Molecular conductance: chemical trends of anchoring groups

Combining density functional theory calculations for molecular electronic structure with a Green function method for electron transport, we calculate from first principles the molecular conductance of benzene connected to two Au leads through different anchoring atoms-S, Se, and Te. The relaxed atomic structure of the contact, different lead orientations, and different adsorption sites are fully considered. We find that the molecule-lead coupling, electron transfer, and conductance all depend strongly on the adsorption site, lead orientation, and local contact atomic configuration. For flat contacts the conductance decreases as the atomic number of the anchoring atom increases, regardless of the adsorption site, lead orientation, or bias. For small bias this chemical trend is, however, dependent on the contact atomic configuration: an additional Au atom at the contact with the (111) lead changes the best anchoring atom from S to Se, although for large bias the original chemical trend is recovered.     

Variability of conductance in molecular junctions

The conductance of molecular junctions, formed by breaking gold point contacts dressed with various thiol functionalized organic molecules, is measured at 293 K and at 30 K. In the presence of molecules, individual conductance traces measured as a function of increasing gold electrode displacement show clear steps below the quantum conductance steps of the gold contact. These steps are distributed over a wide range of molecule-dependent conductance values. Histograms constructed from all conductance traces therefore do not show clear peaks either at room or low temperatures. Filtering of the data sets by an objective automated procedure only marginally improves the visibility of such features. We conclude that the geometrical junction to junction variations dominate the conductance measurements.     

Fullerene-based anchoring groups for molecular electronics

We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum we have determined the conductance and stability of single-molecule junctions of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and has a low-bias conductance of 3 x 10(-4) G0. Compared to 1,4-benzenedithiol the fullerene-anchored molecule exhibits a considerably lower conductance spread. In addition, the signature of the new compound in histograms is more significant than that of 1,4-benzenediamine, probably owing to a more stable adsorption motif. Statistical analyses of the breaking of the junctions confirm the stability of the fullerene-gold bond.     

Oscillation of conductance in molecular junctions of carbon ladder compounds

The electrical conductances of dithiolates of polyacene (PA(n)DTs) and polyphenanthrene (PPh(n)DTs), which are typical carbon ladder compounds, are calculated by means of the Landauer formulation combined with density functional theory, where n is the number of benzene rings involved. Surface Green function used in the Landauer formulation is calculated with the Slater-Koster parameters. Attention is turned to the wire-length dependence of the conductances of PA(n)DTs and PPh(n)DTs. The damping of conductance of PA(n)DTs is much smaller than that of PPh(n)DTs because of the small HOMO-LUMO gaps of PA(n)DTs. PA(n)DTs are thus good molecular wires for nanosized electronic devices. Conductance oscillation is found for both molecular wires when n is less than 7. The electrical conductance is enhanced in PA(n)DTs with even-numbered benzene rings, whereas it is enhanced in PPh(n)DTs with odd-numbered benzene rings. The observed conductance oscillation of PA(n)DTs and PPh(n)DTs is due to the oscillation of orbital energy and electron population. Other pi-conjugated oligomers (polyacetylene-DT, oligo(thiophene)-DT, oligo(meso-meso-linked zinc(II) porphyrin-butadiynylene)-DT, oligo(p-phenylethynylene)-DT, and oligo(p-phenylene)-DT) are also studied. In contrast to PA(n)DTs and PPh(n)DTs, the five molecular wires show ordinary exponential decays of conductance.     

Dithiocarbamate anchoring in molecular wire junctions: a first principles study

Recent experimental realization [J. Am. Chem. Soc., 127 (2005) 7328] of various dithiocarbamate self-assembly on gold surface opens the possibility for use of dithiocarbamate linkers to anchor molecular wires to gold electrodes. In this paper, we explore this hypothesis computationally. We computed the electron transport properties of 4,4'-bipyridine (BP), 4,4'-bipyridinium-1,1'-bis(carbodithioate) (BPBC), 4-(4'-pyridyl)-peridium-1-carbodithioate (BPC) molecule junctions based on the density functional theory and nonequilibrium Green's functions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The broadening effect along with the extension of the pi conjugation from the molecule to the gold electrodes lead to enhanced electrical conductance for BPBC molecule. The conductance enhancement factor is as large as 25 at applied voltage bias 1.0 V. Rectification behavior is predicted for BPC molecular wire junction, which has the asymmetric anchoring groups.     

Influence of functional groups on charge transport in molecular junctions

Using density functional theory (DFT), we analyze the influence of five classes of functional groups, as exemplified by NO(2), OCH(3), CH(3), CCl(3), and I, on the transport properties of a 1,4-benzenedithiolate (BDT) and 1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis demonstrates how ideas from functional group chemistry may be used to engineer a molecule's transport properties, as was shown experimentally and using a semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show that the qualitative change in conductance due to a given functional group can be predicted from its known electronic effect (whether it is sigma/pi donating/withdrawing). However, the influence of functional groups on a molecule's conductance is very weak, as was also found in the BDA experiments. The calculated DFT conductances for the BDA species are five times larger than the experimental values, but good agreement is obtained after correcting for self-interaction and image charge effects.     

