http://dspace.dtu.ac.in:8080/jspui/handle/123456789/31 2026-04-28T02:42:01Z 2026-04-28T02:42:01Z ANEESHA http://dspace.dtu.ac.in:8080/jspui/handle/repository/22682 2026-03-05T06:40:23Z 2025-11-01T00:00:00Z

Title: SYNTHESIS AND INVESTIGATION OF TRANSITION METAL DICHALCOGENIDES: NANOSTRUCTURES AND THEIR APPLICATIONS Authors: ANEESHA Abstract: Transition-metal dichalcogenide (TMD) quantum dots (QDs) represent a rapidly advancing class of nanomaterials distinguished by tunable photoluminescence (PL), high surface reactivity, chemical stability, and biocompatibility, yet achieving water- stable QDs with strong emission remains challenging. This thesis systematically examines the hydrothermal synthesis of WS2 and MoSe2 QDs and correlates their physicochemical properties with optical behavior to develop high-performance fluorescent probes for detecting industrial, environmental, and pharmaceutical contaminants. Alkaline-synthesized WS2 QDs (pH ≈ 11) resolved the instability associated with acidic exfoliation, producing highly dispersible, blue-emitting QDs with multiexponential lifetimes and strong structural stability. These QDs were comprehensively characterized using XRD, FTIR and HR-TEM, while their optical responses were evaluated through steady-state and time-resolved spectroscopy, collectively confirming their crystalline integrity, defect-mediated emissive states and robust aqueous stability. The WS2 QDs demonstrated exceptional sensitivity toward hydrogen peroxide (0.33 nM–594 μM, LoD 1.7×10-6 M) through reversible redox cycling between W(IV) and W(VI), enabling nearly complete signal recovery across multiple cycles. Complementarily, hydrothermally prepared MoSe₂ QDs characterized by strong 260 nm absorption and blue excitation-dependent emission served as highly sensitive probes for 2,4,6-trinitrophenol (TNP), showing linear Stern–Volmer behaviour (3.3–99 nM) and an LoD of 1.43 nM dominated by the inner filter effect. Together, these findings highlight the tunability and responsiveness of TMD QDs toward oxidative and nitroaromatic pollutants. Further extending their sensing versatility, WS₂ QDs were applied for detecting heavy-metal ions and pharmaceutical contaminants. For Cr6+, the QDs exhibited strong and reversible PL quenching driven primarily by static surface complexation, supported by XPS evidence of partial oxidation of W and S atoms. The platform delivered ultralow detection limits (39.5–154 pM across different water matrices), a broad linear range (0.66–660 nM), and excellent recyclability over more than 18 redox cycles, establishing it as one of the most sensitive WS2-based Cr6+ sensors to date. WS2 QDs also enabled sensitive, label-free detection of Vitamin B12 via a static-dominated ix ANEESHA mechanism (0.33–565 nM range, 555 pM LoD), and amoxicillin via combined static– dynamic quenching supported by FTIR-verified adsorption and minor lifetime changes (0.33–607 nM, 1.24 nM LoD). Collectively, this research demonstrates that hydrothermally synthesized WS2 and MoSe2 QDs form a robust, regenerable, and environmentally benign sensing platform capable of detecting oxidants, metals, nitroaromatics, vitamins, and antibiotics. The study provides crucial insight into structure, property and function relationships in TMD QDs, and establishes their strong potential for real-sample monitoring, multimodal sensing, catalytic remediation, and emerging biophotonic applications.

2025-11-01T00:00:00Z SHARMA, VISHAKHA Sinha, R. K. (SUPERVISOR) Kalra, Yogita CO-SUPERVISOR) http://dspace.dtu.ac.in:8080/jspui/handle/repository/22681 2026-03-05T06:07:52Z 2026-02-01T00:00:00Z

