Figure 3: High-resolution transmission electron microscopy (HRTEM) analysis of cationic distribution in two-dimensional doped La2CuO4.
(a) HAADF–STEM image of two-dimensionally doped La2CuO4 showing the microstructure of a superlattice (R=8, N=7) grown on the LaSrAlO4 (001) substrate. The alternation of brighter and darker areas reflects the superlattice structure, in which Sr-doped regions (dark) are separated by undoped La2CuO4 (bright). This is clearly shown by the maxima envelope of the image-intensity profile, integrated perpendicular to the growth direction (b, dark blue line). Scale bar, 2 nm. In b, the intensity oscillations of the intensity profile due to the different contrast of each atomic layer (green line) can be seen. A magnified image of the region highlighted in red in a is shown in e, in which the dotted yellow line corresponds to the layer having maximum Sr content. Scale bar, 1 nm. (c) [Sr]/[La] ratio, extracted from an EDXS line scan across the region shown in a. Asymmetric Sr distribution, smeared in the growth direction, is detectable. Sr-L and La-L lines were used for quantifying the Sr concentration, and the integrated signals of Sr and La were calibrated using the substrate region where [La]/[Sr] is equal to unity (see Supplementary Fig. 8 for the net counts of Sr-L and La-L EDXS lines). Here error bars are the square root of the intensity. A similar Sr asymmetric profile results from the integration of the Sr-L2,3 EELS line profiles, as shown in d, which has been acquired across four Sr-containing layers (blue line in a). Here the error bars (square root of the intensity) are smaller than the symbols. From the EELS analysis, the average Sr number per formula unit FU (x in La2−xSrxCuO4), for each (La,Sr)O-CuO2-(La,Sr)O ‘constituting block’ in proximity of the Sr-containing layers, as depicted in f, was obtained (the s.d. is represented by the error bars).
