Michel Martin

Influence of dioxygen on the junction properties of metallophthalocyanine based devices

The electrical properties of Au/PcZn/M devices are studied both in the dark and under illumination. The nature of the semitransparent metallic electrode M is varied. When the sandwiches are made and studied entirely under vacuum (10−7–10−8 Torr), no rectifying effect is observed in the dark and the open circuit voltage under illumination (Voc) shows no correlation with the work function of M. On the contrary, when the organic layer is exposed to air, a strong rectifying effect is found and Voc seems to be correlated with the work function of the semitransparent metallic electrode. A chemical scheme is proposed to rationalize these results.

Single-electron transistor made of multiwalled carbon nanotube using scanning probe manipulation

We positioned semiconducting multiwalled carbon nanotube, using an atomic force microscope, between two gold electrodes at SiO2 surface. Transport measurements exhibit single-electron effects with a charging energy of 24 K. Using the Coulomb staircase model, the capacitances and resistances between the tube and the electrodes can be characterized in detail.

Characterization of metallophthalocyanine‐metal contacts: Electrical properties in a large frequency range

The dark electrical properties of metallophthalocyanine‐metal contacts are studied in an extended frequency range (10−3–105 Hz). The measurements show that the standard Schottky model cannot be applied to metallo‐organic semiconductors. Three different contributions to the admittance may be distinguished. Over the very first few angstroms of the semiconductor extends a surface‐charge layer associated with a high capacitive term. The surface‐charge capacitance is only slightly dependent upon superimposed dc voltages. The so‐called space‐charge region extends below the surface‐charge layer over approximately 2000–4000 A. The properties of this space‐charge region are, however, dramatically different from those found in monocrystalline inorganic semiconductors. The corresponding capacity is almost independent of any dc superimposed voltage. At the same time, the resistance of the space‐charge region is highly dependent on externally applied dc voltages. The I‐V relationship seems to indicate a Frenkel–Poole ...

Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy

We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substrates using an atomic force microscope. The method utilizes the noncontact mode both for locating and moving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of “seeing” the moving particle in real time. Our method avoids well sticking problems that typically hamper the manipulation in the contact mode.

Theoretical Basis of Field-Flow Fractionation

In field-flow fractionation (FFF), macromolecular or particulate species (including colloids and particles up to about 50 μm) are separated in thin flow channels under the influence of a transverse external field. The separation relies on the non-uniform distribution of the species molecules or particles in the channel cross-section. In the ideal case corresponding to molecules or particles of negligible size and a uniform applied field, the migration toward one of the channel walls is countered by Brownian motion resulting in an exponential transverse concentration profile. This characterizes the Brownian, or normal, mode of operation in FFF. Interactions of species with nonuniform fields, transverse gradients, the flow of carrier fluid, and/or the channel walls may result in quite different transverse concentration profiles. These profiles distinguish the non-Brownian modes of operation.

Magnetization reversal measurements of size-selected iron oxide particles produced via an aerosol route

We report first measurements of the magnetization reversal of monodisperse 30u2009nm and 50u2009nm ferromagnetic Fe3O4 particles. These particles are produced in a carrier gas as an aerosol by spray pyrolysis. After production and size selection, they are precipitated on a silicon chip with a niobium SQUID (superconducting quantum interference device) incorporated on its surface. By changing a magnetic field in the plane of the SQUID, we can measure the magnetization reversal of the particles by the flux they induce into the SQUID. The angular dependence of this reversal is determined by rotating the magnetic field around the SQUID. Scanning electron microscope (SEM) images have confirmed the particle size and revealed the position of the collected particles. If the particle concentration is too high, we cannot detect changes in the magnetic moment of a single particle, but measure the magnetic properties of the whole assembly. If only a few particles are found on the SQUID loop the angular dependence of the magnetic reversal of a single particle can be measured; this result is compared with a simple model of magnetization reversal.

Ferromagnetic tunnel contacts to graphene: Contact resistance and spin signal

We report spin transport in CVD graphene-based lateral spin valves using different magnetic contacts. We compared the spin signal amplitude measured on devices where the cobalt layer is directly in contact with the graphene to the one obtained using tunnel contacts. Although a sizeable spin signal (up to ∼2 Ω) is obtained with direct contacts, the signal is strongly enhanced (∼400 Ω) by inserting a tunnel barrier. In addition, we studied the resistance-area product (R.A) of a variety of contacts on CVD graphene. In particular, we compared the R.A products of alumina and magnesium oxide tunnel barriers grown by sputtering deposition of aluminum or magnesium and subsequent natural oxidation under pure oxygen atmosphere or by plasma. When using an alumina tunnel barrier on CVD graphene, the R.A product is high and exhibits a large dispersion. This dispersion can be highly reduced by using a magnesium oxide tunnel barrier, as for the R.A value. This study gives insight in the material quest for reproducible a...

Determination of Thermodiffusion Parameters from Thermal Field-Flow Fractionation Retention Data

Field-flow fractionation (FFF) encompasses a broad family of methods of separation and characterization of supramolecular species. They are all based on the application of a field force perpendicularly to the direction of the flow of a carrier liquid in a thin parallel plate channel. The principle of the method is described and the general relationships between retention time and analyte properties are presented. Then, the specific characteristics (such as range of applications, relaxation times, flow stability) of thermal field-flow fractionation (thermal FFF), which is the FFF method that is based on the application of a temperature gradient, are discussed. Thermal FFF basically relies on the Soret effect and the thermal FFF retention time is related to the Soret coefficient, S T, of the polymer sample in the carrier liquid. However, the extraction of S T from retention time is rendered complicated by the temperature dependence of all relevant physico-chemical parameters (thermal conductivity, viscosity, density, and Soret coefficient itself). The various methods which have been developed to take into account some or all of these dependences are described. One of the most complete method, the so-called linear γ method, is applied to the determination of the Soret coefficients of a series of polystyrene standards in ethylbenzene, and to their temperature dependence. The resulting data are further analyzed to obtain the molar mass dependence of S T. This dependence can be expressed as a power law. It is found that the prefactor and the exponent of this power law both depend on temperature and have an optimum in the investigated temperature range. Furthermore, this exponent appears to be somewhat larger than the exponent of the power law relating the ordinary mass diffusion coefficient to the molar mass. Hence, depending on temperature, the thermodiffusion coefficient of polystyrene in ethylbenzene might slightly depend on molar mass.