Effects of Irradiation on the Graphite Density of States Analyzed by

Aug 21, 2008 - The use of the second derivative of these spectral regions allowed the identification of the main spectral components of the density of...
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J. Phys. Chem. C 2008, 112, 14412–14416

Effects of Irradiation on the Graphite Density of States Analyzed by Photoelectron Spectroscopy Giorgio Speranza* and Luca Minati FBK-IRST, I-38050 PoVo (Trento), Italy ReceiVed: May 5, 2008

The effect of Ar+ irradiation on the electronic structure of highly oriented pyrolytic graphite was examined by X-ray photoelectron spectroscopy. The valence band and core-valence-valence Auger lines, were investigated. The use of the second derivative of these spectral regions allowed the identification of the main spectral components of the density of state (DOS). A comparison of the virgin- and the irradiated-graphite double differentiated spectra enabled us to follow the effect of the Ar+ bombardment on the DOS components. A new interpretation of the electron rearrangement in the irradiated graphite and a comparison with those from amorphous carbon films are given. On the basis of our results we suggest the use irradiated graphite as a standard for the quantification of sp2, sp3 hybrids in C-based materials. 1. Introduction Historically the interest in irradiation effects on graphite derives from the application of this material as a moderator in thermal nuclear reactor. A long list of both experimental and theoretical works has been devoted to the study of the formation of vacancies and protrusions, the stress induced by interstitials and volume expansions caused by thermal spikes,1–11 the collision mechanisms and the production of defects,12-18 and the modifications of the electrical, optical, and chemical properties of the material after irradiation.19,20 The knowledge of the disorder effects at the atomic level is also important since it helps in understanding the role of defects in carbon nanosystems.7,21-23 In this respect scanning tunneling micrscopy (STM) has recently gained popularity due to the possibility of investigating the surface defects at the atomic scale.10,19,24-28 On the other hand STM allows mapping only surface states in the proximity of the Fermi level (in the simplified model of Selloni et al.,29 the voltage applied to the STM tip is typically in the (5 V range). It is then desirable to create a careful description of the defect influence on the electronic structure of graphite by using other DOS-sensitive probes. Here we study the effect of Ar+ irradiation of highly oriented pyrolytic graphite (HOPG) by using X-ray excited valence and Auger photoelectrons. Both of these spectral regions derive from the occupied DOS though the second one involving two valence electrons can be represented by a fold of the set of states composing the DOS (see ref 30) with a consequent loss of spectral definition. Interpretation of the Auger line shape requires recovering the unfolded DOS and the reduction of the spectrum on the “onehole” binding energy (BE) scale. In spite of the complexity required for the Auger inspection, there are at least two reasons for considering this spectral line: (i) in irradiated graphite valence band (VB) spectra are strongly affected by the presence of the argon peaks; (ii) VB and core-valence valence (KVV) Auger spectra show different cross-sections for s- and p-deriving features of the DOS. Then the combined use of these two * To whom correspondence should be addressed. Tel.: ++39 0461 314487. Fax: +39 0461 810851. E-mail: [email protected].

spectral regions allows a better interpretation of the DOS changes induced by the Ar+ bombardment. A common approach to extract information from Auger spectra is the use of the first derivative proposed many years ago,31-33 which is, in fact, very sensitive to line-shape changes, thus providing both structural and chemical information.33-37 On the other hand the analysis of the first derivative suffers from several problems, such as an inaccurate determination of the features’ positions and the poor estimate of their relative intensities.38 To solve these problems, alternative analytical methods were proposed to obtain more details from Auger spectra.39-43 Among them the negative of the second derivative (SD) has the advantage that peaks’ positions correspond to that of the spectral components, and it allows one to extract spectral features otherwise not visible.44,45 In previous papers we demonstrated that applying this method is possible to interpret the Auger spectrum of the pristine graphite on the basis of its band structure.46,47 Here we start from this description and extend the investigation to Ar+-irradiated graphite and amorphous carbon films produced by plasma processes with the aim to suggest irradiated graphite as a standard for a more reliable quantification of the sp2, sp3 hybrids in C-based materials. 2. Materials and Methods 2.1. Instruments and Sample Preparation. Before each data acquisition, the HOPG from Goodfellow (Goodfellow Cambridge Ltd. U.K.) was freshly cleaved under vacuum. The spectra were acquired in a Scienta ESCA 200 instrument equipped with a 200 mm hemispherical analyzer calibrated with respect to the Ag Fermi edge and a monochromated Al KR (1486.6 eV) X-ray source. Pass energy was set at 300 eV, and the correspondent energy resolution was ∼0.5 eV. The chamber base pressure was in the low 10-8 Pa. Ar+ irradiation was performed by using a Leybold IQE 12/38 ion gun operated at 3850 eV, producing a current density of ∼1 nA/cm2. After Ar+ bombardment, VB and KVV spectra together with survey and O1s core line were acquired to monitor the surface quality in terms of complete absence of oxygen. A series of five experiments were acquired at different irradiation times. For reference, spectra were acquired also on disordered poly-

