Scanning Tunneling Microscopy Break Junction Research Articles

Molecular conductance versus inductive effects of axial ligands on the electrocatalytic activity of self-assembled iron phthalocyanines: The oxygen reduction reaction

The construction of self-assembled iron phthalocyanine (FePc) systems on gold electrodes modified by self-assembled monolayers (SAMs) is becoming an interesting strategy for obtaining electrocatalytic molecular building blocks for the oxygen reduction reaction (ORR). In this work, we have measured the conductance of pyridiniums axial ligands at the single molecule level using the scanning tunneling microscope-based break-junction method (STM-Break Junction) to study the role of the axial ligand on the activity of the self-assembled FePc systems on a gold electrode surface. The electron-pulling effect of pyridinium axial ligands is known to increase the electrocatalytic activity of FePc for the oxygen reduction reaction (ORR). We have used these systems as a platform for carrying out a comparative study for understanding the real influence of the proximal axial ligands. Further, these ligands act as molecular wires between the gold electrode surface and the FePc molecule. The pyridinium molecules were synthesized following a series of structural variations using a basic molecular backbone. From conductance measurements obtained for each pyridinium molecule, it was possible to establish that electron transport through each pyridinium does not influence the activity of FePc for ORR in alkaline media. In addition, the DFT calculations shows that the axial ligand in FePc modifies its catalytic activity by decreasing the binding energy of O2 to the Fe site.

Can One Define the Conductance of Amino Acids?

We studied the electron-transport properties of ten different amino acids and one dimer (di-methionine) using the mechanically controlled break-junction (MCBJ) technique. For methionine and cysteine, additional measurements were performed with the scanning tunneling microscope break-junction (STM-BJ) technique. By means of a statistical clustering technique, we identified several conductance groups for each of the molecules considered. Ab initio calculations revealed that the observed broad conductance distribution stems from the possibility of various binding geometries which can be formed during stretching combined with a multitude of possible conformational changes. The results suggest that it would be helpful to explore different experimental techniques such as recognition tunneling and conditions to help identify the nature of amino-acid-based junctions even further, for example, with the goal to establish a firm platform for their unambiguous recognition by tunneling break-junction experiments.

Synthetic Control of Quantum Interference by Regulating Charge on a Single Atom in Heteroaromatic Molecular Junctions.

A key area of activity in contemporary molecular electronics is the chemical control of conductance of molecular junctions and devices. Here we study and modify a range of pyrrolodipyridines (carbazole-like) molecular wires. We are able to change the electrical conductance and quantum interference patterns by chemically regulating the bridging nitrogen atom in the tricyclic ring system. A series of eight different N-substituted pyrrolodipyridines has been synthesized and subjected to single-molecule electrical characterization using an STM break junction. Correlations of these experimental data with theoretical calculations underline the importance of the pyrrolic nitrogen in facilitating conductance across the molecular bridge and controlling quantum interference. The large chemical modulation for the meta-connected series is not apparent for the para-series, showing the competition between (i) meta-connectivity quantum interference phenomena and (ii) the ability of the pyrrolic nitrogen to facilitate conductance, that can be modulated by chemical substitution.

Metal-Single-Molecule-Semiconductor Junctions Formed by a Radical Reaction Bridging Gold and Silicon Electrodes.

Here we report molecular films terminated with diazonium salts moieties at both ends which enables single-molecule contacts between gold and silicon electrodes at open circuit via a radical reaction. We show that the kinetics of film grafting is crystal-facet dependent, being more favorable on ⟨111⟩ than on ⟨100⟩, a finding that adds control over surface chemistry during the device fabrication. The impact of this spontaneous chemistry in single-molecule electronics is demonstrated using STM-break junction approaches by forming metal-single-molecule-semiconductor junctions between silicon and gold source and drain, electrodes. Au-C and Si-C molecule-electrode contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 1.1 s, which is 30-400% higher than that reported for conventional molecular junctions formed between gold electrodes using thiol and amine contact groups. The high stability enabled measuring current-voltage properties during the lifetime of the molecular junction. We show that current rectification, which is intrinsic to metal-semiconductor junctions, can be controlled when a single-molecule bridges the gap in the junction. The system changes from being a current rectifier in the absence of a molecular bridge to an ohmic contact when a single molecule is covalently bonded to both silicon and gold electrodes. This study paves the way for the merging of the fields of single-molecule and silicon electronics.

