S. Dhara | Homi Bhabha National Institute (original) (raw)

Papers by S. Dhara

Journal of Experimental Nanoscience, 2013

Journal of Nanoscience and Nanotechnology, 2009

Confined acoustic and optical phonons in Si nanoclusters embedded in sapphire, synthesized using ... more Confined acoustic and optical phonons in Si nanoclusters embedded in sapphire, synthesized using ion-beam implantation are investigated using Raman spectroscopy. The l = 0 and l = 2 confined acoustic phonons, found at low Raman shift, are analyzed using complex frequency model and the size of the nanoparticles are estimated as 4 and 6 nm. For the confined optical phonon, in contrast to expected red shift, the Raman line shape shows a substantial blue shift, which is attributed to size dependent compressive stress in the nanoparticles. The calculated Raman line shape for the stressed nanoparticles fits well to data. The sizes of Si nanoparticles obtained using complex frequency model are consistent with the size estimated from the fitting of confined optical phonon line shapes and those found from X-ray diffraction and TEM.

Bulletin of Materials Science, 1995

Medium resolution magneto-optic Faraday rotation measurement setup has been reported. Measurement... more Medium resolution magneto-optic Faraday rotation measurement setup has been reported. Measuremental setup reported is made up of simple polarized laser source, beam expander, analyzer and a bisected cross polarized p n detector for single ended measurements. Result of CoO modified yttrium iron garnet (YIG) films showed a rotation of 3 deg//~m which is comparable to earlier reported Co-doped YIG films.

Journal of Raman Spectroscopy, 2009

Journal of Nanoparticle Research, 2013

ABSTRACT Understanding the mechanism of nanoparticle (NP) induced toxicity in microbes is of pote... more ABSTRACT Understanding the mechanism of nanoparticle (NP) induced toxicity in microbes is of potential importance to a variety of disciplines including disease diagnostics, biomedical implants, and environmental analysis. In this context, toxicity to bacterial cells and inhibition of bioﬁlm formation by GaN NPs and their functional derivatives have been investigated against gram positive and gram negative bacterial species down to single cellular level. High levels of inhibition of bioﬁlm formation ([80 %) was observed on treatments with GaN NPs at sub-micro molar concentrations. These results were substantiated with morphological features investigated with ﬁeld emission scanning electron microscope, and the observed changes in vibrational modes of microbial cells using Raman spectroscopy. Raman spectra provided molecular interpretation of cell damage by registering signatures of molecular vibrations of individual living microbial cells and mapping the interplay of proteins at the cell membrane. As compared to the untreated cells, Raman spectra of NP-treated cells showed an increase in the intensities of characteristic protein bands, which conﬁrmed membrane damage and subsequent release of cellular contents outside the cells. Raman spectral mapping at single cellular level can facilitate understanding of the mechanistic aspect of toxicity of GaN NPs. The effect may be correlated to passive diffusion causing mechanical damage to the membrane or ingress of Ga3? (ionic radius*0.076 nm) which can potentially interfere with bacterial metabolism, as it resembles Fe2? (ionic radius*0.077 nm), which is essential for energy metabolism.

Crystal Growth & Design, 2012

Self-catalytic growth of GaN nanotips and nanoparticles, grown by chemical vapor deposition techn... more Self-catalytic growth of GaN nanotips and nanoparticles, grown by chemical vapor deposition technique, are investigated. Three important parameters, comprised of incubation time, anisotropy of diffusion, and rate-limiting factors of Ga and N adatoms migration over polar and nonpolar surfaces, are found to play significant roles in determining the final morphology of these nanostructures. Nucleation of GaN nanotips takes place under Ga-rich conditions. As the reaction proceeds, the stochiometry changes occur as a result of a shift in Ga-rich to N-rich conditions on the surface. In all of these cases, the growth continues to be in vapor−solid mode. The conical shape of the nanotips is explained in terms of differential growth in the reduced surface diffusion of Ga under N-rich conditions on polar surfaces (0001) relative to nonpolar surfaces (101̅ 0). Nanoparticles are grown initially in N-rich conditions with significantly shorter incubation times. A mechanistic approach that expounds evolution of nanotips and nanoparticles is elucidated in details using crystallographic and electronic structural studies.

Biosensors and Bioelectronics, 2013

We demonstrate a very simple and generic protocol for ultrasensitive in-situ label-free detection... more We demonstrate a very simple and generic protocol for ultrasensitive in-situ label-free detection of DNA hybridization using third generation poly(amidoamine)dendrimer (G3-PAMAM) functionalized GaN nanowires (NWs). PAMAM modified GaN NWs provides large density of docking site to immobilize significant number of probe (p-) DNA covalently. These p-DNA/PAMAM/GaN NWs sensor probes are employed to achieve an ultra-high detection limit down to attomolar level concentration of complementary target (t-) DNA. Comparative in-situ studies on single/triple base-pair mismatched, g-irradiated and complementary t-DNA in the hybridization process reveal selectivity and specificity of the p-DNA/PAMAM/GAN NWs sensor probe over a wide range, 10 À 8 to 10 À 19 M, of analyte concentration. During the hybridization process, there is a substantial change in t-DNA concentration dependent interfacial polarization resistance during electrochemical impedance measurement, which forms the basis of the present DNA biosensor. This novel methodology for specific DNA sequence detection, as compared with the existing methods, is found to be very robust, highly sensitive, and reproducible.

Applied Physics Letters, 2005

Applied Physics Letters, 2008

We report recrystallization of epitaxial (epi-) GaN(0001) film under indentation. Hardness value ... more We report recrystallization of epitaxial (epi-) GaN(0001) film under indentation. Hardness value is measured ~10 GPa, using a Berkovich indenter. 'Pop-in' burst in the loading line indicates nucleation of dislocations setting in plastic motion of lattice atoms under stress field for the recrystallization process. Micro-Raman studies are used to identify the recrystallization process. Raman area mapping indicates the crystallized region. Phonon mode corresponding to E 2 (high) ~570 cm-1 in the as-grown epi-GaN is redshifted to stress free value ~ 567 cm-1 in the indented region. Evolution of A 1 (TO) and E 1 (TO) phonon modes are also reported to signify the recrystallization process.

Applied Physics Letters, 2010

Indentation at very low load rate showed region of constant volume with releasing load in crystal... more Indentation at very low load rate showed region of constant volume with releasing load in crystalline ͑c-͒Si, indicating a direct observation of liquidlike amorphous phase which is incompressible under pressure. Signature of amorphization is also confirmed from load dependent indentation study where increased amount of amorphized phase is made responsible for the increasing elastic recovery of the sample with increasing load. Ex situ Raman study confirmed the presence of amorphous phase at the center of indentation. The molecular dynamic simulation has been employed to demonstrate that the effect of indentation velocities has a direct influence on c-Si during nanoindentation processes.

Plasmonics, 2014

Photoresponse of Au nanoparticle functionalized semiconducting GaN (Au-GaN) nanowires is reported... more Photoresponse of Au nanoparticle functionalized semiconducting GaN (Au-GaN) nanowires is reported for an optical switching using 532 excitation. Wide band gap GaN nanowires are grown by catalyst assisted chemical vapour deposition technique and functionalized with Au in the chemical route. Au-GaN nanowires show surface plasmon resonance (SPR) mode of Au nanoclusters around 550 nm along with characteristic band for GaN around 365 nm. An optical switching is observed for Au-GaN nanowires with a sub-band gap excitation of 532 nm suggesting possible role of surface plasmon polariton assisted transport of electron in the system. Role of band conduction is ruled out in the absence of optical switching using 325 nm excitation which is higher in energy that the reported band gap of GaN ~ 3.4 eV (365 nm) at room temperature. A finite amount of interband contribution of Au plays an important role along with the inter-particle separation. The switching device is also successfully tested for a single GaN nanowire functionalized with Au nanoclusters. A resistivity value of 0.05 -cm is measured for surface plasmon polariton assisted electrical transport of carrier in the single GaN nanowire.

Journal of Materials Chemistry C, 2013

Manipulation of surface architecture of semiconducting nanowires with a control in surface polari... more Manipulation of surface architecture of semiconducting nanowires with a control in surface polarity is one of the important objectives for nanowire based electronic and optoelectronic devices for commercialization. We report the growth of exceptionally high structural and optical quality nonpolar GaN nanowires with controlled and uniform surface morphology and size distribution, for large scale production. The role of O contamination (~1-10 5 ppm) in the surface architecture of these nanowires is investigated with the possible mechanism involved. Nonpolar GaN nanowires grown in O rich condition show the inhomogeneous surface morphologies and sizes (50-150 nm) while nanowires are having precise sizes of 40(+5) nm and uniform surface morphology, for the samples grown in O reduced condition. Relative O contents are estimated using electron energy loss spectroscopy studies. Size-selective growth of uniform nanowires is also demonstrated, in the O reduced condition, using different catalyst sizes. Photoluminescence studies along with the observation of single-mode waveguide formation, as far field bright violet multiple emission spots, reveal the high optical quality of the nonpolar GaN nanowires grown in the O reduced condition.

