Carbon

Carbon • Non-Metals

Primary XPS region: C1s
Overlapping regions: Ru3d, Sr3p_1/2, K2p

Contents of carbon section:

• Adventitious Carbon Contamination
• Polymers
• Graphite, Graphene and Diamond
• Inorganic Carbon (e.g. Carbonates and Carbides)

Adventitious Carbon Contamination

Binding energies of common chemical states:

Charge referenced to adventitious C1s, C-C peak at 284.8eV.

• Most samples that have been exposed to the atmosphere will have a detectable quantity of adventitious carbon contamination (typically with a layer thickness of 1-2nm).
• Adventitious carbon may be removed by argon sputtering.
  • Use lowest energy argon beam available (e.g., 200eV or 500eV) or argon clusters to prevent/minimize chemical damage to the underlying material.

Adventitious carbon contamination is commonly used as a charge reference for XPS spectra.

• C1s spectrum for contamination typically has C-C, C-O-C, and O-C=O components.
• The C-C component may be set to a binding energy of 284.8eV, by default.
  • Not always a valid charge reference value (e.g., C1s peak for adventitious carbon on native oxide of aluminum appears at ~286eV).

A few nanometers of adventitious carbon on a sample can dramatically affect the survey spectrum of that sample.

• Quantification may be inaccurate when using peaks of significantly different energy.
• The intensity of higher binding energy peaks may be reduced relative to their counterparts at lower binding energy.
  • For example, the Na1s peak (1,071eV) may be weaker than expected, or completely disappear, but the Na KLL peak (497eV) remains strong.
• Inelastic scatter of electrons through the adventitious carbon layer may cause a large change in background shape when compared with the spectrum from a clean sample.

Binding energies of common chemical states:

• Acquisition of valence band can be useful for “fingerprint” identification of polymers.
  • Particularly useful in cases where C1s XPS data cannot easily distinguish between polymers, e.g. polyethylene and polypropylene.
     

• Some polymers are sensitive to prolonged X-ray exposure.
  • Different polymers have different degradation rates.
  • Minimize analysis time where possible.
    • C1s regions for polymers can often have acquisition times of <1min.
  • Acquire regions of interest first (e.g., C1s, O1s) before running survey, if possible.
  • If the sample surface is chemically uniform, it may be preferable to rapidly acquire data from many points and then integrate the data post-acquisition.
    • Overall acquisition time is then the sum of every point (giving good signal-tonoise statistics) but the amount of X-ray damage at each individual is negligible.
    • Good method for analyzing self-assembled monolayers.
  • Charge neutralization will be required.
  • Argon cluster source preferred for depth profiling of polymers.
    • Some polymers may be sputtered successfully with 200eV argon monomers.
    • Polymer materials containing F or Cl are often degraded by Ar⁺ monomer sputtering with loss of F or Cl.

• C-C peak at 284.8eV is frequently used a charge correction reference because of its convenience.
  • Not always reliable, possibly changing with nature of the surface hydrocarbon, film thickness and nature of the sample.
• C1s spectra for polymers tend to have symmetric peak components.
• Extended delocalised electrons in a sample (e.g. aromatic rings) can result in satellite structure, several eV to higher binding energy of the main peak.
  • π-π* satellite is seen 6eV from main C1s peak in polyethylene terephthalate, for example.
  • π-π* satellite can be confused with potassium’s K2p_3/2 peak.
    • Check for presence of K2p_1/2 and K2s peaks for confirmation of potassium.
       

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Binding energies of common chemical states:

• Acquire C1s, C KLL, and valence band regions.
  • Maximizes ability to distinguish between sp² and sp³ carbon.
  • C KLL acquisition conditions: Pass energy=100eV, step size=0.5eV, 100 scans, scan range binding energy=1,190eV-1,246eV.
• Charge neutralization may be required, especially for functionalized graphene/graphite or diamond.

• The C1s spectrum from a sample with high concentration of sp² carbon will have a broad, asymmetric tail towards higher binding energy.
  • One or more satellite features, several eV from the main C1s peak, will also be observed in samples with high sp² concentration.
• For high concentrations of sp³-bonded carbon, the C1s peak will have a more symmetric shape and will also be slightly shifted to higher binding energy.

Functionalized graphene will have a complex C1s spectrum, containing sp² and sp³ components.

• Fit the sp² component with asymmetric shape (preferably derived from a real graphite or graphene sample), including symmetric loss peaks where necessary.
• The sp³ carbon and functionalized carbon should be fitted with symmetric peak shapes.
  • The sp³ carbon peak should be 1eV to the higher binding energy side of the sp² component.