Potential-Induced High-Conductance Transport Pathways through Single-Molecule Junctions.

Employing single molecules as electronic circuit building blocks is one promising approach to electronic device miniaturization. We report single-molecule junction formation where the orientation of molecules can be controlled externally by the working electrode potential. The scanning tunneling microscopy break junction (STM-BJ) method is used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic acid (TPA) molecules between the Au(111) electrode and the STM tip to measure the single-molecule conductance through the junction. When the Au(111) electrode is at negative potentials (with respect to the zero-charge potential), a highly ordered and flat-oriented superstructure forms, allowing for direct contact between the π system of the benzene ring of the molecules and the Au(111) electrode, leading to junction formation with no anchoring group involvement. Our first-principles nonequilibrium Green's function (NEGF) computation shows a flat configuration yields a conductance that is 3 orders of magnitude larger than for a molecule vertically connected to the electrodes via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ± 0.02 G0 are experimentally measured with the flat configurations of TFTPA and TPA, respectively. These values are at least 2 orders of magnitude higher than the experimental values previously reported for the conductance of TPA bridged through carboxylic acid anchoring groups (3.8 × 10-4-3.2 × 10-3 G0). In contrast, a positively charged surface triggers an order-disorder transition eliminating the high-conductance states, most likely because the formation of the flat-oriented junction is prevented. The dependence of TFTPA conductance on the electrode potential (electrode Fermi level) suggests a LUMO mediated transport mechanism. Calculation confirms the lack of an effect of the addition of an electron-withdrawing group are investigated.

Modularized Tuning of Charge Transport through Highly Twisted and Localized Single-Molecule Junctions

Although most molecular electronic devices and materials consist of a backbone with a planar structure, twisted molecular wires with reduced inter-ring π-orbital overlap offer a unique opportunity for the modularized fabrication of molecular electronic devices. Herein we investigate the modularized tuning of the charge transport through the localized molecules by designing highly twisted molecules and investigating their single-molecule charge transport using the scanning tunneling microscopy break junction technique. We find that the anthracenediyl-core molecule with a 90° inter-ring twist angle shows an unexpectedly high conductance value, which is five times higher than that of the phenylene-core molecule with a similar configuration, whereas the conductance of the delocalized planar molecule with an anthracenediyl core or a phenylene core is almost the same. Theoretical calculations revealed that highly twisted angles result in weak interactions between molecular building blocks, for which molecule orbitals are separated into localized blocks, which offers the chance for the modularized tuning of every single block. Our findings offer a new strategy for the design of future molecular devices with a localized electronic structure.

Comparative Study of Single Molecular Junctions with Para-Phthalic Acid and Meta-Phthalic Acid Binding to Different Metal Electrodes.

In this paper, single molecular junctions of Para-phthalic acid and Meta-phthalic acid with Au electrodes were studied by STM break junction approach. Conductance values of 10-3.55 G0 and 10-3.70 G0 were found for Para-phthalic acid and Meta-phthalic acid, respectively. The conductance order between Para-phthalic acid and Meta-phthalic acid with Au is different from that with Cu, which can be contributed to the different coupling between molecules and electrodes; different through-space interaction is proposed for such phenomenon between Cu and Au electrodes. Furthermore, the breaking off distances can reflect the length of molecules. The current work presents the important role of electrode in single molecular junctions with different position anchoring groups.

Enhanced coupling through π-stacking in imidazole-based molecular junctions.

