Topmost Surface Layer Research Articles

Impact of strain on the surface properties of transition metal carbide films: First-principles study

The effect of in-plane lattice strain on the atomic and electronic properties of low-index transition metal (M=Ti, Nb, and Ta) carbide surfaces is studied by first-principles molecular dynamics calculations using a pseudopotential plane-wave technique. The most stable cubic rock-salt phase is considered for carbides. The first-principle study of various [(001), (110), and metal-terminated (111)] carbide surfaces reveals that both compressive and tensile strains strongly affect surface relaxation and electronic properties (work function values and band structures). The most stable (001) carbide surfaces exhibit rumpling between transition metal and carbon atoms in the topmost surface layers, which depends on the applied strain. The work function (WF) for the metal-terminated (111) surfaces varies monotonically, rather strongly depending on the applied strain (the range of variation reaches about 1 eV), while the WF for the (001) surface varies nonmonotonically with a much smaller resulting variation over the wide range of the applied strains. Surface energy calculations show that surface stability is also governed by the applied strain.

Diffusion and chemical composition of TiN xO y thin films studied by Rutherford Backscattering Spectroscopy

Thickness and chemical composition of the TiN xO y thin films deposited by reactive magnetron sputtering from Ti target at controllable oxygen flow rate were determined by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He + ions. The films were deposited on carbon foils and amorphous silica (a-SiO 2) substrates at 25 °C and 250 °C. The estimated film thickness is of 75-100 nm. The O/Ti atomic ratio in the films increases up to 1.5 with increasing oxygen flow rate, while that of N/Ti decreases from about 1.1 for TiN to 0.4 at the highest oxygen flow rate. Substantial out-diffusion of carbon from the substrate is observed which is independent of the substrate temperature. Films grown onto a-SiO 2 substrates can be treated as homogeneous single layers without interdiffusion. It is more difficult to determine the nitrogen and oxygen content due to superposition of RBS signals arising from film and substrate. RBS analysis of the depth profile indicates that for the investigated films the carbon diffusion and oxidation not only at the topmost surface layers but over the bulk of the films were found. Comparison with XPS results indicates substantial oxygen adsorption at the surface of TiN x thin films obtained at zero oxygen flow rate.

FE Analysis of the Effects of Cutting Speed on Thermomechanical Responses of AISI 4142 Steel in High Speed Cutting

The high-speed metal cutting process is analyzed by finite element (FE) method in order to understand the effects of the cutting speed on the thermomechanical responses of workpiece materials. The reliability of numerical simulation is firstly validated by comparing the simulated cutting force with experimental data. Then a series of FE simulations are carried out to reveal the effects of the cutting speed on three key cutting state variables. The cutting force varies with the cutting speed and shows a minimum inflexion at 10 m/s. The maximum temperature in the secondary deformation zone increases gradually with the cutting speed and finally tends to a steady value. The residual stress decreases with the cutting speed as a whole. Thus high speed cutting can improve surface machining precision of product. Besides, it is found that the high residual stress mainly concentrates in the topmost surface layer with a depth of 0.1 mm and sharply decreases to a low level beyond the layer.

Structure of monolayer silica on Mo(1 1 2) investigated by rainbow scattering under axial surface channeling

The structure of a monolayer crystalline silica film grown on a Mo(1 1 2) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane (“axial surface channeling”) the corrugation of the interaction potential leads to an azimuthal out-of plane scattering with an intensity enhancement for the maximum deflection angle, the so called “rainbow”. From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg–Gd–Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10–20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg–Gd–Y alloy was proposed.

Electron correlation contribution to the [formula omitted]/ceria(1 1 1) interaction

We have investigated the electron correlation contribution to the interaction energy of the N 2 O /ceria(1 1 1) system at the CCSD(T) level. N 2 O binds either with the N-end towards the surface with an interaction energy E int = - 0.23 eV or with the O-end with E int = - 0.27 eV . In the former case almost the entire binding energy is due to electron correlation effects, in the latter these effects contribute with about 60%. Analyses of the interaction energy contributions show that most of the electron correlation part originates from the interaction of N 2 O with the O ions in the topmost surface layer.

Ab initio calculations of CO physisorption on ceria(1 1 1)

Applying the method of increments, we have performed MP2 and CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 1) surface. Our calculations predict an interaction energy of −0.28 eV. We have compared our calculations to previous CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 0) surface and found a difference in the interaction energy that is related to the different structure of the two surfaces. On the ceria(110) surface only 30% of the interaction energy originate from electron correlation effects, but on the ceria(111) surface almost the entire binding energy (80%) is due to electron correlation effects. Analyses of the interaction energy contributions show that most of the electron correlation part originates from the interaction of CO with the O ions in the topmost surface layer.

