Thermal expansion of thin C-60 films

The thermal expansion coefficient alpha and structure of C-60 films with th ickness t similar to 3-10 nm were investigated in the temperature interval from room to liquid- nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C-60 films condensed in vacuum on a (100) NaC l cleavage surface and oriented in the (111) plane. A surface- induced size effect in the thermal expansion coefficient was observed. It was establish ed that as t decreases alpha(f) increases and is described well by the rela tion af alpha(f) = 17.10-6 K-1 + 8.3.10-5 nm K-1 t(-1). This relation was u sed to estimate the linear expansion coefficient alpha(s) of the C-60 surfa ce in the (111) plane as alpha(s) = 60.10(-6) K-1, which is several times l arger than the bulk value. The experimental results agree satisfactorily wi th the theoretical calculations of the mean- square displacements of molecu les located in a region near the surface. (C) 1998 American Institute of Ph ysics. [S10637761(98)02411-1].