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Electronic structure of Mo2C(0001) studied by resonant photoemission spectroscopy
Solid State Communications 131 (2004) 245–249 www.elsevier.com/locate/ssc
Electronic structure of Mo2C(0001) studied by resonant photoemission spectroscopy M. Sugiharaa, K. Ozawaa, K. Edamotoa,*, S. Otanib a
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-00333, Japan b National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 7 February 2003; received in revised form 29 May 2003; accepted 15 April 2004 by S. Ushioda Available online 25 May 2004
Abstract The electronic structure of a-Mo2C(0001) has been investigated using photoemission spectroscopy with photon energies in the Mo 4p ! 4d transition threshold region. A broad Fano-like resonance peaked at , 46 eV is observed in the whole area of the valence band, indicating that the contribution of the Mo 4d orbitals is included in the whole area of the valence band. The Mo 4d contribution to the valence band is found to become more dominant with decreasing binding energy. It is proposed that, though the valence band is mostly composed of Mo 4d – C 2p hybrid states, the band near the Fermi level (0 – 1 eV) is mostly composed of Mo 4d orbitals. q 2004 Elsevier Ltd. All rights reserved. PACS: 68.35. 2 p; 73.20. 2 r Keywords: A. Surface and interface; D. Electronic state (localized); E. Photoelectron spectroscopies
The early transition metal carbides (TMCs) have attracted much attention because they have an interesting combination of useful properties such as extreme hardness, a high melting point and metallic conductivity [1]. In practical applications, they have been used as stable field electron emitters, coating materials, catalysts, etc. Because the surface properties play an essential role in all these applications, it has become important to investigate the surface properties of TMCs. The surface properties of Mo2C, which is one of the group VI TMCs, are in particular of interest, because Mo2C has high catalytic activities for a variety of reactions, such as the hydrogenation of benzene, ethylene, and carbon monoxide [2,3]. The characterization of the Mo2C surface has been tried for an a-Mo2C(0001) surface using lowenergy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy [4– 8], and it has been * Corresponding author. Tel.: þ81-3-5734-2238; fax: þ 8135734-2655. E-mail address: [email protected] (K. Edamoto). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.04.049
proposed that the composition and the atomic arrangement of the surface are substantially dependent on the cleaning procedure [4– 8]. As to the electronic structure, we recently performed a photoemission spectroscopy study of the Mo2C(0001) surface, and proposed that the valence band of Mo2C(0001) consists of Mo 4d band, Mo 4d – C 2p hybrid band, and C 2s band, on the basis of the analysis of hndependence of the spectra measured with the photon energies around the Cooper minimum region of Mo 4d photoionization cross sections [8]. The catalytic activities of a material should be closely related to the surface electronic structure particularly around the Fermi level ðEF Þ; and thus a detailed study on the surface electronic structure of Mo2C in the valence band region is urgently called for. It is known that resonant photoemission spectroscopy is a powerful method to obtain detailed information on the valence electronic structure of a material [9]. In this communication, we report the results of the hn-dependent photoemission spectroscopy study of the valence electronic structure of Mo2C(0001). The energy of the incident photons is varied around the threshold of Mo 4p ! 4d transition, and the Mo
246
M. Sugihara et al. / Solid State Communications 131 (2004) 245–249
4d contribution to the various regions of the valence band is discussed on the basis of the analysis of the resonant behavior of the valence band emissions. The photoemission spectroscopy measurements using synchrotron radiation were conducted on the Beam Line 3B of the Photon Factory, High Energy Accelerator Research Organization (KEK), where the synchrotron radiation was dispersed by a grazing incidence monochromator. An electron energy analyzer of 1508 hemispherical sector type with an acceptance angle of ^ 18 was used for all photoemission spectroscopy measurements. The spectra presented below are normalized by photon flux estimated from the photocurrent of the final stage mirror. The base pressure in the vacuum system was 8 £ 10211 Torr. An a-Mo2C single crystal was grown by the floating zone method at the National Institute for Materials Science [10]. The a-Mo2C crystal has an orthorhombic crystal structure, in which the Mo atoms form a