Na-ion batteries are considered to be environmentally favourable alternatives to Li-ion batteries, particularly in the extremely large-scale application of grid storage, given the abundance of Na. Among them, sodium layered oxides are very important cathode materials with good sodium mobility and high specific capacity. Layered transition metal oxides Na_XMeO₂ consist of layers of MeO₆ octahedra to preserve 2D transport channels for direct Na ion diffusion. For metal selections, Me can be Fe, Mn, Co, Cr, Ni, Ti, V and their combinations. Interestingly, single transition metal oxides show worse battery performance than binary or ternary transition metal systems, making composition studies imperative. Metal combinations can achieve tuneable properties with structure, electrochemical performance, and cycling stability.

In this application note, air/moisture stability is effectively and systematically investigated with XPS, paving the way for material stability to be modified through rational design.

This applications note demonstrates the use of both conventional (Al Kα) and higher photon energy (Ag Lα) excited X-ray Photoelectron Spectroscopy (XPS) to characterise the solid electrolyte interphase layer formed on a graphite anode during charge/discharge cycles.  Samples were prepared in a glove box at the manufacturing site and transported to the XPS analysis laboratory using an air sensitive sample transport accessory.

Nitrogen-doped GaAs (GaAs_1-xN_x) is finding application in high efficiency solar cells and vertical cavity surface emitting lasers (VCSELs). Nitrogen doping has been shown to cause perturbation of the host band structure. Determination of chemical state and nitrogen concentration is pivotal in defining the material’s electronic structure. X-ray photoelectron spectroscopy (XPS) is a commonly used technique for surface characterisation of this dilute N doped material.
In this application note we contrast the use of Al Kα and Ag Lα as excitation sources for XPS analysis of GaAs_1-xN_x. The advantage of using the higher photon energy excitation source to ‘remove’ overlapping Auger transitions with the N 1s core-level photoemission peak is demonstrated. Also introduced are advantages relating to the increased sampling depth provided by using the Ag Lα excitation for characterisation of a thin-film material.

Small area X-ray Photoelectron Spectroscopy in the 10’s of micron range has become an important tool for the surface analyst. Two approaches to producing small area XPS information are currently used in commercial XPS instrumentation. One approach employs a micro-focused X-ray probe to limit the sample area illuminated by X-rays during measurement. The second approach, utilised in the AXIS range of spectrometers, is to use a system of electrostatic lenses and apertures to limit the acceptance area of the analyser. In both cases the analysed area on the sample surface is determined by measuring the distance required to receive a 20 to 80% signal increase when scanning the spot across a sharp edge. This is standard practice as described in the literature. 

Published work [1] has highlighted the fact that signal can be obtained over an area many times greater than the quoted spot size when recording XPS data using X-ray microprobe technology. This investigation utilises an approach similar to the aforementioned work to determine quantitatively the total area measured during small area XPS measurements using the virtual probe approach.

[1] U. Scheithauer, Surf. Int. Anal., 40, 706-709 (2008).