## 2 Results  
The shape and size of the NCs were characterized by TEM measurements, evidencing (see Figure 1) a parallelepiped shape, with an average width of 5.0 \(\pm\) 1.5 nm and an average length of 21 \(\pm\) 7 nm. The as-prepared thin film shows a photoluminescence quantum yield (PLQY) as high as 88%, evidencing the high optical quality of the NCs. Figure 1: **(a)** TEM image of the CsPbBr\({}_{3}\) NCs evidencing a parallelepiped shape; **(b)** distribution of the NCs width (columns) and best fit Gaussian curve; and **(c)** distribution of the NCs length (columns) and best fit Gaussian curve. The typical PL spectrum of Sample #F in V (see Figure 2a) shows an asymmetric line shape, with a PL peak at about 2.40 eV (517 nm) and a full width at half maximum (FWHM) of 128 meV (28 nm). The line shape can be well reproduced by a best fit with two Gaussian bands peaked at 2.3873 and 2.405 eV and with a FWHM of 149 and 50 meV, respectively. Similar spectra have been reported in the literature for CsPbBr\({}_{3}\) NCs films, ascribing the broad band at low energy to excitons bound to defects (BE) and the narrow band at higher energy to free excitons (FE) [30; 31]. The PL intensity presents a quite stable value during continuous laser irradiation (see Figure 3a) with signal variation within 3% around the average values. When the sample atmosphere is changed from V to WA, we observe (see inset of Figures 2a and 3a) an intensity increase by a factor 1.3 (about 30%), and the signal attests itself to the new value within the WA recovering time (less than 5 s). To evaluate the generality of this result the same experiment has been performed also on different positions on the sample, always finding a relative PL intensity increase of about 30%. Despite the apparently unchanged PL spectrum line shape (see Figure 2b), we observe that the atmosphere variation also affects the BE and FE band peak energy and their FWHM. In particular (see Figure 2b and Table 1), both bands show a small, but evident, red-shift in WA of 16.8 and 11.1 meV for the BE and FE emission band, respectively, and become narrower. These effects are fully reversible and the PL spectrum intensity, the band peak energy, and the FWHM come back to the initial values when the sample chamber is evacuated, thus changing back the atmosphere from WA to V.  
Figure 2: PL spectrum of Samples #F and #A in Vacuum (V) and Wet-Air (WA) atmosphere: (**a**) Sample #F in V; (**b**) Sample #F in WA; (**c**) Sample #A in V; and (**d**) Sample #A in WA. The black dots are the experimental data, and the red, dark cyan, and blue lines are the best fit curves of the BE, FE, and total emission, respectively. All spectra are normalized to 1 for clarity. The insets show the non-normalized PL spectra in V and WA of Sample #F (inset of frame (**a**)) and Sample #A (inset of frame (**c**)), evidencing the clear intensity increase in WA. To probe the role of the NCs surface quality on the interaction with the environment, the same experiment was also performed on a sample aged by exposure to WA in dark for one month (Sample #A). The long exposure of CsPbBr\({}_{3}\) to WA results in an irreversible degradation, mainly due to surface decomposition and generation of surface trap states [29]. Sample #A in V shows (see Figure 2c) a line shape similar to the one of Sample #F, again resulting from the superposition of a broad band at low energy and a narrower band at higher energy ascribed to BE and FE, respectively. Both bands are slightly red-shifted (see Table 1) by about 15 meV for the BE and 10 meV for the FE with respect to the corresponding bands of Sample #F, and are narrower. When the sample atmosphere is switched to WA, we observe (see inset of Figures 2c and 3b) a PL intensity increase qualitatively similar to the one of Sample #F, but with a significantly different quantitative intensity variation.