XPS is the most widely used surface analysis technique that provides both qualitative and quantitative insights on the chemical composition and chemical states, i.e. the chemical environment of an element, in a non-destructive manner. XPS is highly surface-sensitive, probing merely the first 10 nanometres of a sample surface with conventional instruments. Argon-ion and argon-clusters can be used to remove the surface layer-by-layer (so-called sputtering), to generate sputter depth profiles.

Since more than a decade our group collaborates on a scientific basis with Thermo Fisher Scientific Application Lab (East Grinstead, England). Since 2009 we are a K-Alpha-Demo-Lab.

Equipment:

• Thermo Fisher Scientific K-Alpha XPS-Spectrometer with integrated glovebox for moisture- and air-free sample transfer

• Thermo Fisher Scientific K-Alpha⁺ XPS-Spectrometer equipped with a Ar-cluster source

• VG ESCA5 multimethod spectrometer equipped with a Thermo Fisher Scientific Alpha 110 electron energy analyser.

Example from the field of electrochemistry and battery research

XPS is frequently applied in electrochemistry for the characterisation of the electrode-electrolyte interface on corrosion processes. Some interfaces form passivation layers that are surface layers that protect the underlying material from parasitic reactions. For instance, many metals form a passivation layer upon reaction with oxygen. On the other hand, continuous corrosion occurs as a result of insufficient passivation and is commonly observed for iron for instance that forms rust in presence of oxygen and moisture. Other metals such as aluminium for a dense and protective oxide layer that curtails any further reaction with oxygen after a certain layer thickness was reached.

As prominent subdomain in electrochemistry, battery research investigates for example the formation of the so-called ‘solid electrolyte interphase’ (SEI) that is responsible for long cycle life in Li-Ion batteries. The SEI is formed during decomposition reactions of the electrolyte at the electrode interface. The resulting products are partly insoluble and deposit on the electrode surface. The resulting layer can prevent further degradation reactions, while also protecting the electrode material underneath on following charge-discharge cycles. In our group we investigate the interphases formed in different electrolyte systems with the aim to understand the SEI formation in more detail and to influence the formation process through deliberate choice of electrolyte additives. In Figure 1, a typical spectrum of the chemical environment of carbon is shown. Different chemical functional groups appear at different binding energies, which allows a classification of into various organic and inorganic components that are present at the interface. Their relative intensities, i.e. the proportion to the overall signal, further allows us to assess the composition of the SEI layer, which has a direct impact on the degree of protection.

In more general terms, XPS is a powerful tool for material characterization of multicomponent systems, especially when in cases of layered structures. As a result of the above mentioned surface sensitivity of XPS, only the topmost layers are visible. In combination with sputter depths profiles, layers can be removed gradually, allowing to track the changes in chemical composition as a function of sputter time. This is shown in figure 2, where the concentration of selected elements changes as more layers are removed.

ToF-SIMS is a complex surface analytical technique that is available in only a number of labs worldwide. It is also a powerful tool to acquire complementary data to XPS, as it provides elemental and molecular information with high lateral and axial (depth) resolution.

In ToF-SIMS a focused, high-energetic primary ion beam is directed onto a surface causing the formation of monoatomic or molecular fragments to be formed where the beam hits the sample surface. These fragments are enter the gas phase and are directed into a time-of-flight mass analyser where fragments are counted and separated by their molecular mass. The fragmentation of compounds is characteristic and thus their mass spectrum allows conclusions about the compound it originated from.

The technique is highly surface sensitive with a information depths of around 5 nm. The lateral resolution is obtained by scanning the ion beam across the sample surface. On structured surfaces, the mass spectrum in different locations of the sample will thus look different, dependent on the chemical composition in that particular spot. ToF-SIMS has a large range of applications, including analysis of polymers, self-assembling monolayers on a substrate and surfaces in technical applications (e.g. abrasion of materials). Another popular application is the depth profile and 3D imaging analysis, using a combination of primary ion beam and sputtering by cesium-, oxygen- or Ar-ion clusters.

For surface sensitive samples, the spectrometer can be accessed via atmosphere contact free sample transport. We perform ToF-SIMS on a rich variety of different topics, including battery research, organic compounds, semi-conductors or hydrogen-diffusion in materials to name but a few examples from our portfolio.

Equipment:

• ION-TOF GmbH ToF.SIMS 5 Spektrometer ausgestattet mit Ar-Clusterionenquelle

Scanning electron microscopy (SEM; Fig. 3) is used to determine the topology of a sample with high spatial resolution. Compared to an (optical) light microscope, that reaches up to 1.500 x magnification, SEM reaches magnifications of up to 100.000 x and higher resolution. For instance, a virus with size between 30 to 250 nm or proteins in the size range of 10 nm can be readily visualized. Because of the interactions of the electrons with matter, an elemental contrast is can also be achieved, allowing to distinguish areas of different chemical compositions. The instrument is equipped with a detector for energy-dispersive X-ray spectroscopy (EDX/EDS) that is commonly used to distinguish the distribution of elements in a sample for easily.

For our battery research we use SEM frequently to analyse (powder) samples in the size range from the low nanometer (nm) to mid micrometer (µm) range. In addition, SEM is a commonly used tool to prepare images of electrode coatings prior and after aging in a battery. Our lab uses ion milling as an additional tool to generate cross-sections of samples to learn details about porosity and (electrode) structure (Fig. 4).

Equipment:

• Zeiss GmbH FE-SEM MERLIN scanning electron microscope

• PECS II, Model 685 (Gatan)