Polyadducts of fullerene C60 with tert-butyl groups

New soluble fullerene derivatives Bu^t _n C₆₀ were synthesized by the reaction of C₆₀M _n (THF) _x (M = Li, Na; n = 4, 6, 8, 10, 12) with tert-butyl bromide ButBr. The thus obtained compounds were characterized by spectroscopic methods (IR and NMR spectroscopy), mass spectrometry (MALDI), elemental analysis, and quantum chemical calculations. HPLC was used for chromatographic separation of the products of the reaction of C₆₀Na _n (THF) _x (n = 4, 6, 8, 12) with Bu^tBr. It was found that Bu^t ₄C₆₀ and Bu_t ⁶C₆₀ decompose on the column. A complex of physicochemical methods of analysis was used for establishing compositions of individual fractions of chromatographic separation of Bu^t ₄C₆₀. The structure of one of the components was determined as 1-tert-butyl-1,2-dihydrofullerene 1,2-Bu^tC₆₀H.     

Reduction-induced switching of single-molecule conductance of fullerene derivatives

The effect of electrochemical reduction on the single-molecule conductance of fullerene C60 derivatives was studied by scanning tunneling microscopy. Three types of C60 derivatives were synthesized, a monoadduct having an amino-terminated linker and two bisadducts having two linkers at different positions (trans2 and trans3). Each C60 derivative was immobilized on a gold surface by an amino-gold linkage, confirmed by infrared reflection-absorption spectroscopy. The immobilized C60 derivatives showed reversible and multiple reduction peaks in the cyclic voltammogram in dimethylformamide (DMF) at almost the same potentials as those in solution, showing the redox properties of the molecules are intact on gold. Single-molecule conductances of the bisadducts, which can span between a scanning tunneling microscopy (STM) tip made of gold and substrate with the two linkers, were determined by the STM break-junction measurements in water and DMF. The conductances were 6.1+/-4.5 nS in water and 4.9+/-1.7 nS in DMF for the trans2 bisadduct and 8.4+/-3.4 nS in water and 7.9 nS+/-2.8 in DMF for the trans3 bisadduct. By using a potential-controlled STM setup, the tunneling current through a single molecule was recorded with sweeping the potentials of the tip and substrate. The trans2 bisadduct showed significant changes in the current when the reductions of the C60 moiety occur. Some current curves showed multiple peaks, and the other curves showed stepwise increase and decrease at the C60 reduction and subsequent reoxidation. Statistical analysis afforded stepwise switching of the conductance as the average behavior and suggested that the electron tunneling through the C60 derivative is enhanced as it accepts electrons.     

Electrochemistry does the impossible: Robust and reliable large area molecular junctions

1. Download : Download high-res image (115KB)
2. Download : Download full-size image

     

Effect of the continuity of the pi conjugation on the conductance of ruthenium-octene-ruthenium molecular junctions

The conductance of a family of ruthenium-octene-ruthenium molecular junctions with different pi conjugation are investigated using a fully self-consistent ab initio approach which combines the nonequilibrium Green's function formalism with density functional theory. Our calculations demonstrate that the continuity of the pi conjugation in the contact region as well as along the molecular backbone affects the junction conductance significantly, showing the advantage of using the ruthenium-carbon double bond as the linkage of conjugated organic molecules.     

Effect of anchoring groups on single-molecule conductance: comparative study of thiol-, amine-, and carboxylic-acid-terminated molecules

We studied the effect of anchoring groups on the conductance of single molecules using alkanes terminated with dithiol, diamine, and dicarboxylic-acid groups as a model system. We created a large number of molecular junctions mechanically and analyzed the statistical distributions of the conductance values of the molecular junctions. Multiple sets of conductance values were found in each case. The I-V characteristics, temperature independence, and exponential decay of the conductance with the molecular length all indicate tunneling as the conduction mechanism for these molecules. The prefactor of the exponential decay function, which reflects the contact resistance, is highly sensitive to the anchoring group, and the decay constant is weakly dependent on the anchoring group. These observations are attributed to different electronic couplings between the molecules and the electrodes and alignments of the molecular energy levels relative to the Fermi energy level of the electrodes introduced by different anchoring groups. For diamine and dicarboxylic-acid groups, the conductance values are sensitive to pH due to protonation and deprotonation of the anchoring groups. Further insight into the binding strengths of these anchoring groups to gold electrodes is obtained by statistically analyzing the stretching length of molecular junctions.     

Molecular rectification and conductance switching in carbon-based molecular junctions by structural rearrangement accompanying electron injection

Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm(2), or about 10(5) e(-)/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e(-)/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude.     

