Surface Analysis - XPS

Why is cooling needed in the X-ray source?

Anodes are usually water cooled because the incident electron energy is converted into heat, therefore by cooling the anode, the source can operate at higher power without melting or degrading the anode.

What is Eb?

Binding energy is the energy a photon must nee or exceed in order to emit a core-electron from an atom. This varies based on the atom or material being studied.

Chemical shift

Change in binding energy Ex: as oxygen is bound to carbon, carbon becomes more positively charged, and therefore will have a higher binding energy because the electron will be more closely bound to the nucleus

When trying to do nondestructive depth profiling, how does the information acquired vary with take-off angle?

Each take-off angle will produce a different set of data about the composition in terms of depth. As the angle is increased in terms of the normal to the surface, the photoelectrons that are emitted from the surface of the sample originate from an increasingly surface localized zone.

How does the X-ray source work?

Fluorescence X-rays are used in XPS. A target or anode is hit with a ~10 keV electron beam, which emits fluorescence. The anodes are typically Aluminum for monochromatic sources and Aluminum and Magnesium for non-chromatic sources.

Why are binding energies in most XPS spectra internally referenced to a standard peak such as C-C/C-H?

For samples not in electrical contact with the spectrometer, an internal reference is used. Typically for polymer or organic samples, a standard peak such as C-C or C-H is used. By internally referencing the EB scale, accurate measurements of other EB values can be made

How does a hemispherical analyzer work?

Hemispherical analyzers are classified as dispersive analyzers because an electrostatic sphere deflects the electrons. It consists of two concentric hemispheres with a negative outer hemisphere and a positive inner hemisphere, with respect to the potential at the center line. The lens system before the spectrometer retards the kinetic energy down to the pass energy of the analyzer. This system projects the analyzed area away from the entrance of the analyzer, which makes the sample more accessible to the X-ray source. There is a range of electron energies that can enter and exit the analyzer without colliding into one of the hemispheres. This range of electron energies is based on the pass energy, the size of the entrance slits, and the angle that the electrons enter the analyzer.

What elements can XPS not detect?

Hydrogen or Helium (no core-electrons)

Why are multiple peaks observed for p and d orbitals?

Multiplet splitting occurs in p and d orbitals, because as the X-ray photon collides with the electron, it may emit either the spin-up or spin-down electron resulting in final states where the two electrons are separate. This leads to multiple observed peaks.

What determined the energy of an XPS source?

The X-ray emission energy is determined by the specific fluorescence line for each anode. Anodes can be made from different elements, and each element has a specific fluorescence line it will emit.

What is pass energy

The center line potential is also known as the pass energy. A constant pass energy is used, which ensures a constant absolute resolution, ΔE, for all photoelectron peaks. The signal intensity will decrease as the pass energy gets smaller. Usually, to get a high resolution scan, a pass energy of 5-25 eV is required. Whereas energy of 100-200 eV is required for lower resolution (survey) scan.

How would your choice of X-ray anode material affect sampling depth and the elements observable in the spectrum?

The higher the energy of the X-ray source, the more kinetic energy the electrons will have because kinetic energy of photoelectrons is equivalent to the difference between the source energy and the binding energy of the electron in the sample. With increased source energy, electrons further down in the sample will be able to escape the sample. Although this could give you more information about elements further within the sample, it does lead to a lower surface sensitivity.

How do we compensate for the charging of insulating samples with XPS?

This can be accomplished by flooding the sample with a monoenergetic source of low energy (<20eV) electrons. The energy of these electrons is varied to obtain a narrower width of photoemission peak. Avoids partial electrical contact with the spectrometer which can cause differential charging of the samples which leads to distorted peaks or peak broadening.

Auger electron

When an electron drops to a lower energy state, the atom can get rid of the excess energy by ejecting an electron from a higher energy level. The energy for an Auger electron is independent of the irradiation energy.

High resolution scan

With a high resolution scan the chemical states of the elements can be determined and the chemical composition can be found because there are enough data points for curve fitting of the small chemical shifts. One should always use a high resolution scan after getting a wide scan to obtain maximal information.

How does XPS work?

X-ray photons irradiate the surface of a material with high enough energy to transfer the energy from the photon to a core-level electron. If the energy of the photon exceeds the binding energy of the electron, then the electron will be emitted. This is an application of the photoelectric effect by Einstein (1905).

Why is XPS surface sensitive?

XPS is surface sensitive because with the typical X-ray source of 1 keV, the X-rays will penetrate 1000nm or more into the sample, while the electrons of this energy can only penetrate approximately 10nm. The electrons excited below the 10nm of the sample do not have enough energy to escape the sample and therefore are not able to emit from the sample. Since XPS measures emitted electrons, it is very surface sensitive.

Eb =

hv - KE hv: X-ray source KE: kinetic energy of the emitted electron

What is the smallest X-ray spot size achievable for a synchrotron-based XPS?

less than 150 nm

What is the smallest X-ray spot size achievable for a stand-alone XPS?

less than 50 μm

Survey scan

or wide scan, is usually taken initially. In a survey scan elements can be identified.