Elastic theory of surface deformation in C60 adsorption

In molecular physisorption, beyond attractive and repulsive forces, a third contribution arises from the elastic deformation of the surface. In this paper the dimple appearing on a gold surface upon physisorption of a C₆₀ molecule is studied and its effect on the adsorption energy is evaluated.The depth and shape of the dimple are studied on the basis of the theory of elasticity. The equilibrium equation is solved in the presence of the body forces originating from a suitable Au-C₆₀ interaction potential and the solution for the deformation field is obtained by means of a two-step procedure. In the first step a curl-free deformation field is obtained, which takes the effects of the body forces into account, but generates mismatch of the boundary conditions. In the second step the solution of the equilibrium problem in the absence of body force is generated, which compensates the previous boundary condition violation. Thanks to linearity, the final solution is obtained as the sum of the results of the two steps. An (approximate) iterative scheme is applied for taking the surface deformation into account in imposing that no forces act on the Au surface.When the distance z₀ of the centre of the C₆₀ cage from the surface is rather large, the dimple is represented by a bulging surface, but, as the C₆₀ approaches the Au surface, the surface recedes and a well formed dimple appears. The corresponding depth at the mechanical equilibrium position (z₀ = 4.8 Å) turns out to be 36.3 pm.