Metal-molecule Contacts Research Articles

Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions.

Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport. Such combined optical and electrical probes have proven useful for the fundamental understanding of metal-molecule contacts and contribute to the development of nanoscale optoelectronic devices including ultrafast electronics and nanosensors. Here, we employ a self-assembled metal-molecule-metal junction with a nanoparticle bridge to investigate correlated fluctuations in conductance and tunneling-induced light emission at room temperature. Despite the presence of hundreds of molecules in the junction, the electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations-a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photoexcited plasmonic junctions. Discrete steps in conductance associated with fluctuating emission intensities through the multiple plasmonic modes of the junction are consistent with a finite number of randomly localized, point-like sources dominating the optoelectronic response. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature long-term survival at room temperature and under an electrical bias of a few volts, allowing for measurements over several months.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Building large-scale unimolecular scaffolding for electronic devices

The fabrication of a future generation of (opto)electronic devices based on molecular components and materials will require careful chemical design, coupled with assembly methods that permit precise spatial orientation and arrangement of the functional molecules within device structures. Although unimolecular electronics are already a laboratory reality, the variation in the arrangement of the molecule within a molecular junction from measurement to measurement is considerable. Consequently, controlling the precise geometry at the molecule–metal contacts is a long-standing and largely unresolved challenge. Here, a strategy to fabricate uniform unimolecular junctions distributed in a regular pattern is reported. A monolayer of zinc metalloporphyrins, peripherally functionalised by bulky dendrons, is used to provide a well-defined array of molecular binding sites with precise spatial distribution. The dendrons are then photochemically cross-linked to form a robust base-layer. Parallel, uniformly-spaced unimolecular structures are subsequently assembled on these binding sites through a layer-by-layer (LbL) strategy. This LbL strategy proceeds through coordination of an α,ω-amino functionalised oligo(phenylene)ethynylene (OPE) molecule to the zinc ions of the metalloporphyrin template base-layer. The structure is then extended through alternating self-assembled layers of (cross-linked) zinc porphyrin and OPE. The coordination interaction between the zinc(II) sites and the bifunctional OPE wire ensures a high degree of registry between the layers and good electrical contact through the extended arrays and offer fine control over the chemical composition.

Probing metal-molecule contact at the atomic scale via conductance jumps

Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic scale devices. Employing two isomers of bipyridine, $4, 4^\prime$ bipyridine and $2, 2^\prime$ bipyridine between gold electrodes, here, we investigate the formation of metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4, 4^\prime$ bipyridine via a conductance jump from the tunneling regime, referred as `jump to molecular contact', being absent for $2, 2^\prime$ bipyridine. Through statistical analysis of the data, along with, molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during breaking process, providing important insights for using a single-molecule in an electronic device.

Hybrid Molecular-Junction Mapping Technique for Simultaneous Measurements of Single-Molecule Electronic Conductance and Its Corresponding Binding Geometry in a Tunneling Junction.

Improving the techniques for single-molecule conductance measurements is important for the progress of molecular electronics. In this report, a novel technique, which is named molecular-junction mapping (MJM) technique, is demonstrated to be able to simultaneously measure the electronic conductance of single molecules and their corresponding conformations in an electrode gap. Measured conductances of a few model molecules yield a much narrower distribution as compared with the results obtained using conventional break-junction technique, indicating that better defined metal-molecule contacts can be achieved using this new technique. In addition, multiple binding states of an alkanedithiol molecule in an electrode gap, which give rise to multiple conductance states, are efficiently revealed by this hybrid technique, with the results being consistent with those in former reports. This newly demonstrated technique opens up a new avenue for the study of single-molecule electronic properties and will instantly add significant assets to the tool library available for researchers in molecular electronics.

Interface properties and electronic structures of aromatic molecules with anhydride and thio-functional groups on Ag (111) and Au (111) substrates**Project supported by the National Natural Science Foundation of China (Grant Nos. 51471185 and 51325204), the National Key Research and Development Program of China (Grant No. 2016YFJC020013), and the National Supercomputing Center in Tianjin.