We demonstrate that imidazole based π-π stacked dimers form strong and efficient conductance pathways in single-molecule junctions using the scanning-tunneling microscope-break junction (STM-BJ) technique and density functional theory-based calculations. We first characterize an imidazole-gold contact by measuring the conductance of imidazolyl-terminated alkanes (im-N-im, N = 3-6). We show that the conductance of these alkanes decays exponentially with increasing length, indicating that the mechanism for electron transport is through tunneling or super-exchange. We also reveal that π-π stacked dimers can be formed between imidazoles and have better coupling than through-bond tunneling. These experimental results are rationalized by calculations of molecular junction transmission using non-equilibrium Green's function formalism. This study verifies the capability of imidazole as a Au-binding ligand to form stable single- and π-stacked molecule junctions at room temperature.

Control over Near-Ballistic Electron Transport through Formation of Parallel Pathways in a Single-Molecule Wire.

This paper reports highly efficient coherent tunneling in single-molecule wires of oligo-ferrocenes with one to three Fc units. The Fc units were directly coupled to the electrodes, i.e., without chemical anchoring groups between the Fc units and the terminal electrodes. We found that a single Fc unit readily interacts with the metal electrodes of an STM break junction (STM = scanning tunneling microscope) and that the zero-voltage bias conductance of an individual Fc molecular junction increased 5-fold, up to 80% of the conductance quantum G0 (77.4 μS), when the length of the molecular wire was increased from one to three connected Fc units. Our compendium of experimental evidence combined with nonequilibrium Green function calculations contemplate a plausible scenario to explain the exceedingly high measured conductance based on the electrode/molecule contact via multiple Fc units. The oligo-Fc backbone is initially connected through all Fc units, and, as one of the junction electrodes is pulled away, each Fc unit is sequentially disconnected from one of the junction terminals, resulting in several distinct conductance features proportional to the number of Fc units in the backbone. The conductance values are independent of the applied temperature (-10 to 85 °C), which indicates that the mechanism of charge transport is coherent tunneling for all measured configurations. These measurements show the direct Fc-electrode coupling provides highly efficient molecular conduits with very low barrier for electron tunneling and whose conductivity can be modulated near the ballistic regime through the number of Fc units able to bridge and the energy position of the frontier molecular orbital.

Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application.

Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.

"Doping" of Polyyne with an Organometallic Fragment Leads to Highly Conductive Metallapolyyne Molecular Wire.

Exploration of highly conductive molecules is essential to achieve single-molecule electronic devices. The present paper describes the results on single-molecule conductance study of polyyne wires doped with the organometallic Ru(dppe)2 fragment, X-(C≡C) n-Ru(dppe)2-(C≡C) n-X. The metallapolyyne wires end-capped with the gold fragments (X = AuL) are subjected to single-molecule conductance measurements with the STM break junction technique, which reveal the high conductance (10-3-10-2 G0; n = 2-4) with the low attenuation factor (0.25 Å-1) and the low contact resistance (33 kΩ). A unique "'doping'" effect of Ru(dppe)2 fragment was found to lead to the high performance as suggested by the hybrid density functional theory-nonequilibrium green function calculation.

Side-Group Effect on Electron Transport of Single Molecular Junctions.

In this article, we have investigated the influence of the nitro side-group on the single molecular conductance of pyridine-based molecules by scanning tunneling microscopy break junction. Single molecular conductance of 4,4′-bipyridine (BPY), 3-nitro-4-(pyridin-4-yl)pyridine (BPY-N), and 3-nitro-4-(3-nitropyridin-4-yl)pyridine (BPY-2N) were measured by contact with Au electrodes. For the BPY molecular junction, two sets of conductance were found with values around 10−3.1 G0 (high G) and 10−3.7 G0 (low G). The addition of nitro side-group(s) onto the pyridine ring resulted in lower conductance of 10−3.8 G0 for BPY-N and 10−3.9 G0 for BPY-2N, respectively, which can be attributed to the twist angle of two pyridine rings. Moreover, the steric hindrance of nitro group(s) also affects the contacting configuration of electrode-molecule-electrode. As a consequence, only one set of conductance value was observed for BPY-N and BPY-2N. Our work clearly shows the important role of side-groups on the electron transport of single-molecule junctions.

