Two-level conductance fluctuations of a single-molecule junction

The conductance of a single-molecule junction in a low-temperature scanning tunneling microscope has been measured at nanosecond time resolution. In a transition region between tunneling and contact the conductance exhibits rapid two-level fluctuations which are attributed to different geometries of the junction. The voltage dependence of the fluctuations indicates that electrons injected into the lowest unoccupied molecular orbital may efficiently couple to molecular vibrations.     

Reversible switching of Kondo resonance in a single-molecule junction

The control of the Kondo effect is of great interest in single-molecule junction due to its potential applications in spin based electronics.Here,we demonstrate that the Kondo effect is reversibly switched on and off in an iron phthalocyanine(FePc)single-molecule junction by using a superconducting Nb tip.In a scanning tunneling microscope-based Nb-insulator-FePc-Au junction,we achieve a reversible switching between the Kondo dip and inelastic electronic tunneling spectra by simply adjusting the tip-sample distance to tune the tunnel coupling at low temperature.Further approaching the tip leads to the picking up of the molecule to the tip apex,which transfers the geometry of the single-molecule junction into a Nb-FePc-insulator-Au type.As the molecule forms an effective magnetic impurity embedded into the superconducting ground states of the Nb tip,the out-gap Kondo dip switched to an in-gap Yu-Shiba-Rusinov state.Our results open up a new route for manipulating the Kondo effect within a single-molecule junction.     

Signatures of molecular magnetism in single-molecule transport spectroscopy

We report single-molecule-transistor measurements on devices incorporating magnetic molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that sequential electron tunneling may enhance the magnetic relaxation of the magnetic molecule.     

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.     

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.     

Electronics and chemistry: varying single-molecule junction conductance using chemical substituents

We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.     

Directional control in thermally driven single-molecule nanocars

With the hope of directing future bottom-up fabrication through bulk external stimuli (such as electric fields) on nanometer-sized transporters, we sought to study controlled molecular motion on surfaces through the rational design of surface-capable molecular structures called nanocars. Here we show that the observed movement of the nanocars is a new type of fullerene-based wheel-like rolling motion, not stick-slip or sliding translation, due to evidence including directional preference in both direct and indirect manipulation and studies of related molecular structures.     

Effects of hydration on molecular junction transport

The study of charge transport through increasingly complex small molecules will benefit from a detailed understanding of how contaminants from the environment affect molecular conduction. This should provide a clearer picture of the electronic characteristics of molecules by eliminating interference from adsorbed species. Here we use magnetically assembled microsphere junctions incorporating thiol monolayers to provide insight into changing electron transport characteristics resulting from exposure to air. Using this technique, current-voltage analysis and inelastic electron tunnelling spectroscopy (IETS) demonstrate that the primary interaction affecting molecular conduction is rapid hydration at the gold-sulphur contacts. We use IETS to present evidence for changing mechanisms of charge transport as a result of this interaction. The detrimental effects on molecular conduction discussed here are important for understanding electron transport through gold-thiol molecular junctions once exposed to atmospheric conditions.     

具有边缘缺陷石墨烯纳米结的自旋输运特性(英文)

利用第一性原理研究了两种具有边缘缺陷石墨烯纳米结的自旋输运,即边界氢原子饱和和未被饱和两种情况。结果表明:边缘缺陷改变了电子的输运行为。对于完整的石墨烯纳米带,两种自旋的电子在费米能级附近是完全简并的;对于含有边缘缺陷的石墨烯纳米结,两种自旋的电子在费米能级附近的很大能量范围内表现出自旋分离。电子局域态密度可进一步说明这种输运行为。这些纳米结可产生与自旋相关的极化电流。特别对于未饱和的缺陷结,在任何偏压下都有较高的自旋滤波效率。     

Nanolithographic control of the spatial organization of cellular adhesion receptors at the single-molecule level

