Praveen Kumar

Ga induced superstructures as templates for lattice matched hetroepitaxial growth of GaN on Si(111) substrate

High quality GaN is grown by plasma assisted molecular beam epitaxy on Ga induced superstructural phases of Si(111)7×7. Three stable surface phases induced by Ga adsorption, viz., (1×1), (6.3×6.3), and (√3×√3)R30°, are employed as templates to grow epitaxial (0001) GaN thin films. GaN grown on Si(√3×√3)R30°-Ga is found to be highly crystalline with intense (0002) x-ray diffraction and photoluminescence peaks with low full width at half maximum, low surface roughness, and stoichiometric surface composition. The high quality of these GaN films formed at a low temperature of 400 °C is explained by the integral (×2) lattice matching between the unit cell of GaN and the (√3×√3) phase. The experiments demonstrate a plausible approach of adsorbate induced surface modifications as templates for III-V hetroepitaxy on Si surfaces.

(7×7) reconstruction as barrier for Schottky-barrier formation at the Ga/Si(111) interface

We report the change in electronic properties of the Ga/Si interface by monitoring the Ga(3d) core-level photoelectron spectra and electron diffraction induced by submonolayer Ga adsorption on Si(111)-7×7 surface. The spectra shows a flat band for submonolayer coverages, attributed to the metallic nature of the Si(111)-7×7 reconstruction and a premetallic band structure of two-dimensional Ga islands. At 1 ML, electron diffraction pattern shows metallic (7×7) to semiconducting (1×1) phase-transition and the spin-orbit split branching ratio of Ga(2p) core level attain the metallic bulk value, and the barrier assumes the Schottky–Mott value while full width half maxima and branching ratio attain bulk values.

Selective electrochemical sensing for arsenite using rGO/Fe3O4 nanocomposites.

Herein, we report rGO/Fe3O4 nanocomposites (NCs) free from noble metals, synthesized by facile one step chemical reduction method, for electrochemical detection of arsenite in water by square wave anodic stripping Voltammetry (SWASV). The synthesized NCs were characterized for its optical, morphological and structural properties. The NCs modified glassy carbon (GCE), NCs/GCE, electrodes showed a higher sensitivity (0.281μA/ppb) and lower LOD (0.12ppb) under optimized experimental conditions. The proposed NCs/GCE electrodes show no interference towards arsenite species in the presence of common cationic interferants, namely, Cu(II), Pb(II), Ni(II), Co(II), Cd(II), Cr(II), Zn(II), etc. In addition, the proposed electrode demonstrates a good stability, reproducibility and potential practical application in electrochemical detection of arsenite.

Waste derivitized blue luminescent carbon quantum dots for selenite sensing in water

Herein, we report an environmental friendly, facile, and completely green synthetic method for producing carbon quantum dots (CQDs) from whey, a major dairy waste. The as-prepared monodispersed diameter CQDs exhibit blue luminescence with noteworthy quantum yield (~11.4%) and excitation dependent emission behaviour. Nuclear magnetic resonance (NMR) analysis reveals the presence of aromatized carbon peaks, leading to polymerized CQDs diameter architecture during whey pyrolysis. The X-ray and selected area electron diffraction patterns confirm their amorphous nature. Further, we demonstrate, these CQDs as an effective sensor probe for selective selenite monitoring in water upon functionalization with appropriate ligand. The functionalized GCQDs probe is shown to detect selenite with high sensitivity in 10-1000ppb detection range. Further it is selective for selenite over other relevant ions (such as Cu2+, As3+, As5+, Pb2+, Ni2+, Se6+, Cl-, Br-, NO3-, NO2- and F-) and displays a sub-ppb detection limit at 1.1% relative standard deviation.

Structural and magnetic properties of Co1−xFexSr2YCu2O7+δ compounds

Here we study the structural and magnetic properties of the Co1−xFexSr2YCu2O7+δ compound (0≤x≤1). X-ray diffraction patterns and simulated data obtained from Rietveld refinement of the same indicate that the iron ion replacement in Co1−xFexSr2YCu2O7+δ induces a change in crystal structure. The orthorhombic Ima2 space group structure of Co-1212 changes to tetragonal P4/mmm with increasing Fe (x≥0.5) ion. The XPS studies reveal that both Co and Fe ions are in mixed states of 3+/4+ for the former and 2+/3+ in case of later. The magnetization with temperature follows Curie–Weiss behavior, in the range of 150–300 K and short magnetic correlations/spin glasslike features below 150 K. The observed magnetic behavior is due to competition of antiferro/ferromagnetic exchange interaction of Co3+ [intermediate spin (IS)]-O–Co3+ (IS)/Co4+ [low spin (LS)] and Fe3+ [high spin (HS)]-O–Fe2+ (LS)/Fe3+ (HS)/Co3+ (IS)/Co4+ (LS) states. Although none of the studied as synthesized samples in Co1−xFexSr2YCu2O7+δ are superconduc...

