Electron
  Microscopy Group
Research: Electron Energy Loss Spectroscopy
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Researchers: Olivia Nicoletti , Sean Collins, Thomas Ostasevicius, Francisco de la Peņa, Giorgio Divitini, Xiaoyu Peng, Caterina Ducati, and Paul Midgley


EELS: electron energy-loss spectroscopy

Electron energy-loss spectroscopy (EELS) measures the loss of energy of the transmitted electrons through interaction with the sample. The EEL spectrum thus includes elastic scattering (zero-loss) events, collective excitations of valence or conduction electrons and single electron excitations, corresponding to losses of energy from zero to thousands of eV. In a typical EEL spectrum the main feature present is the zero-loss peak (ZLP) that results from unscattered or elastically scattered electrons. This corresponds to electrons that have lost no energy in the interaction with the specimen. Those transmitted electrons that have lost energy that is suficiently small to be indistinguishable from the zero-loss, within the energy spread of the incident beam (such as due to excitation of phonons), also contribute to the ZLP. Next to the ZLP the inelastic scattering regime starts with the plasmon losses, which include losses due to the volume (or bulk) plasmon, the infinite-surface plasmon and local surface plasmon resonances. In this region, depending on the material, inter and intra-band transitions and Cerenkov radiation can also be found. At higher energies, single-electron excitations take place, giving rise to inner-shell ionization edges which can show fine structure linked with the electronic energy levels (ELNES) and the atomic local arrangement (EXELFS). EELS can therefore probe qualitatively and quantitatively chemical (composition), electrical (energy bands and bonding) and optical (plasmons) properties of a material.



EELS @ EM Cambridge

Optical properties of metal nanoparticles

Researchers: Dr Olivia Nicoletti , Sean Collins, Thomas Ostasevicius, and Dr Francisco de la Peņa

High resolution low-loss EELS: experiments

Most of the EELS research at the Electron Microscopy group at University of Cambridge is focused on the study of collective and single-electron excitations in the low-loss EELS part of the spectrum, corresponding to energy losses below 10 eV. The energy resultion of the electron beam is pushed down to ca. 160 meV via the use of a highly monochromated (S)TEM beam in our FEI Titan. EM Cambridge also collaborates with several international electron microscopy laboratories to obtain the best possible low-loss EELS data. Such collaborations include low-loss STEM EELS at Centre for Electron Nanoscopy, DTU (Lyngby, Denmark), low-loss EELS EFTEM at StEM, MPI (Stuttgart, Germany), high energy resolution low-loss EELS at Microscopy of Nanoscale Materials, McMaster University (Hamilton, Canada) and fast acquisition low-loss EELS at FEI NanoPort (Eindhoven, The Netherlands).

Collective and single-electron excitations in the low-loss EEL spectrum are studied for noble metals, in particular gold and silver nanoparticles. In particular, our group research focuses on the study of the fundamental physics underlining localised surface plasmon resonances (LSPRs) in these nanoparticles, which are resonant collective coherent oscillations of the conduction electrons at the interface between metal nanoparticles and their external dielectric environment. In particular, our studies are directed towards the understanding of the propagation and dispersion properties of LSPRs on metal nanoparticles, their intrinsic symmetry, their interaction with a substrate and with the pump-and-probe (S)TEM electron beam. This research has a strong experimental component, but relies heavily also on electrodynamic simulations and on the development of new data analysis techniques.

EELS simulations

EM Cambridge has several codes available for in-house calculations on the optical properties of nanoparticles: DDSCAT, e-DDA, DDEELS with the inclusion of the substrate package, in collaboration with Dr Nicolas Geuquet (previously at the University of Namur - Namur, Belgium), MNPBEM with the inclusion of the EELS package, in collaboration with Prof Ulrich Hohenester of the University of Graz (Graz, Austria). We have also developed a MATLAB toolbox for the calculation of EELS spectra of metal spheres, via the use of analytical solutions (a full description can be found in the relevant paper by Sean Collins and Prof Paul A. Midgley).

EM Cambridge is always looking for interesting new samples to study with low-loss EELS. If interested in setting up a collaboration, feel free to email Dr Olivia Nicoletti at on214@cam.ac.uk.



Photo-active materials

Researchers: Xiaoyu Peng, Dr Giorgio Divitini, Dr Caterina Ducati and Dr Francisco de la Peņa

EM Cambridge has developed several in-situ illumination TEM holders, capable of introducing different light sources in the TEM column. We currently have available an LED TEM holder and a laser injection TEM holder. These holders are mainly used to study the properties of photo-active materials (such as titanium dioxide) at the nanoscale under photon illumination, both in our FEI Tecnai F20 and in our new FEI Titan. EELS is particularly useful to characterize photo-active materials as, under illumination, they experience a change in the density of states near their bandgap, which results in a change of the relative intensity of the low-loss EELS peaks.

EM Cambridge in-situ illumination TEM holders are available for external users who might wish to study their samples under photon illumination condition in a (S)TEM. If interested in setting up a collaboration, feel free to email Dr Caterina Ducati at cd251@cam.ac.uk.



EELS data analysis: HYPERSPY    

EELS data analysis is carried out using the Hyperspy toolbox developed by Dr Francisco de la Peņa and collaborators. Hyperspy is a very versatile toolbox that allows users to perform easily highly sophisticated types of data analysis. It is often used at EM Cambridge to perform several types of blind source separation on EELS data sets, such as principal component analysis, independent component analysis and non-negative matrix factorization.

Hyperspy workshops are run for labs that might wish an introduction to the toolbox. If interested, feel free to email Dr Francisco de la Peņa at fjd29@cam.ac.uk.



ESTEEM2 framework program: our contribution

EM Cambridge is part of the European Network for Electron Microscopy (ESTEEM2), an integrated infrastructure of electron microscopy facilities providing access for the academic and industrial research community in the physical sciences to some of the most powerful characterization techniques available at the nanoscale.

The EELS research at ESTEEM2 has the aim to increase and strengthen the collaboration between the network laboratories actively working on EELS. In particular, EM Cambridge aims to establish collaborations with:

-- StEM (MPI, Stuttgart, Germany), on the application of new types of data analysis for EELS data sets (see Hyperspy section);

-- FELMI-ZFE (TU Graz, Graz, Austria), on the 3D visualization of LSPRs of gold nanocages;

-- LMA (University of Zaragoza, Zaragoza, Spain)

-- LPS (CNRS Paris-Orsay, France)

Dr Mathieu Kociak (LPS CNRS Paris-Orsay) explaining the route to ESTEEM2 collaborations building: already established collaborations (full lines) as opposed to collaborations to be built within the ESTEEM2 framework lifetime (dashed lines).

(GRA = TU Graz, DRE = TU Dresden, STU = StEM at MPI Stuttgart, ORS = LPS CNRS Paris-Orsay, LJU = Institut Jozef Stefan, Lubjana, ZAR = University of Zaragoza, CAM = University of Cambridge, TOU = CEMES-CNRS Toulouse, ANT = University of Antwerp).