334 Chang-Min Kwak et al.
Co-capped LAO tips. This difference occurs presumably because of apex asymmetry on uncapped LAO tips, and can lead to local kinetic field dissociation (Seol et al., 2016). The measured stoichiometric ratios of La:Al:O in the
uncapped and Co-capped tips were 1:1.1:1.9 and 1:1:2.2, respectively, which differ from the expected ratios of 1:1:3. It is deduced that the metal atoms in oxides are commonly less
bound than oxygen atoms and thus can evaporate at low fields (Karahka et al., 2015). Moreover, the deficiency of oxygen can be explained by thermal desorption of neutral oxygen species, which would not be detected as ions (Devaraj et al., 2013). Considering this concept and the multiple events, we found that the Co capping reduces the deviation from the nominal stoichiometric ratio of the LAO. To improve data reliability, the difference in evapora- tion field between capping materials and oxides should be
suggest for the first time that the field evaporation strength of LAO is quite similar to that of Co metal, and is higher than that of Ni. However, further studies are required to consider the effects of numerous variables, such as change of band structure, bonding strength in each phase, and the temperature rise at the tip surface (Gault et al., 2008). Overall, we prove that the combination of “capping
considered. The Ni-capping layer has a lower evaporation field than does LAO, so evaporation of LAO oxides is impeded in the core of the tips (Fig. 3), whereas Co-capping layers evaporated concurrently with the core regions of the oxide (Fig. 4). As a result, the Co-capped LAO tips yield a profound improvement in the mass-resolving power, an increase in signal-to-noise ratios, and a decrease in the fraction of multiple events. Based on the stepwise APT with TEM approach, we
effects” and “stepwise APT with TEM,” which can be applic- able to anymaterials, allows for direct observation of tip shape evolution during APT analysis of oxide materials, and gives improvement in understanding the interactions of ultrafast laser illumination with oxide tips, and provides a way to identify the relative strength of the evaporation field of oxides.
CONCLUSION
We used correlative microscopy by stepwise analysis of APT with TEM observation to investigate how metallic-capping layers affect the evaporation of LAO tips during laser irra- diation. During the APT analysis of uncapped LAO tips, the localized laser absorption at the tip surface induced differ- ences in radius of curvature between the illuminated side and the shadow side, and generated an inhomogeneous heat distribution during field evaporation. These effects lead to long thermal tails in the mass spectrum, which reduce the reliability of APT data. However, the use of a metallic- (Ni or Co) capping layer increases the thermal diffusivity and homogenizes the heat distribution during the first nano- seconds after each laser pulse. The presence of the metallic- capping layer improved the mass resolution and reduced the variance in the time of flights of ion species. The field evaporation sequences of metallic-capped LAO tips suggest
that the evaporation field of LAO is quite similar to that of Co metal, and higher than that of Ni. This result suggests that the evaporation field of oxide materials can be estimated by comparing with the evaporation field of different capping metals. The variation of the reliability of APT data with the choice of capping layers cannot be explained by the capping material’s thermal diffusivity but by the difference in the field evaporation between capping material and oxide. Therefore, the most important criterion when selecting the capping layer for analysis of an oxide material is the difference in the evaporation field of the capping material and the oxide.
ACKNOWLEDGMENTS
This work was financially supported by National Institute for Nanomaterials Technology (NINT) in POSTECH and National Research Foundation of Korea (NRF) grants funded by the Korean Government (MEST) (No. 2011- 0020252). The authors also express gratitude to Dr. Y.-T. Kim (POSTECH) for some of the APT experiments.
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