Arindam Ghosh

Electrons in high-mobility graphene devices have demonstrated great potential in establishing an electronic analogue of relativistic quantum fluid in solid-state systems. Over the past decade, complex device geometries have been employed to enable experimental detection of viscous electronic flow; however, the observations have been found to be sensitive to the device architecture and fabrication process, raising questions about the signature of electron hydrodynamics itself. Here, we present a study on multiple ultraclean graphene field-effect transistors (FETs) in a rectangular four-terminal device architecture. Using electrical transport measurements, we have examined variation of the electrical resistance of FETs in the doped regime as a function of the carrier density and temperature. Our results reveal strong device-dependent variability, attributed to competing momentum-conserving and momentum-relaxing scattering mechanisms. Further, we have proposed a phenomenological method for analyzing the results, yielding transport parameters consistent with recent theory and experiments.

Long-range moiré patterns in twisted WSe₂ enable a built-in, moiré-length-scale ferroelectric polarization that can be directly harnessed in electronic devices. Such a built-in ferroic landscape offers compelling means to enable ultralow-voltage and non-volatile electronic functionality in two-dimensional (2D) materials; however, achieving stable polarization control without charge trapping has remained a persistent challenge. Here, we demonstrate a moiré-engineered ferroelectric field-effect transistor (FeFET) utilizing twisted WSe₂ bilayers that leverage atomically clean van der Waals interfaces to achieve efficient polarization-channel coupling and trap-suppressed, ultralow-voltage operation (subthreshold swing of 64 mV dec^-1). The device exhibits a stable non-volatile memory window of 0.10 V and high mobility, exceeding the performance of previously reported 2D FeFET and matching that of advanced silicon-based devices. In addition, capacitance-voltage spectroscopy, corroborated by self-consistent Landau-Ginzburg-Devonshire modeling, indicates ultrafast ferroelectric switching (∼0.5 μs). These results establish moiré-engineered ferroelectricity as a practical and scalable route toward ultraclean, low-power, and non-volatile 2D electronics, bridging atomistic lattice engineering with functional device architectures for next-generation memory and logic technologies.

Enhancing the electrical conductivity of organic semiconductors at low dopant concentrations while preserving morphology remains a critical challenge for solution-processed optoelectronic devices. We show that tetracyanoquinodimethane (TCNQ)-based dopants increase the conductivity of poly(3-hexylthiophene) (P3HT) films to >0.01 S/cm at concentrations as low as 3 wt %, over 2 orders of magnitude higher than barely conducting neat films. The comparable performance of TCNQ relative to the higher electron-affinity dopant F4TCNQ demonstrates that transport is not dictated solely by dopant electron affinity but also influenced by the subtle interplay between dopant-induced structural disorder and ionized polymer-dopant Coulomb interactions. Two-dimensional electronic and pump-probe spectroscopies reveal uniform film morphology with no preferential doping in the crystalline domains, absence of strongly bound ion-pairs, and rapid polaron pair formation with exciton quenching. The results suggest that polaron-induced backbone planarization extends intrachain hole delocalization, thereby enhancing π-π overlap and interchain electronic coupling within crystalline domains.

Hydrodynamic electrons in high-mobility graphene devices have demonstrated great potential in establishing an electronic analogue of relativistic quantum fluid in solid-state systems. One of the key requirements for observing viscous electron flow in an electronic channel is a large momentum-relaxation path, a process primarily limited by electron-impurity/phonon scattering in graphene. Over the past decade, multiple complex device geometries have been successfully employed to suppress momentum-relaxing scattering mechanisms; however, experimental observations have been found to be sensitive to the device fabrication process and architecture, raising questions about the signature of electron hydrodynamics itself. Here, we present a study on multiple ultra-clean graphene field-effect transistors (FETs) in a simple, rectangular four-terminal device architecture. Using electrical transport measurements, we have characterised the pristine quality of the graphene FETs and examined the variation of electrical resistance in the doped regime as a function of carrier density and temperature. Our results reveal strong device-dependent variability even in the most simple architecture that we attribute to competing momentum-conserving and momentum-relaxing scattering mechanisms, as well as coupling to contacts. Further, we have proposed a phenomenological method for analysing the results, which yields transport parameters in accordance with recent experiments. This simple experimental strategy and analysis can serve as an efficient tool for extracting the viscous electronic contribution in state-of-the-art high-mobility graphene FETs.

Pristine graphene, owing to its remarkable electrical and structural properties, has garnered significant interest as a sensing platform for such applications. In this study, we evaluate the use of 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) chemistry to functionalize pristine graphene for the detection of antigens, essential for food safety and public health. However, our investigation reveals critical limitations in the reproducibility of both electrical and spectroscopic measurements post-functionalization. The unpredictable activation of pristine graphene surfaces via EDC-NHS coupling leads to inconsistent antigen immobilization, resulting in a high variability in sensor performance. Our studies, supported by Raman spectroscopy, indicate the absence of functionalization of the graphene surface via EDC-NHS chemistry. Our findings suggest that optimizing the functionalization protocol or employing alternative surface modification techniques, such as defect engineering or using linker molecules, is necessary to achieve consistent and reliable detection. This study sheds light on the challenges of using EDC-NHS chemistry with pristine graphene and provides a roadmap for improving sensor reproducibility in the detection of penicillin and cephalexin.

A persistent challenge in transition metal dichalcogenide (TMD)-based transistors is the formation of a Schottky Barrier (SB) at the metal–TMD interface which introduces substantial contact resistance and degrades device performance. Minimizing the barrier height and hence contact resistance—ideally to near-zero—is essential for realizing high-performance two dimensional (2D) material-based field-effect transistors. Here, we present a non-invasive photodoping strategy that leverages ultraviolet irradiation to induce localized n-type doping near the contact region, in hBN/TMD field-effect transistors. This targeted doping with UV exposure significantly reduces the SB, leading to a remarkable improvement in device performance. We demonstrate this with hBN/MoS2 transistors, where we achieve a barrier height reduction of ∼100 meV, resulting in a seventy-fold increase in on-state current and a twenty-fold increase in mobility. We further demonstrate the generality of this approach by applying it to other TMD transistors, such as hBN/MoSe2 and hBN/WSe2 hybrids, all of which exhibit similar performance enhancements. These results outline a portable, broadly applicable, and scalable contact engineering strategy for next-generation 2D electronic devices.

The Rashba effect, which plays a crucial role in fundamental materials physics and potential spintronics applications, has been engineered in diverse systems, including semiconductor quantum wells, oxide heterostructures, metallic surfaces, topological insulators, ferroelectrics, etc. However, generating it in systems that preserve bulk inversion symmetry (BIS), for example, in bulk metals, has not been possible so far. We demonstrate a strategy to introduce and tune Rashba spin-orbit interaction (SOI) to unprecedented magnitudes in inversion-symmetric solids by incorporating ultrasmall silver nanoparticles in bulk gold. The near-identical lattice constants of Ag and Au allow dense packing of the Ag/Au hetero-interfaces without compromising the global BIS. By varying the density of embedded nanoparticles, we generate Rashba SOI in a bulk metal with coupling strength ~15 meV∙Å, higher than any known system preserving BIS globally, and show up to ~20 times increase in the spin-orbit scattering rate. We argue that the combined effect of charge transfer at the interfaces and polaronic localization enhances the SOI.