arXiv subjects

Sraboni Dey

Publications and source records attributed to Sraboni Dey.

6 recordsLinked to original sources

arXiv2026

Investigating Sulfur Vacancy Passivation in Monolayer MoS2 FETs via Optically Coupled Low-Frequency Electrical Noise Spectroscopy

Transition metal dichalcogenide monolayers are promising materials for electronic and photonic applications, yet the performance of chemical vapour deposition grown films is severely limited by native sulphur vacancies that introduce mid-gap trap states, degrade carrier mobility, and elevate electrical noise. Here we investigate octane thiol passivation of sulphur vacancies in monolayer MoS2 field effect transistors, combining x-ray photoelectron spectroscopy, photoluminescence, and Raman scattering with electrical transport and optically coupled low-frequency noise spectroscopy. Thiol treatment reduces the sulphur vacancy concentration from 7.5% to 5%, which increases the channel resistance 35-fold while restoring gate switching with an on/off ratio of 10^4 and improving field-effect mobility from 1 to 5 cm^2/Vs. Low frequency noise spectroscopy directly quantifies the defect suppression: the Hooge parameter drops by more than two orders of magnitude after passivation. Gate-dependent noise confirms carrier mobility fluctuation as the dominant dark noise mechanism, while optical excitation drives a crossover to carrier number fluctuation dominated noise, reflecting preferential interaction of photogenerated carriers with residual vacancy states via generation-recombination trapping, a mechanistic distinction inaccessible to gate- bias measurements alone. Density functional theory calculations corroborate these findings, showing suppression of vacancy-induced mid-gap states by more than 50% and partial restoration of the intrinsic bandgap. These results establish optically coupled low-frequency noise spectroscopy as a sensitive, low-cost, and non-destructive tool for quantifying defect passivation in TMDC-based devices.

cond-mat.mtrl-sci↗

arXiv2026

Epsilon-Near-Zero Materials based Photonic Architectures for Absorption and Emission Control

Engineering optical responses at surfaces, including reflection, absorption, transmission, and emission, is crucial for advanced photonic, energy, thermal-management, and optoelectronic applications. This thesis investigates epsilon-near-zero (ENZ) materials, particularly indium tin oxide (ITO) and titanium nitride (TiN), for controlling light-matter interactions through engineered optical coatings and nanostructures. It also explores the enhancement of optical absorption and emission in two-dimensional (2D) materials such as monolayer molybdenum disulphide (MoS2), whose atomic-scale thickness inherently limits light interaction. The research integrates theoretical analysis, finite-element simulations, nanofabrication, and experimental characterization to develop and validate ENZ-based optical platforms. An ITO-based multilayer coating is demonstrated to provide step-function-like reflectivity, with low reflectance in the visible-to-near-infrared range and high reflectance beyond a tunable cut-in wavelength, offering potential for spectrally selective energy management. Building on this, an ITO-based grating structure is developed to achieve broadband and angularly robust near-infrared absorption, demonstrating its potential for thermal-emission applications. TiN thin films are further demonstrated as an effective platform for enhancing the absorption and emission of monolayer MoS2 without complex nanostructuring. Complementary studies investigate substrate-induced strain and ion-irradiation-induced defects as approaches for tailoring the electronic and optical properties of MoS2. Overall, this work establishes ENZ materials based engineered surfaces as versatile platforms for spectral and light-matter interaction control, contributing to efficient, tunable, thermally stable, and scalable solutions for energy, thermal management, and 2D-material-based optoelectronic applications.

physics.optics↗

arXiv2026

Machine Learning Assisted Reconstruction of Local Electronic Structure of Non-Uniformly Strained MoS2

