【作者】 王鹏；

【导师】 李宏年；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2012， 博士

【摘要】 围绕着高碳富勒烯(C70、C84)的电子态,本文利用光电子能谱(PES)、X射线吸收谱(XAS)、扫描隧道显微镜(STM)和低能电子衍射(LEED)等实验手段并结合密度泛函理论(DFT)计算对C70、C84/Ag(111)界面,C70、C84的光电离截面振荡,以及Eu-C70化合物进行了研究。Ag(111)表面C70单层(1 ML C70/Ag(111))的PES和XAS数据表明,电荷转移量(从Ag至C70)为2.6-2.9 e/分子,在各种富勒烯/金属界面体系中仅次于C60/Cu(111)界面。Ag(111)衬底的屏蔽对C70电子结构的影响非常显著,屏蔽大大减弱(甚至消除)了C70的原位Hubbard能,使样品呈金属性。C70与Ag(111)衬底的费米能级对齐。C70多层膜、单层膜的功函数(分别为4.53 eV和4.52 eV)和Ag(111)的功函数(4.50 eV)很接近,表明C70吸附引起的真空能级移动可忽略不计。K掺杂1 ML C70/Ag(111)的实验结果显示每个分子能继续接受～9个电子。在整个掺杂过程中样品都是金属性的。1 ML C70/Ag(111)的STM研究揭示出C70形成一阶公度的(13×13)R±13.9。结构,晶格匹配原则在此失效。C70分子在衬底表面取直立取向,并显示出随温度变化的亮/暗对比度,即Ag(111)表面出现一些凹坑(原子空缺)。类似的原子空缺,或广义地称为衬底重构,广泛存在于富勒烯/金属界面,但此前不知其原因。我们通过综合考虑1 ML C70/Ag(111)的电子结构和几何结构,用带电分子间的库仑排斥统一解释了凹坑的形成原因。对1 ML C8/Ag(111)进行了类似的研究。主要由于作为气相分子存在时C84和C70的能级结构就有很大差异,1 ML C84/Ag(111)的电荷转移为1.0-1.6e,明显小于1 ML C70/Ag(111)。K掺杂表明C84分子最多只能接受～6个电子(包括衬底的电荷转移)。衬底的屏蔽仍使C84单层在K掺杂前后都呈金属性。C84与Ag(111)的费米能级仍是对齐的,但C84吸附引起真空能级明显移动。在Ag(111)表面,无论C84分子之间的结合还是C84-衬底的结合都强于C70和C60的情形,表现为STM观察到的岛状生长方式、单层膜的非公度R30°结构,以及直至550℃都不能完全脱附。光电离截面不仅与占据态(PES的初态)有关,也与高激发态以及电离态(PES的终态有关)有关。对于分子体系,无论初态还是终态都和分子构型有联系。本文测量了C84在13.4 eV到129.3 eV(光子能量)之间的价电子光电离截面振荡,结合文献中已报道的关于C60和C70的数据,提出一种由光电离截面振荡数据得出富勒烯分子直径的简单方法。这种方法只需采用余弦函数拟合光子能量100 eV以下,间隔2-3 eV的振荡数据就能得到富勒烯直径的满意结果。在对Eu-C70化合物(20 nm厚的薄膜)的PES研究中,我们观察到Eu 6s电子与C70 HOMO-m(n=6-10)电子的杂化,这应该就是Eu9C70铁磁耦合的机理。我们还观察到在EuxC70(x=0-9)体系中电荷转移存在一极限(<6e),这个极限不仅远小于18e(即认为每个Eu原子的2个6s电子都转移给C70),也明显小于在K掺杂的1 ML C70/Ag(111)上观察到的～12e。基于Eu原子与金属Eu电子结构的差异以及6s-n杂化,电荷转移极限可以得到满意的解释。这部分工作还预示高富勒烯较深的能级(相对于HOMO)上的电子在除了输运之外的其它物理性质中可能起重要作用。更多还原

