Joseph A. Gardella, &.
Department of Chemlsby and Surface Science Center State Univmlty of New York, Buffalo Buffalo, NY 14214
X-ray photoelectron spectroscopy (XPSor ESCA) is one of the many electron spectroscopies particularly useful for the chemical analysis of surfaces. The technique, based on the photoemission of electrons induced by soft X-rays, is widely used for detailed surface analytical problem solving hecause it allows multiple-element detection, provides chemical bonding and state information from chemical shifts, and provides quantitative information.
optics, and multichannel solid-state techniques for charged-particle detection, however, the technique is still evolving, leading into new areas of application. Of particular importance to the surface analytical chemist are the combinations of instrumentation improvements that lead to better signal-tonoise (SiN) ratio and speed. One result is more sensitivity (i.e., a higher slope of the calibration curve) and lower detection limits for quantitative analysis. In addition, the precision of quantitative analysis is improved because the error caused by instrumental noise is lower. These results allow analysis of increasingly complex samples and detection of a greater dynamic range of
in limited fields of application. We will focus on three areas of instrumentation development improvements in the source, electron energy analyzer, and detector technology. We will not cover advances in data acquisition speed and analysis methodology that have been driven by advances in computer hardware and software. Developments in electron spectroscopy leading to new quantitative capabilities were described in a previous article (1). Specifically, this INSTRUMENTA. TION article will focus on the added capabilities gained by the use of multichannel electron detectors, highthroughput energy analyzers, and high- (energy and spatial) resolution X-ray sources. Improved efficiency in all of these units has led to higher S/N ratio and greater speed of -analysis. Hercules (2) first noted that, of the most widelv used aoolied UHV surface analytical methods; ESCA was slow and had a lower S/N ratio. Thus improvements in ESCA instrumentation that gain speed are especially desirable. The instrumental improvements that will be discussed have led to the following advances:
INS~RLJAAEN~.A~lOA/ * Many scientists believe that of the methods of electron spectroscopyused for surface analysis, both ESCA and Auger electron spectroscopy are fairly mature in terms of development of information content and applicability in chemical analysis. They are widely used for problem solving in a variety of technological fields, including electronic materials and devices, metallurgical and materials engineering, catalysis, corrosion, and adhesion. Because of rapidly improving instrumentation in X-ray and electron sonrces, electron 00052700/8910361589A1$01SO10 @ 1989 American Chemlcal Society
concentrations. These advances, combined with ESCA's capability to determine multiple elements in a single samole and nrovide chemical information. have extended the technique to new areas of interfacial science for ultrahigh-vacuum (UHV) analysis. This article will emphasize recent developments in ESCA instrumentation and methodology that have allowed this technique to be more widely used. AU of the methods discussed here have been available to photoelectron spectroscopists for some time, but only
-
lower X-ray damage, use of lower flux X-ray sources, higher resolution conditions, smaller analysis areas, ease of application of low S i N ratio experiments, and faster data acquisition. In considering each of these ad-
ANALYTICAL CHEMlSTRlf. VOL. 61. NO. 9, MAY 1. 1989 * 589A
Hemispherical analyzer
1
or
Singlechannel detector
f r.gure 1. Block diagram of an ESCA experiment.
