Fabio Comin

Epitaxial growth of crystalline, diamond‐like films on Si (100) by laser ablation of graphite

Pulsed laser evaporation has been used to deposit ultrathin (<1 nm) carbon films on Si (100) in an ultrahigh vacuum environment. Auger spectroscopy studies revealed a layer‐by‐layer growth up to the fourth layer. Within this coverage range, the electronic structure of the carbon atoms evolves from carbidic to diamond‐like. Above two layers the topmost one consists exclusively of carbon atoms as evidenced by low‐energy ion scattering experiments. Scanning tunneling microscopy shows that the films are crystalline and that the surface lattice is hexagonal.

Quantitative non-contact dynamic Casimir force measurements

We show that the Casimir force (CF) gradient can be measured with no contact involved. Results of the CF measurement with systematic uncertainty of 3% are presented for the distance range of 100–600 nm. The statistical uncertainty is shown to be due to the thermal fluctuations of the force probe. The corresponding signal-to-noise ratio equals unity at the distance of 600 nm. Direct contact between surfaces used in most previous studies to determine absolute distance separation is here precluded. Use of direct contact to identify the origin of distances is a severe limitation for studies of the CF on structured surfaces as it deteriorates irreversibly the studied surface and the probe. This force machine uses a dynamical method with an inserted gold sphere probe glued to a lever. The lever is mechanically excited at resonant frequency in front of a chosen sample. The absolute distance determination is achieved to be possible, without any direct probe/sample contact, using an electrostatic method associated to a real time correction of the mechanical drift. The positioning shift uncertainty is as low as 2 nm. Use of this instrument to probe a very thin film of gold (10 nm) reveals important spatial variations in the measurement.

Probing the elastic properties of individual nanostructures by combining in situ atomic force microscopy and micro-x-ray diffraction

Atomic force microscopy (AFM) and micro-x-ray diffraction are combined to investigate nanostructures during in situ indentation. This technique allows the determination of elastic properties of individual nanoscale objects, particularly here SiGe∕Si(001) self-assembled islands. Using this novel technique it was possible to select a specific island, align it in the microfocused beam, and apply a pressure onto it, using the AFM tip. Simultaneously, the x-ray diffuse scattering map from the island and the surrounding substrate was recorded in order to probe the lattice parameter change during indentation. An elastic reduction of the island lattice parameter of up to 0.6% was achieved.

Nanomanipulation by atomic force microscopy of carbon nanotubes on a nanostructured surface

We have investigated atomic force microscopy (AFM) induced displacement of carbon nanotubes (CNT) on a nanostructured surface. Evidence is given that nano-dots act as pinning centers for CNT. We show that adhesion between nano-objects and mechanical properties of nanotubes are the basic mechanisms that control the interaction of mobile and deformable nano-objects with static nano-dots under the mechanical constraint applied by the AFM tip.

Mechanical mode dependence of bolometric backaction in an atomic force microscopy microlever.

Two backaction (BA) processes generated by an optical cavity-based detection device can deeply transform the dynamical behavior of an atomic force microscopy microlever: the photothermal force or the radiation pressure. Whereas noise damping or amplifying depends on the optical cavity response for radiation pressure BA, we present experimental results carried out under vacuum and at room temperature on the photothermal BA process which appears to be more complex. We show for the first time that it can simultaneously act on two vibration modes in opposite directions: Noise on one mode is amplified, whereas it is damped on another mode. Basic modeling of photothermal BA shows that the dynamical effect on the mechanical mode is laser spot position-dependent with respect to mode shape. This analysis accounts for opposite behaviors of different modes as observed.