【Abstract】 Focusing on the electronic states of high fullerenes C70 and C84, we have investigated the (C70, C84)/Ag(111) interfaces, valence photoemission intensity oscillations and Eu-C70 fullerides with photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), scanning tunneling microscopy (STM), low energy electron diffraction (LEED) measurements and density functional theory (DFT) calculations.PES and XAS data of a C70 monolayer on the surface of Ag(111) (1 ML C70/Ag(111)) reveal a large amount of charge transfer,2.6-2.9 e per C70 molecule, which is only surpassed by the C60/Cu(111) interface among the various fullerene/metal interfaces. The screening effect of Ag(111) on the electronic structure of C70 is remarkable, which greatly reduces or even eliminates the on-site Hubbard energy, and makes the monolayer show metallic properties. The energy levels of C70 align with the Fermi level of the Ag(111) substrate. A close resemblance between the work functions of the C70 multilayer (4.53 eV), monolayer (4.52 eV) and the Ag(111) surface (4.50 eV) discloses that the vacuum level shift caused by the C70 adsorption is negligible. Potassium doping indicates that 1 ML C70/Ag(111) can still accommodate about 9 electrons and that the sample keeps metallic at any doping level. STM studies reveal that the 1 ML C70/Ag(111) forms the first-order commensurate ((?)13×(?))R±13.9°structure, and that the rule of lattice match is invalid here. C70 molecules take the up-right orientation on Ag(111) surface, and present temperature-dependent bright/dim contrast, which implies the pit formed at the C70/Ag(111) interface. The pit, generally say, the substrate reconstruction, is widely observed at the fullerene/metal interfaces, but its origin is still unknown. By considering both the electronic and geometric structures of 1 ML C70/Ag(111), we provide a unify interpretation of the pits at the various fullerene/metal interfaces.A similar study has been carried out on C84/Ag(111) system. Owing to the substantial difference between the energy level structures of C70 and C84 molecules, the amount of charge transfer for the 1 ML C84/Ag(111) is determined to be within the range of 1-1.6 e per molecule, which is much lower than the case of 1 ML C70/Ag(111). Potassium doping indicates that 1 ML C84/Ag(111) can accommodate～6 electrons at most (including the electrons transferred from the substrate). As a result of the substrate screening effect, the C84 monolayer also shows metallic properties before and after K doping. The energy levels of C84 also align with the Fermi level of the Ag(111) surface, but here the shift of the vacuum level caused by C84 adsorption is obvious. On the Ag(1111) surface, both the C84-C84 and C84-Ag interactions are stronger than the cases of C60 and C70, which are indicated by the Volmer-Weber type growth manner, the non-commensurate R30°structure of the 1 ML C84/Ag(111), and the fact that a annealing at 550℃can not yet completely desorbe all C84 molecules.Photoionization cross section is not only related to the occupied states (initial states of PES), but also to the ionized states (final states of PES). For molecular systems, both the initial and final states are closely related to the molecular configurations. In this thesis, we have measured the valence photoemission intensity oscillations of C84 in the photon energy range from 13.4 eV to 129.3 eV. Combining with the reported oscillating data of C60 and C70 in the literature, we raise a very simple method to elucidate the sizes of fullerenes from the oscillating data. This method uses cosine functions to fit the oscillations, and only those oscillating data measured with the photon energies smaller than 100 eV (with an interval of 2-3 eV) are needed to give satisfactory results of the molecular diameters.In the PES study of Eu-C70 fullerides, we observed substantial hybridizations between the Eu 6s states and the HOMO-n (n=6-10) orbitals of C70, which should play important role in understanding the ferromagnetic mechanism for Eu9C70. We also found a charge transfer limitation (< 6 e) in the EuxC70 (x=0-9) system. This limitation is much smaller than the 18 e as expected with all 5d6s electrons of Eu transferred to C70, also lowers than the case of K doping 1 ML C70/Ag(111) (～12 e). Based on the 6s-πhybridizing states and the electronic structure differences between Eu atom and Eu metal, the charge transfer limitation can be well understood. Our work also reveals that the deeper molecular levels of fullerenes can be crucial to some physical properties such as magnetism, in comparison with the LUMO and LUMO+1 levels that determine the transport properties.更多还原