Gauthier Torricelli

Switching Casimir forces with phase-change materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of similar to 100 nm between Au and Ag-In-Sb-Te (AIST) surfaces were achieved on crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches, by stimulating the phase-change transition via localized heat sources.

Casimir force between a metal and a semimetal

We present here measurements of the Casimir force gradient in the 60–300 nm range using a commercial Atomic Force Microscope operating in Ultra High Vacuum (UHV). The measurements were carried out in the sphere-plate geometry between a Au sphere and plates consisting of two different classes of material, that is a metal (Au) and a semimetal (HOPG). The variation in the optical properties of the materials produces clearly observed differences in the Casimir force as predicted by calculations based on the quantum theory of optical networks and the Lifshitz theory.

Size-selecting effect of water on fluorescent silicon clusters

Silicon clusters were produced by gas aggregation in vacuum and co-deposited with water vapour onto a cold target where the water vapour froze. Melting of the ice yielded fluorescent silicon nanoparticles suspended in water which were investigated by photoluminescence spectroscopy (PL) and atomic force microscopy (AFM). The PL spectrum showed a prominent band at 420 nm and other, less intense bands at shorter wavelengths. No fluorescence was observed below 275 nm. The shortest wavelength observed was related to a silicon cluster diameter of 0.9 nm using a simple particle-in-a-box model. Drops of the suspension were also deposited on freshly cleaved HOPG and investigated by AFM. The images showed single and agglomerated clusters with heights of typically 0.6 up to 2 nm. The sizes displayed by our measurements are not correlated to the average sizes that result from gas aggregation, indicating a size-selecting effect of the water suspension. The cluster-cluster interaction in water is governed by repulsion due to thermal energy and attraction due to van der Waals forces. For very small clusters repulsion dominates; at 3 nm diameter the two forces are balanced. We identify this stable phase of small clusters as the origin of exceptionally stable fluorescence.

Tuning the effective coupling of an AFM lever to a thermal bath

Fabrication of Nano-Electro-Mechanical-Systems (NEMS) of high quality is nowadays extremely efficient. These NEMS will be used as sensors and actuators in integrated systems. Their use however raises questions about their interface (actuation, detection, read out) with external detection and control systems. Their operation implies many fundamental questions related to single particle effects such as Coulomb blockade, light matter interactions such as radiation pressure, thermal effects, Casimir forces and the coupling of nanosystems to external world (thermal fluctuations, back action effect). Here we specifically present how the damping of an oscillating cantilever can be tuned in two radically different ways: i) through an electro-mechanical coupling in the presence of a strong Johnson noise, ii) through an external feedback control of thermal fluctuations which is the cold damping closely related to Maxwells demon. This shows how the interplay between MEMS or NEMS external control and their coupling to a thermal bath can lead to a wealth of effects that are nowadays extensively studied in different areas.

Measurement of the Casimir effect under ultrahigh vacuum: Calibration method

In this article, the authors present a strategy to measure the Casimir effect with an atomic force microscopy in an ultrahigh vacuum system. The key parameters including the absolute distance, the contact potential difference, and the calibration factor of the probe are determined by electrostatic interaction without contact. The strategy has been developed with the main purpose of performing a reliable relative measurement, that is, comparison of the Casimir force between different surfaces. As an example of the method, the authors estimate the accuracy and the precision of measurements performed on a Au sample.

A MEMS-based high frequency x-ray chopper

Time-resolved x-ray experiments require intensity modulation at high frequencies (advanced rotating choppers have nowadays reached the kHz range). We here demonstrate that a silicon microlever oscillating at 13 kHz with nanometric amplitude can be used as a high frequency x-ray chopper. We claim that using micro-and nanoelectromechanical systems (MEMS and NEMS), it will be possible to achieve higher frequencies in excess of hundreds of megahertz. Working at such a frequency can open a wealth of possibilities in chemistry, biology and physics time-resolved experiments.

Fluorescence of silicon nanoparticles suspended in water: reactive co-deposition for the control of surface properties of clusters

Fluorescent silicon nanoparticles have been produced in a two‐step process in ultra high vacuum. First, silicon clusters were produced in the gas phase in a molecular beam. At the end of the cluster beam machine the cluster were co‐deposited with water onto a cold target. Melting of the ice yields a suspension that fluoresces at 420 nm when excited with ultraviolet light. The fluorescence intensity remains constant over a period of more than a year. Photo‐absorption and photo‐luminescence spectra provide evidence of a Si/SiO2 core‐shell structure having a silicon core size of at least 1.4 nm in diameter and oxygen deficient O‐Si‐O defects as the origin of the deep‐blue fluorescence. Furthermore, the fluorescent suspension was deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG). AFM images recorded in UHV showed networks of agglomerated clusters, their smallest units having a diameter of typically 0.7 nm.

X-ray pushing of a mechanical microswing

We report here for the first time the combination of x-ray synchrotron light and a micro-electro-mechanical system (MEMS). We show how it is possible to modulate in real time a MEMS mass distribution to induce a nanometric and tunable mechanical oscillation. The quantitative experimental demonstration we present here uses periodic thermal dilatation of a Ge microcrystal attached to a Si microlever, induced by controlled absorption of an intensity modulated x-ray microbeam. The mechanism proposed can be envisaged either for the detection of small heat flux or for the actuation of a mechanical system.

Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing

Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers. The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained. This high resolution experiment (the relative Bragg peak shift is 310) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns).