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Hydrogen ion desorption from amorphous carbon films induced by resonant core electron excitations
Nuclear Instruments and Methods in Physics Research B 268 (2010) 127–130
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Hydrogen ion desorption from amorphous carbon films induced by resonant core electron excitations Yutaka Mera a,*, Shijin Liang a, Takayuki Fujiwara a, Kiichiro Ishizaki a, Takuhiro Kakiuchi b, Kazuhiko Mase b, Eiichi Kobayashi b, Koji Okudaira c, Koji Maeda a a b c
Department of Applied Physics, The University of Tokyo, Hongo, Bukyo-ku, Tokyo 113-8656, Japan Photon Factory, High Energy Accelerator Research Organization (KEK-PF), Tsukuba, Ibaraki 305-0801, Japan Graduate School of Advanced Integration Science, Chiba University, Inage-ku, Chiba 263-8522, Japan
a r t i c l e
i n f o
Article history: Received 10 April 2009 Received in revised form 8 October 2009 Available online 10 November 2009 Keywords: Photo-stimulated desorption Hydrogen Tetrahedral amorphous carbon Electronic excitation XAS Soft X-ray Synchrotron radiation
a b s t r a c t In order to get insight into the mechanism of structural change in tetrahedral amorphous carbon (ta-C) films that is induced by soft X-ray illumination at photon energies near the carbon core edge, the desorption of ions from ta-C films, as a possible process taking place concurrently with the photo-induced restructuring, was studied by time-of-flight (TOF) measurements of photo-ions as a function of photon energy. The results show that (1) the main ions detected are H+, (2) the desorption efficiency spectra exhibit a resonant peak at 286–287 eV which is common to all detected ions, and (3) is 3 eV lower than the resonant peak in the efficiency spectrum of photo-induced restructuring. These rule out the hypothesis that it is the photo-induced C–H bond rupture that causes the resonant soft X-ray-induced restructuring in ta-C films. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon is a peculiar element which can form various structures in different dimensions with a wide variety of physical properties. Among such carbon solids, amorphous carbon is an intriguing material because it may be possible to modify the local structures and hence the local properties variably. Concerning amorphous carbon, it is known that thermal treatments do not induce any significant structure change when the annealing temperature is below 400–700 °C [1]. Such high structural stability of carbon solids under ambient conditions is due to the generally strong chemical bonds between carbon atoms. However, some studies have shown that carbon materials are able to transform with an unexpected ease to different structures by the assistance of electronic excitations, leading to non-thermal agitation of the electronic system of the solid. Recently, some of the present authors found that considerable structural changes are induced in tetrahedral amorphous carbon (ta-C) films by electron irradiation [2] and by soft X-ray illumination that excites 1s core electrons in carbon atoms [3]. In both cases, the electronic stimulation induces the development of graphitic ordering primarily absent in the as-
* Corresponding author. Address: 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel./fax: +81 3 5841 6852. E-mail address: [email protected] (Y. Mera). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.172
grown films. Similar effects of soft X-ray illumination are observed also in the desorption of ions from the ta-C films. This fact rises the hypothesis that the structural change is a consequence of ion desorption. The present experiment aimed to examine this hypothesis in the resonant core-excitation induced restructuring in ta-C by studying the ion desorption from the films under intense soft X-ray illumination in the C1s core-excitation energy range.