International Journal of Hydrogen Energy, 2015

ABSTRACT TiO2 (P25) decorated reduced graphene oxide (rGO) is synthesized by γ-radiolytic techniq... more ABSTRACT TiO2 (P25) decorated reduced graphene oxide (rGO) is synthesized by γ-radiolytic technique using water-ethanol solvent medium. Visible light absorption was confirmed by UV–Visible spectroscopy. Photoluminescence (PL) study revealed a decline in electron-hole recombination rate signalled by a sharp fall in luminescence of TiO2. Under such scenario, electron transfer from TiO2 conduction band to the conjugated sheet becomes a preferred pathway in the presence of rGO, which is further confirmed by photocurrent measurement. TiO2 -rGO composite with 1 wt. % rGO was found to be the best composition in terms of visible light absorption, while retaining TiO2 crystallinity. Transmission electron microscopy and Raman spectroscopic studies confirmed the coating of rGO sheet with TiO2 nanoparticles. TiO2 -rGO was found to show about 150 times higher photocatalytic H2 generation under 250 W UV–Visible light irradiation than pristine TiO2. In addition to this, TiO2 -rGO composite shows hydrogen production yield of 35 μmol/h.g under visible light (λ = 400 nm) irradiation. Thus γ-radiolysis can be considered to be a viable technique, which helps in synthesizing photocatalytically active nanocomposite with high H2 evolution rate. In the light of these interesting aspects, a detail investigation is carried out to study the effect of rGO concentration and illumination time on photocatalytic yield.

Science of Advanced Materials, 2013

Modulation of surface wettability is demonstrated using SnO 2 nanoparticles (NPs) upon surface fu... more Modulation of surface wettability is demonstrated using SnO 2 nanoparticles (NPs) upon surface functionalization. Quantum sized SnO 2 NPs synthesized by soft chemical route are annealed under ambient conditions at different temperatures to increase grain size as revealed by field emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. These NPs were treated in a cyclohexane medium with octadecyltrichlorosilane (OTS) to create an organic/inorganic interface. Atomic force microscopy reveals increased roughness of functionalized films with increasing size of SnO 2 NPs, whereas Raman studies illustrate ordered OTS molecules on SnO 2 surfaces forming a hetero-structure interface. Water contact angle measurements on spin coated plain SnO 2 NPs films show distinct hydrophilic nature with a contact angle (CA) of 15 ± 2. Creation of an interface with OTS with these films show hydrophobicity with a minimum contact angle of 137 for the quantum sized SnO 2 NPs. Surface wettability is further modulated by introducing enhanced surface roughness with increasing size of SnO 2 NPs and clusters in the OTS treated films. Super hydrophobicity (CA > 150) is clearly demarcated after silanization of SnO 2 NPs of size of 12 nm and above. Within the frame work of Cassie-Baxter model, super hydrobhobicity is explained based on surface roughness and geometrical architectures.

Radiation Physics and Chemistry, 1998

International Conference on Nanoscience, Engineering and Technology (ICONSET 2011), 2011

GaN nanostructures with different morphologies are grown on few layer graphene (FLG) as template,... more GaN nanostructures with different morphologies are grown on few layer graphene (FLG) as template, using chemical vapor-deposition technique in a self catalytic process using the large surface energy of graphene. Raman and photoluminescence studies reveal wurtzite GaN phase. Morphologies of these nanostructures varied depending on the number of layers in each template. Photoluminescence study reveals that growth occurs without deterioration of FLG layers and no incorporation of carbon in GaN nanostructures Keywords-gallium nitride; GaN; graphene; nanostructures; Raman spectroscopy; photoluminescence I.

Solid State Communications, 2008

Raman spectroscopic investigations are carried out on ZnO nanoparticles for various photon energi... more Raman spectroscopic investigations are carried out on ZnO nanoparticles for various photon energies. Intensities of E 1-LO and E 2 modes exhibit large changes as the excitation energy varied from 2.41 to 3.815 eV, signifying substantially large contribution of Frohlich interaction to the Raman polarizability as compared to deformation potential close to the resonance. Relative strength of these two mechanisms is estimated for the first time in nanoparticles and compared with those in the bulk.

Applied Physics Letters, 2014

Role of "O" defects in sensing pollutant with nanostructured SnO 2 is not well understood, especi... more Role of "O" defects in sensing pollutant with nanostructured SnO 2 is not well understood, especially at low temperatures. SnO 2 nanoparticles were grown by soft chemistry route followed by subsequent annealing treatment under specific conditions. Nanowires were grown by chemical vapor deposition technique. A systematic photoluminescence (PL) investigation of "O" defects in SnO 2 nanostructures revealed a strong correlation between shallow donors created by the in-plane and the bridging "O" vacancies and gas sensing at low temperatures. These SnO 2 nanostructures detected methane (CH 4), a reducing and green house gas at a low temperature of 50 C. Response of CH 4 was found to be strongly dependent on surface defect in comparison to surface to volume ratio. Control over "O" vacancies during the synthesis of SnO 2 nanomaterials, as supported by X-ray photoelectron spectroscopy and subsequent elucidation for low temperature sensing are demonstrated. V