A recent webinar explored the relationship between SEM/EDS and XPS as complementary techniques for surface and materials characterization.

Rather than focusing on one technique in isolation, the discussion emphasized how differences in interaction physics, sampling depth, and signal generation lead to different — but equally valid — perspectives on the same material.

The following points summarize the main technical conclusions from the session.

1. There is no single technique that solves all characterization problems

A central theme throughout the webinar was that materials characterization is inherently multi-technique.

No method provides:

• complete spatial resolution
• full chemical information
• and depth-resolved data

This is not a limitation of individual instruments, but a consequence of the underlying physics of how signals are generated and detected.

2. Sampling depth is the key parameter when comparing techniques

The most important distinction between SEM/EDS and XPS is how deep into the material they probe:

• SEM/EDS: information from microns (depending on accelerating voltage)
• XPS: information from the top few nanometers (~10 nm)

As a result:

• Subsurface features may dominate EDS signals
• XPS reflects only the outermost surface

This explains why the same sample can yield very different results depending on the technique used.

3. SEM can approach surface sensitivity — but not surface chemistry

At low landing voltages, SEM imaging becomes highly surface sensitive:

• Secondary electrons originate from the top ~10 nm
• Further filtering (energy-selective detection) increases surface specificity

However, SEM remains fundamentally:

• an imaging technique with contrast mechanisms
• not a direct probe of chemical composition or bonding

Surface contrast in SEM may arise from:

• composition
• topography
• charging
• surface potential

Without complementary analysis, interpretation remains ambiguous.

4. XPS provides chemically specific information from the outermost surface

XPS is uniquely suited for analyzing:

• elemental composition
• chemical states (e.g. oxidation, bonding)
• quantitative surface composition

Its surface sensitivity arises from the limited escape depth of photoelectrons, governed by the inelastic mean free path.

This enables analysis of the top few nanometers — the region that often determines:

• adhesion
• corrosion behavior
• catalytic activity

5. Differences between techniques are not contradictions — they are complementary views

Examples discussed in the webinar demonstrated that:

• A buried structure can be visible in EDS but invisible in XPS
• A surface contamination layer may dominate XPS but be negligible in EDS

These differences reflect what each technique is sensitive to, not inconsistencies.

Understanding these differences is essential for correct interpretation.

6. Geometry and topography influence both SEM and XPS results

Both techniques are affected by sample geometry:

• In XPS:
• X-ray shadowing can reduce signal in certain regions
• This may create misleading chemical maps
• In SEM/EDS:
• Detector position can lead to shadowing in elemental maps

This reinforces the importance of combining:

• imaging (SEM)
• and analytical techniques (EDS, XPS)

to avoid misinterpretation.

7. XPS can provide limited depth information, but only near the surface

Depth-related information in XPS can be obtained through:

• variation of photon energy
• angle-resolved measurements
• signal attenuation models

This allows:

• thickness estimation of ultra-thin films
• identification of buried interfaces within ~10–15 nm

However:

• XPS is not suitable for thicker layers
• Other techniques (e.g. SEM/EDS, GD-OES, XRF) are required for deeper analysis

8. Lateral resolution remains a key limitation of XPS

While XPS can perform imaging and mapping:

• Typical spatial resolution is in the tens of microns

This contrasts with SEM:

• Nanometer-scale resolution

As a result:

• XPS provides averaged chemical information over larger areas
• SEM is needed for precise localization of features

9. Both techniques face challenges when analyzing nanoscale materials

For XPS:

• Sampling depth becomes comparable to particle size
• Quantification assumes lateral uniformity
• Charging and sample preparation become critical

For SEM/EDS:

• Detection of very thin coatings (~1–2 nm) is difficult
• Sensitivity depends strongly on detector geometry
• Quantitative mapping at this scale is challenging

This highlights the need for careful method selection — and sometimes additional techniques.

10. Practical workflows benefit from combining techniques

The webinar consistently pointed toward correlative analysis:

• SEM → morphology and feature localization
• EDS → elemental composition at larger depth
• XPS → surface chemistry and chemical states

Each technique answers a different part of the same question.

Conclusion

The key message from the webinar is straightforward:

Understanding materials — especially surfaces — requires understanding what each technique actually measures.

• SEM can reveal extremely fine surface features
• XPS explains the chemistry of those features
• Differences between results are expected and informative

Rather than choosing between techniques, the goal is to use them together to build a consistent, multi-scale understanding of the material.