ORC Seminar Series

“Mapping electron excitations in the near IR-UV range using sub-nm resolved STEM-EELS spectrum imaging”

Speaker: Mathieu Kociak, Laboratoire de Physique des Solides

Date: 18 November 2009

Time: 2pm

Venue: Mountbatten Seminar Room

Abstract:

For decades, the electron energy loss spectroscopy (EELS) in a transmission electron microscope has been used to explore electronic and electromagnetic excitations of solids. In particular, the  low-loss energy domain (from few eV to 50 eV) has been exploited for studying dielectric properties of materials. However, so far, only excitations in the UV range and above were investigated due to severe limitations in the detection of lower energy spectral features hidden by the strong contribution of the transmitted beam to the measured spectrum. Recently, significant improvements occurred either instrumentally for significantly reducing the zero-loss tail intensity in the experimentally measured spectrum (instrumental development of monochromators) or for retrieving by a posteriori data processing the spectral information hidden by this zero-loss tail (optimization of deconvolution techniques). In parallel, the EELS spectrum-imaging mode in a scanning transmission electron microscope (STEM) allows to record the variation of the EELS signal at a sub-nanometer scale (typically, in our STEMVG HB501 the nominal probe diameter is 0.5 nm and the accuracy in position is 0.2 nm). Therefore, by combining the spectrum-imaging approach with the above mentioned a posteriori deconvolution techniques it is now possible to probe with unprecedented spatial resolution spectral features that were so far only measurable with optical techniques. As an example, fig.1 displays the spatial variation of the visible spectral range plasmon modes along a line scan of 64 points joining an apex of the triangular particle of figure 1a to the opposite side of the triangle. With the support of well adapted models to simulate the optical response of nano-objects and the associated local low-loss EEL spectra (models both based on a classical dielectric continuum description or discrete dipole approximation), these new possibilities open the route to the exploration of a large variety of new problematics in nanophysics.

Some examples will be reviewed:

- The optical response of individual silver nanoparticles [1]. More specifically, from the mapping of the different plasmon excitations in the visible range, the interplay between local effects (local electromagnetic field enhancement) and long range effects (symmetry of the excitation modes) will be discussed.

-The optical response of new types of gold nanoparticles, namely nanostars.

-The optical response of Split Ring Resonators.

The analogies and differences between EELS and optical measurements will be also stressed, emphasizing that the quantity measured in both EELS and optical near field microscopy, namely the Local Electromagnetic Density of States, is the same [2].

Finally, I will discuss a new class of spectroscopy combining the spatial resolution of the fast electrons and the spectral resolution of the photons [3].

[1] J. Nelayah et al., Nature Physics, 3, 348 (2007)

[2] J. Garcia de Abajo and M. Kociak, Phys. Rev. Lett., 100, 106804 (2008)

[4] Rodriguez-Lorenz et al., JACS, 131, 4616 (2009)

[3] J. Garcia de Abajo and M. Kociak, New Journal of Physics, 10, 073035 (2008)


FIG. 1.A) High Annular dark Field image of a flat equilateral triangular NP on cleaved mica. B) EELS spectra measured at the three distinct positions on the triangular NP shown in A). C) Map of the intensity of the lowest surface plasmon mode (1.75 eV). This mode is most intense at the tips (a) and absent at (b) and (c).

Abstract:

Mathieu Kociak is an  Associate Researcher at the Solid State Physics Laboratory (LPS/CNRS) in Orsay (France).

He graduated with a Ph.D at the University of Paris, and received the Guinier Prize (best french Thesis in Physics) in 2001 for his work on "Superconductivity and plasmons in nanotubes".

He then stayed in the university of Meijo, Japan, where he studied the correlated atomic structure and transport properties of individual carbon nanotubes.

He became associate researcher in 2003. Since then, he worked essentially on the correlation between physical and structural properties of individual nano-object, including carbon and boron-nitride nanotubes and metallic nanoparticles, using different experimental tools (Transmission electron microscopy and spectroscopies, raman diffusion, transport measurements). His current interest focuses on the nanooptics of metamaterials and the development of novel spectroscopies  combining fast electrons and photons.

 

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