File Name: urface analy iby auger and x ray photoelectronpectro copy .zip
X-ray photoelectron spectroscopy XPS is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material elemental composition or are covering its surface, as well as their chemical state , 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 ion-beam etching. 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 ion implantation.
XPS belongs to the family of photoemission spectroscopies in which electron population spectra are obtained by irradiating a material with a beam of X-rays.
Material properties are inferred from the measurement of the kinetic energy and the number of the ejected electrons. When laboratory X-ray sources are used, XPS easily detects all elements except hydrogen and helium. Detection limit is in the parts per thousand range, but parts per million ppm are achievable with long collection times and concentration at top surface.
XPS is routinely used to analyze inorganic compounds , metal alloys ,  semiconductors ,  polymers , elements , catalysts ,     glasses , ceramics , paints , papers , inks , woods , plant parts, make-up , teeth , bones , medical implants , bio-materials,  coatings ,  viscous oils , glues , ion-modified materials and many others. Somewhat less routinely XPS is used to analyze the hydrated forms of materials such as hydrogels 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.
This equation is essentially a conservation of energy equation. It is a constant that rarely needs to be adjusted in practice. In , Heinrich Rudolf Hertz discovered but could not explain the photoelectric effect , which was later explained in by Albert Einstein Nobel Prize in Physics Two years after Einstein's publication, in , P.
Other researchers, including Henry Moseley , Rawlinson and Robinson, independently performed various experiments to sort out the details in the broad bands. Siegbahn received the Nobel Prize for Physics in , to acknowledge his extensive efforts to develop XPS into a useful analytical tool.
A typical XPS spectrum is a plot of the number of electrons detected at a specific binding energy. Each element produces a set of characteristic XPS peaks. These peaks correspond to the electron configuration of the electrons within the atoms, e. The number of detected electrons in each peak is directly related to the amount of element within the XPS sampling volume. To generate atomic percentage values, each raw XPS signal is corrected by dividing the intensity by a relative sensitivity factor RSF , and normalized over all of the elements detected.
Since hydrogen is not detected, these atomic percentages exclude hydrogen. XPS is widely used to generate an empirical formula because it readily yields excellent quantitative accuracy from homogeneous solid-state materials. Absolute quantification requires the use of certified or independently verified standard samples, and is generally more challenging, and less common.
Relative quantification involves comparisons between several samples in a set for which one or more analytes are varied while all other components the sample matrix are held constant.
Quantitative accuracy depends on several parameters such as: signal-to-noise ratio , peak intensity, accuracy of relative sensitivity factors, correction for electron transmission function, surface volume homogeneity, correction for energy dependence of electron mean free path, and degree of sample degradation due to analysis.
Quantitative precision the ability to repeat a measurement and obtain the same result is an essential consideration for proper reporting of quantitative results. Detection limits may vary greatly with the cross section of the core state of interest and the background signal level. In general, photoelectron cross sections increase with atomic number. The background increases with the atomic number of the matrix constituents as well as the binding energy, because of secondary emitted electrons.
For example in the case of gold on silicon where the high cross section Au4f peak is at a higher kinetic energy than the major silicon peaks, it sits on a very low background and detection limits of 1ppm or better may be achieved with reasonable acquisition times. Conversely for silicon on gold, where the modest cross section Si2p line sits on the large background below the Au4f lines, detection limits would be much worse for the same acquisition time.
Detection limits are often quoted as 0. Degradation depends on the sensitivity of the material to the wavelength of X-rays used, the total dose of the X-rays, the temperature of the surface and the level of the vacuum. Metals, alloys, ceramics and most glasses are not measurably degraded by either non-monochromatic or monochromatic X-rays. Some, but not all, polymers, catalysts, certain highly oxygenated compounds, various inorganic compounds and fine organics are.
Non-monochromatic X-ray sources produce a significant amount of high energy Bremsstrahlung X-rays 1—15 keV of energy which directly degrade the surface chemistry of various materials. This level of heat, when combined with the Bremsstrahlung X-rays, acts to increase the amount and rate of degradation for certain materials.
