The Rate of Charge Tunneling through Self‐Assembled Monolayers Is Insensitive to Many Functional Group Substitutions

Abstract

This paper characterizes the rates of charge transport by tunneling across a series of molecules—arrayed in self-assembled monolayers—containing a common head group and body (HS(CH2)4CONH(CH2)2-) and structurally varied tail groups (-R). These molecules are assembled in junctions of the structure AgTS/SAM//Ga2O3/EGaIn. Over a range of common aliphatic, aromatic, and heteroaromatic organic tail groups, changing the structure of R does not significantly influence the rate of tunneling.

Charge Transport Across Insulating Self-Assembled Monolayers: Non-equilibrium Approaches and Modeling To Relate Current and Molecular Structure

“…We have investigated the transport through a series of self-assembled monolayers with varied tail groups, as measured by Yoon et al 13 DFT-NEGF calculations confirm the modest current variation observed experimentally. DFT calculations show that the HOMO is largely located on the thiol group in these molecules, and this is the closest orbital to the Fermi energy of the electrodes.…”

“…11 Another example is the exponential decay of the conductance in alkane self-assembled monolayers (SAMs), which does not exist in n-polyene chains where a chain of C-C single bonds (CH 2 ) 2n is replaced by an extended conjugated chain (CH = CH 2 −) n . 12 Yoon et al, 13 reported a systematic experimental study in SAMs which suggested that large changes in molecular structure (e.g. changing a cyclohexyl group for a phenyl group) need not induce significant changes in the conductance of molecular monolayers.…”

“…More specifically, they measured the current densities through a series of SAMs based on different molecules located between a silver electrode and Ga 2 O 3 /EGaIn electrode, where EGaIn denotes eutectic gallium and indium (a liquid metal alloy 14 ) and Ga 2 O 3 is a spontaneously formed, electrically conducting surface oxide layer (normally 0.7 nm thick) on the EGaIn electrode. The structure of the molecules making up the SAMs is HS(CH 2 ) 4 CONH(CH 2 ) 2 R where R is one of the tail groups Figure 1: Structure of the molecules used by Yoon et al 13 to form the self-assembled monolayers on the Ag surface. The structure of all molecules 1-13 is HS(CH 2 ) 4 CONH(CH 2 ) 2 − R where R is the tail group.…”

Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n-Alkanethiolates

“…We chose the internal amide for the BIPH-terminated SAM for ease of synthesis; no difference in J(V) between an internal amide (−CONH−) and ethylene unit (−CH 2 CH 2 −) has been demonstrated in previous studies. 22,26,27 SAMs of SC 11 PYR, SC 7 CONHC 2 BIPH, and SC 11 PHEPY do not rectify currents at ±1.0 V on Ag TS substrates ( Figure S3): Table 1 summarizes the values for rectification for those compounds. In contrast, a SAM composed of SC 11 PHE on Ag TS showed a rectification ratio (log|r + | mean = ∼ 1.8; r + = ∼ 64) similar to that of (and with the same polarity as) the SAM of SC 11 BIPY (Figure 1).…”

“…(iii) It was designed to examine examples of rectification reported in the literature and to search for common conceptual threads to connect them. This paper describes a molecular rectifier (Figure 1a) that shows a large, reproducible, unambiguous rectification ratio (| J(+1.0 V)|/|J(−1.0 V)| = ∼ 85) having polarity opposite to that of the Fc n -based junctions; this junction has the structure Ag TS / S(CH 2 ) 11 BIPY//Ga 2 O 3 /EGaIn (Figure 1a), where TS denotes a template-stripped substrate; 30 BIPY is a 4-methyl-2,2′-bipyrid-4′-yl group; and Ga 2 O 3 /EGaIn is the eutectic gallium−indium alloy (EGaIn) top electrode (specifically, what we call an "unflattened" conical tip) 22,24,26 covered with a self-limiting thin film of gallium oxide (Ga 2 O 3 , ostensibly ∼0.7 nm, but undoubtedly thicker in portions of the junctions where it is buckled). 24,31 We determined experimentally that rectification (Figure 1b,c) in this system stems from the BIPY terminal Journal of the American Chemical Society groups in the SAM, and not from other features of the EGaInbased junction.…”