What is X-ray photoelectron spectroscopy and what does it measure?

X-ray photoelectron spectroscopy (XPS) is a non-destructive surface analytical technique that measures elemental composition, chemical state, and electronic state of materials within the top 10 nanometers. X-rays strike atoms, ejecting core shell electrons whose kinetic energies are measured. By calculating binding energies from these measurements, researchers determine the material's surface chemistry and composition.

How does the photoelectric effect enable XPS signal generation?

When a core shell electron absorbs a highly energetic X-ray photon, it gains enough energy to escape its orbit as a photoelectron. The excess energy beyond the work function threshold becomes the electron's kinetic energy. This photoelectric effect, which earned Albert Einstein a Nobel Prize, underpins XPS signal generation and allows researchers to determine original binding energies from measured kinetic energies.

What information can be extracted from comparing XPS peaks to reference samples?

Subtle shifts in binding energies relative to reference compounds reveal the chemical state of elements in the sample. The intensity ratio between peaks indicates surface composition. These comparisons allow researchers to identify elemental composition, detect contaminants, and determine electronic states. XPS is particularly valuable for characterizing semiconductor films and analyzing material surfaces for microelectronics manufacturing.

How does pass energy affect XPS spectrum quality and resolution?

Pass energy determines the energy at which all photoelectrons enter the spectrometer, setting constant resolution across the spectrum. High pass energy increases photoelectron flux and signal-to-noise ratio but reduces resolution. Low pass energy improves resolution but decreases signal-to-noise ratio. Researchers must balance these trade-offs based on their analytical requirements.

Why is a survey spectrum collected before high-resolution XPS analysis?

A survey spectrum covers a broad energy range to identify all photoelectron emission peaks from the sample surface. This initial scan reveals which elements are present and their approximate binding energy locations. Once a region of interest is identified from the survey spectrum, researchers can then collect high-resolution spectra of specific elements for detailed chemical analysis.

What sample handling procedures are essential for XPS analysis?

Proper cleanliness is critical for ultra-high vacuum XPS systems. Samples must be handled with powder-free nitrile gloves and tweezers, stored in covered glass containers, and transported safely. Sample holders should be cleaned thoroughly with isopropyl alcohol before use. The load-lock chamber must be vented and pumped to specific pressure ranges (10^-7 to 10^-8 millibar) before analysis begins.

What types of materials can XPS analyze and why is it valuable for research?

XPS analyzes diverse materials including metal alloys, ceramics, polymers, semiconductors, and biological samples. For microelectronics, XPS characterizes thin semiconductor films and detects contaminants to improve manufacturing. XPS also enables researchers to relate novel semiconductor properties to their chemistry, critical for developing new materials. Additionally, XPS analyzes fossilized biological samples to reveal evolutionary and environmental information.

X-ray Photoelectron Spectroscopy

0 views09:20 min

The following procedure applies to a specific XPS instrument and its associated software, and there may be some variations when other instruments are used.

The sample is a thin film of Pt (3 atomic layers thick) grown on a single layer of graphene, which is supported on a commercial silica (SiO₂) glass slide. The graphene (which is a single layer of carbon) was grown on Cu and then transferred to the glass substrate. The Pt atomic layers were then deposited by electrodeposition methods.
To load the samples, first vent the loadlock to get the sample holder. Make sure to follow cleanliness rules for UHV systems. These include: no bare skin, hair, or moisture in contact with samples or sample holder. Use clean tweezers to handle your sample. The door of the loadlock will spring open after the loadlock chamber has vented to atmospheric pressure (~5 minutes). Take out the sample holder.
The sample is held by the spring clips which should be on the stage. Wipe down the stage surface and the clips you are using with alcohol and dry completely.
a.Open the loadlock door and put the sample holder back on the transfer arm. The holder will only fit correctly one way.
b.Close the door and pump down the load lock for about 10 minutes, although you may need extra pumpdown time for some samples (e.g., if it is highly porous, a powder, or contains some unevaporated solvent) and then transfer the sample into the analysis chamber. Note the analysis chamber pressure when the sample transfer occurs. It should be in the mid-to-low 10^-7 mbar range then drop rapidly into the 10^-8 mbar range when the valve between the two chambers closes.
Check the analysis chamber pressure. It should be in the mid 10^-8 mbar range or below when an experiment is started.
Set the pass energy. The pass energy is the energy with which all the photoelectrons will enter the spectrometer and ensure that all the features measured in the spectrum have the same energy resolution. This is because the energy resolution of the spectrometer, ΔE, scales with kinetic energy of the photoelectrons, and so having a constant energy for all electrons entering the detector sets a constant resolution for the whole spectrum. The higher the pass energy, the higher the flux of electrons entering the detector and, thus, better signal-to-noise ratio (s/n), but this comes at a cost of worse energy resolution (larger ΔE). Conversely, a lower pass energy ensures a better energy resolution but at a cost of lower flux and, therefore, a lower s/n.
Collect a survey spectrum. Here your goal is to capture all the various types of electrons that are being ejected from the sample, and be able to, therefore, survey the elemental content of your sample. The detector (spectrometer) will therefore have to be set up to try and capture as many of these types of electrons as possible. You can do this by setting the widest energy scan range for the spectrometer. The specific software controls will vary for the different commercial XPS systems. The survey spectrum allows you to inspect all the photoelectron emissions within your sample prior to doing a high-resolution scan on specific emissions.
For our representative spectrum we have an SiO₂ supported architecture, that also contains C and Pt. The Pt, Si, C and O core-level peaks are labeled in the survey spectrum (Figure 1). Due to the ubiquity of water, oxygen and hydrocarbon molecules in air, a certain amount of these molecules are always expected to be physically or chemically adsorbed on the surface of any sample, and so a C and O signal is almost always expected.
Collect the characteristic core level spectra of your material. Here as the representative spectrum, we show the 4f spin-orbit split peaks for Pt.