Showing papers by "Samit K. Ray published in 2017"
TL;DR: The electroluminescence characteristics are demonstrated for the first time, by forming a heterojunction of stabilizer free WS2 QDs and ZnO thin films, and may pave the way to fabricate prototype futuristic efficient light emitting devices.
Abstract: Sonication induced vertical fragmentation of two-dimensional (2D) WS2 nanosheets into highly luminescent, monodispered, zero-dimensional (0D) quantum dots (QDs) is reported. The formation of 0D structures from 2D sheets and their surface/microstructure characterization are revealed from their microscopic and spectroscopic investigations. Size dependent optical properties of WS2 nanostructures have been explored by UV–vis absorption and photoluminescence spectroscopy. Interestingly, it is observed that, below a critical dimension (∼2 nm), comparable to the Bohr exciton radius, the tiny nanocrystals exhibit strong emission. Finally, the electroluminescence characteristics are demonstrated for the first time, by forming a heterojunction of stabilizer free WS2 QDs and ZnO thin films. The signature of white light emission in the light emitting device is attributed to the adequate intermixing of emission characteristics of WS2 QDs and ZnO. The observation of white electroluminescence may pave the way to fabrica...
92 citations
TL;DR: In this paper, an ultrasound sensitive piezoelectric nanogenerator (PNG) made of a few layers of 2'D-MoS2-incorporated electrospun poly(vinlydine fluoride) (PVDF) nanofiber webs (NFW) is described for the first time.
Abstract: With the widespread use of wearable electronics, portable and flexible energy harvesting devices with a high sensitivity have attracted considerable interest. Herein, an ultrasensitive piezoelectric nanogenerator (PNG) made of a few layers of 2 D-MoS2-incorporated electrospun poly(vinlydine fluoride) (PVDF) nanofiber webs (NFW) is described for the first time. As a result of the semiconducting properties and piezoelectric functionalities of 2 D-MoS2, the resultant piezoelectric performance of PNG can be modulated, which leads to a material suitable for wearable electronics to power devices and to fabricate self-powered biomedical nanosensors for diagnosis, such as heartbeat monitoring, pressure mapping from footsteps, and speech signal abnormality. We have demonstrated that our PNG has a 70 times improvement in acoustic sensitivity than nanosensors made of neat PVDF NFW and are able to charge a capacitor quickly (e.g., 9 V is charged within 44 s). As a result of the ultrafast charging performance and external low-impact detection capability of 2 D-MoS2-modulated PNG, this paves the way to design cost-effective self-powered wearable electronics and robotics.
82 citations
TL;DR: The control over various forms of energy and carrier transport through the unconstrained dimension makes Si and Ge nanowires a promising platform to manufacture advanced solid-state devices.
Abstract: Remarkable progress has been made in the field of one-dimensional semiconductor nanostructures for electronic and photonic devices. Group-IV semiconductors and their heterostructures have dominated the years of success in microelectronic industry. However their use in photonic devices is limited since they exhibit poor optical activity due to indirect band gap nature of Si and Ge. Reducing their dimensions below a characteristic length scale of various fundamental parameters like exciton Bohr radius, phonon mean free path, critical size of magnetic domains, exciton diffusion length etc result in the significant modification of bulk properties. In particular, light emission from Si/Ge nanowires due to quantum confinement, strain induced band structure modification and impurity doping may lead to the integration of photonic components with mature silicon CMOS technology in near future. Several promising applications based on Si and Ge nanowires have already been well established and studied, while others are now at the early demonstration stage. The control over various forms of energy and carrier transport through the unconstrained dimension makes Si and Ge nanowires a promising platform to manufacture advanced solid-state devices. This review presents the progress of the research with emphasis on their potential application of Si/Ge nanowires and their heterostructures for electronic, photonic, sensing and energy devices.
72 citations
TL;DR: The hybrid heterostructure interfaces between atomically thin MoS2 nanocrystals (NCs) on Si platform for their potential applications towards next-generation electrical and optical devices are investigated.