slightly distorted hcp lattice [11]. Thus, the existing body of literature on this material employed a hexagonal notation, and we will also use the hexagonal notation in this communication to be consistent with them. The crystal was cut at an orientation of (0001), which is equivalent to (100) in the ortborhombic system, by spark erosion into a disk of ,1 mm thickness and subsequently polished mechanically. The Mo2C(0001) surface was cleaned in the vacuum chamber by several cycles of Arþ ion bombardment (1 keV for 60 min) and annealing (, 1600 K). The Mo2C(0001) is a polar surface whose first layer should be either a metal layer or a carbon layer, and our previous study showed that the clean surface prepared with the procedure described above is terminated with a C-layer in which the number of the C atoms is nearly equal to that of the Mo atoms in the second layer (a MoClike surface) [8]. It was also shown that the surface gives a complex LEED pattern composed of (1 £ 1) orthorhombic spots, (2 £ 2)R158 spots, and (4/3 £ 4/3)R58 spots, and thus the surface is covered with at least three types of domains [8]. Details about the cleaning procedure and the estimation of the surface atomic composition of Mo2C(0001) were discussed in our previous paper [8]. Fig. 1 shows normal-emission spectra of Mo2C(0001) taken at various photon energies. The background obtained by the Shirley procedure [12] has been subtracted from each raw data. The band is observed at 0 – 7 eV independently of the photon energy, and the band is attributed to the valence band mostly composed of Mo 4d and C 2p orbitals [8]. All spectra show clear cut-off at the Fermi level, indicating the metallic nature of Mo2C. In this work, our main interest is placed on the elucidation of the electronic structure of Mo2C(0001) in the valence band region, which should be closely related to the bonding nature in the crystal and to the chemical reactivity of the surface. Fig. 1 shows that the valence band consists of two dominant bands and a hump; the lower binding energy band peaked at , 1.5 eV and the higher binding energy band peaked at ,3.6 eV, which are separated by a dip at , 2.5 eV, independently of the photon
Fig. 1. Change in the normal-emission spectrum of Mo2C(0001) as a function of photon energy.
energy, and a hump at 6 – 7 eV. The intensities of both dominant bands and the hump are enhanced at hn ¼ 45 – 50 eV; though the lower binding energy band seems to be relatively more enhanced than other bands in this energy region. In Fig. 2, we plot the intensities at some points in the valence band as a function of photon energy. We plot the intensities at the shoulder at the Fermi edge (0.3 eV), at the energy in the peak region of the lower binding energy band (1.5 eV), at the energy in the peak region of the higher binding energy band (3.6 eV), and at the energy in the hump region (6.6 eV), which are labeled as A, B, C, and D,
Fig. 2. Photoemission intensities at the regions A (open circles), B (filled triangles), C (open squares), and D (open triangles) as a function of photon energy.
M. Sugihara et al. / Solid State Communications 131 (2004) 245–249
respectively. The positions of A– D are indicated by vertical bars in Fig. 1. In Fig. 2, the intensities are normalized by those in the spectrum measured at hn ¼ 62 eV; where the Mo 4p ! 4d resonance effect on the photoionization cross sections, which will be discussed below, is mostly removed. As shown in Fig. 2, the intensities of the regions A, B, C, and D have similar hn-dependence; the intensities have a minimum at 36 –40 eV and a maximum at , 46 eV, and the intensities decrease gradually with increasing hn at . 46 eV. The intensities at the other binding energies in the valence band are confirmed to have similar hn-dependence to those at regions A– D. The observed hn-dependence is characteristic of the Fano line shape [13], and the maximum of the photoemission intensity is interpreted as originating from the interference between the normal Mo 4d photoemission process and the process induced by the photoninduced excitation, Mo4p6 4dn þ hn ! Mo4p5 4dnþ1 followed by the emission of a Mo 4d electron through a super-Coster-Kronig decay, Mo4p5 4dnþ1 ! Mo4p6 4dn21 þ e2 : Lince et al. made a resonant photoemission study for MoS2, and found that the Mo 4d-derived non bonding band shows a resonance due to the Mo 4p ! 