SUBLIMATION OF HIGHER FULLERENES AND THEIR INTERACTION WITH SILICON(100) SURFACE

The heat of sublimation of C-60, C-70, C-78, C-84, and C-96 is measure d by modulated molecular beam mass spectrometry (MMBMS). For a mixture of these fullerenes the heat of sublimation is 39 +/- 5, 43 +/- 5, 47 +/- 18, 59 +/- 6, and 65 +/- 11 kcal/mol, respectively, and did not c hange with the variation in composition. The partial pressure of C-84 is estimated to be P-84 = 7.0 x 10(12) exp[-59.4 kcal/mol/RT] Torr. Th e scattering of C-70 and C-84 from Si(100) is also investigated by MMB MS. The behavior of C-84 is quite similar to that of C-60 reported ear lier By Hamza, Balooch, and Moalem (Surf. Sci. 1994, 317, L1129) excep t that the onset of surface decomposition of the fullerene occurs at 8 50 g, 150 K lower than for C-60. When the surface temperature reaches 1000 K, total decomposition of the incident C-84 is observed. The beha vior of C-70 is surprisingly different from both C-60 and C-84 Twenty percent of the incident C-70 decomposes at 1000 K, and the remaining 8 0% does not decompose until a target temperature of 1200 K is reached. At surface temperatures below 800 K, for the interaction of C-70 and C-84 with silicon, the dominant surface processes are adsorption with near perfect sticking probability, followed by surface diffusion to th e edge of the specimen and desorption. The ratio of desorption rate co nstant kd, to diffusion coefficient, D-s, for C-70 and C-84 was calcul ated to be 2.7 x 10(16) exp(-40 (kcal/mol/RT) and 7.8 x 10(7) exp(-22 (kcal/mol/RT) cm(-2), respectively.