Controlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM–semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS₂, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal–organic frameworks is currently underway, which further underlines the high potential of this approach.

中文翻译：

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嵌入式偶极子作为表面工程多功能工具的概念

控制表面和界面的物理和化学性质在物理化学的各个领域具有根本意义，也​​是现代纳米技术的一个关键问题。实现这种控制的一个非常有前途的策略是使用自组装单分子层 (SAM)，它是有序的棒状分子阵列，通过合适的锚定基团与基底结合，并在分子骨架的另一端携带功能性尾基团. 除了各种其他应用外，SAM 还经常用于有机电子学中，通过控制界面水平对齐来进行界面的静电工程。这通常通过在 SAM-半导体界面引入偶极尾基团来实现。然而，这种方法也改变了该界面的化学特性，例如，影响后续层的生长。避免这种并发症的一种策略是将极性基团嵌入到 SAM 形成分子的骨架中。这允许解开电子界面工程和进一步层的成核，从而可以独立优化两者。这种新颖的概念已成功地在不同应用相关基材（例如金、银和氧化铟锡）上的脂肪族和芳香族 SAM 上得到证明。例如，将不同方向的酯和嘧啶基团嵌入到 SAM 形成分子的骨架中会导致显着的功函数变化。然后可以通过生长由具有相反取向的极性基团的分子组成的混合单层，在很宽的能量范围内对其进行微调。在这样的系统中，功函数的变化伴随着 X 射线光电子能谱中峰的明显变化，这表明静电触发的核心能级位移与公认的化学位移一样重要。这说明了 X 射线光电子能谱 (XPS) 作为探测单层内局部静电能的工具的潜力，并且在像这里研究的系统中，使 XPS 成为研究二元 SAM 的组成和形态的有力工具。所有这些实验观察都可以通过模拟进行合理化，模拟表明嵌入偶极基团的组装在单层内引入了潜在的不连续性，从而使偶极子上方和下方的能级相对于彼此移动。在分子和单层电子学中，嵌入式偶极子 SAM 可用于控制转换电压和电流整流。在基于有机和二维半导体的器件中，例如 MoS2_如图2所示，即使在柔性基板上，它们也可以将接触电阻降低几个数量级，而不会对薄膜的生长产生不利影响。通过改变嵌入偶极部分的方向，还可以使用相同的电极材料 (Au) 构建 p 型和 n 型有机晶体管。目前正在将嵌入式偶极子概念从混合接口扩展到金属-有机框架等系统，这进一步强调了这种方法的巨大潜力。