Energetics and kinetics of Ti clustering on neutral and charged C ₆₀ surfaces

Using ab initio spin density functional theory, we investigate the energetics and kinetics of Ti clustering on both neutral and charged C ₆₀ surfaces. We compare the formation energy of sparsely dispersed zero-dimensional (0D), compact single-layered two-dimensional (2D), and clustered three-dimensional (3D) Ti_N configurations as a function of cluster size (N≤12) and further study the transformation kinetics between them. We find that 0D configuration is always less stable than that of 2D and 3D configurations and 0D to 2D transformation involves in a single Ti diffusion process with kinetic barrier of ≤0.7 eV. On the other hand, there exists a critical cluster size (N_C) of N_C=5, below which 2D layers are preferred to 3D clusters. Hole- or B-doping greatly enhance the Ti-fullerene interaction and lead to stronger dispersion of Ti atoms. Even so, for moderate charge doping (less than seven holes) the critical size of Ti atoms on neutral C₆₀ surprisingly remains unchanged or only slightly increases to N_C =6 by B-doping. However, we find that the formation of 3D clusters may be hindered by a high kinetic barrier related to the process of single Ti atoms climbing up a single Ti layer. This barrier is ∼1 eV or even 1.47 eV for B-doped C₆₀ surfaces which is high enough to stabilize larger 2D structures (N≥N_C) at low temperatures. These findings may prove to be instrumental in stabilizing transition metal coated nanostructures and especially homogeneously Ti-coated fullerenes, which are believed to be a very promising material for hydrogen storage.