Electronic and vibronic excitations of semiconductor surfaces studied by electron energy loss spectroscopy
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Electron spectroscopy
Vibronic spectroscopy
Electron spectroscopy
Vibronic spectroscopy
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High-resolution electron energy loss spectroscopy was used to investigate the electronic properties of (111)-oriented Au ultrathin films grown on Cu(111). The loss spectrum showed several features which were ascribed to both single and collective excitations. In particular we distinguished features assignable to dispersionless single-particle transitions and the dispersing electron-hole continuum as well as the ordinary and s-like surface plasmon, and the multipole surface plasmon.
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Electron spectroscopy
Localized surface plasmon
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Superstructure
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High-resolution electron-energy-loss spectroscopy (HREELS) is used to study \ensuremath{\delta}-doped GaAs samples with varying silicon dopant concentration. The \ensuremath{\delta}-doped layers were grown by molecular-beam epitaxy at a substrate temperature of 600 \ifmmode^\circ\else\textdegree\fi{}C and the silicon atoms were incorporated 20 nm underneath the surface. Characteristic loss structures are observed in the HREEL spectra and are attributed to the excitation of quasi-two-dimensional (2D) plasmon modes confined to the \ensuremath{\delta}-doped region. A three-layer model of a confined free-electron gas is presented to understand the nature of the quasi-2D plasmon loss features. The experimental loss spectra are compared with simulations calculated self-consistently for free-electron density profiles in \ensuremath{\delta}-doped GaAs. The spread of the dopant atoms and the level of electrically active dopants are fitting parameters. A significant difference is observed between profiles resulting from doping concentrations in the ${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and in the ${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ range. In the samples with low doping levels all silicon atoms are incorporated as electrically active centers and the dopants segregate asymmetrically in direction of growth. In the samples with high doping levels autocompensation and an additional Si-pair diffusion process appear where the dopant diffuses symmetrically around its original location.
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We explore the incident energy dependence of the electronic excitation spectra of Au and Ag films in scanning probe energy loss spectroscopy (SPELS) and also high resolution electron energy loss spectroscopy. We show that the spectra obtained in SPELS depend strongly on the incident electron beam energy. In the case of Au, interband transitions mask the surface plasmon unless the field emission voltage is reduced to ∼100 V, whereas there is a clear surface plasmon peak above 300 V for Ag.
Localized surface plasmon
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The excitation of surface plasmons on individual silver nanowires is studied by high-resolution electron energy loss spectroscopy in a transmission electron microscope, and the results are compared to ensemble optical spectra. The transverse and longitudinal modes of these nanostructures were selectively resolved, confirming the plasmon peak shift versus nanowire length.
Localized surface plasmon
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The confinement of surface plasmon modes in flat nanoparticles gives rise to plasmonic breathing modes. With a vanishing net dipole moment, breathing modes do not radiate, i.e., they are optically dark. Having thus escaped optical detection, breathing modes were only recently revealed in silver nanodisks with electron energy loss spectroscopy in an electron microscope. We show that for disk diameters >200 nm, retardation induced by oblique optical illumination relaxes the optically dark character. This makes breathing modes and thus the full plasmonic mode spectrum accessible to optical spectroscopy. The experimental spectroscopy data are in excellent agreement with numerical simulations.
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We review the use of optical spectroscopy, electron-induced radiation emission (cathodoluminescence), and electron energy-loss spectroscopy to study localized surface plasmon excitations in sub-wavelength noble-metal nanoparticles prepared via lithography or colloidal chemistry. These techniques provide information about plasmon excitations by recording different physical processes, specifically, light scattering exerted by the particles on externally incoming light, radiation emission produced by interaction with an electron beam, and energy loss suffered by those electrons. We provide a theoretical description of a study of the spectral features and spatially resolved maps of nanoparticle plasmon modes at the single-particle level by using these techniques. Numerical modeling is carried out for each set of experimental measurements in order to interpret the results.
Cathodoluminescence
Localized surface plasmon
Electron spectroscopy
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