Fig. 3: Tuning piezoelectricity of nonstoichiometric Cr_1+σSe₂ nanoflakes and emerging ferroelectricity in nonstoichiometric Fe_1+αTe₂ nanoflakes.

a OOP amplitude images (up), phase images (middle) and intrinsic amplitude images (bottom) of a nonstoichiometric Cr_1+σSe₂ nanoflake under different drive voltages. b Corresponding OOP amplitude evolution curve with drive AC voltage. Orange, pink, blue and prasinous lines are assigned to the resonance-amplified OOP amplitude of the sample, resonance-amplified OOP amplitude of the substrate, original intrinsic OOP amplitude of the sample and linearly fitted intrinsic OOP amplitude of the sample as a function of drive AC voltage, respectively. c Evolution of resonance-amplified OOP amplitudes of nonstoichiometric Cr_1+σSe₂ nanoflakes with different thicknesses. d Effective d₃₃ of nonstoichiometric Cr_1+σSe₂ nanoflakes with different values of σ. e Height image and different AC voltage-driven intrinsic amplitude images of a nonstoichiometric Cr_1+σSe₂ nanoflake. f Corresponding intrinsic IP amplitude in e as a function of the drive AC voltage of substrate and nanoflake sample. Red, blue and dark blue lines represent the IP intrinsic amplitude of the substrate and IP intrinsic amplitude of sample before and after fitted, respectively. g Local OOP ferroelectric switching spectra under direct current (DC) off state of a nonstoichiometric Fe_1+αTe₂ nanoflake. h Phase images of a nonstoichiometric Fe_1+αTe₂ nanoflake after sequential lithography by using different patterns. The black and white dotted line frames indicate +6 V and -6V lithography area, respectively. i Local IP ferroelectric hysteresis loops under DC off state of a nonstoichiometric Fe_1+αTe₂ nanoflake. j–k IP amplitude (j) and IP phase (k) images of a nonstoichiometric Fe_1+αTe₂ nanoflake after electrical field writing along two perpendicular lines. The black line and white line indicate first +6 V and then ‒6 V lithography. All the error bars indicate standard deviation.