Department of Physics
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Item Thin film of porphyrin for heavy metal ion sensing(Wiley, 2025-05) Gupta, Raj KumarThin film science and technology is one of the important fields of research and development. The reduction in the dimension of bulk material by approaching a limit of 2-dimensional (2D) system through the fabrication of ultrathin films of the material provides a remarkable increase in surface-to-volume (S-V) ratio as compared to its bulk form. Such an enhancement in surface-to-volume ratio increases the activities of material enormously and thereby material properties like catalysis, reactivity, and adhesion are enhanced remarkably. This research study deals with the development of ultrathin film of 5,10,15,20-tetraphenylporphyrin molecule-based sensor which is capable of addressing the problem of water pollution caused by heavy toxic metal ions such as cadmium, cobalt, lead, and mercury. In sensing applications, in addition to the physicochemical properties of the molecules, the nature of aggregation governs the sensing performance. Under the controlled environment of a laboratory, the ultrathin film of tetraphenylporphyrin deposited on a transducer surface with different assemblies of molecules and their sensing performance are highlighted in this chapterItem Active and passive electrode matrix optimization technique to improve humidity sensing performance of MoS2-based devices: unfolding an innovative avenue(IEEE, 2025-05) Gupta, Raj KumarActive electrodes” that are primarily used for the collection of carriers and “passive electrodes” that actively take part in target gas molecule dissociation (gas-induced carrier generation but not collection), owing to their catalytic nature, were judiciously coupled and optimized in a matrix for the first time with an aim to enhance humidity sensing performance of the MoS2-based devices. Intrigued by the design concept of electroencephalogram (EEG) electrode configurations, the present endeavor uniquely adapted the electrode matrix for gas-sensing scenarios. MoS2 nanoflowers were synthesized through hydrothermal deposition, while Pd electrodes, which were used both as active and passive ones, were deposited by electron beam evaporation using a suitably designed metal shadow mask. The innovation of this study lies in the strategic incorporation of catalytic Pd-based electrodes (both as active and passive), where two active electrodes ( 2×2 mm) facilitated signal transmission to the measurement unit, while multiple passive electrodes ( 0.5×0.5 mm) enabled carrier generation through catalytic dissociation of the target gas. The optimum number of passive electrodes was identified to be six offering the highest response magnitude (RM). The optimized sensor was tested across a relative humidity (RH) range of 8%–84%, demonstrating an RM of 54.4% at 84% RH. To provide deeper insight into the sensing mechanism, a theoretical model was developed to quantitatively correlate the RM with RH levels. Comparison with the existing resistive humidity sensors demonstrated the superior performance of the developed sensor, making it a strong candidate for applications in industrial humidity control, healthcare, smart IoT systems, and environmental monitoring.Item Improving the longevity of plasmonic SERS-active substrates using functionalized graphene for trace TNT detection(Elsevier, 2025-10) Gupta, Raj KumarSilver (Ag) nanostructures have, by far, been the most effective SERS substrates investigated over the last two decades. However, the oxidation-induced instability limits the practical applications of many Ag-based SERS substrates. In this work, we tackle this problem by creating Ag nanostructures (AgNSs) using femtosecond laser ablation and then covering them with octadecylamine functionalized graphene (ODA-Gr) and reduced graphene oxide (r-GO). The laser-induced periodic surface structures (LIPSS) created on silver surfaces, which strengthen localized plasmonic fields, maximized the SERS activity. While r-GO and pristine graphene have been explored for surface modification of SERS substrates, integration of amine-functionalized graphene offers an unexplored route to synergistically maximize the chemical and electromagnetic enhancements. By introducing octadecylamine groups, we increase both TNT adsorption affinity and protect the Ag nanostructures from oxidation, resulting in unprecedented substrate longevity and detection sensitivity. Our findings show that the ODA-Gr-coated AgNSs outperformed the AgNSs (100 nM) and AgNSs/r-GO (10 nM) in terms of sensitivity, reaching a detection limit of 1 nM for trinitrotoluene (TNT). In addition, the ODA-Gr coating dramatically extended the lifespan of the substrate, maintaining ∼54 % of its original SERS intensity after 120 days, as opposed to ∼32 % for r-GO-coated AgNSs and ∼8 % for bare AgNSs under open-air conditions. Long-term stability and improved adsorption efficiency are facilitated by the combined chemical and electromagnetic enhancement mechanisms in the case of AgNSs/ODA-Gr. These results demonstrate the potential of amine-functionalized graphene-coated AgNSs as a reliable and sensitive SERS platform for explosives detection.Item Impact of ZnO nanosphere on ionic conductivity and dielectric relaxation of nematic liquid crystal(Springer, 2025-05) Gupta, Raj Kumar; Manjuladevi, V.Liquid crystal (LC) devices, including displays, switchable optical components, and sensors, are widely used today. The fundamental physical properties of LCs can be controlled through external physical fields, allowing for dynamic configuration, and can also be modified by incorporating nanoparticles. Ions present in a minuscule amount can affect the reorientation of liquid crystals through the screening effect at the interfaces. Dielectric relaxation, which impacts switching time through the dipole dispersion phenomenon, gets altered by the incorporation of nanoparticles in the bulk regime. Therefore, understanding the effects of nanoparticle dispersion in liquid crystals is crucial for both existing and emerging technologies that rely on LC materials. Here, we present the dielectric spectroscopy results of ZnO nanosphere-incorporated nematic liquid crystal (E7), using an electric field as an external stimulus. In this study, we have synthesized the ZnO nanospheres and dispersed these ZnO nanospheres in E7 at various concentrations. We found that 0.02 wt.