Electronic structure and magnetism of ultrathin transition metal-graphene layered systems

本研究项目的主题是金属衬底上石墨烯层的结构和电子性质的研究，通过（i）在石墨烯和衬底之间插入贵金属和过渡金属以及（ii）在石墨烯覆盖层顶部吸附相应的金属进行改性。石墨烯层，即六边形有序的碳原子的单层，在最近的过去已经吸引了相当大的兴趣，这是由于作为二维Dirac-费米子的特征的带的几乎垂直的费米能级交叉所引起的电子性质的特殊性。这些性质受到化学环境的强烈影响，即受到电荷转移、自旋极化和键合维度的影响，并且在文献中讨论了石墨烯基结构作为自旋过滤器和自旋场效应晶体管的可能应用。另一方面，甚至石墨烯的单层也充当用于表面和薄膜的化学钝化的有效保护层，而石墨烯的表面可以用作用于金属簇生长的纳米网基底。所有这些方面都将在本项目的框架内进行调查。石墨烯层将在超高真空条件下通过吸附在非反应性W（110）、Rh（111）和Ni（111）表面上的丙烯的热裂解原位制备。随后，石墨烯层将通过贵金属（Cu、Ag、Au）和后过渡金属（Fe、Co、Ni）的热沉积、随后热退火来改性，其中作为温度的函数，预期金属层在石墨烯和衬底之间的插入或在石墨烯表面顶部上的有序膜和簇的生长。将通过扫描隧道显微镜（STM）和电子衍射进行结构表征。电子结构将通过角度和自旋分辨光电发射和X射线吸收（近边结构，NEXAFS，以及磁二向色性，XMCD），特别是侧重于层状结构的磁性进行研究。这些实验研究将伴随着理论研究：在密度泛函理论（DFT）能带结构的基础上，将计算不同样品几何形状的局域态密度、磁矩和总能量。

Subject of the present research-project is an investigation of the structural and electronic properties of graphene-layers on metallic substrates, modified (i) by intercalation of noble- and transition metals between graphene and substrate and (ii) adsorption of respective metals on top of the graphene overlayer. Graphene layers, i.e. monolayers of hexagonally ordered carbon atoms, have attracted considerable interest in the recent past due to peculiarities of the electronic properties caused by almost vertical Fermi-level crossings of bands as characteristic for two-dimensional Dirac-fermions. These properties are strongly influenced by chemical environment, i.e. by charge-transfer, spin polarization, and dimensionality of bonding, and possible applications of graphene-based structures as spin filters and spin field-effect transistors were discussed in the literature. On the other hand, even monolayers of graphene act as effective protection layers for the chemical passivation of surfaces and thin films, while the surface of graphene may be used as nanomesh substrate for the growth of metallic clusters. All these aspects will be investigated in the framework of the present project. Graphene layers will be prepared in situ under ultra-high vacuum conditions by thermal cracking of propene adsorbed on non-reactive W(110), Rh(111), and Ni(111) surfaces. Subsequently, the graphene layers will be modified by thermal deposition of noble (Cu, Ag, Au) and late transition metals (Fe, Co, Ni) followed by thermal annealing, where as a function of temperature either intercalation of metal layers between graphene and substrate or growth of ordered films and clusters on top of the graphene surface are expected. Structural characterization will be made by means of scanning tunnel microscopy (STM) and electron diffraction. The electronic structure will be studied by means of angle- and spin-resolved photoemission and x-ray absorption (near-edge structure, NEXAFS, as well as magnetic dichroism, XMCD) focusing particularly on the magnetic properties of the layered structures. These experimental investigations will be accompanied by theoretical studies: On the basis of density functional theory (DFT) band structures, local densities of states, magnetic moments and total energies will be calculated for different sample geometries.