Title: MODELING, DESIGN AND APPLICATIONS OF OPTICAL METASURFACES Authors: SHARMA, VISHAKHA; Sinha, R. K. (SUPERVISOR); Kalra, Yogita CO-SUPERVISOR) Abstract: Metasurfaces, ultrathin arrays of engineered subwavelength resonators, have emerged as powerful platforms for precise control of light, enabling compact and multifunctional devices for imaging, sensing, communication, and secure information processing. This thesis, “Modeling, Design and Applications of Optical Metasurfaces”, advances the field through systematic exploration of achiral and chiral architectures, moving from fundamental design principles to multifunctional applications, and from reflection-based to transmission-based operation. The work begins with achiral metasurfaces, where a dielectric metalens inspired by the human eye is developed using concentric cylindrical resonators. The design reduces the aspect ratio of the meta-atoms, ensuring enhanced structural robustness and fabrication feasibility. Attention then shifts to chiral metasurfaces under oblique incidence, where multilayer and perovskite- based structures achieve strong circular and linear dichroism, including triple-band responses for biosensing applications such as hemoglobin, glucose, and cancer cell detection. To overcome the limitations of oblique excitation, intrinsic chirality under normal incidence is realized using semi-circular ring arrays, producing near-perfect absorption with high circular dichroism across telecommunication bands. These structures also enable secure image encryption, demonstrating potential in optical communication and information security. VISHAKHA SHARMA vii Building further, reflective and transmissive chiral metasurfaces are developed for multifunctional control. A spin-multiplexed reflective metalens achieves polarization-selective focusing, while an all-dielectric meta-coded firewall with graphene tunability enables polarization-gated transmission for secure authentication and friend-or-foe recognition. In conclusion, the thesis establishes a coherent progression from achiral to advanced chiral metasurfaces, integrating concepts of absorption, dichroism, multiplexing, and tunability. The results highlight metasurfaces as versatile and scalable platforms for next-generation technologies spanning communication, defense, biomedical diagnostics, and photonic encryption.

2026-02-01T00:00:00Z NARWAN, MEGHA http://dspace.dtu.ac.in:8080/jspui/handle/repository/22657 2026-02-13T05:30:52Z 2026-01-01T00:00:00Z

