Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces?

Femtosecond pump-probe second-harmonic generation (SHG) and transient linear reflectivity measurements were carried out on polycrystalline Cu, Ag and An in air to analyze whether the electron temperature affects Fresnel factors or nonlinear susceptibilities, or both. Sensitivity to electron temperatures was attained by using photon energies near the interband transition threshold. We find that the nonlinear susceptibility carries the electron temperature dependence in case of Ag and Au, while for Cu the dependence is in the Fresnel factors. This contrasting behavior emphasizes that SHG is nota priori sensitive to electron dynamics at surfaces or interfaces, notwithstanding its cause.     

Concentration and mean free path of the conduction electrons in scandium,yttrium and lanthanides

Measurements were carried out of the thickness dependence of the room-temperature electrical resistivity of vacuum-deposited polycrystalline films of scandium, yttrium and rare earth metals. Inserting these results into the size-effect relation giving the conductivity of thin deposits, and considering all the factors which can affect the mechanism of electron scattering at the surfaces, both the density and the bulk mean free path of the charge carriers were determined.     

Effects of secondary electron emission on plasma characteristics in dual-frequency atmospheric pressure helium discharge by fluid modeling

A one-dimensional(1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In the simulation, electrons,ions of He~+ and He_2~+, metastable atoms of He*and metastable molecules of He*_2 are included. It is found that the secondary electron emission coefficient significantly influences plasma density and electric field as well as electron heating mechanisms and ionization rate. The particle densities increase with increasing SEE coefficient from 0 to 0.3 as well as the sheath's electric field and electron source. Moreover, the SEE coefficient also influences the electron heating mechanism and electron power dissipation in the plasma and both of them increase with increasing SEE coefficient within the range from 0 to 0.3 as a result of increasing of electron density.     

Contact potential measurements on “clean” CdS surfaces

Contact potential difference (cpd) measurements were carried out on {112̄} surfaces of high conductivity CdS cleaved in ultrahigh vacuum. A modified Kelvin method was employed. A change in cpd upon illumination with white light (surface photovoltage) was observed, indicating the presence of intrinsic surface states in these surfaces. Long exposure (about 24 hr) to water vapor (2 × 10⁻⁸ torr) caused an increase of about 1.0 V in cpd and a reduction in surface photovoltage. This increase in cpd was attributed primarily to a decrease in the electron affinity of the surfaces brought about by the molecular dipole moment of physically adsorbed water molecules. The “clean” surfaces exhibited no appreciable affinitity for spectroscopically pure oxygen, except under white light illumination apparently due to the increase of the density free electrons at the surface.     

Synthesis and optical nondestructive evaluation of GaN nanorods on silicon surfaces with gold catalyst

Nanometric gallium-nitride rods were grown on a silicon (1 1 1) substrate through a chemical vapor deposition process with gold particles as the catalyst. Randomly distributed gallium-nitride rods of 20–200 nm in diameter and of various densities and lengths were formed under different deposition conditions. Characterization analyses, such as scanning electron microscopy and optical reflection spectroscopy, have been carried out on samples containing gallium-nitride rods different in size, shape, length and density. While the scanning electron microscopy shows directly the images of the sample surfaces, the optical spectroscopy provides a nondestructive evaluation of the sample surfaces, especially helpful for checking the uniformity of the samples.     

Theory of surface atom mean square displacements in chromium

The temperature dependence of the mean square displacements for atoms on the (100) and (110) surfaces of chromium has been studied theoretically using the harmonic approximation. Calculations were carried out for crystals with periodic boundary conditions in two directions and free boundary conditions in the third direction using a Born-van Karman model with central interactions up to third nearest neighbors and non-central angular stiffness interaction between nearest and next nearest neighbors. The values of the force constants were chosen to give good fits to the elastic constants and to the bulk phonon dispersion curves in the three major directions [100], [110] and [111]. The ratio of the mean-square displacements at a surface to that in the bulk was calculated as a function of temperature for both the (100) and (110) surfaces. The theoretical, results are compared with the available experimental data from low-energy electron diffraction.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

Inter- and intra-agglomerate fracture in nanocrystalline nickel

In situ tensile straining transmission electron microscopy tests have been carried out on nanocrystalline Ni. Grain agglomerates (GAs) were found to form very frequently and rapidly ahead of an advancing crack with sizes much larger than the initial average grain size. High-resolution electron microscopy indicated that the GAs most probably consist of nanograins separated by low-angle grain boundaries. Furthermore, both inter- and intra-GA fractures were observed. The observations suggest that these newly formed GAs may play an important role in the formation of the dimpled fracture surfaces of nanocrystalline materials.     

