Charge Transport through Single-Molecule Junctions with σ-Delocalized Systems.

Single-molecule junctions, formed by a single molecule bridging a gap between two metal electrodes, are attracting attention as basic models of ultrasmall electronic devices. Although charge transport through π-conjugated molecules with π-delocalized system has been widely studied for a number of molecular junctions, there has been almost no research on charge transport through molecular junctions with a σ-delocalized orbital system. Compounds with hexa-selenium-substituted benzene form a σ-delocalized orbital system on the periphery of the benzene ring. In this study, we fabricated single-molecule junctions with the σ-delocalized orbital systems arising from lone-pair interactions of selenium atoms and clarified their electronic properties using the break-junction method. The single-molecule junctions with the σ-orbital systems show efficient charge transport properties and can be one of the alternatives to those with conventional π-orbital systems as minute electronic conductors.

Intermolecular and Electrode-Molecule Bonding in a Single Dimer Junction of Naphthalenethiol as Revealed by Surface-Enhanced Raman Scattering Combined with Transport Measurements.

Electron transport through noncovalent interaction is of fundamental and practical importance in nanomaterials and nanodevices. Recent single-molecule studies employing single-molecule junctions have revealed unique electron transport properties through noncovalent interactions, especially those through a π-π interaction. However, the relationship between the junction structure and electron transport remains elusive due to the insufficient knowledge of geometric structures. In this article, we employ surface-enhanced Raman scattering (SERS) synchronized with current-voltage (I-V) measurements to characterize the junction structure, together with the transport properties, of a single dimer and monomer junction of naphthalenethiol, the former of which was formed by the intermolecular π-π interaction. The correlation analysis of the vibrational energy and electrical conductance enables identifying the intermolecular and molecule-electrode interactions in these molecular junctions and, consequently, addressing the transport properties exclusively associated with the π-π interaction. In addition, the analysis achieved discrimination of the interaction between the NT molecule and the Au electrode of the junction, i.e., Au-π interactions through-π coupling and though-space coupling. The power density spectra support the noncovalent character at the interfaces in the molecular junctions. These results demonstrate that the simultaneous SERS and I-V technique provides a unique means for the structural and electrical investigation of noncovalent interactions.

Unique Electrical Signature of Phosphate for Specific Single-Molecule Detection of Peptide Phosphorylation.

Single-molecule measurements of biomaterials bring novel insights into cellular events. For almost all of these events, post-translational modifications (PTMs), which alter the properties of proteins through their chemical modifications, constitute essential regulatory mechanisms. However, suitable single-molecule methodology to study PTMs is very limited. Here we show single-molecule detection of peptide phosphorylation, an archetypal PTM, based on electrical measurements. We found that the phosphate group stably bridges a nanogap between metal electrodes and exhibited high electrical conductance, which enables specific single-molecule detection of peptide phosphorylation. The present methodology paves the way to single-molecule studies of PTMs, such as single-molecule kinetics for enzymatic modification of proteins as shown here.

Visualization of Thermal Transport Properties of Self-Assembled Monolayers on Au(111) by Contact and Noncontact Scanning Thermal Microscopy.

Thermal transport properties of patterned binary self-assembled monolayers (SAMs) on Au(111) were examined using scanning thermal microscopy (SThM) with both contact and noncontact methods. We fabricated two-dimensional (2D) patterns with two separate domains of n-hexadecanethiol/benzenethiol, benzenethiol/n-butanethiol, or n-hexadecanethiol/n-butanethiol. In the experimental setup, the efficiency of thermal transport from a SThM tip to the SAM surface can be evaluated in terms of the temperature change at the SThM tip. In the contact regime, where a SThM tip physically contacts the SAM surface, direct thermal transport through the SAM and radiation-based thermal transport through the space where SAMs exist may contribute to a drop in temperature at the tip. In the noncontact regime, thermal transport relies on radiation-based heat dissipation from the heated tip to the SAMs. 2D mapping of the spatial temperature distribution on SAMs reflects the difference in thermal transport properties of the two SAM domains. We found that the contact method is effective for visualizing the temperature contrast, which reflects the thermal transport properties of the constituent molecules when the domains of the SAMs have a similar height, while the noncontact method allows visualization of the temperature distribution, which is related to the height of each domain of the SAMs, rather than the chemical structures of the constituent molecules. Combination of contact and noncontact SThM enables 2D imaging of thermal transport properties and topographic imaging simultaneously and represents a new technique for investigating the thermal properties of materials surfaces, which is essential for nanoscale thermal management.

