STM study of C$_{70}$ and $C_{60}$/$C_{70}$ mixture on Ag(100)

The adsorption structure of C$_{60}$ on Ag(100) is a very interesting one because it is a uniaxial incommensurate aperiodic (111) phase with extensive adsorbate-induced reconstruction [1]. Peculiar molecular ordering arises therein due to delicate balance of competitive interactions. In contrast, few adsorption studies were conducted for the next abundant fullerene, C$_{70}$, which has an ellipsoidal shape. We have recently studied C$_{70}$ and C$_{60}$/C$_{70}$ mixtures of controlled ratios on Ag(100). We compare structures and molecular ordering of these two systems to that of C$_{60}$/Ag(100), and seek possible structural phase transitions therein. Our results show that C$_{70}$/Ag forms a commensurate phase i.e., c(6$\times $4). The mixture adlayers evolve from c(6x4) to (111) with increasing C$_{60}$ ratios, as evidenced by low energy electron diffraction. In many cases, uni-directionally aligned bright lines were observed, e.g., at a C$_{60}$:C$_{70}$ ratio of 2:1. Such ordering cannot be explained by the model developed for C$_{60}$/Ag, and thus its origin remains controversial. Sequential deposition of C$_{60}$ and C$_{70}$ shows that C$_{60}$ intermix readily into the C$_{70 }$layer but not vice versa. This behavior suggests that kinetics of fullerene diffusion and intermixing may be crucial to determine the mixture structure. [1] W. W. Pai and C. L. Hsu, Phys. Rev. B \textbf{68}, 121403 (2003); \textit{ibid. }\textbf{68}, 245414 (2003)