Molecular Break Junction Research Articles

Conformation-dependent conductance through a molecular break junction

Ab initio molecular dynamics simulations have been performed of a gold-1,4-benzenedithiol (BDT)-gold nanojunction under mechanical stress. For three different pulling rates between 10 and 40 m s(-1), it is found that the nanowire always ruptures between the second and third Au atom from the thiol sulfur. Larger rupture forces and longer extensions are required at higher pulling rates and vice versa. The electrical conductance was calculated along a pulling trajectory using the DFT-NEGF method to study the effect of thermal and stress-induced structural changes on the electrical transport properties. While the mechanically induced stretching of the junction is seen to lower the time-averaged conductance, thermal conformational changes are capable of altering the conductance by one order of magnitude. No single geometric quantity could be identified as the main contributor to the conductance fluctuations. Small modulations, however, can be explained in terms of C=C double bond vibrations in the BDT molecule. The dependence of the conductance on different geometric variables has further been investigated systematically by performing constrained geometry optimizations along a number of angle and dihedral coordinates. The largest changes in the conductance are observed when the Au-S-C angle and the Au-S-C-C dihedral are simultaneously constrained.

Single-Molecule Electrochemical Gating in Ionic Liquids

The single-molecular conductance of a redox active molecular bridge has been studied in an electrochemical single-molecule transistor configuration in a room-temperature ionic liquid (RTIL). The redox active pyrrolo-tetrathiafulvalene (pTTF) moiety was attached to gold contacts at both ends through -(CH(2))(6)S- groups, and gating of the redox state was achieved with the electrochemical potential. The water-free, room-temperature, ionic liquid environment enabled both the monocationic and the previously inaccessible dicationic redox states of the pTTF moiety to be studied in the in situ scanning tunneling microscopy (STM) molecular break junction configuration. As the electrode potential is swept to positive potentials through both redox transitions, an ideal switching behavior is observed in which the conductance increases and then decreases as the first redox wave is passed, and then increases and decreases again as the second redox process is passed. This is described as an "off-on-off-on-off" conductance switching behavior. This molecular conductance vs electrochemical potential relation could be modeled well as a sequential two-step charge transfer process with full or partial vibrational relaxation. Using this view, reorganization energies of ~1.2 eV have been estimated for both the first and second redox transitions for the pTTF bridge in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIOTf) ionic liquid environment. By contrast, in aqueous environments, a much smaller reorganization energy of ∼0.4 eV has been obtained for the same molecular bridge. These differences are attributed to the large, outer-sphere reorganization energy for charge transfer across the molecular junction in the RTIL.

Noise and transport characterization of single molecular break junctions with individual molecule

We studied the noise spectra of molecule-free and molecule-containing mechanically controllable break junctions. Both types of junctions revealed typical 1/f noise characteristics at different distances between the contacts with square dependence of current noise power spectral density on current. Additional Lorentzian-shape (1/f2) noise components were recorded only when nanoelectrodes were bridged by individual 1,4-benzenediamine molecule. The characteristic frequency of the revealed 1/f2 noise related to a single bridging molecule correlates with the lock-in current amplitudes. The recorded behavior of Lorentzian-shape noise component as a function of current is interpreted as the manifestation of a dynamic reconfiguration of molecular coupling to the metal electrodes. We propose a phenomenological model that correlates the charge transport via a single molecule with the reconfiguration of its coupling to the metal electrodes. Experimentally obtained results are in good agreement with theoretical ones and indicate that coupling between the molecule metal electrodes is important aspect that should be taken into account.

Gold–sulfur bond breaking in Zn(II) tetraphenylporphyrin molecular junctions

Quantized conductance measured from single tetraphenylporphyrin molecules with and without a central zinc(II) [Zn(II)] atom was measured using a molecular break junction (MBJ) method. From the conductance histograms we observed an additional 1.7Å stretch for two-state conductance in a single Zn(II) tetraphenylporphyrin (ZnTPP) molecule as compared to single state conductance in a free tetraphenylporphyrin (TPP) molecule, i.e., no central Zn(II) atom. First-principles density functional calculations, using an electrode–molecule–electrode model, are completed to provide insight into the mechanisms attributed to bond stretching, and eventual bond breaking, to better understand the additional 1.7Å of stretching observed with ZnTPP.

