Kausik Majumdar

Two-dimensional (2D) materials (Graphene, 2D transition metal dichalcogenides (MX ₂ ), hBN) have emerged as exciting materials for discovering novel physical phenomena, and have great potential for electronic and optoelectronic devices, as well as energy and catalysis applications. Further, materials such as Cr ₂ X ₂ Te ₆ (X = Si, Ge), VSe ₂ , Fe ₃ GeTe ₂ , CrX ₃ (X = Cl, Br, I) and MPX ₃ (M = Mn, Fe, V, Zn, Co, Ni, Cd, Mg; X = S, Se) have emerged as fascinating platforms for 2D magnetism. The heterostructures of these 2D materials, and the twist angle between layers are opening new possibilities of studying the effect of Moiré potential, exotic topological phases, magnetic-proximity effects, and strong correlations. At the same time, the research is leading towards many application-based directions such as in excitonics, valleytronics, straintronics, and quantum technologies. In this context, growth of novel 2D materials and heterostructures, as well as large scale growth becomes critically important. This roadmap aims to provide a comprehensive overview of 2D materials research in India and across the globe. The roadmap is broadly divided into six themes: 1. Topology in 2D Materials, Magnetism, Exotic Physics and Emergent Phenomena; 2. Moiré Physics in 2D Heterostructures; 3. Excitonic Physics and Optical properties, including Defects; 4. Transport Phenomena in 2D Materials; 5. 2D Materials Synthesis and Growth; and 6. 2D Materials Electrical Devices and Applications. This roadmap highlights the cutting edge-research being done by Indian and global researchers, emphasizes the challenges, and suggests possible technological solutions.

Van der Waals heterostructures grown by chemical vapor deposition (CVD) offer a scalable platform for optoelectronic applications. However, understanding and accurately assigning their excitonic spectral features remain challenging. This is due to variable interfacial quality arising from interfacial disorder and interlayer coupling, as well as overlapping spectral signatures. Here, we synthesize continuous, large-area (up to 600 μm) MoS2/WS2 vertical heterostructures using two distinct single-step CVD processes. This approach enables a systematic comparison of excitonic behavior across varying interface qualities. Temperature-dependent photoluminescence, supported by ultra-low-frequency Raman spectroscopy, resolves the evolution of distinct interlayer exciton (ILE) spectral features. In heterobilayers, the ILE evolves from localized states in the presence of interfacial disorder to delocalized states stabilized by uniform interlayer coupling. Notably, we identify a hybrid exciton, arising from strong interlayer electronic hybridization, centered at ∼1.90 eV between the neutral A-excitons of MoS2 and WS2. Importantly, hybrid exciton formation correlates with a suppression of ILE radiative emission. By systematically disentangling disorder-trapped, delocalized, and hybrid excitonic regimes, this work provides clear experimental evidence of interface-driven excitonic transitions. These findings highlight the critical role of interfacial quality in governing excitonic behavior in scalable MoS2/WS2 heterostructures.

Moiré-trapped dipolar interlayer excitons in heterobilayers offer a rich platform to explore interaction-driven phenomena. Extending to three layers and beyond leads to highly intriguing multipolar excitons─a superposition state of vertically aligned phase-coherent excitons. However, in experiments, unintentional twist-angle mismatches among layers may degrade the strength and homogeneity of the vertical Coulomb coupling. Here we propose that the supermoiré effect in a hetero-trilayer comes to the rescue by creating periodic pockets of vertically aligned atomic registries that facilitate the formation of quadrupolar excitons trapped in such pockets. Using a WS2/WSe2/WS2 stack, we show interactions between multiple confined levels of the top and bottom moiré interfaces, creating electric field tunable multilevel hybridized bright (symmetric) and dark (antisymmetric) quadrupolar states. Our work underscores the critical role of the supermoiré effect in quadrupolar excitons. The discovery of reduced sensitivity on precise angle alignment will ignite exploration of complex excitonic states in multilayered heterostructures.

The ability to tune the degree of interaction among particles at the nanoscale is highly intriguing. The spectroscopic signature of such interaction is often subtle and requires special probes to observe. To this end, inter‐layer excitons trapped in the periodic potential wells of a moiré superlattice offer rich interaction physics, specifically due to the presence of both attractive and repulsive components in the interaction. Here we show that the Coulomb force between two inter‐layer excitons switches from repulsive to attractive when the length scale reduces from inter‐moiré‐pocket to intra‐moiré‐pocket in a WS ₂ /WSe ₂ hetero‐bilayer ‐ thanks to the complex competition between direct and exchange interaction. The finding is a departure from the usual notion of repelling inter‐layer excitons due to layer polarization. This manifests as the simultaneous observation of an anomalous superlinear power‐law of moiré exciton and a stabilization of moiré trapped biexciton. The experimental observation is facilitated by placing the hetero‐bilayer on a polymer‐nanopillar/gold‐film stack which significantly reduces the inhomogeneous spectral broadening by selectively probing a smaller ensemble of moiré pockets compared with a flat sample. This creates an interesting platform to explore interaction among moiré trapped excitons and higher order quasiparticles.