vances, we can compare ESCA instrumentation growth with that of Fourier transform (FT)instrumentation in IR spectroscopy. In the case of FT-IR instrumentation, the S/N ratio and speed increased, allowing far broader application of the method, especially in the area of quantitative analysis. In the examples discussed here, the lower X-ray damage means more applications to previously sensitive samples such as polymers and biological materials. Lower flux X-ray sources, such as those emitting higher energy photons or those that are focused, allow analysis of microstructure in three dimensions without destructive sputtering, or in conjunction with it. The application of hqher energy resolution conditions, where losses in total signal previously precluded application, has opened up more chemical state information from chemical shifts. Using focused X-rays or lensing to analyze small areas can be accomplished routinely for 150-pm spot sizes, and newer instrumentation promises 10 pm, with 1p n easily on the horizon. In addition, low S/N ratio experiments such as shallow takeoff angle-dependent measurements for shallow sampling depths, or rapid data acquisition for time-dependent measurements, now are more routine; previously they were limited because of damage or noise considerations. In general, we can now acquire data faster. In the mid-l970s, data acquisition for a.single sample took at least 2 4 hand sometimes longer. Currently data acquisition takes only a few minutes, and better resolution is achieved. Moreover, recent developments promise times of less than a few seconds. 590A
a
However, whereas FT methods of IR analysis decreased data acquisition time to the point of introducing rapid kinetica and flow experiments, this advance has not yet occurred for ESCA, although slow kinetica can be monitored (3). Ins$umentationde~ Three areas of instrumentation development can he considered as three major experimental components in the typical block diagram of an ESCA experiment (Figure 1).In thii section we will describe the advances in the development and application of multichannel detection, high-throughput kinetic energy analysis, and high- (energy and spatial) resolution X-ray sources. Multichannel detection. Historically, the development of multichannel detection in ESCA analysis was driven by the need to use efficient signal collection from low X-ray flux monochromatized sources. Sieghahn and FellnerFeldegg’s ESCA design, including a focusing X-ray monochromator, was realized in the Hewlett Packard (HP) Model 5950 instruments (4, 5), which were outfitted with Vidicon detectors coupled with TV camera detection. Although the instruments were efficient, quantitative analysis was severely limited in dynamic range because of signal saturation effects in the Vidicon. In the mid-19708 Surface Science Laboratories (now Surface Science Instruments), the offspring of the H P ESCA development group, developed an advanced multichannel detector (MCD) based on a Chevron type design, with a resistive anode strip position computer (6).This detedor could be used for en-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1. 1989
ergy or spatial resolution of energetic particles. Other types of MCDs were also developed, includingthe so-called fast response ESCA (or FRESCA) developed at Xerox Webster Research Laboratories in the late 19708 (7,8).Placing the MCD in the exit plane of the hemispherical analyzer replaces a slit and single-channel electron multiplier (SCD), which looks at a single channel or bandpass of electron kinetic energies from the analyzer. The MCD simultaneously collects 2n channels of electron energies, gaining speed or S/N ratio from the traditional Fellgett’s advantage (9) by a fador based on the number of resolution elements. With wider use of monochromatized X-ray sources, and now with any application where small spots are analyzed, MCDs increasingly are being used to resolve energy and position. Future developments involving an increase in resolvable energyhpatial channels are expected. Advances are also being made in single-channel detection efficiency and gain (IO),80 that there are reasons for retaining the simpler and more rugged SCDs, which still allow more gain in S/N ratio than does older instrumentation. Kinetic energy analysis. In ESCA, the kinetic energy analysis of electrons takes place using two components: a lenshetardation unit and a kinetic energy filter. The two standard means of scanning electron kinetic energy, fixed analyzer transmission (FAT) and fixed retardingratio (FRR), involve the combination of both units, discussed by Armstrong and eo-workers (1).The hasic aspects of various kinetic energy filter designs have been descrihed by B m i e (11) and Rivisre (12)in hooks edited by Briggs and Briggs and Seah. Although various designs were initially developed for the energy filters, two have been widely used the cylindrical mirror analyzer (CMA) and the hemispherical sedor analyzer. Generally, the CMA is hest suited when high throughput is desired at lower energy resolution. Thus it is widely used in Auger analysis, where chemical shift information is generally not used (and therefore higher energy resolution is not needed), hecause it is so complex (13). (Note that high-resolution Auger spectrometers using hemispherical analyzers are being developed hy VG Scientific.) It can he shown (11)that the hemisphere is the most efficient filter for charged particles where high throughput at high-energy resolution is required. Whereas CMAs have been used-especially in double-pass configurations for ESCA instruments com-
