Cvetko Dean, Fratesi Guido, Kladnik Gregor, Cossaro Albano, Brivio Gian Paolo, Venkataraman Latha, Morgante Alberto
Faculty of Mathematics and Physics, University of Ljubljana, Slovenia.
ETSF and Dipartimento di Fisica, Università degli Studi di Milano, Italy.
Phys Chem Chem Phys. 2016 Aug 10;18(32):22140-5. doi: 10.1039/c6cp04099c.
Charge transfer rates at metal/organic interfaces affect the efficiencies of devices for organic based electronics and photovoltaics. A quantitative study of electron transfer rates, which take place on the femtosecond timescale, is often difficult, especially since in most systems the molecular adsorption geometry is unknown. Here, we use X-ray resonant photoemission spectroscopy to measure ultrafast charge transfer rates across pyridine/Au(111) interfaces while also controlling the molecular orientation on the metal. We demonstrate that a bi-directional charge transfer across the molecule/metal interface is enabled upon creation of a core-exciton on the molecule with a rate that has a strong dependence on the molecular adsorption angle. Through density functional theory calculations, we show that the alignment of molecular levels relative to the metal Fermi level is dramatically altered when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level. We also calculate charge transfer rates as a function of molecular adsorption geometry and find a trend that agrees with the experiment. These findings thus give insight into the charge transfer dynamics of a photo-excited molecule on a metal surface.
金属/有机界面处的电荷转移速率会影响有机电子器件和光伏器件的效率。对发生在飞秒时间尺度上的电子转移速率进行定量研究往往很困难，特别是因为在大多数系统中分子吸附几何结构是未知的。在这里，我们使用X射线共振光电子能谱来测量吡啶/Au(111)界面上的超快电荷转移速率，同时还控制分子在金属上的取向。我们证明，在分子上产生一个芯激子时，能够实现跨分子/金属界面的双向电荷转移，其速率强烈依赖于分子吸附角。通过密度泛函理论计算，我们表明当分子上产生一个芯空穴时，分子能级相对于金属费米能级的排列会发生显著变化，使得最低未占据分子轨道部分下降到金属费米能级以下。我们还计算了作为分子吸附几何结构函数的电荷转移速率，并发现了与实验相符的趋势。因此，这些发现深入了解了光激发分子在金属表面的电荷转移动力学。
