Conductance channels of a platform molecule on Au(111) probed with shot noise

Michael Mohr, Torben Jasper-Toennies, Alexander Weismann, Thomas Frederiksen, Aran Garcia-Lekue, Sandra Ulrich, Rainer Herges, and Richard Berndt

Phys. Rev. B 99, 245417 – Published 24 June 2019

Abstract

The shot noise of the current $I$ through junctions to single trioxatriangulenium cations $({TOTA}^{+})$ on Au(111) is measured with a low-temperature scanning tunneling microscope using Au tips. The noise is significantly reduced compared to the Poisson noise power of $2 e I$ and varies linearly with the junction conductance. The data are consistent with electron transmission through a single spin-degenerate transport channel and show that ${TOTA}^{+}$ in a Au contact does not acquire an unpaired electron. Ab initio calculations reproduce the observations and show that the current involves the lowest unoccupied orbital of the molecule and tip states close to the Fermi level.

Received 20 May 2019

DOI:https://doi.org/10.1103/PhysRevB.99.245417

Physics Subject Headings (PhySH)

Adsorbed biomolecules Electrical conductivity Electronic structure of atoms & molecules Noise

Density functional calculations Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Michael Mohr¹, Torben Jasper-Toennies¹, Alexander Weismann¹, Thomas Frederiksen^2,3, Aran Garcia-Lekue^2,3, Sandra Ulrich⁴, Rainer Herges⁴, and Richard Berndt¹

¹Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, D-24098 Kiel, Germany
²Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, E-20018 Donostia-San Sebastián, Spain
³IKERBASQUE, Basque Foundation for Science, E-48013 Bilbao, Spain
⁴Otto-Diels-Institut für Organische Chemie, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany

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Issue

Vol. 99, Iss. 24 — 15 June 2019

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Images

Figure 1
(a) Model of the calculated gas-phase structure of trioxatriangulenium (TOTA). (b) Constant-current STM topograph of a single TOTA molecule, which appears 2.2 Å high at the sample voltage $V = 129$ mV and the tunneling current $I = 28$ pA. A $〈 110 〉$ direction of the Au substrate is indicated by an arrow. (c) Constant-height $d I / d V$ map of the LUMO of a single TOTA molecule recorded at $V = 0.6$ V. (d) Spectrum of the differential conductance $(d I / d V)$ of TOTA recorded above the center of the molecule. The feedback loop was opened at 500 pA and 2 V.
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Figure 2
Measured conductance $G$ of a TOTA junction vs vertical displacement $Δ z$ of the STM tip. The tip was centered above the molecule and a sample voltage $V = 0.129$ V was used. $Δ z = 0$ corresponds to the tip position at $I = 40$ pA. Contact with the same tip to an fcc area of the bare substrate occurs at $\approx 8 Å$ . The data shown were measured during seven approaches and retractions of the tip. Each approach or retraction consisted of overlapping pieces that extended over approximately two decades in conductance. The reproducibility of the experimental result is indicated by the resulting line thickness.
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Figure 3
Noise power $Δ S (I)$ measured on a TOTA molecule. Markers show experimental data with uncertainty margins. The uncertainties were obtained from 400 measurements between 100 and 120 kHz. The line is the result of a fit $Δ S = F [S_{0} coth (S_{0} / S_{θ}) - S_{θ}]$ . The upper axis shows junction voltages.
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Figure 4
Experimental Fano factors $F$ (red circles) and corresponding channel transmissions $τ_{1}$ (blue diamonds) and $τ_{2}$ (gray diamonds) vs the conductances $G$ of trioxatriangulenium junctions. The data were measured from seven different contacts with possibly different atomic-scale structures of the tips. A dotted line shows the Fano factors expected from a model assuming a single spin degenerate transport channel $(1 - G / G_{0})$ . Uncertainty margins are defined as 95% confidence intervals. The uncertainties of $G$ are negligible, i.e., smaller than the marker sizes.
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Figure 5
Transport simulations for a TOTA junction. (a) Supercell containing one TOTA molecule on $6 \times 6$ Au(111) surface. Contact with a Au tip is through a bond to the central carbon atom. (b) Projected density of states onto the atoms of the tip and the molecule. Black, red, and green curves correspond to decreasing tip heights (steps of 0.5 Å) in the contact regime. The dashed blue curve corresponds to TOTA relaxed on the surface without interactions with the tip, revealing the LUMO resonance close to the Fermi level. (c) Electron transmission functions $T (E)$ and (d) Fano factors along with the $k$ -averaged channel transmissions $τ_{1}$ and $τ_{2}$ for the same three contact geometries.
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