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Physics of XPS

X-ray photoelectron spectroscopy is a technique used for the analyzing the elemental composition of the surface of materials. It is also known as also known as Electron Spectroscopy for Chemical Analysis (ESCA). It works by irradiating a sample with an x-ray beam and then quantifying the kinetic energy and number of electrons that are ejected from the material.

Figure 1 below illustrates the steps that occur during the XPS process. This technique works by having a high energy X-ray beam and using it to excite the surface of the desired sample. If the incoming X-ray has enough energy and is absorbed by an atom in the surface, and innermost electron will be ejected. This phenomenon is known as the photoelectric effect. Because the energy of an x-ray with a particular wavelength is known, the ejected photoelectron has a kinetic energy that can be calculated to be equal to the energy of the incident x-ray minus the binding energy and the work function of the element. By detecting and measuring the energy of this electron that is unique for each element, one is able to determine the composition of the sample.

Figure 1- XPS process. Incoming x-ray beam excites the atoms and as result emits a photoelectron.

Example of XPS

In this paper by the NTT Basic Research Laboratories of Japan, a new approach to increase the quality of graphene is explored. This carbon based material is used in electronics, and new methods of fabrication are being sought that improve its quality at a feasible cost. The course of action in this study was to grow graphene using gas-source molecular beam epitaxy (MBE). The growth took place at high temperatures, decreasing the growth rate, and increasing the material's quality.

The MBE growth took place in a ultra high-vacuum growth system (< 10-9 Torr) equipped with an ethanol-gas supply system with a Tungsten filament. The gas flow rate was set at 0.3 standard cubic centimetres per minute (sccm), thus a pressure of 2×10-3 Torr was obtained during growth. The W-filament temperature was set at 2000 degrees centigrade. Several growths took place at substrate temperatures between 600 and 915 degrees centigrade. The MBE growth time was 4 hours for every growth. The substrate used was graphene formed on n-type SiC(0001), around 1.3 monolayers thick.

The growth system used is connected to an analysis system equipped with a monochromatized Al Ka source and a photoelectron analyser. This allows the samples to be transferred at ultrahigh vacuum conditions between the systems. The XPS analysis was made before and after MBE growth. This took place at room temperature, with a pressure <2 × 10-9 Torr, and the takeoff angle of photoelectrons of 25 degrees from the surface. The XPS analyses results are displayed in Figure 2 below.

Figure 2- Photoelectron spectra for C 1s before (a) and after (b) MBE at 915 degrees centigrade. The constituent layers are denoted as SiC, graphene (G) and the interface (I).

A comparison between the peaks in the spectra shows that after MBE the surface is composed mostly of graphene, indicating that growth was achieved. The quality of the material was determined by measuring the film crystallinity. The conclusion of this study was that high temperature growth results in higher quality graphene.

References

•Wikipedia- XPS
http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy

•Fumihiko, M. (2011). Molecular beam epitaxial growth of graphene and ridge-structure networks of graphene. JOURNAL OF PHYSICS D: APPLIED PHYSICS, Retrieved from stacks.iop.org/JPhysD/44/435305

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