4d transition at , 42 eV [14], which is qualitatively agreement with the present study. Recently we measured core-level photoemission spectra of Mo2C(0001), and observed a peak of the Mo 4p states at 36.5 eV. Thus, the resonance maximum is located about 10 eV above the Mo 4p excitation threshold expected from the core-level photoemission study. This delayed resonance is commonly observed for many transition metal compounds [9,15]. The reason of this delay is still in dispute, and one of the possible reasons of this delay is the Coulomb interaction between the 4p and 4d electrons, which has been proposed by Kurahashi et al. [16]. In addition to the large maximum at 46 eV, the intensities of the regions A– D all show a slight hump at , 53 eV (Fig. 2). We attribute the hump to the resonance induced by the interference between the normal Mo 4d photoemission process and the process induced by the excitation of a Mo 4p electron to unoccupied Mo 5sp states Mo4p6 4dn þ hn ! Mo4p5 4dn 5ðspÞ1 followed by the deexcitation through the emission of a Mo 4d electron Mo4p5 4dn 5ðspÞ1 ! Mo4p6 4dn21 þ e2 : Similar resonance involving Mo 4p ! 5sp transition has been observed in the resonance photoemission study of MoS2 by Lince et al. [14], in which the resonance maximum is observed at 51 eV. In this work, we measured normal-emission spectra of a single crystal surface, and thus the hn-dependence of the spectra should reflect the bulk band structure along the axis
247
normal to the surface. In other words, the hn-dependent change in the spectral intensities plotted in Fig. 2 should be caused from the effect of bulk band dispersions as well as from the resonance effect. However, since the spectral line shape remains two-band structure independently of the photon energy and the band dispersion of each peak is small, density-of-state effect is thought to be dominant in the photoemission process in the photon energy range of 37 – 62 eV. The peak intensity could vary as a function of hn even if the density-of-state effect is dominant, and thus it is necessary to check the effect of the band dispersion on the plots in Fig. 2. The work function of Mo2C(0001) is estimated to be 5.0 eV from the width of UP spectra. Assuming that the bottom of the valence band is 8 eV below EF (Fig. 1), the initial states of the points A, B, C and D in normal-emission spectra scan the regions in the reciprocal ˚ 21 along space of 2.2– 3.6, 2.1 – 3.5, 2.0 –3.5 and 1.8 –3.3 A the axis normal to the surface, respectively, at hn ¼ 32 – 62 eV: The G point in the second Brillouin zone locates at ˚ 21 in this axis, which corresponds to hn of 40.3, 41.5, 2.66 A 43.6 and 46.6 eV for the A, B, C and D points, respectively. Thus, if the effect of the band dispersion contributes to the hn-dependent change in the spectral intensity, the hndependence of the intensity at A, B, C and D should show a symmetric behavior with respect to the point at hn ¼ 40:3; 41.5, 43.6 and 46.6 eV, respectively. None of the intensities of A – D shows such a symmetric behavior as shown in Fig. 2. Thus, we conclude that the Mo 4d resonance effect is dominant in the hn-dependence of the spectral intensity at least in the photon energy region of 32 – 62 eV. In this work, we did not eliminate the contribution of the emissions excited by the second-order beam. Therefore, the spectral intensities at A– D may include the contribution of the Mo 4p emission (36.5 eV) excited by the second-order beam at hn # 37 eV and the contribution of the Mo 4s emission (, 63 eV [17]) at hn ¼ 55 – 62 eV: The contribution of the emissions excited by the second-order beam is considered to give a smooth background in spectra in the other photon energy region, and thus is mostly subtracted through the background subtraction process. Thus, the effect of the second-order beam is thought to have little contribution in the resonance maximum region. As shown in Fig. 2, the intensities of the region A– D all show a resonant behavior characteristic of the photoionization cross section of Mo 4d orbitals. Since the intensities of the whole area of the valence band show the resonance characteristic of the Mo 4d photoionization, it is concluded that the Mo 4d contribution is included in the whole area of the valence band of Mo2C(0001). This is in contrast with the case of the valence band of MoS2, where it has been found that the higher binding energy part of the valence band does not include the Mo 4d contribution [14]. Fig. 2 also shows that, the resonant effect is most prominent in the Fermi edge region (A), suggesting that the contribution of the Mo 4d component to the valence band is in particular important around EF :
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Fig. 3. Upper panel: comparison of normal-emission spectra of Mo2C(0001) measured at hn ¼ 47 and 60 eV. Lower panel: (a) spectral difference between the normal emission spectra measured at 47 and 60 eV; (b) difference between the raw spectrum and the curve (a). In (b), solid and dashed curves correspond to the spectra measured at hn ¼ 47 and 60 eV, respectively.
