Manu Jaiswal

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.

Graphene oxide (GO) is among the most promising platforms for studying confined water and for engineering applications. The lamellar stacking of GO flakes, tunable sub-nanometer interlayer spacings, confining wall chemistry, and hydrophilicity favour a remarkable modulation in the arrangement and motion of water molecules. This promise comes with a challenge: hierarchical microstructure and structural and morphological disorder are ubiquitous in GO. The interpretation of several experimental outcomes is therefore quite challenging, and this also renders the simulations made on idealised systems rather inadequate. Here, we present a detailed overview of the recent developments in the structure and dynamics of confined water in GO. Sorption properties providing phenomenological insight into water's interactions and dielectric properties related to water's arrangement and reorientation are included within the scope. Comparisons with the parent graphene system are made throughout to place the results in the context of the defining features of GO. Particular emphasis is given to the super-permeation of water in GO, whose origin has been the subject of extensive debate and recent developments. A critical account of several experimental efforts on water in the flow and sorption-diffusion regimes is presented along with the findings from simulation studies. Finally, environmental applications that leverage the sorption capacity and rapid water transport in GO are discussed. The ion rejection performance, along with the challenges for utilising GO in desalination, is presented. Furthermore, the latest developments in atmospheric water harvesting and dehumidification applications of GO are discussed.

Graphene oxide (GO) holds tremendous promise for applications like filtration and ion-conductive membranes, multifunctional protective coatings, flexible electronics, and electrodes in lithium-ion batteries. The mechanical properties of GO can show large variations, and understanding these is essential for these applications. Here, we study the in-plane elastic modulus of 100 μm-thick free-standing GO films with different hydrated states and lithium salt loading. The moisture uptake was characterized with IR spectroscopy, sorption studies, and temporal evolution of the XRD peaks. The Young’s modulus of the film varied from E ≈ 12 to 1 GPa as the film’s hydration state was tuned from dry to a moisture-saturated state. The value of E for the Li-salt-loaded GO (Li-GO) films varied from ≈13 to 0.06 GPa when tuning the hydration state from dry to moisture-saturated, i.e., a reduction by an average factor of ≈212. Hydration of the Li-GO films reduced their tensile strength by 2 orders of magnitude. We attribute this wide tunability of mechanical properties partly to the stiffening effect arising from the strong Li⁺ ion: GO electrostatic interactions in the dry state. Furthermore, the synergistic effect of intercalated Li⁺ ions and moisture enhanced the hydration’s softening effect, attributable to increased water uptake enabled by the alkali metal cations.

Ultrafast water transport through the lamellar arrangement of flakes in graphene oxide is well established, but its origin has been debated in the literature. Here, we comprehensively examined water transport through graphene oxide membranes with sorption and permeation experiments across the entire relative humidity interval, with varied membrane thicknesses and flake sizes. A series of experiments is necessitated since the diffusion process is influenced by the geometric tortuosity of pathways, degree of nano-confinement and associated surface interactions. The ratio of in-plane and out-of-plane Fickian diffusion coefficients, D_2D/D_out ∼ 10², which indicates that the water transport pathways have a tortuosity with an average step-size that is significantly smaller than the flake size. This is attributed to the presence of pinholes on the basal plane of graphene oxide. The thermodynamic correction to the Fickian D_2D is used to estimate the Maxwell-Stefan diffusion coefficient for in-plane transport of confined water, D_MS, which is a fundamental parameter representing molecular mobility. D_MS increases monotonically with interlayer spacing of graphene oxide, which suggests a decrease in the frictional forces exerted by the confining walls on the diffusing molecules. Moreover, D_MS < 1.0 × 10⁹ m²/s is lower than the self-diffusion constant of bulk water and is typical of hydrophilic nanoporous systems. The large permeation rate is thus reconciled as a consequence of large sorption capacity, despite a modest D_MS. Finally, we also examined transport scenarios with single-sided condensation of water on the graphene oxide surface. A comparison of the permeation rates with the sorption-diffusion experiments is presented.

The chemical vapour deposition of micron-size, horizontal, uniform, high-aspect ratio MoO₂@MoS₂ core–shell nanorods with a multilayer MoS₂ shell, grown on sapphire and silica substrates, is demonstrated. Alongside, the growth of 2D MoS₂ nanoribbons is also observed, mainly on the silica substrates. The optical and vibrational properties were probed using photoluminescence and Raman spectroscopy. The Raman analysis of the core–shell nanorods is particularly challenging due to overlapping vibrational modes. Further, the resonant Raman condition is simultaneously fulfilled for the MoS₂ shell and the MoO₂ core. A comprehensive vibrational analysis of MoO₂@MoS₂ core–shell nanorods highlights the unique resonance Raman features that emerge from the interplay between the crystalline MoO₂ core and the multilayer MoS₂ shell. The resonant mode enhancements are found to be selective, unlike the scenario in the materials considered individually. A pronounced blue shift (40–70 meV) in the neutral A-exciton photoluminescence peak is observed, whose plausible origin is discussed. Our findings demonstrate that the structural geometry and interfacial interaction significantly tailor the optical and vibrational properties of MoO₂@MoS₂ core–shell nanorods.