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INSlRUMENlAllON bined with Auger-all ESCA manufacturers except Shimadzu currently market instruments based on the hemispherical analyzer. The need to acquire data and resolve chemical shifts of a few tenths of an electron volt in ESCA makes the hemisphere the optimum choice. Recently, Scienta Analytica introduced an extremely large hemispherical analyzer (60-cm diameter, as opposed to the 30-cm diameter typically found in most commercial instruments) to obtain higher performance. When compared with smaller diameter hemispheres, this yields either higher throughput at an equivalent resolution or higher resolution a t equivalent throughput. However, it is the former capability that will improve the S B ratio or speed, because most current applications are not limited in resolution by instrumentation. Major developments have occurred in the design of the lendretardation unit. Originally, lenses were placed in front of the energy analyzer to allow longer focal distances to be used for electron collection. This allowed multiple-probe and analyzer units to focus on the same sample for multitechnique instruments. Currently the role of the lenshetardation unit is dual: to focus electrons collected a t the entrance planehlit of the energy analyzer and to retard the electron energies to a range where higher overall energy resolution pan be gained. Advanced methods of electron optics design have yielded commercially available multiple lens units with apertures. These additional lenses have allowed the focusing role to be expanded to one where high throughput of electrons collected from a small area or spot can he analyzed (1617). Thus, even without the use of f o c d X-ray sources (discussed below), small areas can be analyzed or microscopically viewed with spatial resolution (commercially available) to spot sizes of 200 q with a standard Perkin-Elmer (PE) Omnifocus lens system (15) and the newly introduced Vacuum Generators’s ESCAscope and Surface Science Instruments’s MicroESCA (which is based on entirely different focusinghollection methods ( I @ ) , each specifying 10 am. Using a modification of Wannherg’s basic design approach that produced the Omnifocus lens, Scienta’s ESCA 300 also specifies areas to a few micrometers. This is accomplished with both a focusing lens and the focused source discussed below. High-resolution X-ray sources. X-ray source capabilities have been ex,tended to high(er) energy and spatial resolution. Again, it is interesting to 592A
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consider the historical development based on the SieghahnD’ellner-Feldegg/HP X-ray monochromator (4,5), which was developed to decrease line width of the X-ray source and to filter unwanted Bremstrahllung and X-ray satellite background. This approach has reached the stage where natural Xray line widths are hardly broadened by the source; line widths of 0.34 eV are measured on the Si 2p core levels with the X-ray monochromator available from PE. However, in the H P design (4, 5), focusing considerations of the Rowland Circle monochromator mounting of the X-ray crystals led to commercially available focused X-ray monochromator output available on Surface Science Instruments’s ESCAs (15).Using a focused electron beam source to the anode allows an initial point source of X-rays before monochromatization. This has permitted the routine excitation and data collection from small spots to a size of 150 am. Thus the higher energy resolution gained from the monochromatized source (traditionally AI Ka radiation) using bent crystals is also spatially focused. Three other advances are likely to be important in the near future with the application of monochromatized X-ray sources. The fmt is the use of alternative anode materials. As discussed helow, many other X-ray sources have been available to supplement information from the typical Mg Ka or AI Ka sources (Table I). Two other sources, Ag La (2984 eV) (17)and Cr Ka (5417 eV), are particularly appropriate for monochromatization, even though they are limited to low fluxesrelative to Mg Ka. Both sources have near integral multiples of energy relative to AI Ka (Ag La = 2.0075 A1 Ka (17,19); Cr Ka = 3.64 AI Ka).With reasonable adjustment of alignment of X-ray monochromator crystals (in the case of Ag, the second-orderreflection can be used directly), the same spacing and c w a ture as those for the AI Ka can be used
ANALYTICAL CHEMISTRY. VOL. 61, NO. 9, MAY 1. 1989
to nmow the X-ray line, eliminate Bremstrahllung background, and fdter X-ray satellites with higher energies. This accesaes more ESCA signals and allows sampling to deeper depths (see below). Secondly, high flux output rotating anode designs, although complex and bulky, promise to increase data yield to the point where data acquisition in the newly introduced Scienta ESCA, using the rotating anode source, can take place in lesa than 1 s at extremely high resolution. Further in the future is the development of smaller spot size focusing optics. Golijanin and Wittry (20) have published encouraging results of X-ray focusing crystal experiments using plastically deformed focusing crystals that promise 1-pm spot sizes, and they also have proposed using this for X-ray microprobe use. The application to ESCA source design is natural and will allow microstructural analysis of smaller and smaller features with very higb resolution. Extended areas of appllcaion The six advantages yielded hy the new instrumentation developments listed at the beginning of this article can be illustrated by a number of examples. Interest in our laboratories (and many others) has extended into materials that are sensitive to X-ray damage (e.g., organic, polymeric, and biological surfaces). Applications to all of these areas are important in biomedical materials, biological adhesion and corrosion problems, and general compwite materials science and engineering. Unique challenges in these fields, some outlined recently by Hercules (ZZ), are posed for the surface analytical cbemist. One specific need is the ability to describe composition, structure, and bonding heterogeneity near or at the surface of materials in the lateral and transverse planes. X-ray damage is a key aspect of anal-
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594,.
