Terahertz-based optoelectronic properties of ZnS quantum dot-polymer composites: For device applications.

Terahertz (THz) technology integration with nanomaterials is receiving excellent attention for next-generation applications, including enhanced imaging and communication. The excellent optical properties in THz domain can lead to preparation of low-cost CMOS camera which can convert THz radiation into optical signal in very efficient manner. In the present study, we have studied the properties of Zinc Sulfide quantum dots (ZnS QDs) embedded with Polyvinyl Alcohol (PVA) composites films using THz Signal at room temperature. The optical characterizations such as refractive index, absorption coefficients and dielectric constants of these samples were measured in the 0.1-2.0 THz range. Additionally, optical impedance, surface roughness, and reflection coefficient in TE and TM mode between 0.1 and 2.0 THz range were determined for these samples based on surface roughness-based reflection and scattering properties. The surface roughness factor was used to measure the optical impedance of the ZnS QDs based polymer films. The measured values of the absorption coefficient at 266 nm are compared with THz radiation, and the refractive indices of these samples range from 1.75 to 2.0. Finally, these samples were subjected to UV light excitation (λexe = 266 nm) of 0.15 ns duration and 400 nm for the fluorescence and corresponding life time measurements. We observed two numbers of fluorescence lines in nanosecond based excited domain whereas 400 nm excitation-based fluorescence life time lies between 13.8-11.39 ns range along with shift in fluorescence lines between 538.7 to 560.7 nm, respectively.

Replica symmetry breaking in a colloidal plasmonic random laser with gold-coated triangular silver nanostructures.

Plasmonic random lasers have drawn significant attention recently due to their versatility, low threshold, and the possibility of achieving tunable and coherent/incoherent outputs. However, in this Letter, the phenomenon of replica symmetry breaking is reported in intensity fluctuations of a rarely used colloidal plasmonic random laser (RL) illumination. Triangular nanosilver scatter particles produced incoherent RL action when used in a dimethylformamide (DMF) environment in a Rhodamine-6G gain medium. The use of gold-coated triangular nanosilver as the scatterer in place of triangular nanosilver offered a dual contribution of scattering and lower photo-reabsorption, which caused a reduction in the lasing threshold energy of 39% compared to that obtained with the latter. Further, due to its long-term photostability and chemical properties, a phase transition from the photonic paramagnetic to the glassy phase is observed experimentally in the RL system used. Interestingly, the transition occurs at approximately the lasing threshold value, which is a consequence of stronger correlation of modal behaviors at high input pump energies.

Utilization of DNA and 2D metal oxide interaction for an optical biosensor.

The efficient monitoring and early detection of viruses may provide essential information about diseases. In this work, we have highlighted the interaction between DNA and a two-dimensional (2D) metal oxide for developing biosensors for further detection of viral infections. Spectroscopic measurements have been used to probe the efficient interactions between single-stranded DNA (ssDNA) and the 2D metal oxide and make them ideal candidates for detecting viral infections. We have also used fully atomistic molecular dynamics (MD) simulation to give a microscopic understanding of the experimentally observed ssDNA-metal oxide interaction. The adsorption of ssDNA on the inorganic surface was found to be driven by favourable enthalpy change, and 5'-guanine was identified as the interacting nucleotide base. Additionally, the in silico assessment of the conformational changes of the ssDNA chain during the adsorption process was also performed in a quantitative manner. Finally, we comment on the practical implications of these developments for sensing that could help design advanced systems for preventing virus-related pandemics.

Transition metal dichalcogenides nanomaterials based piezocatalytic activity: recent progresses and outlook.

Piezoelectric materials have drawn significant attention from researchers in the recent past as the piezo-potential, induced by applied external stress, generates an electric field, which paves the way for the creation and transfer of electrons and holes. After the theoretical prediction of the existence of the piezoelectric effect in transition metal dichalcogenides (TMDCs) semiconductors, intense research efforts have been made by various researchers to demonstrate the effect experimentally. In addition 2D TMDCs exhibit layer-dependent tunable electronic structure, strongly bound excitons, enhanced catalytic activity at their edges, and novel spin/pseudospin degrees of freedom. The edge sites and activated basal planes of 2D TMDCs are shown to be highly active toward catalysis of the hydrogen evolution reaction (HER). However, as compared to electrocatalytic or even photocatalytic performances, TMDC materials exhibit poorer piezocatalytic activity, in general. Therefore, a numbers of research strategies have been made to intensify the piezoelectric effect by synthesizing different types of TMDC nanostructures, by coupling the piezoelectric effect with the photocatalytic effect, by doping with other materials, etc. This review discusses various techniques of synthesis of TMDCs nanostructures and the recent progresses in applications of TMDC nanomaterials in piezocatalysis. In the present article, the piezocatalytic dye degradation performances and HER activity using different TMDCs have been reviewed in detail. Different methods of increasing the piezocatalytic activity of various TMDCs nanostructures have been illustrated. Here, it has also been attempted to systematically summarize and provide an outlook of the charge transfer behaviour and catalytic mechanisms in large varieties of TMDC piezocatalysts and piezo-photocatalysts. In addition, advanced applications of TMDC piezocatalytic materials as piezoelectric nanogenerator, piezocatalytic dye degradation, piezo-phototronic dye degradation and HER studies have been highlighted.

Self-Assembly of Solvent-Stabilized Au Nanocluster as Efficient Förster Resonance Energy-Transfer Initiator for White Light Generation.

Aggregation-induced enhancement (AIE) in the photoluminescence quantum yield (PLQY) from 12.5 to 51% in the N,N-dimethylformamide (DMF)-stabilized Au nanocluster (AuNC) system is reported here. The self-assembling of AuNC has been achieved via hydrogen bonding interaction, which is further utilized in designing the AuNC_DCM system for realizing a Förster resonance energy transfer (FRET)-based white LED (WLED), having CIE coordinates of (0.35, 0.29). The solution-processed fabrication strategy used, has given us the liberty to optimize its components for optimal full-spectrum light output. The CIE coordinates of the designed WLED have been improved further to (0.33, 0.32), with a high color rendering index of 93 and correlated color temperature of 5620 K by incorporating a green emitter, namely nitrogen-doped graphene quantum dots (NGQD), in the AuNC_DCM system. The excellent spectral quality of the as-designed WLED and the repeatability of the proposed fabrication method will make the developed AuNCs_DCM FRET conjugate useful in practical photonic applications.

Syntheses of flower and tube-like MoSe2 nanostructures for ultrafast piezocatalytic degradation of organic dyes on cotton fabrics.

The synthesis of few-layered transition metal dichalcogenides (TMDCs) with abundant exposure of the active site, viz., is an important key to achieve excellent dye degradation performance. Here, we have reported synthesis and ultrafast dye degradation performance of flowers-like MoSe2 nanostructure (FMN) with ~230 nm in diameter and its transformation to tube-like MoSe2 microstructure (~1 µm in length) by tuning the solvothermal reaction time. The piezoelectric devices are developed using the FMNs delivers the highest open-circuit voltage of ~ 2.12 V, which is ~21 times higher than that of the developed device with the tube-like MoSe2 microstructure. The piezoelectric property of the synthesized samples has been judiciously utilized further for ultrafast degradation of organic dyes within 60-120 s only under the low-frequency (40 kHz) ultrasonication vibration in the dark. The estimated dye degradation efficiencies of the FMNs-based piezocatalyst are found to be ∼86% and 85% for degradation of Rhodamine B (RhB) and methylene blue (MB) dye within the 60 s, respectively. Also, the FMN has exhibited an excellent piezocatalytic dye degradation capability for RhB-MB dye mixture and dye loaded on a cotton fabric with an efficiency of ~98% (60 s) and 84% (120 s), respectively. The piezocatalytic dye degradation mechanism of FMNs has also been explained theoretically.

Unusual higher-order nonlinear optical properties in Au-coated triangular Ag-Au nanostructures.

Here, we report hitherto unobserved local field (LF)-assisted pump wavelength-dependent nonlinear optical (NLO) effects of three-photon (3PA)-induced four-photon absorption (4PA) at 532 nm and two-photon-induced 3PA at 730 nm in triangular-shaped core-shell Ag-Au nanoparticles (TrAg@Au) by femtosecond Z-scan. The shell thickness-dependent enhancement in the LF is observed by a COMSOL simulation. The intensity-dependent interplay between saturable and reverse-saturable absorptions along with switching of nonlinear (NL) phase is reported at 730 nm, showing the superiority of TrAg@Au in optical switching (OS). The optical limiting (OL) threshold (Fth) of 5.9(6.5)mJ/cm2 at 730 (532) nm boost their potential over benchmarked materials.

Reversible temperature-dependent photoluminescence in semiconductor quantum dots for the development of a smartphone-based optical thermometer.

Photoluminescence (PL) intensity-based non-contact optical temperature sensors are in great demand due to their non-contact nature, rapid response, sensitivity, as well as thermal and chemical stability at different environmental conditions. However, herein, reversible temperature dependent PL emission quenching properties of chemically synthesized Mn2+-doped ZnS QDs (MZQDs) have been advantageously utilized for achieving the development of a smartphone-based optical thermometer. The temperature dependent variations of PL have been studied by taking MZQDs in various forms, such as in aqueous dispersion, powder form, and a polymer-encapsulated thin film. The origin of the PL quenching of MZQD in the polymer film has been cross-verified through temperature-dependent electrical conductivity measurement and the movement of charge carriers has also been confirmed by the first-principles DFT simulation. Through thermal cycling experiments on QD-encapsulated polymer film and by utilizing an indigenously-developed Android App based on color coordinates, a novel smartphone-based colorimetric imaging method for the measurement of temperature has been demonstrated in this work. The synthesized smart QDs might be suitable candidates for temperature sensing and the colorimetric thermometer probe may be utilized in various photonics applications as a smart optical sensor for daily life applications.

Forster resonance energy transfer assisted white light generation and luminescence tuning in a colloidal graphene quantum dot-dye system.

The development of white light-emitting LED (WLED) is now one of the important demands of present society due to its energy saving feature. However, in this work, an ingenious strategy has been employed to develop Forster resonance energy transfer (FRET) based and rare-earth material free luminescent duo (LD) for the generation of white light. A colloidal LD, consisting of nitrogen-doped graphene quantum dots (N-GQDs) and DCM dye (DCM@N-GQDs), is employed for demonstrating the FRET. The band-gap is engineered by nitrogen (N) doping in GQDs, and an energy transfer efficiency of ~30% for white light generation is attained. The widely tunable PL emission from blue to the red region has been obtained by changing the D-A ratio. Therefore, the present work has provided an alternate approach to widen the light emission band of a conventional laser dye, which is otherwise restricted within a limited region of the visible spectrum. FRET-based WLED (F-WLEDs) with colour rendering index of 70 and correlated colour temperature of 4690 K have also been fabricated. The F-WLED exhibits an emission overlapping of 56% with the solar spectrum (AM 1.5) in visible region, which is doubled in compared to that of a commercial WLED. The present report of rapid synthesis of highly luminescent N doped GQDs and the strategy used here for generation of FRET based colloidal DCM-GQDs luminescent duo may also be extended further with other suitable laser dyes for further widening as well as tuning the spectral range of light emissions of different commercially available laser dyes and GQDs for their different photonic applications.

Antimicrobial Synergy of Silver-Platinum Nanohybrids With Antibiotics.

Various bacterial pathogens are responsible for nosocomial infections resulting in critical pathophysiological conditions, mortality, and morbidity. Most of the bacterial infections are associated with biofilm formation, which is resistant to the available antimicrobial drugs. As a result, novel bactericidal agents need to be fabricated, which can effectively combat the biofilm-associated bacterial infections. Herein, for the first time we report the antimicrobial and antibiofilm properties of silver-platinum nanohybrids (AgPtNHs), silver nanoparticles (AgNPs), and platinum nanoparticles (PtNPs) against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. The AgPtNHs were synthesized by a green route using Dioscorea bulbifera tuber extract at 100°C for 5 h. The AgPtNHs ranged in size from 20 to 80 nm, with an average of ∼59 nm. AgNPs, PtNPs, and AgPtNHs showed a zeta potential of -14.46, -1.09, and -11.39 mV, respectively. High antimicrobial activity was observed against P. aeruginosa and S. aureus and AgPtNHs exhibited potent antimicrobial synergy in combination with antibiotics such as streptomycin, rifampicin, chloramphenicol, novobiocin, and ampicillin up to variable degrees. Interestingly, AgPtNHs could inhibit bacterial biofilm formation significantly. Hence, co-administration of AgPtNHs and antibiotics may serve as a powerful strategy to treat bacterial infections.

Tailoring of structural and photoluminescence emissions by Mn and Cu co-doping in 2D nanostructures of ZnS for the visualization of latent fingerprints and generation of white light.

There has been a recent demand for the development of luminescent materials for visualizations of latent fingerprints (LFPs) for achieving enhanced security. Also recently, there has been a new research trend in the development of 2D materials from non-layered semiconductors with strong luminescence properties in the visible region. The conventional growth process of luminescent materials limits their capacity of tuning the structure and light emission efficiency. However, multi-atom doping provides an additional degree of freedom to tune the basic morphologies and optical properties of luminescent semiconductors by controlling the defect levels. Here, by using a simple chemical technique, multi-atom (Cu and Mn) doped rarely reported 2D nanosheets of zinc sulphide (ZnS) have been grown. Thus, a stable high fluorescence efficiency of ∼62% in the visible region has been realized for the visualization of LFPs. Furthermore, near-white light emission has been demonstrated by coating the synthesized materials with a suitable doping concentration on a commercially available UV-LED chip. The proposed technique may be utilized further to build up other 2D nanostructured materials for multifunctional applications in solid state lighting, LFPs and forensic science.

Gold nanostructures for the sensing of pH using a smartphone.

Recently, metal nanostructures have been found to be capable of recognizing small changes in their surrounding environment, which can be utilized as significant sensing tools. In this study, we demonstrated colorimetric sensing of pH by gold nanostructures (GNs) using a simple smartphone. An indigenously developed Android app based on the CIELab 1931 analysis, which could run in a smartphone, was used for the precise determination of the pH value of liquid media. The pH value of an unknown solution obtained from the developed Android app was also compared with that obtained from the conventional ratiometric technique and a commercial pH meter. In another endeavor, it was found that the synthesized GNs demonstrated a high energy transfer efficiency from a donor (namely, the rhodamine 6G, (Rh 6G)) dye. This property of the GNs can be utilized further in the future for studying different bimolecular activities within the human body. It was found that the photoluminescence (PL) of Rh 6G was quenched when it was kept in the vicinity of the synthesized GNs, which was explained in terms of the Förster energy transfer mechanism. Thus, the present study will open up a plethora of opportunities for researchers to employ the nanostructures of gold and other metals in developing low-cost and Internet of Things (IoT)-based sensing devices using only a smart phone.

Resonance energy transfer-assisted random lasing in light-harvesting bio-antenna enhanced with a plasmonic local field.

Thanks to the advent of the random laser, new light applications have opened up, ranging from biophotonic to security devices. Here, by using the well-known but unexplored light-harvesting bio-pigment of butterfly pea (Clitoria ternatea, CT) flower extract, generation of continuous-wave (CW) random lasing at ∼660 nm has been demonstrated. Furthermore, a wavelength tunability of ∼30 nm in the lasing emission was obtained by utilizing the resonance energy transfer (RET) mechanism in a gain medium with a binary mixture of CT extract and a commercially available methylene blue (MB) dye as the gain medium. In the CT extract-dye mixture, the bio-pigments are acting as donors and the MB dye molecules are acting as acceptors. Amplification in intensity of the lasing emission of this binary system has further been achieved in the presence of optimized concentrations of metal (Ag)-semiconductor (ZnO) scattering nanoparticles. Interestingly, the lasing threshold has been reduced from 128 to 25 W cm-2, with a narrowed emission peak just after loading of the Ag nanoplasmon in the ZnO-doped binary gain medium. Thanks to the strong localized electric field in the metal nanoplasmon, and the multiple scattering effects of ZnO, the lasing threshold was reduced by approximately four times compared to that of the gain medium without the use of scatterers. Thus, we believe that our findings on wavelength-tunable, non-toxic, biocompatible random lasing will open up new applications, including the design of low-cost biophotonic 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 60 min 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.

Colloidal N-Doped Graphene Quantum Dots with Tailored Luminescent Downshifting and Detection of UVA Radiation with Enhanced Responsivity.

Luminescent downshifting (LDS) materials are in great demand for their applications in light conversion devices. In this work, by an ingenious chemical approach of in situ doping, N-doped graphene quantum dots (N-GQDs) have been synthesized with tailored green photoluminescence (PL) under ultraviolet (UV) light excitation. The incorporation of N atoms in the form of pyridinic and graphitic C-N bonding into the sp2-hybridized graphitic framework of N-GQDs has led to tailored LDS via PL emissions. The LDS property of synthesized N-GQDs has been advantageously utilized to demonstrate enhanced responsivity (R) of a low-cost commercially available photoconductive cell (PC) for detection of UVA radiation through an indigenous technique. The linear optical responses of samples are optimized by varying the concentration and the dispersing medium. Also the N-GQDs are shown to be photostable in poly(vinyl alcohol) (PVA) hydrogel. A 60% enhancement in photocurrent of the PC-based photodetector under UV radiation has been obtained here by using N-GQDs/PVA as LDS material. Thus, detection of UVA radiation with a high specific detectivity (D*) of 9 × 1013 Jones and responsivity (R) of 3 A W-1 has been demonstrated, which might open the opportunity of using this material in future energy conversion devices.

Quasi-monocyclic near-infrared pulses with a stabilized carrier-envelope phase characterized by noncollinear cross-correlation frequency-resolved optical gating.

Precise characterization of noncollinear optical parametric amplifier idler pulses that have bandwidths of more than an octave with a center wavelength at 990 nm was demonstrated. The method employed was cross-correlation frequency-resolved optical gating with broadband sum-frequency mixing to take advantage of the idler's angular dispersion. Compression to near the transform limit was achieved to produce quasi-monocyclic near-infrared pulses by use of a deformable membrane mirror.

Sellmeier dispersion for phase-matched terahertz generation in ZnGeP2.

A Sellmeier dispersion of zinc germanium diphosphide (ZnGeP2) crystal has been formulated to determine the phase-matching characteristics of the crystal for the generation of coherent tunable terahertz radiation by difference-frequency mixing techniques. The results computed with the formulated Sellmeier dispersion provide an excellent fit to the experimental data.