The upper panel of Fig. 3 shows a comparison of normalemission spectra measured at hn ¼ 47 eV (resonance region) and 60 eV (off-resonance region). It is clearly shown that the resonance effect is more prominent in the lower binding energy band in the valence band, indicating that the Mo 4d contribution to the valence band is dominant in the lower binding energy band, The difference between the spectra measured at hn ¼ 47 and 60 eV is shown in the lower panel of Fig. 3 (curve (a)). Fig. 1 shows that each peak in the valence band shows a small dispersion, suggesting that the spectrum reflects the density of states (DOS) of Mo2C(0001) to a high degree in this photon energy region. Thus the difference spectrum (Fig. 3(a)) can be approximately attributed to the Mo 4d component in a spectrum, since only the Mo 4d emission is enhanced at the resonance region. The obtained difference spectrum clearly shows that the Mo 4d contribution to the valence band is dominant in the lower binding energy band. In order to resolve the C 2p component, we took a difference between a raw spectrum and the Mo 4d component (curve (a)), and the results corresponding to the spectra at hn ¼ 47 and 60 eV are shown as solid and dashed lines, respectively, in the lower panel of Fig. 3 (curves (b)). Before taking the difference, the intensity of the curve (a) is adjusted so that the intensity at the Fermi edge, where the Mo 4d contribution is supposed to be dominant, becomes equal to that in the raw spectrum. Fig. 3(b) shows that the difference curves corresponding to the spectra at hn ¼ 47 and 60 eV obtained by the procedure
described above are essentially identical to each other, and thus the curves (b) can be viewed as the C 2p component in the valence band approximately. Fig. 3(b) shows that the C 2p component is included in the whole area of the valence band and the contribution is especially dominant at 1 – 5 eV. Though the obtained curves in Fig. 3(a) and (b) may not strictly reflect the DOS of Mo 4d and C 2p components, the results described above qualitatively show that the most part of the valence band of Mo2C(0001) is composed of Mo 4d – C 2p hybrid band. It is also deduced from Fig. 3(a) and (b) that the Mo 4d component becomes more dominant with decreasing binding energy in the valence band, and the valence band around EF (0 – 1 eV) is mostly composed of Mo 4d orbitals. The proposed valence band structure is consistent with the result of our previous photoemission study [8], in which it has been shown that the emission of the valence band in the lower binding energy part is suppressed more prominently relative to the higher binding energy part in the Cooper minimum region of the Mo 4d photoionization cross sections, indicating that the Mo 4d component is more dominant in the lower binding energy part. Recently Clair et al. calculated the bulk DOS of a-Mo2C using density functional theory and proposed that the valence band of Mo2Cconsists of Mo 4d band (0 – 4 eV) and Mo 4d – C2p hybrid band (4 – 7 eV) [18]. Their result is qualitatively agreement with the present photoemission results that the band near EF is mostly composed of Mo 4d orbitals, but our photoemission study suggests that the band mostly composed of Mo 4d orbitals is restricted in narrower binding energy region around EF (0 –1 eV).
Acknowledgements We are pleased to thank the staff of the Photon Factory, High Energy Accelerator Research Organization, particularly Prof. Y. Azuma, for their excellent support. This work has been performed under the approval of the Photon Factory Advisory Committee (Proposal No. 2000G017).
References [1] Y. Kumashiro, Electric Refractory Materials, Marcel Dekker, New York, 2000. [2] I. Kojima, E. Miyazaki, J. Catal. 89 (1984) 168. [3] J.S. Lee, K.H. Lee, Y.J. Lee, J. Phys. Chem. 96 (1992) 362. [4] T.P. St. Clair, S.T. Oyama, D.F. Cox, S. Otani, Y. Ishizawa, R.-L. Lo, K. Fukui, Y. Iwazawa, Surf. Sci. 426 (1999) 187. [5] R.-L. Lo, K. Fukui, S. Otani, Y. Iwasawa, Surf. Sci. 440 (1999) L857. [6] R.-L. Lo, K. Fukui, S. Otani, S.T. Oyama, Y. Iwasawa, Jpn. J. Appl. Phys. 38 (1999) 3818. [7] K. Fukui, R.-L. Lo, S. Otani, Y. Iwasawa, Chem. Phys. Lett. 325 (2000) 275.
