Replacing −CH₂CH₂– with −CONH– Does Not Significantly Change Rates of Charge Transport through Ag^TS-SAM//Ga₂O₃/EGaIn Junctions

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 −CH2CH2– 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 AgTS-S(CH2) m X(CH2) n H//Ga2O3/EGaIn (TS = template stripped, X = −CH2CH2– 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.

Replacing AgTSSCH2‐R with AgTSO2C‐R in EGaIn‐Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

“…In principle, one approach to manipulating the shape of the tunneling barrier, and thus to influencing the rate of charge transport, is to introduce functional groups into the structure of the SAM that are capable of influencing this topography, and thus the rate or mechanism of charge transport. [25][26][27][28][29] Using Ga 2 O 3 /EGaIn top-electrodes, however, we found previously that the tunneling current is insensitive to the incorporation of several functional groups familiar in organic chemistry 4,8 (e.g., an amide, -CONH-or -NHCO-) in the backbone of the molecules in the SAM, or a variety of functional groups (both aliphatic and aromatic) that are not electrochemically active at the terminus of the SAM ostensibly in contact with the Ga 2 O 3 film. bidentate ionic binding coordination of the carboxylate to the surface.…”

“…focused predominately on the influence of backbone substituents [1][2][3][4] or terminal functional groups [5][6][7][8] on rates of charge transport. The effect of changing the group (which we call the "anchoring group") that links the SAM to the metal substrate has not been explored in detail; 9,10 only a few studies have approached this issue at the single-molecular level using scanning tunneling microscopy [11][12][13][14] or conducting atomic force microscopy.…”

Odd–Even Effects in Charge Transport across n-Alkanethiolate-Based SAMs

“…18 Using EGaIn-based top electrodes in molecular junctions, we have studied odd−even effects in charge transport across n-alkanethiolate SAMs on Ag TS substrates. 17,11 Using statistical tools, we showed that the data for current densities for SAMs with n even and n odd belong to separate data sets. Theory of Charge Tunneling across SAMs.…”

“…10−14 (Here Ag TS is a template-stripped silver substrate, S(CH 2 ) n is the n-alkanethiolate SAM covalently bonded to the Ag TS substrate, T is the terminal functional group, and EGaIn is the liquid alloy of gallium and indium, covered with a thin, electrically conductive surface oxide mostly Ga 2 O 3 that forms spontaneously upon exposure to air. 15,16 ) Those studies showed that alterations in the structure of the insulating organic layer, S(CH 2 ) n T, for example, changing the anchoring group of the SAMs 10 or introducing polar organic groups either into the backbone of a polyethylene chain 11 or at the van der Waals T//Ga 2 O 3 interface, 12−14 do not influence the rates of charge transport at a level that is statistically significant (less than a factor of 3). In surprising contrast, we and others observed that the addition of one CH 2 group to the alkyl chaina change that, in essence, simply interchanges an exposed terminal methyl group for an ethyl groupappears to be sufficient to make a small but statistically significant difference in the electronic properties of the junction.…”

Tunneling across SAMs Containing Oligophenyl Groups

“…1 Among the exceptions are the observation of a small "odd-even effect" in charge transport across n-alkanethiolates on gold, [2][3][4] the observation of a substantial reduction in current density when fluorine is present at the SAM//Ga 2 O 3 interface, 5 and the observation of rectification of current when T is a redox active group such as ferrocenyl [6][7][8] or bipyridyl. 9 Having studied the influence of the structure of saturated n-alkyl groups on charge tunneling extensively, [10][11][12][13][14][15] we turned our attention to understanding the relationship between the structure of polyaromatics (molecules that result in a reduction in the height of the tunneling barrier relative to that characterizing aliphatics 16 ) and the rates of charge transport. We measured rates of charge transport across SAMs of oligophenylthiols (M/SPh n ), -methanethiols (M/SCH 2 Ph n ), and -acetylenes (M/C≡CPh n ), where n = 1-3 and M = gold and silver metal electrodes ( Figure 1).…”