In those, a quartz monochromator system diffracts the Bremsstrahlung X-rays out of the X-ray beam, which means the sample is only exposed to one narrow band of X-ray energy. These are the intrinsic X-ray line widths; the range of energies to which the sample is exposed depends on the quality and optimization of the X-ray monochromator. Because the vacuum removes various gases e. This type of degradation is sometimes difficult to detect. Measured area depends on instrument design.
The minimum analysis area ranges from 10 to micrometres. Instruments accept small mm range and large samples cm range , e. The limiting factor is the design of the sample holder, the sample transfer, and the size of the vacuum chamber. Large samples are laterally moved in x and y direction to analyze a larger area. XPS detects only electrons that have actually escaped from the sample into the vacuum of the instrument.
In order to escape from the sample, a photoelectron must travel through the sample. Photo-emitted electrons can undergo inelastic collisions, recombination, excitation of the sample, recapture or trapping in various excited states within the material, all of which can reduce the number of escaping photoelectrons.
These effects appear as an exponential attenuation function as the depth increases, making the signals detected from analytes at the surface much stronger than the signals detected from analytes deeper below the sample surface.
Thus, the signal measured by XPS is an exponentially surface-weighted signal, and this fact can be used to estimate analyte depths in layered materials. The ability to produce chemical state information, i. The local bonding environment is affected by the formal oxidation state, the identity of its nearest-neighbor atoms, and its bonding hybridization to the nearest-neighbor or next-nearest-neighbor atoms.
For example, while the nominal binding energy of the C 1 s electron is Chemical-state analysis is widely used for carbon. Chemical state analysis of the surface of a silicon wafer reveals chemical shifts due to different formal oxidation states, such as: n-doped silicon and p-doped silicon metallic silicon , silicon suboxide Si 2 O , silicon monoxide SiO , Si 2 O 3 , and silicon dioxide SiO 2.
An example of this is seen in the figure "High-resolution spectrum of an oxidized silicon wafer in the energy range of the Si 2 p signal". The main components of an XPS system are the source of X-rays, an ultra-high vacuum UHV chamber with mu-metal magnetic shielding, an electron collection lens, an electron energy analyzer, an electron detector system, a sample introduction chamber, sample mounts, a sample stage with the ability to heat or cool the sample, and a set of stage manipulators.
The most prevalent electron spectrometer for XPS is the hemispherical electron analyzer. They have high energy resolution and spatial selection of the emitted electrons. Sometimes, however, much simpler electron energy filters - the cylindrical mirror analyzers are used, most often for checking the elemental composition of the surface. This type consists of two co-axial cylinders placed in front of the sample, the inner one being held at a positive potential, while the outer cylinder is held at a negative potential.
Only the electrons with the right energy can pass through this setup and are detected at the end. The count rates are high but the resolution both in energy and angle is poor.
Electrons are detected using electron multipliers : a single channeltron for single energy detection, or arrays of channeltrons and microchannel plates for parallel acquisition. These devices consists of a glass channel with a resistive coating on the inside.
A high voltage is applied between the front and the end. An incoming electron is accelerated to the wall, where it removes more electrons, in such a way that an electron avalanche is created, until a measurable current pulse is obtained. The resulting wavelength is 8. When working under practical, everyday conditions, high energy-resolution settings will produce peak widths FWHM between 0. The energy width of the non-monochromated X-ray is roughly 0. A breakthrough has been brought about in the last decades by the development of large scale synchrotron radiation facilities.
Here, bunches of relativistic electrons kept in orbit inside a storage ring are accelerated through bending magnets or insertion devices like wigglers and undulators to produce a high brilliance and high flux photon beam. The beam is orders of magnitude more intense and better collimated than typically produced by anode-based sources. Synchrotron radiation is also tunable over a wide wavelength range, and can be made polarized in several distinct ways. This way, photon can be selected yielding optimum photoionization cross-sections for probing a particular core level.
The high photon flux, in addition, makes it possible to perform XPS experiments also from low density atomic species, such as molecular and atomic adsorbates.
The number of peaks produced by a single element varies from 1 to more than Tables of binding energies that identify the shell and spin-orbit of each peak produced by a given element are included with modern XPS instruments, and can be found in various handbooks and websites. Before beginning the process of peak identification, the analyst must determine if the binding energies of the unprocessed survey spectrum eV have or have not been shifted due to a positive or negative surface charge.
