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Ultraviolet photoelectron spectroscopy study of electronic states and deuterium adsorption on carbon nanowalls
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Chemical Physics Letters 450 (2008) 360–364 www.elsevier.com/locate/cplett
Ultraviolet photoelectron spectroscopy study of electronic states and deuterium adsorption on carbon nanowalls Ikuo Kinoshita a
a,*
, Saori Hayashi a, Hirofumi Yoshimura a, Hiroshi Nakai b, Masaru Tachibana a
International Graduate School of Arts and Science, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan b Ishikawajima-Harima Heavy Industries Co., Ltd., 1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan Received 27 April 2007; in final form 14 November 2007 Available online 21 November 2007
Abstract Electronic structure of carbon nanowalls (CNW) was measured by ultraviolet photoelectron spectroscopy (UPS) in comparison with highly oriented pyrolitic graphite (HOPG). While the interlayer band of CNWs was observed at same energy of that in HOPG, the r and p bands were observed as are in carbon nanotubes. Electronic effects by deuterium adsorption were investigated by combination of UPS and temperature programmed desorption (TPD) measurements. Deuterium adsorption preferably occurred in the boundary regions between crystallites in CNWs and enhanced the density of states associated with them. 2007 Elsevier B.V. All rights reserved.
1. Introduction Much attention is being devoted to carbon-based nanosized materials such as fullerenes, carbon nanotubes, and nanographenes. Their unique forms and inherent quantum size effects that cannot be attained in bulk materials present new mechanical, chemical, and electrical properties. Recently another two-dimensional carbon nanostructure called carbon nanowalls (CNWs) has been fabricated by Wu et al. [1]. The images of scanning electron microscope (SEM) for CNWs show that they stand vertically on the substrate forming wavy figures [2,3]. Raman spectra revealed that CNWs consist of small sized crystallites, which have high degree of graphitization [3,4] and the basal plane in the crystallites were found to be aligned vertically on the substrate. The structural characteristics of CNWs were also clarified by recent TEM observation [5]. Electronic structures of carbon-based materials have been investigated by photoelectron spectroscopy for graph-
*
Corresponding author. Fax: +81 45 787 2413. E-mail address: [email protected] (I. Kinoshita).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.11.038
ite [6], carbon nanotubes [7], and nanographenes [8]. It is indispensable to know the similarities and differences of electronic structure of CNWs comparing with the other kind of carbon materials in order to bring out functional properties for various applications such as electro-chemical energy storage and field electron emitter. Also investigation of hydrogen adsorption on CNWs is expected to bring additional information to the proceeding studies of hydrogen storage in carbon materials [9]. In this Letter, we report on the electronic structure of CNWs and the electronic effects caused by adsorption of atomic deuterium based on the results of ultraviolet photoelectron spectroscopy (UPS) and temperature programmed desorption (TPD) measurements. 2. Experimental CNWs were grown on Si(1 0 0) substrate. The Si(1 0 0) substrate was prepared by normal cleaning processes to be covered by oxide films. After flashing to 1000 K the silicon substrate in the growth chamber, CNWs were synthesized by plasma-enhanced chemical vapor deposition (PECVD) that has been described in detail elsewhere [3].
I. Kinoshita et al. / Chemical Physics Letters 450 (2008) 360–364
3. Results and discussion Fig. 1 depicts photoelectron spectra of CNW and HOPG. The spectra were plotted as a function of binding energy of electrons with respect to Fermi level. In the spectra of HOPG two peaks at 13.73 eV (A) and 5.35 eV (B) are observed. The peaks A is assigned to the final state feature related to the interlayer band that has large charge densities between carbon planes [13]. The peak B is assigned to p bands from the calculated band structure of graphite [14,15]. In the spectra of CNW, three structures were observed at the binding energies of 13.73 eV (A), 10.26 eV (C), and 3.22 eV (D). It can be recognized that the spectra reveal three remarkable features. The first feature is that
Normal Emission He I (21.22eV)
B Intensity (arb. units)
CH4, H2, and Ar were used as materials gases. During the growth procedure the substrate was heated up to 1000 K and the pressure was maintained at 7.2 · 103 Torr. For each grown CNW sample Raman spectrum and SEM image were measured. The dimensional definition of size has been schematically illustrated elsewhere [3]. From the SEM images the average length, height, and thickness of CNWs in the present experiment were estimated 1 lm, 0.5 lm, and 10 nm, respectively. The intensity ratio of D band to G band (ID/IG) in the Raman spectrum was 2.4. To carry out the UPS measurements the CNW samples were transferred in air into an ultrahigh vacuum (UHV) chamber and were cleaned by heating up to 1100 K. The UHV chamber was made of l-metals and the background pressure was 3 · 1010 Torr. For the UPS measurements a conventional He gas-discharge lamp was used as the source of unpolarized light at photon energy of 21.22 eV (He I) and a spherical deflection analyzer were used as the electron detector. To better understanding of photoelectron spectra for CNWs, spectra of highly oriented pyrolitic graphite (HOPG) were also measured. In order to compare with the CNWs, which stands right on the substrates, the HOPG sample was held vertically in the UHV chamber to direct an axis within the basal plane toward the electron analyzer, i.e., the c-axis of the basal plane of HOPG was at right angle (90 degree) to the electron analyzer as the measurements of CNWs done. On atomic hydrogen and deuterium adsorption to the CNWs, hydrogen or deuterium was dissociated and dosed to the samples through a hot tungsten filament placed about 3 cm away in front of the samples. TPD measurements were also carried out to study the deuterium adsorption to the CNWs with a quadrupole mass spectrometer equipped in the UHV chamber. Sample temperature was monitored by a chromel-constantan thermocouple. The thermocouple was welded to clip-like tantalum wire and attached in the center on the back of the silicon substrate [10]. The temperature was calibrated by comparing with the referenced values of desorption peaks of hydrogen and deuterium from bare Si measured elsewhere [11,12].
