For steady-state and transient optical characterization and calibration, the same instruments and procedures were used here as our previous study (25). As an excitation source, an Ar+ laser (Lexel 95) with a 514.5-nm line was used in steady-state PL, and the power density was adjusted by neutral density filters and was simultaneously monitored by a photodiode power sensor (ThorLabs S120C). It should be noted that the ratio of laser power on the diode to incident power onto the sample was 50 so that the low laser power can be accurately measured. A 50× MD Plan (Olympus) objective lens (numerical aperture, 0.55) was used for all measurements. A Si CCD detector (Andor iDus BEX2-DD) on a spectrometer (f = 340 mm) with a grating of 150 g/mm was used to collect the steady-state PL spectra, and the dark background of the CCD was measured and subtracted each time from the acquired signal. Calibration for the external sample PL efficiency was performed using the wavelength-dependent instrument function and the collection efficiency of the instrument, which was previously described in our work (5). The absolute internal PL QY was extracted from the measured external quantum efficiency using the quantitative absorption at the pump wavelength and by the fraction of light within an escape cone (1/4n2, where n is the refractive index of the medium). Time-resolved measurements were performed with a 514.5-nm line source (2-nm bandwidth, 20- to 30-ps pulse duration) selected from a supercontinuum laser (Fianium WhiteLase SC-400) using a double monochromator. The signal was subsequently detected using a single-photon counting avalanche photodiode (ID Quantique) with a time-correlated single-photon counting module (Becker-Hickl GmbH). In situ PL imaging was performed using the 514.5-nm Ar+ laser, with a diffuser as an excitation source. A fluorescence microscopy setup with a 470-nm LED as an excitation source was used for the centimeter-scale film imaging, and the images were acquired using a commercial camera with 58-mm lens (f/5.6) after removing a cold filter where the excitation power density on the sample plane was ~0.2 W cm−2. A 550-nm colored glass long-pass filter was used to block the excitation signal for PL imaging and PL spectroscopy measurements.

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