Fluorination, and Tunneling across Molecular Junctions

This paper describes the influence of 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 self-assembled 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 CH2-1 ) and difluoromethylene ( β =1.15 ± 0.02 n CF2 -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.


INTRODUCTION
Studies 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 single-molecule studies, with currents). A convenient, semi-quantitative 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][2][3][4][5][6][7][8][9][10][11] In this = ! ( ) !!" = ! ( )10 ! !" !.!"! (1) 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][15][16][17][18] We have studied this type of system using 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][20][21][22][23][24][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 Nijhuis and Zhang calculated that the T//Ga 2 O 3 interface was the highest region in the tunneling barrier. 26 One possible, testable hypothesis based on these experimental and theoretical studies might be that decreasing the strength of the T//Ga 2 O 3 interface might decrease the tunneling current density by increasing the height of the tunneling barrier at its highest point (i.e., at that interface).
The work we describe here was designed to test this possibility by replacing C-H bonds in the terminal group T with C-F bonds, and comparing tunneling current densities. Replacing hydrogen (C-H bonds) with less polarizable fluorine (C-F bonds) often changes the structural, chemical, and electronic properties of hydrocarbons. 27,28 For example, electronegative fluorine influences the frontier orbital energy of alkanes, lowers their surface energy and polarizability, and disrupts interchain packing and van der Waals interactions between chains in a SAM. 29,30 We explored the influence of the extent of fluorination of n-alkyl SAMs on the rate of charge transport across large-area junctions of the form Ag TS O 2 C(CH 2 ) n (CF 2 ) m T//Ga 2 O 3 /EGaIn (n, m = 0, 2, 4, 6, 8) at ±0.5 V. We varied the number of methylene (CH 2 ) and difluoromethylene (CF 2 ) groups in the backbone of the SAM, and changed the terminal function groups (T) of the SAM from H to F, and from CH 3 to CF 3 , in order to test two possibilities: i) In the backbone of the molecules, CH 2 and CF 2 might contribute differently to the height of the tunneling barrier, since electronegative fluorine substituents could lower the energy of the highest occupied molecular orbital (HOMO) of the n-alkyl SAMs, increase β, and decrease the tunneling current density. ii) In the terminal group T, replacing C-H bonds with C-F bonds might decrease the strength of the van der Waals interaction across the T//Ga 2 O 3 interface, raise the tunneling barrier in this region, decrease J 0 (V), and decrease the tunneling current density.
Replacing C-H bonds with C-F bonds in the terminal group T did, in fact, significantly lower the tunneling current density in a number of these compounds. Our specific focus in this work concerned the mechanism and origin of this effect. Experimentally, we observed that CH 2 and CF 2 contribute similarly (although perhaps distinguishably; see below) to the effective height of the tunneling barrier, and that the C-F//Ga 2 O 3 interface is responsible for low tunneling currents of some fluorinated hydrocarbons (e.g., perfluoroalkanes). The values of the attenuation factor, β in eq. 1, for CH 2 (β = 1.05 ± 0.02 n CH 2 -1 ) and CF 2 (β = 1.15 ± 0.02 n CF 2 -1 ) are similar, although distinguishable. The extrapolated current density (at −0.5 V) of the perfluorinated nalkyl SAMs (log|J CF 3 //Ga 2 O 3 | = 1.3 ± 0.2 for HO 2 C(CF 2 ) n CF 3 when n = 0) and the homologous hydrocarbons (log|J CH 3 //Ga 2 O 3 | = 2.8 ± 0.2) differ significantly and in a way that suggests that the difference is primarily attributable to the T//Ga 2 O 3 interface. We observed similar offsets of J 0 for para-substituted (T = CH 3 or CF 3 ) ω-tolyl-alkanoates (HO 2 C(CH 2 ) n (C 6 H 4 )T) and oligophenyl carboxylates (HO 2 C(C 6 H 4 ) n T). In all of these compounds, the rates of charge transport across the intermolecular forces between fluorocarbons than between hydrocarbons. 35 Fluorocarbons, 5 which have lower polarizability than hydrocarbons, 32,34 are functional groups that have been used to manipulate the chemical and electronic properties of metals and metal oxides. [36][37][38][39][40][41] SAMs with R F groups exhibit both hydrophobic and lipophobic properties, and are more thermally stable and chemically inert than SAMs based on analogous hydrocarbons (R H ). 36,42,43 In contrast to R H chains (-(CH 2 ) n -), which generally show linear, trans-extended packing in SAMs, the backbone of R F chains (-(CF 2 ) n -) form a helix-like structure that creates a slightly larger footprint for the individual R F molecules. [44][45][46] Both R H and R F molecules form densely packed SAMs.
SAMs have been used to modify the electronic properties of the interface between the electrodes and the organic active layers in organic electronics. 41 For example, Blom et al.
demonstrated that SAMs of n-alkanethiols can be used to decrease the work functions of gold and silver, whereas SAMs of perfluorinated n-alkanethiols can be used to increase these work functions (the R H and R F groups introduce opposite dipoles at the surface of the electrodes). 47 Cho and Tao reported that the work function of silver and aluminum can also be tuned using carboxylate-based SAMs; 48 depending on the length and the extent of fluorination in the structure of n-alkanoates, the work function of SAM-modified silver can be shifted from 4.6 eV (bare Ag) to 5.7 eV (R F -bound Ag). [48][49][50] This work also confirmed that the carboxylate anchoring group binds in a bidentate form to the surface of Ag (where a layer of native silver oxide possibly exists at the interface between the metal and the carboxylate) with an angle of inclination of the R F chain of ca. 28 o to the surface normal. 48