361
D A x15 CNW C
HOPG D CNW
B
HOPG –10 Binding Energy (eV)
0
Fig. 1. Photoelectron spectra of CNWs and HOPG. The spectra were plotted as a function of binding energy of electrons with respect to Fermi level. The HOPG sample was hold vertically to direct an axis within the basal plane toward the electron analyzer.
the intense peak (A) due to the interlayer band of CNWs is observed at the same energy as HOPG. The interlayer band was not observed in the spectrum of single-walled carbon nanotubes (SWNTs) [16], because of the lack of the two-dimensional interlayers in SWNTs. The observation of the interlayer band at the common energy suggests that CNWs contain two-dimensional interlayers as HOPG does. The suggestion is in good agreement with the indication of crystallites with high degree of graphitization in CNWs, which was shown in the previous reports of Raman spectroscopy [3]. The second feature is that the structure corresponding to the peak B of HOPG was missing in the spectra of CNWs. HOPG is a crystalline material with uniform c-axis orientation. On the other hand, CNWs consist of crystallites with random c-axes restricted in parallel to the substrate as the polarization dependence of Raman spectroscopy revealed [4]. The uniformity in HOPG and the randomness in CNWs of c-axes orientations might be thought to make the existing p bands detected and undetected, respectively. However, in the vertically aligned configuration of the present measurements for both of HOPG and CNWs, the spectra are reflected in allowed transitions at all wave number parallel to the basal plane. Moreover, with unpolarized light, the transition dipole to excite the electrons in the band does not need to be uniform to form the peak B in the spectrum. Therefore, the result suggests that the p band in the crystallites is not undetected but is perturbed. The p band missing allows the two possible models to speculate about the structure of CNWs. One is the different geometries from the HOPG. In this model the crystallites
I. Kinoshita et al. / Chemical Physics Letters 450 (2008) 360–364
CNW Normal Emission He I (21.22eV) Intensity (arb. units)
of CNWs are made of curving structures. However, this model contradicts the experimental results suggesting that CNWs consist of crystallites with highly graphitization. The alternative model is that CNWs consist of small sizes of good graphitized flat crystallites and the neighboring crystallites position with folding angles. In this model the interlayer band is formed between the highly graphitized basal planes and at exactly the same energy as HOPG. The p bands in the small sizes of crystallites are considered to be influenced and disturbed by the neighboring crystallites. The previous Raman spectroscope measurements also revealed that CNWs consist of small sizes of crystallites. The third feature is that peaks C and D were observed at almost same energies as were referenced in the photoelectron spectra for multi-walled carbon nanotubes (MWCTs) [7]. The reported photoelectron spectra for aligned and unaligned MWCTs with He I showed two peaks corresponding to the peaks C and D in the present spectra of CNWs. In contrast to the highly graphitized crystallites in the CNWs, this identical observation indicates that CNWs contains similar electronic structures of MWCTs supposed to be with hybridization of r and p electronic states due to curvatures. It seems to be conflicting in a sense that the spectra show both graphite- and tube-like features. However, the CNWs are considered to consist of boundary regions surrounding the small sizes of crystallites. In the growth processes of crystallites, the termination of the honeycomb structures and the conjunction between them in the boundary possibly arrange the carbon atoms in folding structures. The folding structure allows the hybridization of the r and p electronic states. In these boundary regions the MWCTlike electronic structure should be formed. Now we move to the effects on the electronic