Vibrational spectra of the monolayer films of hexagonal boron nitride and graphite on faceted Ni(755)

Surface Science 427–428 (1999) 97–101

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Vibrational spectra of the monolayer films of hexagonal boron nitride and graphite on faceted Ni(755) E. Rokuta a, *, Y. Hasegawa a, A. Itoh a, K. Yamashita a, T. Tanaka a, S. Otani b, C. Oshima a a Department of Applied Physics, Waseda University, 4-3-1 Okubo, Shinjuku-ku, Tokyo 169, Japan b National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305, Japan

Abstract Hexagonal boron nitride (h-BN ) and graphite films on faceted Ni(755) have been investigated by means of vibrational spectroscopy, accompanied by low energy electron diffraction and Auger electron spectroscopy. A faceted surface consisting of the (111) face and high-Miller index face has been grown by the adsorption of monolayer films. Vibrational spectra of the monolayer h-BN have exhibited two peaks at 90 and 97 meV, which originate from the optical phonons with out-of-plane displacement (TO ) of the films adsorbed on the faceted (111) and the other high) index faces, respectively. These peaks indicate that the bond strength between h-BN films and a Ni face depends on the Ni face orientation. On the contrary, only one peak of TO mode has appeared in the monolayer graphite on the ) same faceted surface. The single peak means a similar strength of the interfacial bond on various surfaces. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Faceted Ni(755); Graphite; Hexagonal boron nitride; Monolayer films; Vibrational spectra

1. Introduction In recent years, much attention has been devoted to various p electron systems such as fullerenes [1], nanotubes [2], and monolayer films of graphite and hexagonal boron nitride (h-BN ) [3,4] and other systems. So far, we have investigated intralayer and interfacial interactions of the epitaxial monolayer films formed on the various low index metal surfaces [5]. The effects of the edges of the films are almost negligible in these studies, since the film sizes are large enough compared to the unit cell of the films. However, as shown in some studies focusing on industrial applications such as rechargeable lithium batteries [6 ], * Corresponding author.

it has been inferred that the film edges play an important role. In this study, we tried to grow epitaxial nano-size films with a ribbon structure on a stepped surface to study the physical properties of the edges, and, as a result, found that the formation of the monolayer films induced a faceted structure. We show the difference in the vibrational spectra of graphite and h-BN on the same faceted surface, and discuss the different interfacing bond strengths on the different Ni faces.

2. Experimental The apparatus used in this study has been described in detail elsewhere [7,8]. Briefly, the

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experiments were conducted in an ultra-high vacuum chamber equipped with a high-resolution electron energy loss (HREEL) spectrometer, a low-energy electron diffraction (LEED) optics, a cylindrical mirror analyzer for Auger electron spectroscopy (AES), and a quadrupole mass spectrometer (QMS ) for reactant gas analysis. In this study, we have used a new power supply for the HREEL spectrometer and its control system and, with a computer constructed in our laboratory, have successfully realized a low electric drift and noise, and the optimal combination of the potentials for all the electrodes has been reproduced instantly with this system. In this paper, all the HREEL spectra were obtained in a specular condition. The substrate used in this study was a Ni (755) surface. The clean (755) surface consists of (111) terraces about 1.3 nm wide, separated by steps along the (100) orientation with a single atom height. One face of the specimen was polished mechanically to optically flat, and then chemically etched. In the UHV chamber of ~2×10−8 Pa, the specimen was finally cleaned by the cyclic treatments of heating at 1050°C by electron bombardment ( EB) and Ar+-ion sputtering. The wellordered clean surface was confirmed by AES, LEED and HREELS. We have measured the specimen temperature by using an optical pyrometer (Minolta TR-630, Japan). Since a light emitted from the hot filament for EB caused overestimation of this temperature below about 650°C, we have also estimated the temperature from a heating power, W , below sample 650°C . In this paper, the heat treatment below 650°C will be represented by both the consumed power and the estimated temperature. The power W enabled reproducible measurement. sample