ysis in ESCA, which, as Hercules ohserved, is the least “damaging/destructive” of the major surface analytical tools (2). Because X-ray sources are used, there is little limitation to conducting or semiconducting samples. Insulators are routinely analyzed by ESCA. However, one still must he concerned with damage to sensitive materials. Damage encompasses many mechanisms, including the primary Xray induced chemical changes and the secondary factors of heating in vacuo, secondary electrons, Bremstrahllung, etc. (22, 23). A monochromatized source (4.5)was first used to determine that most damage observed in ESCA experiments was not a result of primary X-ray flux. Rather, it resulted from a cornhination of the secondary factors listed, most of which involve instrumental factors not related to X-ray damage (22).Although most solids are stable under the X-rays in a typical ESCA experiment, many materials degrade with heating or secondary electrons in vacuo before spectral data can he acquired. A typical example is poly(viny1chloride) (PVC). PVC is very sensitive to radiation of any sort, and the primary degradation process involves dehydrochlorination. Thomas and Chang (24) showed that this can he evaluated in ESCA using both the ratio of carbon to chlorine and the ratio of the two equal intensity peaks in the carbon 1s envelope. Their results show that some of the sensitivity to damage is caused hy oxygen-containing additives, which accelerate the kinetics of degradation. This concern is exemplified in our work to perform nondestructive ESCA depth profile measurements on multicomponent polymers, to determine the distribution and extent of surface excesses of one component in the mixture, or to determine microdomain heterogeneity near the air interface. Blends of various homopolymers with PVC have been studied hy using angledependent ESCA to provide the depth profile (25,26) of surface excess of one component. With modern collection and filtering and multichannel detection-even with a standard X-ray source-what took Thomas and Chang a minimum of 15 min of acquisition time for a single analysis angle can he accomplished in 2-3 min (25,26).This has allowed the collection of data for three separate angle-dependent measurements in less than 10 min total, ensuring against detecting chemical effects induced by damage. Other groups have studied the danage induced hy focused X-rays from the small spot source (27). They concluded that the filtered Bremstrahl-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9. MAY 1, 1989
lung, the lack of secondary electrons, and the separation of the X-ray source from the sample (thus eliminating heat transfer) drastically reduce damage even in very sensitive polymers. Even with the X-rays focused into a small spot, damage is limited to that region of a sample so that analysis of adjacent regions is affected minimally. Thus the added S / N ratio can he used to decrease total analysis time and to limit damage to a variety of samples previously considered too delicate for ESCA analysis. The added S/N ratio can obviously be used to address the use of weaker sources. This was the initial reason for the multichannel Vidicon detector in the HP instrument ( 4 , 5 ) .However, one burgeoning area is that of higher energy photon sources (I7,19,28).The desire to use other sources of X-rays is twofold. The higher energy X-ray produces electrons with higher kinetic energy, which can travel from deeper regions because of attenuation effects. This produces deeper sampling depths, the so-called energy-dependent measurements. Additionally, higher energy core-level orbitals can he excited, allowing use of Auger parameter analysis (28,29) not accessible with a standard Mg K a or AI K a source. Application of this approach previously was limited hy the lack of flux from these sources, some of which are listed in Table I. Castle’s group (19) has worked on the application of alternative X-ray sources, with special emphasis on added chemical information from Auger parameter measurements, which is very useful for metallurgical and corrosion problems. In our laboratory we attempted to expand the use of the Ti K n X-ray source (hu = 4510.9 eV) to polymers (30, 31). The Ti K a source is particularly useful because of the desire to sample much deeper into polymers, and the sampling depth estimated from empirical electron attenuation length relationships is approximately twice that of the Mg Ka sampling for the carbon 1s photoelectron (30, 31). Although use of the Ti K a source was attempted earlier hy Clark (321, the S/ N ratio limited the full use of high resolution and introduced problems with analysis of polymers and other materials sensitive to damage. Our results, taken with a multichannel detectorbased instrument, have allowed the collection of data with analysis times of less than 1 h, thus minimizing damage in sensitive polymers. A table of accessible Auger lines for third-row elements of importance in high-performance polymers (phosphazenes, siloxanes, sulfur-containing polymers) and