Electrochemical behavior and microstructural characterization of 1026 Ni-B coated steel

Ni-B coatings have been deposited on the surfaces of commercial steels (SAE-1026). The depositions were carried out using the electroless plating technique employing a nickel chloride solution with borane-dimethylamine as the reducing agent. These specimens were subsequently heat treated at different temperatures (300-500 °C) and different periods of time. The obtained coating thickness was in the order of approximately 1.5 μm. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterize the structure and superficial morphology of the coatings. Phases like Ni, Ni₃B and Ni₄B₃ were observed through X-ray diffraction and confirmed by differential scanning calorimeter (DSC) studies. Some of the precipitated phases have been structurally characterized. The corrosion behavior of the coated surfaces was carried out by electrochemical impedance spectroscopy (EIS) using electrolytic sodium chlorine solutions with pH 2 and 7. The EIS results showed an active corrosion mechanism in acid solution while diffusion-reaction phenomena are predominant in neutral solution.     

A study of Fermi surfaces of the α-phase Cu-Ge and Cu-Si alloys by positron annihilation

Angular correlation measurements on the fcc solid solutions of Cu?Ge and Cu?Si alloys have been carried out on single crystals with four crystallographic orientations by a crossed-slit geometry. The results on both the alloys are nearly the same. The 〈111〉-neck radius and the 〈100〉-radius of the Fermi surfaces increase almost linearly with increasing electron concentration in accord with previous results on Cu?Al and Cu?Zn alloys. The Fermi surface does not touch the square faces of the Brillouin zone at the solubility limit. The results agree fairly well with a calculation based on the sinking-conduction band model.     

Effect of pulsed laser radiation on the structure and electrical properties of ferroelectric ceramics

Ceramic samples of barium titanate and solid solution of barium-strontium titanate subjected to pulsed laser irradiation have been studied by impedance spectroscopy. The measurements have been carried out in the frequency range 10²?C10⁶ Hz and at temperatures in the range from 20 to 450°C. The experimental data are represented by the dispersion of the electric modulus. The activation energy of relaxation processes in the paraelectric phase has been calculated. Scanning electron microscopy has been used to obtain micrographs of the sample surfaces and the data on the elemental composition.     

Comparison of escape depths between Auger electron and disappearance potential spectroscopy electron

The escape depths of the characteristic electrons of the Auger electron and the quasielastically reflected electron were determined by Auger electron spectroscopy (AES) and disappearance potential spectroscopy (DAPS), respectively, for a Cr overlayer onto Ti and Fe substrates. For the case of Cr on Fe, in-situ measurements of AES and DAPS were carried out. From the results, the mean free paths of 455, 575 and 710 eV electrons through Cr were obtained as 9.6, 13 and 15 Å, respectively. The attenuation length of a 2.5 keV primary electron of AES through Cr was also obtained and the value was 62 Å. In addition, the mean free paths of electrons with the same energy depend on the scattering materials of Cr, Mo and W (material dependence). The phenomena are useful for a quantitative electron spectroscopy of surfaces.     

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO₂ laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.     

Effect of Abrasives on Three-Body Abrasive Wear Behaviour of Particulate-Filled Polyamide66/Polypropylene Nanocomposites

Polyamide66/polypropylene (PA66/PP) blend, graphite (Gr)-filled PA66/PP composite and nanoclay (NC)-filled PA66/PP nanocomposites were prepared by twin screw extrusion and injection molding. Three-body abrasive wear behaviour of the injection moulded composites was carried out using a rubber wheel abrasion wear tester. In this study, angular silica sand and quartz particles of size ranging from 200 to 250 μm were used as dry and loose abrasives. The tests were carried out for 150, 300, 450 and 600 m abrading distances at a constant load of 36 N. It was observed that inclusion of particulate fillers in PA66/PP have significant influence on wear under varied abrading distances for different abrasive particles. Further, it was found that NC-filled PA66/PP nanocomposite exhibited lower wear rate compared to Gr filled ones for different abrasive particles. In addition, the worn surfaces of the samples were examined by scanning electron microscopy (SEM) and the morphology was also discussed.     

An example of chemistry-morphology interaction: making up for the geometric and energetic heterogeneities of the (1 0 0) surface of single crystalline silicon by high-temperature treatments in H2

A lot of work has been carried out to prepare chemically homogeneous (1 0 0) silicon surfaces. The hydrogen-terminated (1 0 0) silicon surfaces are the most promising ones, especially in view of their remarkable environmental stability. The simplest way to produce hydrogen-terminated surfaces (attack in water solution of HF of a sacrificial, thermally grown, oxide) results in strongly heterogeneous rough surfaces (although with prevailing dihydride terminations). These surfaces can, however, be flattened and homogenized by treating them in H₂ at high temperature (>850 C). The morphological and chemical changes undergone by the surface during the treatment are studied X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunnelling microscopy, infrared absorption spectroscopy in the attenuated total reflection mode, reflection high energy electron diffraction and thermal programmed desorption, and the mechanisms responsible for them are discussed.     

