Photoluminescence Quenching in N-Heterocyclic Carbene Functionalized Tungsten Dichalcogenide Mixed-Dimensional Heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for quantum optoelectronic devices due to their pronounced valley physics and excitonic properties. One approach to modulate these properties is to modify 2D TMDs with carbon-based ligands.¹ Among carbon-based ligands, the strong σ-donor character, stability, and structural tunability of N-heterocyclic carbenes (NHCs) has prompted investigations of NHC coordination to various atomically flat metal surfaces and metal nanoparticles. Here, we discuss the optical properties of WS₂ and WSe₂ monolayers functionalized with NHCs via a solvent-free route and an air-stable precursor. The deposition of NHCs on the 2D TMDs results in a significant room-temperature photoluminescence (PL) quenching. Moreover, low-temperature PL measurements (T = 3.9 K) show quenching of the WS₂ and WSe₂ excitonic emission and a shift of the defect bands to lower energies. The extent of both effects is tunable by altering the N-substituents of the NHCs as well the thickness of the deposited organic layer. The resulting mixed-dimensional heterostructures are further characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and time-of-flight secondary ion mass spectrometry to elucidate the nature of the functionalization process. The observed PL quenching is consistent with a charge transfer process from defect states introduced by chalcogen vacancies to midgap states introduced by the carbene layer, as predicted from first-principles density functional theory calculations. Overall, these results demonstrate that carbene functionalization is an effective pathway for modifying the optical properties of 2D TMDs.<br/><br/><b><u>References:</u></b><br/><br/>(1) M. I. B. Utama, H. Zeng, T. Sadhukhan, A. Dasgupta, et al., Nature Communications 2023, 14 (2193).