Charge transport across dynamic covalent chemical bridges (original) (raw)
Related papers
Journal of Physics-condensed Matter, 2008
It is now becoming clear that the characteristics of the whole junction are important in determining the conductance of single molecules bound between two metal contacts. This paper shows through measurements on a series of seven conjugated molecular bridges that contact separation is an important factor in determining the electrical response of the molecular junction. These data are obtained using the I(t) method developed by Haiss et al since the scanning tunnelling microscope tip to substrate separation can be controlled through choice of the set-point (I0) current and calibrated with current-distance curves and knowledge of the terminal to terminal length of the molecular wire. The contact gap separation dependence is interpreted as arising from tilting of these molecules in the junction and this model is underpinned by ab initio transport computations. In this respect we make the general observation that conductance increases rather dramatically at higher tilt angle away from the normal for conformationally rigid molecular wires and that this increase in conductance arises from increased electronic coupling between the molecular bridge and the gold contacts.
Promising anchoring groups for single-molecule conductance measurements
Phys. Chem. Chem. Phys., 2014
The understanding of the charge transport through single molecule junctions is a prerequisite for the design and building of electronic circuits based on single molecule junctions. However, reliable and robust formation of such junctions is a challenging task to achieve. In this topical review, we present a systematic investigation of the anchoring group effect on single molecule junction conductance by employing two complementary techniques, namely scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques, based on the studies published in the literature and important results from our own work. We compared conductance studies for conventional anchoring groups described earlier with the molecular junctions formed through p-interactions with the electrode surface (Au, Pt, Ag) and we also summarized recent developments in the formation of highly conducting covalent Au-C s-bonds using oligophenyleneethynylene (OPE) and an alkane molecular backbone. Specifically, we focus on the electron transport properties of diaryloligoyne, oligophenyleneethynylene (OPE) and/or alkane molecular junctions composed of several traditional anchoring groups, (dihydrobenzo[b]thiophene (BT), 5-benzothienyl analogue (BTh), thiol (SH), pyridyl (PY), amine (NH 2 ), cyano (CN), methyl sulphide (SMe), nitro (NO 2 )) and other anchoring groups at the solid/liquid interface. The qualitative and quantitative comparison of the results obtained with different anchoring groups reveals structural and mechanistic details of the different types of single molecular junctions. The results reported in this prospective may serve as a guideline for the design and synthesis of molecular systems to be used in molecule-based electronic devices.
Probing the chemistry of molecular heterojunctions using thermoelectricity
Nano …, 2008
Thermopower measurements offer an alternative transport measurement that can characterize the dominant transport orbital and is independent of the number of molecules in the junction. This method is now used to explore the effect of chemical structure on the electronic structure and charge transport. We interrogate junctions, using a modified scanning tunneling microscope break junction technique, where: (i) the 1,4-benzenedithiol (BDT) molecule has been modified by the addition of electron-withdrawing or -donating groups such as fluorine, chlorine, and methyl on the benzene ring; and (ii) the thiol end groups on BDT have been replaced by the cyanide end groups. Cyanide end groups were found to radically change transport relative to BDT such that transport is dominated by the lowest unoccupied molecular orbital in 1,4-benzenedicyanide, while substituents on BDT generated small and predictable changes in transmission.
Polarizability as a Molecular Descriptor for Conductance in Organic Molecular Circuits
Journal of Physical Chemistry C, 2016
We explore a connection between the static molecular polarizability and the molecular conductance that arises naturally in the description of electrified molecular interfaces and that has recently been explored experimentally. We have tested this idea by using measured conductance of few different experimental design motifs for molecular junctions and relating them to the molecular polarizability. Our results show that, for a family of structurally connected molecules, the conductance decreases as the molecular polarizability increases. Within the limitations of our model, this striking result is consistent with the physically intuitive picture that a molecule in a junction behaves as a dielectric that is polarized by the applied bias, hence creating an interfacial barrier that hinders tunneling. The use of the polarizability as a descriptor of molecular conductance offers significant conceptual and practical advantages over a picture based on molecular orbitals. To further illustrate the plausibility of this idea, we have used Simmons tunneling model that incorporates image charge and dielectric effects to describe transport through a barrier that represents the molecular junction. In such a model, the barrier height depends on the effective dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the molecular polarizability via the classical Clausius-Mossotti relation. Despite the simplicity of our model, it sheds light on a hitherto neglected connection between molecular polarizability and conductance, and paves the way for further experimental, conceptual and theoretical developments.
