Molecular rectification is a particularly attractive phenomenon to examine in studying structure−property relationships in charge transport across molecular junctions, since the tunneling currents across the same molecular junction are measured, with only a change in the sign of the bias, with the same electrodes, molecule(s), and contacts. This type of experiment minimizes the complexities arising from measurements of current densities at one polarity using replicate junctions. This paper describes a new organic molecular rectifier: a junction having the structure Ag TS /S(CH 2 ) 11 -4-methyl-2,2′-bipyridyl//Ga 2 O 3 /EGaIn (Ag TS : template-stripped silver substrate; EGaIn: eutectic gallium−indium alloy) which shows reproducible rectification with a mean r + = |J(+1.0 V)|/|J(−1.0 V)| = 85 ± 2. This system is important because rectification occurs at a polarity opposite to that of the analogous but much more extensively studied systems based on ferrocene. It establishes (again) that rectification is due to the SAM, and not to redox reactions involving the Ga 2 O 3 film, and confirms that rectification is not related to the polarity in the junction. Comparisons among SAM-based junctions incorporating the Ga 2 O 3 /EGaIn top electrode and a variety of heterocyclic terminal groups indicate that the metal-free bipyridyl group, not other features of the junction, is responsible for the rectification. The paper also describes a structural and mechanistic hypothesis that suggests a partial rationalization of values of rectification available in the literature.

This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (n odd and n even ) using junctions with the structure M TS /SAM//Ga 2 O 3 /EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on Au TS and Ag TS demonstrated a statistically significant odd−even effect on Au TS , but not on Ag TS , that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on Au TS with n odd and n even belonging to two separate sets, and while there is a significant difference between the values of injection current density, J 0 , for these two series (log|J 0Au,even | = 4.0 ± 0.3 and log|J 0Au,odd | = 4.5 ± 0.3), the values of tunneling decay constant, β, for n odd and n even alkyl chains are indistinguishable (β Au,even = 0.73 ± 0.02 Å ). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J 0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.

This paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag) TS /A(CH 2 ) n H//Ga 2 O 3 /EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O 2 CR),and Au/acetylene (Au/CtCR), where R is an n-alkyl group. Values of J 0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J 0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).

Abstract:This paper compares rates of charge transport by tunneling across junctions with the structures Ag TS X(CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn (n = 1 -8 and X = -SCH 2 -and -O 2 C-); here Ag TS was 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 (J 0 , A/cm 2 ) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates.

This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiolterminated derivatives of oligo(ethylene glycol) (HS(CH 2 CH 2 O) n CH 3 ; HS(EG) n CH 3 ); these SAMs are positioned between gold bottom electrodes and Ga 2 O 3 /EGaIn top electrodes and are of the form: SAMs of oligo(ethylene glycol)s using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. According to calculations using density functional theory (DFT), these orbitals-localized orbitals predominately on the backbone oxygen atoms-are lower in energy (E MO = -6.8--7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, E MO : ~-5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (E MO < -8.5 eV for orbitals associated with CH 2 ), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG) n CH 3 are, in this mechanism, good conductors (by hole tunneling), but good insulators (by electron and/or hole drift conduction)-an unexpected observation that suggests SAMs derived from these or electronically similar molecules as a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of β for SAMs of S(EG) n CH 3 rests on the 3 possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.4

This paper describes rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga 2 O 3 /EGaIn as the top electrode. It compares the injection current, J 0 , and the attenuation constant, β, of the simplified Simmons equation, across oligophenyl groups (R = Ph n ; n = 1, 2, 3), with three different anchoring groups (thiol, HSR; methanethiol, HSCH 2 R; 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 SPh n , and C≡CPh n on gold show similar values of β and log|J 0 | (β = 0.28 ± 0.03 Å -1 and log|J 0 | = 2.7 ± 0.1 for Au/SPh n ; β = 0.30 ± 0.02 Å -1 and log|J 0 | = 3.0 ± 0.1 for Au/C≡CPh n ). The introduction of a single intervening methylene (CH 2 ) group, between the anchoring sulfur atom and the aromatic units to generate SAMs of SCH 2 Ph n , increases β to ~0.6 Å -1 on both gold and silver substrates. (For nalkanethiolates on gold the corresponding values are β = 0.76 Å -1 and log|J 0 | = 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 J 0 ; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J 0 .

This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form Ag TS O 2 C(CH 2 ) n (CF 2 ) m T//Ga 2 O 3 /EGaIn, where T is methyl (CH 3 ) or trifluoromethyl (CF 3 ). Alkanoate-based selfassembled monolayers (SAMs) having perfluorinated groups (R F ) show current densities that are lower (by factors of 20− 30) than those of the homologous hydrocarbons (R H ), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)n CH 2 −1 ) and difluoromethylene (β = (1.15 ± 0.02)n CF 2 −1 ) are similar (although the value for (CF 2 ) n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl-and -phenyl-alkanoates suggests that the C−F//Ga 2 O 3 interface is responsible for the lower tunneling currents for CF 3 . The decrease in the rate of charge transport in SAMs with R F groups (relative to homologous R H groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga 2 O 3 interface, and/or to weak van der Waals interactions at that interface. ■ INTRODUCTIONStudies of charge tunneling through metal−molecule−metal (MMM) junctions have focused predominately on testing hypotheses that correlate the chemical and electronic structure of the molecules with current densities (or in the case of singlemolecule studies, with currents). A convenient, semiquantitative theoretical framework around which to organize trends relating measurable parameters (e.g., the length of a (CH 2 ) n group) to experimental data (e.g., current densities at a fixed applied voltage) has been the simplified Simmons equation (eq 1). 1−11 In this approximation, the tunneling barrier is approximated as rectangular, with width d, and a height related to the attenuation factor β. 12,13 J(V) is current density (A/cm 2 ) at an applied bias V, and J 0 is loosely interpretable as the injection current for a hypothetical junction with d = 0. Changes in the topography of the barrier, the energies of the frontier orbitals, molecular dipoles, and polarizabilities of the insulating molecules in the junctions, are ignored or considered as part of J 0 . 14−18We have studied this type of system using self-assembled monolayer (SAM)-based junctions of the structure Au TS or Ag TS /A−R 1 −M−R 2 −T//Ga 2 O 3 /EGaIn; previous papers describe these studies. 11,19−25 We have used a variety of polar, aromatic, and aliphatic groups for the "anchoring" (A), "middle" (M), and "terminal" (T) groups. One of the unexpected implications of these studies has been that increasing the strength of the interaction across the T//Ga 2 O 3 interface does not seem to increase the tunneling current density of n-alkyl SAMs; 20 decreasing this strength does, however, seem to decrease the tunneling current. The topography of these tunneling barriers seems to be dominated by the electronic structure of the insulating alkyl chains. A theoretical study by Nij...

Junctions with the structure Ag TS /S(CH 2 ) n T// Ga 2 O 3 /EGaIn (where S(CH 2 ) n T is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga 2 O 3 . Introducing a series of Lewis acidic/basic functional groups (T = −OH, −SH, −CO 2 H, −CONH 2 , and −PO 3 H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm 2 , that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen-or ionic bonds at the T//Ga 2 O 3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J 0 , for T = −CO 2 H, and T = −CH 2 CH 3 −two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)−suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.