Toledano Tal, Sazan Haim, Mukhopadhyay Sabyasachi, Alon Hadas, Lerman Keti, Bendikov Tatyana, Major Dan T, Sukenik Chaim N, Vilan Ayelet, Cahen David
Department of Materials & Interfaces, Weizmann Institute of Science , Rehovot 76100, Israel.
Langmuir. 2014 Nov 18;30(45):13596-605. doi: 10.1021/la503536f. Epub 2014 Nov 5.
A distinct odd-even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H2C═CH-(CH2)n-phenyl, with n = 2-5, were adsorbed onto Si-H and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π-π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd-even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics.
报道了由直接键合在Si(111)上的烷基苯基分子单层诱导的一种独特的奇偶效应，该效应作用于电学性质。n = 2 - 5的H2C═CH-(CH2)n-苯基单体吸附在Si-H上，并形成了具有50 - 60% Si(111)表面原子结合密度的高质量单层。分子动力学模拟表明，结合距离足够近，能够实现有效的π-π相互作用，因此，烷基间隔中具有奇数或偶数亚甲基的单体具有明显不同的堆积和环取向。通过接触角、椭偏仪、傅里叶变换红外光谱和X射线光电子能谱实验证实了分子倾斜的奇偶交替，其与模拟结果在定量上非常吻合。对于具有偶数亚甲基的分子，环平面和烷基间隔长轴的取向比具有奇数亚甲基的分子更垂直于衬底平面。有趣的是，具有偶数亚甲基的分子比具有奇数亚甲基的分子形成的有效更薄的单层具有更好的导电性。我们将此归因于芳香环上电子密度相对于最短隧穿路径的取向变化，这增加了通过奇数单层进行电子传输的势垒。分子电荷传输对芳香部分取向的高灵敏度可能与更好地控制有机电子学中界面的电子性质有关。