Nano Letters, 2008
We describe a method to determine whether a measured current in a break junction is due to electron tunneling via space or conduction through a molecule bridged between two electrodes. By modulating the electrode separation, we monitor both the DC and the AC components of the current. The AC component indicates if a molecule is connected to the electrodes while the DC component is the transport current through the molecule. This method allows us to remove the tunneling background from conductance histograms and unambiguously measure the I- V characteristic of single molecules. Furthermore, it provides valuable information about the electromechanical properties of single molecules.
Dependence of single-molecule junction conductance on molecular conformation
Nature, 2006
The conductance of a single metal-molecule-metal junction depends not only on the chemical nature of the molecule used, but also on its conformation 1-4 . In the simple case of a biphenyl-two phenyl rings linked by a single C-C bond-conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. A number of different techniques have measured the conductance of metal-molecule(s)-metal junctions 5-12 .
Correlation of breaking forces, conductances and geometries of molecular junctions
Scientific Reports, 2015
Electrical and mechanical properties of elongated gold-molecule-gold junctions formed by tolane-type molecules with different anchoring groups (pyridyl, thiol, amine, nitrile and dihydrobenzothiophene) were studied in current-sensing force spectroscopy experiments and density functional simulations. Correlations between forces, conductances and junction geometries demonstrate that aromatic tolanes bind between electrodes as single molecules or as weakly-conductive dimers held by mechanically-weak p 2 p stacking. In contrast with the other anchors that form only S-Au or N-Au bonds, the pyridyl ring also forms a highly-conductive cofacial link to the gold surface. Binding of multiple molecules creates junctions with higher conductances and mechanical strengths than the single-molecule ones. M olecular junctions created by single molecules trapped between two probes enable the creation of nanoscale structures with unique mechanical, electrical, optical and quantum properties 1-3 . A common strategy to form molecular junctions is based on the approach and contact of a sharp nanoprobe to a second probe in the presence of the molecules of interest, followed by the subsequent separation of the probes. This strategy is widely employed in mechanically controllable break junction (MCBJ) or scanning tunnelling microscopy-based break junction (STMBJ) 1-3 experiments, and in force spectroscopy 4-6 to characterise electrical and mechanical properties, respectively. In particular, conductance studies with small organic molecules revealed unique correlations between molecular structure and junction conductances. These studies explored the influence of molecular length and conjugation 7-12 , torsion angle 13-15 , electrode material 16,17 , and anchoring group 18-23 , on junction conductance. To date, generic force spectroscopy experiments primarily address supramolecular bonds 24-28 , and almost no studies of mechanical properties of covalently-bound junctions were reported 4 .
Single-Molecule Conductance of Functionalized Oligoynes: Length Dependence and Junction Evolution
We report a combined experimental and theoretical investigation of the length dependence and anchor group dependence of the electrical conductance of a series of oligoyne molecular wires in single-molecule junctions with gold contacts. Experimentally, we focus on the synthesis and properties of diaryloligoynes with n = 1, 2, and 4 triple bonds and the anchor dihydrobenzo[b]thiophene (BT). For comparison, we also explored the aurophilic anchor group cyano (CN), amino (NH 2 ), thiol (SH), and 4-pyridyl (PY). Scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics. The BT moiety is superior as compared to traditional anchoring groups investigated so far. BT-terminated oligoynes display a 100% probability of junction formation and possess conductance values which are the highest of the oligoynes studied and, moreover, are higher than other conjugated molecular wires of similar length. Density functional theory (DFT)-based calculations are reported for oligoynes with n = 1−4 triple bonds. Complete conductance traces and conductance distributions are computed for each family of molecules. The sliding of the anchor groups leads to oscillations in both the electrical conductance and the binding energies of the studied molecular wires. In agreement with experimental results, BT-terminated oligoynes are predicted to have a high electrical conductance. The experimental attenuation constants β H range between 1.7 nm −1 (CN) and 3.2 nm −1 (SH) and show the following trend: β H (CN) < β H (NH 2 ) < β H (BT) < β H (PY) ≈ β H (SH). DFT-based calculations yield lower values, which range between 0.4 nm −1 (CN) and 2.2 nm −1 (PY).