Fig. 1: Crystal-glass transformation and microstructural investigation.

a Schematic diagram of the crystal structure of (S-NEA)₂PbBr₄ viewed along the crystallographic b-axis. Atoms shown are Pb (orange), Br (purple), C (grey) and N (green). H atoms are omitted for clarity. b Differential scanning calorimetry (DSC) traces of (S-NEA)₂PbBr₄ recorded under argon at a heating/cooling rate of 10 °C min⁻¹. The first heating upscan (navy) shows a melting endotherm at T_m = 176 °C, with an enthalpy of melting ∆H_m = 51 J g⁻¹ (shaded). The second heating upscan (red) displays a glass transition at T_g = 68 °C and a crystallisation exotherm at T_x = 115 °C, with an enthalpy of crystallisation ∆H_x = −28 J g⁻¹ (shaded). Inset: magnified view of the glass transition region in the second heating cycle. c Powder X-ray diffraction patterns of crystalline (navy) and glassy (red) (S-NEA)₂PbBr₄, with symmetry-allowed Bragg reflections indicated by grey ticks. The intensity of the glass pattern is scaled by a factor of 10 to highlight the broad features. Inset: optical microscope image of the glass sample; scale bar, 1 mm. d–f Atomic-resolution imaging of crystalline (S-NEA)₂PbBr₄ using scanning transmission electron microscopy (STEM): d High-angle annular dark-field (HAADF) STEM image showing the periodic lattice fringes with interplanar spacings of ca. 2 nm; e Processed STEM image of an individual nanosheet oriented along the [1\(\bar{1}\)0] zone axis, judged from (f) the corresponding fast Fourier transform (FFT) pattern. STEM imaging and energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the glass, a_g(S-NEA)₂PbBr₄: g, h HAADF-STEM images; i EDS elemental maps showing the distribution of Pb (yellow), Br (blue), and C (red). Scale bar, 20 nm (d); 10 nm (e, h); 1 nm⁻¹ (f); 50 nm (g, i). Source data are provided as a Source Data file.