HR-TEM and DF-TEM experiments were conducted using FEI Tecnai G2 F20 X-TWIN TEM and FEI Titan 80-300 TEM, operated at either 80 or 300 kV. Acquisition times for DF-TEM images were 3 to 10 s. The spatial resolution of DF-TEM imaging is 1 nm. A Gatan 628-0500 in situ heating holder system was used to increase the sample temperature up to 900°C. All DF-TEM images were taken at the field of view where no image change was observed for more than 1-min exposure, preventing any change by the beam exposure during the measurement.
The angle-averaged spatial autocorrelation function 〈G(r)〉 of the DF-TEM images is the statistical correlation of two points separated by distance r and was calculated following the method of Giraldo-Gallo et al. (26 (link)). The characteristic length scale L from 〈G(r)〉 represents ξ in the disordered phase, although it approximately corresponds to the inverse square root of the paired dislocation density in the quasi-ordered crystalline phase. Each data point inFigs. 2 and 4 was calculated from the correlation functions of multiple (>10) images acquired from the membranes of the same thickness.
The angle-averaged spatial autocorrelation function 〈G(r)〉 of the DF-TEM images is the statistical correlation of two points separated by distance r and was calculated following the method of Giraldo-Gallo et al. (26 (link)). The characteristic length scale L from 〈G(r)〉 represents ξ in the disordered phase, although it approximately corresponds to the inverse square root of the paired dislocation density in the quasi-ordered crystalline phase. Each data point in