First Principles Calculations Of M_nC(M=Fe, Co, Ni, Cu, N≤6) Clusters

After the discoveries of metallocarbohedrenes (met-cars) and carbonnanotubes (CNTs), more and more interests have been attracted for study-ing the clusters, which are composed of metal(M) and C atoms, both exper-imentally and theoretically. In this thesis, M_nC(M=Fe, Co, Ni, Cu, n=1-6)clusters are studied systematically by a first principles method based den-sity functional (DFT) theory.Chapter one introduces the properties and status in quo of clusters,transitional metal (TM) clusters and CNTs, as well as the clusters consistedof TM and C atoms.In chapter two we describe in detail the method used in this thesis: afirst principles method based on DFT within the framework of plane-wavebasis sets and ultrasoft pseudopotentials. The calculation programme ofPWscf code is also introduced.The report of a systematic study of M_nC(M=Fe, Co, Ni, Cu, n=1-6)clusters is given in chapter three. The ground state geometrical structures,the evolution of binding energies, and magnetic moment are analyzed.The results of structural optimization show that, in general, the struc-tures of the system have no consistent evolution, however, the structuresof TM_nC(TM=Fe,Co,Ni)clusters are similar to those of correspond-ingly pure TM_n+1 clusters. But for Cu_nC clusters, except Cu₂C, doping of a C atom makes the two-dimensional geometries of Cu_n(n=3-6) clusterschange into three-dimensional structures. By analyzing the binding ener-gies we find that doping of a C atom improves the stabilities of M_n clusters.According to the second-order energy difference and HOMO-LUMO energygap, Fe₄C, Ni₂C and Cu₄C clusters have relatively higher stabilities, whichcan be predicted as magic clusters. The interaction between the C atomand M_n clusters is the important issue in this thesis, so the fragmentationenergies of two channels, i.e., loss of a M atom (DM) and loss of the C atom(DC), are calculated. The results show that the binding energies of C-M_nare nearly twice as large as those of M-M_n-1C and the binding energy ofC-Cu_n are considerably less than those of C-TM_n. It indicates the bind-ing strengths of Fe_n-C, Co_n-C and Ni_n-C are stronger than that of Cu_n-C,which plays an important role in the M-C growth mechanisms.In the last chapter, summarizations of our works are given.