Odd–Even Effects in Charge Transport across <i>n</i>-Alkanethiolate-Based SAMs

Baghbanzadeh, Mostafa; Simeone, Felice C.; Bowers, Carleen M.; Liao, Kung-Ching; Thuo, Martin M.; Baghbanzadeh, Mahdi; Miller, Michael S.; Carmichael, Tricia Breen; Whitesides, George M.

doi:10.1021/ja509436k

Cited by 102 publications

(175 citation statements)

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“…50 We also approximated d as the length of the transextended molecule from the anchoring atom to the distal hydrogen atom to calculate β in units of Å -1 ; this analysis yields β (EG)n = 0.24 Å -1 (Figure 2b), which is comparable to that describing tunneling across SAMs of oligophenyls on gold (β (CH2)n = 0.28 Å -1 ). 17 S8).…”

Section: Linear Regression Analyses Of the Values Of Versus D Yieldementioning

confidence: 94%

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

et al. 2017

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This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiolterminated derivatives of oligo(ethylene glycol) (HS(CH 2 CH 2 O) n CH 3 ; HS(EG) n CH 3 ); these SAMs are positioned between gold bottom electrodes and Ga 2 O 3 /EGaIn top electrodes and are of the form: SAMs of oligo(ethylene glycol)s using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. According to calculations using density functional theory (DFT), these orbitals-localized orbitals predominately on the backbone oxygen atoms-are lower in energy (E MO = -6.8--7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, E MO : ~-5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (E MO < -8.5 eV for orbitals associated with CH 2 ), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG) n CH 3 are, in this mechanism, good conductors (by hole tunneling), but good insulators (by electron and/or hole drift conduction)-an unexpected observation that suggests SAMs derived from these or electronically similar molecules as a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of β for SAMs of S(EG) n CH 3 rests on the 3 possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.4

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Section: Linear Regression Analyses Of the Values Of Versus D Yieldementioning

confidence: 94%

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

et al. 2017

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“…7,12 Understanding the odd−even effect has consequences for the design and development of SAM-based technologies such as molecular tunneling junctions, field-effect transistors, and molecular diodes among many others. 1,5,11,22,25 The odd−even effect in wettability in n-alkanethiolate SAMs is believed to be caused by both the varying tilting angles of terminal CH 3 −CH 2 groups (structural effect) and the dipole of the SAMs (electronic effect). 2,35 Molecules with an even number (SAM E ) of carbons have the terminal −CH 2 CH 3 oriented along the surface normal while odd-numbered (SAM O ) molecules orient away from the surface normal, as shown in Figure 1a,b.…”

Section: ■ Introductionmentioning

confidence: 99%

Limits to the Effect of Substrate Roughness or Smoothness on the Odd–Even Effect in Wetting Properties of n-Alkanethiolate Monolayers

et al. 2015

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This study investigates the effect of roughness on interfacial properties of an n-alkanethiolate self-assembled monolayer (SAM) and uses hydrophobicity to demonstrate the existence of upper and lower limits. This article also sheds light on the origin of the previously unexplained gradual increase in contact angles with increases in the size of the molecule making the SAM. We prepared Au surfaces with a root-mean-square (RMS) roughness of ∼0.2-0.5 nm and compared the wetting properties of n-alkanethiolate (C10-C16) SAMs fabricated on these surfaces. Static contact angles, θ(s), formed between the SAM and water, diethylene glycol, and hexadecane showed an odd-even effect irrespective of the solvent properties. The average differences in subsequent SAM(E) and SAM(O) are Δθ(s|n  – (n+1)|) ≈ 1.7° (n = even) and Δθ(s|n – (n+1)|) ≈ 3.1° (n = odd). A gradual increase in θ(s) with increasing length of the molecule was observed, with values ranging from water 104.7-110.7° (overall Δθ(s) = 6.0° while for the evens Δθ(s)(E) = 4.4° and odds Δθ(s)(O) = 3.5°) to diethylene glycol 72.9-80.4° (overall Δθ(s) = 7.5° while for the evens Δθ(s)(E) = 2.9° and odds Δθ(s)(O) = 2.4°) and hexadecane 40.4–49.4° (overall Δθ(s) = 9.0° while for the evens Δθ(s)(E) = 3.7° and odds Δθ(s)(O) = 2.1°). This article establishes that the gradual increase in θ(s) with increasing molecular size in SAMs is due to asymmetry in the zigzag oscillation in the odd-even effect. Comparison of the magnitude and proportion differences in this asymmetry allows us to establish the reduction in interfacial dispersive forces, due to increasing SAM crystallinity with increasing molecular size, as the origin of this asymmetry. By comparing the dependence of θ(s) on surface roughness we infer that (i) RMS roughness ≈ 1 nm is a theoretical limit beyond which the odd-even effect cannot be observed and (ii) on a hypothetically flat surface the maximum difference in hydrophobicity, as expressed in θ(s), is ∼3°.

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“…changes to the anchoring group, A, and the terminal group, T) difficult to detect. 1 Among the exceptions are the observation of a small "odd-even effect" in charge transport across n-alkanethiolates on gold, [2][3][4] the observation of a substantial reduction in current density when fluorine is present at the SAM//Ga 2 O 3 interface, 5 and the observation of rectification of current when T is a redox active group such as ferrocenyl [6][7][8] or bipyridyl. 9 Having studied the influence of the structure of saturated n-alkyl groups on charge tunneling extensively, [10][11][12][13][14][15] we turned our attention to understanding the relationship between the structure of polyaromatics (molecules that result in a reduction in the height of the tunneling barrier relative to that characterizing aliphatics 16 ) and the rates of charge transport.…”

Section: Introductionmentioning

confidence: 99%

Tunneling across SAMs Containing Oligophenyl Groups

et al. 2016

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This paper describes rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga 2 O 3 /EGaIn as the top electrode. It compares the injection current, J 0 , and the attenuation constant, β, of the simplified Simmons equation, across oligophenyl groups (R = Ph n ; n = 1, 2, 3), with three different anchoring groups (thiol, HSR; methanethiol, HSCH 2 R; and acetylene, HC≡CR) that attach R to the template-stripped gold and silver substrates. The results demonstrate that the structure of the molecules between the anchoring group (-S-or -C≡C-) and the oligophenyl moiety significantly influences charge transport. SAMs of SPh n , and C≡CPh n on gold show similar values of β and log|J 0 | (β = 0.28 ± 0.03 Å -1 and log|J 0 | = 2.7 ± 0.1 for Au/SPh n ; β = 0.30 ± 0.02 Å -1 and log|J 0 | = 3.0 ± 0.1 for Au/C≡CPh n ). The introduction of a single intervening methylene (CH 2 ) group, between the anchoring sulfur atom and the aromatic units to generate SAMs of SCH 2 Ph n , increases β to ~0.6 Å -1 on both gold and silver substrates. (For nalkanethiolates on gold the corresponding values are β = 0.76 Å -1 and log|J 0 | = 4.2). As a generalization, based on this and other work, it seems that increasing the height of the tunneling barrier in the region of the interfaces increases β, and may decrease J 0 ; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J 0 .

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Odd–Even Effects in Charge Transport across n-Alkanethiolate-Based SAMs

Cited by 102 publications

References 50 publications

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

Limits to the Effect of Substrate Roughness or Smoothness on the Odd–Even Effect in Wetting Properties of n-Alkanethiolate Monolayers

Tunneling across SAMs Containing Oligophenyl Groups

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