This is most often done by looking for two peaks that are due to the presence of carbon and oxygen. Charge referencing is needed when a sample suffers a charge induced shift of experimental binding energies to obtain meaningful binding energies from both wide-scan, high sensitivity low energy resolution survey spectra eV , and also narrow-scan, chemical state high energy resolution spectra. If, by chance, the charging of the surface is excessively positive, then the spectrum might appear as a series of rolling hills, not sharp peaks as shown in the example spectrum.
Charge referencing is performed by adding a Charge Correction Factor to each of the experimentally measured peaks. Since various hydrocarbon species appear on all air-exposed surfaces, the binding energy of the hydrocarbon C 1s XPS peak is used to charge correct all energies obtained from non-conductive samples or conductors that have been deliberately insulated from the sample mount. The peak is normally found between The Conductive materials and most native oxides of conductors should never need charge referencing.
Conductive materials should never be charge referenced unless the topmost layer of the sample has a thick non-conductive film. The charging effect, if needed, can also be compensated by providing suitable low energy charges to the surface by the use of low-voltage eV electron beam from an electron flood gun, UV lights, low-voltage argon ion beam with low-voltage electron beam eV , aperture masks, mesh screen with low-voltage electron beams, etc.
The process of peak-fitting high energy resolution XPS spectra is a mixture of scientific knowledge and experience. The process is affected by instrument design, instrument components, experimental settings and sample variables.
Peak fitting results are affected by overall peak widths at half maximum, FWHM , possible chemical shifts, peak shapes, instrument design factors and experimental settings, as well as sample properties:. From the theoretical point of view, the photoemission process from a solid can be described with a semiclassical approach, where the electromagnetic field is still treated classically, while a quantum-mechanical description is used for matter.
The one—particle Hamiltonian for an electron subjected to an electromagnetic field is given by:. Fermi's Golden Rule uses the approximation that the perturbation acts on the system for an infinite time. This approximation is valid when the time that the perturbation acts on the system is much larger than the time needed for the transition. The photoemission event leaves the atom in a highly excited core ionized state, from which it can decay radiatively fluorescence or non-radiatively typically by Auger decay.
Unrivalled large area spectroscopic performance allows photoelectron spectra to be acquired. Fast, high spatial resolution XPS imaging reveals the lateral distribution of surface chemistry and aids further characterisation with selected area analysis. Unattended sample holder transfer and exchange during analysis is achieved through coordination of the Flexi-lock sample magazine and sample analysis chamber autostage. Efficient collection of photoelectrons combined with high transmission electron optics ensures unrivalled sensitivity and resolution at large analysis areas. As well as conventional scanned acquisition, spectra may be acquired in fast, unscanned snap-shot mode in less than a second making use of the channel Delay-Line Detector DLD. The fundamental requirement of any spectrometer is the best possible energy resolution. High energy resolution, where the spectrometer does not contribute to the broadening of the photoemission peaks, is critical for the accurate measurement of small chemical shifts.
The surface of NaCl crystals outside and in the crater was examined using an x-ray photoelectron spectrometer. The comparative analysis of.
X-ray photoelectron spectroscopy XPS is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material elemental composition or are covering its surface, as well as their chemical state , 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 ion-beam etching. 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 ion implantation. XPS belongs to the family of photoemission spectroscopies in which electron population spectra are obtained by irradiating a material with a beam of X-rays.
Since the interaction between solid materials and their surrounding media, whether gaseous or liquid, occurs at the surface, analytical techniques capable of providing information from the interaction region are fundamental in understanding the processes that are occurring. X-ray Photoelectron Spectroscopy XPS is one such technique, and is capable of analysing both conducting and insulating materials. During analysis, the surface is irradiated by soft X-rays and the energy of the emitted photoelectrons measured. The energy of these electrons is determined by the atomic number of the emitting element, and is sensitive to changes in the number of electrons in the valence band, so that surface chemical state information is obtained. XPS is now a mature technique, the first commercial instruments having become available as long ago as , and judged by the number of scientific publications, it is not only the most popular of the surface analytical techniques, but also the fastest growing.
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