Abstract: We report a theoretical and experimental investigation of the hybrid heterostructure interfaces between atomically thin MoS2 nanocrystals (NCs) on Si platform for their potential applications towards next-generation electrical and optical devices. Mie theory-based numerical analysis and COMSOL simulations based on the finite element method have been utilized to study the optical absorption characteristics and light–matter interactions in variable-sized MoS2 NCs. The size-dependent absorption characteristics and the enhancement of electric field of the heterojunction in the UV-visible spectral range agree well with the experimental results. A lithography-free, wafer-scale, 2D material on a 3D substrate hybrid vertical heterostructure has been fabricated using colloidal n-MoS2 NCs on p-Si. The fabricated p-n heterojunction exhibited excellent junction characteristics with a high rectification ratio suitable for voltage clipper and rectifier applications. The current–voltage characteristics of the devices under illumination have been performed in the temperature range of 10–300 K. The device exhibits a high photo-to-dark current ratio of ~3 × 103 and a responsivity comparable to a commercial Si photodetector. The excellent heterojunction characteristics demonstrate the great potential of MoS2 NC-based hybrid electronic and optoelectronic devices in the near future.
42 citations
TL;DR: The use of liquid exfoliated MoS2 nanoflakes as the sensing layer in a handheld, resistive toxic gas sensor to detect toxic gases like ammonia and NO holds promises to developing cost-effective, reliable and highly sensitiveMoS2 based ammonia sensors.
Abstract: Various issues like global warming and environmental pollutions have led to the research of toxic gas detection worldwide. In this work, we have tried to develop a molybdenum disulfide (MoS2) based gas sensor to detect toxic gases like ammonia and NO. MoS2, an inorganic analog of graphene, has attracted lots of attention for many different applications recently. This paper reports the use of liquid exfoliated MoS2 nanoflakes as the sensing layer in a handheld, resistive toxic gas sensor. The nanoflakes were exfoliated from MoS2 bulk powder using a sonication based exfoliation technique at room temperature. The successful exfoliation of the nanoflakes was characterized using different techniques e.g., optical microscopy, atomic force microscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy, x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy and ultraviolet-visible spectrophotometry. The characterization results showed that few-layered nanoflakes have successfully been exfoliated. The MoS2 nanoflakes showed reasonable sensing towards ammonia and NO. In order to explore the effect of particle size on ammonia sensing, the MoS2 flakes were also exfoliated using different sonication times. We also observed that various factors like presence of vacancy sites, ambient oxygen, humidity, different contact electrodes have significant effect on the sensing characteristics. In fact, the response of the sensing layer against 400 ppm of ammonia increased from 54.1% to ∼80% when it was UV-ozone treated. This work holds promises to developing cost-effective, reliable and highly sensitive MoS2 based ammonia sensors.
38 citations
TL;DR: In this paper, the authors report on the tuning of luminescence response of few layered graphitic carbon nitride (g-C3N4) nanosheets by reducing the functional groups though chemical reduction.
Abstract: We report on the tuning of luminescence response of few layered graphitic carbon nitride (g-C3N4) nanosheets by reducing the functional groups though chemical reduction. The nanosheets of g-C3N4 have been obtained from its bulk counterpart through liquid phase exfoliation, while the attached functional groups have been removed through sodium borohydride treatment. X-ray photoelectron and micro Raman studies indicate the improved aromatization through the removal of the functional groups, while structural defect formation has been realized at higher reduction levels. It has been found that the increase in sp2 C–N cluster size as a result of improved aromatization leads to the enhanced absorption through π → π* transition. Consequently, the luminescence response has been found to increase at the lower reduction level. On the other hand, near-ultraviolet emission and suppressed visible emission has been witnessed at higher reduction levels. The improvement in near-ultraviolet emission originates from the mod...
32 citations
TL;DR: This novel concept of ZnO nanorod array photodetectors on a natural silk protein platform provides an opportunity to realize integrated flexible and self-powered bio-photonic devices for medical applications in near future.
