Using Electron Energy Loss Spectroscopy (EELS) for High-Speed Composition and Chemical Analysis

The GIF Continuum™ series epitomizes the next generation of electron energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) systems from Gatan.

Gatan’s exclusive CMOS detector technology extends the capabilities of EELS and EFTEM data fidelity and acquisition speed of the GIF Continuum.

Spectral acquisition rates of up to 8000 spectra per second can be achieved with almost 100% efficiency and low added noise thanks to proper detector optimization in combination with an enhanced scintillator/fiber optic imaging stack.

Such components also enhance data quality and productivity over previous CCD-based GIF spectrometers. The GIF Continuum builds on the analytical possibilities of EELS and EFTEM.

Where high-quality data is crucial for observing energy-loss near edge fine structure (ELNES), the reduced-noise electronics refine the sensitivity of EELS when employed for trace element analysis and chemical analysis.

Moreover, the high spectral acquisition rates are converted into rapid collection of comprehensive spectrum images, eradicating the restrictions of only examining small sub-regions to evade artifacts such as sample drift and beam damage.

The recently designed energy-selecting slit mechanism facilitates the collection of individual spectra covering energy ranges up to 3000 eV, producing spectrum images richer in data than ever before.

These technical benefits also enhance energy filtered images and diffraction patterns, making in-situ EFTEM and in-situ 4D STEM viable techniques when utilizing the GIF Continuum.

Core-Loss EELS Analysis of Piezoelectric Ceramics with the GIF Continuum

Piezoelectric ceramics, including PbZrxTi1-xO3 (PZT), are central materials systems in sensor and transducer applications.

The chemical oxidation state and atomic coordination of elements in piezoelectric materials considerably influence piezoelectric properties, and EELS is the perfect characterization technique for supplying this information.

However, using EELS to analyze samples can be challenging due to the mix of light and heavy elements in the compound. The primary edges have extremely high energy-loss features, with the Zr L and Pb M edges located at 2222 eV and 2484 eV, respectively.

Furthermore, the Ti L edge at 456 eV and O K edge at 532 eV was removed from the Zr and Pb edges by almost 2000 eV, making acquisition all the more difficult. It is almost impossible to capture spectra with a satisfactory signal-to-noise ratio at such high energy losses and over such an extensive energy loss range using CCD-based systems from the previous generation.

However, the enhanced capabilities of the GIF Continuum are employed to circumvent these hurdles and offer a complete analysis of a Si/SrTiO3/PbZr0.5Ti0.5O3 (Si/STO/PZT) structure.

HAADF STEM image of Si/STO/PZT. The green box labeled Spectrum Image shows the region analyzed with EELS. The blue box highlights the integration region for EELS data, shown in Figure 2.

Figure 1. HAADF STEM image of Si/STO/PZT. The green box labeled Spectrum Image shows the region analyzed with EELS. The blue box highlights the integration region for EELS data, shown in Figure 2. Image Credit: Gatan Inc.

In Figure 1, the HAADF STEM image displays the Si/STO/PZT stack. The green region labeled Spectrum Image signals the EELS analysis region (138 x 120 pixels in size, 5 ms exposure per pixel).

Individual EELS spectrum integrated from the blue region in Figure 1. The Ti L and O K edges are shown separately from the background subtracted high energy-loss edges for clarity.

Figure 2. Individual EELS spectrum integrated from the blue region in Figure 1. The Ti L and O K edges are shown separately from the background subtracted high energy-loss edges for clarity. Image Credit: Gatan Inc.

Figure 2 demonstrates an integrated spectrum across the blue region in Figure 1. This spectrum stretches across an energy range from 200 – 3200 eV and includes all elemental edges of the Si/STO/PZT sample.

Furthermore, each of the edges in the spectrum, even in the case of >1800 eV, has enough signal-to-noise ratio to accurately conduct any required data processing, such as fine structure analysis, multiple linear least squares (MLLS) fitting and elemental quantification and mapping.

Si K and Sr L EELS edges extracted from the data. Note the distinct fine structure and edge shift of the oxidized silicon.

Figure 3. Si K and Sr L EELS edges extracted from the data. Note the distinct fine structure and edge shift of the oxidized silicon. Image Credit: Gatan Inc.

For instance, mapping using MLLS fitting in effect splits the overlapping Si K and Sr L edges, as displayed in Figure 3. Also, the energy shift and clear shape change of the Si K edge in Figure 3 uncovers the presence of a SiOx layer in the Si/STO/PZT stack. 

EELS composition mapping of Ti, Sr, Si, O, and Pb. Figure 1 shows the location of this mapped region.

Figure 4. EELS composition mapping of Ti, Sr, Si, O, and Pb. Figure 1 shows the location of this mapped region. Image Credit: Gatan Inc.

Mapping of the Ti L, O K, Si K, Sr L, Zr L and Pb M edges was then conducted to show the elemental distribution in the Si/STO/PZT structure. As the spectra acquired at each pixel contain all elemental edges, individual raw element maps can be easily combined to generate the comprehensive elemental distribution map displayed in Figure 4.

EELS fine structure of the Ti4+ L2,3 edges in STO and PZT.

Figure 5. EELS fine structure of the Ti4+ L2,3 edges in STO and PZT. Image Credit: Gatan Inc.

This map was obtained with an extremely short (5 ms) exposure time per pixel and a total collection time of less than 90 seconds. No data enhancement process or filtering was required to enhance the overall quality.

In addition to periodic changes in Ti concentration that correlate with image contrast changes seen in Figure 1, the map uncovers the SiOx region as an interlayer at the Si/STO interface.

Figure 5 demonstrates a close examination of the Ti L2,3 edges in the STO and PZT regions. Each edge shows a strong crystal field splitting. The fine structure also exhibits slight shifts in peak maxima and shifts to the peak intensity. These variations signal the different atomic arrangements of Ti4+ in STO and PZT.

The Distinction of the GIF Continuum

The GIF Continuum expands the analytical possibilities with EELS significantly, as seen in the spectrum imaging of the Si/STO/PZT stack. The newly redesigned energy slit promises appropriate alignment of spectra in the optical path and increases the energy range that an individual spectrum can cover to 3000 eV.

Moreover, the GIF Continuum’s new detector stack technology substantially increases sensitivity while reducing noise and offers high-quality spectra when utilizing low beam currents or rapid acquisition times.

A data-rich analysis of the Si/STO/PZT structure can be obtained by utilizing the GIF Continuum. The benefits of the GIF Continuum go as far as all current samples of interest for EELS characterization and also to samples where EELS analysis was not feasible using previous generation spectrometer technology.

This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.

For more information on this source, please visit Gatan, Inc.

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