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International audience; Using supramolecular chemistry to functionalise graphene for photonic applications is a challenging issue due to graphene’s capacity to quench any emission from molecules adsorbed on its surface. To overcome this problem, we propose the use of molecular dyads to form ordered self- assemblies on graphene-like substrates. These dyads are designed to reduce surface quenching by positioning the emissive component out-of-the plane of the substrate. We use a zinc porphyrin and a phthalocyanine as molecular pedestals to immobilise the dyads onto the graphene thanks to a nanoporous network; and a perylenetetracarboxylic diimide, as the emissive component. This approach has been recently reported, however; we have found that the formation of these dyads is an intricate process, that requires an in-depth study of the solution phase before its study on a graphene surface. We demonstrate that two types of dyads can be formed in solution, depending on the supramolecular interactions that dominate the equilibrium, and the type of molecular pedesal used. A metal–ligand association was observed between the perylene and the porphyrin pedestal, whilst the phthalocyanine leads to a dyad formed via $\pi-\pi$ interactions. We also conclude that scanning tunneling microscopy is not a reliable technique to characterise the on-surface assemblies, due to a strong probe–molecule interaction. Other spectroscopic techniques; such as epifluorescence micro-spectroscopy coupled with atomic force-microscopy, were investigated, however we found it is ambitious to rely solely on these techniques, to correlate observations from the nano to the micrometric scale.

International audience; Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity, and topological properties. Their physical properties can be controlled by doping in electric double‐layer field‐effect transistors (FET). However, for the case of single layer NbSe2, FET doping is limited to ≈1 × 1014 cm−2, while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing angle‐resolved photoemission spectroscopy, scanning tunneling microscopy, quasiparticle interference measurements, and first‐principles calculations it is shown that a misfit compound formed by sandwiching NbSe2 and LaSe layers behaves as a NbSe2 single layer with a rigid doping of 0.55–0.6 electrons per Nb atom or ≈6 × 1014 cm−2. Due to this huge doping, the 3 × 3 charge density wave is replaced by a 2 × 2 order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, this work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.

International audience; Surface-confined host–guest chemistry at the air/solid interface is used for trapping a functionalized 3D Zn–phthalocyanine complex into a 2D porous supramolecular template allowing the large area functionalization of an sp2-hybridized carbon-based substrate as evidenced by STM, resonance Raman spectroscopy, and water contact angle measurements.

International audience; Here is reported a novel strategy of non-covalent functionalization of graphene to avoid the electronic coupling between this semi-metal and directly adsorbed optically active molecules. Graphene-confined supramolecular host–guest recognition is used to elaborate an emitting hybrid platform. It is shown that the cavities of an on-monolayer graphene nanoporous self-assembled network are able to trap zinc phthalocyanine molecules coordinated to an emitting axial ligand. As a result, the emission of the hybrid system exhibits the same features as the isolated molecular emitter, demonstrating that the fluorescence is not quenched by graphene and that the well-controlled inter-chromophore distance prevents any interaction between the dyes. Furthermore, an in-depth modelling study confirms the weak interaction between the out of plane emitting moieties and the monolayer graphene. This work opens a new avenue for the realization of innovative light-responsive graphene-based devices in nanophotonics and optoelectronics.

Very few studies concern water-soluble nanocrystals self-assembled in crystalline 3D superlattices called supracrystals. Furthermore, the control of the crystalline structure of nanocrystals known as nanocrystallinity has not been yet achieved with water-soluble nanocrystals. Here we produce, selectively, 5 nm Au single-domain (SD) and polycrystalline (POLY) water-soluble nanocrystals. These nanocrystals self-assembled in face-centered-cubic (fcc) supracrystals. The supracrystal stiffness evolves with the nanocrystallinity, the nanocrystal surface charge, as well as the steric effect of the coating agent. The optical properties of SD and POLY nanoparticles and those of the related supracrystals are also presented. In addition, a nanocrystallinity segregation event was observed upon drying-assisted self-assembly of aqueous stoichiometric mixtures of SD and POLY NCs, as in the case of their hydrophobic counterparts.

