F. Soria

On the detection and size classification of nanometer-size metal particles on amorphous substrates

Abstract The detection and size analysis of small metal aggregates supported on amorphous substrates becomes increasingly difficult when the particle size approaches that of the phase contrast background structures of the support. Standard conventional transmission electron microscopy (CTEM), even with subsequent analog image analysis, becomes inconclusive or fails for particles less than 1 nm in diameter, and clear differentiation of the particles from the background can only be made with considerable effort, typically involving several micrographs taken from the same specimen area under different imaging conditions. The TEM image contrast transfer mechanism is briefly reviewed with emphasis on practical conclusions. In the 1–2 nm particle size range, particles can be distinguished from the amorphous background only when focus, astigmatism, and specimen drift are optimally controlled. Operating at 100 keV and with Cs = 2.2 mm, a deviation of 100 nm from the optimum focus condition was, for example, found to increase the “apparent” diameter of 1.3 nm particles by as much as 0.4 nm. Furthermore, the electron exposure required to perform such microscopy may severely complicate the situation by radiation- induced changes in specimen and support. These changes may be intensified by specimen contaminants and an improperly controlled environment. Compared to standard high resolution imaging practice, operation at lower magnifications, lower acceleration voltage, larger illumination aperture, and closer to the Gaussian focus is recommended for routine imaging of very small particles on amorphous substrates.

A leed I(E) analysis based on an island model for the interaction of oxygen with Al{111} surfaces

The low-energy electron diffraction (LEED) technique has been used to study the structures which appear during the first stages (≤200 L) of the interaction of oxygen with Al{111} surfaces at room temperature (RT). LEED dynamical programs have been used to compute I(E) theoretical curves for different structures (islands, clean surface, oxygen underlayer, oxygen overlayer). An island model obtained by linear combinations of those structures has been tested comparing the theoretical curves to the experimental ones measured at several oxygen exposures using the R -factor technique. The clean Al{111} surface layer, d bulk 12 = 2.338 A, has been found to be dilated by (3 ± 2)%, d 12 = 2.41 ± 0.05 A. An Al{111}1×I-O underlayer structure at 25 L O 2 exposure has been found, providing a consistent explanation for all experimental results at this coverage. During the adsorption of the first 1/3 monolayer (≤30 L), oxygen penetrates through the three-fold fcc hollows of the Al{111} surface, occupying underlayer positions at 0.3 ± 0.1 A below the first Al plane, in islands which have their top Al layer contracted by about 15%. Above this coverage, oxygen starts to occupy fcc hollows in overlayer positions, and at about 75 L the simultaneous formation of oxide-like alumina begins, probably on the borders of the depressed terraces. At 100 L the maximum coverage of the Al{111}1×I-O overlayer structure is reached, with an interplanar distance d 12 = 0.7 (+0.12, −0.04) A. This result agrees with the determination made by surface-extended X-ray-absorption fine structure (SEXAFS), and recent theoretical calculations. At coverages around half a monolayer, at about 50 L, a new d 12 value is obtained, d 12 = 0.4 (+0.10, −0.05) A, and its meaning is discussed.

CuO nanowires for inhibiting secondary electron emission

Copper oxide nanowires (NWs) grown on copper to avoid the secondary electron emission were investigated. Optimal temperatures for NW growth were found to be in the range 700–800 K. NW surface coverage of 102 µm−2 is required to strongly reduce the secondary electron yield. A total secondary electron emission coefficient below 1 was obtained for NW aspect ratio higher than 103.

Bulk Fermi surface determination by tuning the photoelectron kinetic energy

The three‐dimensional Fermi surface of a Cu single crystal has been measured by angle‐resolved photoemission (ARUPS). Well‐defined two‐dimensional (2D) cuts through the bulk fcc Cu Brillouin zone have been mapped by adjusting the incident photon energy of a tunable light source. This method, conceived of as an extension of the traditional ARUPS, allows the direct visualization of Fermi surface contours from experimental two‐dimensional patterns. The surface shape of constant energy at the Fermi level and its link with the translational bulk symmetries could be clearly seen by the determination of the Fermi photoelectron momentum distribution along several Brillouin zones. In addition, the 2D sections of the Fermi surface measured by photoemission have been well reproduced by a semiempirical tight binding calculation.

