Study On Structure And Magnetism Of Atoms And Clusters Supported On Graphene

The transition metal (TM) film, nanostructures, atomic chains, and clusters suppored on the the surface of materials has great scientific and technical value because these nanostructures are small enough to show quantum phenomena which can induce many incredible electronic and magnetic properties. Clusters array regularly on the substrate is promising to be the next generation microelectronics, high density recording, and nano-catalyzed material.The transition metal clusters with strong spin-orbit coupling (SOC) exhibit attractive physical properties in spintronics applications. However, for commonly inert metallic substrates, due to the very strong d-d hybridization between transition metal clusters and the substrates, it is difficult to obtain desired magnetic moment or magnetic anisotropy energy (MAE) through depositing clusters on these substrates. Graphene, sp2 hybridized carbon atoms arranged in a honeycomb close crystal structure, has increasingly become the center of attention because of its unusual transport and electronic properties. So it is possible to get ideal result by deposting the transition metal atom or clusters on the sp2 hybridezed graphene due to the avoiding of the d-d hybridization.In this paper, graphene and single vacancy graphene was chosed as supporting substrate. The transition metal atom or clusters with strong SOC was chosed as particles supported on substrate. The interaction between clusters and substrates, the structural stability the system, the impact of SOC on electronic structure of the system, and the MAE of the system are theoretically investigated by using the VASP package based on density functional theory (DFT). We predicted that clusters adsorbed on graphene could be unique functional nanomaterials.Firstly, the lowest energy structure of a single Co atom, Co dimer, Co trimer, Co tetrahedral adsorbed on grahene and the MAE of IrmCon(m=0-1, n=1-4) on graphene were investigated. Through the search of transition state of the Co atoms and Co dimer migrating on perfect graphene without defects, we determined quantitatively the migration barrier of Co atoms and Co dimer on graphene and found that the average life expectancy of Co atom and Co dimer on the most stable position was only 5.46×10-6s, which means that the single Co atom and Co dimer on the graphene can slide easily to assemble a trimer or tetramer. The study of the MAE of Co3 and Co4 freely or supported on graphene found that both the supporting of graphene and the doping of Ir can increase the MAE of Co3 and Co4. These findings might be able to help us to find materials with greater MAE.Secondly, as a dimer with a strong MAE on perfect graphene is unstable, we introduce single vacancies (SV) in the graphene to fix the dimers. Based on the search of dimers of Co group (including Co, Rh, Ir) elements, we obtain highly stable system of Ir dimer on the SV graphene with the axis of the Ir dimer exactly perpendicular to the graphene plane. The high energy gradient in the SV graphene makes it possible to move the dimer from the perfect graphene area into the centre of SV. Such system has giant MAE value as large as 25.7 meV per Ir atom. Furthermore, we discuss the possible origin of the giant MAE in such system, and propose a feasible avenue to assemble Ir dimers on the graphene with SV defects for actual applications.Finally, we studied the peculiar physical properties of 5d single atoms adsorbed on SV graphene. We find two systems with both large magnetism and large MAE by caculating all the magnetism and MAE of 5d atoms on SV graphene, i.e. the system of W atom supported by the SV graphene (W/SVGr) and the system of Au atom supported by the SV graphene (Au/SVGr). A further study found that the easy axis of W/SV is in the graphen plane and easy axis of the Au/SVGr is perpendicular to the graphene plane, which indicates that it is difficult for W/SVGr to magnetize spontaneously, while the spontaneous magnetization may occur in Au/SVGr. The study of the RKKY interaction between the magnetic moments of Au/SVGr indicate that exchange coupling of the system is relevant with the distance and which sublattice the SV located. By the study of the formation process of W clusters on the SV graphene, we found that a uniform array of clusters with odd number of atoms will be formed on the SV graphene preferentially.