Water Splitting Induced by Visible Light at a Copper-Based Single-Molecule Junction.

Water splitting is an essential process for converting light energy into easily storable energy in the form of hydrogen. As environmentally preferable catalysts, Cu-based materials have attracted attention as water-splitting catalysts. To enhance the efficiency of water splitting, a reaction process should be developed. Single-molecule junctions (SMJs) are attractive structures for developing these reactions because the molecule electronic state is significantly modulated, and characteristic electromagnetic effects can be expected. Here, water splitting is induced at Cu-based SMJ and the produced hydrogen is characterized at a single-molecule scale by employing electron transport measurements. After visible light irradiation, the conductance states originate from Cu/hydrogen molecule/Cu junctions, while before irradiation, only Cu/water molecule/Cu junctions were observed. The vibration spectra obtained from inelastic electron tunneling spectroscopy combined with the first-principles calculations reveal that the water molecule trapped between the Cu electrodes is decomposed and that hydrogen is produced. Time-dependent and wavelength-dependent measurements show that localized-surface plasmon decomposes the water molecule in the vicinity of the junction. These findings indicate the potential ability of Cu-based materials for photocatalysis.

Organometallic Molecular Wires with Thioacetylene Backbones, trans-{RS-(C≡C)n }2 Ru(phosphine)4 : High Conductance through Non-Aromatic Bridging Linkers.

In this work, the design, synthesis, and single-molecule conductance of ethynyl- and butadiynyl-ruthenium molecular wires with thioether anchor groups [RS=n-C6 H13 S, p-tert-Bu-C6 H4 S), trans-{RS-(C≡C)n }2 Ru(dppe)2 (n=1 (1R ), 2 (2R ); dppe: 1,2-bis(diphenylphosphino)ethane) and trans-(n-C6 H13 S-C≡C)2 Ru{P(OMe)3 }4 3hex ] are reported. Scanning tunneling microscope break-junction study has revealed conductance of the organometallic molecular wires with the thioacetylene backbones higher than that of the related organometallic wires having arylethynylruthenium linkages with the sulfur anchor groups, trans-{p-MeS-C6 H4 -(C≡C)n }2 Ru(phosphine)4 4n (n=1, 2) and trans-(Th-C≡C)2 Ru(phosphine)4 5 (Th=3-thienyl). It should be noted that the molecular junctions constructed from the butadiynyl wire 2R , trans-{Au-RS-(C≡C)2 }2 Ru(dppe)2 (Au: gold metal electrode), show conductance comparable to that of the covalently linked polyynyl wire with the similar molecular length, trans-{Au-(C≡C)3 }2 Ru(dppe)2 63 . The DFT non-equilibrium Green's function (NEGF) study supports the highly conducting nature of the thioacetylene molecular wires through HOMO orbitals.

Structure and Electron Transport at Metal Atomic Junctions Doped with Dichloroethylene.

The front cover artwork was provided by the group of Prof. Nishino, Tokyo Institute of Technology. The image depicts the investigation of the structure and electron transport of the Au, Ag, Cu, Ni, Fe, and Pd atomic junctions doped with dichloroethylene. Read the full text of the Article at 10.1002/cphc.201900988.

Structure and Electron Transport at Metal Atomic Junctions Doped with Dichloroethylene.

