Showing papers by "Debnarayan Jana published in 2015"

Optical properties of P and Al doped silicene: a first principles study

Abstract: Here we study the optical properties of two dimensional pure, as well as doped, buckled silicene nanosheets using density functional theory in the long wavelength limit. Optical properties were studied by varying the concentration of substituted aluminium (Al), phosphorus (P) and aluminium–phosphorus (Al–P) atoms in silicene nanosheets. It has been observed that unlike graphene, no new electron energy loss spectra (EELS) peak occurs irrespective of doping type for parallel polarization. However, for perpendicular polarization two new, small yet significant, EELS peaks emerge for P doping. The origin of these new EELS peaks may be explained through the buckling effect of stable silicene. In addition, the calculations have revealed that the maximum values of the absorption coefficient of the doped system are higher than the pristine one. The study on reflectivity modulation with doping concentration has indicated the emergence of some strong peaks having the robust characteristic of a doped reflective surface for both polarizations of the electromagnetic field. Moreover, for all doped systems, the reflectivity modulation is restricted to low energy ( 8 eV) for parallel and perpendicular polarization respectively. Although no significant changes are noticed in the maximum values of optical conductivity with doping concentration in perpendicular polarization, a sudden jump appears for the Al–P codoped system at an 18.75% doping concentration. All these theoretical observations are expected to shed light on fabricating opto-electronic devices using silicene as the block material.

Role of Zn-interstitial defect states on d0 ferromagnetism of mechanically milled ZnO nanoparticles

Abstract: Ferromagnetism in the nanostructures of undoped oxide semiconductors has become an exciting problem nowadays for its potential future applications in spintronics. In order to elucidate the room temperature d0 ferromagnetism of oxide semiconductors, we have investigated the changes in magnetic property of ZnO nanoparticles with the reduction of size by mechanical milling. We have observed that ferromagnetic ordering appears in the sample when the particle size decreases from 39 ± 1 nm to 30 ± 1 nm. This observation strongly supports the idea of the effect of specific grain boundaries in nanoparticles. The results of Raman scattering also support this observation. From photoluminescence spectra shifted green emissions have been found for ferromagnetic samples. This indicates clearly two different origins for green emissions that are strongly related to the changes in magnetic property. Observations from electron spin resonance spectra suggest that zinc related interstitial defects are significant to give rise to this ferromagnetic coupling. An impurity level formed by the interstitial defects at the surfaces could satisfy the Stoner criteria for the occurrence of band ferromagnetism for these samples.

First principles Raman study of boron and nitrogen doped planar T-graphene clusters

Abstract: Tetragonal graphene (TG) is one of the theoretically proposed dynamically stable graphene allotropes. In this study, the Raman spectra, IR spectra and some electronic properties of pristine and doped (single boron (B) and nitrogen (N)) TG have been investigated by first-principles based density functional theory (DFT) at the B3LYP/6-31G(d) level. Formation energy computation indicates that for the pristine structures, stability increases with increasing cluster size. In addition, for a particular cluster size, single B doping introduces some distortion in the system while single N doping increases the stability of it. The Raman spectrum of the N doped system is dominated by a single peak but for the B doped system several intense lines are found. For all the structures low intensity similar breathing-like modes have been observed. Besides, relatively low (high) intensity Raman lines are found for single B (N) doping compared to those of the pristine one. The vibrational study also reveals the existence of a prominent phonon Raman line for pristine clusters which hardly changes its position and nature of vibration with varying cluster size. So this mode can be used for identification of pristine TG structures. Unlike pristine TG, the doped structures possess non-zero finite dipole moments due to asymmetry in charge distribution. Large values of the HOMO–LUMO gap as well as the absence of DOS at the Fermi level lead to the semiconducting nature of all the structures. All these theoretical predictions from DFT calculations may shed light on experimental observations involving TG systems.

