Godhuli Sinha

Recyclable SERS Substrates Based on Au-Coated ZnO Nanorods

Vertically aligned Au-coated ZnO nanorods (Au-ZnO NRs) were investigated as cheap, efficient and recyclable SERS-active substrates. The ZnO NRs were prepared through a simple, low-temperature hydrothermal route and made SERS-active through deposition of gold nanoislands by sputtering at room temperature. Optimized samples were able to detect methylene blue over a wide range of low concentrations (from 1 × 10(-4) to 1 × 10(-12) M), with good reproducibility. The photocatalytic properties of Au-ZnO NRs were exploited to recycle these substrates through UV-assisted cleaning. The experimental results showed that these substrates are characterized by high reproducibility and long shelf life, which make them promising as SERS platforms for multiple detection of different molecular species.

Enhanced Magnetic and Dielectric properties of Eu and Co co-doped BiFeO3 nanoparticles

Bi1−xEuxFe1−yCoyO3 (x = 0, 0.01; y = 0, 0.01) nanoparticles, having an average size of 13 nm, were prepared by a simple sol gel route. Strong electronegativity of Eu3+ and smaller oxidation-reduction potential of Co3+/Co2+ (0.55 eV) than Fe3+/Fe2+ (1.3 eV) increase the concentration of Fe3+ ions with doping. Distinct magnetic hysteresis and complete saturation of magnetisation indicate the presence of ferromagnetic phase. The successful co-doping of Eu and Co into BiFeO3 (BFO) lattice dramatically enhances the saturation magnetization (Ms) and coercivity (Hc) by about 20 times than that of pure BiFeO3. A large value of dielectric constant of about 650, low loss (<0.001), and small leakage current density (1.79 × 10−8 A/cm2) are observed for the co-doped sample.

Hybrid Colloidal Au-CdSe Pentapod Heterostructures Synthesis and their Photocatalytic Properties

In this report, we present a self-driven chemical process to design exclusive Au/CdSe pentapod heterostructures with Au core and CdSe arms. We have analyzed these heterostructures using high-resolution transmission electron microscope (HRTEM), high angle annular dark field-scanning transmission electron microscopic (HAADF-STEM), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) studies. Microscopic studies suggest that pentapod arms of CdSe are nucleated on the (111) facets of Au and linearly grown only along the [001] direction. From the XPS study, the shifting of peak positions in the higher binding energy region for Au/CdSe heterostructures compared to Au nanoparticles has been found which indicates the charge transfer from CdSe to Au in heterostructures. The steady state and time resolved spectroscopic studies unambiguously confirm the electron transfer from photoexcited CdSe to Au, and the rate of electron transfer is found to be 3.58×10⁸ s⁻¹. It is interesting to note that 87.2% of R6G dye is degraded by the Au/CdSe heterostructures after 150 min UV irradiation, and the apparent rate constant for Au/CdSe heterostructures is found to be 0.013 min⁻¹. This new class of metal-semiconductor heterostructures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

Bright white light emitting Eu and Tb co-doped monodisperse In2O3 nanocrystals

Bright light emitting monodisperse In2−(x+y)EuxTbyO3 (x = 0.05, 0.1, 0.15, 0.2 and y = 0.05, 0.1, 0.15) nanocrystals were successfully synthesized by a versatile hot-injection colloidal route. The X-ray diffraction and X-ray photoelectron spectroscopy studies clearly showed that Eu and Tb were incorporated into the In2O3 lattice. Surface adsorbed NO3− and Cl− anions of Eu3+ and Tb3+ precursors assist the formation of flower like morphology through the oriented attachment process. Optical absorption spectra have been interpreted in terms of 4f → 5d (Eu, Tb) and 2p(O) → 5d (Eu, Tb) interband transitions. The observation of characteristic emission peaks related to Eu3+ and Tb3+ by indirect excitation suggests that In2O3 is an efficient sensitizer. Proper tuning of the compositions, 15% of Eu and 10% of Tb, gives the most intense white light and excellent chromaticity co-ordinates (0.35, 0.35). The shortening of lifetime with Eu and Tb doping improves the energy transfer from the host In2O3 to dopant Eu3+ and Tb3+ ions.

Controlled growth of well-aligned GaS nanohornlike structures and their field emission properties.

Here, we report the synthesis of vertically aligned gallium sulfide (GaS) nanohorn arrays using simple vapor-liquid-solid (VLS) method. The morphologies of GaS nano and microstructures are tuned by controlling the temperature and position of the substrate with respect to the source material. A plausible mechanism for the controlled growth has been proposed. It is important to note that the turn-on field value of GaS nanohorns array is found to be the low turn-on field 4.2 V/μm having current density of 0.1 μA/cm(2). The striking feature of the field emission behavior of the GaS nanohorn arrays is that the average emission current remains nearly constant over long time without any degradation.

