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XPS works

X-ray photoelectron spectroscopy (XPS) is a surface characterization technique that can analyze a sample to a depth of 2 to 5 nanometers (nm). Kai Siegbahn, who won the Nobel Prize in physics in 1981 for his research, developed XPS in the 1960s. XPS reveals which chemical elements are present at the surface and the nature of the chemical bond that exists between these elements. It can detect all of the elements except hydrogen and helium.

Photoemission principle: When an x-ray (red arrow) bombards a sample (left), some electrons (yellow spheres) become excited enough to escape the atom (right).

XPS is conducted in ultrahigh vacuum (UHV) conditions, around 10-9 millibar (mbar). Atmospheric pressure is about 1 bar, which means that the number of atoms of gas in a UHV chamber is one-trillionth that of air per unit of volume. The ambient atmosphere that a sample is exposed to can change its properties. For example, at a pressure of 10-6 mbar, background gas constituents (O2, H2O, etc.) can react with the surface of a sample in several seconds. Even under optimal instrumental conditions this is too little time in which to conduct an experiment. However, at UHV pressures it takes hours before a sample significantly degrades, thus enabling accurate surface interrogation using XPS.

Irradiating a sample with x-rays of sufficient energy results in electrons in specific bound states to be excited. In a typical XPS experiment, sufficient energy is input to break the photoelectron away from the nuclear attraction force of an element. Two key features are derived from XPS data. The first is that even photo-ejected electrons from core levels have slight shifts depending on the outer valence configuration of the material examined. The second is that the specific energy of an elemental core level transition occurs at a specific binding energy that can uniquely identify (and in favorable cases quantify) the element.

In a typical XPS spectrum some of the photo-ejected electrons inelastically scatter through the sample enroute to the surface, while others undergo prompt emission and suffer no energy loss in escaping the surface and into the surrounding vacuum. Once these photo-ejected electrons are in the vacuum, they are collected by an electron analyzer that measures their kinetic energy. An electron energy analyzer produces an energy spectrum of intensity (number of photo-ejected electrons versus time) versus binding energy (the energy the electrons had before they left the atom). Each prominent energy peak on the spectrum corresponds to a specific element. In the spectrum below, there is a peak at 284.6 electronvolts (eV), which corresponds to carbon, and a peak at 532.5 eV, which corresponds to oxygen; therefore, this sample contains carbon (C) and oxygen (O).

Besides identifying elements in the specimen, the intensity of the peaks can also tell how much of each element is in the sample. Each peak area is proportional to the number of atoms present in each element. The specimenŐs chemical composition is obtained by calculating the respective contribution of each peak area.

By applying relative sensitivity factors and appropriately integrating peak areas, it can be determined that the sample below is 25 percent oxygen and 75 percent carbon.

By studying the energy of the carbon peak, it can also be determined if the surface of this material corresponds to a C-O single bond (ethers, alcohols) or a much stronger C=O double bond (carboxylates, ketones). Core level shifts are important in determining valence states in metals, transition metal oxides, and actinide materials.

Information for this article was taken from Thermo Electron Corp.'s website on XPS (www.lasurface.com)


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