Browsing by Author "Bhola N. Pal"

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A Lithography-Free Fabrication of Low-Operating Voltage-Driven, Very Large Channel Length Graphene Field-Effect Transistor with NH3Sensing Application
(Institute of Electrical and Electronics Engineers Inc., 2020) Nitesh K. Chourasia; Abhishek Kumar Singh; Suyash Rai; Anand Sharma; P. Chakrabarti; Anchal Srivastava; Bhola N. Pal
Large-area-based field-effect transistor (FET) gas sensor has the potential to provide a larger sensing area for a chemical analyte. So far, graphene FETs (GFETs) are mostly fabricated by expensive lithographic techniques with a minimum channel length. We have demonstrated a simple way to fabricate a very large channel length of 0.45 mm GFET using ion-conducting dielectric with thermally evaporate source/drain electrodes and has been demonstrated for an application of ambient atmosphere ammonia gas sensing. Ion-conducting Li5AlO4 gate dielectric has reduced operating voltage up to 2.0 V with good current saturation. The chemical vapor deposition (CVD) grown uniform monolayer of graphene has been used as an active channel layer of FET. The fabricated device has been tested for different concentrations of ammonia in ambient environment conditions at 25 °C temperature, which indicates that the Dirac point voltage of the device varies up to 0.8 V when the concentration of ammonia has been changed from 0 to 3 ppm. Moreover, this study also reveals that this GFET is capable of detecting ammonia up to the concentration level of 0.1 ppm. © 1963-2012 IEEE.
Application of a microwave synthesized ultra-smooth a-C thin film for the reduction of dielectric/semiconductor interface trap states of anoxide thin film transistor
(Royal Society of Chemistry, 2022) Nila Pal; Baishali Thakurta; Rajarshi Chakraborty; Utkarsh Pandey; Vishwas Acharya; Sajal Biring; Monalisa Pal; Bhola N. Pal
In high-κ dielectric-based thin-film transistors (TFTs), tailoring the surface of the gate dielectric layer is a crucial issue for the improvement of the device performance. Herein, a simple solution-processed ultra-smooth amorphous-carbon (a-C) film is applied as a surface modification layer on the top of the high-κ ion-conducting Li-Al2O3 dielectric of a bottom gated SnO2 TFT. The a-C film minimizes the surface roughness of the gate dielectric and forms a strong coordination bond between the doped nitrogen of the a-C film and tin (Sn) of the upper lying SnO2 semiconducting channel, which lowers the gate leakage current, carrier scattering and trap state density at the dielectric/semiconductor interface successfully. As a consequence, the TFT with an a-C interface shows an improvement in the carrier mobility by 6.7 times with a higher ON/OFF ratio and a lower subthreshold swing (SS) by 3.8 times. An optimized device with an a-C gate interface shows a saturation carrier mobility, ON/OFF ratio and SS value of 21.1 cm2 V−1 s−1, 7.0 × 104, and 147 mV dec−1, respectively. Moreover, a significant improvement in the cycling electrical stability has been observed which is an outcome of a reduced trap state of an a-C modified TFT. © 2022 The Royal Society of Chemistry.
Enhancement of ammonia gas sensitivity and selectivity by depleted layer of PBTTT-C14/MoS2-QDs heterojunction based thin film transistor
(Elsevier B.V., 2023) Shipra Gupta; Bhola N. Pal; Rajiv Prakash
A highly sensitive, room temperature operating ammonia (NH3) gas sensor has been fabricated by using PBTTT-C14/MoS2-QDs heterojunction based organic thin film transistor (OTFT). Thin film of PBTTT-C14/MoS2-QDs has been fabricated by low cost and high yield ‘floating film transfer method’ (FTM). Initially, hydrothermally synthesized MoS2 quantum dots (QDs) were added in PBTTT-C14 polymer to make a composite ink, which was used for film fabrication. This polymer/QDs based film has been used as a channel of a top contact-bottom gated organic thin film transistor. Various concentration of NH3 ranging from 0.5 to 50 ppm has been exposed on the channel to determine the sensitivity and the selectivity of the device. For comparison, a reference OTFT has been fabricated under the same condition by using pristine PBTTT-C14 thin film. It has been observed that the response of the polymer/QDs heterojunction device is ∼ 85 % at 50 ppm which is ∼ 3 times higher compared to reference device in higher concentration range of NH3. This enhancement becomes possible due to the NH3 sensitive depletion layer of polymer/QDs heterojunction. This OTFT sensor shows high selectivity to the NH3 gas with good linearity to the concentration, which can fulfill basic requirements for practical utilization. © 2023 Elsevier B.V.
