HR-EELS from Gatan: A Milestone in EELS Technology
Electron Energy Loss Spectroscopy (EELS) is a powerful analytical technique that allows researchers to study the physical and chemical properties of materials across a broad range of elements and with spatial and spectral resolution beyond that available with most other micro-analytical techniques. Recently, there has been increased demand for an EELS spectrometer with energy (spectral) resolution of 100 meV or less. Combined with a TEM designed to provide exceptional electron source stability, monochromated electron beam and high spatial resolution, such a system allows researchers to perform advanced studies of phase, molecular and interband structures at nanoscale spatial resolution.
Gatan, Inc. has cooperated with FEI Electron Optics to create a system incorporating a new high-resolution spectrometer (HR-EELS from Gatan) to be used with a 200 kV Tecnai™ (S)TEM equipped with a monochromator and improved high tension (from FEI EO). It was expected that such a system would achieve better than 100 meV energy resolution and would reveal information on EELS fine edge structures previously not attainable on any commercially available instrument.
The first HR-EELS has been installed at the Technical University of Delft on a monochromated Tecnai™ TF20. Early results show detailed structure of a Boron-Nitride K-edge peak with a full width at half maximum of 0.30 eV! In comparison, Gatan's ENFINA energy loss spectrometer (the latest version of the industry standard parallel EELS) on a TEM equipped with a LaB6 filament only resolves this peak to 1.5 eV. So, already the system (TEM and HR-EELS) has achieved performance beyond the original expectation.
At the recent SALSA (Strategies and Advances in Atomic Level Spectroscopy and Analysis) Workshop held in Guadeloupe May 5-9, 2002, results obtained with the new spectrometer on the monochromated TEM, showing fine structure resolution of transition metal oxides (Hofer et al) and diamond (Zandbergen et al.) were presented.
Spectrometer energy-resolution is a function of its design, electron optical aberrations, electronic instabilities, and operating conditions. Improving the spectrometer design and adding multipoles to correct for higher order aberrations (up to 4th order) yield important increases in optical performance. Improved isochromaticity of the HR-EELS now makes it possible to use entrance apertures as large as 5mm. This simplifies the acquisition of data with large collection angles. Positioning the entrance apertures for the new spectrometer higher up in the system yields an increase in aperture size compared to the present Gatan Imaging Filter 2000 series, from 3 mm to 5.4 mm. At the same time there is a 14-fold improvement in non-isochromaticity. An additional benefit is that the increase in aperture size decreases the internal magnfication contribution of the GIF by a factor of 1.8 X from the TEM screen to the spectrometer detector.
Totally new spectrometer electronics significantly improve instrument stability over long-term measurements. Longer exposures are of particular importance with a monochromated electron source because the total beam current is typically low. A 30-fold improvement compared to the GIF2000 series prism current stability has been achieved. Total system drift is about 2 eV/hour or 30 meV for a one-minute exposure. In addition, the noise in the prism current has been improved by a factor of 5. In combination with the monochromated 200 kV TEM, better control over laboratory environmental factors (e.g. temperature changes, fields and vibrations) the HR-EELS yields energy resolution of 100 meV for a 1 second exposure.
Further improvements can be obtained at lower operating voltages, due to a reduction in noise and electron energy spread at lower kV. For example, at 80 kV an energy resolution of 80 meV has been realized with an exposure of 3 seconds.
Presentations and publications of an increasing amount of applications and theories underlying EELS fine structure and low energy-loss analysis will appear in the near future. Be sure to check here for updated information.
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