Influence of halogen substitutions on rates of charge tunneling across SAM-based large-area junctions

Kong, Gyu Don; Jang, Hyeon Jae; Liao, Kung-Ching; Yoon, Ho‐Geun

doi:10.1039/c5cp00145e

Cited by 29 publications

(56 citation statements)

References 29 publications

(39 reference statements)

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“…These authors also showed that ε r of conjugated SAMs is less sensitive to the presence of highly polarizable atoms than non‐conjugated SAMs, which explains why we observed a factor of 4 increase of ε r . This notion also explains why Yoon et al only observed marginal effects in the tunneling rates in EGaIn junctions of the form Ag TS –S(CH 2 ) n ( p ‐C 6 H 5 X)//GaO x /EGaIn with n = 0 or 1 and X = H, F, Cl, Br, and I (these authors did not measure ε r ) . In addition, these short and conjugated SAMs hybridize strongly with the bottom‐electrode masking molecular effects …”

Section: The Properties Of the Samsmentioning

confidence: 98%

Tuning the Tunneling Rate and Dielectric Response of SAM‐Based Junctions via a Single Polarizable Atom

et al. 2015

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Section: The Properties Of the Samsmentioning

confidence: 98%

Tuning the Tunneling Rate and Dielectric Response of SAM‐Based Junctions via a Single Polarizable Atom

et al. 2015

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“…[41] In contrast to these results, for junctions with a conjugated backbone, Au-S(Ph) m X//GaO x /EGaIn (Ph = phenyl, X = F, Cl, Br, or I, m = 1 or 2), the identity of X did not change the charge transport characteristics of the junctions. [42,170] Figure 15a shows that the J(V) curves are essentially independent of X, except for X = H for which the currents are a factor of 10 larger than for the halogenated SAMs. [42] The value of R C obtained from IS is independent of X, and ε r only marginally increases from 2.3 to 3.2 in sharp contrast to the fourfold increase of ε r for S(CH 2 ) 11 X SAMs (Figure 14d).…”

Section: Collective Electrostatic 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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“…Such an influence, in the case of the terminal halogen atom, was reported for several aromatic SAMs, based on the static conductance data. 34,35,37 Whereas a terminal substitution is regretfully not possible within the molecular design suitable for RAES-CHC experiments, the side substitutions, performed for the benzonitrile-based SAMs (Figure 10) do not affect the ET dynamics noticeably, if one compares the average values for τ ET (π 1 *) and τ ET (π 3 *) with those for the nonsubstituted NC-PT SAM (Figure 4a), viz. 9 ± 3 fs and 31.5 ± 6 fs, respectively.…”

Section: ■ Direct Coupling To the Substratementioning

confidence: 99%

“…Apart from the negligible effect of the molecular dipole, the spectra of the benzonitrile-based SAMs are relevant in context of possible influence of halogen substitution, and fluorine in particular, on the electric transport properties of aromatic monolayers. Such an influence, in the case of the terminal halogen atom, was reported for several aromatic SAMs, based on the static conductance data. ,, Whereas a terminal substitution is regretfully not possible within the molecular design suitable for RAES-CHC experiments, the side substitutions, performed for the benzonitrile-based SAMs (Figure ) do not affect the ET dynamics noticeably, if one compares the average values for τ ET (π 1 *) and τ ET (π 3 *) with those for the nonsubstituted NC-PT SAM (Figure a), viz. 9 ± 3 fs and 31.5 ± 6 fs, respectively …”

Section: Effect Of Molecular Dipole and Substitutionmentioning

confidence: 99%

Femtosecond Charge Transfer Dynamics in Monomolecular Films in the Context of Molecular Electronics

Zharnikov

2020

Acc. Chem. Res.

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Conspectus A key issue of molecular electronics (ME) is the correlation between the molecular structure and the charge transport properties of the molecular framework. Accordingly, a variety of model and potentially useful molecular systems are designed, to prove a particular function or correlation or to build a prototype device. These studies usually involve the measurements of the static electric conductance properties of individual molecules and their assembles on solid supports. At the same time, information about the dynamics of the charge transport (CT) and transfer in such systems, complementary in the context of ME and of a scientific value on its own, is quite scarce. Among other means, this drawback can be resolved by resonant Auger electron spectroscopy (RAES) in combination with core hole clock (CHC) approach, as described in this Account. The RAES-CHC scheme was applied to a variety of aliphatic and aromatic self-assembled monolayers (SAMs), adsorbed on Au(111) over the thiolate and selenolate docking groups. Electron transfer (ET) from a suitable terminal tail group to the substrate, across the molecular framework, was monitored, triggered by resonant excitation of this group (nitrile in most cases) by narrow-band X-ray radiation. This resulted in the quantitative data for the characteristic ET time, τET, in the femtosecond domain, with the time window ranging from ∼1 fs to ∼120 fs. The derived τET exhibit an exponential dependence on the molecular length, mimicking the behavior of the static conductance and suggesting a common physical basis behind the static CT and ET dynamics. The dynamic decay factors, β ET, for the alkyl, oligophenyl, and acene molecular “wires” correlate well with the analogous parameters for the static CT. Both τET and β ET values exhibit a distinct dependence on the character of the involved molecular orbital (MO), demonstrating that the efficiency and rate of the CT in molecular assemblies can be controlled by resonant injection of the charge carriers into specific MOs. This dependence as well as a lack of correlation between the molecular tilt and τET represent strong arguments in favor of the generally accepted model of CT across the molecular framework (“through-bond”) in contrast to “through-space” tunneling. Comparison of the SAMs with thiolate and selenolate docking groups suggests that the use of selenolate instead of thiolate does not give any gain in terms of ET dynamics or molecular conductance. Whereas a certain difference in the efficiency of the electronic coupling of thiolate and selenolate to the substrate cannot be completely excluded, this difference is certainly too small to affect the performance of the entire molecule to a noticeable extent. The efficient electronic coupling of the thiolate docking group to the substrate was verified and the decoupling of the electronic subsystems of the substrate and π-conjugated segment by introduction of methylene group into the backbone was demonstrated. No correlation between the molecular dipole or fluorine substi...

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Influence of halogen substitutions on rates of charge tunneling across SAM-based large-area junctions

Cited by 29 publications

References 29 publications

Tuning the Tunneling Rate and Dielectric Response of SAM‐Based Junctions via a Single Polarizable Atom

Tuning the Tunneling Rate and Dielectric Response of SAM‐Based Junctions via a Single Polarizable Atom

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

Femtosecond Charge Transfer Dynamics in Monomolecular Films in the Context of Molecular Electronics

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