Browsing by Author "Kumar, Ashok"

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2D layered transition metal dichalcogenides (MoS2): Synthesis, applications and theoretical aspects
(Elsevier, 2018) Singh, Arun Kumar; Kumar, P.; Late, D.J.; Kumar, Ashok; Patel, S.; Singh, Jai
Recently, graphene and other two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely explored due to their unique optical, mechanical, electrical and sensing properties for versatile electronic and optoelectronic applications. The atomically thin layers of TMDC materials have shown potential to replace state-of-the-art silicon-based technology. Graphene has already revealed an excess of new physics and multifaceted applications in several areas. Similarly, mono-layers of TMDCs such as molybdenum disulfide (MoS2) have also shown excellent electrical and optical properties possessing a direct band-gap of ∼1.8 eV combined with high mechanical flexibility. In contrast to semi-metallic graphene, the semiconducting behavior of MoS2 allows it to overcome the deficiencies of zero-band-gap graphene. This review summarizes the synthesis of 2D MoS2 by several techniques, i.e., mechanical and chemical exfoliation, RF-sputtering, atomic layer deposition (ALD) and chemical vapor deposition (CVD), etc. Furthermore, extensive studies based on potential applications of MoS2 such as the sensor, solar cells, field emission and as an efficient catalyst for hydrogen generation has been included. Theoretical aspects combined with the experimental observations to provide more insights on the dielectric, optical and topological behavior of MoS2 was highlighted.
2D layered transition metal dichalcogenides (MoS2): Synthesis, applications and theoretical aspects
(Elsevier Ltd, 2018) Singh, Arun Kumar; Kumar, P.; Late, D.J.; Kumar, Ashok; Patel, S.; Singh, Jai
Recently, graphene and other two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely explored due to their unique optical, mechanical, electrical and sensing properties for versatile electronic and optoelectronic applications. The atomically thin layers of TMDC materials have shown potential to replace state-of-the-art silicon-based technology. Graphene has already revealed an excess of new physics and multifaceted applications in several areas. Similarly, mono-layers of TMDCs such as molybdenum disulfide (MoS2) have also shown excellent electrical and optical properties possessing a direct band-gap of ?1.8 eV combined with high mechanical flexibility. In contrast to semi-metallic graphene, the semiconducting behavior of MoS2 allows it to overcome the deficiencies of zero-band-gap graphene. This review summarizes the synthesis of 2D MoS2 by several techniques, i.e., mechanical and chemical exfoliation, RF-sputtering, atomic layer deposition (ALD) and chemical vapor deposition (CVD), etc. Furthermore, extensive studies based on potential applications of MoS2 such as the sensor, solar cells, field emission and as an efficient catalyst for hydrogen generation has been included. Theoretical aspects combined with the experimental observations to provide more insights on the dielectric, optical and topological behavior of MoS2 was highlighted.
2D-HfS 2 as an efficient photocatalyst for water splitting
(Royal Society of Chemistry, 2016) Singh, Deobrat; Gupta, Sanjeev K.; Sonvane, Yogesh; Kumar, Ashok; Ahuja, Rajeev
Two dimensional monolayer nanostructures for water splitting solar photocatalysts are drawing more attention due to their extraordinary properties. Using first principles calculations we have systematically investigated the structural, electronic and vibrational properties of corresponding HfS2 monolayers in both hexagonal (1H) and trigonal (1T) phases. The most stable adsorption configurations and adsorption energies are calculated. The adsorption energy of H2O on the substrate is 646.53 kJ mol−1 for the 1H-phase and 621.65 kJ mol−1 for the 1T-phase of HfS2. This shows that H2O molecules have a stronger interaction with the HfS2 substrate. The calculated redox potentials of H2O splitting lie properly astride the valence and conduction bands, suggesting that the monolayers of 1H- and 1T-HfS2 show the same characteristics as a photocatalyst for water splitting. Furthermore, we also calculated that the optical band gaps for the 1H and 1T phases of HfS2 are 2.60 eV and 3.10 eV, respectively. We have also calculated Raman spectrum signatures of the monolayer 1H and 1T-phase of the in-plane vibrational mode of the Hf and S atoms (E1g) and the out-of-plane vibrational mode of S atoms (A1g and A2u). Our work suggests that a lot more research and attention in this field is needed for the practical application of the material as visible light active photocatalysts.
