Local heating in metal-molecule-metal junctions

We report a technique that can be utilized as a nanoscale thermometer and its application to metal-molecule-metal junctions at room temperatures. We find that a molecular junction heats up to 463 K at 1 V. We also revealed an onset bias of approximately 0.04 V for heat generation via electron-phonon scattering by examining inelastic electron tunneling spectroscopy. The present results suggest the importance of attaining an optimal thermal link at molecule-electrode interfaces for providing practically sufficient electrical durability.     

Local electron heating in nanoscale conductors

The electron current density in nanoscale junctions is typically several orders of magnitude larger than the corresponding one in bulk electrodes. Consequently, the electron-electron scattering rate increases substantially in the junction. This leads to local electron heating of the underlying Fermi sea in analogy to the local ionic heating that is due to the increased electron-phonon scattering rates. We predict the bias dependence of local electron heating in quasi-ballistic nanoscale conductors and its effect on ionic heating and discuss possible experimental tests of our results.     

Precision control of single-molecule electrical junctions

There is much discussion of molecules as components for future electronic devices. However, the contacts, the local environment and the temperature can all affect their electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures. Here we report an approach to monitor the electrical properties of single-molecule junctions, which involves precise control of the contact spacing and tilt angle of the molecule. Comparison with ab initio transport calculations shows that the tilt-angle dependence of the electrical conductance is a sensitive spectroscopic probe, providing information about the position of the Fermi energy. It is also shown that the electrical properties of flexible molecules are dependent on temperature, whereas those of molecules designed for their rigidity are not.     

Electron transfer dynamics of bistable single-molecule junctions

We present transport measurements of single-molecule junctions bridged by a molecule with three benzene rings connected by two double bonds and with thiol end-groups that allow chemical binding to gold electrodes. The I-V curves show switching behavior between two distinct states. By statistical analysis of the switching events, we show that a 300 meV mode mediates the transition between the two states. We propose that breaking and reformation of a S-H bond in the contact zone between molecule and electrode explains the observed bistability.     

Real-time conductivity analysis through single-molecule electrical junctions

Conductance through single-molecule junctions, consisting of nanoparticle/molecule/nanoparticle units between nanoscale planar electrodes, was monitored in real time during several process sequences, including dielectrophoretic directed self-assembly and post-assembly modification. Assembly faults are directly detected in real time when non-ideal assembly conditions result in molecular junction failure and nanoparticle fusion in the junction. The real-time conductivity measured through the junction was sensitive to ambient conditions, and changes persisted over several days of exposure. Atomic layer deposition of Al(2)O(3) was used to encapsulate and isolate the molecular junctions, and the effect of the deposition process sequence on current through the junction was evaluated in real time. Results indicate that the current measured during atomic layer deposition is sensitive to the chemical oxidation and reduction reactions proceeding in the 1-2?nm confined region between assembled nanoparticles.     

Vibrational and electronic heating in nanoscale junctions

Understanding and controlling the flow of heat is a major challenge in nanoelectronics. When a junction is driven out of equilibrium by light or the flow of electric charge, the vibrational and electronic degrees of freedom are, in general, no longer described by a single temperature. Moreover, characterizing the steady-state vibrational and electronic distributions in situ is extremely challenging. Here, we show that surface-enhanced Raman emission may be used to determine the effective temperatures for both the vibrational modes and the electrons in the current in a biased metallic nanoscale junction decorated with molecules. Molecular vibrations show mode-specific pumping by both optical excitation and d.c. current, with effective temperatures exceeding several hundred kelvin. Anti-Stokes electronic Raman emission indicates that the effective electronic temperature at bias voltages of a few hundred millivolts can reach values up to three times the values measured when there is no current. The precise effective temperatures are model-dependent, but the trends as a function of bias conditions are robust, and allow direct comparisons with theories of nanoscale heating.     

Charge transport characteristics of diarylethene photoswitching single-molecule junctions

We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.     

