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Oligomeric Gold-Thiolate Units Define the Properties of the Molecular Junction between Gold and Benzene Dithiols Mikkel Strange,† Olga Lopez-Acevedo,‡ and Hannu H€ akkinen*,†,‡ †
Department of Chemistry and ‡Physics, Nanoscience Center, University of Jyv€ askyl€ a, FI-40014 Jyv€ askyl€ a, Finland
ABSTRACT Understanding the structure and conductance of the molecular junction between benzene dithiolates (BDT)and gold electrodes has posed a classic unsolved problem for high-level theoretical work for over a decade. Recent breakthroughs for the gold-thiolate interface in thiolate-passivated gold clusters and in Au(111)/self-assembled monolayers (SAMs) motivated us to reanalyze the properties of Au-BDT-Au junctions. We show that distinct molecular Au(SR)2 and Au2(SR)3 units, which are known to exist at the nanoparticle-thiolate and Au-SAM interfaces, define the properties of the electrode-molecule junction. These units can form multiple contacts. The junction can be stretched by more than 1 nm whereby alternating gold-thiolate chains are spontaneously formed in ab initio molecular dynamics simulations. The calculated conductance values for the BDT junctions agree with a wide range of reported experimental data. Our results give a solid ground for further theoretical studies of molecular junctions between gold and a wide variety of organic molecules containing dithiols. SECTION Nanoparticles and Nanostructures
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ingle, well-defined molecular junctions have been envisioned as cheap, fast, and robust building blocks in electronic devices since 1970s.1-3 Advances in the experimental techniques, such as scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJs), have allowed for exploration of electron transport properties of individually contacted atoms and molecules for over a decade. Despite the conceptual simplicity of singlemolecule junctions, challenges for their theoretical understanding involve unsolved questions concerning the effects of the crystallographic structure of electrodes, alignment of the proper quantum states of the molecule and electrode due to the chemistry of the metal-molecule bonding, and the detailed atomic structure of the molecule-electrode interface. These issues are critical to solve in order to understand factors that contribute to the reproducibility and robustness of the electrical conductance of single-molecule junctions. The first experimental attempt to form and measure the conductance of a single-molecule junction considered a benzene dithiol (BDT) molecule between gold leads.4 The junction was created by the MCBJ technique with a gold wire covered by a self-assembled monolayer (SAM) of BDT molecules. The experiment reported a characteristic I-V curve of the contact with a 0.7 eV central gap; dI/dV showed two steps on both polarities of the bias voltage, yielding estimates for the conductance values as 0.045 μS and 0.075 μS. In units of one conductance quantum, G0 = 2e2/h (e is the electron unit charge and h is the Planck constant), these values correspond to 5.8 10-4 G0 and 9.7 10-4 G0, respectively. This
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prototype experiment inspired considerable experimental and theoretical effort to characterize molecular contacts between gold leads and a number of molecules. On the experimental side, gold-thiolate contacts have been studied both by the STM and MCBJ techniques. Generally, with both methods, stretching the contact first yields conductance values in multiples of G0, indicating direct metal-metal contact with welldefined conductance channels, which form when the crosssection of a metallic contact narrows down to the range of the Fermi wavelength.5 Below the last metallic ∼1 G0 plateau, experiments have found numerous low-conductance steps in the range of 10-2 to 10-5 G0.6-9 These steps are associated with conductance through a small number of, or possibly individual, molecules. The conductance values at steps of individual conductance traces show statistical order or disorder depending on the experimental conditions. The first prototype for theoretical studies of the gold-thiolate contact involved a single BDT molecule sandwiched between two jellium system leads describing the leads.10 Although the calculation yielded an I-V curve with a qualitative shape recorded in the Reed et al. experiment,4 the calculated conductance values were consistently too high by several orders of magnitude. Since then, the detailed atomic environment of the BDT-terminating sulfur, the orientation of the phenyl ring with respect to the crystallographic direction of the model Received Date: March 7, 2010 Accepted Date: April 23, 2010 Published on Web Date: April 28, 2010
