Electron transport through single π-conjugated molecules bridging between metal electrodes

Charge Transport in Conjugated and Saturated Hydrocarbons: Comparing Ballistic and Cotunneling Contributions

The comparison between electrical transport in CnH2n+2S2 alkane and CnHn+2S2 alkene (n = 4, 6, 8, 10) is studied by using a generalized Breit-Wigner approach and considering coherent transport mechanisms and eventual changes in the state of charge (i.e., cotunneling processes) for both molecules. In general, the conductance of alkanes tends to be smaller than that of similar-sized alkenes. However, cotunneling processes have an important participation in the overall transport in the case of alkanes but not for the alkene family. The progressive changes in both the eigenenergies of the relevant frontier molecular orbitals of the charged species and their spatial localization play decisive roles in the observed differences. While the molecular orbitals of the charged species of the conjugated molecules are hardly affected by the applied voltage, their saturated counterparts are quite sensitive to the external field. With this, successive avoided-crossing events between the molecular orbitals of the single-charged alkane molecules can lead to the appearance of nonballistic conduction channels that make no negligible contributions to the molecular transport.

The effect of spin polarization on the electron transport of molecular wires with diradical character

Some of the most promising materials for application in molecular electronics and spintronics are based on diradical chains. Herein, the proposed relation between increasing conductance with length and diradical character is revisited using ab initio methods that account for the static electron correlation effects. Electron transmission was previously obtained from restricted single determinant wavefuntions or tight-binding approximations, which are unable to account for static correlation. Broken Symmetry Unrestricted Kohn-Sham Density Functional Theory (BS-UKS-DFT) in combination with electron transport analysis based on electron deformation orbitals (EDOs) reflects an exponential decay of the electrical conductance with length. Also, other important effects such as quantum interference are correctly accounted for, leading to a decrease of the conductance as the diradical character increases. As a proof-of-concept, the electrical conductance obtained from BS-UKS-DFT and CASSCF(2,2) wavefunctions were compared in diradical graphene strips in the frame of the pseudo-π approach, obtaining very similar results.

Extent of conjugation in diazonium-derived layers in molecular junction devices determined by experiment and modelling

This paper shows that molecular layers grown using diazonium chemistry on carbon surfaces have properties indicative of the presence of a variety of structural motifs. Molecular layers grown with aromatic monomers with thickness between 1 and ∼15 nm display optical absorption spectra with significant broadening but no change in band gap or onsets of absorption as a function of layer thickness. This suggests that there is no extended conjugation in these layers, contrary to the conclusions of previous work. Density-functional theory modelling of the non-conjugated versions of the constituent aromatic monomers reveals that the experimental trends in optical spectra can be recovered, thereby establishing limits to the degree of conjugation and the nature of the order of as-grown molecular layers. We conclude that the absence of both shifts in band gap and changes in absorption onset is a consequence of resonant conjugation within the layers being less than 1.5 monomer units, and that film disorder is the main origin of the optical spectra. These findings have important implications for understanding charge transport mechanisms in molecular junction devices, as the layers cannot be expected to behave as ideal, resonantly conjugated films, but should be viewed as a collection of mixed nonresonantly- and resonantly-conjugated monomers.

Through-Space Conjugation: An Effective Strategy for Stabilizing Intramolecular Charge-Transfer States

Intramolecular charge transfer (ICT) has significant impacts on organic optoelectronic materials, photochemistry, biotechnology, and so on. However, it is hard to stabilize the ICT state because of the rapid nonradiative charge recombination process, which often quenches light emission. In this work, we use new foldamers of the protonated pyridine-modified tetraphenylethene derivatives that possess through-space conjugation (TSC) characters as the models to study the impact of TSC on the ICT state. Steady and transient spectroscopies illustrate that the lifetime of the ICT state in the molecule with strong TSC can be much longer than those of molecules without TSC, giving rise to a higher fluorescence quantum yield. By combining the theoretical calculations, we demonstrate that the strong TSC can stabilize the ICT state and slow the charge recombination rate by more efficiently dispersing charges. This is a conceptually new design strategy for functional optoelectronic materials that require more stable ICT states.

High-conductance contacts to functionalized molecular platforms physisorbed on Au(1 1 1)

The conductances of molecules physisorbed to Au(1 1 1) via an extended [Formula: see text] system are probed with the tip of a low-temperature scanning tunneling microscope to maximize the control of the junction geometry. Inert hydrogen, methyl, and reactive propynyl subunits were attached to the platform and stand upright. Because of their different reactivities, either non-bonding (hydrogen and methyl) or bonding (propynyl) tip-molecule contacts are formed. The conductances exhibit little scatter between different experimental runs on different molecules, display distinct evolutions with the tip-subunit distance, and reach contact values of 0.003-0.05 G ₀. For equal tip-platform distances the contact conductance of the inert methyl is close to that of the reactive propynyl. Under further compression, the inert species, hydrogen and methyl, are found to be better conductors. This shows that the current flow is not directly correlated with the chemical interaction. Atomistic calculations for the methyl case reproduce the conductance evolution and reveal the role of the junction geometry, forces and orbital symmetries at the tip-molecule interface. The current flow is controlled by orbital symmetries at the electrode interfaces rather than by the energy alignment of the molecular orbitals and electrode states. Functionalized molecular platforms thus open new ways to control and engineer electron conduction through metal-molecule interfaces at the atomic level.

