BITS Faculty Publications
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Item MoSe2/single-walled carbon nanotube n-p heterojunction for broadband photodetection and ambient stability improvement with surface engineering(Elsevier, 2025-07) Hazra, ArnabTransition metal dichalcogenides (TMDCs) have a tremendous capability of visible/infrared light absorption and conversion to electrical signals. On the other hand, single-walled carbon nanotubes (SWCNTs) are popularly used as efficient carrier transporting layers rather than light absorption and carrier generation. In the current study, we are reporting an excellent heterojunction of MoSe2 (TMDC) and SWCNT for a wide range (532–1064 nm) photodetection. Hydrothermally grown 2D sheets (∼34 nm) assembled nanoflower-like spongy structure of MoSe2 was interlinked by semiconducting SWCNTs and utilized for broadband detection. The p-n heterojunction between SWCNT and MoSe2 facilitates the excess hole transfer from MoSe2 to SWCNT under illumination that is eventually responsible for high-performance photoresponse with a responsivity of 152.6 mA/W and detectivity of 1.66 × 1010 Jones in 1064 nm light. Unfortunately, the photoresponse of MoSe2/SWCNT heterojunction was deteriorated nearly after four weeks due to the strong reactivity of MoSe2 chalcogenide towards ambient oxygen. The Raman and UV–visible-NIR absorbance spectra ensured substantial transformation of MoSe2 to MoOx in the air. To achieve essential ambient stability, the MoSe2/SWCNT surface was encapsulated with a thin transparent layer of polymethyl methacrylate. The surface-engineered photodetector exhibited extremely stable photoresponse after eight weeks, but the photoresponsivity was declined almost ten times.Item Modulation of 1d ZNO nanostructures for selective formaldehyde sensing: the role of surface energy, polarization, and adatom kinetics toward asymmetric growth(Elsevier, 2025-07) Hazra, Arnab; Gangopadhyay, SubhashisThe precise morphological control of highly dense, crystalline, and asymmetric one-dimensional (1D) metal oxide semiconductor nanostructures is of high practical importance for developing high-performance chemiresistive gas sensors. In this study, various 1D ZnO nanostructures, including nanobelts, nanowires, nanorods, and nanoneedles, were uniformly grown on glass substrates via the controlled thermal oxidation of thin Zn films under ambient air conditions. Comprehensive structural, chemical, optical, and electrical characterizations were conducted to investigate their properties and growth mechanism. Thermal oxidation of thin Zn films initiates only above 400 °C, whereas a transition towards asymmetric growth of 1D ZnO nanostructures starts to occur at 600 °C. Different nanoscale morphologies such as nanobelts (t = 58 nm, w = 460 nm), nanowires (d = 48 nm), nanorods (d = 280 nm), and nanoneedles (d = 85–120 nm) are obtained after thermal oxidation of [600 °C, 5h], [700 °C, 1h], [700 °C, 5h] and [800 °C, 5h], respectively. Chemiresistive gas sensing performance of these 1D nanostructures was evaluated against different volatile organic compounds (VOCs) in a static vapor mode, demonstrating exceptional sensitivity and selectivity for formaldehyde detection. The nanoneedle-based sensor exhibited superior sensitivity (36 %) with rapid response (28 s), while the nanobelt-based sensor demonstrated excellent selectivity. Notably, the nanowire-based sensor with highly porous morphology achieved an ultra-low detection limit, well below 50 ppb. The growth mechanism was explained based on surface free energy, surface polarization, and adatom diffusion kinetics. Additionally, gas sensing performance was analyzed in relation to surface-to-volume ratio, crystal defect states and crystal plane orientation. Mechanism behind controlled electron transport through the nano-junction of 1D nanostructure is also discussed. All these findings establish a growth mechanism of 1D ZnO nanostructures as promising candidates for advanced gas sensing applications.Item Two dimensional material based sensor for health care applications(CRC Press, 2025) Hazra, ArnabFollowing the discovery of graphene, there has been extensive research on 2D materials, leading to a wide range of applications in the healthcare industry due to their unique properties, such as a high surface-to-volume ratio, nanometre-size layered structure, and bandgap tuning capabilities. In this chapter, we have categorized 2D material-based health sensors into three types: (i) Flexible sensors, (ii) Chemical sensors and (iii) Biosensors. Flexible sensors involve stretching or compressing 2D materials on flexible substrates to vary their resistance or capacitance, enabling the detection of body movements and heart rate. The deposition of mono or a few layers of 2D materials on thin, flexible substrates allows for conformal contact-like properties, facilitating proper and easy placement on the skin of the human body. Chemical sensors produce measurable electric signals when a molecule sits on its active layer. Using 2D material as the active layer enhances their sensitivity due to their thin nature and high surface-to-volume ratio. In biosensors, 2D materials are not used for directly sensing biomolecules due to incompatibility with inorganic molecules. Instead, bioreceptor layers are utilized on 2D material to detect the presence of biomolecules. The presence of 2D material