Contrary to the common belief that the light-induced halide ion segregation in a mixed halide alloy occurs within the illuminated area, we discover that the anion segregation in a (FA,MA)Pb(Br_xI_1-x)₃ thin film is a nonlocal effect of which the Br ion are expelled from the illuminated area, forming a ring with the size proportional to the illumination beam size, from μm up to well over mm, depending on the beam size. Specifically, we find that under illumination, within the illumination area, the photoluminescence (PL) peak is red shifted from the initial wavelength; while concurrently, the alloy PL peak is strongly enhanced in a ring area circling the illuminated area. Furthermore, the process is reversible, but non-monotonically, exhibiting an ultra-low-frequency, damped oscillations between the ring and center in terms of PL intensity and position. This finding may be the first observation of an ionic plasma oscillation in solids. Interestingly, right after removing the illumination, a voltage about 0.4 V between the ring and the center is established. Our findings suggest that the “ion-segregation” phenomenon, commonly viewed as an adverse effect, could have novel applications, such as ionic patterning, self-destructive memory, and energy storage (particularly a battery directly chargeable by sunlight).

X. X. Sun, Y. Zhang, and W. K. Ge, Light: Science & Applications volume 11, 262 (2022)