Surface bio-modification of poly(hydroxybutyrate-co-hydroxyhexanoate) and its aging effect

In this work, poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHHx) film was fabricated by a solution-casting method and subsequently was modified by NaOH treatment to improve the surface hydrophilic property. Surface properties including hydrophilicity, surface appearance and functional groups were characterized by water contact angle measurement, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed the hydrophilicity of PHBHHx film was obviously improved by the NaOH treatment due to the topography changes promoted by the NaOH-etching and the introduction of polar groups included hydroxyl and carboxyl on the topmost surface layers. However, the modified film exhibited an aging effect: the hydrophilicity decreases with time elapsed during storage. It was found that the aging rate was strongly dependent on the crystallinity of the film and the storage environment. The sample with high-crystallinity lost hydrophilic property slower than that with low-crystallinity. Hydrophilic and low-temperature environment also prevented the modified PHBHHx from fast losing of the hydrophilicity.

Ultrafast laser ablation of graphite

We have studied single-shot femtosecond laser ablation of graphite by combining a variety of experimental techniques including Raman spectroscopy, atomic force microscopy as well as time of flight spectrometry. The comprehensive analysis reveals insights into the ablation process by exploring the surface structure, the fluence dependence, and the structural dynamics of the detachment. The results show formation and detachment of charged carbon products (such as graphene nanoflakes) from the surface. Time-resolved measurements of ion yields and velocities reveal strong quenching and revival of Coulomb explosion as a function of delay time in the range of 100--200 fs, suggesting a displacive motion between the topmost surface layers which regulates the optical properties of the system.

X-ray photoelectron spectroscopic study on surface reaction on titanium by laser irradiation in nitrogen atmosphere

The surface reaction on titanium due to pulsed Nd:YAG laser irradiation in a nitrogen atmosphere was investigated using X-ray photoelectron spectroscopy (XPS). The laser, with a wavelength of 532 nm (SHG mode), was irradiated on a titanium substrate in an atmosphere-controlled chamber, and then the substrate was transported to an XPS analysis chamber without exposure to air. This in situ XPS technique makes it possible to clearly observe the intrinsic surface reaction. The characteristics of the surface layer strongly depend on the nitrogen gas pressure. When the pressure is 133 kPa, an oxynitride and a stoichiometric titanium nitride form the topmost and lower surface layers on the titanium substrate, respectively. However, only a nonstoichiometric titanium oxide layer containing a small amount of nitrogen is formed when the pressure is lower than 13.3 kPa. Repetition of laser shots promotes the formation of the oxide layer, but the formation is completed within a few laser shots. After the initial structure is formed, the chemical state of the surface layer is less influenced by the repetition of laser shots.

Hydrogen adsorption on model tungsten carbide surfaces

The potential of tungsten carbide to replace platinum as an anode electrocatalyst has attracted considerable attention due to its activity towards hydrogen oxidation, passivity in acidic electrolyte, and resistance to carbon monoxide poisoning. As a first step towards understanding the mechanisms underlying these qualities, we have performed density functional calculations on slab models of the tungsten carbide surface. We have calculated the adsorption energies of hydrogen on the different available bonding sites, and made a comparison to known values for platinum. The variation of bond strength with surface coverage has been investigated, and used to construct model surface Pourbaix diagrams. The nature and structure of the passivated surface is still an open question. It is possible that the tungsten dissolves out of the top-most surface layers, leaving a surface enriched in carbon. We have studied the effect of additional layers of carbon on the adsorption behaviour of hydrogen. While the most stable surface in its bare form provides binding sites that adsorb carbon with an energy of around 4.0 eV, about 60% higher than the adsorption energy of hydrogen on platinum, a carbonaceous tungsten carbide surface provides a maximum binding energy of 2.49–2.69 eV, much closer to the optimum value for efficient catalysis.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys

The reaction of oxygen at low pressure with the Sn/Pd(110) system has beenexamined by photoelectron spectroscopy using synchrotron radiation. Thec(2 × 2) and(3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers(ML) Sn coverage and a 1.75 ML Sn overlayer on the Pd(110) surface afterflashing to 470 K were studied. The Sn 4d core level is strongly affected byO2 adsorption while the Pd 3d core level shows very little change other than a decreasein intensity. Starting with a 10 L dose of oxygen, prominent changes in thespectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4dcore levels indicates that oxidation proceeds with the formation of well-definedstates of Sn, which were identified as a Pd–Sn–O interface layer, SnO andSnO2 oxides. The valence band spectra confirm this assignment. TheSn2+ andSn4+ component signals originate from the topmost surface layer, i.e. tin atoms in more highlyoxidized states constitute the topmost surface layer on top of the Pd–Sn–O interface.The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxideformation; the Sn–O phase formation is accompanied by Pd–Sn bond dissociation.