Abstract: Silk protein has been used as a biopolymer substrate for flexible photonic devices. Here, we demonstrate ZnO nanorod array hybrid photodetectors on Au nanoparticle-embedded silk protein for flexible optoelectronics. Hybrid samples exhibit optical absorption at the band edge of ZnO as well as plasmonic energy due to Au nanoparticles, making them attractive for selective UV and visible wavelength detection. The device prepared on Au-silk protein shows a much lower dark current and a higher photo to dark-current ratio of ∼105 as compared to the control sample without Au nanoparticles. The hybrid device also exhibits a higher specific detectivity due to higher responsivity arising from the photo-generated hole trapping by Au nanoparticles. Sharp pulses in the transient photocurrent have been observed in devices prepared on glass and Au-silk protein substrates due to the light induced pyroelectric effect of ZnO, enabling the demonstration of self-powered photodetectors at zero bias. Flexible hybrid detectors have been demonstrated on Au-silk/polyethylene terephthalate substrates, exhibiting characteristics similar to those fabricated on rigid glass substrates. A study of the performance of photodetectors with different bending angles indicates very good mechanical stability of silk protein based flexible devices. This novel concept of ZnO nanorod array photodetectors on a natural silk protein platform provides an opportunity to realize integrated flexible and self-powered bio-photonic devices for medical applications in near future.
30 citations
TL;DR: The results provide a new paradigm for intercalant impurity-free metal nanoparticle assisted exfoliation of n-type few-layer WS2, with the nanoparticles playing a dual role towards the realization of 2D materials based broadband heterojunction optoelectronic devices by inducing chemical doping as well as tunable plasmon enhanced absorption.
Abstract: Chemical doping and plasmonic enhanced photoresponsivity of two dimensional (2D) n-WS2/p-Si heterojunctions are demonstrated for the first time Novel PVP coated Ag0 intercalation induced synthesis has led to the formation of impurity-free, chemically doped few-layer n-WS2 with reversed conductivity following the Maxwell–Wagner–Sillars interfacial effect The resultant composite film exhibits excellent stability and tunable plasmonic absorption due to silver nanoparticles of different sizes A sharp band-edge absorption of the hybrid material indicates the presence of spin–orbit coupled direct band gap transitions in WS2 layers, in addition to a broader plasmonic peak attributed to Ag nanoparticles Stabilized Ag-nanoparticle (∼4–6 nm) embedded electron rich n-WS2 has been used to fabricate plasmon enhanced, silicon compatible heterojunction photodetectors The detectors exhibited superior properties, possessing a photo-to-dark current ratio of ∼103, a very high responsivity (80 A W−1) and an EQE of 2000% under 10 V bias with a broad spectral photoresponse in the wavelength range of 400–1100 nm The results provide a new paradigm for intercalant impurity-free metal nanoparticle assisted exfoliation of n-type few-layer WS2, with the nanoparticles playing a dual role towards the realization of 2D materials based broadband heterojunction optoelectronic devices by inducing chemical doping as well as tunable plasmon enhanced absorption
26 citations
TL;DR: In this article, an n-ZnO/p-Si membrane heterojunction flexible photodetector with a peak responsivity of 0.20 AW−1 with a detectivity of 4.8 × 1011 cm Hz 1/2 W−1 was presented.
Abstract: We report the characteristics of an n-ZnO/p-Si membrane heterojunction flexible photodetector sensitive to UV and visible illumination. A piezo-phototronic effect has been observed for the deposited ZnO thin films on flexible silicon membranes. Si membranes as low as ~3.0 µm thick have been fabricated by the alkaline etching of Si wafers followed by ZnO deposition using RF sputtering for realizing the heterostructure. A peak responsivity of 0.20 AW−1 with a detectivity of 4.8 × 1011 cm Hz1/2 W−1 is found at ~490 nm for zero bias. Strain induced piezo-potential developed in ZnO thin films is found to modulate the transport property of the photo generated carriers, resulting in the enhanced performance of the device. With a gradual increase in the external tensile stress, the photocurrent increases by 22%. The accompanying COMSOL analysis displays the piezopotential distribution developed in ZnO films on application of an external stress to the heterojunction, which is in close agreement with the experimental data.
