Abstract

The results of a study of the unoccupied electronic states of ultrathin films of bis-carboxyphenyl-phthalide (DCA-DPP) and bis-methylphenyl-phthalide (DM-DPP) up to 8 nm thick are presented. The studies are carried out by total current spectroscopy (TCS) in the energy range from 5 to 20 eV above E_F during thermal vacuum deposition of these organic films on the surface of highly ordered pyrolytic graphite (HOPG). The values of E_vac relative to E_F, i.e., the work function of electrons of the DM-DPP films at a film thickness of 5–8 nm are found to be 4.3 ± 0.1 eV. The values of the work function of electrons of the DCA‑DPP films are found to be 3.7 ± 0.1 eV. The structure of the maxima of unoccupied electronic states of DCA-DPP and DM-DPP films in the studied energy range is established. A comparison of the obtained properties of DCA-DPP and DM-DPP films with the properties of films of molecules of unsubstituted diphenylphthalide (DPP) is presented. Thus, the –CH₃ substitution of the DPP molecule has almost no effect on the height of the potential barrier between the film and the HOPG surface, while –COOH substitution of the DPP molecule leads to an increase in the height of the potential barrier between the film and the HOPG substrate surface by 0.5–0.6 eV. Substitution of DPP molecules with –COOH functional groups and, thus, the formation of DCA-DPP molecules lead to a shift of two maxima of the fine structure of the total current spectra located at energies in the range from 5 to 8 eV above E_F by about 1 eV towards the lower electron energies.

中文翻译：

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高阶热解石墨表面上取代的二苯邻苯二甲酸酯薄膜的空位电子态和势垒

摘要

给出了对厚达8 nm的双羧基苯基邻苯二甲酸酯（DCA-DPP）和双甲基苯基邻苯二甲酸酯（DM-DPP）的超薄膜的未占据电子态的研究结果。这些研究是通过在高阶热解石墨（HOPG）的表面上热真空沉积这些有机膜的过程中，通过在E _F之上5至20 eV的能量范围内的总电流光谱法（TCS）进行的。值ë _VAC相对于Ë _˚F即，发现DM-DPP薄膜在5–8 nm的厚度下电子的功函数为4.3±0.1 eV。发现DCA‑DPP薄膜的电子功函值为3.7±0.1 eV。建立了在所研究的能量范围内DCA-DPP和DM-DPP薄膜的未占据电子态的最大值的结构。将获得的DCA-DPP和DM-DPP薄膜的性能与未取代的二苯基邻苯二甲酸酯（DPP）分子薄膜的性能进行了比较。因此，–CH ₃DPP分子的取代几乎对薄膜与HOPG表面之间的势垒高度没有影响，而DPP分子的–COOH取代导致薄膜与HOPG衬底之间的势垒高度增加表面0.5-0.6 eV。用–COOH官能团取代DPP分子，从而形成DCA-DPP分子导致位于E _F上方5至8 eV能量处的总电流谱的精细结构的两个最大值移动向较低的电子能量下降大约1 eV。