Abstract
This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a -CH(2)CH(2)- group with a -CONH- group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure Ag(TS)-S(CH(2))(m)X(CH(2))(n)H//Ga(2)O(3)/EGaIn (TS = template stripped, X = -CH(2)CH(2)- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS~CONH~R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.
Highlights
Understanding charge transport through organic molecules and supramolecular structures is important in fields from biology1-5 to materials science.6-18 In biology, understanding the flow of electrons in redox biochemistry requires understanding the relation between molecular structure and rates of charge transport
This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM
Incorporation of the amide group does, increase the yields of working junctions. These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g. HS~CONH~R) using an amidebased coupling provide practical routes to molecules useful in studies of molecular electronics
Summary
Understanding charge transport through organic molecules and supramolecular structures is important in fields from biology to materials science. In biology, understanding the flow of electrons in redox biochemistry requires understanding the relation between molecular structure and rates of charge transport. In biology, understanding the flow of electrons in redox biochemistry requires understanding the relation between molecular structure and rates of charge transport. In materials science, it is important in evaluating the potential of tunneling devices based on organic matter for use in electronics: the concept of "wave-function engineering" – that is, designing and shaping tunneling barriers by molecular design – has been an influential and theoretically attractive, but practically unproven, starting point for a number of concepts proposed for molecular electronics. The current consensus in the field of molecular electronics is that charge transport in SAMs of insulating organic molecules proceeds via non-resonant, through-bond tunneling.20,23,34,42,57-64 This behavior is typically modeled by a simple form of the Simmons equation (Equation 1).. We19,21-23 and others have previously reported that J through SAMs of nalkanethiols is approximately log-normally distributed (albeit often with long, asymmetrical tails and significant outliers), rather than normally distributed, and have suggested that variations from junction to junction in thickness and in the number or type of defects in the SAM and electrodes would lead to a normal distribution in the effective thickness, d, of the SAM. Since J is exponentially dependent on a normally distributed parameter (eq 1), J itself should be log-normally distributed
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