M. Sugihara et al. / Solid State Communications 131 (2004) 245–249 [8] M. Sugihara, K. Ozawa, K. Edamoto, S. Otani, Solid State Commun. 121 (2002) 1. [9] L.C. Davis, J. Appl. Phys. 59 (1986) R25. [10] S. Otani, Y. Ishizawa, J. Cryst. Growth 154 (1995) 202. [11] E. Parthe, V. Sadagopan, Acta Crystallogr. 16 (1963) 202. [12] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [13] U. Fano, Phys. Rev. 124 (1961) 1866. [14] J.R. Lince, S.V. Didziulis, J.A. Yarmoff, Phys. Rev. B 43 (1991) 4641.
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[15] K. Edamoto, M. Yamazaki, T. Noda, K. Ozawa, S. Otani, Surf. Sci. 498 (2002) 343. [16] M. Kurahashi, M. Yamamoto, M. Mabuchi, S. Naito, J. Phys.: Condens. Matter 7 (1995) 4763. [17] S. Hu¨fner, Photoelectron Spectroscopy, Springer, Berlin, 1995. [18] T.P. Clair, S.T. Oyama, D.F. Cox, Surf. Sci. 511 (2002) 294.
Electronic structure of Mo2C(0001) studied by resonant photoemission spectroscopy M. Sugiharaa, K. Ozawaa, K. Edamotoa,*, S. Otanib a
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-00333, Japan b National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 7 February 2003; received in revised form 29 May 2003; accepted 15 April 2004 by S. Ushioda Available online 25 May 2004
Abstract The electronic structure of a-Mo2C(0001) has been investigated using photoemission spectroscopy with photon energies in the Mo 4p ! 4d transition threshold region. A broad Fano-like resonance peaked at , 46 eV is observed in the whole area of the valence band, indicating that the contribution of the Mo 4d orbitals is included in the whole area of the valence band. The Mo 4d contribution to the valence band is found to become more dominant with decreasing binding energy. It is proposed that, though the valence band is mostly composed of Mo 4d – C 2p hybrid states, the band near the Fermi level (0 – 1 eV) is mostly composed of Mo 4d orbitals. q 2004 Elsevier Ltd. All rights reserved. PACS: 68.35. 2 p; 73.20. 2 r Keywords: A. Surface and interface; D. Electronic state (localized); E. Photoelectron spectroscopies
The early transition metal carbides (TMCs) have attracted much attention because they have an interesting combination of useful properties such as extreme hardness, a high melting point and metallic conductivity [1]. In practical applications, they have been used as stable field electron emitters, coating materials, catalysts, etc. Because the surface properties play an essential role in all these applications, it has become important to investigate the surface properties of TMCs. The surface properties of Mo2C, which is one of the group VI TMCs, are in particular of interest, because Mo2C has high catalytic activities for a variety of reactions, such as the hydrogenation of benzene, ethylene, and carbon monoxide [2,3]. The characterization of the Mo2C surface has been tried for an a-Mo2C(0001) surface using lowenergy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy [4– 8], and it has been * Corresponding author. Tel.: þ81-3-5734-2238; fax: þ 8135734-2655. E-mail address: [email protected] (K. Edamoto). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.04.049
proposed that the composition and the atomic arrangement of the surface are substantially dependent on the cleaning procedure [4– 8]. As to the electronic structure, we recently performed a photoemission spectroscopy study of the Mo2C(0001) surface, and proposed that the valence band of Mo2C(0001) consists of Mo 4d band, Mo 4d – C 2p hybrid band, and C 2s band, on the basis of the analysis of hndependence of the spectra measured with the photon energies around the Cooper minimum region of Mo 4d photoionization cross sections [8]. The catalytic activities of a material should be closely related to the surface electronic structure particularly around the Fermi level ðEF Þ; and thus a detailed study on the surface electronic structure of Mo2C in the valence band region is urgently called for. It is known that resonant photoemission spectroscopy is a powerful method to obtain detailed information on the valence electronic structure of a material [9]. In this communication, we report the results of the hn-dependent photoemission spectroscopy study of the valence electronic structure of Mo2C(0001). The energy of the incident photons is varied around the threshold of Mo 4p ! 4d transition, and the Mo