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quantitative sensitivity factors is reported for typical polymer-containing elements (30,31). One particularly interesting application involves the energy-dependent depth profiling to complement the angle-dependent measurements on multicomponent polymers. Building a depth profile from both angle- and energy-dependent measurements is complicated by the fact that sampling depths are based on a multiple of the inelastic mean free path, or attenuation length, of electrons through the material. Thus the sampling depth quoted are in fact integral values, witlthe signal contribution from the topmost attenuation length more important than the second and third attenuation lengths. However, adding information with greater sampling depths can complement more surface-sensitive methods, where the profile of constituents involves species segregated from the surface. Table I1 shows the results of angledependent and Ti Ka analysis of a segmented poly(ether urethane) (Figure 2) block copolymer (30,32) where the nitrogen-containing urethane hard block component is segregated from the topmost interface, and the determination of the nitrogen profile is complicated by low concentrations in the region sampled by the Mg Km.The values of sampling depths quoted are three times the attenuation length, determined empirically from tabulated data (30,32). The absolute values of sampling depth are estimated; however, the relative values are extremely important, especially given the nitrogen concentration at deeper depths. Confidence in the distrihution and detection of the urethane (nitrogen) profile is increased when the Ti Ka data are considered. The use of higher energy resolution conditions usually involves both th, monochromatization of X-rays (4, 5 26) and the proper filtering in the lens/ retardation unit and kinetic energy filter (26). The development of X-ray monochromators has continued to pro590A
vide instruments with narrower and narrower line width specifications. The standard for the fwhm of silver 3d51Zis now 0.50 eV, as compared with typlcal specifications for nonmonocbromatized radiation for the silver 3&n line at 0.80 eV. The practical ramifications of the narrower lines are important in analy-
ais of insulators, where charge-based broadening contributes to overlapping lines from chemical shifts, reducing the utility of functional group analysis from curve resolution. The use of hgher energy resolution is illustrated in Figure 3. ESCA sulfur 2p lines for a highly ordered rigid rod polymer, poly(bishenzothiazole)(PBT), are shown as a function of analysis angle (E0, 45'. and 90'). The high resolution is obviously able to partially resolve the spin orbit splitting of the 2 ~ 3 , and ~ 2p1/2 lines. At shallow angles of analysis (shallower sampling depths), the line broadens. Without the high resolution, this slight change would not be detected, yet this broadening effect can be attributed to the disordered structure near the surface of the polymer, where slight amounts of amorphous material would be situated. The uee of focusing sources and double-pass lens systems (14-27) bas re-
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ANALYTKAL CHEMISTRY. VOL. 61, NO. 9. MAY 1, 1989
duced the achievable sample size and initiated studies in ESCA microscopy and microprobi. Specialized instrumentation is now available that incorporates either focused sources or smallarea lensing systems for microprobe
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and now even microscope measurements. An important reason for obtaining ESCA information from small spots is to compare capabilities with other microprobe measurements. For instance, although X-ray fluorescence
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measurements are widely used in microprobe experiments, the lack of sensitivity to low Z elements is a major limitation. The surface sensitivity (Xray fluorescence samples to micrometer depths versus the Angstrom sampling depths in ESCA) and ability to analyze insulators and sensitivity to elements involved in organic bonding, as well as the chemical-state sensitivity, would he extremely valuable if limited to small analysis regions in the lateral plane. We recently completed a small spot ESCA study of cross-sectioned and fractured urinary tract stones (33).The phenomenon of mineralization of materials in biological environments is extremely important in biocompatihility, oil field s c a l i i , milk proeessing, and other technologies, but it is moat obvious in the etiology of urinary stones. The structure of the stone is one of concentric rings that differ in color and hue. Our interest is to determine the mineral composition and phases, interfacial presence of proteinaceous material, and presence of impurities as a function of the microstructure. Preliminary results are shown in Figure 4, which is a plot of the change in concentration of various species as a function of position across the crws section. Very different distributions of nitrogen-containing species, assignable to proteinaceous material, and carbonate and oxalate mineral species were detected. The impact of this type of microstructural information a c r w interfaces should be very broad in fields of biomedical materials development and biotechnology. Extension of this approach to micro-
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ANALYTICAL CHEMISTRY. VOL. 61, NO. 9, MAY 1, 1989
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INSTRUMENTATION
Figure 5. ESCA image of the surfaceof weathered steel after peeling alkyl polymer protective coating.
