Cochrane K A, Schiffrin A, Roussy T S, Capsoni M, Burke S A
Department of Chemistry, University of British Columbia, Vancouver British Columbia, Canada V6T 1Z1.
Department of Physics and Astronomy, University of British Columbia, Vancouver British Columbia, Canada V6T 1Z1.
Nat Commun. 2015 Oct 6;6:8312. doi: 10.1038/ncomms9312.
Organic semiconductor devices rely on the movement of charge at and near interfaces, making an understanding of energy level alignment at these boundaries an essential element of optimizing materials for electronic and optoelectronic applications. Here we employ low temperature scanning tunneling microscopy and spectroscopy to investigate a model system: two-dimensional nanostructures of the prototypical organic semiconductor, PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) adsorbed on NaCl (2 ML)/Ag(111). Pixel-by-pixel scanning tunneling spectroscopy allows mapping of occupied and unoccupied electronic states across these nanoislands with sub-molecular spatial resolution, revealing strong electronic differences between molecules at the edges and those in the centre, with energy level shifts of up to 400 meV. We attribute this to the change in electrostatic environment at the boundaries of clusters, namely via polarization of neighbouring molecules. The observation of these strong shifts illustrates a crucial issue: interfacial energy level alignment can differ substantially from the bulk electronic structure in organic materials.
有机半导体器件依赖于界面处及界面附近电荷的移动，因此了解这些边界处的能级排列是优化用于电子和光电子应用材料的关键要素。在此，我们采用低温扫描隧道显微镜和光谱技术来研究一个模型体系：吸附在NaCl(2 ML)/Ag(111)上的典型有机半导体PTCDA（3,4,9,10-苝四羧酸二酐）的二维纳米结构。逐像素扫描隧道光谱技术能够以亚分子空间分辨率绘制这些纳米岛的占据和未占据电子态图谱，揭示边缘分子与中心分子之间存在显著的电子差异，能级移动高达400 meV。我们将此归因于团簇边界处静电环境的变化，即通过相邻分子的极化。这些强烈移动的观测结果说明了一个关键问题：有机材料中的界面能级排列可能与体电子结构有很大不同。
