Detailed analysis of un-reacted boron. The region surrounded by residual boron was examined by high-angle annular dark-field scanning TEM. (a) Many bright spots corresponding to Mg-rich phase were observed, as indicated by arrows. (b, c) Representative high-resolution BF-STEM image of the Mg-rich nanoparticles and its fast Fourier transform. These Mg-rich nanoparticles were found to be the MgB2 crystalline phase. (d) Within the nanoparticles, a small Mg-K EELS peak can be observed.