Molecule Metal Junctions Research Articles

Exploring Electrical Conductivity of Thiolated Micro‐ and Nanoparticles of Gallium

Nano‐/microparticles of gallium (Ga), as a low‐melting‐point metal, are extensively used in the fields of soft electronics and sensors to provide thermal and electrical conductivity. However, a passivating oxide layer can be formed on the surface of Ga nano‐/microparticles during the synthesis process. This oxide layer is removed by a secondary sintering step, especially mechanical sintering, which is generally not a controllable process, and compromises the integrity of the system. Herein, thiol molecules, 1‐butanethiol, thiophenol, and 4‐mercaptopyridine, that can functionalize the surface of Ga via sonication to reduce the oxidation of Ga surface are used. The resulting particles exhibit electrical conductivity based on metal–molecule junctions without the requirement for a sintering step. In particular, 4‐mercaptopyridine functionalized, thiolated Ga particles exhibit higher electrical conductivity compared to the other three thiolated Ga systems as the organic material conjugation provides conductive pathways for the mix. Subsequently, using these particle systems, soft devices are developed that can be used for gas, exhalation, and flex sensing. This study provides insights into the possibility of creating combinations of organic molecules with liquid metal‐based nano‐/microparticles to generate electrically conductive mixes and the prospects of fabricating multifunctional sensors.

Hard-Soft Chemistry Design Principles for Predictive Assembly of Single Molecule-Metal Junctions.

The achievement of atomic control over the organic-inorganic interface is key to engineering electronic and spintronic properties of molecular devices. We leverage insights from inorganic chemistry to create hard-soft acid-base (HSAB) theory-derived design principles for incorporation of single molecules onto metal electrodes. A single molecule circuit is assembled via a bond between an organic backbone and an under-coordinated metal atom of the electrode surface, typically Au. Here, we study molecular composition factors affecting the junction assembly of coordination complexes containing transition metals atoms on Au electrodes. We employ hetero- and homobimetallic lantern complexes and systematically change the coordination environment to vary the character of the intramolecular bonds relative to the electrode-molecule interaction. We observe that trends in the robustness and chemical selectivity of single molecule junctions formed with a range of linkers correlate with HSAB principles, which have traditionally been used to guide atomic arrangements in the synthesis of coordination complexes. We find that this similarity between the intermolecular electrode-molecule bonding in a molecular circuit and the intramolecular bonds within a coordination complex has implications for the design of metal-containing complexes compatible with electrical measurements on metal electrodes. Our results here show that HSAB principles determine which intramolecular interactions can be compromised by inter molecule-electrode coordination; in particular on Au electrodes, soft-soft metal-ligand bonding is vulnerable to competition from soft-soft Au-linker bonding in the junction. Neutral donor-acceptor intramolecular bonds can be tuned by the Lewis acidity of the transition metal ion, suggesting future synthetic routes toward incorporation of transition metal atoms into molecular junctions for increased functionality of single molecule devices.

Atomistic Control of Metal–Molecule Junctions for Efficient Photo-Induced Uphill Charge Transfer

Metalloporphyrins were immobilized at single crystalline gold surfaces of (111), (100), and (110) orientations via imidazole-terminated alkanethiols. This method enabled us to simultaneously contro...

Quantum Point Contact Single-Nucleotide Conductance for DNA and RNA Sequence Identification.