2. Experimental The ta-C films were prepared by depositing accelerated carbon ions on a Si substrate by a filtered cathodic vacuum arc method [4] basically under hydrogen-free condition. The films were in situ exposed to microwave plasma in Ar and O2 mixture gas to remove a few surface graphitic layers. Details of the sample preparation are described elsewhere [5]. The electron energy loss spectra (EELS) of the films grown at the acceleration energy of 100 eV exhibited no significant p* peak indicating the absence of graphitic structures in the interior of the films. The soft X-ray experiments were conducted at an undulator beam line BL-13C at KEK-PF in Tsukuba operated in the single-bunch mode using monochromatic soft X-rays of 1010 photons/s cm2 in intensity. The p-polarized light having passed through a grating monochromator with an energy resolution of E/DE = 700 was guided to a sample with an incident
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Y. Mera et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 127–130
angle of 84°. X-ray absorption spectra (XAS) were measured by total electron yield (TEY, the current draining from the sample due to photoelectrons) divided by the beam intensity (monitored by the current flowing out of a gold mesh placed in the upstream of the sample). The scale of photon energy was corrected so as to assume the p* peak at 284.5 eV. For the present experiments, the sample faced to a custom-designed analyzer consisting of a time-of-flight (TOF) spectrometer coaxially installed in a cylindrical mirror analyzer (CMA) for sorting photo-electrons according to their energy [6]. The mass of desorbed ions was analyzed by the TOF spectrometer which was triggered by the RF pulse synchronous to the electron bunch in the radiation ring so that the spectra exhibit peaks only for ions that are emitted from the sample in coincidence with the pulsed illumination repeated with a period of 624 ns. The desorption efficiency in a relative unit was assessed by dividing the counting rate of photo-ions by the sample current which represents the light intensity. To acquire the efficiency spectrum for the photo-stimulated desorption as a function of photon energy used for soft X-ray illumination, we conducted two types of measurements: specific ions discriminated by TOF spectra were counted individually, and all the ions collected by the analyzer were counted irrespective of the ion species. The kinetic energy distribution of ions emitted on soft X-ray illumination were measured by the CMA. All the experiments were conducted at room temperature under ultra high vacuum (UHV) conditions (<3 107 Pa).
3. Results and discussions Fig. 1 shows a typical TOF spectrum obtained when the sample was illuminated with photons of 290 eV energy near the C1s edge. The spectrum shown is a superposition of TOF spectra shifted by multiples of 624 ns, the period of photon pulses. The indicated ion species are those assigned by careful analysis of systematic
peak shifts with the voltages applied to the analyzer electrodes. þ The main observed ions were H+(m/e = 1), Hþ 2 (m/e = 2), CH3 (m/ e = 15), F+(m/e = 19), CO+(m/e = 28), NO+(m/e = 30), and Oþ 2 (m/ e = 32), besides some other ions of larger masses whose species could not be assigned definitely. No signature was found for desorption of C+(m/e = 12) and Cþ 2 (m/e = 24) ions for all photon energies ranging from 280 eV to 300 eV around the C1s core edge. The marks in Fig. 2 show the desorption efficiency spectrum of H+ ions measured three times by varying the photon energy for illumination. Fig. 2 also shows for comparison the desorption efficiency spectrum of total ion (blue line) dominated by H+ and the Xray absorption spectrum (XAS) (broken curve). The desorption efficiency spectrum exhibits a sharp resonant structure near the C1s core absorption edge, although there is only a faint shoulder structure at this energy in the XAS spectrum. This peak was commonly observed in the efficiency spectra (not shown) for other ion species separately deduced from the TOF spectra under illumination with various photon energies. The kinetic energy distribution of H+ photo-ions differed slightly depending on the photon energy hm used for illumination: it peaked at 5.0 eV for hm = 297 eV, but it peaked at 5.5 eV for hm = 287 eV, and at 6.0 eV for hm = 282 eV. The secondary event immediately induced by illumination at hm = 297 eV is a normal Auger process (the initial photo-ionization of C1s level by the soft X-ray followed by filling of the core hole by generating multiple holes in the valence states) which should not be resonant in nature. In fact, the efficiency spectrum shown in Fig. 2 is featureless in the energy range above 294 eV where the only triggered events are normal Auger processes. The increase of the kinetic ion energy on illumination with lower photon energy suggests that the photo-stimulated desorption at such energies is enhanced by a spectator Auger process in which the electron resonantly excited to an empty level of anti-bonding nature stays at the level assisting the bond instability caused by the succeeding Auger process that generates multiple holes [7].
800 mass=1 + H
mass=(27-)28 + CO mass=(29-)30 + NO
600
+
Ion Counts
reflection of H mass=(31-)32 + O2
mass=19 + F
400
mass=15 + CH3
mass=100-150
mass=2 + H2
200
0
0
100
200
300
400
mass=50-51
500
600
TOF (ns) Fig. 1. A typical time-of-flight (TOF) spectrum of ions emitted from a ta-C film illuminated with 290 eV photons in the intensity of 1010 photons/s cm2. Dominant ions are H+.
