Efficient Molecular Rectification in Metal-Molecules-Semimetal Junctions.

Molecular rectification is expected to be observed in metal-molecule-metal tunnel junctions in which the resonance levels responsible for their transport properties are spatially localized asymmetrically with respect to the leads. Yet, effects such as electrostatic screening and formation of metal induced gap states reduce the magnitude of rectification that can be realized in such junctions. Here we suggest that junctions of the form metal-molecule(s)-semimetal mitigate these interfacial effects.

Electron transport through two interacting channels in Azurin-based solid-state junctions.

The fundamental question of "what is the transport path of electrons through proteins?" initially introduced while studying long-range electron transfer between localized redox centers in proteins in vivo is also highly relevant to the transport properties of solid-state, dry metal-protein-metal junctions. Here, we report conductance measurements of such junctions, Au-( monolayer ensemble)-Bismuth (Bi) ones, with well-defined nanopore geometry and ~10 proteins/pore. Our results can be understood as follows.

Disordered Ballistic Bismuth Nano-waveguides for Highly Efficient Thermoelectric Energy Conversion.

Junctions based on electronic ballistic waveguides, such as semiconductor nanowires or nanoribbons with transverse structural variations in the order of a large fraction of their Fermi wavelength, are suggested as highly efficient thermoelectric (TE) devices. Full harnessing of their potential requires a capability to either deterministically induce structural variations that tailor their transmission properties at the Fermi level or alternatively to form waveguides that are disordered (chaotic) but can be structurally modified continuously until favorable TE properties are achieved. Well-established methods to realize either of these routes do not exist.

Large Seebeck Values in Metal-Molecule-Semimetal Junctions Attained by a Gateless Level-Alignment Method.

Molecular junctions are potentially highly efficient devices for thermal energy harvesting since their transmission properties can be tailored to break electron-hole transport symmetry and consequently yield high Seebeck and Peltier coefficients. Full harnessing of this potential requires, however, a capability to precisely position their Fermi level within the transmission landscape. Currently, with the lack of such a "knob" for two-lead junctions, their thermoelectric performance is too low for applications.

Molecular ensemble junctions with inter-molecular quantum interference.

We report of a high yield method to form nanopore molecular ensembles junctions containing ~40,000 molecules, in which the semimetal bismuth (Bi) is a top contact. Conductance histograms of these junctions are double-peaked (bi-modal), a behavior that is typical for single molecule junctions but not expected for junctions with thousands of molecules. This unique observation is shown to result from a new form of quantum interference that is inter-molecular in nature, which occurs in these junctions since the very long coherence length of the electrons in Bi enables them to probe large ensembles of molecules while tunneling through the junctions.

Thermometry of Plasmonic Heating by Inelastic Electron Tunneling Spectroscopy (IETS).

The electronic and lattice heating accompanying plasmonic structures under illumination is suggested to be utilized in a broad range of thermoplasmonic applications. Specifically, in molecular electronics precise determination of the temperature of illuminated junctions is crucial, because the temperature-dependent energy distribution of charge carriers in the leads affects the possibility to steer various light-controlled conductance processes. Existing optical methods to characterize the local temperature in all these applications lack the spatial resolution to probe the few nanometers in size hot spots and therefore typically report average values over a diffraction limited length scale.

Pump-Probe Noise Spectroscopy of Molecular Junctions.

The slow response of electronic components in junctions limits the direct applicability of pump-probe type spectroscopy in assessing the intramolecular dynamics. Recently the possibility of getting information on a sub-picosecond time scale from dc current measurements was proposed. We revisit the idea of picosecond resolution by pump-probe spectroscopy from dc measurements and show that any intramolecular dynamics not directly related to charge transfer in the current direction is missed by current measurements.

Bismuth nanowires with very low lattice thermal conductivity as revealed by the 3ω method.

Thermoelectric materials transform temperature gradients to voltages and vise versa. Despite their many advantages, devices based on thermoelectric materials are used today only in a few applications, due to their low efficiency, which is described by the figure of merit ZT. Theoretical studies predict that scaling down these materials to the nanometric scale should enhance their efficiency partially due to a decrease in their lattice thermal conductivity.

Large anisotropic conductance and band gap fluctuations in nearly round-shape bismuth nanoparticles.

Unlike their bulk counterpart, nanoparticles often show spontaneous fluctuations in their crystal structure at constant temperature [Iijima, S.; Ichihashi T. Phys.

Very high thermopower of Bi nanowires with embedded quantum point contacts.

Quantum confinement effects in bismuth (Bi) nanowires (NWs) are predicted to impart them with high thermopower values and hence make them efficient thermoelectric materials. Yet, boundary scattering of charge carriers in these NWs operating in the diffusion transport regime mask any quantum effects and impede their use for nanoscale thermoelectric applications. Here we demonstrate quantum confinement effects in Bi NWs by forming in their structure ballistic quantum point contacts (QPCs) leading to exceptionally high thermopower values (S > 2 mV/K).