The Electrical Properties of Porphyrin Single Molecule Wires

Measurement of the electrical properties of porphyrin single molecular wires sandwiched between metal contacts is now an experimental reality and such methods have contributed to understanding charge flow through and the current-voltage response of porphyrin wires. We have exploited scanning-tunneling-microscopy-based methods such as the STM break junction method and the non-contact I(s) technique (I=current and s=distance between STM tip and substrate) for achieving this feat. In both these techniques single molecule junctions are formed by bringing a gold STM tip into contact or very close to the gold substrate surface. As the tip is rapidly retracted the conductance of the porphyrin molecule junction or the complete current-voltage response can be recorded. When combined with statistical analysis this provides a strong platform for investigating the electrical properties of porphyrin junctions. This presentation will review some of our findings on the electrical properties of porphyrin single molecule wires as well as presenting our latest data recorded using the STM break junction method, with a focus on the mechanochemical properties of 5,15-diaryl porphyrins with thiol end groups. Topics to be discussed include:(1) The mechanochemical properties of 5,15-diaryl porphyrins with thiol end groups. (2) Single molecule junction formation mechanisms and evolution during junction stretching.(3) The influence of contacting groups on porphyrin electrical junctions.(4) Long range electron transport in porphyrin oligomers, their current-voltage response and unusual voltage dependent length decay of conductance.(5) Mechanisms of charge transport in porphyrin single molecule wires.REFERENCES to our work on porphyrin single molecule electronics(1) G. Sedghi, K. Sawada, L. J. Esdaile, M. Hoffmann, H. L. Anderson, D. Bethell, W. Haiss, S. J. Higgins, and R. J. Nichols, Single Molecule Conductance of Porphyrin Wires with Ultralow Attenuation, Journal of the American Chemical Society 130, 8582 (2008).(2) G. Sedghi, V.M. Garcia-Suarez, L.J. Esdaile, H.L. Anderson, C.J. Lambert, S. Martin, D. Bethell, S.J. Higgins, M. Elliott, N. Bennett, J.E. Macdonald and R.J. Nichols, Long-range electron tunnelling in oligo-porphyrin molecular wires, Nature Nanotechnology, 6, 517-523, (2011).(3) G. Sedghi, L.J. Esdaile, H.L. Anderson, S. Martin, D. Bethell, S.J. Higgins and R.J. Nichols, Comparison of the Conductance of Three Types of Porphyrin-Based Molecular Wires: beta,meso,beta-Fused Tapes, meso-Butadiyne-Linked and Twisted meso-meso Linked Oligomers, Advanced Materials, 24, 653-, (2012).(4) Edmund Leary, Cécile Roche, Hua-Wei Jiang, Iain Grace, M. Teresa González, Gabino Rubio-Bollinger, Yaoyao Xiong, Qusiy Al-Galiby, Maria A. Lebedeva, Kyriakos Porfyrakis, Nicolás Agrait, Andrew Hodgson, Simon J. Higgins, Colin J. Lambert, Harry L. Anderson, and Richard J. Nichols, JACS submitted 2017. Figure 1

Conductance Measurement of Pyrazine Molecular Junction with Cu and Ag Electrodes.

We have measured the conductance of pyrazine molecular junction contacting with Cu and Ag electrodes by using an electrochemical jump-to-contact based scanning tunneling microscopy break junction (ECSTM-BJ). While conductance values of 10-2.8 and 10-3.7 G0 are measured for pyrazineCu electrode, 10-2.1 and 10-3.3 G0 are found for pyrazine-Ag contact. The result shows that the conductance of pyrazine with Ag electrode is larger than that with Cu electrode, which can contribute to the different efficiency of electron transport along the molecular junction between Ag and Cu electrodes. The current work shows the important role for the electrode material in electron transport.

Ferrocene-Alkynyl Conjugated Molecular Wires: Synthesis, Characterization, and Conductance Properties.