3. Results Fig. 1 shows LEED patterns of the clean Ni(755) surface and the surface covered with monolayer graphite. As can be seen in a typical LEED pattern of a stepped surface [9], bright and sharp dominant spots following less-bright adja-

cent spots shown in Fig. 1a represent a wellordered (755) surface. The formation of the monolayer graphite films changes the LEED pattern drastically; the stepped (755) surface changed into the faceted one. The movement of the spots with the changing electron energy indicates the existence of the two specular spots. One is the 111 face, and the other is the high-Miller index face oriented from (111) surface by 45°. This structural change was always observed in the various experimental conditions. The area ratio of the (111) face to the higher-index face was estimated to be five at the largest from the average intensity ratios of these specular spots, which is consistent with the orientation angle and area ratio of the macroscopic (755) surface. The faceted surface covered with graphite is depicted schematically in Fig. 1c. As for the monolayer h-BN adsorption, the same spontaneous structure change took place as well; a LEED pattern exhibited the same faceted surface. The AES measurement for coverage analysis clarified a saturated adsorption of BN with stoichiometric boron and nitrogen atoms. Fig. 2 represents HREEL spectra of the faceted Ni(755) covered with monolayer graphite and h-BN, which were grown by a 100 L borazine and a 10 L benzene exposure, respectively, at W =1.0 W (about 450°C ). Any loss of peaks sample due to BMH or NMH stretching vibrations did not appear (not shown in Fig. 2) during this treatment, which means that dehydrogenation occurred completely. We observed loss of two peaks at 90 and 97 meV which are characteristic energies of the transverse optical phonons with out-of-plane displacements ( TO ) in the spectra. In addition, ) we detected a broad loss peak centered at 180 meV due to optical phonons with in-plane displacement. These features in the spectrum indicate that h-BN films covered the specimen’s surface [5]. Two different energies of the TO phonons indicate two ) adsorption states of the h-BN films. The broad loss peak at the higher energy region suggests poor crystallinity of the films on the faceted surface. In the spectrum of monolayer graphite on the faceted surface, on the contrary, we observed loss of only one peak due to TO phonons at 89 meV. ) The observed energy is the same value as the energies measured from the dispersion curves of

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Fig. 1. Typical LEED patterns of (a) a clean Ni(755) and (b) a faceted surface covered with monolayer graphite. Incident electron energy is 90.0 eV for both patterns. Each arrow indicates the moving direction of a spot as the incident energy was changed. (c) The corresponding faceted structure model is drawn schematically.

monolayer graphite films on flat Ni(111) and (100) surfaces studied by Aizawa et al. [10]. In their HREEL spectra, no TO phonons were detected ) in the specular direction and, hence, the observable phonon peaks in Fig. 1 originate from the impact scattering mechanism in an off-specular reflection caused by the facet. Namely, our scattering geometry is specular to the macroscopic (755) surface, and is off-specular to the microscopic faceted faces. ˚ −1 for The corresponding wave vector was 0.08 A the (111) faces. Fig. 3 shows the HREEL spectra of the surface covered with monolayer h-BN measured before (a) and after (b–d) heating the specimen. Heating up to W =3.0 W (about 620°C ) did not essensample

tially change the measured spectra. A drastic change occurred in spectrum (d ) measured after heating at 720°C. The loss peak at 97 meV disappeared, and the other peak at 92 meV became intense and sharp. Furthermore, a sharp loss peak at 172 meV caused by optical phonons with in-plane displacements appeared, following a peak due to the second harmonic of TO phonons. The ) drastic change at 720°C also happened in a following AES measurement; the peak-to-peak intensity ratios of boron and nitrogen to nickel remain nearly constant at around 0.2 up to about 620°C, keeping stoichiometry. At 720°C, both values decreased to a half, remaining stoichiometric with h-BN.

