Lorentz Center - Transport through Single Molecules from 7 Mar 2005 through 12 Mar 2005
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    Transport through Single Molecules
    from 7 Mar 2005 through 12 Mar 2005




Monday, 7 March 9:15 am.


"Experimental approaches in transport through single molecules"


Herre van der Zant

Delft, Netherlands


There are several methods to contact single molecules. Each of them has its advantages and disadvantages. A reliable method suitable for a large variety of molecules does not exist. Scanning-probe techniques and mechanical break junctions offer mechanical control, but the incorporation of a third, gate electrode is difficult. Three-terminal molecular junctions have been made using standard (e-beam) fabrication techniques. In combination with electro-chemical methods, atomic control over the gap distance has been obtained, but the electrochemistry itself is complex. In the electromigration technique a small wire is broken by passing a large current through it.  This widely used technique has the disadvantage that small gold grains are formed near the gap region masking single molecule behavior. A recent development is the use of nanotube as leads to contact small molecules, but in this case the nanotube itself can add spectroscopic features as well. Despite these shortcomings, many exciting experiments on single molecule transport have been performed. An overview will be given, highlighting the experimental approaches.


Tuesday, 8 March 9:00 am.


"Introduction to the theory of electron transport in molecular systems"


Juan-Carlos Cuevas

Karlsruhe, Germany


With the recent advances in nanofabrication techniques it has become possible to manipulate single atoms and molecules and to investigate the electronic transport through them. This has posed an exciting theoretical challenge, namely the understanding of the conduction mechanisms at the molecular scale. During the last years there has been a great effort to describe quantitatively the electrical conduction in molecular circuits combining ideas coming from different fields such as mesoscopic physics and quantum chemistry. In this tutorial I will review some of the basic concepts and theoretical techniques that are presently used to describe the transport through single-molecule junctions. To be precise, I will discuss at a simple level three main issues:

(i) Description of the elastic current: Landauer formalism, Green's functions and ab initio calculations.

(ii) Role of the correlation effects: Coulomb blockade.

(iii) Inelastic interactions: signatures of the vibration modes in the molecular conduction.


Tuesday, 8 March 2:00 pm.


"What can we learn about a naojunction from current and voltage"


Nicolás Agraït

Madrid, Spain


In this tutorial I will review the wealth of information that can be extracted from the conductance of atomic-sized metal contacts and molecules at low temperatures. The voltage dependence of the conductance (or differential conductance) reflects the energy dependence of elastic and inelastic scattering processes making possible to determine important properties of the junction such as the vibrational modes (related to the structure) and the number and transmission of the individual quantum channels. Additionally, correlations between the electrons give rise to zero-bias anomalies. 


Wednesday, 9 March 9:00 am.


"Carbon Nanotubes Nanocomposites – Quasi 1-Dimensional Structures for Electron Transfer"


Dirk M. Guldi, Jeff Ramey

Universität Erlangen-Nürnberg, Germany

Aminur Raman

University of Notre Dame, USA

Norbert Jux

Universität Erlangen-Nürnberg , Germany

Nikos Tagmatarchis, Maurizio Prato

Università di Trieste, Italy


Nanoscale carbon materials (i.e., fullerenes and nanotubes) are attractive platforms for applications in photovoltaics.  These nanoscale materials have extended, delocalized π-electron systems, which, in combination with photoexcited electron donors, may make them useful for managing charge transfer within novel, ultra-high efficient photoelectrochemical cells for water splitting and reduction of CO2 to fuels.

Single wall carbon nanotubes (SWNT) may also serve as the electron acceptor component in donor-acceptor ensembles, just as fullerenes have been the electron acceptors in much recent research. Notably, the expected electrical conductivity behavior associated with the tubular structure and good chemical stability opens new promising scenarios for their use as “molecular wires” with high surface areas in the design of electro- and photoactive ensembles. However, there are several obstacles in the way of integrating SWNT into functional donor-acceptor constructs and their use in practical applications.  Controlled modification of their surface with multifunctional groups – chromophores, electron donors, biomolecules, etc. – is required to fully realize their potential.

I will discuss recent advances in the design, synthesis, purification, characterization, and examination of the potential for applications of new multifunctional SWNT materials as two- or three-dimensional architectures for high mechanical strength and electron donor-acceptor chemistry. 

