Damjan Vengust

Dispersion and purification of Mo6S3I6 nanowires in organic solvents

Sedimentation measurements have been performed on dispersions of Mo6S3I6 nanowires in a range of common solvents. By far the best solvents were N,N-dimethylformamide (DMF) and acetone. Stable dispersions of purified Mo6S3I6 nanowires in DMF, with concentrations as high as 0.06gl−1, could be produced. Detailed analysis of the sedimentation curves showed that the material consisted of three phases, two insoluble phases and one which could be stably dispersed. We associate the insoluble phases with pseudospherical impurities and insoluble nanowires. The sedimenting nanowires tend to be in the form of large diameter bundles, in contrast to the dispersed phase which consists of nanowires arranged in smaller bundles. The average diameters of the nanowire bundles stably suspended in DMF are 12±14nm. The measured sedimentation time constants of the insoluble nanowires agree very well with theory allowing us to calculate the solid-fluid interaction parameter, β, to be very close to 105Pa. The material could be pur...

Transport properties of Mo6S3I6 nanowire networks

We present measurements of resistivity on Mo6S3I6 nanowire networks under different conditions. The room-temperature values of conductivity of as-grown nanowire networks are on the order of σ300K=0.04S∕m and show linear current-voltage characteristics, indicating that—in agreement with band structure calculations—there is a finite density of states at the Fermi level. The conductivity of pristine samples is thermally activated, closely following three-dimensional variable range hopping (VRH) behavior of the form σ=σ0exp−(T0∕T)β, where β=1∕4. Removal of interstitial iodine from the network by annealing in a vacuum gives rise to a cross over to one-dimensional VRH with β=1∕2 and a concurrent increase in room-temperature conductivity. The introduction of water vapor leads to a decrease in conductivity and reveals that the resistivity of the network is sensitive to interstitial water molecules.

Nanowire transformation and annealing by Joule heating.

Joule heating of bundles of Mo(6)S(3)I(6) nanowires, in real time, was studied using in situ TEM probing. TEM imaging, electron diffraction, and conductivity measurements showed a complete transformation of Mo(6)S(3)I(6) into Mo via thermal decomposition. The resulting Mo nanowires had a conductivity that was 2-3 orders higher than the starting material. The conductivity increased even further, up to 1.8 x 10(6) S m( - 1), when the Mo nanowires went through annealing phases. These results suggest that Joule heating might be a general way to transform or anneal nanowires, pointing to applications such as metal nanowire fabrication, novel memory elements based on material transformation, or in situ improvement of field emitters.

Field emission of point-electron source Mo6S3I6 nanowires

Th efi eld emission (FE) properties of nanowires made from the recently synthesized nanowire material Mo6S3I6 are reported. A single nanowire was mounted on an indium-coated nickel holder by dielectrophoresis in isopropyl alcohol. A careful activation or conditioning in a vacuum of 10 −7 mbar was shown to be indispensable in order to extract relatively stable FE currents in excess of 1 µ Ao riginating from only a few sites at the end of the nanowire. Measurements were performed in an FE microscope with an additional plane mesh, which enables us to record I –U characteristics in diode mode, or to observe the emission patterns in triode mode. With FE currents around 5–7 µ At he emitter degrades gradually, and to some extent irreversibly. The degradation mechanism is not a reversal of the instant site build-up.

Inorganic Molecular-Scale MoSI Nanowire-Gold Nanoparticle Networks Exhibit Self-Organized Critical Self-Assembly

We investigate for the first time the topological characteristics of large molecular-scale inorganic networks self-assembled in solution using the unique sulfur-bonding chemistry of conducting MoSI molecular wires and gold nanoparticles (GNPs). The network self-assembly is shown to display power-law distribution of graph edges, indicating an intrinsic tendency to self-organize into scale-invariant critical state, without any external control parameter. We discuss the electronic transport properties of such networks particularly with regard to the possibility of data processing.