10.1021/jp803941q CCC: $40.75  2008 American Chemical Society Published on Web 08/21/2008

Irradiation Effect on Graphite Density of States

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14413 TABLE 1: Assignments and Positions of the Second Derivative Peaks of HOPG and I-HOPG Obtained from VB and Auger Spectraa HOPG KVV assignment

HOPG I-HOPG VB VB

Q-2u(π)

2.7

Γ+3g(σ) Q+2g(σ) Γ-2u(π) Q+1u(σ) Q+1g(σ)

5.1 8.0 10.0 13.6 16.5 19.0 20.8

Γ+1g(σ)

1.1 2.7 5.3 7.5 b

13.4 16.5 18.0 19.8

I-HOPG KVV

Ek

one-hole BE

Ek

one-hole BE

280.2

2.2

280.9

1.8

275.3 268.9 262.5 258.6 253.7 246.7 239.4

4.5 7.7 10.9 12.9 15.3 18.8 22.7

274.7 269.5 263.6 258.1 254.1 246.3 241.3

4.8 7.4 10.4 13.1 15.1 19.0 21.5

a

Figure 1. Negative of the double differentiated VB spectrum from HOPG together with their assignment.

crystalline graphite (DPG) scraped in air just before insertion in the analysis chamber. Finally two plasma (13.56 MHz) assisted chemical vapor deposited (PACVD) amorphous carbon films were analyzed to compare the spectra of the irradiated HOPG with those from disordered carbon structures. The two samples were deposited using cyclohexane (C6H12) as a precursor gas at room temperature (sample a-C1) while the second (a-C2) is at 400 °C. 2.2. Data Manipulation. All the spectra were analyzed following the indications given in ref 30. Shirley background subtraction was performed on both VB and Auger bands. Spectra were then normalized to a common intensity. Extrinsic energy losses were deconvoluted utilizing those associated with the C1s peak after a proper rescaling of its intensity (see refs 30 and 46 for details). One of the major drawbacks of deconvolution is the corruption of the original signal due to a sensible decrease of the signal-to-noise ratio (SNR). To optimize the signal quality, both fast Fourier transform (FFT) and van Cittert algorithm were utilized to perform the deconvolution. Although deconvolution using the FFT algorithms is straightforward and allows data filtering before going back to the energy domain, the van Cittert method was used since it leads to a better white noise which may be suppressed by proper filtering. Relevant for the quality of the results is the filtering procedure for noise rejection. Both infinite- and finite-impulse response (IIR Butterworth, FIR) filters were implemented in MATLAB having negligible ripple in the pass-band and a high noise rejection in the stop band. After careful estimation of their performances a 61-coefficient FIR filter having -60 dB attenuation in the stop-band was chosen since it ensures zero-phase distortion. Gaussian windowing was applied to the original data to avoid discontinuities at the spectrum edges, and a proper zeropadding was applied to recover the constant retard of the FIR filter. Finally, the manipulation of the VB requires an additional careful procedure to suppress the Ar components. They are, in fact, the most intense structures of the I-HOPG, which, in the VB-SD, mask and distort the other features. The suppression of the Ar components was performed by replacing the peaks with linear segments matched to the original spectrum at their extremes to avoid improper oscillation of the SD induced by filtering. 3. Results Figure 1 presents the negative of the second derivative of HOPG together with the feature assignment. The trend of the

In this second case the energy values are given also in the “one-hole” BE scale. b In irradiated HOPG, Γ-2u(π) is overlapped to Ar 2p peak and then removed.

