Cn films (n=50, 52, 54, 56, and 58) on graphite: cage size dependent electronic properties.
Cn films (n=50, 52, 54, 56, and 58) on graphite: cage size dependent electronic properties.
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石墨上的 Cn 薄膜（n=50、52、54、56 和 58）：电子特性取决于笼尺寸。
DOI：
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发表时间：
2006
影响因子：
4.4
通讯作者：
M. Kappes
中科院分区：
文献类型：
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作者：
D. Löffler;S. Jester;P. Weis;A. Böttcher;M. Kappes
在超高真空条件下，以6 eV的平均动能将质量选择的Cn ~+（50 ≤ n<60;偶数n）软着陆在高度取向的热解石墨表面，制备了新型半导体材料。在所有情况下，Cn膜生长根据沃尔默-韦伯机制：表面最初装饰的二维分形岛，在以后的沉积阶段成为三维树枝状土丘。我们推断，Cn聚集是由反应位点，包括相邻的五边形（或七边形）的个别笼子。与固体C60相比，所产生的共价笼-笼键负责膜的异常高的热稳定性。完整的Cn升华的表观活化能范围从2.2 eV的C58到2.6 eV的C50来自热脱附光谱。所有的Cn膜表现出一个共同的价带紫外光电子能谱的光谱特征位于一个广泛的最高占据分子轨道（HOMO）衍生带（EB约2.5 eV）的中心周围。该特征已被分配给在聚合物结构中彼此共价连接的Cn单元。在实验精度范围内，相同的功函数（4.8 eV）被确定为厚膜的所有Cn研究。相比之下，“HOMO”电离势是笼尺寸依赖性的，并且显著低于C60获得的电离势。C58表现出最低的HOMO（6.5eV）。通过在薄膜顶层沉积少量的Cs原子，确定了Cn薄膜的带隙。HOMO-最低未占分子轨道衍生的带隙在0.8 eV（C52）和1.8 eV（C50）之间进行了观察，相比之下，固体C60的带隙为1.5 eV。
Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.