Extended Data Fig. 3: Nano-spectroscopy across a grain boundary of the FA0.83Cs0.17Pb(I1–xClx)3 sample.
a, Topography map of two grains with lateral extensions on the order of 100 nm. b, Peak amplitude of the scattered near field Ê2 (tEOS = 0 fs) as a function of tip position with the characteristic signal enhancement in grain boundaries. c, Relative spectral amplitude and phase at five positions across a grain boundary. The colours represent the position markers in a. We attribute the signal enhancement within the grain boundary to a topographic artifact since it shows no change in spectral shape, but only in signal strength. We have chosen a location where we transition from a grain in the perovskite α-phase (blue sphere) to a PbI2-covered grain (green sphere). This transition is clearly reflected in the corresponding spectral response: All spectra show the phonon peak around 1 THz characteristic of the α-phase FA0.83Cs0.17Pb(I1–xClx)3. This is also true at the grain boundary. However, as the tip moves further onto the PbI2 protrusion, the amplitude of this feature decreases in favour of the characteristic spectral response of PbI2. Because the PbI2 layer studied here is thinner (~30 nm) than the one discussed in Fig. 1 (~60 nm) the low-frequency phonon of the α-phase contributes more strongly to the spectral response. The data are shown as mean values of 10 measurements per spectrum and the error bars indicate the corresponding standard error.
