X-ray photoelectron spectroscopy (XPS) is also known as electron spectroscopy for chemical analysis (ESCA). It uses an analytical instrument to obtain the chemical composition of a material surface and works on the principle of the photoelectric effect. When a beam of X-rays with photons of energy, hν, is irradiated on a solid surface, it ejects electrons from the inner shells of atoms present on the sample surface. The kinetic energy (KE) of the ejected electrons (photoelectrons) is measured. The binding energy (BE ) of the photoelectrons is then calculated using the following relationship inEq. (4.7):
BE¼ hν − KE: ð4:7Þ
In the above equation, BE is the binding energy of the photoelectrons, hν is the energy of the photons (where h is Planck’s constant (6.626 10–34Js) andν is the frequency of the radiation), and KE is the kinetic energy of the photoelectrons. Since the binding energies of electrons coming from the inner shells of different atoms are readily available in the scientific literature, it is easy to identify the type of elements present on the surface.
XPS consists of an X-ray source, an ultrahigh vacuum chamber; an electron energy analyzer, and a computer to process the data (Figure 4.11). X-ray sources such as aluminum Kαwith photon energy of 1486.7 eV and magnesium Kαwith photon energy of 1253.6 eV are typically used in XPS. The samples to be analyzed are placed in an ultrahigh vacuum chamber to avoid potential contamin- ation on the sample surface and to maximize the photoelectrons reaching the analyzer effectively. An electron energy analyzer measures the kinetic energy of the photoelectrons. Software in the attached computer then processes the data and converts the kinetic energy to binding energy.
XPS detects the presence of all elements except hydrogen and helium. It also provides information regarding the chemical bonding of an element. The binding
energies of electrons coming from an atom can be different depending on its bonding to the neighboring elements. For example, a carbon atom can bind to an oxygen atom through a single bond or double bond. Table 4.2 provides the binding energies of carbon at different bonding states.
Typically, the XPS spectrum is presented with BE on the y-axis and intensity of the peaks on the x-axis. As an example,Figure 4.12shows the chemical structure offlufenamic acid (FA), an anti-inflammatory drug, andFigure 4.13shows a high resolution XPS C 1s spectrum of FA. The high resolution C 1s spectrum is deconvoluted intofive components, with the peaks of the components, C 1s (1), C 1s (2), C 1s (3), C 1s (4), and C 1s (5) observed at 285 eV, 286 eV, 289.5 eV, 291.2 eV, and 293.1 eV, respectively. The peaks of C 1s (1), C 1s (2), C 1s (3), C 1s (4), and C 1s (5) are assigned to carbon atoms in C―C, C―O, C═O, π!π* shake-up satellite from the aromatic rings, and C―F3bonds, respectively.
The sampling depth of X-rays in XPS is 1–10 nm and so, it provides infor- mation regarding the top 1–10 nm of a material surface. The detection limit varies from 0.1 atom% to 1.0 atom%, and the minimum analysis area varies from 10μm to 200µm depending on the design of the instrument. Although XPS is a surface sensitive technique, depth profiling can provide elemental information regarding the material’s subsurface. During depth profiling, a chemically inert beam of energetic ions is used to etch the sample surface and the new surface thus obtained is analyzed using XPS. This process is continued to obtain elemental composition for up to a depth of 1 µm. Thus, XPS depth profiling is very valuable in quantifying elements as a function of sample depth.
Computer X-Y Recorder Amplifier and Ratemeter Electron Detector Source Slit Sample X-Ray Anode X-Ray Monochromator Crystal hv Lens System Energy Analyzer − + Figure 4.11
Schematic of an X-ray photoelectron spectrometer. (Sibilia, J. P.,A Guide to Materials
Characterization and Chemical Analysis, page no. 169 (1988). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)
Figure 4.14 shows an example of an XPS depth profile for a nitinol surface which was finished under different methods: (a) mechanically polished (MP); chemically etched (CE); chemically etched and boiled in water (CEWB).1 The nickel concentration is slightly lower for CEWB at the top surface when compared Table 4.2 Binding energies of carbon at different bonding states
Chemical species Binding energy (eV)
C―C 285 C―O 286.5 C═O 289 C―F 290 C―F2 292 C―F3 293 296 293.1 eV 291.2 eV 289.5 eV 286 eV 285 eV Intensity (Arbitr ar y Units) 294 292 290
Binding Energy (eV)
288 286 284 282
Figure 4.13
High resolution XPS C 1s spectrum of flufenamic acid. F F F H N HO O Figure 4.12
to that of CE. However, there is no major difference observed in the concentration of titanium between CEWB and CE. The depth profile of MP showed that the nickel concentration is greater at the top surface when compared to that of CE and CEWB. The concentration of titanium is lower for MP when compared to that of CE and CEWB. Thus, XPS depth profiling is very useful in studying the concen- tration of elements at different depths of the material surface.
XPS provides information regarding chemical composition of a material surface.
It detects all elements except hydrogen and helium, and provides chem- ical bonding information of an element.
XPS depth profiling provides elemental information regarding the sub- surface for up to a depth of 1µm.
Other methods of evaluating the subsurface include the use of angle-resolved XPS. As grazing take-off angles provide more surface information compared to
0 0 10 20 30 40 50 60 70 5 10 Surface Depth (nm) Atomic Concentration (%) 15 Ti2p mp Ti2p CeWb Ni2p3 mp Ti2p Ce Ni2p3 CeWb Ni2p3 Ce 20 Figure 4.14
XPS depth profiles of Ni and Ti for mechanically polished (MP), chemically etched (CE), and chemically etched and boiled in water (CEWB). (Reprinted fromBiomaterials, 30, Shabalovskaya, S. A., Rondelli, G. C., Undisz, A. L.et al., The electrochemical characteristics of native Nitinol surfaces, 3662–3671, Elsevier (2009), with permission from Elsevier.)
angles close to the surface normal, reduction in the photoelectron take-off angle allows XPS information to be obtained from decreasing depths. The use of angle- resolved XPS permits the estimation of thefilm thicknesses and thickness of the contaminated layer.