Several nanoscale electronic methods have been proposed for high-throughput single-molecule nucleic acid sequence identification. While many studies display a large ensemble of measurements as "electronic fingerprints" with some promise for distinguishing the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine, and uracil), important metrics such as accuracy and confidence of base calling fall well below the current genomic methods. Issues such as unreliable metal-molecule junction formation, variation of nucleotide conformations, insufficient differences between the molecular orbitals responsible for single-nucleotide conduction, and lack of rigorous base calling algorithms lead to overlapping nanoelectronic measurements and poor nucleotide discrimination, especially at low coverage on single molecules. Here, we demonstrate a technique for reproducible conductance measurements on conformation-constrained single nucleotides and an advanced algorithmic approach for distinguishing the nucleobases. Our quantum point contact single-nucleotide conductance sequencing (QPICS) method uses combed and electrostatically bound single DNA and RNA nucleotides on a self-assembled monolayer of cysteamine molecules. We demonstrate that by varying the applied bias and pH conditions, molecular conductance can be switched ON and OFF, leading to reversible nucleotide perturbation for electronic recognition (NPER). We utilize NPER as a method to achieve >99.7% accuracy for DNA and RNA base calling at low molecular coverage (∼12×) using unbiased single measurements on DNA/RNA nucleotides, which represents a significant advance compared to existing sequencing methods. These results demonstrate the potential for utilizing simple surface modifications and existing biochemical moieties in individual nucleobases for a reliable, direct, single-molecule, nanoelectronic DNA and RNA nucleotide identification method for sequencing.

Temperature dependent tunneling conductance of single molecule junctions

We perform temperature dependent conductance measurements on sub-nanometer sized single molecules bound to gold electrodes using a scanning tunneling microscope-based break junction technique in Ultra-High Vacuum (UHV). We find a threefold increase in the conductance of amine-terminated conjugated molecules when the temperature increases from 4 K to 300 K in UHV. Furthermore, the conductance measured at 300 K in UHV is consistent with solution-based measurements under ambient conditions where the transport mechanism corresponds to off-resonant electron tunneling across the molecule. Our measurements indicate that at 300 K, conductance is largely independent of pressure or solvent around the junction. In addition, our data unambiguously show that temperature can affect the tunneling conductance of single molecule-metal junctions. We show that the structure of the metal electrodes that form in these junctions varies systematically with temperature, and hypothesize that this changing structure of the interface alters electron tunneling probability and propose a mechanism to explain our findings.

Large Magnetoresistance in Single-Radical Molecular Junctions

Organic radicals are promising building blocks for molecular spintronics. Little is known about the role of unpaired electrons for electron transport at the single-molecule level. Here, we examine the impact of magnetic fields on electron transport in single oligo(p-phenyleneethynylene) (OPE)-based radical molecular junctions, which are formed with a mechanically controllable break-junction technique at a low temperature of 4.2 K. Surprisingly huge positive magnetoresistances (MRs) of 16 to 287% are visible for a magnetic field of 4 T, and the values are at least 1 order of magnitude larger than those of the analogous pristine OPE (2-4%). Rigorous analysis of the MR and of current-voltage and inelastic electron-tunneling spectroscopy measurements reveal an effective reduction of the electronic coupling between the current-carrying molecular orbital and the electrodes with increasing magnetic field. We suggest that the large MR for the single-radical molecular junctions might be ascribed to a loss of phase coherence of the charge carriers induced by the magnetic field. Although further investigations are required to reveal the mechanism underlying the strong MR, our findings provide a potential approach for tuning charge transport in metal-molecule junctions with organic radicals.

Stability and Destabilization Processes in the Formation of Ferrocene-Based Metal–Organic Molecule–Metal Nano-Junctions

The realization of functional metal–molecule junctions relies on our understanding and control of the fundamental interactions leading to or preventing the formation of nanojunctions. In this work, focusing on the promising double-decker organometallic compound ferrocene, deposited on a Cu(111) surface, we show that destabilization or dissociation of these compounds can occur depending on the nature of the metal atom deposited on the ferrocene. Specifically, at variance with the more common deposition of Cu atoms, Fe atoms can give rise to a disassembly of the Fe(C5H5)2 structure in a barrierless way, potentially realizing catalytic active sites. Conversely, the deposition of Co can occur only upon overcoming an activation barrier of about 0.85 eV. In this case, the ferrocene does not dissociate, but the Co atom results fully inserted in the upper C5H5 ring of the organometallic molecule, thus becoming a new six-member structure that eventually allows for a selective extraction of deposited ferrocene mole...

