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.

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.

High sensitivity NH3 gas sensor with electrical readout made on paper with perovskite halide as sensor material.

In this paper we report a cheap, paper electronics based solid state gas sensor to detect NH3 gas selectively with a detection capability of better than 1 ppm. The sensor uses perovskite halide CH3NH3PbI3 (MAPI) as the active sensor material grown on a paper. This paper based sensor works at room temperature. The current through the paper sensor increases by one order on exposure to only 10 ppm NH3 gas. The calibrated sensitivity is ~55% for 1 ppm of NH3 gas in Nitrogen or Air. The current noise limited resolution estimated to be ~10 ppb. This work establishes perovskite halide as a new solid state gas sensing material that can reach sub ppm sensitivity using simple paper electronics. Use of paper and also solution method used to grow the active material makes the sensor cost effective and easy to manufacture. This type of disposable high sensitive paper sensor can be used for detection of NH3 as a marker in exhaled breathes for non-invasive diagnosis. The sensor formed on the paper, since it supports unheated operation, needs less than few nanowatt power for its operation.

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.

Continuous transition from weakly localized regime to strong localization regime in Nd0.7La0.3NiO3 films.

We report an investigation of metal-insulator transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd0.7La0.3NiO3 films grown on crystalline substrates of LaAlO3 (LAO), SrTiO3 (STO), and NdGaO3 (NGO) by pulsed laser deposition. The film grown on LAO experiences a compressive strain and shows metallic behavior with the onset of a weak resistivity upturn below 2 K which is linked to the onset of weak localization contribution. Films grown on STO and NGO show a cross-over from a positive temperature coefficient (PTC) resistance regime to negative temperature coefficient (NTC) resistance regime at definite temperatures. We establish that a cross-over from PTC to NTC on cooling does not necessarily constitute a MIT because the extrapolated conductivity at zero temperature [Formula: see text] though small (<10 S cm-1) is finite, signaling the existence of a bad metallic state and absence of an activated transport. The value of [Formula: see text] for films grown on NGO is reduced by a factor of 40 compared to that for films grown on STO. We show that a combination of certain physical factors makes substituted nickelate (that are known to exhibit first-order Mott type transition), undergo a continuous transition as seen in systems undergoing disorder/composition driven Anderson transition. The MC measurement also supports the above observation and shows that at low temperatures, there exists a positive MC that arises from the quantum interference which co-exists with a spin-related negative MC that becomes progressively stronger as the electrons approach a strongly localized state in the film grown on NGO.

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.

Proposed Bose-Einstein condensation of magnons in nanostructured films of Gd at low temperature and its manifestations in electrical resistivity and magnetoresistance.

In this paper we report the observation of a proposed Bose-Einstein condensation (BEC) of magnons in a temperature range of around 15-20 K in nanostructured films of Gd with grain sizes that are much larger than the size range where superparamagentism is expected. The observation was carried out using magnetic as well as high precision resistivity and magnetoresistance (MR) measurements performed to low temperatures. We observe that the experimental observations depend crucially on one parameter, namely softening of the spin wave stiffness parameter D at BEC and the resistivity as well as MR can be related quantitatively to magnetic measurements through the temperature variation of the constant D in the vicinity of the transition. This paper establishes that the BEC reported before in nanocrystalline Gd can be extended to a somewhat larger size range.

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.

Weak localization and the approach to metal-insulator transition in single crystalline germanium nanowires.

We study the low-temperature electronic transport properties of single germanium nanowires (NWs) with diameters down to 45 nm to investigate the weak localization (WL) behavior and approach to metal-insulator transition (MIT) within them. The NWs (single crystalline) we investigate lie on the metallic side of the MIT with an extrapolated zero temperature conductivity [Formula: see text] in the range 23 to 1790 [Formula: see text] cm)-1 and show a temperature-dependent conductivity which below 30 K can be described by a 3D WL behavior with Thouless length [Formula: see text] and [Formula: see text]. From the observed value of [Formula: see text] and the value of the critical carrier concentration n c, it is observed that the approach to MIT can be described by the scaling equation [Formula: see text] with [Formula: see text], which is a value expected for an uncompensated system. The investigation establishes a NW size limit for the applicability of 3D scaling theories.

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.

Enhancement of Efficiency of a Solar Cell Fabricated on Black Si Made by Inductively Coupled Plasma-Reactive Ion Etching Process: A Case Study of a n-CdS/p-Si Heterojunction Cell.

We show that a significant enhancement of solar cell efficiency can be achieved in cells fabricated on black Si made using inductively coupled plasma-reactive ion etching (ICP-RIE). The ICP-RIE-fabricated black Si results in an array of vertically oriented defect-free Si nanocones (average height ∼150 nm; apex diameter ∼25 nm) exhibiting an average reflectance ≤2% over most of the relevant solar spectral range. The enabling role of the ultralow reflectance of the nanostructured black Si has been demonstrated using a heterojunction solar cell fabricated by depositing a n-type CdS film on p-Si nanocones followed by a transparent conducting coating of Al-doped ZnO (AZO). The fabricated n-CdS/p-Si heterojunction exhibits promising power conversion efficiency close to 3%, up from a mere efficient 0.15% for a similar cell fabricated on a planar Si. The effect of the fabrication process for the black Si on solar cell performance has been investigated through the measurements of carrier lifetime and surface recombination velocity. The accompanying model and simulation analysis shows that the conical structure leads to the effective dielectric constant varying smoothly from the value of the air at the top to the value of Si at the base over the length of the nanocone, leading to a substantial reduction of its reflectance.

