Rajeev Gupta

Bio: Rajeev Gupta is an academic researcher from Rajasthan Technical University. The author has contributed to research in topics: Materials science & Raman spectroscopy. The author has an hindex of 33, co-authored 231 publications receiving 3704 citations. Previous affiliations of Rajeev Gupta include University College of Engineering & Post Graduate Institute of Medical Education and Research.

Laser-Induced Fano Resonance Scattering in Silicon Nanowires

Abstract: We have studied the impact of excitation laser power density on the Raman spectrum of small-diameter (5−15 nm) silicon nanowires At low power densities, a Lorentzian line is observed at 520 cm-1, the same value as that of the zone center LO (TO) phonon in bulk silicon With increasing laser illumination, the Raman band downshifts and asymmetrically broadens on the low-frequency side Our results contradict the traditionally accepted notion that a downshifted and asymmetrically broadened line in Si nanowires is due to quantum confinement effects Rather, we suggest that the downshifting can be due to a laser heating effect of the nanowire and that the asymmetric line shape is due to a Fano interference between scattering from the k = 0 optic phonon and electronic continuum scattering from laser-induced electrons in the conduction band

Surface Optical Phonons in Gallium Phosphide Nanowires

Abstract: Raman scattering studies of crystalline GaP nanowires reveal a strong additional peak in the first order spectrum that can be clearly assigned to surface optical (SO) phonons. The frequency of this SO peak is found to be sensitive to the dielectric constant of the surrounding medium in which the nanowire is embedded. A theory for the SO phonons in cylindrical wires is presented, the SO mode dispersion ω(q) and the experimental peak frequency are then used to predict the wavelength of the dominant Fourier component of the surface potential responsible for activating the SO mode. Interestingly, this SO phonon wavelength is found to agree with the wavelength of diameter modulation observed for some nanowires.

Raman study of stoichiometric and Zn-doped Fe 3 O 4

Abstract: We report here our Raman studies on Fe3–xZnxO4(x = 0,0.015,0.03) across the Verwey transition in the temperature range 20–300 K. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Zn doping. Allen's formula has been used to estimate the strength of electron-phonon interaction from the observed lineshape parameters. These estimates show that there is strong electron-phonon coupling in this system and is highest for the T mode in comparison to A1g and T modes.

Diameter-selective resonant Raman scattering in double-wall carbon nanotubes

Abstract: Double-wall carbon nanotubes (DWNT's) have been studied by Raman scattering using different excitation wavelengths and their spectra compared to those of single wall nanotubes (SWNT's) and ${\mathrm{C}}_{60}$-SWNT peapods. Raman scattering from the radial and tangential vibrational modes of very small diameter $d\ensuremath{\sim}0.6--0.9 \mathrm{nm}$ secondary (interior) semiconducting tubes within the DWNT can be unambiguously identified with 647.1 and 1064 nm excitations. The frequency of the tangential displacement vibrational modes identified with these secondary (interior) tubes is found to be downshifted by $\ensuremath{\sim}7 {\mathrm{cm}}^{\ensuremath{-}1}$ relative to that of the larger primary (exterior) tubes that exhibit a diameter $d\ensuremath{\sim}1.3--1.6 \mathrm{nm}.$ This downshift strongly suggests that at small tube diameters (i.e., $d\ensuremath{\sim}0.7 \mathrm{nm}),$ the associated wall curvature of the nanotube may require an admixture of ${\mathrm{sp}}^{3}$ character in the C-C interaction. Our results also show that the value ${\ensuremath{\gamma}}_{0}=2.90 \mathrm{eV}$ for the nearest C-C tight binding integral is consistent with the resonant enhanced Raman scattering from DWNT's.

Chemically doped double-walled carbon nanotubes: cylindrical molecular capacitors.

Abstract: A double-walled carbon nanotube is used to study the radial charge distribution on the positive inner electrode of a cylindrical molecular capacitor. The outer electrode is a shell of bromine anions. Resonant Raman scattering from phonons on each carbon shell reveals the radial charge distribution. A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.

Chemistry of Carbon Nanotubes

Abstract: Department of Materials Science, University of Patras, 26504 Rio Patras, Greece, Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, 116 35 Athens, Greece, Institut de Biologie Moleculaire et Cellulaire, UPR9021 CNRS, Immunologie et Chimie Therapeutiques, 67084 Strasbourg, France, and Dipartimento di Scienze Farmaceutiche, Universita di Trieste, Piazzale Europa 1, 34127 Trieste, Italy

Graphene: The New Two-Dimensional Nanomaterial

Abstract: Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high-temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half-integer quantum Hall effect and ballistic electron transport. This two-dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two-layer and few-layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.