Adsorption of nucleobases on different allotropes of phosphorene
(American Institute of Physics, 2019) Jakhar, M; Kumar, Ashok; Srivastava, S; Parida, P; Tankeshwar, K.
There has been tremendous interest in low-dimensional quantum systems during past two decades, fueled by a constant stream of striking discoveries and also by the potential for, and realization of, new state-of-the-art electronic device architectures. In this paper, our work includes the structural, electronic and optical properties of nucleobase (Adenine(A), Cytosine(C), Guanine(G), Thymine(T)) adsorbed on different allotropes of phosphorene (α, β, γ). From the optical absorption spectra of different nucleobases when adsorbed on the surface of phosphorene, we could optically probe different Nucleobases. As phosphorene shows different spectra for different nucleobases, it behaves as a bio-sensor to detect various nucleobases. © 2019 Author(s).
Alloyed monolayers of Cu, Ag, Au and Pt in hexagonal phase: A comprehensive first principles study
(Elsevier Ltd, 2018) Kapoor, Pooja; Kumar, Arun; Sharma, Munish; Kumar, Jagdish; Kumar, Ashok; Ahluwalia, P. K.
We present density functional theory (DFT) based comprehensive study of two-dimensional (2D) alloyed monolayers of noble metals (AgCu, AgPt, AgAu, AuCu, AuPt and CuPt) in hexagonal phase within numerical atomic orbitals and plane wave basis sets methods. The monolayers considered exhibit positive phonon frequencies suggesting them to be dynamically stable. The Pt containing alloyed monolayers have superior structural stability (binding energy and tensile strength) and exhibit metallic and ferromagnetic character amongst all the alloyed monolayers. Interestingly, alloying of Au monolayer with Cu and Ag show semiconducting behavior whereas alloyed AgCu monolayer posseses Dirac-cone like features at high symmetry points. These distinct features in electronic structures of alloyed 2D monolayers have been captured in STM like set up. An anisotropic behavior has been observed in dielectric spectra for all the considered structures. Tunneling characteristics show NDR region for Pt containing alloyed monolayers. The considered alloyed monolayers may potentially be useful as a building blocks for the applications in nano- and opto-electronics. ? 2017 Elsevier B.V.
Armchair and zigzag nanoribbons of gold and silver: A DFT study
(American Institute of Physics Inc., 2018) Kapoor, Pooja; Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.
This paper presents the results from a DFT-based computational study of structural and electronic properties of zigzag and armchair edge shaped nanoribbons of gold and silver in hexagonal phase. The cohesive energy of the considered nanoribbons are found to be more than the corresponding 2D counterpart, thereby, suggesting Au and Ag nanoribbons to be more stable in 1D as compared to 2D. All nanoribbons are found to be metallic with a modulation in quantum ballistic conductance with length and edge type of the nanoribbon. Au nanoribbons are found to have higher conductance than Ag nanoribbon. There is increase in conductance with increase in length of nanoribbon. ? 2018 Author(s).
DWT-SVD Based robust digital image watermarking using adaptive median filter
(Central University of Punjab, 2013) Kumar, Ashok; Brar, Sukhreet Singh
Feature Matching is one of the central issues of model-based recognition and an important component of most object recognition systems. The feature matching technique can be applied in image alignment, 3D secure reconstruction, motion tracking, object recognition and in pharmacophore based molecular alignment. Even though numerous algorithms exist for feature recognition and search yet the efficiency of these have considerable room for improvement. This has necessitated a continuous search for better performing algorithms. The combination of geometric hashing technique and tolerance limits for a given set of known interactions can be used for the pharmacophore modelling based search. The algorithm proposed is precise as compared with the previous existing algorithms. This algorithm is better in its runtime complexity.