structure caused by atomic deuterium adsorption. Fig. 2 shows photoelectron spectra depending on the amount of deuterium exposure. The inset shows the expanded spectra of clean surface after flashing to 1060 K (s), exposed in atomic deuterium of 100 L (h), 400 L (4), and 1000 L (·). Exposures and measurements were carried out at 340 K of substrate temperature. These spectra were normalized by the intensities at the binding energy of 13.73 eV, which changed in a fraction of 5% with exposure and flashing. With the atomic deuterium exposures, photoelectron intensity between binding energies of 4 and 12 eV increases. The increase was almost saturated with 100 L of exposure, but still kept showing positive correlation with the amount of exposure of atomic deuterium until 1000 L. The same exposures of hydrogen and deuterium were carried out on HOPG, however, this kind of enhancement was not observed. It is found that from the comparison with HOPG, this enhancement of density of states (DOS) is due to the preferable deuterium adsorption on the peculiar sites of CNWs which are excluded in the HOPG. Electronic effects caused by adsorption of atomic hydrogen on graphite surfaces have been reported [17], where the same kind of DOS
Intensity (arb. units)
362
–10 –5 Binding Energy (eV)
–20
–10 Binding Energy (eV)
0
Fig. 2. Deuterium exposure dependence of photoelectron spectra. The inset shows the expanded spectra of clean surface after flashing to 1060 K (s), exposed by atomic deuterium in 100 L (h), 400 L (4), and 1000 L (·).
enhancement was seen for graphite surface after hydrogen plasma treatment in photoelectron spectra. The study revealed chemisorption of atomic hydrogen and atomic vacancy formation on the basal plane of graphite. Plasma treatments are considered to make the graphite surface rough. Hence, this kind of enhancement occurred with the adsorption of hydrogen or deuterium in boundary regions of CNWs. The DOS enhancement is considered to be due to the formation of bonding orbitals between the carbon and deuterium atoms. The correlation of the DOS enhancements with boundary region of CNWs is confirmed from the results of photoelectron spectra as shown in Fig. 3, which depicts photoelectron spectra depending on flashing temperature after exposure in atomic deuterium. The spectra were measured for clean sample after flashing to 1060 K (s), and for sample exposed in atomic deuterium of 600 L at 340 K (h), followed by flashing to 680 K (4), and 950 K (·). Every measurement was carried out after waiting for the sample temperature cool down to 340 K following each procedure. TPD spectrum of deuterium from CNWs is also shown in the inset. The amount of deuterium exposure was 300 L and heating rate was 3 K/sec. The mass signals were plotted after substituted by background signals. Photoelectron intensity at the binding energy between 4 eV and 12 eV increased after atomic deuterium exposure as also seen in Fig. 2. This DOS enhancement was kept even after flashing to 680 K. However, after flashing to 950 K the intensity reduced and went back to that in clean sample. In the TPD spectrum, desorption peaks of deuterium appeared slightly around 450 K and mainly around 800 K. From the correlation of UPS and TPD, it is found
CNW Normal Emission He I (21.22eV)
CNW D exposure 300L 3K/s
400
600 800 1000 Temperature (K)
Before exposure D exposure 100 L D exposure 400 L D exposure 1000 L
CNW Normal Emission He I (21.22eV) –10
363
Intensity (arb. units)
Intensity (arb. units)
D 2 TPD msss signal
I. Kinoshita et al. / Chemical Physics Letters 450 (2008) 360–364
–5
–17.0
0
Fig. 3. Flashing temperature dependence of photoelectron spectra after exposure in atomic deuterium. The spectra were measured for clean sample after flashing to 1060 K (s), and for sample exposed in atomic deuterium of 600 L at 340 K (h), followed by flashing to 680 K (4), and 950 K (·). The inset shows TPD of deuterium from CNWs. The amount of deuterium exposure was 300 L and heating rate was 3 K/sec.
