X-Ray Photoelectron Spectroscopy Surface Analysis Techniques

X-ray photoelectron spectroscopy and related techniques for surface analysis

Achieving a comprehensive understanding of sample surfaces requires the optimal utilization of X-ray photoelectron spectroscopy (XPS) and related surface analysis techniques. While single point XPS analysis provides valuable insights, additional analytical methods can offer deeper comprehension of surface composition and chemistry. Explore these integrated approaches to construct a comprehensive surface profile.

X-ray photoelectron spectroscopy of small areas

Small area or selected area X-ray photoelectron spectroscopy (SAXPS) is used for analyzing small features on a solid surface, such as surface blemishes or particles. This technique maximizes the detected signal from a specific area while minimizing the signal contribution from the surrounding area.

Insulator analysis

In cases where the surface is electrically insulating, the accumulation of positive charge on the surface significantly impacts the XPS spectrum. Charge compensation counteracts this effect by supplying electrons from an external source, neutralizing the surface charge. This process stabilizes and regulates the charging to maintain the surface in a nearly neutral state, within a few electron volts.

XPS depth profiling

Depth profiling involves the controlled removal of material using an ion beam, followed by data collection at each etching step. This technique provides a high-resolution composition profile, allowing the examination of composition changes from the surface to the bulk. Depth profiles are useful for studying phenomena like corrosion, surface oxidation, and the chemistry of material interfaces.

Monatomic and gas cluster ion-depth profiling

The Thermo Scientific MAGCIS Dual Beam Ion Source for XPS Instruments enables depth profiling analysis and surface cleaning of soft and hard materials on a single XPS instrument. New gas cluster ion sources enable the analysis of several classes of materials previously inaccessible to XPS depth profiling.

XPS imaging

X-ray photoelectron spectroscopy (XPS) is not only used for identifying points or small features on a surface but also for imaging the surface of a sample. This is useful in understanding the distribution of chemistries across a surface, for finding the limits of contamination, or examining the thickness variation of an ultra-thin coating. Two approaches, mapping (serial acquisition) and parallel imaging (parallel acquisition), are used to obtain XPS images.

Angle-resolved XPS

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) collects electrons at varying emission angles, enabling electron detection from different depths. This technique provides valuable insights into the thickness and composition of ultra-thin films.

Hard X-ray photoelectron spectroscopy

Employing X-ray sources with higher energy levels than the standard Al K-alpha source can enhance XPS analysis. This is commonly referred to as HAXPES. The higher photon energy enables deeper analysis and also allows access to core levels that are otherwise inaccessible. It can also be combined with X-ray induced Auger features to generate Wagner plots, aiding in the interpretation of surface chemical states.

Correlative Imaging and Surface Analysis Workflow

Harness the combined powers of SEM and XPS with the Thermo Scientific Correlative Imaging and Surface Analysis (CISA) Workflow. By integrating datasets from our XPS and SEM instruments, you can gain deeper insights into your samples.

 

To gain a comprehensive understanding of a sample, analysis of different instruments is often necessary. While imaging a sample in an SEM with EDX provides composition information, it may not reveal crucial surface chemistry. On the other hand, XPS offers surface chemistry details but may lack high-resolution imagery to explain the interplay between chemistry and structure. The CISA Workflow bridges this gap, enabling comprehensive sample understanding.

Additional surface analysis techniques

Bar chart showing common properties and processes as a function of material depth or thickness, measured by the number of atomic layers.
Bar chart showing common properties and processes as a function of material depth or thickness, measured by the number of atomic layers.

Ion scattering spectroscopy

Ion scattering spectroscopy (ISS) or low-energy ion scattering (LEIS) is a highly surface-sensitive technique that probes the elemental composition of the first atomic layer of a surface. It utilizes a beam of noble gas ions that scatter from the surface, and the kinetic energy of the scattered ions is measured.

 

Conservation of momentum, along with information known about the energy of the incident beam, the mass of the ion, the scattering angle, and the energy of the scattered ion, can be used to calculate the mass of the surface atom.

 

ISS is effective because it interacts only with the outermost surface layer, making it valuable for studying surface segregation and layer growth and complementing the composition information obtained from XPS.

Reflected electron energy loss spectroscopy

Reflected electron energy loss spectroscopy (REELS) is a technique used to probe the electronic structure of the material at the surface. It works in a similar fashion to ISS, but in this case, the incident particle is an electron, and it is the scattered electron beam that is measured.

Energy losses in the incident electrons, resulting from electronic transitions in the sample, are measured in the REELS experiment. REELS allows for the measurement of properties like electronic band gaps and relative energy levels of unoccupied molecular orbitals. Additionally, REELS has the advantage of being able to detect hydrogen in some cases, which is not possible with XPS.

Typical analysis depth ranges for surface analysis techniques used to complement XPS, including ISS, REELS, UPS, Raman, and AES
Typical analysis depth ranges for surface analysis techniques used to complement XPS, including ISS, REELS, UPS, Raman, and AES

UV photoelectron spectroscopy

UV photoelectron spectroscopy (UPS) is a similar technique to XPS, but it utilizes UV photons instead of X-ray photons to excite photoelectrons from the surface. As UV photons have lower kinetic energy, the detected photoelectrons are from the lower binding energy levels involved in bonding. Although this complex region has overlapping peaks, it serves as a distinctive fingerprint for compounds.

Comparing UPS data with XPS data is often beneficial. UPS complements REELS data in understanding electronic properties by providing information on the highest energy occupied bonding states.

 

The width of a photoelectron spectrum can be used to measure the work function on suitable samples using a relatively short full range scale (0–22 eV or 40 eV) rather than 0–1487 eV for Al Ka X-rays).

Materials classes used for surface analysis techniques used to complement XPS, including ISS, REELS, UPS, Raman, and AES
Materials classes used for surface analysis techniques used to complement XPS, including ISS, REELS, UPS, Raman, and AES

Raman spectroscopy

Raman spectroscopy provides insights into molecular bonding in materials. This technique is very sensitive to structural changes. It involves the scattering of photons from a laser source, covering a range of wavelengths from infrared to UV. A portion of the incident photons undergo Raman scattering, interacting with vibrational modes in the sample and losing energy. The resulting scattered photons are detected to generate a spectrum.

 

Raman spectroscopy offers a larger depth of analysis compared to other techniques discussed. Its complementary information is particularly valuable for understanding polymers, where bulk and surface information are complementary, as well as nanomaterials like graphene and carbon nanotubes, where depth scales are correlated.

Auger electron spectroscopy

Auger electron spectroscopy (AES) is a technique that employs a focused electron beam to excite the sample and analyze surface composition. It relies on the Auger emission process, which occurs when an atom relaxes after electron emission. This relaxation involves filling a shell vacancy with an electron from another orbital, resulting in the release of excess energy and the emission of another electron.

 

While Auger features can be observed in XPS spectra, AES provides elemental and some chemical state information, serving as a valuable complement to XPS analysis. Notably, AES offers superior spatial resolution compared to XPS.

For Research Use Only. Not for use in diagnostic procedures.