Dissecting contact mechanics from quantum interference in single-molecule junctions of stilbene derivatives

Electronic factors in molecules such as quantum interference and cross-conjugation can lead to dramatic modulation and suppression of conductance in single-molecule junctions. Probing such effects at the single-molecule level requires simultaneous measurements of independent junction properties, as conductance alone cannot provide conclusive evidence of junction formation for molecules with low conductivity. Here, we compare the mechanics of the conducting para-terminated 4,4'-di(methylthio)stilbene and moderately conducting 1,2-bis(4-(methylthio)phenyl)ethane to that of insulating meta-terminated 3,3'-di(methylthio)stilbene single-molecule junctions. We simultaneously measure force and conductance across single-molecule junctions and use force signatures to obtain independent evidence of junction formation and rupture in the meta-linked cross-conjugated molecule even when no clear low-bias conductance is measured. By separately quantifying conductance and mechanics, we identify the formation of atypical 3,3'-di(methylthio)stilbene molecular junctions that are mechanically stable but electronically decoupled. While theoretical studies have envisaged many plausible systems where quantum interference might be observed, our experiments provide the first direct quantitative study of the interplay between contact mechanics and the distinctively quantum mechanical nature of electronic transport in single-molecule junctions.     

In situ formation of highly conducting covalent Au-C contacts for single-molecule junctions

Charge transport across metal-molecule interfaces has an important role in organic electronics. Typically, chemical link groups such as thiols or amines are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal-carbon coupling has been shown through C60, benzene and π-stacked benzene, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe(3))-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique. Gold atoms at the electrode displace the SnMe(3) linkers, leading to the formation of direct Au-C bonded single-molecule junctions with a conductance that is ～100 times larger than analogous alkanes with most other terminations. The conductance of these Au-C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au-C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal-organic contacts.     

Local effects across core junctions in sandwich panels

Sandwich beams and panels with symmetric faces and cores of varying stiffness are investigated. The paper presents a theoretical and experimental study of the local effects that occur in the vicinity of intersections between cores of different stiffness in such sandwich panels. These local effects manifest themselves by a significant rise of the bending stresses in the faces in the vicinity of the core junctions. Closed-form estimates of the stress/strain fields induced by local effects are presented for sandwich beams and panels loaded in cylindrical bending. The accuracy of the derived closed-form estimates is verified experimentally for the case of a sandwich beam in three-point bending.     

Local lattice instability and ionic transport in high-temperature superconductors

Ionic transport, a characteristic feature of superionic conductors, is analyzed microscopically for the RBa₂Cu₃O{7-Σ} (R = rare earth) high-temperature superconductors. The electronic correlation effects are considered to be responsible for the formation of a local double-well potential for apical oxygen, which in its turn results in a decrease of the activation energy for interstitial chain oxygen defects. The behavior of the double well upon reduction of the oxygen content is discussed and its possible manifestation in the low-frequency dynamical response is considered.     

Local nanoscale heating modulates single-asperity friction

We demonstrate measurement and control of single-asperity friction by using cantilever probes featuring an in situ solid-state heater. The heater temperature was varied between 25 and 650 °C (tip temperatures from 25 ± 2 to 120 ± 20 °C). Heating caused friction to increase by a factor of 4 in air at ～ 30% relative humidity, but in dry nitrogen friction decreased by ～ 40%. Higher velocity reduced friction in ambient with no effect in dry nitrogen. These trends are attributed to thermally assisted formation of capillary bridges between the tip and substrate in air, and thermally assisted sliding in dry nitrogen. Real-time friction measurements while modulating the tip temperature revealed an energy barrier for capillary condensation of 0.40 ± 0.04 eV but with slower kinetics compared to isothermal measurements that we attribute to the distinct thermal environment that occurs when heating in real time. Controlling the presence of this nanoscale capillary and the associated control of friction and adhesion offers new opportunities for tip-based nanomanufacturing.     

Direct binding of a redox protein for single-molecule electron transfer measurements

An electron transfer protein is engineered with two thiol groups introduced at different positions in the molecular structure to allow robust binding to two gold electrodes. Atomic force microscopy and scanning tunneling microscopy single-molecule studies show that the engineered proteins: (1) bind to a gold electrode in defined orientation dictated by the thiol-pair utilised, and (2) have a higher conductance than the wild-type proteins indicating a more efficient electron transmission due to the strong gold-thiol contacts.     