that the main desorption of deuterium around 800 K cancelled the DOS enhancements. The peak around 450 K is considered to be due to desorption from the terrace site of crystallites in CNWs in accordance to the previous thermal desorption measurements for HOPG [18]. On the other hand, the peak around 800 K is considered to be due to desorption from the boundary regions. In the work of thermal desorption of hydrogen from C:H films [19], higher peak around 800 K was attribute to sp3 hybridized CHx (x = 1–3). The same conclusion was reported in the study of thermal desorption of hydrogen from carbon nanosheets [20]. These results confirmed that the enhancement of DOS is due to the deuterium adsorption strongly bound to the dominant sites supposed to be boundary regions in the CNWs. Finally the work function change of CNWs with deuterium adsorption is shown. Fig. 4 shows the low-energy cutoff of the photoelectron spectra as a function of binding energy relative to the Fermi level. A sample bias of 3 V was applied to determine the work function of the sample. The work function of CNW before exposure was 4.3 eV (s). Gradual reduction of work function was measured with the increased amount of atomic deuterium exposure of 100 L (h), 400 L (4), and leaded to lowering by 0.2 eV to a value of 4.1 eV at 1000 L (·). Ruffieux et al. have reported the work function changes of single-walled nanotubes (SWNTs) and graphite with adsorption of atomic hydrogen [21]. The work functions of SWNTs and graphite were 4.4 eV and 4.5 eV, respec-
–16.0 Binding Energy (eV)
Binding Energy (eV)
Fig. 4. The low-energy cutoff of the photoelectron spectra as a function of binding energy relative to the Fermi level. A sample bias of 3 V was applied to determine the work function of the sample. The work function of CNW before exposure was 4.3 eV.
tively, and lowered by 0.3 eV to 4.1 eV and 0.4 eV to 4.1 eV, respectively. It is recognized from their results and the present results of CNWs that the work functions of these three kinds of carbon-based structures decrease in order of graphite, SWNTs, and CNWs and reduce to the common value of 4.1 eV with adsorption of hydrogen or deuterium. The work function change by hydrogen adsorption on a surface of the carbon-based structures is considered to have two factors. One is due to the polarized C–H bond [17,21]. The adsorbed hydrogen atoms make a new surface dipole layer with positive charge on the vacuum side resulting in the reduction of work functions. The other is due to the structural distortion by the relaxation of carbon atoms. In the case of CNWs, deuterium atoms adsorbed on the dominant boundary regions, in which the carbon atoms may already have relaxed to their minimum energy positions. The work function change of 0.2 eV is supposed to be due to the C–H bond. While in the case of HOPG, the work function change of 0.4 eV for graphite surface is supposed to include the factor of carbon layer distortion in addition to that of the C–H bond. Provided that the work function change originating from the C–H bond is evaluated 0.2 eV for all these kinds of carbon based structures, the change due to distortion of the flat carbon layers in HOPG could be evaluated around 0.2 eV. 4. Conclusion We have presented photoelectron spectra of CNWs in comparison with HOPG and electronic effects with deute-
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rium adsorption. The observed intense interlayer band indicates that the CNWs consist of crystallites with high degree of graphitization. While, the delocalized p bands in the small sizes of crystallites are perturbed by the neighboring crystallites in boundary regions. In addition, electronic structure similar to that of MWCTs is included in the CNWs suggesting that the electronic structure of dominant boundary regions hybridizes r and p electronic states and has sp3 character. On the base of UPS and TPD measurements, adsorption of deuterium is mainly occurred on the boundary regions resulting in strong bounds and DOS enhancements. Acknowledgement This work was supported by Strategic Research Project (K18030) in Yokohama City University. References [1] Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater. 14 (2002) 64 (Weinheim, Ger.). [2] Y. Wu, B. Yang, B. Zong, H. Sun, Z. Shen, Y. Feng, J. Mater. Chem. 14 (2004) 469. [3] S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibana, J. Appl. Phys. 97 (2005) 104320. [4] Z. Ni, H. Fan, Y. Feng, Z. Shen, B. Yang, Y. Wu, J.Chem. Phys. 124 (2006) 204703.
