The proposed protocol 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 via off-resonant tunneling. The recipe is simple. The measured current-voltage curve $I = I(V)$ should be recast as a curve of $V^{ 5/3}/I$ versus $V$. This curve exhibits two maxima: one at positive bias ($V = V_{ p+}$), another at negative bias ($V = V_{ p-}$). The values $V_{ p +} > 0$ and $V_{ p -} < 0$ at the two peaks of the curve for $V^{ 5/3}/I$ at positive and negative bias and the corresponding values $I_{ p +} = I(V_{ p+}) > 0$ and $I_{ p -} = I(V_{ p-}) < 0$ of the current is all information needed as input. The arithmetic average of $V_{ p +}$ and $\vert V_{ p -}\vert$ in volt provides the value in electronvolt of the MO energy offset $\varepsilon_0 = E_{ MO} - E_F$ relative to the electrode Fermi level ($\vert \varepsilon_0\vert = e (V_{ p +} + \vert V_{ p -}\vert )/2$). The value of the (Stark) strength of the bias-driven MO shift is obtained as $\gamma = (4/5) (V_{ p +} - \vert V_{ p -} \vert) / (V_{ p +} + \vert V_{ p -} \vert) $. Even the low-bias conductance estimate, $ G = (3/8) (I_{ p +} / V_{ p +} + I_{ p -} / V_{ p -})$, can be a preferable alternative to that deduced from fitting the $I$-$V$ 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 $\sigma$ and delocalized $\pi$ electrons, and} various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).
Categories: cond-mat.mes-hall physics.chem-ph