Orbital-resolved visualization of single-molecule photocurrent channels
Orbital-resolved visualization of single-molecule photocurrent channels
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DOI：
10.1038/s41586-022-04401-0
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发表时间：
2022-03-31
期刊：
影响因子：
64.8
通讯作者：
Kim, Yousoo
中科院分区：
文献类型：
--
作者：
Imai-Imada, Miyabi;Imada, Hiroshi;Kim, Yousoo
由于其在利用光能方面的核心作用，激发分子的光致电子转移(PET)已被广泛研究(1)(-6)。然而，尽管微观光电流测量方法(7)(-11)使该过程的效率与局部特征相关联成为可能，但空间分辨率一直不足以在分子水平上解决它。然而，最近的工作表明，当扫描隧道显微镜(STM)与由可调谐激光(12)(，13)驱动的局域等离子体激元场相结合时，单分子可以被有效地激发和探测。在这里，我们使用这种方法，通过探测通过STM尖端的第一激发态隧穿的电子，以原子尺度的分辨率直接可视化通过单个自由碱酞菁(FBPc)分子轨道的光电流通道。我们发现，光电流的方向和空间分布敏感地依赖于偏置电压，并且即使在平均光电流接近于零的电压下也能检测到反向流动的光电流通道。此外，我们看到了PET和光致发光之间竞争的证据(12)，并发现我们可以通过以三维、原子精度定位STM尖端来控制激发的分子主要是通过PET还是光致发光来松弛。这些观察结果表明，可以通过调节激发态分子轨道的耦合来促进或抑制特定的光电流通道，从而为通过原子级电子和分子界面几何工程来提高能量转换效率提供了新的视角。
Given its central role in utilizing light energy, photoinduced electron transfer (PET) from an excited molecule has been widely studied(1)(-6). However, even though microscopic photocurrent measurement methods(7)(-11) have made it possible to correlate the efficiency of the process with local features, spatial resolution has been insufficient to resolve it at the molecular level. Recent work has, however, shown that single molecules can be efficiently excited and probed when combining a scanning tunnelling microscope (STM) with localized plasmon fields driven by a tunable laser(12)(,13). Here we use that approach to directly visualize with atomic-scale resolution the photocurrent channels through the molecular orbitals of a single free-base phthalocyanine (FBPc) molecule, by detecting electrons from its first excited state tunnelling through the STM tip. We find that the direction and the spatial distribution of the photocurrent depend sensitively on the bias voltage, and detect counter-flowing photocurrent channels even at a voltage where the averaged photocurrent is near zero. Moreover, we see evidence of competition between PET and photoluminescence(12), and find that we can control whether the excited molecule primarily relaxes through PET or photoluminescence by positioning the STM tip with three-dimensional, atomic precision. These observations suggest that specific photocurrent channels can be promoted or suppressed by tuning the coupling to excited-state molecular orbitals, and thus provide new perspectives for improving energy-conversion efficiencies by atomic-scale electronic and geometric engineering of molecular interfaces.