25 citations
TL;DR: Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeOx film in a simple W/GeOx/W structure and understanding of switching mechanism through redox reaction in H2O2/sarcosine sensing (or changing Ge°/Ge4+ oxidation states under external bias) have been reported for the first time.
Abstract: Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeOx film in a simple W/GeOx/W structure and understanding of switching mechanism through redox reaction in H2O2/sarcosine sensing (or changing Ge°/Ge4+ oxidation states under external bias) have been reported for the first time. Oxidation states of Ge0/Ge4+ are confirmed by both XPS and H2O2 sensing of GeOx membrane in electrolyte-insulator-semiconductor structure. Highly repeatable 1000 dc cycles and stable program/erase (P/E) endurance of >106 cycles at a small pulse width of 100 ns are achieved at a low operation current of 0.1 µA. The thickness of GeOx layer is found to be increased to 12.5 nm with the reduction of polycrystalline grain size of <7 nm after P/E of 106 cycles, which is observed by high-resolution TEM. The switching mechanism is explored through redox reaction in GeOx membrane by sensing 1 nM H2O2, which is owing to the change of oxidation states from Ge0 to Ge4+ because of the enhanced O2− ions migration in memory device under external bias. In addition, sarcosine as a prostate cancer biomarker with low concentration of 50 pM to 10 µM is also detected.
24 citations
TL;DR: The proposed method significantly lowers the laser intensity required for optical power-dependent doping, resulting in prevention of damage to the sample due to local heating, and could be very useful for tuning graphene plasmons on the widely used Si/SiO2 substrates for various photonic device applications.
Abstract: The novel opto-chemical doping effect in Ag nanoparticle-decorated monolayer graphene grown by chemical vapor deposition has been investigated using Raman spectroscopy for the first time. We used both noble metal nanoparticles and optical excitation, in a hybrid opto-chemical route, to tune the doping level in graphene. Metal nanoparticle-induced chemical effects and laser power-induced substrate effects alter the doping nature of graphene from p- to n-type. Compared with earlier studies, the proposed method significantly lowers the laser intensity required for optical power-dependent doping, resulting in prevention of damage to the sample due to local heating. Some other interesting observations are the enhanced peak intensity in the Raman spectrum of graphene, enhancement of the D-band intensity and the introduction of G-band splitting. This novel, cheap and easily implemented hybrid optical-chemical doping strategy could be very useful for tuning graphene plasmons on the widely used Si/SiO2 substrates for various photonic device applications.
TL;DR: In this article, a novel sensing technique was proposed based on adsorption, which was found to be more effective in terms of reproducibility and reusability of the sensor than the conventional electrochemical sensing technique.
Abstract: Nickel oxide, and non-stoichiometric and Cu-doped variants (NiO, Ni2O3 and CuxNi(1−x)O) possessing porous coral-like nanostructures, were prepared by a facile, low temperature, hydrothermal approach. Structural and morphological characterization was performed by X-ray diffractometry, X-ray photoelectron spectroscopy and FESEM imaging. The Cu-doped oxide was found to possess lattice strain which resulted in the induction of surface defects. The prepared materials were used as sensing materials for toxic Cr(VI) ions in aqueous medium. A novel sensing technique was proposed based on adsorption, which was found to be more effective in terms of reproducibility and reusability of the sensor than the conventional electrochemical sensing technique. The sensing mechanism was explained based on the phenomenon of adsorption. This makes the efficiency of the sensor surface-dependent rather than chemical reactivity-dependent, thereby making it a non-destructive sensing technique. The Cu-doped NiO nanostructure (10% Cu doping) was found to show maximum sensitivity (252.62 at 1 ppm) and high selectivity, together with a low response time (∼2 s), towards Cr(VI) ions relative to Cd(II), As(V) and Pb(II) ions. This was due to its enhanced surface properties compared with its un-doped variants. The effect of Cu doping on the nanostructure morphology and consequently on sensor efficiency was also studied. The limit of detection was found to be 1 ppm (1 mg L−1) of Cr(VI) ions in aqueous solution. This material, along with the technique proposed, can thus be advantageous for the cost-efficient monitoring of water quality and drinking water standards.
