Abstract
This paper compares rates of charge transport by tunneling across junctions with the structures Ag(TS) X(CH2 )2n CH3 //Ga2 O3 /EGaIn (n=1-8 and X= SCH2 and O2 C); here Ag(TS) is template-stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient (β, Å(-1) ) and the injection current (J0 , A cm(-2) ) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing Ag(TS) SCH2 -R with Ag(TS) O2 C-R (R=alkyl chains) had no significant influence on J0 (ca. 3×10(3) A cm(-2) ) or β (0.75-0.79 Å(-1) )-an indication that such changes (both structural and electronic) in the Ag(TS) XR interface do not influence the rate of charge transport. A comparison of junctions comprising oligo(phenylene)carboxylates and n-alkanoates showed, as expected, that β for aliphatic (0.79 Å(-1) ) and aromatic (0.60 Å(-1) ) SAMs differed significantly.
Highlights
The rate of charge transport by tunneling through SAMs decays exponentially with increasing distance between the top and bottom electrodes
Using Ga2O3/EGaIn top-electrodes, we found previously that the tunneling current is insensitive to the incorporation of several functional groups familiar in organic chemistry[4, 8] in the backbone of the molecules in the SAM, or a variety of functional groups that are not electrochemically active at the terminus of the SAM ostensibly in contact with the Ga2O3 film.[8]
The details of the bonds at the AgTSO2C‒R interface are quite different from those at the AgTSSCH2‒R interface,[31, 33] this study suggests that nature of the coordination between the metal of bottom electrode (AgTS) and the SAM does not significantly influence the rate of tunneling
Summary
The rate of charge transport by tunneling through SAMs decays exponentially with increasing distance between the top and bottom electrodes. One approach to manipulating the shape of the tunneling barrier, and 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 the rate or mechanism of charge transport.[25,26,27,28,29] Using Ga2O3/EGaIn top-electrodes, 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 Ga2O3 film.[8]. The structure of n-alkanoate SAMs on Ag is comparable to that of n-alkanethiolates; the tilt angle of the alkyl chains is 15 ‒ 25o (from the surface normal), and they form a p(2x2) overlayer with a lattice spacing of 5.8 Å, indicating a densely packed monolayer,[34, 36] as summarized in Table S1 (see the supplementary information)
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