Figure 2. Example of integral VB spectra from pristine (solid line) and irradiated graphite (dots) together with their second derivative (with opposite sign). In the I-HOPG spectrum, Ar peaks are removed and gray bands refer to spectral regions where they normally fall.

double differentiated VB from HOPG is in substantial agreement with previously published data.46,47 The positions of the HOPGSD peaks are summarized in column 2 of Table 1. A comparison of VB spectra from pristine and I-HOPG in their integral and negative double differentiated form is shown in Figure 2. The efficiency of Ar peaks suppression performed on the I-HOPG spectrum can be appreciated. The gray zones represent the spectral regions where the Ar components fall. In these regions the SD does not show any remarkable oscillation, confirming the goodness of the Ar peak removal plus the edge matching procedure. Concerning I-HOPG, it is immediately clear that the irradiation induces strong changes in the DOS structure as it appears from the SD. Starting from the VB top, the π band initially represented by only one peak at 2.8 eV is now split by two components, one at the same BE as the original, the second more proximal to the Fermi level. A good correspondence of the SD oscillations is observed at 5.1 eV, while a slight shift occurs around 8 eV. No information can be recovered around 10 eV where the Ar 3p is located. Again good agreement is present in the range of 13-17 eV, while, in the remaining spectral portion until the VB bottom, the two SDs show pronounced differences. The peaks of VB-SD measured from I-HOPG are listed in the third column of Table 1. We compare now the Auger spectra from HOPG and I-HOPG. The integral and double differentiated spectra are

14414 J. Phys. Chem. C, Vol. 112, No. 37, 2008

Figure 3. Example of integral KVV Auger spectra from pristine HOPG (solid line) and I-HOPG (dots) with their second derivative (with opposite sign).

Figure 4. Example of integral KVV Auger spectra from DPG and (solid line) and I-HOPG (dots) together with their second derivative (with opposite sign).

shown in Figure 3. The SD peaks on the “two- and one-hole” BE scale48,49 are reported in columns 4-7 of Table 1. Relevant is the appearance of a small but important protuberance at ∼283.5 eV in the integral Auger spectrum of I-HOPG. This spectral change corresponds to a significant shift of the π band toward the Fermi edge. In the remaining part the two Auger spectra show oscillations which are slightly shifted without a defined rule. For this reason the spectrum is not immediately interpretable on the basis of the band structure obtained from the virgin graphite. Important in this respect is the comparison of the Auger lines from I-HOPG and from disordered polycrystalline graphite (DPG) together with their negative double differentiated spectra. This is shown in Figure 4. The spectral difference between the high-kinetic shoulders of the Auger lines is now almost disappeared. Concerning the SD oscillations, the peaks are now well superimposed on the whole energy range. Only a slight difference is present at the Auger bottom where the last SD oscillation from I-HOPG falls at lower kinetic energy. 4. Discussion The ability of the second derivative to single out components embedded in poorly structured spectra was recently demonstrated. In this respect, the signal preprocessing is essential to obtain meaningful results. The performance of the data filtering clearly appears when the second derivative

