http://dspace.dtu.ac.in:8080/jspui/handle/123456789/31 2026-07-02T14:25:44Z http://dspace.dtu.ac.in:8080/jspui/handle/repository/22880 Title: DEVELOPMENT OF 2D MOLYBDENUM DISULFIDE BASED NO2 GAS SENSOR Authors: KUMAR, RAMESH; SINGH, VINOD ( SUPERVISOR ); KUMAR, MAHESH ( CO- SUPERVISOR) Abstract: The rapid increase in environmental pollution and industrial activities has led to a growing demand for efficient, reliable, and cost-effective gas sensing technologies. Toxic gases such as nitrogen dioxide (NO₂) and hydrogen (H₂) pose significant risks to human health, environmental safety, and industrial operations. In this context, the present thesis focuses on the development and investigation of nanostructured molybdenum disulfide (MoS₂) thin films for gas sensing applications, with particular emphasis on understanding their structural properties and sensing behavior toward NO₂ and H₂ gases. The research begins with the synthesis of MoS₂ thin films using a combination of electron beam evaporation and chemical vapor deposition (CVD) techniques. Molybdenum (Mo) thin films of varying thicknesses were initially deposited on suitable substrates and subsequently sulfurized under controlled conditions to obtain high-quality MoS₂ layers. Systematic optimization of synthesis parameters such as film thickness, annealing temperature, and sulfurization temperature was carried out to achieve uniform, crystalline, and reproducible thin films. Among the different samples studied, the 20 nm thick MoS₂ films exhibited superior crystallinity, structural stability, and surface uniformity, making them suitable for gas sensing applications. Comprehensive structural and morphological characterization was performed using X ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The XRD analysis confirmed the formation of polycrystalline hexagonal-phase MoS₂, while Raman studies validated the presence of characteristic vibrational modes corresponding to in plane and out-of-plane lattice vibrations. Surface morphology analysis revealed the formation of nanosheet and flower-like structures, providing a high surface-to-volume ratio and abundant active sites for gas adsorption. A significant contribution of this work is the investigation of MoS₂–hydrogen interaction using in-situ X-ray diffraction. This approach enabled real-time monitoring of structural changes in MoS₂ under controlled hydrogen gas environments, eliminating sample-to-sample variations. The study revealed that exposure to hydrogen gas leads to noticeable changes in diffraction peak intensity, indicating structural modifications and possible lattice strain effects. These findings provide deeper insights into the interaction mechanisms between hydrogen molecules and layered MoS₂ structures. Furthermore, the gas sensing performance of MoS₂ thin films toward NO₂ gas was systematically investigated as a function of operating temperature and gas concentration. The fabricated sensor exhibited n-type semiconducting behavior, with an increase in electrical resistance upon exposure to the oxidizing NO₂ gas due to electron withdrawal. The sensing response improved significantly with increasing temperature, achieving optimal performance at 150 °C. The sensor demonstrated a RAMESH KUMAR ix maximum response of approximately 14.2% at 20 ppm concentration, along with a response time of about 102 seconds and a recovery time of 94 seconds. Additionally, the sensor response increased with increasing NO₂ concentration, indicating strong adsorption and efficient charge transfer processes. Despite the promising performance, certain limitations such as reduced recovery at higher gas concentrations and potential structural defects were observed. These findings highlight the need for further improvements in selectivity, stability, and environmental adaptability of MoS₂-based sensors. Overall, this thesis establishes a strong correlation between synthesis conditions, structural properties, and gas sensing performance of MoS₂ thin films. The integration of advanced synthesis techniques with in-situ characterization provides a comprehensive understanding of gas–material interactions. The results demonstrate that nanostructured MoS₂ is a highly promising material for next-generation gas sensors, with potential applications in environmental monitoring, industrial safety, and clean energy systems. 