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. Here, disordered bismuth (Bi) waveguides are reported, which are both formed and structurally tuned by electromigration until their efficiency as TE devices is maximized. In accordance with theory, the conductance of the most efficient TE waveguides is in the sub quantum of conductance regime. The stability of these structures is found to be substantially higher than other actively studied devices such as single molecule junctions.

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. Here we report that the requested capability can be realized by using junctions with a semimetal lead and molecules with a tailored effect of their monolayers on the work function of the semimetal. The approach is demonstrated by junctions with monolayers of alkanethiols on bismuth (Bi). Fermi-level tuning enables in this case increasing the Seebeck coefficient by more than 2 orders of magnitude. The underlying mechanism of this capability is discussed, as well as its general applicability.

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. Here we demonstrate that inelastic electron tunneling spectroscopy of molecular junctions based on thiol-alkyl chains can be used to precisely measure the temperature of metal nanoscale gaps under illumination. The nature of this measurement guarantees that the reported temperature indeed characterizes the confined volume in which heat is produced by the relaxation of hot carriers. Using a simple model, we suggest that the accuracy of the method enables also one to semiquantify the energy distribution of the hot carriers.

Real-time detection of redox events in molecular junctions.

Redox molecular junctions are promising systems for nanoelectronics applications, and yet they are still only marginally understood. The study of these systems has so far been conducted in solution, utilizing "electrolyte gating" to control their redox states and, as a result, their steady-state transistor-like conductance behavior. Here we explore redox junctions under vacuum at 77 K, and report real time detection of redox events in junctions of the type Au-6-thiohexanethiolferrocene-Au. Redox events are revealed as a two-level fluctuating signal in current-time traces with potential-dependent amplitude and frequency. Using a theoretical model for signals with a telegraph-like noise, the current-time traces are analyzed to extract the various molecular parameters which define the dynamics of the system. The presented method, which can be applied to other types of redox molecules, offers a new approach to study the unexplored territory of molecular dynamics in molecular junctions.

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). The power factor, S(2)G, of the QPCs is maximized at G ~ 0.25G(0) (where G(0) is the quantum of conductance) within agreement with a one-band model with step edge characteristics.

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. Rev. Lett.1985, 56, 616; Ajayan, P. M.; Marks L. D. Phys. Rev. Lett.1988, 60, 585; Ben-David, T.; Lereah, Y.; Deutscher, G.; Penisson, J. M.; Bourret, A.; Korman, R.; Cheyssac, P. Phys. Rev. Lett.1997, 78, 2585]. This phenomenon takes place whenever the net gain in the surface energy of the particles outweighs the energy cost of internal strain. The configurational space is then densely populated due to shallow free-energy barriers between structural local minima. Here we report that in the case of bismuth (Bi) nanoparticles (BiNPs), given the high anisotropy of the mass tensor of their charge carriers, structural fluctuations result in substantial dynamic changes in their electronic and conductance properties. Transmission electron microscopy is used to probe the stochastic dynamic structural fluctuations of selected BiNPs. The related fluctuations in the electronic band structure and conductance properties are studied by scanning tunneling spectroscopy and are shown to be temperature dependent. Continuous probing of the conductance of individual BiNPs reveals corresponding dynamic fluctuations (as high as 1 eV) in their apparent band gap. At 80 K, upon freezing of structural fluctuations, conductance anisotropy in BiNPs is detected as band gap variations as a function of tip position above individual particles. BiNPs offer a unique system to explore anisotropy in zero-dimension conductors as well as the dynamic nature of nanoparticles.

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.

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. The magnitude of rectification is used to accurately determine the effective oscillating potentials in the junctions induced by the irradiating laser. Since the gap size of the junctions used in this study is precisely determined by the length of the embedded molecules, the oscillating potential can be used to calculate the plasmonic enhancement of the electromagnetic field in the junctions. With a set of junctions based on alkyl thiolated molecules with identical HOMO-LUMO gap and different lengths, an exponential dependence of the plasmonic field enhancement on gap size is observed.

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. Under such conditions, each molecule within the ensembles becomes an interference path that modifies via its tunneling phase the electronic structure of the entire junction. This new form of quantum interference holds a great promise for robust novel conductance effects in practical molecular junctions.

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. For wavelengths that are not absorbed by Au, the results are consistent with a photoassisted transport mechanism in which conductance depends both on the electronic structure of the leads and on the interaction of the transporting electrons with oscillating electric fields originating from excitation of local plasmons. For wavelengths absorbed by Au, photoinduced mechanism is suggested to be the dominant transport mechanism. The results demonstrate optical control of ballistic transport in MQPCs and are also important for future interpretation of light effects on the conductance of single-molecule junctions.

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. We propose a pump-probe dc shot noise spectroscopy as a suitable alternative. Numerical examples of time-dependent and average responses of junctions are presented for generic models.

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. Here we report on a new method to form MQPCs with quantized conductance values in the range of 1-4G0. The contacts appear to be stable at room temperature for hours and can be deterministically switched between conductance values, or reform in case they break, using voltage pulses. The method enables us to integrate MQPCs within nanoscale circuits to fully harness their unique advantages.

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. All the Raman-active modes in our system show similar heating as a function of bias at room temperature, which suggests fast vibrational relaxation processes inside the junctions. These results demonstrate the power of direct spectroscopic probing of heating and cooling processes in nanostructures.

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. Goals for future experimental work are outlined.

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. These results will help to unify data obtained from disparate device structures and to provide an improved basis for designing future molecular electronic devices.

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.