Strain heterogeneity and magnetoelastic behaviour of nanocrystalline half-doped La, Ca manganite, La0.5Ca0.5MnO3.

Elastic and anelastic properties of La0.5Ca0.5MnO3 determined by resonant ultrasound spectroscopy in the frequency range ∼100-1200 kHz have been used to evaluate the role of grain size in determining the competition between ferromagnetism and Jahn-Teller/charge order of manganites which show colossal magneto resistance. At crystallite sizes of ∼75 and ∼135 nm the dominant feature is softening of the shear modulus as the charge order transition point, Tco (∼225 K), is approached from above and below, matching the form of softening seen previously in samples with 'bulk' properties. This is consistent with a bilinear dominant strain/order parameter coupling, which occurs between the tetragonal shear strain and the Jahn-Teller (Γ3(+)) order parameter. At crystallite sizes of ∼34 and ∼42 nm the charge ordered phase is suppressed but there is still softening of the shear modulus, with a minimum near Tco. This indicates that some degree of pseudoproper ferroelastic behaviour is retained. The primary cause of the suppresion of the charge ordered structure in nanocrystalline samples is therefore considered to be due to suppression of macroscopic strain, even though MnO6 octahedra must develop some Jahn-Teller distortions on a local length scale. This mechanism for stabilizing ferromagnetism differs from imposition of either an external magnetic field or a homogeneous external strain field (from a substrate), and is likely to lead both to local strain heterogeneity within the nanocrystallites and to different tilting of octahedra within the orthorhombic structure. An additional first order transition occurs near 40 K in all samples and appears to involve some very small strain contrast between two ferromagnetic structures.

Single Si nanowire (diameter ≤ 100 nm) based polarization sensitive near-infrared photodetector with ultra-high responsivity.

We report the fabrication and optical response of boron-doped single silicon nanowire-based metal-semiconductor-metal photodetector. Typical single nanowire devices with diameter of ∼80-100 nm and electrode spacing of ∼1 µm were made using electron-beam lithography from nanowires, grown by a metal-assisted chemical etching process. A high responsivity, of the order of 10(4) A W(-1), was observed even at zero bias in a single nanowire photodetector with peak responsivity in the near-infrared region. The responsivity was found to increase with increasing bias and decreasing nanowire diameter. Finite element based optical simulation was proposed to explain the diameter dependent performance of a single nanowire. The observed photoresponse is sensitive to the polarization of exciting light source, allowing the device to act as a polarization-dependent near-infrared photodetector.

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.

Effect of size reduction on the structural and magnetic order in LaMnO(3+δ) (δ ≈ 0:03) nanocrystals: a neutron diffraction study.

We report a structural transition from the orthorhombic to the rhombohedral phase upon size reduction in nanocrystalline LaMnO(3+δ) (δ ≈ 0:03) as revealed through neutron diffraction studies. The transition occurs when the average particle (crystallite) size is taken below ~50 nm without change of δ, which is fixed at around 0.03 as measured by a number of characterization tools. The change in the crystallographic structure is accompanied by a change in the magnetic order, where the canted antiferromagnetic order with moments in the basal (ab) plane for the bulk changes to collinear ferromagnetic order with spins along the c-axis for the nanocrystals. The spontaneous ferromagnetic moment ~3 µ(B) and the transition temperature of 260 K in LaMnO(3+δ) nanocrystals are similar to those found in La0:67Ca0:33MnO3 which has a much higher Mn(4+) content. The likely origin is traced to change in magnetic exchange interactions due to change in Mn-O bond lengths which become almost identical in the MnO6 octahedron in the rhombohedral structure in the absence of Jahn-Teller distortion. The study provides an example of structural and magnetic phase transition driven purely by size reduction and with no change in the chemical constituents.

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.

Electric field driven destabilization of the insulating state in nominally pure LaMnO3.

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO3 single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO3 and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (Eth) that shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves a large change in resistance, it softens on heating and is eventually absent above 280 K. When the Mn(4+) content is increased by Sr substitution up to x = 0.1, the observed transition, although observable in a certain temperature range, softens considerably. This observation has been explained as a bias driven percolation type transition between two co-existing phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics as f = fo(E)exp(-Δ/kBT) with the activation energy Δ ≈ 200 meV, and the pre-factor fo(E) is a strong function of the field that leads to a rapid enhancement of f on application of field, leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO3 that can lead to the bias driven percolation type transition.

Tuning the instability in static mode atomic force spectroscopy as obtained in an AFM by applying an electric field between the tip and the substrate.

We have investigated experimentally the role of cantilever instabilities in determination of the static mode force-distance curves in presence of a dc electric field. The electric field has been applied between the tip and the sample in an atomic force microscope working in ultra-high vacuum. We have shown how an electric field modifies the observed force (or cantilever deflection)-vs-distance curves, commonly referred to as the static mode force spectroscopy curves, taken using an atomic force microscope. The electric field induced instabilities shift the jump-into-contact and jump-off-contact points and also the deflection at these instability points. We explained the experimental results using a model of the tip-sample interaction and quantitatively established a relation between the observed static mode force spectroscopy curves and the applied electric field which modifies the effective tip-sample interaction in a controlled manner. The investigation establishes a way to quantitatively evaluate the electrostatic force in an atomic force microscope using the static mode force spectroscopy curves.