Sarthak Das

Among the transition metal dichalcogenides, single layer MoS ₂ has drawn great attention due to its direct bandgap of approximately 1.9 eV at room temperature, exhibiting extremely high excitonic photoluminescence in the visible range, making it a promising material for opto-electronic applications. In this work, we synthesized MoS ₂ on a Si/SiO ₂ substrate using a single-step atmospheric pressure chemical vapor deposition process, successfully yielding bilayer to few layered MoS ₂ films. The layer dependence of the as-grown MoS ₂ layers was analyzed from the peak energy difference (Δ) between the in-plane (E _2g ) and out-of-plane (A _1g ) phonon modes in the Raman spectra to quantify the specific number of layers within the MoS ₂ flake (2–15 layers). Room-temperature photoluminescence characteristics of the excitonic emission exhibited a redshift in energy by 0.04 eV as the number of layers increased in MoS ₂ . An increase in the number of layers exhibited a decreasing dispersion of trion binding energy from a standardized fitting procedure of the PL spectra under appropriate boundary conditions. Henceforth, we observe a thickness dependent inversely related interplay between trion binding energy and electron density ( n ) due to defects in MoS _2, which is crucial for future opto-electronic applications.

Quasi-one-dimensional (quasi-1D) layered nanoribbons (NRs) are being extensively investigated for their potential applications in next-generation nanoelectronics and optoelectronics. Understanding electrical conduction in such NRs is essential for designing the devices. In this work, we provide insights into hopping conduction mechanisms in NR transistors based on titanium trisulfide (TiS3), a quasi-1D layered semiconductor with distinctive anisotropic properties. We report both linear and nonlinear current–voltage characteristics over a temperature range of 5–300 K. The thermal evolution of the nonlinear characteristics exhibits power-law scaling and collapses onto a single universal curve when scaled by temperature. Our findings suggest that the nonlinear conduction is dominated by rare-chain hopping, having its origins in the disorders in TiS3.

One-dimensional (1D) nanoribbons (NRs) constitute rapidly advancing nanotechnology with significant potential for emerging applications such as quantum sensing and metrology. TiS3 nanoribbons exhibit strong in-plane crystal anisotropy, enabling robust 1D confinement and resilience to edge disorder. Nevertheless, charge transport in 1D TiS3 remains relatively unexplored, particularly at low temperatures, where high contact resistance impacts device performance and fundamentally limits its applications. Here, we engineer electrical contacts between a bulk metal and a 1D NR and explore the low-temperature characteristics of the 1D field-effect devices. We report ohmic contacts for 1D TiS3 with temperature-independent contact resistances as low as 2.7 ± 0.3 kΩ·μm, enabling the study of charge transport at low temperatures (down to 35 mK) and clear observation of the Coulomb blockade effect. We demonstrate single-electron transport in 1D TiS3 and perform excited state spectroscopy and magnetospectroscopy, extracting an out-of-plane electron g-factor, g = 1.8 ± 0.3.

Developing future electronics will require aggressive scaling of the channel material thickness while maintaining device performance. Two-dimensional (2D) semiconductors are promising candidates to sustain further device scaling, but despite more than two decades of intense research, experimental performance continues to lag theoretical expectations. Here, we develop an oxygen-free approach to fabricate 2D field-effect transistors and push the electrical transport toward the theoretical phonon-limited intrinsic mobility. This approach achieves record carrier mobilities of 91 and 132 cm² V^-1 s^-1 for mono- and bilayer MoS₂ transistors on silicon oxide substrates. Statistical analysis of over 60 MoS₂ and WS₂ devices confirms that oxygen-free fabrication enhances device performance by more than an order of magnitude across key figures of merit. While previous studies suggest that 2D transition metal dichalcogenides such as MoS₂ and WS₂ are relatively stable in air, we show that even short-term ambient exposure can degrade their performance. We identify oxygen as a crucial factor in limiting transistor performance through irreversible chemisorption. This study emphasizes the criticality of avoiding oxygen exposure and offers guidance for device manufacturing that impacts fundamental research and practical applications of 2D materials.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

Excitons are key to the optoelectronic applications of van der Waals semiconductors with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation is complicated by their inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic control of valley polarization in charged biexciton (quinton) states of monolayer tungsten disulfide, achieving up to a sixfold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the quinton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible non-monotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a coherent interplay between neutral and charged exciton states.

Excitons are key to the optoelectronic applications of van der Waals semiconductors with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation is complicated by their inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic control of valley polarization in charged biexciton (quinton) states of monolayer tungsten disulfide, achieving up to a sixfold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the quinton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible non-monotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a coherent interplay between neutral and charged exciton states.

Strong Coulomb interactions in monolayer semiconductors allow them to host optically active large many-body states, such as the five-particle state, charged biexciton. Strong nonlinear light absorption by the charged biexciton under spectral resonance, coupled with its charged nature, makes it intriguing for nonlinear photodetection─an area that is hitherto unexplored. Using the high built-in vertical electric field in an asymmetrically designed few-layer graphene encapsulated 1L-WS₂ heterostructure, here we report a large, highly nonlinear photocurrent arising from the strong absorption by two charged biexciton species under zero external bias (self-powered mode). Time-resolved measurement reveals that the generated charged biexcitons transfer to the few-layer graphene in a time scale of sub-5 ps, indicating an ultrafast intrinsic limit of the photoresponse. By using single- and two-color photoluminescence excitation spectroscopy, we show that the two biexcitonic peaks originate from bright-dark and bright-bright exciton-trion combinations. Such innate nonlinearity in the photocurrent due to its biexcitonic origin, coupled with the ultrafast response due to swift interlayer charge transfer, exemplifies the promise of manipulating many-body effects in monolayers toward viable optoelectronic applications.

The monolayers of semiconducting transition metal dichalcogenides host strongly bound excitonic complexes and are an excellent platform for exploring many-body physics. Here we demonstrate a controlled kinetic manipulation of the five-particle excitonic complex, the charged biexciton, through a systematic dependence of the biexciton peak on excitation power, gate voltage, and temperature using steady-state and time-resolved photoluminescence. With the help of a combination of the experimental data and a rate equation model, we argue that the binding energy of the charged biexciton is less than the spectral separation of its peak from the neutral exciton. We also note that while the momentum-direct radiative recombination of the neutral exciton is restricted within the light cone, such restriction is relaxed for a charged biexciton recombination due to the presence of near-parallel excited and final states in the momentum space.