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Y. Mera et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 127–130
1st run 2nd run 3rd run TIY/TEY XAS-TEY
60
16
14
40
12 30 10 20 ∗
π
10
-3
18
50
+
H Ion Count Rate/Sample Current
20x10
8
Total Ion Count Rate/Sample Current
70
6 0 280
285
290
295
300
Photon Energy (eV) +
Fig. 2. H ion desorption efficiency spectra measured by TOF experiments (marks), total ion counting (blue) and XAS (dotted black) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The absence of C+ and C2+ ions in the TOF spectra means that carbon atoms are not desorbed from the surface although atomic displacements leading to structural changes must be induced inside the film by the soft X-ray illumination. In the core-excitation-induced restructuring in ta-C films [3], the excitation spectrum exhibits a characteristic peak at 289 eV either attributed to a resonant excitation of a C1s core exciton [8,9] or to an excitation to the anti-bonding C–H r* state [10–15]. Since it is well established in crystalline diamond and graphite that the formation of core exciton causes a temporary but substantial displacement of carbon atoms from the lattice position [16–20], the core exciton formation is a possible mechanism for the core-excitation-induced restructuring [3]. However, desorbed H+ ions were observed as the dominant ions in photo-stimulated desorption from the ta-C film although the film was grown with no intentional introduction of hydrogen in the film deposition process. Therefore, a plausible mechanism other than the spectator Auger process or core exciton formation is the rupture of C–H bonds that triggers the resonant photo-induced restructuring (not desorption). Experimentally, however, the peak position in the efficiency spectrum for the photo-induced desorption is 286–287 eV, considerably lower by 3 eV than the characteristic 289 eV peak in the excitation spectrum for photo-induced restructuring. Thus, the discrepancy of the peak position in the efficiency spectra disfavors the C–H bond rupture mechanism for the resonant core-excitation-induced restructuring in ta-C films. Nevertheless, the C1s XAS spectra associated with C–H bonds can vary depending on carbon coordination [21] and interaction with the substrate [22]. One might speculate that the origin of the faint shoulder in the surface-sensitive XAS spectra at 286–287 eV is the transition between the C1s core state to the anti-bonding state of C–H at least on the sample surface. However, the peak energy in the desorption efficiency spectra does not depend on the ion species, which also disfavors the hypothesis
that the primary excitation is associated with carbons bonded with hydrogen atoms. 4. Summary The photo-stimulated desorption from tetrahedral amorphous carbon films was studied in the energy range of soft X-rays near the C1s core edge. The time-of-flight spectra of the photo-ions indicate that the dominant desorbing ions are H+ with no trace of elemental carbon ions such as C+ and C2+. The desorption efficiency spectra exhibited a sharp resonant peak at 286–287 eV near the C1s edge. The peak energy is 3 eV lower than that in the core-excitation-induced restructuring in ta-C films, which disfavors the hypothesis that the resonant photo-restructuring is caused by rupture of C–H bonds. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2007G004). The authors acknowledge a financial support by Grant-in Aid from MEXT of Japan. One of the authors (S. Liang) is grateful for the scholarship support from MEXT of Japan.
References [1] R.U.A. Khan, S.R.P. Silva, in: E.S.P. Siva (Ed.), Properties of Amorphous Carbon, 2003, p. 250 (INSPEC). [2] S. Liang, Y. Mera, K. Maeda, Surf. Sci. 593 (2005) 161. [3] K. Maeda, S. Liang, Y. Harada, Y. Kitajima, S. Shin, Y. Mera, Proc. SPIE 65860C-1 (2007). [4] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, Phys. Rev. B 48 (1993) 7234.
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[5] S. Liang, T. Banno, Y. Mera, M. Kitajima, K. Ishioka, Y. Harada, Y. Kitajima, S. Shin, K. Maeda, New Carbon Mater. 23 (2008) 241. [6] E. Kobayashi, K. Mase, A. Nambe, J. Seo, S. Tanaka, T. Kakiuchi, K.K. Okudaira, S. Nagaoka, M. Tanaka, J. Phys.: Condens. Matter 18 (2006) S1389. [7] K. Mase, M. Nagasono, S. Tanaka, T. Urisu, E. Ikenaga, T. Sekitani, K. Tanaka, Surf. Sci. 390 (1997) 97. [8] A. Gutiérrez, M.F. Lopez, Europhys. Lett. 31 (1995) 299. [9] J. Díaz, S. Anders, X. Zhou, E.J. Moler, S.A. Kellar, Z. Hussain, J. Elec. Spec. Rel. Phenom. 101–103 (1999) 545. [10] G. Comelli, J. Stoehr, C.J. Robinson, W. Jark, Phys. Rev. B 38 (1988) 7511. [11] M. Jaouen, G. Tourillon, J. Delafond, N. Junqua, G. Hug, Diam. Relat. Mater. 4 (1995) 200. [12] M. Lübbe, D.A. Evans, T.U. Kampen, K. Okano, D.R.T. Zahn, Diam. Relat. Mater. 7 (1998) 247. [13] C.S. Lee, J.-K. Shin, K.Y. Eun, K.-R. Lee, K.H. Yoon, J. Appl. Phys. 95 (2004) 4829.
[14] G.A. Abba, P. Papakonstantinou, J.A. McLauphlin, Appl. Phys. Lett. 87 (2005) 251918. [15] S.C. Ray, K.P. Krishna Kumar, H.M. Tsai, J.W. Chiou, C.W. Pao, W.F. Pong, M.-H. Tsai, B.-H. Wu, C.-R. Sheu, C.-C. Chen, F.C.-N. Hong, H.-H. Cheng, A. Dalakyan, Thin Film Solids 516 (2008) 3374. [16] Y. Ma, P. Skytt, N. Wassdahl, P. Glans, J. Guo, J. Nordgren, Phys. Rev. Lett. 71 (1993) 3725. [17] F. Mauri, R. Car, Phys. Rev. Lett. 75 (1995) 3166. [18] S. Tanaka, Y. Kayanuma, Solid State Commun. 100 (1996) 77. [19] K.A. Mader, S. Baroni, Phys. Rev. B 55 (1997) 9649. [20] Y. Harada, T. Tokushima, Y. Takata, T. Takeuchi, Y. Kitajima, S. Tanaka, Y. Kayanuma, S. Shin, Phys. Rev. Lett. 93 (2004) 017401. [21] K. Weiss, H. Öström, L. Triguero, H. Ogasawara, M.G. Garnier, L.G.M. Pettersson, A. Nilsson, J. Elec. Spec. Rel. Phenom. 128 (2003) 179. [22] C. Wöll, J. Synchrotron Radiat. 8 (2001) 129.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Hydrogen ion desorption from amorphous carbon films induced by resonant core electron excitations Yutaka Mera a,*, Shijin Liang a, Takayuki Fujiwara a, Kiichiro Ishizaki a, Takuhiro Kakiuchi b, Kazuhiko Mase b, Eiichi Kobayashi b, Koji Okudaira c, Koji Maeda a a b c
Department of Applied Physics, The University of Tokyo, Hongo, Bukyo-ku, Tokyo 113-8656, Japan Photon Factory, High Energy Accelerator Research Organization (KEK-PF), Tsukuba, Ibaraki 305-0801, Japan Graduate School of Advanced Integration Science, Chiba University, Inage-ku, Chiba 263-8522, Japan
a r t i c l e
i n f o
Article history: Received 10 April 2009 Received in revised form 8 October 2009 Available online 10 November 2009 Keywords: Photo-stimulated desorption Hydrogen Tetrahedral amorphous carbon Electronic excitation XAS Soft X-ray Synchrotron radiation
a b s t r a c t In order to get insight into the mechanism of structural change in tetrahedral amorphous carbon (ta-C) films that is induced by soft X-ray illumination at photon energies near the carbon core edge, the desorption of ions from ta-C films, as a possible process taking place concurrently with the photo-induced restructuring, was studied by time-of-flight (TOF) measurements of photo-ions as a function of photon energy. The results show that (1) the main ions detected are H+, (2) the desorption efficiency spectra exhibit a resonant peak at 286–287 eV which is common to all detected ions, and (3) is 3 eV lower than the resonant peak in the efficiency spectrum of photo-induced restructuring. These rule out the hypothesis that it is the photo-induced C–H bond rupture that causes the resonant soft X-ray-induced restructuring in ta-C films. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon is a peculiar element which can form various structures in different dimensions with a wide variety of physical properties. Among such carbon solids, amorphous carbon is an intriguing material because it may be possible to modify the local structures and hence the local properties variably. Concerning amorphous carbon, it is known that thermal treatments do not induce any significant structure change when the annealing temperature is below 400–700 °C [1]. Such high structural stability of carbon solids under ambient conditions is due to the generally strong chemical bonds between carbon atoms. However, some studies have shown that carbon materials are able to transform with an unexpected ease to different structures by the assistance of electronic excitations, leading to non-thermal agitation of the electronic system of the solid. Recently, some of the present authors found that considerable structural changes are induced in tetrahedral amorphous carbon (ta-C) films by electron irradiation [2] and by soft X-ray illumination that excites 1s core electrons in carbon atoms [3]. In both cases, the electronic stimulation induces the development of graphitic ordering primarily absent in the as-
* Corresponding author. Address: 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel./fax: +81 3 5841 6852. E-mail address: [email protected] (Y. Mera). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.172
grown films. Similar effects of soft X-ray illumination are observed also in the desorption of ions from the ta-C films. This fact rises the hypothesis that the structural change is a consequence of ion desorption. The present experiment aimed to examine this hypothesis in the resonant core-excitation induced restructuring in ta-C by studying the ion desorption from the films under intense soft X-ray illumination in the C1s core-excitation energy range.