Here, we report on hierarchical mechanical behavior of 500-nm-thick Co-nanocrystal 3D superlattices (supracrystals) induced by either the crystalline structure (nanocrystallinity) or the length of the coating agent of Co nanocrystals. Increasing the nanocrystal shape anisotropy of Co nanocrystals through the control of their nanocrystallinities induces a higher level of ordering with both translational and orientational alignment of nanocrystals within the supracrystals. The hierarchy in ordering at various scales, i.e., from the atomic lattice within the nanocrystals to the nanocrystal superlattices within supracrystals, is correlated with marked changes in the Young’s modulus of supracrystals: From 0.7 ± 0.4 to 1.7 ± 0.5 and to 6.6 ± 1.5 GPa as the crystalline structure of Co nanoparticles changes from amorphous-Co to e-Co and to hexagonal compact packing (hcp)-Co, respectively. Moreover, for supracrystals of 7 nm amorphous Co nanoparticles, the Young’s modulus decreases by one order of magnitude from 0.7 ± 0.4 to 0.08 ± 0.03 GPa upon reducing the alkyl chain length of the ligands coating the Co nanoparticles from C18 (oleic acid) to C12 (lauric acid). The hierarchical mechanical behavior is rationalized using a dimensional model of the stress-strain relationship in supracrystals.

International audience; We propose a novel approach to trap 2 nm Pt nanocrystals using nanoporous two-dimensional supramolecular networks for cavity-confined host-guest recognition process. This will be achieved by taking advantage of two features of supramolecular self-assembly at surfaces. First, its capability to allow to the formation of complex 2D architectures, more particularly nanoporous networks, through non-covalent interactions between organic molecular building-blocks. Second, the ability of the nanopores to selectively host and immobilize a large variety of guest species. In this paper, for the first time, we will use isotropic honeycomb networks and anisotropic linear porous supramolecular networks to host 2nm Pt nanocrystals.

The Young's modulus of three-dimensional self-assembled Ag nanocrystals, as so-called supracrystals, is correlated with the type of coating agent as well as the nanocrystal morphology. The Young's moduli of supracrystals of icosahedral Ag nanocrystals are measured in the range of tens to hundreds of megapascals revealing an extremely soft mechanical behavior. The alkylamine molecules used as coating agents weakly interact with the Ag nanocrystal surface favoring translational and orientational ordering of atomic lattice planes of nanocrystals. Under such experimental conditions, both the average distance between nanocrystals and the increase of the nanocrystal diameter control the measured Young’s modulus: it increases with decreasing interparticle distance and increasing nanocrystal diameter. When dodecylamine (C12NH2) molecules are replaced by dodecanethiol (C12SH), the orientational ordering between nanocrystals, produced from the same batch as C12NH2, disappears by inducing a drop in the Young’s modulus. This is attributed to the formation of a “skin” at the nanocrystal surface causing a transition from shaped to spherical nanocrystals. Finally, by comparing with various studies performed in our group with Co and Au nanocrystals, we explain the formation of such extremely soft materials with Ag nanocrystals by both the strength of the binding between nanocrystals and coating agents and the ligand–ligand interactions.

Here mechanical properties of face cubic centered colloidal crystals obtained out of equilibrium by solvent evaporation of coated Au nanocrystals suspension, called supracrystals, are reported as a function ligand chain length (n) and interparticle edge-to-edge distance within the supracrystals (δpp) for two nanocrystal sizes (d). Young’s modulus (E*) and hardness (H) are independent of δpp and of the supracrystal morphology. Both E* and H are in the range of few tenths of a MPa to a few GPa. Tuning of δpp by 50% is achieved by controlling the solvent vapor pressure (Pt) during the evaporation process. For any nanocrystal size, at Pt = 0, E* and H values markedly increase with increasing n from 12 to 14. At Pt = 39% and 75%, such dependency disappears. This trend differs from classical nanocomposite materials and is attributed to a change in the conformation of flexible ligands with n and to free thiol-containing molecules trapped in the supracrystal lattices.

Here, 5 nm Ag nanocrystals are deposited, using the same procedure, on various substrates differing by their rms roughness, wetting properties and nanoparticle-substrate interactions leading, consequently, to different nanocrystal orderings. Theoretical calculations are carried out to understand how these parameters influence the size of the nanocrystal organizations on the substrate surface. When these nanocrystal arrays are subjected to an oxygen plasma treatment, the nanocrystals perfectly assembled in hexagonal networks remain intact, while the nanocrystals that are not well-packed coalesce to form larger particles independently on the used substrate. This phenomenon is observed on the entire substrate surface. This procedure gives an innovative way of using oxygen plasma generated by the reactive ion etching technique, as a new method to reveal defects in 2D Ag nanocrystal self-assemblies.

