Self-assembled monolayers of decanethiol on Au(111)/mica

We report here a preparation for thin gold films on mica substrates. We have investigated the influence of the substrate temperature and the evaporation rate on the morphology of the films. After careful outgasing of the substrate, 100 nm of Au is evaporated onto the mica surface maintained at high temperature. After slow cooling, ex situ characterizations are performed using AFM and STM. For our purposes, the best compromise between roughness and grain size is found to occur for an evaporation rate of 2 ?s^-1 onto a mica substrate maintained at 460 C. We have used these substrates for STM and AFM study of decanethiol self-assembled monolayers (SAMs). We present results for gold samples immersed for a few seconds in decanethiol solutions, revealing an incomplete organization of the films. The organization process is discussed through comparison between AFM and STM data recorded on the SAMs. Then we present molecular resolution STM pictures of ordered SAMs for longer immersion times. Received 25 May 1999     

Comparison of tunneling through molecules with Mott-Hubbard and with dimerization gaps

In order to study the tunneling of electrons through an interacting, 1D, dimerized molecule connected to leads, we consider the persistent current in a ring embedding this molecule. We find numerically that, for spinless fermions, a molecule with a gap mostly due to interactions, i.e. a Mott-Hubbard gap, gives rise to a larger persistent current than a molecule with the same gap, but due only to the dimerization. In both cases, the tunneling current decreases exponentially with the size of the molecule, but more slowly in the interacting case. Implications for molecular electronic are briefly discussed. Received: 17 November 1997 / Revised: 16 January 1998 / Accepted: 16 January 1998     

Molecular resolution of an organic monolayer by dynamic AFM

We report molecularly resolved dynamic AFM-measurements of a monolayer of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) epitaxially grown on Ag(110). Preparation and experiments were performed under UHV conditions. The frequency-modulation AFM technique was applied. Received: 26 February 1998 / Revised: 13 May 1998 / Accepted: 8 June 1998     

Determination of the interactions in confined macroscopic Wigner islands: theory and experiments

Macroscopic Wigner islands present an interesting complementary approach to explore the properties of two-dimensional confined particles systems. In this work, we characterize theoretically and experimentally the interaction between their basic components, viz., conducting spheres lying on the bottom electrode of a plane condenser. We show that the interaction energy can be approximately described by a decaying exponential as well as by a modified Bessel function of the second kind. In particular, this implies that the interactions in this system, whose characteristics are easily controllable, are the same as those between vortices in type-II superconductors.     

Shrinking of ultrathin polyelectrolyte multilayer capsules upon annealing: A confocal laser scanning microscopy and scanning force microscopy study

Heating-induced morphological changes of micrometer size capsules prepared by step-wise deposition of oppositely charged polyelectrolytes onto melamine formaldehyde (MF) latex particles and biological cells with subsequent dissolution of the core have been investigated by confocal laser scanning microscopy (CLSM) and scanning force microscopy (SFM). For poly(styrenesulfonate-Na salt)/poly(allylamine hydrochloride) polyelectrolyte capsules a remarkable heating-induced shrinking is observed. An increase of the wall thickness corresponding to the capsule diameter decrease is found. The morphology of these microcapsules after temperature treatment is characterized. The thickening of the polyelectrolyte multilayer is interpreted in terms of a configurational entropy increase via polyanion-polycation bond rearrangement. Received 20 January 2000     

A mechanism for cutting carbon nanotubes with a scanning tunneling microscope

We discuss the local cutting of single-walled carbon nanotubes by a voltage pulse to the tip of a scanning tunneling microscope. The tip voltage ( V) is the key physical quantity in the cutting process. After reviewing several possible physical mechanisms we conclude that the cutting process relies on the weakening of the carbon-carbon bonds through a combination of localized particle-hole excitations induced by inelastically tunneling electrons and elastic deformation due to the electric field between tip and sample. The carbon network releases part of the induced mechanical stress by forming topological defects that act as nucleation centers for the formation of dislocations that dynamically propagate towards bond-breaking. Received 6 April 2000     

Can one hear the shape of an electrode? II. Theoretical study of the Laplacian transfer

The flux across resistive irregular interfaces driven by a force deriving from a Laplacian potential is computed on a rigorous basis. The theory permits one to relate the size of the active zone to the derivative of the spectroscopic impedance with respect to the surface resistivity r through: . It is shown that the macroscopic transfer properties through a system of arbitrary shape are determined by the characteristics of a first-passage interface-interface random walk operator . More precisely, it is the distribution of the harmonic measure (or normalized primary current) on the eigenmodes of this linear operator that controls the transfer. In addition, it is also shown that, whatever the dimension, the impedance of a weakly polarizable electrode for any irregular geometry scales under a homothety transformation as L^d-1, L being the size of the system and d its topological dimension. In this new formalism, the question addressed in the title is transformed in a open mathematical question: “Knowing the distribution of the harmonic measure on the eigenmodes of the self-transport operator, can one retrieve the shape of the interface?” Received 3 November 1998     

Implementation of a sapphire cell with external electrodes for laser excitation of a forbidden atomic transition in a pulsed E-field

We demonstrate the production of an electric field inside a high temperature cesium vapor cell with external electrodes. This external control of the electric field, which is not possible with a glass cell in presence of a cesium vapor, is achieved using a cell made of sapphire, and is of particular interest for our ongoing Parity Violation experiment. We describe the main components and the implementation on the set-up, including the pulsed high voltage generator. With pulse duration not exceeding 200 ns the system provides a reversible longitudinal E-field of up to 2 kV/cm in the vapor at a density of ∼ 2×10¹⁴ at/cm³ without discharge. Atomic signals attest the application of the electric field in the cell, with the predicted value. Further improvements obtained with sapphire cells are also presented. Received 15 September 2000     

