Christian Joachim

Conductance of a Single Conjugated Polymer as a Continuous Function of Its Length

The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching.

Controlled Room-Temperature Positioning of Individual Molecules: Molecular Flexure and Motion

Two-dimensional positioning of intact individual molecules was achieved at room temperature by a controlled lateral “pushing” action of the tip of a scanning tunneling microscope. To facilitate this process, four bulky hydrocarbon groups were attached to a rigid molecule. These groups maintained sufficiently strong interactions with the surface to prevent thermally activated diffusional motion, but nevertheless allowed controllable translation. Simulations demonstrated the crucial role of flexure during the positioning process. These results outline the key role of molecular mechanics in the engineering of predefined properties on a molecular scale.

Properties of large organic molecules on metal surfaces

Abstract The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.

Calculation of the benzene on rhodium STM images

Abstract Avoiding the Bardeen—Tersoff—Hamann approximation we present calculations of the scanning tunneling microscope (STM) image of benzene or Rh(111). The tunneling current between the tip and the substrate through the benzene is calculated from the generalized Landauer formula. The electronic structure of the complete system made of the tip, of the substrate and of the benzene adsorbate is obtained with its molecular orbitals from an extended Huckel molecular orbital calculation. Experimental and calculated images are in good agreement. This leads to a detailed analysis of the image using the molecular orbital approach in through-space and through-bond tunneling processes.

Voltage-dependent conductance of a single graphene nanoribbon

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry.

An electromechanical amplifier using a single molecule

We describe the first single-molecule electromechanical amplifier, the active element of which is one fullerene molecule (C60) with a junction length and width of ∼ 1 nm. Forces in the nano-Newton range are generated by a metallic tip pressing on the molecule in its adsorbed state. This working nanodevice is based on the modulation of virtual resonance tunneling through the molecule by electromechanical deformation of the C60 cage.

Controlled clockwise and anticlockwise rotational switching of a molecular motor

The design of artificial molecular machines often takes inspiration from macroscopic machines. However, the parallels between the two systems are often only superficial, because most molecular machines are governed by quantum processes. Previously, rotary molecular motors powered by light and chemical energy have been developed. In electrically driven motors, tunnelling electrons from the tip of a scanning tunnelling microscope have been used to drive the rotation of a simple rotor in a single direction and to move a four-wheeled molecule across a surface. Here, we show that a stand-alone molecular motor adsorbed on a gold surface can be made to rotate in a clockwise or anticlockwise direction by selective inelastic electron tunnelling through different subunits of the motor. Our motor is composed of a tripodal stator for vertical positioning, a five-arm rotor for controlled rotations, and a ruthenium atomic ball bearing connecting the static and rotational parts. The directional rotation arises from sawtooth-like rotational potentials, which are solely determined by the internal molecular structure and are independent of the surface adsorption site.

Molecular wires: guiding the super-exchange interactions between two electrodes

After discussing experimental works on the measurement of the conductance of a metal–molecule–metal tunnel junction with many down to a few molecules and down to only one molecule in the junction, we propose a comprehensive way of understanding the electron transport phenomenon through such a junction. The dependence of the junction conductance on the length of the molecule(s) is explained starting from the quantum super-exchange electron transfer phenomenon up to the effective mass of the tunnelling electrons in the coherent limit. This super-exchange mechanism results from the electronic coupling between the two electrodes introduced by the molecule(s). The molecular wire guides this interaction better than the electronic coupling through vacuum between the two electrodes of the junction. Dephasing and thermal effects during the electron transfer events along the molecular wire are described using a density matrix formalism. The implication of our understanding of this through junction electronic transport is described starting from hybrid molecular electronics towards mono-molecular electronics.

Molecular ‘OR’ and ‘AND’ logic gates integrated in a single molecule

Based on the N electrodes elastic scattering quantum chemistry (NESQC) technique, an intramolecular circuit simulator is presented for the design of electronic logic functions integrated inside a single molecule interconnected to the N electrodes. Using molecular rectifier groups, a molecule-OR and a molecule-AND are designed, their current-voltage characteristics calculated and their logic response presented. Both the OR and AND molecules have approximatively the targeted function. The running current of the OR gate, 10 fA, is quite low and the AND gate works only in an output voltage mode. This forbids the design of larger logic functions inside a single molecule with molecular rectifiers.

Step-by-step rotation of a molecule-gear mounted on an atomic-scale axis

Gears are microfabricated down to diameters of a few micrometres. Natural macromolecular motors, of tens of nanometres in diameter, also show gear effects. At a smaller scale, the random rotation of a single-molecule rotor encaged in a molecular stator has been observed, demonstrating that a single molecule can be rotated with the tip of a scanning tunnelling microscope (STM). A self-assembled rack-and-pinion molecular machine where the STM tip apex is the rotation axis of the pinion was also tested. Here, we present the mechanics of an intentionally constructed molecule-gear on a Au(111) surface, mounting and centring one hexa-t-butyl-pyrimidopentaphenylbenzene molecule on one atom axis. The combination of molecular design, molecular manipulation and surface atomic structure selection leads to the construction of a fundamental component of a planar single-molecule mechanical machine. The rotation of our molecule-gear is step-by-step and totally under control, demonstrating nine stable stations in both directions.