Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

Abstract

The use of single-molecule junctions for various functions constitutes ac entral goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy-level alignment (ELA), that is,the offset potential between the electrode Fermi level and the frontier molecular orbitals.Examples manifesting ELA are rare owingt oe xperimental challenges and the large energy barriers of typical model compounds.I nt his work, singlemolecule junctions of organometallic compounds with five metal centers joined in acollinear fashion were analyzed. The single-molecule i-V scans could be conducted in ar eliable manner,and the E FMO levels were electrochemically accessible. When the electrode Fermi level (E F )i sc lose to the frontier orbitals (E FMO )o ft he bridging molecule,l arger conductance was observed. The smaller j E F ÀE FMO j gap was also derived quantitatively,u nambiguously confirming the ELA. The mechanism is described in terms of atwo-level model involving co-tunneling and sequential tunneling processes.Electron transport is an essential theme across major disciplines and is of utmost importance to the success of molecule-based electronic devices.F or further improving such systems,a nu nderstanding of the interfacial transport processes,which can be analyzed in terms of the energy-level alignment (ELA), is crucial.[1] As mall barrier height (f B ), that is,the difference between the electrode Fermi level (E F ) and the frontier molecular orbitals (E FMO ), is expected to lead to facile transport. Thedegree of ELA for thin-film devices is typically unveiled by ultraviolet photoelectron spectroscopy (UPS).[1] However,f or ultimate miniaturization down to the single-molecule level, the relatively large beam size of UPS renders it unsuitable for probing local structures.M oreover, each single-molecule junction confers only one f B .The lack of systematic changes in f B makes the correlation with electrontransport efficiencydifficult. To this end, E FMO can be shifted with ag ate electrode by the field effect with single-molecule transistors (SMTs).[2] Unfortunately,S MT studies are very limited owing to fabrication difficulties.S canning tunneling microscopy (STM) based break junctions have been shown to be ac onvenient method for creating single-molecule junctions, [3] and the tuning of E F and f B is achievable through electrochemical control (hereafter referred to as EC-STM BJ) by driving the potential of the working electrode (E wk ) against that of the reference electrode. [4][5][6][7][8] Thus far, the effect of f B on the single-molecule conductance has only been proposed sporadically.[6-9] Most EC-STM BJ studies were performed at fixed E wk values and unable to monitor the system upon E F approaching E FMO .EC studies of single-molecule conductance focus mostly on organic redox moieties. [5,[7][8][9][10] Examples of organometallic compounds are rare and limited to those with one [11][12][13] or two [14,15] metal centers,s uch as ferrocenedicarbo...

Improving Gating Efficiency of Electron Transport through Redox‐Active Molecular Junctions with Conjugated Chains

“…Among them, redox‐active molecules could be converted between the oxidized and reduced states upon different electrochemical potentials, and have been widely studied to illustrate the electrochemical gating at single‐molecule level, such as ferrocene, pyrrolo‐tetrathiafulvalene, anthraquinone, viologen, transition metal complex and redox‐active DNA or proteins . Their conductance could switch on as the energy level of electrodes shifted to resonance with molecules.…”

“…While the energy levels of electrodes move close to the energy level of HOMO, typically near equilibrium redox potential, sequential tunneling process (hopping mechanism) could happen. Electron hops onto the redox center through either the reduced or oxidized state of the molecule at a Franck−Condon type transition, which could enhance current flow together with the co‐tunneling process. As the potentials further increase, the molecule become charged state, the energy levels of electrodes move out HOMO and electron transport is predominantly through frontier molecular orbitals like HOMO‐1 again …”

“…The peak value of one‐dimension conductance histograms first slightly increase from 10 −4.00 to 10 −3.90 G 0 as the gate potentials increasing from −0.3 to 0.4 V, and then soar to 10 −3.75 G 0 at 0.5 V, which is near the redox potential of Fc + /Fc in CVs. It is reported that the redox reaction moves or adds electrons from or to the molecules, which makes charge to hop through the oxidized or reduced molecules and sequential tunneling happens under a bias . Thus this process contributes to this switch‐on phenomenon.…”

Anti-resonance features of destructive quantum interference in single-molecule thiophene junctions achieved by electrochemical gating

“…Since the quantum interference in the charge transport through single-molecule junctions is energy dependent, the continuous tuning of the electrode potential offers an ideal strategy for realizing interference-based single-molecule electrochemical transistors (ECTs) [16][17][18][19][20][21][22] , and the high gating efficiency and relatively large gate voltage windows provide opportunities for gating single-molecule junctions between resonances associated with molecular energy levels and anti-resonances associated with DQI 18,23 .…”

Defective Octanuclear Nickel Complex With Pyrazine and Naphthyridine Modulated N 2 (Pyrazin-2-Yl)-N 7 -(2-(Pyrazin-2- Ylamino)-1,8-Naphthyridin-7-Yl)-1,8-Naphthyridine-2,7-Diamine Ligand

“…In the past decade, numerous metal string complexes (also called "Extended Metal Atom Chain complexes" -EMACs) have been synthesized with various metals (Cu, Rh, Co, Ru, Ni, and Cr) and their magnetic and electronic properties extensively studied [1][2][3][4][5][6]. The chemistry of metal string complexes is of great interest in the field of molecular wire development [7]. Metal string complexes are known to be valuable for their interesting magnetic properties, as well as superconductivity [8].…”