2. Experimental The ta-C films were prepared by depositing accelerated carbon ions on a Si substrate by a filtered cathodic vacuum arc method [4] basically under hydrogen-free condition. The films were in situ exposed to microwave plasma in Ar and O2 mixture gas to remove a few surface graphitic layers. Details of the sample preparation are described elsewhere [5]. The electron energy loss spectra (EELS) of the films grown at the acceleration energy of 100 eV exhibited no significant p* peak indicating the absence of graphitic structures in the interior of the films. The soft X-ray experiments were conducted at an undulator beam line BL-13C at KEK-PF in Tsukuba operated in the single-bunch mode using monochromatic soft X-rays of 1010 photons/s cm2 in intensity. The p-polarized light having passed through a grating monochromator with an energy resolution of E/DE = 700 was guided to a sample with an incident
128
Y. Mera et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 127–130
angle of 84°. X-ray absorption spectra (XAS) were measured by total electron yield (TEY, the current draining from the sample due to photoelectrons) divided by the beam intensity (monitored by the current flowing out of a gold mesh placed in the upstream of the sample). The scale of photon energy was corrected so as to assume the p* peak at 284.5 eV. For the present experiments, the sample faced to a custom-designed analyzer consisting of a time-of-flight (TOF) spectrometer coaxially installed in a cylindrical mirror analyzer (CMA) for sorting photo-electrons according to their energy [6]. The mass of desorbed ions was analyzed by the TOF spectrometer which was triggered by the RF pulse synchronous to the electron bunch in the radiation ring so that the spectra exhibit peaks only for ions that are emitted from the sample in coincidence with the pulsed illumination repeated with a period of 624 ns. The desorption efficiency in a relative unit was assessed by dividing the counting rate of photo-ions by the sample current which represents the light intensity. To acquire the efficiency spectrum for the photo-stimulated desorption as a function of photon energy used for soft X-ray illumination, we conducted two types of measurements: specific ions discriminated by TOF spectra were counted individually, and all the ions collected by the analyzer were counted irrespective of the ion species. The kinetic energy distribution of ions emitted on soft X-ray illumination were measured by the CMA. All the experiments were conducted at room temperature under ultra high vacuum (UHV) conditions (<3 107 Pa).
