Enhanced piezoelectric response in BTO NWs-PVDF composite through tuning of polar phase content.

We have fabricated a flexible, environment friendly piezoelectric nanogenerator (PENG) based on the ferroelectric Polyvinylidene fluoride (PVDF) composite incorporated with Barium titanate (BaTiO3) nanowires (NWs) of piezoelectric coefficientd33 = 308 pm V-1. The single-layered PENG can deliver output power density of 10µW cm-2and an output voltage of 2 V with a nominal mechanical load of 1 kPa. BaTiO3(BTO) NWs of different concentrations were incorporated into PVDF to tune the polar phase content, internal resistance, and optimize the output power. We show that there exists a critical value of BTO NWs loading of 15 wt%, beyond which the piezoelectric energy harvesting characteristics of the PVDF nanocomposites decrease. The oxygen vacancies present in the BTO NWs surface attract the fluorine ions of PVDF chain and favour the formation ofßphase. The enhanced value of dielectric constant and dielectric loss of BTO-PVDF samples in the low frequency region suggest strong interfacial polarization in the composite system. The fabricated PENG can charge a super-capacitor up to 4 V within 35 s. The origin of the high power output from the BTO (15 wt%)-PVDF composite is attributed to the combined effect of enhanced polar phase content, strong interfacial polarization, and reduced internal resistance. This study provides an effective pathway in enhancing the performance of BTO-PVDF based piezoelectric energy harvesters.

Piezoelectric Nanogenerators based on Lead Zirconate Titanate nanostructures: an insight into the effect of potential barrier and morphology on the output power generation.

The high internal resistance of the perovskite materials used in Nanogenerators (NGs) lowers the power generation. It severely restricts their application for mechanical energy harvesting from the ambient source. In this work, we demonstrate a flexible Piezoelectric NG (PENG) with an improved device structure. Hydrothermally grown one-dimensional Lead Zirconate Titanate (Pb(ZrTi)O3) of different morphologies are used as the generating material. The morphology of the PZT nanostructures, engineered from nanoparticles to needle-shaped nanowires to increase the surface to volume ratio, provides effective mechanical contact with the electrode. The reduction of the internal resistance of the PENG has been achieved by two ways: (i) fabrication of interdigitated electrodes (IDE) to increase the interfacial polarization and (ii) lowering of Schottky barrier height (SBH) at the junction of the PZT nanostructure and the metal electrode by varying the electrode materials of different work functions. We find that lowering of the SBH at the interface contributes to an increased piezo voltage generation. The flexible nano needles-based PENG can deliver output voltage 9.5 V and power density 615µW cm-2on application low mechanical pressure (∼1 kPa) by tapping motion. The internal resistance of the device is ∼0.65 MΩ. It can charge a 35µF super-capacitor up to 5 V within 20 s. This study provides a systematic pathway to solve the bottlenecks in the piezoelectric nanogenerators due to the high internal resistance.

ZnO/Si nanowires heterojunction array-based nitric oxide (NO) gas sensor with noise-limited detectivity approaching 10 ppb.

We report a ZnO/Silicon nanowire (ZnO/Si NWs) heterojunction array-based NO gas sensor operating at room temperature with an extremely high response (noise limited response ∼10 ppb). The sensor shows very high selectivity towards NO gas sensing and limited perturbation in response due to the presence of moisture. The sensor has been fabricated by using cost-effective chemical processing that is compatible with wafer-level processing. The vertically aligned Si NWs array has been made by an electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The electrical characteristic of the model was to fit the I-V data before and after exposure to gas and essential changes in electrical parameters were obtained. This was then explained based on a proposal for the mechanism of gas sensing. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW.

Self-powered single semiconductor nanowire photodetector.

Self-powered photodetectors have been fabricated from a single germanium nanowire (NW) in the metal-semiconductor-metal (MSM) device configuration. The self-powered devices show a high photoresponse (responsivity âˆ¼ 103-105 A W-1) in the wavelength range 300-1100 nm. It has been established from I-V characteristics that asymmetry exists in the Schottky barrier height (SBH) at the two MS contacts. We have used simulation to establish that the asymmetric SBH at the metal contacts in an MSM device is a major cause for the 'built-in' axial field that leads to separation of a light generated electron-hole pair in the absence of an applied bias. Thus, even in the absence of external bias, the photogenerated carriers can be separated, which then diffuse to the appropriate electrodes driven by the 'built-in' axial field. We also point out the physical origins that can lead to unequal barrier heights in seemingly identical NW/metal junctions in a MSM device.