Electron transport and thermoelectric performance of defected monolayer MoS2
(Elsevier, 2019) Sharm, Munish a; Kumar, Ashok; Ahluwalia, P. K.
Electronic and thermoelectric properties of a two-dimensional MoS2 monolayer containing atomic defects are investigated using density functional theory. All the atomic defects have been found to exhibit endothermic nature. Electronic structure of MoS2 shows tuneability of band gap with the atomic defects. The MoS2 vacancy in pristine monolayer makes it magnetic and narrow band gap semiconductor. The spin-polarized character of the monolayer with defects is clearly captured by the tunneling current calculated in the STM-like setup. A relatively low thermal conductivity has been observed in monolayers with defects as compared to pristine form resulting in enhanced room temperature figure of merit as high as 6.24 and 1.30 respectively. The results presented open up a new window for the use of monolayer MoS2 in electronic devices, thermal management and thermoelectric devices
Electronic and optical properties of boron-based hybrid monolayers
(IOP Publishing Ltd, 2021-06-24T00:00:00) Katoch, Neha; Kumar, Ashok; Kumar, Jagdish; Ahluwalia, P.K.; Pandey, Ravindra
Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 1012 cm?2 which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in the x-direction is accompanied with that in the visible region along the y-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications. � 2021 IOP Publishing Ltd Printed in the UK
Electronic properties and mechanical strength of ?-phosphorene nano-ribbons
(American Institute of Physics Inc., 2016) Swaroop, Ram; Bhatia, Pradeep; Kumar, Ashok
We have performed first principles calculations to find out the effect of mechanical strain on the electronic properties of zig-zag edged nano ribbons of ?-phosphorene. It is found that electronic band-gap get opened-up to 2.61 eV by passivation of the edges of ribbons. Similarly, the mechanical strength is found to be increase from 1.75GPa to 2.65GPa on going from unpassivated nano ribbons to passivated ones along with the 2% increase in ultimate tensile strain. The band-gap value of passivated ribbon gets decreased to 0.43 eV on applying strain up to which the ribbon does not break. These tunable properties of ?-phospherene with passivation with H-Atom and applying mechanical strain offer its use in tunable nano electronics.
Electronic Properties and Mechanical Strength of β- Phosphorene Nano-ribbons
(AIP Publishing, 2016) Swaroop, Ram; Bhatia, Pradeep; Kumar, Ashok
We have performed first principles calculations to find out the effect of mechanical strain on the electronic of zig-zag edged nano ribbons of β-phosphorene. It is found that electronic get opened-up to 2.61 eV by of the edges of ribbons. Similarly, the mechanical strength is found to be increase from 1.75 GPa to 2.65 GPa on going from unpassivated nano ribbons to passivated ones along with the 2% increase in ultimate tensile strain. The value of passivated ribbon gets decreased to 0.43 eV on applying strain up to which the ribbon does not break. These tunable of β-phospherene with with H-atom and applying mechanical strain offer its use in tunable nano electronics.
Electronic properties and STM images of vacancy clusters and chains in functionalized silicene and germanene
(Elsevier B.V., 2017) Jamdagni, Pooja; Kumar, Ashok; Sharma, Munish; Thakur, Anil; Ahluwalia, P. K.
Electronic properties and STM topographical images of X (=F, H, O) functionalized silicene and germanene have been investigated by introducing various kind of vacancy clusters and chain patterns in monolayers within density functional theory (DFT) framework. The relative ease of formation of vacancy clusters and chain patterns is found to be energetically most favorable in hydrogenated silicene and germanene. F- and H-functionalized silicene and germanene are direct bandgap semiconducting with bandgap ranging between 0.1?1.9?eV, while O-functionalized monolayers are metallic in nature. By introducing various vacancy clusters and chain patterns in both silicene and germanene, the electronic and magnetic properties get modified in significant manner e.g. F- and H-functionalized silicene and germanene with hexagonal and rectangle vacancy clusters are non-magnetic semiconductors with modified bandgap values while pentagonal and triangle vacancy clusters induce metallicity and magnetic character in monolayers; hexagonal vacancy chain patterns induce direct-to-indirect gap transition while zigzag vacancy chain patterns retain direct bandgap nature of monolayers. Calculated STM topographical images show distinctly different characteristics for various type of vacancy clusters and chain patterns which may be used as electronic fingerprints to identify various vacancy patterns in silicene and germanene created during the process of functionalization. ? 2016 Elsevier B.V.