provides a planar conducting channel for current transport in biosensors. The sensors discussed in this chapter primarily have field effect transistor device structures and exhibit responses in terms of current and resistance for monitoring human health indicators such as heart rate, glucose level, pH level and early detection of life-threatening diseases such as tuberculosis, cancer and more. This chapter delves into the applications, reliability, scalability, challenges and future applications of 2D materials such as graphene, transition metal dichalcogenides (TMDCs), MXenes and black phosphorus in healthcare systems.Item Formation of all tin oxide p–n junctions (SNO–SNO2) during thermal oxidation of thin sn films(Wiley, 2024-12) Hazra, Arnab; Gangopadhyay, SubhashisMetastable stannous oxide (SnO) phase of p-type semiconductor and all tin oxides p–n junctions of SnO–SnO2 nanostructures are formed by controlled thermal oxidation of thin tin films. High purity Sn is deposited on quartz substrates using a vacuum-assisted thermal evaporation technique. Afterwards, controlled thermal oxidation at different temperatures is performed in air ambient condition (150–800 °C). Various surface characterization techniques have been employed to analyze the structure, morphology, chemistry, optical, and electronic properties of these SnOx films. P-type SnO phase is found to be thermodynamically stable at lower oxidation temperatures (250–400 °C), while n-type SnO2 phase starts to appear above 500 °C. Highly uniform and dense SnO nanospheres along with few 1D nanorods are observed after oxidation at 400 °C. Mixed oxide phases of p–n junctions with a sudden decrease in electrical conductivity is observed for 500 °C film. Significantly lower surface conductivity of mixed oxide phase indicates the formation of depletion layers between p-type SnO and n-type SnO2 nanograins. A transition from SnO layer to SnO2 layer is also observed above 600 °C. Overall, SnOx-based nanostructures would be a potential candidate for solar cells, p-channel thin film transistors, p–n junction diodes and gas sensors.Item TiO2-GO Field Effect Transistors for Amplified Ethanol Sensing(IEEE, 2020) Hazra, ArnabIn this work we have proposed p-TiO 2 -GO nanocomposite field effect transistor based ethanol sensor. p-type TiO 2 nanoparticles was prepared by sol-gel method and mixed with 2 wt% aquas solution of graphene oxide (GO) and sonicated for 30 min. The nanocomposite was prepared in combination of 5 vol% p-TiO 2 nanoparticles with 95 vol% GO. The morphological and structural characterizations of developed nancomposite were carried out with field emission scanning electron microscopy (FESEM) and Raman spectroscopy techniques, respectively. The p-TiO 2 -GO field effect transistor (FET) sensor showed a response magnitude of 6% in terminal structure when V GS =0 and 41% as in three terminal structure when V GS =0.65 V in the exposure of 100 ppm ethanol at 100°C. The p-TiO 2 -GO FET showed maximum ~7 times amplification in sensitivity due to application of positive gate voltage.Item Growth and Characterization of ZnO Nanostructures: Materials for CO and Ethanol Sensing(Springer, 2021) Hazra, Arnab; Choudhary, Sumita; Gangopadhyay, SubhashisControlled growth of ZnO-based nanostructures, starting from a vertical nanowall surface morphology to laterally grown highly anisotropic nanorods/wires formation has successfully been achieved by controlled thermal oxidation of thin Zn films for a temperature range of 100–700 °C. The as-grown ZnO nanorods were further used for carbon monoxide gas sensing at low temperatures (down to 150 °C) as well as ethanol vapour sensing at room temperatures. Thin films of Zn were deposited on glass and silicon substrate at room temperature, using a vacuum-assisted thermal evaporation technique. Structure, morphology and chemical property of ZnO layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. The XRD and SEM results are in very good correlation and showed vertical growth morphology of ZnO nanowall/sheet structures at a relatively lower oxidation temperature up to 400 °C. However, at higher oxidation temperature, lateral growths started to dominate over the vertical growth. Oxidation at 700 °C appeared with laterally grown one-dimensional (1D) ZnO nanowires/rods of high density. Raman spectroscopy and XPS results suggested that the vertical growth is mainly initiated by the metallic Zn film morphology, whereas the lateral growth is strongly dominated by the oxide (ZnO) formation. Finally, laterally grown ZnO nanorods could successfully sense CO gas and ethanol vapour. A drastic enhancement in CO gas sensitivity for a concentration of 230 ppm was clearly observed in dynamic gas flow mode even for a wide range of operating temperature.Item Efficient acetone sensing by Pd nanoparticle loaded graphene Field Effect Transistor(IEEE, 2021) Hazra, ArnabIn this work we have reported Pd/GO FET nanocomposite field effect transistor (FET) based acetone sensor. Pd nanoparticle loaded graphene oxide (GO) was prepared by one step spray coating technique at room temperature. The morphological and structural characterizations of developed pure GO and Pd/GO samples were performed with field emission scanning electron microscopy (FESEM), Raman spectroscopy and UV-Vis spectroscopy techniques. The effect of gate voltage on sensors at different temperature range (25- 