Replacing AgTSSCH2‐R with AgTSO2C‐R in EGaIn‐Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

Liao, Kung-Ching; Yoon, Ho‐Geun; Bowers, Carleen M.; Simeone, Felice C.; Whitesides, George M.

doi:10.1002/anie.201308472

Cited by 44 publications

(81 citation statements)

References 45 publications

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“…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,3H-perfluoroalkanoates, perfluoroalkanoates, 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-extended (CH 2 ) n conformations), we do not estimate the width of the tunneling barrier (d in eq 1) from an estimation of the extended length of the molecules in nm, but instead empirically from the number of carbon atoms being considered. The current density across junctions containing fluorinated SAMs decreased exponentially with an increasing length of the fluorinated n-alkyl chain ( Figure S1 summarizes histograms of log|J(−0.5 V)| derived from the measurements).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Fluorination, and Tunneling across Molecular Junctions

et al. 2015

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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...

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Section: ■ Results and Discussionmentioning

confidence: 99%

Fluorination, and Tunneling across Molecular Junctions

et al. 2015

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“…The preparation of aromatic SAMs on Ag TS followed a previously reported literature procedure and is outlined in Supporting Information. 44,53,54 We measured and compared Figure 2 shows a plot of log|J(−0.5 V)| versus the number of methylene groups for these two analogous series. The length of the methylene chain (C n ) was estimated in Å; the length of a dimethylene (−CH 2 CH 2 −; C 2 ) unit is ∼2.54 Å.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…We concluded previously that replacing Ag TS /SCH 2 R with Ag TS /O 2 CR in EGaIn-based junctions does not significantly change rates of charge transport. 44 SAMs of oligophenyl thiolates (SPh n ) and −methylenethiolates (SCH 2 Ph n ), and of O 2 CPh n and SPh n , are, however, not comparable. In a separate study, 37 we show that the methylene group in SCH 2 Ph n prevents delocalization of electron density between the oligophenyl (−Ph n ) and the conducting electrodes (Au and Ag).…”

Section: ■ Experimental Designmentioning

confidence: 98%

“…39−42 We previously studied the attenuation factors of aliphatic and aromatic SAMs using junctions with the structure Met TS /A−R−H//Ga 2 O 3 /EGaIn, where Met TS is template-stripped gold or silver, A is a thiolate (−S−), acetylene (−CC−), a methylenethiolate (−SCH 2 −), or a carboxylate (−O 2 C−) group that links ("anchors") organic moieties (R = (CH 2 ) n or (C 6 H 4 ) m ) to the surface of metal electrodes, and EGaIn is a eutectic gallium−indium alloy covered by a thin film of gallium oxide; we have characterized this junction in a series of papers. 43−47 In the previous study involving organic carboxylates (O 2 CR), 44 we found that the attenuation factor of n-alkanoates (β = 0.79 ± 0.02 Å −1 ) is higher than that of oligophenyl carboxylates (β = 0.60 ± 0.03 Å −1 ), but the injection current (log|J 0 (−0.5 V)| = 3.5 ± 0.2) appears to be insensitive to the identity of these two types of hydrocarbons and their interfaces with electrodes. The difference in β for aliphatic and aromatic carboxylates is in agreement with predictions based on molecular orbital (MO) theory.…”

Section: ■ Backgroundmentioning

confidence: 99%

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Molecular Series-Tunneling Junctions

et al. 2015

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Charge transport through junctions consisting of insulating molecular units is a quantum phenomenon that cannot be described adequately by classical circuit laws. This paper explores tunneling current densities in self-assembled monolayer (SAM)-based junctions with the structure Ag TS / O 2 C−R 1 −R 2 −H//Ga 2 O 3 /EGaIn, where Ag TS is templatestripped silver and EGaIn is the eutectic alloy of gallium and indium; R 1 and R 2 refer to two classes of insulating molecular units(CH 2 ) n and (C 6 H 4 ) m that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R 1 and R 2 in the junction does not alter the overall rate of charge transport. By using the Ag/O 2 C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R 1 and R 2 units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R 1 and R 2 ; these differences are not influenced by the order of R 1 and R 2 in the SAM. In an electrical potential model that rationalizes this observation, R 1 and R 2 contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag TS /O 2 C and H//Ga 2 O 3 interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by J(V) = J 0 (V) exp(−β 1 d 1 − β 2 d 2 ), where J(V) is the current density (A/cm 2 ) at applied voltage V and β i and d i are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width d and a height related to the attenuation factor β.

