Topics in..
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Chemical Instrvmentcltion
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Edited by GALEN W. WING, Seton Hall University, So. Orange, N. J. 07079
nym (for electron spectroscopy for chemical analysis) given to the method by Kai Sieghhahn and associates who developed its theory and instrumentation ( 1 , 2, 3). The x-ray excitation method also is referred to as x-ray photoelectron spectroscopy (XPS), induced electron emission (IEE), and photoelectron spectroscopy of inner shells (PESIS) ( 4 ) . When UV-encitatian is used, the method is generally called ohatoeleetron s ~ e e t r o s c o,~(PES) v but also rs rer'erred tu as ulrravlulet phuruelertrun spwrosrupv (UPS) and phutoelertrun rpeerroscopy of outer ahells I'ESOS, \5r
A third related area in which electrons are generally used as the ionizing radiation is Auger spectroscopy (6). In this paper, only instrumentation designed primarily for ESCA is discussed. The value of the ESCA method stems from its general applicability to all the elements in the periodic table above helium and from its ability to determine the midation states of the elements. In principle the energies of all atomic electron levels, from the inner K-shell to the outer valence levels, can he studied. Since the binding energies unambiguously define a particular atom in a given electronic environment, the elements in a sample as well as their oxidation states can be determined. The relatively small variation of the binding energy as a function of the electronic distribution about an atom is known as the "chemical shift," and it is the ease of its determination by ESCA that is respansible far the rapid growth of the method. Other important features include the small required sample size (of the order of a microgram or less), its generally nondestructive nature, and the extremely shallow sampling depth in the case of solids (from 5 to 100 A). In addition, its applicability to the second row elements, including carbon, nitrogen, and oxygen, is making ESCA an important structural t w l for organic materials. Indicative of the growing importance of ESCA is the appearance of the subject far the first time in the 1972 Analytical Chemistry Fundamental Reviews (7). (Continued on page A206)
the Mobil Chemical Company to head the Analytical and Physical Chemistry Department in the Metuchen, N. 'J. Research Laboratory. From 1966-68 he was manager of the Central Coatings Laboratory in Mobil's Coatings Division. He has been a t Northwestern since 1968. Dr. Lucchesi is a native of Chicago and received a B.S. in chemistry from the University of Illinois in 1950. He is active in the Chicago ACS and SAS Sections. He is a past chairman of the Chicago SAS, and presently i s the chairman-elect o f t h e Chicago ACS topical group on analytical cheiistiy. He is a member of the governing hoard of the newly-formed Federation of Analytical Chemistry and Spectroscopy ,Societies (FACSS). Dr. Lucchesi's research interests include polymer analysis and characterization, titrimetry, and x-ray and ESCA spectroscopy. He has over thirty papers in analytical chemistry and applied spectroscopy.
Dr. Joseph E. Lester is a physical chemist whose research interests are spectroscopic studies of surfaces and sur.rce reaction kinetics. His undergraduate atudies were done a t Rice University (B.A., 1964) and his graduate training in chemistry was a t the University of California, Berkeley (Ph.D., 1968). He has been on the faculty of Northwestern University since 1967. Dr. Lester is a member of the American Chemical Society, American Physical Society, Sigma Xi, Phi Beta Kappa and Phi Lambda Upsilon.
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hese articles are intended to s e w the readers of THIS JOURNAL by calliny attention lo new developments in the theoy, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and ezplanations of topics that are of practical importance to those whu use, or leach ihe use of, modem instrumen~o&ma n d in&umental techniques. The editor invites correspondence from prospective contributors.