Optical properties of transition metal atom adsorbed graphene: A density functional theoretical calculation

Abstract: Electronic and optical properties of 3d-transition metal adsorbed graphene system, theoretically studied in the framework of density functional theory, reveals significant modification compared to the pristine system. Due to adsorption of transition metal, the emergence of closely separated electronic bands leads to substantial amount of low energy optical absorption below 2.0 eV photon energy. Very significant enhancement of static dielectric constant and large value of reflectivity in the low optical energy regime has been identified for different adsorbed systems. In the different 3d-transition metal adsorbed systems, particularly up to the half filled d-shell transition metal atom, pronounced emergence of optical absorption line in the deep ultraviolet regime beyond 30.0 eV photon energy is observed.

Magnetic phase characterization of nanocrystalline La2NiMnO6 using alternating current conductance

Abstract: The signature of various disordered phases is inferred from the measurement of the real part of alternating current conductance Σ(T, f) of a nanocrystalline double perovskite La2NiMnO6. The system exhibits a paramagnetic insulating (PMI) phase at high temperatures, a ferromagnetic insulating (FMI) phase at low temperatures, and a Griffiths-like phase in the intermediate temperature range. In these three phases, Σ(T, f) shows qualitatively similar variation with frequency f. At a fixed temperature T, Σ(T, f) remains constant to its Ohmic value Σ0 up to a certain frequency, known as the onset frequency fc and increases with increasing f beyond fc. Scaled appropriately, Σ(T, f) versus f data corresponding to these three regimes fall on the same master curve indicating the universal nature of the scaling behaviour of alternating current conductance. This onset frequency fc scales with Σ0 as fc∼Σ0xf with xf as the nonlinearity exponent. This exponent xf shows a gradual crossover from 1.025 ± 0.006 in FMI phase...

A first principles approach to magnetic and optical properties in single-layer graphene sandwiched between boron nitride monolayers

Abstract: The dependence of the stability of single-layer graphene (SLG) sandwiched between hexagonal boron nitride bilayers (h-BN) has been described and investigated for different types of stacking in order to provide the fingerprint of the stacking order which affects the optical properties of such trilayer systems. Considering the four stacking models AAA-, AAB-, ABA-, and ABC-type stacking, the static dielectric functions (in case of parallel polarizations) for AAB-type stacking possesses maximum values, and minimum values are noticed for AAA. However, AAA-type stacking structures contribute the maximum magnetic moment while vanishing magnetic moments are observed for ABA and ABC stacking. The observed optical anisotropy and magnetic properties of these trilayer heterostructures (h-BN/SLG/h-BN) can be understood from the crystallographic stacking order and inherent crystal lattice symmetry. These optical and magnetic results suggest that the h-BN/SLG/h-BN could provide a viable route to graphene-based opto-electronic and spintronic devices.

Dielectric Behavior in B1 and B2 Phases Composed of Unsymmetrical Bent Shaped Liquid Crystal Molecules

Abstract: Detail dielectric measurements were carried out on two different liquid crystalline (LC) samples formed by bent core (BC) molecules, showing B1 and B2 phases in the temperature range 100–150°C. The thermal variation of real part of dielectric permittivity (ϵ′) represents a tiny dip only in the saturated B1 phase which might arising from the dipole orientation disorder due to the inherent phase instability. The experimental data have been used to determine different relaxation parameters. Evidently only one relaxation mode has been detected in the dielectric relaxation spectrum for both phases, which is attributed to the long axis molecular rotation process.

