P. Kumbhakar

Enhanced nonlinear optical properties of graphene oxide–silver nanocomposites measured by Z-scan technique

Nonlinear optical properties (NLO) of a graphene oxide–silver (GO–Ag) nanocomposite have been investigated by the Z-scan setup at Q-switched Nd:YAG laser second harmonic radiation i.e., at 532 nm excitation in a nanosecond regime. A noteworthy enhancement in the NLO properties in the GO–Ag nanocomposite has been reported in comparison with those of the synthesized GO nanosheet. The extracted value of third order nonlinear susceptibility (χ3), at a peak intensity of I0 = 0.2 GW cm−2, for GO–Ag has been found to be 2.8 times larger than that of GO. The enhancement in NLO properties in the GO–Ag nanocomposite may be attributed to the complex energy band structures formed during the synthesis which promote resonant transition to the conduction band via surface plasmon resonance (SPR) at low laser intensities and excited state transition (ESA) to the conduction band of GO at higher intensities. Along with this photogenerated charge carriers in the conduction band of silver or the increase in defect states during the formation of the GO–Ag nanocomposite may contribute to ESA. Open aperture Z-scan measurement indicates reverse saturable absorption (RSA) behavior of the synthesized nanocomposite which is a clear indication of the optical limiting (OL) ability of the nanocomposite.

A Novel Technique of Synthesis of Highly Fluorescent Carbon Nanoparticles from Broth Constituent and In-vivo Bioimaging of C. elegans

Here we have demonstrated a novel single step technique of synthesis of highly fluorescent carbon nanoparticles (CNPs) from broth constituent and in vivo bioimaging of Caenorhabditis elegans (C. elegans) with the synthesized CNPs has been presented. The synthesized CNPs has been characterized by the UV-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy (TEM) and Raman studies. The sp2 cluster size of the synthesized samples has been determined from the measured Raman spectra by fitting it with the theoretical skew Lorentzian (Breit-Wigner- Fano (BWF)) line shape. The synthesised materials are showing excitation wavelength dependent tunable photoluminescence (PL) emission characteristics with a high quantum yield (QY) of 3xa0% at a very low concentration of CNPs. A remarkable increase in the intensity of PL emission from 16xa0% to 39xa0% in C. elegans has also been observed when the feeding concentration of CNPs to C. elegans is increased from 0.025xa0% to 0.1xa0% (w/v). The non-toxicity and water solubility of the synthesized material makes it ideal candidate for bioimaging.

Solvatochromism in highly luminescent environmental friendly carbon quantum dots for sensing applications: Conversion of bio-waste into bio-asset

Recently studies on synthesis and fluorescence based sensing in biocompatible carbon quantum dots (CQDs) have become a widely spoken topic of research due to the several advantageous properties of CQDs in compared to semiconductor quantum dots. In this work, we have reported the rarely reported solvatochromism along-with a high photoluminescence (PL) quantum yield (PLQY) of 22%. Samples have been synthesized by using a simple process of hydrothermal carbonization of a naturally occurring bio-waste i.e. Aegle marmelos leaves powder. The linear absorption and PL emission characteristics of CQDs have been studied in different solvent environments to explore the origin of the observed excitation dependent PL emissions characteristics of the sample. The interesting solvatochromic PL (SPL) behavior of CQDs are observed at an excitation wavelength of 325nm by dispersing them in different polar protic and aprotic solvents, which suggest their possible applications as a replacement of solvatochromic dye molecules for sensing applications. Different polarity functions and molecular-microscopic solvent polarity parameter (ETN) are used to calculate the change in dipole moment (Δδ) of the solute-solvent system and the origin of SPL in CQDs has been explained. The SPL behavior of CQDs has been utilized for fluorescence sensing of organic liquids (Ethanol and Tetrahydrofuran) in water. Whereas, the photo-induced electron transfer mediated quenching in PL of aqueous dispersion of CQDs has led to development of turn off fluorescence Fe3+ ion sensor with a detection limit of 0.12μM. Therefore, this work may open a new avenue of conversion of a bio-waste into a fluorescent bio-asset.

Mn2+ doped ZnS quantum dots in ferroelectric liquid crystal matrix: Analysis of new relaxation phenomenon, faster optical response, and concentration dependent quenching in photoluminescence

Phase transitional, dielectric, electro-optical, polarizing optical microscopic, photoluminescence (PL), and Fourier transformed infrared (FTIR) spectroscopic measurements have been carried out on ZnS:Mn quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC). A new dielectric relaxation mode has been envisaged in FLC material due to the presence of 0.25u2009wt.u2009% ZnS:Mn (40u2009mol.u2009%) QDs. The characteristics of the new mode have been compared with those of the soft mode. A significant fastening of the electro-optical response (∼75%) has been observed in the case of 0.25u2009wt.u2009% ZnS:Mn (20u2009mol.u2009%) QDs doped FLC material. The induction of the new relaxation mode is attributed to the flexoelectric tilt fluctuations. The induced flexoelectric polarization in the FLC medium at the vicinity of QDs might be responsible for the enhanced spontaneous polarization in the FLC/QDs mixtures. Quenching in PL for the FLC/QDs mixtures has been observed, which strongly depends on Mn content in QDs. The change in FTIR spec...