LXX. Electron Spectroscopy Instrumentation Claude A. Lucchesi and Joseph E. Lester, Department of Chemistry, Northwestern University, Evanston, lliinols 60201
INTRODUCTION Electron spectroscopy is a technique for studying the energy distribution of electrons ejected from a material which is irradiated with a source of ionizing radiation, such as x-rays, ultraviolet radiation, or electrons. Modern developments revolve amund the use of soft x-rays and vacuum ultraviolet sources, and it is convenient to distinguish two types of electron spectroscopy based on these excitation saurces. When x-ray excitation is used, the technique is commonly called ESCA, the acro-
Dr. Claude A. Lucchesi is Lecturer in Chemistry and Director of Analytical Services for the Chemistry Department a t Northwestern University. He started his professional career with the Shell Development Company in Houston immediately after receiving the Ph.D. from Northwestern University in 1954. In 1956 he went to the Sherwin-Williems Company where he became Director of the Analytical Research Department in Chicago. When Mobil Oil Corporation formed its chemical subsidiary in 1961, Dr. Lucchesi joined
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Chemicd Instrumentation Historical Background The photoelectric emission of electrons from a substance irradiated with x-rays was known and examined by Robinson in England and M. deBroglie in France over 50 years ago. In this early work polychromatic radiation mas used and the energy distribution of the emitted electrons was recorded photographically by dispersing the electrons in a homogeneous magnetic field. The electron energy distributions obtained in this way were characterized by long tails with edges a t the high energy end. The edge positions were not well defined, and because of its generally poor
precision compared with x-ray absorption and emission techniques, very little further work was done with photoelectron spectroscopy until the 1950's when Siegbahn and coworkers developed the high rerolutlun mapnrtic speutn,mater 111. Its delayed development rs easily understood in view of the . great rxverirmmml difficulties in making electrical measurements of electron energies with a precision comparable to the optical equivalent so easily obtained by optical spectroscopy. In the words of the late Ralph MUller: "To do these things electrically is obviously doing it the hard way, but now genius has prevailed and we have a potent challenge to conventional spectroscopy." (8) Experimentally, the basic requirement was measurement of electron energies with high resolution and high precision. These
critical conditions were realized in 1954 when Seigbahn and his group produced a large dispersion magnetic spectrometer of the double focusing type t o study the energy spectrum of electrons expelled by xrays. With this instrument, photoelectron lines which could be resolved from the edge of the long tails obtained in the early studies were observed. It was found tbat the line corresponded t o the binding energy of the relevant inner shell and that the position of a line could be measured with a relative precision of better than one part in 10'. By 1967, when the monumental contributions of the Uppsala group were published in book form (I), a t least five electron spectrometers had been developed by Siegbahn and coworkers. During this time, Turner and his group in England were developing UV-source instrumentation which was described in 1963 (9, 10). The early instruments were of the magnetic double focussing type and required large Helmholtz coils to compensate far the earth's magnetic field. These were unwieldly, required a large area for installation, and lacked the flexibility of the present commercial instruments which are all hased an electrostatic energy analyzers. The first commercial ESCA spectrometer was manufactured by Varian and placed on the market in 1969. At present there are a t least six major companies marketing ESCA units in the United States: AEI, duPont, Hewlett-Packard, McPberson, Vacuum Generators, and Varian. One company, Advanced Research Instrument Systems Inc. (Austin, Tex.), offers companents for electron spectrometers. It is estimated that by the end of 1972, about 30 commercial ESCA spectrometers were in use in the United States. Except for the Du Pont instrument, the cost of the commercial systems is about $100,WO. The DuPont unit is about $40,000. Basic Principles ESCA is based on the precise measurement of the kinetic energy of the ejected electrons. The binding energy of an electron in the atomic system before its ejection (EB), can be obtained from its measured kinetic energy in the spectrometer (EK) via the energy conservation equation for the photoemission process:
where C is a correction factor for the particular system and includes the work function, 4, of the spectrometer, and Ehu is the energy of the monoenergetic excitation source. For x-ray excitation En. is usually the energy of the K-alpha doublet of aluminum (1487 eV) or magnesium (1254 eV). For solids the reference level for Eg is conveniently taken as the Fermi level; for gases i t is the vacuum level (3). To determine the absolute binding energy, C must be known, but in practice it is sufficient tbat C be a constant. The value of C may be calculated relative to a reference element, such as carbon or gold. The EB calculated from the measured EK and from the value of C is used to identify an element. Frequently, only a relative binding energy (chemical shift) is desired in (Continued onpageA212) A206
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Chemical instrumentation chemical studies. For example, the chemical shift of a metal oxide relative to the metal may he calculated from the measured kinetic energies: A E ~ =~ EX,m.tal, ~ ~ . - E ~ i o x i d c ) (2) It is AEoXjd,,the chemical shift, which gives chemical structure information. The calculation of chemical shifts has been discussed by James (3) and Hercules (2). Binding energies may be determined with a n electron spectrometer which is diagrammed in simplified schematic form in Figure 1. The x-ray source, sample chamber, energy analyzer, and electron multiplier detector must he maintained a t a nressure of less than 10-5 ttor. The entire
Figure 1. Block diagram of an electron spectrometer.
