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Scientific activities

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Master Internship proposal 2018-2019 / Proposition de stage de M2 2018-2019

 

 

Current Research

Development of ultra-sensitive plasmonic sensors (click for details)

 

Nanoparticles of different metals (gold, silver, palladium, etc.) have particular optical properties, related to the so-called localized surface plasmon resonances, which are collective oscillations of the conduction electrons confined within the particles, as illustrated in Fig.1. For example, plasmon resonance for Au is in the visible optical range, and gives a red or purple color to Au particles instead of the usual yellow color (Fig.2).

 

 

Fig.1 Localized surface plasmon resonance of a metal nanoparticle : collective oscillation of conductions electrons induced by the electric field of light

Fig.2 Solution of gold nanoparticles in suspension

 

 

This resonance is very sensitive to the immediate environment of the particles and can be strongly affected when the particles interact with molecules or ions. Thanks to this very high sensitivity, gas or biological sensors based on nanoparticles of gold, gold-based alloys or other metals [1] are being developed (see Fig. 3). We have developed an original optical technique, the reflectance anisotropy spectroscopy (RAS), which allows us to achieve a higher sensitivity than conventional plasmonic sensors, making it possible to demonstrate the adsorption of very small quantities of molecules. We used it to study the reaction of hydrogen with gold nanoparticles [2] or to make a prototype hydrogen sensor [3]. For this purpose, specific samples with optical dichroism are prepared by evaporation of gold and / or palladium on a glass substrate (fig 4).

 

 

Fig. 3 Shift of the plasmon resonance induced by the adsorption of molecules

Fig. 4 Scanning electron micrograph of gold nanoparticles

 

 

 

An illustration of the sensing sensitivity down to a few ppm of H2 is shown in Fig. 5. The top panel shows the modification of the RAS signal measured in real time, when switching the gas from pure Ar to H2 diluted in Ar and back, for decreasing concentration of H2, from pure (100%) to highly diluted (0.0002%, i.e 2 part per million). On the left part, from 100% to 4%, the change of signal is large, and corresponds to the formation of the dense ’beta’ phase of Pd hydride. On the right part, at and below 0.25%, the change of signal is much smaller, and corresponds to the formation of the dilute ’alpha’ phase of Pd hydride. At intermediate concentration of H2 (1%), there is coexistence of the two phases. The zoom shown in the bottom panel shows that, except for the intermediate case, the kinetics for the detection of H2 is fast, even at very low concentration, showing that this method provides a very competitive H2 sensor. More details can be found in 2018 PhD defense by William Watkins

 

Fig.5 Effect of H2 diluted in Ar on the RAS signal measured on an anisotropic plasmonic Pd sample. Up : change of the RAS signal measured as a function of switching between pure H2 in Ar and pure Ar, for decreasing concentrations of H2, from pure H2 (100%) to 2 ppm. Middle : zoom on three cycles (16%, 1% and 0.015%), the first and last ones showing a fast saturation Bottom : scheme of PdHx for dense, dilute and intermediate phases

 

 

 

 

References

 

 
Formation and properties of monolayers of Si (« silicene ») (click for details)

 

2D-Xenes (where X = Si, Ge, Sn ...) are a class of two-dimensional materials from the group IVA that possess electronic properties different from those of the corresponding 3D bulk. Calculations have shown that silicene and germanene, i.e. Si or Ge atoms arranged in a honeycomb structure, could possibly exist and display electronic, transport or magnetic properties similar to graphene’s one. With the prospect of device engineering, silicene and germanene present a better compatibility with silicon-based microelectronics. Moreover, the spin-orbit coupling leads to the opening of a small band gap (a few meV) for these materials. However, contrary to graphene, there is no layered material from which 2D-Xenes could be exfoliated. Thus, intensive research has been made in order to synthesize these 2D materials while trying to maintain their intrinsic properties.

 

At the Institute of NanoSciences of Paris, we study the structure and growth mechanisms of silicene and germanene obtained by material evaporation under UHV on various substrates (Ag, Al, layered materials). We use in-situ scanning tunneling microscopy (STM), grazing incidence X-ray diffraction (GIXD) and optical investigations, together with density functional theory (DFT) calculations (in collaboration).

A complete overview of our results can be found in the page :Structure, optical properties, and growth mechanisms of 2D-Xenes

 

  • Optical investigation of so-called « silicene » monolayers

Theoretical calculations have shown that the optical properties of free-standing silicene are similar to the ones of graphene, and are dominated by the presence of the Dirac cone at the Fermi level, around the K point, and by interband transitions around the M point.

However, the Si monolayers when grown on Ag(110) or Ag(111) are in strong interaction with the sustrate, and their electronic properties are completely modified with respect to what is expected for the free-standing silicene. Consequently, the optical response is also different, and appears to be rather similar to amorphous or disordered silicon, which indicates that the Si layers are sp3 hybridized and not sp2.

  • « Silicene » multilayers

Optical measurements, combined with Auger electron spectroscopy and X-ray diffraction show that Si multilayers grown on Ag(111) have actually the bulk-Si (diamond-like) structure, and are covered by a thin layer of Ag which gives the well-known root(3)xroot(3) R30° reconstruction.

See details in page : 3. Silicene thin films : are they silicene ?.  

Optical results to be given soon.

 

References

 
Catalytic properties of Au and Au-alloys nanoparticles
  • Catalytic Properties
  • Oxidation of CO
  • Transmission Electron Microscopy
  • Real-time investigation of plasmon resonance in Au/oxide catalysts during reaction
  • Environmental Scanning Tunneling Microscopy of Au and AuCu Nanoparticles
 
Optical response of silicon surfaces
  • Intrinsic optical response of Si(100) and Si(111) surfaces.
 
Adsorption and molecular layers on Silicon
  • Dissociative / non-dissociative adsorption mode of molecules (oxygen, ethylene, benzene...) on Si(100)
  • Monitoring the adsorption kinetics
  • Monte-Carlo Simulation
  •  

    Methods

     

    Selected talks and posters

     



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