Replacing AgTSSCH 2 ‒R with AgTSO 2 C‒R in EGaIn-Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport
Kung-Ching Liao,1 Hyo Jae Yoon,1 Carleen M. Bowers,1 Felice C. Simeone,1 and George M. Whitesides1,2,3*

1

Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 United States,

2

Wyss Institute for Biologically Inspired Engineering, Harvard University, 60 Oxford Street, Cambridge, Massachusetts 02138 United States, and
3

Kavli Institute for Bionano Science & Technology, Harvard University, 29 Oxford Street, Massachusetts 02138 United States

*Corresponding author, email: gwhitesides@gmwgroup.harvard.edu

Abstract: This paper compares rates of charge transport by tunneling across junctions with the structures AgTSX(CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn (n = 1 ‒ 8 and X = ‒SCH 2 ‒ and ‒O 2 C‒); here AgTS 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/cm2) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing AgTSSCH 2 ‒R with AgTSO 2 C‒R (R = alkyl chains) had no significant influence on the value of J 0 (ca. 3 × 103 A/cm2) or on the value of 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 the value of β for aliphatic (0.79 Å-1) and aromatic (0.60 Å-1) SAMs differed significantly.

1

Studies of charge tunneling through self-assembled monolayer (SAM)-based junctions have focused predominately on the influence of backbone substituents1-4 or terminal functional groups5-8 on rates of charge transport. The effect of changing the group (which we call the “anchoring group”) that links the SAM to the metal substrate has not been explored in detail;9, 10 only a few studies have approached this issue at the single-molecular level using scanning tunneling microscopy11-14 or conducting atomic force microscopy.15-17 Here we explore the replacement of the anchoring group using a large-area (50 μm2 on average) SAM-based junction having the structure AgTS‒X‒(CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn,18, 19 where X is the anchoring group for the SAM; AgTS is a template-stripped silver substrate;20 EGaIn is a liquid metal, eutectic gallium‒indium alloy; and Ga 2 O 3 is a thin semiconducting film that forms spontaneously on the surface of EGaIn in air.18 We prepared analogous junctions, compositionally different only in the replacement of AgTSSCH 2 ‒R with AgTSO 2 C‒R, and compared the rates of charge transport by tunneling through these two junctions. The similarity in these rates establishes that the rate of charge transport across the SAM-based tunneling junction is surprisingly insensitive to changes in the composition of the interface between the AgTS and the SAM. While the study of organothiolates in molecular electronics is limited by the availability and stability of thiols, a large variety of carboxylic acids are commercially available, or easily accessible by straightforward synthetic routes. The ability to study charge transport through junctions of the structure AgTSO 2 CR//Ga 2 O 3 /EGaIn makes mechanistic studies of tunneling more accessible experimentally, and provides a new system that helps to clarify the role of the interfaces to electrodes in tunneling junctions. Background. The rate of charge transport by tunneling through SAMs decays exponentially with increasing distance between the top and bottom electrodes. In studies analogous to those in 2

many other systems, we determined that junctions of the structure AgTSSCH 2 ‒ (CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn (across a range of molecular lengths and structures) obey the simplified Simmons equation (1),21-24 where J (A/cm2) is the measured current density; β (Å-1) is the tunneling decay coefficient; we take d (Å) to be the extended length of molecule. The injection current, J 0 , represents the current density when d = 0; that is, the value of J for a ������ = ������0 ������ −������������ (1)

hypothetical system consisting only of the top and bottom electrodes, and the metal‒SAM interfaces.24 Values of β characterize tunneling junctions having a range of alkyl structures are similar; values of J 0 vary among different types of junctions, for reasons that are at least partially understood.19 In an electrode‒SAM‒electrode junction, charge crosses a tunneling barrier whose energetic topography is not exactly known, but which describes the space (including the SAM, the interfaces between the SAMs and the electrodes, and any surface films on the electrodes) between the two metallic junctions. In principle, one approach to manipulating the shape of the tunneling barrier, and thus 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 thus the rate or mechanism of charge transport.25-29 Using Ga 2 O 3 /EGaIn top-electrodes, however, we found previously that the tunneling current is insensitive to the incorporation of several functional groups familiar in organic chemistry4, 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 Ga 2 O 3 film.8

