Electronic states of monolayer graphite formed on TiC(111) surface

Surface Science 291 (1993) 93-98 North-Holland

surface science

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Electronic states of monolayer graphite formed on T i C ( l l l ) surface Ayato Nagashima, Kenji Nuka, Hiroshi Itoh, Takeo Ichinokawa, Chuhei Oshima Department of Applied Physics, Waseda University, 3-4-10kubo, Shinjuku, Tokyo 169, Japan

and Shigeki O t a n i National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received 28 December 1992; accepted for publication 3 March 1993

Electronic states of a monolayer graphite formed on TiC(111) surface have been investigated by means of XPS, UPS, and work-function measurement. The chemical shift of C ls peaks in XPS spectra has inhibited a large electron transfer from the substrate to the monolayer graphite. On the other hand, the band structure of the graphite overlayer has altered from that of bulk graphite. The work-function measurement has suggested an electron redistribution in the graphite layer. These results indicate that the electronic states of the graphite monolayer are modified not by the charge transfer, but mainly by the orbital hybridization between the graphite monolayer and the substrate, which differs from graphite intercalation compounds.

1. Introduction It has been long known that a graphite overlayer is grown on various solid surfaces by segregation of carbon from the interior of the crystal a n d / o r dissociation of hydrocarbon [1-5]. However, its physical and chemical properties have not received much attention for a long time, because the electronic states in the graphite overlayer have not been expected to differ largely from those in the graphite crystal. Recently, a graphite overlayer with a thickness of one atomic layer has been successfully formed by controlling the reaction temperature of the surface with hydrocarbon gases [6]. In addition, its phonon anomalies have been detected for some cases by high-resolution electron energy loss spectroscopy [7,8]. Lattice-dynamical analysis has shown that the C - C bondings of the monolayer graphite (MG) on (111) surfaces of transition metal carbides considerably weaken, whereas the interlayer bondings between the C atoms and the substrate strengthen. This bond weakening also leads to an increase in the lattice constant of the

graphite layer, of which the value is larger than that of alkali-metal graphite intercalation compounds (AGIC's) [9,10]. Since the clean TiC(111) surface is a Titerminated polar surface [11], the graphite overlayer contacts with the metal atoms. If the bond weakening is caused only by charge transfer, as in the case of A G I C [12], a substantial number of electrons should transfer to the anti-bonding ~-* states of the graphite layer from the substrate. These electrons would change the position of the C ls energy level and raise the Fermi level of the graphite. The purpose of this paper is to explore the electronic states of the graphite monolayer which exhibits phonon anomalies and the expansion of the lattice constant. 2. Experimental The experiments were done in a two-level vacuum chamber equipped with a low-energy electron diffraction ( L E E D ) optics and a gas inlet in the upper stage, and an electron energy analyzer, an X-ray source and an ultra-violet discharge

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94

A. Nagashima et al. / Electronic states o f monolayer graphite formed on TiC(l l l ) surface

lamp in the lower stage. The 150 ° hemispherical analyzer is rotatable around the specimen within an angular accuracy of +0.5 °. A characteristic X-ray line of M g K a , 1253.6 eV, was used for X-ray photoelectron spectroscopy (XPS), and unpolarized H e I, He II resonance lines were used for ultraviolet photoelectron spectroscopy (UPS). The vacuum system was evacuated by a diffusion pump with a liquid-nitrogen cooled trap, an ion sputtering pump, and a titanium-sublimation pump. After 24-hour baking at temperatures up to 200°C, a base pressure of ~ 1 x 10 -8 Pa was routinely obtained. One face of the T i C ( l l l ) specimen was mechanically polished to a mirror finish with diamond fine powders, and finally cleaned in ultrahigh vacuum by flash heating using electron back bombardment. After several hearings up to 1500°C, the L E E D pattern showed sharp diffraction spots on a low background, corresponding to a 1 x 1 structure as shown in fig. la. No impurities such as oxygen and contaminated carbon were found in the XPS spectra. The graphite overlayer was grown by reaction of ethylene gas with the substrate at l l00°C. After an exposure of 600 langmuirs, the L E E D pattern changed from fig. la to fig. lb. Besides the original diffraction spots indicated by " T " , new diffraction spots " G " and " S " appear in fig. lb; the spots " G " correspond to diffraction due to the graphite monolayer, and the other ones " S " are attributed to double diffraction. The lattice constant of the graphite layer determined from fig. lb is 2.52 A, which is larger than that of bulk graphite by 2%. Both the sharp spots and the low background in the L E E D pattern indicate that the graphite overlayer has good crystalline quality in comparison with that of a single crystal of kish graphite used in this experiment. o

Fig. 1. The LEED pattern for T i C ( l l l ) clean surface (a), and a monolayer graphite formed on T i C ( I l l ) ( b ) .