First-principles calculations for several aromatic molecules with anhydride and thio groups on Ag (111) and Au (111) reveal that the self-assembly structures and the interface properties are mainly determined by the functional groups of aromatic molecules. Detailed investigations of the electronic structures show that the electrons in molecular backbone are redistributed and charge transfer occurs through the bond between the metal and the functional groups after these molecules have been deposited on a metal substrate. The interaction between Ag (111) (or Au (111)) and aromatic molecules with anhydride functional groups strengthens the π bonds in the molecular backbone, while that between Ag (111) (or Au (111)) and aromatic molecules with sulfur weakens the π bonds. However, the intrinsic electronic structures of the molecules are mostly conserved. The large-sized aromatic backbone has less influence on the nature of electronic structures than the small-sized one, either at the interface or at the molecules. These results are useful to build the good metal–molecule contact in molecule-based devices.

Is the Suzuki-Miyaura Cross-Coupling Reaction in the Presence of Pd Nanoparticles Heterogeneously or Homogeneously Catalyzed? An Interfacial Surface-Enhanced Raman Spectroscopy Study.

Pd-catalyzed Suzuki-Miyaura C-C cross-coupling is very central in chemistry. The question of whether the catalysis by using Pd nanoparticles (NPs) is heterogeneous (on the Pd surface) or homogeneous (by soluble Pd released from the NP surface) remains under fundamental physicochemical debate. This work reports on the in situ characterization of the Suzuki-Miyaura cross-coupling reactions by using surface-enhanced Raman spectroscopy. We find clear evidence of heterogeneous catalysis on the Pd surface. In contrast, the soluble Pd species leaching into the solution cannot catalyze the reaction, indicating a direct contact of the aryl halides with the metal surface is a prerequisite. Accordingly, the surface ligands and charge of the Pd NPs, which determine the molecule-metal contact, are very important in the couplings. By a simple exchange of the surface ligand or a surface modification of the support material, the catalytic activity of Pd NPs is improved due to the enhanced electric attraction between the metal and the reactant molecules.

First-Principles Study of the Adsorption Behavior of Triptycene Molecular Tripods on Au(111): Site Selectivity and Unambiguous Molecular Orientation

Through a first-principles theoretical study, thiol tripods with a rigid C3-symmetric triptycene are proposed as excellent building blocks to achieve well-defined molecule–metal contact. We designe...

Molecular flexure and atom trapping with sexiphenyl molecules by scanning tunneling microscope manipulation

Molecular flexure, and molecule-metal contact of para-sexiphenyl molecules on a Ag(111) surface are investigated by using low temperature scanning tunneling microscopy, and molecular manipulations. Atom trapping with sexiphenyl molecules is realized by laterally manipulating the molecules onto individual silver atoms and up to three silver atoms have been trapped. We also demonstrate breaking of a silver dimer into individual silver atoms by atom trapping. STM manipulation experiments show that the molecule-metal complexes formed by the atom trapping are mechanically stable. Moreover, Lateral manipulation of a single sexiphenyl across a Ag(111) atomic step highlights how the molecule moves across step-edges; the molecule can easily conform across the step and it recovers original configuration after the manipulation.

Roles of vacuum tunnelling and contact mechanics in single-molecule thermopower

Molecular junction is a chemically-defined nanostructure whose discrete electronic states are expected to render enhanced thermoelectric figure of merit suitable for energy-harvesting applications. Here, we report on geometrical dependence of thermoelectricity in metal-molecule-metal structures. We performed simultaneous measurements of the electrical conductance and thermovoltage of aromatic molecules having different anchoring groups at room temperature in vacuum. We elucidated the mutual contributions of vacuum tunnelling on thermoelectricity in the short molecular bridges. We also found stretching-induced thermoelectric voltage enhancement in thiol-linked single-molecule bridges along with absence of the pulling effects in diamine counterparts, thereby suggested that the electromechanical effect would be a rather universal phenomenon in Au-S anchored molecular junctions that undergo substantial metal-molecule contact elongation upon stretching. The present results provide a novel concept for molecular design to achieve high thermopower with single-molecule junctions.