We have investigated the structure and electron transport at dichloroethylene-doped metal atomic junctions at low temperatures (20 K) in ultra-high vacuum, using Fe, Ni, Pd, Cu, Ag, and Au. The metal atomic junctions were fabricated using the mechanically controllable break junction technique. After introducing the dichloroethylene (DCE), the conductance behavior of Fe, Ni, and Pd junctions was considerably changed, whereas little change was observed for Cu, Ag, and Au. For the Pd and Cu junctions, a clear peak was observed in their conductance histograms, showing that the single-molecule junction was selectively formed. To investigate the structure of the metal atomic junctions further, their plateau lengths were analyzed. The length analysis revealed that the Au atomic wire was elongated, and the metal atomic wires were formed for the other transition metals: those that do not normally form metal atomic wires without DCE doping, as DCE adsorption stabilized the metal atomic states. There is a strong interaction between DCE and the metals, where DCE supports the formation of the metal atomic wire for Fe, Ni, and Pd.

Selective formation of molecular junctions with high and low conductance states by tuning the velocity of electrode displacement.

A single-molecule junction of 1,4-di(4-pyridyl)benzene (DPB) was prepared in a nano-gap between two Au electrodes using the scanning tunnelling microscopy-based break junction method (STM-BJ). Electric conductance and current versus bias voltage (I-V) measurements during the pulling and pushing processes of DPB single-molecule junctions revealed that high (H) and low (L) conductance states formed in both the pulling and pushing processes. Analysis of the I-V curves based on a single-level model indicated that the difference in conductivity of the H and L states mainly arises from high and low metal-molecule electric coupling in the junction. We demonstrated the controllable formation of H and L conductance states by simply tuning the velocity of electrode displacement in the pushing process. In the pulling process, both H and L states formed regardless of the velocity (v) of electrode displacement, while in the pushing process, H and L states could be selectively fabricated by using low (v = 16 nm s-1) and high (v = 64 nm s-1) velocities of displacement, respectively. This study provides a simple approach to selectively fabricate high and low conductance states by fine tuning of the electrode displacement.

Electric-Field-Controllable Conductance Switching of an Overcrowded Ethylene Self-Assembled Monolayer.

Molecular isomerism has been discussed from the viewpoint of the tiniest switch and memory elements in electronics. Here, we report an overcrowded ethylene-based molecular conductance switch, which fulfills all the essential requirements for implementation into electronic devices, namely, electric-field-controllable reversible conductance change with a molecular-level spatial resolution, robust conformational bistability under ambient conditions, and ordered monolayer formation on electrode surfaces. The conformational state of this overcrowded ethylene, represented by a folded or twisted conformer, is susceptible to external environments. Nanoscopic measurements using scanning tunneling microscopy techniques, together with theoretical simulations, revealed the electronic properties of each conformer adsorbed on Au(111). While the twisted conformer prevails in the molecularly dispersed state, upon self-assembly into a monolayer, a two-dimensional network structure of the folded conformer is preferentially formed due to particular intermolecular interaction. In the monolayer state, folded-to-twisted and its reverse isomerization can be controlled by the modulation of electric fields.

Tuneable single-molecule electronic conductance of C60 by encapsulation.

It has been demonstrated that the single-molecule transport properties of fullerene C60 can be modulated by encapsulating endohedral species, i.e. Li+ and H2O, which exhibit different degrees of van der Waals interactions with the C60 cage. Single-molecule junctions were prepared between the gaps of Au electrodes using a break junction technique. Encapsulation of H2O inside the cage caused a slight decrease in the electronic conductivity relative to that of pristine C60. This is in sharp contrast to Li+ encapsulation, which results in a twofold-to-fourfold increase in the conductivity. The electronic couplings between the C60 cage and the Au electrodes were weakly dependent on the endohedral species in the cage, though the molecular orbital energy levels were remarkably modulated upon encapsulation.

Highly Reproducible Formation of a Polymer Single-Molecule Junction for a Well-Defined Current Signal.