Density Functional Theory Study for the Stability and Ionic Conductivity of Li2O Surfaces

The energetic, electronic, and defect properties of two low-index Li2O surfaces were studied theoretically at density functional level. In agreement with previous theoretical studies, it was found that the (111) surface is more stable than (110). For both surfaces, a slight shift of the position of the valence band maximum and conduction band minimum with respect to the bulk Li2O was found. The formation of an isolated cation vacancy and a cation Frenkel defect in the Li2O (111) surface were studied as a function of defect concentration. The defect formation energy is ∼10% smaller on the (111) surface than in the bulk. Possible pathways for Li+ diffusion in the Li2O (111) surface were investigated. The activation energy for local hopping processes in the topmost surface layer is significantly smaller than in the Li2O bulk, which is in agreement with experimentally observed increased conductivity in nanocrystalline Li2O materials.

Reduction kinetics of porous zinc oxide pellet with CO–N2 gas mixture

Effect of purity on reduction kinetics of porous zinc oxide bearing pellets in a 2 : 1 nitrogen–carbon monoxide gas mixture has been studied at 1273 K, and the experimental results have been compared with the data available in literature. Two types of materials have been selected for experimentation: laboratory grade 99% pure zinc oxide and industrial grade zinc calcine containing 78% zinc oxide. In each experiment, a single pellet with an average porosity of 60% has been subjected to reduction inside a vertical reactor under a constant flow rate of N2–CO gas mixture. Subsequently, the reduced pellets have been studied under a scanning electron microscope and an image analyser for determination of the extent of reaction at different depths of the pellet. The topmost surface layer of the reacted pellet has been found to be friable indicating the presence of loosely bound particles. Image analysis study has established that the individual zinc oxide particle of any single pellet at different depths from top surface of the pellet has not undergone any appreciable size change during the course of reaction. Such evidence suggests that no appreciable reduction has taken place below the top surface of the pellets. Assuming the process to be topochemical and irreversible with reaction rate as the rate controlling step, the rate constants have been estimated and found to be the same for both the materials, agreeing well with those reported in literature for a non-porous pure zinc oxide pellet.

Plasmas and atom beam activation of the surface of polymers

Wetting properties of polyethylene terephthalate (PET) and low-density polyethylene polymers have been investigated after treatment with a microwave (MW) plasma discharge at low pressure and a dielectric barrier discharge at atmospheric pressure. Experiments have also been carried out in situ with an atom source installed in an x-ray photoemission spectrometer (XPS). The water contact angle measured on both polymers experienced a significant decrease after activation, but a progressive recovery up to different values after ageing. Standard chemical analysis by XPS showed that the plasma and oxygen beam treatments produced an increase in the concentration of –C(O)x functional groups at the outermost surface layers of the treated polymers. Besides, the oxygen distribution between the topmost surface layer and the bulk has been obtained by non-destructive XPS peak shape analysis. Atomic force microscopy analysis of the surface topography showed that, except for PET treated with the MW plasma and the atom beam, the surface roughness increased after the plasma treatments. Wetting angle variations, oxygen content and distribution, surface roughness and evolution of these properties with time are comparatively discussed by taking into account the basic processes that each type of activation procedure induces in the outmost surface layers of the treated polymers.

Nanostructured black cobalt coatings for solar absorbers

AbstractNanocrystalline black cobalt electrically deposited onto a steel substrate from aqueous solution was investigated. The influence of electrolyte composition and operating parameters on the appearance and optical properties of the coat was studied. The deposition conditions that ensure the highest solar absorptance were optimized. The chemical composition of fabricated thin films before and after annealing at 400 °C was determined by energy dispersive X‐ray analysis (EDS) and XPS technique. The crystal structure analysis showed that the bulk composition of the films was mainly cobalt oxide. The surface analysis reveals that the topmost surface layers of the films are made of different cobalt compounds confirming the multivalence state of Co on the surface with an oxidation state of ≥ + 2. Scanning electron microscope (SEM) observation indicated that the surface morphology was changed from dendritic structure to lamellar at higher current density. The black cobalt film showed soft magnetic characteristicsand excellent optical properties to transform solar energy into thermal energy. Copyright © 2008 John Wiley & Sons, Ltd.