3

Most studies using EGaIn-based tunneling junctions have focused on SAMs of nalkanethiolates on Au or Ag. This focus on systems based on the Ag‒SR anchoring group has limited our understanding of the role of the interface between the Ag (or Au) and the SAM to a single chemical and electronic structure. Here, we replaced n-alkanethiolates with n-alkanoates in order to examine the effect of the bottom-interface on the value of J 0 for a AgTS‒ SAM//Ga 2 O 3 /EGaIn junction. n-Alkanoic acids form highly-ordered monolayers on metal surfaces;30 in particular, monolayers composed of long-chain n-alkanoates on Ag exhibit nearly crystalline packing of the hydrocarbon backbone, all-trans methylene conformations, and a bidentate ionic binding coordination of the carboxylate to the surface.30-33 Previous reports31, 33 showed that the carboxylate moiety coordinates through ionic interactions with the surface of the Ag, and a native oxide of Ag possibly (or in our view probably) exists at the interface between the metal and the carboxylate. SAMs of n-alkanethiolates on Ag(111) have a (√7 X √7)R10.9o cell with 4.4 Å nearest neighbor spacing.34 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). Results and discussion Preparation of SAMs. We prepared SAMs with commercially available n-alkanoic acids, CH 3 (CH 2 ) 2n CO 2 H where n = 1 – 8 (i.e., the number of methylene groups). The preparation of nalkanoate SAMs on AgTS followed previously reported literature procedures.30, 35 Freshly prepared AgTS substrates were introduced into a solution of 1 mM n-alkanoic acid in anhydrous n-hexadecane for 3 hr. After incubation at ambient conditions, we rinsed the SAM-bound Ag substrates three times with anhydrous hexane and dried the substrates under a gentle stream of 4

nitrogen. We characterized the surface of SAMs of n-alkanoates on AgTS using contact angle measurements. The monolayers exhibited low wettability: static contact angles for wetting by water and n-hexadecane were 113 ± 7o and 45± 3o, respectively. These values are consistent with previous reports by Tao et al.30 and Lin et al.32 Measurements of Tunneling Currents. We measured J(V) for junctions of the form AgTSO 2 C‒ (CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn over the range of ±0.5 V as a function of the length of alkyl chain (n = 1 – 8); we did not observe rectification of current (Figure 1b). Each curve of log|J(V)|versus V was generated with 430 – 720 data from at least 20 different junctions on three to four samples, and the yield of working junctions was ≥ 84% (Table 1). As observed in many previous studies, the binned J values (100 bins in the range of log|J| = -6 to 4, with units of A/cm2; the width of each bin is log|J|~0.1) had a distribution that was approximately log-normal; this form justified fitting each histogram of log|J| with a Gaussian curve. From these fittings, we obtained mean values of log|J| (log|J| mean ) and standard deviations (σ log ) of the corresponding Gaussian fits (Figure 2); log|J| mean is indistinguishable from the log median value (log|J| median ) of log|J| determined in each histogram (Table 1). Values of σ log ranged from 0.1 to 0.5; these values are similar to those observed in the junctions of n-alkanethiolate SAMs.36 As expected from the simplified Simmons equation, the rate of charge transport across junctions containing SAMs of n-alkanoates followed an exponential decrease with increasing length of the n-alkyl groups (n=1 to 8 for CH 3 (CH 2 ) 2n CO 2 ). Figure 1c shows a plot of log|J| versus calculated length (Å) which includes the length of terminal H‒C bond but excludes the length of Ag‒O bond, which we considered to be a part of the AgTS‒SAM interface (in this view [O 2 ]C(CH 2 ) 2n CH 3 and [S]CH 2 (CH 2 ) 2n CH 3 are compatible structure). The linear-least square fit of the full set of data (CH 3 (CH 2 ) 2n CO 2 H, n = 1 – 8) yielded the log-injection current, log|J 0 | = 3.5 ± 0.2 A/cm2

5

(coefficient of determination, R2 = 0.99). The slope derived from the plot of ln|J| versus the length of molecules provided the tunneling decay coefficient, β = 0.79 ± 0.02 Å-1. Comparisons of J(V) data from n-alkanoate and n-alkanethiolate36 SAMs on Ag (Table 1) indicate that these junctions have a statistically indistinguishable tunneling decay coefficient (β = 0.79 ± 0.02 Å-1 for n-alkanoates; β = 0.75 ± 0.02 Å-1 for n-alkanethiolates, both with even numbers of carbon atoms) and injection current (log|J 0 | = 3.5 ± 0.2 A/cm2 derived from nalkanoates; log|J 0 | = 3.2 ± 0.3 A/cm2 derived from n-alkanethiolates). The similarities in β and J 0 imply that any difference in the contribution of the Ag-thiolate and the Ag-oxygen interface to the shape of the tunneling barrier is not detectable by our methods, although these two interfaces are chemically and electronically quite different. We conclude therefore, that the AgTS‒X‒R interface does not contribute to the features of the tunneling barrier that influence tunneling current. While we and Tao et al. have not yet defined whether mono- or multi-layers of native silver oxide are sandwiched between the Ag metal and the carboxylate, the J‒V measurements suggest that this film, if any, is conductive, and makes no contribution to the resistance of the junction. Several studies have predicted or reported a distinct electronic influence from different metal-molecule interfaces used in junction measurements.11, 37-40 For example, Zimbovskaya and Pederson41 examined different metal-molecule interfaces theoretically, and concluded that different modes of binding at the interfaces might influence the conductance of junctions. Chu et al.42 reported that the current through molecules with an Au‒amine junction is larger by a factor of 10 than that with an Au‒thiolate linkage, and attributed this observation to the difference of the electronic interactions between the gold and the anchoring group. Our findings, however, indicate that replacing AgSCH 2 ‒R with AgO 2 C‒R (a large change in the structure of the bottom 6

