Centre d’Élaboration de Matériaux et d’Études Structurales

Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate. This allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy. Thereby direct images of the unperturbed molecular orbitals of the individual pentacene molecules are obtained. Elastic scattering quantum chemistry calculations substantiate the experimental findings.

The first use of non-centrosymmetric Janus Au-TiO(2) photocatalysts in efficient, plasmon-enhanced visible-light hydrogen generation is demonstrated. The intense localization of plasmonic near-fields close to the Au-TiO(2) interface, coupled with optical transitions involving localized electronic states in amorphous TiO(2) brings about enhanced optical absorption and the generation of electron-hole pairs for photocatalysis.

Experiments on individual molecules using scanning probe microscopies have demonstrated an exciting diversity of physical, chemical, mechanical, and electronic phenomena. They have permitted deeper insight into the quantum electronics of molecular systems and have provided unique information on their conformational and mechanical properties. Concomitant developments in experimentation and theory have allowed a diverse range of molecules to be studied, varying in complexity from simple diatomics to biomolecular systems. At the level of an individual molecule, the interplays of mechanical and electronical behavior and chemical properties manifest themselves in an unusually clear manner. In revealing the crucial role of thermal, stochastic, and quantum-tunneling processes, they suggest that dynamics is inescapable and may play a decisive role in the evolution of nanotechnology.

Abstract The cholesteric‐liquid‐crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective – the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization‐selectivity rule. These limits must be exceeded for innovative applications like polarizer‐free reflective displays, broadband polarizers, optical data storage media, polarization‐independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric‐liquid‐crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization‐selectivity rule and go beyond the reflectance limit.

Using the Green’s dyadic method, we investigated numerically the heat generation in gold nanoparticles when illuminated at their plasmonic resonance. Two kinds of structures are discussed—colloidal-like nanoparticles and lithographic planar nanostructures—putting special emphasis on the influence of the object’s morphology at a constant metal volume. The mechanism of heating is explained and discussed by mapping the heating power density inside the structures. This work aims at giving an intuitive and original understanding of the relative heating efficiency of a wide set of morphologies and could stand for a basis recipe to design optimized plasmonic nanoheaters.

PART ONE: LEVERS AND NOISE 1. Mechanical properties of levers 2. Resonance enhancement 3. Sources of noise PART TWO: SCANNING FORCE MICROSCOPES 4. Tunneling detection systems 5. Capacitance detection systems 6. Homodyne detection systems 7. Heterodyne detection systems 8. Laser-Diode feedback detection systems 9. Polarization detection systems 10. Deflection detection systems PART THREE: SCANNING FORCE MICROSCOPY 11. Electric force microscopy 12. Magnetic force microscopy 13. Atomic force microscopy References Index

Monodispersed nanoparticles of cobalt have been prepared by an original method using the decomposition under hydrogen of an organometallic precursor in the presence of a stabilizing polymer. Two colloids (Coll-I and Coll-II) have been obtained by changing the organometallic concentration in the polymer. Observation by high-resolution transmission electronic microscopy (HRTEM) showed Co particles well isolated and regularly dispersed in the polymer with a very narrow size distribution centered around 1.5 nm (Coll-I) and 2 nm (Coll-II) diameter. These particles are superparamagnetic above the blocking temperature 9 K (Coll-I) and 13.5 K (Coll-II). The particle size deduced from the analyses of the magnetic susceptibilities and magnetization curves are consistent with those measured by HRTEM. Magnetization at 5 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom for both samples, where $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=1.94\ifmmode\pm\else\textpm\fi{}0.05$ ${\ensuremath{\mu}}_{B}$ for the smallest particles. High-field magnetization measurements, up to 35 T, increases nearly linearly with the applied field. This is equivalent to an increase of the mean magnetic moment with $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=2.1\ifmmode\pm\else\textpm\fi{}0.1$ ${\ensuremath{\mu}}_{B}$ at 35 T for the smallest particles. The effective magnetic anisotropies are found to be larger than that of the bulk materials and decrease with increasing particle size. This set of data allows us to conclude that the enhanced magnetization, its increase with applied magnetic field, and the enhanced effective magnetic anisotropy are associated with the large influence of the surface atoms and are more significant with decreasing size.

Visualizing Bond Order Bond lengths in conjugated molecules closely reflect individual bond order and are usually determined by diffraction methods. It is valuable to know bond order for rationalizing aromaticity, and reactivity and for chemical structure determination. Gross et al. (p. 1326 ; see the Perspective by Perez and the cover) differentiated the bond orders in individual molecules in the fullerene C 60 and in polyaromatic hydrocarbons by imaging with noncontact atomic force microscopy (AFM). The molecules were adsorbed onto a copper surface, and the AFM tip was decorated with a CO molecule, which was used to measure tip frequency shifts above the bonds and their apparent lengths. Multiple bonds appeared brighter in the images because of stronger Pauli repulsion, and their shorter length was amplified by bending of the CO at the tip apex.

Diffusion of atoms in a crystalline lattice is a thermally activated process that can be strongly accelerated by defects such as grain boundaries or dislocations. When carried by dislocations, this elemental mechanism is known as "pipe diffusion." Pipe diffusion has been used to explain abnormal diffusion, Cottrell atmospheres, and dislocation-precipitate interactions during creep, although this rests more on conjecture than on direct demonstration. The motion of dislocations between silicon nanoprecipitates in an aluminum thin film was recently observed and controlled via in situ transmission electron microscopy. We observed the pipe diffusion phenomenon and measured the diffusivity along a single dislocation line. It is found that dislocations accelerate the diffusion of impurities by almost three orders of magnitude as compared with bulk diffusion.

