Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers
Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers
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化学改性烷硫醇自组装单层有机分子的能级排列
DOI：
10.1021/acs.jpcc.7b07955
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Nakajima Atsushi
中科院分区：
文献类型：
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作者：
Shibuta Masahiro;Ogura Munehisa;Eguchi Toyoaki;Nakajima Atsushi
我们用双光子光电子能谱无损地解析了沉积在硫代烷基单分子膜(SAM)上的富勒烯C60分子的未占据能级。由于表面偶极层的形成，通过对烷基链上的氢原子进行氟取代，功函数(WF)从硫代烷基硫酸酯SAM(H-SAM)的4.3 eV增加到氟取代SAM(F-SAM)的5.7 eV。当C60被沉积在H-SAM和F-SAM上时，C60的空能级/被占能级的能级位置被钉扎在真空级(费米能级(EF)+WF)上。由于能级的排列，在F-SAM上，C60的最高占据分子轨道Ef的相对能量几乎等于最低未占据分子轨道的Ef，这意味着F-SAM上的C60薄膜表现出p型和n型(双极)电荷输运性质，而C60是典型的n型半导体。即使在厚度至少达到∼5 nm的多层C60薄膜中，能量仍然保持不变，这表明由自组装膜诱导的偶极层对分子覆盖层是健壮的。这种对有机薄膜能级的光谱研究将对有机薄膜器件的进一步发展具有重要意义。
We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C60molecules deposited on alkanethiolate self-assembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C60is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C60are pinned to the vacuum level (Fermi level (EF) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy fromEFof the highest occupied molecular orbital of C60almost equals that of the lowest unoccupied molecular orbital, implying that the C60film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C60is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C60films at least up to ∼5 nm in thickness, showing that the dipole layers induced by SAMs are robust against molecular overlayers. Such a spectroscopic study on the energy levels for organic films will be of importance for further development of organic thin film devices.