Comparison of X-ray-excited Auger Lineshapes of Graphite, Polyethylene and Diamond

SURFACE AND INTERFACE ANALYSIS, VOL. 24, 517-521 (1996)

Comparison of X-ray-excited Auger Lineshapes of Graphite, Polyethylene and Diamond

A. P. Dementjev* and M. N. Petukhov IRTM, RRC Kurchatov Institute, Kurchatov sq., Moscow 123182, Russia

The experimental x-ray-excited Auger lineshapes for graphite, diamond and polyethylene have been obtained. The comparison of N(E) C KVV spectral features of graphite, diamond and polyethylene has also been carried out. The coincidence of C KVV Auger lineshape and energy position between diamond and polyethylene spectra is observed and the reasons discussed. The features of the CKVV Auger lineshapes for graphite, polyethylene and diamond provide a useful identification of sp2 and sp' states in carbon solids.

INTRODUCTION ethylene,'g*20 and on the comparison of XPS and AES data for polymeric compounds.20

Auger electron spectroscopy is often capable of yielding information concerning the chemical state of atoms in the near-surface region of a solid. Its characteristics of surface sensitivity permit its wide application to prob- lems of chemical identification and concentration analysis,'-4 and basic to many of these applications is the assumption of 'finger print'. The C KVV Auger lineshapes for graphite (sp' hybridization) and diamond (sp3 hybridization) have different characteristic features, and derivative N'(E) CKVV spectra of diamond and gra~hi te~.~- ' ' are widely used to identify sp2 and sp3 states of carbon atoms in solids. Discussion and theoretical interpretation of N(E) C KVV Auger spectra for a set of carbon solids can be found in Ref. 4.

The interpretation of CKVV Auger spectra for sp2 hybridization has been performed using the graphite spectrum as an example. The comparison of experimental and theoretical N(E) CKVV spectra of graphite shows their close b e h a ~ i o u r . ~ . ' ~ - ~ ~ Th e N(E) CKVV Auger spectra of d i a m ~ n d ~ , ~ , ~ . ' and polyethylene' 5*16 are extensively investigated spectra for sp3 hybridization of carbon in solids. However, there are essential differences in the experimental data for diamond, whreas data for polyethylene are rather stable.15*16 It was shown in Refs 5 and 10 that the dispersion of experimental data for diamond depends on the excitation type of the Auger process, and in addition some difficulties exist in preparing the diamond surface.

In the present work, the x-ray-excited CKVV Auger lineshapes of graphite, polyethylene and diamond have been obtained. Comparison analysis of these Auger spectra has also been performed to eliminate charcteris- tic features. Because there is not a grain of evidence to suggest the correctness of the spectrum obtained for diamond, the polyethylene C KVV spectrum has been considered as a guide to recognize the diamond C KVV lineshape. This assumption is based on the similarity of XPS valence-band spectra for diamond' 7,' and poly-

* Author to whom correspondence should be addressed.

CCC 0142-2421/96/080517-05 0 1996 by John Wiley & Sons, Ltd.

EXPERIMENTAL ~ ~~

The XPS and X-ray-excited AES (XAES) data were obtained using a MK 11 VG Scientific spectrometer. Photoelectron and Auger processes were excited using an A1 Ka source with a photon energy of 1486.6 eV, and the vacuum in the analysis chamber was maintained at 5 x lo-'' Torr. Spectra were collected in the analyzer constant energy mode, with a pass energy E , = 20 eV for XPS and E, = 50 eV for XAES analysis. The C 1s and 0 1s XPS spectra were acquired with a 0.1 eV step size; a 0.3 eV step size was used for CKVV XAES spectra. The Auger spectra were obtained with an overall collecting time t, = 20-30 min and a signal-to- noise ratio of 500.

Samples of graphite, polyethylene and diamond were investigated. A calibration of C KVV Auger spectra was made by taking into account the charging effect and, according to the C 1s energy position, &,(C 1s) = 284.4 eV for and Eb(C Is) = 284.6 eV for poly- eth~1ene.l~ The graphite sample was in good contact with the spectrometer and the measured E,(C 1s) = 284.6 eV agrees well with data in Refs 12, 13 and 18.

Background subtraction was made on the basis of Shirley's method by using a standard VG Scientific program. Derivative Auger spectra were obtained by a standard procedure as in Ref. 8; a seven point convo- lute was used for derivation smoothing.

It should be mentioned that the electron kinetic energy of the CKVV spectra ( E , = 240 eV) coincides with the kinetic energy position of the C 1s level due to 0 Ka excitation2' As a rule, the Mg anode has a deeper oxidative layer than the A1 anode, so the use of A1 Ka excitation for C KVV Auger emission is preferred over Mg Ka.

