A Density Functional Study Of A_nC₂ (A=Fe, Co, Ni, Cu) Clusters

Clusters have the size between those of atoms and macroscopical sys-tems, and have many novel properties in structural, physical and chemicalaspects, as well as widely potential applications and far-reaching signifi-cance in the fields of manufacture and development of new materials, somany experimental and theoretical researches have been done on them.More focus has been put on transition metal clusters, because they haveimportant roles in the surfaces, chemical catalysts, magnetic properties,and other aspects of applications. In the process of growing single-walledcarbon nanotubes (SWCNTs), the commonly used catalysts such as tran-sition metal particles play vital roles, and can decompose carbon feedstock,and although Cu is not commonly used as catalyst to growth SWCNTs,it is one of the best catalyst for carbon fiber growth. So the clusters com-posed of transition metal and carbon atoms have been attracting people'sgreat attention. Through studing of small clusters composed of transitionmetal and carbon atoms, researchers expected to provide insight into thefunction of the transition metal atoms in the process of growth of nano-materials such as carbon nanotubes.With the rapid progress in computational physics, now it is possibleto study the geometry, electronic structure and many other properties ofsmall clusters from first-principles calculations, and get more and accurateinformation.Geometrical structures, binding energies and magnetic moments ofA_nC₂ (A=Fe,Co,Ni,Cu, n = 1 - 3) clusters were studied by using the den-sity functional theory with the plane wave basis sets and ultrasoft pseu- dopotentials.In chapter one we introduce the properties and research status ofclusters, transition metal clusters, carbon nanotubes and the clusters com-posed of transition metal atoms and carbon atoms. The background ofcluster research and the reasons of studing clusters composed of transitionmetal atoms and carbon atoms are also listed in chapter one.In chapter two we describe the details of the theory and compu-tational methods employed in this thesis. The development of densityfunctional theory, the plane wave basis sets and ultrasoft pseudopotentialsmethods, the cluster research methods and the PWscf module of Quantum-ESPRESSO package used in this thesis are introduced detailedly.In chapter three density functional theory approach are used to cal-culate the ground state geometrical structures and their first isomers ofA_nC₂ (A=Fe,Co,Ni,Cu,n = 1 - 3) clusters.In chapter four we calculate the energetic properties and magneticproperties of A_nC₂ (A=Fe,Co,Ni,Cu, n = 1 - 3) clusters.In chapter five we summarize our main conclusions, and also give awork plan in the near future.We summarized our results as follows:(1)All the ground state geometrical structures of these metal-carbonclusters are found to be planar except Fe₃C₂ and Co₃C₂ which show trig-onal bipyramid structures with the symmetry of Cs and C_3v, respectively.The ground state geometrical structures of AC₂ clusters have same config-uration which show planar triangle structures with the symmetry of C_2v.Our predicted structures for TM₂C₂ (TM= Fe,Co,Ni) clusters are still pla-nar structures and which can be obtained by adding another TM atom to one side of the triangular, though the ground state geometry of Cu2C₂ isalso a planar structure but different from the TM₂C₂ that the two copperatoms located far away from each other. The ground state geometries ofFe₃C₂ and Co₃C₂ show trigonal bipyramid structures with the symmetry ofCs and C_3v, respectively. The ground-state structures of Ni₃C₂ and Cu₃C₂are predicted to have fanlike geometries with the symmetry of C_2v in whichthe two carbon atoms lie in the center of the chain.(2)The doping of two carbon atoms enhances the average bindingenergies of the A_n clusters remarkably, and improves stability of pure A_nclusters.(3)The interaction strength between C₂ and TM_n (TM=Fe,Co,Ni) inTM_nC₂ clusters are much stronger than the interaction strength betweenC₂ and Cu_n in Cu_nC₂ clusters.(4)The doping of carbon atoms decreases the total magnetic momentsof the host TM_n clusters, however, the doping of carbon atoms doesn'tchange the total magnetic moments of the Cu_n clusters.