[5] K. Kobayashi, M. Tanimura, H. Nakai, A. Yoshimura, H. Yoshimura, K. Kojima, M. Tachibana, J. Appl. Phys. 101 (2007) 094306. [6] P. Oelhafen, P. Pfluger, E. Hauser, H. Gu¨ntherodt, Phys. Rev. Lett. 44 (1980) 197. [7] S. Suzuki et al., Phys. Rev. B 63 (2001) 245418. [8] I. Kinoshita, D. Ino, K. Nagata, K. Watanabe, N. Takagi, Y. Matsumoto, Phys. Rev. B 65 (2002) 241402(R). [9] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [10] H. Nishino, W. Yang, Z. Dohnalek, V.A. Ukraintsev, W.J. Choyke, J.T. Yates, J. Vac. Sci. Technol. A 15 (1997) 182. [11] J.H. Kang, S.K. Jo, J. Lee, B. Gong, D. Lim, J.M. White, J.G. Ekerdt, Phys. Rev. B 59 (1999) 13170. [12] K.W. Kolasinski, W. Nessler, K. Bornscheuer, E. Hasselbrink, Surf. Sci. 331 (1995) 485. [13] D. Marchand, C. Fre´tigny, M. Lague¨s, F. Batallan, Ch. Simon, I. Rosenman, R. Pinchaux, Phys. Rev. B30 (1984) 4788. [14] G.S. Painter, D.E. Ellis, Phys. Rev. B 1 (1970) 4747. [15] J.C. Boettger, Phys. Rev. B 55 (1997) 11202. [16] S. Suzuki, C. Bower, Y. Watanabe, O. Zhou, Appl. Phys. Lett. 76 (2000) 4007. [17] P. Ruffieux, O. Gro¨ning, P. Schwaller, L. Schlapbach, P. Gro¨ning, Phys. Rev. Lett. 84 (2000) 4910. [18] T. Zecho, A. Gu¨ttler, X. Sha, B. Jackson, J. Ku¨ppers, J. Chem. Phys. 117 (2002) 8486. [19] A. Schenk, B. Winter, J. Biener, C. Lutterloh, U.A. Schubert, J. Ku¨ppers, J. Appl. Phys. 77 (1995) 2462. [20] X. Zhao, R.A. Outlaw, J.J. Wang, M.Y. Zhu, G.D. Smith, B.C. Holloway, J. Chem. Phys. 124 (2006) 194704. [21] P. Ruffieux, O. Gro¨ning, M. Bielmann, P. Mauron, L. Schlapbach, P. Gro¨ning, Phys. Rev. B 66 (2002) 245416.
Chemical Physics Letters 450 (2008) 360–364 www.elsevier.com/locate/cplett
Ultraviolet photoelectron spectroscopy study of electronic states and deuterium adsorption on carbon nanowalls Ikuo Kinoshita a
a,*
, Saori Hayashi a, Hirofumi Yoshimura a, Hiroshi Nakai b, Masaru Tachibana a
International Graduate School of Arts and Science, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan b Ishikawajima-Harima Heavy Industries Co., Ltd., 1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan Received 27 April 2007; in final form 14 November 2007 Available online 21 November 2007
Abstract Electronic structure of carbon nanowalls (CNW) was measured by ultraviolet photoelectron spectroscopy (UPS) in comparison with highly oriented pyrolitic graphite (HOPG). While the interlayer band of CNWs was observed at same energy of that in HOPG, the r and p bands were observed as are in carbon nanotubes. Electronic effects by deuterium adsorption were investigated by combination of UPS and temperature programmed desorption (TPD) measurements. Deuterium adsorption preferably occurred in the boundary regions between crystallites in CNWs and enhanced the density of states associated with them. 2007 Elsevier B.V. All rights reserved.
1. Introduction Much attention is being devoted to carbon-based nanosized materials such as fullerenes, carbon nanotubes, and nanographenes. Their unique forms and inherent quantum size effects that cannot be attained in bulk materials present new mechanical, chemical, and electrical properties. Recently another two-dimensional carbon nanostructure called carbon nanowalls (CNWs) has been fabricated by Wu et al. [1]. The images of scanning electron microscope (SEM) for CNWs show that they stand vertically on the substrate forming wavy figures [2,3]. Raman spectra revealed that CNWs consist of small sized crystallites, which have high degree of graphitization [3,4] and the basal plane in the crystallites were found to be aligned vertically on the substrate. The structural characteristics of CNWs were also clarified by recent TEM observation [5]. Electronic structures of carbon-based materials have been investigated by photoelectron spectroscopy for graph-
*
Corresponding author. Fax: +81 45 787 2413. E-mail address: [email protected] (I. Kinoshita).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.11.038
ite [6], carbon nanotubes [7], and nanographenes [8]. It is indispensable to know the similarities and differences of electronic structure of CNWs comparing with the other kind of carbon materials in order to bring out functional properties for various applications such as electro-chemical energy storage and field electron emitter. Also investigation of hydrogen adsorption on CNWs is expected to bring additional information to the proceeding studies of hydrogen storage in carbon materials [9]. In this Letter, we report on the electronic structure of CNWs and the electronic effects caused by adsorption of atomic deuterium based on the results of ultraviolet photoelectron spectroscopy (UPS) and temperature programmed desorption (TPD) measurements. 2. Experimental CNWs were grown on Si(1 0 0) substrate. The Si(1 0 0) substrate was prepared by normal cleaning processes to be covered by oxide films. After flashing to 1000 K the silicon substrate in the growth chamber, CNWs were synthesized by plasma-enhanced chemical vapor deposition (PECVD) that has been described in detail elsewhere [3].
