Person:

Bowers, Carleen

This paper compares rates of charge transport by tunneling across junctions with the structures AgTSX(CH2)2nCH3 //Ga2O3 /EGaIn (n=1–8 and X= [BOND]SCH2[BOND] and [BOND]O2C[BOND]); here AgTS 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 AgTSSCH2-R with AgTSO2C-R (R=alkyl chains) had no significant influence on J0 (ca. 3×103 A cm−2) or β (0.75–0.79 Å−1)—an indication that such changes (both structural and electronic) in the AgTSXR 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.

This paper describes rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga2O3/EGaIn as the top electrode. It compares the injection current, J0, and the attenuation constant, β, of the simplified Simmons equation, across oligophenyl groups (R = Phn; n = 1, 2, 3), with three different anchoring groups (thiol, HSR; methanethiol, HSCH2R; and acetylene, HC≡CR) that attach R to the template-stripped gold and silver substrates. The results demonstrate that the structure of the molecules between the anchoring group (-S- or -C≡C-) and the oligophenyl moiety significantly influences charge transport. SAMs of SPhn, and C≡CPhn on gold show similar values of β and log|J0| (β = 0.28 ± 0.03 Å-1 and log|J0| = 2.7 ± 0.1 for Au/SPhn; β = 0.30 ± 0.02 Å-1 and log|J0| = 3.0 ± 0.1 for Au/C≡CPhn). The introduction of a single intervening methylene (CH2) group, between the anchoring sulfur atom and the aromatic units to generate SAMs of SCH2Phn, increases β to ~0.6 Å-1 on both gold and silver substrates. (For n-alkanethiolates on gold the corresponding values are β = 0.76 Å-1 and log|J0| = 4.2). As a generalization, based on this and other work, it seems that increasing the height of the tunneling barrier in the region of the interfaces increases β, and may decrease J0; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J0.

This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called “EGaIn” electrodes), and in measurements of current density, J(V) ((A/cm^2)), across self-assembled monolayers (SAMs) incorporated into (Ag/SR//Ga_2O_3)/EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheres—air, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or water—were surveyed using both “rough” conical-tip electrodes, and “smooth” hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave (log |J|mean = −2.4 \pm 0.4) for the conical tip, and (log |J|mean = −0.6 \pm 0.3) for the drop electrode (and, thus, (\Delta log |J| \approx 1.8)); this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the (Ga_2O_3) film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of (log |J(V)|), and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from ∼70% to ∼100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl ((CH_3)^-) or carboxyl ((CO_2H)^-) terminated SAMs) over the range typically encountered in the laboratory (20%–60% (RH)).

This paper compares rates of charge transport by tunneling across junctions with the structures (Ag^{TS}X(CH_2)_{2n}CH_3 //Ga_2O_3 /EGaIn \space) (n=1–8 and (X= -SCH_2-) and (O_2C-)); 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 ((\beta), Å(^{−1})) and the injection current ((J_0), A cm(^{−2})) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing (Ag^{TS}SCH_2-R) with (Ag^{TS}O_2C-R) (R=alkyl chains) had no significant influence on (J_0) (ca. (3\times10^3 A cm^{−2})) or (\beta) (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 (\beta) for aliphatic (0.79 Å(^{−1})) and aromatic (0.60 Å(^{−1})) SAMs differed significantly.