Nanoscale transport of surface excitons at the interface between ZnO and a molecular monolayer

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

Figure 1

(a) Schematic of the experimental setup showing the near-field tip with excitation/collection mode and the sample geometry. (b) Band-gap diagram of the sample. The band gaps $E_g$ are marked by the dotted arrows and have the following values: 1: 20 nm ZnO film, $E_g=3.37\mathrm{eV}$; 2: 350 nm $\mathrm{Z}{\mathrm{n}}_{0.88}\mathrm{M}{\mathrm{g}}_{0.12}\mathrm{O}$ barrier, $E_g=3.7\mathrm{eV}$; 3: 10 nm $\mathrm{Z}{\mathrm{n}}_{0.85}\mathrm{M}{\mathrm{g}}_{0.15}\mathrm{O}$ barrier, $E_g=3.75\mathrm{eV}$; 4: 180 nm ZnO buffer, $E_g=3.37\mathrm{eV}$. The photon energies in the experiments are indicated by the solid colored arrows (3.81 eV: cw excitation; 3.54 eV: pulsed excitation). (c) Sketch of the potential roughness $\Delta E$ and spatial extension of the exciton wave function. (d)–(f) Visualization of (d) the creation, (e) the time evolution by recombination and diffusion, and (f) the detection of the carrier distribution $n(r,t)$ with the NSOM tip.

Figure 2

(a) Map of 20 PL spectra along a 1 $\mu \mathrm{m}$ scan line at $T\le 10\mathrm{K}$ (excitation at 3.81 eV). (b) Spectrum of an isolated DX peak obtained by subtracting spectrum 2 from spectrum 1. (c) Black: Near-field PL spectrum along line 1 in (a) $\left(T\le 10\mathrm{K}\right)$. Gray: PL spectrum at 50 K. The thin dashed line is an extrapolation of the SX PL band. (d) Vertical near-field PL scan along a lateral surface of the sample structure ($T\le 10\mathrm{K}$, excitation at 3.81 eV). The different layers are marked on the right. The data show that SX PL originates exclusively from the top of the sample.

Figure 3

Time-resolved PL intensity recorded in the near-field (nf) and the far-field (ff) at (a) a photon energy of 3.365 eV [SX peak in Fig. 2, $T\le 10\mathrm{K}]$. The solid black lines represent numerical fits to the measured kinetics and the dashed line shows the instrument response (IR). (b) Same as (a) at 3.361 eV (DX peak with underlying SX contribution). (c) Comparison of the rise times at 3.361 and 3.365 eV. (d) PL transients at $T=50\mathrm{K}$ at the photon energies indicated [cf. Fig. 2].