Image processing enhancement of high-resolution TEM micrographs of nanometer-size metal particles

Abstract The high-resolution TEM detectability of lattice fringes from metal particles supported on substrates is impeded by the substrate itself. This effect appears in both amorphous and crystalline substrates. In the former case, it is due to substrate-generated phase contrast features; in the latter case it is due to the intermixing of fringes from particle and substrate. Single value decomposition (SVD) and Fourier filtering (FFT) methods were applied to standard high resolution micrographs to enhance lattice resolution from particles as well as from crystalline substrates. SVD produced good results for one direction of fringes, and it can be implemented as a real-time process. Fourier methods are independent of azimuthal directions and allow separation of particle lattice planes from those pertaining to the substrate, which makes it feasible to detect possible substrate distortions produced by the supported particle. This method, on the other hand, is more elaborate, requires more computer time than SVD and is, therefore, less likely to be used in real-time image processing applications.

Initial stages of oxidation of GaAs(111)2×2-Ga surfaces

Abstract The initial interaction of oxygen at room temperature with GaAs(111)2 × 2-Ga surfaces has been studied by quantitative Auger analysis and low-energy electron diffraction, under different electron irradiation and gas ionization conditions. Oxygen fills first the non-vacancy overlayer sites with a preferential bond to the Ga atoms. This adsorption phase is characterized by the absence of chemical shifts in the Ga Auger peaks that involve core levels. The oxidation stage begins with the occupation of the underlayer sites below the first Ga-As bilayer. For coverages lower than 2 monolayers oxygen adsorption and incorporation takes place without any loss of Ga or As atoms of the surface layers. Electron irradiation and gas ionization of the oxygen-covered surface increase the kinetics up to two orders of magnitude, but no changes in the adsorption sites and/or occupation sequence have been detected.

Chemisorption of Si on Al(111) surfaces: A local-chemical-bond analysis from Auger transition density of states

Abstract Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L 2,3 VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.

Kinetics of electron‐stimulated oxidation of GaAs(1̄1̄1̄)

Kinetics and current dependence of the electron‐stimulated oxidation (ESO) of GaAs (111) surfaces are obtained by means of a quantitative Auger numerical formalism for oxide layers applied to new experimental data. The kinetics presents two stages: a linear growth with time, and a saturation region. The rate of growth and the saturation thickness for the oxide layer show simultaneous maxima as a function of the current density. The amount of oxygen adsorbed in the preoxidation stage (two monolayers of atomic height) and its independence of the current density is also obtained. The minimum diameter of the oxide spots made with our conventional Auger gun is about 20 μm. The possibility of a classical interpretation of ESO effects in GaAs (111) looks controversial.

Surface and bulk reconstruction of Pt(111) 1 × 1

A new structural analysis of the Pt(111) 1 × 1 surface by low energy electron diffraction gives an unexpected lattice parameter compression of ao=3.904 A with respect to the crystallographic value of 3.924 A, yielding a Pendry R-factor of 0.078. This implies a 0.5% contraction for both the in-plane lattice parameter, ap=2.761 A, and bulk vertical interplanar distances, db=2.25 A, as compared to the crystallographic values of 2.775 and 2.265 A, respectively. The topmost surface layer exhibits an expansion of 0.9%, 2.285 A, and the second a contraction of 0.9%, 2.245 A. Deeper layers are bulk layers.

Thermal effects on the growth of SiO2 on GaAs(100) by reduction of native oxides

The reactivity of Si with oxidized GaAs surfaces has been recently proposed as a new pathway to form SiO2/GaAs interfaces. In this work, we study the maximum amount of native oxides which can react with evaporated Si atoms, leading to a complete reduction of the GaAs–oxides. Photoemission techniques, using synchrotron radiation and conventional x‐ray sources, were used to monitor in situ the interface formation. The completion of the native oxides reduction is limited to layers around 5 A thick at room temperature, though substrate heating at 550 K during Si deposition allows this limitation to be overcome. This temperature seems to be high enough to activate the diffusion of the Si monomers to the inner interface so that the reduction process can continue. The reduction process stops when Si monomers cannot diffuse through the Si–oxide layer and Si–Si bonds are formed on top of the oxide layer.