As such, the data obtained must be used cautiously, and care should be taken to avoid over-analyzing data. The peaks may then be assigned to particular species, but the peaks may not correspond with species in the sample. Computer programs are used to deconvolute the elemental peak. However, the identification of different species is discretionary. High resolution scans of a peak can be used to distinguish among species of the same element. Additionally, the depth to which XPS can analyze depends on the element being detected. The width of the peaks in the spectrum consequently depends on the thickness of the sample and the depth to which the XPS can detect therefore, the values obtained vary slightly depending on the depth of the atom. Electrons from atoms deeper in the sample must travel through the above layers before being liberated and detected, which reduces their kinetic energies and thus increases their apparent binding energies. Furthermore, each peak represents a distribution of observed binding energies of ejected electrons based on the depth of the atom from which they originate, as well as the state of the atom. Also, elements with relatively low atomic percentages close to that of the detection limit or low detection by XPS may not be seen in the spectrum. For this reason, XPS can provide only relative, rather than absolute, ratios of elements in a sample. Limitationsīoth hydrogen and helium cannot be detected using XPS. Deconvoluted high resolution F1s spectrum of F-DWNTs (O. Deconvoluted high resolution C1s spectrum of F-DWNTs (O. and show high resolutions scans of C1s and F1s peaks, respectively, from, along with the peak designations.
Computer software is used to fit peaks within the elemental peak which represent different states of the same element, commonly called deconvolution of the elemental peak. Elements of the same kind in different states and environments have slightly different characteristic binding energies. Subsequently, high resolution scans of the peaks can be obtained to give more information. shows a sample survey XP scan of fluorinated double-walled carbon nanotubes (DWNTs). Elements with low detection or with abundances near the detection limit of the spectrometer may be missed with the survey scan. Initially, a survey XP spectrum is obtained, which shows all of the detectable elements present in the sample.
Comparing the areas under the peaks gives relative percentages of the elements detected in the sample. units), since only relative intensities provide relevant information. Often, intensity is reported as arbitrary units (arb. Intensity may be measured in counts per unit time (such as counts per second, denoted c/s). XPS data is given in a plot of intensity versus binding energy. The binding energies are used to identify the elements to which the peaks correspond. Detectors have accuracies on the order of ☐.1 eV. X-ray photoelectron (XP) spectra provide the relative frequencies of binding energies of electrons detected, measured in electron-volts (eV). Since XPS is a surface technique, the orientation of the material affects the spectrum collected. XPS analyzes material between depths of 1 and 10 nm, which is equivalent to several atomic layers, and across a width of about 10 µm. Ultra-high vacuum conditions are used in order to minimize gas collisions interfering with the electrons before they reach the detector. The photoelectrons ejected from the material are detected and their energies measured. Monochromatic aluminum ( hν = 1486.6 eV) or magnesium ( hν = 1253.6 eV) K α X-rays are used to eject core electrons from the sample. Samples are usually placed on 1 x 1 cm or 3 x 3 cm sheets. A common method of XPS sample preparation is embedding the solid sample into a graphite tape. Furthermore, XPS samples must be prepared carefully, as any loose or volatile material could contaminate the instrument because of the ultra-high vacuum conditions. Basics of XPS Sample preparationĪs a surface technique, samples are particularly susceptible to contamination. These orbital energies are characteristic of the element and its state. By Koopman’s theorem, which states that ionization energy is equivalent to the negative of the orbital energy, the energy of the orbital from which the electron originated is determined. Using the equation with the kinetic energy and known frequency of radiation, the binding energy of the ejected electron may be determined. The kinetic energies of the resulting core electrons are measured. In photoelectron spectroscopy, high energy radiation is used to expel core electrons from a sample.