L. Forro

Field emission from single-wall carbon nanotube films

We report on the field emission properties of single-wall carbon nanotube films, with emphasis on current–versus–voltage (I–V) characteristics and current stability. The films are excellent field emitters, yielding current densities higher than 10u2009mAu2009cm−2 with operating voltages that are far lower than for other film emitters, but show a significant degradation of their performances with time. The observed deviations from the Fowler-Nordheim behavior in the I–V characteristics point to the presence of a nonmetallic density of states at the tip of the nanotubes.

High mobility n‐type charge carriers in large single crystals of anatase (TiO2)

Resistivity, thermopower, and Hall‐effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n‐type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.

Infrared and Raman studies of the Verwey transition in magnetite

We present infrared and Raman measurements of magnetite (Fe3O4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T-V = 120 K. The structural aspect of the Verwey transition is disclosed by the appearance of additional infrared-active and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm(-1) change abruptly at the transition. For T<T-V, we observe fine structure in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. The effective mass of the mobile carriers is estimated to be m*approximate to 100m, where m is the electronic mass.

Electronic properties of doped fullerenes

The most abundant fullerene molecule, C60, has just the right combination of size, chemical stability, and activity to serve as the building block for a large variety of solids with fascinating properties. After a short overview of the related carbon compounds, first the structures of the C60 molecule and the pure crystalline C60 are summarized. Experimental and theoretical works on the vibrational and the electronic properties of the molecule and the solid are reviewed. Next, some of the relevant concepts widely used in solid state phyics, like electron-phonon and electron-electron interactions, Hunds rule and Jahn-Teller distortion are discussed in the context of doped fullerides. The structural and electronic instabilities of the conducting C60 compounds are reviewed in detail. Finally, a few open questions are discussed.

Tunable Polaronic Conduction in Anatase TiO2

Oxygen vacancies created in anatase TiO(2) by UV photons (80-130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.

Capacitive nanoelectromechanical switch based on suspended carbon nanotube array

We present the fabrication and high frequency characterization of a capacitive nanoelectromechanical system (NEMS) switch using a dense array of horizontally aligned single-wall carbon nanotubes (CNTs). The nanotubes are directly grown onto metal layers with prepatterned catalysts with horizontal alignment in the gas flow direction. Subsequent wetting-induced compaction by isopropanol increases the nanotube density by one order of magnitude. The actuation voltage of 6 V is low for a NEMS device, and corresponds to CNT arrays with an equivalent Young’s modulus of 4.5–8.5 GPa, and resistivity of under 0.0077u2002Ω⋅cm. The high frequency characterization shows an isolation of −10 dB at 5 GHz.

Uniformly Dispersed Deposition of Colloidal Nanoparticles and Nanowires by Boiling

Uniformly dispersed deposition of various nanoparticles (gold and silver of 20–50nm) and nanowires (ZnO and VOx) on different surfaces (Si, GaAs, mica, and steel) was obtained by boiling colloid drops on preheated substrates. Our results suggest a deposition mechanism based on the formation of microdrops induced by boiling. Indeed, microdrops produced by an ink-jet printing system resulted in uniformly dispersed deposits when the substrate was heated to high temperatures. We demonstrate that boil-deposited gold nanoparticles could be used for the catalytic growth of ZnO nanowires as well as for the manipulation of individual particles by means of an atomic force microscope.

Versatile force–feedback manipulator for nanotechnology applications

A nanoscale manipulation system has been designed and built through the integration of a force–feedback haptic device and a commercial atomic force microscope. The force–feedback interaction provides a very intuitive, efficient and reliable way for quick manipulation of nanoscale objects. Unlike other nanomanipulators, ours allows the user to feel the actual tip–sample interaction during the manipulation process. Various modes of manipulation have been implemented and evaluated. As a proof of concept, we show a contact-mode nanomanipulation of a carbon nanotube and a noncontact manipulation of silicon beads. In addition to nanomanipulation itself, all relevant signals can be recorded during the manipulation process which allows quantitative interpretation of nanomechanics experiments.

Pressure dependence of the large-polaron transport in anatase TiO2 single crystals

We have measured the pressure and temperature dependences of the resistivity and the thermoelectric power of anatase, TiO2. The resistivity varies with T 3 at high temperatures, and its absolute value is in the 1 Ωcm range. Below 60 K, the resistivity is activated. Most surprisingly, the activation energy shows non-monotonic pressure dependence. The thermoelectric power has a very high value and its temperature dependence resembles that of polaronic materials. We suggest a large polaronic model to describe the temperature and pressure dependence of the two transport coefficients.

In situ viscometry by optical trapping interferometry

We demonstrate quantitative in situ viscosity measurements by tracking the thermal fluctuations of an optically trapped microsphere subjected to a small oscillatory flow. The measured power spectral density of the sphere’s positions displays a characteristic peak at the driving frequency of the flow, which is simply proportional to the viscosity, when measured in units of the thermal power spectral density at the same frequency. Measurements are validated on different water-glycerol mixtures, as well as in a glycerol gradient, where no a priori knowledge of the solution is used to determine the glycerol concentration.