Speranza and Minati is applied to HOPG. Remarkable oscillations of the VB-SD occur at energies correspondent to those of HOPG theoretical band structure (see ref 50 and references therein). The same filtering was applied to VB coming from DPG and I-HOPG. In this second case Ar peak suppression is not a trivial procedure. For this reason we cannot exclude distortions in those regions (gray bars of Figure 2). The main differences observed in the HOPG- and I-HOPGSDs are located at the bottom and at the top of the DOS. Other authors observed DOS changes near to the Fermi level, analyzing the Auger first derivative spectrum.51-53 The band gap narrowing was interpreted as due to distortion in the pz-pz alignment caused by the bombardment. More recently atomic force microscope (AFM) and scanning tunneling microscopy were used to study the formation of surface protrusions in graphite after Ar+ bombardment.10,11,25,26,28 Comparing AFM and STM measurements, some authors demonstrated that the hillocks observed on the HOPG after Ar+ bombardment are not due to surface corrugation but rather to increased electron density in correspondence to defects. Besides experimental evidence, also theoretical works were carried out to describe the effect of defects on the graphite DOS.55,56 Lee et al.54 using extended Huckel molecular orbital were able to show that lattice vacancies create an enhancement of charge density, resulting in an enhanced chemical reactivity. This evidence is confirmed by other theoretical calculations.9,57,58 Our experimental results are in perfect agreement with the theoretical models, giving direct evidence of the increased DOS at the Fermi level. After irradiation the π band splits in two components: one at the same energy as in virgin graphite; the second closer to the Fermi edge (see Table 1). The theoretical values of the energy splitting between these two π bands range from 1.1 to ∼1.7 eV,9,58,59 which well-correlate with our experimental value of 1.6 eV. Slight differences among the component positions are found in the range of 5-17 eV. In this range the Ar+ bombardment seems to not deeply modify the electronic structure of graphite. A different situation is found at the bottom of the VB. In I-HOPG the σs band shows two structures at 18 and 19.8 eV, which do not superimpose with the σs band of HOPG. The presence of an Ar-suppressed region in the I-HOPG-VB prevent us from identifying other possible DOS features at higher BE. We can obtain this information from the Auger spectra. Differently from the VB spectra, Figure 3 shows us that the double differentiated Auger spectra from HOPG and I-HOPG are rather different. To interpret the origin of the SD peaks from I-HOPG, we will refer to Figure 5. In I-HOPG the first SD peak correspondent to the π band is shifted at higher BE. This component may derive only from the self-convolution of the two π bands found in the VB-SD. And in fact on the one-hole BE scale, this component falls in the middle of the two features at 1.7 eV (see Figure 5). Interestingly, in the range of 3-15 eV the main peaks of the Auger SD from I-HOPG are well overlapped to those of VB-SD from HOPG. This means that, in the presence of Ar+ irradiation, Auger transitions involving the same DOS feature (self-convolutions) dominate with respect to those deriving from mixed bands (cross-convolutions). It is as if the Ar irradiation, introducing strong perturbations in the DOS, were able to decouple the VB energy states, lowering the probability of mixed Auger transitions:

|〈xi|Hˆ|xj〉|2 < |〈xi|Hˆ|xi〉|2,

i*j

This effect, never described for graphite, was observed in polycrystalline diamond.38 The component at 15.1 eV of I-HOPG Auger SD does not have a parent feature in the

Irradiation Effect on Graphite Density of States

Figure 5. SD of VB and KVV spectra obtained from pristine and irradiated HOPG. Dashed bars indicate the position of the SD peaks obtained from I-HOPG to enlighten overlaps of the SD features.

VB-SD of HOPG. Probably it is the result of self- and, in a major part, cross-convolutions that in this region give the main contribution. The final two components at the DOS bottom may be related to the two σs components considering the distortion induced by the hole-hole interaction. Comparing the HOPG VB- and Auger-SDs, we estimate a hole-hole interaction equal to 1.9 eV in good agreement with literature.30,60 Concerning the I-HOPG we find 1.7 eV a slightly lower value with respect to that of HOPG. Two main effects may explain this: (i) the lower coupling between the VB electronic states from which the Auger process originates (in this case the two holes originate on the same atomic orbital and can recombine more easily, decreasing the correlation effects); (ii) more localized electric charge induced by irradiation leads to a reduction of the charge mobility and then to a reduction of screening effects, i.e., hole-hole interaction. Thanks to the Auger SD we can exclude the presence of other DOS features falling in the onehole region where Ar 3s components were suppressed. Finally the complete overlap between the Auger spectra from I-HOPG and DPG enables us to state that, apart from the π bands, the origin of the DOS changes observed in I-HOPG with respect to HOPG are due to disorder rather than irradiation. For the implications in the characterization of amorphous carbon films,61 it is interesting to assess if the irradiation process leads to a disordered structure comparable with that of amorphous carbon films obtained by plasma processes. This can be obtained by comparing their VB- and Auger-SDs as illustrated in Figure 6. Starting from a-C1 we find the same SD components of I-HOPG apart from the π-doublet that here reduces to only one peak and the lack of the σ feature at 18 eV. Concerning a-C2, all the p-components of I-HOPG-SD are preserved although they are shifted to higher BE. A perfect correspondence is then found in the range of 10-20 eV. This is not surprising since the production conditions strongly influence the a-C properties such as the optical gap. Concerning the Auger-SDs, both a-C1 and a-C2 exhibit a good correlation with VB-SDs. The Auger-SDs of a-C1 and a-C2 are very similar to each other and replicate that of I-HOPG. Differences are limited to the feature relative to the π component that in a-C2 falls in a different position since deriving from a VB doublet placed at higher BE with respect to I-HOPG. Moreover in a-C1 the Γ+3 g(σ) is scarcely visible, and the a-C2 DOS bottom ends with a single well-marked component.

