422 Stella Pedrazzini et al.


field evaporation of the specimen. For example, the promi- nent diagonal lines can be explained by the delayed eva- poration of both ions with respect to the application of the laser pulse. These plots can also be used to identify mid-flight ion dissociation events, some of which can potentially lead to the systematic “loss” of chemical species (Saxey, 2011). This technique was first pioneered by Saxey in (2011) on GaN samples in a LEAP 3000X Si, where the atoms are ionized, leave the tip of the sample and follow a straight flight path directly to the detector. The present work was, however, performed on a LEAP 3000X HR equipped with a reflectron


lens, which improves mass resolution by deflecting the ion flight path differently according to their kinetic energy. Figure 7b shows the different ways in which ion dissociation is visible in a reflectron atom probe, when compared with a straight flight path atom probe on the same GaN sample. When the ions are deflected through a reflectron lens, the full ion dissociation track is no longer visible, and is replaced by an ellipsoidal trace with negative slope. Ellipsoidal traces with negative slope are visible in the


YBCO data shown in Figure 7a, and are indicative of mid- flight ion dissociation (highlighted by the arrows). This is


because the time that it takes daughter ions of the dissocia- tion to reach the detector is either increased or reduced, because of the time spent in flight in the early stages of their trajectories as part of a larger complex ion with a different overall mass-to-charge-state. Generally, the daughter ion with smaller mass-to-charge-state will arrive later than expected. Hence, the mass-to-charge-state ratio measured in the APT experiment for these ions will be distorted to larger values. The opposite is true for the daughter ion with the larger mass-to-charge-state ratio. Figure 7 provides possible evidence for dissociation occurring more at the lowest laser energies, as expected as the applied voltage is higher and, therefore, the local field at the specimen tip is greater and the number of multiple events is higher (as shown in Fig. 6). For instance, the mass-to-charge state value of the multiple events suggests that the molecular ion Cu2O2+ dissociates into CuO+ and Cu+, but neither of those ions would be lost as they are both charged. No other ion dissociation events were evident from the correlation histograms. This suggests that the loss of individual species due to ion dissociation should not be a problem in the present analysis.


SUMMARY AND CONCLUSIONS


The present study has proved that APT can be used to measure chemical compositions (and in particular oxygen stoichiometry) in the superconducting perovskite YBCO, though the interpretation of the results requires a detailed and systematic standardization of the experimental para- meters. Ion identification and range widths proved to have a substantial effect on chemical composition, and the use of full-width-half-maximum ranges were found to be most accurate in the present work. Decreasing the laser energy was shown to improve the mass resolution by decreasing thermal “tails” in the mass


spectra and improving the visibility of smaller peaks. Due to the improvement in mass resolution, the oxygen and copper content could be measured more accurately. There is however a trade-off: mass resolution is improved with


decreasing laser energy but, likely due to its low evaporation field, some evidence suggests that Ba could be evaporating off-pulse. The barium content was therefore more accurate at higher laser energies. The reduced detectability of Ba due to off-pulse evaporation artificially inflates themeasured content of the other elements, when expressed in units of at%. It is also noteworthy that the number of multiple events


increased with decreasing laser energy, which increases the uncertainty of the measurement. An intermediate value of 0.3–0.4 nJ is therefore suggested for the study of YBCO. The detection loss of neutral species due to complex ion dissociation was not observed to be an issue at any of the laser energies tested. Background corrections were required due to the insu-


lating nature of the sample, which caused field evaporation to continue even after the laser pulse, translating in the mass- to-charge state spectrum as thermal “tails” extending from the peaks. The thermal tails were fitted to exponentially decaying functions which were then subtracted from overlapping peaks. The final measured composition obtained was


Y7.9Ba10.4Cu24.4O57.2 at%. For data sets exceeding 1M ions the counting error should be small enough to allow the composition to be accurate within ±0.01 at%. This opens up new prospects for studying the local chemistry of nanoscale defects created by radiation damage, and explaining their influence on the superconducting properties.


ACKNOWLEDGMENTS


Dr. Daniel Haley is gratefully acknowledged for helping with a part of the Matlab script and the C++ programused in the present work. Dr. Tayebeh Mousavi is gratefully acknowl- edged for performing the superconductivity measurements. Funding is acknowledged from the UK’s Engineering and Physical Sciences Research Council (EPSRC) under grant EP/K029770/1. PDE also acknowledges support from the US Department of Energy, Office of Science, Fusion Energy Sciences. The authors also thank the Bulk Superconductivity Group in the University of Cambridge for providing the samples which were used in the present study.


REFERENCES


BABU, N.H., JACKSON, K.P., DENNIS, A.R., SHI, Y.H., MANCINI, C., CARDWELL, D.A. & DURRELL, J.H. (2012). Growth of large size Y1Ba2Cu3O7 single crystals using the Top Seeded Melt Growth process. Supercond Sci Technol 25(7), 75012.


BACHHAV, M., DANOIX, F., HANNOYER, B., BASSAT, J.-M. & DANOIX,R. (2013). Investigation of O-18 enriched hematite (α-Fe2 O3)by laser assisted atom probe tomography. Int J Mass Spectrom 335, 57–60.


BADWAL, S.P.S., GIDDEY, S., MUNNINGS,C.&KULKARNI, A. (2014). Review of progress in high temperature solid oxide fuel cells. J Aust Ceram Soc 50(1), 23–37.


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