Gehring Pascal, Sowa Jakub K, Hsu Chunwei, de Bruijckere Joeri, van der Star Martijn, Le Roy Jennifer J, Bogani Lapo, Gauger Erik M, van der Zant Herre S J
Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands.
IMCN/NAPS, Université Catholique de Louvain, Louvain-la-Neuve, Belgium.
Nat Nanotechnol. 2021 Apr;16(4):426-430. doi: 10.1038/s41565-021-00859-7. Epub 2021 Mar 1.
Theoretical studies suggest that mastering the thermocurrent through single molecules can lead to thermoelectric energy harvesters with unprecedentedly high efficiencies. This can be achieved by engineering molecule length, optimizing the tunnel coupling strength of molecules via chemical anchor groups or by creating localized states in the backbone with resulting quantum interference features. Empirical verification of these predictions, however, faces considerable experimental challenges and is still awaited. Here we use a novel measurement protocol that simultaneously probes the conductance and thermocurrent flow as a function of bias voltage and gate voltage. We find that the resulting thermocurrent is strongly asymmetric with respect to the gate voltage, with evidence of molecular excited states in the thermocurrent Coulomb diamond maps. These features can be reproduced by a rate-equation model only if it accounts for both the vibrational coupling and the electronic degeneracies, thus giving direct insight into the interplay of electronic and vibrational degrees of freedom, and the role of spin entropy in single molecules. Overall these results show that thermocurrent measurements can be used as a spectroscopic tool to access molecule-specific quantum transport phenomena.
理论研究表明，掌握通过单分子的热电流可带来效率空前高的热电能量采集器。这可通过设计分子长度、经由化学锚定基团优化分子的隧道耦合强度或在主链中创建具有量子干涉特征的局域态来实现。然而，对这些预测进行实证验证面临相当大的实验挑战，目前仍有待验证。在此，我们使用一种新颖的测量方案，该方案可同时探测作为偏置电压和栅极电压函数的电导和热电流。我们发现，所得热电流相对于栅极电压具有强烈的不对称性，在热电流库仑菱形图中有分子激发态的迹象。仅当速率方程模型同时考虑振动耦合和电子简并性时，这些特征才能被重现，从而直接洞察电子和振动自由度之间的相互作用以及自旋熵在单分子中的作用。总体而言，这些结果表明热电流测量可作为一种光谱工具来获取特定于分子的量子输运现象。
