Fig. 2: Characterizations of charge redistribution and piezoelectric performance.

a Bi 4 f and b I 3 d XPS spectra of BIO and BF1.5. c The planar average charge density differences of BF. Yellow denotes electron accumulation; blue indicates electron depletion. The iso-surface of density was set to 0.002 e Å− 3. d The butterfly amplitude loop and phase curves of BF1.5. Surface piezo-potentials of e BIO and f BF1.5.