Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor 417
to confirm that at 0.4 nJ the expected Y:Ba:Cu:O ratios of 2:1:1:5 in the Y-211 phase and 1:2:3:7 in the Y-123 phase both deviated slightly from the expected stoichiometries due to minor Ba and O deficiencies. The chemical composition profile across the Y-123 and Y-211 phases measured as a function of distance from the interface is shown in Figure 2b. No Ce was detected within the Y-211 phase, the Y-123 phase or at the interface between them (however, due to the small data set, counting errors were comparatively large at±0.15 at %, which may have prevented the detection of Ce, if present below this threshold). Theories suggests that the mechanism of refinement of
generated at the sample tip in the atom probe. For this rea- son, the sample yielded a relatively small data set (300,000 ions), making it unsuitable for calibration analysis of the effect of experimental parameter variation on the chemical composition. The mass spectrum did however, contain sufficient ions
plotted as shown in Figure 3b. As expected increasing the laser energy, thereby reducing the applied electric field, results in a relative increase in the detection of lower charge states. The reconstruction of the calibration sample is shown in Figure 4a. This data set was used to calibrate the experimental
conditions. It is assumed that the analysis volume incorpo- rated by Figure 4a (~80×80× 300nm) is homogeneous Y1Ba2Cu3O7−δ. Note that no abrupt changes in local ion density or apparent composition are detected the data set in Figure 4a as the laser energy is changed. Figures 4b and 4c superimpose the normalized mass spectra at the lowest and highest energies, 0.05 and 0.6 nJ, respectively. A close-up of a section of the mass spectra, between 30 and 38 Da, illustrates specific differences in mass resolution resulting from chan- ges to the applied laser energy. The mass spectra shown were normalized using the O2 highest peak in both spectra.
+ peak at 32 Da, which was the
Y-211 particle size involves the formation of Y2O3 nanoparticles in solid state reactions between the super- conducting matrix and Y-211 and CeO2 to form BaCeO3 (Vilalta et al., 1997). In the present data set no evidence of Y2O3 was found. The APT analytical conditions were optimized on a
much larger 17 million-ion data set collected wholly from the Y-123 matrix. A plot summarizing the experimental condi- tions across the course of this analysis is shown in Figure 3a. The laser pulsing energies and resulting voltage adjustments (to maintain a constant ions-detected-per-pulse detection rate) were plotted as a function of the instantaneous total number of ions detected during the calibration experiment. The applied voltage directly determines the intensity of the electric field to which the sample is exposed. If the field changes, the distribution of charge-states at which ions are detected will change as a result. As such the charge-state ratios, i.e. the ratio of the detected frequency of different charge states of the same type of ion, can be used as an approximate metric to quantify the electric field (Kingham, 1982). Therefore, the charge state ratios for every ion were
1000 2000 3000 4000 5000
0
Voltage Laser Energy
0.0 3.0x106 Cu+ /Cu2+ 6.0x106 9.0x106 /CuO2+ 1.2x107
Number of Ions CuO+
Y2 1.5x107 +/Y3+ YCuO2+
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
DISCUSSION
The following discussion and analysis are based on the pre- sumption that the chemical composition of bulk super- conducting crystal samples is close to the nominal stoichiometry of the Y1Ba2Cu3O7−δ compound, which is 7.7 at% Y, 15.3 at% Ba, 23.1 at% Cu and 53.9 at% O, assuming δ→0. This claim was substantiated by the excel- lent superconducting properties previously shown in Figure 1c. Further factors, such as analytical conditions, which can influence the accuracy of chemical composition and stoichiometry measured through APT, are analyzed and discussed in the following sections. An interesting future development would include a round robin experiment in order to establish how that would have an impact on the comparison of data from different instruments and different labs if we are to make quantitative comparisons on the same materials, such as the recent work performed by Marquis et al. (2016).
ab 10 1 0.1 0.01 0.00.1 0.20.3 /YCUO3+ YO+ 0.40.5
Laser Energy (nJ) /YO2+
YO 2 0.6 +/YO2 2+
Figure 3. a: Selected laser energy and its effect on the experimental voltage required to field-evaporate ions at a constant detection rate from the atom probe needle during an experiment designed to study the effect of experimental running conditions on detected stoichiometry. b: Measured ionic charge state ratio as a function of applied laser energy.
Voltage (V)
Laser Energy (nJ) Ratio
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