Multiwalled carbon nanotubes (MWCNTs) were used as nanotemplates for the dispersion and stabilization of FePt nanoparticles (NPs). Pre-formed capped FePt NPs were connected to the MWCNTs external surface via covalent binding through organic linkers. Free FePt NPs and MWCNTs-FePt hybrids were annealed in vacuum at 700 degrees C in order to achieve the L1(0) ordering of the FePt phase. Both as prepared and annealed samples were characterized and studied using a combination of experimental techniques, such as Raman and Mossbauer spectroscopies, powder X-ray Diffraction (XRD), magnetization and transmittion electron microscopy (TEM) measurements. TEM measurements of the hybrid sample before annealing show that a fine dispersion of NPs along the MWCNTs surface is achieved, while a certain amount of free particles attached to each other in well connected dense assemblies of periodical or non-periodical particle arrangements is also observed. XRD measurements reveal that the FePt phase has the face-centered cubic (fcc) disordered crystal structure in the as prepared samples, which is transformed to the face-centered tetragonal (fct) L1(0) ordered crystal structure after annealing. An increase in the average particle size is observed after annealing, which is higher for the free NPs sample. Superparamagnetic phenomena due to the small FePt particle size are observed in the Mossbauer spectra of the as prepared samples. Mossbauer and magnetization measurements of the MWCNTs-FePt hybrids sample reveal that the part of the FePt particles attached to the MWCNTs surface shows superparamagnetic phenomena at RT even after the annealing process. The hard magnetic L1(0) phase characteristics are evident in the magnetization measurements of both samples after annealing, with the coercivity of the hybrid sample over-scaling that of the free NPs sample by a factor of 1.25.

International audience; sing a complementary pair of simple alkylthiolates with hydrogen-bonding moieties, functionalized Au-55 clusters could be selectively deposited onto self-assembled monolayers on gold that carry the opposite functionality. The deposition can be readily controlled by the medium in which the clusters are dissolved and by the density of the functionalities in the self-assembled monolayers, and yields single clusters as well as two-dimensional cluster assemblies on the surface. The clusters are sufficiently strongly bound to give structures that are stable at ambient temperature, and that allow scanning tunneling spectroscopy on single clusters on the surface.

We report a study of acid-terminated self-assembled monolayers of alkanethiols of different length, 11-mercaptoundecanoic acid (11-MUA) and 16-mercaptohexadecanoic acid (16-MHDA), on Au(111). Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle techniques were used for characterization, and the results were compared with those obtained from n-alkanethiols of similar chain length, providing a detailed description of the two-dimensional crystalline structure. Molecular resolution STM images show that 11-MUA forms a dense-packed monolayer arranged in a (square root 3 x square root 3)R30 degrees structure with a c(2 x 4) superlattice, where the simple hexagonal phase, the c(2 x 4) superlattice, and nonordered areas coexist. 16-MHDA assembles in a uniform monolayer with similar morphology to that of 11-MUA, but molecular resolution could not be reached in STM due to both the hydrophilicity of the acid groups and the poor conductivity of the thick monolayer. Nevertheless, the monolayer thicknesses estimated by XPS and electrochemistry and the highly blocking character of the film observed by electrochemistry as well as the low water contact angle are consistent with 16-MHDA molecules forming a compact monolayer on the Au(111) substrate with fully extended alkyl chains and acid groups pointing away from the surface. The results obtained for 16-MHDA were reproducible under different preparation conditions such as the addition or omission of acetic acid to the ethanolic solution. Contrary to other reports, we demonstrate that ordered acid-terminated self-assembled monolayers are obtained with the same preparation conditions as those of the methyl-terminated ones, without any additional treatment.

International audience; We report on the elastic modulus of 3D superlattices of single domain epsilon-Co nanocrystals self-assembled in face-centered cubic structures, so-called supracrystals. The effects of nanocrystal size, probe size, and supracrystal morphology on the elastic properties of supracrystals are investigated by nanoindentation measurements using an atomic force microscope. We show that elastic moduli of supracrystals of single domain epsilon-Co nanocrystals are invariant on varying the nanocrystal size and supracrystal morphology (films or faceted shape) produced through heterogeneous (solvent evaporation) growth process. A mechanical model is proposed to analyze elastic properties of supracrystals that differ from classical composite materials. These results are explained in terms of nanocrystallinity, that is, the crystalline structure of nanocrystals, which is a key parameter in achieving supracrystals of different nanocrystal sizes with robust elastic properties.

Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in tern-is of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.

The layer covering a (1 1 0) face of Nb crystal annealed at 1500–2200 K in UHV has been studied by photoemission spectroscopy with synchrotron radiation and scanning tunneling microscopy (STM). This overlayer resulting from segregation of the oxygen dissolved in Nb bulk presents a complex structure with several kinds of Nb oxides. Analysis of photoemission spectra of 3d Nb core levels reveals that: (i) the oxide stoichiometry is mainly ranging from NbOx ≈ 1.2 to NbOx ≈ 0.8; (ii) the oxide thickness corresponds to 1.4 ± 0.3 monolayer of Nb atoms. As observed in STM images, the structure of this oxide overlayer consists of a quasi-periodic arrangement of NbOx ≈ 1 nanocrystals. Each nanocrystal is characterized by a prominent stick formed by 10 ± 1 egg-shaped bumps aligned along a close-packed NbO〈1 1 0〉 direction. These bumps are related to Nb atoms adsorbed as a close-packed row on the outmost (1 1 1) oxygen plane of a NbO nanocrystal, and to chemical bonds involving these Nb atoms and the underlying atoms. The origin of such an atomic structure is discussed.

The oxide/metal interface induced by surface segregation of oxygen during the annealing of a Nb single crystal in UHV has been studied by photoemission spectroscopy with synchrotron radiation. With 260 and 350 eV photons, four well-resolved peaks A, B, C, D are found in spectra within the 200–210 eV range of binding energy. One couple of peaks (A and C) is associated with 3d5/2 and 3d3/2 core levels of Nb atoms in the metal while the other one (B and D), shifted by 1.4 eV when compared to A and C, corresponds to 3d levels of oxidized Nb atoms. The metal peak A at 202.3 eV is formed by three 3d5/2 components: a peak due to a metallic state (202.1 eV) and two components shifted by 0.2 and 0.5 eV, which are attributed to Nb6O and Nb4O compounds due to interstitial atoms of oxygen, respectively. The estimated concentration of the interstitial oxygen atoms in the nanometer-thick metal skin underlying the NbO/Nb interface corresponds to a large enrichment when compared to the one in the Nb bulk.

It is common practice in the literature to reduce the field emission properties of film field emitters to a few parameters to make possible a comparison between different samples. In particular, the macroscopic field needed to extract a given current density and the field enhancement factor are often used. The purpose of this contribution is to attract attention to the fact that such comparisons have to be done with utmost care, as the values of these parameters depend on the experimental configuration, and especially on the inter-electrode distance, used for the measurements. Current and future solutions are considered and discussed.

International audience; We study the influence of the size and nanocrystallinity of dodecanethiol-coated gold nanocrystals (NCs) on the stiffness of 3D self-assembled NC superlattices (called supracrystals). Using single domain and polycrystalline NCs as building blocks for supracrystals, it is shown that the stiffness of supracrystals can be tuned upon change in relative amounts of single and polycrystalline NCs.

The elastic properties of highly ordered three-dimensional colloidal crystals of gold nanocrystals (called supracrystals) are reported. This study is based on the simultaneous growth of two kinds of gold nanocrystal supracrystals that range in size from 5 nm to 8 nm: interfacial supracrystals and precipitated supracrystals. The elastic properties are deduced from nanoindentation measurements performed with an atomic force microscope. The Young's modulus of the interfacial supracrystals, which grow layer-by-layer and form well-defined films, is compared to that of precipitated supracrystals, which are produced by homogeneous growth in solution. For the precipitated supracrystals, characterized by a thickness larger than 1 μm, the Oliver and Pharr model is used to determine the elastic moduli, which are in the gigapascal range and decrease with increasing nanocrystal size. For the interfacial supracrystals, with 300 nm average thickness, a second model (plate model) is applied in addition to the Oliver and Pharr model. These two models confirm independently that the interfacial films are very soft with Young's modulus in the range of 80–240 MPa. This result reveals a totally new feature of nanocrystal solids, never emphasized before. It is shown that these changes in the Young's modulus are related to the supracrystal growth mechanism.

