Himanshu Srivastava

Growth and characterization of α-Fe2O3 nanowires

Hematite (α-Fe2O3) nanowires have been synthesized on a large surface area by thermal oxidation of iron foil in an ozone-rich environment. The effects of annealing time, temperature, and oxidizing environment on the growth of nanowires have been systematically studied. The samples were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). It was found that annealing in ozone-rich environment for 2–4 h at 700 °C yielded the best results in terms of number density and diameter of nanowires. The average diameter of the nanowires was found to be 85 nm. It was found that nanowires are bicrystal in nature with a length around 4 μm, which grows uniquely along the [110] direction.

Water-vapour-assisted growth of ZnO nanowires on a zinc foil and the study of the effect of synthesis parameters

Dense and large-aspect-ratio ZnO nanowires have been synthesized by thermal oxidation of a Zn foil under the flow of moist nitrogen. The effects of annealing temperature and time have been systematically studied in detail. It was observed that the length, diameter and number density increase with time and temperature before they saturate at the optimum synthesis condition of 600 °C annealing temperature and 16 h annealing time. The nanowires have been characterized by a scanning electron microscope, a transmission electron microscope, UV–Vis absorption and room temperature photoluminescence. The nanowires grow uniquely along the [1 1 0] direction up to ~14–16 µm length and are of good crystalline and optical quality. It was found that the formation of nanowires enhances greatly in the presence of water vapour. We have also shown by TEM observations that nanowires grow by surface diffusion of Zn ions from root to tip.

Transient absorption and higher-order nonlinearities in silver nanoplatelets

We show that the imaginary parts of higher-order optical nonlinearities and their decay times can be determined by a time-intensity domain analysis of the conventional transient absorption data. Using this method we have measured the values and decay times of third, fifth and seventh-order nonlinear susceptibilities of silver nanoplatelets in water. The origin of these higher-order nonlinearities is explained using a two-temperature model.

X-ray enhancement in a nanohole target irradiated by intense ultrashort laser pulses

In this paper, we present a comparative study of the laser energy absorption, soft x-ray emission (in the water window region: 2.3–4.4 nm) and hard x-ray emission (in the 2–20 keV range) from planar aluminum and nanohole alumina of 40 nm average diameter, when irradiated by Ti:sapphire laser pulses. The laser pulse duration was varied from 45 to 500 fs, and the focused intensity on the target ranged from ∼3 × 1016 W/cm2 to 3×1017 W/cm2. The x-ray yield enhancement from the nanoholes shows an increased coupling of the laser energy to the target. The effect of laser pulse duration on the x-ray emission was also studied, where a resonance like phenomenon was observed. The laser energy absorption measurements in the nanoholes showed a marginal enhancement in absorption as compared to planar Al. The integrated keV x-ray yield, from nanohole alumina and planar Al, at an intensity of 3 × 1017 W/cm2, was 25 and 3.5 μJ, respectively. The results can be explained by considering the hydrodynamic expansion of the las...

Tuning the localized surface plasmon resonance of silver nanoplatelet colloids

The effect of femtosecond laser irradiation on silver nanoplatelet colloids is described. It is shown that irradiation with a femtosecond laser of appropriate fluence can be used to tune the localized surface plasmon resonances of triangular silver nanoplatelets by a few tens of nanometres. This peak shift is shown to be caused by the structural modifications of the particle tips. We have also shown that post-preparation addition of poly-vinyl pyrrolidone to the nanocolloid arrests the peak shift.

Enhancement of Kα emission through efficient hot electron generation in carbon nanotubes on intense laser pulse irradiation

Near complete absorption of the energy of intense ultra-short laser pulses (45 fs, intensity ∼1.6 × 1016 to 2.5 × 1017 W/cm2) is observed in carbon nanotubes deposited on a planar molybdenum substrate. The hollow structure of the nanotube plasma facilitates resonant electric field enhancement during its ionization phase. This resonantly enhanced localized field at a density much larger than the critical density nc leads to efficient hot electron generation, which results in enhanced Kα emission of Mo at 17.5 keV. It is observed that for nanotubes, depending on the degree of hollowness, there is an optimum laser intensity for maximum x-ray enhancement compared to a planar uncoated target.

Aggregated nanoplatelets: optical properties and optically induced deaggregation

A study of aggregation and laser-induced deaggregation of silver nanospheres and nanoplatelets in colloidal form is presented. Changes in the extinction spectrum caused by aggregation are explained using a two-particle approximation. In the case of platelets, controlled laser irradiation is shown to reverse the aggregation process.

In-situ energy dispersive x-ray diffraction study of the growth of CuO nanowires by annealing method

The in-situ growth of CuO nanowires was studied by Energy Dispersive X-ray Diffraction (EDXRD) to observe the mechanism of growth. The study was carried out for comparison at two temperatures—at 500 °C, the optimum temperature of the nanowires growth, and at 300 °C just below the temperature range of the growth. The in situ observation revealed the successive oxidation of Cu foil to Cu2O layer and finally to CuO layer. Further analysis showed the presence of a compressive stress in CuO layer due to interface at CuO and Cu2O layers. The compressive stress was found to increase with the growth of the nanowires at 500 °C while it relaxed with the growth of CuO layer at 300 °C. The present results do not support the existing model of stress relaxation induced growth of nanowires. Based on the detailed Transmission Electron Microscope, Scanning Electron Microscope, and EDXRD results, a microstructure based growth model has been suggested.

Giant magnetocaloric effect near room temperature in the off-stoichiometric Mn–Co–Ge alloy

We report a giant magnetocaloric effect near room temperature in an off-stoichiometric Mn-Co-Ge alloy, across the magnetostructural transition. The isothermal entropy change accompanying this transition has a peak value of nearly 40 J/kg-K near 297 K and a refrigerant capacity of 270 J/kg with the hot end at 302.5 K and cold end at 293.5 K. We also present an experimental protocol to avoid spurious peaks in the magnetocaloric effect across a sharp first order magnetostructural transition, not confined to Mn-Co-Ge alone, where metastability during the transition could influence the measured magnetization and thus the estimated entropy change. The estimated entropy change in the present off-stoichiometric Mn-Co-Ge alloy is possibly the highest reported value near room temperature in undoped Mn-Co-Ge alloys and underlines the potential of the alloy for technological applications in room temperature magnetic refrigeration.

Topotaxial growth of α-Fe2O3 nanowires on iron substrate in thermal annealing method

A detail cross-sectional transmission electron microscopy of as-grown α-Fe2O3 nanowire sample, synthesized on iron substrate by thermal annealing method, was carried out to understand the mechanism of growth in this system. Iron undergoes sequential oxidation to form a layered structure of Fe/FeO/Fe3O4/α-Fe2O3. α-Fe2O3 nanowires grow on to the top of α-Fe2O3 layer. It was found that subsequent oxide layers grow topotaxially on the grains of iron, which results in a direct orientation relationship between the α-Fe2O3 nanowire and the parent grain of iron. The results also showed that the grains of α-Fe2O3 layer, which were uniquely oriented in [110] direction, undergo highly anisotropic growth to form the nanowire. This anisotropic growth occurs at a twin interface, given by (−11−1), in the α-Fe2O3 layer. It was concluded that the growth at twin interface could be the main driving factor for such anisotropic growth. These observations are not only helpful in understanding the growth mechanism of α-Fe2O3 na...