Invasibility of riparian plant communities was estimated by the percentage of alien species found along the Adour River (Southwest France) and along Lockout Creek, McKenzie River, and Willamette River (Central Cascades, Oregon, U.S.A.). At the patch scale, the invasibilities of riparian plant communities were compared between one exceptionally rich site of the Adour River and patches selected in the Hoh and Dungeness watersheds (Olympic Peninsula, Washington, U.S.A.). Alien species represented 24% of 1396 species for the Adour and 30% of 851 species for the McKenzie. They represented 24% of 148 species for the Hoh drainage and 28% of 200 species for the Dungeness drainage. Similar trends were found along the Adour River and along the McKenzie River for changes in total number of species per site and in percentages of alien species per site. These trends may be related to the intermediate disturbance regimes and to the physical structure of the riparian corridors. Climatic and human factors are also involved in these longitudinal changes. Positive linear relationships were found between the total number of species and the percentage of aliens observed in each site. At the patch scale, most of the sampled communities contained alien species. Although mature vegetative patches appeared to be invasible, young communities contained more alien species than older ones. For entire corridors, a positive linear relationship was found between total species richness and percentage of alien species in each patch type for the richest site of the Adour River. This may be partially explained by landscape features considered in a successional context. We suggest the use of empirical rules, and stress the importance of riparian systems for monitoring the conservation of local and regional species pools are suggested.

The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching.

The field of molecular electronics comprises a fundamental set of issues concerning the electronic response of molecules as parts of a mesoscopic structure and a technology-facing area of science. We will overview some important aspects of these subfields. The most advanced ideas in the field involve the use of molecules as individual logic or memory units and are broadly based on using the quantum state space of the molecule. Current work in molecular electronics usually addresses molecular junction transport, where the molecule acts as a barrier for incoming electrons: This is the fundamental Landauer idea of "conduction as scattering" generalized to molecular junction structures. Another point of view in terms of superexchange as a guiding mechanism for coherent electron transfer through the molecular bridge is discussed. Molecules generally exhibit relatively strong vibronic coupling. The last section of this overview focuses on vibronic effects, including inelastic electron tunneling spectroscopy, hysteresis in junction charge transport, and negative differential resistance in molecular transport junctions.

Nickel nanoparticles of tunable shape have been synthesized in THF, in the presence of hexadecylamine (HDA) or trioctylphosphineoxide (TOPO), in mild conditions and characterized by HREM and SQUID measurements. The formation of nanorods is promoted by a high amine content in the reaction medium. In contrast to what is observed for TOPO-protected nickel particles, the saturation magnetization of HDA-capped nanoparticles is comparable to that of bulk nickel, which demonstrates that the coordination of an amine ligand does not alter the magnetic properties of nickel.

We report the first study of electrical contact with an individual molecule (${\mathrm{C}}_{60}$). Using a scanning tunneling microscope tip, the electrical current $I$ flowing as a function of tip displacement $s$ towards the molecule is investigated [ $I\left(s\right)$ characteristics]. The tunneling current increases approximately exponentially with tip displacement in the tunnel regime, but this behavior changes significantly as contact is established. From the $I\left(s\right)$ data and calculations for ${\mathrm{C}}_{60}$ we determine an apparent electrical resistance of 54.80 M $\ensuremath{\Omega}$ for the junction at ``tip contact.'' In the Landauer formalism, this value is a measurement of the electronic transparence $2.35\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ of the molecule under the tip.

Negatively charged graphene layers from a graphite intercalation compound spontaneously dissolve in N-methylpyrrolidone, without the need for any sonication, yielding stable, air-sensitive, solutions of laterally extended atom-thick graphene sheets and ribbons with dimensions over tens of micrometers. These can be deposited on a variety of substrates. Height measurements showing single-atom thickness were performed by STM, AFM, multiple beam interferometry, and optical imaging on Sarfus wafers, demonstrating deposits of graphene flakes and ribbons. AFM height measurements on mica give the actual height of graphene (ca. 0.4 nm).

We report muon spin rotation measurements on the $S=1/2$ (${\mathrm{Cu}}^{2+}$) paratacamite ${\mathrm{Zn}}_{x}{\mathrm{Cu}}_{4\ensuremath{-}x}(\mathrm{OH}{)}_{6}{\mathrm{Cl}}_{2}$ family. Despite a Weiss temperature of $\ensuremath{\sim}\ensuremath{-}300\text{ }\text{ }\mathrm{K}$, the $x=1$ compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagom\'e Heisenberg antiferromagnet. We find that the limit between a dynamical and a partly frozen ground state occurs around $x=0.5$. For $x=1$, we discuss the relevance to a singlet picture.

Two-dimensional positioning of intact individual molecules was achieved at room temperature by a controlled lateral “pushing” action of the tip of a scanning tunneling microscope. To facilitate this process, four bulky hydrocarbon groups were attached to a rigid molecule. These groups maintained sufficiently strong interactions with the surface to prevent thermally activated diffusional motion, but nevertheless allowed controllable translation. Simulations demonstrated the crucial role of flexure during the positioning process. These results outline the key role of molecular mechanics in the engineering of predefined properties on a molecular scale.

A detailed experimental and theoretical investigation of the processes involved in the manipulation of individual specially designed porphyrin-based molecules by scanning tunneling microscopy at low temperature is presented. On a stepped Cu(211) surface, the interaction between tip and molecule was used to locally modify in a reversible way the internal configuration of a single molecule, thus drastically changing the tunneling current passing through it. Model calculations confirm that this manipulation realizes the principle of a conformational molecular switch.

Short chains and complex networks of interconnected Au nanoparticle chains (see Figure) are produced by a simple template-free approach. Optical spectroscopy and computer simulations show that surface plasmons from individual non-contacting nanoparticles are strongly coupled in the resulting 1D superstructures. These chains may provide a unique way to fabricate complex subwavelength optical waveguides.