Department of Physics

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14 Ultrathin films of nanomaterials: a lyotropic liquid crystalline system and its sensing application
(De Gruyter, 2021) Gupta, Raj Kumar; Manjuladevi, V.
The ultrathin films of nanomaterials provide remarkable features that can be potentially employed for efficient device fabrication. In this chapter, an overview of ultrathin films of nanomaterials at different interfaces, along with their sensing capabilities, is discussed. From the symmetry point of view, the ultrathin films and sensing applications of the nanoparticles with spherical geometry (eg amphiphilic gold and TiO2 nanoparticle), one-dimensional system (single-walled carbon nanotube) and two-dimensional systems (functionalized graphene) are highlighted.
3D Graphene-Based Optical Sensors
(Springer, 2023-07) Gupta, Raj Kumar; Manjuladevi, V.
3D graphene (3DG) has been utilized as a functional material for the development of gas and chemical sensors. The sensor based on optical phenomena such as surface plasmon resonance (SPR) offers label-free measurements at a very high resolution and sensitivity. It is essential for any sensor to exhibit a very high analyte adsorption capability and good perceptibility to measure changes in electrical and optical properties due to such adsorption. Although SPR is extremely sensitive, 3DG is one of the potential functional materials which can exhibit a high analyte adsorption capability. Therefore, the use of 3DG as a functional layer in SPR devices can ensure a next-generation sensor. The Kretschmann configuration in angular interrogation-based SPR sensors may offer a sensitivity of the order of 10–8 RIU. In the SPR sensor, the extent of the plasmonic field over the metallic surface is limited to a few hundred nanometres. The excellent adsorption capability of 3D graphene can be employed for bio-sensing applications. However, due to the bulk nature of 3DG, the plasmonic field during SPR-based sensing decays rapidly into the porous structure of the 3DG, and therefore, the measurement becomes unresponsive even after the efficient adsorption of the analytes. Therefore, the layer of 3D graphene can be grown over the gold surface by some bottom-up deposition mechanism with control over the thickness. The bottom-up deposition mechanism may yield monolayer, bilayer, and twisted bilayer graphene which also exhibit excellent bio-sensing merits.
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 Kumar
Active 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.
AFM studies on Langmuir-Blodgett films of cholesterol
(Springer, 2004) Gupta, Raj Kumar
The Langmuir monolayer of cholesterol at the air-water interface exhibits a condensed phase in which the cholesterol molecules are aligned normal to the water surface. We have transferred the monolayer from water surface to different substrates by Langmuir-Blodgett (LB) technique and have studied their assembly by atomic force microscope (AFM). Our studies reveal that the aggregation of cholesterol molecules on hydrophobic surfaces leads to interesting structures. The cholesterol molecules assemble into a uniform film, elongated domains and uniformly distributed torus-shaped domains (doughnuts) for one, two and four cycles of deposition, respectively. Beyond four cycles, the molecules adsorb and desorb by an equal amount resulting in no further deposition. The formation of uniformly distributed doughnuts can be attributed to the hydrophobic interaction and reorganization of the molecules due to successive adsorption and desorption during deposition cycles. Our studies on hydrophilic surfaces show that cholesterol cannot form more than one layer of deposition.
Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals
(Taylor & Francis, 2019-02) Manjuladevi, V.; Gupta, Raj Kumar
The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir‒Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.
Alignment Of Liquid Crystalson Lb Films Of Stearic Acid
(CIB Tech, 2012) Gupta, Raj Kumar; Manjuladevi, V.
We formed the Langmuir monolayer of stearic acid at the air-water (A-W) interface. The Langmuir-Blodgett (LB) films were deposited on solid substrates at different surface pressures. The liquid crystal cells were prepared over such LB films. We found that these cells exhibit a tendency to align in planar orientation for the liquid crystal. We found that the liquid crystal alignment improves in cells prepared with substrates deposited at higher surface pressure like 25mN/m. We present the results on the liquid crystal alignment as a function of number of layers of LB films deposited on the substrates at various surface pressures. We propose that LB films of stearic acid can be used as an aligning agent for liquid crystals.
