arXiv subjects

Joy Mitra

Publications and source records attributed to Joy Mitra.

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

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↗

arXiv2025

Leveraging Epsilon Near Zero phenomena for on-chip photonic modulation

Epsilon-near-zero (ENZ) systems exhibit unconventional electromagnetic response close to their zero permittivity regime. Here, we explore the ability of ultrathin ENZ films to modulate the transmission of radiation from an underlying quantum emitter through active control of the carrier density of the ENZ film. The achievable on/off switching ratio is shown to be constrained by the material's loss parameter, particularly in the ENZ regime, where transmissivity increases with higher material loss. The finite loss in real materials limit the more extraordinary potential of ideal near-zero-index systems. Along with an in-depth discussion on the material parameters vis-a-vis the underlying physics, this work provides avenues to overcome the shortcomings of finite loss in real materials. These findings are intended to guide material development and offer valuable insights for designing on-chip optical modulators and beam steering devices operating in the near-infrared regime.

physics.optics↗

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↗

arXiv2021

Tailoring Infrared Absorption and Thermal Emission with Ultrathin-film Interferences in Epsilon-Near-Zero Media

Engineering nanophotonic mode dispersions in ultrathin, planar structures enables significant control over infrared perfect absorption (PA) and thermal emission characteristics. Here, using simulations, the wavelength and angular ranges over which ultrathin, low loss, epsilon-near-zero (ENZ) films on a reflecting surface most efficiently absorb and re-radiate are identified, and the design parameters that tailor the ENZ mode dispersion within these limits are investigated. While the absorption is spectrally limited to wavelengths where the refractive index ($n$) lies below unity, the angular limits are determined by the ENZ material dispersion in this range. A model of ultrathin-film interference is developed to provide physical insight into the absorption resonances in this regime, occurring well below the conventional quarter-wavelength thickness limit. Driven by non-trivial phase shifts incurred on reflection at the $n<1$ surface, these resonant interferences are shown to be universal wave phenomena in planar structures having appropriate index contrast, extending beyond ENZ materials. Selective choice of material, film thickness and loss allows fine-tailoring the mode dispersions, enabling wide variation in spectral range ($ \sim 0.1 - 1.0 μm$) and precise directional control of spectrally and angularly narrow-band PA and thermal radiation, paving the way towards efficient ENZ-based infrared optical and thermal coatings.

physics.optics↗