Substrate Induced Thermal Decomposition of Perfluoro-Pentacene Thin Films on the Coinage Metals
Substrate Induced Thermal Decomposition of Perfluoro-Pentacene Thin Films on the Coinage Metals
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DOI：
10.1021/jp307316r
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发表时间：
2012-11-15
影响因子：
3.7
通讯作者：
Witte, Gregor
中科院分区：
文献类型：
--
作者：
Schmidt, Christian;Breuer, Tobias;Witte, Gregor
利用温度依赖性 X 射线光电子能谱 (XPS) 和近边 X 射线吸收精细结构光谱 (NEXAFS) 研究了通过有机分子束沉积在造币金属铜、银和金的 (111) 取向表面上生长的全氟并五苯 (PFP) 薄膜的热稳定性和化学稳定性。在真空条件下，PFP 多层在 425 K 时完全解吸，而与 Au(111) 表面接触的分子在高达 500 K 时仍保持完整。相比之下，与 Cu(111) 接触的 PFP 明显变形，并在多层热解吸时已经部分脱氟。在 440 K 左右的温度下，PEP 也会在 Ag(111) 上发生明显的脱氟，而进一步加热会导致完全裂解和脱氟。在规则阶梯状的银表面上进行的额外测量表明，阶梯是在较低温度下促进脱氟的活性位点。范德华校正密度泛函 (DFT-D) 计算表明，虽然 PFP 在所有三种金属表面上的吸附都很弱，但其脱氟能垒降低，特别是在铜和银上，从而反映了它们的催化活性。计算进一步表明，脱氟分子与基材共价结合，导致分子主链显着弯曲。本研究强调了在理论上分析分子/金属相互作用时考虑化学反应的重要性，并表明氟化芳香族分子虽然提供了有趣的电子特性，但由于催化效应，实际上在与银等一些电极表面接触时表现出有限的稳定性。
The thermal and chemical stability of perfluoropentacene (PFP) thin films grown by organic molecular beam deposition onto the (111)-oriented surfaces of the coinage metals copper, silver, and gold have been studied by means of temperature dependent X-ray photoelectron spectroscopy (XPS) and Near-Edge X-ray absorption fine structure spectroscopy (NEXAFS). Under vacuum conditions, PFP multilayers are completely desorbed at 425 K while molecules in contact with the Au(111) surface remain intact up to 500 K. By contrast, PFP that is in contact with Cu(111) is distinctly distorted and becomes partially, defluorinated already upon thermal desorption of rnultilayers. A pronounced defluorination of PEP also takes place on Ag(111) at temperatures around 440 K, while further heating causes a complete cracking and defluorination. Additional measurements carried out on a regularly stepped silver surface demonstrate that steps are active sites that promote defluorination already at lower temperatures. van der Waals corrected density-functional (DFT-D) calculations show that PFP, though being weakly adsorbed on all three metal surfaces, exhibits a reduced energy barrier for defluorination, in particular on copper and silver, thus reflecting their catalytic activity. The calculations reveal further that defluorinated molecules are covalently bound to the substrate, leading to a notable bending of the molecular backbone. The present study highlights the importance of also considering chemical reactions when theoretically analyzing molecule/metal interactions and indicates that fluorinated aromatic molecules, though offering interesting electronic properties, actually exhibit a limited stability in contact with some electrode surfaces like silver due to catalytic effects.