2026-05-01T00:00:00Z http://dspace.dtu.ac.in:8080/jspui/handle/repository/22875 Title: ADVANCING MEMS GYROSCOPES: DESIGN, ANALYSIS AND THERMAL MANAGEMENT FOR ENHANCED FIGURE OF MERIT Authors: SHAVETA; Chaujar, Rishu (SUPERVISOR); Bhan, R. K. ( CO- SUPERVIOR) Abstract: Microelectromechanical systems (MEMS) have become a cornerstone of modern sensing technologies owing to their unique combination of miniaturization, low power consumption, batch fabrication capability, and high sensitivity. This thesis presents a comprehensive investigation of MEMS-based sensors, with a particular emphasis on MEMS gyroscopes as inertial sensors for navigation, intelligent systems, and defence-related applications. The work begins with a broad overview of MEMS sensors, covering pressure, acceleration, and angular rate sensors; bolometers; magnetic sensors; humidity and flow sensors; optical sensors; biosensors; and microphones. The historical evolution of MEMS technology, its biological inspiration, fabrication techniques, and material considerations are discussed in detail. In addition, the emerging role of MEMS in quantum technologies is explored, highlighting their increasing use in quantum sensing, communication, and atomic-scale devices. Building on this foundation, the thesis focuses on MEMS gyroscopes operating on the Coriolis principle and develops a detailed theoretical framework to describe their dynamic behaviour and performance. Key performance parameters, including sensitivity, bandwidth, noise, and quality factor, are systematically analysed. Rather than optimizing these parameters individually, the work emphasizes the importance of maximizing an integrated performance measure. To this end, a unified design methodology is proposed to enhance an amended Figure of Merit (FOM) that simultaneously accounts for sensitivity, bandwidth, and noise. Analytical vi models are validated using CoventorWare and MATLAB/Simulink simulations, demonstrating close agreement with theoretical predictions within 5%. Under identical operating conditions, the optimized thick sense mass configuration achieves a 52-fold improvement in FOM, expressed in units of m Hz/dps²·mm². Furthermore, a new empirical relationship between sensitivity and bandwidth is proposed, offering additional insight into design trade-offs. The effect of temperature on thermomechanical noise is also incorporated to improve the realism of performance prediction. To further enhance miniaturization without sacrificing performance, a novel Vertical Sense Mass (VSM) MEMS gyroscope architecture is introduced. The proposed VSM design employs deep reactive ion etching (DRIE) to realize thick proof masses in the out-of-plane direction. This approach enables a 30% reduction in sense mass area and a corresponding 36% reduction in overall sensor footprint compared to conventional planar sense mass designs. Despite this reduction in size, the VSM architecture delivers a substantial performance enhancement, with the overall Performance Metric (PM) increasing from 70.7 mHz/dps²·µm² for the planar design to 1090 mHz/dps²·µm² for the VSM design. Detailed fabrication process flows are presented, and the successful experimental realization of thick proof mass structures using DRIE confirms the practical feasibility of the proposed architecture. Recognizing damping as a fundamental limitation in miniaturized MEMS gyroscopes, this thesis presents a comprehensive comparative analysis of energy dissipation mechanisms in both PSM and VSM architectures under identical sense mass areas. The study systematically examines air damping, thermoelastic damping, material damping, anchor loss, viscous damping, and acoustic damping. The results indicate that residual air damping remains a dominant loss mechanism even under vacuum packaging. While the overall trends of individual damping mechanisms are similar for both architectures, the net quality factor (QTotal) of the VSM design is approximately eight times higher than that of the planar design. Temperature dependent analysis further shows that the VSM architecture maintains a 2.7-times higher quality factor across the operating temperature range. In addition, the VSM design exhibits higher sense displacement up to a quality factor of 100, approximately 20 times the bandwidth across all Q values, and a noise reduction factor of 3.3 compared to the planar counterpart. Sensitivity analysis accounting for fabrication imperfections reveals a maximum variation in QTotal of ±12.8%, indicating acceptable robustness. The proposed VSM design is further vii validated through comparison with state-of-the-art reported designs and available experimental results. Finally, the thesis addresses