Abstract: We demonstrate reversible, light-controlled conductance switching of molecular devices based on photochromic diarylethene molecules. These devices consist of ordered, two-dimensional lattices of gold nanoparticles, in which neighboring particles are bridged by switchable molecules. We independently confirm that reversible isomerization of the diarylethenes employed is at the heart of the room-temperature conductance switching. For this, we take full advantage of the possibility to use optical spectroscopy to follow molecular switching in these samples.
Abstract: We report on a photoconductive gain effect in two-dimensional arrays of gold nanoparticles, in which alkane molecules are inserted. The nanoparticle arrays are formed by a self-assembly process from alkanethiol-coated gold nanoparticles, and subsequently they are patterned on a Si/SiO2 chip by a microcontact printing technique. We find that the photoconductance of the arrays is strongly enhanced at the frequency of the surface plasmon of the nanoparticles. We interpret the observation as a bolometric enhancement of the conductance of the nanoparticle arrays upon excitation of the surface plasmon resonance.
Abstract: If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-pi coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.
Abstract: We study the electrical conductance of octanedithiol molecular junctions using a mechanically controllable break-junction setup. The stability of the system allows control of whether the electrodes get into contact before each new molecular junction formation or not (contact and non-contact modes). We find three characteristic conductance values for octanedithiol. Well-defined peaks in the conductance histograms at multiples of 1.2Ã10â5 G0 suggest that this value corresponds to the conductance of a single molecular junction conductance. Reproducible features are also observed at 4.5Ã10^(â5) and 2.3Ã10^(â4) G0. The first value has the strongest statistical weight, whereas the second is only observed in the non-contact mode. We propose that these two values reflect the formation of several molecular junctions in parallel between the electrodes.
Abstract: We have studied the 1/f voltage noise of gold nanocontacts in electromigrated and mechanically controlled break junctions having resistance values R that can be tuned from 10 Ohms (many channels) to 10 kOhms (single-atom contact). The noise is caused by resistance fluctuations as evidenced by the S_V propto V^2 dependence of the power-spectral density S_V on the applied dc voltage V. As a function of R the normalized noise S_V/V^2 shows a pronounced crossover from \propto R^3 for low-Ohmic junctions to \propto R^1.5 for high-Ohmic ones. The measured powers of 3 and 1.5 are in agreement with 1/f noise generated in the bulk and reflect the transition from diffusive to ballistic transport.
Abstract: We investigate the importance of anchoring end-groups in conjugated oligomers for the formation of molecular junction networks. Oligo(phenylene ethynylene) with a single (OPE-MT) and two (OPE-DT) thiol end-groups have been inserted into self-assembled octanethiol-capped gold nanoparticle arrays by taking advantage of molecular exchange. Comparing the exchange for tens of devices, we observe significantly different final conductances for devices comprising monothiol- and dithiolated compounds. Our experimental results support the picture that OPE-DT covalently bridge neighboring nanoparticles via AuâS bonds at both ends of the conjugated oligomer to form interlinked networks of molecular junctions.
Abstract: We determine and compare, at the single molecule level and under identical environmental conditions, the electrical conductance of four conjugated phenylene oligomers comprising terminal sulfur anchor groups with simple structural and conjugation variations. The comparison shows that the conductance of oligo(phenylene vinylene) (OPV) is slightly higher than that of oligo(phenylene ethynylene) (OPE). We find that solubilizing side groups do neither prevent the molecules from being anchored within a break junction nor noticeably influence the conductance value.
Abstract: We describe a method to detect and count transient burstlike signals in the presence of a significant stationary noise. To discriminate a transient signal from the background noise, an optimum threshold is determined using an iterative algorithm that yields the probability distribution of the background noise. Knowledge of the probability distribution of the noise then allows the determination of the number of transient events with a quantifiable error (wrong-positives). We apply the method, which does not rely on the choice of free parameters, to the detection and counting of transient single-molecule fluorescence events in the presence of a strong background noise. The method will be of importance in various ultra sensing applications.