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system also must be shielded from tlie earth's magnetic field. The sample is inserted into the vacuum system and is irradiated with monoenergetic x-rays which cause ejection of electrons with discrete kinetic energies. The ejected electrons are then sorted out in the energy analyzer and those with a selected kinetic energy are detected in the electron multiplier. The pulses from the multiplier are amplified and routed to either the Y-input of an analog display system or to a digital data acquisition system. Energy scanning is accomplished hy continuously or incrementally changing the analyzer voltage which controls the X-input of the readout device. In this way, a spectrum of relative electron intensity versus electron kinetic energy is obtained. The commercial spectrometers to he discussed differ in the exact nature of the components shown in Figure 1. The type of spectrum obtained with a n ESCA spectrometer is shown in Figure 2. I t is a spectrum of H ~ C O ( S C N powder )~ taken with an AEI ES-100 spectrometer, as a wide analog scan from 240 to 1490 eV kinetic energy. In terms of binding energy, calculated with Equation 1, this range corresponds to binding energies from 1244 to -6 eV because En. = 1487 eV (Al KO radiation) and C = 3 eV for the particular spectrometer. The reason for scanning such a wide region is that a t least one energy level from almost every element in the periodic table yields a peak in this range (see Appendix 1 of Ref. I). Accordingly, characteristic peaks assignable to cobalt, nitrogen, mercury, carbon and sulfur are observed. Also observed are several
Figure 2. Wide-scan photoelectron trum of H g c o l S c N ) ~
spec-
unexpected peaks which correspond to the oxygen is, aluminum 2s, and aluminum 2p photoelectron peaks and Auger electron peaks from carbon, magnesium, and aluminum ( 6 ) . The oxygen peak probably is due to adsorbed water or to a surface hydroxide on the crystallites. The aluminum and magnesium peaks are due t o the aluminum sample mounting plate (which contains some magnesium). The surface of the plate was not completely covered by the sample. The general shape of the spectrum can be explained by the Fact that each peak of characteristic energy has associated with i t a tail extending to lower kinetic energy. This tail is due t o electrons which have lost energy by inelastic collisions within the solid sample after their initial ejection
Figure 3. Scans of selected characteristic regions far the elements of Hgco(ScN)~. from the atom by the incident photon. Thus, the haekground on the left of the Hg 4f doublet is much higher than that on the rizht. In some soectra the inelastic electnm features e r h i h ~ tdistmpmhable strur. ture aisorwpd w i t h rlw deetrnnir properties of the rdid I I,. 'There ~i alsc, a ronrribution t o the haekground counting rate from the electrons ejected from the sample by the background radiation (Bremsstrah-
scans of narrow energy regions. The vertical lines in Figure 3 correspond to the expected error from counting statistics for each voltage counted. The smooth curve is drawn by a computer program with a parabolic fit to sequential sets of data points that are ineremented through the data. From the precise energy positions of the peaks one can obtain the chemical shift of the element involved. For instance, the position of the nitrogen 1s peak a t a binding energy of 398 eV correlates very well with nitrogen being in a formal oaidation state of -1. In order to make this assignment one must refer t o a catalog of reference nitrogen spectra or to correlation charts of the type prepared by Hercules ( 2 ) . One should be cautious about making structural assignments on the basis of small ESCA binding energy shifts since factors such as crystal potential energy differences and sample charging can cause apparent shifts of the order of magnitude of the observed chemical shifts. Numerous examples of the use of chemical shift data in structure determination have been given by Siegbahn et el. ( I ) . In addition to elemental identification and oxidation state information, the characteristic peaks provide a means for obtaining concentrations. The integral of a peak is directly proportional to the quantity of the species giving rise to the peak. For example, the ratio of two areas of the same element with different chemical shifts is a measure of the relative amounts of the element in the two oxidation states. When different elements are involved, quantitative information may be obtained by comparing the areas with suitable standards. The use of ESCA for quantitative analysis has been discussed by James (3). Hercules (2, l l ) , Larson ( I l a ) , and Wagner (12).