3. Results and discussion

3.1. C Is core level shift

Fig. 2 shows the C ls peaks in the XPS spectra of a clean T i C ( l l l ) surface, M G / T i C ( l l l ) , and kish graphite. T h e r e are two peaks in the spec-

trum of M G / T i C ( l l l ) . For convenience, we shall label the higher and lower binding energy peaks as "graphite peak" and "TIC peak", respectively. In order to confirm that the graphite peak comes from carbon atoms in the overlayer, we per-

A. Nagashima et al. / Electronic states o f monolayer graphite formed on TiC(lll) surface

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285 280 Binding Energy (eV) Fig. 2. C ls region of XPS spectra of a TiC(111) clean surface, the graphite monolayer on TiC(Ill), and bulk graphite.

formed an angle-resolved XPS m e a s u r e m e n t by rotating the sample about its vertical axis. Fig. 3 shows the emission angle d e p e n d e n c e of the intensities of these two peaks, and the intensity ratio of the graphite p e a k to the TiC peak. The TiC p e a k has maximum intensities at emission angles of about 20 °, 40 ° and 55 °, which correspond to the forward scattering in the TiC crystal [13]. As the emission angle gets closer to the

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95

grazing angle, the intensity ratio increases steeply, which shows that the graphite peak in the spectrum originates from the overlayer. It should be noted that the C ls p e a k of the graphite monolayer is located very close to that of the kish graphite, departing from that of TiC. In the graphite crystal, of course, no electrons are transferred from carbon atoms to carbon atoms. In the TiC crystal, on the other hand, a large amount with a charge of 0.2 electrons per C atom ( e / C atom) moves from the Ti atoms to the carbon atoms [14], which causes the observed chemical shift in fig. 2. Hence, the smaller chemical shift of C ls of M G from bulk graphite indicates that such a large charge ( ~ 0.2 e / C atom) is not transferred from the substrate to the graphite overlayer. In other words, the XPS resuits show that the C - C bond weakening in the monolayer graphite is not caused by electron transfer. This is in contrast to the case of A G I C , in which a charge of 0.1-0.2 ( e / C atom) is transferred from the intercalants to the carbon atoms [12]. 3.2. Valence band structure

Fig. 4 shows typical angle-resolved UPS spectra measured in the F M direction excited by H e I and H e II resonance lines. In fig. 5, the dispersion relations of the peaks and shoulders of the spectra in the fig. 4 are plotted along the F M and F K axes of the two-dimensional Brillouin zone of graphite. The ~" band indicated by a shaded area shows symmetry around the M point. Because the unit cell in the graphite crystal is composed of double layers, the ~ band splits into two branches in the bulk [15]. In this experiment, on the other hand, only one ~- branch was observed, which is further experimental evidence for one atomic layer thickness of the graphite overlayer. A similar p h e n o m e n o n has been observed for the graphite layer on Ni(100) [16]. N e a r the M and K, points, the binding energies of the observed ~r band of the graphite monolayer have shifted to a position deeper than those of bulk graphite by more than 2 eV [17], while their changes near the F point have been much smaller. As a consequence, its bandwidth be-

A. Nagashima et al. / Electronic states of monolayer graphite formed on TiC(111) surface

96

comes remarkedly narrow, which was also observed in the M G / N b C ( l l l ) [18]. These phenomena show that the rigid band model is not valid for the electronic structure of the graphite monolayer, which is very different from the case of A G I C [19]. Hence, we must consider a mechanism other than charge transfer, to understand both the weakening of the intralayer bonding and the strengthening of the interlayer bonding. Recently, an energy band calculation of M G on the Ti-terminated T i C ( l l l ) surface was performed by Kobayashi et al. [20]. The calculated bands which mainly contain the rr character in the graphite are also indicated in fig. 5 by the dotted curves. The theoretical dispersion relation of the 7r band agrees well with the present experimental data. Kobayashi et al. have pointed out two important results in their calculation. First, the electron transfer from the substrate to the graphite layer is small; at the most, 0.01 e / C atom, which is consistent with the small chemical i i , , , i , , i , i =- Band