Abstract: Applying ac voltages, we trapped gold nanoparticles between micro-fabricated electrodes under well-defined conditions. We demonstrate that the nanoparticles can be controllably fused together to form homogeneous gold nanowires with pre-defined diameters and conductance values. Whereas electromigration is known to form a gap when a dc voltage is applied, this ac technique achieves the opposite, thereby completing the toolkit for the fabrication of nanoscale junctions.
Abstract: New cruciform structures 1â4 were synthesized to investigate a new single molecule switching mechanism arising from the interplay between the molecule and the electrode surface. These molecular cruxes consist of two rod-type substructures, namely an oligophenylenevinylene and an oligophenyleneethynyl. While the oligophenylenevinylene rods are functionalized with acetyl protected sulfur anchor groups, the oligophenyleneethynyl rods provide terminal pyridine units. The hypothesized switching mechanism should arise from the electrochemical potential dependent coordination of the pyridine unit to the electrode surface. The assembly of the oligophenylenevinylene substructure was based on a Wittig reaction whereas its perpendicular oligophenyleneethynyl rod was assembled by SonogashiraâHagihara coupling reactions. Preliminary transport investigations with molecular cruciforms 2 and 4 in a mechanical controllable break junction in a liquid environment displayed the trapping of single molecules between two gold electrodes via the terminally sulfur functionalized oligophenylenevinylene rod.
Abstract: Well ordered nanoparticle arrays were prepared on Si/SiO2 surfaces from alkanethiol-coated Au nanoparticles via self-assembly and micro-contact printing. We study the insertion of conjugated molecular species within the nanoparticle arrays via spectroscopic and electrical transport measurements. Upon exchange of the alkanethiol chains with the conjugated oligomers, the conductance of the network increases by one to 3 orders of magnitude. In addition, the absorption spectra in the visible light range show a red-shift of the surface plasmon resonance (SPR). The latter shift, which is due to the difference in permittivity between alkanes and conjugated oligomers, can be understood within Mie and MaxwellâGarnett theory. Finally, infrared absorption spectra provide direct spectroscopic evidence that the conjugated oligomers can be not only inserted but also, subsequently, fully removed from the nanoparticle arrays via place-exchange. The reversibility of the exchange process demonstrates the potential of these structures as a platform for molecular electronics.
Abstract: The authors have developed a fast, yet highly reproducible method to fabricate metallic electrodes with nanometer separation using electromigration (EM). They employ four terminal instead of two-terminal devices in combination with an analog feedback to maintain the voltage U over the junction constant. After the initialization phase (U<~0.2 V), during which the temperature T increases by 80â150 °C, EM sets in shrinking the wire locally. This quickly leads to a transition from the diffusive to a quasiballistic regime (0.2 V<~U<~0.6 V). At the end of this second regime, a gap forms (U>~0.6 V). Remarkably, controlled electromigration is still possible in the quasiballistic regime.
Abstract: A new molecular wire suitably functionalized with sulfur atoms at terminal positions and endowed with a central redox active TTF unit has been synthesized and inserted within two atomic-sized Au electrodes; electrical transport measurements have been performed in STM and MCBJ set-ups in a liquid environment and reveal conductance values around 10â2G0 for a single molecule.
Abstract: We propose an objective and robust method to extract the electrical conductance of single molecules connected to metal electrodes from a set of measured conductance data. Our method roots in the physics of tunneling and is tested on octanedithiol using mechanically controllable break junctions. The single molecule conductance values can be deduced without the need for data selection.
Abstract: Molecular electronics is attracting increasing research attention, primarily supported by the possibilities offered by synthetic chemistry for the tailoring of single molecules to achieve specific electronic functions. Whereas various experimental approaches have been devised to form and electrically study molecular junctions, the integration of individual junctions into functional electronic circuits remains a demanding task, requiring innovative approaches in fabrication philosophy and circuit structure. We show here that an approach combining the self-assembly and microcontact printing of ligand-protected metallic nanoparticles, followed by an in situ ligand-exchange reaction, allows the preparation of stable 2D networks of molecular junctions.A significant decrease in resistance (up to three orders of magnitude) after the exchange of alkanethiol ligands with conjugated, double-ended organic wires (thiolated oligo(phenylene ethynylene), OPE) confirms a proper interlinking of neighboring nanoparticles. We also demonstrate that the formation of the molecular junctions is reversible, making nanoparticle networks a promising platform for the development of molecular electronic circuits. The flexibility of this approach lets us envisage the realization of more complex networks, for instance, by intermixing ensembles of bimodal nanoparticles of different materials.