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Fig. 2. HREEL spectra of the monolayer films of h-BN and graphite on the faceted Ni(755).

4. Discussion Here, we discuss the different bond strength between the h-BN film and the different surfaces from the following experimental facts. 1. The faceted structure was formed on the (755) surface by the formation of monolayer graphite and h-BN. The surface consists of the (111) face and higher-index face. 2. While the two characteristic peaks of TO ) phonons appeared in the spectrum at the full coverage, a single peak was detected at 92 meV after heating at 720°C. The heating treatment decreased the AES signals for both boron and nitrogen by half. 3. In previous work, we have found the TO ) phonons at 91 meV of h-BN on Ni(111), which is lower than the bulk value (100 meV ). This softened phonon comes from the weakening

Fig. 3. A HREEL spectrum of monolayer h-BN on Ni(755) (a), followed by spectra obtained after heating (b–d). The spectra denoted by (b–d) are in order of increasing heating temperature; (b) W =1.5 W, (c) 3.0 W (about 620°C ), and sample (e) 5.0 W (720°C ). AES intensity ratios of boron and nitrogen to nickel were noticed beside the corresponding HREEL spectra.

BMN bonds of p electron, owing to the orbital mixing between the film and Ni(111) [5]. 4. The vibrational energy of the TO phonons of ) h-BN films on Pd and Pt(111) are almost same as a bulk one [5]. The monolayer h-BN film interacts with Pd and Pt(111) with Van der Waals force [4], and the lattice grows on the substrate in an incommensurate manner. Because of the small lattice mismatching, on the other hand, the h-BN films adsorbed in a commensurate manner on Ni(111), which results in a strong interfacing orbital mixing. The above facts indicate clearly that the observed peak at 90 meV in Fig. 1 stems from the BN film on the (111) face, and 97 meV from the other high-index face. The AES and HREELS

E. Rokuta et al. / Surface Science 427–428 (1999) 97–101

data in Fig. 3 explicitly indicate that only the BN film with the 97 meV TO phonons vanished at ) 720°C, while the films exhibiting the lower phonon energy (92 meV in Fig. 3) remain at the same temperature. Namely, only the films on the (111) faces survived heating at 720°C because of the chemical interfacial bonds discussed before. On the contrary, the films on the high-index faces vanished from the face due to the weak interfacial bond. The weak bond strength between h-BN and the high-index faces is consistent with the case of Pd and Pt (111); bulk-like TO phonons are ) detected in the films on Pd and Pt(111) associated with the weak interfacial bond, commonly. At present, we assume that the orbital mixing of the p band and d bands mentioned above, and hence the bond strength between the films and Ni faces depends on their lattice matching. It should be, however, confirmed by systematic experiments by using surfaces of Ni other than (111). The situation of the graphite is different from that of the h-BN. As shown in Fig. 1, the monolayer graphite films on both (111) and high-index faces exhibit only one TO phonon, which indi) cates almost the same interfacial bond on the different Ni faces. What caused the difference? The reason is not clear now, but two different points in these films are noticeable: 1. The spatial distributions of p electrons: while the p electrons in the graphite occupy the orbitals of the constituent two carbon atoms equivalently, the h-BN p-electrons are localized at the nitrogen atom. 2. The band width of p electrons: the p band width of graphite is 10 eV, and that of h-BN is 6 eV, because of the large band gap. Owing to the wide p bands of graphite, the orbital can

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easily mix with the similar wide d bands of Ni surfaces. Deeper understanding requires an advanced theoretical study such as that performed on the films on Ni(111) [11].

5. Summary In this study, we have clarified that a formation of monolayer films of h-BN and graphite induces a faceted structure on a Ni(755) surface. Differing from the HREEL spectrum of monolayer graphite films, the spectrum of h-BN showed the two energetically different TO phonons. One of them, ) with softened energy, was more stable against heating. This TO phonon comes from the films ) adsorbed on (111) faces.

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