Ligand-free attachment of plasmonic Au nanoparticles on ZnO nanowire to make a high-performance broadband photodetector using a laser-based method.

We report a new strategy for ligand-free attachment of plasmonic Au nanoparticles on the surface of a ZnO nanowire to make high-performance broadband photodetectors using a pulsed laser ablation technique in a liquid medium. The photoresponse of the ZnO-based photodetector is enhanced and the photodetection limit is broadened from UV to visible, which can be controlled by varying the concentration of Au nanoparticles attached to the ZnO surface. This Au nanoparticle concentration can be tuned by varying the number of laser pulses used in the ablation process. We found that the responsivity of the detector is 10 mA W-1 for [Formula: see text] and increases to as much as 0.4 A W-1 for λ ≤ 400 nm for the maximum Au concentration. The enhanced responsivity was found to be linked to increased absorption over a broad spectral range arising from direct and indirect plasmonic processes due to Au nanoparticle attachment, and the enhanced absorption also leads to a large increment in photocurrent generation. We also found that the attachment of Au nanoparticles makes the relaxation of the photocurrent (persistence) considerably faster in both the UV and visible regions of the spectrum and that the persistence directly depends on the concentration of Au nanoparticles attached to the ZnO nanowire. This single-step pulsed laser ablation-based nanoparticle attachment process can be further used to make other plasmonic nanoparticle-decorated nanowire devices.

First principles study of bimetallic Ni13-nAgn nano-clusters (n = 0-13): Structural, mixing, electronic, and magnetic properties.

Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni13-nAgn nano-clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg12 alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with the Ag environment causes reduction of Ni moment.

Structural instability and phase co-existence driven non-Gaussian resistance fluctuations in metal nanowires at low temperatures.

We report a detailed experimental study of the resistance fluctuations measured at low temperatures in high quality metal nanowires ranging in diameter from 15-200 nm. The wires exhibit co-existing face-centered-cubic and 4H hcp phases of varying degrees as determined from the x-ray diffraction data. We observe the appearance of a large non-Gaussian noise for nanowires of diameter smaller than 50 nm over a certain temperature range around ≈30 K. The diameter range ∼30 nm, where the noise has maxima coincides with the maximum volume fraction of the co-existing 4H hcp phase thus establishing a strong link between the fluctuation and the phase co-existence. The resistance fluctuation in the same temperature range also shows a deviation of [Formula: see text] behavior at low frequency with appearance of single frequency Lorentzian type contribution in the spectral power density. The fluctuations are thermally activated with an activation energy [Formula: see text] meV, which is of same order as the activation energy of creation of stacking fault in FCC metals that leads to the co-existing crystallographic phases. Combining the results of crystallographic studies of the nanowires and analysis of the resistance fluctuations we could establish the correlation between the appearance of the large resistance noise and the onset of phase co-existence in these nanowires.

Single CuTCNQ charge transfer complex nanowire as ultra high responsivity photo-detector.

We report ultra large photo responsivity ℜ (ratio of photo-generated current to absorbed power) in a single nanowire (NW) device made from a single strand of a nanowire (diameter ~30nm and length ~200nm) of an organomettalic semiconducting charge transfer complex material of CuTCNQ. The device shows responsivity of 8x10(4) A/Watt at 1 volt applied bias with an enhancement over the dark current exceeding 10(5) at zero bias. The observed photo current has a spectral dependence that strongly follows the main absorption peak (close to 405 nm) showing the primary role of absorbed photo-generated carriers.

Nanoscopic oxygen control of functional oxide nanoparticles by electro-chemical route at ambient temperature.