246
M. Sugihara et al. / Solid State Communications 131 (2004) 245–249
4d contribution to the various regions of the valence band is discussed on the basis of the analysis of the resonant behavior of the valence band emissions. The photoemission spectroscopy measurements using synchrotron radiation were conducted on the Beam Line 3B of the Photon Factory, High Energy Accelerator Research Organization (KEK), where the synchrotron radiation was dispersed by a grazing incidence monochromator. An electron energy analyzer of 1508 hemispherical sector type with an acceptance angle of ^ 18 was used for all photoemission spectroscopy measurements. The spectra presented below are normalized by photon flux estimated from the photocurrent of the final stage mirror. The base pressure in the vacuum system was 8 £ 10211 Torr. An a-Mo2C single crystal was grown by the floating zone method at the National Institute for Materials Science [10]. The a-Mo2C crystal has an orthorhombic crystal structure, in which the Mo atoms form a slightly distorted hcp lattice [11]. Thus, the existing body of literature on this material employed a hexagonal notation, and we will also use the hexagonal notation in this communication to be consistent with them. The crystal was cut at an orientation of (0001), which is equivalent to (100) in the ortborhombic system, by spark erosion into a disk of ,1 mm thickness and subsequently polished mechanically. The Mo2C(0001) surface was cleaned in the vacuum chamber by several cycles of Arþ ion bombardment (1 keV for 60 min) and annealing (, 1600 K). The Mo2C(0001) is a polar surface whose first layer should be either a metal layer or a carbon layer, and our previous study showed that the clean surface prepared with the procedure described above is terminated with a C-layer in which the number of the C atoms is nearly equal to that of the Mo atoms in the second layer (a MoClike surface) [8]. It was also shown that the surface gives a complex LEED pattern composed of (1 £ 1) orthorhombic spots, (2 £ 2)R158 spots, and (4/3 £ 4/3)R58 spots, and thus the surface is covered with at least three types of domains [8]. Details about the cleaning procedure and the estimation of the surface atomic composition of Mo2C(0001) were discussed in our previous paper [8]. Fig. 1 shows normal-emission spectra of Mo2C(0001) taken at various photon energies. The background obtained by the Shirley procedure [12] has been subtracted from each raw data. The band is observed at 0 – 7 eV independently of the photon energy, and the band is attributed to the valence band mostly composed of Mo 4d and C 2p orbitals [8]. All spectra show clear cut-off at the Fermi level, indicating the metallic nature of Mo2C. In this work, our main interest is placed on the elucidation of the electronic structure of Mo2C(0001) in the valence band region, which should be closely related to the bonding nature in the crystal and to the chemical reactivity of the surface. Fig. 1 shows that the valence band consists of two dominant bands and a hump; the lower binding energy band peaked at , 1.5 eV and the higher binding energy band peaked at ,3.6 eV, which are separated by a dip at , 2.5 eV, independently of the photon
Fig. 1. Change in the normal-emission spectrum of Mo2C(0001) as a function of photon energy.
energy, and a hump at 6 – 7 eV. The intensities of both dominant bands and the hump are enhanced at hn ¼ 45 – 50 eV; though the lower binding energy band seems to be relatively more enhanced than other bands in this energy region. In Fig. 2, we plot the intensities at some points in the valence band as a function of photon energy. We plot the intensities at the shoulder at the Fermi edge (0.3 eV), at the energy in the peak region of the lower binding energy band (1.5 eV), at the energy in the peak region of the higher binding energy band (3.6 eV), and at the energy in the hump region (6.6 eV), which are labeled as A, B, C, and D,
Fig. 2. Photoemission intensities at the regions A (open circles), B (filled triangles), C (open squares), and D (open triangles) as a function of photon energy.