Single-photon emitters (SPEs) are integral parts of several quantum technology implementations. Over the past decade or so, monolayers of transition metal dichalcogenides (TMDCs) have emerged as promising candidates for SPE platforms with attractive characteristics. To move ahead, it is necessary to understand the atomistic origin of SPEs in TMDCs─a topic that is highly debated with contradicting proposals. In this work, we critically review these existing proposals to elucidate their origin. Further, we perform a critical trend analysis for different figures of merit of TMDC-based SPEs, and we propose a characterization methodology to streamline the reporting process. Finally, we review several quantum technology implementations, where solid-state SPEs are being used, and identify the advancements required in TMDC-based SPEs for their successful adoption in these technologies.

A Single-Photon Detector (SPD) can detect extremely low intensity of electromagnetic waves, down to a single photon. Driven by the rapid developments in quantum information science and an increasing demand for ultra-low-light sensing across various domains, there is a need for transformative advancements in the design and development of SPDs. In this context, low-dimensional platforms, including quantum dots, superconducting nanowires, and layered materials, have emerged as crucial frontiers of research. This Review explores the state-of-the-art of different low-dimensional SPD platforms, focusing on the engineering physics across their device architectures, performance parameters, and application potential. By critically comparing the performance and addressing current challenges inherent to each low-dimensional platform, this Review aims to outline future research directions to advance next-generation SPD technologies.

The ability to tune the degree of interaction among particles at the nanoscale is highly intriguing. The spectroscopic signature of such interaction is often subtle and requires special probes to observe. To this end, inter-layer excitons trapped in the periodic potential wells of a moiré superlattice offer rich interaction physics, specifically due to the presence of both attractive and repulsive components in the interaction. Here we show that the Coulomb force between two inter-layer excitons switches from repulsive to attractive when the length scale reduces from inter-moiré-pocket to intra-moiré-pocket in a WS$_2$/WSe$_2$ hetero-bilayer - thanks to the complex competition between direct and exchange interaction. The finding is a departure from the usual notion of repelling inter-layer excitons due to layer polarization. This manifests as the simultaneous observation of an anomalous superlinear power-law of moiré exciton and a stabilization of moiré trapped biexciton. The experimental observation is facilitated by placing the hetero-bilayer on a polymer-nanopillar/gold-film stack which significantly reduces the inhomogeneous spectral broadening by selectively probing a smaller ensemble of moiré pockets compared with a flat sample. This creates an interesting platform to explore interaction among moiré trapped excitons and higher order quasiparticles.

Although infrared detection is of high technological and strategic importance, the narrow-bandgap materials used for infrared detection often suffer from poor air stability and pose environmental hazards. Hot electron-based detectors avoid such issues by using conventional wide bandgap semiconductors and exploiting intraband transition. However, hot electron infrared detectors usually suffer from poor quantum efficiency. By photoexciting MoS₂ conduction electrons over a thin barrier layer, here we show that a reversal of the role of the emitter and collector results in a >1000-fold enhancement in the photoresponse compared with a conventional metal/2D semiconductor Schottky diode. We reveal that electron-electron scattering plays a key role in the device performance, which can be effectively tuned by a gate voltage. The photodetector exhibits a nearly flat response up to a measurement wavelength of 1800 nm with a responsivity of 42 mA/W (@1550 nm) at room temperature. We demonstrate an operating frequency of 30 kHz @1550 nm excitation (100 kHz @633 nm). The detector chip is integrated with post-processing electronics in a printed circuit board, making it readily useable for system-level applications─a demonstration of heterogeneous integration of 2D materials with conventional electronics.

A monolayer semiconductor transferred on nanopillar arrays provides site-controlled, on-chip single photon emission, which is a scalable light source platform for quantum technologies. However, the brightness of these emitters reported to date often falls short of the perceived requirement for such applications. Also, the single photon purity usually degrades as the brightness increases. Hence, there is a need for a design methodology to achieve an enhanced emission rate while maintaining high single photon purity. By using WSe₂ on high-aspect-ratio (∼3, at least 2-fold higher than previous reports) nanopillar arrays, here we demonstrate >10 MHz single photon emission rate in the 770-800 nm band that is compatible with quantum memory and repeater networks (Rb-87-D1/D2 lines) and satellite quantum communication. The emitters exhibit excellent purity (even at high emission rates) and improved out-coupling due to the use of a gold back reflector that quenches the emission away from the nanopillar.