Surface resistance of single-crystal copper in the millimeter-wave region at room temperature

Measurements of the surface resistance of plane surfaces of single-crystal copper were carried out at 35 GHz. The results indicate an anomaly of the skin effect at room temperature. The investigated surfaces were prepared in stress-free processes, annealed, and manipulated in purified hydrogen and argon.     

On the correlation of geometrical structure and electronic properties at clean semiconductor surfaces

In their LEED patterns most clean semiconductor surfaces show extra spots in addition to the normal beams as expected for a continuation of the bulk lattice up to the topmost layer. This means, that these surfaces are reconstructed. The (111) surfaces of silicon and germanium crystals exhibit another interesting feature: the cleaved surfaces show a metastable superstructure which irreversibly converts to a new modification upon heat treatments above about $\text{1}\text{3}$ of the melting temperatures. The structural changes are correlated with changes in the electronic properties of the surfaces. This has been shown by investigations of surface conductivity, surface photoconductivity, surface photovoltage, internal reflection, electron energy-loss spectroscopy, photoemission, and work function. Recent theoretical studies have also revealed a distinct correlation between geometrical arrangements of the surface atoms and the corresponding band structures of the surface states. However, the atomic positions of the surface atoms used in these calculations are only a model since a detailed analysis of the LEED-intensity-versus-energy curves have not been carried out up to now.     

Microstructure evolution during surface alloying of ductile iron and austempered ductile iron by electron beam melting

Alloying and microstructural modification of surfaces by electron beam has become popular to tailor the surface properties of materials. Surface modification of as-received ductile iron, Ni-plated ductile iron and Ni-plated austempered ductile iron was carried out by electron beam melting to improve the surface properties. Martensitic structure evolved in the heat affected zone and ledeburite structure was produced in the molten zone of the ductile iron. Microhardness of the melted specimens enhanced considerably as compared to the as-received samples. However the microhardness of melted Ni-plated samples is lower than that of the unplated specimens. X-ray diffraction clearly revealed the formation of an austenite and Fe₃C phases in the electron beam molten zone. The broadening of peaks suggests refinement of the microstructure as well as internal stresses generated during electron beam melting.     

Adsorption of atomic oxygen on cubic PbTiO3 and LaMnO3 (001) surfaces: A density functional theory study

We present and discuss density functional theory calculations addressing the electronic structure and energetics of isolated oxygen ad-atoms at the (001) surfaces of PbTiO₃ (PTO) and LaMnO₃ (LMO) cubic perovskites. Both AO- and BO₂-type of surface terminations are considered for each perovskite. Difference electron density analysis has been carried out for each surface to probe local electronic charge redistribution upon oxygen adsorption. Our results show that the (001) surfaces of the two perovskites behave quite differently towards oxygen adsorption. In the case of the PTO (001) surfaces, the adsorbate oxygen atom was found to form a peroxide-type molecular species along with a surface lattice oxygen atom on both PbO- and TiO₂-terminated surface facets. On the other hand, the most stable oxygen adsorption site for the LMO (001) surfaces corresponds to the one expected from a natural continuation of the perovskite lattice. Moreover, the dissociative adsorption of molecular oxygen varies from being only slightly exothermic on the PTO (001) facets to being highly exothermic on the LMO (001) facets. The AO-terminated facets, in general, showed a stronger binding to the adsorbed oxygen.     

Evaluation of a bioluminescence method, contact angle measurements and topography for testing the cleanability of plastic surfaces under laboratory conditions

Detection of adenosine triphosphate (ATP) by bioluminescence is used, for instance, in the food industry and in hospitals to assess the hygiene status of surfaces. The aim of this laboratory study was to investigate the feasibility of the ATP method for estimating the cleanability of resilient floor coverings from biological soil. The surfaces were worn using a Soiling and Wearing Drum Tester, and soiled and cleaned with an Erichsen Washability and Scrubbing Resistance Tester. In the laboratory test carried out with the bioluminescence method, most of the new and worn floor coverings that were biologically soiled were cleaned efficiently. According to this study, the semiquantitative ATP screening method can be used for hygiene monitoring of flooring materials. No correlation was found between cleanability and contact angles or surface topography measured using a profilometer. However, by revealing local irregularities and damage on surfaces, scanning electron micrographs appeared useful in explaining differences in cleanability.