M. Sugihara et al. / Solid State Communications 131 (2004) 245–249
respectively. The positions of A– D are indicated by vertical bars in Fig. 1. In Fig. 2, the intensities are normalized by those in the spectrum measured at hn ¼ 62 eV; where the Mo 4p ! 4d resonance effect on the photoionization cross sections, which will be discussed below, is mostly removed. As shown in Fig. 2, the intensities of the regions A, B, C, and D have similar hn-dependence; the intensities have a minimum at 36 –40 eV and a maximum at , 46 eV, and the intensities decrease gradually with increasing hn at . 46 eV. The intensities at the other binding energies in the valence band are confirmed to have similar hn-dependence to those at regions A– D. The observed hn-dependence is characteristic of the Fano line shape [13], and the maximum of the photoemission intensity is interpreted as originating from the interference between the normal Mo 4d photoemission process and the process induced by the photoninduced excitation, Mo4p6 4dn þ hn ! Mo4p5 4dnþ1 followed by the emission of a Mo 4d electron through a super-Coster-Kronig decay, Mo4p5 4dnþ1 ! Mo4p6 4dn21 þ e2 : Lince et al. made a resonant photoemission study for MoS2, and found that the Mo 4d-derived non bonding band shows a resonance due to the Mo 4p ! 4d transition at , 42 eV [14], which is qualitatively agreement with the present study. Recently we measured core-level photoemission spectra of Mo2C(0001), and observed a peak of the Mo 4p states at 36.5 eV. Thus, the resonance maximum is located about 10 eV above the Mo 4p excitation threshold expected from the core-level photoemission study. This delayed resonance is commonly observed for many transition metal compounds [9,15]. The reason of this delay is still in dispute, and one of the possible reasons of this delay is the Coulomb interaction between the 4p and 4d electrons, which has been proposed by Kurahashi et al. [16]. In addition to the large maximum at 46 eV, the intensities of the regions A– D all show a slight hump at , 53 eV (Fig. 2). We attribute the hump to the resonance induced by the interference between the normal Mo 4d photoemission process and the process induced by the excitation of a Mo 4p electron to unoccupied Mo 5sp states Mo4p6 4dn þ hn ! Mo4p5 4dn 5ðspÞ1 followed by the deexcitation through the emission of a Mo 4d electron Mo4p5 4dn 5ðspÞ1 ! Mo4p6 4dn21 þ e2 : Similar resonance involving Mo 4p ! 5sp transition has been observed in the resonance photoemission study of MoS2 by Lince et al. [14], in which the resonance maximum is observed at 51 eV. In this work, we measured normal-emission spectra of a single crystal surface, and thus the hn-dependence of the spectra should reflect the bulk band structure along the axis
247
normal to the surface. In other words, the hn-dependent change in the spectral intensities plotted in Fig. 2 should be caused from the effect of bulk band dispersions as well as from the resonance effect. However, since the spectral line shape remains two-band structure independently of the photon energy and the band dispersion of each peak is small, density-of-state effect is thought to be dominant in the photoemission process in the photon energy range of 37 – 62 eV. The peak intensity could vary as a function of hn even if the density-of-state effect is dominant, and thus it is necessary to check the effect of the band dispersion on the plots in Fig. 2. The work function of Mo2C(0001) is estimated to be 5.0 eV from the width of UP spectra. Assuming that the bottom of the valence band is 8 eV below EF (Fig. 1), the initial states of the points A, B, C and D in normal-emission spectra scan the regions in the reciprocal ˚ 21 along space of 2.2– 3.6, 2.1 – 3.5, 2.0 –3.5 and 1.8 –3.3 A the axis normal to the surface, respectively, at hn ¼ 32 – 62 eV: The G point in the second Brillouin zone locates at ˚ 21 in this axis, which corresponds to hn of 40.3, 41.5, 2.66 A 43.6 and 46.6 eV for the A, B, C and D points, respectively. Thus, if the effect of the band dispersion contributes to the hn-dependent change in the spectral intensity, the hndependence of the intensity at A, B, C and D should show a symmetric behavior with respect to the point at hn ¼ 40:3; 41.5, 43.6 and 46.6 eV, respectively. None of the intensities of A – D shows such a symmetric behavior as shown in Fig. 2. Thus, we conclude that the Mo 4d resonance effect is dominant in the hn-dependence of the spectral intensity at least in the photon energy region of 32 – 62 eV. In this work, we did not eliminate the contribution of the emissions excited by the second-order beam. Therefore, the spectral intensities at A– D may include the contribution of the Mo 4p emission (36.5 eV) excited by the second-order beam at hn # 37 eV and the contribution of the Mo 4s emission (, 63 eV [17]) at hn ¼ 55 – 62 eV: The contribution of the emissions excited by the second-order beam is considered to give a smooth background in spectra in the other photon energy region, and thus is mostly subtracted through the background subtraction process. Thus, the effect of the second-order beam is thought to have little contribution in the resonance maximum region. As shown in Fig. 2, the intensities of the region A– D all show a resonant behavior characteristic of the photoionization cross section of Mo 4d orbitals. Since the intensities of the whole area of the valence band show the resonance characteristic of the Mo 4d photoionization, it is concluded that the Mo 4d contribution is included in the whole area of the valence band of Mo2C(0001). This is in contrast with the case of the valence band of MoS2, where it has been found that the higher binding energy part of the valence band does not include the Mo 4d contribution [14]. Fig. 2 also shows that, the resonant effect is most prominent in the Fermi edge region (A), suggesting that the contribution of the Mo 4d component to the valence band is in particular important around EF :
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Fig. 3. Upper panel: comparison of normal-emission spectra of Mo2C(0001) measured at hn ¼ 47 and 60 eV. Lower panel: (a) spectral difference between the normal emission spectra measured at 47 and 60 eV; (b) difference between the raw spectrum and the curve (a). In (b), solid and dashed curves correspond to the spectra measured at hn ¼ 47 and 60 eV, respectively.
The upper panel of Fig. 3 shows a comparison of normalemission spectra measured at hn ¼ 47 eV (resonance region) and 60 eV (off-resonance region). It is clearly shown that the resonance effect is more prominent in the lower binding energy band in the valence band, indicating that the Mo 4d contribution to the valence band is dominant in the lower binding energy band, The difference between the spectra measured at hn ¼ 47 and 60 eV is shown in the lower panel of Fig. 3 (curve (a)). Fig. 1 shows that each peak in the valence band shows a small dispersion, suggesting that the spectrum reflects the density of states (DOS) of Mo2C(0001) to a high degree in this photon energy region. Thus the difference spectrum (Fig. 3(a)) can be approximately attributed to the Mo 4d component in a spectrum, since only the Mo 4d emission is enhanced at the resonance region. The obtained difference spectrum clearly shows that the Mo 4d contribution to the valence band is dominant in the lower binding energy band. In order to resolve the C 2p component, we took a difference between a raw spectrum and the Mo 4d component (curve (a)), and the results corresponding to the spectra at hn ¼ 47 and 60 eV are shown as solid and dashed lines, respectively, in the lower panel of Fig. 3 (curves (b)). Before taking the difference, the intensity of the curve (a) is adjusted so that the intensity at the Fermi edge, where the Mo 4d contribution is supposed to be dominant, becomes equal to that in the raw spectrum. Fig. 3(b) shows that the difference curves corresponding to the spectra at hn ¼ 47 and 60 eV obtained by the procedure
described above are essentially identical to each other, and thus the curves (b) can be viewed as the C 2p component in the valence band approximately. Fig. 3(b) shows that the C 2p component is included in the whole area of the valence band and the contribution is especially dominant at 1 – 5 eV. Though the obtained curves in Fig. 3(a) and (b) may not strictly reflect the DOS of Mo 4d and C 2p components, the results described above qualitatively show that the most part of the valence band of Mo2C(0001) is composed of Mo 4d – C 2p hybrid band. It is also deduced from Fig. 3(a) and (b) that the Mo 4d component becomes more dominant with decreasing binding energy in the valence band, and the valence band around EF (0 – 1 eV) is mostly composed of Mo 4d orbitals. The proposed valence band structure is consistent with the result of our previous photoemission study [8], in which it has been shown that the emission of the valence band in the lower binding energy part is suppressed more prominently relative to the higher binding energy part in the Cooper minimum region of the Mo 4d photoionization cross sections, indicating that the Mo 4d component is more dominant in the lower binding energy part. Recently Clair et al. calculated the bulk DOS of a-Mo2C using density functional theory and proposed that the valence band of Mo2Cconsists of Mo 4d band (0 – 4 eV) and Mo 4d – C2p hybrid band (4 – 7 eV) [18]. Their result is qualitatively agreement with the present photoemission results that the band near EF is mostly composed of Mo 4d orbitals, but our photoemission study suggests that the band mostly composed of Mo 4d orbitals is restricted in narrower binding energy region around EF (0 –1 eV).
Acknowledgements We are pleased to thank the staff of the Photon Factory, High Energy Accelerator Research Organization, particularly Prof. Y. Azuma, for their excellent support. This work has been performed under the approval of the Photon Factory Advisory Committee (Proposal No. 2000G017).
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