I. Kinoshita et al. / Chemical Physics Letters 450 (2008) 360–364
3. Results and discussion Fig. 1 depicts photoelectron spectra of CNW and HOPG. The spectra were plotted as a function of binding energy of electrons with respect to Fermi level. In the spectra of HOPG two peaks at 13.73 eV (A) and 5.35 eV (B) are observed. The peaks A is assigned to the final state feature related to the interlayer band that has large charge densities between carbon planes [13]. The peak B is assigned to p bands from the calculated band structure of graphite [14,15]. In the spectra of CNW, three structures were observed at the binding energies of 13.73 eV (A), 10.26 eV (C), and 3.22 eV (D). It can be recognized that the spectra reveal three remarkable features. The first feature is that
Normal Emission He I (21.22eV)
B Intensity (arb. units)
CH4, H2, and Ar were used as materials gases. During the growth procedure the substrate was heated up to 1000 K and the pressure was maintained at 7.2 · 103 Torr. For each grown CNW sample Raman spectrum and SEM image were measured. The dimensional definition of size has been schematically illustrated elsewhere [3]. From the SEM images the average length, height, and thickness of CNWs in the present experiment were estimated 1 lm, 0.5 lm, and 10 nm, respectively. The intensity ratio of D band to G band (ID/IG) in the Raman spectrum was 2.4. To carry out the UPS measurements the CNW samples were transferred in air into an ultrahigh vacuum (UHV) chamber and were cleaned by heating up to 1100 K. The UHV chamber was made of l-metals and the background pressure was 3 · 1010 Torr. For the UPS measurements a conventional He gas-discharge lamp was used as the source of unpolarized light at photon energy of 21.22 eV (He I) and a spherical deflection analyzer were used as the electron detector. To better understanding of photoelectron spectra for CNWs, spectra of highly oriented pyrolitic graphite (HOPG) were also measured. In order to compare with the CNWs, which stands right on the substrates, the HOPG sample was held vertically in the UHV chamber to direct an axis within the basal plane toward the electron analyzer, i.e., the c-axis of the basal plane of HOPG was at right angle (90 degree) to the electron analyzer as the measurements of CNWs done. On atomic hydrogen and deuterium adsorption to the CNWs, hydrogen or deuterium was dissociated and dosed to the samples through a hot tungsten filament placed about 3 cm away in front of the samples. TPD measurements were also carried out to study the deuterium adsorption to the CNWs with a quadrupole mass spectrometer equipped in the UHV chamber. Sample temperature was monitored by a chromel-constantan thermocouple. The thermocouple was welded to clip-like tantalum wire and attached in the center on the back of the silicon substrate [10]. The temperature was calibrated by comparing with the referenced values of desorption peaks of hydrogen and deuterium from bare Si measured elsewhere [11,12].
361
D A x15 CNW C
HOPG D CNW
B
HOPG –10 Binding Energy (eV)
0
Fig. 1. Photoelectron spectra of CNWs and HOPG. The spectra were plotted as a function of binding energy of electrons with respect to Fermi level. The HOPG sample was hold vertically to direct an axis within the basal plane toward the electron analyzer.
the intense peak (A) due to the interlayer band of CNWs is observed at the same energy as HOPG. The interlayer band was not observed in the spectrum of single-walled carbon nanotubes (SWNTs) [16], because of the lack of the two-dimensional interlayers in SWNTs. The observation of the interlayer band at the common energy suggests that CNWs contain two-dimensional interlayers as HOPG does. The suggestion is in good agreement with the indication of crystallites with high degree of graphitization in CNWs, which was shown in the previous reports of Raman spectroscopy [3]. The second feature is that the structure corresponding to the peak B of HOPG was missing in the spectra of CNWs. HOPG is a crystalline material with uniform c-axis orientation. On the other hand, CNWs consist of crystallites with random c-axes restricted in parallel to the substrate as the polarization dependence of Raman spectroscopy revealed [4]. The uniformity in HOPG and the randomness in CNWs of c-axes orientations might be thought to make the existing p bands detected and undetected, respectively. However, in the vertically aligned configuration of the present measurements for both of HOPG and CNWs, the spectra are reflected in allowed transitions at all wave number parallel to the basal plane. Moreover, with unpolarized light, the transition dipole to excite the electrons in the band does not need to be uniform to form the peak B in the spectrum. Therefore, the result suggests that the p band in the crystallites is not undetected but is perturbed. The p band missing allows the two possible models to speculate about the structure of CNWs. One is the different geometries from the HOPG. In this model the crystallites
I. Kinoshita et al. / Chemical Physics Letters 450 (2008) 360–364
CNW Normal Emission He I (21.22eV) Intensity (arb. units)