La0.67Ca0.33MnOδ nanoparticles of approximate size ∼ 4 nm have been prepared by the chemical solution deposition method to investigate effect of oxygen stoichiometry in the nanoparticles without changing their sizes. Electrochemical oxidation method has been used to change the oxygen stoichiometry [Formula: see text] at room temperature, which unlike conventional methods to change oxygen stoichiometry by heating in controlled ambience, does not lead to any significant change in size. This has allowed us to investigate the effects of stoichiometry variations in the nanoparticles with no change in size. The unit cell volume, lattice constants and orthorhombic strains of the as prepared sample (with [Formula: see text] = 2.74) are changed by incorporation of oxygen by electrochemical oxidation which in turns affects the magnetic properties. In addition, oxidation leads to change in oxygen stoichiometry of the magnetically "dead" surface layer on the nanoparticles which also affects their magnetization and coercive field.

Inverse magnetocaloric and exchange bias effects in single crystalline La0.5Sr0.5MnO3 nanowires.

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La0.5Sr0.5MnO3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (T(L) ~ 42 K) in the magnetic entropy change (-ΔS(M)) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at T(L), the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (T(N) ~ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.

Ultrafast dynamics of excitons in semiconductor quantum dots on a plasmonically active nano-structured silver film.

The excited state dynamics of core-shell type semiconductor quantum dots (QDs) of various sizes in close contact with a plasmonically active silver thin film has been demonstrated by using picosecond resolved fluorescence spectroscopy. The non-radiative energy transfer from the QDs to the metal surface is found to be of Förster resonance energy transfer (FRET) type rather than the widely expected nano-surface energy transfer (NSET) type. The slower rate of energy transfer processes compared to that of the electron transfer from the excited QDs to an organic molecule benzoquinone reveals an insignificant possibility of charge migration from the QDs to the metallic film.

Thermal fluctuation spectroscopy of DNA thermal denaturation.

We have developed the technique of thermal fluctuation spectroscopy to measure the thermal fluctuations in a system. This technique is particularly useful to study the denaturation dynamics of biomolecules like DNA. Here we present a study of the thermal fluctuations during the thermal denaturation (or melting) of double-stranded DNA. We find that the thermal denaturation of heteropolymeric DNA is accompanied by large, non-Gaussian thermal fluctuations. The thermal fluctuations show a two-peak structure as a function of temperature. Calculations of enthalpy exchanged show that the first peak comes from the denaturation of AT rich regions and the second peak from denaturation of GC rich regions. The large fluctuations are almost absent in homopolymeric DNA. We suggest that bubble formation and cooperative opening and closing dynamics of basepairs causes the additional fluctuation at the first peak and a large cooperative transition from a partially molten DNA to a completely denatured state causes the additional fluctuation at the second peak.

Observation of Peierls transition in nanowires (diameter approximately 130 nm) of the charge transfer molecule TTF-TCNQ synthesized by electric-field-directed growth.

We report the growth of nanowires of the charge transfer complex tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) with diameters as low as 130 nm and show that such nanowires can show Peierls transitions at low temperatures. The wires of sub-micron length were grown between two prefabricated electrodes (with sub-micron gap) by vapor phase growth from a single source by applying an electric field between the electrodes during the growth process. The nanowires so grown show a charge transfer ratio approximately 0.57, which is close to that seen in bulk crystals. Below the transition the transport is strongly nonlinear and can be interpreted as originating from de-pinning of CDW that forms at the Peierls transition.

The effect of intrinsic instability of cantilever on static mode atomic force spectroscopy.

We show that the static force spectroscopy curve taken in an atomic force microscope is significantly modified due to presence of intrinsic cantilever instability which occurs as a result of its movement in a nonlinear force field. This instability acts in tandem with such instabilities as water bridge or molecular bond rupture and makes the static force spectroscopy curve (including 'jump-off-contact') dependent on the step size of data collection. A theoretical model has been proposed to explain the data. We emphasize the necessity of taking care of this fundamental instability of the microcantilever in calculating the adhesive force and also in the interpretation of data taken using an atomic force microscope.

Dynamics of light harvesting in ZnO nanoparticles.

