A. K. Tyagi

Conductivity landscape of highly oriented pyrolytic graphite surfaces containing ribbons and edges

We present an extensive study on electrical spectroscopy of graphene ribbons and edges of highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). We have addressed in the present study two main issues (1) How does the electrical property of the graphite (graphene) sheet change when the graphite layer is displaced by shear forces? and (2) How does the electrical property of the graphite sheet change across a step edge? While addressing these two issues we observed (1) variation of conductance among the graphite ribbons on the surface of HOPG. The top layer always exhibits more conductance than the lower layers, (2) two different monolayer ribbons on the same sheet of graphite shows different conductance, (3) certain ribbon/sheet edges show sharp rise in current, (4) certain ribbons/sheets on the same edge shows both presence and absense of the sharp rise in the current, and (5) some lower layers at the interface near a step edge shows a strange dip in the current/conductance (depletion of charge). We discuss possible reasons for such rich conducting landscape on the surface of graphite.

Enhanced conductivity in graphene layers and at their edges

We have observed that the conductivity of graphene sheets is higher whenever they are loosely bound to the underlying bulk graphite. We also observe that certain edges of the graphene layers show sharp rise in current when biased, indicating enhanced electronic density of states spatially localized near those edges. In certain edges, we do not observe this phenomenon. These two observations, i.e., enhancement of conductivity of loosely bound layers and sharp rise in current at the edges are discussed with possible reasons and invoking recent theoretical predictions.

Raman spectroscopic and photoluminescence investigations on laser surface modified α-Al2O3 coatings

Abstract Laser surface modification of plasma sprayed alumina coatings were carried out for obtaining desired structural and microstructural transformations. Appearance of Raman modes was used to detect the transformation of Al2O3 from γ to α phase. Photoluminescence due to the natural occurrence of Cr3+ ions was used to probe the densification and enhancement of the surface quality of the coatings brought about by laser treatment.

Multiphonon Raman scattering in GaN nanowires

UV Raman scattering studies show longitudinal optical (LO) mode up to fourth order in wurtzite GaN nanowire system. Frohlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ∼3nm.

Plasmon-mediated, highly enhanced photocatalytic degradation of industrial textile dyes using hybrid ZnO@Ag core–shell nanorods

Hybrid ZnO@Ag core–shell heterojunction nanorods were synthesized using a novel, facile two-step process based on hydrothermal and seed mediated growth techniques. The material was characterized by UV-visible spectroscopy, Fourier transform-infrared spectroscopy (FT-IR), room temperature photoluminescence spectroscopy (RTPL), Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The hybrid ZnO@Ag core–shell nanorods were comprised of one-dimensional (1D) ZnO nanorods serving as a core material, over which surface-doped Ag nanoclusters (∼2.5 nm) were anchored as a heterogeneous shell. The presence of oxygen vacancies and Zn interstitials were confirmed by RTPL and Raman spectroscopic analysis. The photocatalytic activity of the hybrid ZnO@Ag core–shell nanorods was studied in comparison to bare ZnO nanorods using standard R6G dye and industrial textile dyes such as Congo red and Amido black 10B under UV and visible light (solar) irradiations. Moreover, the material was tested for real time industrial textile effluents under ambient conditions and was found to be highly efficient. The enhanced photocatalytic property observed for ZnO@Ag hybrid core–shell nanorods is attributed to a phenomenal increase in oxygen related defects in the core that generate photo-induced charge carriers and the presence of plasmonic Ag nanoclusters in the shell, which act as a sink for the photo-induced charge carriers.

Direct label free ultrasensitive impedimetric DNA biosensor using dendrimer functionalized GaN nanowires

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, γ-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(-8) to 10(-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.

Nanocrystalline and metastable phase formation in vacuum thermal decomposition of calcium carbonate

Abstract Well characterised, polycrystalline powders of commercially procured CaCO 3 were thermally decomposed in the vacuum as well as in the flowing gas atmosphere for the purpose of studying solid state transformations. The characterisation of the end product CaO, obtained from the thermal decomposition, revealed contrasting features in the powder X-ray diffractograms. While the flowing gas method, conducted inside a thermogravimetric analyser (TGA), indicated formation of stable microcrystalline calcia, the decomposition under dynamic vacuum revealed formation of metastable-nanocrystalline calcia. The latter study was carried out in an evolved gas analysis-mass spectrometry (EGA-MS) facility. Experiments were also conducted inside the high temperature XRD (HTXRD) machine. The paper attempts to bring out possible mechanisms responsible for formation of these end products with such glaring structural contrast. Non-equilibrium conditions prevalent under dynamic vacuum condition as well as misfit strain energy available from CaCO 3 /CaO interface are presumed to be the reason behind such metastable transformations. Kinetic analysis of the transformation revealed prevalence of nucleation and growth phenomena. Corresponding Arrhenius factors were also calculated.

Laser induced structural and microstructural transformations of plasma sprayed Al2O3 coatings

Abstract Air plasma sprayed Al2O3 coatings on metallic substrates suffer from porosities and metastable polymorphs. In this study, laser treatment employed to improve the coating properties resulted in transformation of the as-sprayed coating predominantly consisting of metastable γ-Al2O3 into thermodynamically stable α-Al2O3 phase with enhanced microhardness and reduced surface roughness.

Surface optical modes in GaN nanowires

We investigate the optical phonons in crystalline GaN nanowires using Raman spectroscopy. Reduced phonon lifetime in the nanostructures is attributed to the increased anisotropy in lattice vibrations. Apart from the group theoretically allowed optical phonons, new phonon modes around 652 cm−1 and 691 cm−1 have been observed. In view of its good agreement with values in GaN, the observed phonon mode is assigned as surface optical (SO) phonon. This could be attributed to the surface modulation along the GaN nanowire diameter and it is quantitatively evaluated with observed surface morphology and the calculated dispersion relation corresponding to SO phonon modes. The modulation in the surface morphology, observed in the present study, is typical of the vapour-liquid-solid growth process. The instability in the surface phonon potential activates the SO phonon modes, which is well explained in this present study.

Humidity-dependent friction mechanism in an ultrananocrystalline diamond film

Friction behaviour of an ultrananocrystalline diamond film deposited by the microwave plasma-enhanced chemical vapour deposition technique is studied in a controlled humid atmosphere. The value of friction coefficient consistently decreases while increasing the humidity level during the tribology test. This value is 0.13 under 10% relative humidity conditions, which is significantly decreased to 0.004 under 80% relative humidity conditions. Such a reduction in friction coefficient is ascribed to passivation of dangling covalent bonds of carbon atoms, which occurs due to the formation of chemical species of hydroxyl and carboxylic groups such as C?COO, CH3COH and CH2?O bonding states.