Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals

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