For example, we reported recently on the photoinduced electron transfer within a novel single wall carbon nanotube – ferrocene nanohybrid (SWNT-Fc), which yields a long-lived charge-separated (SWNT)•–-(Fc)•+ state.  The presence of SWNT•–, detected by laser flash photolysis, has been confirmed by time-resolved pulse radiolysis and steady-state bulk electrolysis.

In complementary work we tested the coulomb complex formation of water-soluble SWNT grafted with poly(sodium 4-styrenesulfonate) (SWNT-PSS) and a 5,10,15,20-tetrakis-(2’,6’-bis-(N-methylene-(4’’-tert-butylpyridinium))-4’-tert-butylphenyl)porphyrin octabromide salt (H2P8+) en route to versatile donor-acceptor ensembles.  Photoexcitation of the porphyrin chromophore in SWNT-PSSn- / H2P8+ is followed by a rapid and efficient intra-ensemble charge separation to generate a charge-separated state that lives for tens of microseconds.


Wednesday, 9 March 2:00 pm.


"Supramolecular Engineering at Surfaces: Control of Matter at the Nanoscale"

J.V. Barth

Ecole Polytechnique Fédérale de Lausanne, Switzerland; University of British Columbia, Vancouver, Canada.


The fabrication of supramolecular architectures using programmed molecular building blocks opens up new vistas for the control of matter and the design of novel functional materials. In recent years significant progress was made in their assembly at solid surfaces, which can be directed and monitored in exquisite detail. Moreover, this approach facilitates integration in environments structured at a higher level. Here we concentrate on molecular control and atomistic understanding of low-dimensional supramolecular nanosystems at metal substrates. Versatile building blocks proved to be species with planar pi-systems. Variable-temperature scanning tunneling microscopy observations provide direct insight into their adsorption, interactions and organization principles. On close-packed noble metals we followed the self-assembly of H-bonded one-dimensional nanogratings, two-dimensional sheets and open honeycomb networks. On the Cu(100) surface the formation and dynamics of mononuclear metal-organic compounds were monitored. Also one-dimensional coordination polymers and nanoporous two-dimensional metallosupramolecular networks with specific topologies and a high structural stability were engineered. Their tailoring and functionalization allows for the steering of molecular organization and sorption, as demonstrated with the accommodation of C60 and other guest molecules. Last not least, metal-organic linkages between magnetic clusters were obtained on nanopatterned substrates.




Keynote lectures


Thursday, 10 March 9:20 am.


Title to be announced


Marcelo Goffman

CEA Saclay, France


Thursday, 10 March 2:00 pm.


Conductance calculations for small molecules


Karsten Jacobsen

Technical University of Denmark, Lyngby, Denmark


The conductance of a several different small molecules have been calculated based on density-functional theory. The calculations are carried out using a Green function technique based on maximally localized and partly occupied Wannier functions. The Wannier functions are convenient for the calculations because they combine high accuracy with a local chemical intuitive representation. The latter aspect allows for a detailed orbital analysis of the mechanisms behind the transport.

We have focused on the structure, dynamics and tranport of reasonably small molecules many of which have also been investigated experimentally. Some of the results are for the following systems.

Hydrogen molecule between Pt electrodes: Different possible structures and vibrational modes are identified as a function of contact strain together with calculations of the coherent transport. The findings support the previous suggestion of a molecule bridging between two electrodes and it is shown that the molecular antibonding state is responsible for the nearly open conductance channel.

CO between Pt electrodes: A single CO molecule is found to bind to Pt contacts with several different geometries. During the breaking of the contact the molecule switches from being bound through the carbon atom to both electrodes to a more bridge-like configuration which has a conductance close to half a quantum unit. This may explain the recently observed half-integer peak in experiments performed in Leiden.

Results for (nitro-)benzene and bipyridine will also be presented with particular focus on the connection between bonding and transport as the bonding geometry and electrode material is changed.


Friday, 11 March 9:00 am.


"Molecular Electronics: The Organic Chemists' Viewpoint"


Martin Bryce

Durham, UK



Friday, 11 March 2:00 pm.


"Wet molecular junctions"


Christian Schönenberger

Basel, Switserland


Molecular electronics is attracting a growing attention today. On the one hand this is due to the recognition that molecules offer (in principle) unprecedented precision down to single atoms and on the other hand due to the sophisticated tools of nanoscience and technology which is penetrating into the molecular size regime.