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induced gold-thiolate structural motifs motivated us to reexamine the Au-BDT-Au junction, which has been the prototype model for metal-molecule junctions since 1997. We studied several models for Au-BDT-Au junctions via DFTstructure optimizations and molecular dynamics by using the real-space DFT code GPAW (see ref 27 and the Supporting Information text). Figure 1 shows three prototypical junctions 2-4 consisting of the Au(SR)2 and Au2(SR)3 units found at the curved Au-sulfur nanoparticle interface, where the radius of curvature of the Au core surface is 0.5 to 1 nm,18-24 conceivable also at MCBJ or STM/surface contacts. We will compare the properties of these contacts to the standard model of the junction, structure 1, where a single BDT is sandwiched between two Au(111) surfaces. All junctions are shown at their respective equilibrium configuration. In 2 and 4, methylthiolate (SMe) was used as the terminating group for the noncontacting part of the junction, to reduce the computational cost. We have verified that this does not change our conclusions. We calculated the conductance by using the nonequilibrium Green's function method and atomic orbital basis sets, implemented recently in GPAW. Conductance calculations are fully converged with respect to basis set and k-point sampling (see ref 14 and Supporting Information text for more details). We note that the equilibrium structures 1A-4A yield a rather wide variation of conductance values ranging between 2.8 10-1 G0 and 8 10-3 G0 (marked by circles in Figure 1). While the mechanical equilibrium of the junction is a natural reference point for theoretical comparisons, in the experiments the reported conductance values are routinely recorded at the last conductance plateau before the contact breaks, i.e., before the vacuum tunneling regime. Therefore, we simulated the mechanical response of the junctions to pulling both by stepwise stretching/quenching and by continuous stretching in DFT molecular dynamics, and calculated the conductance for numerous strained configurations (Figure 1). Junctions 1 and 3 show a mechanically stiff response (they sustain only 1-2 Å strain before breaking) and rather high conductance that increases as the junctions are stretched, reaching even the level of 0.7-0.9 G0 before the junction breaks. It is quite evident that none of these models produces conductance values in the range that has been reported from the experiments. On the contrary, junctions 2 and 4 show extremely interesting mechanical and electrical properties. Both can be stretched considerably before breaking, by more than 1 nm. The conductance of 2 settles on a plateau at about 0.01 G0, while 4 yields fluctuating values below 10-4 G0. We were able to correlate the conductance of junction 2 to the structural changes upon elongation (Figure 2). The maximum conductance of this junction, close to 0.1 G0, is achieved when the bond between the sulfur and the tip Au atom underneath (atom Au* as marked in Figure 2) is stretched. When the S-Au* bond breaks, the conductance decreases steadily and reaches the level of about 0.01 G0 when the interaction between the tip atom Au** (see Figure 2) and the oxidized AuI atom in the RS-AuI-SR unit weakens. After that, a long molecular wire Au(tip)-SRAuI-BDT-AuI-SR-Au(tip) is formed, which can be stretched significantly just by straightening the interatomic bonds while
Figure 1. Structures of the four considered junctions 1-4 at mechanical equilibrium (top) and their conductance values as the junctions are manipulated (bottom). The conductance values at equilibrium are labeled by 1A-4A. Values obtained upon stretching are to the right of the corresponding equilibrium values. Typical conductance values reported for Au-BDT-Au contacts from experiments are denoted by the horizontal dashed lines. The color code in the structure figures is as follows: Au, orange-yellow; S, bright yellow; C, dark gray; H, gray.
electrodes, and the dynamical effects have been shown to play an important role in modifying the calculated conductance values.11-15 However, theory is still, in general, unable to reproduce the measured low conductance values, leading to a puzzling situation. Gold-sulfur bonding is known to create ubiquitous molecular interfaces in SAMs on smooth gold surfaces16 and for thiolate-stabilized gold nanoclusters.17 The latter system can be considered to have a highly curved gold-SAM interface. Since 2007, considerable experimental and theoretical breakthroughs have allowed for a detailed understanding of the atomic structure of this interface, induced by the strong Au-S surface-covalent bond.18-23 Four crystallographically determined thiolate-protected gold nanoclusters with 25 to 102 gold atoms have up to now been characterized.18-21 Density functional theory (DFT) studies have shown that all of these clusters feature a metallic gold core that is protected by a number of oligomeric Aux(SR)xþ1 (x = 1,2) units; in other words, a significant ratio of the gold atoms in the cluster is not in the metal core but in the oxidized AuI state covalently bound with thiolates.22,23 A similar structural motif is predicted for a slightly larger cluster with 144 Au atoms.24 In a recent DFT study, Au(SR)2 units were shown to reproduce the c(4 2) SAM structure on Au(111),25 and those units along with longer polymeric -SR-Au-SR-Au-SR- chains were shown to yield a structural fit to grazing incidence X-ray diffraction data on long-chain alkyl SAMs on Au(111).26 The recent success in understanding the surface-covalent Au-S bond and the
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Figure 2. Conductance of junction 2 as a function of the electrode displacement from the mechanical equilibrium. The insets (from left to right) highlight three important structural changes occurring during the elongation, correlating with the behavior of the conductance data: maximal strain of the S-Au* bond corresponding to the maximal conductance, breaking of the S-Au* bond, reflected in an immediate drop of the conductance, and subsequent elongation and breaking of the AuI-Au** bond. The conductance settles on a value of about 0.01 G0 after the AuI-Au** bond is broken. Further elastic stretching of the formed RS-AuI-BDT-AuI-SR unit does not affect the conductance since the BDT is decoupled from the gold electrodes.
