EELS and EDS Analysis
Figure 1 : ADF STEM survey image. The EELS SI was taken across the region inside the green box that extends over the entire image.
analysis at the atomic level across the entire area shown in the ADF STEM image in Figure 1 . T e region was divided into 1,050×1,050 pixels, and EELS spectra were acquired at a rate of 500 spectra/second across the entire area. Each EELS spectrum was acquired using an exposure time of 1.5 ms. Even with such a short exposure time, the system is so sensitive that high-energy edges such as the Sr L 2,3 at 1,940 eV have good SNR, as shown in Figure 2 . Given the very short exposure time used for the experiment, the spectrometer was set with the highest possible binning both in the non-dispersive (vertical at 130×) and the energy (horizontal at 8×) directions. Binning along the vertical direction does not aff ect the energy resolution, but there is a huge improvement in sensitivity and spectral readout speed. Notwithstanding the improve- ments in sensitivity, 8× binning along the horizontal (that is, energy) direction strongly deteriorates the energy resolution as the total number of energy channels reduces from 2,048 down to 256. However, in the case of composi- tional analysis, the signal is extracted over a wide energy integration range, thus the energy resolution is not very important. Figure 3a shows a colorized composite of the EELS elemental maps for Ti, Mn, La, and Sr (oxygen is omitted for clarity). Here it is possible to see that the interface becomes fairly irregular in the lower portion of the map as indicated in
Figure 2 : Background subtracted EELS spectrum of the Sr L 2,3 -edges at 1,940 eV. Each EELS spectrum was acquired with only 1.5 ms exposure time. Despite the short exposure time per spectrum, the Sr L 3 and L 2 white lines can be observed.
image of atomic columns at the STO/LMO/STO/LMO interface. T e interface appears abrupt in the upper portion of the image but is quite rough with the presence of terraces in the lower portion of the image indicating some growth in homogeneities. T e high sensitivity of the CCD in the GIF Quantum allows the acquisition of EELS at high speed with good signal-to-noise ratio (SNR), and the high stability of the micro- scope installation at IBM allows compositional distribution
46
the ADF STEM image ( Figure 1 ). However, by zooming in on one of terraces along the interface, it is possible to observe elemental diff usion that in some areas across the STO/LMO interface extends up to 2 monolayers, as indicated by the 2 arrows in Figure 3b . In addition, the interface seems to maintain its crystalline structure with the presence of atomic columns containing both La and Sr in one case and Mn and Ti in the other case. It is important to mention that no spatial driſt correction was employed during the acquisition of the spectrum imaging (SI). Although some spatial distortion and minor driſt can be seen, the single, continuous pass acquisition ensures fi delity of the composition maps. Combined EELS and EDS results from fast elemental mapping analysis . EELS and EDS elemental maps were taken from a smaller region of the same interface of Figure 1 . In this case, EELS data were acquired in single-range mode, and three diff erent signals (ADF, EDS, and EELS core-loss) were recorded simultaneously. To ensure adequate counts in the EDS data, the spectrum integration time per pixel was increased to 10 ms compared to the 1.5 ms of the fi rst example. T e EELS spectrum was acquired from 300 eV to 2,300 eV, and the EDS spectra were acquired simultaneously, integrating counts over the entire pixel time employed for the EELS acquisition. T e entire 180×80 pixel spectrum imaging dataset was acquired in just over 2 minutes.
Elemental maps were constructed from spectrum images for relevant EELS edges (Sr-L at 1,940 eV, Ti L at 456 eV,
www.microscopy-today.com • 2015 July
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84