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“…The reaction of organic thiols (RSH) with group Ib metals (Au and Ag) to generate SAMs with composition Au/AgSR is the reaction most commonly used to prepare SAMs, 1 although reactions that generate organosilanes on silicon 2 (SiR) and organic carboxylates on silver 3 (AgO 2 CR) have attractive properties, and a number of other precursors have been surveyed. There have also been scattered descriptions of SAMs formed on gold from solutions of alkynes 4 (HCC(CH 2 ) n CH 3 , n = 3, 5, 7, 9, 11, and 13), ethynylbenzene 5 (HCCC 6 H 5 ) or nalkylmercury(II) tosylates 6 (CH 3 (CH 2 ) n HgOTs, n = 4 and 18) on Au(111).…”

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Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate

et al. 2014

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Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC C(CH 2 ) n CH 3 , n = 5, 7, 9, and 11) with Au(111) at 60°Cwere characterized using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties, hydrophobicity, high surface order, and packing density, also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment; trace quantities of O 2 lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM by oxidation of the −CC− group (most likely catalyzed by the gold substrate in the presence of O 2 ).T hin organic films based on self-assembled monolayers (SAMs) 1 are ubiquitous in surface science. The reaction of organic thiols (RSH) with group Ib metals (Au and Ag) to generate SAMs with composition Au/AgSR is the reaction most commonly used to prepare SAMs, 1 although reactions that generate organosilanes on silicon 2 (SiR) and organic carboxylates on silver 3 (AgO 2 CR) have attractive properties, and a number of other precursors have been surveyed. There have also been scattered descriptions of SAMs formed on gold from solutions of alkynes 4 (HCC(CH 2 ) n CH 3 , n = 3, 5, 7, 9, 11, and 13), ethynylbenzene 5 (HCCC 6 H 5 ) or nalkylmercury(II) tosylates 6 (CH 3 (CH 2 ) n HgOTs, n = 4 and 18) on Au(111). Although the potential interest of SAMs having metal−CCR bonds is high, since they offer a new type of metal−organic bond, most of these studies have used preparations analogous to those employed with n-alkanethiols and have generated SAMs that do not seem to be highly ordered and, thus, are perhaps unsuitable for detailed studies of the physical chemistry of the surface. In particular, there are no procedures that describe the formation of SAMs that are highly ordered in two dimensionsa key requirement for high-quality surface science. The most recent analyses of n-alkyl-based SAMs on Au(111) indicate a "liquid-like" structure of the monolayer, 6 and XPS analyses of SAMs formed from alkynes 4,5 suggest that these SAMs are sensitive to oxidation at an undefined point in their formation; that is, oxidation occurs either during or after SAM formation (for example, by reaction of the AuCCR bond with O 2 ). Contact angle analyses of increasing lengths of alkynes (HCC(CH 2 ) n CH 3 , n = 5, 7, 9, and 11) also suggest 4 that the quality of these SAMs is lower than those based on n-alkanethiols.Although SAMs have enabled studies of wetting, 7,8 adhesion, 9,10 and charge transport 3,11−13 (...

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Replacing Ag^TSSCH₂‐R with Ag^TSO₂C‐R in EGaIn‐Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

Cited by 44 publications

References 45 publications

Fluorination, and Tunneling across Molecular Junctions

Fluorination, and Tunneling across Molecular Junctions

Molecular Series-Tunneling Junctions

Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate

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