Electronic properties of phosphorene/graphene heterostructures: Effect of external electric field
(American Institute of Physics Inc., 2016) Kaur, Sumandeep; Kumar, Ashok; Srivastava, Sunita; Tankeshwar, K.
We report the electronic properties of electrically gated heterostructures of black and blue phosphorene with graphene. The heterostructure of blue phosphorene with graphene is energetically more favorable than black phospherene/graphene. However, both are bonded by weak interlayer interactions. Graphene induces the Dirac cone character in both heterostructure which shows tunabilities with external electric field. It is found that Dirac cone get shifted depending on the polarity of external electric field that results into the so called self induced p-type or n-type doping effect. These features have importance in the fabrication of nano-electronic devices based on the phosphorene/graphene heterostructures. ? 2016 Author(s).
Electronic Properties of Phosphorene/MoSe 2 Vertical Hetero-structures
(AIP Publishing, 2017) Kaur, Sumandeep; Kumar, Ashok; Srivastava, Sunita; Tankeshwar, K.
We report three structurally different and stable phases of MoSe2 namely h-MoSe2 (trigonal prismatic phase), t-MoSe2 (distorted octahedral coordinated phase) and o-MoSe2 (consisting of repeated octagon pairs) and their hetero-structures with black phosphorene. The MoSe2-octa phase possesses graphene-like character i.e. cone feature at the All the considered hetero-structures are energetically equally favorable. The h-MoSe2/black-P is found to be a in nature while on the other hand t-MoSe2/black-P and o-MoSe2black-P are metallic. These novel hetero-structures may be useful in the of nano-electronic based on phosphorene hetero-structures.
Electronic properties of ultrathin 2D and 1D alloyed nanostructures of stanene
(American Institute of Physics Inc., 2017) Sachdeva, Geeta; Kumar, Chandra; Tankeshwar, K.; Kumar, Ashok
Electronic structure of two dimensional (2D) and one dimensional (1D) ultrathin alloyed nanostructures of stanene has been investigated within the framework of state-of-the-art density functional theory (DFT). Ultrathin stanene is Dirac semimetal with linear dispersion of bands at K-point, whereas ?0.2 eV energy gap get induced in alloyed stanene, thereby, offers wide variety of application at nanoscale. Furthermore, the mechanical strength of alloyed stanene increases from 1.6 GPa in pristine monolayer to 2.2 GPa in alloyed nanostructure. Various topologies of 1D nanostructures are found to metallic in nature with calculated ballistic conductance in the range 2G0 to 4 G0. Our theoretical predictions may be useful for experimentalist to fabricate devices based on ultrathin nanostructures of alloyed stanene. ? 2017 Author(s).
Electronic properties of ZnPSe3-MoS2 Van der Waals heterostructure
(American Institute of Physics Inc., 2018) Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.
We present a comparative study of electronic properties of ZnPSe3-MoS2 heterostructure using GGA-PBE functional and DFT-D2 method within the framework of density functional theory (DFT). Electronic band structure for the considered heterostructure shows a direct band gap semiconducting character. A decrease in band gap is observed with the heterostructuring as compared to their constituent pristine monolayers. The alignment of valance band maxima and conduction band minima on different layers in heterostructure indicate the physical separation of charge carriers. A work function of 5.31 eV has been calculated for ZnPSe3-MoS2 heterostructure. These results provide a physical basis for the potential applications of these ZnPSe3-MoS2 heterostructure in optoelectronic devices. ? 2018 Author(s).
Electronic structure engineering of various structural phases of phosphorene
(Royal Society of Chemistry, 2016) Kaur, Sumandeep; Kumar, Ashok; Srivastava, Sunita; Tankeshwar, K.