Single-molecule devices attract much interest in the development of nanoscale electronics. Although a variety of functional single molecules for single-molecule electronics have been developed, there still remains the need to implement sophisticated functionalization toward practical applications. Given its superior functionality encountered in macroscopic materials, a polymer could be a useful building block in the single-molecule devices. Therefore, a molecular junction composed of polymer has now been created. Furthermore, an automated algorithm was developed to quantitatively analyze the tunneling current through the junction. Quantitative analysis revealed that the polymer junction exhibits a higher formation probability and longer lifetime than its monomer counterpart. These results suggest that the polymer provides a unique opportunity to design both stable and highly functional molecular devices for nanoelectronics.

Fluctuation in Interface and Electronic Structure of Single-Molecule Junctions Investigated by Current versus Bias Voltage Characteristics.

Structural and electronic detail at the metal-molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still remain elusive. On the single-molecule scale, the metal-molecule interface structures and relevant charge transport properties are subject to fluctuation, which contain the fundamental science of single-molecule transport and implication for manipulability of the transport properties in electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal-molecule interface in single-molecule junctions, based on current-voltage ( I- V) measurements in combination with first-principles simulation. Contrary to conventional molecular conductance studies, this I- V approach provides a correlated statistical description of both the degree of electronic coupling across the metal-molecule interface and the molecular orbital energy level. This statistical approach was employed to study fluctuation in single-molecule junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4'-bipyridine (BPY), and fullerene (C60). We demonstrate that molecular-dependent fluctuation of σ-, π-, and π-plane-type interfaces can be captured by analyzing the molecular orbital (MO) energy level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C60 features unique distance dependence and molecular-dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrast, the MO level of C60 with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular-dependent fluctuation in the junctions.

In situ observation of the formation process for free-standing Au nanowires with a scanning electron microscope.

We have developed a simultaneous electronic and structural characterization method for studying the formation process for Au nanowires. The method is based on two-probe electronic transport measurement of free-standing Au nanowires and simultaneous structural characterization using scanning electron microscopy (SEM). We measured the electronic currents during the electromigration (EM)-induced narrowing process for the free-standing Au nanowires. A free-standing Au nanowire with a desired conductance value was fabricated by EM. Simultaneous SEM and conductance measurements revealed the EM-induced narrowing process for the Au wires, in which material transfer in the nanowires caused growth towards the positively biased electrode and contact failure at the negatively biased electrode. The narrowed free-standing Au nanowires were stable and could be maintained for more than 10 h without their conductance changing. These results indicate the high stability of the EM-processed Au nanowires compared to Au nanowires fabricated by mechanical elongation or the breaking of Au nanocontacts.

Atomic structure of water/Au, Ag, Cu and Pt atomic junctions.

Much progress has been made in understanding the transport properties of atomic-scale conductors. We prepared atomic-scale metal contacts of Cu, Ag, Au and Pt using a mechanically controllable break junction method at 10 K in a cryogenic vacuum. Water molecules were exposed to the metal atomic contacts and the effect of molecular adsorption was investigated by electronic conductance measurements. Statistical analysis of the electronic conductance showed that the water molecule(s) interacted with the surface of the inert Au contact and the reactive Cu ant Pt contacts, where molecular adsorption decreased the electronic conductance. A clear conductance signature of water adsorption was not apparent at the Ag contact. Detailed analysis of the conductance behaviour during a contact-stretching process indicated that metal atomic wires were formed for the Au and Pt contacts. The formation of an Au atomic wire consisting of low coordination number atoms leads to increased reactivity of the inert Au surface towards the adsorption of water.

Controlling the formation process and atomic structures of single pyrazine molecular junction by tuning the strength of the metal-molecule interaction.

The formation process and atomic structures were investigated for single pyrazine molecular junctions sandwiched by three different Au, Ag, and Cu electrodes using a mechanically controllable break junction technique in ultrahigh vacuum conditions at 300 K. We demonstrated that the formation process of the single-molecule junction crucially depended on the choice of the metal electrodes. While single-molecule junction showing two distinct conductance states were found for the Au electrodes, only the single conductance state was evident for the Ag electrodes, and there was no junction formation for the Cu electrodes. These results suggested that metal-molecule interaction dominates the formation process and probability of the single-molecule junction. In addition to the metal-molecule interaction, temperature affected the formation process of the single-molecule junction. The single pyrazine molecular junction formed between Au electrodes exhibited significant temperature dependence where the junction-formation probability was about 8% at 300 K, while there was no junction-formation at 100 K. Instead of the junction formation, an Au atomic wire was formed at the low temperature. This study provides insight into the tuning of the junction-forming process for single-molecule junctions, which is needed to construct device structures on a single molecule scale.