of CNWs are made of curving structures. However, this model contradicts the experimental results suggesting that CNWs consist of crystallites with highly graphitization. The alternative model is that CNWs consist of small sizes of good graphitized flat crystallites and the neighboring crystallites position with folding angles. In this model the interlayer band is formed between the highly graphitized basal planes and at exactly the same energy as HOPG. The p bands in the small sizes of crystallites are considered to be influenced and disturbed by the neighboring crystallites. The previous Raman spectroscope measurements also revealed that CNWs consist of small sizes of crystallites. The third feature is that peaks C and D were observed at almost same energies as were referenced in the photoelectron spectra for multi-walled carbon nanotubes (MWCTs) [7]. The reported photoelectron spectra for aligned and unaligned MWCTs with He I showed two peaks corresponding to the peaks C and D in the present spectra of CNWs. In contrast to the highly graphitized crystallites in the CNWs, this identical observation indicates that CNWs contains similar electronic structures of MWCTs supposed to be with hybridization of r and p electronic states due to curvatures. It seems to be conflicting in a sense that the spectra show both graphite- and tube-like features. However, the CNWs are considered to consist of boundary regions surrounding the small sizes of crystallites. In the growth processes of crystallites, the termination of the honeycomb structures and the conjunction between them in the boundary possibly arrange the carbon atoms in folding structures. The folding structure allows the hybridization of the r and p electronic states. In these boundary regions the MWCTlike electronic structure should be formed. Now we move to the effects on the electronic structure caused by atomic deuterium adsorption. Fig. 2 shows photoelectron spectra depending on the amount of deuterium exposure. The inset shows the expanded spectra of clean surface after flashing to 1060 K (s), exposed in atomic deuterium of 100 L (h), 400 L (4), and 1000 L (·). Exposures and measurements were carried out at 340 K of substrate temperature. These spectra were normalized by the intensities at the binding energy of 13.73 eV, which changed in a fraction of 5% with exposure and flashing. With the atomic deuterium exposures, photoelectron intensity between binding energies of 4 and 12 eV increases. The increase was almost saturated with 100 L of exposure, but still kept showing positive correlation with the amount of exposure of atomic deuterium until 1000 L. The same exposures of hydrogen and deuterium were carried out on HOPG, however, this kind of enhancement was not observed. It is found that from the comparison with HOPG, this enhancement of density of states (DOS) is due to the preferable deuterium adsorption on the peculiar sites of CNWs which are excluded in the HOPG. Electronic effects caused by adsorption of atomic hydrogen on graphite surfaces have been reported [17], where the same kind of DOS
Intensity (arb. units)
362
–10 –5 Binding Energy (eV)
–20
–10 Binding Energy (eV)
0
Fig. 2. Deuterium exposure dependence of photoelectron spectra. The inset shows the expanded spectra of clean surface after flashing to 1060 K (s), exposed by atomic deuterium in 100 L (h), 400 L (4), and 1000 L (·).
enhancement was seen for graphite surface after hydrogen plasma treatment in photoelectron spectra. The study revealed chemisorption of atomic hydrogen and atomic vacancy formation on the basal plane of graphite. Plasma treatments are considered to make the graphite surface rough. Hence, this kind of enhancement occurred with the adsorption of hydrogen or deuterium in boundary regions of CNWs. The DOS enhancement is considered to be due to the formation of bonding orbitals between the carbon and deuterium atoms. The correlation of the DOS enhancements with boundary region of CNWs is confirmed from the results of photoelectron spectra as shown in Fig. 3, which depicts photoelectron spectra depending on flashing temperature after exposure in atomic deuterium. The spectra were measured for clean sample after flashing to 1060 K (s), and for sample exposed in atomic deuterium of 600 L at 340 K (h), followed by flashing to 680 K (4), and 950 K (·). Every measurement was carried out after waiting for the sample temperature cool down to 340 K following each procedure. TPD spectrum of deuterium from CNWs is also shown in the inset. The amount of deuterium exposure was 300 L and heating rate was 3 K/sec. The mass signals were plotted after substituted by background signals. Photoelectron intensity at the binding energy between 4 eV and 12 eV increased after atomic deuterium exposure as also seen in Fig. 2. This DOS enhancement was kept even after flashing to 680 K. However, after flashing to 950 K the intensity reduced and went back to that in clean sample. In the TPD spectrum, desorption peaks of deuterium appeared slightly around 450 K and mainly around 800 K. From the correlation of UPS and TPD, it is found
CNW Normal Emission He I (21.22eV)
CNW D exposure 300L 3K/s
400
600 800 1000 Temperature (K)
Before exposure D exposure 100 L D exposure 400 L D exposure 1000 L
CNW Normal Emission He I (21.22eV) –10
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Intensity (arb. units)
Intensity (arb. units)
D 2 TPD msss signal
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–5
–17.0
0
Fig. 3. Flashing temperature dependence of photoelectron spectra after exposure in atomic deuterium. The spectra were measured for clean sample after flashing to 1060 K (s), and for sample exposed in atomic deuterium of 600 L at 340 K (h), followed by flashing to 680 K (4), and 950 K (·). The inset shows TPD of deuterium from CNWs. The amount of deuterium exposure was 300 L and heating rate was 3 K/sec.