3. Results and discussions Fig. 1 shows a typical TOF spectrum obtained when the sample was illuminated with photons of 290 eV energy near the C1s edge. The spectrum shown is a superposition of TOF spectra shifted by multiples of 624 ns, the period of photon pulses. The indicated ion species are those assigned by careful analysis of systematic
peak shifts with the voltages applied to the analyzer electrodes. þ The main observed ions were H+(m/e = 1), Hþ 2 (m/e = 2), CH3 (m/ e = 15), F+(m/e = 19), CO+(m/e = 28), NO+(m/e = 30), and Oþ 2 (m/ e = 32), besides some other ions of larger masses whose species could not be assigned definitely. No signature was found for desorption of C+(m/e = 12) and Cþ 2 (m/e = 24) ions for all photon energies ranging from 280 eV to 300 eV around the C1s core edge. The marks in Fig. 2 show the desorption efficiency spectrum of H+ ions measured three times by varying the photon energy for illumination. Fig. 2 also shows for comparison the desorption efficiency spectrum of total ion (blue line) dominated by H+ and the Xray absorption spectrum (XAS) (broken curve). The desorption efficiency spectrum exhibits a sharp resonant structure near the C1s core absorption edge, although there is only a faint shoulder structure at this energy in the XAS spectrum. This peak was commonly observed in the efficiency spectra (not shown) for other ion species separately deduced from the TOF spectra under illumination with various photon energies. The kinetic energy distribution of H+ photo-ions differed slightly depending on the photon energy hm used for illumination: it peaked at 5.0 eV for hm = 297 eV, but it peaked at 5.5 eV for hm = 287 eV, and at 6.0 eV for hm = 282 eV. The secondary event immediately induced by illumination at hm = 297 eV is a normal Auger process (the initial photo-ionization of C1s level by the soft X-ray followed by filling of the core hole by generating multiple holes in the valence states) which should not be resonant in nature. In fact, the efficiency spectrum shown in Fig. 2 is featureless in the energy range above 294 eV where the only triggered events are normal Auger processes. The increase of the kinetic ion energy on illumination with lower photon energy suggests that the photo-stimulated desorption at such energies is enhanced by a spectator Auger process in which the electron resonantly excited to an empty level of anti-bonding nature stays at the level assisting the bond instability caused by the succeeding Auger process that generates multiple holes [7].
800 mass=1 + H
mass=(27-)28 + CO mass=(29-)30 + NO
600
+
Ion Counts
reflection of H mass=(31-)32 + O2
mass=19 + F
400
mass=15 + CH3
mass=100-150
mass=2 + H2
200
0
0
100
200
300
400
mass=50-51
500
600
TOF (ns) Fig. 1. A typical time-of-flight (TOF) spectrum of ions emitted from a ta-C film illuminated with 290 eV photons in the intensity of 1010 photons/s cm2. Dominant ions are H+.
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Y. Mera et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 127–130
1st run 2nd run 3rd run TIY/TEY XAS-TEY
60
16
14
40
12 30 10 20 ∗
π
10
-3
18
50
+
H Ion Count Rate/Sample Current
20x10
8
Total Ion Count Rate/Sample Current
70
6 0 280
285
290
295
300
Photon Energy (eV) +
Fig. 2. H ion desorption efficiency spectra measured by TOF experiments (marks), total ion counting (blue) and XAS (dotted black) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The absence of C+ and C2+ ions in the TOF spectra means that carbon atoms are not desorbed from the surface although atomic displacements leading to structural changes must be induced inside the film by the soft X-ray illumination. In the core-excitation-induced restructuring in ta-C films [3], the excitation spectrum exhibits a characteristic peak at 289 eV either attributed to a resonant excitation of a C1s core exciton [8,9] or to an excitation to the anti-bonding C–H r* state [10–15]. Since it is well established in crystalline diamond and graphite that the formation of core exciton causes a temporary but substantial displacement of carbon atoms from the lattice position [16–20], the core exciton formation is a possible mechanism for the core-excitation-induced restructuring [3]. However, desorbed H+ ions were observed as the dominant ions in photo-stimulated desorption from the ta-C film although the film was grown with no intentional introduction of hydrogen in the film deposition process. Therefore, a plausible mechanism other than the spectator Auger process or core exciton formation is the rupture of C–H bonds that triggers the resonant photo-induced restructuring (not desorption). Experimentally, however, the peak position in the efficiency spectrum for the photo-induced desorption is 286–287 eV, considerably lower by 3 eV than the characteristic 289 eV peak in the excitation spectrum for photo-induced restructuring. Thus, the discrepancy of the peak position in the efficiency spectra disfavors the C–H bond rupture mechanism for the resonant core-excitation-induced restructuring in ta-C films. Nevertheless, the C1s XAS spectra associated with C–H bonds can vary depending on carbon coordination [21] and interaction with the substrate [22]. One might speculate that the origin of the faint shoulder in the surface-sensitive XAS spectra at 286–287 eV is the transition between the C1s core state to the anti-bonding state of C–H at least on the sample surface. However, the peak energy in the desorption efficiency spectra does not depend on the ion species, which also disfavors the hypothesis
that the primary excitation is associated with carbons bonded with hydrogen atoms. 4. Summary The photo-stimulated desorption from tetrahedral amorphous carbon films was studied in the energy range of soft X-rays near the C1s core edge. The time-of-flight spectra of the photo-ions indicate that the dominant desorbing ions are H+ with no trace of elemental carbon ions such as C+ and C2+. The desorption efficiency spectra exhibited a sharp resonant peak at 286–287 eV near the C1s edge. The peak energy is 3 eV lower than that in the core-excitation-induced restructuring in ta-C films, which disfavors the hypothesis that the resonant photo-restructuring is caused by rupture of C–H bonds. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2007G004). The authors acknowledge a financial support by Grant-in Aid from MEXT of Japan. One of the authors (S. Liang) is grateful for the scholarship support from MEXT of Japan.