Site-Selection in Single-Molecule Junction for Highly Reproducible Molecular Electronics.

Adsorption sites of molecules critically determine the electric/photonic properties and the stability of heterogeneous molecule-metal interfaces. Then, selectivity of adsorption site is essential for development of the fields including organic electronics, catalysis, and biology. However, due to current technical limitations, site-selectivity, i.e., precise determination of the molecular adsorption site, remains a major challenge because of difficulty in precise selection of meaningful one among the sites. We have succeeded the single site-selection at a single-molecule junction by performing newly developed hybrid technique: simultaneous characterization of surface enhanced Raman scattering (SERS) and current-voltage (I-V) measurements. The I-V response of 1,4-benzenedithiol junctions reveals the existence of three metastable states arising from different adsorption sites. Notably, correlated SERS measurements show selectivity toward one of the adsorption sites: "bridge sites". This site-selectivity represents an essential step toward the reliable integration of individual molecules on metallic surfaces. Furthermore, the hybrid spectro-electric technique reveals the dependence of the SERS intensity on the strength of the molecule-metal interaction, showing the interdependence between the optical and electronic properties in single-molecule junctions.

Determination of the number of atoms present in nano contact based on shot noise measurements with highly stable nano-fabricated electrodes.

A highly stable experimental setup was developed for the measurement of shot noise in atomic contacts and molecular junctions to determine the number of atoms or molecules present. The use of a nano-fabricated mechanically controllable break junction (MCBJ) electrode improved the overall stability of the experimental setup. The improved stability of the system and optimization of measurement system enabled us to comprehensively investigate the shot noise as well as charge transport properties in Au atomic contacts and molecular junctions. We present a solid proof that the number of atoms (cross sectional atom) in the Au atomic contacts was exactly one. In the atomic contacts, contribution from the additional channels was under the detection limit. Furthermore, the effect of molecular adsorption on the charge transport in the Au atomic contact was investigated. Additional transport channels were opened by exposing pyrazine molecules to the Au contacts, which gave rise to an increase in the Fano factor in the shot noise.

Evaluation of the energy barrier for failure of Au atomic contact based on temperature dependent current-voltage characteristics.

We investigated the mechanical stability of single gold atomic contacts at an applied bias voltage of 0-1 V using a nano-fabricated mechanically controllable break junction technique at 300-400 K under ambient conditions. The single atomic contact shows the quantized conductance (G0 = 2e(2)/h) and can carry considerably large current, which results in the current-induced failure of the contact. The contact failure behaviour under the applied bias conditions was studied by statistical analysis of the current-voltage (I-V) curves of the single Au contacts. We demonstrated that, at the elevated temperature of 300-400 K, the current-induced local heating effect is negligibly small and current-induced forces in the contact are responsible for the observed failure of the single gold contacts under the high bias voltage conditions (>0.4 V). Furthermore, based on the temperature dependence of the contact failure behaviour in the I-V curves, the energy barrier of the contact-failure was evaluated to be ca. 0.1 V.

Nanoantennas as biomarkers for bacterial detection.

Understanding the biology of bacteria is critical for exploiting their beneficial properties and for preventing and treating bacterial diseases. Nanobioscience is an area that has recently seen major scientific progress. Here, we demonstrate that a raspberry-shaped nanostructure with a high density of gold nanoparticles acts like an excellent antenna due to its optical properties, which permit sensitive detection and analysis of bacterial cells. By using antibodies, these nanoantennas can be engineered to recognize only specific bacterial species. This system provides a new technique that will allow for more sensitive detection of specific bacteria.