metal‒SAM interface) has no significant influence on the rates of charge transport across nalkyl-based SAMs. Junctions comprising SAMs of aromatics. Tao and coworkers reported the formation and characterization of SAMs of biphenyl-4-carboxylic acid43 and p-terphenyl-4-carboxylic acid33 on Ag. Their report showed that the oligo(phenylene)carboxylate binds perpendicularly (from the surface normal) through a symmetric ionic coordination to the surface of Ag.43 We incorporated SAMs of oligo(phenylene)carboxylic acids into junctions of the structure AgTSO 2 C(C 6 H 4 ) n H//Ga 2 O 3 /EGaIn (n = 1 ‒ 3) and characterized rates of charge transport across them (Figure 3a; Table 2). Figure 4 shows the histograms of log|J| at -0.5 V and relative Gaussian fittings for benzoic acid, biphenyl-4-carboxylic acid, and p-terphenyl-4-carboxylic acid. We found a narrow distribution of current density with a small range of σ log (0.2 ‒ 0.3) for each SAM. The value of log|J 0 | (3.0 ± 0.2 A/cm2) of the AgTSO 2 C(C 6 H 4 ) n H//Ga 2 O 3 /EGaIn junctions was estimated from a linear least squares fit (R2 = 0.98) of the plot of log|J| versus the calculated length of the molecules (Å); β was 0.60 ± 0.2 Å-1. The tunneling decay coefficient of n-alkanoate-based SAMs (0.79 Å-1) is higher than that of oligophenylene-containing SAM (0.60 Å-1), and suggests (as have other studies10, 29, 44, 45) that in these oligophenylene-based SAMs i) the shape of the tunneling barrier is influenced both by the width of the tunneling barrier and the electronic structure of the molecules forming the SAM; ii) charge transport by tunneling through poly-aromatic SAMs is more rapid than through nonconjugated SAMs (a conclusion in agreement with a number of other studies). The value of log|J 0 |, derived from the junctions comprising SAMs of oligophenylene carboxylates (log|J 0 | = 3.0 ± 0.2 A/cm2), is indistinguishable to that of n-alkanoates (log|J 0 | = 3.5 ± 0.2 A/cm2) at the precision of our measurements. 7

Conclusion Within the constraints of the accuracy and uncertainties of our system, the Ag‒SAM interface has no significant influence on the rates of tunneling across an n-alkane-based tunneling barrier, since the tunneling currents through junctions of n-alkanoate and of nalkanethiolate SAMs are indistinguishable in our experiments. Although the details of the bonds at the AgTSO 2 C‒R interface are quite different from those at the AgTSSCH 2 ‒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. The tunneling decay coefficient for n-alkanoates and for n-alkanethiolates on silver are indistinguishable (0.75‒0.79 Å-1), and are similar to previous literature reports for junctions with different structures.19, 46-48 The coefficient for oligophenylene carboxylates (β = 0.60 Å-1) is, as expected from prior work, significantly lower than that of alkyl-based SAMs. We conclude that the AgTSO 2 CR//Ga 2 O 3 /EGaIn junction provides a versatile and convenient experimental system with which to investigate factors that may influence rates of charge transport through SAM-based junctions, and to understand the mechanisms underlying these influences. The use of carboxylate anchoring groups promises to simplify the study of tunneling junctions greatly by eliminating the instability (due to oxidation, desulfurization, and other processes) and multiple chemical incompatibilities of the commonly-studied structures based on organic thiolates. Moreover, carboxylates are more flexible than thiols from the vantage of physical-organic studies, because they are commercially available, stable (especially to oxidation), and easily handled and purified.

