Dimplegrinder

The
sample was cut into blocks and mounted face to face in a brass
ring with epoxy glue. The TEM specimen was ground to a thickness of
100 μm and dimpled down to 15 μm at the disc center (Dimple
grinder, Gatan, Inc., Warrendale, PA). The TEM specimen was finally
ion-milled (PIPS, Precision Ion Polishing System, Gatan, Inc., Warrendale,
PA) using 3 kV Ar⁺ ions at an incidence angle of 8°
until perforation. Detailed structural investigations of the sample
were performed using a 200 kV transmission electron microscope with
field emission electron gun (JEM-2010F, Jeol Ltd., Tokyo, Japan).

The SrTiO₃ single crystal was purchased from Hefei Kejing Materials Technology Co. Ltd. The STEM sample was prepared using the standard procedure, including mechanical grinding, dimpling, and ion milling. First, a 500-μm-thick single-crystalline sample was mechanically grinded to about 30 μm using diamond lapping films with different grit sizes. Then, the sample was stuck to a copper ring and dimpled to about 20 μm on the Gatan Dimple Grinder. Diamond slurry was used for dimpling, and aluminum oxide suspension was used for polishing. Last, the dimpled sample was milled by Ar⁺ ion on the Gatan PIPS. The accelerating voltage was set to 5 kV, and the glancing angles were ±5°. After Newton’s rings appeared, 3, 2, and 1 kV were used successively until a hole formed. The glancing angles were decreased to ±4°. Last, 0.5 kV was used to reduce the thickness of the amorphous layer.

Microstructure
analyses were performed on thermally etched (1150 °C in air for
15 min) cross sections using a scanning electron microscope (SEM;
Thermo Fisher Quanta 650 ESEM, Massachusetts, USA) with a thermionic
electron source.
Samples for scanning transmission electron
microscopy (STEM) were prepared by cutting a 3 mm disk from the ceramic
pellet, mechanical thinning to ∼100 μm, dimpling to ∼20
μm in the disc center (Dimple grinder, Gatan Inc., Warrendale),
and finally, ion milling to perforation using 3.8 keV Ar ions at an
angle of 8° from both sides (PIPS 691, Gatan Inc., Pleasanton,
USA). After perforation, the energy was gradually lowered, finally
to 500 eV for 5 min to minimize the thickness of the amorphous surface
layer. STEM analyses were performed using a probe-corrected atomic-resolution
microscope (JEOL ARM200 CF, Jeol Ltd., Tokyo, Japan) operated at 200
kV and equipped with a high-angle annular dark-field (HAADF) detector
with inner and outer semiangles of 68 and 180 mrad, respectively.
EELS spectra were acquired using a Gatan DualEELS Quantum ER spectrometer.
Samples for STEM analyses were coated with 2 nm of amorphous carbon
(PECS 682, Gatan Inc., Pleasanton, USA) to prevent charging under
the electron beam.