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Fig. 17.11. Electron energy loss function, Im(-l/e), the real part of the dielectric function e,(ai,q), and the imaginary part of the dielectric function f2('°< l) f°r RhjQo (x = 0,3, and 6) plotted vs. electron energy ha> up to 40 eV and for a momentum transfer of q = 0.15 A'1 [17.21], a heavy mass (m*/m0 ~ 4) charge carrier plasmon and a Fermi energy of ~0.25 eV, consistent with a narrow width for the -derived band for 60. The peak at ~1.1 eV is identified with tlu tlg interband transitions in Rb3C60, consistent with calculated and experimentally observed optical properties for K3C60 (see §13.4.3) [17.69]. Similar transitions near 1 eV

are also observed for Rb3C70, 60, and Rb6C70. Regarding the 7r plasmon and the (it + a) plasmon, many similarities are found between these features among the various RbxC60 and Rb,C70 films [17.21]. Whereas some differences appear in the detailed spectral features for the ir electron structures in the RbxC60 and RbxCV0 systems, the a electron structures appear to be unchanged upon doping with Rb.

17.3. Auger Electron Spectroscopy

Auger electron spectroscopy (AES) is a surface science technique which yields a unique spectrum for each chemical species and therefore is useful for identifying the chemical species on the surface. The intensity of the Auger lines for a given chemical element further gives the concentration of that species on the surface. Scanning Auger spectroscopy allows a map to be made of the sample surface regarding its stoichiometry. In AES experiments, a high-energy incident photon excites an electron from a core level to an ionization state, inducing a transition from a higher-lying electron state to fill the core hole level and the simultaneous emission of an electron whose energy is measured. The energy of the emitted electron is determined by the energy levels of the atom, and since the atomic energy levels for each atom are unique, the AES spectrum gives a unique identification of each atom. Each Auger line for a given species is labeled by three symbols, such as KLM, indicating the atomic shell of the core level (K), of the electron filling the core level (L), and of the emitted Auger electron that is detected (M). We discuss in this section the use of the AES technique to provide information on the surface composition, as well as on many-body interactions used to determine the repulsive Hubbard U interaction energy for two holes on the same C^q molecule.

An example of the analysis of the surface composition of a C60 film on an Si substrate is shown in Fig. 17.12. By monitoring the intensity of the Auger electron peak for carbon (272 eV) and for the substrate [e.g., Si (100) or Si (111) with an Si peak at 90 eV] (see Fig. 17.12), characterization information can be obtained on the surface composition of samples used in other surface science studies, such as the ratio of the carbon/silicon AES line intensities as a function of surface temperature [17.70,71]. At low-temperature, below the desorption temperature for C60, the C/Si ratio is high (~1.2). Once desorption of C60 occurs (at ~600 K), the carbon sur-

Fig. 17.12. Series of Auger electron spectra (AES) at 2 keV primary beam energy for as-deposited Cm on an Si (100) (1 x 1) surface at room temperature (RT) and the C^/Si (100) surface annealed to 650, 900, 1150, and 1250 K. The silicon AES feature appears at 90 eV and the carbon feature at 272 eV. The surface is heated at 3 K/s to the annealing temperature and is then rapidly cooled to less than 350 K, at which temperature each AES spectrum is taken. The various spectra are diplaced for clarity [17.70].

Electron Energy (eV)

Electron Energy (eV)

Fig. 17.12. Series of Auger electron spectra (AES) at 2 keV primary beam energy for as-deposited Cm on an Si (100) (1 x 1) surface at room temperature (RT) and the C^/Si (100) surface annealed to 650, 900, 1150, and 1250 K. The silicon AES feature appears at 90 eV and the carbon feature at 272 eV. The surface is heated at 3 K/s to the annealing temperature and is then rapidly cooled to less than 350 K, at which temperature each AES spectrum is taken. The various spectra are diplaced for clarity [17.70].