We report the tailoring of the electronic structures of various structural phases of phosphorene (?-P, ?-P, ?-P and ?-P) based homo- and hetero-bilayers through in-plane mechanical strains, vertical pressure and transverse electric field by employing density functional theory. In-plane biaxial strains have considerably modified the electronic bandgap of both homo- and hetero-bilayers while vertical pressure induces metallization in the considered structures. The ?-P homo-bilayer structure showed the highest ultimate tensile strength (UTS ? 6.21 GPa) upon in-plane stretching. Upon application of a transverse electric field, the variation in the bandgap of hetero-bilayers was found to be strongly dependent on the polarity of the applied field which is attributed to the counterbalance between the external electric field and the internal field induced by different structural phases and heterogeneity in the arrangements of atoms of each surface of the hetero-bilayer system. Our results demonstrate that the electronic structures of the considered hetero- and homo-bilayers of phosphorene could be modified by biaxial strain, pressure and electric field to achieve the desired properties for future nano-electronic devices. ? the Owner Societies 2016.
Electronic, Mechanical, and Dielectric Properties of Two-Dimensional Atomic Layers of Noble Metals
(Springer New York LLC, 2017) Kapoor, Pooja; Kumar, Jagdish; Kumar, Arun; Kumar, Ashok; Ahluwalia, P. K.
We present density functional theory-based electronic, mechanical, and dielectric properties of monolayers and bilayers of noble metals (Au, Ag, Cu, and Pt) taken with graphene-like hexagonal structure. The Au, Ag, and Pt bilayers stabilize in AA-stacked configuration, while the Cu bilayer favors the AB stacking pattern. The quantum ballistic conductance of the noble-metal mono- and bilayers is remarkably increased compared with their bulk counterparts. Among the studied systems, the tensile strength is found to be highest for the Pt monolayer and bilayer. The noble metals in mono- and bilayer form show distinctly different electron energy loss spectra and reflectance spectra due to the quantum confinement effect on going from bulk to the monolayer limit. Such tunability of the electronic and dielectric properties of noble metals by reducing the degrees of freedom of electrons offers promise for their use in nanoelectronics and optoelectronics applications. ? 2016, The Minerals, Metals & Materials Society.
Energetics and Electronic Properties of Pt Wires of Different Topologies on Monolayer MoSe 2
(AIP Publishing, 2016) Jamdagni, Pooja; Kumar, Ashok; Thakur, Anil; Pandey, Ravindra; Ahluwalia, P. K.
The energetics and electronic properties of different of Pt wires including linear, zigzag and ladder structures on MoSe2 have been investigated in the framework of The predicted order of stability of Pt wire on MoSe2 is found to be: linear > ladder > zigzag. Pt wires induce states near the of MoSe2 that results into metallic characteristics of Pt-wire/MoSe2 assembled system. signifies most of the contribution from Pt atoms near the Fermi energy of assembled wire/MoSe2 system. These findings are expected to be important for the of based on MoSe2 layers for flexible nanoelectronics.
Energetics and electronic structure of novel hybrid dumbbell monolayers
(American Institute of Physics, 2019) Kaur, S; Singh, J; Kumar, Ashok; Srivastava, S; Tankeshwar, K.
We report three new hybrid monolayers (C6P4, C6N4 and N6P4) of group-IV and group-V elements in dumbbell structure using density functional theory calculations. C6P4, C6N4 possess sp2 as well as sp3 hybridization in their honeycomb dumbbell structure while N6P4 possess only the sp3 hybridization in its non-honeycomb but dumbbell structure. The magnitude of cohesive energy of these hybrid monolayers suggests that C6N4 is the most favorable monolayer to be formed. We found that C6P4 is metallic while C6N4 and N6P4 are semiconductors. Also, we report as a representative case, the systematic structural phase transition from LHD-C to a new phosphorous allotrope which has been suggested to exists in our cohesive energy calculations. The reported monolayers join the family of two dimensional materials and may possess application in nanoelectronic devices. © 2019 Author(s).