(a) Polarized optical microscopy image of the center of a rossette-like BiFeO3 single crystal with the (001) plane parallel to the surface. Bulk domains are distinguishable using standard birefringence. (b) PFM image (amplitude) of the same 40 μm × 40 μm area. A distribution of stripe domains is observable on the whole area, over imposed to the bulk domains. Domain walls are observable as a change in the preferred stripe orientations. The inset shows the FFT image of the PFM amplitude. The stripe domains are running along only 4 preferred directions in the whole crystal. The PFM image was taken with an Olympus AC240TM conductive tip (2 N/m, Pt coating) at a fixed frequency f = 280 kHz and Vac = 2.5 V.
PFM image (amplitude) of an area close to a bulk domain wall. The red arrow shows the position of the bulk domain wall. The inset shows the FFT image of the PFM amplitude. The stripe domains of the same quadrant show a relative angle orientation of 128°, while the relative rotation with respect to the other quadrant in this case is 37.5°. The PFM image was taken with an Olympus AC240TM conductive tip (2 N/m, Pt coating) in DART mode, at a contact resonance frequency of frc ∼ 290 kHz and Vac = 1 V.
DART PFM measurements: (a) topography and (b) deflection images of 1.5 μm2 area of BiFeO3 surface. Calculated RMS roughness is 288.37 pm. Amplitude DART PFM image of (c) out-of-plane and (d) in-plane polarization. Phase PFM image of (e) out-of-plane and (f) in-plane polarization. The in-plane signal is measured in the y-axis direction. Measurements were done with a conductive μmasch DPER14 tip (6 N/m, PtIr coating), at Vac = 2.5 V and a contact resonance frequency of frc ∼ 665 kHz for the out-of-plane signal and 1.27 MHz for the in-plane one.
DART PFM measurements of a 1 μm × 1 μm area measured at T = 300 °C (a), (b) and at room temperature (c)–(e). (a) Amplitude PFM image of out-of-plane polarization and (b) phase PFM image with a contrast signal of 180°. Striped nanodomains still exist over the skin phase transition temperature observed at 275 °C. (c) Topography image of a wider area of 1.8 μm × 1.8 μm on the same region, and the corresponding amplitude PFM image (d) and phase PFM image (e) of the out-of-plane polarization, measured at room temperature for comparison. To perform this measurements, a Nanosensors EFM conductive tip (2 N/m, PtIr coating) in DART mode, at a contact resonance frequency of frc ∼ 335 kHz and Vac = 3 V.
(a) Topography image of an area of 10 × 10 μm showing several cracks produced on the surface after reiteration of thermal cycles. (b)Section profiles of yellow line and red line of previous image, respectively. The average thickness of the cracks is measured to be around 5 nm. (c) Scheme of the structure of the bulk BiFeO3 single crystal. There is a skin layer of about 5 nm (purple layer), and underneath we find a thicker subsurface layer of several hundreds of nanometers with complex nanodomains structure (striped region) overimposed to the bulk domain configuration of the single crystal.
Topography of two different cracks (a), (b) in 3 μm × 3 μm image area; (c) and (d) are the corresponding out-of-plane amplitude of PFM signal; (e)and (f) are the out-of-plane PFM signal phase. The range is 180°. (g) and (h) are the KPFM images of the two cracks, with a voltage range of 400 mV for (g) and 25 mV for (h). For sample (a) measurements were done with a conductive μmasch DPER14 tip (6 N/m, PtIr coating), in DART mode, at a contact resonance frequency of frc ∼ 746 kHz and Vac = 3.5 V, while for sample (b) measurements were done with an Olympus AC240TM conductive tip (2 N/m, Pt coating) in DART mode, at a contact resonance frequency of frc ∼ 336 kHz and Vac = 2 V. In both cases, KPFM was done in lift mode by applying an Vac = 1 V to the tip.
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