Highly Sensitive Broadband Photodetector Based on PbI2-Passivated CdS:Mn Quantum Dots with a Spectrally Flat Response
(American Chemical Society, 2022) Piyali Maity; Satya Veer Singh; Santanu Das; Anup K. Ghosh; Bhola N. Pal
This paper has demonstrated the role of manganese (Mn) doping in cadmium sulfide (CdS) quantum dots (QDs) and their surface passivation for the fabrication of highly sensitive broadband photodetectors. These photodetectors have been fabricated in a p-n heterojunction geometry using the Mn-doped CdS (CdS:Mn) QDs with TiO2 nanoparticles. The thin film of CdS:Mn QDs has been passivated with lead iodide (PbI2) for faster charge transport and broadening of the spectral response of the device. Our detailed X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy indicate a partial PbS formation, which also causes spectral broadening. In addition, this passivation process enables us to enhance the photosensitivity of the device with a spectrally flat response of quantum efficiency throughout the visible range spectrum (350-700 nm). This work also demonstrated that the photosensitivity of the device gradually increases with Mn doping with a faster photoresponse. The highest detectivity of the device was obtained with 4% Mn-doped dots with a value of ∼3.9 × 1012 Jones under -0.5 V external bias with a photoresponse time of 0.2 s, indicating its very high detectivity with a fast response. © 2022 American Chemical Society.
Lithography-free fabrication of low operating voltage and large channel length graphene transistor with current saturation by utilizing Li+of ion-conducting-oxide gate dielectric
(American Institute of Physics Inc., 2020) Nitesh K. Chourasia; Vijay K. Singh; Anand Sharma; Anchal Srivastava; Bhola N. Pal
The large channel length graphene field-effect transistor (GFET) can outperform its competitors due to its larger active area and lower noise. Such long channel length devices have numerous applications, e.g., in photodetectors, biosensors, etc. However, long channel length graphene devices are not common due to their semi-metallic nature. Here, we fabricate large channel length (up to 5.7 mm) GFETs through a simple, cost-effective method that requires thermally evaporated source-drain electrode deposition, which is less cumbersome than the conventional wet-chemistry based photolithography. The semiconducting nature of graphene has been achieved by utilizing the Li+ ion of the Li5AlO4 gate dielectric, which shows current saturation at a low operating voltage (∼2 V). The length scaling of these GFETs has been studied with respect to channel length variation within a range from 0.2 mm to 5.7 mm. It is observed that a GFET of 1.65 mm channel length shows optimum device performance with good current saturation. This particular GFET shows a "hole"mobility of 312 cm2 V-1 s-1 with an on/off ratio of 3. For comparison, another GFET has been fabricated in the same geometry by using a conventional SiO2 dielectric that does not show any gate-dependent transport property, which indicates the superior effect of Li+ of the ionic gate dielectric on current saturation. © 2020 Author(s).
