Imaging Time-Dependent Electronic Currents through a Graphene-Based Nanojunction

Pohl, Vincent; Steinkasserer, Lukas Eugen Marsoner; Tremblay, Jean Christophe

doi:10.1021/acs.jpclett.9b01732

Cited by 22 publications

(30 citation statements)

References 63 publications

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“…We calculated the current density in a cylindrical coordinate system, which allow us to color the arrows by the relative cylindrical vectorcomponent 9 " |9 ;⃑| . The conversion from cartesian to cylindrical coordinates is trivial, and is included for reference in SI part H. Finally, as we and other have recently noted, 45,56,[97][98][99][100] ballistic current density calculations can have serious issues with current conservation over the molecule due to using a finite local basis set and approximated core-potentials; we provide some examples of this problem in SI part G.…”

Section: Methodsmentioning

confidence: 99%

Three Distinct Torsion Profiles of Electronic Transmission through Linear Carbon Wires

2020

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The one-dimensional carbon allotrope carbyne, a linear chain of sp-hybridized carbon atoms, is predicted to exist in a polyynic and a cumulenic structure. While molecular forms of carbyne have been extensively characterized, the structural nature is hard to determine for many linear carbon wires that are made in-situ during pulling experiments. Here, we show that cumulenes and polyynes have distinctively different low-bias conductance profiles under axial torsion. We analyze the change of the electronic structure, Landauer transmission, and ballistic current density of the three types of closed-shell molecular carbynes as a function of the torsion angle. Both polyynic, odd-carbon cumulenic, and even-carbon cumulenic carbon wires exhibit helical frontier molecular orbitals when the end-groups are not in a co-planar configuration. This helical conjugation effect gives rise to strong ring current patterns around the linear wires. Only the transmission of even-carbon polyynic wires follows the well-known cosine-squared law with axial torsion that is also seen in biphenyl-type systems. Notably, the transmission of even-carbon cumulenic carbon wires rises with axial torsion from co-planar towards perpendicular orientation of the end-groups. The three distinct transmission profiles of polyynes, odd-carbon cumulenes, and even-carbon cumulenes may allow for experimental identification of the structural nature of linear carbon wires. Their different electron transport properties under axial torsion furthermore underline that, in the molecular limit of carbyne, three different subclasses of linear carbon wires exist. File list (3)download file view on ChemRxiv torsion_chemrxiv3.pdf (6.17 MiB) download file view on ChemRxiv SI_chemrxiv3.pdf (9.24 MiB) download file view on ChemRxiv files.zip (21.38 MiB)

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Section: Methodsmentioning

confidence: 99%

Three Distinct Torsion Profiles of Electronic Transmission through Linear Carbon Wires

2020

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“…Refs. 45,[49][50][51] Yet, these cases so far have been limited to aliphatic compounds, 51 and the past success of atom-to-atom pathway analysis for describing and elucidating IET spectra and single-molecule conductance properties of conjugated systems as studied here suggests that our conclusion is reasonable, and that if a correlation between electron transport pathways and IET spectra for molecules with delocalized vibrational modes can be made, it is at least not straightforward.…”

mentioning

confidence: 78%

“…[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Circular pathways are closely linked with destructive quantum interference in molecular junctions, [39][40][41] and engineering pathways can be employed as a means of manipulating the transport properties of molecular systems. 42 While a number of theoretical approaches exist for extracting pathway information, 9,14,15,20,39,[43][44][45][46][47][48][49][50][51] access by experimental methods is limited. An experimental technique which is able to provide pathway information is inelastic tunneling spectroscopy (IETS), 1,[52][53][54][55][56] in which signatures of tunneling electrons exciting molecular vibrations are recorded.…”

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confidence: 99%

The Limits of Inelastic Tunneling Spectroscopy for Identifying Transport Pathways

Deffner¹,

Herrmann²

2020

Preprint

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Inelastic Electron Tunneling Spectroscopy (IETS) is a powerful tool to study the properties of molecular junctions. In particular, it is considered useful for extracting information on electron transport pathways. We explore the limits of this approach by comparing computed interatomic transmission pathways with IETS intensities for different molecular junctions, employing a new efficient implementation for evaluating IETS intensities via the mode-tracking algorithm. We find that while a correlation be- tween pathways and IETS intensities indeed holds when vibrations are clearly localized on atoms off the transport pathway, there is no such correlation for molecules with less localized vibrations, even if transport pathways only sample part of the molecule, and even if a statistical analysis over the vibrational modes is made. This could indicate that the significance of IETS signals for transport pathways is limited to molecules with localized vibrational modes.

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“…For example, if a fraction η of a charge q oscillating along the z-axis over a distance 2a lacks a corresponding current, the predicted maximum longitudinal field (along z) is E L,max = 2ηω 2 qa/(c 2 r). It is tempting to speculate that this kind of oscillations could happen in graphene-based nanojunctions or carbon nanowires [21,22,[44][45][46].…”

Section: Discussionmentioning

confidence: 99%

Are Current Discontinuities in Molecular Devices Experimentally Observable?

Minotti

Modanese

2021

Symmetry

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An ongoing debate in the first-principles description of conduction in molecular devices concerns the correct definition of current in the presence of non-local potentials. If the physical current density ??j=(−ieℏ/2m)(Ψ*∇Ψ−Ψ∇Ψ*) is not locally conserved but can be re-adjusted by a non-local term, which current should be regarded as real? Situations of this kind have been studied for example, for currents in saturated chains of alkanes, silanes and germanes, and in linear carbon wires. We prove that in any case the extended Maxwell equations by Aharonov-Bohm give the e.m. field generated by such currents without any ambiguity. In fact, the wave equations have the same source terms as in Maxwell theory, but the local non-conservation of charge leads to longitudinal radiative contributions of E, as well as to additional transverse radiative terms in both E and B. For an oscillating dipole we show that the radiated electrical field has a longitudinal component proportional to ωP^, where P^ is the anomalous moment ∫I^(x)xd3x and I^ is the space-dependent part of the anomaly ?I=∂tρ+∇·j. For example, if a fraction η of a charge q oscillating over a distance 2a lacks a corresponding current, the predicted maximum longitudinal field (along the oscillation axis) is EL,max=2ηω2qa/(c2r). In the case of a stationary current in a molecular device, a failure of local current conservation causes a “missing field” effect that can be experimentally observable, especially if its entity depends on the total current; in this case one should observe at a fixed position changes in the ratio B/i in dependence on i, in contrast with the standard Maxwell equations. The missing field effect is confirmed by numerical solutions of the extended equations, which also show the spatial distribution of the non-local term in the current.

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Imaging Time-Dependent Electronic Currents through a Graphene-Based Nanojunction

Cited by 22 publications

References 63 publications

Three Distinct Torsion Profiles of Electronic Transmission through Linear Carbon Wires

Three Distinct Torsion Profiles of Electronic Transmission through Linear Carbon Wires

The Limits of Inelastic Tunneling Spectroscopy for Identifying Transport Pathways

Are Current Discontinuities in Molecular Devices Experimentally Observable?

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