Evaluation of I(V) curves in scanning tunneling spectroscopy of organic nanolayers
Evaluation of I(V) curves in scanning tunneling spectroscopy of organic nanolayers
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DOI：
10.1103/physrevb.75.235432
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发表时间：
2007-06-01
影响因子：
3.7
通讯作者：
Fritz, T.
中科院分区：
文献类型：
--
作者：
Wagner, C.;Franke, R.;Fritz, T.
我们想验证用于无机样品的扫描隧道光谱（STS）评估方法是否也适用于有机纳米层，或者是否需要修改。由于传统的方法是针对大量样品和低电压的情况而导出的，因此出现了这个问题。由于基质上的有机吸附物呈现出具有更复杂结构的样品，并且进一步以eV范围的大间隙为特征，因此这个问题的答案并不先验明确。在讨论了相关的量，即样本态密度（DOS）和局部态密度之后，我们演示了在Wentzel-Kramers-Brillouin近似中使用简单且众所周知的一维隧道结模型来计算处理超薄有机层的几个STS结果的样本DOS。在随后的讨论中，我们得出结论，该模型适用于“穿过”有机分子的轨道介导的隧道过程，并且可以用于评估此类STS测量。从尖端-样品距离的估计出发，讨论了在STS中检测最高占据分子轨道以下电子态的可能性。通过几个例子，我们说明了归一化微分电导率作为STS I(V)曲线评估方法的弱点，并提出了一种新的归一化算法作为解决问题的方法。
We want to verify if the use of scanning tunneling spectroscopy (STS) evaluation methods developed for inorganic samples can be justified also for the case of organic nanolayers, or if modifications are necessary. This question arises since the traditional approaches are derived for the case of bulk samples and low voltages. Since an organic adsorbate on a substrate presents a sample with a more complex structure and is further characterized by a large gap in the eV range, the answer to this question is not a priori clear. After discussing relevant quantities, i.e., the sample density of states (DOS) and local density of states, we demonstrate the use of the simple and well-known model of a one-dimensional tunnel junction in Wentzel-Kramers-Brillouin approximation in order to calculate the sample DOS for several STS results from literature dealing with ultrathin organic layers. In a subsequent discussion, we conclude that the model is applicable to the orbital-mediated tunneling process "through" organic molecules and that it can be used to evaluate such STS measurements. Emanating from an estimation of the tip-sample distance, the possibility of detecting electronic states below the highest occupied molecular orbital in STS is discussed. With several examples, we illustrate a weakness of the normalized differential conductivity as a method of STS I(V) curve evaluation and propose a new normalization algorithm as a solution to the problem.