Design and control of electron transport properties of single molecules

Abstract

We demonstrate in this joint experimental and theoretical study how one can alter electron transport behavior of a single melamine molecule adsorbed on a Cu (100) surface by performing a sequence of elegantly devised and well-controlled single molecular chemical processes. It is found that with a dehydrogenation reaction, the melamine molecule becomes firmly bonded onto the Cu surface and acts as a normal conductor controlled by elastic electron tunneling. A current-induced hydrogen tautomerization process results in an asymmetric melamine tautomer, which in turn leads to a significant rectifying effect. Furthermore, by switching on inelastic multielectron scattering processes, mechanical oscillations of an N-H bond between two configurations of the asymmetric tautomer can be triggered with tuneable frequency. Collectively, this designed molecule exhibits rectifying and switching functions simultaneously over a wide range of external voltage.hydrogen tautomerization ͉ melamine molecules ͉ rectifying effect ͉ switching property E lectron transport is a fundamental process that controls physical properties and chemical activities of molecular and biological systems. Over the years, different electron transport behaviors of a variety of molecules have been observed, and much effort has been made to elucidate the underlying mechanisms (1-12). Despite of these recent advances, it remains a great challenge to actively control the electron transport in a molecule and to systematically change its behavior from one type to another since this requires not only precise control of molecular structure, but also accurate activation of different electron tunneling processes. Controlling electron transport at the molecular level has important consequences for many applications, such as molecular electronics (13-26), biosensors (27), and solar cells (28,29). For certain molecules, change of electron transport properties could take place due to their specific response to the change of molecular conformation or orientation (19-25), chemical reactions (26), and tautomerization (18). The latter experiment (18) has attracted considerable attention owning to the facts that the switching process involved does not result in drastic molecular conformation changes as often occurring in mechanical molecular switches induced by cis-trans isomerization of azobenzene (23-25), making the process potentially more relevant to applications in memory devices.Although many studies have been conducted over the years, only a limited number of special molecules can be chosen for such experiments. In this joint experimental and theoretical study, we demonstrate the possibility of changing the electron transport behavior of an ordinary molecule, melamine, with the help of surface chemistry and scanning tunneling microscope (STM) in a controllable manner. It is shown that the involvement of a dehydrogenation process can make the molecule standing on a Cu (100) surface and behaving like a conducting molecule. By applying a high-voltage pulse, the energeti...

Ab initiotheory for current-induced molecular switching: Melamine on Cu(001)

“…In addition to the C1/C2 switching, the stability of the C1 and C2 tautomers at all measured voltages with respect to switching back to the lowest energy state (GS), i.e., the forbidden tauromerization to the GS, also remains an open question. 5 Here we report on an investigation of the microscopic origin of the switching behavior of melamine on Cu(001) obtained with a combination of ab initio calculations and a resonance model for energy exchange between ions and electrons. In order to evaluate the barrier height of the conformational change at an applied voltage, we apply our newly developed nudged elastic band (NEB) method 6 within the nonequilibrium Green's function (NEGF) formalism.…”

“…Switching between C1 and C2 via rotation of the N-H bond (the barrier is ∼1.0 eV; Ref. 5) is then possible by applying a bias voltage larger than 0.5 V. Although the switching mechanism has been investigated and quantified with a resonance model, ad hoc parameters have often been used to reproduce the experimental rate constants. In such an approach, nonequilibrium electronic effects on the assumed resonant state are not taken into account.…”

“…5 Once a bias pulse of ∼2.2 V is applied to the molecule, the lowest energy structure (GS) changes irreversibly to either the C1 structure (with an energy barrier of ∼1.9 eV; Ref. 5) or the C2 structure by STM-induced tautomerization. Switching between C1 and C2 via rotation of the N-H bond (the barrier is ∼1.0 eV; Ref.…”

“…As a result, some experimental features remain unexplained by the theory, e.g., the noninteger power law dependence of the switching rate on the electric current. 5 These features should be addressed if one wants to design novel efficient molecular switches or tailor specific chemical reactions at a surface. In addition to the C1/C2 switching, the stability of the C1 and C2 tautomers at all measured voltages with respect to switching back to the lowest energy state (GS), i.e., the forbidden tauromerization to the GS, also remains an open question.…”

Manipulation of the electronic structure by reversible dehydrogenation of tetra(p-hydroxyphenyl)porphyrin molecules

“…This is done in many cases by inducing conformational changes in a molecular system via external stimuli such as light 5 , short-range force, an electric field 6 or electron injection, e.g. from a STM tip [7][8][9][10][11][12][13][14] . Another route to achieve a molecular "switch" is in the sense of interconversion between different electronic or magnetic properties by controlled local breaking of chemical bonds via a STM tip, especially dehydrogenation reactions.…”