Electron Energy-Loss Spectroscopy for Nannomaterials

Abstract

Electron energy-loss spectroscopy (EELS) using high-energy incident electrons has been known to be a powerful tool for studies of electronic structure in solids. The main energy-loss mechanisms in thin solid samples are due to single-particle excitations such as interband and core-level transitions, and collective excitations such as bulk plasmons and surface plasmons. In EELS, the intensity of inelastically scattered incident electrons could be measured as a function of energy-losses and scattering angles (reciprocal space) under the condition of parallel beam illumination. Energy-loss spectra obtained as a function of scattering angles can yield the dispersion relations, i.e., the excitation energy vs. wave vector, of the corresponding excitations. The angular (wave vector) dependence of the EELS spectra plays an important role for us to fully understand the elementary excitations in solids. High resolution spatially-resolved EELS spectra can be obtained from different areas of interest in a sample when a convergent beam electron nanoprobe is utilized. In this case, the spatial resolution is determined primarily by the electron probe size. This is readily accomplished when EELS is performed in conjunction with a modern scanning transmission electron microscope (STEM) which can deliver an electron probe ∼ 0.2 nm or smaller. The combination of EELS with a STEM is a very powerful tool for the studies of local electronic structures on an atomic-scale for complex materials. This is particularly suitable for studies of nanomaterials in which inhomogeneity of sizes, shapes and structures often can not be avoided. The combination of STEM/EELS can circumvent this difficulty and allows us to study each nano-object individually. It should be noted, however, that the use of nanoprobes under convergent beam conditions for spatially resolved EELS inevitably collected electrons inelastically scattered through a wide angle. The measured scattering cross-section is, therefore, angle-integrated in this case. In most solids, the character of collective plasmon excitations normally can be easily distinguished from single particle excitations such as interband and core-level transitions. This is particularly true for metals well described by the nearly free electron-gas model in which the energies of these two types of excitations are well separated. Aluminum which exhibits a bulk plasmon peak at 15 eV and an interband transition at 1.5 eV is an excellent example in this category. The situation,