Christian A. Martin

Fullerene-based anchoring groups for molecular electronics.

We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum we have determined the conductance and stability of single-molecule junctions of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and has a low-bias conductance of 3 x 10(-4) G0. Compared to 1,4-benzenedithiol the fullerene-anchored molecule exhibits a considerably lower conductance spread. In addition, the signature of the new compound in histograms is more significant than that of 1,4-benzenediamine, probably owing to a more stable adsorption motif. Statistical analyses of the breaking of the junctions confirm the stability of the fullerene-gold bond.

Large tunable image-charge effects in single-molecule junctions

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metals Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

Lithographic mechanical break junctions for single-molecule measurements in vacuum: possibilities and limitations

We have investigated electrical transport through the molecular model systems benzenedithiol, benzenediamine, hexanedithiol and hexanediamine. Conductance histograms under different experimental conditions indicate that measurements using mechanically controllable break junctions in vacuum are limited by the surface density of molecules at the contact. Hexanedithiol histograms typically exhibit a broad peak around 7?10?4?G0. In contrast to recent results on scanning tunnelling microscope (STM) based break junctions in solution we find that the spread in single-molecule conductance is not reduced by amino anchoring groups. Histograms of hexanediamine exhibit a wide peak around 4?10?4?G0. For both benzenedithiol and benzenediamine we observe a large variability in low-bias conductance. We attribute these features to the slow breaking of the lithographic mechanically controllable break junctions and the absence of a solvent that may enable molecular readsorption after bond breaking. Nevertheless, we have been able to acquire reproducible current?voltage (I?V) characteristics of benzenediamine and benzenedithiol using a statistical measurement approach. Benzenedithiol measurements yield a conductance gap of about 0.9?V at room temperature and 0.6?V at 77?K. In contrast, the I?V characteristics of benzenediamine-junctions typically display conductance gaps of about 0.9?V at both temperatures.

Transition voltage spectroscopy and the nature of vacuum tunneling.

Transition voltage spectroscopy (TVS) has been proposed as a tool to analyze charge transport through molecular junctions. We extend TVS to Au-vacuum-Au junctions and study the distance dependence of the transition voltage V(t)(d) for clean electrodes in cryogenic vacuum. On the one hand, this allows us to provide an important reference for V(t)(d) measurements on molecular junctions. On the other hand, we show that TVS forms a simple and powerful test for vacuum tunneling models.

A nanoelectromechanical single-atom switch.

We have exploited the electromechanical properties of gated mechanical break junctions to form single-atom relays. The gate voltage can be used to reversibly switch between a monatomic contact with a conductance around 2e(2)/h and the tunneling regime. In tunneling, the source-drain conductance varies smoothly with gate voltage. The characteristics of the devices can be understood within a simple continuum model. It indicates that the elastic properties of the substrate facilitate the electromechanical tuning and that the details of the switching depend sensitively on the nanoscale geometry of the electrode tips.

Sandwich-type gated mechanical break junctions

We introduce a new device architecture for the independent mechanical and electrostatic tuning of nanoscale charge transport. In contrast to previous gated mechanical break junctions with suspended source-drain electrodes, the devices presented here prevent an electromechanical tuning of the electrode gap by the gate. This significant improvement originates from a direct deposition of the source and the drain electrodes on the gate dielectric. The plasma-enhanced native oxide on the aluminum gate electrode enables measurements at gate voltages up to 1.8 V at cryogenic temperatures. Throughout the bending-controlled tuning of the source-drain distance, the electrical continuity of the gate electrode is maintained. A nanoscale island in the Coulomb blockade regime serves as a first experimental test system for the devices, in which the mechanical and electrical control of charge transport is demonstrated.

Influence of the Chemical Structure on the Stability and Conductance of Porphyrin Single‐Molecule Junctions

