Surface Energy Loss Functions Research Articles

Indirect bulk plasmon generation by electrons reflected above the solid surface.

We demonstrate that the spatial dispersion of the dielectric response provides the mechanism to excite the bulk plasmon in a solid by incident electrons not penetrating into the bulk, which is the process inhibited in the nondispersive models. In this case the bulk plasmon generation is described by the surface energy-loss function, but not by the bulk one -Im[1/\ensuremath{\epsilon}(q,\ensuremath{\omega})], while the latter must be used when the probing electrons do not penetrate the bulk. Comparison of the indirectly excited bulk plasmon, obtained for the surface of aluminum, with experimental spectrum shows that the effect is sufficiently strong to be accounted for in the interpretation of the reflection electron-energy-loss spectroscopy data of surface structures.

Surface energy-loss function of a semi-infinite spatially dispersive solid.

Surface energy-loss function of a semi-infinite spatially dispersive solid.

Phase modulated ellipsometry from the ultraviolet to the infrared: In situ application to the growth of semiconductors

Nanocomposite TiO2–SiO2 thin films with different compositions (from 0 to 100 mol% TiO2) were deposited by sol–gel dip-coating method on silicon substrate. Crystal structure, chemical bonding configuration, composition and morphology evolutions with composition were followed by Raman scattering, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy respectively. The refractive index and the extinction coefficient were derived in a broad band wavelength (250–900 nm) from spectroscopic ellipsometry data with high accuracy and correlated with composition and microstructure. Results showed an anatase structure for 100% TiO2 with a grain size in 6–10 nm range. Whereas, the inclusion of SiO2 enlarges the optical band gap and suppresses the grain growth up to 4 nm in size. High TiO2 dispersion in SiO2 matrix was observed for all mixed materials. The refractive index (at λ = 600 nm) increases linearly with composition from 1.48 (in 100% SiO2) to 2.22 (in 100% TiO2) leading to lower dense material, its dispersion being discussed in terms of the Wemple-DiDomenico single oscillator model. Hence, the optical parameters, such optical dispersion energies E0 and Ed, the average oscillators, strength S0 and wavelength λ0 and the ratio of the carrier concentration to the effective mass N/m∗ have been derived. The analysis revealed a strong dependence on composition and structure. The optical response was also investigated in term of complex optical conductivity (σ) and both volume and surface energy loss functions (VELF and SELF).

Optical Properties of Thermally Deposited Bismuth Telluride in the Wavelength Range of 2.5-10 µm

The optical constants (the refractive index n, the absorption index k and the absorption coefficient α) of Bi2 Te3 thin films were determined in the wavelenght range of 2.5 to 10 μm. The shape of the absorption edge in Bi2Te3 thin films has been determined from transmittance and reflectance measurements. The edge is of the form expected for direct transition corresponding to Eg = 0.21 eV. The optical constants were used to determine the high frequency dielectric constant eo = 58, the optical conductivity σ = + σ2 as well as the volume and surface energy loss functions. All these parameters were used to get some information about the intraband and interband transitions.

Phonon surface loss function of ionic-crystal films: A comparison between microscopic and macroscopic approaches

The surface loss function is an important quantity in the description of reflection electron-energy-loss spectroscopy (EELS) in near-specular geometry. In this paper, lattice-dynamics calculations of the surface energy-loss function associated with optical excitations in ionic-crystal slabs are performed. Test calculations are carried out for both NaF isolated films and NaF slabs onto a semi-infinite substrate. In the latter case, a continuum dielectric theory is used to describe the response of the substrate, emphasis being on long-wavelength optical phonons. We compare the results of the microscopic formulation of the surface loss function of the film with those deduced from the so-called dielectric theory, where a bulk-like dielectric response of the slab is assumed. The legitimacy of the use of a bulk response is a central question in applications of the dielectric theory to a very thin slab. It is shown that the latter provides a valid description of the surface energy-loss function for the wave vectors predominantly probed in EELS when the film thickness exceeds ~ 2 nm (about 10 atomic planes). For very thin, relaxed films (less than 8 atomic layers), the dielectric theory fails in producing the correct frequency positions of the surface energy-loss peaks; by contrast the intensities of the loss peaks are more accurately predicted.

Surface energy-loss function for the inelastic scattering of electrons from a metal substrate with an overlayer of adsorbed alkali-metal atoms.

We investigate the spectrum of elementary excitations of the conduction electrons associated with an overlayer of sodium atoms adsorbed on a metal surface. We compute the surface energy-loss function for the small-angle backscattering of electrons as function of coverage (up to two full monolayers of sodium atoms are considered). The ground-state problem is dealt with by using Lang's uniform-background model of alkali-metal-atom--metal-surface chemisorption. The dynamical response of the electron gas is treated in the random-phase approximation. Previous theoretical work has, for the most part, assumed that an overlayer of atomic thickness responds as bulk matter. Our microscopic calculation reveals that for one-monolayer coverage the overlayer's response in fact deviates significantly from bulklike behavior. For two-monolayer coverage the response is bulklike, with a prominent collective (plasmon) mode. The quantum-mechanical nature of the overlayer response is displayed in detail. A case is made for the need that an angle-resolved electron-energy-loss experiment be performed, using an s-p-bonded metal, such as aluminum, as the substrate (instead of a transition metal).

Surface plasmon modes and energy loss function of indium thin films on mica substrates

Thin indium films of thickness ranging from 15 to 100 nm are deposited onto crystalline mica substrates under a pressure of 1.333 × 10−3 Pa. Using ellipsometry the real and imaginary parts of dielectric constant and energy loss function −1m (1/e) and −Im (1/(e + 1)) are calculated. Low energy peaks are interpreted in terms of interband transitions while high-energy peaks are interpreted in terms of surface plasmon oscillations. Dunne Indiumschichten mit einer Dicke zwischen 15 und 100 nm werden auf kristallinen Glim-mersubstraten unter einem Druck von 1,333 × 10−3 Pa aufgebracht. Mittels Ellipsometrie werden Real- und Imaginiirteil der Dielektrizitatskonstante und Energieverlustfunktionen — Im (1/e) und — Im (1/(e + 1)) berechnet. Die Niederenergiemaxima werden mit Interbandubergungen inter-pretiert, wahrend die Hochenergiemaxima mit Oberflachenplasmonoszillationen erklart werden.