EELS and EDS Analysis
for the same elements (Sr Lα at 1.81 keV, Ti Kα at 4.51 keV, O K at 0.52 keV, Mn Kα at 5.90 keV, and La Lα at 4.65 keV) using an empirical Kramers background subtraction followed by MLLS fi tting of each peak family represented by a set of Gaussian peaks [ 16 ]. X-ray maps using the O K line were unsatisfactory because of the overlap between the O K and Ti L peaks, the low fl uores- cence yield, and absorption eff ects that strongly aff ect low-energy X rays [ 4 , 8 , 17 ]. Figures 4 shows the Ti, O, Mn, La, and Sr elemental maps obtained using EELS, whereas Figure 5 shows the same elemental maps taken with EDS X-ray signals. Qualitatively, there is less noise and higher contrast in all the EELS maps compared to those obtained using EDS. The O K elemental maps using EELS and EDS are shown in Figures 4 b and 5 b, respectively. The EELS data do not completely resolve individual oxygen atoms, but they do show the distributed oxygen framework. As mentioned above, the EDS data show no oxygen atom locations. Figures 6 a and 6 b show colorized elemental maps of Ti, Mn, La, and Sr and La obtained using EDS and EELS, respectively. The EELS map appears sharper and shows stronger contrast; the EDS data are noise-limited due to the low number of counts in each spectrum. It is important to note that the EELS and EDS data shown are the raw output and have not been filtered or de-noised.
Investigating chemistry across the interface . EELS can provide chemical bonding information when analyzing the core-loss region of the EELS spectrum. Bonding and coordination infl uences the shape of the near-edge fi ne structure and can shiſt the edge threshold energy resulting in a so-called chemical shiſt . T ese advantages are countered by the presence of plural scattering that can blur the shape of the near-edge structure and energy driſt that prevents accurate measure- ments of the small chemical shiſt . Both these problems can be corrected using the low-loss spectrum acquired under the same conditions as the core-loss spectrum [ 3 , 9 , 10 ]. T is acquisition capability is called DualEELS and allows nearly simultaneous collection of diff erent regions of the EELS spectrum under the same experimental conditions.
Figure 3 : a) EELS colorized elemental maps across the STO/LMO/STO/LMO of Sr L 2,3 -edges at 1,940 eV in red, Ti L 2,3 -edges in green, Mn L 2,3 -edges at 640 eV in blue, and La M 4,5 -edges at 832 eV in purple. The contrast shown by these elemental maps is very high despite the short exposure time per spectrum. There is some roughness and elemental diffusion that can be observed across the interfaces. b) Enlarged view of the region in the black box of Figure 3a.
O K at 532 eV, Mn L at 640 eV, and La M at 832 eV), and data were extracted using MLLS fi tting [ 3 , 14 , 15 ]. In the case of EDS, elemental maps were extracted using the X-ray lines
48
To analyze compositional and bonding changes in this sample, data were acquired at higher energy resolution from the interface region using DualEELS mode. T e spectrometer was confi gured with a dispersion of 0.25 eV/channel to give a measured energy resolution of 0.75 eV but an energy range from 0 to 900 eV (using DualEELS mode). T is setup allows fi ne structure analysis of oxygen and transition metal
www.microscopy-today.com • 2015 July
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