8

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Acknowledgements We thank Dr. Mathieu Gonidec for his technical contribution. This work was supported by a subcontract from Northwestern University from the Department of Energy (DE-SC0000989) on materials and measurements of charge transport. Author contributions K.-C.L. and G.M.W. conceived this project. K.-C.L. performed the experiments of carboxylates; F.C.S. performed the experiments of n-alkanethiolates. All authors analysed the junction measurements, discussed the results, and contributed to the manuscript. Additional Information Supplementary information including Table S1 is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to G.M.W. Competing financial interests The authors declare no competing financial interests.

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Figure 1. a, A cartoon representation of the AgTSXR//Ga 2 O 3 /EGaIn junction (X = ‒SCH 2 ‒ and ‒O 2 C‒); b, log-current density (log|J|) versus bias (V) plots for the AgTSO 2 C(CH 2 ) n CH 3 //Ga 2 O 3 /EGaIn junctions with various chain lengths (4 to 18 carbons including the top methyl group and the bottom anchoring group), as indicated in the figure; c, a plot of log-current density (log|J|) against the chain length of n-alkanoates (including the terminal C‒H bond), given in number of carbons at -0.5 V. The linear-least square fits for nalkanoates (dotted line) and for n-alkanethiolates (solid line) and the results of electrical measurement are inserted in the figure.

14

Figure 2. Histograms for values of log|J| data derived from n-alkanoates at -0.5 V. Each histogram is fitted with a Gaussian curve (black curve).

15

Figure 3. a, A cartoon representation of the junction structure comprising oligo(phenylene)carboxylate SAMs. The carboxylate forms a bidentate coordination to the surface of Ag; b, a plot of log-current density (log|J|) against the calculated length (Å), including the terminal C‒H bond, of oligo(phenylene)carboxylates, given in the number of phenylene units at -0.5 V. The linearleast square fits (dotted line) and the results of electrical measurement are inserted in the figure.

16

Figure 4. Histograms of log|J| for a, benzoate; b, biphenyl-4-carboxylate; and c, p-terphenyl-4carboxylate in the AgTSO 2 C(C 6 H 4 ) n H//Ga 2 O 3 /EGaIn junctions (n= 1 ‒ 3) at -0.5 V. Each histogram is fitted with a Gaussian curve (black curve).

17

Table 1. Summary of data derived from n-alkanoates (C n O 2 ) and n-alkanethiolates36 (C n S) at -0.5 V to illustrate the similarity in the charge transport characteristics among the two homologous SAMs. n-alkanoates CnO2 C 4 O2 C 6 O2 C 8 O2 C 10 O
2

n-alkanethiolates log|J| median (A/cm2) 1.85 0.95 -0.05 -0.85 -1.65 -2.65 -3.55 -4.15 log|J| mean ± σ log (A/cm2) 1.9 ± 0.10 1.0 ± 0.20 -0.06 ± 0.06 -0.8 ± 0.20 -1.6 ± 0.20 -2.7 ± 0.10 -3.5 ± 0.20 -4.1 ± 0.50 -3.5 ± 0.20 (log|J 0 |) β = 0.79 ± 0.02 Å-1 CnS C4S C6S C8S C 10 S C 12 S C 14 S C 16 S C 18 S C0S log|J| mean ± σ log (A/cm2) 1.6 ± 0.5 0.9 ± 0.3 0.2 ± 0.3 -1.1 ± 0.3 -1.5 ± 0.5 -2.2 ± 0.3 -3.2 ± 0.3 -3.9 ± 0.3

Number of Samples 3 3 4 4 3 4 3 4

Working Yield traces Junctions (%) 20 20 30 29 27 32 24 32 95 90 97 91 90 97 91 84 432 467 721 668 637 719 556 717

C 12 O
2

C 14 O
2

C 16 O
2

C 18 O
2

C0O2

3.2 ± 0.3 (log|J 0 |) β = 0.75 ± 0.02 Å-1

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Table 2. Summary of data derived from oligo(phenylene)carboxylates (AgTS‒O 2 C‒ (C 6 H 4 ) n H//Ga 2 O 3 /EGaIn, n = 1 ‒ 3) at -0.5 V. Oligo(phenylene)carboxylates Molecular Number of Working Yield traces n Lengths (Å) Samples Junctions (%) 1 2 3 C0O2 6 10 14 3 3 3 18 12 15 72 80 79 343 253 300 log|J| median (A/cm2) 1.45 0.15 -0.55 log|J| mean ± σ log (A/cm2) 1.5 ± 0.2 0.2 ± 0.2 -0.6 ± 0.3 3.0 ± 0.2 (log|J 0 |) β = 0.60 ± 0.02 Å-1

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Graphical Abstract

This manuscript describes a comparative study on the role of the metal‒SAM interface in the charge tunneling process. We investigate the rate of charge transport through EGaIn-based junctions comprising self-assembled monolayers (SAMs) of organo-thiolates and carboxylates.

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