The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

Wang, Zhengjia; Oyola‐Reynoso, Stephanie; Thuo, Martin M.

doi:10.3390/coatings5041034

Cited by 32 publications

(49 citation statements)

References 58 publications

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“…The roughness of the stationary electrode(s), in both sandwich and trench configurations is critical for monolayer uniformity 312,313 and the resulting transport characteristics (see Fig. 4D).…”

Section: Surface Roughnessmentioning

confidence: 99%

Large-Area, Ensemble Molecular Electronics: Motivation and Challenges

2017

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We review charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research and to single-molecule junctions (1mJ) in addition to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MolEl, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead, we focus on how to analyze the net electrical performance of molecular junctions, from a functional device perspective. Finally, we point to creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes. This review is complemented by a another review that covers metal-molecule-semiconductor junctions and their unique hybrid effects.

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Section: Surface Roughnessmentioning

confidence: 99%

Large-Area, Ensemble Molecular Electronics: Motivation and Challenges

2017

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“…[58,152] Such odd-even effects are caused by changes in both the electronic and supramolecular structure of the SAMs. Odd-even effects have been reviewed before [44,53,187,188] and are the result of a fixed M-S-C (M = metal) bond angle where the addition of subsequent CH 2 units follow a zig-zag pattern since CCC bond angles are fixed at 109.5°. [187,189] The result is that the terminal group of the monolayer appears to be "ethyl" or "methyl" like, in the example of S(CH 2 ) n CH 3 , depending on the value of n as indicated by the black arrows in Figure 17a.…”

Section: Separating Interface From Molecular Effects: Odd-even Effectsmentioning

confidence: 99%

The Unusual Dielectric Response of Large Area Molecular Tunnel Junctions Probed with Impedance Spectroscopy

Chen

Nijhuis

2021

Adv Elect Materials

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workings of charge transport in exquisite detail in areas including molecular plasmonics, [9] spintronics, [10,11] bioelectronics, [12] sensing, [13] catalysis, [14,15] energy storage, [12,16] memory, [17][18][19] and flexible electronics. [2,3,[20][21][22] Control over the dielectric behavior of nanoscale systems is important for a wide range of applications including supercapacitors, [23] organic field effect transistors (OFETs), [24][25][26][27] flexible and stretchable electronics, [25,28] optoelectronics, [29] and memory. [30] For example, for OFETs, it is important to develop gate materials with high dielectric constants yet with low leakage (via tunneling) currents. [24,25] In principle, molecular junctions are interesting because they allow us to study the dielectric properties of materials at the molecular length scale, but their dielectric behavior remains, to date, virtually unexplored despite promising features. [31][32][33][34] For instance, it has been predicted that, due to their well-organized nature, self-assembled monolayers (SAMs) can have very high relative dielectric constants ε r of ≈20, [32] while usually bulk organic materials have values of ε r in the range of 2-4. [35,36] Theoretically it has been shown that ε r can increase with the thickness of SAMs helping to avoid the adverse decrease of the capacitance in OFETSs with increasing gate dielectric thickness [26,31,34,35] which, in turn, can be used to reduce leakage currents. The dielectric behavior of SAMs are also important to modulate work functions, [37][38][39][40] or to reduceThe dielectric behavior of organic materials at molecular dimensions can be vastly different than their bulk counterparts and therefore it is important to investigate and control the dielectric response of molecular-scale materials for a large variety of applications. Large-area molecular junctions are explored to study the charge transport mechanisms with unprecedented detail and to demonstrate novel functionalities. Therefore, large-area molecular junctions are, in principle, a promising platform to study the dielectric effects at the molecular scale. This review summarizes recent progress on the measurements and understanding of the dielectric behavior of molecular systems and how they behave differently from their bulk properties. This review introduces, briefly, the concepts of impedance spectroscopy (IS) and how this technique can be applied to study the dielectric response of solid-state large-area molecular junctions. This analysis gives new insights in the factors that determine the dielectric constant of monolayers (including collective electrostatic effects and how the dielectric constant increases with monolayer thickness), how IS gives new insights into the role of defects, and the factors that contribute to the molecule-electrode contact resistance. This review ends with an outlook highlighting interesting future directions.

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“…Organic self-assembled monolayers (SAMs) formed on semiconductor, metal or metal oxide surfaces are an integral component of molecular electronic devices, for electrical insulation, charge storage, and charge transport. − A recent series of investigations of the charge transport properties of large-area, solid-state tunnel junctions comprised of SAMs of the redox-active ferrocenylalkanethiolates (Fc(CH 2 ) n S) , or insulating n -alkanethiolates (CH 3 (CH 2 ) n S) − have sparked renewed interest in the odd–even (or parity) effect, − which refers to the alternation of a material’s structure and/or property due to an odd or even number of repeat units in the molecule. Parity effects in solid condensed phases often arise from differences in the intermolecular interactions and molecular packing.…”

Section: Introductionmentioning

confidence: 99%

“…Clearly, it is essential to optimize the supramolecular organization of the SAM via the parity of the repeat units in its backbone to harness its full functionality. The perception that due to the large body of published work everything is known about the structural characteristics of SAMs, including the odd–even effect, is false. , Early structural characterizations of SAMs did not consider the role played by the roughness of the metal surface on the SAM order, thereby leading to contradictory results on the existence of a parity effect, examined a limited number of odd–even homologues, − or focused on chains of n odd . Since the presence of multiple electrodes and interfaces makes it challenging to elucidate the contribution of the inherent supramolecular structure of the organic SAM to the junction properties, recent investigations of the odd–even effect have focused on SAMs chemisorbed to a metal electrode or substrate, probing either the structure of the SAM/ambient interface (near edge X-ray fine structure spectroscopy, photoemission spectroscopy, static contact angle, sum frequency generation spectroscopy, quartz crystal microbalance) or the electronic structure of the SAM/metal interface (ultraviolet photoelectron spectroscopy).…”

Section: Introductionmentioning

confidence: 99%

Molecular Origin of the Odd–Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

et al. 2020

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Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH3(CH2)nSAu, n = 6-19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level, by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit dependencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH2 stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH3 stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH3. The correlations between the different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects on the electrical characteristics of tunnel junctions.

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The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

Cited by 32 publications

References 58 publications

Large-Area, Ensemble Molecular Electronics: Motivation and Challenges

Large-Area, Ensemble Molecular Electronics: Motivation and Challenges

The Unusual Dielectric Response of Large Area Molecular Tunnel Junctions Probed with Impedance Spectroscopy

Molecular Origin of the Odd–Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

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