Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

Abstract

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SC n (with n = number of carbon atoms) incorporated in junctions having structure Ag TS -SAM//Ga 2 O 3 /EGaIn leads to a value for the injection tunnel current density J 0 (i.e., the current flowing through an ideal junction with n = 0) of 10 3.6±0.3 A·cm −2 (V = +0.5 V). This estimation of J 0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1−18. This value of J 0 is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the ). A comparison of the characteristics of conical Ga 2 O 3 / EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs. ■ INTRODUCTIONMeasurements, using a number of techniques, of rates of charge transport by tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates on silver and gold substrates show an interesting, puzzling, and unresolved mixture of consistency and inconsistency. Rates of tunneling across these SAMs follow the simplified Simmons equation,( 1) with the falloff in current density J(V) (A·cm ). Using mercury drops as top-electrodes, measurements of rates of tunneling across n-alkanes anchored to heavily doped silicon surfaces led to β = 0.9 ± 0.2 nC −1 , similar to the values observed for nalkanethiolates on Au and Ag substrates. , observed in large-area junctions using, as top-electrodes, conductive polymers, 13 Hg-drops supporting an insulating organic film (Hg-SAM), 14−16 and Ga 2 O 3 /EGaIn tips. 17−20 Why is there high consistency in values of β, but broad inconsistency in values of J 0 (V) within these systems?A priori, at least four factors might contribute to differences in J 0 (V) among methods of measurements:(i) In large-area junctions, assuming that the effective electrical contact area (A elec )the area through which current actually passescoincides with the geometrical contact area (A geo ) estimated by optical microscopy could result in errors in the conversions of values of current into current densities. Contact between surfaces occurs only through asperities distributed on the surfaces, which are always rough to some extent; in addition, only a fraction of the true, physical contact area is conductive.21−24 Estimations of the effective contact area from measurements of adhesion and friction between surfaces indicate that values of A elec /A geo vary in the range 10 −2 −10 −4 , depending on the hardness of the materials, the heights, widths, and number of asperities on both surfaces, and loads applied to the contacts. 22,23,25−27

Tunneling across SAMs Containing Oligophenyl Groups

“…Since the structural elements in this region differ from those in the region where there are data (the region of Ph n ) extrapolation may be inappropriate for S(Ph) n and C≡C(Ph) n , although the correction of this extrapolation is well-validated for nalkanethiolates on gold and silver. 2,12 …”

“…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).…”

“…11,12 SAMs of SPh n and SCH 2 Ph n have been characterized extensively. They form structures that are highly-ordered and densely-packed on both silver and gold substrates.…”

Influence of Environment on the Measurement of Rates of Charge Transport across AgTS/SAM//Ga2O3/EGaIn Junctions

“…39, 40 Junctions comprising Ga 2 O 3 /EGaIn and HOPG result in an ohmic metal/semiconductor interface; that is, the current density, J, varies linearly with the applied voltage. 1 By using a freshly formed HOPG surfacegenerated by mechanical cleavage of the top layers of a piece of graphite glued to a metal shimwe could create a clean, flat, electrically conducting surface (root mean square (rms) roughness of 0.3−0.5 nm, according to SPI technical specifications). By using graphite electrodes, we were able to attribute the majority of the variations observed as a result of changes in the environment to the Ga 2 O 3 film, rather than to some other part of the junction.…”

“…Simeone et al 1 have compared a liquid drop of Hg and a conical tip Ga 2 O 3 /EGaIn electrode in measurements of a Fe/ FeO X substrate, where the FeO X was the most-resistive component of the junction. They created electrodes with equivalent nominal contact areas, and found that the current density of the conical tip electrode was ∼10 −3 × lower than the value measured from the Hg-drop electrode.…”