Session A14: Focus Session: Transport Properties of Nanostructures I: Single-Molecule Junctions

Metallic conductance in single-molecule junctions

Through break junction techniques it has become possible to attach metallic wires to individual molecules. The actual presence of the molecule, its identity, and the numbers of molecules involved in the transport in many cases need to be deduced from measurements of current and voltage only. In parallel, several implementations of non-equilibrium Greens function approaches in combination with density functional theory have been developed in order to compute the properties of molecular junctions. For a sensitive test of the computational models there is a need for more detailed experimental observations on well-characterized model systems. In our experiments we focus on such model systems. They are simple, small molecules which have the advantage that they are relatively easy to handle in computations and they allow for more precise experimental tests at low temperatures. The molecules we have studied include H$_{2}$, H$_{2}$O, CO, CO$_{2}$, C$_{6}$H$_{6}$ (benzene), and C$_{60}$ contacted between Pt leads. The molecular levels hybridize strongly with the Pt metal giving rise to a high conductance. The presence of the molecules can be confirmed by the detection of vibration modes. These modes are visible in the differential conductance, dI/dV, as a function of voltage bias as fine steps at the energies $eV=\hbar \omega _n $ corresponding to those of the vibration modes. Further test involve isotope-substituted molecules and stretching of the molecular bridge in order to detect shifts in the energy of the modes. The conductance of a single-molecule bridge cannot be uniquely distinguished from bridges due to several parallel channels. However, by measuring shot noise, i.e. the intrinsic noise in the electron current, it is possible to show that the current is carried by a single molecule. For other molecules the measurement gives evidence that the molecule provides two, or sometimes more channels for conductance. The results are compared with state of the art non-equilibrium DFT calculations.   

Fullerene-based anchoring groups for molecular electronics: insights from theory

Recent experiments [1] have explored the idea of using C60 as anchoring groups to increase the stability of single molecule junctions. To further explore this concept, we have performed elaborated electronic structure calculations based on the density functional theory. First, the influence of dispersive interactions on the location of C60 with respect to the electrode surface is carefully investigated. Second, the transmission of C60 and BDC60 [=1,4-bis(fullero[c]pyrrolidin-1-yl)benzene] junctions is obtained. We find that a mismatch of the chemical potential of Au electrodes and frontier molecular orbitals of C60 generates a tunneling barrier. As a consequence, BDC60 acts as a sequence of three weakly coupled quantum dots. Specifically, the small conductance values ($\sim 10^{-3}$ -- 10$^{-4}$ G$_0$) observed experimentally [1], arise from the small broadening of the HOMO level of the inner molecule capped by C60 moieties. Third, electrode materials with a smaller work function (e.g. Ag instead of Au) are discussed, which might provide better matching to C60 and therefore establish more favorable conditions for electron transfer. \medskip \newline \noindent [1] C.A.\ Martin, D.\ Ding, J.K.\ S{\o}rensen, T.\ Bj{\o}rnholm, J.\ van Ruitenbeek, H.S.J.\ van der Zant, JACS {\bf 130}, 13198-13199 (2008).   

Transport properties of touching molecules

Advances in the understanding of transport through individual molecules make the next critical issue to characterize charge transport from one single molecule to another one. This talk addresses recent results on electron transport between two touching C$_{60}$ molecules [Phys. Rev. Lett. 103, 206803 (2009)]. The tip of a scanning tunneling microscope was used to pick up single C$_{60}$ molecules from C$_{60}$ islands prepared on Au or Cu substrates. With such C$_{60}$-tips contact experiments were performed on both clean surfaces and on other C$_{60}$ molecules. We found that while the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of a C$_{60}$-C$_{60}$ contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C$_{60}$-C$_{60}$ distances, and identify the essential roles of the molecule-metal interface and the intermolecular link on the contact conductance. The theoretical analysis thereby provides insight into the limiting factors for transport in C$_{60}$ junctions.   

Simultaneous Force and Conductance Measurements of Single Molecule Junctions

We present simultaneous conductance and breaking force measurements of single molecular junctions formed using a modified conductive atomic force microscope. Breaking forces are determined for large data sets of over 10000 measurements and the data is analyzed using a novel approach which allows an unbiased and statistically significant determination of single bond-breaking forces. We confirm our experimental setup and analysis techniques by a comparison of the determined single Au-Au bond breaking force to the accepted experimental and theoretical value from literature. We then apply the same method to study the breaking forces for molecular junctions bonded with amine, methylsulfide and pyridine linkers, which form donor-acceptor bonds with under-coordinated Au atoms. We find that the molecule breaking force depends both on the linker as well as its chemical nature. Comparison to the Au-Au breaking force allows the conclusion that for each molecule studied the junctions break at the N-Au and S-Au bond as is expected from theoretical calculations.   

