Instrumentation for XPS

Rgure 14.3 Schematic diagram of an XPS instmment using an X-ray monochromator and a multichannel detector. 

Modem XPS instruments are typically equipped with a specially shaped crystal monochromator that, in addition to decreasing the bandwidth, also focuses the X-ray beam. Currently, it is possible to produce X-ray beams that are less than 10 pm in diameter with this approach. XPS instruments of this type provide much higher sensitivity for small-area XPS analysis than those using an aperture to select the analysis area. 

The CMA as shown is used for AES. For XPS, two CMAs in series are used to obtain the required energy resolution. This design is called a double-pass CMA. The transmission of electrons through a double-pass CMA is good, but the resolution is poorer than that obtained using the concentric hemispherical analyzer (CHA) described subsequently. 

Other analyzers have been designed based on the magnetic deflection of electrons, but in general, these have not been very successful because of the difficulty of maintaining a uniform magnetic field. Electrostatic systems are used in all commercial instrumentation. 

The instrument is calibrated regularly with known conductive standards such as gold or copper to establish the linearity of the energy scale and its position. 

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X-ray ray photoelectron spectroscopy (XPS) characterizes the chemical state and elemental abundance of the near-surface by measuring the kinetic energy and intensity of photoelectrons excited by irradiation of a sample (Raeburn et al. 1997b). Excellent reviews of the principles and instrumentation for XPS can be found in Hochella (1988), Turner and Schreifels (2000), and Tonner et al. (1999). 

Measurement of specific activity. The half-life of a nuclide can be readily calculated if both the number of atoms and their rate of decay can be measured, i.e., if the activity A and the number of atoms of P can be measured, then X is known from A = XP. As instrumentation for both atom counting and decay counting has improved in recent decades, this approach has become the dominant method of assessing half-lives. Potential problems with this technique include the accurate and precise calibration of decay-counter efficiency and ensuring sufficient purity of the nuclide of interest. This technique provides the presently used half-lives for many nuclides, including those for the parents of the three decay chains, U, U (Jaffey et al. 1971), and Th. 

Solid metal hydrides specifically have been reviewed here, but XPS and UPS can serve as tools to study vapors or volatile liquids. Much of the original work with these two methods involved organic molecules only later were solid surfaces studied. Therefore, they should always be considered as helpful analytical instruments for examining the bonding chemistry of organometallic compounds. This symposium covered mainly organometallic hydrides, and they are prime candidates for photoelectron spectroscopy study. 

The usefulness of UV-excitation for these experiments was already stressed on page 2. In many cases UPS and XPS give valuable complementary information S2), therefore an instrument for band structure analysis should permit different modes of excitation of both gases and solids. However, in the present article we have to restrict ourselves to studies on solids by means of x-ray photons. 

All components of the photoelectron spectroscopy instrumentation have continued to evolve over the last decade. New commercial sources for XPS with the anode at high positive potential have an order of magnitude improvement in photon flux over the older grounded anode designs. Analyzers with electron lens prefocusing are far superior to unmodified hemispherical, parallel plate, or cylindrical... 

The transmission electron microscopy was done with a 100-kV accelerating potential (Hitachi 600). Powder samples were dispersed onto a carbon film on a Cu grid for TEM examination. The surface analysis techniques used, XPS and SIMS, were described earlier (7). X-ray photoelectron spectroscopy was done with a Du Pont 650 instrument and Mg K radiation (10 kV and 30 mA). The samples were held in a cup for XPS analysis. Secondary ion mass spectrometry and depth profiling was done with a modified 3M instrument that was equipped with an Extranuclear quadrupole mass spectrometer and used 2-kV Ne ions at a current density of 0.5 /zA/cm2. A low-energy electron flood gun was employed for charge compensation on these insulating samples. The secondary ions were detected at 90° from the primary ion direction. The powder was pressed into In foil for the SIMS work. 

Instruments for electron spectroscopy are offered by several instrument manufacturers. These products differ considerably in types of components, configurations, and costs.. Some are designed for a single type of application, such as XPS, and others can be adapted to AES and UPS by purchase of suitable accessories. All are expensive ( 300,(KK) to > 10 ),... 

A KRATOS XSAM800 instrument with a MgKa X-ray source (1253.6 eV proton energy, no monochromator) was used to analyse clean membranes and surface deposits. The pass energy was 40 eV in fixed analyser transmission (FAT) mode. No sample preparation was required for XPS. Results of this analysis are presented in Chapter 7. 

Ion Gun. An ion gun is used in XPS and Auger instruments for two purposes (1) to clean the sample surface of any external contamination layer and (2) to sputter atoms from the surface in order to obtain a depth profile analysis, discussed under applications. One type of ion gun uses a heated hlament to ionize inert gas atoms. The ions are accelerated by a potential placed on the ionization chamber and are focused to strike the sample surface. The ions remove atoms from the sample surface by collision. The rate of removal of surface atoms is controlled by the kinetic energy of the ions and by the namre of the surface atoms sputtering rates may be as high as 10 nm/min. 

XPS analyses was performed on a Quantum 2000 Scanning SK Microprobe instrument. For the XPS analyses the samples were mounted on a sample plate, introduced into the XPS chamber and evacuated to <2x10 Pa. Elemental surveys and narrow scans were conducted with A1 KD X-rays on the uncalcined samples after which the temperature was increased to 573 K and 873 K (heating rate = 20 K min" ). Semi-quantitative data were calculated from the survey scans. The peak area ratios reported in this paper are fractions, which were calculated from the areas under the XPS peaks. The narrow scan data were used to determine probable compounds for the Co and C peaks. This was accomplished from a Gauss-Lorenzian peak fit of the appropriate photoelectron peaks. Wide spectra were recorded to obtain a semi-quantitative analyses of all elements present on the surface (except H and He) and high-resolution narrow spectra were recorded to identify the oxidation states and/or compounds. 

As for XPS and AES (Eq. 8 and Eq. 13, respectively) K is a constant containing the various instrumental and other factors that are maintained approximately fixed during an analysis. 

XPS and surface FT-IR measurements. X-ray photoelectron spectroscopy (XPS, Perkin-Elmer Physical Electronics PHI 5300) was also used to extract information about the surfece species. The XPS instrument is equipped with a monochromatic A1 Kq X-ray source (1486.6 eV) and hemispherical analyzer. The sample was prepared for XPS analysis by sprinkling the powders on an adhesive tape so as to obtain uniform and complete coverage. 

Concerning to the experimental apparatus for XPS and NEXAFS analysis, as the time goes by the general development of apparatus never stops. As for traditional X-ray source using instrumentations, the development of analyzer with higher luminosity and high resolution received a strong impulse in the early nineties when the Scienta 200 and 300 analyzers were developed. So far, since several years the most scientifically advanced laboratories around the world have adopted this analyzer used in conjunction with the formal instrumentation or better taken as a part for a custom or similar instrument. The majority of the beamlines at synchrotron radiation plants around the world have adopted such analyzer. 

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