We use an environmental scanning transmission electron microscope (ESTEM) equipped with electron energy loss spectroscopy (EELS) and a monochromated electron source to perform energy loss measurements on metallic nanoparticles (NPs) exposed to local gaseous environments at varying pressures. In particular, we characterize the effect of exposure to CO or H₂ on the surface plasmon resonance of a gold NP. By addressing various sites around the perimeter of a triangular NP (edge length ~20 nm) with the electron beam in STEM mode, the energy loss spectrum resulting from site-specific excitation of surface plasmon resonance is probed with a spatial resolution of ~1 nm and energy resolution of ~100 meV. Local gas adsorption is evidenced by peak shifts in the energy loss spectrum, which are found to be positive for CO and negative for H₂. Strong site selectivity is evident, with CO and H₂ adsorbing preferentially at the edge and corner sites, respectively. To characterize the sign and magnitude of the energy shifts, finite-difference time-domain (FDTD) simulations of electron-beam excitation of the NP are performed using a specialized model in which the local electron concentration is allowed to vary spatially over the particle volume. This is a result of both the inhomogeneous spatial distribution of the adsorbate and its degree of electronegativity.

Date Published: 28 August 2014
PDF: 6 pages
Proc. SPIE 9169, Nanoimaging and Nanospectroscopy II, 91690Z (28 August 2014); doi: 10.1117/12.2060553

Published in SPIE Proceedings Vol. 9169:
Nanoimaging and Nanospectroscopy II
Prabhat Verma; Alexander Egner, Editor(s)