Probing charge transfer dynamics in self-assembled monolayers by core hole clock approach

Zharnikov, Michael

doi:10.1016/j.elspec.2015.05.022

Cited by 27 publications

(84 citation statements)

References 93 publications

(188 reference statements)

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“…Such a behavior is inconsistent with the experimental situation for two reasons: first, at least in the present model system, the pinning situation is solely a consequence of the artificial collective electrostatic effects resulting from unrealistically high excitation densities in the simulations. Second, even if the core hole locally shifted unoccupied states below the Fermi level (for example, because of a much smaller energy gap of the adsorbate), for many interfaces, the time scales of the photoelectron experiments 133 , 134 would be such that the (partial) filling of these states with electrons would be too slow to affect the measured kinetic energies of the escaping electrons (and, thus, the core-level binding energies).…”

Section: Resultsmentioning

confidence: 99%

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

2020

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Core-level energies are frequently calculated to explain the X-ray photoelectron spectra of metal-organic hybrid interfaces. The current paper describes how such simulations can be flawed when modeling interfaces between physisorbed organic molecules and metals. The problem occurs when applying periodic boundary conditions to correctly describe extended interfaces and simultaneously considering core hole excitations in the framework of a final-state approach to account for screening effects. Since the core hole is generated in every unit cell, an artificial dipole layer is formed. In this work, we study methane on an Al(100) surface as a deliberately chosen model system for hybrid interfaces to evaluate the impact of this computational artifact. We show that changing the supercell size leads to artificial shifts in the calculated core-level energies that can be well beyond 1 eV for small cells. The same applies to atoms at comparably large distances from the substrate, encountered, for example, in extended, upright-standing adsorbate molecules. We also argue that the calculated work function change due to a core-level excitation can serve as an indication for the occurrence of such an artifact and discuss possible remedies for the problem.

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Section: Resultsmentioning

confidence: 99%

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

2020

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Section: Key Referencesmentioning

confidence: 89%

“…Phenom . 2015 , 200, 160–173 . The technical and methodical aspects of the approach are considered, also in connection with the complementary spectroscopic tools; the results for a variety of representative systems are presented and discussed. Werner, P.; Wächter, T.; Asyuda, A.; Wiesner, A.; Kind, M.; Bolte, M.; Weinhardt, L.; Terfort, A.; Zharnikov, M. Electron Transfer Dynamics and Structural Effects in Benzonitrile Monolayers with Tuned Dipole Moments by Differently Positioned Fluorine Atoms.…”

Section: Key Referencesmentioning

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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“…Zharnikov also observed an exponential decay of CT time with increasing length of alkyl chains in SAMs of Au–S–(CH 2 ) n –CN. 62 However, when the orbital participating in the transfer/transport is highly delocalized (resonance III), the CT rate decay does not behave in the same way as the static current and CT can be highly efficient. In molecular junctions, however, normally the applied voltage is low (to avoid electrical breakdown) and therefore tunneling in a molecular junction does not involve such high-lying orbitals (such as LUMO+2 for resonance III reported here) although they potentially are much more efficient in CT than the lower-energy orbitals.…”

Section: Results and Discussionmentioning

confidence: 99%

Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces

et al. 2021

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Charge transfer (CT) dynamics across metal–molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal–sulfur bond to the carbon–sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule–metal interface.

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Probing charge transfer dynamics in self-assembled monolayers by core hole clock approach

Cited by 27 publications

References 93 publications

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

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

Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces

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