X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 50-60 atoms, 0.01 um, 5-10 nm of any surface. It belongs to the family of <a href="/facts/Photoemission_spectroscopy/UPvrQ8zD">photoemission spectroscopies</a> in which electron population <a href="/facts/Spectrum/wPUjkpqI">spectra</a> are obtained by irradiating a material with a beam of <a href="/facts/X-ray/wseP79cN">X-rays</a>. XPS is based on the <a href="/facts/Photoelectric_effect/gIDcdm5U">photoelectric effect</a> that can identify the elements that exist within a material (elemental composition) or are covering its surface, as well as their <a href="/facts/Chemical_state/wVgcbY7x">chemical state</a>, and the overall electronic structure and density of the electronic states in the material. XPS is a powerful measurement technique because it not only shows what elements are present, but also what other elements they are bonded to. The technique can be used in line profiling of the elemental composition across the surface, or in depth profiling when paired with <a href="/facts/Ion_beam/e23WW7uC">ion-beam etching</a>. It is often applied to study chemical processes in the materials in their as-received state or after cleavage, scraping, exposure to heat, reactive gasses or solutions, ultraviolet light, or during <a href="/facts/Ion_implantation/PHMokNUf">ion implantation</a>.
Chemical states are inferred from the measurement of the <a href="/facts/Kinetic_energy/aJRxUd4p">kinetic energy</a> and the number of the ejected <a href="/facts/Electron/L0KBo5cW">electrons</a>. XPS requires high vacuum (residual gas pressure p ~ 10−6 Pa) or <a href="/facts/Ultra-high_vacuum/K3Xu5K7I">ultra-high vacuum</a> (p < 10−7 Pa) conditions, although a current area of development is ambient-pressure XPS, in which samples are analyzed at pressures of a few tens of millibar.
When laboratory X-ray sources are used, XPS easily detects all elements except <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> and <a href="/facts/Helium/yRul93TG">helium</a>. The <a href="/facts/Detection_limit/Qd2cAguT">detection limit</a> is in the <a href="/facts/Parts_per_thousand/PozRuoLM">parts per thousand</a> range, but <a href="/facts/Parts_per_million/PozRuoLM">parts per million</a> (ppm) are achievable with long collection times and concentration at top surface.
XPS is routinely used to analyze <a href="/facts/Inorganic_compound/TfFfzxSz">inorganic compounds</a>, <a href="/facts/Metal_alloy/I7w7WQWR">metal alloys</a>, <a href="/facts/Polymer/eF32E50K">polymers</a>, <a href="/facts/Element_(chemistry)/0jducgWD">elements</a>, <a href="/facts/Catalyst/LPBS7TQC">catalysts</a>, <a href="/facts/Glass/hezRsAGf">glasses</a>, <a href="/facts/Ceramic/9JKudpqX">ceramics</a>, <a href="/facts/Paint/Q176AsIG">paints</a>, <a href="/facts/Paper/FUETwwKV">papers</a>, <a href="/facts/Ink/Y7Wj0YQS">inks</a>, <a href="/facts/Wood/z13NpHPl">woods</a>, plant parts, <a href="/facts/Cosmetics/rb1x6P8c">make-up</a>, <a href="/facts/Teeth/Wc6IqwVf">teeth</a>, <a href="/facts/Bone/kwJaR6IQ">bones</a>, <a href="/facts/Implant_(medicine)/3PKvy4I2">medical implants</a>, bio-materials, <a href="/facts/Coating/fGt3R3VC">coatings</a>, viscous <a href="/facts/Oil/fsgX2IRM">oils</a>, <a href="/facts/Glue/shTfIJ0E">glues</a>, ion-modified materials and many others. Somewhat less routinely XPS is used to analyze the <a href="/facts/Hydrate/dev82l69">hydrated</a> forms of materials such as <a href="/facts/Hydrogel/kYMJum9P">hydrogels</a> and biological samples by freezing them in their hydrated state in an ultrapure environment, and allowing multilayers of ice to sublime away prior to analysis.

X-ray photoelectron spectroscopy open-in-new

X-ray photoelectron spectroscopy