Assessment of Water Quality Parameters in Real-Time Environment
(Springer, 2020-10) Gupta, Raj Kumar; Gupta, Karunesh Kumar
Assessment of drinking water quality has been an important issue nowadays as the water available is severely polluted and can be the cause of diseases like cholera, diarrhea, dysentery, etc. The traditional methods for water quality monitoring require a high-labor-cost and tine consumption as these methods include a sample collection followed by lab-based chemical testing. In addition, the chemicals used in the testing are toxic and of high-cost. So, there is a need for real-time monitoring and chemical-free testing of water quality parameters. This paper presents a real-time water quality monitoring system based on the Raspberry Pi 3 development board and a Python framework. The water quality parameters utilized for water quality monitoring are temperature, pH, oxidation reduction potential, electrical conductivity, and dissolved oxygen and E. coli. The water quality sensors were interfaced with the designed embedded platform. Finally, the acquired parameters were compared with the benchmark equipment for validation.
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.
Correlation of Volumetric Vaporsorption and Vapor Sensing Phenomenon of Flower-Like MoS2-Based Sensor
(IEEE, 2023-03) Manjuladevi, V.; Gupta, Raj Kumar
An innovative approach is reported with an aim to effectively correlate room temperature alcohol vapor (methanol, ethanol, and 2-propanol) sensing performance of flower-like MoS2 sphere-based devices with the corresponding vacuum volumetric vaporsorption study. Hydrothermally derived flower-like MoS2 sphere-based resistive devices were tested to detect the above species and the steady-state characteristics of the sensor (response magnitude and baseline drift) were directly and effectively correlated with the corresponding physisorption measurements carried out using Quantachrome Autosorb iQ. Response magnitude of the sensing layers having different porosity and effective surface area was duly predicted from the volume of adsorbed vapor species on the respective surfaces. Moreover, the baseline drift of the sensors was directly correlated with the amount of target species that remain chemisorbed. Considering rising and falling (transient characteristics) edges of the dynamic response curve, it was found that response time is predominantly governed by the physisorption phenomena, while the recovery is determined by both the mechanism (physidesorption and chemidesorption), though later having relatively lower influence. The highest response magnitude ( ∼ 77%) was achieved for sensor originated from the highest deposition time (24 h) toward methanol. It was duly correlated with its highest Brunauer–Emmett–Teller (BET) surface area (10 m2 /g) as well as its highest methanol adsorption at room temperature (116 cm3/g) among the lot. The highest baseline drift ( 12∘) of 24-h deposited device was explained in the light of the highest open-loop gap (30 cm3/g).
Cost Effective Fabrication and Current-Voltage Characteristics of ZnO Homojunction Based n-p-n Bipolar Junction Transistor
(IEEE, 2023-09) Gupta, Raj Kumar; Manjuladevi, V.
This is the first report on successful fabrication of ZnO homojunction based n-p-n bipolar junction transistor (BJT) employing an easy cost-effective route and exploring its DC current (I)–voltage (V) characteristics, in all the three modes (common base, common emitter and common collector). Sol-gel grown undoped n-type layers was used to form emitter and collector regions; while the base was constituted of Sodium (Na) doped sol-gel grown p-type layer. The carrier concentrations of the emitter ( 2.2×1018 cm−3), base ( 1.35×1016 cm−3) and collector ( 2.58×1017 cm−3) were tuned by precisely tailoring the molarity of the precursor and the dopant concentrations. The doping/carrier concentration profile of emitter, base and collector was authenticated through EIS measurement. After carrying out the input and output I-V characteristics, DC current gain of the fabricated transistor were found to be 0.958 ( α) for common base, 24 ( β) for common emitter and 24.6 ( γ) for common collector mode, respectively. However, defects (due to vacancies and interstitials), and surface states were found to play the pivotal role (rather than bulk) in determining the leakage current (high) which eventually limits the gain.