International audience; Well-defined superlattices of colloidal nanocrystals, called supracrystals, are expected to have interesting physical properties. While the electronic properties of thin supracrystals have been extensively studied in the planar configuration, little is known about electron transport through micrometer-thick supracrystals. Here, we investigate the electronic properties of supracrystals made of Au nanocrystals with diameters of 5, 6, 7 and 8 nm using scanning tunneling microscopy/spectroscopy at low temperatures. The current-voltage characteristics show power-law dependences with exponents varying strongly with supracrystal thicknesses from 30 nm to a few microns. The crystallinity of these nanocrystals, called nanocrystallinity, is exclusively single domain for 5 nm nanocrystals and a mixture of single and polycrystalline phase for 6, 7 and 8 nm nanocrystals. We observed that supracrystals made of 5 nm nanocrystals have a different behavior than supracrystals made of 6, 7 and 8 nm nanocrystals and this might be related to the nanocrystallinity. These results help us to better understand the electron transport mechanism in such miniscule structures built from a bottom-up approach.

For self-assembled nanocrystals in three-dimensional (3D) superlattices, called supracrystals, the crystalline structure of the metal nanocrystals (either single domain or polycrystalline) or nanocrystallinity is likely to induce significant changes in the physical properties. Previous studies demonstrated that spontaneous nanocrystallinity segregation takes place in colloidal solution upon self-assembling of 5 nm dodecanethiol-passivated Au nanocrystals. This segregation allows the exclusive selection of single domain and polycrystalline nanoparticles and consequently producing supracrystals with these building blocks. Here, we investigate the influence of nanocrystallinity on different properties of nanocrystals with either single domain or polycrystalline structure. In particular, the influence of nanocrystallinity on the localized surface plasmon resonance of individual nanocrystals dispersed in the same dielectric media is reported. Moreover, the frequencies of the radial breathing mode of single domain and polycrystalline nanoparticles are measured. Finally, the orientational ordering of single domain nanocrystals markedly changes the supracrystal elastic moduli compared to supracrystals of polycrystalline nanocrystals.

International audience; Scanning tunneling microscopy and spectroscopy at 5 K have been used to determine the electronic properties of 7-nm dodecanethiol-passivated Au nanocrystals in three different configurations: isolated nanocrystal, self-organized thin films (few nanocrystal layers), and large three-dimensional well-defined thick films (over 30 nanocrystal layers) called supracrystals. The electronic properties of both thin and thick well-ordered supracrystals are analyzed in scanning tunneling spectroscopy geometry through dI/dV curves and conductance mapping at different bias voltages. The single particles exhibit a typical dI/dV curve with a Coulomb gap of similar to 360 meV and a Coulomb staircase. The dI/dV curve of the thin supracrystals presents a Coulomb blockade feature similar to 100 meV narrower in width than that of the single nanocrystal but without well-defined staircase. On the contrary, the thick supracrystals exhibit a dI/dV curve showing a large Coulomb gap with a Coulomb-staircase-like structure. Generally, the conductance mapping is found to be very homogeneous for both supracrystals. Nevertheless, for some bias voltages, inhomogeneities across individual nanocrystals appear. Additionally, some of these inhomogeneities seem to be related to the supracrystal surface morphology. Finally, these slight variations in the conductance mapping across individual nanocrystals embedded in the supracrystal are discussed in terms of high degree of nanocrystal ordering, low nanocrystal size distribution, and nanocrystal crystallinity.

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or supracrystals. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air–liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties.

International audience; We investigated the electronic properties of highly ordered three-dimensional colloidal crystals of gold nanocrystals (7 +/- 0.4 nm), called supracrystals. Two kinds of Au supracrystals with typical thicknesses of 300 nm and 5 mu m, respectively, are probed for the first time with scanning tunneling microscopy/spectroscopy at 5 K revealing similar power law behavior and showing homogeneous conductance with the fingerprint of isolated nanocrystal. Potential applications evading the size-related risks of nanocrystals could be then considered.

Carbon nanotubes have been grown by thermal chemical vapour deposition using C2H2 as source gas at low temperature (6301C) and low pressure (5-20 mbar) on borosilicate glass substrates similar to the ones used for flat panel displays. The field emission properties of the films have been investigated. The type of catalyst and the growth pressure were found to be important parameters to control the emission properties. r 2002 Elsevier Science B.V. All rights reserved.

International audience; In this contribution, we describe the synthesis of a star shaped ethynylpyrimidine having long alkoxyl side chains using Suzuki cross-coupling reactions. This compound presents interesting blue light emission fluorescence as well as self-assembling properties on graphite: a chiral system is obtained starting from a nonchiral molecule. This preliminary work indicates that pyrimidine derivatives could be good candidates for the development of novel functional organic materials.