Column size effects of DER fluids

The static yield stress of dielectric electrorheological (DER) fluids of infinite column state and chain state are calculated from the first principle method. The results indicate that the column surface contributions to ER effects is very small and both states will give correct results to the real DER fluids. Received 29 January 2001 and Received in final form 30 April 2001     

Optics of embedded semiconductor nano-objects using a hybrid model: bare versus dressed polarizabilities

The influence of the surrounding semiconducting matrix upon the optical response of embedded nano-objects (quantum dots) has been investigated. This system can be described by means of a hybrid model, where the full response is a combination of a macroscopic electrostatic response term and a dynamic response term, obtained quantum mechanically. The result is a modified discrete dipole model, where excess discrete dipoles having an excess polarizability with respect to a uniform background identical to the dielectric host material represent the response. In this model all electrodynamic interactions are screened by the host material. The electrostatic response is obtained by approximating the quantum dots by embedded dielectric oblate ellipsoids. Closed expressions for the electrostatic response of these ellipsoids have been derived. The electrodynamic nature of the dynamic quantum mechanical polarizability term however is unclear. It is not certain whether this polarizability is dressed or bare. Therefore we have investigated in detail the consequences of both options. Although there is no real qualitative difference between them, the difference is so large that experiment can easily discriminate between both. Results should be easily measurable anyhow.     

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Single crystal RFe₂(110) films were grown by molecular beam epitaxy to a total thickness of 1000 ? at different substrate temperatures ranging from 450 ^° C to 660 ^° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe₂(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound. Received 29 June 2000     

Thermodynamics of the Casimir effect

A complete thermodynamic treatment of the Casimir effect is presented. Explicit expressions for the free and the internal energy, the entropy and the pressure are discussed. As an example we consider the Casimir effect with different temperatures between the plates (T) resp. outside of them (T'). For T'<T the pressure of heat radiation can eventually compensate the Casimir force and the total pressure can vanish. We consider both an isothermal and an adiabatic treatment of the interior region. The equilibrium point (vanishing pressure) turns out instable in the isothermal case. In the adiabatic situation we have both an instable and a stable equilibrium point, if T'/T is sufficiently small. Quantitative aspects are briefly discussed. Received 24 February 1999 and Received in final form 26 April 1999     

Destabilizing effect of a surface electric field generated by ionic adsorption on the molecular orientation of nematic liquid crystals

The destabilizing effect of a surface electric field, produced by selective ionic adsorption, on the molecular orientation of a nematic-liquid-crystal sample is analyzed for a cell in the shape of a slab of thickness d. The electric-field distribution considered in the analysis is the one obtained in the limit in which essentially all the positive ions are adsorbed. Because of the coupling of this surface field with the nematic director, the surface anchoring energy depends on the thickness of the sample as well as on the adsorption energy characterizing the surfaces. A relation connecting the threshold field for the destabilization of the homeotropic pattern to the adsorption energy and to the thickness of the sample is established in closed form, after solving a set of two coupled non-linear equations determining the electric-field distribution across the sample. It is shown that the values of surface electric field generated by adsorbed ions that can lead to a destabilization of the homeotropic alignment can be attained by real samples.     

DC conductivity of a suspension of insulating particles with internal rotation

We analyse the consequences of Quincke rotation on the conductivity of a suspension. Quincke rotation refers to the spontaneous rotation of insulating particles dispersed in a slightly conducting liquid and subject to a high DC electric field: above a critical field, each particle rotates continuously around itself with an axis pointing in any direction perpendicular to the DC field. When the suspension is subject to an electric field lower than the threshold one, the presence of insulating particles in the host liquid decreases the bulk conductivity since the particles form obstacles to ion migration. But for electric fields higher than the critical one, the particles rotate and facilitate ion migration: the effective conductivity of the suspension is increased. We provide a theoretical analysis of the impact of Quincke rotation on the apparent conductivity of a suspension and we present experimental results obtained with a suspension of PMMA particles dispersed in weakly conducting liquids.     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

The interplay between surface micro-topography and -mechanics of type I collagen fibrils in air and aqueous media: An atomic force microscopy study

Calf skin type I collagen fibrils were regenerated from acidic solution and imaged with contact mode atomic force microscopy in air, water, and buffer solution. When imaged in air at a contact force of 20-150 nN, collagen fibrils exhibited a distinct transverse banding pattern with a period of 65 nm, consisting of high ridges and shallow grooves. The force dependence of the images suggests that such banding pattern is attributed to the transverse contraction of the fibril upon dehydration during sample preparation, which reflects the tangential mass density across the fibril. Imaging in water and phosphate buffer solution at a contact force of 15-80 nN revealed hydrated collagen fibrils with smooth surfaces. The rigidity of the collagen fibrils decreased considerably upon hydration. Scanning the cantilever tip in an aqueous medium at a contact force of 90-280 nN enabled us to probe subunit arrangement in the bulk region of the collagen fibril. The results indicate that the molecular assembly in the hydrated fibril is akin to that in the intact form. The image resolution was improved by stabilizing the collagen molecules through crosslinking with glutaraldehyde, which served to resolve microfibril-like structure on the fibril surface. Received 28 March 2000 and Received in final form 15 June 2001