face depletion shows a C/Si ratio of ~0.4, associated with the residual C60 molecules that are tightly bound to the clean Si (100) surface [17.70,71], The AES C/Si ratio for an Si (111) surface is a factor of 2 higher than for an Si (100) surface, indicative of the larger number of binding sites for Ceo on Si (111)- In the temperature range 900-1150 K, the C60 molecules were reported to rupture, yielding carbon atoms and clusters on the surface which start reacting with the Si surface above 1150 K to form SiC [17.72], The weakly bound carbon atoms desorb more readily than the C60 itself, thereby enhancing the C/Si ratio that is measured by desorption. The increase in the C/Si ratio in the AES detector occurs at a slightly lower temperature for an Si (111) substrate, indicative of the higher activity and larger C60 surface concentration of the Si (111) surface relative to Si (100) [17.71], Above 1150 K, only residual C impurities remain on the Si (100) and Si (111) surfaces, as SiC forms and diffuses into the bulk yielding a low C/Si surface ratio that is nearly temperature independent [17.70,71]. These results can be explained by the fact that the bonding of C60 to a clean (100) Si surface is much stronger (~56 kcal/mol) than for C60 to itself (32 kcal/mol), so that the heating of C60 on an Si (100) surface yields a strongly adhering C60 monolayer above about 600 K. Although Si is not a metal, strong bonding between C and Si occurs, in accordance with the strong C-Si bond in SiC and the high density of dangling bonds on the Si surface. At high temperatures above 1200 K, only a few carbon atoms remain on the Si (100) surface [17.70,72],

A detailed observation of cage opening of CMI at 1020 K has been observed on an Si (111) (7 x 7) surface using a variety of surface science techniques, including scanning tunneling microscopy (STM) in an ultrahigh vacuum (UHV) chamber, temperature-programmed desorption (TPD), and Auger electron spectroscopy (AES) [17.71]. At 1020 K, the carbon clusters appear under STM examination with an open cup-like structure and at yet higher temperatures, almost all of the carbon desorbs from the surface. The STM studies (see §17.4 and §17.9) suggest that C60 cages are mobile on Si surfaces at high T and tend to agglomerate, as also occurs for the fragments after the cages are opened. The thermal opening of the C60 cages may provide an explanation for the enhanced diamond nucleation on an Si surface [17.73,74] (see §20.3.1).

Auger spectroscopy is commonly used to monitor whether the growth pattern of C60 on a metal surface is layer-by-layer or islandic. In this application of AES, Auger compositional maps are made during the adsorption process. For example, by monitoring the Rh (302 eV) Auger line, the layer-by-layer adsorption of C60 on a Rh (111) surface was established [17.54],

Another closely related example is the use of the AES technique to monitor the interaction of C60 with metal surfaces such as aluminum, where it is found that the C60 adheres much more strongly to an A1 surface than to another C60 molecule [17.75], Because of this strong adhesion, a complete C60 monolayer tends to form before the second C60 layer is initiated in the growth of a C60 film on Al, and correspondingly, multiple C60 layers are easily desorbed from a metal surface such as Al to leave a continuous adsorbed C60 monolayer for T = 570 K. This strongly bound C60 layer passivates the Al surface, inhibiting A1203 formation [17.75] (see §20.2.6). Similar behavior is found for C70 on an Al surface, with the C70 multilayers leaving the surface at 620 K. Both C60 and C70 fullerene monolayers are stable on a clean Al surface until ~650 K (as shown in Fig. 17.13 for C60 on Al) and above this temperature the fullerenes begin to diffuse rapidly into the bulk metal [17.75]. Further study of desorption from clean, oriented single crystal faces of Al would be of great interest.

Auger electron spectroscopy has also been applied to study many-body effects in fullerenes, based on an analysis of the Auger lineshape [17.76,77]. From a many-body standpoint, the magnitude of the on-site Coulomb repulsion energy U between two holes on the same Cgg molecule can be determined from a high-resolution KW Auger spectrum, such as that shown in

Fig. 17.13. Plot of the carbon (272 eV) to aluminum (68 eV) AES peak height ratio as a function of surface anneal temperature from 570 to 825 K for a single Q,, monolayer on a clean Al surface [17.75].

mx) 650 700 750 »00 Surface Anneal Temperature (K)

mx) 650 700 750 »00 Surface Anneal Temperature (K)

Fig. 17.13. Plot of the carbon (272 eV) to aluminum (68 eV) AES peak height ratio as a function of surface anneal temperature from 570 to 825 K for a single Q,, monolayer on a clean Al surface [17.75].

. Auger

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