Artur Erbe

Benzenedithiol: A Broad-Range Single-Channel Molecular Conductor

More than a decade after the first report of single-molecule conductance, it remains a challenging goal to prove the exact nature of the transport through single molecules, including the number of transport channels and the origin of these channels from a molecular orbital point of view. We demonstrate for the archetypical organic molecule, benzenedithiol (BDT), incorporated into a mechanically controllable break junction at low temperature, how this information can be deduced from studies of the elastic and inelastic current contributions. We are able to tune the molecular conformation and thus the transport properties by displacing the nanogap electrodes. We observe stable contacts with low conductance in the order of 10(-3) conductance quanta as well as with high conductance values above ∼0.5 quanta. Our observations show unambiguously that the conductance of BDT is carried by a single transport channel provided by the same molecular level, which is coupled to the metallic electrodes, through the whole conductance range. This makes BDT particularly interesting for applications as a broad range coherent molecular conductor with tunable conductance.

Nanomechanical resonator shuttling single electrons at radio frequencies.

We observe transport of electrons through a metallic island on the tip of a nanomechanical pendulum. The resulting tunneling current shows distinct features corresponding to the discrete mechanical eigenfrequencies of the pendulum. We report on measurements covering the temperature range from 300 down to 4.2 K. We explain the I-V curve, which unexpectedly differs from previous theoretical predictions, with model calculations based on a master equation approach.

Charge transport characteristics of diarylethene photoswitching single-molecule junctions.

We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.

Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single‐Molecule Junctions

A combined experimental and theoretical study is presented revealing the influence of metal-molecule coupling on electronic transport through single-molecule junctions. Transport experiments through tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current-voltage characteristics to a single-level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal-molecule coupling. The values found for these parameters are rationalized with the help of density-functional-theory-based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.

Mechanical mixing in nonlinear nanomechanical resonators

The physics of nonlinear dynamics has been studied in detail in macroscopic mechanical systems like the driven classical pendulum. By now, it is possible to build mechanical devices on the nanometer scale with eigenfrequencies on the order of several 100 MHz. In this work, we want to present how to machine such nanomechanical resonators out of silicon-on-insulator wafers and how to operate them in the nonlinear regime in order to investigate higher-order mechanical mixing at radio frequencies. The nonlinear response then is compared in detail to nth-order perturbation theory and nonperturbative numerical calculations.

Direct Measurement of Electrical Transport Through G-Quadruplex DNA with Mechanically Controllable Break Junction Electrodes†

The need for miniaturization of devices for future nanoelectronic applications has led to the search for new constituents in molecular electronics. DNA is particularly interesting for applications in nanoelectronics circuits owing to its inherent properties, such as the predictable size and selfassembly of the stacked nucleobase pairs. 2] In recent years, charge transport in double-stranded DNA (dsDNA) has attracted considerable attention because of its potential use in building blocks for future nanoelectronic circuits. The onedimensional nanowire conformation of DNA and its unique self-assembly ability 2] can also be used in biochemical sensors. Theoretical studies suggested rather high conductance of DNA in the case of optimal and undisturbed overlap of the electronic orbitals of the p electrons. Previously, several experimental groups reported high conductance of particular DNA molecules, whereas other experiments showed predominantly that the conductance of DNA was very low, which is presumably due to variation in the contact geometries 12] and its variable sequence and flexible conformation. However, it was recently reported that short dsDNA with a G–C sequence is more conductive than that with a A–T sequence. Certain guanine-rich DNA sequences, such as those found in telomeres at the end of chromosomes, can form stable four-stranded structures, which result from the stacking of several G-quartets folding into quadruplex structures. Furthermore, potential G-quartet-forming sequences have been found to be enriched in promoters of proto-oncogenes. Herein we present direct transport measurements on a G-quadruplex covalently wired between two gold electrodes realized by the mechanically controllable break junction (MCBJ) technique. The G-quadruplex shows a rather high conductance. Interestingly, when the distance of both electrodes was reversibly varied over a several-nanometer span, this conductance behavior persists reproducibly. These unprecedented properties make G-quadruplexes interesting candidates for nanoelectronic applications in which varied distances between electrodes need bridging without loss of conductance. Apart from the usual helical double-stranded form, DNA with particular sequences can also form stable four-stranded structures with repeated guanine bases. These structures result from the stacking of several G-quartets (planes of four guanines held together by eight hydrogen bonds; Figure 1a). The G-quartet stacking can be further stabilized by cations (typically K or Na) located between two quartets. From various sequences, both intramolecular and intermolecular G-quadruplexes or G-wires with lengths of up to micrometers can be formed. For the human telomeric sequence that was used in our experiments, different structures are reported that containing either parallel, antiparallel, or even mixed parallel-antiparallel folding of the strands.

A mechanically flexible tunneling contact operating at radio frequencies

We report on a nanomachined electromechanical resonator applied as a mechanically flexible tunneling contact. The resonator was machined out of a single-crystal silicon-on-insulator substrate and operates at room temperature with frequencies up to some 73 MHz, transferring electrons by mechanical motion.

Influence of Laser Light on Electronic Transport through Atomic-Size Contacts

This Letter reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable break-junction technique. We concentrate on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, and on the wavelength, the intensity, and the position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.

Nanostructured silicon for studying fundamental aspects of nanomechanics

An oil pump has a split frame with the pump mechanism situated in a chamber in one of the frames or between the split frames. An inlet conduit extends from an inlet port to the low pressure side of the pump and an outlet conduit extends from the high pressure side of the pump to an outlet port. An excess pressure relief path is provided to permit fluid flow from the outlet conduit to the inlet conduit when the outlet conduit pressure exceeds the inlet conduit pressure by a predetermined difference. This is accomplished by providing a bypass channel formed in one frame and fluidly intersecting the outlet conduit, and a valve cavity formed in the other frame and fluidly intersecting the bypass channel and the inlet conduit. Valve means are situated in the valve cavity for isolating the bypass channel from the inlet conduit during normal pressure differences between the inlet and outlet conduits. Upon the occurrence of excess pressure in the outlet conduit, the valve opens to permit fluid flow from the outlet conduit to the inlet conduit.

Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser

We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the systems quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.