Growth and field emission properties of vertically aligned molybdenum–sulfur–iodine nanowires on molybdenum and quartz substrates

Vertically aligned bundles of molybdenum–sulfur–iodine nanowires (MoSIx) were grown on a molybdenum foil, thin wire, and quartz substrate. By choosing different growth parameters we have been able to vary the surface density from ∼0.01 to 0.5 bundless/μm2 and the length of the nanowire bundles from ∼30 μm to 15 mm. In the case of growth on molybdenum, resistivity measurements show that a good electrical contact exists between the bundles and the metal substrate. The nanowires are found to have excellent field emission properties, competitive with state-of-the-art carbon nanotube large-area field emitters.

Unlocking the Functional Properties in One-Dimensional MoSI Cluster Polymers by Doping and Photoinduced Charge Transfer

To improve functionalization of MoSI cluster polymers we have studied the effects of adsorption doping on the electrical transport, bundling, and optical absorption spectra. Doping results both in enhanced conductivity and aggregated bundles in dispersion. The different electronic properties of different bundle diameters can be ascribed to self-doping during the synthesis. Furthermore, doping shifts the characteristic absorption peaks and transfers oscillator strength to lower energies. Femtosecond optical spectroscopy shows that the spectral signature of adsorption and self-doping indeed originates from the population of electronic levels that are empty or absent in the undoped sample. The large spectral shifts and long lifetimes of photoinduced charges suggest efficient localization.

Deuterium influence on the field emission from inorganic nanowires

We report the initial conditioning procedure and achieved properties of nanowires composed of molybdenum sulfur iodine and directly grown on the top of a macroscopic molybdenum wire. Such a wire with nanowires, which were mostly perpendicularly grown to the surface, was applied as a radial field emitter with a 20 mm2 geometric area positioned inside a cylindrical metal anode. A stable current density of ∼2 mA/cm2 was achieved at a relatively low macroscopic electric field. Simultaneous recording of gases released from the anode exhibits linear relationship with the dissipated power. The most unexpected result was a reversible field emission current dependence induced by varying the deuterium pressure within the range from 10−8 to 10−4 mbar. The maximum increase for a factor of 7 was registered. Possible mechanisms responsible for the observed phenomena are discussed.

Molybdenum carbide nanowires: facile synthesis, a new hybrid phase and their use as transparent electrodes

Several synthetic routes exist for the production of bulk molybdenum carbides whereas production of larger quantities of different molybdenum carbide phases in the form of nanowires and nanowire networks is still challenging. Here we report a novel route for synthesis of molybdenum carbide nanowires by carburisation of Mo6S2I8 nanowire bundles. Gram quantities of molybdenum carbide nanowires composed of mainly MoC and Mo2C phases were obtained and further reduced to single phase Mo2C nanowires. By controlling the reaction conditions during the transformation, we can modify the ratio between MoC and Mo2C phases in nanowires and produce a novel hybrid inorganic/organic nanomaterial that we identified as molybdenum carbide nanowires densely covered with carbon nanofibres. We used the same approach on nanowire networks and demonstrate a new use of molybdenum carbide in the form of nanowire networks as transparent electrodes. The sheet resistance of such robust and air-stable electrodes is around 1050 Ω sq−1 at room temperature and their transmittance between 93% and 95% in the range of 200–900 nm. The electrodes are thus suitable for electro-optic applications, especially where high transparency in the UV region is required.

A facile route to self-assembled Hg//MoSI nanowire networks

Nanotechnology crucially depends on new molecular-scale materials with tunable properties. In molecular electronics, building blocks have been reduced to single molecules, while connectors have largely remained at the mesoscopic scale. As a result, the behaviour of such devices is largely governed by interface effects and hence, currently, attention is focused on finding suitable molecular-scale alternatives. In this paper we discuss a new generation of one-dimensional inorganic nanostructures aimed at to replacing the mesoscopic connectors currently used in the electronics industry. We demonstrate how chemical functionalisation of nanowires consisting of molybdenum, sulphur and iodine in conjunction with very low concentrations of molecular mercury leads to one-dimensional systems which can be easily connected opening up new pathways to controlled deposition and interface formation.