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Electron Microscopy of Nanoscale Materials

G.A. Botton1, C. Bock2, M. Bugnet1, Z. Mi3, P. Prabhudev1, G. Radtke4, D. Rossouw1, X. Sun5, S.Y. Woo1, G.Z. Zhu1.

Principal Investigators

Gianluigi A. Botton
Department of Materials Science and Engineering, and BIMR, McMaster University

(1) McMaster University, Hamilton, ON
(2) National Research Council, Ottawa, ON
(3) Université Pierre et Marie Curie, Paris
(4) McGill University, Montreal, (Qué)
(5) Western University, London, ON

Introduction

The Canadian Centre for Electron Microscopy, a national facility operated by the BIMR, was established through funding in the 2004 National Competition from the Canada Foundation for Innovation (CFI) and Ontario Government. While electron microscopy has been at the core of the BIMR for decades, the CCEM started operations in 2006 and formally opened 2008, with the commissioning of the two aberration-corrected FEI Titan microscopes. The CCEM facility is been used by over 300 research groups spread across the country accessing the broad suite of instruments from the variable-pressure SEM to the aberration-corrected and monochromated microscopes. Here we demonstrate the application of the CCEM instruments to a range of materials application highlighting a small subset of research carried out at the BIMR, namely related to sub-Angstrom resolution and sub-0.1eV microscopy and spectroscopy.

Sub-wavelength spectroscopy of plasmonic structures

The monochromated electron beam of the aberration-corrected microscopes has allowed the detailed study of plasmonic response of nanostructures. Achieving 0.06eV energy resolution with a sub-nm size probe, the CCEM instruments have been used to study surface plasmon resonances in metallic nanoscale wires that have potential use in photonic information transfer. Using the electron beam, surface plasmon polaritons are excited and a standing wave pattern is formed which can be simultaneously detected with the same electrons when analyzed with a high-resolution spectrometer.

Figure 1. Electron energy loss spectra of Ag nanowires recorded with a monochromatic electron beam with 60meV energy resolution [1].

Figure 2. Energy-filtered plasmon resonance maps of metallic Ag nanowires [1].

This electron excitation is directly related to the photonic density of states of the sample and is very effectively probed with an electron beam smaller than 1nm. As demonstrated by Rossouw et al, in Physical Review Letters, the energy loss spectrum shows multiple excitations in energy range down to 0.17eV (figure 1), the lowest energy features ever observed with an electron beam [1]. By selecting the energy window for a given energy loss, the spatial resolution of the standing wave, of a given energy has also been mapped. The results show that, using the CCEM instrumentation, we can resolve optical excitation down to the mid-infra red regime and this by spatially resolving energy modes extended in space by few 10’s of nm (Figure 2). These features would hence be not detectable with photon-based techniques because some of the excitations would not couple to light (i.e. they are dark-modes) and would be significantly smaller than the free-space wavelength of light.

Structure of Surfaces revealed by TEM

Electron microscopy provides structural information derived from projection of a sample.

This normally means that the signal, in particular any spectroscopic information is “averaged” over the trajectory electron beam transmitted through the sample.

Here we have demonstrated that, thorough an elegant extraction of quantitative information in a “thickness” series, it is in fact possible to derive spectroscopic information from few atomic layers of the sample thus probing the intrinsic nature of reconstructed surfaces in materials spectroscopic signature of the surface atomic layer (red) and the bulk SrTiO3(black) [2]

Figure 3a) Structural model of a SrTiO3 c(2x4) reconstructed surface

Figure 3b) Electron energy loss spectra of the Ti L23 edge derived from the “thickness-series” method providing the spectroscopic signature of the surface atomic layer (red) and the bulk SrTiO3(black) [2]

The method has been demonstrated on samples of SrTiO3 annealed in conditions favoring the development of a c(2x4) reconstruction. The work published in Nature has shown that a distinct spectroscopic signature of the two first monolayer of the surface (Figure 3a ) that, based on simulations with current structural models, is consistent with the presence of a distorted octahedral coordination for the Ti cations and the presence of square pyramidal sites for the very topmost atomic layer Figure 3b). This methodology highlights the potential for quantitative TEM and electron energy loss spectroscopy to be a very sensitive probe of surface state of matter.

Structure of Novel Alloy Nano-Catalysts

The commercialization of proton-exchange membrane fuel cells is limited, in great part by the cost of Pt catalysts. We have worked in the development of novel alloy nanocatalysts used for next-generation fuel cells. These catalysts are based on the Pt-Fe alloy family and have shown excellent specific activity and durability. [3]. Here we have applied aberration-corrected electron microscopy to study the exact structure and composition of Pt-Fe nanoparticles in their pristine state and follow electrochemical cycling [4, 5]. Our work published in Nanoscale and ACS Nano shows that the nanoparticles form an ordered core- disordered shell structure that is persistent even following electrochemical cycling (figure 4). These results demonstrate the sensitivity of the aberration-corrected technique to structural order at the atomic scale and explain the improved specific activity due to the presence of a Pt-rich shell few atomic layers thick.

Figure 4) Experimental High Angle Annular-Dark Field STEM image of a Pt-Fe alloy nanoparticle (left) demonstrating the order PtFe core and a disordered Pt-rich shell. Example of the structural model of the nanoparticle with the Pt atoms (gray colour) and Fe atoms (in gold colour).

Acknowledgements

We are very grateful for the support of the Natural Sciences and Engineering Research Council for funding the research work. The CCEM was established through funding from Canada Foundation for Innovation, the Ontario Research Fund, and McMaster University.

References

Principal publication:
[1] PRL and Nanoletters
[2] Nature
[3] chemcatchem
[4] Nanoscale
[5] ACS nano.