The graphite spectrum was obtained from the plane of highly oriented pyrolytic graphite (HOPG). For the HOPG samples a layer were peeled off in two ways; in the outer air just prior to insertion into the vacuum system and in the vacuum conditions inside a prep- aration chamber. The 0 1s line in the XPS spectra was

Received 27 February 1996 Accepted 7 May 1996

518 A. P. DEMENTJEV AND M. N. PETUKHOV

absent and the C KVV lineshape was unchanged for both cases. A clean polyethylene surface was obtained by splitting of a double polyethylene skin in the outer air. Oxygen was not detected at this surface.

A surface of an artificial diamond specimen 3 x 3 x 1 mm3 was prepared by polishing with a diamond paste and ultrasonically washing in acetone immediately before insertion into the spectrometer, as in Ref. 11. Oxygen lines for the untreated diamond surface are present both in the valence-band spectrum [Eb(O 2s) = 24 eV] and in the wide spectrum [Eb(O 1s) = 532 eV].22 The level of contamination may be estimated by the C ls/O 1s ratio or by the intensity of the 0 2s peak. In Ref. 17 the C ls/O 1s ratio rose from 4 to 13 after the cleaning procedure and the valence-band spectrum showed no evidence of oxygen contamination. Accord- ing to our XPS measurements, the C ls/O 1s ratio was 3 for untreated diamond and 10 for a cleaned diamond surface.

RESULTS A N D DISCUSSION ~ ~

The raw Auger spectra of graphite, polyethylene and diamond after correction for the charging effect are shown in Fig. 1. The raw polyethylene spectrum in Fig. 1 is in good agreement with that in Refs 15 and 20. In Ref. 20, the C KVV Auger spectrum of polyethylene was obtained using x-ray monochromation. This technique eliminates the influence of 0 Ka radiation due to the presence of an oxidative layer on the surface of the x-ray source.z1 The similarity of polyethylene spectra in Ref. 20 and in Fig. 1 allows the suggestion that 0 Ka radiation did not deform the C KVV Auger line-shapes in Fig. 1.

Figure 2 demonstrates the raw diamond spectra before and after the cleaning procedure, corrected for the charging effect. The main intensities of both spectra differ dramatically in lineshapes, whereas the low- energy features ( E , < 250 eV) have a similar structure. The Auger lineshape for untrated diamond looks like that for polymers containing C-0 bonds.20 Taking into account valence-band and wide spectra analysis for

1 CKVV

240 260 280 Electron kinetic energy, eV

Figure 1. The raw Auger spectra of graphite, diamond and polyethylene after correction for the charging effect.

untreated _____- C KVV

240 260 280

Electron kinetic energy, eV

Figure 2. The raw Auger spectra of untreated and cleaned diamond after correction for the charging effect.

the untreated diamond surface (presence of 0 2s and 0 1s peaks)22 it is possible to conclude that the main contamination of the diamond surface is chemisorbed oxygen, which significantly changes Auger lineshape.

Figure 3 shows the N(E) CKVV Auger spectra of graphite and polyethylene after background subtraction. The Auger spectrum for graphite in Fig. 3 coincides well with that in Ref. 12 and differs from the lineshape in Refs 13 and 14 by the incorporation of the left feature (E, - 240 eV). The authors of Refs 13 and 14 assigned this feature to plasmon losses and excluded it from consideration. The N ( E ) CKVV spectrum of polyethylene in Fig. 3 and the Auger spectra for polyethylene in Refs 15 and 16 have identical spectrum structures A, B, C, D and E (as in Ref. 15) and different relative intensities of the left features of spectrum due to a distinction in background correction. The Auger spectra for polyethylene (sp3 hybridization) and graphite (sp2 hybridization) in Fig. 2 have three general differences :

B ." E:

2 .- d 2 0 -

I . , , , , , , , , I , , . .I, , , . I , , , , , , , 240 260 280


Figure 3. The N(E) Auger lineshapes of graphite and polyethylene after background subtraction. The energy positions of some maxima in the folded partial electron densities of up and II states of graphite are shown in the bottom of the f i g ~ r e . ' ~ The features of the polyethylene spectrum are identified as A, B, C, D and E.16

AUGER LINESHAPES OF GRAPHITE, PE AND DIAMOND 519

(1) the right shoulder of the graphite spectrum lies in the higher kinetic energy range of Auger electrons (7 eV at half-magnitude);

(2) the maximum of main intensities of both spectra have different energy positions and the main intensity of polyethylene spectrum has an apparently split structure identified above as A and B;

(3) the left shoulder of the polyethylene spectrum has two intrinsic features C and D, whereas that for the graphite spectrum has only one feature (the extreme left feature of polyethylene spectrum D and the left feature of graphite spectrum have the same energy positions);

(4) the right edge of the polyethylene spectrum has the weak characteristic feature E.

