A bias of -10 V was applied to the sample to measure the secondary electron cut-off energy. Both spectra with and without bias was obtained. LEIPS was also conducted inside PHI VersaProbe 4 with an electron gun with band-pass filter of 4.77 eV. LEET spectra were simultaneously collected using a microcurrent meter. The PL spectra was measured with a Horiba FluoroLog-3 system with a Xe arc lamp. The PL lifetimes were measured with the same system but with a pulsed 375 nm laser. The absorption spectra were collected on the Agilent Cary 6000i UV/Vis/NIR system with transmission mode.