scispace - formally typeset
Search or ask a question

Showing papers by "Samit K. Ray published in 2018"


Journal ArticleDOI
TL;DR: The fabrication of flexible solid state supercapacitor devices using WS2 nanocrystals is reported in this article, where the charge storage performance of WS2 quantum dots and nanosheets is compared in a solid state symmetric two electrode system for the first time.
Abstract: The fabrication of flexible solid state supercapacitor devices using WS2 nanocrystals is reported in this paper. The charge storage performance of WS2 quantum dots (QDs) and nanosheets is compared in a solid state symmetric two electrode system for the first time. A lithium bromide-assisted lithium intercalation and sonication method has been employed to synthesize WS2 nanosheets and QDs. The formation of defect-rich QDs of size varying from 1 to 3 nm by fragmentation of WS2 nanosheets is confirmed through different spectroscopic and microscopic techniques. The fabricated solid state devices using WS2 QDs show a high areal specific capacitance (28 mF cm−2) with high energy density (1.49 μW h cm−2), at a current density of 0.1 mA cm−2. The superior performance of the QD-based devices as compared to that of the nanosheets is attributed to the large number of defect states present in the WS2 QDs. Fabricated solid state devices exhibit good flexibility and superior retention of specific capacitance (80% after 10 000 cycles), indicating WS2 as a potential candidate for future unconventional energy storage devices.

39 citations


Journal ArticleDOI
TL;DR: In this article, high uniform and dense GeO2 nanowires with Ge as the core were grown on Si (001) substrates by vapor-liquid-solid technique with an aim to utilize geO2 as the active material for photodetectors.
Abstract: Highly uniform and dense GeO2 nanowires with Ge as the core were grown on Si (001) substrates by vapor–liquid–solid technique with an aim to utilize GeO2 as the active material for photodetectors. X-ray photoelectron spectroscopy was carried out to investigate the composition and interfaces of the resultant Ge/GeO2 NWs. The observed broad and visible photoluminescence emission from as-grown core–shell NW heterostructure is attributed to oxygen-related defect states in the GeO2 shells. Single Ge-GeO2 NW based metal–semiconductor–metal photodetectors were fabricated using nanolithography techniques. Self-driven (zero bias) detectors have been found to be responsive to a broadband spectrum from 350 to 900 nm with a peak responsivity (∼0.6 × 104 A/W) and detectivity (∼3.8 × 1012 Jones). The high gain in photocurrent has been explained using a back-to-back metal–semiconductor junction model in single NW. The device also shows sensitivity to polarization-dependence of light source. Finite element method (FEM) b...

36 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive study employing Raman spectroscopy is reported on N and Li implantation effects in ZnO, showing that the deep acceptor level may be at approximately 300 ǫmeV above the valence band consistent with previous results.

28 citations


Journal ArticleDOI
01 Dec 2018-Carbon
TL;DR: In this article, a one-pot, facile and rapid synthesis technique was used to synthesize high luminescent, crystalline graphene quantum dots (GQDs) of homogenous size and shape with high yield.

26 citations


Journal ArticleDOI
TL;DR: Improved piezo-phototronic properties originated from the plasmonic properties of Au loaded g-C3N4 and the piezoelectric characteristics of c-axis oriented ZnO films may be used for future flexible photonic devices.
Abstract: We have studied the piezo-phototronic induced enhancement in the photo-response of CdS/ZnO heterojunctions attached with plasmonic Au nanoparticle loaded 2D-graphitic carbon nitride (g-C3N4) The hybrid g-C3N4/CdS/ZnO heterojunction favours the charge carrier separation through the formation of a step-like band alignment Furthermore, the integration of plasmonic Au loaded g-C3N4 nanosheets on the conventional CdS/ZnO heterojunction facilitates improved visible light absorption properties The heterojunction device on a flexible platform under the application of a strain (∼0017%) exhibits ∼102 times higher photoresponse over the control sample at a constant bias of ∼2 V The variation in the photo-response under different bending conditions has been explained in terms of the improved charge transport through the modified energy bands at the interface of ZnO The improved piezo-phototronic properties originated from the plasmonic properties of Au loaded g-C3N4 and the piezoelectric characteristics of c-axis oriented ZnO films may be used for future flexible photonic devices

24 citations


Journal ArticleDOI
02 Nov 2018
TL;DR: In this paper, a detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of the excitons and biexcitons in layered transition metal dichalcogenides (TMDs) at room temperature.
Abstract: Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS2 (mono-to-quad) dispersion using broadband (350–750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS2 has similar spectral features as monolayer WS2 in terms of saturation absorptions (SA) and excited state absorptions (ESA). Similar to monolayer TMDs, we are able to identify excitons and biexcitons in multi-layered 2D stratum of WS2 as well as calculate the biexciton binding energies ( 69 meV and 66 meV), which are in excellent agreement with earlier theoretical predictions. Furthermore, using many-body physics, we demonstrate that the excitons in layered WS2 behave like Wannier–Mott excitons and explain their origins via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of the excitons and biexcitons in layered WS2. Indeed, our results unravel the complex optical response of layered TMDs, which should lead to numerous technological applications for developing excitonic quasiparticle-based valleytronic devices and ultrafast biexciton lasers at room temperature.

22 citations


Journal ArticleDOI
TL;DR: An organic salt composed of 4,4′-dinitro-2,2′,6,6′-tetracarboxybiphenyl and acridine has been shown to act as host to accommodate large polyaromatic guest molecules.
Abstract: An organic salt composed of 4,4′-dinitro-2,2′,6,6′-tetracarboxybiphenyl and acridine has been shown to act as host to accommodate large polyaromatic guest molecules. Carboxylate–pyridinium hydrogen bonds, π–π and cation···π interactions are found to play a pivotal role in these host–guest complexes. All the complexes are found to exhibit distinct colors and photoluminescence which correlate with the charge-transfer interactions between acridinium ion and guest molecules, and ionization potential or donor ability of the guest molecules.

19 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reported on the enhancement of the visible luminescence of Au-ZnO nanocomposites synthesized via a simple photoreduction method.

18 citations


Journal ArticleDOI
TL;DR: In this article, a hierarchical NiO nanostructure sensing layer was used as highly sensitive formaldehyde sensor in the presence of light and the response was found to be 292% with a fast response of 24 s and recovery time of 42 s.
Abstract: Indoor air quality (IAQ) monitoring is essential to maintain healthy human life. Among different pollutants affecting IAQ, formaldehyde is one of the toxic volatile organic compounds (VOC) that needs to be monitored. In this paper, hierarchical NiO nanostructure sensing layer was used as highly sensitive formaldehyde sensor in the presence of light. The response was found to be 292% in the presence of 190 ppm formaldehyde with a fast response ~24 s and recovery ~42 s time. The sensor was found to be highly specific towards formaldehyde compared to other VOCs. The LOD calculated for the sensor was as low as 910 ppb. The optimum operating temperature was found to be ~300°C, much less than the conventional NiO-based sensors (~600°C). The sensing layer was found to be optically active due to the induced defects in the NiO nanostructure. Thus, the fabricated light-assisted sensor shows potential for future commercial formaldehyde sensor.

14 citations


Journal ArticleDOI
TL;DR: The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail.
Abstract: The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail. The metallic 1T phase NCs were prepared by the Li+ intercalation–deintercalation exfoliation techniques followed by prolonged sonication. The effect of ex situ thermal annealing on MoS2 polymorphs and their transformation from the 1T to 2H phase has been extensively monitored by the X-ray photoelectron, Raman, and optical absorption spectroscopy techniques. Electrical conductivity measurements have also been carried out to probe the phase transition of the synthesized NCs. The temperature-dependent (10–350 K) electrical charge transport properties of variable-sized NCs have been investigated to probe the scaling of conductivity and activation energy with size, which are yet to be reported experimentally. The charge transport mechanisms through the NC assembly for different temperature regions have been modeled and it is observed that the electron trans...

13 citations


Journal ArticleDOI
01 Aug 2018-Carbon
TL;DR: In this paper, a delay-dependent dynamic switching of optical nonlinearity in terms of saturation absorption (SA) and excited state absorption (ESA) was achieved with an optical pump and white light super-continuum probe only above a threshold pump intensity.

Journal ArticleDOI
21 Feb 2018
TL;DR: In this paper, a chloroform sensor using functionalised reduced graphene oxide (RGO) as a sensing layer is presented, where the thiol group is covalently attached on the basal plan of RGO film by a simple one-step aryl diazonium chemistry to improve its selectivity.
Abstract: This paper presents a highly selective chloroform sensor using functionalised reduced graphene oxide (RGO) as a sensing layer. Thiol group is covalently attached on the basal plan of RGO film by a simple one-step aryl diazonium chemistry to improve its selectivity. Several spectroscopic techniques like X-ray photoelectron, Raman and Fourier transform infrared spectroscopy confirm successful thiol functionalization of RGO. Finally, the fabricated chemiresistor type sensor is exposed to chloroform in the concentration range 200–800 ppm (parts per million). The sensor shows a 4.3% of response towards 800 ppm chloroform. The selectivity of the sensor is analyzed using various volatile organic compounds as well. The devices show enhanced response and faster recovery attributed to the physiosorption of chloroform onto thiol functionalized graphene making them attractive for 2D materials based sensing applications.

Journal ArticleDOI
TL;DR: In this paper, the authors reported synthesis of ordered hierarchical CuxNi(1−x)O nanostructure (average diameter of (400-600 nm))-based sensing layer for chemoresistive toxic gas sensing application.
Abstract: Chemoresistive toxic gas sensors nowadays are being used exhaustively for maintaining healthy air quality. In this paper, we report synthesis of ordered hierarchical CuxNi(1−x)O nanostructure (average diameter of (400–600 nm))-based sensing layer for chemoresistive toxic gas sensing application. The selectivity of the sensor was found to be temperature tunable. It was sensitive to humidity (response 212% with 90% RH) at room temperature and volatile organic compounds (VOCs) [response towards Ethanol (28%), Methanol (34%), Acetone (25%), and Toluene (44%) at 550 ppm] at 300 °C. The cross sensitivity of the device for VOCs at 300 °C was addressed by performing a low computationally complex clustering technique to define the zone of clear selectivity for each VOC. It was also observed that on sampling the continuous signal with Fast Fourier Transform increases the clustering efficiency by removing the background noise from the sensing transients. The response and recovery times of the sensor (for all analytes) ranged in tens of seconds with good repeatability and stability. Thus, the sensor device so fabricated is expected to serve as a commercially viable temperature tunable and selective gas sensor.

Journal ArticleDOI
TL;DR: The fabrication of ZnO/CdS self-biased heterojunction photodetectors on soft lithographically patterned PEDOT:PSS layers with grating geometry is reported, observing enhanced photoluminescence on patterned devices, in comparison to devices fabricated on flat PEDots.
Abstract: Nanopatterning of the active layer with feature size comparable to the wavelength of visible light is a popular strategy for improving the performance of optoelectronic devices, as these structures enhance the optical path length by light trapping due to combined contribution of multiple scattering, diffraction, and antireflection. Here, we report the fabrication of ZnO/CdS self-biased heterojunction photodetectors on soft lithographically patterned PEDOT:PSS layers with grating geometry. The present study combines the robustness of inorganic devices along with the convenience of easy patterning capability of an organic PEDOT:PSS layer. Patterns with two different line widths (L P = 350 nm, and Lp = 750 nm) have been used in this study to understand the influence of feature dimension on the device performance. We observe enhanced photoluminescence on patterned devices, in comparison to devices fabricated on flat PEDOT:PSS films, which is attributed to the increased interfacial area between the organic and inorganic layers. The spectral response [R( λ )] and specific detectivity [D * ( λ )] are found to be higher for the devices with Lp = 350 nm as compared to other devices due to enhanced absorption within the structures due to confinement of light, which also results in reduced reflectance in devices with Lp = 350 nm.

Journal ArticleDOI
TL;DR: In this paper, the growth dynamics of Ge adatoms under different surface stress regimes of the patterned dielectric to control the selective growth of self-assembled Ge nanostructures on silicon was investigated.

Journal ArticleDOI
TL;DR: A study was conducted in seven geothermal springs located in Bakreswar, District Birbhum, India, in order to assess numerous geochemical processes which were responsible for chemical composition of thermal and mineralized water as mentioned in this paper.
Abstract: A study was conducted in seven geothermal springs located in Bakreswar, District Birbhum, West Bengal, India, in order to assess numerous geochemical processes which were responsible for chemical composition of thermal and mineralized water. The study area lies over the Sone, Narmada, and Tapti lineament of Precambrian Chotanagpur Gneissic Complex. Water chemistry has been carried out based on reaction stoichiometry and geo-statistical tools to identify geochemical process. Piper and Gibbs diagram suggest that the spring water belongs to Ca2+-Mg2+-HCO3− + CO32− water type and are controlled by rock dominance. Dissolution and precipitation of calcite, dolomite, gypsum, and fluorite minerals were identified as principle source of major ions in seven geothermal spring water. Principle component analyses revealed that major ions of spring water are derived from geogenic processes such as weathering, dissolution, and precipitation of various minerals. Overall results suggest that major ions of the spring’s water are derived from natural origin because no evidence of anthropogenic sources was observed during the study period. This study has also revealed that water quality of spring’s water is not suitable for drinking purposes and quite suitable for irrigation because of high abundance of Na+, K+, Cl−, and HCO3− ions.


Proceedings ArticleDOI
01 Oct 2018
TL;DR: In this paper, a ZnO nanorod-based humidity sensor prototype using Arduino Uno microcontroller platform was developed for Internet of Things (IOT) using field emission electron microscopy and X-ray diffraction.
Abstract: Here we report development of a zinc oxide (ZnO) nanorods based humidity sensor prototype using Arduino Uno microcontroller platform. A commercial humidity sensor (DHT11) was also connected with the microcontroller to calibrate ZnO sensor. ZnO nanorods were grown hydrothermally on indium tin oxide (ITO) coated PET substrate. Nanorods were characterised with field emission electron microscopy and X-ray diffraction. Sensors were exposed to 50%–80% relative humidity (RH) at room temperature. The sensor response was found to be 0–76%. Such low cost simple prototype development can open up opportunities for future realization of internet of things (IOT).

Proceedings ArticleDOI
01 Oct 2018
TL;DR: A low-temperature, wet-chemical process for the synthesis of single wall carbon nanotubes (SWCNTs) films on large area, transparent, flexible substrates were developed and its application to ammonia detection at room temperature were demonstrated as discussed by the authors.
Abstract: A low-temperature, wet-chemical process for the synthesis of single wall carbon nanotubes (SWCNTs) films on large area, transparent, flexible substrates were developed and its application to ammonia detection at room temperature were demonstrated. The carboxyl groups were attached in the sidewalls of carbon nanotubes due to wet-chemical process which helps to improve the sensor performance. The transmittance measurement showed that the film is ninety percent transparent to visible spectrum with respect to air. Selectivity measurements were also performed. The sensor responses were same for both the fat and bend film. The present SWCNTs based sensors could potentially be used to develop low cost, flexible, foldable and disposable sensor system for the monitoring of toxic gases like ammonia in environment.

Proceedings ArticleDOI
01 Dec 2018
TL;DR: In this article, a broadband absorption of gold nanoparticle-decorated chlorophyll b was extracted from spinach, and a broad absorption spectrum ranging from 315 nm to 1025 nm was obtained.
Abstract: This paper reports a broadband absorption of gold nanoparticle-decorated chlorophyll b. Chlorophyll b was extracted from spinach. Au nanoparticles were synthesized by chemical method. Structural and optical characterization of chlorophyll-b and Au nano-particles were also studied. Current-voltage characteristics of chlorophyll-b and Au nanoparticle-decorated chlorophyll b were also studied by fabricating devices using etched ITO glass. Chlorophyll-b has a strong absorption in the blue region at 435 nm and in the red region at 665 nm, whereas Au nanoparticle has its plasmonic peak at 535 nm. Au nanoparticle-decorated chlorophyll b gives a broad absorption spectrum ranging from 315 nm to 1025 nm due to the interaction between chlorophyll b molecules with Au nano-particles.

Proceedings ArticleDOI
21 May 2018
TL;DR: In this paper, a simple sono-chemical exfoliation technique was used to obtain confined nano-crystals of MoS2 of average diameter 2 nm, interdispersed in the flakes and study the effect of quantum confinement on this layered semiconductor.
Abstract: We study transient absorption response of few-layered MoS2 nano-flakes in dispersion, mainly focusing on its high energy exciton (commonly known as C exciton). We use a simple sono-chemical exfoliation technique to obtain confined nano-crystals of MoS2 of average diameter 2 nm, inter-dispersed in the flakes and study the effect of quantum confinement on this layered semiconductor. We emphasize on the interplay between exciton bleaching and excited state absorption upon a blue-detuned pumping. The relaxation times for the exciton are found and for the nano-crystals the radiative relaxation process is found to be slower as compared to that of the nano-flakes.

Proceedings ArticleDOI
04 May 2018
TL;DR: Using an optical pump and a time delayed white light super continuum probe, delay dependent switching is achieved between saturation absorption and reverse saturation absorption above a threshold pump intensity in reduced graphene oxide (RGO2) as discussed by the authors.
Abstract: Using an optical pump and a time delayed white light super continuum probe, delay dependent switching is achieved between saturation absorption (SA) and reverse saturation absorption (RSA) above a threshold pump intensity in reduced graphene oxide (RGO2). RGO2 is obtained using photo-thermal reduction and chemical reduction respectively. The wavelength regime which experience switching can be varied by changing the degree of reduction. At 415 nm pump, the threshold intensity to obtain switching property decreases to 9 GW/cm 2 for RGO2 from 18 GW/cm 2 in graphene oxide(GO) and the tunability range shifts from 471-526 nm for as grown GO to 519-623 nm in maximally reduced RGO2.Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm 2 ) than RGO2 (18.2 GW/cm 2 ), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. Decay dynamics of scattering processes show faster relaxation of electron in RGO than in GO. Results are accounted using a model band diagram based on amorphous-carbon model.

Journal ArticleDOI
TL;DR: In this paper, a laboratory synthesized pristine graphene based sensor device was developed for tea quality discrimination by sensing the volatile aroma compounds, responsible for its quality, and the sensing performance was visualized by principal component analysis.

Posted Content
TL;DR: In this paper, a solution processed Carbon nano-dots (CNDs)/n-Si heterojunction showing broadband spectral response with a peak responsivity of ~ 125 A/W in UV (~300 nm) wavelength was reported.
Abstract: Carbon nanostructures technology has recently emerged as a key enabler for next-generation optoelectronic devices including deep UV detectors and light sources which is promising in health and environment monitoring Here, we report the fabrication of solution processed Carbon nano-dots (CNDs)/n-Si heterojunction showing broadband spectral response with a peak responsivity of ~ 125 A/W in UV (~300 nm) wavelength The surface topography and chemical information of synthesized CNDs via a facile synthesis route have been characterized showing the presence of surface chemical states resulting broad optical emission The CNDs/n-Si photo diodes exhibit very low dark current (~500 pA), excellent rectification ratio (~5*10^3), and very good photo-modulation in UV region Given the solution-processing capability of the devices and extraordinary optical properties of CNDs, the use of CNDs will open up unique opportunities for future high-performance, low-cost DUV photo detectors

Proceedings ArticleDOI
01 Oct 2018
TL;DR: In this paper, a NiO/FexNi(1-x)O based p-p heterojunction device has been fabricated via environment friendly hydrothermal approach, which was subjected to controlled VOC vapors (e.g. 2-propanol, toluene and formaldehyde) at operating temperatures varying from 150°C to 350°C.
Abstract: Air quality monitoring is quite essential for maintaining sound human health. In this work, NiO/FexNi(1-x)O based p-p heterojunction device has been fabricated via environment friendly hydrothermal approach. The device was subjected to controlled VOC vapors (e.g. 2-propanol, toluene and formaldehyde) at operating temperatures varying from 150°C to 350°C. It was observed that the sensor can be made selective towards three VOCs (2-propanol, toluene and formaldehyde) by setting the operating temperature at 200°C, 250°C and 300°C respectively. The response was highest for formaldehyde (2.1 times with 76 ppm) and was found to be reasonably fast (response time 58 sec and recovery time 46 sec). Thus, it was possible for selectively sensing different VOCs using a single sensor device by varying the operating temperature which would lead to a smart sensing technology in the future.

Proceedings ArticleDOI
17 May 2018
TL;DR: In this article, an optical Tamm structure is fabricated by depositing silver thin film on onedimensional photonic crystals (1DPhCs), constituting twelve pairs of alternating quarter wave thick SiO2 and TiO2 thin films.
Abstract: Tamm plasmon polaritons (TPPs) are excellent candidates for photonic device application for their intriguing properties and simple fabrication design. In this study, optical Tamm structures are fabricated by depositing silver thin film on onedimensional photonic crystals (1DPhCs), constituting twelve pairs of alternating quarter wave thick SiO2 and TiO2 thin films. Carbon quantum dots (CQDs) were incorporated in the TiO2 matrix of the final four pairs of the 1DPhCs. TPPs are observed in the reflectance spectra of the samples with and without CQDs. With the help of transfer matrix method electric field intensity distribution profile is obtained. It is observed that the electric field is confined and enhanced at the metal-1DPhC interface and decays within the 1DPhC. Comparison of PL emission from samples with and without CQDs in the last four layers are presented. Enhanced PL emission from CQDs corresponding to the TPP mode and suppression of emission within the photonic stop band is demonstrated.

Proceedings ArticleDOI
TL;DR: A simple fabrication of 2D photonic crystal with hexagonal lattice system of air-column in a high-dielectric medium based on 3D photolithography technique and interaction of quantum emitters embedded in a microcavity is presented in this paper.
Abstract: A simple fabrication of 2D photonic crystal with hexagonal lattice system of air-column in a high-dielectric medium based on 3D photolithography technique and interaction of quantum emitters embedded in a microcavity is presented.