Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor 419 If all the peaks had the same shape and width, then the


composition would be independent of range width. The difference in composition measured with different range widths is a result of the asymmetry of the peaks, due to the thermal tails and the changes in peak widths corresponding to different ions. The peak width, in laser-assisted measure- ments, will depend on the field required to evaporate each ionic species from the sample apex (Gault et al., 2012). This effect causes the peaks to change across the reconstruction. The selected widths are therefore not tailored to a specific experimental condition, but are being applied to the average of a range conditions, i.e. the best performing ranging


approach is effectively the most robust across the changing field conditions. Because no information could be obtained for the evaporation field of every type of ion detected in the present mass spectra, as a compromise, full-width-half- maximum was selected for the following analysis. The mea- sured chemical composition from the whole atom probe run at the selected range width was Y7.4Ba16.2Cu23.8O52.4 at% and contained traces of ~0.17 at% Ce, whereby Ce was identified as the CeO2+ peak at 78 Da.


Thermal Tails


Samples which are thermally insulating when illuminated with a laser pulse will dissipate heat more slowly. As a result, the specimen remains at a higher temperature for a longer time, and hence the field evaporation process can continue for several nanoseconds after the application of each laser pulse (Bunton et al., 2007). This delayed evaporation man- ifests in the acquired time-of-flight spectrum as a “thermal tail” which extends from the main peak to longer times. When the time-of-flight spectrum is converted into a mass- to-charge spectrum, this effect is evident through tails which extend from the main peak to higher effective masses. During analysis of the spectrum, thermal tails can raise the effective background and hide smaller peaks, potentially causing the introduction of errors into any chemical quan- tification. Figure 4b clearly illustrates a reduction of thermal tail size in the whole spectrum when the laser energy is reduced. Figure 4c shows the small 65Cu2+ peak just after the large O2


+ peak, which at higher laser energies would be


hidden in the thermal tail. This demonstrates that, for the present samples, a reduction in laser energy from 0.6 to 0.05 nJ can lead to an effective improvement in mass resolution, and hence more confidence in the ranging of the mass spectrum. This effect can be explained as the reduction in laser


energy is expected to lead to a lower maximum temperature reached at the sample apex. The temperature change caused by the laser has been both experimentally estimated and modeled in previous studies on a variety of materials (Liu & Tsong, 1984), and is known to be dependent upon the laser energy and the individual specimen properties (Liu & Tsong, 1984; Kelly et al., 2014). If the maximum temperature is reduced, the time required for the surface to quench will also decrease. This, in turn, will cause ion evaporation to occur


within a narrower time range after each individual pulse, reducing the “thermal tail” effect in both the time-of-flight spectrum and therefore also in the final mass spectrum.


Effect of Laser Energy


could be correlated to its lower evaporation field when com- paredwith Cu orY(Gault et al., 2012). However, it is important to note that evaporation fields of elements have only been measured in their elemental state, they could be different in the case of compounds. Barium could be field-evaporating throughout the experiment in a way which is uncorrelated with the laser pulses. In this case the ions would leave the tip and reach the detector, however their time-of-flight could not be measured and they would therefore be unidentifiable, hidden in the mass spectrum background noise. This sugges- tion is further supported by the evidence presented in Figure 5b, which proves that at higher laser energies (which means lower applied voltage, therefore lower field) more Ba is detected. Further evidence supporting this theory is provided by the


The effect of altering laser energy on composition is shown in Figure 5a. Each data point represents a 600,000 ion sec- tions of the 17 million-ion data set. The entire Ba content of the analyzed volume was detected in the form of Ba2+ ions at 65–69 Da, which was expected based on the post-ionization theory developed by Haydock and Kingham (Kingham, 1982). However, this also means that the overlap with the thermal tail of the Cu+ ions at 63 and 65Da (Fig. 5b) can cause an overestimate of the Ba content of the sample. The compositions given in Figure 5c were therefore back- ground corrected by fitting an exponentially decaying func- tion to the thermal tails and subtracting them using a custom Matlab code developed by London et al. (2015). London et al. (2015) developed a model for fitting the peaks in their mass spectrum, by using an exponentially corrected Gaussian function for the peak shape, with an added exponentially decaying function on one side as the thermal tail, as descri- bed in further depth in their paper. At a laser energy below 0.25 nJ, the overall composition of the sample is approxi- mately constant, but a significant Ba deficiency is seen at every laser energy, as seen in Figure 5b. One suggested reason for the measured Ba deficiency


rise in background rate with decreasing laser energy. The background rate was measured in a region where no peaks or thermal tails were present, between 4–11 Da. At low laser energy (between 0.05 and0.15 nJ) the background rate was 8.1×10−5/ amu, whereas at high laser energy (between 0.45 and 0.6 nJ) it was 7.6×10−5/amu, when measured on a data set divided into bins of 0.05 Da. The background rate showed an effective increase by 6% at lower laser energy, which can be indicative of off-pulse evaporation by one or more chemical species.


Event Multiplicity


Subsequent to each laser (or high voltage) pulse, the micro- scope acquires signals fromthe detector for a duration dictated by the pulses repetition rate, in what is referred to as an event. During an event, if two or more ions are detected, then all ions


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