A DFT study on surface-enhanced Raman spectroscopy of aromatic dithiol derivatives adsorbed on gold nanojunctions

A computational study on aromatic dithiol derivatives (HS-Ar-X-Ar-SH, X=O, S, Se, NH, CH₂, NN, CHCH, CC) interacting with gold cluster(s) was presented to investigate the chemical enhancement mechanism related to surface-enhanced Raman spectroscopy (SERS) for molecular junctions. Density functional theory (DFT) were performed on derivatives molecules as well as their single-end-linked (SEL) or double-end-linked (DEL) complexes for geometric, spectra, electronic and excitation properties, leading to discussions on dominant factor during SERS process. The resulted enhancement factors of SEL and DEL complexes exhibited specific dependency on linking atom or functional group between two phenyls, which was in accordance with the variation of polarizabilities and molecule-cluster transition energy.

High surface coverage of a self-assembled monolayer by in situ synthesis of palladium nanodeposits

Nascent metal|monolayer|metal devices have been fabricated by depositing palladium, produced through a CO-confined growth method, onto a self-assembled monolayer of an amine-terminated oligo(phenylene ethynylene) derivative on a gold bottom electrode. The high surface area coverage (85%) of the organic monolayer by densely packed palladium particles was confirmed by X-ray photoemission spectroscopy (XPS) and atomic force microscopy (AFM). The electrical properties of these nascent Au|monolayer|Pd assemblies were determined from the I-V curves recorded with a conductive-AFM using the Peak Force Tunneling AFM (PF-TUNA™) mode. The I-V curves together with the electrochemical experiments performed rule out the formation of short-circuits due to palladium penetration through the monolayer, suggesting that the palladium deposition strategy is an effective method for the fabrication of molecular junctions without damaging the organic layer.

Single-Molecule Conductance Studies of Organometallic Complexes Bearing 3-Thienyl Contacting Groups

The compounds and complexes 1,4-C6 H4 (C≡C-cyclo-3-C4 H3 S)2 (2), trans-[Pt(C≡C-cyclo-3-C4 H3 S)2 (PEt3 )2 ] (3), trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 (dppe)2 ] (4; dppe=1,2-bis(diphenylphosphino)ethane) and trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 {P(OEt)3 }4 ] (5) featuring the 3-thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3-5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break-junction (STM-BJ) and STM-I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM-I(s) methods, and a higher feature by the STM-BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi-parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non-resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2-5.

Transport properties of a three-shell icosahedral matryoshka cluster: a first-principles study

The molecular junction based on three-shell icosahedral matryoshka cluster with huge magnetic moment exhibits robust spin-filtering effect, which highlights it for promising applications in molecular devices.

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.

The electron and spin polarized transport in wide-voltage-ranges through colbaltporphyrin-based molecular junctions

The electron and spin polarized transport properties of Co benzene-porphyrin-benzene (BPB) molecule coupled to gold (Au) nanowires in a wide voltage range (0-3.0 V) are investigated. By successively removing the front-end Au atoms, we construct Au nanowires with different molecule-electrode contact symmetries. Multiple negative differential resistance (NDR) peaks emerge at different bias voltage regions. It is found that the low-voltage NDR effect at 0.4 V can only be found in the junctions with S-Au top bindings. High-bias NDR effects intrinsic to central molecule at 2.8 V are observed in all the six structures. In particular, both the electron and spin polarized current-voltage (I-V) curves depend strongly on the contact configurations between Co-BPB molecule and the Au electrodes. And the top-binding may result in spin dependent transport properties and will be the priority selection in the design of molecular devices.

Rudnev A, Moreno-García P, Baghernejad M, Huang C, Hong W, Wandlowski T. Promising anchoring groups for single-molecule conductance measurements

Qualitative and quantitative comparison of the results obtained with different anchoring groups reveals structural and mechanistic details of the different types of single molecular junctions.

Mechanically controllable bi-stable states in a highly conductive single pyrazine molecular junction

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.

Single molecule bridging between metal electrodes

Single molecular junctions, in which a single molecule bridges between metal electrodes, have attracted wide attention as novel properties can appear due to their peculiar geometrical and electronic characters. The single molecular junction has also attracted attention due to its potential application in ultrasmall single molecular electronic devices, where single molecules are utilized as active electronic components. Thus, fabrication of single molecular junctions as well as understanding and controlling their properties (e.g. conductance, optical and magnetic properties) have become long-standing goals of scientists and engineers. This review article focuses on the experimental aspects of single molecular junctions, with primary focus on the electron transport mechanism.

Trimethylsilyl-terminated oligo(phenylene ethynylene)s: an approach to single-molecule junctions with covalent Au-C σ-bonds

A new and efficient approach using cleaving of trimethylsilyl groups to create covalent Au-C anchoring sites has been developed for single-molecule junction conductance measurements. Employing the mechanically controllable break junction (MCBJ) technique in liquid, we demonstrate the formation of highly conducting single molecular junctions of several OPE derivatives. The created junctions are mechanically stable and exhibit conductances around one order of magnitude higher than those of their dithiol analogues. Extended assembly and reaction times lead to oligomerization. Combined STM imaging and gap-mode Raman experiments provide structure evidence to support the formation of covalent Au-C contacts and further oligomerization.