thermal robustness, a major challenge that affects the reliability and accuracy of MEMS gyroscopes in real-world operating environments. A novel packaging level thermal management strategy is proposed through the integration of a thermally optimized substrate that establishes a controlled temperature offset between the sensor and the package base. When combined with a thermally engineered structural design that minimizes heat flow into the sense mass, this approach achieves a device temperature reduction of approximately 25 °C, as confirmed by transient thermal analysis. The improved thermal isolation leads to significant reductions in temperature-dependent variations of sense deflection and scale factor. Specifically, sense deflection variation is reduced from 2.7 × 10⁻⁶ µm/°C to 8.6 × 10⁻⁸ µm/°C, while scale factor temperature sensitivity decreases from 232 ppm/°C to 6 ppm/°C. Additional improvements are observed in noise reduction and bandwidth stabilization. Stress analysis confirms enhanced structural integrity, and etching experiments validate the feasibility of the thermally optimized substrate. Overall, this thesis presents a holistic, fabrication-aware, and quantitatively validated approach to MEMS gyroscope development, integrating architectural innovation, performance optimization, damping mitigation, and thermal management. The outcomes of this work significantly advance the state of MEMS gyroscope technology and provide robust design guidelines for the development of compact, high-performance, and thermally stable inertial sensors suitable for next-generation navigation, autonomous, and defence systems. 2026-05-01T00:00:00Z http://dspace.dtu.ac.in:8080/jspui/handle/repository/22737 Title: ANALYTICAL AND NUMERICAL SIMULATION OF WAVES AND INSTABILITIES IN STRONGLY COMPLEX PLASMA Authors: MOR, HARENDER; Sharma, Suresh C. (SUPERVISOR); Segwal, Kavita (CO-SUPERVISOR) Abstract: The central objective of this thesis is to examine the fundamental nature of electrostatic wave phenomena in magnetized dusty plasma systems. A detailed understanding of dusty plasma behavior is essential for interpreting a wide range of physical processes occurring in both controlled laboratory experiments and natural space environments. In contrast to ordinary electron–ion plasmas, dusty plasmas contain micron- or submicron-sized solid particles immersed within the plasma, a feature that fundamentally alters the collective dynamics of the system. The presence of these dust grains introduces additional degrees of freedom, leading to richer and more complex wave and instability characteristics. The investigation begins with an exploration of the basic physical principles underlying dusty plasma systems, focusing on the coupled interactions among charged dust particles, electrons, and ions under the influence of an external magnetic field. Particular attention is given to the generation, propagation, and stability of electrostatic waves, as well as to the physical mechanisms responsible for their excitation. Through this approach, the complex nature of electrostatic wave dynamics and instability formation in dusty plasmas is systematically analyzed. The theoretical insights obtained from this study are relevant to a broad spectrum of applications, including plasma-based materials processing, geophysical and atmospheric phenomena, astrophysical systems, microelectronic fabrication, fusion research, and solar wind dynamics. The overarching objective is to elucidate how the interplay between charged dust grains, the surrounding plasma medium, and the magnetic field collectively governs the behavior of electrostatic waves. To achieve these goals, several theoretical frameworks are developed using fundamental plasma fluid description. The governing equations namely the continuity and momentum equations, and Poisson’s equation are employed to derive analytical expressions for wave frequencies and instability growth rates. The dependence of these quantities on key plasma parameter is then examined in detail. Generalised Hydrodynamic Model (GHD) has been used for the description of strongly coupled dust grains as they are in a fluid like state for a wide range of strong coupling parameter. The effects of strong coupling among dust particles on the dispersion characteristics of longitudinal mode and the novel shear mode have been studied. Furthermore, the effect of cylindrical geometry, magnetic field, collissions among dust grains have been analysed. The evolution of Rayleigh Taylor instability (RTI) in a strongly correlated dusty plasma driven by an ion beam has been investigated through Generalized Hydrodynamic( GHD) Model. The instability analysis focuses on effects of beam velocity, beam density, density scale length and strong coupling parameters on the instability growth rate and frequency. The ion beam enhances the pressure imbalance across the interface, thereby driving the RTI, while the viscoelastic response of the strongly coupled medium introduces a stabilizing influence. It has been observed that the growth rate of ion beam driven RTI is suppressed with increasing coupling strength and magnetic field, while the instability grows with higher beam density and velocity enhances the instability, density scale length and beam density. These results provide insight into controlling beam-induced RTI in complex plasmas and are relevant to inertial confinement fusion (ICF), astrophysical dusty plasmas. vi The parallel velocity shear instability (PVSI) or the Kelvin Helmholtz instability is a prominent instability in various astrophysical and laboratory situations. The instability analysis has been done in the linear regime. The combined effect of ion beam and polarisation of dust grains have been studied in the presence of dust charge fluctuation. A theoretical model has been developed to incorporate all this factors. The effect of beam parameters and dust polarisation on the instability has been analysed. 2026-01-01T00:00:00Z http://dspace.dtu.ac.in:8080/jspui/handle/repository/22703 Title: STUDIES ON RARE EARTH DOPED LUMINESCENT MATERIALS FOR APPLICATIONS IN GENERAL ILLUMINATION AND BIO-PHOTONICS Authors: BAJAJ, RAJAT; RAO, A. S. (SUPERVISOR); PRAKASH, G. VIJAYA (CO-SUPERVISOR) Abstract: Research in the field of luminescent materials doped with rare earth (RE) and transition metal (TM) ions has taken giant strides due to the rapid development of technologies such as solid-state lighting and other display technologies. RE ions doped glassy materials also find applications in lasers, optoelectronic devices, and civil-military applications such as infrared detectors, infrared fairings, nuclear imaging, and detection. Photo luminescent glass applies these distinctive properties to photonics, lighting, and photovoltaics by applying Downconversion light from UV to visible or near-infrared (NIR) light, and it is suitable for display devices, smart windows, lasers, and optical fibers & w-LEDs, among many other applications. RE doped glasses useful for optoelectronics devices have been fabricated and characterized by many researchers because of their high transparency, low production cost, ease of shaping, and relatively high thermal stability. Rare earth activated glasses, and nanophosphor are important groups of engineering materials that can be used in a wide range of multifunctional and industrial applications. The advantages of glasses and glass ceramics have attracted a lot of attention. Inorganic glasses, which belong to the unique family of amorphous solid-state materials, are usually thermally stable and form over a wide range of glass-former concentrations. Since Snitzer first demonstrated the laser action of Nd3+ ions in barium crown glass in 1961, a great deal of work has been done on rare-Earth-doped glasses for solid-state lighting (SSL) applications, broadband optical amplifiers, up-conversion luminescence temperature sensors, and near-IR STUDIES ON RARE EARTH DOPED LUMINESCENT MATERIALS FOR APPLICATIONS IN GENERAL ILLUMINATION AND BIO-PHOTONICS vii lasers. Additionally, optical fibers that emitted infrared light were made using a variety of precursor glasses. In the current work, we have created a good glassy system (using the melt quench method) called alkali zinc alumino borosilicate (AZABS) glass doped with varying concentrations of europium ions. We have characterized them spectroscopically to gain insight into their suitability for general illumination such as w-LEDs and other related SSL device applications, all against the backdrop of the various scientific patronages of chemical species like H3BO3, SiO2, Al2O3, ZnO, and Na2CO3. Also, the spectral properties of RE doped chlorides, oxides, fluorides and phosphate have been studied extensively to understand their suitability as potential luminescent applications. However, most of the systems are sensitive to moisture and thus are not quite suitable for bio labeling except the fluoride compounds with a formula such as AREF4 (A = alkali, RE = rare earth F = Fluoride) [38,39]. Among AREF4 host