Jan M. van Ruitenbeek

Quantum properties of atomic-sized conductors

Abstract Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to single atom upon further stretching. For some metals it is even possible to form a chain of individual atoms in this fashion. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. The properties of the contact are then dominantly determined by the nature of this atom. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics. Properties investigated include multiple Andreev reflection, shot noise, conductance quantization, conductance fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum effects show up in the mechanical properties of the contacts, as seen in the force and cohesion energy of the nanowires. We review this research, which has been performed mainly during the past decade, and we discuss the results in the context of related developments.

The signature of chemical valence in the electrical conduction through a single-atom contact

Fabrication of structures at the atomic scale is now possible using state-of-the-art techniques for manipulating individual atoms, and it may become possible to design electrical circuits atom by atom. A prerequisite for successful design is a knowledge of the relationship between the macroscopic electrical characteristics of such circuits and the quantum properties of the individual atoms used as building blocks. As a first step, we show here that the chemical valence determines the conduction properties of the simplest imaginable circuit—a one-atom contact between two metallic banks. The extended quantum states that carry the current from one bank to the other necessarily proceed through the valence orbitals of the constriction atom. It thus seems reasonable to conjecture that the number of current-carrying modes (or ‘channels’) of a one-atom contact is determined by the number of available valence orbitals, and so should strongly differ for metallic elements in different series of the periodic table. We have tested this conjecture using scanning tunnelling microscopy and mechanically controllable break-junction techniques, to obtain atomic-size constrictions for four different metallic elements (Pb, Al, Nb and Au), covering a broad range of valences and orbital structures. Our results demonstrate unambiguously a direct link between valence orbitals and the number of conduction channels in one-atom contacts.

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.

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

Size-dependent magnetisation of Pd clusters and colloids

Abstract Magnetic susceptibility measurements of three different Pd clusters/colloids with diameters ranging from 25–150 A are presented, and a comparison with bulk Pd is made. We observe a dramatic reduction of the susceptibility with particle size, and a corresponding decrease of the temperature dependence of χ. A simple model is presented explaining the reduction as a result of a drop in the local density of states from the center towards the surface of the cluster. The ensuing size dependence of the susceptibility results from the variation with cluster size of both the fraction of surface atoms and the Stoner enhancement factor.

Observing “quantized” conductance steps in silver sulfide: Two parallel resistive switching mechanisms

We demonstrate that it is possible to distinguish two conductance switching mechanisms in silver sulfide devices at room temperature. Experiments were performed using a Ag2S thin film deposited on a wide Ag bottom electrode, which was contacted by the Pt tip of a scanning tunneling microscope. By applying a positive voltage on the silver electrode, the conductance is seen to switch to a state having three orders of magnitude higher conductance, which is related to the formation of a conductive path inside the Ag2S thin film. We argue this to be composed of a metallic silver nanowire accompanied by a modification of the surrounding lattice structure. Metallic silver nanowires decaying after applying a negative voltage allow observing conductance steps in the breaking traces characteristic for atomic-scale contacts, while the lattice structure deformation is revealed by gradual and continuously decreasing conductance traces.