Gating the Charge State of Single Molecules by Local Electric Fields

Abstract

The electron acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer-complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence/absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation/reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.A goal of molecular electronics is to use single molecules as transistors [1][2][3][4]. To achieve this paradigm a high degree of charge localization in the device is required [5]. Charge localization favors that a molecular device can only change its electron occupation in integer numbers, leading to discrete changes of its transconductance [6,7]. However, a molecule contacted by metal electrodes generally shows non-integer charge states, in which the electron density is redistributed throughout the bond with the leads [5,8]. To achieve a large degree of charge localization, the interaction of the molecule with the metal leads has to be sufficiently weak. This is best achieved by introducing an "effective tunneling barrier" to electronically decouple molecular states from the metal. Several STM studies on single molecules adsorbed on atomically clean and well-structured metal surfaces have shown that this is achievable using thin insulating layers (ionic or oxide thin films) separating the molecule from the surface [9-12, 18, 19]. A comparable level of decoupling has been observed when combining two different molecules in a monolayer directly adsorbed on a metal surface [13,14]. In this case, the proximity of the metal surface screens more efficiently charges localized at the molecule, leading to a reduction of Coulomb charging energy U and, hence, of the energy cost to change the electron occupation of the molecule [15]. Here, we find a similar degree of charge localization on the electron acceptor molecule tetracyanoquinodimethane (TCNQ) mixed with the electron donor tetramethyl-tetrathiafulvalene (TMTTF) in a stoichiometrically ordered monolayer. The methylation of the TTF backbone reduces the interaction of the molecular ad-layer with the metal substrate in comparison to the parent compound TTF-TCNQ [14,16], while maintaining ground states with integer charge occupation.A second requirement for molecules acting as electronic building blocks is the tunability of their charge state which would allow us to control the molecular transconductance [17]. To change the charge state one needs an external handle (i.e. a potential) that shifts the alignment of molecular levels around the leads' chemical potential. A successful approach consists in using the local electric field induced by the proximity of a metal STM tip to induce such shift [20][21][22][23][24]. In this case, the critical field f...

Forces from periodic charging of adsorbed molecules

“…1(b)). From these and other characteristic observations being analogous to several previous works, [70][71][72][73][74][75] it could be concluded that the molecules are charged in the field of the scanning-probe junction. It was concluded further that without applied bias a localized state of the molecule was close enough to the substrate's Fermi level, such that at a given threshold electric field the localized state can be shifted across the Fermi level of the substrate, which will change the (average) occupation of the state and hence the charge state.…”

Controlling the Charge State and Redox Properties of Supported Polyoxometalates via Soft Landing of Mass-Selected Ions

“…1-4 Control over the charge state of atoms and molecules on surfaces have been achieved experimentally [2][3][4][5][6][7][8][9] and examined theoretically. [10][11] For example, the charging/discharging of nanoparticles on substrates for use as efficient nanoscale memory devices, [12][13] selective adsorption of CO and NH 3 by mass-selected molybdenum sulfide cations on gold surfaces 14 as well as the improved catalytic activity towards the oxidation of adsorbed CO molecules by partially charged gold clusters on metal oxide supports, 1, 15 illustrate the wide range of possibilities.…”

Scanning Quantum Dot Microscopy

“…In the experiment, the electronic levels of the QD are gated with respect to the Fermi level E F of the tip by applying a bias voltage to the tip-surface junction [ Fig. 1 [6][7][8][9][10]. In this way, the charge state of the QD can be changed, e.g., if the bias voltage V applied to the junction reaches a critical value V − that aligns one of the QD's occupied electronic levels with E F , this level is depopulated [ Fig.…”