Raman spectra and DFT calculations of thiophenol molecules adsorbed on a gold surface.

We report the calculation of Raman modes of thiophenol molecules adsorbed on a real gold surface. The calculated Raman spectra strongly depend on the absorption configuration of the molecule on the metallic surface, a feature that should be carefully taken into account in the interpretation of the surface enhanced Raman spectra (SERS). The calculated Raman spectra are compared with experimental SERS measurements, the best accordance being obtained for a tilted configuration of the absorbed molecule. The present study supports the necessary combination of computational approaches with SERS measurements to predict the type of molecular adsorption configurations on metallic surfaces.

Forming Weakly Interacting Multilayers of Graphene Using Atomic Force Microscope Tip Scanning and Evidence of Competition between Inner and Outer Raman Scattering Processes Piloted by Structural Defects.

We report on an alternative route based on nanomechanical folding induced by an AFM tip to obtain weakly interacting multilayer graphene (wi-MLG) from a chemical vapor deposition (CVD)-grown single-layer graphene (SLG). The tip first cuts and then pushes and folds graphene during zigzag movements. The pushed graphene has been analyzed using various Raman microscopy plots- AD/ AG × EL4 vs ΓG, ω2D vs Γ2D, Γ2D vs ΓG, ω2D+/- vs Γ2D+/-, and A2D-/ A2D+ vs A2D/ AG. We show that the SLG in-plane properties are maintained under the folding process and that a few tens of graphene layers are stacked, with a limited number of structural defects. A blue shift of about 20 cm-1 of the 2D band is observed. The relative intensity of the 2D- and 2D+ bands have been related to structural defects, giving evidence of their role in the inner and outer processes at play close to the Dirac cone.

Near-field chemical mapping of gold nanostructures using a functionalized scanning probe.

We report on photochemical and photophysical properties produced by Surface Plasmon Resonance (SPR) on metallic nanograins by means of high resolution Functionalized Tip-Enhanced Raman Spectroscopy (F-TERS). This technique relies on a sharp gold tip functionalized with Raman-active molecules to be scanned relatively to plasmonic hot-spots on a surface. We describe the local variation of plasmon-induced Raman enhancement on the surface of nanostructures that also affects the photochemistry while the quantitative interpretation of peak intensities requires the consideration of surface topography near the tip apex. Our F-TERS maps show Raman modes of hot electron reduction of 4-nitrothiophenol (4-NTP) molecules on the tip and indicate at least partial photochemical dimerization. An apparent photo-induced reversibility of this dimerization can be conservatively explained by a local topography feature that we simulate in a finite element environment. Our experimental results reveal a spatial resolution of approximately 10 nm, corresponding to a few hundred 4-NTP molecules exposed to the near-field.

Vibrational modes of aminothiophenol: a TERS and DFT study.

We report Tip Enhanced Raman Spectroscopy (TERS) mapping and Density Functional (DFT) calculations of aminothiophenol (ATP) grafted on a gold surface. The TERS mapping has demonstrated Raman modes of (ATP) and its dimerised derivative Dimercaptoazobenzene (DMAB). This feature confirms that the plasmon activated chemical reaction of ATP has occurred during TERS measurements. In some specific part of the samples some unidentified Raman modes are observed. We suggest that they could come from intermediate species formed during the conversion of ATP into DMAB. These modes are compared with calculated Raman spectra of some possible intermediate species. These results confirm the high potentiality of TERS measurements for nanochemistry.

A near field optical image of a gold surface: a luminescence study.

This paper addresses recent experimental findings about luminescence of a gold tip in near-field interaction with a gold surface. Our electrochemically etched gold tips show a typical, intrinsic luminescence that we exploit to track the plasmon resonance modeled by a Lorentzian oscillator. Our experimental device is based on a spectrometer optically coupled to an atomic force microscope used in tuning fork mode. Our measurements provide evidence of a strong optical coupling between the tip and the surface. We demonstrate that this coupling strongly affects the luminescence (intensity, wavelength and FHWM) as a function of the tip position in 2D maps. The fluctuation of these parameters is directly related to the plasmonic properties of the gold surface and is used to qualify the optical near field enhancement (which subsequently plays the predominant role in surface enhanced spectroscopies) with a very high spatial resolution (typically around 20 nm). We compare these findings to the independently recorded near-field scattered elastic Rayleigh signal.

A surface enhanced Raman spectroscopy study of aminothiophenol and aminothiophenol-C60 self-assembled monolayers: evolution of Raman modes with experimental parameters.

P-aminothiophenol (PATP) is a well-known molecule for the preparation of self-assembled monolayers on gold via its thiol functional group. After adsorption, it has been demonstrated that this molecule is anchored to gold through its thiol group, and standing nearly upright at the surface with the amino functional group on top. This molecule has been extensively studied by surface enhanced Raman spectroscopy but its exact SERS spectrum remains unclear. Here, we demonstrate that it can be strongly affected by at least two experimental parameters: laser power and layer density. Those features are discussed in terms of a dimerization of the PATP molecules. The free amino group affords the adsorption of other molecules such as C(60). In this case, a complex multilayer system is formed and the question of its precise characterisation remains a key point. In this article, we demonstrate that surface enhanced Raman spectroscopy combined with x ray photoelectron spectroscopy can bring very important information about the organization of such a self-assembled multilayer on gold. In our study, the strong evolution of Raman modes after C(60) adsorption suggests a change in the organization of aminothiophenol molecules during C(60) adsorption. These changes, also observed when the aminothiophenol layer is annealed in toluene, do not prevent the adsorption of C(60) molecules.