Conductance changes in inelastic electron tunnelling junctions during infusion doping

The conductance of inelastic electron tunnelling (IET) junctions, typically Al/aluminium oxide/dopant/Pb (about 3000 Å thick), was measured during infusion doping. The measurements, necessarily rapid and accurate, were made by employing a digital multimeter and microcomputer. These measurements indicate an increase in the insulating barrier thickness during doping, which has been confirmed by corresponding changes in the junction capacitance. An experimental technique was also developed, allowing contamination-free infusion-doped IET spectra to be obtained routinely.A model is proposed which to a first approximation predicts the behaviour of junction conductance as a function of time during doping. Thus the dynamics of the infusion doping process have been clarified.     

Local heating effect on film flow structure

Studies of fundamental regularities governing film flow regimes are of interest for wide range of practical problems appearing in projecting and optimization of technological plants in energetic, chemical industry and other branches of industry, including space technologies. The present work is devoted to theoretical study and numerical modeling of processes in film flow of fluid on inclined surface with local heat source. Experimental researches carried out at the Institute of Thermophysics SB RAS [1] show that the effect of thermocapillarity under certain conditions can significantly influence the regime of film flow. Forming of “roller” of fluid is observed in the experiments in the area with high gradient of film surface temperature. If the temperature (or surface tension) gradient exceeds certain critical level then the periodical 3-D flow structure appears. The main quantity of fluid is gathered in periodical streams (or “fingers”). Between the streams the thickness of film decreases significantly [2]. The authors’ previous theoretical results described 2-D regime of locally heated film flow [3, 4, 5]. Those results allow us to state the following hypothesis: 2-D flow structure becomes unstable and 3-D perturbations grow as the local arrest of liquid is achieved due to thermocapillary effect (in the frame of reference moving with the heat source) [6, 7]. The results of linear stability analysis and numerical modelling are presented.     

Probing molecule-metal bonding in molecular junctions by inelastic electron tunneling spectroscopy

We present first-principles calculations for the inelastic electron tunneling spectra (IETS) of three molecules, 1-undecane thiol (C11), alpha,omega-bis(thioacetyl)oligophenylenethynylene (OPE), and alpha,omega-bis(thioacetyl)oligophenylenevinylene (OPV), sandwiched between two gold electrodes. We have demonstrated that IETS is very sensitive to the bonding between the molecule and electrodes. In comparison with experiment of Kushmerick et al. (Nano Lett. 2004, 4, 639), it has been concluded that the C11 forms a strong chemical bond, while the bonding of the OPE and OPV systems are slightly weaker. All experimental spectral features have been correctly assigned.     

Local heating of a cylinder with an inclusion

Exact and approximate solutions are presented for the stationary heat-conduction problem for a cylinder with a foreign inclusion for a discontinuous boundary condition of the first kind. Limits of applicability are set for the approximate solutions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 4, pp. 648–654, September, 1987.     

Point cathode electron gun using electron bombardment for cathode tip heating: numerical study of heating condition for high brightness operation

The cathode tip heating in a point cathode electron gun has been studied with numerical methods. In this gun, a straightened tungsten wire of 0.1 mm diameter is used as the cathode, and the tip part of it is locally heated by electron bombardment. The heating improves the cathode life and the gun is operated at higher cathode temperature. The gun operation requires heating conditions that confines the cathode evaporation to the tip part and keeps the temperature variation of the cathode during the evaporation small. The time variations of the temperature distribution and the cathode shape were calculated for different heating conditions. The methods and results are described, and the heating condition that is suitable for the high brightness operation of this type of gun is discussed.     

Beam heating of UO2 thin foils in an electron microscope

Parallel striations in thin foils of UO₂ single crystals introduced by pulse beam heating in the electron microscope have been studied by electron microscopy and electron diffraction. The observations are consistent with the view that both parallel (110) surfaces of the thin foil transform during heating into sets of parallel {111} faces.