The N ( E ) Auger lineshape for diamond was studied but there are essential differences in the

experimental Auger lineshape for diamond. It is well known that electron irradiation during AES measurements often damages the diamond sample surface." Pepper'' investigated modification of the diamond surface by an electron beam. Using a moving diamond sample to reduce electron beam damage, Pepper obtained an Auger spectrum of the non-damaged diamond surface. Mizokawa et observed electron beam damage for natural diamond, diamond-like carbon film and graphite. They demonstrated more pro- nunciation of 'diamond' features in the XAES spectrum, which implies that the XAES spectrum is probably more representative of the true diamond Auger spectrum than the AES spectrum with electron excitation.

The raw Auger spectrum for cleaned diamond in Fig. 1 resembles the spectrum obtained in Ref. 5 and differs from the spectrum in Ref. 8, which is like the raw spectrum for contaminated diamond in Fig. 2. The Auger spectrum for diamond in Fig. 1 is very similar to that for polyethylene; this fact was not noted in the previous literature. Nevertheless, it is not surprising that the Auger spectra of diamond and polyethylene coincide very closely, because XPS valence-band spectra of d i a m ~ n d ' ~ , ' ~ and polyethylene' 9,20 have very close structures. According to data in Ref. 20, in spite of sig- nificant valence-band differences, the left features (C and D) and high-energy feature E are present in Auger lineshapes for a wide range of polymers containing C-C and C-H bonds, whereas only the structure of the main intensities in these C KVV spectra slightly changes.

A comparison of Auger lineshapes for diamond and polyethylene after background subtraction is present in Fig. 4. The XAES spectrum for diamond in Fig. 4 resembles the spectrum obtained in Ref. 5 and differs from the spectrum in Ref. 15. Both spectra in Fig. 4 contain the split main intensity (A and B), two characteristic features in the left shoulder (C and D) and the small feature near 280 eV (E).

The N'(E) CKVV spectra of graphite and diamond are shown in Fig. 5 to compare with other articles dealing only with derivative spectra. The derivative graphite spectrum in Fig. 5 agrees well with that in Refs 5-9. The derivative diamond spectrum in Fig. 5 is similar to the spectrum in Ref. 5 but differs from N'(E) spectra in Refs 6-11 (in Refs 6-7 and 9-11, electron excitation was used). The coincidence of energy posi-

A C KVV

___-__ -Diamond

D c /

240 260 280


Figure 4. The N(E) Auger lineshapes of diamond and polyethylene after background subtraction. The features of both spectra are identified as A, B, C, D and E.16

tions of the extreme left feature of diamond spectrum D and the left feature of graphite spectrum (Fig. 1) is usable to calibrate electron beam-excited C KVV Auger spectra.

The CKVV Auger spectrum of graphite has been widely studied and there is a known series of theoretical and experimental investigations of graphite Auger line- ~hape.~."- '~ A th eoretical lineshape is generated by self-folding of the one-electron density of states (DOS).23 According to the (as, a,, n)-configuration of DOS for graphite,13 the contributions of folds in order of increasing kinetic energy are: uS*as, as*ap, tss*n, ap*ap, ap*n, ~*n. ' ' - '~ Following this theoretical description, the maximum of the aP*tsp fold locates at 269 eV, and the right shoulder of the Auger spectrum for graphite (E, > 275 eV) contains ap*n and n*n folds (Fig. 3). Each experimental CKVV Auger spectrum extends to much lower electron kinetic energy than the theoretical spectrum that uses only the DOS self-

Houston et a1." incorporated the effects of hole-hole interactions to describe this extension; more- over, they corrected the theoretical description by

1 N(E) C KVV __Graphite

. I


Figure 5. The first-derivative XAES spectra for graphite and diamond obtained by numerical differentiation of the raw N ( E ) Auger spectra.

520 A. P. DEMENTJEV AND M. N. PETUKHOV

Graphite

c - 0

y

? h - >r. 8 ’ P

- . .. .-

d .-

240 260 280


Figure 6. The separation of n-like diamond surface states by the normalized graphite spectrum. The D-G line is the difference between the diamond spectrum and the normalized graphite spectrum. The insert shows the magnified right edges of the spectra.

dynamic initial-state screening effects. The authors in Ref. 12 drew attention to the characteristic feature near 240 eV and provided strong evidence that this feature is not a plasmon satellite, because the width of this feature is much narrower than that of the Auger line. They related the above-mentioned feature to dynamic final- state effects.