lattices, KREF4 (K stands for potassium) especially have attracted much more attention because of the high reflective index and low phonon energy that make them excellent host matrix for both DS as well as UC processes For the current thesis project, the rare earth-doped luminous material's industrial and multipurpose uses are chosen based on the previously described benefits. Enhancing the luminous qualities of rare earth ion-doped glasses and nanophosphors for use in biomedical and general illumination applications is the main goal of the research. Several chapters are meeting the research's objectives. Each chapter is meant to be read independently. Chapter 1: A clearer introduction, the reason for the problem, the motivation for the research, and a review of recent literature are all included in the first chapter. This chapter starts with a brief introduction, the origin of the problem, the motivation of the research work, and an overview of the current literature. This chapter provides an introduction to different types of glasses and nanophosphors, the components involved in their formation, and their specific viii properties. Furthermore, the importance of borosilicate glasses and nanophosphor are discussed in detail. Based on the characteristics required for glass host, 35B2O3.20SiO2.15Al2O3.15ZnO. -15Na2CO3 glass composition selected to synthesize, transparent, thermally mechanically stable glass with exceptional photonic properties, which can directly be applicable in general illumination. This approach has involved a thorough exploration of the properties of the many chemical components present in the host glass. The usefulness of RE ions doped in glasses for use in photonic devices has been studied further. Also, discussed the Eu doped KYF4 (Potassium Yttrium Fluoride) nanophosphors as promising host material for biomedical application. Chapter 2: The experimental process used for producing RE-doped glasses and nanophosphor and the methods for evaluating their luminous properties are the main topics of the second chapter. There is also a detailed discussion of the melt-quench process, which is used to create as-prepared glasses. This chapter describes the use of many sophisticated experimental techniques, such as X-ray diffraction (XRD), UV-VIS spectrophotometer, Temperature Dependent PL (TDPL) Spectroscopy, PL Decay Spectroscopy, SEM, and Up-conversion processes, to study various properties, including thermal, structural, photoluminescent, and colorimetric properties. Chapter 3: In this chapter, Dy3+ doped Alkali Zinc Alumino Borosilicate (AZABS) glasses have been prepared via melt quenching technique. A series of AZABS glasses of varying concentrations of dysprosium (Dy3+) (0.1 mol% -2.5 mol%) was prepared. It was found that under UV excitation, 0.5 mol% Dy3+ doped glass exhibited maximum luminescence intensity. Subsequent photoluminescence studies like emission/excitation spectra, temperature dependent photoluminescence and decay kinetics were also performed. Dexter theory was applied to study the energy transfer mechanism between the dopant ions in the glass matrix. Positive and ix encouraging results from all the photoluminescence studies for Dy3+ doped AZABS glasses confirm that these as-prepared glasses can be used as prospective materials in general illumination. [Part of this work has been published in Journal of Materials Science: Materials in Electronic, 33 (2022) 4782–4793 (Impact Factor = 2.8)] Chapter 4: Samarium (Sm3+) ions doped AZABS glasses were synthesized via quick melt quench technique. Various spectroscopic studies like optical absorption, photoluminescence (PL) emission, PL excitation, temperature-dependent PL and PL decay kinetics were performed on the as prepared glass system. Under 402 nm excitation, three sharp bands at wavelengths 563, 599 and 645 nm corresponding to transitions 4G5/2 → 6H5/2, 6H7/2 and 6H9/2 respectively can be seen in the PL emission spectra. The 0.25 mol% Sm3+ glass has the highest intensity for these emissions. The lanthanide interaction in the glass matrix is dipole-dipole in nature as was proven from Dexter’s analysis. The direct bandgap of 0.25 mol% Sm3+ doped AZABS glass was calculated to be 2.88 eV. The lifetimes of the as prepared glasses range from 1.93 ms for the lowest concentration of Sm3+ to 0.75 ms for the highest. From temperature dependent PL studies, the activation energy for 0.25 mol% Sm3+ doped AZABS glass was found to be 0.19 eV which shows high thermal stability of this glass. We propose to utilize these Sm3+ doped AZABS glasses for general illumination such as w-LEDs and solid-state lighting (SSL) applications. [Part of this work has been published in Luminescence 28 (2023), 428-436, (Impact Factor = 3.2)] Chapter 5: In this chapter we discuss the synthesis of thermally stable borosilicate glasses doped with europium ions having chemical composition 35B2O3.20SiO2.