ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 9, pp. 1930–1934. © Pleiades Publishing, Ltd., 2012. Original Russian Text © M.M. Brzhezinskaya, L.A. Pesin, V.M. Morilova, E.M. Baitinger, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 9, pp. 1808–1812.
ATOMIC CLUSTERS
Manifestation of Auger Processes in C1sSatellite Spectra of SingleWalled Carbon Nanotubes M. M. Brzhezinskayaa, L. A. Pesinb, V. M. Morilovab, and E. M. Baitingerc a
HelmholtzZentrum Berlin für Materialien und Energie, HahnMeitnerPlatz 1, Berlin, 14109 Germany b Chelyabinsk State Pedagogical University, pr. Lenina 69, Chelyabinsk, 454080 Russia c Bundesanstalt für Materialforschung und Prüfung, Unter den Eichen 87, Berlin, 12205 Germany email:
[email protected] Received February 10, 2012
Abstract—Using the equipment of the Russian–German beamline of the synchrotron radiation at the BESSY II electron storage ring, satellite spectra accompanying the C1s core lines in the cases of singlewalled carbon nanotubes and highly ordered pyrolytic graphite have been measured with a high energy resolution. The Auger spectra corresponding to shaking of the valence system of carbon by the core vacancy have been found and investigated. The Auger spectra of the studied singlewalled carbon nanotubes and highly ordered pyrolytic graphite are caused by annihilation of the excited π* electron with a hole in the π subband. It has been established that the electron states in the conduction band have 3π* (Γ, K, M) symmetry; i.e., they cor respond to flat 3π* subband, which is localized by 12–13 eV above the Fermi level. It has been revealed that the general regularities of the distribution of electron states in the valence system insignificantly change dur ing its shakeup by the excited core. DOI: 10.1134/S1063783412090065
1. INTRODUCTION The use of the synchrotron radiation substantially extends the possibilities of photoelectron spectroscopy [1, 2]. This is also referred to the investigation of the electron structure of carbon nanotubes [3]. Satellites that exist near core lines of carbon in Xray photoelec tron spectra (XPS) are of special interest for research ers [4]. Satellites near core lines appear as a result of shakeup of the atomic electron system by the core vacancy. The mechanism of this process is rather com plicated [5]. Its essence is in that, after the formation of the core vacancy, valence electrons are transferred from ground state i with energy Ei under the effect of its field to a virtual orbital f with energy Ef. The band toband transitions, which are described by the expression P(i
f) =
∫ ϕ* ϕ dV , i
f
(1)
are called monopole transitions since all quantum numbers that characterize the initial state of the i atomic system are retained. Satellite spectra carry important information on the electron structure of objects and are used, in particular, to investigate car bon nanotubes. Main physical regularities of the formation of shakeup satellites near core lines in carbon materials with lowered dimensionality were originally consid ered using the example of the benzene ring, which is the main element of the structure of nanotubes [6].
The relative intensity of satellites for fullerene and nanotubes is 2–7% of the intensity of the C1s spec trum itself [7]. This article is devoted to the experimental study of the features of satellite spectra near the C1s line in sin glewalled carbon nanotubes (SWNTs). The main maxima and features of satellite spectra are caused by bandtoband transitions as well as by collective vibra tions of π and π + σ electron–hole pairs [8]. However, in this work, we studied the Auger spectra, which accompany satellites with the formation of valence vacancies. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Singlewalled nanotubes were synthesized by the electric arcdischarge method with the use of the nickelyttrium catalyst. Amorphous carbon and metal catalyst were removed from primary condensation products containing 15–20 wt % SWNTs by multiple oxidation in air at temperatures up to 550°C, which alternated with washing in hydrochloric acid [9]. As a result of purification, nanotubes in a form of the SWNT powder with the content of the main substance of about 80–85% were obtained. Purified nanotubes had a narrow distribution over the diameter near the average value of 1.5 nm and resided in the SWNT pow der in a strongly aggregated sate in a form of bars, microcrystal films, and coatings having the polycrys
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MANIFESTATION OF AUGER PROCESSES IN C1sSATELLITE SPECTRA
3. DESCRIPTION OF THE RESULTS Satellite SWNTs spectra are partially presented in Fig. 1. The dependences of photoemission intensity on the binding energy for three photon energies of PHYSICS OF THE SOLID STATE
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(a)
Intensity, arb. units
SWNTs
350 eV
380 eV 400 eV
π
0
π+σ
20
40 60 Binding energy, eV
80
100 (b) SWNTs
Intensity, arb. units
