Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals.

Size-dependent photoluminescence Stokes shifts (Δ) universally exist in CsPbX (X = Cl, Br, or I) perovskite nanocrystals (NCs). Δ values, which range from ∼15 to 100 meV for NCs with average edge lengths () from approximately 13 to 3 nm, are halide-dependent such that Δ(CsPbI) > Δ(CsPbBr) ≳ Δ(CsPbCl). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr NC Stokes shifts (⟨Δ⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble ( = 6.8 ± 0.8 nm) Δ values (Δ ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to Δ. This excludes polaron formation as being responsible for Δ. Altogether, the results point to an origin for Δ, intrinsic to the size-dependent electronic properties of individual perovskite NCs.