% of ZnO nanosphere dispersed in the E7 mixture exhibits a reduction of ionic conductivity as well as relaxation frequency.Item Conventional and nonconventional materials for Langmuir monolayer and LB film studies(Elsevier, 2025) Gupta, Raj Kumar; Manjuladevi, V.Langmuir monolayer can be considered as a classical two-dimensional system for studying thermodynamics and molecular interactions at the air-water (a/w) interface. In general, the amphiphilic organic molecules exhibiting a proper balance between hydrophobicity and hydrophilicity can yield a stable Langmuir monolayer at the a/w interface. Such monolayers exhibit variety of surface phases which are important not only from fundamental understanding but also for device applications. These monolayers in these phases can be transferred onto solid substrates layer-by-layer in a highly controlled manner using the Langmuir–Blodgett (LB) technique. Several amphiphilic molecules such as fatty acids, cholesterol and derivatives, lipids, liquid crystals, and polymers were widely studied by forming Langmuir monolayer and Langmuir–Blodgett (LB) films. The field of Langmuir monolayer and LB films is not limited by the requirement of amphiphilicity of the molecules, however there are variety of technologically important hydrophobic materials viz. nanomaterials, liquid crystals, and polymers can form stable Langmuir monolayer at the a/w interface and thereby can be deposited through the LB technique for device applications. In this chapter, we will present an extensive review on such conventional materials and non-conventional materials forming Langmuir monolayer and LB films and some of their applications.Item Tuning liquid crystal properties with 0D carbon dots: exploring the impact of functionalization of carbon dots(RSC, 2025) Manjuladevi, V.; Gupta, Raj KumarFunctionalization of nanomaterials is an efficient way to prevent the aggregation of particles and improve the dispersibility in solvents. However, we propose that if unfunctionalized nanoparticles are capable of forming stable dispersions in solvents and miscible in the LC matrix without aggregation or accumulation at interfaces, they could be a better alternative than their functionalized counterparts for improving the physical properties of NLC. In this study, the effect of functionalization of nanomaterials on various physical properties, such as dielectric, electro-optic and conductivity properties, of nematic liquid crystals is investigated. To explore the validity of our hypothesis, we investigated the properties of NLC, 7CB incorporated with carbon dots and octadecylamine-functionalized carbon dots. Dielectric permittivity and elastic constant measurements suggested that quantum dots were rearranged in the nematic matrix in such a way to minimize the free energy of the composite, and functionalization did not significantly affect the global ordering of NLC molecules. We also observed that the conductivity of C-dot composites decreased when compared to pure NLC but increased with the dispersion of ODA C-dot in NLC compared to pure NLC. It was observed that the ligand molecules of the functionalized quantum dots did not add to the conductivity of the dispersions but act as a trap for ionic impurities, and the partial release of these impurities upon interactions of the ligand shell with the uniaxial nematic host could be the source for the increased conductivity. This study is expected to impart substantial insights into designing high-performance nanocomposites of LCs for device applications.Item Facile hydrothermal synthesis of α-MnO2 nanorods for low-cost, scalable and stable photoresponsive devices(RSC, 2025-09) Gupta, Raj Kumar; Manjuladevi, V.Morphologically well-defined α-MnO2 nanorods were synthesized through a facile one-pot hydrothermal method. The structural, morphological and electronic characteristics were systematically investigated using XRD, XPS, FESEM, FTIR, UV-vis and ultraviolet photoelectron spectroscopy. Analysis confirmed the formation of pure α-phase MnO2, exhibiting n-type semiconducting behavior with an indirect bandgap of ∼1.24 eV (from Tauc analysis) and a work function of 4.53 eV, as determined from UPS. XPS confirms the coexistence of Mn3+, along with Mn4+, in the synthesized MnO2. To evaluate the optoelectronic properties, a simple photoresponsive device was fabricated with an in-plane geometry, where the ITO substrate was patterned via a straightforward etching process to define lateral electrodes, followed by drop-casting α-MnO2 nanorods across the active region. The device exhibited a distinct photoresponse under varying illumination conditions (dark, red, and green lasers at different intensities). Under 532 nm green laser excitation, the photocurrent increased by ∼34%, attributed to enhanced charge carrier separation and electron–hole recombination. The fabricated device demonstrated robust stability over repeated measurement cycles, with response and recovery times of 76.5 s and 77.5 