that the main desorption of deuterium around 800 K cancelled the DOS enhancements. The peak around 450 K is considered to be due to desorption from the terrace site of crystallites in CNWs in accordance to the previous thermal desorption measurements for HOPG [18]. On the other hand, the peak around 800 K is considered to be due to desorption from the boundary regions. In the work of thermal desorption of hydrogen from C:H films [19], higher peak around 800 K was attribute to sp3 hybridized CHx (x = 1–3). The same conclusion was reported in the study of thermal desorption of hydrogen from carbon nanosheets [20]. These results confirmed that the enhancement of DOS is due to the deuterium adsorption strongly bound to the dominant sites supposed to be boundary regions in the CNWs. Finally the work function change of CNWs with deuterium adsorption is shown. Fig. 4 shows the low-energy cutoff of the photoelectron spectra as a function of binding energy relative to the Fermi level. A sample bias of 3 V was applied to determine the work function of the sample. The work function of CNW before exposure was 4.3 eV (s). Gradual reduction of work function was measured with the increased amount of atomic deuterium exposure of 100 L (h), 400 L (4), and leaded to lowering by 0.2 eV to a value of 4.1 eV at 1000 L (·). Ruffieux et al. have reported the work function changes of single-walled nanotubes (SWNTs) and graphite with adsorption of atomic hydrogen [21]. The work functions of SWNTs and graphite were 4.4 eV and 4.5 eV, respec-
–16.0 Binding Energy (eV)
Binding Energy (eV)
Fig. 4. The low-energy cutoff of the photoelectron spectra as a function of binding energy relative to the Fermi level. A sample bias of 3 V was applied to determine the work function of the sample. The work function of CNW before exposure was 4.3 eV.
tively, and lowered by 0.3 eV to 4.1 eV and 0.4 eV to 4.1 eV, respectively. It is recognized from their results and the present results of CNWs that the work functions of these three kinds of carbon-based structures decrease in order of graphite, SWNTs, and CNWs and reduce to the common value of 4.1 eV with adsorption of hydrogen or deuterium. The work function change by hydrogen adsorption on a surface of the carbon-based structures is considered to have two factors. One is due to the polarized C–H bond [17,21]. The adsorbed hydrogen atoms make a new surface dipole layer with positive charge on the vacuum side resulting in the reduction of work functions. The other is due to the structural distortion by the relaxation of carbon atoms. In the case of CNWs, deuterium atoms adsorbed on the dominant boundary regions, in which the carbon atoms may already have relaxed to their minimum energy positions. The work function change of 0.2 eV is supposed to be due to the C–H bond. While in the case of HOPG, the work function change of 0.4 eV for graphite surface is supposed to include the factor of carbon layer distortion in addition to that of the C–H bond. Provided that the work function change originating from the C–H bond is evaluated 0.2 eV for all these kinds of carbon based structures, the change due to distortion of the flat carbon layers in HOPG could be evaluated around 0.2 eV. 4. Conclusion We have presented photoelectron spectra of CNWs in comparison with HOPG and electronic effects with deute-
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rium adsorption. The observed intense interlayer band indicates that the CNWs consist of crystallites with high degree of graphitization. While, the delocalized p bands in the small sizes of crystallites are perturbed by the neighboring crystallites in boundary regions. In addition, electronic structure similar to that of MWCTs is included in the CNWs suggesting that the electronic structure of dominant boundary regions hybridizes r and p electronic states and has sp3 character. On the base of UPS and TPD measurements, adsorption of deuterium is mainly occurred on the boundary regions resulting in strong bounds and DOS enhancements. Acknowledgement This work was supported by Strategic Research Project (K18030) in Yokohama City University. References [1] Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater. 14 (2002) 64 (Weinheim, Ger.). [2] Y. Wu, B. Yang, B. Zong, H. Sun, Z. Shen, Y. Feng, J. Mater. Chem. 14 (2004) 469. [3] S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibana, J. Appl. Phys. 97 (2005) 104320. [4] Z. Ni, H. Fan, Y. Feng, Z. Shen, B. Yang, Y. Wu, J.Chem. Phys. 124 (2006) 204703.
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