In the present talk, I will report on research with “wet junctions”, meaning molecular systems which are kept in a liquid environment while measuring the electrical properties. This work started some time ago with carbon nanotubes when we realized that an electrochemical environment, i.e. an electrolyte, allows to gate the nanotube in an efficient way. Moreover, it became clear that devices composed of a single (or a few) molecule(s) consist to a great extent of surface only. Hence, the environment plays a key role. While it is important to study electrical properties of molecules in vacuum and at low temperatures, from the perspective of applications in e.g. sensing, an understanding of the effect of an “active” chemical environment is crucial.

The work on carbon nanotubes has been continued into studies of small molecules for which we exploit the break-junction approach. We have explored the effect of different solvents in the electrical conductance of single-atom Au contacts and tunnel junction. I will also demonstrate measurements on a C60 variant which is anchored on one side and measured in DMSO and toluene. A large liquid effect is found.

This work has been conducted in the framework of the Swiss NCCR on Nanoscience within a collaboration between Basel (Schönenberger et al.) and the ETHZ (Diederich et al.). Support by the NCCR, the Swiss NFS and ESF (SONS) are gratefully acknowledged.


Saturday, 12 March 9 am.


"Scanning tunneling spectroscopy of single magnetic adatoms and complexes at surfaces"


Peter Wahl

MPI Stuttgart, Germany


The spin of individual magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies. Starting from a study of the behaviour of single cobalt adatoms on noble metal surfaces by low temperature STM and STS, we demonstrate the ability to manipulate the spin state and the associated magnetic properties of individual adatoms by the controlled attachment of molecular ligands. The spin state of the resulting complexes is determined via the Kondo resonance, which appears in STS spectra. Spectroscopic results are complemented by a study of the spatial dependence of the Kondo resonance. This novel imaging mode allows to map the spin centers of magnetic molecules with atomic precision. Spin detection and mapping via the Kondo Effect is compared to other SPM-related techniques offering spin contrast.




Brief contributions



"Liquid gating of single molecules in break junctions"


Teresa Gonzalez

Basel, Switserland


either on Thursday or on Friday,



"Electrical transport through DNA molecules under stretching"


Ning Kang

Konstanz, Germany


Theoretical calculations have demonstrated that the charge transport through the DNA will be strongly influenced by conformational transitions. To probe this effect, we have measured the resistance of DNA molecules under stretching with the help of the mechanically controllable break junction technique (MCB). Using the break junctions, we are able to fabricate electrodes with nanometer separation and fine-tune the tunneling gap between electrodes. In our experiments, we used 10-nm-long (30 base pairs) DNA with thiol groups at both ends, and stretched continuously the trapped molecules by means of MCB. A discrete two-level resistance switching behavior is observed when changing the distance of the electrodes.



"C60 in gated nanogaps"


Sergey kubatkin

Göteborg, Sweden


I shall describe our recent unpublished experiments with deposition of fullerenes in metallic nanogaps prepared on top of electrostatic gate made of oxidized aluminium.



"One-way switching of photochromic molecules on gold"


Sense Jan van der Molen, D. Dulic, T. Kudernac, H.T. Jonkman, J.J.D. de Jong, T.N. Bowden, J. van Esch, B.L. Feringa, and  B.J. van Wees

 Groningen, The Netherlands



Single-molecule electronics provides an interesting pathway for further miniaturization of electronic devices. We concentrate on creating a molecular switch that can be manipulated with light. For this, we investigate photochromic molecules (1,2-bis[5’-(5”-acetylsulfanylthien-2”-yl)-2’-methylthien-3’-yl]) that are synthesized in the chemical department of our institute. In solution, these organic molecules can be switched reversibly between a conjugated, ‘conductive’ state and a non-conjugated, ‘insulating’ state by using light of the proper wave lengths (conjugated to non-conjugated: l » 550 nm; non-conjugated to conjugated: l » 330 nm).

