Nanoscale Transport of Surface Excitons at the Interface between ZnO and a Molecular Monolayer

TL;DR

This study uses near-field optical microscopy to investigate surface exciton transport at the ZnO/molecular monolayer interface, revealing inhomogeneous energy distribution and quantifying exciton diffusion at nanoscale resolution.

Contribution

It provides the first nanoscale measurement of surface exciton transport in a hybrid ZnO-molecular system using NSOM.

Findings

Surface excitons exhibit inhomogeneous energy distribution.

Surface exciton diffusion coefficient measured as 0.30 cm²/s at T<10K.

Spectral diffusion indicates diffusive exciton transport on 50 nm scale.

Abstract

Excitons play a key role for the optoelectronic properties of hybrid systems. We apply near-field scanning optical microscopy (NSOM) with a $100\,\text{-nm}$ spatial resolution to study the photoluminescence of surface excitons (SX) in a $20\,\text{nm}$ thick ZnO film capped with a monolayer of stearic acid molecules. Emission from SX, donor-bound (DX), and - at sample temperatures $T>20\,\text{K}$ - free (FX) excitons is separated in steady-state and time-resolved photoluminescence spectra. The $4\,\text{meV}$ broad smooth envelope of SX emission at $T<10\,\text{K}$ points to an inhomogeneous distribution of SX transition energies and spectral diffusion caused by diffusive SX transport on a $50\,\text{nm}$ scale with a SX diffusion coefficient of $D(T<10 K)=0.30\,\text{cm$^2$/s}$.