The charge transport properties of single superconducting tin nanowires encapsulated by multiwalled carbon nanotubes have been investigated by multiprobe measurements. The multiwalled carbon nanotube protects the tin nanowire from oxidation and shape fragmentation and therefore allows us to investigate the electronic properties of stable wires with diameters as small as 25 nm. The transparency of the contact between the Ti/Au electrode and nanowire can be tuned by argon ion etching the multiwalled nanotube. Application of a large electrical current results in local heating at the contact which in turn suppresses superconductivity. Nano Lett

International audience; Controlled patterning of highly oriented pyrolitic graphite with well‐defined 3D nanostructures is realized by steered uniaxial alignment of multilayered [2.2]paracyclophane‐based building blocks bearing functional clips (see picture). Their in‐plane self‐assembly allows exact positioning of vertical structural elements with precisely defined nanoscale spacing.

A 3:1 site-differentiated [4Fe-4S] cluster is immobilized on a thiol-functionalized self-assembled monolayer (SAM) on Au(111) by thiol-thiolate exchange chemistry. Fe 2p signals observed by X-ray photoelectron spectroscopy support the presence of [4Fe-4S] clusters at the SAM surface; further evidence comes from the detection of n-Bu4N+ and cluster-derived species by secondary-ion mass spectrometry. The immobilizing interaction is sufficiently strong to allow the study of [4Fe-4S] clusters at solid-liquid interfaces.

We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene thin films. It is found that O2 and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O2 at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface.

The growth of multiwall carbon nanotubes is characterized in situ in a chemical vapor deposition reactor by measuring the current extracted by field emission from the growing nanostructures. The lengthening of the nanotubes provokes an increase of the emitted current at constant applied voltage, and the use of a phosphor screen allows to observe the individual emitters during the growth. A simple model permits furthermore to estimate the growth rate. The nanotubes grow with a closed cap under 10 - 2 -10 - 4 mbar of C 2 H2 at 700 °C with a growth rate over I μm/s that increases with the C 2 H 2 pressure. The growth of the nanotubes is neither simultaneous nor homogeneous over the cathode and involves a different activation time for every emitter.

The overlayer covering a (110) face of a Nb crystal annealed at 1500--2000 K in UHV has been studied by Auger electron and photoemission spectroscopies, low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). This layer, which results from the surface segregation of oxygen dissolved in Nb bulk, corresponds to a thin niobium oxide with a ${\mathrm{NbO}}_{x\ensuremath{\approx}1}$ stoichiometry as shown by photoemission with synchrotron radiation. Both LEED and STM investigations show the complex structure of the oxide overlayer with two orientations rotated by 109\ifmmode^\circ\else\textdegree\fi{}. LEED diagrams reveal the epitaxial relationship between lattices of the oxide overlayer and the metal. From STM observations, each domain in the oxide layer consists of a quasiperiodic arrangement of strictly parallel sticks. Analysis of all the results shows that each stick characterizes a small NbO crystal with a typical $3.5\ifmmode\times\else\texttimes\fi{}1.4{\mathrm{nm}}^{2}$ size. Therefore, the oxide layer can be described as a side to side arrangement of these NbO nanocrystals (fcc) on Nb(110) (bcc). Two kinds of epitaxial relationship between these two lattices are found: (i) the relative arrangement of NbO nanocrystals is determined by the underlying Nb(110) lattice; (ii) each nanocrystal develops an epitaxy relationship with the metal surface since the NbO lattice presents one (111) plane parallel to Nb(110) and one NbO 〈110〉 direction is parallel to one Nb 〈111〉 direction (Kurdjumov-Sachs-type alignment). The origin of this structure is discussed in terms of NbO/Nb misfit.

Growing carbon nanotubes were observed during the chemical vapor deposition process by field emission microscopy. The experiments were performed in a configuration similar to the one used to realize a luminescent tube, with a wire as nanotube support and cathode placed in the center of symmetry of a glass tube with a phosphor layer that served as anode. To activate the growth, the wire was resistively heated to 700°C and a partial pressure of 10−2…4 mbar of C2H2 was introduced in the chamber. 10 μm‐long nanotubes grew after an activation time that was as short as 10 s with a growth rate of typically 1 μm ⋅ s−1.