s, respectively, at room temperature. A maximum responsivity of 8.66 mA W−1 at 4 V bias was achieved under 17.2 mW cm−2 green laser illumination, along with an external quantum efficiency (ηEQE) of 2.018% at room temperature. The device shows superior performance at elevated temperatures, demonstrating a response time of ∼12 seconds. At 160 °C temperature, the device shows a responsivity of 240.29 mA W−1 at 4 V bias and an ηEQE of 56.2%, highlighting its applications in next-generation optoelectronic devices. Temperature-dependent measurements confirmed the role of thermally activated carrier transport, revealing enhanced photocurrent at elevated temperatures due to increased carrier mobility. This study establishes α-MnO2 nanorods as a promising platform for cost-effective and stable photoresponsive devices, highlighting the role of band alignment and temperature-activated dynamics in advancing next-generation MnO2-based photodetectors and energy-harvesting systems.Item Synergistic detection of E. coli using ultrathin film of functionalized graphene with impedance spectroscopy and machine learning(Springer Nature, 2025-04) Gupta, Raj KumarBacterial detection and classification are critical challenges in healthcare, environmental monitoring, and food safety, demanding selective and efficient methods. This study presents a novel, label-free approach for E. coli detection using ultrathin Langmuir-Blodgett films of octadecylamine functionalized (ODA)-functionalized graphene on gold electrodes, with a detection range spanning colony-forming units/mL (CFU/mL). Electrochemical impedance spectroscopy (EIS) was performed on six bacterial strains, representing Gram-negative and Gram-positive classes, to evaluate selectivity. The method achieved a remarkably low detection limit of 2.5 CFU/mL for E. coli, with its EIS spectra exhibiting distinct features compared to other bacterial strains. The pronounced differences enabled perfect classification using the Bagging Classifier, achieving no false positives. Machine learning (ML) algorithms applied to raw impedance data improved detection precision and reliability, enabling automated and accurate analysis. These findings establish a robust framework for rapid and selective E. coli detection, crucial for ensuring food and water safety. The integration of ML significantly improves detection accuracy, reduces analysis time, and minimizes human error, paving the way for scalable, cost-effective diagnostic tools for diverse biological and environmental applications.Item Ultrathin langmuir-blodgett film of functionalized single-walled carbon nanotubes for enhanced acetone sensing(Elsevier, 2024-05) Manjuladevi, V.; Gupta, Raj KumarAcetone is one of the important volatile organic compounds which needs to be detected from the industrial effluent and as a bio-marker from breath of diabetic patients. Here, ultrathin Langmuir-Blodgett (LB) film of octadecylamine- functionalized single-walled carbon nanotubes (ODA-CNT) was formed on interdigitated gold electrodes and was employed for acetone vapor sensing using the impedance spectroscopy (IS) at room temperature. The LB film deposited in liquid-like phase of the Langmuir monolayer of ODA-CNT revealed aligned nanotubes. Being a reducing agent, acetone interacts readily with the CNTs through the transfer of electrons. Using IS, multiparameter were measured with respect to both frequency and acetone vapor concentration. Principal component analysis (PCA) of the IS data revealed capacitance as the most suitable parameter for acetone vapor sensing using the ODA-CNT film. A very low limit of detection (0.5 ppm) and a wide detectable concentration range (1–300 ppm) for acetone sensing using the LB film of ODA-CNT was obtained. The 2D calibration map revealed that the sensing performance of acetone using LB film was much better as compared to that of drop-cast film of ODA-CNT and LB film of pristine CNTs. The superior sensing performance of the LB film of ODA-CNT is attributed to the uniform and aligned nature of the nanotubes, leading to a coherent behavior due to interaction with the acetone molecules. This study shows the potential of LB films of ODA-CNT for sensitive and low level detection of acetone vapor at room temperature.Item Optical anisotropy and surface phases of cholesterol derivative monolayer at air–water interface(Elsevier, 2024-11) Manjuladevi, V.; Gupta, Raj KumarCholesterol and its derivatives play crucial roles in regulating processes of biological membranes. Langmuir monolayer can mimic a bio-membrane which can be used to investigate the molecular interaction governing the important physical phenomena. The cholesterol derivative exhibiting mesophases (CHLC molecules) was synthesized and spread at the air–water (A/W) interface to investigate the surface behavior. The monolayer exhibited a variety of surface phases such as gas, liquid expanded (LE), low density liquid like (L1) and liquid condensed (L2) phases. Treating the CHLC molecules to be rod-like, the average tilt of the molecules with respect to the surface normal in these phases are found to be different. The tilt angle decreases systematically from LE to L2 phase. The optical anisotropy of the ultrathin Langmuir-Blodgett (LB) films of CHLC molecules in these phases was measured using the surface plasmon resonance (SPR) spectroscopy. The high tilted molecules in the ultrathin LB film displayed a high value of optical anisotropy. The ultrathin film of CHLC molecules at different interfaces was investigated using Brewster angle microscopy, X-ray reflectivity (XRR), SPR spectroscopy and atomic force microscopy. This study is useful for the systems where the physical phenomena are governed by tilt of the molecules.