Physikalisches Institut and DFG-Center for Functional Nanostructures, Karlsruhe Institute of Technology , Wolfgang-Gaede-Straße 1, 76131 Karlsruhe, Germany.
ACS Nano. 2014 Apr 22;8(4):3294-301. doi: 10.1021/nn404257v. Epub 2014 Mar 11.
In the field of molecular electronics, thin films of molecules adsorbed on insulating surfaces are used as the functional building blocks of electronic devices. Control of the structural and electronic properties of the thin films is required for reliably operating devices. Here, noncontact atomic force and Kelvin probe force microscopies have been used to investigate the growth and electrostatic landscape of pentacene on KBr(001) and KCl(001) surfaces. We have found that, together with molecular islands of upright standing pentacene, a new phase of tilted molecules appears near step edges on KBr. Local contact potential differences (LCPD) have been studied with both Kelvin experiments and density functional theory calculations. Our images reveal that differently oriented molecules display different LCPD and that their value is independent of the number of molecular layers. These results point to the formation of an interface dipole, which may be explained by a partial charge transfer from the pentacene to the surface. Moreover, the monitoring of the evolution of the pentacene islands shows that they are strongly affected by dewetting: Multilayers build up at the expense of monolayers, and in the Kelvin images, previously unknown line defects appear, which reveal the epitaxial growth of pentacene crystals.
在分子电子学领域，吸附在绝缘表面上的分子薄膜被用作电子器件的功能构建块。为了可靠地操作器件，需要控制薄膜的结构和电子特性。在这里，我们使用非接触原子力和 Kelvin 探针力显微镜研究了 pentacene 在 KBr(001) 和 KCl(001) 表面上的生长和静电势。我们发现，除了直立 pentacene 的分子岛之外，在 KBr 的台阶边缘附近还出现了一个新的倾斜分子相。我们使用 Kelvin 实验和密度泛函理论计算研究了局部接触电位差（LCPD）。我们的图像显示，不同取向的分子显示出不同的 LCPD，并且它们的值与分子层数无关。这些结果表明形成了界面偶极子，这可以通过 pentacene 向表面的部分电荷转移来解释。此外，pentacene 岛的演化监测表明，它们受到去湿的强烈影响：多层以牺牲单层为代价而建立，并且在 Kelvin 图像中出现了以前未知的线缺陷，这揭示了 pentacene 晶体的外延生长。