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Fig. 5. Binding energies of the observed peaks and shoulders in the photoemission spectra excited by He I (open squares), and H e II (open circles) resonance lines are plotted against the wave vector parallel to the surface. T h e shaded area presents the well-known shape of the ~" band, fitted to our experimental results. T h e theoretical dispersion relations [20] are also indicated by the dotted curves.

shift of the C ls p e a k in fig. 2. Second, the hybridization of the ~" and rr* orbitals with the d orbitals of Ti atoms forms the new covalent bonding states, resulting in the narrow bands. Conceming the hybridized bands, we observed three dispersionless branches in addition to the rr band in fig. 5. The 3 eV branch results from the bulk electronic states of the TiC substrate [11,21], however, the other branches are new states due to the overlayer formation, which correspond to the hybridized d bands. The covalent bondings are, of course, much stronger than the van der Waals bondings in the bulk graphite, and the new electronic states presumably weaken the in-plane C - C bonding with donation of electrons from the ~- states and back-donation into the ~r* states. This is analogous to the mechanism of C - O bond weakening of carbon monoxide chemisorbed on transitionmetal surfaces [22,23]. Therefore, this orbital hybridization could be the main origin of the anomalous phonon structure and the lattice constant expansion in the monolayer graphite.

B i n d i n g E n e r g y (eV) Fig. 4. Angle-resolved UPS spectra of the monolayer graphite on T i C ( I l l ) m e a s u r e d in the F M direction. Emission angles m e a s u r e d to the surface normal are also denoted. Approximate positions of the ~" band are indicated by dashed lines.

3.3. Work function measurement Table 1 shows a list of measured work functions for some clean surfaces and graphite-

A. Nagashima et at / Electronic states of monolayer graphite formed on TiC(lll) surface

Table 1 Work function of some clean surfaces and graphite-covered surfaces Substrate

Clean surface (eV)

TiC(Ill) Ni(111) Ru(0001) Ir(111) Graphite

4.7 [11] 5.3 [24] 5.4 [25] 5.8 [26] 4.6

Graphite-covered surface (eV) 4.4 4.3 [24] 4.5 [25] 4.5 [26]

References between square brackets.

covered surfaces. Though the work functions of the substrates differ largely from each other, the graphite-covered surfaces have almost the same value. This fact shows that the work functions of the covered surfaces are determined mainly by the electronic states of the graphite overlayer. In table 1, it should be r e m a r k e d that all of the graphite covered surfaces have slightly smaller values than bulk graphite, which is consistent with the above argument of the orbital hybridization for the following reason. In general, a work function ~b is described as follows: ¢ = Aqb - / x ,

(1)

where Ark is the surface potential barrier resulting from the surface electric dipole, and /x is the chemical potential of the electrons. W h e n the orbital hybridization between the graphite layer and the substrate forms covalent bondings, redistribution of the electrons occurs; the electrons in the vacuum side of the graphite overlayer move to the interface side, and this electron redistribution produces an electric dipole perpendicular to the surface, which may cause a decrease in the work function.

4. Conclusions By means of XPS, UPS and work-function measurement, we have investigated the electronic states of the monolayer graphite formed on the T i C ( I l l ) surface. T h e results are summarized as follows.

97

(1) F r o m the chemical shift of the C ls peaks in XPS spectra, the charge transfer from the substrate to the graphite layer was estimated to be much smaller than in the case of stage-1 A G I C ' s ( ~ 0.2 e / C atom). Thus, the charge transfer is not the main origin of the reported C - C bond weakening. (2) The binding energies of the Tr band of the graphite monolayer shift to a higher energy position n e a r the edge of the Brillouin zone. Therefore, the bandwidth becomes narrow in comparison with that of bulk graphite. This fact indicates that the rigid band model is not valid for describing the electronic structure of the graphite monolayer. (3) T h e observed work functions imply a redistribution of electrons in the graphite layer, correlated with the orbital hybridization between the overlayer and the substrate. From these results, we have concluded that the orbital hybridization between the graphite layer and the substrate is the origin of the C - C bond weakening in the monolayer graphite on the TiC(111) surface. This is in striking contrast to the case of A G I C ' s , of which the physical properties are changed mainly by charge transfer. All the present experimental data concerning the XPS, UPS and work-function m e a s u r e m e n t agree with the recent theoretical calculations [20].

Acknowledgement We are grateful to Dr. Ooiwa and the other m e m b e r s in U L V A C - P H I for their technical support in this experiment, and to Professor T. Ohsaka for his encouragement.

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A. Nagashima et al. / Electronic states of monolayer graphite formed on TiC(lll) surface

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