Title: STRUCTURAL AND OPTICAL PROPERTIES IN LANTHANIDE (LN3+= ER3+/YB3+/HO3+) DOPED BI0.5NA0.5TIO3 CERAMICS Authors: NARWAN, MEGHA Abstract: The development of lead-free ferroelectric materials has gained considerable attention due to their potential in energy storage, sensing, and optoelectronic applications. Among them, bismuth sodium titanate (Bi₀.₅Na₀.₅TiO₃, BNT) is a promising candidate owing to its favorable ferroelectric and structural properties. The functional performance of BNT can be significantly tailored through rare-earth doping, which modifies its electrical, luminescent, and sensing characteristics. This thesis outlines the effect of Er3+substituted bismuth sodium titanate ceramics with the chemical composition Bi0.5-xErxNa0.5TiO3 (x =0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) were synthesized using conventional solid-state technique. The influence of Er3+ ions on structural, optical, ferroelectric, and temperature sensing properties have been investigated. The prepared ceramic powders were initially heated at a calcination temperature 850◦C to form single phase Bi0.5-xErxNa0.5TiO3 and finally sintered at temperature 1050◦C. Formation of pure phase composition with rhombohedral crystal structure is confirmed through X-ray diffraction studies. The typical FTIR bands near 540, 860, 910 cm-1 confirmed the presence of Ti–O stretching of octahedral groups in the perovskite structure. The decent squared shaped saturated P-E hysteresis are obtained under an electric field of 60 ≤E ≤70 kV/cm, and the loops become slimmer at higher Er3+ concentrations (x =0.04). The efficiency of energy storage density increases with Er3+ doping and an improved recoverable energy storage (Wr =2.73 J/cm3) and a higher efficiency (η =70.77%) are obtained for Er content, x =0.04. The photoluminescence spectra were recorded at two excitation wavelengths (488 nm and 980 nm). Two distinct green emission bands (529 nm and 550 nm) and one weak red emission band (670 nm) were observed at both excitation wavelengths. Increasing Er3+ content beyond (x >0.03) leads to significant quenching of light emission due to cross relaxation process and non- radiative relaxations. The pump power dependency revealed that two photons were involved in the light upconversion process. The time- viii resolved fluorescence spectroscopy confirmed the decrease in lifetime with increasing Er3+ concentration. The absolute and relative sensitivity of the prepared ceramic at Er3+ concentration (x =0.03) were found to be 0.47% K-1 at 523 K and 1.1% K-1 at 303 K, respectively. These optical and electrical properties open the possibility of realizing multifunctionality in the field of energy storage and opto- electronic applications. In continuation, a series of Er3+/Yb3+ co-doped Bi0.5-x-yErxYbyNa0.5TiO3 ferroelectric ceramic is prepared using traditional solid-state technique to investigate the structural, optical and sensing properties. The XRD analysis confirms the rhombohedral geometry and no extra peaks shows the solubility of the dopant ions. SEM images exhibited a dense micro structure with well-defined grain boundaries. The FTIR vibrational bands observed at 550cm−1 and 820cm−1 depict the typical characteristics of perovskite structure caused by the expansion of the octahedral group of Ti–O bonds. The Tauc plot displays the energy band gaps (Eg) in a range of 2.93 eV to 2.88 eV as a function of the Yb3+ concentration. The photoluminescence spectra were measured at two wavelengths of excitation at 489 nm and 980 nm for all the BNT ceramic compositions. It has been observed that the two intense green bands and one visible red band appeared at 530 nm, 549 nm and 662 nm, respectively. The dependency of pump power on UCL spectra is observed with varying pump powers at 980 nm excitation. The two photons that are involved in the UCL process are confirmed by this investigation. The time-resolved fluorescence spectroscopy reveals that the efficiency in energy transfer between the dopant ions increased for all the co-doped BNT ceramic compositions. The absolute (Sab) and relative (Sr) sensitivity of BE3Y3 ceramic composition are 0.54% K−1 and 1.24% K−1 at 523 K and 303 K, respectively. Further, the Ho3+ions are systematically inserted on the A-site of Bi0.5-xHoxNa0.5TiO3 lead-free ferroelectric ceramic via solid-state method. The x-ray diffraction spectra (XRD) shows the rhombohedral structure of Bi0.5- xHoxNa0.5TiO3. Fourier transform infrared (FTIR) spectroscopy shows two vibrational bands at 537 cm-1 and 832 cm-1, due to the stretching vibrations of Ti-O bonds in the octahedral units of the perovskite structure. The diffuse reflectance spectra (DRS) showed three bands at 454, 542, and 646 nm transit from 5I8→5G6, 5I8→5F4/5S2, and 5I8→5F5, respectively. The band gap varies and maximum for 0.03 concentration due to the local structural instability within ix the lattice by Ho3+ions. The photoluminescence (PL) emission spectra are traced under 452 nm, and one intense green band at 548 nm and two at 655 (orange) and 750 nm (red) were observed. In upconversion luminescence (UCL) spectra, four emission wavelengths were obtained at 490, 525, 552, and 660 nm. The orange emission band is highly intense, whereas other color bands are relatively weak. Pump power dependence on UCL spectra is analyzed using concentration (x =0.03). The decay profile for green and orange bands showed an average lifetime of 12.99 μs and 9.43 μs, respectively. The PE loops become slimmer, and Pr values decrease with increasing concentration. The energy storage efficiency (η%) of the ceramic is increasing with dopant concentration and comes out to be 90 %. The maximum absolute and relative sensitivity observed for the BNT (x =0.03) is 0.29 % K-1 and 0.22 % K-1 at 303 K, respectively. This study demonstrates the novel observation of orange emission in Ho3+doped Bi0.5Na0.5TiO3 ceramics without any sensitizers which is not usual behavior as Ho3+ typically exhibits green or red emission. This unusual luminescence highlights distinct site symmetry and energy level interactions within the lead-free BNT matrix, validating the uniqueness of the work. Lastly this study presents the application of BNT, the development and characterization of flexible piezoelectric nanocomposite films based on bismuth sodium titanate (BNT) embedded in a polyvinylidene fluoride (PVDF) matrix for efficient mechanical energy harvesting. BNT ceramic powder was synthesized via a conventional solid-state reaction route and subsequently incorporated into PVDF at varying concentrations (0 – 25wt% BNT) using a drop casting method. Structural analyses confirmed the successful formation of phase-pure rhombohedral BNT and enhancement of the electroactive β- phase in PVDF upon BNT addition. Morphological evaluation using FESEM revealed uniform dispersion of BNT up to 20wt% BNT, beyond which agglomeration was observed. FTIR and XRD studies substantiated the β-phase promotion in the composites, while polarization–electric field (P–E) loop measurements demonstrated improved ferroelectric behavior with increasing BNT content, peaking at 20wt% BNT. Notably, the piezoelectric voltage and current outputs were maximized at 20wt% BNT, registering 25V and 12μA, respectively, under mechanical excitation. A practical demonstration using the composite film to power LEDs confirmed its potential as a x flexible nanogenerator. These findings highlight the suitability of BNT/PVDF composites as promising candidates for next-generation, lead-free, wearable energy harvesting devices. In conclusion, Er³⁺,Er³⁺/Yb³⁺ and Ho³⁺doped Bi₀.₅Na₀.₅TiO₃ ceramics were successfully synthesized and systematically examined for their structural, ferroelectric, luminescent, and sensing characteristics. The results confirmed phase-pure rhombohedral structures, enhanced energy storage efficiency, and notable upconversion emissions with composition-dependent behavior. Particularly, Er³⁺ and Er³⁺/Yb³⁺ co-doping improved energy storage and temperature sensitivity, while Ho³⁺ substitution revealed an unusual orange emission, underscoring unique site symmetry effects. Furthermore, extending the study to BNT/PVDF nanocomposites demonstrated their effectiveness as flexible, lead-free piezoelectric generators with promising output performance. Overall, these findings establish BNT-based materials as multifunctional candidates for future energy storage, optoelectronic, and wearable energy-harvesting applications.