PERFORMANCE CRITERIA In a photoelectron spectrometer the signal intensity, I,(E) in counts per second,
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Chemical Instrumentation obtained a t energy E for a n element A in a homogeneous solid sample may be given by:
IAE)
%
I o u n C ~ l [-lexp(-dll)] l
R (3)
where lo =intensity of the incident manoenergetic x-ray beam a t the surface of the sample. CA =photoelectron cross-section far a given shell of element A far incident photons of energy hu. Ca = concentratmn of A a t depth d. 1 = mean escape depth for electrons of energy E. d = effective sample thickness. R = instrumental and geometric factors. The factors CA and 1 depend upon the sample and upon the energy of the ineident x-rays. lo is a function of the x-ray tube power, and R contains the instrumental parameters, such as the irradiated sample area, slit area, acceptance angle and energy analyzer transmission coefficient. Unfortunately, in addition to IA(E) defined in Equation 3, there is a background caused by contributions to the electron intensity a t energy E which are not related to the concentration of A. As indicated earlier, predominant sources of these background electrons are inelastic scattering of electrons of higher initial kmetic energy and generation of photoelectrons by the Bremsstrahlung radiation from the x-ray source. Since the number of inelastic electrons of a given energy is extremely difficult t o calculate, and since the parameters entering Equation 3 may he unavailable, empirical measures are used to abtain L ( E ) and t o describe the performance of a photoelectron spectrometer. From Equation 3 i t may be seen that the instrumental parameters affecting the ESCA experiment are: (a) the x-ray energy and intensity, ( b ) the effective sample area and thickness, and (c) the retardation optics and analyzer transmission factors. The effect of these parameters on the experiment may be translated into the usual instrument obsemahles-sensitivity and detection limit, resolution, and reproducihility-by measurements with a reference material, such as elemental gold. The peak counting rate (I,) for a photoelectron line represents a n effective measure of psrameters ( b ) and (c); the x-ray photon source and power are a measure of (a). The full width a t half maximum (FWHM) of the photoelectron line depends on the width of the exciting x-ray line and on ( e ) . In addition, the hackground and the noise must be measured in order to extract the signal from the spectrum and to estimate detection limits.
Definitions Spectrometer resolution is defined as the minimum value of the full width a t half maximum (FWHM) of the photoelectron peak of same reference element, the peak maximum being measured from the low binding energy side of the peak. The background is defined as the counting rate A216
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(IB) a t an energy, E', which is either a t the peak position or is displaced about 3 to 5 eV from the peak energy, E, an the low binding energy side. The noise is defined as the rwt-mean-square fluctuation in counts (RMS) a t E' (or E). Thus, the following may he defined for a counting time
t:
SIB = Signal/Baekgraund
=
(Ip- IdlI8
(4)
In specifying the above three quantities, the scanning rate or data acquisition time, x-ray source and power and resolution should he stated (1%).
Sensitivity and Detection Limit The above definition of sensitivity is a compromise between the present practice of electron spectrometer manufacturers and the generally accepted definition of sensitivity as the slope of a calibration curve, i.e., a ratio of a change in signal output, AI, to the increment of material in suitable units (for example, ACA) that produced the change in signal (13, 14). The spectrometer manufacturers give the peak counting rate, I,, as the instrument sensitivity. Some, but not all of the manufacturers, also provide values for FWHM, a-ray power, SIN, and SIB. These quantities (for carbon 1s) were used for a comparison of sensitivities hy Karasek (15). The same quantities are used in this paper, but in addition, values of sensitivity calculated according t o Equation 6 are provided. These data are Listed in Table 1 for elemental gold, silver, and graphite. The sensitivities shown were calculated for the FWHM given in the table by suhtracting the background intensity (obtained from the SIB value) from the peak intensity and dividing by 1W%. The detection limit is, of course, related t o the sensitivity (13). For a particular element in a given sample, i t is a function of the sample material (matrix) as well as of the spectrometer. It is generally defined as the minimum amount (micro method) or concentration (trace method) required to observe a signal. To be meaningful, the definition of detection limit must he given in terms of the number of counts, N g , a t the background position. The criterion commonly used in x-ray fluorescence spectrometry is that the net count (peak less background), N, a t the characteristic peak position of element A must equal or exceed 3 d N B in order to consider that a detectable amount of A is present (16). With this criterion, one should be wrong only one time in forty if he reports a detectable amount of A when N = 3 d N ~ I.t is suggested that the 3 d N ~criterion is appropriate for ESCA. In discussing detection limits, i t must be emphasized that ESCA is a surface method which probes the first few tens of Angstroms immediately under the surface.