TL;DR: In this article, a detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of excitons and biexcitons in WS$2.
Abstract: Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel exciton-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for real device applications. We explore the ultrafast excitonic processes in WS$_{2}$ dispersion using broadband femtosecond pump-probe spectroscopy at room temperature. We detect the biexcitons experimentally and calculate their binding energies, in excellent agreement with earlier theoretical predictions. Using many-body physics, we show that the excitons act like Weiner-Mott excitons and explain the origin of excitons via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of excitons and biexcitons in WS$_{2}$. Indeed, our results demonstrate the potential for excitonic quasiparticle-controlled TMDs-based devices operating at room temperature.
TL;DR: A lower reflectance and higher minority carrier life time along with a larger surface-to-volume ratio resulted in an enhanced power conversion efficiency for smaller sized CZTS nanocrystals.
Abstract: CZTS nanocrystals have been synthesized via a new facile and environmentally friendly route using olive oil at a relatively low temperature. Nanocrystals synthesized using olive oil have a smaller average size in comparison to those synthesized with a conventional solvent-like ethylenediamine. Nanocrystals with an average diameter of 40, 20 and 6 nm have been extracted from the olive oil at different centrifugation speeds of 500, 1000 and 2000 rpm, respectively. The photovoltaic characteristics of p-CZTS/n-Si heterojunctions fabricated using the synthesized colloidal quaternary nanocrystals are demonstrated. The device fabricated with smallest sized CZTS nanocrystals, having an average diameter of ∼6 nm, exhibits an enhancement in power conversion efficiency of 61% in comparison to that of the device fabricated with the nanocrystals of 40 nm in diameter. A lower reflectance and higher minority carrier life time along with a larger surface-to-volume ratio resulted in an enhanced power conversion efficiency for smaller sized CZTS nanocrystals.
TL;DR: It is proposed that the π*→π transition in g-C3N4 can trigger surface plasmon oscillation in Au, which subsequently increases the excitation process in the nanosheets and results in enhanced emission in the green region of the photoluminescence spectrum.
Abstract: We demonstrate surface plasmon induced enhancement and tunablilty in optical emission properties of two dimensional graphitic carbon nitride (g-C3N4) nanosheets through the attachment of gold (Au) nanoparticles. Raman spectroscopy has revealed surface enhanced Raman scattering that arises due to the combined effect of the charge transfer process and localized surface plasmon induced enhancement in electromagnetic field, both occurring at the nanoparticle–nanosheet interface. Photoluminescence studies suggest that at an optimal concentration of nanoparticles, the emission intensity can be enhanced, which is maximum within the 500–525 nm region. Further, the fabricated electroluminescent devices reveal that the emission feature can be tuned from bluish-green to red (~160 nm shift) upon attaching Au nanoparticles. We propose that the π*→π transition in g-C3N4 can trigger surface plasmon oscillation in Au, which subsequently increases the excitation process in the nanosheets and results in enhanced emission in the green region of the photoluminescence spectrum. On the other hand, electroluminescence of g-C3N4 can induce plasmon oscillation more efficiently and thus can lead to red emission from Au nanoparticles through the radiative damping of particle plasmons. The influence of nanoparticle size and coverage on the emission properties of two dimensional g-C3N4, nanosheets has also been studied in detail.
TL;DR: The observed electroluminescence in p-i-n devices fabricated on Ge1-x Sn x island samples above a threshold bias of 4 V makes them attractive for future Si based optical devices.
