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Fig. 17.14. Comparison of the carbon CKVV Auger spectrum of solid CM with the self-convolution of the photoelectron spectrum after subtraction of a smoothing curve from the original Auger spectrum shown in the inset. The central curve is obtained from the self-convolution of the photoelectron spectrum after also subtracting a smooth curve and shifting the whole spectrum to higher energies by 1.6 eV. This energy shift is identified with the Hubbard U interaction energy [17.9,10].

Fig. 17.14. In this Auger process, the valence (V) electron emitted from the surface into the electron collector and the valence electron (V) dropping down to the carbon K hole level are both excited by the incident photon [17.9]. Analysis of the KW Auger lineshape indicates a similar electron correlation energy for C60, benzene, and graphite [17.76]. By studying the Auger and photoemission spectra on the same sample, analysis of the KW Auger spectrum in Fig. 17.14 shows that the spectrum must be shifted by an energy U = 1.6 ± 1 eV to match the self-convolution of the photoelectron spectra (labeled PES®PES in Fig. 17.14), thereby giving the density of valence states for the two valence electrons (V) of the Auger KW process. These two valence electrons are excited when an electron in the K shell is ionized by the incident x-ray. This interpretation of the KW Auger spectrum thus shows that the Coulomb repulsion energy U is much larger than the bandwidth of either the valence or conduction bands [17.9]. It should be noted that the nearest neighbor correlation energy V has also been determined (V = 0.3 eV) by the AES technique [17.78,79],

17.4. Scanning Tunneung Microscopy

Scanning tunneling microscopy has provided an excellent technique for studying the epitaxial growth of C60 films on various substrates and for identifying the surface crystal structures, crystalline domain sizes, and the common defect structures that occur in epitaxial fullerene films [17.2]. It is found that STM is an especially powerful technique for studying the interaction between fullerene molecules and surfaces and between fullerene molecules adsorbed on various substrates, as is discussed in §17.4.1 and more extensively in §17.9. Also of importance is the use of STM to identify the crystal structure of nanometer dimension crystallites prepared from higher-mass fullerenes (§17.9.5). The STM tip has also been used to manipulate individual C60 molecules on the surface. The imaging of individual carbon atoms on the CM molecules using STM has been difficult to achieve because of the rotational degrees of freedom of fullerenes near 300 K, although images showing internal structure have been obtained (see §17.9). It has also been found that the addition of C60 to an STM tip increases the resolution of the STM instrument [17.80], as is further discussed in §20.5.2.

An STM tip has also been used for electron beam irradiation of a C60 surface to stimulate the polymerization of C60 [17.81]. The same STM tip operating at a lower voltage was used to probe the polymerized material (see §7.5.2) and to write fine lines on a fullerene surface. In addition, STM has been used to carry out tunneling experiments on K3C60 and Rb3C60 to measure the temperature dependence of the superconducting energy gap [17.82,83], and these results are summarized in §15.4. The STM technique has also been used extensively to study the structure of carbon nanotubes (see §19.6.1) [17.84,85],

17.4.1. STM Studies of the Fullerene-Surface Interaction

Scanning tunneling microscopy provides an excellent tool for studying the interaction between fullerenes and surfaces and has been used extensively for such studies (see §17.9). The STM results show that C60 interacts strongly with metal surfaces, such as A1 [17.75], Cu (111) [17.86], Ag (111) [17.87], Au (111) [17.88], Au (001) and Au (100) [17.89], and weakly with oxide surfaces [17.72,90-93]. The interaction with these metal surfaces is much stronger than the C60-C60 interaction, but the opposite applies to these oxide surfaces. Although a range of values are given in the literature for the binding energies of C60 to specific surfaces, the values given in

Table 17.2

Desorption energy and desorption temperature for Cm from various substrates.

Table 17.2

Desorption energy and desorption temperature for Cm from various substrates.

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