The ability to control the placement of individual molecules promises to enable a wide range of applications and is a key challenge in nanoscience and nanotechnology. Many biological interactions, in particular, are sensitive to the precise geometric arrangement of proteins. We have developed a technique which combines molecular-scale nanolithography with site-selective biochemistry to create biomimetic arrays of individual protein binding sites. The binding sites can be arranged in heterogeneous patterns of virtually any possible geometry with a nearly unlimited number of degrees of freedom. We have used these arrays to explore how the geometric organization of the extracellular matrix (ECM) binding ligand RGD (Arg-Gly-Asp) affects cell adhesion and spreading. Systematic variation of spacing, density, and cluster size of individual integrin binding sites was used to elicit different cell behavior. Cell spreading assays on arrays of different geometric arrangements revealed a dramatic increase in spreading efficiency when at least four liganded sites were spaced within 60 nm or less, with no dependence on global density. This points to the existence of a minimal matrix adhesion unit for fibronectin defined in space and stoichiometry. Developing an understanding of the ECM geometries that activate specific cellular functional complexes is a critical step toward controlling cell behavior. Potential practical applications range from new therapeutic treatments to the rational design of tissue scaffolds that can optimize healing without scarring. More broadly, spatial control at the single-molecule level can elucidate factors controlling individual molecular interactions and can enable synthesis of new systems based on molecular-scale architectures.     

A single-molecule potentiometer

Controlling electron transport through a single-molecule device is key to the realization of nanoscale electronic components. A design requirement for single molecule electrical devices is that the molecule must be both structurally and electrically connected to the metallic electrodes. Typically, the mechanical and electrical contacts are achieved by the same chemical moiety. In this study, we demonstrate that the structural role may be played by one group (for example, a sulfide) while the electrical role may be played by another (a conjugated chain of C═C π-bonds). We can specify the electrical conductance through the molecule by modulating to which particular site on the oligoene chain the electrode binds. The result is a device that functions as a potentiometer at the single-molecule level.     

Triple junction transport and the impact of grain boundary width in nanocrystalline Cu

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.     

Rapid measurement of single-molecule conductance

A new device for measuring the conductance values of single-molecule junctions which are covalently bound to two electrodes is presented. The system works by repeatedly bringing two electrodes into and out of contact in a solution of molecules while measuring the current between the two electrodes during withdrawal. When molecules connect the two electrodes, steps occur in the current transient, and a statistical analysis provides the most probable conductance value for a single-molecule junction. This system provides an order of magnitude increase in speed over previous devices used for single-molecule conductance measurements, and the applicability of this tool is demonstrated in array based measurements as well as in biologically relevant samples where the conductances of single amino acid residues are measured.     

Two-channel sequential single-molecule measurement

Experimental results are given which demonstrate the sequential detection of single molecules with a measurement efficiency of near unity. IR140 dye molecules are detected in sequential probe volumes within a flowing stream through a 9 μm i.d. capillary. The measurement of single molecules was confirmed by means of autocorrelation, photobleaching, visual observation, and cross-correlation analysis. The number of single molecules photobleached prior to being measured in the second probe region was in excellent agreement with the bulk studies described by a photodestruction curve. A cross-correlation peak with a temporal delay corresponding to the interprobe volume transit time and a width in agreement with parabolic flow give a clear indication of sequential detection. The near unity measurement efficiencies for both channels indicates the great potential for rapid quantitative analysis of dilute solutions.     

Thermoelectricity in single-molecule devices

Harvesting waste energy from heat sources such as electronic devices through exploiting the thermoelectric effect is becoming increasingly attractive as information technique associated devices are now making up over 10% of the worldwide electricity consumption [Mills MP. The cloud begins with coal – big data, big network, big infrastructures and big power; 2013. Available from: https://www.tech-pundit.com/wp-content/uploads/2013/07/CloudBeginsWithCoal.pdf]. While solid-state thermogenerators have been developed and have shown promising results, their price in addition to other characteristics make them unattractive for a wide use [Zhang Q, Sun Y, Xu W, et al. Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently. Adv Mater. 2014;26(40):6829–6851]. Owing to those limitations, molecular structures have emerged as a possible alternative for fabricating cheap, efficient thermogenerators. Single-molecule devices are not prone to the mechanisms that limit the performance of bulk devices and could potentially be used to build ultra-high-efficiency thermoelectric power generators [Venkatasubramanian R, Siivola E, Colpitts T, et al. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature. 2001;413(6856):597–602]. Since the pioneering work by Reddy et al. in measuring single molecules there has been a steady rise in interest in this field [Reddy P, Jang SY, Segalman RA, et al. Thermoelectricity in molecular junctions. Science. 2007;315(5818):1568–1571]. While a molecular device structure allowing for cheap and scalable fabrication as well as exhibiting high efficiency has not been identified yet, there have been intriguing results in theory and experiment, showing what would be necessary to make molecules a viable alternative. Furthermore, molecular electronics has been shown to permit sensing applications and give additional insight into molecular transport.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.