Reexamination of the Giant Oscillator Strength Effect in CdSe Nanoplatelets

CdSe nanoplatelets have large extinction coefficients and, at low temperatures, very short photoluminescence lifetimes. To explain these observations, the giant oscillator strength (GOST) effect has been hypothesized to exist in such semiconductor nanoplatelets. In principle, suppression of phonon scattering can increase the area of coherent exciton motion up to the full size of the nanoplatelet, increasing oscillator strength proportional to its size. Yet at high temperatures, the measured area of exciton motion as estimated from various photophysical methods is much smaller than the size of nanoplatelets and insensitive to nanoplatelet size. To examine the emergence of this discrepancy, this work uses temperature-dependent measurements of steady-state absorption, transient optical bleaching, and the optical Stark effect. Although the excitonic oscillator strength (and size) does increase somewhat at reduced temperature, a large difference remains between the area of coherent exciton motion and the entire nanoplatelet area. These measurements indicate that the area of coherent exciton center-of-mass motion does not increase sufficiently to explain observed rapid radiative lifetimes in CdSe nanoplatelets at 3 K. Instead, the data are consistent with localization of excitons to areas much smaller than whole NPLs.