Role of Cobalt Doping in CdS Quantum Dots for Potential Application in Thin Film Optoelectronic Devices
(American Chemical Society, 2021) Piyali Maity; Shiv Kumar; Ravi Kumar; S.N. Jha; D. Bhattacharyya; S.R. Barman; Sandip Chatterjee; Bhola N. Pal; Anup K. Ghosh
Colloidal quantum dots (QDs) are promising materials for optoelectronic devices. In this paper, monodispersed and environment stable cobalt (Co)-doped CdS QDs have been synthesized and characterized for potential application in thin film optoelectronic devices. The Rietveld refinement profiles of X-ray diffraction data reveal that both undoped and Co-doped CdS QDs exhibit a zinc blende structure without any impurity phase. X-ray photoemission spectroscopy has been used for electronic structure and valence state analysis. The detailed information about the doping, coordination number, and local geometry has been studied with XANES and EXAFS measurements. Analysis of Raman spectra reveals that the intensity of longitudinal optical (LO) modes varies considerably due to short-range structural disorder. Absorption spectra also show the creation of a new doping band (DB) near the NIR region in Co-doped CdS QDs which is not observed for doping of many other transition metals. The width of this DB increases with an increase in the doping concentration, and enhancement of photoconductivity of the thin film heterojunction of the samples has been obtained. Evolution of the new DB and enhancement of the photocurrent upon Co doping make the prepared quantum dots very promising materials to exploit for fabricating UV−vis/NIR thin film optoelectronic devices.(Figure presented) © 2021 American Chemical Society.
Selective near-infrared (NIR) photodetectors fabricated with colloidal CdS:Co quantum dots
(Royal Society of Chemistry, 2019) Piyali Maity; Satya Veer Singh; Sajal Biring; Bhola N. Pal; Anup K. Ghosh
Herein, cobalt-doped cadmium sulphide CdS (CdS:Co) quantum dots (QDs) were synthesized by an organometallic synthesis route using different doping concentrations of Co ranging from 1 to 8%. Optical absorption data indicate the appearance of a new NIR absorption peak of the QDs due to Co doping; moreover, the intensity of the peak increases with the doping concentration; this NIR absorption peak originates from the existence of a doping band located in close vicinity to the valence band inside the band gap. The electrical conductivities of the CdS:Co thin films 'in the dark' show increasing conductivity with doping concentration; this supports the enhancement of the carrier concentration in the valence/conduction band. In addition to the dark current, the photosensitivities of these CdS:Co QDs thin film increase gradually with doping, and significant enhancement was observed in the zinc oxide (ZnO) CdS:Co QD heterojunction structure. Lateral ZnO/CdS:Co heterojunction photodetectors with different doping concentrations of Co2+ show selective NIR sensitivity, which is not realized in the case of undoped CdS. The highest detectivity was observed for the 8% doped CdS:Co heterojunction photodetector, with the detectivity of 3.1 × 1011 Jones with illumination of 820 nm wavelength light under 5.0 V external bias, which is significantly high for an NIR photodetector. © 2019 The Royal Society of Chemistry.
Single quantum dot rectifying diode with tunable threshold voltage
(Royal Society of Chemistry, 2017) Gopal S. Kenath; Piyali Maity; Yogesh Kumar; Hemant Kumar; Vinod K. Gangwar; Sandip Chaterjee; Satyabrata Jit; Anup K. Ghosh; Bhola N. Pal
An ambient atmosphere single quantum dot (QDs) rectifying diode with tunable threshold voltage has been fabricated using cobalt (Co) doped CdS QDs with a device structure of ITO/ZnO/QDs. Current-voltage (I-V) characterization of this device has been tested using ambient atmosphere scanning tunnelling microscope (STM). The scanning tunnelling spectra (STS) shows a very high rectification behavior of this single dot based device with a ratio of 103. The threshold voltage of this device decreases with increase in doping concentration of QDs. Reduction of this turn-on voltage occurs due to the formation of additional energy band of Co impurity within the band gap of QDs that exist closer to the valance band (VB) of CdS. Existence of this additional energy band has also been observed in the UV-VIS absorption data of Co doped CdS, which introduces an additional absorption peak in the near infrared region. This impurity band is fully populated at room temperature and the width of this band increases with doping concentration, which is the key for the tunability of threshold voltage. This finding has been explained with one empirical model of relative band shifting of semiconductor-QDs-tip interfaces with positive and negative substrate bias. © 2017 The Royal Society of Chemistry.