Gold Evaporation for Charge Correction in X-Ray Photoelectron Spectroscopy C. R. Ginnard and W. M. Riggs Central Research Department, E. 1. du Ponf de Nemours and Company, Wimingfon, Del. 19898
The analytical significance of chemical shifts in X-ray photoelectron spectroscopy (XPS or ESCA) (1, 2 ) lies in the fact that the nature of a chemical species can be established by accurately determining the inner level electron binding energies ( E b ) of the atoms in the species. Numerous ESCA studies have shown that chemical effects such as oxidation state (3, 4 ) , electronegativity of substituent atoms ( 5 ) , and type of bonding between atoms ( 6 ) affect electron binding energies. From an electrostatic viewpoint, changes in the effective atomic charge of an element will change the energy required to remove an electron from a core level. The kinetic energy of the ejected electrons, the quantity ultimately measured in electron spectroscopy, reflects these changes in the core energy levels. As a result of the photoemission process non-conducting materials can develop a positive static charge near the surface which retards electrons that leave the sample, thereby decreasing their kinetic energy ( 7 ) .This results in apparent binding energies which are higher than the true values and limits the amount of chemical information which can be extracted from the data. Most studies which correlate atomic charge properties of a non-conducting material with ESCA data rely on an appropriate compensation method to determine absolute binding energies. The compensation methods used generally fall into two categories. The first involves reducing the charge to a low value (cO.1 eV). Attempts to dispel sample charge by flooding the sample environment with low energy electrons (8) or by preparing extremely thin samples on conductive supports (9) have met with some success. These techniques, however, require elaborate equipment or are limited to select systems. A second approach monitors the sample charge using an internal standard. By measuring the apparent binding energy of the standard with respect to its true energy, the amount of charge can be determined. Known energy levels from adventitious “pump oil” ( I ) , admixed materials ( I O ) , vacuum-evaporated noble metals ( 1 1 ) , or elements present within the sample (12) have frequently been used for such corrections. Gold evaporation has gained some favor over other charge-correction methods (13) because of the ease of sample preparation and chemical inertness of the reference material. In systems where gold may react with the surface-ie., with some cya(1) Kai Siegbahn et ab, “ESCA,” Almquist and Wiksells, Uppsala, 1967. (2) D. M. Hercules, Anal. Cbem., 42(1), 20A (1970). (3) D. N. E. Buchannan, M. Robbins, H. J. Guggenheim, G. K. Wertheim, and V. G. Lambrecht, Jr., Solidstate Commun., 9, 583 (1971). (4) W. M. Riggs, Anal. Cbem., 44, 830 (1972). (5) C. R. Ginnard and W . M. Riggs, Anal. Chem., 44, 1310 (1972). (6) C. A. Toiman, W. M. Riggs, W. J. Linn, C. M. King, and R. C. Wendt,
lnorg. Cbem., in press. (7) W. P. Dianis and J. E. Lester, Anal. Cbem., 45, 1416 (1973). (8) D. A. Huchital and R. T. McKeon, Appl. Pbys. Lett., 20, 158 (1972). (9) M. E. Counts, J. S. C. Jen, and J. P. Wightman. J. Phys. Cbem., 7 7 ,
1024 (1973). Stec, W. E. Morgan, R. G. Albridge, and J . R. Van Wazer, horg. Chem., 11, 219 (1972). (11) D. J. Hnatowich, J. Hudis, M. L. Periman, and R. C. Ragaini. J. Appl. Pbys., 42, 4883 (1971). (12) J. J. Ogiivie and A. Woiberg, Appl. Spectrosc., 26, 401 (1972). (13) Northeastern ESCA Users Group Meeting, Summit, N.J., November 9, (10) W. J.
1971.
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nide and halide salts-the gold evaporation technique has been shown to be inapplicable (14). Fortunately, gold remains in the metallic state when vacuum deposited onto most materials. A more detailed discussion of calibration procedures including effects of sample charging has appeared recently (15). A fundamental assumption of the methods utilizing added standards is that the reference material is in electrical equilibrium with the sample and exactly “follows” the charge on the sample. In this study, we have evaluated this assumption for the evaporation of gold onto polyethylene (PE) and polytetrafluoroethylene (PTFE). A study of P E and P T F E is particularly useful because these materials exhibit large differences in sample charging and should demonstrate the effectiveness of gold evaporation over a wide range of charging conditions.
EXPERIMENTAL T h e photoelectron spectra were obtained with a D u P o n t electron spectrometer using Lee’s (16) electrostatic retarding field energy analyzer. Details of this instrument have been previously described (17). T h e instrument was calibrated such t h a t t h e Au 4f7/2 electron binding energy of a gold reference was 83.3 eV. Recent work (15) indicates t h a t a value of 83.8 eV is more accurate and t o be preferred for spectrometer calibration. However, since t h e measurements cited here are all relative, the 0.5 eV adjustment has no effect on t h e conclusions drawn. Instrumental resolution was 1.7 eV, FWHM, demonstrated on t h e same electron line. An aluminum X-ray anode provided an excitation energy of 1486.6 eV. T h e ESCA peak positions were located t o t h e nearest 0.1 eV. Peak intensity was taken to be t h e peak height measured from t h e peak maximum t o a background under t h e peak approximated by a straight line extrapolation of background values a t t h e high and low E b sides of the Au 4f doublet. Gold evaporation was carried o u t a t high vacuum (lo@ Torr) in t h e reaction chamber of t h e instrument by heating a gold-coated tungsten filament. Samples consisted of 1-mm thick X 12-mm diam. circular disks cut from freshly prepared polymer films. Three samples of each film were examined. Each sample was given a number of exposures to t h e evaporator and ESCA analysis was performed after each deposition. T h e evaporation/ESCA analysis procedure was terminated when t h e increase in t h e Au 4f;/2 peak was less than 5% between evaporations. This provided a series of ESCA spectra which could be related t o t h e increasing surface coverage of gold on the sample.
RESULTS AND DISCUSSION Condensation of small amount of gold vapor on a substrate proceeds by a nucleation process which results in the formation of electrically isolated islands of gold on the s u r face (18). Additional condensation causes coalescence of these islands to form a thick (
