Burying the Inverted Surface Dipole: Self-Assembled Monolayers Derived from Alkyl-Terminated Partially Fluorinated Alkanethiols

Marquez, Maria D.; Zenasni, Oussama; Rodriguez, Daniela; Yu, Tianlang; Sakunkaewkasem, Siwakorn; Figueira, Fabiana Toro; Czader, Arkadiusz; Baldelli, Steven; Lee, T. Randall

doi:10.1021/acs.chemmater.9b02887

Cited by 10 publications

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“…Controlling and understanding the charge tunneling rate across a molecular framework and, in an ideal case, variation of this rate by external stimuli represent major challenges of molecular electronics. − The great synthetic freedom allows us to vary both chemical composition of potential “device” molecules and their chemical and physical properties, such as, e.g., the proneness to reduction, oxidation, or protonation; possibility of quantum interference; and the availability of a certain dipole moment. − The effect of the latter parameter on the charge transport properties of molecular systems represents a controversial issue, discussed in different contexts, such as its influence on the tunneling rate, molecule–electrode coupling, tuning of the transition voltage, dipole-induced rectification, etc. − The standard way to vary the dipole moment for a molecule, assembled on an electrode and coupled to it via a suitable anchor, is its decoration with a dipolar tail group, − even though the embedding of dipolar groups into the molecular backbone became increasingly popular recently. − A simplest version of a dipolar tail group is represented by a single atom, such as a halogen one, possessing intrinsic electronegativity and polarizability. Indeed, the work function (WF) of both aliphatic and aromatic , self-assembled monolayers (SAMs) composed of such molecules can be varied via a single polarizable terminal atom, viz., F, Cl, Br, and I.…”

Section: Introductionmentioning

confidence: 99%

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

et al. 2020

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We studied charge tunneling rates across the single-component and binary self-assembling monolayers (SAMs) composed of 4-methyl-4′-mercaptobiphenyl (CH3-BPT) and 4-trifluoromethyl-4′-mercaptobiphenyl (CF3-BPT) on Au(111). The composition of the binary SAMs could be flexibly tuned, accompanied by gradual variation of the work function between ∼4.45 eV (CH3-BPT) and ∼5.5 eV (CF3-BPT). The tunneling rate across the single-component CH3-BPT SAM was found to be notably higher (by 1.5–2 orders of magnitude) than in the CF3-BPT case, while that across the binary SAMs varied progressively with their composition, between the values for the single-component monolayers and could, consequently, be fine-tuned. The observed behavior was tentatively explained by the higher projected density of states at the position of the terminal tail groups in the CH3-BPT case compared to CF3-BPT and by the appearance of an internal electrostatic field in the SAMs, leading to a change and renormalization of the energy-level alignments within the junction upon contact of the SAMs to the top EGaIn electrode. The extent of the latter effect depends primarily on the characteristics of the strongly confined two-dimensional (2D) sheet of dipolar tail groups at the SAM/top electrode interface. The height of the respective injection barrier is, however, unaffected by these characteristics, as follows from the values of the transition voltage, which do not change notably with the SAM composition. Analysis of the presented as well as literature data suggests that the position of a dipolar group in SAM-forming molecules has a significant impact on the performance of the respective SAM in the context of molecular electronics.

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Section: Introductionmentioning

confidence: 99%

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

et al. 2020

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“…7 Furthermore, increasing the length of the hydrocarbon chain atop the terminal dipole, effectively burying the dipole into the film, caused the contact angle value to start anew, with the fluorocarbons serving as a surrogate substrate. 8 These findings provided unequivocal evidence for the influence of surface dipoles on the macroscopic wettability of surfaces. 4,5,7,8 Despite the many studies analyzing the structure of fluorinated SAMs in comparison to their contact angle wettability, 9,10 there remains insufficient information regarding the liquid structure at the SAM interface, perhaps due to the limited number of techniques with the capacity to survey buried interfaces.…”

Section: ■ Introductionmentioning

confidence: 80%

“…Self-assembled monolayers (SAMs) derived from the adsorption of alkanethiols on gold provide conformationally ordered organic thin films with a precise orientation of the terminal functional groups. − Studies undertaken to determine SAM structure and organization when in contact with liquids have shown the composition and orientation of the terminal group strongly influences the wettability of the SAMs. − Recently, Zenasni et al, using selectively fluorinated SAMs on gold with oppositely oriented dipoles at the chain termini, concluded that an inversion of the terminal dipole in a monolayer film can cause a reversal of the odd–even wettability effect with polar contacting liquids . Furthermore, increasing the length of the hydrocarbon chain atop the terminal dipole, effectively burying the dipole into the film, caused the contact angle value to start anew, with the fluorocarbons serving as a surrogate substrate . These findings provided unequivocal evidence for the influence of surface dipoles on the macroscopic wettability of surfaces. ,,, Despite the many studies analyzing the structure of fluorinated SAMs in comparison to their contact angle wettability, , there remains insufficient information regarding the liquid structure at the SAM interface, perhaps due to the limited number of techniques with the capacity to survey buried interfaces.…”

Section: Introductionmentioning

confidence: 99%

“…Furthermore, increasing the length of the hydrocarbon chain atop the terminal dipole, effectively burying the dipole into the film, caused the contact angle value to start anew, with the fluorocarbons serving as a surrogate substrate . These findings provided unequivocal evidence for the influence of surface dipoles on the macroscopic wettability of surfaces. ,,, Despite the many studies analyzing the structure of fluorinated SAMs in comparison to their contact angle wettability, , there remains insufficient information regarding the liquid structure at the SAM interface, perhaps due to the limited number of techniques with the capacity to survey buried interfaces. Only a handful of studies have shown the interaction of liquids in proximity to polar interfaces, such as the use of SFG to study the dipoles in water reorienting in the presence of charged surfactants, and the use of noncontact atomic force microscopy to study ethanol selectively wetting polar functional groups in a patterned surface …”

Section: Introductionmentioning

confidence: 99%

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Surface Dipoles Induce Uniform Orientation in Contacting Polar Liquids

et al. 2020

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Molecular interactions at the solid−liquid interface have a profound impact on the macroscale wetting properties of surfaces. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and is used to determine the orientation of molecules at the interface. This study utilized SFG spectroscopy to study the interactions between the polar liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF 3 (CH 2 ) 17 SH and CD 3 (CF 2 ) 6 (CH 2 ) 10 SH on gold; these SAMs exhibit strong oriented molecular dipoles at their chain termini. We used SFG spectroscopy to examine the C−N and C−H stretching regions, 2000−2300 and 2800−3050 cm −1 , respectively, to probe the solid−liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive-or negative-going peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the orientation (CH 3 up or down) of the acetonitrile molecules with respect to changes in the direction of the FSAM dipole. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided direct evidence for the dipole-influenced wettability behavior of these unique model surfaces.

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“…Here, we primarily focus on work performed recently in the groups of the authors. It should, however, also be mentioned that in the literature there are several other examples of embedded-dipole SAMs. − Moreover, in the discussion we largely concentrate on thiolate SAMs on gold in view of the broad use of these systems both in basic research and in applications …”

Section: Introductionmentioning

confidence: 99%

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering

2022

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Conspectus Controlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM–semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS 2 , they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces ...

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Burying the Inverted Surface Dipole: Self-Assembled Monolayers Derived from Alkyl-Terminated Partially Fluorinated Alkanethiols

Cited by 10 publications

References 66 publications

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

Surface Dipoles Induce Uniform Orientation in Contacting Polar Liquids

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering

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