talline structure [10]. Dispersion of the SWNT pow der in the aqueous solution of the aqueous solution of the surfactant allowed us to separate the main impu rity of coarse graphite particles from nanotubes. Thus, we obtained the highpurity SWNTs with the content of the main substance of more than 98–99 wt % and a small impurity of ultradispersed particles of graphitized ash less than 20 nm in size. The crystal of highly oriented pyrolytic graphite (HOPG) with degree of perfection SPI1 Grade was purchased at SPI Supplies. XPS investigations were performed using the equipment of the Russian–German beamline of the BESSY II electron storage ring (Berlin) [11]. The XPS samples were prepared in air. The powders of single walled nanotubes under study were rubbed into the corrugated surface of the strip of metal indium 7 × 7 mm2 in size so that to provide the uniform coating of the substrate coating. The sample was fastened on a special holder, which was transferred from the load lock with a pressure of residual gases of ~10–8 Torr into the preparation chamber with the pressure of residual gases of ~1 × 10–9 Torr using a system for sample trans fer. Then, using the second transfer, the sample was transferred into a measuring chamber of the spectrom eter with ultrahigh vacuum of 2 × 10–10 Torr and fas tened their on the manipulator. The sample was arranged at an angle of ~45° to the incident SR beam. We also performed the comparative experiments for a sample of highly ordered pyrolytic graphite (HOPG) was mounted directly on a holder. The photoelectron spectra of C1s satellites and valence band were measured in a detection mode of normal photoemission using the measurement station of the Russian–German laboratory with a 180° hemi spherical analyzer Phoibos 150 produced by Specs with the spectral resolution of 50–200 meV. The energy calibration of the analyzer was performed by the 4f7/2, 5/2photoelectron spectra of gold. The mono chromator was calibrated by means of recording the ground photoelectron lines of C1s spectra excited by the radiation reflected from the diffraction grating in the first and second diffraction orders. The photon energy was varied from 325 to 1030 eV. Reference pho toelectron spectra were recorded for all the samples at the excitation energy of 1030 eV in the binding energy range of 0–900 eV in order to control their chemical composition. They showed no presence of contamina tions in the samples. No noticeable effects of charging the samples irradiated by the intense beam of the monochromatized SR were observed during the mea surements.
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400 eV
350 eV 5
15 25 Kinetic energy, eV
35
Fig. 1. Dependences of intensity of photoelectron satellite spectra of singlewalled carbon nanotubes on (a) the bind ing energy and (b) the kinetic energy of photoelectrons. Numbers near the spectral curves correspond to the pho ton energy. The Auger components of the spectra are shaded. The features in the spectra associated with the excitation of plasmons are denoted as π and π + σ.
350, 380, and 400 eV are presented in Fig. 1a, and those on the kinetic energy of photoelectrons are shown in Fig. 1b. The energy origin in Fig. 1a is taken to be 284.8 eV, which corresponds to the maximum of the C1s spectrum. Collective (plasma) excitations in Fig. 1a are marked by letters π and π + σ, respectively, for π plas mons and π + σ plasmons. Intensity of π + σplasma
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Auger spectra
SWNTs
Intensity, arb. units
Intensity, arb. units
HOPG
VB
SWNTs
8
16 24 Kinetic energy, eV
Auger
32 0
4
8 Energy, eV
12
16 (b)
Fig. 2. Auger spectra of singlewalled carbon nanotubes and graphite (dotted line).
losses depends on the photon energy and increases as the SR energy increases. The shaded portions of the spectrum correspond the Auger emission and envelope the range of kinetic energies from 15 to 30 eV (Fig. 1b). Their shape and location on the axis of kinetic energies are indepen dent of the photon energy. The HOPG test object also shows a similar shape of satellite spectra. In order to confirm this fact, two Auger spectra, namely, of the SWNTs and HOPG (dotted line) are presented in Fig. 2. The Auger spectra presented in Fig. 2 are obtained after the subtraction of the background and normaliz ing over the vertical scale. Both the approximately identical width of the spectra and the presence of two peaks at kinetic energies of ~13 and ~21 eV are com mon for these spectra. However, the ratio of relative intensities of these peaks is different, namely, the first one dominates in the Auger spectrum of SWNTs, while the second peak dominates in the Auger spec trum of the HOPG. The procedure of inverse of the selfconvolution for the Auger spectra of SWNTs and the test object for the comparison in the valence band (VB) was per formed. The intensity distribution in the valence band of SWNTs is measured at photon energy of 77 eV, while that for HOPG was measured at energy of 125 eV. The results are presented graphically in Fig. 3, where solid lines denote the inverted Auger spectra, while dotted lines denote the valence states. We superposed both types of the spectra by the location of the zero level corresponding to the Fermi energy. This superposition along the vertical scale is performed over the height of the corresponding peak.