We have explored light harvesting of the complex of ZnO nanoparticles with the biological probe Oxazine 1 in the near-infrared region using picosecond-time-resolved fluorescence decay studies. We have used ZnO nanoparticles and Oxazine 1 as a model donor and acceptor, respectively, to explore the efficacy of the Förster resonance energy transfer (FRET) in the nanoparticle-dye system. It has been shown that FRET from the states localized near the surface and those in the bulk of the ZnO nanoparticles can be resolved by measuring the resonance efficiency for various wavelengths of the emission spectrum. It has been observed that the states located near the surface for the nanoparticles (contributing to visible emission at lambda approximately 550 nm) can contribute to very high efficiency (>90%) FRET. The efficiency of light harvesting dynamics of the ZnO nanorods has also been explored in this study and they were found to have much less efficiency (approximately 40%) for energy transfer compared to the nanoparticles. The possibility of an electron transfer reaction has been ruled out from the picosecond-resolved fluorescence decay of the acceptor dye at the ZnO surface.

Frequency dependent enhancement of heat transport in a nanofluid with ZnO nanoparticles.

In this paper we report an observation of an unusual frequency dependent enhancement of the heat transport parameter (C(p)kappa, C(p) = heat capacity and kappa = thermal conductivity) of a nanofluid containing ZnO nanoparticles of an average size of 10 nm (volume fraction

Growth of atomically smooth films of metal-arachidates by Langmuir-Blodgett technique.

In this paper, we report synthesis of atomically smooth Ni-arachidate films using Langmuir-Blodgett technique. The interaction between arachidic acid monolayer and nickel ion has been investigated as a function of subphase pH by measuring the compression isotherms at the air/water interface. As the pH is increased, the compressible liquid (L2) phase is observed over a smaller range of surface pressure (pi) and area/mol (A) until at high enough pH the L2 phase is altogether absent. A further increase in pH does not result in any additional change in the isotherm. This is a general trend observed for all bivalent cations but the disappearance of L2 phase occurs at different pH values for different ions. The atomically smooth monolayers are deposited when the pressure is beyond the L2 phase (typically approximately 35 mN/m). Along with the isotherms, monolayer of nickel arachidate deposited on mica, were studied thoroughly using Atomic force Microscopy. Atomic force microscope images show atomically smooth and defect free surface of nickel arachidate transferred on mica substrate. The images allow proper determination of the lattice vectors.

Synthesis and physical properties of ordered arrays of nanowires of complex functional oxides.

We have synthesized ordered arrays of nanowires of complex oxides. The synthesis was done using nanoporous templates of anodized alumina (AAO). The synthesis involves sol-gel route using Polyethylene glycol. The synthesized nanowires can be made down to diameters of 35-40 nm. In this paper we report on nanowires of ferromagnetic La0.67Ca0.33MnO3 (LCMO). The nanowires characterized by a number of techniques, were found to be largely single crystalline and they have the nearly same crystal structure and stoichiometry as those of the bulk. We found LCMO nanowires retain ferromagnetism down to this size range (approximately 35-40 nm) with Tc (260 K) which is comparable to that of bulk.

Critical phenomena in magnetic nanowires.

In this paper we report the first experimental study of critical phenomena in case of magnetic nanowires of nickel near the ferromagnetic-paramagnetic transition from the electrical transport properties. Nickel nanowire arrays, prepared by potentiostatic electrodeposition of nickel inside pores of nanoporous anodic alumina template were well characterized by X-ray Diffraction, Transmission electron microscopy and Energy dispersive Spectroscopy. Precise electrical resistance measurement of the nanowire arrays of wire diameter 20 nm have been done in the temperature range between 300 K to 700 K. We see a drop in the Curie temperature as observed from the resistivity anomaly. We analyzed the resistance data near the critical region and extracted the critical exponent alpha directly from the resistance. We observed a decrease in the critical part of the resistivity including a decrease in the magnitude of the critical exponent alpha and severe modification in the correction to scaling.

Investigation of very low-frequency noise in ferromagnetic nickel nanowires.

In this paper we report very low-frequency (0.1 mHz-1.0 Hz) resistance fluctuation (noise) in Nickel nanowires of diameter 20 nm in the temperature range 77 K-300 K. The wires are one-dimensional magnetic systems since the diameter is less than the domain wall width. We found a clear signature of deviation of the spectral power from 1/f noise at low frequencies showing excess fluctuations with very slow dynamics. The magnitude of resistance fluctuations increases by an order of magnitude when the diameter of the wires is reduced below the domain wall width of nickel. The excess resistance fluctuation has been linked to thermally activated magnetization reversal and the associated domain wall motion.