References [1] R.U.A. Khan, S.R.P. Silva, in: E.S.P. Siva (Ed.), Properties of Amorphous Carbon, 2003, p. 250 (INSPEC). [2] S. Liang, Y. Mera, K. Maeda, Surf. Sci. 593 (2005) 161. [3] K. Maeda, S. Liang, Y. Harada, Y. Kitajima, S. Shin, Y. Mera, Proc. SPIE 65860C-1 (2007). [4] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, Phys. Rev. B 48 (1993) 7234.
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[5] S. Liang, T. Banno, Y. Mera, M. Kitajima, K. Ishioka, Y. Harada, Y. Kitajima, S. Shin, K. Maeda, New Carbon Mater. 23 (2008) 241. [6] E. Kobayashi, K. Mase, A. Nambe, J. Seo, S. Tanaka, T. Kakiuchi, K.K. Okudaira, S. Nagaoka, M. Tanaka, J. Phys.: Condens. Matter 18 (2006) S1389. [7] K. Mase, M. Nagasono, S. Tanaka, T. Urisu, E. Ikenaga, T. Sekitani, K. Tanaka, Surf. Sci. 390 (1997) 97. [8] A. Gutiérrez, M.F. Lopez, Europhys. Lett. 31 (1995) 299. [9] J. Díaz, S. Anders, X. Zhou, E.J. Moler, S.A. Kellar, Z. Hussain, J. Elec. Spec. Rel. Phenom. 101–103 (1999) 545. [10] G. Comelli, J. Stoehr, C.J. Robinson, W. Jark, Phys. Rev. B 38 (1988) 7511. [11] M. Jaouen, G. Tourillon, J. Delafond, N. Junqua, G. Hug, Diam. Relat. Mater. 4 (1995) 200. [12] M. Lübbe, D.A. Evans, T.U. Kampen, K. Okano, D.R.T. Zahn, Diam. Relat. Mater. 7 (1998) 247. [13] C.S. Lee, J.-K. Shin, K.Y. Eun, K.-R. Lee, K.H. Yoon, J. Appl. Phys. 95 (2004) 4829.
[14] G.A. Abba, P. Papakonstantinou, J.A. McLauphlin, Appl. Phys. Lett. 87 (2005) 251918. [15] S.C. Ray, K.P. Krishna Kumar, H.M. Tsai, J.W. Chiou, C.W. Pao, W.F. Pong, M.-H. Tsai, B.-H. Wu, C.-R. Sheu, C.-C. Chen, F.C.-N. Hong, H.-H. Cheng, A. Dalakyan, Thin Film Solids 516 (2008) 3374. [16] Y. Ma, P. Skytt, N. Wassdahl, P. Glans, J. Guo, J. Nordgren, Phys. Rev. Lett. 71 (1993) 3725. [17] F. Mauri, R. Car, Phys. Rev. Lett. 75 (1995) 3166. [18] S. Tanaka, Y. Kayanuma, Solid State Commun. 100 (1996) 77. [19] K.A. Mader, S. Baroni, Phys. Rev. B 55 (1997) 9649. [20] Y. Harada, T. Tokushima, Y. Takata, T. Takeuchi, Y. Kitajima, S. Tanaka, Y. Kayanuma, S. Shin, Phys. Rev. Lett. 93 (2004) 017401. [21] K. Weiss, H. Öström, L. Triguero, H. Ogasawara, M.G. Garnier, L.G.M. Pettersson, A. Nilsson, J. Elec. Spec. Rel. Phenom. 128 (2003) 179. [22] C. Wöll, J. Synchrotron Radiat. 8 (2001) 129.