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14415

Figure 6. SD of VB and KVV spectra obtained from irradiated HOPG and two plasma deposited carbon films (a-C1, a-C2). Dashed bars indicate the position of the SD peaks obtained from I-HOPG to enlighten overlaps of the SD features.

To conclude, even if this study is restricted to only two samples produced by PACVD, we can assert that irradiation of HOPG induces a degree of disorder that, to a certain extent, mimics that found in the analyzed amorphous carbon films. During the growth, the carbon film surface is exposed to a heavy ion bombardment. We can reasonably expect distortions, strained configuration, vacancies, and dangling bonds which are similar to those in I-HOPG. This is a very important result since here we demonstrate that irradiated graphite may be used as a standard to better estimate sp2, sp3 hybrids in a-C materials. 5. Conclusion In this work we analyzed the effects of irradiation on HOPGDOS. In particular we were able to obtain details of the irradiation-induced DOS changes by double differentiating VB and Auger spectra. Comparison of SD from pristine HOPG with those obtained from I-HOPG enlightens the formation of a new π band, the origin of which is linked to the increase of charge density at the vacancy sites. The remaining modifications introduced by Ar+ bombardment are mainly ascribable to disorder. The increased charge localization induced by irradiation leads to a decrease of the correlation effects. As a result, the hole-hole interaction which in HOPG was estimated as 1.9 eV decreases to 1.7 eV in I-HOPG. Finally the VB- and Auger-SDs enabled us to put in evidence that DOS features of I-HOPG are very similar to those from amorphous carbon films produced by PECVD. This justifies the use of irradiated graphite as a standard for more reliable quantifications of sp2, sp3 hybrids in a-C films. References and Notes (1) Kelly, B. T. J. Vac. Sci. Technol. 1986, A 4, 1171. (2) Birch, M. Brocklehurst, J. E. A ReView of the BehaVior of Graphite under the Conditions Appropriate for Protection of the First Wall of a Fusion Reactor; Springfields Nuclear Power Dev. Lab., United Kingdom At. Energy Auth.: Salwick Preston, Lancashire, UK, 1987; North Div. Rep. ND-R, U.K. At. Energy Auth., Nd-R-1434(s), 113 pp. (3) Brocklehurst, J. E. Kelly, B. T. A ReView of Irradiation-Induced Creep in Graphite under CAGR Conditions; Springfields Nuclear Power Dev. Lab., United Kingdom At. Energy Auth.: Salwick Preston, Lancashire, UK, 1989; North Div. Rep. ND-R, U.K. At. Energy Auth., ND-R-1406(s), 96 pp. (4) Hoffman, A.; Prawer, S.; Kalish, R. Phys. ReV. 1992, B45, 12736. (5) Tang, Z.; Hasegawa, M.; Shimamura, T.; Nagai, Y.; Chiba, T.; Kawazoe, Y.; Takenaka, M.; Kuramoto, E.; Iwata, T. Phys. ReV. Lett. 1999, 82 (12), 2532.

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