Bâldea Ioan
Theoretical Chemistry, Heidelberg University, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany.
Phys Chem Chem Phys. 2024 Mar 13;26(11):8724-8733. doi: 10.1039/d4cp00217b.
The protocol put forward in the present paper is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs off-resonant tunneling. The recipe is simple. The measured current-voltage curve = () should be recast as a curve of /. This curve exhibits two maxima: one at positive bias ( = ), another at negative bias ( = ). The values > 0 and < 0 at the two peaks of the curve for / at positive and negative bias and the corresponding values = () > 0 and = () < 0 of the current is all information needed as input. The arithmetic average of and || in volt provides the value in electronvolt of the MO energy offset = - relative to the electrode Fermi level (|| = ( + ||)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as = (4/5)( - ||)/( + ||) sign (). Even the low-bias conductance estimate, = (3/8)(/ + /), can be a preferable alternative to that deduced from fitting the - slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this "five-thirds" protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).
本文提出的方案旨在满足实验人员合理的需求，即能够可靠且轻松地从分子结的电流 - 电压测量中提取微观参数。该方案适用于电荷传输由单个能级（分子轨道，MO）主导且发生非共振隧穿的结。方法很简单。测得的电流 - 电压曲线(I = I(V))应重新表示为(I/V)的曲线。该曲线有两个最大值：一个在正偏压处（(V = V_+)），另一个在负偏压处（(V = V_-)）。(I/V)曲线在正、负偏压处两个峰值的(V_+ > 0)和(V_− < 0)值以及相应的电流值(I_+ = I(V_+) > 0)和(I_− = I(V_−) < 0)都是作为输入所需的全部信息。(V_+)和(\vert V_−\vert)在伏特下的算术平均值给出相对于电极费米能级的MO能量偏移(\Delta = E - E_F)的电子伏特值（(\vert\Delta\vert = (V_+ + \vert V_−\vert)/2)）。偏压驱动的MO位移的（斯塔克）强度值可通过(S = (4/5)(V_+ - \vert V_−\vert)/(V_+ + \vert V_−\vert) \text{sign}(V_+))获得。即使是低偏压电导估计值(g = (3/8)(I_+/V_+ + I_−/\vert V_−\vert))，在低偏压下曲线有噪声的情况下，也可能是从拟合(I - V)斜率推导得出的估计值的更好替代方案。为了证明这个“五分之三”方案的可靠性和通用性，我展示了它在使用金属和非金属电极制造的分子隧道结、具有局域化(\sigma)电子和离域化(\pi)电子的分子种类以及各种技术（机械控制断裂结、STM断裂结、导电探针AFM结和大面积结）方面的广泛适用性。