In situ observation of the elastic deformation of a single epitaxial SiGe crystal by combining atomic force microscopy and micro x-ray diffraction

An in situ combination of atomic force microscopy and micro x-ray diffraction was developed to study the elastic behavior of nanosized objects. This technique offers the means to locally access the Young elastic moduli and Poisson ratios of individual nanostructures. Here, we investigated the elastic behavior of a single self-assembled 450 nm high SiGe island. As pressure was applied on the island, the resonance frequency of the atomic force microscope tuning fork was tracked together with the x-ray diffraction stemming from this individual crystal. The change in the tip-island contact stiffness could be derived from the variation in the resonance frequency of the tuning fork, whereas the island mean lattice parameter was inferred from the center of mass of the island’s Bragg scattering. From this information, the reduced elastic modulus of the tip-island system could be directly determined, which is in very good agreement with literature values. The pressure needed to compress the island lattice to the S...

Local detection of X-ray spectroscopies with an in-situ Atomic Force Microscope

The in situ combination of Scanning Probe Microscopies (SPM) with X-ray microbeams adds a variety of new possibilities to the panoply of synchrotron radiation techniques. In this paper we describe an optics-free AFM/STM that can be directly installed on synchrotron radiation end stations for such combined experiments. The instrument can be used just for AFM imaging of the investigated sample or can be used for detection of photoemitted electrons with a sharp STM-like tip, thus leading to the local measure of the X-ray absorption signal. Alternatively one can can measure the flux of photon impinging on the sharpest part of the tip to locally map the pattern of beams diffracted from the sample. In this paper we eventually provide some examples of local detection of XAS and diffraction.

Incomplete Melting of the Si(001) Surface: A Photoelectron Diffraction Study

A high-energy photoelectron diffraction study of Si(001) at different temperatures reveals that at about 1400 K a surface phase transition occurs. It has been interpreted as being an incomplete melting. The thickness of the liquid film is estimated to be about 2 A (or two atomic layers) on the basis of single-scattering cluster calculations.

Why do atomic force microscopy force curves still exhibit jump to contact

The force between two particles as a function of distance is one of the most fundamental curves in physics. Here, we describe how the force feedback microscope can routinely measure the tip-surface interaction in the entire range of distances with a sensitivity of 1 pN and in different media. The method allows to measure simultaneously the force, force gradient, and damping from solely the knowledge of the lever spring constant. The jump to contact is avoided and thus it is possible to follow the brutal nucleation of a water bridge between the tip and the surface.The force between two interacting particles as a function of distance is one of the most fundamental curves in science. In this regard, Atomic Force Microscopy (AFM) represents the most powerful tool in nanoscience but with severe limits when it is to probe attractive interactions with high sensitivity. The Force Feedback Microscope (FFM) described here, removes from AFM the well known jump to contact problem that precludes the complete exploration of the interaction curve and the study of associated energy exchanges. The FFM makes it possible to explore tip-surface interactions in the entire range of distances with a sensitivity better than 1 pN. FFM stands out as a radical change in AFM control paradigms. With a surprisingly simple arrangement it is possible to provide the AFM tip with the right counterforce to keep it fixed at any time. The counterforce is consequently equal to the tip-sample force. The force, force gradient and damping are simultaneously measured independently of the tip position. This permits the measurement of energy transfer in thermodynamic transformations. Here we show some FFM measurement examples of the complete interaction force curve and in particular that the FFM can follow the nucleation of a water bridge by measuring the capillary attractive force at all distances, without jump to contact despite the large attractive capillary force. Real time combination of the measured parameters will lead to new imaging modalities with chemical contrast in different environments.

Trace element analysis on Si wafer surfaces by TXRF at the ID32 ESRF undulator beamline.

Synchrotron radiation total-reflection X-ray fluorescence (SR-TXRF) has been applied to the impurity analysis of Si wafers using a third-generation synchrotron radiation undulator source. A lower limit of detectability (LLD) for Ni atoms of 17 fg (1.7 x 10(8) atoms cm(-2)) has been achieved with an optical set-up based on an Si(111) double-crystal monochromator and a horizontal sample geometry. These first results are very promising for synchrotron radiation trace element analysis since we estimate that it is possible to lower the LLD by a factor of about 25 by employing appropriate optics and detectors. The use of a crystal monochromator opens new possibilities to perform absorption and scattering experiments (NEXAFS and X-ray standing-wave methods) for chemical and structural analysis of ultratrace elements.