Fabrication of organic conductive wires and molecular break junction

Break junction have been proposed as one of methods for making a molecular-scale nano-gaps between the two metallic electrodes. Here we had made organic metallic wire bridges being consist of tetrathiafulvalene-tetracyanouinodimethane (TTF-TCNQ) organic charge transfer complex, and then gradually disconnected the bridged molecular wires by a Joule heating to make a molecular-size gap or single-molecular junction. The observed conductance of the initially high conductive wires decreased by a direct current (DC) Joule heating. The conductance exhibited several different behaviors with the stage of the thermal desorption of molecules, and finally disconnected. Significant increase and decrease of the conductance were observed just before the disconnection. After the DC treatment and before the disconnection of the wires, we switched to alternation current (AC) Joule heating to make a single-molecular junction which would work as a single-molecular transistor. During the AC Joule heating, many discrete steps were found in the time evolution of the conductance. The steps were approximately 1 pS or it's multiple, and the duration of the step were nearly the same (approx. 300 - 400 s) in the final stage of the AC Joule heating. We conclude that the step height corresponds to the single-molecular conductance and the duration reflects the probability of single TCNQ molecular desorption.

The Impact of E−Z Photo-Isomerization on Single Molecular Conductance

The single molecule conductance of the E and Z isomers of 4,4'-(ethene-1,2-diyl)dibenzoic acid has been determined using two scanning tunneling microscopy (STM) methods for forming molecular break junctions [the I(s) (I = current and s is distance) method and the in situ break junction technique]. Isomerization leads to significant changes in the electrical conductance of these molecules, with the Z isomer exhibiting a higher conductance than the E isomer. Isomerization is achieved directly on the gold surface through photoirradiation, and the STM is used to determine conductance before and after irradiation; reversible switching between the two isomers could be achieved through irradiation of the surface bound species at different wavelengths. In addition, three groups of molecular conductance values [A ("low"), B ("medium"), and C ("high")] have been measured for these carboxylate-terminated molecules. The origin of these conductance groups as well as the increase of the conductance for the Z isomer have been analyzed by comparing the length of the molecules extended in the gap, derived from molecular modeling, with the experimentally observed break-off distance for both isomers.

Probing the Molecule−Electrode Interface of Single-Molecule Junctions by Controllable Mechanical Modulations

Presented here is an in-depth study to exploit the molecule−electrode interface effects on electronic transport properties in stabilized molecular junctions under controlled mechanical modulations. By monitoring and analyzing the conductance and force changes corresponding to the modulations, we isolated the effects of both the molecule terminating groups and the different contact configurations on electronic transport properties. The experimental results can be understood by our calculations developed based on simple and straightforward models. The results would be helpful not only to resolve the discrepancies of the recent scanning tunneling microscopy molecular break junction experiment results but also to add new insights into the understanding of the electronic transport mechanisms of molecular junctions.

Fabrication and Electronic Characterization of Single Molecular Junction Devices: A Comprehensive Approach

We demonstrated a new comprehensive method to combine scanning probe microscopy (SPM) nanolithography and modified SPM break junction techniques to fabricate and characterize single molecular break junction devices. By patterning alkanedithiol and alkanediamine molecules in the alkanethiol template and measuring the conductance of the two kinds of molecular junctions, we have shown the following: (1) the new "stretch-hold" approach produced four groups of conductance values for each molecular junction, for the first time realizing the less populated conductance values that correspond to different contact configurations; (2) the electronic transport mechanism for such molecular junctions is electronic tunneling with similar decay constants for each conductance group of the same kind of molecules. The conductance differences among different groups are due to the molecule-electrode contact configuration difference, which was shown by the extrapolated contact resistances. This new approach also allows one to eliminate, or at least minimize, the variations of experimental conditions and enables the measurement of multiple molecules under the same experiment with exactly the same experimental conditions.

Charge Transport in Single Au | Alkanedithiol | Au Junctions: Coordination Geometries and Conformational Degrees of Freedom

Recent STM molecular break-junction experiments have revealed multiple series of peaks in the conductance histograms of alkanedithiols. To resolve a current controversy, we present here an in-depth study of charge transport properties of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM measurements unambiguously confirm features showing more than one set of junction configurations. On the basis of quantum chemistry calculations, we propose that certain combinations of different sulfur-gold couplings and trans/gauche conformations act as the driving agents. The present study may have implications for experimental methodology: whenever conductances of different junction conformations are not statistically independent, the conductance histogram technique can exhibit a single series only, even though a much larger abundance of microscopic realizations exists.

The effect of molecular conformation on single molecule conductance: measurements of π-conjugated oligoaryls by STM break junction

Measurements of molecular break junction reveal quantitatively the correlation between the single-molecule conductance and the conformation of pi-conjugated molecules with 6-18 conjugated double bonds.

Prediction of increased tunneling current by bond length stretch in molecular break junctions

We use ab initio calculations of the tunneling current through a 1,4-phenylenedimethanethiol (XYL) molecule adsorbed between Au(1 1 1) electrodes to show that there are circumstances under which tunneling currents can be increased by bond stretching. The effect is dependent upon the geometry of the gold–sulfur bond and does not occur for the physisorbed thiol, that is, a thiol with the hydrogen atom still in place. However, we predict that when the hydrogen atom is removed, causing the sulfur atom to be strongly bound to the gold surface, stretching this bond away from equilibrium will actually increase the tunneling current.

Quantum inelastic electron–vibration scattering in molecular wires: Landauer-like versusGreen’s function approaches and temperature effects

In this paper, we consider the problem of inelastic electron transport in molecular systemsin which both electronic and vibrational degrees of freedom are considered on the quantumlevel. The electronic transport properties of the corresponding molecular nanojunctions areobtained by means of a non-perturbative Landauer-like multi-channel inelasticscattering technique. The connections between this approach and other Green’sfunction techniques that are useful in particular cases are studied in detail. Thevalidity of the wide-band approximation, the effects of the lead self-energy and thedynamical polaron shift are also studied for a wide range of parameters. As a practicalapplication of the method, we consider the effects of the temperature on theconductance properties of molecular breakjunctions in relation to recent experiments.

Molecular break-junctions : Interaction with light and conductance switching

In this work, we analyzed electronic transport across metal-molecule-metal bridges using gold mechanically-controlled break-junctions. In particular, we studied the influence of illumination on these contacts as well as stochastic conductance fluctuations which can be detected on molecular junctions. When visible light is applied to tunnel junctions, i.e. bare break-junctions, the conductance increases. This can be explained by thermal expansion of the electrodes of the break-junction. Molecular contacts also exhibit an increase of the conductance where the mechanism is not yet clear. Without illumination, we measured the conductance of molecular junctions for extended periods of time, this can be performed for several days. We observed stochastic fluctuations of the conductance where one specific value of fluctuation is preferred. This preferred value is independent of the conductance value of the junction. Such fluctuations were observed on several samples comprising different molecules and might be identified with single molecules breaking and re-establishing chemical contact.

Theoretical study of the nonlinear conductance of Di-thiol benzene coupled to Au(1 1 1) surfaces via thiol and thiolate bonds

Di-thiol–benzene (DTB) is one of the most intensively studied systems, both experimentally and theoretically, for electron transport in molecules. Despite this, there persists a gap of three orders of magnitude between the measured and most reliable calculated conductances. In this paper, we present state of the art calculations of the electron transport through DTB coupled to Au(111) surfaces using our newly developed method TranSIESTA. The method is based on density functional theory (DFT) and determines the self-consistent electronic structure of a nanostructure coupled to 3-dimensional electrodes with different electrochemical potentials, using a full atomistic description of both the electrodes and the nanostructure. We find qualitative differences with other more approximative theoretical approaches, although we confirm that the theoretical conductance for a perfectly contacted DTB molecule is several orders of magnitude higher than the value obtained in molecular break junction (MBJ) experiments. We discuss the formation of the molecular contact in the MBJ, and calculate the current–voltage (I–V) characteristics of DTB for geometries which do not include perfect thiolate bonds to one or both of the electrodes.