Correlative characterization of molecular two-dimensional van der Waals material heterostructures on the nanometer scale

In the design of nanoscale materials, hybrid van der Waals heterostructures that integrate the excitonic landscape of atomically thin transition metal dichalcogenide (TMDC) semiconductors with molecular electric dipoles offer enhanced control over light-matter interactions and charge carrier dynamics. Even minor deviations in homogeneity can profoundly affect their optoelectronic properties and, consequently, device performance, necessitating stringent quality control capable of probing structural and compositional divergences down to the nanoscale. However, the reliable characterization of such complex, multilayered systems, remains challenging due to the interplay of chemical, structural, and optical inhomogeneities across different length scales. In this study, we examine a trilayer heterostructure consisting of chemical vapor deposition (CVD) graphene (G), a self-assembled layer of Rhodamine 6G (R6G), and a transferred monolayer MoS₂ (G/R6G/MoS₂), incorporating regions of a tri- and multilayer MoS₂ as well. Comprehensive structural and optical characterization was performed to identify possible inhomogeneities, employing photoluminescence (PL) spectroscopy, Raman spectroscopy, Kelvin probe force microscopy (KPFM), and scattering-type scanning near-field optical microscopy (s-SNOM). Analytical methods indicate that the TMDC layer has almost uniform molecular coverage and preserved crystallinity. Importantly, near-field optical imaging demonstrates the propagation of exciton-polaritons in MoS₂, with a clear redshift of the polariton wavelength upon R6G integration, signifying substantial modulation of the local dielectric environment and excitonic response. These findings underscore the tunability of hybrid 2D molecular-inorganic interfaces and their promise for advanced applications in nanophotonic devices, excitonic circuitry, and quantum optoelectronics.