Abstract: Self-assembled strained Ge1−x Sn x islands on Si (100) have been grown at a low temperature using molecular beam epitaxy. The in-built strain and fraction of Sn in the islands have been estimated using x-ray photoelectron spectroscopy and high resolution x-ray diffraction study of grown samples. No-phonon assisted transition in the optical communication wavelength range of 1.4–1.8 μm has been observed in the Ge1−x Sn x island samples. The direct band gap transition intensity is found to increase with a growth in Sn concentration, with this increase in intensity sustained up to a temperature of 130 K in Ge1−x Sn x islands. The observed electroluminescence in p-i-n devices fabricated on Ge1−x Sn x island samples above a threshold bias of 4 V makes them attractive for future Si based optical devices.
TL;DR: In this article, a facile one-pot approach to synthesize graphene-Ag0 hybrid plasmonic nanostructures exhibiting superior optical properties was reported. And the synergistic effect of Ag and graphene in the hybrid nanostructure results enhanced Raman and photoluminescence (PL) in the visible wavelength.
Abstract: We report a facile one pot approach to synthesize graphene-Ag0 hybrid plasmonic nanostructures exhibiting superior optical properties. The Ag nanoparticles (NPs) (average particle size ~25 nm) are found to be highly stabilized within the graphene matrix probably due to the favorable d-π interaction among the vacant d-orbitals of Ag0 and the π-electrons cloud of graphene moiety. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been performed to characterize the hybrid nanostructures. The synergistic effect of plasmonic Ag and graphene in the hybrid nanostructures results enhanced Raman and photoluminescence (PL) in the visible wavelength (~520 nm). Non-linear absorption (NLA) property in femtosecond regime has been studied for this hybrid nanostructure. It is also observed that the two photon absorption (TPA) coefficient of this hybrid increases from 0.0127 cm/GW to 0.0155 cm/GW when the pulse energy is increa...
TL;DR: In this article, the authors demonstrate the properties of self-assembled Ge QDs, which were grown on Si substrates by solid source molecular beam epitaxy driven by the Stranski-Krastanov method.
01 Dec 2017
01 Oct 2017
TL;DR: In this paper, hierarchical NiO nanostructured thin film devices are used as highly selective and sensitive formaldehyde sensor in presence of light, the response was obtained as 291.7% at 190 ppm.
Abstract: Indoor air quality (IAQ) monitoring is quite essential to maintain healthy human life. Among different pollutants effecting IAQ, formaldehyde is one toxic VOC that needs to be monitored. In this work, hierarchical NiO nanostructured thin film devices are used as highly selective and sensitive formaldehyde sensor in presence of light. The response in presence of light was obtained as 291.7% at 190 ppm with a fast response time of ∼ 24 s and recovery time of ∼ 42 s (at 190 ppm of formaldehyde). The sensor was found to be highly specific towards formaldehyde compared to other VOCs. The optimum operating temperature was ∼ 300°C, much less than the conventional NiO based sensors (∼600°C). The sensing layer was made optically active by inducing defects which resulted in the dependence of the sensing response on light irradiation to a considerable amount (∼82.4% in dark and ∼291.7% in light for 190 ppm formaldehyde). Thus, the fabricated light assisted sensor shows potential to develop future commercial formaldehyde sensor.
30 Jan 2017
21 Dec 2017
TL;DR: In this article, the authors reported synthesis of Au-SnO 2 nanocomposites and their integration on micro-hotplates through dip pen nano-lithography to realize a resistive acetone sensor device.
Abstract: In this paper we report synthesis of Au-SnO 2 nanocomposites, and their integration on micro-hotplates through dip pen nano-lithography to realize a resistive acetone sensor device. The devices are power efficient (8.2°C temperature increase requires for 1 milli watt of power). The devices were characterized exposing acetone in presence of both dry and humid (40% RH) air. The response varies between 3.8 times (with 250 ppm) and 5.5 times (1000 ppm), and did not show much deterioration in presence of humidity.