The use of porphyrin molecules as building blocks of functional molecular devices has been widely investigated. The structural flexibility and well-developed synthetic chemistry of porphyrins allows their physical and chemical properties to be tailored by choosing from a wide library of macrocycle substituents and central metal atoms. Nature itself offers magnificent examples of processes that utilize porphyrin derivatives, such as the activation and the transport of molecular oxygen in mammals and the harvesting of sunlight in plant photosynthetic systems. In order to exploit the highly desirable functionality of porphyrins in artificial molecular devices, it is imperative to understand and control the interactions that occur at the molecule–substrate interface. Such interactions largely depend on the electronic and conformational structures of the adsorbed molecules, which can be studied using techniques such as scanning tunneling microscopy, UV photoemission spectroscopy, and X-ray photoemission spectroscopy, and on a theoretical level with density functional calculations. Recent studies on conjugated rod-like molecules have shown that molecular conductance measurements can be significantly affected by the binding geometry, coupling of the p orbitals to the leads, or p–p stacking between adjacent molecules. Herein, we present the results of a study of the interaction of laterally extended p-conjugated porphyrin molecules with electrodes by means of timeand stretching-dependent conductance measurements on molecular junctions. We further investigate strategies to reduce interactions of the molecular p electrons with the metal electrodes by modifying the chemical structure of the porphyrin molecules. We used the series of molecules represented in Figure 1a– c to examine the influence of the molecular structure on the formation of porphyrin single-molecule junctions. Since the thiol group is most commonly used to contact rod-like molecules to form straight molecular bridges, we first compared 5,10,15,20-tetraphenylporphyrin without thiol termination (H2-TPP; Figure 1a) to a nearly identical molecule with two thiol groups on opposite sides of the molecule (5,15di(p-thiophenyl)-10,20-di(p-tolyl)porphyrin (H2-TPPdT); Figure 1b). To investigate the influence of the molecular backbone geometry on the junction formation we further studied a thiol-terminated porphyrin molecule with two bulky pyridine axial groups attached through an octahedral Ru ion ([Ru{5,15-di(p-thiophenyl)-10,20-diphenylporphyrin}(py)2] (Ru-TPPdT); Figure 1c). As a consequence of steric hindrance, the pyridine groups in Ru-TPPdT reduce the direct interaction of the metal electrodes with the p face of the porphyrin. A similar strategy was used previously. Prior to electrical characterization, the molecules were deposited using self-assembly from solution. To study the conductance of these molecules we used lithographic mechanically controllable break junctions (MCBJs) in vacuum at room temperature. The layout of an MCBJ device in a threepoint bending mechanism is shown in Figure 1d. Details concerning the synthesis of the molecules and the experimental procedures are given in the Supporting Information. Sets of 1000 consecutive breaking traces from individual junctions were analyzed numerically to construct “trace histograms” of the conductance (log10G versus the electrode displacement d). This statistical method maps the breaking dynamics of the junctions beyond the point of rupture of the last monatomic gold contact (defined as d= 0), which has a conductance of one quantum unit G0= 2e h. Areas of high counts represent the most typical breaking behavior of the molecular junctions. Figure 2 presents trace histograms as well as examples of individual breaking traces for acetone as reference, H2-TPP, H2-TPPdT, and Ru-TPPdT. For all three porphyrin molecules as well as for the reference sample several junctions were measured (see the Supporting Information). Herein, we only show a typical set of junctions. In the junction which was exposed to pure acetone (Figure 2a), the Au bridge initially gets stretched until a plateau around the conductance quantum (G G0) is observed (only visible in the individual traces shown in [*] M. L. Perrin, F. Prins, Dr. C. A. Martin, Prof. Dr. H. S. J. van der Zant, Dr. D. Dulic Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft (The Netherlands) E-mail: d.dulic@tudelft.nl

Driving change in the battery industry.

High-capacity silicon anodes could improve the performance of lithium-ion batteries for electric vehicles, but their cyclability has been limited. Christian Martin analyses recent progress in nanoscale engineering that addresses this shortcoming.

A versatile low-temperature setup for the electrical characterization of single-molecule junctions

We present a modular high-vacuum setup for the electrical characterization of single molecules down to liquid helium temperatures. The experimental design is based on microfabricated mechanically controllable break junctions, which offer control over the distance of two electrodes via the bending of a flexible substrate. The actuator part of the setup is divided into two stages. The slow stage is based on a differential screw drive with a large bending range. An amplified piezoceramic actuator forms the fast stage of the setup, which can operate at bending speeds of up to 800 μm/s. In our microfabricated break junctions this is translated into breaking speeds of several 10 nm/s, sufficient for the fast acquisition of large statistical datasets. The bandwidth of the measurement electronics has been optimized to enable fast dI/dV spectroscopy on molecular junctions with resistances up to 100 MΩ. The performance of the setup is demonstrated for a π-conjugated oligo(phenylene-ethynylene)-dithiol molecule.

Charge transport in a zinc–porphyrin single-molecule junction

Summary We have investigated charge transport in ZnTPPdT–Pyr (TPPdT: 5,15-di(p-thiolphenyl)-10,20-di(p-tolyl)porphyrin) molecular junctions using the lithographic mechanically controllable break-junction (MCBJ) technique at room temperature and cryogenic temperature (6 K). We combined low-bias statistical measurements with spectroscopy of the molecular levels in the form of I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, having an observable effect on the trace histograms and the current–voltage (I(V)) measurements. Both methods show that multiple, stable single-molecule junction configurations can be obtained by modulating the interelectrode distance. In addition we demonstrate that different ZnTPPdT–Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.