Pming was induced by exposure to 3.5% kcl s~tutionfw 20 days. (a) Oxygen 1s photoelectron siglai image; (b) carbon 1s photoelectron signal image; and (c) Na K U AW signal image. showing Na menballon amund mrmian pn. ( R o t a s ulurtesy of VG ScienWic Applications Labaalw.)
scopic analysis of even smaller areas (-10-pm spot size) is illustrated in Figure 5, which shows images of ESCA elemental signals from a pitting corrosion sample. The important images contained here are the sodium and chlorine images, which show localization around the ring features (-80 pm). It is impossible to image sodium concentrations in a scanning Auger experiment because sodium tends to be quite mobile. This type of imaging information should be important in many fields (e.g., composite materials, microelectronics, and superconductivity) where
microphase heterogeneity is extremely important and dependent on chemicalstate resolution. The general improvement in S/N ratio and speed is very important to other routine low-signal experiments. The greatest impact is made on the shallow angle analysis discussed earlier. Here, very low S/N ratio often limits application to complex materials or to materials with low concentration species. This limitation is now less of a concern, and the added S/N ratio is being used for more routine angle-dependent depth profiling.
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ANALYTICAL CHEMISTRY. VOL. 61. NO. 9, MAY 1, 1989
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Just as the development of FT instrumentation broadened the applications of IR spectmcopy, the gain in S/N ratio and speed made possible by improved ESCA instrumentation has allowed more routine use of this surface analytical tool. This improvement in speed also suggests the use of ESCA in process analysis and control, where extremely rapid and automatic feedback is necessary. Faster data acquisition generally means more productivity in the analytical laboratory, and with
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9. MAY 1. 1989
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INSTRUMENTATION
Laboratory Heating Products I
more routine application, ESCA can now be integrated into process control. Although most instrument manufacturers seem to be focusing on higher performance instruments, they have responded to requests for instruments t o handle more and more nontraditional ESCA samples. In fact, instruments that accommodate u p t o 8-in.-diameter wafers or 100 sample sites on an 8-in. platter are now available. This suggests that process control with surface analysis by ESCA is indeed just on the hori-
zon. The author thanks the many people who helped in the preparation of this article. Recent results were mntributed by Lawrence Sdvati, Jr.; George L. Grebe; Roland Chin; Kevin J. Hook Robert J. Schmitt; Jaseph H. Wandasa; Terrence G . Vargo; James J. Sehmidt; Michael B. Clark, Jr.; Cindy BurLhardt; &?I Mittlefehldt; Helen Lee; Paula Cornelio; Cara Weitzsaker; Norma Hernandez de Getics: Allison Campbell; Carl Richardson; George Nanmllsa; Jean-Jacques Pireaur; Ruth Siordk Robert Chaney; and John Hammond. Finaneidsupportvma provided by NSF (Division of Materials Research and Division of Industrial Siences), NATO, ONR, USPHS-BRSG, New York State Science and Technology Foundation, the Gelb Foundation, and Perkin-Elmer.
(19) Edgell, M. J.; Paynter, R. W.; Castle, J. E. J . Electron Spectroscopy andRehted Phenomena 1985,37,241. (20) Golijanin, D., M.; Wittry, D. B. Microbeams Anal 81s 1987; San Francisco Press, he.: {sn 6ancim, 1987; pp. 51, 54. (21)Hercules, D. M. Anal. Chem. 1986,58, 1177 A. (22) Wagner, C. D. Surf. Interface Aml. 1984,2,90. (23) Klein, J.C.;Li,C.-P.;Hereules,D.M.; Black, J. F. Appl. Spectmse. 1984, 38, 790
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(24) Chang, H. P.; Tbomaa, J. H., 111. J.