A novel series of 1,2,3-substituted ferrocene-based wires a1-a2 and b4-b5 have been synthesized by using an iterative Pd-mediated Sonogashira cross-coupling methodology. The molecular structures of a2 and b3 were determined by single-crystal X-ray analysis. Electrochemical data showed that there was a strong electronic communication among the ferrocenyl moieties in b1-b5. The UV absorption spectra indicated that replacing the 1,1'-substituted ferrocene unit with a 1,2,3-substituted ferrocene moiety causes delocalization of electrons in the extended π orbitals. The self-assembled monolayers of wire a1 and a2 on Au surfaces have been comprehensively characterized by electrochemistry and scanning tunneling microscopy break junction. The data demonstrated that 1,2,3-substituted ferrocene-based wires reduced the intermolecular π-π stacking, and furthermore solved the rotation problem in the 1,1'-substituted ferrocene-based wires.

Detecting Mechanochemical Atropisomerization within an STM Break Junction.

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in C2Cl4D2). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au-porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S-S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au-S and Au-Au bonds, but higher than that of the Au-SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound.

Precise tuning of single molecule conductance in an electrochemical environment.

Tuning of molecular conductance in a liquid environment is a hot topic in molecular electronics. In this article, we explore a new concept where the Fermi level positions of the metallic ends are varied simply by modifying the electroactive salt concentration in solution. We rely on the electrochemical scanning tunneling microscope break junction method that allows the construction in solution of copper atomic contacts that can be then bridged by single molecules. The experimental conductance evolution is first confronted with an analytical formulation that allows the deduction of the molecule's LUMO position and electronic coupling factors. These parameters are in close agreement with those obtained by independent DFT calculations.

Photoconductance from Exciton Binding in Molecular Junctions

We report on a theoretical analysis and experimental verification of a mechanism for photoconductance, the change in conductance upon illumination, in symmetric single-molecule junctions. We demonstrate that photoconductance at resonant illumination arises due to the Coulomb interaction between the electrons and holes in the molecular bridge, so-called exciton-binding. Using a scanning tunneling microscopy break junction technique, we measure the conductance histograms of perylene tetracarboxylic diimide (PTCDI) molecules attached to Au-electrodes, in the dark and under illumination, and show a significant and reversible change in conductance, as expected from the theory. Finally, we show how our description of the photoconductance leads to a simple design principle for enhancing the performance of molecular switches.

Investigation of the geometrical arrangement and single molecule charge transport in self-assembled monolayers of molecular towers based on tetraphenylmethane tripod

Multipodal molecular platforms were designed recently to establish a well-defined contact between molecular electronic components and metallic electrodes to manufacture working devices based on molecular electronics. In this work, we use electrochemical techniques, scanning tunneling microscopy break junction technique and theoretical approaches combining density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism to investigate the geometrical arrangement and single molecule charge transport in self-assembled monolayers (SAMs) of molecular towers anchored by tetraphenylmethane tripod on Au (111) surface. The effect of the molecular length as well as the role of the position of anchoring groups was addressed. Electrochemical double-layer capacitance measurements and reductive desorption studies combined with theoretical modeling clearly demonstrated that the molecular towers form densely packed SAMs, in which the individual molecules are attached to the Au (111) surface by the tripodal base and the principal molecular axis is directed away from the electrode surface, providing thus desired orientation. Temperature resolved single molecule conductance measurements combined with DFT/NEGF calculations showed that the electric charge is transported by tunneling via highly conductive molecular junctions formed by the tripodal base. Our combined experimental and theoretical work demonstrates that tetraphenylmethane tripods are suitable platforms to bear functional groups serving as molecular electronic components.

Statistical I–V measurements of single-molecule junctions with an asymmetric anchoring group 1,4-aminobenzenethiol**Invited talk at 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8–12 November 2016, Ha Long City, Vietnam.

Molecular diodes are an interesting topic in the field of single-molecule electronics. Rectification of molecules such as 1,4-aminobenzenethiol (ABT) having different contact areas was reported. However, a more statistical approach is necessary to clarify the rectification of the ABT single-molecule junctions. In this research, we statistically measured the single molecular conductance and I–V characteristics of ABT single-molecule junctions using the scanning tunneling microscope break junction (STM-BJ) method. Two single molecular conductances caused to difference of bridging geometries were observed in the conductance measurements. Statistically significant rectification was not observed for ABT junctions. We concluded rectification does not appear only due to the difference of two anchoring groups in case of a small conjugate molecule such as ABT.