Dependence of the conductance change on the molecular orbitals in Ag and Au electrodes

The conductance change in single-molecule junctions due to the molecular orbitals and the metal’s Fermi energy was investigated by using a scanning tunneling microscopy break-junction technique with Ag and Au electrodes. 4,4′-diaminobiphenyl and 4,4′-dicyanobiphenyl as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) junctions were used. The amine-linked HOMO-conducting junction had a lower conductance, and cyano-linked LUMO-conducting junction had a higher conductance for Ag electrode compared to Au electrode. These results are attributed to the difference between the metal electrode Fermi energy and the molecular orbital level in the metal-molecule junction. Furthermore, 2,7-diaminofluorene exhibited a higher conductance but the identical molecular plateau length for the Ag electrodes compared to that of 4,4′-diaminobiphenyl indicating that the twist angle of the molecular backbone affects the conductance.

Thiophene-based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes.

Molecule-metal junctions are inevitable for the realization of single-molecule electronics. In this study, we developed new tripodal anchors with electron-rich aromatic rings to achieve robust contact with gold electrodes, an effective hybridization of the π orbital with gold electrodes (π channel), and hole transport through π-channel hybridization. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements of the monolayers indicated that the thiophene-based tripodal molecule exhibits anchoring characteristics as expected. The electrical conductance of thiophene-anchored bistripodal molecules using the scanning tunneling microscope (STM)-based break junction technique confirmed the formation of molecular junctions. The Seebeck coefficient of this compound estimated from thermoelectric voltage measurements using a STM was determined to be a positive value, which indicates that the charge carriers are holes. On the contrary, the corresponding pyridine-anchored molecules showed electron transport. These results reveal the versatility of π-channel tripodal anchors for the control of charge-carrier type in single-molecule electronics.

Linker‐dependent Junction Formation Probability in Single‐Molecule Junctions

We compare the junction formation probabilities of single‐molecule junctions with different linker molecules by using a scanning tunneling microscope‐based break‐junction technique. We found that the junction formation probability varies as SH > SMe > NH2 for the benzene backbone molecule with different types of anchoring groups, through quantitative statistical analysis. These results are attributed to different bonding forces according to the linker groups formed with Au atoms in the electrodes, which is consistent with previous works. Our work allows a better understanding of the contact chemistry in the metal–molecule junction for future molecular electronic devices.

Identifying Highly Conducting Au–C Links through Inelastic Electron Tunneling Spectroscopy

We use inelastic electron tunneling spectroscopy first-principles simulations to identify the different chemical bonds present at metal–molecule junctions. We unambiguously identify the nature of these bonds from two distinctive features in the calculated spectra: (i) the presence (or absence) of active vibrational modes and (ii) the dependence of vibrational frequencies on electrode separation. We use this method to present a study of the vibrational properties of alkanes bound to the electrodes via highly conducting Au–C links. In the experiment, these links were formed from molecules synthesized with trimethyl-tin (SnMe3) terminations, where the SnMe3 groups were removed in situ at the junction, in a process involving both breaking and formation of bonds [Cheng, Z.-L.; Skouta, R.; Vazquez, H.; Widawsky, J. R.; Schneebeli, S.; Chen, W.; Hybertsen, M. S.; Breslow, R.; Venkataraman, L. Nat. Nanotechnol. 2011, 6, 353–357]. We obtain the vibrational fingerprint of these direct Au–alkane links and extend thi...

Metal–organic molecule–metal nano-junctions: a close contact between first-principles simulations and experiments

The realization of metal–molecule junctions for future electronic devices relies on our ability to assemble these heterogeneous objects at a molecular level and understand their structure and the behavior of the electronic states at the interface. Delocalized interface states near the metal Fermi level are a key ingredient for tailoring charge injection, and such a delocalization depends on a large number of chemical, structural and morphological parameters, all influencing the spatial extension of the electron wavefunctions. Our large-scale dynamical simulations, combined with experiments, show that a double-decker organometallic compound (ferrocene) can be deposited on a Cu(111) surface, providing an ideal system to investigate the adsorption, the interface states and localized spin states at a metal–organometallic interface. Adsorbed ferrocene is shown to have a peculiar pattern and realizes a 2D-like interface state strongly resembling Shockley’s surface state of Cu. By a subsequent deposition of single metal atoms on the adsorbed ferrocene, we analyze the sensitivity of the interface state to local modifications of the interface potential. This provides an insight into adsorption, spin configuration and charge redistribution processes, showing how to tune the electron behavior at a metal–molecule interface.

