Khalid Lahlil

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about t...

Organic Functionalization of Luminescent Oxide Nanoparticles toward Their Application As Biological Probes

Luminescent inorganic nanoparticles are now widely studied for their applications as biological probes for in vitro or in vivo experiments. The functionalization of the particles is a key step toward these applications, since it determines the control of the coupling between the particles and the biological species of interest. This paper is devoted to the case of rare earth doped oxide nanoparticles and their functionalization through their surface encapsulation with a functional polysiloxane shell. The first step of the process is the adsorption of silicate ions that will act as a primary layer for the further surface polymerization of the silane, either aminopropyltriethoxysilane (APTES) or glycidoxypropyltrimethoxysilane (GPTMS). The amino- or epoxy- functions born by the silane allow the versatile coupling of the particles with bio-organic species following the chemistry that is commonly used in biochips. Special attention is paid to the careful characterization of each step of the functionalization process, especially concerning the average number of organic functions that are available for the final coupling of the particles with proteins. The surface density of amino or epoxy functions was found to be 0.4 and 1.9 functions per square nanometer for GPTMS and APTES silanized particles, respectively. An example of application of the amino-functionalized particles is given for the coupling with alpha-bungarotoxins. The average number (up to 8) and the distribution of the number of proteins per particle are given, showing the potentialities of the functionalization process for the labeling of biological species.

Near-field optical patterning on azo-hybrid sol–gel films

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

Novel photorefractive sol-gel materials

We have developed new photorefractive media based on hybrid organic-inorganic materials containing a charge transporting (CT) molecule either as side-chain or main-chain substituents on the silica backbone. Second order nonlinear optical (NLO) chromophores were introduced either as side chain or as guest units. These materials were prepared by the sol-gel process in the form of thin films of a few μm-thick. NLO and photorefractive properties have been evaluated using electro-optic measurements, two beam coupling experiments and photoconductivity measurements.

Evidence of Two Distinct Mechanisms Driving Photoinduced Matter Motion in Thin Films Containing Azobenzene Derivatives

Photoinduced matter motion in thin films containing azobenzene derivatives grafted to a polymer backbone is investigated by means of near-field probe microscopy. We evidence the existence of two different photomechanical processes which produce mass transport. One is governed by the light intensity pattern and the other by the light polarization pattern. The intensity-driven mechanism is found to critically depend on the polymer matrix while the polarization-driven mechanism occurs with almost the same efficiency in different materials. Depending on the relationship between the polarization and intensity patterns, the two processes may either compete or cooperate giving rise to a nontrivial directional mass transport process.

Real-time near-field imaging of photoinduced matter motion in thin solid films containing azobenzene derivatives

We present a study of the formation of surface relief gratings in thin solid films containing azobenzene derivatives upon illumination with an interference pattern. This study is based on near-field microscopy techniques that provide real-time imaging of both the photomechanical response of the material and light excitation profile. We demonstrate that the material deformation follows two distinct regimes characterized by different kinetics, a different phase relative to the light intensity pattern, and a specific dependence on light polarization.

Optically Anisotropic Thin Films by Shear‐Oriented Assembly of Colloidal Nanorods

Device-scale thin films of highly oriented (in-plane) colloidal nanorods are made available by using a simple coating process involving thixotropic rod gel suspensions. Application of this process to LaPO₄ nanorods leads to films exhibiting outstanding anisotropic optical properties, such as a remarkably large birefringence (Δn = 0.13) associated with high transparency, and sharply polarized fluorescence spectra when doped with europium.

Influence of the Molecular Design on the Antifouling Performance of Poly(ethylene glycol) Monolayers Grafted on (111) Si

Various poly(ethylene glycol) monomethyl ether moieties were grafted onto hydrogenated silicon surfaces in order to investigate the influence of the molecular design on the antifouling performance of such coatings. The grafted chains were either oligo(ethylene oxide) chains (EG)(n)OMe bound to silicon via Si-O-C covalent bonds, or hybrid alkyl/oligo(ethylene oxide) chains C(p)(EG)(n)OMe bound via Si-C covalent bonds (from home-synthesized precursors). Quantitative IR spectroscopy gave the molecular coverage of the grafted layers, and AFM imaging demonstrated that a proper surfactinated rinse yields C(p)(EG)(n)OMe layers free of unwanted residues. The protein-repellent character of these grafted layers (here, toward BSA) was studied by IR and AFM imaging. C(p)(EG)(n)OMe layers exhibit a lower surface concentration than (EG)(n)OMe layers, because of the presence of a solvent in the grafting solution; they however demonstrate high resistance against BSA adsorption for high values of the n/p ratio and a higher stability than (EG)(n)OMe. This behavior is consistently explained by the poor ordering capability of the alkyl part of the layer, contrary to what is observed for similar layers on Au, and the key role of an entangled arrangement of the ethylene oxide chains which forms when these chains are long enough.

Surface and volume gratings investigated by the moving grating technique in sol-gel materials

Abstract We analyze the various contributions to optically induced gratings in an azo-hybrid sol–gel material. This is done by a time-resolved grating translation technique, by polarization and laser intensity dependent experiments, and by index matching with an oil film. We characterize surface relief and anisotropy (index and absorption) gratings. In particular, we measure the phase shift of the surface relief modulation with respect to the fringe interference pattern.

Hole mobilities in sol–gel materials

New silylated precursors with hole transporting units are prepared by modification of different active molecules (carbazole, oxadiazole and tetraphenylphenylenediamine derivatives) using sol-gel precursors. Absorption and photoluminescence spectra show that the electronic structures are not significantly modified by the functionalization. Field dependence of the hole mobility of the different sol-gel layers is measured using the time-of-flight technique. The highest hole mobility is observed for the layer having tetraphenylphenylenediamine units: 5.7 × 10—5 cm2.V—1.s—1 at a field strength of E = 5 × 105 V.cm—1. For the best carbazole compound, the mobility is found to be about twentyfold lower at the same field. Further experiments are required to test these new materials as hole transporting layers in photorefractive and electroluminescent devices. Copyright