Ordering and stabilization of C60 films on the ()R30° Sn/Pt(111) surface alloy
Introduction
The growth of ordered C60 films by modification of the substrate surface has been the objective of many recent studies [1], [2], [3], [4], [5]. Ordered C60 films have enhanced conductivity and stable electrical properties [6]. This is of importance particularly in the light of the interesting electrical properties of C60[7], doped fullerides [8], and superfullerides [9], which may find applications in electrical devices [10]. Charge transfer interactions between C60 and metal [11], [12], [13] and semiconductor substrates [14] have been of great interest in light of understanding adsorbate–substrate bonding, and also in explaining the anomalous conductivity of C60 on some metal surfaces [15], [16]. On Pt(111), C60 grows as a disordered film at 300 K because of strong chemisorption [17] that leads to low adsorbate mobility on the surface [5]. It has been proposed that annealing to 900 K enhances the adsorbate mobility and results in an ordered C60 film, as seen by the appearance of C60 domains in low energy electron diffraction (LEED) of C60/Pt(111) [17], [18]. However, since Pt(111) catalyzes the graphitization of C60 at 900 K, graphite domains appear along with C60 domains in the LEED results [17], and the integrity of C60 in these films is uncertain.
In previous studies it was found that the alloying of Pt(111) with Sn to form the ()R30° Sn/Pt(111) surface alloy [19] (henceforth called the alloy), chemically deactivates Pt(111) [20]. The main objective of this work was to chemically modify the Pt–C60 interface by alloying Pt(111) with Sn, and study the deposition and growth of ordered C60 films on this less reactive alloy surface. We find that Sn arrests the charge transfer from Pt(111) to C60 on deposition at 300 K. Upon annealing C60 on the alloy to higher temperatures, we find that Sn inhibits graphitization of C60 on Pt(111) in the 900–1100 K temperature range, such that ordering of C60 occurs on this alloy surface without decomposition. To date, no studies of C60 growth on bimetallic alloy surfaces have been reported and hence this work provides a useful reference for future studies in this field.
The interaction of Sn with C60 is also of interest. Superconductivity up to 37 K of Sn-doped C60 has been reported [21]. Vibrational studies of this compound have identified features in the spectra that differ distinctly from that of pristine C60[22]. Further, the resistivity of C60 films is known to drop sharply in the presence of an Sn layer [23]. Whereas photoemission spectroscopy results [24] suggest no charge transfer interactions between Sn and C60, both absorption and luminescence studies [25] show absorption band broadening and new bands of the C60 film attributed to Sn intercalation. The structure and properties of the Sn-doped C60 phase have not been studied. It has been suggested that the reaction of Sn with C60 is slow and limited by diffusion of Sn into the C60 lattice [24]. In previous studies, the Sn-doped phase was usually present with an excess amount of pristine C60, and this complicated interpretation of results. This is especially true when using surface-sensitive probes, such as photoemission for fulleride films [26]. A related objective of the work reported herein was to study the interaction of only a few monolayers of C60 with Sn from the surface alloy, in order to minimize the effects of unreacted C60 on the results.
Homogenous decomposition of C60 molecules in the gas phase [27] and solid phase [28], [29] at high temperatures has been studied. Isolated C60 molecules fragment at 1700 K and this is lowered to about 1200 K for solid C60[28], [29]. Although the onset temperature for fragmentation of 1 ML C60 on various metal surfaces is difficult to ascertain, extensive decomposition of C60 clearly occurs by 900 K on Pt(111) [17] and 850 K on Ni(110) [18] and Rh(111) [13]. No intermediates were identified in the vibrational spectra of the conversion of C60 to graphite on Pt(111), Ni(110) or Rh(111). In the current study, Sn on Pt(111) stabilizes the ordered C60 monolayer until 1100 K, and graphitic domains appear in LEED only at 1200 K. In order to characterize any intermediates in the decomposition of C60 to graphite, this work presents vibrational spectra of C60 films annealed to 1200 K on the alloy surface.
Section snippets
Experimental methods
The experiments were conducted in a three-level ultrahigh-vacuum chamber with a base pressure of 2×10−10 Torr, as has been described previously [17]. The top level was equipped with a double-pass cylindrical mirror analyzer (CMA) which was used for Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The middle level was equipped with LEED optics and a quadrupole mass spectrometer (QMS) for temperature-programmed desorption
Adsorbate–surface charge transfer interactions
HREELS spectra for adsorption of C60 on Pt(111) and the alloy at 300 K are shown in Fig. 1, Fig. 2 respectively. Multilayer C60 films (5 ML) on Pt(111) in Fig. 1 showed dipole-active peaks at 533 cm−1 [T1u(1)], 1215 cm−1 [T1u(3)] and 1467 cm−1 [T1u(4)], whereas the dipole inactive Hg(4) mode appeared at 756 cm−1[30], [31], [32]. The peak for the T1u(1) mode shifted from 511 cm−1 at 7(C60)=0.3 ML to 533 cm−1 at 7(C60)=5 ML. The shift of this vibrational mode has been correlated in theoretical [30] and
Discussion
The alloying of Pt(111) with Sn to form the ()R30° structure results in comparatively small electronic changes in the Pt valence band [37], and thus the influence of Sn on the chemistry of Pt(111) has been attributed mostly to site blocking effects that deactivate the surface for reaction with many organic molecules [39]. However, some electronic effect of alloyed Sn at the Pt(111) surface was also seen, for example in NO chemisorption studies on the Sn/Pt(111) alloy [40]. In studies of
Conclusions
The major conclusions of this work can be stated as follows.
(1) Vibrational spectra from HREELS show strong interactions between C60 and Pt(111) that lead to charge transfer from Pt of about two electrons per C60 molecule for 7(C60)=0.3 ML, and one electron per C60 molecule for 7(C60)=1.0 ML. These charge transfer interactions are inhibited by alloying of Pt(111) with Sn to form the ()R30° Sn/Pt(111) surface alloy.
(2) Ordered structures of C60 can be formed on Pt(111) by annealing multilayer C
Acknowledgments
We acknowledge support of this work by the Divisions of Chemistry and Materials Research of the National Science Foundation (NSF).
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