Measurement and control of detailed electronic properties in a single molecule break junction

The lack of detailed experimental controls has been one of the major obstacles hindering progress in molecular electronics. While large fluctuations have been occurring in the experimental data, specific details, related mechanisms, and data analysis techniques are in high demand to promote our physical understanding at the single-molecule level. A series of modulations we recently developed, based on traditional scanning probe microscopy break junctions (SPMBJs), have helped to discover significant properties in detail which are hidden in the contact interfaces of a single-molecule break junction (SMBJ). For example, in the past we have shown that the correlated force and conductance changes under the saw tooth modulation and stretch-hold mode of PZT movement revealed inherent differences in the contact geometries of a molecular junction. In this paper, using a bias-modulated SPMBJ and utilizing emerging data analysis techniques, we report on the measurement of the altered alignment of the HOMO of benzene molecules with changing the anchoring group which coupled the molecule to metal electrodes. Further calculations based on Landauer fitting and transition voltage spectroscopy (TVS) demonstrated the effects of modulated bias on the location of the frontier molecular orbitals. Understanding the alignment of the molecular orbitals with the Fermi level of the electrodes is essential for understanding the behaviour of SMBJs and for the future design of more complex devices. With these modulations and analysis techniques, fruitful information has been found about the nature of the metal-molecule junction, providing us insightful clues towards the next step for in-depth study.

Energy level alignment and quantum conductance of functionalized metal-molecule junctions: Density functional theory versus GW calculations

We study the effect of functional groups (CH3*4, OCH3, CH3, Cl, CN, F*4) on the electronic transport properties of 1,4-benzenediamine molecular junctions using the non-equilibrium Green function method. Exchange and correlation effects are included at various levels of theory, namely density functional theory (DFT), energy level-corrected DFT (DFT+Σ), Hartree-Fock and the many-body GW approximation. All methods reproduce the expected trends for the energy of the frontier orbitals according to the electron donating or withdrawing character of the substituent group. However, only the GW method predicts the correct ordering of the conductance amongst the molecules. The absolute GW (DFT) conductance is within a factor of two (three) of the experimental values. Correcting the DFT orbital energies by a simple physically motivated scissors operator, Σ, can bring the DFT conductances close to experiments, but does not improve on the relative ordering. We ascribe this to a too strong pinning of the molecular energy levels to the metal Fermi level by DFT which suppresses the variation in orbital energy with functional group.

What Determines the Sign Reversal of Magnetoresistance in a Molecular Tunnel Junction?

The observations of both positive and negative signs in tunneling magnetoresistance (TMR) for the same organic spin-valve structure have baffled researchers working in organic spintronics. In this article, we provide an answer to this puzzle by exploring the role of metal-molecule interface on TMR in a single molecular spin-valve junction. A planar organic molecule sandwiched between two nickel electrodes is used to build a prototypical spin-valve junction. A parameter-free, single-particle Green's function approach in conjunction with a posteriori, spin-unrestricted density functional theory involving a hybrid orbital-dependent functional is used to calculate the spin-polarized current. The effect of external bias is explicitly included to investigate the spin-valve behavior. Our calculations show that only a small change in the interfacial distance at the metal-molecule junction can alter the sign of the TMR from a positive to a negative value. By changing the interfacial distance by 3%, the number of participating eigenchannels as well as their orbital characteristics changes for the antiparallel configuration, leading to the sign reversal in TMR.

Statistical Investigation of Current-Voltage Characterization in Single Molecule-Metal Junctions

We show statistical measurements of single molecule metal contacts using the mechanically controllable break junction technique. The measurements are carried out in a solvent, in order to allow in situ binding of the molecules to the metallic contacts during the measurements. Statistics is gathered by opening and closing the junctions repeatedly and recording current voltage characteristics at various stages of the opening and closing curves. By modeling the data with a single level model we can extract parameters such as the position of the molecular energy level, which carries the current, and the coupling between the metal and the molecule. In rst experiments we use this method to characterize di erent anchoring groups, which mediate the mechanical and electrical coupling between the metallic electrodes and the molecules. We use tolane molecules, which are structurally simple, as model systems for this purpose.