Notes: Cited in Nature, News&Views, August 31st, 2006
Abstract: We present electronic transport measurements through thiolated C60 molecules in a liquid environment. The molecules were placed within a mechanically controllable break junction using a single anchoring group per molecule. On varying the electrode separation of the C60-modified junctions, we observed a peak in the conductance traces. The shape of the curves is strongly influenced by the environment of the junction as shown by measurements in two distinct solvents. In the framework of a simple resonant tunnelling model, we can extract the electronic tunnelling rates governing the transport properties of the junctions.
Abstract: In this Communication, we present a break junction setup with an integrated liquid cell, which allows an exploration of the influence of solvents on the electronic properties of atomic contacts. The setup is used to study the variation of the electrical conductance G of Au junctions with their elongation in the regimes of tunneling (low conductance) and of true metallic contact (high conductance). As solvents, we have used deionized water, dichloromethane (DCM), dimethylsulfoxide (DMSO), octane, and toluene. The last four are of particular interest since they are potential organic solvents for molecules relevant in molecular electronics. In addition, these solvents cover a broad range of polarities. We also compare the results with reference measurements obtained in vacuum and air.
Abstract: Here we describe a hybrid material composed of a single-stranded DNA (ssDNA) molecule, a 1.4 nm diameter gold nanoparticle, and a fluorophore that is highly quenched by the nanoparticle through a distance-dependent process. The fluorescence of this hybrid molecule increases by a factor of as much as several thousand as it binds to a complementary ssDNA. We show that this composite molecule is a different type of molecular beacon with a sensitivity enhanced up to 100-fold. In competitive hybridization assays, the ability to detect single mismatch is eightfold greater with this probe than with other molecular beacons.
Abstract: The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa2Cu3O7 films was studied by measuring their ac sheet impedance Z over a broad range of frequencies Ï. With decreasing temperature the dissipative component of Z exhibits, at a temperature T*(Ï) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T*(Ï), appears to be frozen at the time scales 1/Ï of the experiments.
Abstract: A controlled handling of single molecules is essential for the fabrication and the investigation of devices based on molecules. We present here the implementation of an electric field based method used to manipulate DNA molecules by means of lithographically patterned metallic electrodes. We optimized the geometry of the lithographic structures to favor a precise positioning of the molecules via dielectrophoresis. This process is combined with an orientation of the molecules parallel to the electric field lines due to their induced dipole moment. The relatively high polarizability of the DNA molecules in solution is essential to achieve these manipulations. We expect this method to be softer than the stretching of molecules using a receding meniscus. The visualization of the molecules was achieved using fluorescence microscopy.
Abstract: The activation energy for plastic vortex motion in YBa2Cu3O7 thin films exposed to a magnetic field is found to exhibit a dimensional crossover at a sample thickness corresponding to a typical vortex elastic length.
Abstract: The complex ac sheet impedance of c-axis oriented YBa2Cu3O7 thin films was measured as a function of temperature in the frequency range 10(2)Hz - 10(5)Hz for film thicknesses varying between 24 Angstrom and 1100 Angstrom. We present the measurements on the thinnest films and examine their consistency with the predictions from various models for the superconducting transition, in particular the Kosterlitz-Thouless (KT) theory for the vortex-antivortex unbinding phase transition.
Notes: Superconducting and Related Oxides: Physics and Nanoengineering III, Davor Pavuna, Ivan Bozovic, Editors, December 1998
3rd Conference on Oxide Superconductor Physics and Nanoengineering, JUL 20-24, 1998 SAN DIEGO, CA
Abstract: If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligophenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.