Two ways of contacting are chosen, both based on the Au-S bond. First, we use the mechanically controllable break junction (MCBJ) technique, which allows us to manipulate the distance between two gold electrodes with sub-Angstrom precision. Second, we study a diluted self-assembled monolayer (SAM), i.e. dodecanethiol containing a low concentration of the switch, using scanning tunneling microscopy (STM). In both experiments, we observe switching from the ‘conductive’ to the ‘insulating’ state. However, we do not observe the reverse process, which should occur upon illumination with UV light. An independent optical absorption experiment using a solution of gold colloids covered by switchable molecules confirms this observation: once connected to gold, the molecule can switch from conjugated to non-conjugated, but it cannot switch back. We attribute this effect to quenching of the excited state of the non-conjugated molecule by the presence of gold [1,2]. Finally, we discuss a possible way out of this problem, using newly synthesized organic molecules.


[1]     D. Dulic, S.J. van der Molen, T. Kudernac, H.T. Jonkman, J.J.D. de Jong, T.N. Bowden, J. van Esch, B.L. Feringa, and  B.J. van Wees,

Phys. Rev. Lett. 91 (20), 207402 (2003)

[2]     R. Hania, A. Pugzlys, T. Kudernac, H.T. Jonkman and K. Duppen

Proceedings of 14th  International Conference on Ultrafast Phenomena,

July 25-30, 2004 Toki Messe, Convention Center Niigata, Japan.



"Coordination complexes with redox active TTF-type ligands"


OUAHAB Lahcène

Rennes, France


Designing molecule-based materials, which possess synergy or interplay between two or more properties such as electrical conductivity with magnetism or spin cross-over …, is currently a challenging target and it have been attracting great interests from chemists and physicists for both application to devices or for fundamental science. For the particular class of materials combining electrical conductivity and magnetic interactions, it is hopped to achieve magnetic coupling between the localized spin of the inorganic part through the mobile electrons of the organic part via the so-called p-d interactions [1]. We present in this contribution some of our results according to the idea consisting in the synthesis of paramagnetic coordination complexes [2,3] containing TTF’s as ligands as well as their radical cation salts [4].


[1] L. Ouahab, T. Enoki, Eur. J. Inorg. Chem., 2004, 931

[2] F. Iwahori et al. Inorg. Chem., 2001, 40, 6541

[3] L. Ouahab et al Synthetic Metal, 2003, 133-134, 505

[4] F. Setifi et al. Inorg. Chem., 2003, 42, 1791




"Intramolecular Electron Transfer in Organic Radical Ions"


Prof. J. Veciana

Barcelona, Spain





"Single Molecule Transport Measurements through Transition Metal Complexes"


Mario Ruben 1,Aitor Landa 1, Heiko Weber 1, Jan Würfel1, Marcel Mayor1

Institute of Nanotechnolgy,Research Centre Karlsruhe, P.O. 3640,D-76021 Karlsruhe 


Electronic transport through single molecules is a driving new direction in the science and technology of nano-metre-scale electronic systems. Especially, the use of the mechanically controlled break junction techniques has been proven to a powerful tool, since it enables a symmetrical connection to both electrodes by well defined covalent, mostly Au-S, bonds. Herein, we describe the use of terpyridine molecules incorporating a transition metal and having attached fully conjugated, conformationally rigid organic linkers in I-V measurements in break junctions.


The class of transition metal–bis(terpyridyl) complexes exhibits a broad variety of highly interesting properties. In particular, the complexes containing Ru2+ have been shown to act as photo- and electrochemically driven molecular switches in a multitude of supramolecular devices. In order to to determine the electronic and photophysical properties of this class of complexes at the single molecule level, we have synthesized taylor-made molecule 1 and investigated its I-V characteristics in a mechanically controlled break junction set-up. 















Thereby, we have found that the spatial symmetric of 1 is reflected in rather symmetric I-V characteristics. The molecule 1 exhibits a single-molecule conductance of G = 10-8 S. Further details of the electronic characteristics will be discussed.

We would like to acknowledge the support by the BMBF-FZK network project MOLMEM (13N8360) of the German Ministry of Education and Research.



"The relationship between bonding and conductance in a bipyridine molecular contact"


Robert Stadler

Lyngby, Denmark


Inspired by recent experiments, where the forces and conductances through bipyridine nano-junctions have been measured simultaneously, we performed total energy and conductance calculations based on density functional theory. Our study focuses on a variation of the surface geometry of the gold leads, to which the molecules are connected, and on the effect that this has on the binding energy and the electron transmission through the molecule. We find that the binding energy depends linearly on the coordination number of the gold atom the molecule is connected to and can be explained in terms of the interaction between the HOMO and the Au-s-states. The conductance is mediated by the LUMO only, which shifts in its energetic position relative to the metal's Fermi level depending on its surface geometry. We explain the resulting conductance behaviour using a simple one-level model.