Intensity, arb. units
HOPG
VB
Auger 0
4
8 Energy, eV
12
16
Fig. 3. Comparison of inverted Auger spectra with the spectra of valence states (dotted line) for (a) SWNTs and (b) HOPG. Vertical thin lines are plotted for the better comparison of the features in the spectra of different nature.
The Auger spectra involve only the πelectron states. For SWNTs (Fig. 3a), accordance between two spec tra of various nature can be considered satisfactory. Vertical lines are plotted here for the better compari son of thin features of two spectra. In the case of graphite (Fig. 3b), the Auger spectra show a certain fine structure near the Fermi level, which is absent in the photoelectron spectrum of valence states (dotted line).
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MANIFESTATION OF AUGER PROCESSES IN C1sSATELLITE SPECTRA (a) Virtual level
(b) π* CB
α
β π
VB
VB σ
C1s
Fig. 4. Schematic diagrams of (a) shakeup process [12] and (b) CVV process. Notations: VB and CB are the valence band and conduction band, respectively; π and σ are subbands, π* is the level in the conduction band, α and β are bandtoband transitions of the CV and VC types, respectively. Thick vertical arrows correspond to electrons, and the circles correspond to holes.
4. DISCUSSION OF THE RESULTS Expression (1) is the general formula for the prob ability of shakeup processes. Figure 4 represents the simplified twostage schematic from [12], which allows us to understand qualitatively the regularities of the shakeup shock of the atomic system during the formation of the core vacancy in it. The state of the atomic system excited by the Xray photon is consid ered starting (Fig. 4a). The escape of the core photo electron is conventionally denoted in Fig. 4a by the declined line. At the first stage, the hole appears at the core level, but the atom still does not emit the valence electron. At the second stage of the shakeup process, the valence electron is escaped to a certain virtual level. In both states, it is assumed that the nucleus charge increases by unity. The mentioned schematic representation is limited since it does not take into account the possibility of formation of other effects, for example, the formation of collective vibrations of the excited system or the Auger processes. Energy losses by the photoelectron with mentioned two steps Calculated energies in the CB of HOPG according to data [13] Point in the Brillouin zone with the marked state symmetry
Energy in the conduction band (EF = 0) in eV
π*(M) σ*(Γ) π*(Γ) 3π*(Γ, K, M) σ *1 (Γ, K, M)
~2 7–8 9.5–11 12–13 20
Note: Conventional notations of highsymmetry points of the recip rocal lattice of HOPG are parenthesized. PHYSICS OF THE SOLID STATE
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lead to an apparent decrease in the energy of the escaped 1s electron. These are the satellite spectra (Fig. 1a). It should be noted that the final state of the shakeup process is the atomic system with the hole in the valence band (VB) and excited electron in the con duction band (CB). Figure 4b shows as the presence of this electron– hole pair stimulates the Auger process with the con ventional CVV formula. The sequence of the CVV process involves annihilation of the electron–hole pair at the first stage (arrow α in Fig. 4b). The second stage involves the nonradiative transfer of the formed energy excess to the electron in the VB. Finally, at the third stage, the bandtoband transition of this excited elec tron to a free level in the CB occurs (arrow β in Fig. 4b). The kinetic energy of these last electrons in the discussed case of Auger processes in SWNTs and HOPG, as it follows from the experiment (Fig. 1b), exceeds 10 eV relative to the Fermi level. Calculated energies in critical points of the HOPG CB according to [13] are presented in the table. According to the tabulated data and experimental results (Fig. 1), the initial states during the annihila tion of the electron–hole pair (α in Fig. 4b) can be only π*(Γ) and/or 3π*(Γ, K, M) free states (the corre sponding level is conventionally denoted in Fig. 4b as π*). Since the shakeup process is characterized