Surface enhanced Raman spectroscopy of organic molecules deposited on gold sputtered substrates.

Aggregates of Au nanoparticles have been extremely easily obtained on glass substrates by physical sputtering under primary vacuum. With such a protocol, we demonstrate that it is possible to control the surface plasmon band absorption. Surface enhanced Raman spectroscopy (SERS) experiments were performed with methylene blue, zinc octacarboxyphthalocyanine, 4-aminothiophenol and cysteamine. The correlation between the absorption band and the wavelength giving the highest SERS intensity is clearly observed for methylene blue, in accordance with the electromagnetic enhancement theory. For the other molecules, effects of the chemical enhancement are also observed. In addition, we noticed a strong influence of the nature of the adsorbed molecule on the enhancement factor for a given wavelength. The origin of this feature is discussed in terms of resonant effects or multipolar surface plasmon modes.

Micrometric granular ripple patterns in a capillary tube.

The oscillatory motion of a fluid carrying micron-sized particles inside a capillary tube is investigated experimentally. It is found that initially uniformly distributed particles can segregate and accumulate to form regularly spaced micron-sized particle clusters. The wavelength of the microclusters is compared to data for macroscale sand-ripple patterns and found to obey the same universal scaling as these. A dimensional analysis is performed that confirms the universality of the experimentally observed scaling. The experimental data for the microripple clusters further suggest the existence of a minimum particle length scale for which patterns can form and below which the Brownian motion associated with the molecules of the matrix fluid inhibits pattern formation.

Effect of a vertically flowing water jet underneath a granular bed.

The response of a granular bed to a punctual, vertically flowing water jet underneath it is studied experimentally, theoretically, and numerically. Experiments show that three regimes depending on the flow rate Q appear to outline the bed's behavior. For sufficiently small Q , the bed remains motionless and acts as a rigid porous medium [regime (i)]. It then becomes deformed when Q is sufficiently increased [regime (ii)]. Finally, the bed "explodes" and a locally fluidized bed limited to a domain above the water jet is observed as Q is increased further [regime (iii)]. This fluidization creates a "chimney" in the bed, roughly cylindrical in shape, inside which the grains are in motion. The flow motion in regime (i) is theoretically modeled as a Darcy flow inside an unbounded granular bed while a numerical model accounting for the boundaries is performed. Results from the theory and computations are compared to experimental data and the effects of the boundaries, the bed's thickness, and the size of the jet on the flow motion inside the bed are underlined. The onset for fluidization [regime (iii)] is explained by assuming a stick-slip behavior of the chimney. Despite the simplistic model, the comparison with experimental data show very good agreement for bony sand granules and relatively good agreement for spherical glass beads.

Some aspects of electrical conduction in granular systems of various dimensions.

We report on measurements of the electrical conductivity in both a 2D triangular lattice of metallic beads and in a chain of beads. The voltage/current characteristics are qualitatively similar in both experiments. At low applied current, the voltage is found to increase logarithmically in good agreement with a model of widely distributed resistances in series. At high enough current, the voltage saturates due to the local welding of microcontacts between beads. The frequency dependence of the saturation voltage gives an estimate of the size of these welded microcontacts. The DC value of the saturation voltage ( approximately 0.4 V per contact) gives an indirect measure of the number of welded contact carrying the current within the 2D lattice. Also, a new measurement technique provides a map of the current paths within the 2D lattice of beads. For an isotropic compression of the 2D granular medium, the current paths are localized in few discrete linear paths. This quasi-one-dimensional nature of the electrical conductivity thus explains the similarity between the characteristics in the 1D and 2D systems.

High speed single charge coupled device Cranz-Schardin camera.

This article describes an ultrahigh speed visualization system based on a miniaturization of the Cranz-Schardin principle. It uses a set of high power light emitting diodes (LEDs) (Golden Dragon) as the light source and a highly sensitive charge coupled device (CCD) camera for reception. Each LED is fired in sequence and images the refraction index variation between two relay lenses, on a partial region of a CCD image sensor. The originality of this system consists in achieving several images on a single CCD during a frame time. The number of images is 4. The time interval between successive firings determines the speed of the imaging system. This time lies from 100 ns to 10 micros. The light pulse duration lies from 100 ns to 10 micros. The principle and the optical and electronic parts of such a system are described. As an example, some images of acoustic waves propagating in water are presented.

Acoustic scattering from complex elastic shells: visualization of S0, A0 and A waves

Acoustic scattering phenomena from complex immersed elastic shells are studied by high-speed Schlieren visualization for spark-generated spherical quasi-Dirac excitations. The situations considered are air-filled cylindrical shells under normal insonification and air-filled cylindrical shells soldered at one end with a hemispherical endcap under axial and non-axial incidences. The results are compared with those provided by the Methode d'Isolement et d'Identification des Resonances (MIIR) method and corresponding analytical theoretical solutions. The combination of these complementary approaches better highlights some of the behaviors of the symmetric S0- and antisymmetric A0-Lamb waves, as well was the Scholte-Stoneley A-wave, on the considered targets. In particular, the influence of the internal solder inhomogeneities on the creation and/or the conversion of these modes is demonstrated.