Browsing by Author "Sharma, Raman"

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    Janus ?-PdXY (X/Y = S, Se, Te) materials with high anisotropic thermoelectric performance
    (Royal Society of Chemistry, 2023-02-21T00:00:00) Jakhar, Mukesh; Sharma, Raman; Kumar, Ashok
    Two-dimensional (2D) materials have garnered considerable attention as emerging thermoelectric (TE) materials owing to their unique density of states (DOS) near the Fermi level. We investigate the TE performance of Janus ?-PdXY (X/Y = S, Se, Te) monolayer materials as a function of carrier concentration and temperature in the mid-range from 300 to 800 K by combining density functional theory (DFT) and semi-classical Boltzmann transport theory. The phonon dispersion spectra and AIMD simulations confirm their thermal and dynamic stability. The transport calculation results reveal the highly anisotropic TE performance of both n and p-type Janus ?-PdXY monolayers. Meanwhile, the coexistence of low phonon group velocity and a converged scattering rate leads to a lower lattice thermal conductivity (Kl) of 0.80 W mK?1, 0.94 W mK?1, and 0.77 W mK?1 along the y-direction for these Janus materials, while the high TE power factor is attributed to the high Seebeck coefficient (S) and electrical conductivity, which are due to the degenerate top valence bands of these Janus monolayers. The combination of lower Kl and a high-power factor at 300 K (800 K) leads to an optimal figure of merit (ZT) of 0.68 (2.21), 0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe monolayers, respectively. To evaluate rational electron transport properties, the effects of acoustic phonon scattering (?ac), impurity scattering (?imp), and polarized phonon scattering (?polar) are included in the temperature-dependent electron relaxation time. These findings indicated that the Janus ?-PdXY monolayers are promising candidates for TE conversion devices. � 2023 The Royal Society of Chemistry.
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    Stability and electronic properties of bilayer graphene spirals
    (Elsevier B.V., 2021-01-19T00:00:00) Thakur, Rajesh; Ahluwalia, P.K.; Kumar, Ashok; Sharma, Raman
    Spiral topology offers many potential applications to next-generation nanoelectronic devices. The ab-initio simulations are used to investigate the stability and electronic properties of the hexagonal and triangular double-layer spiral (DLS). A room temperature molecular dynamics (MD) simulation reveals that the AA stacking of triangular DLS (t-DLS) is thermodynamically stable, however, the AA stacking of hexagonal DLS (h-DLS)is found to get distorted. When h-DLS and t-DLS are subjected to tensile strain the h-DLS behaved elastically, however, the t-DLS is extremely brittle. Both h-DLS and t-DLS are observed to be metallic in an equilibrium state. On applying an electric field, the h-DLS remains metallic, whereas, the t-DLS becomes a semiconductor. The bandgap of t-DLS is observed to open up even for a small magnitude of electric field. Furthermore, we also found that the triangular-shaped bilayer spiral topology gives rise to an intrinsic Rashba splitting. Our study opens up new and innovative ideas for investigating the spiral-shaped nano-structures. � 2021 Elsevier B.V.
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    Twisted helical armchair graphene nanoribbons: mechanical and electronic properties
    (Springer Science and Business Media Deutschland GmbH, 2021-05-08T00:00:00) Thakur, Rajesh; Ahluwalia, P.K.; Kumar, Ashok; Sharma, Munish; Sharma, Raman
    Abstract: The Hydrogen and Fluorine planar armchairs graphene nanoribbons (H & F AGNRs), subjected to twist deformation within fixed periodic boundary conditions. H-AGNRs is highly elastic in nature, though passivation with Fluorine does induce the plasticity when twisted beyond threshold torsional strain. This plasticity attributes to the wider bond length distribution suggests distortion of benzo-rings. The bandgap response to the effective strain of narrow GNRs N= 6 , 7 , and 8 get arranged as (i) monotonously increasing for q= 0 , 2 and (ii) decreasing for q= 1 ; here, q= mod(N, 3) in effective strain space (?2?2). The effective strain space is found to be more appropriate for gauging the response of torsional strain. This trend has also been observed for Fluorine passivated AGNRs; however, because of higher sensitive response to torsional strain, the bandgap of N= 7 F-AGNRs drops from Eg? 0.95 eV to Eg? 0.05 eV at extreme torsional strain forming Dirac cone at � K allows dissipationless transport to charge carriers of high kinetic energy at low bias. Graphic abstract: [Figure not available: see fulltext.] � 2021, The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature.