Showing papers by "Samit K. Ray published in 2019"

MoSe2–Cu2S Vertical p–n Nanoheterostructures for High-Performance Photodetectors

Abstract: Heterostructures based on atomically thin two-dimensional layered transition metal dichalcogenides are highly promising for optoelectronic device applications owing to their tunable optical and electronic properties. However, the synthesis of heterostructures with desired materials having proper interfacial contacts has been a challenging task. Here, we develop a colloidal synthetic route for the design of MoSe2–Cu2S nanoheterostructures, where the Cu2S islands grow vertically on top of the defect sites present on the MoSe2 surface, thereby forming a vertical p–n junction having plasmonic characteristics. These MoSe2–Cu2S nanoheterostructures are used to fabricate photodetectors with superior photoresponse characteristics. The fabricated device exhibits a broad-band spectral photoresponse over the visible to near-infrared range with a peak responsivity of 410 mA W–1 at −2.0 V and over 3000-fold photo-to-dark current ratio. The superior device performance of MoSe2–Cu2S over only MoSe2 devices is due to the...

Ethylenediamine-Assisted High Yield Exfoliation of MoS2 for Flexible Solid-State Supercapacitor Application

Abstract: A high yield synthesis technique of few-layer MoS2 by ethylenediamine (EDA)-assisted liquid phase exfoliation is reported. In the cyclic two-step process, EDA is adsorbed initially followed by sonication with propylene carbonate. The combination of organic solvents results in the exfoliation of micrometer sized few-layer two-dimensional (2D) MoS2 nanosheets within a short duration. Controlled gradual centrifugations separate 2D MoS2 of different sizes ranging from nanocrystals to micrometer sized flakes. The method differs from other sonication-induced fragmentation processes and results in larger sized nanosheets up to ∼2.0 μm of lateral dimension having a thickness of ∼2–3 nm with high yield. Optical properties of the 2D nanosheets are studied by UV–vis absorption and Raman spectroscopy. Finally, the exfoliated MoS2 is employed to fabricate flexible solid-state supercapacitors, which shows superior performance compared to bulk MoS2-based devices, indicating its potential for energy storage applications.

P-type β-MoO 2 nanostructures on n-Si by hydrogenation process: synthesis and application towards self-biased UV-visible photodetection.

Abstract: We report on the synthesis and UV-vis photodetection application of p-type MoO2 nanostructures (NSs) on Si substrate. β-MoO2 NSs have been synthesized from previously grown α-MoO3 structures/n-type Si via a hydrogenation process at 450 °C. After hydrogenation, the α-MoO3 structures were completely converted into β-MoO2 NSs without the presence of sub-oxidized phases of molybdenum oxide. The as-grown NSs exhibited very good p-type electrical conductivity of ≈2.02 × 103 S-cm-1 with hole mobility of ≈7.8 ± 1.3 cm2-V-1-Sec-1. To explore optoelectronic properties of p-type β-MoO2 NSs, we have fabricated a p-MoO2/n-Si heterojunction photodetector device with Au as the top and Al as the bottom contacts. The device exhibits peak photoresponsivity of ≈0.155 A W-1 with maximum detectivity ≈1.28 × 1011 cm-Hz1/2-W-1 and 44% external quantum efficiency around ≈436 nm, following the highest photoresponse (I ph/I d ≈ 6.4 × 102) and good response speed (rise time ∼29 ms and decay time ∼38 ms) at -1.5 V. Importantly, this device also shows good self-powered high-speed (rise time ∼47 ms and decay time ∼70 ms) photodetection performance with peak responsivity and detectivity of ≈45 mA W-1 and ≈4.05 × 1010 cm-Hz1/2-W-1, respectively. This broadband UV-visible light detection feature can be attributed to the coordinated effects of MoO2 band-edge absorption, interfacial defects and self absorption in Si. The photodetection behavior of the device has been understood by proposed energy-band diagrams with the help of an experimentally derived work function, band gap and valence band maximum position of MoO2 NSs.

Förster Resonance Energy Transfer Mediated Charge Separation in Plasmonic 2D/1D Hybrid Heterojunctions of Ag–C3N4/ZnO for Enhanced Photodetection

Abstract: We report on the events of Forster resonance energy transfer (FRET) and direct transfer (DT) of electrons in two-dimensional graphitic carbon nitride (g-C3N4) nanosheets-based plasmonic hybrid hete...

Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties.

Abstract: Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI3 cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr3 nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr3 NWs, indicating its potenti...

Light Trapping-Mediated Room-Temperature Gas Sensing by Ordered ZnO Nano Structures Decorated with Plasmonic Au Nanoparticles.

Abstract: An ordered array of 1D ZnO nanorods obtained by colloidal templating is shown to dramatically enhance the sensing response of NO x at room temperature by confining light and creating periodic structures. The sensitivity is measured for a concentration varying from 2 to 10 ppm (response 53% at 10 ppm) at room temperature under white light illumination with ≈225 nm hole diameter. In contrast, structures with ≈450 nm hole size show better sensing under (response 98% at 10 ppm) elevated temperatures in dark conditions, which is attributed to the increased surface chemical interactions with NO x molecules due to the porous nature and enhanced accessible surface area of ZnO nanorods. Further, the decoration of ZnO Nanorods with gold nanoparticles shows enhanced sensor performance (response 130% at 10 ppm) due to localized surface plasmon resonance under white light illumination. The findings may lead to new opportunities in the visible light-activated room-temperature NO x sensors for healthcare applications.

Photoresponse characteristics of MoS2 QDs/Si nanocone heterojunctions utilizing geometry controlled light trapping mechanism in black Si.

Abstract: A unique light trapping mechanism associated with nano-conical textured black Si templates has been utilized to achieve improved photoresponse in MoS2QDs/Si heterojunctions over a wide wavelength range from visible to near infrared. Black Si templates have been fabricated by a simple and cost effective metal assisted chemical etching technique followed by spin-coating of colloidal MoS2 quantum dots (QDs) to form the heterojunction. A peak responsivity of as high as ∼1.39 A W-1 at ∼665 nm for a bias of 5 V has been achieved. The responsivity value is higher as compared to recently published results having similar device structure. The combination of MoS2 QDs and black Si has resulted in a broader spectral response with enhanced optical absorption in the nano-conical heterojunction devices. Finite element based optical simulation results revealed the superiority of MoS2 QDs/Si nano-conical heterojunctions due to improved light trapping. The results appear attractive for next generation Si CMOS compatible broad band photodetectors using two dimensional semiconductors.

Solution Processed Highly Responsive UV Photodetectorsfrom Carbon Nanodot/Silicon Heterojunctions

Abstract: Carbon nanostructures technology has recently emerged as a key enabler for next-generation optoelectronic devices working in the deep UV region due to their excitonic absorption. Here, we report th...

Geometry Controlled White Light Emission and Extraction in CdS/Black-Si Conical Heterojunctions

Abstract: The light emission characteristics of Si nanocrystallites on a chemically etched black-Si surface, coupled with its excellent light extraction feature of nanocone geometry is reported for potential...

Optical Tamm state aided room-temperature amplified spontaneous emission from carbon quantum dots embedded one-dimensional photonic crystals

Abstract: Optical Tamm state (OTS) aided room-temperature amplified spontaneous emission (ASE) from carbon quantum dots (CQDs) embedded all-dielectric one-dimensional photonic crystals (1DPhCs) is presented. 1DPhCs, constituting twelve pairs of alternating quarter wave thick SiO2 and TiO2 thin films are fabricated by sol-gel synthesis route. The 1DPhCs are covered with ~50 nm thick silver thin film to obtain the Tamm structures. CQDs are prepared using organic precursors (onion pulp) and incorporated in the TiO2 matrix of the final four pairs of the 1DPhCs. OTSs are observed in the reflection spectra at a detection angle of ~15°, at ~648 nm and ~618 nm, for the samples with and without CQDs respectively. Comparisons of enhancement of photoluminescence from samples with and without CQDs are presented. ASE at ~648 nm corresponding to the OTS of the CQDs incorporated Tamm structure, and suppression of emission within the photonic stop-band is demonstrated.

Facile and time-resolved chemical growth of nanoporous CaxCoO2 thin films for flexible and thermoelectric applications

Abstract: CaxCoO2 thin films can be promising for widespread flexible thermoelectric applications in a wide temperature range from room-temperature self-powered wearable applications (by harvesting power from body heat) to energy harvesting from hot surfaces (e.g., hot pipes) if a cost-effective and facile growth technique is developed. Here, we demonstrate a time resolved, facile and ligand-free soft chemical method for the growth of nanoporous Ca0.35CoO2 thin films on sapphire and mica substrates from a water-based precursor ink, composed of in-situ prepared Ca2+-DMF and Co2+-DMF complexes. Mica serves as flexible substrate as well as sacrificial layer for film transfer. The grown films are oriented and can sustain bending stress until a bending radius of 15 mm. Despite the presence of nanopores, the power factor of Ca0.35CoO2 film is found to be as high as 0.50 x 10-4 Wm-1K-2 near room temperature. The present technique, being simple and fast to be potentially suitable for cost-effective industrial upscaling.

Qualitative behavior of three species food chain around inner equilibrium point: spectral analysis

Abstract: This work deals with analytical investigation of local qualitative temporal behavior around inner equilibrium point of a model for three species food chain, studied earlier by Hastings and Powel and others. As an initial step towards the spectral analysis of the model, the governing equations have been split into linear and nonlinear parts around arbitrary equilibrium point. The explicit parameter dependence of eigenvalues of Jacobi matrix associated to the linear part have been derived. Analyzing these expressions in conjunction with some pedagogical analysis, a lot of predictions on stable, unstable or chaotic change of species have been highlighted. Agreement of predictions of this work with available numerical or semi-analytical studies suggest the utility of analytical results derived here for further investigation/analysis of the model as desired by earlier works.

Liquid Exfoliated NiO Nanosheets for Trace Level Detection of Acetone Vapors

Abstract: Acetone, a colorless volatile organic compound (VOC), is exhaled in large concentrations in the breath of diabetic patients. In this paper, a simple liquid exfoliation technique-based few-layered NiO (111) plane exposed nanosheets (diameter 125 nm) were synthesized [in dimethylformamide (DMF) as solvent] and used as highly selective trace level (15–450 ppm) acetone sensors. The sensors were found to be fast (response and recovery time of 82 and 80 s, respectively) and repetitive with a maximum response of 196% (for 450 ppm acetone). The limit of detection of the device is 830 ppb. The operating temperature of the sensor was optimized at 200 °C in comparison to other reported NiO-based VOC sensors mainly due to high surface energy aiding oxygen adsorption at a lower temperature. An “assistive binding model” is proposed to explain sensor performance in the presence of humid air. The model showed positive sensitivity and binding coefficients ( ${s}_{a} = {0.242}$ ppm−1 and ${k}_{{{aw}}} = {2.04} \times 10^{-{3}}$ ), proposing a synergistic effect of water on the sensor response for acetone vapors.

Selective photoresponse of plasmonic silver nanoparticle decorated Bi 2 Se 3 nanosheets

Abstract: The plasmon-enhanced photoresponse properties of a Ag-nanoparticle-decorated-Bialt;subagt;2alt;/subagt;Sealt;subagt;3alt;/subagt;-nanosheet (AGBS)/p-Si heterojunction device have been studied. The Ag nanoparticles, the Bialt;subagt;2alt;/subagt;Sealt;subagt;3alt;/subagt; nanosheets, and the AGBS nanocomposite are synthesized chemically. Microscopic investigations, ultimately of the AGBS nanocomposite, reveal that the Bialt;subagt;2alt;/subagt;Sealt;subagt;3alt;/subagt; nanosheets of thickness ~ 20 nm and lateral dimension ~ 1 ?m are decorated with Ag nanoparticles of sizes 20 - 40 nm in the nanocomposite. The X-ray diffraction pattern of AGBS shows that apart from being in the metallic state, Ag in AGBS is also in the form of compounds with Bi, Se, and additionally O. This observation is further complemented by the X-ray photoelectron spectrum, which shows the presence of Agalt;supagt;0alt;/supagt; and Agalt;supagt;+alt;/supagt; states of Ag in AGBS. The UV-visible absorption spectra show the plasmonic peak of the Ag nanoparticles to occur at 420 nm; the peak is shifted to ~ 500 nm in AGBSa#13; due to the modified dielectric environment of the nanoparticles. The AGBS/p-Si heterojunction shows excellent photoresponse properties, with a responsivity of 0.28 A/W, a fairly high detectivity of 4×10alt;supagt;10alt;/supagt; Jones, and an EQE of 71% under 10 V reverse bias at 500 nm wavelength. The plasmon enhanced photoresponse at the selective wavelength makes this material attractive for high performancea#13; optoelectronic devices.

Occurrence and diversity of microalgae in phytoplankton collected from freshwater community ponds of Hooghly District, West Bengal, India

Abstract: A study on diversity, seasonal occurrence, distribution percentage of microalgal taxa and physico-chemical parameters of five community ponds, located in Chinsurah town, Hooghly district of West Bengal, has been carried out. Correlation between occurrence of microalgal genera and some parameters of environment, physico-chemical nature of natural water bodies were explored by Canonical Correspondence Analysis (CCA). Diversity indices have been calculated using PAST software program. A total of 23 microalgal taxa belonging to four algal classes were recorded and the study indicated that the microalgal diversity vary with variation in physico-chemical parameters of water and light intensity. Out of these genera, occurrence of Chodatella sp. in late summer and Tetrallantos sp. and Synechocystis sp. in winter season were specific. In CCA, multiple variables (dissolved oxygen, water temperature, electrical conductivity, pH, light intensity and inorganic phosphorous) played a significant role in occurrence of microalgal taxa. Observation concluded that the Chlorophycean members were dominant throughout the study period and the Shannon–Wiener diversity index was highest for a site with large number of Chlorophycean member. This study will help in future to assess water quality.

Morphological variations in geographically isolated populations of Bryum coronatum Schwager (Bryaceae, Bryales)

Abstract: Four populations of Bryum coronatum Schwager (Bryaceae, Bryales), collected from widely separated geographic regions - Sambalpur, Odisha; Lataguri, West Bengal; Gangtok and Changu, Sikkim; were studied with regard to variation in taxonomically significant morphological features as well as their dimension of both gametophytic and sporophytic features. In this study special emphasis has been put on spore dimension and spore ornamentation pattern as revealed by Scanning Electron Microscopy (SEM). Dimension of taxonomically significant morphological parameters were measured for four populations and were subjected to analysis of variance (One way ANOVA) and Principal component analysis (PCA). ANNOVA results showed significant variation for all features except leaf length and upper leaf cell breadth. PCA results corroborated ANNOVA result only for upper leaf cell breadth. Spore ornamentation as revealed through SEM, is found to be bacculate in all the populations. CCA graph revealed that summer, winter temperature and monsoon rainfall influence leaf size, lamina cell size, capsule size, peristome size and spore size. With the help of above mentioned observations and statistical analysis the morphological variations, due to phenotypic plasticity have been quantified to reach at a meaningful conclusion. Taxonomic significance of spore dimension and wall ornamentation is also discussed.