RESULTS AND DISCUSSION
We prepared SAMs on template-stripped silver using commercially available fluorinated n-alkanoic acids, following a previously reported procedure (Figure 1a). 48 SAMs were formed by 6 introducing freshly prepared Ag TS substrates 51  Although the structure and surface energy of R F and R H SAMs differ significantly, the contributions of CH 2 and CF 2 groups to the height of the tunneling barrier are apparently slightly different in their contributions to the value of β. Figure 1b shows a plot of log|J(-0.5 V)| versus the number of CH 2 and/or CF 2 groups for junctions comprising SAMs of 2H,2H,3H,3Hperfluoroalkanotes, perfluoroalkanotes, and previously studied n-alkanoates. 19 Because the R F and R H SAMs may adopt different molecular structures (e.g., helical (CF 2 ) n versus trans- None of these junctions rectified current. A linear-least squares fitting for each series yielded a slope (R 2 ≥ 0.99) and an intercept at the y axis.
Values of β for changes in the length of (CF 2 ) n chain (β = 1.15 ± 0.02 n CF 2 -1 ) and corresponding (CH 2 ) n chains (β = 1.05 ± 0.02 n CH 2 -1 ) were apparently slightly different in SAMs of perfluoroalkanoates and n-alkanoates. In the Simmons equation, 12 between the values of β for CF 2 and CH 2 to differences in their frontier orbital energies. It is possible that CF 2 has a lower HOMO energy (by 1 eV) than that of CH 2 , 34 but the work function of R F -bound Ag is also increased by approximately 1 eV (relative to bare Ag). 48 The simultaneous shifts in the HOMO energy of molecules and the work function of the SAM-bound electrode make φ of CF 2 and CH 2 indistinguishable (to the level of granularity that we can detect). A study using inelastic electron tunneling spectroscopy reported a similar conclusion. 29 The C-F//Ga 2 O 3 interface is responsible for low tunneling currents of R F SAMs. The  Table 1). The difference in J 0 (up to a factor of ~30) between the R F and R H 8 SAMs, and the similarity in β for compounds that interchange CH 2 and CF 2 groups, suggest that the C-F//Ga 2 O 3 interface is responsible for the reduction in J 0 . To verify the influence of the C-F//Ga 2 O 3 interface on the rates of charge transport, we replaced only the distal CH 3 Figure 4) also agrees with our observation of low tunneling currents for SAMs terminated with a CF 3 group ( Figure 1 and Table 1). The results from these compounds demonstrate that the C-F//Ga 2 O 3 interface is part of the tunneling barrier, as expected from prior work, 20 and is-in these compounds-responsible for the reduction in J 0 .
We have explored previously the influence of terminal groups T-ranging from aliphatic, simple aromatic, polar, and Lewis-acidic and -basic functional groups that form van der Waals, hydrogen, and/or ionic interactions at the T//Ga 2 O 3 interface-on the shape of the tunneling barrier. [20][21][22] A comparison of rates of charge transport to n-alkanes (using a standard set of reference compounds) suggested that increasing the strength of the interaction-from a van der Waals interface to a hydrogen-bonded and/or ionically bonded interface-did not significantly influence the height of the barrier at the T//Ga 2 O 3 interface. Among these molecules, SAMs of S(CH 2 ) 4 CONH(CH 2 ) 2 CF 3 seemed to be the exception; we measured a lower J(V) (by a factor of 3) relative to that of a standard n-alkanethiolate of similar length. 21 We do not entirely understand why this factor is not the factor of 20-30 measured here, but note that the terminal group of S(CH 2 ) 4 CONH(CH 2 ) 2 CF 3 in contact with the Ga 2 O 3 /EGaIn electrode might not be exclusively trifluoromethyl (CF 3 ). Our recent study of odd/even effects 25

CONCLUSIONS
The key experimental result of this study is its demonstration that changing a terminal methyl We have compared tunneling currents of partially and completely fluorinated nalkanoates and the homologous n-alkanoates using EGaIn-based junctions. We conclude that i) the influence of CH 2 and CF 2 groups on the shape of the tunneling barrier is similar (although marginally distinguishable by these measurements). This observation suggests that fluorination in the methylene groups of n-alkanes has only a small effect (at the scale that we can detect) on the apparent height (φ) (as approximated by eq. 1) of the tunneling barrier, perhaps because both the HOMO energy of R F SAMs and the work function of R F -bound Ag are shifted, in ways that partially cancel. ii) By contrast, fluorination of the terminal group has a large (x 20−30) effect and decreases tunneling currents. We conclude that the C-F//Ga 2 O 3 interface is the part of the tunneling barrier that is responsible for the reduction of rates of charge transport across R F SAMs.
The mechanism(s) of reduction of tunneling transport across the R F //

Supporting information
Detailed experimental procedure, histograms of current densities, and summary of junction measurements. This material is available free of charge via the Internet at http://pubs.acs.org.