X-ray photoelectron spectroscopic study of silicon surface passivation in alcoholic iodine and bromine solutions

We report an X-ray photoelectron spectroscopic (XPS) study of silicon surfaces passivation using alcoholic solutions of iodine and bromine where different behavior with two systems is observed. The minority carrier lifetime determined by microwave photoconductive decay method showed better surface passivation for iodine-alcoholic system with HF preconditioning step. The iodine–ethanol (I–E) passivated samples show strong Si–I bonding (two times) in Si core level spectra for the samples without oxide (25.5%) compared with oxide (10.2%) counterpart. However, bromine–ethanol (B–E) passivated samples show higher Si–Br bonding strength in the samples with oxide (24.7%) compared to without oxide specimens (12.0%). This may be the reason of difference in passivation behavior of I–E and B–E systems. Higher O–Br bonding in O core level spectra of B–E passivated samples with oxide (35.8%), compared to without oxide (20.7%), results in comparable lifetime values in both with and without preconditioning. To understand the effect of solvent on the passivation, experiments are performed using iodine–methanol (I–M) and bromine–methanol (B–M) solutions and XPS analysis shows similar Si–I, Si–Br, and O–Br bonding trends.

Phase dependence of secondary electron emission at the Cs-Sb-Si (111) interface

The multi‐alkali antimonides adsorption on Si (111) surface has drawn much attention of several surface science studies due to its importance in both, fundamental and technological aspects of night vision devices & photocathodes. We report the formation of alkali metal antimonide ternary interface on Si(111)‐7×7 surface and in‐situ characterization by X‐ray Photoelectron Spectroscopy (XPS). The results show that Cs adsorption on clean Si(111) surface follows the layer‐by‐layer (Frank van der Merwe) growth mode at low flux rate, while Sb grows as islands (Volmer‐Weber) on Cs/Si surface. The changes in the Si (2p) and Cs (3d) core level spectra show the formation of a ternary interface (Sb/Cs/Si) at room temperature, which is further confirmed by changes in the density of states in the valence band spectra. The temperature controlled desorption of ternary interface, by monitoring the chemical species remnant on the surface after annealing at different temperatures, reveal that the Sb islands desorb at 750° C, which implies a stronger Cs‐Si bond to Cs‐Sb bond. The work function changes from 3.9 eV to 0.8 eV for Cs adsorption on Si, which further reduces to 0.65 eV after Sb adsorption on the Cs/Si interface. The changes in work function corresponds to the compositional and chemical nature of the interface and thus indicate that the secondary electron emission is an extremely phase dependent phenomena.The multi‐alkali antimonides adsorption on Si (111) surface has drawn much attention of several surface science studies due to its importance in both, fundamental and technological aspects of night vision devices & photocathodes. We report the formation of alkali metal antimonide ternary interface on Si(111)‐7×7 surface and in‐situ characterization by X‐ray Photoelectron Spectroscopy (XPS). The results show that Cs adsorption on clean Si(111) surface follows the layer‐by‐layer (Frank van der Merwe) growth mode at low flux rate, while Sb grows as islands (Volmer‐Weber) on Cs/Si surface. The changes in the Si (2p) and Cs (3d) core level spectra show the formation of a ternary interface (Sb/Cs/Si) at room temperature, which is further confirmed by changes in the density of states in the valence band spectra. The temperature controlled desorption of ternary interface, by monitoring the chemical spec...

Formation of low-dimensional GaN on trenched Si(5 5 12), probed by STM and XPS

We report the formation of self-assembled nanostructures of GaN, with controlled size and shape on the trenched planar Si (5 5 12) surface. Adsorbing low coverages of Ga on Si (5 5 12) forms 1D arrays of Ga adatoms. The Ga adsorbed Si surface is annealed to 300 °C, which results in the formation of Ga 2D nanoparticles (NPs). These Ga NPs were exposed to various fluence of energetic 2 keV ions followed by annealing which yields GaN nanostructures self-assembled along the direction. These studies were performed in ultrahigh vacuum using in situ scanning tunneling microscopy and ex situ x-ray photoelectron spectroscopy, to observe the structural and chemical evolution of the interface.

Growth and Annealing Influence on Structural and Optical Properties of Nanostructured ZIO Thin Films

Zinc‐Indium‐Oxide (ZIO) thin films were deposited on glass substrate with varying concentrations (ZnO:In2O3‐100:0, 90:10, 70:30 and 50:50 wt%) at room temperature by flash evaporation technique. These deposited ZIO film were annealed at 450 °C in vacuum. These films were characterized to study the effect of annealing on the structural and optical properties. Atomic force microscopy (AFM) images manifest the surface morphology of these ZIO thin films. The apparent growth of surface features revealed the formation of nanostructured ZIO thin films. The optical transmittance found to be decreased however blue shift has been observed after annealing.