"Characterization of electromigration-induced gold nanogaps"


Trouwborst M.L., Van der Molen S.J., Dulic D., Van Wees B.J.

Groningen, the Netherlands



One of the challenges in single-molecule electronics, is the development of stable electrodes, with gaps in the (sub)nanometer range. In the past years, some creative solutions have been found for manufacturing such nanojunctions. One of the fastest methods is fabrication with the help of electromigration [1]. We have created electromigration-induced gaps, approximately 0 to 2 nm in width, and have investigated the breaking process.

Our samples are fabricated using electron-beam lithography, and consist of a gold wire with a constriction in the middle with a thickness of 12 nm and a with of 100 nm. The gaps are formed by sending a current through the wire. When the current density exceeds a certain value, the electromigration process will start, finally leading to the formation of a gap.

We have investigated the electromigration process at constant voltage and made a comparison with simulations performed by Mahadevan et.al. [2] We have found that the data fits well with stimulation, except for the initial part and the part just before breaking. For the first part, we observe a resistance decrease prior the narrowing of the gap. We ascribe this initial resistance decrease to a pushing away of impurities and vacancies in the bulk, by the high current density in the wire. This behavior is especially present at the room temperature data. For the data at 4.2 Kelvin, this “cleaning” effect has not been observed. This is as expected since at lower temperatures it is harder for the impurities to diffuse.


For the last part of the electromigration process, the conductance is expected to decrease logarithmically with time. However, we observe steps in the conductance just prior the breaking. This behavior is expected, when considering a small number of atoms present in the constriction, starting to dominate the total resistance. The observed conductance steps are in the order of 1-8 times the conductance quantum 2*e2/h.

These conductance plateaus are investigated at different voltages. At voltages higher than 0.8 Volt, the conductance is no longer quantized, but starts to vary at non-integer values of 2*e2/h. This value for the critical voltage is also observed by Mizobata et al [3], and is not yet completely understood.  


[1] H. Park et al, Appl. Phys. Lett. 75, 301 (1999)

[2] M. Mahadevan, R.M. Bradley, Phys. Rev. B. 59, 11037 (1999)

[3] J. Mizobata et al,  Phys. Rev. B. 68, 155428 (2003)


email: M.L.Trouwborst@rug.nl


Fig.1 Electromigration sample.

Constriction in the middle is ~12×100 nm (h×w)





"Application of nanoparticle arrays in molecular electronics"


Jianhui Liao
University of Basel, Switzerland

A great challenge in molecular electronics is to overcome the huge size mismatch between a molecule and an electronic device. An alternative approach is to use colloidal particles as a media to interconnect molecules to external leads. Herein, 2D nanoparticle arrays were prepared by self-assembly on water surface and acted as breadboards for mesurement of molecules. Molecular wires (OPE and OPV) were inserted to connect neighboring nanoparticles by molecular exchange. This method can give average information of molecular transport properties.




"The electron conduction of photosynthetic protein complexes embedded in a membrane"


Thijs Aartsma

Universiteit Leiden, Netherlands



"Simulating electron transport in atomic-scale systems from first principles"


Mads Brandbyge
Technical University of Denmark, Lyngby, Denmark

We have developed a first principles method [1] for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes with an applied voltage bias. Our method is based on the density functional theory as implemented in the well tested SIESTA package [2]. With our code, dubbed TranSIESTA, we fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The electron density at finite bias is calculated using non-equilibrium Greens functions.

In this talk we concentrate on our results for the transport channels of single atom wide metal contacts. We consider contact systems of different metals connecting perfect bulk electrodes where detailed experimental data is available [3]. The odd-even behaviour of the conductance of monovalent atomic chains as a function of chain-length is investigated [4] as well as the effect of impurities present in the chains. We discuss the forces acting on the contact atoms due to the non-equilibrium situation in the electronic subsystem, i.e. in the presence of an electronic current [5].


[1] M. Brandbyge, J. Taylor, K. Stokbro, J-L. Mozos, and P. Ordejon, Phys. Rev. B 65, 165401 (2002).

