Interplay of Fermi Level Pinning, Marcus Inverted Transport, and Orbital Gating in Molecular Tunneling Junctions

Abstract

This Letter examines the interplay of important tunneling mechanismsFermi level pinning, Marcus inverted transport, and orbital gatingin a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2′-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus–Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

Interstitially Mixed Self-Assembled Monolayers Enhance Electrical Stability of Molecular Junctions

“…The Supporting Information contains histograms of log| J ( V )| and log| r | (Figures S17 and S18). The pure SC 11 BIPY SAM presents a high rectification ratio, log| r + | mean = 2.0 at ±1.0 V. ,,, Whereas as-prepared SC 8 –SC 11 BIPY mixed SAM (i.e., that formed via the traditional coadsorption method) showed a reduced log| r + | value (log| r + | mean = 1.7), the imSAM 2nd retained a high rectification ratio (log| r + | mean = 2.0) (Figure a). This is rather surprising given that a considerable amount of nonrectifying SC 8 molecules (∼40% according to the XPS analysis shown in Figure b) existed in the imSAM 2nd .…”

Modulated Rectification of Carboxylate-Terminated Self-Assembled Monolayer Junction by Humidity and Alkali Metal Ions: The Coupling and Asymmetric Factor Matter

“…Meanwhile, it seems that the electroactive state responsible for rectification is pinned in the junction, irrespective of the K + /Na + counterion or water. Though the pinning of the energy level at the molecule–electrode interface has been well documented − and shown to be pivotal in determining the properties of molecular devices, ,,− its origin at COOX–water–GaO x /GaIn is yet to be clarified due to the lack of the detailed electronic structure at the interface, especially the ill-defined GaO x layer. The molecule–GaO x /GaIn interface, therefore, is indeed a great challenge and a great opportunity that deserves further exploration for controllable molecular devices.…”

“…We placed the sample on the sample stage and connected the grounded Au surface to the negative port of the source meter (6430 Sub-Femtoamp Remote SourceMeter, Keithley). The eutectic gallium-indium (EGaIn, 75.5% Ga and 24.5% In) in a microinjector was used as the top electrode and connected to the port of the source meter, which is a common method to build a molecular junction. ,,− The liquid metal forms a GaO x (average thickness of 0.7 nm) oxide layer when exposed to air. − We used a syringe to squeeze out the EGaIn droplet and brought it in contact with a clean gold surface and then slowly moved the syringe upward to form a conical EGaIn tip, followed by gently making contact with the samples (contact area ∼ 300 μm 2 ). A bias voltage was applied to the EGaIn tip, the J – V curve was scanned from 0 → +1.5 → −1.5 → 0 V with a step size of 200 mV, and the delay time between each step of applying the bias voltage was 0.08 s. For each new tip, we formed three junctions in 3–4 positions on one sample.…”

On the theory of charge transport and entropic effects in solvated molecular junctions

“…38,39 This inverted region has been recently observed experimentally in charge transport through molecular junctions. 32,40 Therefore, in the last part of this work, we demonstrate how lifetime broadening can be incorporated into the Marcus-Levich-Dogonadze-Jortnertype description of molecular conduction.…”