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, D_e, to be (4.2 ± 0.9) × 10⁻⁹ m² s⁻¹, the relaxation time, τ_ex, to be 0.21 ± 0.04 ms and the exciton diffusion length, l_D, to be 0.95 ± 0.02 μm for SrAl₂O₄:Eu,Dy.