Accurate determination of plasmonic fields in molecular junctions by current rectification at optical frequencies.

Current rectification, i.e., induction of dc current by oscillating electromagnetic fields, is demonstrated in molecular junctions at an optical frequency.

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.

Electrical detection of surface plasmon polaritons by 1G0 gold quantum point contacts.

Electrical detection of surface plasmons polaritons (SPPs) is essential for realization of integrated fast nanoscale plasmonic circuits. We demonstrate electrical detection of SPPs by measuring their remote gating effect on 1G(0) metal quantum point contacts (MQPC) made of gold. Gating is argued to take place by a photoassisted transport mechanism with nonmonotonic behavior of its magnitude as a function of distance between the MQPCs and the position of SPPs creation.

Measurement of electronic transport through 1G0 gold contacts under laser irradiation.

Metal quantum point contacts (MQPCs) with dimensions comparable to the de Broglie wavelength of conducting electrons reveal ballistic transport of electrons and quantized conductance in units of G(0) = 2e(2)/h. We measure the transport properties of 1G(0) Au contacts under laser irradiation. The observed enhancement of conductance appears to be wavelength-dependent, while thermal effects on conductance are determined to be negligible.

Detection of heating in current-carrying molecular junctions by Raman scattering.

As the scaling of electronic components continues, local heating will have an increasing influence on the stability and performance of nanoscale electronic devices. In particular, the low heat capacity of molecular junctions means that it will be essential to understand local heating and heat conduction in these junctions. Here we report a method for directly monitoring the effective temperature of current-carrying junctions with surface enhanced Raman spectroscopy (SERS) that involves measuring both the Stokes and anti-Stokes components of the Raman scattering.

Fabrication of highly stable configurable metal quantum point contacts.

Metal quantum point contacts (MQPCs), with dimensions comparable to the de Broglie wavelength of conducting electrons, reveal ballistic transport of electrons and quantized conductance in units of G0=2e(2)/h. While these contacts hold great promise for applications such as coherent controlled devices and atomic switches, their realization is mainly based on the scanning tunneling microscope (STM) and mechanically controlled break junction (MCBJ), which cannot be integrated into electronic circuits. MQPCs produced by these techniques have also limited stability at room temperature.

Segmented metal nanowires as nanoscale thermocouples.

Segmented Au-Ni nanowires are demonstrated to be highly effective thermocouples with a spatial resolution of a few nanometers and a temporal resolution in the microsecond range. The performance of the devices is characterized by a self-heating procedure in which an ac heating current with frequency ω is applied on the wires while monitoring the resulting thermoelectric voltage V(TH) at 2ω using a lock-in technique. An analytical model is developed that enables one to determine the time response of the thermocouples from plots of V(TH) as a function of ω.

Fabrication of very high aspect ratio metal nanowires by a self-propulsion mechanism.

A novel synthesis method of very high aspect ratio metal nanowires is described. The synthesis utilizes a nanoporous membrane as a template and self-electrophoresis as a directed force that continuously push formed nanowires out of the pores in a rate that is identical to the rate of their elongation. As a result, while the pores of membranes are only 6 microm long, the formed nanowires could be more than 100 microm long.

Single-molecule electrical junctions.

The objective of this review is to describe current experimental research of single-molecule electrical junctions in the context of various theoretical frameworks, with emphasis on the application of single-electron transistor theory to molecular junctions. Molecule quantum dots are at least an order of magnitude smaller than semiconductor quantum dots, which allows the study of many of the same mesoscopic and many-body effects at far higher temperatures. We discuss processes such as cotunneling, sequential tunneling, and incoherent tunneling, as well as the Kondo effect, Zeeman splitting, and the Coulomb diamond.

Effect of local environment on molecular conduction: isolated molecule versus self-assembled monolayer.

Developing a fundamental understanding of molecular conduction in different device environments is essential to the advance of molecular electronics. We show through a quantitative comparison of two types of junctions with the same molecule - one based on an isolated individual molecule and the other on a self-assembled monolayer - that intrinsic differences in the conduction per molecule as large as several orders of magnitude can exist simply as a function of the presence or absence of neighboring molecules. This behavior can be understood on the basis of thermal and electrostatic effects that depend critically on the local molecular environment.

Thermally activated conduction in molecular junctions.

We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased.

Effect of molecule-metal electronic coupling on through-bond hole tunneling across metal-organic monolayer-semiconductor junctions.

Using Hg/alkyl-chain-monolayer/p-Si devices we find that the type of contact between the chains and the electrodes (chemical bonding or not) is of critical importance for electronic transport across the junctions. As the semiconductor is p-type, the transport is that of holes. In agreement with theory we find that holes tunnel more efficiently through alkyl chains than do electrons.