Electron Spectroscopy and Related Phenomena 1982,26,203. (25) Schmidt, J. J.; Gardella, J. A,, Jr.; Salvati, L.,Jr., submitted for publication in M"Pmmnlor,rlon
(26) Clark, M. B., Jr.; Burkbardt, C. B.; Gardella, J. A., Jr.. submitted for publication in Macromolecules. (27) Chanev, R. L., Surface Science Instrumenta, Inc.,personal communication. (28) Wagner, C. D. J . Vac. Sei. Technol. 19'18.15.518.
References (1) Nebesny, K. W.; Mascbboff, B. L.; Armstrong, N. R.Aml. Chem. 1989.62,469 A481 A. (2) Hercules, D. M. Anal. Chem. 19'18,50, 7.94 A
Pan, D. H.; Prest, M. W., Jr. J . Appl. 'h s 1985,58,15. zelly,M. A.;Tyler, C. E. HewlettPaekard Journal 19'12,24,2. (5) Fellner-Feldegg, H.;Gelius, U.; Wannberg, B.; Nilsson, A. G.;Basilier, E.; Siegbahn, K. J. Electron Spectroscopy and Related Phenomena 19'14.5,643. (6) Lampton, M.; Carlson, C. Rev. Sci. Instrum. 1975.5,1093. (7) Thomas, H. R.,. Po!= Materials, Inc., personal communlcahon, March 1982. (8) Ford, W., Montana State University, personal ,a** communication, September (S[j&ett,P.
R. A., submitted for publication'in Calci-'
fied Tissue International.
B. J.Phys.Radium 1958.19,
LO,.
(10) Kurz, E. A. American Laboratory March 1979.27. (11) Barrie, A. In Handbook of X-ray and
BriskHeat offers surface heatid solulions up to 1W F for cornoiex heatina reauirements to& in many b i r c h and devebpment applications, tmm atstom pdwts and blankets to --my mantles, heatingtapes, mpmture controllers. J( write fur a tree bmchure. --.I
Ultmuiolet Photoelectron Spectroscopy; Brig@, D., Ed.; Heyden: London, 1977; pp. 79-119. (12) Rivihre, J. C. In Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy: Briggs, D.; Seah, M. P., Eda.; Wiley and Sons: Chichater, U.K., 1983;pp. 17-85. (13)Joshi,A.;Davis,L. E.;Palmburg, P. N. In Methods of Surface Analysis; Czanderna, A. W., Ed.; Elsevier: Amsterdam, 1975;pp. 159-222. (14) Wannberg, B.; Skallermo, A. J . Electron Spectroscopy and Related Phenome m 1977,IO,45. (15) Perkin-Elmer P h sical Electronics Technical Bulletin W1,February 1986,Eden Prairie, MN. (16) Cbaney. R. L. Surf. Interface A w l . 1987.10,36. (17) Yates, K. H.; West, R. H. Surf. Interface A w l . 1983.5,133. (18) Bramson, G.; Porter, H.; Turner, D. Nnture 1981,290,556.
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CIRCLE 18 ON READER SERVICE CARD
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, I989
Joseph A. Gardella, Jr., is a n associate professor of chemistry and codirector of t h e IndustrylUniuersity Cooperatiue Research Center for Biosurfaces a t t h e State Uniuersity of New York, Buffalo. H e receiued undergraduate degrees i n chemistry and philosophy f r o m Oakland Uniuersity (Rochester, MII and his Ph.D. from t h e University of Pittsburgh in 1981. His research efforts focus on the study o f organic and polymeric surfaces and interfaces with ion beam methods, t h e determination o f surface structurelcompositionproperty relationships i n multicomponent polymer materials, and t h e understanding of biologylmaterial interfacial chemistry. He is sometimes passionate about the philosophy of science, politics, and t h e three B'S of sport: bowling, basketball, and baseball.
