Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals

Size-dependent photoluminescence Stokes shifts (ΔEs) universally exist in CsPbX3 (X = Cl–, Br–, or I–) perovskite nanocrystals (NCs). ΔEs values, which range from ~15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔEs(CsPbI3) > ΔEs(CsPbBr3) ≳ ΔEs(CsPbCl3). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr3 NC Stokes shifts ((ΔEs) = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔEs values (ΔEs ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔEs. This excludes polaron formation as being responsible for ΔEs. Overall, the results point to an origin for ΔEs, intrinsic to the size-dependent electronic properties of individual perovskite NCs.