Exciton Dynamics and Anisotropy in 2D Metal Organochalcogenolate Semiconductors

Abstract

Silver phenylselenolate (AgSePh) is a novel hybrid organic-inorganic two-dimensional (2D) semiconductor that belongs to the broader class of metal organochalcogenolates (MOCs). Since its blue-emitting excitonic properties were discovered in 2018, AgSePh has attracted attention from the scientific community. From a fundamental science perspective, AgSePh provides an excellent platform for exploring many-body interactions among quasiparticles (such as excitons, phonons, and photons) due to its large exciton binding energy, strong exciton-lattice interactions, and natural photonic cavity structure. From a technological standpoint, its narrow blue emission, a tunable bandgap through composition control, chemical robustness, in-plane anisotropy, and low-cost, scalable synthetic methods make AgSePh promising candidate for photonic and optoelectronic applications. However, we do not yet fully understand how its excitonic properties arise at a fundamental level. The central aim of this thesis is to elucidate the correlation between structure, inorganic composition, organic ligands, and excitonic properties in these novel hybrid 2D semiconductors. First, we present the synthesis, structural and optical properties of 2D AgEPh (E = S, Se, Te) single crystals, colloidal nanocrystals, and thin films. Importantly, the growth of millimeter-sized single crystalline 2D AgEPh (E = S, Se, Te) enables their crystal structure determination via single crystal X-ray diffraction: AgSPh in P2₁, AgSePh in P2₁/c, and AgTePh in P2₁/c. Second, we explore the underlying mechanism of light emission in AgSePh and AgTePh. Despite having the same crystal structure, these compounds exhibit strikingly different excitonic properties: AgSePh shows narrow photoluminescence (PL) with a minimal Stokes shift, while AgTePh exhibits broad PL with a large Stokes shift. Using time-resolved and temperature dependent optical spectroscopy, combined with sub-gap photoexcitation studies, we demonstrate that the exciton dynamics in AgSePh films are dominated by the interaction of free-excitons with extrinsic defect states, whereas the dynamics in AgTePh are dominated by intrinsic exciton selftrapping behavior. Third, we study alloying between AgEPh. we demonstrate that AgSePh and AgTePh form homogeneous alloys with tunable excitonic properties across all compositions, whereas AgSPh and AgSePh/AgTePh exhibit a miscibility gap. These observations are elucidated by density functional theory calculations and correlated with crystallographic information. Fourth, using polarization-resolved micro-absorption, reflectance, and photoluminescence spectroscopy, combined with the GW plus Bethe-Salpeter equation calculations, we reveal multiple low-lying excitons with in-plane anisotropy in AgSePh and AgTePh. This showcases the richness of excitonic physics in these materials, which arises from their low-symmetry crystal structures. Finally, we show that the electronic and excitonic structure of AgSePh can be engineered through organic functionalization, resulting in giant excitonic anisotropy and a completely different absorption spectrum in 2D AgSePh-F₂(2,3). This divergence in excitonic properties is attributed to the semi 1D Ag chains in AgSePh-F₂(2,3), in contrast to hexagonal 2D Ag network in AgSePh. This finding can be generalized to other blue-emitting 2D AgSePh-R compounds which exhibit either AgSePh-like or AgSePh-F₂(2,3)-like absorption spectra. Overall, this thesis advances the understanding of the structure-composition-excitonic property relationships in these emerging hybrid semiconductors, paving the way for future investigations into this exciting material family.