(15-x)Al2O3.15ZnO. 15Na2CO3.xEu2O3 (x = 0.5 to 2.5 mol%) using melt quench process. A broad hump without any sharp peaks observed in the XRD spectrum recorded for an undoped glass confirm its glassy x nature. The DSC & TGA has been conducted on an undoped glass to understand thermal stability and aggregate weight loss. The absorption spectral features recorded for the as prepared glasses are used to estimate optical band gap. In the process of understanding the effective usage of the as prepared glasses in visible red photonic device applications, the spectral features such as photoluminescence (PL) excitation, PL emission and PL decay were recorded and analyzed. Under 393 nm sharp excitation, all the glass samples are showing red emission corresponds to 5D0 → 7F2 transition (612 nm) and whose intensity continuously increasing with Eu3+ ion concentration up to 2.5 mol%. The red to orange color ratio (R/O) estimated from the recorded PL spectral features varies from 3.62 to 3.92 within the variation limits of Eu3+ ions from 0.5 to 2.5 mol% indicates relatively low symmetry around Eu3+ ions in the as prepared glasses. Relatively higher R/O ratio also reveals that the nature of bonding between Eu3+ ions and the surrounding ligands as covalent. The Judd-Ofelt theory has been applied to the emission spectral features to understand the nature of bonding between the doped RE ion and its surrounding ligands along with the radiative properties of the doped RE ion. Activation energy (0.175 eV) and percentage loss (82%) in PL intensity estimated for 2.5 mol% of Eu3+ ions through temperature dependent PL (TDPL) studies reveal the superiority in thermal stability of the as prepared glasses. The PL, TDPL, PL decay studies conducted along with CIE coordinates estimated allows us to contemplate that, the as prepared glasses are quite useful in fabricating thermally stable visible red photonic devices. [Part of this work has been communicated in Journal of Non-Crystalline Solids 575 (2022) 121184 (Impact Factor = 3.2)] Chapter 6: This chapter deals with the synthesis of cubic phase KYF4:Eu3+ nanophosphors via wet chemical route. Morphological studies such as XRD, SEM and EDAX mapping were done to ascertain shape, size and composition of the as prepared nanophosphors. Debye Scherrer formula applied to the XRD spectral features of the as prepared nanophosphors reveals the xi average size in the range 3 - 4 nm. The JCPDS data analysis for KYF4:Eu3+ nanophosphors confirm cubic structure with lattice constant a = b = c = 5.448Å and α = β = γ = 90°. The SEM image mapping clearly demonstrates the uniform distribution of all the constituent elements such as potassium, yttrium, fluorine and europium. Up-conversion (UC) studies carried out using 800 nm spitfire femtosecond laser produces peaks at 576, 590, 612, 650, 700 nm pertaining to the transitions 5D0 → 7Fj (J = 0, 1, 2, 3, 4) respectively. In addition to this, three higher order peaks are also observed at 523 531, 552 nm pertaining to 5D1 → 7Fj (J = 0, 1, 2) transitions respectively. Down-shifting (DS) studies under 393 nm and 405 nm excitation were also recorded to understand the utility of the as prepared phosphors for lighting applications. These nanophosphors are capable of emitting visible emissions under UV/NIR excitations. The powder dependence studies conducted on UC and DS reveal the excitation process as two photons and single photon respectively. DS temperature dependent PL spectral revels good thermal stability for the as prepared phosphor. The interesting results obtained allow us to contemplate that the as prepared KYF4:Eu3+ nanophosphors are useful for bio-imaging (through UC) as well as lighting applications (through DS). [Part of this work has been published in Journal of Alloys and Compounds, 885 (2021), 160893, (Impact Factor = 5.8)] Chapter 7: An overview of the overall study effort and the particular conclusions drawn from the data are presented in sixth chapter of this dissertation. This chapter also looks at future directions and social impact for this study and how it could be used to inform future research directions. 2025-05-01T00:00:00Z