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Chemical Instrumentation To the extent that the surface is representative of the hulk, one can talk about the determination of bulk concentration. In terms of bulk concentrations, ESCA is a rather insensitive method with a limit of detection of the order of 0.1%. In terms of the amount of an element that can be detected in the surface layers of a s A d snmplv. F'SCA is 8 w r y wnzilive trchniqw with a limit id detection of rhe order of 10 nanograms. An example cited by the Uppsala group indicates a signal intensity of 20 counts per second (above a n unspecified background) for iodine atoms in an iodostearic acid molecular double layer which contained about 10 nanograms of iodine ( 1 ) . The iodine in the iadostearic acid surface layer is less than 1014 atams/cm2 mole/em2) hut is more than 30% by weight. Thus, although ESCA is quite sensitive to the amount of an element present, it certainly is not a trace method.
Resolution The sharpness of the electron peaks (FWHM) is a measure of the resolution of the electron spectrometer, but resolution also refers to the ability of the method to distinguish closely spaced peaks. In general, photoelectron peaks are narrower than the corresponding x-ray emission lines, and in most cases, vary from 1 to 3 eV (FWHM). Since the maximum chemical shift for a given element is of the order of 10 eV, ESCA is not a high resolution method in that the ratio of the chemical shift to a typical value of FWHM is about 10. For comparison, a similar ratio for praton nuclear magnetic resonance is about 1000; for I3C NMR i t is about 5000. Nevertheless, the FWHM resolution of about 1.0 eV for mast commercial spectrometers makes it possible to resolve electron energies which differ by about 0.5 to 1.0 eV, and thus, to measure chemical shifts which are of interest to most chemists. In general, to achieve the above resolution at an acceptable sensitivity, i t is necessary to use two-directional electron focusing in either a magnetic or electrical field. The best reported resolution for the commercially available instruments is given in Table 1. Several terms used in conjunction with discussions of resolution merit mention: resolving power, relative energy resolution, and relative momentum resolution. All three terms refer to the dispersive ability of the spectrometer, i.e., the degree to which two electrons of closely spaced energies may be separated as a function of energy. The latter two terms are specific expressions of resolving pawer-for a magnetic field in the ease of relative momentum resolution (p/Ap) and for an electrostatic field in the case of relative energy resolution (EIAE). In commercial electron spectrometers electrostatic fields are used, and the quantity affecting the spectrometer utility in chemical analysis is AE which is the FWHM of the electron peak in eV. Since the determination of chemical shifts is limited by the attainable resolu-
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Table 1. System ~erformanceSpecificationsa
Sensitivity Parameters Peak Intensity FWHM Element (eounts/sec) (eV) AEI ES-2W DuPont 650 Hewlett-Packard ESCA-5950A McPherson ESCA-36 Vacuum Generators, Ltd. ESCA-2 Varran VIEE-15
Au 4f Ag 3d C 1s Au 4f Ag 3d C Is Au 4P Ag 3d C 1s Au 4f Ag 3d C 1s Au 4f Ag 3d C Is Au 4f Ag 3d C 1s
140,OW 25,OW 40,000 300,000 100,000 60,000 120,000 26,000 12,000 75,000 18,700 14,500 30,000
1.30 0.90 1.0 1.20 1.15 1.15 0.8 0.87 0.8 1.35 0.88 1.0 2.36
Powere (watts)
500 500
500 (Al) 504 (Al) 26M) 2400 1000
na 7 13 17
5W 350 350 350 400 (All I 750 (Al) 800(Al) 750 750 1MX)
SIN
SIB 11 12 70 7 8 10 120 33 200 14 19 53 na
350 500 500 (20 sec/eV) 2500 (20secjeV) 1250 (20 sec/eV) 1250 (20 sec/eV) 15W 350 270 470 (22 seelev) na
nad
10,000 1,000,000 24,000 11,700
1.83 1.6 0.88 1.0
Peak Position Sensi- Resolutivity tion0 Aecu- Reproducracy ihility (cps/%) (el') (el') (eV)
na na na na
13W 230 390 2600 1200 250 120 7W 180 140 8800
220 110
0.86 0.9 0.51 0.88 -.