Native defects and optical properties of Ar ion irradiated ZnO

Abstract: MeV Argon (Ar) ion irradiation turns white coloured ZnO to yellowish (fluence 1 × 10 14 ions/cm 2 ) and then reddish brown (1 × 10 16 ions/cm 2 ). At the same time the material becomes much more conducting and purely blue luminescent for the highest fluence of irradiation. To get insight on the defects in the irradiated samples Ultraviolet-visible (UV-vis) absorption, Raman, and photoluminescence (PL) spectroscopy and Glancing Angle X-Ray Diffraction (GAXRD) measurements have been carried out. Enhancement of overall disorder in the irradiated samples is reflected from the GAXRD peak broadening. UV-vis absorption spectra of the samples shows new absorption bands due to irradiation. Complete absorption in the blue region of the spectrum and partial absorption in the green and red region changes the sample colour from white to reddish brown. The Raman peak representing wurtzite structure of the ZnO material (~ 437 cm -1 ) has decreased monotonically with the increase of irradiation fluence. At the same time, evolution of the 575 cm -1 Raman mode in the irradiated samples shows the increase of oxygen deficient disorder like zinc interstitials (IZn) and/or oxygen vacancies (VO) in ZnO. PL spectrum of the yellow coloured sample shows large reduction of overall luminescence compared to the unirradiated one. Further increase of fluence causes an increase of luminescence in the blue region of the spectrum. The blue-violet emission can be associated with the interstitial Zn (IZn) related optical transition. The results altogether indicates IZn type defects in the highest fluence irradiated sample. Large changes in the electrical resistance and luminescent features of ZnO using Ar ion beam provides a purposeful way to tune the optoelectronic properties of ZnO based devices. Copyright © 2015 VBRI press.

Kardar-Parisi-Zhang universality class of a discrete erosion model

Abstract: We investigate a novel (d + 1)-dimensional discrete erosion model for d = 1, 2 and 3. The dynamics of the model is controlled by the physically motivated erosion mechanism. The coarse grained nature of this erosion process has been well compared with the Kardar–Parisi–Zhang (KPZ) equation. The kinetic roughening of the discrete model shows the same scaling behavior as that of the KPZ equation in the dimensions d = 1, 2. Moreover, in this present discrete model in (3 + 1)-dimension almost smooth interface has been obtained with vanishingly small roughness exponent, indicating the model belongs to the weak coupling regime of KPZ universality class.

Conductance of disordered graphene sheets: A real space approach

Abstract: In this work the conducting properties of graphene lattice (buckled as well as planar) having different concentrations of defects are studied with the help of real space block recursion method introduced by Haydock et al. Since the defects are completely random, reciprocal space based methods which need artificial periodicity are not applicable here. Different resonant states appear because of the presence of topological and local defects which are calculated within the framework of Green function. Except random voids, in all other density of states (DOS) spectra there are signatures of Breit–Wigner and Fano resonance at occupied and unoccupied regime respectively. Although Fano resonance states are not prominent for graphene with random voids, however Stone–Wales (SW) type defect can naturally introduce their resonance states. The appearance of localized states depends strongly on the concentration of defects.

- Electronic and Optical Properties of Boron- and Nitrogen-Functionalized Graphene Nanosheet

Abstract: Graphyne, another similar structure of graphene, consists of double-and triple-bonded units of carbon atoms. The presence of triple bonds in graphyne can have intriguing features, having other geometry instead of hexagonal as possessed by graphene.

Observation of nonuniversal scaling exponent in a novel erosion model

Abstract: In this present numerical work, we report a discrete erosion kind of model in (1 + 1)-dimension. Erosion and re-deposition phenomena with probabilities p and q(= 1 - p) are considered as two tunable parameters, which control the overall kinetic roughening behavior of the interface. Redeposition or diffusion dominated erosion like kinetic roughening model gives rise to nonuniversal growth exponent, which varies continuously with respect to erosion probability. However, universal character is restored for the roughness exponent with the value of 0.5 in (1 + 1)-dimension with respect to p. Due to nonuniversal nature of growth exponent, we observe a significant modification to the scaling behavior of surface width with respect to erosion probability. For low erosion probability (≲ 0.1) a power law like divergence has been observed of the correlation growth time. This can be argued as limiting behavior of a generalized functional behavior of crossover time with erosion probability.

Spatial Ergodicity and Configuration averaging in disordered systems

Abstract: We examine the concept of spatial ergodicity in disordered systems. Our aim is to understand why experimenters in some situations observe the configuration average of global properties in a single sample. We describe two techniques : one of which directly calculates the spatial average of a single sample, and another which explicitly obtains the configuration average over many samples with different samples. For graphenes with random voids we