Phase modulated scanning near field optical luminescence imaging as a probe of exciton dark lifetime and diffusivity in persistently luminescent micro- and nanocrystals
Abstract
Mesoscopically structured semiconductors with long-range exciton diffusion lengths are desirable for producing high efficiency photovoltaics. However, the experimental determination of the exciton diffusion length with nanoscale precision remains rare. Here, we introduce phase modulated scanning near-field optical luminescence (PM-SNOL) imaging, a new method for assessing the exciton diffusion length and relaxation time of optical materials simultaneously. Preliminary testing is executed on strontium aluminate doped with europium and dysprosium (SrAl2O4:Eu,Dy); an insulating material known to exhibit persistent luminescence. Our results demonstrate that transport between SrAl2O4:Eu,Dy and ITO nanoparticles is feasible at the metal–insulator interface without the necessity of device fabrication and performing high voltage measurements. The basic principles of PM-SNOL are explained using a model based on semiclassical quasiparticle transport for spherical crystals. We determine the exciton diffusivity, De, to be (4.2 ± 0.9) × 10−9 m2 s−1, the relaxation time, τex, to be 0.21 ± 0.04 ms and the exciton diffusion length, lD, to be 0.95 ± 0.02 μm for SrAl2O4:Eu,Dy.