Yves Borensztein

Tailoring Anisotropic Interactions between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations

We investigated composite films of gold nanoparticles (NPs)/liquid crystal (LC) defects as a model system to understand the key parameters, which allow for an accurate control of NP anisotropic self-assemblies using soft templates. We combined spectrophotometry, Raman spectroscopy, and grazing incidence small-angle X-ray scattering with calculations of dipole coupling models and soft sphere interactions. We demonstrate that dense arrays of elementary edge dislocations can strongly localize small NPs along the defect cores, resulting in formation of parallel chains of NPs. Furthermore, we show that within the dislocation cores the inter-NP distances can be tuned. This phenomenon appears to be driven by the competition between soft (nano)sphere attraction and LC-induced repulsion. We evidence two extreme regimes controlled by the solvent evaporation: (i) when the solvent evaporates abruptly, the spacing between neighboring NPs in the chains is dominated by van der Waals interactions between interdigitated capping ligands, leading to chains of close-packed NPs; (ii) when the solvent evaporates slowly, strong interdigitation between the is avoided, leading to a dominating LC-induced repulsion between NPs associated with the replacement of disordered cores by NPs. The templating of NPs by topological defects, beyond the technological inquiries, may enable creation, investigation, and manipulation of unique collective features for a wide range of nanomaterials.

Optical spectroscopy study of hydrogenation of the Si(111)-7 × 7 surface

Abstract Deposition of foreign atoms on silicon surfaces gives rise to a variety of phenomena, which are the subject of very active research. Fundamentally and technologically important, the chemisorption of hydrogen at Si(111)-7 × 7 surface is still under debate due to its complex structure, the existence of different adsorption sites and the etching in the process of adsorption. We present both experimental and theoretical optical spectroscopy results on the atomic hydrogen adsorption at the Si(111)-7 × 7 surface. The real-time optical, namely differential reflectivity, measurements were performed by means of an in-situ spectrometer with 60° angle of incidence. Structural changes upon H adsorption have been monitored by LEED. Microscopical calculation of the optical properties of the full scale Si(111)-7 × 7 DAS structure was done within a sp3s∗ tight-binding approach. In order to model the structural changes due to H adsorption, we calculated the relative difference in reflectivity ΔR R between the clean Si(111)-7 × 7 and hydrogenated Si(111)-1 × 1:H surfaces. The surface optical response has been decomposed into the various components, due to surface-to-surface, surface-to-bulk, bulk-to-surface and bulk-to-bulk states transitions. The microscopic origin of the different contributions is discussed in detail. The comparison with the theoretical results allowed us to clarify the origin of the main optical peaks observed experimentally at the hydrogenated Si(111)-7 × 7 surface.

Resolving the Controversial Existence of Silicene and Germanene Nanosheets Grown on Graphite

The highly oriented pyrolytic graphite (HOPG) surface, consisting of a dangling bond-free lattice, is regarded as a potential substrate for van der Waals heteroepitaxy of two-dimensional layered materials. In this work, the growth of silicon and germanium on HOPG is investigated with scanning tunneling microscopy by using typical synthesis conditions for silicene and germanene on metal surfaces. At low coverages, the deposition of Si and Ge gives rise to tiny and sparse clusters that are surrounded by a honeycomb superstructure. From the detailed analysis of the superstructure, its comparison with the one encountered on the bare and clean HOPG surface, and simulations of the electron density, we conclude that the superstructure is caused by charge density modulations in the HOPG surface. At high coverages, we find the formation of clusters, assembled in filamentary patterns, which indicates a Volmer-Weber growth mode instead of a layer-by-layer growth mode. This coverage-dependent study sets the stage for revisiting recent results alleging the synthesis of silicene and germanene on the HOPG surface.

Mechanism of hydrogen adsorption on gold nanoparticles and charge transfer probed by anisotropic surface plasmon resonance

The adsorption of hydrogen on Au nanoparticles (NPs) of size of the order of 10 nm has been investigated by use of localised surface plasmon resonances (LSPR) in the NPs. The samples, formed by Au NPs obtained by oblique angle deposition on glass substrates, display a strong optical dichroism due to two different plasmon resonances dependent on the polarisation of light. This ensured the use of Transmittance Anisotropy Spectroscopy, a sensitive derivative optical technique, which permitted one to measure shifts of the LSPR as small as 0.02 nm upon H adsorption, which are not accessible by conventional plasmonic methods. The measured signal is proportional to the area of the NPs, which shows that H atoms diffuse on their facets. A negative charge transfer from Au to H is clearly demonstrated.

Ultrasensitive and fast single wavelength plasmonic hydrogen sensing with anisotropic nanostructured Pd films

Abstract Anisotropic nanostructured porous Pd films are fabricated using oblique angle deposition in vacuum on a glass substrate. They display a dichroic response, due to localised surface plasmon resonances (LSPR) within the nanoparticles forming the film, dependent on the incident light polarisation. Ultrasensitive hydrogen sensing is reached by using these films in conjunction with a differential optical technique derived from the reflectance anisotropy spectroscopy. The evolution of the samples’ optical responses is monitored during the formation of Pd hydride in both the dilute α-phase and the dense β-phase, whilst the samples are exposed to different concentration of H2 in Ar (from 100% H2 to a few ppm). The measurements are performed at a single wavelength in the visible range and at 22u202f°C. The results show that a quantitative measurement of the hydrogen concentration in a carrier gas can be measured throughout the concentration range. The limit of detection is 10u202fppm and the time for detecting the presence of H2 in the carrying gas is below one second at concentration down to 0.25% of H2 in Ar. Furthermore, the optical anisotropy of the samples and its evolution with exposure to H2 are correctly reproduced with an effective medium theory.

Transition from silicene monolayer to thin Si films on Ag(111): comparison between experimental data and Monte Carlo simulation

Scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low energy electron diffraction have been used to follow the growth of Si films on Ag(111) at various temperatures. Using a simple growth model, we have simulated the distribution of film thickness as a function of coverage during evaporation, for the different temperatures. In the temperature regime where multilayer silicene has been claimed to form (470–500 K), a good agreement is found with AES intensity variations and STM measurements within a Ag surfactant mediated growth, whereas a model with multilayer silicene growth fails to reproduce the AES measurements.