the conductance does not change anymore. This is the key mechanism that produces the remarkable flexibility of junction 2. DFT molecular dynamics simulations directly confirmed these observations from the quasi-static stretching simulations and shed light onto dynamic fluctuations in junctions 1 and 2 (Figure 3, see also the note of available animations of the molecular dynamics simulations in Supporting Information). It is interesting to note that, when junction 1 breaks, the impulse from the breaking S-Au bond(s) makes the BDT molecule “tumble” back to the other electrode where it finds a stable configuration lying flat on the surface with two S-Au interactions (Figure 3a). Interestingly, the Au(SR)2 units in junction 2 naturally leave an attached Au atom (AuI) to the molecule after junction rupture (see Figure 3b, the rightmost inset), which has also been observed experimentally for thiol molecules stripped off from an Au elecrode.29 The electronic structure of bulk-like Au electrodes is characterized by a wide and featureless 6s6p band in the energy region -2 eV < E - EF < 2 eV (EF is the Fermi energy); consequently, electron transport through the junction is dominated by the resonances with relevant BDT frontier orbitals that are able to hybridize with the electrode states via sulfur p-states. We have analyzed the molecular resonances in the junctions in the subspace of the atomic orbitals of the C6H4 moiety of the BDT. This analysis clearly shows that the relevant orbitals are the HOMO-1 and LUMO (HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital). The shapes of these orbitals can also be recognized as the LUMO and LUMOþ1 shapes of the isolated BDT molecule. Figure 4 shows the transmission functions T(E) of junctions 1-4 at the mechanical equilibrium (upper left panel) and at the maximal stretch (upper right panel). Our results both for the shape of T(E) and for the conductance G = T(EF) for
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Figure 3. Time evolution of junctions 1 and 2 in DFT molecular dynamics simulations: (a) junction 1, (b) junction 2. Significant changes in the potential energy upon elongation (beyond thermal fluctuations) take place, and these can be correlated to structural changes (bond breakings) as shown by selected snapshots and arrows. Junction 1 is rather rigid; breaking of the contact takes place around 4.5 ps, and the BDT molecule “tumbles back” to one of the electrodes, making a new S-Au bond in a flat adsorption configuration. Junction 2 can be elongated significantly. Two important structural changes are reflected by snapshots at around 4 and 8.5 ps; these correspond to the ones also observed in the quasi-static relaxations of the contact as a function of stretch, analyzed in Figure 2. We used pulling speeds of 50 and 100 m/s for 1 and 2, respectively, slightly lower than those used recently in DFT molecular dynamics simulations of Au-alkanedithiol junctions.28
junction 1 are in an excellent agreement with the previously reported ones.30 Junctions 1-3 have a rather strong coupling of both HOMO-1 and LUMO to the electrodes at equilibrium, which produces broad features and a relatively high conductance at EF, located in the energy gap between these states. When contacts 1 and 3 are being stretched, the HOMO-1 resonance remains broad and moves up in energy, partially crossing EF. This effect induces the increase of conductance as a function of elongation. A similar phenomenon takes place in a lesser extent initially for junction 2. However, when the S-Au* and AuI-Au** bonds are broken (Figures 2 and 3), the upward shift of the transmission resonance stops just below EF, and the resonance remains very sharp, since the HOMO-1 of BDT is electronically decoupled from the Au electrode states. Thiolate-stabilized gold nanoclusters form in solution when neutral gold atoms reduced from gold salts nucleate into
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Figure 4. Analysis of the electronic structure of junctions 1-4. Upper panels: transmission functions (logarithmic scale) of junctions 1-4 at the mechanical equilibrium (left) and at the maximal stretching before breaking (right). Lower panels: the corresponding transmission functions of junction 2 replotted in the linear scale, compared to state-resolved transmission functions of HOMO-1 and LUMO of the C6H4 moiety. The HOMO does not contribute to the conductance around the Fermi energy EF, thus the total transmission is to a large extent given as a sum of transmissions through HOMO-1 and LUMO (the small deviations from the sum are caused by weak interference effects). Note the broad transmission functions of both HOMO-1 and LUMO at the mechanical equilibrium (left), signaling a strong electronic coupling of BDT to the gold electrode states, the significant shift of the transmission peak close to EF upon stretching, and sharpening of that state to HOMO-1 resonance just below EF in the maximally stretched configuration (right), due to the decoupling of BDT states from the electrodes. The insets show the shapes of the relevant orbitals.
locally varying radii of curvature. Depending on the experimental conditions, junctions that are close to our model 4 could also form, providing the measured low conductance values (
