Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria.
ACS Nano. 2010 Nov 23;4(11):6735-46. doi: 10.1021/nn102360d. Epub 2010 Nov 3.
The electronic structure of mixed self-assembled monolayers (SAMs) on Au(111) surfaces is modeled using slab-type density-functional theory calculations. The studied molecules have a dipolar character induced by polar and electron donating or accepting tail-group substituents. The resulting electronic structure of mixed layers is found to differ qualitatively from a simple superposition of those of the respective pure layers. Specifically, the positions of the frontier electronic states are shifted relative to the metal Fermi level, with the sign and magnitude of that shift depending on the dipole moment of the molecules and the mixing ratio in the film. This appears counterintuitive considering previous investigations, in which it has been shown that, for densely packed layers, tail-group substituents have no impact on the interfacial energy-level alignment. The seeming contradiction can be lifted by considering the local electrostatic interactions within the films in both mixed and homogeneous monolayers. Beyond that, we show that mixed SAMs provide an efficient tool for continuously tuning substrate work functions over a range that far exceeds that accessible by merely changing the coverage of homogeneous layers, with the net effect depending linearly on the mixing ratio in agreement with recent experimental findings.
采用基于平板的密度泛函理论计算方法，对金（111）表面混合自组装单分子层（SAM）的电子结构进行建模。所研究的分子具有由极性和供电子或受电子尾基团取代基诱导的偶极子特性。研究发现，混合层的电子结构在质量上不同于各自纯层的简单叠加。具体而言，前沿电子态的位置相对于金属费米能级发生了偏移，这种偏移的符号和大小取决于分子的偶极矩和膜中的混合比。考虑到先前的研究，这似乎违反直觉，在先前的研究中，已经表明对于密集堆积的层，尾基团取代基对界面能级排列没有影响。通过考虑混合和均相单层中膜内的局部静电相互作用，可以消除这种矛盾。除此之外，我们还表明，混合 SAM 是一种有效的工具，可在远远超过仅通过改变均相层覆盖率可达到的范围内，连续调整衬底功函数，其净效果与实验结果一致，呈线性依赖于混合比。