[2] J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J. Junquera, P. Ordejon, and D. Sanchez-Portal, J. Phys. C 14, 2745 (2002).

[3] S. K. Nielsen, M. Brandbyge, K. Hansen, K. Stokbro, J. M. Van Ruitenbeek, and F. Besenbacher, Phys. Rev. Lett. 89, 66804 (2002).

[4] Y. J. Lee,  M. Brandbyge,  M. J. Puska,  J. Taylor, K. Stokbro,  and R. M. Nieminen, Phys. Rev. B  69, 125409 (2004).

[5] M. Brandbyge, K. Stokbro, J. Taylor, J. L. Mozos, P. Ordejon, Phys. Rev. B 67, 193104 (2003).




"A Statistical Approach for the Investigation of Charge Carrier Transport through Single Molecules"


Emanuel Lörtscher*, Heiko B. Weber+ and Heike Riel*

* IBM Research GmbH, Rueschlikon, Switzerland

+ University of Erlangen-N¨urnberg, Germany



A fundamental understanding of conductance through metal-molecule-metal junctions is a key challenge of molecular electronics and a requirement for future applications. However, for any implementation into circuits, the contacting of single molecules by symmetric electrodes remains challenging and charge carrier transport through single molecules can be dominated by the nature of the contact between molecule and electrodes.

Statistical investigation of charge carrier transport through metal-moleculemetal junctions is performed by a mechanically controllable break-junction (MCBJ) technique [1][2]. Our system is designed to acquire data automatically during the stepwise opening and closing of the junction. This approach requires an excellent mechanical stability which is testified by consistently observable quantized conductance of quantum point contacts at room temperature. Analysis tools provide the basis for statistical examination of comprehensive data sets to perform e.g. conductance spectra at different voltage points.

A model system consisting of one, two, three and four phenyl units terminated symmetrically by acetyl protected thiol groups [a] has been investigated. We report an extensive data set of reproducible and stable current-voltage curves reflecting the characteristics of the individual molecule. Conductance studies at different temperatures will be compared to results achieved by other techniques [3,4] and theoretical predictions.


[a]Courtesy of the Institut fuer Nanotechnologie, Forschungszentrum Karlsruhe, Germany

[1] J. Moreland and J. Elkin, J. Appl. Phys. 58, 3888 (1985).

[2] C. J. Muller, J. M. van Ruitenbeek, and L. de Jongh, Physica C 191, 485 (1992).

[3] S. Hong, R. Reifenberger, W. Tian, S. Datta, J. Henderson, and C. P. Kubiak, Superlattices and Microstructures 28, 289 (2000).

[4] M. A. Reed, C. Zhou, C. Muller, T. Burgin, and J. Tour, Science 278, 252 (1997).



"An AFM to measure low temperature IV-curves of single molecules"

 Roel Smit
Universidad Autónoma de Madrid, Spain



Inelastic scattering in atomic junctions

Carlos Untiedt
Universidad de Alicante, Spain


"Inelastic transport in atomic gold wires - a first principles approach"


Thomas Frederiksen

Technical University of Denmark, Lyngby, Denmark


We describe a method for calculating the conductance of nanosized systems taking into account inelastic effects related to local vibrations in the junction. This method combines Density Functional Theory (DFT) for the electronic structure and vibrational modes with non-equilibrium Green's functions (NEGF) for the steady current and power flow. The electron-vibration interaction is treated within the self-consistent Born approximation (SCBA). The scheme is applied to atomic gold wires for which comparison with experimental results can be made. We show results for a four-atom wire under two different states of strain, and discuss mode selectivity as well as signatures of phonon heating.



"Theoretical description of electrical conduction through platinum wires"


Magnus Paulsson
Technical University of Denmark, Lyngby, Denmark


We present calculations of the elastic and inelastic conductance through short atomic wires of platinum. All relevant physical properties, i.e., phonon frequencies, electron-phonon coupling and elastic conductance, are calculated using Density Functional Theory. The inelastic conductance is then calculated using the Self Consistent Born Approximation (SCBA). We show that the elastic conductance fit the experimental measured conductance and compare the inelastic conductance to the known example of gold wires.