Crystallization and optical properties of finite sized β-Ga2O3 in sol–gel derived Ga2O3:SiO2 nanocomposites

Gallium oxide nanoparticles embedded in silica matrix with different molar ratios were synthesized by the sol–gel method. Powdered nanocomposite samples were annealed at 400, 500 and 900 °C. The gallium oxide nanoparticles (2–5 nm) crystallized in the β-phase at a very low temperature (~400 °C) as against the expected temperature (>700 °C), indicating a depression of crystallization temperature under the present condition. This may be a signature of the behaviour of confined nanosized particles. The indications of only Ga–O bonds and Si–O–Si bonds in FTIR spectra and peaks of gallium, oxygen and silicon in energy dispersive x-ray analysis (EDAX) confirmed the non-existence of any impurity. Room temperature photoluminescence study of the samples shows a strong blue emission at ~460 nm.

Self-catalytic growth and field-emission properties of Ga2O3 nanowires

Ga2O3 nanowires with very narrow width (~30 nm) were fabricated from precursor gallium metal via a self-catalytic vapour–liquid–solid method using sol–gel derived Ga2O3 thin films as substrates. The morphological evolution of Ga2O3 nanostructures has been analysed by scanning electron microscopy and transmission electron microscopy. The field-emission (FE) properties of Ga2O3 nanowires are recorded and the turn-on field is found to be 1.88 V µm−1. It is shown from the I–t plot that the emission current remains nearly constant over 2 h at the pre-set current value of 1 µA. The average emission current at the stabilized value is seen to be fairly constant suggesting that the Ga2O3 nanowires are potentially important for applications in FE based devices.

Gallium nitride quantum dots in a nitrogen-bonded silica gel matrix

GaN:SiO2 nanocomposites were successfully synthesized by a two-step process. Firstly, an intermediate xerogel was obtained by the sol?gel method and, in the second step, annealing the powdered samples at 900??C for 5?h in flowing NH3 atmosphere produced the nanocomposites. X-ray diffraction and high-resolution transmission electron microscopic studies confirmed that the confined GaN nanoparticles (2?12?nm) were crystallized as a mixture of hexagonal and cubic phases. The optical band gap was observed to be enhanced slightly (3.69 eV) from the bulk value (3.39?eV) for the sample with the lowest molar ratio of GaN (10:90) in the nanocomposites. Fourier transform infrared spectroscopic studies indicated the formation of Ga?N bond along with probable signatures of Si?O, Si?N and Si?N?O bonds. The nanocomposites showed photoluminescence peaks at 370 and 571?nm as a result of excitation at 310?nm. Raman study of the samples shows peaks corresponding to the hexagonal and cubic phases of GaN.

Optical properties of nanocrystalline α-GaO(OH) thin films

α-GaO(OH) thin films obtained by the sol–gel process have been characterized. An x-ray diffraction study confirmed that the films were crystalline with orthorhombic structure. The average crystallite size of the thin films was about ~2–3 nm as revealed by transmission electron microscope and x-ray diffraction studies. The optical transparency of the film was about 95% in the visible region. The films exhibited an optical semiconducting band gap of 5.27 eV, which was much higher than those of α-Ga2O3 and β-Ga2O3. The type of the optical transition was determined: it was found to be allowed direct in nature. No degradation of the films was observed after more than a year of exposure to an ambient atmosphere.

Experimental and Numerical Study of Submonolayer Sputter Deposition of Polystyrene Fragments on Silver for the Storing Matter Technique

In static secondary ion mass spectrometry (SIMS), quantification and high ionization probabilities are difficult to obtain. The Storing Matter technique has been developed to circumvent these issues and has already been applied to deposit inorganic and organic samples. For organic samples, the effect of fragmentation during sputter deposition and changing coverage on time-of-flight (TOF)-SIMS mass spectra has not been investigated. In this work, polystyrene (PS) was sputter deposited on silver using an argon ion beam in order to investigate these parameters and to get a better control of the whole process. For this purpose, we introduce a multitechnique characterization approach for the submonolayer deposition of PS. Experimental methods (TOF-SIMS, X-ray photoelectron spectroscopy (XPS)) were used in combination with simulations (density functional theory (DFT) calculations) in order to obtain information about the molecular and structural changes and the interactions of organic matter with the metal surface. Alterations of the PS surface and PS sputter deposit as a function of surface coverage and Ar(+) ion fluence are addressed. A major finding is that this approach can be used to identify surface reactions between different fragments on the collector surface. Indeed, in the dynamic regime, the ratio of large to small fragments is increasing although the fragmentation during the sputter deposition should lead to increasingly smaller fragments. Hence, for Storing Matter, the coverage on the collector must be kept low in order to minimize the reactions between fragments and to preserve the information on the original sample.