1.0 0.9 -
0.1 0.1 0.1 0.04 0.1 0.1
0.05 0.05 0.03 0.015 0.05 0.03
o Data obtained from manufacturers; 0 Best reported system resolution; X-ray power with magnesium target except where indicated to he Al; 6 Not available; ' Known to he cleaned by Ar+ bombardment; r For silicon 2p doublet.
tion, it is worthwhile discussing in detail the factors which contribute to the width of the photoelectron line. These factors are: (a) the width of the incident x-ray line (AE,), ( b ) the width contributed by the atomic levels of the sample (AE.) and (e) the spectrometer aberration and slit widths (AE,). The overall resolution, AE, is approximately related to the three factors in the following way: + '&")' + (AE*)Z(7) (AE)Z (aEx)'t Fm the K-atomic level of elements in the
second period of the periodic table, the inherent line widths are of the order of a few tenths of a n electron volt (I). The spectrometer contribution, AE,, is typically less than 0.4 eV and the exciting x-ray width (AE,) is about 0.8 eV for the Kalpha doublet of magnesium. Substitution of these values into Equation 7 yields an overall line width, AE, of -0.95 eV which is attainable routinely with commercial spectrometers. AS may he seen fmm the above, the major contribution of AE is the inherent
width of the excitation source. Accordingly, one of the active research areas in ESCA instrumentation involves the development of a n effective means for monochromatizing a n excitation source capahle of core ionization. Sieghahn has described a means for reducing the source contribution to the width by a technique which involves crystal dispersion of the xradiation (I). One result of source monochromatization is to decrease AE, to a sufficiently small value (-0.2 eV) so that the (Continued onpageA220)
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Table 2. Commercial Electron Spectrometersa
Company and Model Function
Energy Analyzer Type Sphere radius (em) Sean Mode Resolving Power (EIAE)" Sweer, Range, . eV Excitation Source X-ray Other Sample Type (Number) Confieuratian Std. size (cm) Temp. Range ('C) Access Ports" Preparation, Detector
Vacuum Pumping Pressure (torr) Data Acquisition Analog Scan Digital Scan
AEI ES-200 Hemispherical, Retarding Lens 12.7 Retarding and hemisphere voltages 400
Hewlett-Packard ESCA-5950A
Du Pant 650 Energy Filter, Retarding Field Retarding Voltage
Hemispherical, Retarding Lens 15.5 Retarding Voltage
McPherson ESCA-36
Vacuum Generators ESCA-2
Hemispherical only
Hemispherical, Retarding Lens
36 Hemisphere Voltage
Retarding Voltage
Mg, A1 None
Solids (41, Gases Flat 0 . 5 1.5 ~ -150 to +350
Solids (1) Flat 0.63 or 1.25 dia. Ambient
A
Solids (3) Flat 0.7 x 1.0 - 180 to +300Z AP
3P
Mg, Al, Cu UV, Electron
Mg. A1 UV. Electron
Solids (8). Gases Flat 1 x 2, - 190 to +hoof
Solids (1) Flat na - 160 to +660g 4P 1,s.
Solids (3)e Cylindrical, Flat 3 ern2 -130 to +200a
Channel Electron Multiplier
Cu-Be Dynode Multiplier
Diffusion na
Sublimation
Yes Optional
No Computer Control
I.S.,S.D., G.D. Channel Electron Multiplier
Channel Electron Multiplier
Channel Multiplier Arrav and
Diffusion
Diffusion