75°C) was investigated by I DS -V GS characteristic. GO and Pd/GO FET sensors showed optimum response at 50 °C temperature with and without applied gate voltage. The response of Pd/GO FET sensor was around 8 % under zero gate voltage (V GS = 0 V) at operating temperature of 50 °C. Due to the application of gate voltage near Dirac point voltage (V GS =V dirac ), both the sensors showed a significant increment in the response magnitude where pure GO exhibited 22 % and Pd/GO exhibited 45 % response in the exposure of 80 ppm acetone at 50°C. The Pd/GO FET sensor showed ~6 times amplification in sensitivity as the consequence of applied gate voltage.Item Statistical Analysis for Selective Identifications of VOCs by Using Surface Functionalized MoS2 Based Sensor Array(MDPI, 2021-06) Hazra, ArnabDisease diagnosis through breath analysis has attracted significant attention in recent years due to its noninvasive nature, rapid testing ability, and applicability for patients of all ages. More than 1000 volatile organic components (VOCs) exist in human breath, but only selected VOCs are associated with specific diseases. Selective identification of those disease marker VOCs using an array of multiple sensors are highly desirable in the current scenario. The use of efficient sensors and the use of suitable classification algorithms is essential for the selective and reliable detection of those disease markers in complex breath. In the current study, we fabricated a noble metal (Au, Pd and Pt) nanoparticle-functionalized MoS2 (Chalcogenides, Sigma Aldrich, St. Louis, MO, USA)-based sensor array for the selective identification of different VOCs. Four sensors, i.e., pure MoS2, Au/MoS2, Pd/MoS2, and Pt/MoS2 were tested under exposure to different VOCs, such as acetone, benzene, ethanol, xylene, 2-propenol, methanol and toluene, at 50 °C. Initially, principal component analysis (PCA) and linear discriminant analysis (LDA) were used to discriminate those seven VOCs. As compared to the PCA, LDA was able to discriminate well between the seven VOCs. Four different machine learning algorithms such as k-nearest neighbors (kNN), decision tree, random forest, and multinomial logistic regression were used to further identify those VOCs. The classification accuracy of those seven VOCs using KNN, decision tree, random forest, and multinomial logistic regression was 97.14%, 92.43%, 84.1%, and 98.97%, respectively. These results authenticated that multinomial logistic regression performed best between the four machine learning algorithms to discriminate and differentiate the multiple VOCs that generally exist in human breath.Item Development of Graphene-Doped TiO2-Nanotube Array-Based MIM-Structured Sensors and Its Application for Methanol Sensing at Room Temperature(MDPI, 2021-07) Hazra, ArnabThis work concerns the development of a good quality graphene doped TiO2 nanotube array sensor for efficient detection of methanol. A pure and graphene doped TiO2 nanotube array was synthesized by electrochemical anodization. Morphological, structural and optical characterizations were performed to study the samples. Both the nanotube samples were produced in Au/TiO2 nanotube/Ti type MIM-structured devices. Pure and graphene-doped TiO2 nanotubes offered a response magnitude of 20% and 28% to 100 ppm of methanol at room temperature, respectively. Response/Recovery time was fast for the graphene doped TiO2 nanotube array (34 s/40 s) compared to a pure TiO2 nanotube array (116 s/576 s) at room temperature. This study confirmed the notable enhancement in methanol sensing due to the formation of local heterojunctions between graphene and TiO2 in the hybrid sample.Item C60-encapsulated TiO2 nanoparticles for selective and ultrahigh sensitive detection of formaldehyde(IOP, 2021-09) Hazra, ArnabThe current study concerns development of fullerene-C60-encapsulated TiO2 nanoparticles hybrid for an efficient detection of volatile organic compounds (VOCs). The nanocomposite was synthesized via chemical route by using hydrated fullerene-C60 and sol-gel derived undoped p-type TiO2 nanoparticles. The nanocomposite was characterized morphologically and structurally comparing with pure C60 clusters and pure TiO2 nanoparticles as the reference materials. The average diameter of the C60-encapsulated TiO2 nanoparticles was 150 nm whereas the average diameters of C60 clusters and pure TiO2 nanoparticles were 161 nm and 18 nm respectively. Therefore, all the materials were implemented in interdigitated electrode based planner structured sensors and tested towards multiple VOCs. However, C60–TiO2 composite exhibited its natural selectivity towards formaldehyde with a very high sensitivity for the concentration range of 1–1000 ppm. C60-encapsulated TiO2 nanoparticles depicted more than double response magnitude (117%) than the pure TiO2 nanoparticle (48%) and pure C60 particles (40%) and appreciably fast response/recovery (12 s/331 s) towards 100 ppm of formaldehyde at 150 °C. However, the efficient VOC sensing was achieved in C60-encapsulated TiO2 sensors possibly due to the extreme reactive surface provided by the oxygen functionalized C60 and easy electronic exchange between ambient and the TiO2 nanoparticles through C60 layers. The combined properties of both C60 and TiO2 lead to the formation of a promising nanocomposite which provided better sensing characteristics than that of the pure materials.