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Abstract Platinum‐based halide perovskites exhibit promising optoelectronic properties along with merits of low‐temperature processing and stability. Current research on Pt halide perovskites is limited to 0D A₂BX₆structure as the ABX₃3D structure is thermodynamically unstable. Herein, the study reports the stabilization of the ABX₃structure into a 2D layered phase, CsPtI₃(DMSO), that is stable up to 181.5 °C. The 2D phase shows an excitonic peak at the absorption edge of 600 nm, indicating quantum confinement. It also exhibits a large Stokes shift due to intersystem crossing (ISC), with a quenched singlet excitonic fluorescence at 610 nm and strong triplet emission at 852 nm. Pt(II) co‐ordinates with dimethyl sulfoxide (DMSO) via σ‐donation of S lone‐pair electrons and π‐ back donation from Pt to S, stabilizing CsPtI₃(DMSO) layered structure. The strong electronic interaction between DMSO and Pt(II) and orbital mixing lead to spin‐orbit‐coupling, facilitating ISC and singlet‐to‐triplet exciton energy transfer. The interaction of Pt and DMSO is further confirmed by addition of thioacetamide (TAA), a strong S‐donor, which retards the formation of 2D layered structure, and directly results in Cs₂PtI₆and Pt. 

[Formula: see text] is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, [Formula: see text] may exhibit quantum spin Hall states. However, growing coalesced ultrathin [Formula: see text] films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of [Formula: see text] films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of [Formula: see text] before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Overall, coalesced ultrathin [Formula: see text] films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.