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

Abstract

The heat of sublimation of C-60, C-70, C-78, C-84, and C-96 is measured 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 change 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. The scattering of C-70 and C-84 from Si(100) is also investigated by MMBMS. The behavior of C-84 is quite similar to that of C-60 reported earlier By Hamza, Balooch, and Moalem (Surf. Sci. 1994, 317, L1129) except that the onset of surface decomposition of the fullerene occurs at 850 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 behavior 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 80% 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 the edge of the specimen and desorption. The ratio of desorption rate constant kd, to diffusion coefficient, D-s, for C-70 and C-84 was calculated 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.