A single atom change turns insulating saturated wires into molecular conductors
Xiaoping Chen, Bernhard Kretz, Francis Adoah, Cameron Nickle, Xiao Chi, Xiaojiang Yu, Enrique del Barco, Damien Thompson, David A. Egger, Christian A. Nijhuis
Abstract
Abstract We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β , by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH 2 ) (10-18) X, current densities ( J ) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å −1 . Impedance measurements show tripled dielectric constants ( ε r ) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β . Density-functional theory shows highly localized, X-dependent potential drops at the S(CH 2 ) n X//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in ε r . Here, we demonstrate experimentally that $$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> <mml:mo>∝</mml:mo> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:msqrt> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>ε</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>r</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msqrt> </mml:math> , suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.