[Audio] Molecular Conduction Workshop 2007

In our recent research, we have been able to measure and characterize the impact of intrinsic molecular properties on the conductance of single molecule circuits formed with amine-gold linkages. In this talk, I will review the experiments and the physical picture of the junction based on the calculations. I will then focus on three recent studies: (i) the impact of substituents on the conductance through diaminobenzene; (ii) the trends in conductance through diaminonaphthalene and diaminoanthracene as a function of the location of the amine link groups [4]; and (iii) comparison of junctions formed with amine links to those formed with methylthiol and dimethylphosphine links.

The interaction of deposited metals with monolayer films is critical to the emerging arena of molecular electronics. We present the results of a thorough study of the interaction of vapor-deposited Au and Ag on alkane films attached to Si substrates.

Experimental advances in electrically and optically probing individual molecules have provided new insights into the behavior of single quantum objects and their interaction with the nanoenvironments without requiring ensemble average. Molecular-scale devices are open quantum systems whose dynamics are intrinsically stochastic and are subject to dissipation and decoherence through system-environment correlation. New concepts and computational techniques may be needed to unravel the rich physics underlying single-molecule measurements. In this talk, I discuss our efforts in developing quantum open systems theory of single-molecule electronics and optics, based on the concept of quantum trajectory which links measurement theory with stochastic dynamics of open quantum systems. .

We first outline the qualitatively different physics involved in the charging-induced current asymmetries in molecular conductors operating in the strongly coupled (weakly interacting) self-consistent field (SCF) and the weakly coupled (strongly interacting) Coulomb Blockade (CB) regimes. The CB regime, dominated by single charge effects, typically requires a computationally demanding many-electron or Fock space description.Our analysis of several molecular Coulomb Blockade measurements reveal that many novel signatures can be explained using a “simpler” orthodox model that involves an incoherent sum of Fock space excitations and hence treats the molecule as a “metallic dot” or an “island”. This also reduces the complexity of the Fock space description by just including various charge configurations only, thus partially underscoring the importance of electronic structure, while retaining the essence of the single charge nature of the transport process.

Molecular electronics faces many problems in practical device implementation, due to difficulties with fabrication and gate-ability. In these devices, molecules act as the main conducting channel. One could imagine alternate device structures where molecules act as quantum dots rather than quantum wires, with assembled molecular adsorbates acting as scattering centers in commercial silicon-based FETs.

Porphyrin molecules are often used for sensor engineering to improve sensitivity and selectivity to specific analytes. It is important to understand how the porphyrin HOMO-LUMO levels deplete surface states during functionalization of solid state sensors. Additionally, the effect of functionalization on the analyte sensing mechanism is significant to overall gas sensor design. We have previously studied the impact of the hemin porphyrin on the sensitivity of an InAs device with respect to NO ...

We review our recent research on role of interactions in molecular transport junctions. We consider simple models within nonequilibrium Green function approach (NEGF) in steady-state regime.

Molecular junctions consisting of a monolayer of fluorene and 10 nm of TiO2 between conducting contacts exhibit a memory effect upon positive polarization of the of the TiO2 for a few milliseconds. The junction conductance increases for a period of several minutes, but can be “erased” by a millisecond negative bias pulse. This “memory” effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to “dynamic doping” of the TiO2 layer by imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.

Solid-state nanopores have emerged as possible candidates for next-generation DNA sequencing devices. In this talk, we will review our recent work in development of solid-state nanopore channels that are selective towards single strand DNA (ssDNA). Nanopores functionalized with a 'probe' of hair-pin loop DNA can, under an applied electrical field, selectively transport short lengths of 'target' ssDNA that are complementary to the probe. Even a single base mismatch between the probe and the target results in longer translocation pulses and a significantly reduced number of translocation events. Our single molecule measurements allow us to separately measure the molecular flux and the pulse duration, providing a tool to gain fundamental insight into the channel-molecule interactions. The results can be explained in the conceptual framework of diffusive molecular transport with particle-channel interactions.

Most recent theoretical studies of electron transport in single-molecule junctions rely on a Landauer approach, simplified to treat electron-electron interactions at a mean-field level within density functional theory (DFT). While this framework has proven relatively accurate for certain systems, such as metallic point contacts, the computed conductance often substantially exceeds the measured values for organic molecules. This disagreement has raised questions about the validity of static DFT, inherently a ground state theory, for computing electronic transport properties.

We will discuss several proposed explanations for the switching and negative differential resistance behavior seen in some molecular junctions. It is shown that a proposed polaron model is successful in predicting both hysteresis and NDR behavior, and the model is elaborated with image charge effects and parameters from electronic structure calculations. This model includes the effects of conformational change, charging, and image charge stabilization.Sina Yeganeh is a graduate student ...