Detection of cadmium ion in aqueous medium by simultaneous measurement of piezoelectric and electrochemical responses
(Elsevier, 2018-09) Manjuladevi, V.; Gupta, Raj Kumar; Gupta, Karunesh Kumar
Cadmium is one of the important heavy metals which poses health hazards due to its consumption through potable water. Cadmium is known to form complexes with amine group and also it has good affinity towards carbon nanotubes. The octadecylamine functionalized single-walled carbon nanotubes (ODACNTs) can be employed for sensing cadmium ion in aqueous medium. A thin film of ODACNTs offers not only strong adsorption properties towards cadmium ion but also provides an enormous gain in surface to volume ratio, and good mechanical and chemical stability. Therefore, a sensing layer of ODACNTs was formed on the gold deposited quartz wafer and the sensing towards cadmium ion in the aqueous medium was performed. An experimental setup was designed to record the electrochemical and piezo-responses simultaneously. The piezo and electrochemical responses were found to be linear in the given concentration range. Interestingly, the piezoresponse modulates systematically and repeatedly from a maximum to minimum value due to voltage sweep during cyclic voltammetry indicating the interfacial phenomenon of adsorption and desorption.
Detection of Cadmium Ion in Potable Water Using Quartz Crystal Microbalance
(Wiley, 2017-12) Gupta, Raj Kumar; Manjuladevi, V.
The presence of heavy metal e.g. cadmium in potable water is one of the major threats on our health. It should be detected and removed from potable water before consumption. The maximum permissible concentration of cadmium ion in drinking water lies in the range of 2–10 ppb. The consumption of excessive cadmium through potable water can lead to cancer, and can affect the organs like kidney and lungs severely. In this article, we report the development of a piezoelectric based sensor for the detection of cadmium ion in water. A functional layer of nanocomposite of single walled carbon nanotubes and beewax was deposited by spin coating technique onto 5 MHz AT-cut quartz crystal wafers and the shift in resonance due to physioadsorption of cadmium from water medium was recorded. We find a very stable linear response curve with the lowest detectable concentration of 5.2 ppb. The change in the morphology of the functional layer before and after adsorption of cadmium is studied using atomic force microscope and field emission scanning electron microscope. The presence of calcium ion in water medium affected the response curves of cadmium sensing. We found a systematic variation in sensitivity towards Cd ion due to the presence of Ca ion in water medium.
Detection of cadmium ion in water using films of nanocomposite of functionalized carbon nanotubes and anionic polymer
(AIP, 2018-05) Manjuladevi, V.; Gupta, Raj Kumar
Presence of cadmium in drinking water is one of the major threats to human health. According to international standards, the maximum permission concentration of cadmium ion in drinking water should be less than 0.002 to 0.010mg/l (2-10 ppb). It is one of the major contaminants in potable water in western part of Indian subcontinent. It is found up to 2.3 to 8.6 mg/l in Rajasthan water. In this article, we report our study on detection of cadmium ion in water employing a sensing layer of nanocomposites of functionalized single walled carbon nanotubes (SWCNTs) and polyacrylic acid (PAA). The film was deposited onto 5 MHz AT-cut quartz crystal of a quartz crystal microbalance (QCM). The response was collected in both static and dynamic mode. We obtained a linear response curve in a given concentration range of cadmium ion indicating the suitability of the functional layer for cadmium ion detection in water. The surface morphology of the functional layer was studied using atomic force microscope before and after sensing.
