Auger electron spectroscopy is an electron spectroscopy technique that uses the detection of the electrons created via Auger relaxation processes. Some of the main applications of Auger electron spectroscopy include elemental composition analysis, spatial and depth profiling of materials and characterization of nanostructures.
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While there are a number of techniques available for profiling surface structures, like scanning electron microscopy and transmission electron microscopy, the advantage Auger spectroscopy has for nanostructure characterization is that it is sensitive to surface modifications in the first few nanometers in a wide range of specimens.
Auger Effect
The Auger effect is where an electron is emitted from an atom following a core-hole recombination process on the same atom. When core or inner valence electrons are excited on an atom, they leave a hole in the electronic structure of the atom. Such electronic configurations are usually unstable, so a more energetic bound electron will relax to fill the core hole. It is the relaxation process that provides the energy for the emission of another electron.
Auger spectroscopy can be performed in both resonant and non-resonant configurations. In both cases, the resulting Auger spectra can be quite complex as there are many different electrons that can undergo core hole recombination resulting in the emission of an Auger electron.
Auger electrons are named for the electronic energy levels involved in the transition. For example, a LMN Auger electron will involve initial ionization from an L shell in the element, relaxation of an electron in the M shell and subsequent emission of the Auger electron from an N shell. To allow for unambiguous assignments, Auger spectra are often recorded at several different incident photon energies to separate contributions from shake-up and shake-off processes during photoionization.
The use of resonant Auger techniques - where the initial excitation energy is tuned to be resonant with a bound transition rather than above the ionization continuum - can be used for more detailed explorations of the electronic structure of systems. For materials, resonant Auger spectroscopy can be used to investigate phenomena such as charge transfer processes at the interfaces and how quickly such processes happen.
In non-resonant Auger experiments, the Auger process leads to doubly-ionized final states with two holes. For resonant Auger processes, it is possible to get either one hole final states (participator decay) or two hole one particle states (spectator decay).
Nanomaterials
The Auger effect can be used as a spectroscopic tool in Auger electron spectroscopy but understanding this effect is also important for understanding the charge dynamics in nanomaterials.
In many nanomaterials, multiple excitons can be performed and interact with each other. This is true for many different nanostructures of perovskite systems, including nanocrystals and layered thin films. Perovskites have been of great interest for use in solar energy conversion devices and photovoltaics research due to their excellent conversion efficiencies.
In these perovskite nanomaterials and many other semiconductor materials, Auger recombination is a critical process as it limits the maximum efficiency of a device. Understanding the mechanisms of the Auger recombination process is key to avoiding design features and material properties that may increase the yield of these unwanted recombination events.
Nanoprobes
The advantage of Auger electron methods for probing nanomaterials is that layers just 1 mm thick can be probed and spatial resolutions of below 10 nm are achievable. Scanning Auger microscopy, sometimes known as an Auger nanoprobe, has been used to characterize many materials, including graphene, nanocomposites and nanowires.
Very limited surface preparation is needed to perform scanning Auger microscopy, so the technique can have a relatively high throughput. As Auger electron methods rely on accurate detection of the kinetic energy of the Auger electrons, experiments are generally performed under a vacuum, so the samples must be able to withstand this and irradiation, potentially with quite high energy photons. Acquisition times are usually quicker with scanning Auger microscopy than with scanning electron microscopy, which does also help reduce beam exposure.
Many nanotechnologies need multiple layers of different materials to form a functional device. Being able to probe layer thicknesses less than 2 mm means that Auger analysis can help reveal information on each layer's elemental composition and properties. Auger electron spectroscopy is a destructive technique, but this can be an advantage for analyzing several layers of material as each layer is eventually ablated, revealing the one below.
Outlook
The complexities in the mechanisms of the Auger process and the resulting spectra can be very advantageous for understanding electron dynamics in condensed matter systems. The theories for Auger spectroscopy are challenging due to the complexity of the electronic configurations involved in each stage of the Auger process, but huge advances have been made in including true ground and excited state information as well as effects such as spin-orbit coupling in models.
There is still a great deal of development work to be done both theoretically and experimentally to reveal fully all the rich information on electron dynamics contained in Auger spectra, which improved experimental resolutions will aid. Alongside this development, Auger electron spectroscopy will remain a powerful tool for elemental analysis and materials profiling.
Continue reading: What is X-Ray Photoelectron Spectroscopy?
References and Further Reading
Raman, S. N., et al. (2011). Auger Electron Spectroscopy and Its Application to Nanotechnology. Microsocopy Today, 19(2), pp. 12–15. https://doi.org/10.1017/S1551929511000083
Püttner, R., et al. (2017). Detailed assignment of normal and resonant Auger spectra of Xe near the L edges. Physical Review A, 96, p. 022501. https://doi.org/10.1103/PhysRevA.96.022501
Garcia-basabe, Y., et al. (2018). Ultrafast interface charge transfer dynamics on P3HT / MWCNT nanocomposites probed by resonant Auger spectroscopy. RSC Advances, 8(46), pp. 26416–26422. https://doi.org/10.1039/c8ra04629h
Franco, C. V., et al. (2021). Auger Recombination and Multiple Exciton Generation in Colloidal Two-Dimensional Perovskite Nanoplatelets: Implications for Light- Emitting Devices. ACS Applied Nano Materials, 4(1), pp. 558–567. https://doi.org/10.1021/acsanm.0c02868
Martinez, E., et al. (2013). Scanning Auger microscopy for high lateral and depth elemental sensitivity. Journal of Electron Spectroscopy and Related Phenomena, 191, pp. 86-91. https://doi.org/10.1016/j.elspec.2013.11.008
Verdozzi, C., et al. (2001). Auger spectroscopy of strongly correlated systems : present status and future trends. Journal of Electron Spectroscopy and Related Phenomena, 117-118, pp. 41–55. https://doi.org/10.1016/S0368-2048(01)00244-4
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