Near white light emission and enhanced photocatalytic activity by tweaking surface defects of coaxial ZnO@ZnS core-shell nanorods

The enhancement in the emission of visible light in the synthesized coaxial ZnO@ZnS core-shell nanorods (CSNR) has led to the development of a nearly white light-emitting photoluminescent material as confirmed by the calculation of Commission Internationale de lEclairage chromaticity coordinates. However, we have fabricated a nearly white light-emitting diode (WLED) by combining a commercial UV LED chip with our CSNR material, and it emits warm white light. The observed increase in the relative intensity of deep level (IDLE) over UV (IUV) photoluminescence emission is attributed to the addition of new defect states during the formation of the shell with a larger thickness as becomes evident from X-ray photoelectron spectroscopy (XPS) study. Thanks to the presence of such defect states, whose effective exploitation enabled us to obtain similar to 93% photodegradation of a test dye, namely, methylene blue, in the presence of core-shell ZnO@ZnS heterostructure within only 25min of irradiation of UV-Visible light. Thus, apart from demonstrating the fabrication of a near WLED, we have successfully demonstrated the enhanced photocatalytic performance by tweaking the surface defects of ZnO nanorods via the formation of coaxial ZnO@ZnS core-shell nanorods with various shell thicknesses. Published by AIP Publishing.

Template-free hydrothermal synthesis of amphibious fluorescent carbon nanorice towards anti-counterfeiting applications and unleashing its nonlinear optical properties

Anisotropic fluorescent carbon nanostructures, namely carbon nanorice (CNR), has been prepared hydrothermally, without using any template, from a commercially available beef extract (BE) bio-precursor at a temperature of 150 °C. The structural and morphological characterizations of the synthesized sample have been carried out using transmission electron microscopy (TEM) and Raman spectroscopy. It has been proposed that the synthesized CNR structures have been formed due to the coalescence of initially formed small spherical carbon nanocrystals during hydrothermal treatment. The third order nonlinear refractive index (n2) of the synthesized CNR has been measured using the spatial self-phase modulation (SSPM) technique, for the first time, and a high optical nonlinearity with an n2 value of the order of 10−6 cm2 W−1 was obtained. The origin of such high nonlinearity is the reorientation of the sp2 graphitic core of CNR in response to intense laser radiation coupled with the thermal lensing effect. A theoretical simulation has been carried out to explain the formation of experimental optical diffraction ring patterns, which have occured due to SSPM of a continuous wave He–Ne laser beam of 632.8 nm wavelength. Furthermore, the synthesized CNR structures have been found to exhibit excitation-dependent tunable photoluminescence (PL) emission with visible blue light emission in liquid dispersion as well as in a PVA matrix, under 365 nm UV light irradiation. This amphibious nature in PL emission characteristics of the synthesised CNR structures has also been utilized in successful demonstration of using CNR structures as an anti-counterfeiting agent and indicates their potential future applications in photonic devices.

In-situ synthesis of rGO-ZnO nanocomposite for demonstration of sunlight driven enhanced photocatalytic and self-cleaning of organic dyes and tea stains of cotton fabrics

Recently, research activities are focused on development of 2D reduced graphene oxide (rGO) based semiconductor nanocomposite materials for boosting up its catalytic applications. In this work, a rarely reported green synthesis approach has been envisioned to synthesize in-situ 2D rGO-ZnO (rGZn) nanocomposites from Apple juice and zinc acetate. Also the composition of the samples has been optimized to achieve high photocatalytic and self-cleaning properties by the formation of reactive oxidation species. The samples are characterized for their microstructural, optical absorption and photoluminescence properties. It has been tested that rGZn nanocomposites are capable of removing a test dye, namely methylene blue (MB) from water and achieved the highest dye degradation efficiency of ∼91% within only 60u2009min under UV-vis light irradiation. A smart cotton fabric (CF) coated with rGZn has been prepared and demonstrated its photocatalytic self-cleaning property by degradation of MB, rhodamine B dyes and tea stains on it even under sunlight irradiation, which is scarcely available in the literature. Therefore, this work may open a new avenue of research for low cost and easy synthesis of rGO-semiconductor nanocomposites with high photocatalytic properties for industrial applications as well as for development of rGO based smart fabric for real-life applications.

Optical Properties of Co2+ Doped ZnS Nanocrystals

ZnS nanocrystals with Co2+ doping have been prepared through a soft chemical route. The undoped ZnS and Co2+ doped ZnS:Co nanocrystals have been analyzed using X‐ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), and ultraviolet‐visible (UV‐VIS) spectrophotometer. Undoped sample exhibits room‐temperature photoluminescence (PL) emission in the blue region with a broad spectral band peaked at ∼397 nm under UV excitation. But from the 1.5% Co2+ doped samples, a strong blue emission peaked at ∼470 nm is observed and further increase in doping leads to considerable blue shift and enhancement in intensity of the PL spectrum.