Chemical mapping at atomic resolution using energy-dispersive x-ray spectroscopy
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Introduction Scanning transmission electron microscopy (STEM) at atomic resolution is now in wide general use, with Z-contrast imaging and electron energy-loss spectroscopy (EELS) being well established.1,2 This review discusses an extension of STEM to obtain atomic-level chemical maps using energy-dispersive x-ray (EDX) spectroscopy. The first such results were published by D’Alfonso and co-workers3 using a test sample of SrTiO3. The specimen, estimated to be 100 nm thick, was illuminated by a 300 kV probe formed using a lens with an aperture semi-angle of 9.6 mrad and with a third-order spherical aberration coefficient Cs = 1.2 mm. This gave an estimated probe size of 0.14 nm and a beam current of 10–20 pA. The EDX detector subtended a collection angle of 0.13 sr. Subsequently, these results were confirmed in an experiment led by Chu et al. using a similar system.4 Since the first results were reported,3,4 there has been considerable progress from an experimental point of view.5–7 Detectors enabling count rates an order of magnitude better than those used in the earlier work have been developed and installed in microscopes that have correction for spherical aberration,8,9 allowing for the use of finer probes. A brief review of aberration correction in electron microscopy can be found in Reference 10.
In this article, we present an overview of these new stateof-the-art systems and illustrate the potential for applications by considering three case studies. First, we consider a crystalline slab of GaAs oriented along and show maps obtained for both K-shell and L-shell ionization in Ga and As. We next consider SrTiO3 and obtain chemical maps by monitoring the x-rays associated with ionization of the constituent elements.5 Last, we study a SrTiO3-PbTiO3 interface. In all cases, firstprinciple calculations are presented that confirm and facilitate the interpretation of the experimental maps. The calculations allow us to discuss issues such as probe channeling and spreading, the contribution from thermal scattering (electrons that have already excited phonons), and delocalization of the images.
Experiment The signal strength in EDX spectroscopy (for a given specimen) is determined by three experimental STEM parameters: the accelerating voltage, the probe current, and the collection efficiency of the detector. For the atomic resolution chemical mapping described in this review, we discuss the last two—in particular, the improvement in probe current using a highbrightness field emission gun electron source (XFEG) in a
Leslie J. Allen, School of Physics, University of Melbourne; [email protected] Adrian J. D’Alfonso, School of Physics, University of Melbourne; [email protected] Bert Freitag, FEI Company, The Netherlands; [email protected] Dmitri O. Klenov, FEI Company, The Netherlands; [email protected] DOI: 10.1557/mrs.2011.331
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 37 • JANUARY 2012 • www.mrs.org/bulletin
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CHEMICAL MAPPING AT ATOMIC RESOLUTION USING ENERGY-DISPERSIVE X-RAY SPECTROSCOPY
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