Showing papers by "Francis Lévy published in 1996"

MoS2 as microtubes

Abstract: We report on the existence of MoS2 hollowed microtubes, several mm in lengths and less than 0.1 μm wall thickness, grown from the vapor phase. Scanning electron microscopy studies reveal that instability of thin weakly bonded sheets against folding causes the tube growth directly or indirectly beyond the formation of turbulent gas flow. Electron diffraction of a single tube wall proves a parallel growth mode of successive layers while the crystal lattices of both tube walls are mutually rotated. A spiral growth mode is proposed.

Preparation and characterization of highly oriented, photoconducting WS2 thin films

Abstract: A novel combination of methods is shown to produce semiconducting WS2 thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS3 +x) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS2 films. X-ray diffraction shows that the films crystallize in the 2H-WS2 phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W18O49 needles andβ-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.

Electrical transport properties of thin-film metal-oxide-metal Nb2O5 oxygen sensors

Abstract: The electrical properties of thin-film metal-oxide-metal Nb 2 O 5 oxygen sensors have been investigated in the temperature range 400–600°C. They are shown to be strongly dependent on the electrode material. With Nb electrodes, the cathode is a good electron injector. At low bias, the oxygen pressure dependence of the conductivity follows P (O 2 ) −1/ n with n equal 1.0 ± 0.2 over a pressure range extending at least from 10 5 to 10 Pa. A model is proposed explaining this behaviour by the presence of surface electron traps associated with chemisorbed oxygen. The films present a very large concentration of chemisorption sites as atmospheric oxygen is able to diffuse into the bulk through channels running between the Nb 2 O 5 chains of the crystal structure. Above a certain threshold voltage, a non-linear regime where the current density is proportional to the square of the applied voltage is observed. This regime is explained by the anodic oxidation of the Nb anode. Cr and Pt electrodes behave like valve contacts: they are blocking at low voltage and injecting at high applied voltage. Structures shorter than 50 μm are shown to be depleted of their oxygen vacancies for electric fields in the range 10 3 –10 4 V cm −1 . The electrical admittance presents a steep non-linear double-injection regime above a certain threshol voltage. In this bias range the current density varies like V n with n ≥ 7. The threshold voltage is sensitive to the ambient and decreases when the sensor is exposed to an atmosphere with low oxygen concentration. It is shown that the migration of ionized oxygen vacancies induced by the electric field actually controls the electronic transport through the metal-oxide-metal structure. These results provide new insights into the capabilities and limitations of Nb 2 O 5 sensors.

Deposition and characterization of refractory ternary phases: the transition metal nitride Ti1 − xMoxNy

Abstract: Differences in the number of valence electrons and atomic mass in the transition metals of ternary nitride phases cause relevant evolutions in the physical properties of these refractory coatings. This report presents results on structural and mechanical investigations in the Ti1 − xMoxNy system. Thin films are deposited by RF reactive magnetron sputtering from two metal targets. Different structures corresponding to a wide range of chemical compositions determined by electron probe microanalysis are obtained by varying the nitrogen partial pressure and the discharge powers. The crystallographic phase is analyzed by X-ray diffraction: for 0≤x≤0.79 and 1.65≤y≤1.04 the samples are single phase face centered cubic. For 0.5≤y≤0.65 the phase transition from γ-Mo2N to face centered cubic occurs. The steepness of the transition is related to the titanium content. The microhardness HK is measured by Knoop indentation. For 0.06≤y≤0.17 the microhardness is higher than for pure molybdenum. For y≈0.5 the highest values are reached for x=1. Increasing y, HK increases and reaches, for 0.40

An ultrasonic and thermal expansion study of the quasi-one-dimensional compound

Abstract: We report ultrasonic and thermal expansion measurements on quasi-one-dimensional . The elastic constant exhibits strong anomalies over a wide temperature range around the Peierls transition. Anisotropic behaviour of the slow shear mode propagating along and perpendicular to the chain direction is observed around . Elastic constants, like electrical resistance, are very sensitive to a small Nb content substituted for Ta. Anomalous behaviour of at low temperatures is reported. We also present measurements of the anisotropic thermal expansion coefficients.

Thermopower of HfTe2 and ZrTe2

Abstract: Thermopower and electrical resistivity of Hf Te 2 and ZrTe 2 , semi-metals with layered structure, were measured. The results on Hf Te 2 are compared with the Hall coefficient by Klipstein et al. The electronic properties of Hf Te 2 are interpreted in terms of a simple 2-band model. By analogy, the properties of ZrTe 2 are understood similarly.

Effect of ion collisions on Langmuir probe measurements in Ti-N deposition by unbalanced magnetron sputtering

Abstract: Langmuir probe measurements are a useful tool to evaluate the influence of plasma parameters on the mechanical properties of sputtered thin films. Ti - N is deposited by unbalanced magnetron sputtering at room temperature. The plasma parameters are determined by a cylindrical Langmuir probe. The deposition parameters are correlated to the plasma characteristics through the determination of the electron mean free path. The ion density is calculated according to the collisional Zakrzewski and Kopiczynski correction to the Laframboise model extended to a two-component gas. In order to evaluate the ion bombardment per condensing atom, the chemical composition and the density of the deposited thin films are determined by accurate electron probe microanalysis. The calculations show how the action of ion - atom collisions on ion collection by the probe affects the evaluation of the ion bombardment.

Phonon thermal conductivity of the inorganic quasi-one-dimensional conductors

Abstract: The scattering mechanisms involved in heat transport of the inorganic quasi-one-dimensional conductors exhibiting charge density waves (CDW) may be investigated through thermal conductivity measurements. In particular, measurements of the thermal conductivity K(T), in the range 1.5–350 K, on (TaSe4)2I, (NbSe4)3.3I, K0.3MoO3 can give information on different types of scatterings: by the soft Kohn-Peierls phonons above the Peierls temperature Tc, by the excitations of the CDW just below Tc and by defects at low temperatures in the vicinity of K(T) maximum in addition to the umklapp scattering. In order to interpret K(T) specific for CDW ground state, we show for comparison the results obtained for (NbSe4)3I, which is structurally similar to (TaSe4)2I and (NbSe4)3.3I, but it does not exhibit CDW.

Internal friction in charge density wave (TaSe4)2 I compound

Abstract: The low frequency anelastic properties of CDW compounds near the Peierls phase transition are studied in a forced sub-resonant pendulum suitable for delicate (TaSe 4 ) 2 I samples. The influence of measurement parameters such as frequency, oscillation amplitude, temperature rate and low temperature cycles, are obtained. The principal effects found are : the Peierls transition, a higher temperature relaxation (∼ 260 K) and a low temperature process (∼50 K).