Maria Anita Rampi

Light‐Powered Electrical Switch Based on Cargo‐Lifting Azobenzene Monolayers

Inspired by the complex molecular machines found in nature, chemists have developed much simpler molecular motors. Among them, several systems incorporating azobenzene have been proposed, which exploit the reversible trans–cis isomerization triggered by light or an electric field for applications such as optical data-storage devices, switchable supramolecular cavities, and sensors. Recently, it has been demonstrated that the photoisomerization process of individual polymer chains incorporating azobenzenes can express mechanical work. In light of these findings, one can foresee self-assembled monolayers (SAMs) of aromatic azobenzenes as molecular systems able to express forces of unprecedented magnitude by exploiting a collective subnanometer structural change. We recently designed a rigid and fully conjugated azobenzene exposing a thiol anchoring group, which was able to form a tightly packed SAM on Au(111) (SAMAZO). Scanning tunneling microscopy (STM) studies revealed that upon light irradiation of the chemisorbed SAMs, a collective isomerization of entire molecular-crystalline domains occurred with an outstandingly high directionality. Based on these results, a cooperative nature of the isomerization of adjacent AZO molecules has been proposed. Furthermore, the joint action of the molecules in the SAM provides an ideal system as a potential “cargo” lifter. Herein, we show that, upon irradiation, azobenzene SAMs incorporated in a junction between an Au(111) surface and a mercury drop are able to 1) lift the “heavy” Hg drop, and 2) reversibly photoswitch the current flowing through the junction (Figure 1). Current–voltage (I–V) characteristics averaged over more than 30 junctions incorporating AZO SAMs in the trans and the cis conformations are shown in Figure 2a. The SAMAZO in the cis conformation was obtained with extremely high yield (98%) upon irradiation by UV light of the SAMAZO initially formed by the trans conformer. The difference in the measured currents, which amounts to about 1.4 orders of magnitude, is in agreement with a through-bond tunneling mechanism described by Equation (1).

Cooperative light-induced molecular movements of highly ordered azobenzene self-assembled monolayers

Photochromic systems can convert light energy into mechanical energy, thus they can be used as building blocks for the fabrication of prototypes of molecular devices that are based on the photomechanical effect. Hitherto a controlled photochromic switch on surfaces has been achieved either on isolated chromophores or within assemblies of randomly arranged molecules. Here we show by scanning tunneling microscopy imaging the photochemical switching of a new terminally thiolated azobiphenyl rigid rod molecule. Interestingly, the switching of entire molecular 2D crystalline domains is observed, which is ruled by the interactions between nearest neighbors. This observation of azobenzene-based systems displaying collective switching might be of interest for applications in high-density data storage.

Alkanethiol self-assembled monolayers as the dielectric of capacitors with nanoscale thickness

Alkanethiol self-assembled monolayers (SAMs) on a mercury surface are used to build a junction consisting of two opposing mercury surfaces with interposed SAMs: Hg-SAM/SAM-Hg. The liquid mercury surface provides a support for the SAM that is smooth, compliant, free of defects, and without the incommensurate lattice properties that characterize solid metal surfaces. The thickness of the dielectric (∼30–90 A) in this junction can be easily changed by using alkanethiols with different lengths. From capacitance measurements, a dielectric constant of 2.7±0.3 is calculated for the SAMs. The conductivity of SAMs on the Hg surface is σ=6±2×10−15 Ω−1 cm−1, a value close to that of bulk polyethylene. The junction sustains an electric field of 6 MV/cm.

Azobenzenes as Light-Controlled Molecular Electronic Switches in Nanoscale Metal−Molecule−Metal Junctions

Conductance switching associated with the photoisomerization of azobenzene-based (Azo) molecules was observed in nanoscopic metal-molecule-metal junctions. The junctions were formed by using a conducting atomic force microscope (C-AFM) approach, where a metallic AFM tip was used to electrically contact a gold-supported Azo self-assembled monolayer. The measured 30-fold increase in conductance is consistent with the expected decrease in tunneling barrier length resulting from the conformational change of the Azo molecule.

A versatile experimental approach for understanding electron transport through organic materials

This paper describes an experimentally simple method for assembling junctions with nanometer-scale, structured organic films positioned between two metal electrodes. These junctions comprise two metal electrodes that sandwich two self-assembled monolayers (SAMs) – that is, metal (mercury)–SAM//SAM–metal (mercury, gold or silver) junctions. The junctions are easy to assemble (because the mercury electrode is compliant) and they are compatible with SAMs incorporating organic groups having a range of structures. This paper describes three different variations on this type of Hg-based junction. The first junction, formed by two contacting mercury drops covered by the same type of SAM, is a prototype system that provided useful information on the structure and electrical properties of the Hg-based junctions. The second junction consists of a Hg drop covered by one SAM (Hg–SAM(1)) in contact with a second SAM supported on a silver film (Ag–SAM(2)) – that is, a Hg–SAM(1)//SAM(2)–Ag junction. This junction allowed systematic measurements of the current that flowed across SAM(2), as a function of structure (for example, using aliphatic or aromatic thiols of different length), and a common SAM(1) of hexadecane thiol. The current density follows the

