Seldenthuis Johannes S, van der Zant Herre S J, Ratner Mark A, Thijssen Joseph M
Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.
ACS Nano. 2008 Jul;2(7):1445-51. doi: 10.1021/nn800170h.
In bulk systems, molecules are routinely identified by their vibrational spectrum using Raman or infrared spectroscopy. In recent years, vibrational excitation lines have been observed in low-temperature conductance measurements on single-molecule junctions, and they can provide a similar means of identification. We present a method to efficiently calculate these excitation lines in weakly coupled, gateable single-molecule junctions, using a combination of ab initio density functional theory and rate equations. Our method takes transitions from excited to excited vibrational state into account by evaluating the Franck-Condon factors for an arbitrary number of vibrational quanta and is therefore able to predict qualitatively different behavior from calculations limited to transitions from ground state to excited vibrational state. We find that the vibrational spectrum is sensitive to the molecular contact geometry and the charge state, and that it is generally necessary to take more than one vibrational quantum into account. Quantitative comparison to previously reported measurements on pi-conjugated molecules reveals that our method is able to characterize the vibrational excitations and can be used to identify single molecules in a junction. The method is computationally feasible on commodity hardware.
在宏观体系中，分子通常通过拉曼光谱或红外光谱利用其振动光谱来识别。近年来，在单分子结的低温电导测量中观察到了振动激发线，它们能提供类似的识别方法。我们提出一种方法，利用从头算密度泛函理论和速率方程的组合，在弱耦合、可门控的单分子结中高效计算这些激发线。我们的方法通过评估任意数量振动量子的弗兰克 - 康登因子，考虑了从激发振动态到激发振动态的跃迁，因此能够预测与限于基态到激发振动态跃迁的计算在定性上不同的行为。我们发现振动光谱对分子接触几何结构和电荷状态敏感，并且通常需要考虑不止一个振动量子。与先前报道的对π共轭分子测量的定量比较表明，我们的方法能够表征振动激发，可用于识别结中的单分子。该方法在通用硬件上计算可行。