A time-of-flight secondary ion mass spectroscopy study of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide RT-ionic liquid

Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has been used to investigate the surface structural transformation of room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid ([EMIM][Tf 2N]), in the temperature range between 300 and 160 K. The intensity of the [EMIM] + cation from the crystal surface becomes about twice as large as that from the glassy and liquid surfaces, whereas the fragment ions from the [Tf 2N] − moiety are almost unchanged upon crystallization. This phenomenon can be ascribed to the steric effect of the cation relative to the counter anion at the topmost surface layer rather than their surface compositions: a specific layered structure of the crystal surface, in which the imidazolium ring of [EMIM] + is aligned parallel to the surface plane, is thought to be responsible for the enhancement of the [EMIM] + ion emission. The smaller [EMIM] + intensity from the glassy and liquid films evidences that the imidazolium ring is not parallel to the surface.

Band structure ofTl/Ge(111)−(3×1): Angle-resolved photoemission and first-principles prediction of giant Rashba effect

We have investigated the atomic and electronic structure of the $\text{Tl}/\text{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ surface using angle-resolved photoelectron spectroscopy (ARPES) and first-principles total-energy-band calculation. The experimental surface bands exhibit semiconducting electronic structure and are well reproduced by the band calculation based on the honeycomb-chain channel structure with Tl adsorbed at ${H}_{3}$ site. Electron density maps of the surface-related states show the formation of the $\text{Ge}=\text{Ge}$ double-bond-like state as observed on the alkali-metal-induced $\text{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ surfaces. In our scalar-relativistic all-electron calculation, the Tl-derived surface bands show a Rashba-type spin splitting in the projected bulk band gap, which is not clearly resolved by ARPES due to a limited energy resolution. They show complicated dispersion due to the hybridization with the bulk states in the projected bulk band region. The lowest unoccupied surface band, which is an antibonding state formed between $\text{Tl}\text{ }6p$ and Ge dangling bonds and is highly localized in the topmost surface layer, shows a large spin-orbit splitting of $\ensuremath{\sim}200\text{ }\text{meV}$.

Freeze-etching and vapor matrix deposition for ToF-SIMS imaging of single cells.

Freeze-etching, the practice of removing excess surface water from a sample through sublimation into the vacuum of the analysis environment, has been extensively used in conjunction with electron microscopy. Here, we apply this technique to time-of-flight secondary-ion mass spectrometry (ToF-SIMS) imaging of cryogenically preserved single cells. By removing the excess water which condenses onto the sample in vacuo, a uniform surface is produced that is ideal for imaging by static SIMS. We demonstrate that the conditions employed to remove deposited water do not adversely affect cell morphology and do not redistribute molecules in the topmost surface layers. In addition, we found water can be controllably redeposited onto the sample at temperatures below -100 degrees C in vacuum. The redeposited water increases the ionization of characteristic fragments of biologically interesting molecules 2-fold without loss of spatial resolution. The utilization of freeze-etch methodology will increase the reliability of cryogenic sample preparations for SIMS analysis by providing greater control of the surface environment. Using these procedures, we have obtained high quality spectra with both atomic bombardment as well as C 60 (+) cluster ion bombardment.

Interaction of Carboxylic Acids and Water Ice Probed by Argon Ion Induced Chemical Sputtering

In this paper, we investigated the interaction of simple carboxylic acids, formic acid (FA), acetic acid (AA), and propionic acid (PA) with thin layers of water ice in the temperature range of 110-190 K in ultrahigh vacuum. The focus, however, is on the AA-ice system. Molecularly thin layers of these systems were prepared on a pre-cooled polycrystalline copper substrate. The interactions and phase changes in the system were monitored with chemical sputtering using low-energy (≤30 eV) Ar + , which probes the topmost surface layers. At 110 K, the deposited AA exists as dimers in its amorphous solid form. At the same temperature, in the presence of water ice, this dimeric form gets converted to chainlike oligomers. Chemical sputtering spectra show distinct features for these two surface species. The data suggest that ion formation reflects the surface structure, implying a unique mechanism for its formation. Detailed studies have been made with amorphous solid water (ASW) and crystalline water (CW) to get a complete understanding of the system. Experiments carried out with AA-D 2 O ice confirmed the proton-transfer mechanism during chemical sputtering. Other studies were conducted with AA-CH 3 OH and AA-CCl 4 systems. Detailed investigations performed to understand the effect of thickness of AA and ice overlayers suggest that the extent of water molecules required to effect the structural transformation in the acid is dependent on the amount of the latter. Dimeric-to-oligomeric transformation does not occur for the PA-ice system. Detection of a structural transition at the very top of ice in molecularly thin films adds additional capabilities to the low-energy ion scattering technique.