"Electronic Interactions in a New p-Extended Tetrathiafulvalene Dimer"

Marta C. Díaz,a Beatriz M. Illescas,a Nazario Martína,* Igor F. Perepichka,b Martin R. Bryce,b,* and Eric Levillain,c

a Universidad Complutense, Madrid, Spain.

bUniversity of Durham, Durham,United Kingdom.

c CNRS UMR 6200, Angers, France


To study the electronic interactions between the two redox centers in extended TTF molecules (exTTF) we synthesized compound D-D in which two exTTF units linked together.



Cyclic voltammetry (CV) of D–D shows two quasi-reversible oxidation waves (Fig. 1). These redox processes can be described either as:

     D0–D0 à D2+–D0 à D2+–D2+ (one 2e-process at each step; two exTTF units interacts electronically) or

     D0–D0 à D·+–D·+ à D2+–D2+ (two 1e-processes at each step; no electronic interactions between the exTTF units).

                To clarify the mechanism of the redox transformations, spectroelectrochemical experiments (SEC) for D–D and the monomeric exTTF D have been performed (Fig. 2). An oxidation of D–D results in a new broad band in the region of 460–540 nm, which is characteristic for the dication D2+ species (as confirmed by comparison with the SEC for the monomeric exTTF D). No long-wavelength absorption in the region of 560–700 nm, where the radical cations of the exTTF expected was observed during the oxidation of DD (Fig. 2). Good isobestic points leave no room for the existence of D·+ species during the oxidation.


Fig. 1. Thin layer CV of compound D–D in
            acetonitrile-dichloromethane, 1:1 v/v,
            0.25 M Bu4NPF6 at 2 mV s–1

Fig. 2. Spectroelectrochemistry of compound D–D in
            acetonitrile, 0.1 M Bu4NPF6.


            Similar evolution of the spectra was observed at the chemical oxidative titration of D–D by NOSbF6. SEC experiments in thin layer conditions demonstrates the spectral features of D2+–D0 and D2+–D2+ species. All these data point to the electrochemical oxidation of compound D–D occuring as two consecutive 2e processes:

D0–D0 à D2+–D0 à D2+–D2+. With this model, we found a good correlation between the experimental CVs in diffusion conditions at different scan rates and their simulations.

            Thus, the oxidation of D–D can be interpreted by two consecutive inverted potentials on each exTTF moiety, which are electronically interacting.



"Nano-meter scale contacts by controlled electromigration"


Kevin O'Neill

Kavli Institute of Nanoscience, Delft University of Technology, Netherlands



Electromigration is currently a favourite technique for making molecular-scale electrodes: a high current in a wire 50nm wide by 10-20nm high can move enough material to create a 1-10nm gap, which may be subsequently bridged by a molecule. Typically considered a nuisance in the electronics industry, electromigration has been successfully used to measure conduction through single molecules. It suffers, however, from lack of yield and controllability, making it necessary to often break thousands of samples for a reasonable yield of credible single-molecule transport data. We discuss briefly how electromigration may be controlled to optimize the resistance of the junction and thus increase device yield, and present data on bare gold electrodes and single molecules.



Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt junction


D. Djukic, C. Untiedt, R.H.M. Smit, and J.M. van Ruitenbeek

Kamerlingh Onnes Laboratorium, Universiteit Leiden, Netherlands

K.S. Thygesen and K.W. Jacobsen

Technical Univiversity Denmark, Lyngby, Denmark


A conducting bridge of a single hydrogen molecule between Pt electrodes is formed in a break junction experiment. It has a conductance near the quantum unit carried by a single channel. Using point contact spectroscopy three vibrational modes are observed and their variation upon stretching and isotope substitution is obtained. The interpretation of the experiment is verified by Density Functional Theory calculations for the stability, vibrational modes and conductance of the PtH2 structure.



Vibrational spectroscomicroscopy of carbon nanostructrures with atomic resolution


L.Vitali, M.Burghard, P.Wahl, M.A.Schneider, K.Kern


Max-Planck-Institute, Stuttgart, Germany



Inelastic Electron Tunneling Spectroscopy (IETS-STM) is an elegant technique to probe the vibrational states of individual molecules adsorbed at surfaces. Moreover it has recently been demonstrated by IETS-STM measurements of HOPG surfaces that the inelastic spectrum is directly proportional to the total density of states.