Interface Tuning of Current-Induced Cooling in Molecular Circuits

We study the effect of the atomistic structure of metal–molecule contacts on the current-induced damping and excitation of vibrations in molecular circuits by means of first-principles calculations. We consider a carbene-based molecule bound to Au electrodes via three different tip terminations: a tetramer, a pyramid, and a chainlike structure. The change in the width and position of molecular levels associated with each of these metal–molecule structures under an applied voltage controls the heating and cooling processes. In blunt tips, where the electronic coupling between molecular and Au bulk states is strong, the cooling efficiency decreases as a function of bias which results in the heating of the most active vibrational modes. On the other hand, in chainlike structures where the coupling is weak, the cooling rate has a nonmonotonic behavior as a function of the applied bias and increases sharply beyond a certain voltage. This results in a current-induced cooling at high bias. These findings open th...

Molecular spin on surface: From strong correlation to dispersion interactions.

A reliable prediction of magnetic properties of surface-supported molecules containing 3d/4f spin carriers has challenged the electronic structure theory for decades. Here we tackle this problem with Hubbard-U corrected van der Waals density functional (vdW-DF), incorporating strong correlation effects of the localized electrons and dispersion interactions involved in the molecule-surface binding. By fitting the spin state energetics of a series of Fe(ii) compounds with varying ligand field strength, we find that the optimal U value for vdW-DF is much smaller than that for the local density approximation (LDA) while quite similar to that for the generalized gradient approximation (GGA). We show that although vdW-DF+U overestimates largely the metal-ligand bond distance, the predicted adiabatic high-spin-low-spin energy splitting ΔEHL is only slightly changed with respect to that obtained using the LDA+U geometries consistent with experiment. Then we use Cu(111)-supported metallocene (M(C5H5)2, M = Fe, and Co) as a prototype example to explore the effects of the molecule-surface interactions. We show that the non-local dispersion interactions, poorly described by LDA and GGA while reasonably captured by vdW-DF, are critical for reproducing ΔEHL at large molecule-surface distances. Besides, we find that ΔEHL is decreased by the molecule-metal contact, which is shown to weaken the local ligand field around the magnetic center.

Comparison of DC and AC Transport in 1.5-7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic Ga-In versus Conducting Probe Atomic Force Microscope Junctions.

We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms.

Rigid multipodal platforms for metal surfaces.

In this review the recent progress in molecular platforms that form rigid and well-defined contact to a metal surface are discussed. Most of the presented examples have at least three anchoring units in order to control the spatial arrangement of the protruding molecular subunit. Another interesting feature is the lateral orientation of these foot structures which, depending on the particular application, is equally important as the spatial arrangement of the molecules. The numerous approaches towards assembling and organizing functional molecules into specific architectures on metal substrates are reviewed here. Particular attention is paid to variations of both, the core structures and the anchoring groups. Furthermore, the analytical methods enabling the investigation of individual molecules as well as monomolecular layers of ordered platform structures are summarized. The presented multipodal platforms bearing several anchoring groups form considerably more stable molecule–metal contacts than corresponding monopodal analogues and exhibit an enlarged separation of the functional molecules due to the increased footprint, as well as restrict tilting of the functional termini with respect to the metal surface. These platforms are thus ideally suited to tune important properties of the molecule–metal interface. On a single-molecule level, several of these platforms enable the control over the arrangement of the protruding rod-type molecular structures (e.g., molecular wires, switches, rotors, sensors) with respect to the surface of the substrate.

Low variability of single-molecule conductance assisted by bulky metal–molecule contacts

A detailed study of the trimethylsilylethynyl moiety, –CCSiMe3 (TMSE), as an anchoring group, using a combination of experiment and DFT is presented.

Understanding the conductance switching of permethyloligosilanes: A theoretical approach.