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

We propose a complementary interpretation of the mechanism responsible for the strong enhancement observed in surface enhanced raman scattering (SERS). The effect of a strong static local electric field due to the Schottky barrier at the metal-molecule junction on SERS is systematically investigated. The study provides a viable explanation to the low repeatability of SERS experiments as well as the Raman peak shifts as observed in SERS and raw Raman spectra. It was found that a strong electrostatic built-in field at the metal-molecule junction along specific orientations can result in 2-4 more orders of enhancement in SERS.

Control of junction resistances in molecular electronic devices fabricated by FIB

A major hurdle to realize molecular electronic devices (MEDs) is to make reliable electrical contacts to a single or a few molecules. Our nano-contact platform with a gap size of less than 25nm with resistances above 1000TΩ was built using combined techniques of photolithography, electron beam lithography and focused ion beam milling. In this study, we have used gold nanoparticles (AuNPs) to bridge the nanoelectrode gaps by dielectrophoretic trapping and thus obtain electrical contacts. The electrodes and/or the nanoparticles were functionalised with 1–2nm long alkane-thiol molecules so that the electronic structure of these molecules determines the properties of the electrical junction. Molecules were introduced both by functionalising the nanogap and the nanoparticles and the results of both functionalisation protocols are compared. Here, we show the nanogap–nanoparticle bridge set-up containing metal–molecule junctions that can be used as a base for the development of molecular electronics containing only a few molecules under ambient conditions. Current–voltage (I–V) characterization of alkanethiol/gold junction showed non-linear response where mean geometric resistance of four different junctions could be tuned from 20GΩ to 20TΩ. The results from the measurements on 1-alkanethiol in such devices is a first step to demonstrate that this platform has the potential to obtain stable electronic devices having relatively small numbers of molecules with reliable metal molecule junction by combing top-down and bottom-up approaches.

Assessment of a nanoparticle bridge platform for molecular electronics measurements

A combination of electron beam lithography, photolithography and focused ionbeam milling was used to create a nanogap platform, which was bridged by goldnanoparticles in order to make electrical measurements and assess the platformunder ambient conditions. Non-functionalized electrodes were tested to determinethe intrinsic response of the platform and it was found that creating devices inambient conditions requires careful cleaning and awareness of the contributionscontaminants may make to measurements. The platform was then used to makemeasurements on octanethiol (OT) and biphenyldithiol (BPDT) molecules byfunctionalizing the nanoelectrodes with the molecules prior to bridging the nanogap withnanoparticles. Measurements on OT show that it is possible to make measurements onrelatively small numbers of molecules, but that a large variation in response canbe expected when one of the metal–molecule junctions is physisorbed, whichwas partially explained by attachment of OT molecules to different sites on thesurface of the Au electrode using a density functional theory calculation. Onthe other hand, when dealing with BPDT, high yields for device creation arevery difficult to achieve under ambient conditions. Significant hysteresis in theI–V curves of BPDT was also observed, which was attributed primarily to voltage inducedchanges at the interface between the molecule and the metal.

Vibrationally mediated control of single-electron transmission in weakly coupled molecule-metal junctions

We propose a mechanism which allows one to control the transmission of single electrons through a molecular junction. The principle utilizes the emergence of transmission sidebands when molecular vibrational modes are coupled to the electronic state mediating the transmission. We will show that if a molecule-metal junction is biased just below a molecular resonance one may induce the transmission of a single electron by externally exciting a vibrational mode of the molecule. The analysis is quite general but requires that the molecular orbital does not hybridize strongly with the metallic states. As an example we perform a density functional theory (DFT) analysis of a benzene molecule between two Au(111) contacts and show that exciting a particular vibrational mode can give rise to transmission of a single electron