Dielectric spectroscopy studies of silver nanorod doped nematic liquid crystal
(Elsevier, 2022) Manjuladevi, V.; Gupta, Raj Kumar
Dielectric spectroscopy provides information on molecular dynamics as well as on important material parameters such as the static dielectric permittivity (ε) and DC electrical conductivity (σ) of the bulk as well as at the interfaces. In this study, we present the dielectric spectroscopy results of silver nanorod doped nematic liquid crystal (NLC) in the frequency range of 20 Hz to 10 MHz. The relaxation frequencies tend to increase owing to the availability of free volume due to addition of nanorods. However, the conductivity (at 1 kHz) is observed to decrease with the increase in the nanorod concentration until aggregation of nanorods is observed in the NLC host. Dielectric strength of doped NLC samples also increases with increasing concentration of nanorods.
Effect of functionalised silver nanoparticle on the elastic constants and ionic transport of a nematic liquid crystal
(Taylor & Francis, 2019-05) Gupta, Raj Kumar; Manjuladevi, V.
Addition of nanomaterial into pure nematic liquid crystals (NLCs) leads to improvement in the various physical properties of the liquid crystal (LC) host. Doping of nanomaterials affects the local molecular arrangement of the LC molecules. Here, we present the results of our investigation on the effect of functionalised silver nanoparticles (f-AgNPs) on the physical properties of the rod-shaped NLC, 4-trans-pentyl-cyclohexyl cyanobenzene (5PCH). The dielectric constant, threshold voltage, elastic constants, birefringence and conductivity measurements were performed on pure 5PCH and its f-AgNPs doped nanocomposites as a function of temperature in planar cell. The magnitude of dielectric anisotropy, elastic constants and birefringence in nanocomposites were enhanced with increasing concentration of f-AgNPs indicating enhancement of order parameter in the nematic medium. Threshold voltage decreases with increasing concentration of f-AgNPs. Both parallel and perpendicular components of conductivity decrease with increasing concentration of f-AgNPs due to the absorption of ion by the doped f-AgNPs. This observed decrease in conductivity in nanocomposites is further confirmed by calculating the ion transportation number and time of flight. The ion transport number i.e ionic contribution present in the LC cell was found to be 0.966 in pure 5PCH, whereas 0.830 in 0.5 wt% of f-AgNPs nanocomposite of 5PCH.
Effect of functionalized carbon nanotube on electro-optic and dielectric properties of a liquid crystal
(Elsevier, 2012-07) Gupta, Raj Kumar; Manjuladevi, V.
The effect of octadecylamine functionalized carbon nanotube (ODACNT) on the electro-optic response and dielectric measurements of a liquid crystal (LC) were investigated. The threshold electric field of ODACNT doped LC 'less as compared to that of pure LC whereas the dielectric anisotropy is enhanced. The dielectric measurements reveal that the ODACNT doped LC exhibits a relaxation frequency of 30 Hz as compared to 3 Hz for pure LC. The tunneling current images obtained using atomic force microscope in spreading resistance mode indicate presence of ODACNT in close proximity of alignment layer. These results clearly indicate that ODACNT is influencing the nematic anchoring as well as the ordering in the bulk due to π − π interaction between LC, ODACNT and the alignment layer.
Effect of Functionalized CNT on Nematic anchoring
(ARXIV, 2015-07) Manjuladevi, V.; Gupta, Raj Kumar
Nematic phase is the most fundamental mesophase exhibited by most of the rod shaped anisotropic liquid crystalline molecules. Nematics are orientationally ordered fluids whose average orientation direction can be manipulated on application of electric and magnetic fields. Carbon nanotube (CNT), a highly shape anisotropic object can find numerous industrial application because of its interesting electronic and mechanical properties. The self-organizing properties of nematics can be used to align CNTs dispersed in them. We have dispersed functionalized CNTs in nematic liquid crystal and carried out many experimental studies. We will present results of electro-optic switching and dielectric measurements on some CNT-LC dispersion. We have observed that addition of functionalized CNTs in a liquid crystal (LC) has led to improvement in the nematic ordering which is evidential from enhancement in dielectric anisotropy (De) measurement. These results indicate that the anchoring energy at alignment layers has been influenced by presence of FCNT in the LC host. The anchoring enhancement can be attributed to p-p electron stacking between the FCNT, LC and the alignment layer.
