Structural and optical characterization

The crystallinity of the samples was determined with a Bruker D8 Discovery X-ray diffractometer (XRD), using CuKα1 radiation and a Ge(111) monochromator. XRR was measured on a PanAlytical Empyrian instrument using CuKα1 radiation without monochromator, while data were analysed using the PanAlyticals X’Pert Reflectivity software. Film thickness and refractive index were determined with a J. A. Woollam alpha-SE spectroscopic ellipsometer in the 380–900 nm range. The ellipsometry data were modelled using a Cauchy model.

PL and PLE measurements were done using two different set-ups. A 280 nm diode and an USB2000+ spectrometer from OceanOptics were used for recording the PL spectra in Fig. 3a, while an Edinburgh Instruments FLS920 fluorescence spectrometer with a 450 W Xe lamp as excitation source and a Hamamatsu R928 PMT for detection was used for the PLE spectra in Fig. 3a and the PL spectra in Fig. 3b, c. PL decay measurements were performed with the third harmonic of a Nd:YAG laser as excitation source of the OpotekHE 355 II with an ~10 ns pulse width and a repetition rate of 20 Hz. The decay was recorded with MultiChannelScaling function incorporated in the Edinburgh Instruments Spectrofluometer. Absorbance data were obtained by combining transmittance and reflectance measurements in a Shimadzu UV3600 photospectrometer using an integrating sphere.

STEM investigations of the samples were conducted after standard sample preparation techniques, by cutting, manual grinding and polishing. Final thinning was done with a Gatan PIPS II ion mill using argon ions, with gradually decreasing voltages and angles and increasing polishing time. Plasma cleaning with a Fishione Model 1020 for ca. 5 min was applied prior to the TEM experiments. STEM was performed with a monochromated FEI Titan G2 60-300 kV TEM, equipped with a CEOS probe-corrector. The microscope was operated at 300 kV with a convergence angle of 20 mrad, where HAADF imaging was done with collection angles 40–200 mrad.