Beyond elemental analysis in the electron microscope: accessing isotopes with in‐situTEM‐SIMScorrelative analysis
Authors: Yedra Lluis , Eswara Santhana , Dowsett David , Wirtz Tom 
Published: December 2016

Despite having superior spatial resolution, the analytical capabilities of Transmission Electron Microscopy (TEM), either Energy Dispersive X‐ray Spectroscopy (EDS) or Electron Energy‐Loss Spectroscopy (EELS) [1], are fundamentally limited in terms of isotopic analysis. Moreover, the analysis of light elements still remains out of reach. However, these analyses count among the advantages of Secondary Ion Mass Spectrometry (SIMS), which also provides high chemical sensitivity and high dynamical range, but offers poor lateral resolution [2]. Therefore, a combination of both techniques is the logical step to complement the strengths of SIMS with a high‐resolution imaging technique such as TEM. As ex‐situ approaches are prone to sample modification artefacts, we have developed an in‐situ combination of TEM and SIMS called the Parallel Ion‐Electron Spectrometry (PIES).

The developed prototype instrument [3] is based on a Tecnai F20 which has had its octagon and pole pieces modified to accommodate a FEI Magnum Ga+focused Ion Beam (FIB) primary column and secondary ion extraction optics. This extraction optics, with extraction efficiency up to 90%, is coupled with a double focusing magnetic‐sector mass spectrometer developed in house. Finally, another needed addition involved the sample holder, which was designed to be biased to high voltages (4.5 kV) and to act as the first electrode in the extraction system.

To highlight the new methodology developed in the instrument, lithium carbonate (Li2CO3) was chosen. Lithium exists in two stable isotopes in nature (6Li and7Li) with very different abundances, 7.5% and 92.5%, respectively. A powder sample of Li2CO3enriched in6Li up to 95% was mixed with a natural‐abundance sample. The goal of the investigation was the distinction of particles according to their isotopic abundance.

The correlative approaches for the information extraction can be regarded as two‐directional. The first option, for samples with areas of interest depending on structure, TEM should be used first for identification of those areas, while SIMS will provide the analysis afterwards. The second option suits samples marked with isotopic (or elemental) labels. SIMS will then be used for localisation of the area of interest, to be further imaged at high resolution with TEM. Using this second approach, mass spectra were acquired for each of the individual materials before mixing them. Mass filtered images were acquired form the mixture. The subsequent imaging with TEM of the same area was used to assign each of the particles to one of the original materials according to their isotopic composition.

As the correlative approach leads to a multimodal imaging of the same regions of interest, and, in order to successfully exploit the results, there is need of correction of imaging artefacts as well as data fusion for obtaining high spatial resolution isotopic images. We will show how these new kinds of datasets can be obtained with this new instrument.