by the monopole bandtoband transitions from the π sub band (shaded rectangle). This completely corresponds to the results presented in Fig. 3. Indeed, the inverted CVV spectra of SWNTs and HOPG correlate well with the states of the valence band determined by the π sub band. 5. CONCLUSIONS In this article, new results of the experimental study of satellite spectra accompanying C1s core lines in sin glewalled carbon nanotubes and graphite are pre sented. The Auger spectra accompanying the shake up of the valence system of carbon by the core vacancy are found and studied. The Auger spectra of studied SWNTs and HOPG are characterized by the conven tional formula CVV. They are caused by annihilation of the excited π* electron with the hole in the π sub band. By means of comparison with the known calcu lations of the electron structure of graphite (the table), it is established that the electron states in the conduc tion band have 3π*(Γ, K, M) symmetry, i.e., they cor respond to the flat 3π* subband, which is localized above the Fermi level by 12–13 eV. The shape of inverted Auger spectra satisfactorily reproduces the shape of valence π states. This indicates that during the shakeup of the valence system by the excited core, the total regularities of the distributed electron states in it change insignificantly. This result completely corre sponds to the computed data of work [6], where it is shown that the account of the influence of the inter
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electron repulsion in the benzene ring leads only to a certain asymmetric broadening of the satellite spec trum compared with the singleelectron approxima tion, and the “tail” of this distribution is directed towards higher energies. This can be qualitatively explained by the fact that the interelectron repulsion promotes an increase in energy of bandtoband tran sition; however, this interaction itself is essential only for energies close to singleelectron ones. ACKNOWLEDGMENTS This work was fulfilled in the framework of the bilateral program “Russian–German Laboratory at BESSY.” REFERENCES 1. K. Codling, W. Gudat, E. E. Koch, A. Kotani, C. Kunz, D. W. Lynch, E. M. Rowe, B. F. Sonntag, and Y. Toyozawa, in Synchrotron Radiation: Techniques and Applications, Ed. by C. Kunz (Springer, Heidelberg, 1979; Mir, Moscow, 1981). 2. D. Briggs, J. C. Riviére, M. P. Seah, S. Hofmann, D. W. Harris, R. S. Nowicki, T. L. Barr, N. S. McIn tyre, T. C. Chan, G. C. Smith, P. Swift, D. Shuttle worth, P. A. M. Sherwood, S. D. Waddington, and C. D. Wagner, in Practical Surface Analysis by Auger and XRay Photoelectron Spectroscopy, Ed. by D. Briggs and M. P. Seah (Wiley, New York, 1983; Mir, Moscow, 1987).
3. M. M. Brzhezinskaya, N. A. Vinogradov, A. Zimina, V. E. Muradyan, Yu. M. Shul’ga, and A. S. Vinogradov, Appl. Phys. A: Mater. Sci. Process. 94, 445 (2009). 4. R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Phys ical Properties of Carbon Nanotubes (Imperial College Press, London, 1998). 5. T. Äberg, Phys. Rev. 156, 35 (1967). 6. M. S. Deleuze, M. G. Giuffreda, J.P. Francois, and L. S. Cederbaum. J. Chem. Phys. 112, 5325 (2000). 7. B. Gao, Z. Wu, and Y. Luo, J. Chem. Phys. 128, 234704 (2008). 8. H. Raether, Excitation of Plasmons and Interband Tran sition by Electrons (Springer, Berlin, 1980), p. 192. 9. A. V. Krestinin, N. A. Kiselev, A. V. Raevskii, A. G. Rya benko, D. N. Zakharov, and G. I. Zvereva, Eurasian Chem. Tech. J. 5, 7 (2003). 10. A. V. Krestinin, A. V. Raevskii, N. A. Kiselev, G. I. Zve reva, O. M. Zhigalina, and O. I. Kolesova, Chem. Phys. Lett. 381, 529 (2003). 11. S. L. Molodtsov, S. I. Fedoseenko, D. V. Vyalikh, I. E. Iossifov, R. Follath, S. A. Gorovikov, M. M. Brzhe zinskaya, Yu. S. Dedkov, R. Puettner, J.S. Schmidt, V. K. Adamchuk, W. Gudat, and G. Kaindl, Appl. Phys. A: Mater. Sci. Process. 94, 501 (2009). 12. B. Brena, S. Carniato, and Y. Luo, J. Chem. Phys. 122, 184316 (2005). 13. R. Tatar and S. Rabii, Phys. Rev. B: Condens. Matter 25, 4126 (1982).
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Translated by N. Korovin
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