Zhang Ziyu, Cao Liang, Chen Xue, Thompson Damien, Qi Dongchen, Nijhuis Christian A
Department of Chemistry, National University of Singapore, 3 Science Drive, 117543, Singapore.
Anhui Province Key Laboratory of Condensed Matter Physics at Extreme Conditions, High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, China.
J Phys Chem C Nanomater Interfaces. 2021 Aug 26;125(33):18474-18482. doi: 10.1021/acs.jpcc.1c04655. Epub 2021 Aug 17.
Charge transfer (CT) dynamics across metal-molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal-sulfur bond to the carbon-sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule-metal interface.
跨金属 - 分子界面的电荷转移（CT）动力学对分子电子器件的性能和功能具有重要影响。利用基于同步加速器的芯孔时钟（CHC）光谱可以精确测量飞秒量级的CT时间，但对于金属功函数以及金属与锚定基团产生的键偶极对CT时间的影响却知之甚少。为了解决这个问题，我们在此测量了通过硫醇盐锚定基团与银、金和铂结合的自组装单分子层上的CT动力学。这些分子具有一个末端二茂铁（Fc）基团，通过不同数量的亚甲基单元连接到一个二苯乙炔（DPA）导线。使用CHC与共振光电子能谱（RPES）测量的CT时间表明，共轭DPA导线的导电速度比类似长度的脂肪族碳导线更快。较短的亚甲基连接体在Fc和DPA之间表现出增强的共轭作用，通过提供更大的轨道混合促进了CT。我们发现，由于从金属 - 硫键到碳 - 硫键的部分电子转移产生了更大的键偶极，从而产生了一个阻碍分子CT的静电场，与银相比，铂上的CT时间增加了近10倍。通过拟合RPES信号，我们区分了来自铁中心和环戊二烯基（Cp）环的电子。由于Cp轨道的离域作用，后者显示出更快的CT速率。我们的研究表明，通过对分子的几个部分以及分子 - 金属界面进行精心设计，可以对跨结的CT速率进行微调。
