## 3 Results and Discussion
Representative SEM images of the as-prepared and after-ball-milling MAPbI3 and PEA2 PbI4 samples are respectively reported in Figures 3 and 4. Figure 3: Scanning electron microscopy (SEM) images, at 5 k\(\lambda\)and 10 k\(\lambda\) magnification, of (**a**,**b**) as-prepared MAPbI3, and that after (**c**,**d**) 120 and (**e**,**f**) 240 h of ball-milling. Figure 2: XRD patterns of PEA2 PbI4 (PEA = phenylethylammonium) as a function of ball-milling time. The as-prepared MAPbI3 (Figure 3a,b) shows the typical cubic-like shape of the crystallites, which have dimensions ranging from 5 to 20-30 \(\upmu\)m. The ball-milling is performed to significantly reduce the quality of the crystal shape, also producing smaller particles which tend to agglomerate (Figure 3c-f). No other remarkable variations concerning the morphology can be observed by increasing the milling time from 120 to 240 h.  
As-prepared PEA2 PbI4 (Figure 4a,b) reveals a lamellar-like morphology, with grains of several microns. Such a peculiar morphology is lost due to the ball-milling, which reduces the size of the crystallites, producing also a broader size distribution (Figure 4c,f). As in the case of MAPbI3, by further prolonging the mechanical treatment (from 204 to 420 h), a dramatic change in the crystal morphology is not found. However, as is clear from the diffraction data, the intrinsic crystallinity is progressively reduced by increasing the ball-milling time, irrespective of the sample morphology and size. For the 3D MAPbI3 sample, the absorption spectra (Figure 5) show an evident shift associated with the ball-milling process. From the as-prepared sample to the 240-h-milled one, there is a \"blue shift\" of about 0.1 eV, with the sample after 120 h of treatment laying in an intermediate region. As discussed above, the high-energy process causes a phase transition from the tetragonal to the cubic perovskite phase. Additionally, it induces an expansion of the lattice. In principle, a reduction of the band gap, passing from a distorted Pb-I geometry (tetragonal) to a symmetric one (cubic), could be caused by an improved interaction between the lead and iodide orbitals. However, in the examined systems, the transition to the cubic phase is also linked to a lattice expansion, leading to an elongation of the  
Figure 4: SEM images, at 5 k\(\times\) and 10 k\(\times\) magnification, of (**a,b**) as-prepared PEA2 PbI4, and that after (**c,d**) 204 and (**e,f**) 420 h of ball-milling. Pb-I bond length and thus to the downshift of the VB [47; 48]. In the cubic phase \"regime\" of the MAPbI3 perovskite, the [PbI6]4- octahedra expansion (Table 1) increases the Pb-I bond length. This in turn reduces the coupling of the Pb s and the I p orbitals, and pushes down the VB, leading to the observed blue shift of the band gap. It is worth noting that the absorption edge retains a relative sharpness, even after long-lasting milling, which is an indirect indication of material order retention, as the low-energy tail of the exciton absorption is reminiscent of Urbach absorption, which is a measure of the disorder [49]. The emissions of the systems could not be obtained, for two main reasons; one is related to the setup used, which is very weakly sensitive above 830 nm, where the emission band likely lays [50], and the second is related to the very weak emission expected in such systems, due to the defects introduced following high-energy milling, as observed below for the more strongly emissive samples (PEA2 PbI4). For PEA2 PbI4 systems, a similar trend for the absorption spectra can be observed (Figure 6), also resulting in the light emission behavior, but with some additional findings. The as-prepared crystalline powder shows a narrow intense emission, characterized by a lifetime in the order of a few nanoseconds (biexponential decay: 2.5 and 10.5 ns), and the emission band presents a shift, relative to the absorption onset, of a few nm (around 8 nm).