On the basis of ab initio density functional theory coupled with non-equilibrium Green's function technique, we have presented a molecular level understanding on the stereoelectronic switching of conducting properties of oligosilane molecules. Su et al. [Nat. Chem. 7, 215-220 (2015)] demonstrated that these types of oligosilane molecules exhibit three stereoconformers which show two distinct conducting profiles. On the basis of break-junction technique, the authors show that manipulating a specific dihedral angle and thereby controlling the length of the molecular contact, it is possible to switch the conducting states minutely. However, their discussions scarce the proper energy level alignment upon which the molecule-lead tunneling amplitude depends. On the basis of electronic structure and non-equilibrium electron transport calculations, we interpret such switching behavior and thus quantify the switching parameter demonstrating how the metal-molecule contact geometry along with the electronic energy level alignment is responsible for such kind of junction process. We also provide the variation of switching parameter and the type of majority carrier with the conjugation length of the oligosilanes.

Symmetry of Single Hydrogen Molecular Junction with Au, Ag, and Cu Electrodes

We present a charge-transport study on a H2 molecule wired to Au, Ag, and Cu electrodes using the mechanically controllable break junction technique at 10 K. Formation of molecular junctions with a H2 molecule bound between the metal electrodes was identified by measuring the vibrational modes of the H2 molecule using inelastic electron tunneling spectroscopy. The H2 molecule exhibited a preferential conductance value of 0.3G0 (G0 = 2e2/h) for Cu electrodes, whereas it displayed variable conductance values below 1G0 for Au and Ag electrodes. The geometrical symmetry of the molecular junctions was characterized in terms of the current versus bias voltage (I–V) characteristics of the junctions, in which H2/Cu junctions and H2/Au, H2/Ag junctions showed asymmetric and symmetric shapes, respectively, at positive and negative bias voltages. The shape of the I–V curve reflects the geometrical symmetry of the metal electrodes (i.e., symmetry of metal–molecule contacts in molecular junctions). We investigated the...

Simultaneous measurement of electrical conductance and thermopower of single benzenedithiol molecular junctions

We have developed a system for the simultaneous measurement of electrical conductance and thermopower of the single benzenedithiol (BDT) molecular junction, which was characterized by inelastic electron tunneling spectroscopy, at low temperature. The simultaneous measurements revealed a negative correlation between the electrical conductance and the thermopower. Strong metal–molecule coupling at the single BDT molecular junction leads to high conductance and low thermopower because of the broadening of the conduction orbital, which explains the negative correlation. The observed fluctuation in conductance and thermopower reflects the change in the metal–molecule contact configuration and molecular orientation.

Organic magnetic tunnel junctions: The role of metal-molecule interface

We report a first-principles theoretical investigation of spin-polarized quantum transport in organic magnetic tunnel junctions (OMTJs) to provide a microscopic understanding on the sign of the tunnel magnetoresistance ratio (TMR). We consider two different OMTJs, formed by sandwiching 1-stearic acid radicals (1-SAR) or 1,18-stearic diacid radicals (1,18-SDR) between two Ni electrodes. Even though the main difference between them is only on one of the Ni/molecule contacts, such a structure difference is found to induce a significant sign change of the TMR. The TMR is negative for 1-SAR at -19.6%, but is positive for 1,18-SDR at 13.7%. By investigating the concept of scattering density of states (SDOS), we found that scattering processes of p electrons at the Ni/molecule interface determines the sign of TMR. Based on spin polarization of the SDOS, we extend the Julliere model to explain both the sign and the value of the TMR qualitatively and semiquantitatively. It is concluded that understanding spin-polarized quantum transport in organic magnetic tunnel junction requires a comprehensive knowledge of the electronic structures of the molecule, the metal electrode, and the metal-molecule contacts. DOI: 10.1103/PhysRevB.86.224419

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron-transport properties of single π-conjugated molecule junctions, which include benzene, fullerene, and π-stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single-molecule junctions. The effect of the metal-molecule contact on the conductance of the single π-conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π-stacked system using π-stacked aromatic molecules enclosed within self-assembled coordination cages is reviewed. The electron transport in the π-stacked systems is found to be efficient at the single-molecule level, thus providing insight into the design of conductive materials.