Spectroscopy of molecular junctions

In this tutorial review we present in detail recent studies in which molecular junctions were simultaneously probed by conductance measurements and optical spectroscopy methods such as electroluminescence (EL) and Raman scattering. The advantages of combining these experimental approaches to improve our understanding of charge transport through molecular junctions are discussed and routes for future developments are suggested.     

Single molecular conductance of tolanes: experimental and theoretical study on the junction evolution dependent on the anchoring group

Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH(2), and CN) at a solid/liquid interface. The combination of current-distance and current-voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH(2) > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively.     

Synthesis, electrochemical, and molecular inclusion properties of 'canopied' trinuclear ruthenium complexes with six anchoring groups on an ITO electrode

New trinuclear Ru complexes bearing both 1,3,5-tris(2,2':6',2'-terpyridyl)benzene and bis(benzimidazol-2-yl)pyridine with six phosphonate anchors were synthesised and immobilised on an ITO electrode. The 'canopied' structure was proved by AFM measurements. The immobilised Ru trinuclear complex revealed a one step three-electron oxidation process for the Ru(II/III) couple at around +0.9 V vs. Ag/AgCl, indicating that the Ru-Ru interaction is small. For the Ru immobilised ITO surface, the electron transfer blocking and the electron mediation of [Fe(CN)(6)](4-) oxidation through the Ru(iii) <--> Ru(ii) catalytic cycle are strongly dependent on the alkyl chain length. The spectroelectrochemistry of the Ru trinuclear complex monolayer on the ITO electrode showed a stable electrochromic response under the potential pulse. The present 'canopied' Ru trinuclear complexes hold a small cavity that encapsulates tetrathiafulvalene molecules, which can be proved by a cyclic voltammogram.     

Orbital interaction mechanisms of conductance enhancement and rectification by dithiocarboxylate anchoring group

We study computationally the electron transport properties of dithiocarboxylate terminated molecular junctions. Transport properties are computed self-consistently within density functional theory and nonequilibrium Green's functions formalism. A microscopic origin of the experimentally observed current amplification by dithiocarboxylate anchoring groups is established. For the 4,4'-biphenyl bis(dithiocarboxylate) junction, we find that the interaction of the lowest unoccupied molecular orbital (LUMO) of the dithiocarboxylate anchoring group with LUMO and highest occupied molecular orbital (HOMO) of the biphenyl part results in bonding and antibonding resonances in the transmission spectrum in the vicinity of the electrode Fermi energy. A new microscopic mechanism of rectification is predicted based on the electronic structure of asymmetrical anchoring groups. We show that the peaks in the transmission spectra of 4'-thiolato-biphenyl-4-dithiocarboxylate junction respond differently to the applied voltage. Depending upon the origin of a transmission resonance in the orbital interaction picture, its energy can be shifted along with the chemical potential of the electrode to which the molecule is more strongly or more weakly coupled.     

Electronic transport through molecular junctions with nonrigid molecule-leads coupling

The Landauer-type formulation of current through a molecular junction with electronic-nuclear coupling introduced by Troisi et al. [J. Chem. Phys. 118, 6072 (2003)] is generalized to account for the dependence of the molecule-leads coupling terms on the nuclear coordinates. Although this electronic-nuclear coupling is external to the molecule there is no need to extend the molecular subspace when projection operators are employed for calculations of the current through the junction. A test case of a conductor with vibrating contacts to the leads is studied numerically. It is demonstrated that contact vibrations lead to inelastic contributions to the current and to characteristic features in the I-V curve and its derivatives, similar to the ones observed for internal (molecular) electronic-nuclear coupling.     

Synthesis of fullerene adducts with terpyridyl- or pyridylpyrrolidine groups in trans-1 positions

Two C(60) hexakis-adducts (2 and 3) were synthesized by using a protection-deprotection strategy. The symmetric fullerene tetrakis-adduct 8 was obtained by anthracene removal from the hexakis-adduct 7. Reaction of 8 with terpyridylglycine or pyridylglycine afforded two hexakis-adducts, 2 and 3. By using the retro-cyclopropanation reaction, the four malonate addends located on the equatorial belt of the hexakis-adducts were removed to afford two trans-1 bis-adducts, 4 and 5, with terpyridyl- or pyridylpyrrolidine groups. The structures of 2 and 3 were confirmed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and (1)H, (13)C, and COSY NMR, and UV-visible spectroscopy. The cyclic voltammograms of fullerene multiadducts 2, 3, and 9 show irreversible reductions. Self-assembled monolayers (SAMs) of 1 and 3 were formed on gold surfaces through nitrogen adsorption. SAMs of 3 represent the first example of a fullerene hexakis-adduct formed on gold surfaces through nitrogen adsorption. Controlled potential electrolyses (CPE) were conducted to prepare trans-1 bis-adducts 4 and 5 modified with terpyridyl and pyridyl groups.