The study of charge transport through organic thin films: mechanism, tools and applications

In this paper, we discuss the current state of organic and molecular-scale electronics, some experimental methods used to characterize charge transport through molecular junctions and some theoretical models (superexchange and barrier tunnelling models) used to explain experimental results. Junctions incorporating self-assembled monolayers of organic molecules—and, in particular, junctions with mercury-drop electrodes—are described in detail, as are the issues of irreproducibility associated with such junctions (due, in part, to defects at the metal–molecule interface).

Redox Site-Mediated Charge Transport in a Hg−SAM//Ru(NH3)63+/2+//SAM−Hg Junction with a Dynamic Interelectrode Separation: Compatibility with Redox Cycling and Electron Hopping Mechanisms

This paper describes the formation and electrical properties of a new Hg-based metal-molecules-metal junction that incorporates charged redox sites into the space between the electrodes. The junction is formed by bringing into contact two mercury-drop electrodes whose surfaces are covered by COO(-)-terminated self-assembled monolayers (SAMs) and immersed in a basic aqueous solution of Ru(NH(3))(6)Cl(3). The electrical behavior of the junction, which is contacted at its edges by aqueous electrolyte solution, has been characterized electrochemically. This characterization shows that current flowing through the junction on the initial potential cycles is dominated by a redox-cycling mechanism and that the rates of electron transport can be controlled by controlling the potentials of the mercury electrodes with respect to the redox potential of the Ru(NH(3))(6)(3+/2+) couple. On repeated cycling of the potential across the junction, the current across it increases by as much as a factor of 40, and this increase is accompanied by a large (>300 mV) negative shift in the formal potential for the reduction of Ru(NH(3))(6)(3+). The most plausible rationalization of this behavior postulates a decrease in the size of the gap between the electrodes with cycling and a mechanism of conduction dominated by physical diffusion of Ru(NH(3))(6)(3+/2+) ions (at larger interelectrode spacing), with a possible contribution of electron hopping to charge transport (at smaller interelectrode spacing). In this rationalization, the negative shift in the formal potential plausibly reflects extrusion of the solution of electrolyte from the junction and an increase in the effective concentration of negatively charged species (surface-immobilized COO(-) groups) in the volume bounded by the electrodes. This junction has the characteristics required for use in screening and in exploratory work, involving nanogap electrochemical systems, and in mechanistic studies involving these systems. It does not have the stability needed for long-term technological applications.

Gating current flowing through molecules in metal–molecules–metal junctions

We have assembled two junctions that incorporate redox sites between Hg electrodes by different interactions. In the first junction, Hg-SAM-R//R-SAM-Hg, the redox site (R) are covalently linked to each electrode in self assembled monolayers (SAM-R). In the second junction, Hg-SAM//R//SAM-Hg, the redox sites dissolved in solution are trapped by electrostatic interaction at the SAM formed at the electrodes. The current flowing through these junctions can be controlled by adjusting the potential applied at the electrodes with respect to the redox potential of the species by using an electrochemical system. The current flowing in these two junctions is mediated by the redox sites through different mechanisms. In particular, the current flowing through the Hg-SAM-R//R-SAM-Hg junction occurs through a self exchange mechanism between the redox sites organized at each electrode, while the current flowing through the Hg-SAM//R//SAM-Hg junction is dominated by a redox-cycling mechanism. The systems described here are easy to assemble, well-characterized, yield reproducible data and make it easy to modify the electrical properties of the junctions by changing the nature of the redox centres. For these characteristics they are well suited for collecting fundamental information relevant to the fabrication of molecular switches.

Electron exchange between two electrodes mediated by two electroactive adsorbates.

Experimental data for electron exchange between two electrodes covered by electroactive films are presented and discussed in terms of the Gerischer model. A model Hamiltonian is proposed for such indirect electron exchange involving two intermediate species. Explicit model calculations are performed for the case in which the coupling between the two adsorbates is weak and determines the overall rate. The calculations agree well with the experimental data, and can be used to determine the energy of reorganization associated with the electron transfer.

Ultrarobust Thin-Film Devices from Self-Assembled Metal-Terpyridine Oligomers

Ultrathin molecular layers of Fe(II) -terpyridine oligomers allow the fabrication of large-area crossbar junctions by conventional electrode vapor deposition. The junctions are electrically stable for over 2.5 years and operate over a wide range of temperatures (150-360 K) and voltages (±3 V) due to the high cohesive energy and packing density of the oligomer layer. Electrical measurements reveal ideal Richardson-Shottky emission in surprising agreement with electrochemical, optical, and photoemission data.