Michel Ramonda

Determination of the nanoscale dielectric constant by means of a double pass method using electrostatic force microscopy

We present a method to determine the local dielectric permittivity of thin insulating layers. The measurement is based on the detection of force gradients in electric force microscopy by means of a double pass method. The proposed experimental protocol is simple to implement and does not need any modification of standard commercial devices. Numerical simulations based on the equivalent charge method make it possible to carry out quantification whatever the thickness of film, the radius of the tip, and the tip-sample distance. This method has been validated on a thin SiO2 sample for which the dielectric permittivity at the nanoscale has been characterized in the literature. We also show how we can quantitatively measure the local dielectric permittivity for ultrathin polymer film of poly(vinyl acetate) and polystyrene.

Discontinuous ion tracks on silicon oxide on silicon surfaces after grazing-angle heavy ion irradiation

Thin silicon oxide layers on silicon have been characterized by atomic force microscopy before and after swift heavy ion irradiation with 0.63MeV∕u Pb ions at grazing angle of incidence. In this letter, the authors report the observation of extended intermittent tracks at the silicon oxide (SiO2) surface. As a result, this raises the question of the discontinuous energy deposition at the nanometric scale. This experimental overlook is of major interest for nanostructuring and surface nanoprocessing as well as with regard to reliability of electronic components and systems.

In-situ determination of the mechanical properties of gliding or non-motile bacteria by atomic force microscopy under physiological conditions without immobilization.

We present a study about AFM imaging of living, moving or self-immobilized bacteria in their genuine physiological liquid medium. No external immobilization protocol, neither chemical nor mechanical, was needed. For the first time, the native gliding movements of Gram-negative Nostoc cyanobacteria upon the surface, at speeds up to 900 µm/h, were studied by AFM. This was possible thanks to an improved combination of a gentle sample preparation process and an AFM procedure based on fast and complete force-distance curves made at every pixel, drastically reducing lateral forces. No limitation in spatial resolution or imaging rate was detected. Gram-positive and non-motile Rhodococcus wratislaviensis bacteria were studied as well. From the approach curves, Young modulus and turgor pressure were measured for both strains at different gliding speeds and are ranging from 20±3 to 105±5 MPa and 40±5 to 310±30 kPa depending on the bacterium and the gliding speed. For Nostoc, spatially limited zones with higher values of stiffness were observed. The related spatial period is much higher than the mean length of Nostoc nodules. This was explained by an inhomogeneous mechanical activation of nodules in the cyanobacterium. We also observed the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium. Both strains left a track of polymeric slime with variable thicknesses. For Rhodococcus, it is equal to few hundreds of nanometers, likely to promote its adhesion to the sample. While gliding, the Nostoc secretes a slime layer the thickness of which is in the nanometer range and increases with the gliding speed. This result reinforces the hypothesis of a propulsion mechanism based, for Nostoc cyanobacteria, on ejection of slime. These results open a large window on new studies of both dynamical phenomena of practical and fundamental interests such as the formation of biofilms and dynamic properties of bacteria in real physiological conditions.

Growth of silicon bump induced by swift heavy ion at the silicon oxide-silicon interface

Thin silicon oxide layers on silicon substrates are investigated by scanning probe microscopy before and after irradiation with 210 MeV Au+ ions. After irradiation and complete chemical etching of the silicon oxide layer, silicon bumps grown on the silicon surface are observed. It is shown that each impinging ion induces one silicon bump at the interface. This observation is consistent with the thermal spike theory. Ion energy loss is transferred to the oxide and induces local melting. Silicon-bump formation is favored when the oxide and oxide-silicon interface are silicon rich.

Force gradient detection under vacuum on the basis of a double pass method

The feasibility of detecting electrostatic gradients in the linear regime is shown under vacuum by combining intermittent contact atomic force microscopy and a double pass method. To achieve our goal, different flexure mode orders were employed. We show that the sensitivity of the frequency or phase shifts to a given gradient was reduced when the order was increased. This behavior is theoretically explained in quantitative agreement with the experiments. Thus, on the basis of different flexure mode orders, gradient detection can now be extended to other forces plus various environments, i.e., under vacuum or controlled atmosphere.

Cantilever calibration for nanofriction experiments with atomic force microscope

The lateral force microscope can provide information on frictional properties on surfaces down to the nanometer scale. Reproducible quantitative measurements require an accurate calibration of the mechanical response of cantilever. In this letter, we propose a fast and nondestructive method to determine the normal and lateral cantilever stiffness, kcantileverN and kcantileverL, used to calculate the normal and friction forces.

Dynamic atomic force microscopy operation based on high flexure modes of the cantilever

We show the interest of the high flexure modes of vibration for amplitude-controlled atomic force microscopy (AFM). In connection with AFM working conditions, we define the stabilization time threshold of the oscillating sensor. We show experimentally that, in both air and vacuum, the stabilization time decreases appreciably when the order of the flexure mode of the cantilever increases. Under ambient conditions, this increases the possible scan speeds by about one order of magnitude. Under vacuum and using standard sensors, the amplitude-controlled conditions are satisfied for harmonics equal to or higher than the second. Morphology imaging is then obtained. Thus, high flexure mode AFM easily extends the well known amplitude-controlled operations from ambient to vacuum environment, which allows new AFM applications.

Viruses Occur Incorporated in Biogenic High-Mg Calcite from Hypersaline Microbial Mats

Using three different microscopy techniques (epifluorescence, electronic and atomic force microscopy), we showed that high-Mg calcite grains in calcifying microbial mats from the hypersaline lake “La Salada de Chiprana”, Spain, contain viruses with a diameter of 50–80 nm. Energy-dispersive X-ray spectrometer analysis revealed that they contain nitrogen and phosphorus in a molar ratio of ~9, which is typical for viruses. Nucleic acid staining revealed that they contain DNA or RNA. As characteristic for hypersaline environments, the concentrations of free and attached viruses were high (>1010 viruses per g of mat). In addition, we showed that acid treatment (dissolution of calcite) resulted in release of viruses into suspension and estimated that there were ~15 × 109 viruses per g of calcite. We suggest that virus-mineral interactions are one of the possible ways for the formation of nano-sized structures often described as “nanobacteria” and that viruses may play a role in initiating calcification.

High-Energy Heavy Ion Irradiation-Induced Structural Modifications: A Potential Physical Understanding of Latent Defects

From annealing experiments performed on both irradiated SiO2-Si structures and MOS devices, swift heavy ions-induced morphological oxide defects are proposed to possibly act as latent defects.

Self-Organization of Silicon Dots Grown by Thermal Decomposition of HSiO3/2 Gels

Silicon crystallites embedded in thin silica films were prepared by the sol-gel route, using triethoxysilane as a precursor. The films of silicon crystallites were either obtained as free-standing films or were deposited on (001)-oriented silicon or glass substrates by a spin coating deposition of a liquid phase that was further heat-treated under static vacuum. It was found out that the processing temperature impacts both the silicon dot size and the density of dots. We also deposited such crystallites on silicon using the thermal decomposition of the dried gels under vacuum. In the latter case, two regimes of the silicon dot growth have been observed depending on the decomposition temperature. Specifically, at T≈800°C (regime I) the round-shaped silicon dots are virtually randomly distributed across the substrate surface, whereas the regime II at T≈1000°C is characterized by growth of the elongated silicon dots arranged in nanochains along the (1±10) axis.