  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$/s}$.