Medium Effects in Single Molecule Electronics

We use STM-based techniques for measuring the electrical properties of metal$\vert $molecule$\vert $metal junctions. For a family of molecules HS(CH$_{2})_{6}$-Ar-(CH$_{2})_{6}$SH (Ar = substituted benzene), we found that the single molecule conductances varied significantly with substituent, being higher for electron-donating substituents [1]. Later, we studied the effect of increasing conjugation on this system by examining oligothiophenes HS(CH$_{2})_{6}$-[C$_{4}$H$_{4}$S]$_{x}$-(CH$_{2})_{6}$SH (x = 1, 2, 3, 5). We found that the conductances of junctions involving these molecules depended upon the medium in which the measurements were made. In fact, for x = 3, the conductance was two orders of magnitude higher in the presence of water than in anhydrous conditions [2]. This presentation will outline these studies, together with the results of transport calculations that rationalise these unusual findings, and will set the results in the context of existing literature on medium effects in single molecule conductance determinations. In collaboration with Edmund Leary and Richard Nichols, University of Liverpool; Colin Lambert, Iain Grace, and Chris Finch, University of Lancaster; and Wolfgang Haiss, University of Liverpool.   

Solvent Effects on the Conductance of 1,4-benzenediamine

We measured the conductance of 1,4-benzenediamine (BDA) by mechanically forming and breaking Au point contacts with a modified STM in a solution of molecules in ambient conditions, using a variety of solvents. Here, we present reliable experimental results which show that the conductance of BDA can be increased by over 50{\%} when dissolved in aromatic organic solvents solely by varying halogen groups on the solvent molecule. The trends in conductance do not correlate with the solvent dielectric constant, dipole moment, or direct solvent-BDA interactions. First-principles density functional theory calculations of solvent molecule binding to gold surfaces are used to discuss mechanisms behind the conductance shift of the BDA molecule.   

Observation of Orbital Gate Modulation in Molecular Junctions

We report the observation of direct gate modulation of molecular orbitals in solid-state molecular junctions where transport current is controlled by an external gate. Resonant-enhanced coupling to the nearest molecular orbital is revealed by electron tunneling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. We contrast molecular structures that have near-resonant coupling to ones that have far-from-resonant coupling. We show this using a multiprobe approach combining a variety of transport techniques that elucidates the transport mechanisms and electronic structure of molecular junctions.   

First-Principles Studies of Aromatic Single-Molecule Junctions: Length Dependence of Conductance and Thermopower

Using a scattering-state approach incorporating self-energy corrections to the junction level alignment, the conductance G and thermopower S of oligophenyldiamine-Au junctions are calculated and elucidated. In agreement with experiment, we find G decays exponentially with the number of phenyls N with decay constant beta = 1.7. A parameter-free self-energy correction, going beyond density functional theory (DFT), is found to be essential for understanding the measured values of both G and beta (Quek et al, Nano Lett 9, 3949 (2009)). The thermopower is found to be sensitive to contact geometry. However, for each contact geometry, S increases linearly with N as found in experiment. DFT overestimates both S and the slope of S versus N, while the self-energy corrections bring both into much better agreement with experiment.   

Statistically-Determined Molecular I-V Curves Measured by STM-Break Junction

We present a study of the current-voltage (I-V) characteristics for a series of amine and pyridine linked single molecule junctions. The junctions are created using the STM-based break-junction technique where an Au point-contact is broken in a solution of the the target molecules in ambient conditions. After the Au point contact breaks, the tip-substrate distance is held temporarily constant in order to achieve a steady metal-molecule-metal junction. The bias across the junction is ramped while current is measured to generate an I-V curve*. We find that the I-V curves can vary greatly from junction to junction for most conjugated molecules. Data from thousands of such I-V curves are compiled into 2D histograms to determine a ``most-likely'' current for a given voltage. We observe that these statistically-defined I-V curves depend on the molecule under study and the magnitude of the voltage ramp. In addition, from the slope of the I-V, we are able to track how the differential conductance increases as a function of applied voltage. *Nanotechnology \textbf{20} (2009) 434009   

Excess Volltage-Dependent Noise in Atomic-Scale Au Contacts

An atomic-scale metal contact shows nonlinear scaling of excess noise power with applied bias. While this excess noise scales more rapidly with voltage than the linear dependence expected from pure shot noise, we do observe clear suppression of the noise power at the first few conductance quanta, $G_{0}\equiv 2e^2/h$, $2G_{0}$, and $3G_{0}$. Using high frequency techniques, data is taken at room temperature in mechanical break junctions. We employ lock-in techniques combined with an rf amplifier chain to measure the excess noise power simultaneously with the dc conductance. We describe the dependence of the noise on bias voltage and analyze the noise vs. conductance histograms. This additional excess noise provides a possible probe for local electronic heating of the junction.