Ultraviolet Photoelectron Spectroscopy (UPS)

Ultraviolet Photoelectron Spectroscopy
He(I) UPS spectra from 10 nm fullerene (C60) film on CaF2 substrate before any surface cleaning (by ion cluster etch), and after initial surface cleaning. As received, the sample showed overlaying amorphous carbon contamination that obscured measurement of the C60 film.

Ultraviolet photoelectron spectroscopy (UPS) is often performed in conjunction with X-ray Photoelectron Spectroscopy (XPS), a powerful surface chemical characterization technique.

Unique to UPS, its selective low-energy signal photoelectrons enables specific measurement of valence-band properties, as well as the electronic work function of the surface.

Strengths
  • Very high surface specificity: information depth 2-4 nm
  • Highly accurate spectral fingerprinting
  • Selective valence-band signal: bonding orbital properties
  • Reliable work function measurements possible
Limitations
  • UPS peaks are not used for elemental quantification: degree of hybridization and extent of BE-shift is too substantial
  • Robust density of electronic states / orbital occupancy typically requires Advanced Modeling
  • Work function measurement requires careful setup to isolate sample low-KE cutoff from detector behavior
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Ultraviolet Photoelectron Spectroscopy (UPS)
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Example Outputs

He(I) UPS spectra from 10 nm fullerene (C60) film on CaF2 substrate before any surface cleaning (by ion cluster etch), and after initial surface cleaning. As received, the sample showed overlaying amorphous carbon contamination that obscured measurement of the C60 film.

He(I) UPS spectra from 10 nm fullerene (C60) film on CaF2 substrate at 2 more etch times. After 900 second etch, the valence electron structure has shifted away from the C60 bonding configuration (shown above).

Instruments Used for UPS
Thermo Scientific Nexsa

Thermo Scientific Nexsa

  • Spot Size: 1 mm
  • Sensitivity: depends on the noble gas species used for the ion beam
  • X-ray Source: monochromated, micro-focused, high-efficiency Al Kα X-ray Anode

View Instrument Spec Sheet

Sample Requirements
  • Solid phase
  • Must be stable under ultra-high vacuum conditions
  • Maximum Sample Dimensions: 60mm (L) x 60mm (W) x 20mm (T)
How UPS Works

In UPS measurements, a high-energy laser beam is applied to the sample surface, and characteristic photoelectrons are emitted via the photoelectric effect. The emitted photoelectrons are then analyzed by energy to produce a spectrum of peaks associated with the quantitative concentration of specific elemental species.

Unlike XPS, UPS uses an ultraviolet-laser source, which limits the amount of energy imparted to the surface atoms.

As a result, UV photoelectrons are selectively emitted from outer (valence-band) orbitals, and the information depth is more surface specific. These features enable more detailed analysis of the valence-band properties and bonding states and provide a direct measurement of work function.

The ability to clean the surface using Argon ion clusters ensures that work function measurements can be done with clean rather than environmentally contaminated surfaces.

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