AXIS Supra⁺ - Applications

XPS and surface characterisation techniques have contributed a wealth of information to battery research.  Batteries and power storage devices are complex structures where the electronic properties and heterogeneous interfaces between components are critical in the operation of the device.  

XPS is an ideal technique to provide fundamental information relating to interfacial properties that arise from surface related chemistries such as chemical composition, elemental or chemical distribution of species, defect sites or functional groups.  The Kratos Supra2⁺ allows these studies to be extended to in-situ and in-operando characterisation of model devices.

Considerable research effort has been applied to Li-ion batteries which are used for high energy-density storage.  Current limitations such as dendritic Li growth and insulating secondary electrolyte interphase layer formation are being addressed by XPS analysis.  Further surface analysis studies are also required to reduce limitations such as stability and charge rate.  

As well as lateral distribution characterised by XPS imaging, the depth distribution of elemental and chemical states can be determined by combining Ar⁺ ion sputter depth profile cycles with XPS.  An example is the use of the recently developed Ar_n⁺ gas cluster ion source (GCIS) which has proved important in the correct determination of Li concentration through solid electrolyte materials.  Use of a conventional monatomic Ar⁺ ion source has been demonstrated to induce Li⁺ ion migration through the electrolyte material towards the electrolyte/electrode interface as a consequence of implanted Ar⁺ ions repelling the small, mobile Li⁺ ions.  This repulsive bulk migration accounts for erroneous determination of Li concentration in monatomic Ar⁺ depth profiles and the increase in Li concentration at the interface with the electrode.

The above example highlights the value of GCIS depth profiling of solid electrolyte materials using the AXIS Supra⁺.  
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The use of high energy X-ray sources gives rise to the acronym hard X-ray photoelectron spectroscopy, HAXPES.  By changing the anode material to silver, Ag Lα X-rays can be used to excite photoelectrons.  The photon energy of Ag Lα is 2984.3 eV, approximately twice that of Al Kα (1486.6 eV).  A significant advantage of this is the ability to use the same monochromator mirror for both photon energies making this optional excitation source very affordable.

An further advantage of using the Ag Lα is the ability to excite additional, higher binding energy core lines from a number of elements.  The higher photon energy also results in an increase in the information depth. This can be demonstrated by considering the inelastic mean free path (IMFP) of C 1s (BE 285 eV).  Using Al Kα X-rays the C 1s IMPF is 3.14 nm at KE 1201 eV whereas for the same core level excited using Ag Lα the IMFP is increased to 5.89 nm at KE 2699 eV.  

A novel application of hard X-ray sources is in the study of buried layers in model gate oxide devices.
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Surface coatings and thin films are of great commercial importance in many industries and are used to enhance or provide required properties to bulk materials specific to their applications. Thin films can range from tens of Angstroms to several microns in thickness.  They find application in areas as diverse as optical anti-reflective coatings, architectural glazing and drug eluting thin films in the biomedical industry. X-ray photoelectron spectroscopy (XPS) is ideally suited to characterising the surface chemistry of these thin films and when combined with sputter depth profiling can be used to determine the elemental and chemical composition as a function of depth through the film.  

The AXIS Supra⁺ instrument uses a monochromated Al Kα excitation source for XPS characterisation of thin films.  Photoelectrons that contribute to the peaks observed in an XPS spectrum are described as elastically scattered.  Surface sensitivity of XPS derives from the distance the photoelectrons can travel through a material without losing energy.  The information depth for Al Kα excited XPS is generally accepted as 10 nm.  This can be made more surface sensitive by rotating the sample relative to the collection lens in an ‘angle resolved’ experiment.  Such data can be used to reconstruct a non-destructive concentration depth profile of the uppermost 10 nm of the material.

Using the Kratos developed Ag Lα higher energy X-ray source the information depth can be extended to 15 – 20 nm.  The increased information depth, relative to Al Kα excitation, is due to the higher kinetic energy of the excited photoelectrons.  The higher photon energy of the Ag Lα excitation also means that a greater number of XPS core-levels can be measured.

Extending the information depth range to the bulk material beyond that accessible by lab. X-ray sources and electron escape depths requires the use of ion sputter sources to remove material prior to XPS analysis.  The AXIS Supra⁺ can be equipped with either a monatomic Ar⁺ ion source or the versatile gas cluster ion source.  The use of massive Ar_n⁺ gas cluster ions as the projectile allows successful sputtering of ‘soft’ organic materials with the retention of chemistry throughout the depth profile.  Being able to select n, the number of atoms in the cluster, and the acceleration energy of the cluster ion gives flexibility to select the energy per atom, or partition energy, appropriate for the material that is being depth profiled.
Suggested applications notes