Joël Chevrier

Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon

Semiconducting β‐FeSi2 is drawing much current research interest because of hoped‐for silicon‐based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron‐diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron‐diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high‐energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible ...

A RHEED Study of Epitaxial Growth of Iron on a Silicon Surface: Experimental Evidence for Kinetic Roughening

In a molecular beam epitaxy (MBE) chamber, iron thin films are found to grow epitaxially on a (111) silicon surface at a temperature of about 50 °C and thus even at large thicknesses (up to 2400 A) as checked in situ by reflection high-energy electron diffraction (RHEED). Electron diffraction also provides a clear evidence for a roughness which develops at the surface during the growth of the crystal. Subsequent annealing at about 200 °C clearly induces an irreversible flattening of this surface. Once the iron surface is made flatter, growth is started on this new surface which develops roughness as the iron thickness increases.

Plasmon enhanced near-field radiative heat transfer for graphene covered dielectrics

It is shown that a graphene layer on top of a dielectric slab can dramatically influence the ability of this dielectric for radiative heat exchange turning a poor heat emitter/absorber into a good one and vice versa. The effect of graphene is related to thermally excited plasmons. The frequency of these resonances lies in the terahertz region and can be tuned by varying the Fermi level through doping or gating. It makes possible the fast modulation of the heat flux by electrical means, which opens up new possibilities for very fast manipulations with the heat flux. The heat transfer between two dielectrics covered with graphene can be larger than that between best known materials and becomes especially efficient below the room temperature.

Composition and chemical bonding of pulsed laser deposited carbon nitride thin films

We studied composition, structure, and growth parameters of amorphous diamond-like carbon (DLC) and carbon nitride (CNx) films deposited by pulsed laser deposition in vacuum and in nitrogen atmosphere. The composition (0⩽N/C⩽0.4), the structural and the electronic properties of the deposited carbon and carbon nitride films were investigated for different laser fluences (1–12 J/cm2). Electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and micro-Raman spectroscopy indicated an increase in sp3-bonded carbon sites in the DLC films and an increase in N-sp3 C bonded sites in the CNx films with increasing deposition laser fluence. Raman spectroscopy also showed the presence of a small amount of C≡N bonds in the CNx films. Furthermore, we observed that keeping the nitrogen pressure constant (P=100 mTorr) the increase in the deposition laser fluence is reflected by an increase in the nitrogen content in the films. All the results have been discussed in the framework of different theoretical models.

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.

Epitaxial Growth of α-FeSi2 on Si(111) at Low Temperature

Epitaxial α-FeSi2 phase has been observed on silicon (111) face after growth at T = (500 ÷ 550) °C by Molecular Beam Epitaxy (MBE). In situ Reflection High-Energy Electron Diffraction (RHEED) enabled us to identify its crystallographic structure. Quantitative measurements of the lattice parameters parallel to the interface were obtained from ex situ X-ray diffraction experiments at grazing incidence. The existence of three domains was also observed. These results are shown to be fully consistent with the growth of tetragonal α-FeSi2. Compared to the bulk orthorhombic β-FeSi2 state which is mainly determined by a solid-state Jahn-Teller effect, this change of the FeSi2 structure appears to be directly induced by the epitaxial growth on the (111) silicon surface. During the growth, the relaxation α-FeSi2 towards β-FeSi2 has been found. During in situ thermal annealing, the transition of another pseudomorphic cubic-FeSi2 toward α-FeSi2 has also been observed. These two irreversible transformations suggest a hierarchy of stability between the various phases of FeSi2 epitaxially grown on silicon (111) from cubic-FeSi2 to α-FeSi2 and finally β-FeSi2.

Nanotechnology and Nanoscale Science: Educational challenges

Nanotechnology has been touted as the next ‘industrial revolution’ of our modern age. In order for successful research, development, and social discourses to take place in this field, education research is needed to inform the development of standards, course development, and workforce preparation. In addition, there is a growing need to educate citizens and students about risks, benefits, and social and ethical issues related to nanotechnology. This position paper describes the advancements that have been made in nanoscale science and nanotechnology, and the challenges that exist to educate students and the public about critical nanoscience concepts. This paper reviews the current research on nanotechnology education including curricula, educational programs, informal education, and teacher education. Furthermore, the unique risks, benefits and ethics of these unusual technological applications are described in relation to nanoeducation goals. Finally, we outline needed future research in the areas of nanoscience content, standards and curricula, nanoscience pedagogy, teacher education, and the risks, benefits, and social and ethical dimensions for education in this emerging field.

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.

Imaging electron wave functions inside open quantum rings

Combining scanning gate microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of the electron probability density |Psi|(2)(x,y) in embedded mesoscopic quantum rings. The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wave function interferences. Simulations of both |Psi|(2)(x,y) and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to |Psi|(2)(x,y).

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.