Gaining insight into molecular tunnel junctions with a pocket calculator without I - V data fitting. Five-thirds protocol.

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 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 | V p- | in volt provides the value in electronvolt of the MO energy offset ε 0 = E MO - E F relative to the electrode Fermi level (| ε 0 | = e ( V p+ + | V p- |)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as γ = (4/5)( V p+ - | V p- |)/( V p+ + | V p- |) sign ( ε 0 ). 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 σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).