Effect of Nanomaterials on Physical Properties of Liquid Crystals
(NOVA Science Publishes, 2015) Gupta, Raj Kumar; Manjuladevi, V.
Liquid crystals are the fourth state of matter whose order lies between that of 3-dimensionally ordered solids and completely disordered fluids. The physical parameters such as threshold voltage, dielectric anisotropy, and elastic constants are the key factors in designing liquid crystal display devices. In this chapter, we have reviewed the effect of nanomaterials viz. carbon nanotubes, quantum dots, and nanoparticles and their derivatives in liquid crystal-nanocomposites on the electro-optic switching, threshold voltage, dielectric anisotropy and elastic constants of the nanocomposites.
Effect of octadecylamine-functionalised SWCNTs on the elastic constants and electro-optic response of a liquid crystal
(Taylor & Francis, 2015-03) Manjuladevi, V.; Gupta, Raj Kumar
For efficient and high-performance liquid crystal (LC) devices, the physical properties of the LC materials can be suitably altered by incorporating nanomaterials like carbon nanotubes (CNTs), graphene and quantum dots. In the present work, the effect of incorporation of octadecylamine-functionalised single-walled carbon nanotubes (ODA-SWCNT) in a LC exhibiting nematic and smectic phases on its physical properties is investigated. The electro-optic threshold voltage in the nematic phase of ODA-SWCNT nanocomposite of LC is enhanced as compared to that of pure LC. The dielectric anisotropy as a function of reduced temperature decreases with an increase in the concentration of ODA-SWCNT. The ratio of bend to splay elastic constants of ODA-SWCNT nanocomposite of LC is enhanced considerably as compared to that of pure LC at low temperature. The electro-optic response rise time decreased and decay time increased at higher concentrations of ODA-SWCNT. The temperature range of smectic phase is increased with an increase in the concentration of ODA-SWCNT. At the lower concentration of ODA-SWCNTs (0.05 wt%), the anchoring of the nanotubes at the LC–electrode interface is favoured. At higher concentration of the ODA-SWCNTs, in addition to the anchoring at the interface, the nanotubes that are dispersed in the LC medium increases the orientational ordering.
The Effect of Relative In-Plane Twisting in Graphene Bilayer on Sensing Using Surface Plasmon Resonance
(Springer, 2022-12) Manjuladevi, V.; Gupta, Raj Kumar
Surface plasmon resonance (SPR) is generally observed by the excitation of surface plasmon polaritons on the metal (Au/Ag) surface. In order to utilize the SPR phenomenon for sensing application, the metal surface is functionalized with suitable ligands. Although such functionalization can enhance the specific adsorption capability of the sensor however due to the large thickness of the ligands, the plasmonic field of the metal surface becomes less sensitive towards the adsorption of analytes. In the next generation SPR-based sensor, graphene can be utilized not only as plasmonic material but also as a suitable ligand for attracting analytes through π-π interaction. In this article, we present our theoretical simulation studies on the observation of the SPR phenomenon using graphene monolayer (MLG), bilayer graphene (BLG), and in-plane twisted layers of BLG (T-BLG) as plasmonic materials deposited over zinc-selenide substrate. The Kretschmann configuration under wavelength interrogation setup was simulated, and SPR wavelength for graphene systems/water interface was estimated. The bio-sensing simulation was performed, and the sensing parameters viz. sensitivity, figure-of-merit (FOM), and plasmonic field for different graphene systems were obtained. Interestingly, the excellent sensing parameters were found in T-BLG system with relative in-plane twist angle near to magic angle viz. 1°. The enhancement is due to strong coupling between the layers twisted at the magic angle. This study demonstrates that the MLG, BLG and T-BLG can be employed as a standalone layer system for not only the generation of plasmonic fields but also enhanced sensing due to its intrinsic interactions with bio-analytes.