Wrinkles and nanobubbles are an integral and often unavoidable part of integrating 2D van der Waals semiconductors into actual device architectures. Despite their ubiquitous nature, quantitative correlation between such spatially non-uniform strain and modifications to the local electronic structure remains challenging. Here, density functional theory is combined with a recurrent neural network to reconstruct the local electronic structure of monolayer MoS2 from strain maps derived from atomic force microscopy (AFM) topography and Raman spectral maps. The analysis reveals that biaxial bending induced strain is significantly more effective than both uniaxial bending or in-plane strain in modifying electronic and dielectric properties. A ~ 0.35% strain induced by biaxial bending results in ~ 22% reduction in band gap and ~ 7% increase in dielectric constant, compared to a ~ 5% reduction in band gap and ~ 1% increase in dielectric constant under comparable uniaxial bending. The modified band structure reveals band edge states that concentrate charge in regions of high curvature or strain. While conductive AFM measurements indicate increased local conductance (carrier density) at wrinkles and nanobubbles, the spatial band gap maps predicted by the model are validated against experimental photoluminescence peak energy maps. The results indicate that strained features like wrinkles and nanobubbles commonly present in real devices influence the band gap, carrier distribution, and dielectric response, which favourably affects electrical transport in such systems. The framework developed here can be readily extended to other 2D materials and heterostructures, offering a computationally efficient route for studying and exploiting strain effects.

cond-mat.mtrl-sci↗

arXiv2024

Engineering band selective absorption with epsilon-near-zero media in the infrared

Band-selective absorption and emission of thermal radiation in the infrared are of interest due to applications in emissivity coatings, infrared sensing, thermo-photovoltaics and solar energy harvesting. The broadband nature of thermal radiation presents distinct challenges in achieving spectral and angular selectivity, which are difficult to address by prevalent optical strategies, often yielding restrictive responses. We explore a tri-layer coating employing a nanostructured grating of epsilon-near-zero (ENZ) material, indium tin oxide (ITO), atop a dielectric (silicon dioxide) and metal (gold) underlayer, which shows wide-angle (0-60 degrees) and band-selective (1800 - 2800 nm) high absorption (> 0.8). Numerical simulations and experimental results reveal that the ENZ response of ITO combined with its localized plasmon resonances define the high absorption bandwidth, aided by the sandwiched dielectric's optical properties, elucidating the tunability of the absorption bandwidth. Thermal imaging in the mid-infrared highlights the relevance of the ENZ grating, emphasizing the potential of this coating design as a thermal emitter. This study offers valuable insights into light-matter interactions and opens avenues for practical applications in thermal management and energy harvesting.

physics.optics↗

arXiv2024

Epsilon near zero metal oxide based spectrally selective reflectors

Epsilon near zero (ENZ) materials can contribute significantly to the advancement of spectrally selective coatings aimed at enhancing efficient use of solar radiation and thermal energy management. Here, we demonstrate a subwavelength thick, multilayer optical coating that imparts a spectrally "step function" like reflectivity onto diverse surfaces, from stainless steel to glass, employing indium tin oxide as the key ENZ material. The coating, harnessing the ENZ and plasmonic properties of nominally nanostructured ITO along with ultrathin layers of Cr and Cr2O3 show 15% reflectivity over the visible to near-infrared and 80% reflectivity (and low emissivity) beyond a cut-in wavelength around 1500 nm, which is tunable in the infrared. A combination of simulations and experimental results are used to optimize the coating architecture and gain insights into the relevance of the components. The straightforward design with high thermal stability will find applications requiring passive cooling.

physics.optics↗

arXiv2023

Mobility enhancement in CVD-grown monolayer MoS2 via patterned substrate induced non-uniform straining

The extraordinary mechanical properties of 2D TMDCs make them ideal candidates for investigating strain-induced control of various physical properties. Here we explore the role of non-uniform strain in modulating optical, electronic and transport properties of semiconducting, chemical vapour deposited monolayer MoS2, on periodically nanostructured substrates. A combination of spatially resolved spectroscopic and electronic properties explore and quantify the differential strain distribution and carrier density on a monolayer, as it conformally drapes over the periodic nanostructures. The observed accumulation in electron density at the strained regions is supported by theoretical calculations which form the likely basis for the ensuing 60x increase in field effect mobility in strained samples. Though spatially non-uniform, the pattern induced strain is shown to be readily controlled by changing the periodicity of the nanostructures thus providing a robust yet useful macroscopic control on strain and mobility in these systems.

cond-mat.mtrl-sci↗