, we report the facile synthesis of NiCo2O4 (NCO) and NiCo2O4-Pd (NCO-Pd) nanosheets by the electrodeposition method. We observed enhanced glucose-sensing performance of NCO-Pd nanosheets as compared to bare NCO nanosheets. The sensitivity of the pure NCO nanosheets is 27.5 ?A ?M-1 cm-2, whereas NCO-Pd nanosheets exhibit sensitivity of 40.03 ?A ?M-1 cm-2. Density functional theory simulations have been performed to qualitatively support our experimental observations by investigating the interactions and charge-transfer mechanism of glucose on NiCo2O4 and Pd-doped NiCo2O4 through demonstration of partial density of states and charge density distributions. The presence of occupied and unoccupied density of states near the Fermi level implies that both Ni and Co ions in NiCo2O4 can act as communicating media to transfer the charge from glucose by participating in the redox reactions. The higher binding energy of glucose and more charge transfer from glucose to Pd-doped NiCo2O4 compared with bare NiCo2O4 infer that Pd-doped NiCo2O4 possesses superior charge-transfer kinetics, which supports the higher glucose-sensing performance. � 2017 American Chemical Society.
ERRATUM: Substrate-induced band gap renormalization in semiconducting carbon nanotubes
(2014) Lanzillo N.A.; Kharche N.; Nayak S.K.
[No abstract available]
An experimental and computational study of enhanced charge storage capacity of chemical vapor deposited Ni3S2-reduced graphene oxide hybrids
(2019) Rout C.S.; Mondal S.; Samal R.; Gangan A.S.; Nayak S.K.; Chakraborty B.
In the present approach, we have successfully synthesized wormlike Ni3S2 and Ni3S2-RGO hybrids on nickel (Ni) foam by a facile and highly reproducible one-step chemical vapor deposition (CVD) method. We have demonstrated that Ni3S2 and Ni3S2-RGO hybrids can be grown directly on the current collector of energy storage devices without using any binder, which may lead to serve industry for large-scale production of the material. We have studied the pseudocapacitive energy storage performance of the CVD grown Ni3S2 and its hybrids with variable concentration of RGO. The Ni3S2-RGO hybrid with 0.5 mg GO showed a maximum areal specific capacitance of 1.4 F�cm?2 at a current density of 1 mA�cm?2 (approximately 1124 Fg?1 at a current density of 1 Ag?1). The enhanced supercapacitive performance of Ni3S2-RGO predominantly due to its high surface area and high conductivity as compared to bare Ni3S2. The electrochemical impedance spectroscopic analysis revealed Ni3S2-RGO possess enhanced charge-transfer characteristics as compared to bare Ni3S2. Furthermore, Density Functional Theory (DFT) simulations infer the strong hybridization between C p orbital and Ni d orbital lead to enhanced electrochemical property and Density of States (DOS) is crucially responsible for the improved charge storage performance of Ni3S2-RGO hybrids. � 2019 Elsevier B.V.
Facile Hydrothermal Synthesis of MnWO4 Nanorods for Non-Enzymatic Glucose Sensing and Supercapacitor Properties with Insights from Density Functional Theory Simulations
(2017) Naik K.K.; Gangan A.S.; Pathak A.; Chakraborty B.; Nayak S.K.; Rout C.S.
Here we report a facile and novel hydrothermal method to grow MnWO4 nanorods and their electrochemical glucose sensing and supercapacitor properties have been investigated. MnWO4 nanorods exhibited good glucose sensing performance with sensitivity of 13.7 ?A?M?1cm?2 in the 5�110 ?M linear range and specific capacitance of 199 F/g at 2 mV/s and 256.41 F/g at 0.4 A/g. First principles simulations have also been carried out to qualitatively support our experimental observations by investigating the bonding and charge transfer mechanism of glucose on MnWO4 through demonstration of Partial Density of States and charge density distributions. Large Density of States near Fermi level and empty d states around 2 eV above Fermi level of Mn d orbital qualify MnWO4 as communicating media to transfer the charge from glucose by participating in the redox reactions. Insight into the electronic structure reveals that there is charge transfer from oxygen p orbital of glucose to d orbital of Mn. Also, the quantum capacitance of MnWO4 electrodes has been presented to justify its supercapacitor performance. The maximum quantum capacitance of 762 ?F/cm2 is obtained which is mostly contributed by the d electrons of Mn. Our experimental data and theoretical insight strongly infer that MnWO4 has the potential to be tailored as efficient and high-performance glucose sensing and energy storage devices. � 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
First-principles study of a vertical spin switch in atomic scale two-dimensional platform
(2019) Sahoo M.R.; Kushwaha A.K.; Pati R.; Ajayan P.M.; Nayak S.K.
High in-plane charge carrier mobility and long spin diffusion length makes graphene a unique material for spin-based devices. However, in a vertical graphene junction, the 2pz orbitals of carbon atoms in graphene can be tuned via suitable magnetic substrates; this would affect the spin injection into graphene. Here, a vertical spin switch has been designed by embedding a single layer of graphene as a tunnel layer between the Ni (1 1 1) substrate. Periodic density functional approach in conjunction with Julliere's model is used to calculate the tunnel magnetoresistance (TMR). Further, single-layered hexagonal Boron Nitride (h-BN) is sandwiched between the graphene and Ni (1 1 1) substrate to understand the role of hybridization at the interface on TMR. Our calculation shows that in contrast to the graphene junction, a much higher TMR value is obtainable in the case of the graphene/h-BN multi-tunnel junction (MTJ). The TMR in graphene junction is found to decrease with the increase of an externally applied electric field, and drops to zero for a field greater than equal to 0.16 V/�. Similar phenomenon was observed in the case of h-BN/graphene MTJ, where TMR value remains unchanged for electric field up to 0.1 V/� beyond which it drops to zero. The change in hybridization and charge-carrier-population at the interface modifies the magnetic exchange interaction and magnetic anisotropy resulting in a spin flip at interface, leads to rapid drop in TMR after a threshold electric field. The high and tunable TMR value suggests h-BN assisted high performance graphene based vertical spin switch. � 2019
Giant quasiparticle bandgap modulation in graphene nanoribbons supported on weakly interacting surfaces
(2013) Jiang X.; Kharche N.; Kohl P.; Boykin T.B.; Klimeck G.; Luisier M.; Ajayan P.M.; Nayak S.K.
In general, there are two major factors affecting bandgaps in nanostructures: (i) the enhanced electron-electron interactions due to confinement and (ii) the modified self-energy of electrons due to the dielectric screening. While recent theoretical studies on graphene nanoribbons (GNRs) report on the first effect, the effect of dielectric screening from the surrounding materials such as substrates has not been thoroughly investigated. Using large-scale electronic structure calculations based on the GW approach, we show that when GNRs are deposited on substrates, bandgaps get strongly suppressed (by as much as 1 eV) even though the GNR-substrate interaction is weak. � 2013 AIP Publishing LLC.