Dayan and Pepper15 and Ramaker4 made attempts to describe theoretically electron beam-excited C KVV Auger spectra of polyethylene and diamond. Dayan and Pepper” pointed out that the entire experimental spectra of diamond and polyethylene could not be understood in terms of existing theories. The self- convolution of DOS is very different in shape from the experimental peak and the experimental results cannot accommodate any of the existing theories on the Auger KVV lineshape.” In Ref. 4 the Auger lineshape for polyethylene was described using the theory of hole- hole correlation effects. To obtain the best fitting of the left shoulder (features C and D in Fig. 4), Ramaker4

240 260 280


Figure 7. Comparison of the N(E) lineshapes of feature E for the polyethylene spectrum and the normalized graphite spectrum. The P-G line is the difference between the polyethylene spectrum and the normalized graphite spectrum. The insert shows the magnified right edges of the spectra.

incorporated the initial-state shake Auger term arising when Auger decay occurs in the presence of a localized valence hole, which was created via a shake-off process during the initial ionization. The right feature of the experimental Auger spectrum for polyethylene near 280 eV (feature E in Fig. 4) Ramaker interpreted in terms of the Auger processes occurring in the presence of a localized electron created by resonant e~citation.~ However, the resonant satellites occur under electron excitation, and the appearance of these under x-ray- excitation of the Auger process is unlikely.

The diamond surface states are very similar to the behavior of 7c bonds of graphite and the surface orbitals form n-bonded chains.24 The surface states of diamond have binding energies similar to graphite n bondsz4 In conjunction with theoretical interpretation of Auger lineshape for graphite,”-14 the op*x and n*n folds of diamond surface states should be arranged in the kinetic energy region E , > 275 eV. Consequently, the experimental Auger spectrum of diamond is the sum of the spectrum for sp2 states and the spectrum for sp3 states. This is supported by the fact that the right shoulder of the diamond spectrum is described by the normalized graphite spectrum (Fig. 6). The area of the normalized graphite spectrum in Fig. 6 relates to sp2 states and allows the concentration of sp2 states in the near-surface region of the sample to be estimated. The estimated ratio of the area for the graphite spectrum in Fig. 6 to the rest of the diamond spectrum is 1/5. The procedure for obtaining the spectra in Fig. 6 provides an estimation of z-like states on the near-surface region of carbon solids. The same procedure was used for the polyethylene spectrum (Fig. 7), but feature E in this case is not described by the normalized graphite spectrum. So, the feature E for the polyethylene spectrum is not directly related to d i k e states and is probably associ- ated with the intrinsic process of Auger decay. It is con- ceivable that the presence of n-like states on the cleaned diamond surface causes a worse resolution of the diamond Auger spectrum features in comparison with polyethylene features, and some of the states in the right feature E of the diamond spectrum may be thought of as originating from the same intrinsic process as in polyethylene.

CONCLUSIONS

The coincidence of the N(E) C KVV spectra of diamond and polyethylene has been observed. Our N(E) and N’(E) CKVV Auger spectra for diamond contrast with previous experimental spectra. Most likely, the Auger spectrum obtained for diamond differs from previous literature data for the following reasons; the diamond surface was carefully cleaned; a signal-to-noise ratio of > 500 was used; and the experimental spectrum was not smoothed.

Taking into account the Auger data for polymeric compounds,20 valence-band and our experimental spectra, it is possible to conclude that a diamond sample having the polyethylene-like C KVV lineshape may be considered to be a well-prepared sample. The Auger lineshape for diamond and poly-

AUGER LINESHAPES OF GRAPHITE, PE AND DIAMOND 521

ethylene (Figs 1-4) is characteristic of sp3 carbon states in solids containing C-C and C-H bonds.

Comparison of graphite, polyethylene and diamond N(E) CKVV Auger spectra shows that the right shoulder of the graphite spectrum lies in the higher kinetic energy range of Auger electrons due to the presence of x-bond folds. The features of the left shoulder (Ek c 250 ev) of the C KVV spectrum indicate sp2 (one feature D) and sp3 (two features C and D) states in compounds containing C-C and C-H bonds. It is possible to

explain the right feature E of the diamond spectrum by folds of x-like surface states, but this explanation is unusable for the feature on the right in the polyethylene spectrum.

The coincidence of energy positions of the extreme left feature of the diamond spectrum (feature D in Fig. 4) and the left feature of the graphite spectrum (Fig. 3) is usable for identification of sp2 and sp3 states in carbon solids, as shown by Fig. 5.

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Comparison of X-ray-excited Auger Lineshapes of Graphite, Polyethylene and Diamond