2026-01-01T00:00:00Z JHA, SAROJ KUMAR http://dspace.dtu.ac.in:8080/jspui/handle/repository/22655 2026-02-12T04:39:57Z 2026-02-01T00:00:00Z

Title: STUDY OF MAGNETIC PROPERTIES AND INTERFACIAL SPIN DYNAMICS IN FERROMAGNETIC / NOBLE METAL MULTILAYER STRUCTURES Authors: JHA, SAROJ KUMAR Abstract: This works covers the magnetic and interfacial dynamics properties of ferromagnetic, antiferromagnetic and nonmagnetic metal based heterostructures, a convincing potential for insulator based spintronic devices. Magnetic insulator based spintronics offers numerous applications in the field of electronics industry. In this sector, nanometre (nm) thick low damping materials are critically needed for fundamental studies like spin pumping and spintronics based device application such as narrow band filters, band-notch (stop) filters, band-pass filters, spin-torque nano-oscillators etc. Yttrium iron garnet (Y3Fe5O12, YIG) a well-known, best suitable candidate among the other ferromagnetic materials. High grade YIG films that are nanometre thick are desperately needed. This works reports experimental and theoretical studies on nanometre (nm) thick yttrium iron garnet (YIG) films. At first, demonstrated the feasibility for deposition of YIG thin film using pulsed laser deposition (PLD) system, single crystalline gadolinium gallium garnet (GGG) (111), substrate was used for the deposition of YIG thin film. Optimization of deposition parameters and annealing process for the achievement of high-quality thin films are examined in depth. The morphology of thin films was analyzed using the atomic force microscope, the structural properties were studied using X-ray diffraction technique and Raman spectroscopy. Magnetic properties of films were investigated using the physical properties measurement system (PPMS) equipped with vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) spectroscopy. Furthermore, looked in to the effect of deposition parameters in the design of reciprocal and non-reciprocal microwave devices, this work demonstrated theoretically as well as experimentally, to study theoretically used HFSS simulation to understand the reciprocal and non- reciprocal EM (Electromagnetic) wave propagation in the range of microwave frequency through different transmission line using YIG thin film as active element. Spin pumping effect carried out in YIG thin film through deposition of thin layer of Ph.D. Thesis (SAROJ KUMAR JHA) vii nonmagnetic metal (Ta) layer and magnetic (Ni) material over the YIG, by controlled oxidation of Ni to NiO, found the significant enhancement in linewidth and other damping parameters, spin mixing conductance and other parameters also calculated to understand the spin pumping efficiency of heterostructure. In the final section of thesis, deposited the series of heterostructures of ferromagnetic and noble metal using the PLD and DC/RF sputtering technique, fabricated the multilayer thin film of normal metal copper (Cu), platinum (Pt), and tungsten (W) over yttrium iron garnet (YIG). To study the spin pumping effect in nanometer thick YIG film, Series of five samples were deposited over YIG. YIG/Pt, YIG/Cu/Pt, YIG/Cu/W, YIG/Cu/Pt/W and YIG/Cu/W/Pt. Spin pumping across the interface has been analyzed using the ferromagnetic resonance (FMR) technique, shows the change in damping parameters in all the structures confirms the spin pumping across the samples from YIG. Our finding broadens the range of possible application of nanometer (nm)-thick yttrium iron garnet (YIG) epitaxial films and yttrium iron garnet (YIG) based multilayers in insulator based spintronics devices, magnonics, etc.

2026-02-01T00:00:00Z