The scanning tunneling microscope is also ideally suited to identify and characterize local defects, such as intermolecular junctions and endings of carbon nanotubes. Indeed the predicted signature of these local topological structures on the tube electronic properties has been measured by scanning tunneling spectroscopy. Despite the electronic characterization of these defects from the theoretical and the experimental point of view, little is known with respect to their vibrational properties. In this work, the capability of IETS-STM to probe vibrational features on the atomic scale have been applied to study single wall carbon nanotubes adsorbed on Au substrates. The radial breathing mode (RBM) of isolated tubes has been identified and its linear dependence on the inverse of the tube diameter is demonstrated. Moreover, the high spatial resolution has allowed the unraveling of changes in the local phonon spectrum related to topological defects.



One-dimensional supramolecular ordering of oxalic amidine derivates on a noble metal substrate

F. Klappenberger, Marta Esther Cañas Ventura, Stéphane Pons, Sylvain Clair, Zhi-Rong Qu, Mario Ruben, Roman Fasel, Klaus Kern, Harald Brune, Johannes V. Barth


Scanning tunneling microscopy investigations of an oxalic amidine derivate on a metal surface are presented. The studies were carried out at low temperatures under ultra high vacuum conditions. The oxalic amidine, which was synthesized as organic molecular beam epitaxy compatible base compound for molecular magnetism, self-assembles in linear nanostructures on the Au(111) surface. This supramolecular arrangement indicates intermolecular hydrogen bonds mediated by NH groups. It implies presumably mesomeric state of the ONH group along the supramolecular lines, giving rise to particular electronic properties. The mesoscopic ordering of the molecular chains is influenced by the substrate chevron reconstruction. In addition, first results on metallosupramolecular arrangements obtained by reticulating of the oxalic amidine species with codeposited Fe atoms are shown.



Metallosupramolecular nanocavities and ribbons on fcc (111) noble metal surfaces


Marta Esther Cañas Ventura 1,2, Stéphane Pons 1, Sylvain Clair 1, Florian Klappenberger 1 , Zhi-Rong Qu 3, Mario Ruben 3, Roman Fasel 2, Klaus Kern 1,4, Harald Brune 1, Johannes V. Barth 1,5


1 École Polytechnique Fédérale de lausanne, Institut de Physique des Nanostructures (IPN), Lausanne, Switzerland

2 Swiss federal Laboratories for materials testing and Research (EMPA), Thun, Switzerland

3 Institute of Nanotechnology, Forschungszentrum Karlsruhe, Karlsruhe, Germany

4 Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany

5 Advanced Materials and Process Engineering Laboratory, University of British Columbia, Vancouver, Canada



In order to advance the understanding and nanoscale control of molecular self-assembly processes at surfaces, systematic investigations on the adsorption of 1,4-benzenedicarboxylic acid (TPA) on single crystal metal surfaces have been carried out. In situ low-temperature scanning tunneling microscopy (STM) studies under ultra high vacuum provide atomistic  insight into the underlying intermolecular and molecule-substrate interactions. The TPA building blocks deposited on Pd (111) produce linear hydrogen-bonded stripes in three orientations (reflecting the substrate symmetry) which do not reticulate. When Fe is subsequently added it decorates the stripe edges and connected networks evolve leading to the formation of nanocavities, which can serve as template for growing new metallic nanstructures. Furthermore, metallosupramolecular ribbons with length of hundreds of nanometers are built when depositing TPA on a pre-patterned Fe/Au (111) surface. In this system the chevron reconstruction of Au (111) plays an important role for nanotexturing.



Electron Transport through the Single Molecule Magnet Manganese-12


Hubert Heersche


Kavli Institute of Nanoscience, Delft University of Technology


The single molecule magnet Manganese-12 (Mn-12) is a well known molecule due to its high spin ground state (S=10). Although the magnetic properties of Mn-12 have been studied extensively, little is known about the interplay between molecular spin and electron transport. We have trapped single Mn12 molecules between two electrodes that are only a few (1-5) nanometers apart. The electrodes are fabricated using a combination of electron beam lithography and electro-migration. For the first time, we measured the transport properties of a molecular magnet in a single molecule transistor geometry. The results will be discussed in the context of recent theoretical calculations.





1. Carlos Arroyo: Au-S-Au junctions

2. J.J. Riquelme:  Distribution of channels in superconducting nanocontacts