Clean Metal Surfaces Research Articles

A Modified Formal Theory on Semi-Relativistic Effects during Electron Emission from Metallic Surfaces upon the Impact of High Energy Particles

In this paper we introduce a formal theory on unveiling relativistic effects during electron emission from clean metallic surfaces upon high charged particle impact using a Jellium-type wave function including suitable spinors. In addition image charge final state electron surface interactions have been initiated in the relativistic region as well as the scattering of the projectile from the multi-center bulk potential. Finally, a semi-relativistic condition is considered in place of the ionization mechanism of scattering from an aluminium semi-infinite solid target by non-relativistic electrons to determine multiple differential cross-section.

Vibrational enhancement of electron emission in CO (a3Π) quenching at a clean metal surface

We have combined a Stark decelerator with a molecular beam-surface scattering setup to accurately measure the absolute electron emission yield, γ, of CO (a(3)Π) quenching at a Au(111) surface with quantum state resolution. We clearly observe an enhanced probability of electron emission when scattering vibrationally excited states of CO from the surface.

211 雰囲気制御切削試験機によるグリースの分解と水素発生挙動の評価(セッション7 機械の評価)

Recent years, bearings frequently suffer from early damage caused by flaking. This is thought to be due to hydrogen embrittlement in connection with hydrocarbon decomposition when greases contact the nascent clean metal surface created under the current severer frictional situations. This study proposes the evaluation method of grease decomposition and hydrogen generation by a controlled atmosphere cutting apparatus. Mass spectrometry of this method provided the evidences of hydrogen generation and hydrocarbon decomposition in relation to the nascent metal surface appearance by cutting. The further investigation demonstrated additives in greases greatly influenced the amount of hydrogen generation. Recent years, bearings frequently suffer from early damage caused by flaking. This is thought to be due to hydrogen embrittlement in connection with hydrocarbon decomposition when greases contact the nascent clean metal surface created under the current severer frictional situations. This study proposes the evaluation method of grease decomposition and hydrogen generation by a controlled atmosphere cutting apparatus. Mass spectrometry of this method provided the evidences of hydrogen generation and hydrocarbon decomposition in relation to the nascent metal surface appearance by cutting. The further investigation demonstrated additives in greases greatly influenced the amount of hydrogen generation.

Alignment of benzene thin films on self-assembled monolayers by surface templating

A key question in the epitaxial growth of materials is whether vapor-deposited molecules acquire an ordered structure that depends on the structure and chemical composition of the underlying substrate. In this paper we examine this question for metallic and chemically-tailored self-assembled monolayer (SAM) substrates, and show that proper selection of the templating substrate can lead to the deposition of nearly 100 layer thick molecular films that retain the alignment of the initial interface. In particular, we have examined benzene growth on gold, alkanethiol and phenoxy-terminated SAMs using a combination of in situ infrared spectroscopy and molecular beam techniques. When benzene molecules stick to a clean metal surface, the molecules in the first layer lie flat. However, after the deposition of a few layers, growth occurs as three-dimensional crystallites, resulting in randomly aligned polycrystalline domains. We found that there need not be an orientational transformation from the thin to thick structure for vapor-deposited molecules. Here, we vapor deposited benzene on phenoxy-terminated self-assembled monolayers (SAMs), and grew thick films of aligned benzene molecules. The alignment of the first layer of benzene molecules on the SAM was retained for more than 80 layers. In contrast, when methyl-terminated SAMs were used as a substrate, the thick film structure was indistinguishable from those with Au substrates.

An atomic study of hydrogen effect on the early stage oxidation of transition metal surfaces

Density functional theory (DFT) and tight-binding quantum chemical molecular dynamics (QCMD) have been applied to analyze the role of interstitial hydrogen in the process of oxygen adsorption to Ni (111) and Cr-doped Ni (111) surfaces and diffusion within these metal surfaces. The DFT calculations demonstrate that the fcc hollow and octahedral sites are the most favorable for hydrogen adsorption on the surface and subsurface, respectively. A clean metal surface has a slight inward relaxation in the topmost layer, whereas the metal atoms show outward relaxation (2%) due to interstitial hydrogen. The adsorption energies of oxygen and OH have decreased to 0.26 and 0.13 eV, respectively, and the metal atomic bond further extended in the range of 1–2% in order to hydrogen remained interstitial site. Hydrogen changes to a negatively charged in the interstitial site by receiving electron. The QCMD results reveal that the oxygen penetration depth increases when hydrogen occupy into interstitial octahedral site. The deeply diffused or interstitial hydrogen receives electrons from the metal. Additionally, interstitial hydrogen initiates the charge transfer and extends the metal atomic bond. The localized process weakens the metal–metal bonds and it makes the surface chemically active for further interaction. This process can help oxygen or other species to diffuse into the structure. As a result, the subsurface hydrogen accelerates the early stage of oxidation initiation.

Inner valence excitations of condensed neon

Applying two-dimensional electron spectroscopy, we investigated bound and continuum excitations of neon films condensed onto clean metal surfaces, in the energy range of the [Ne 2s] singly and [Ne 2p2] doubly excited states. Precise energy values of excitons and ionization potentials were obtained for the surface and for the bulk and compared to data for the isolated neon atom. In addition, valuable information on the mobility and decay of excitons in this energy range was accessible with our technique.

Electrochemical Scanning Tunneling Microscopy

The electrochemical scanning tunneling microscope was the first tool for the investigation of solid-liquid interfaces that allowed in situ real space imaging of electrode surfaces at the atomic level. Therefore it quickly became an important addition to the repertoire of methods for the determination of the local surface structure as well as the dynamics of reactions and processes taking place at surfaces in an electrolytic environment. In this short overview we present several examples to illustrate the powerful capabilities of the EC-STM, including the observation of clean metal surfaces as well as the adsorption of thin metal layers, specifically adsorbed anions and non-specifically adsorbed organic cations. In several cases the electrode potential has a significant influence on structure and reactivity of the surface that can be explained by the observations made with the EC-STM.

Plasmon-assisted electron-electron collisions at metallic surfaces

We present a theoretical treatment for the ejection of a secondary electron from a clean metallic surface induced by the impact of a fast primary electron. Assuming a direct scattering between the incident, primary electron and the electron in a metal, we calculate the electron-pair energy distributions at the surfaces of Al and Be. Different models for the screening of the electron-electron interaction are examined, and the footprints of the surface- and the bulk-plasmon modes are determined and analyzed. The formulated theoretical approach is compared with the available experimental data on the electron-pair emission from Al.

Trapping surface electrons on graphene layers and islands

We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.

Evolution of Crystallographic Texture in Pure Iron and Commercial Steels by γ to α Transformation

A novel method has been discovered for controlling the crystallographic orientation of pure iron using the γ to α phase transformation. When pure iron with clean metal surfaces undergoes the γ to α phase transformation, it develops a strong cube-on-face texture ({100}<0vw>) with the grain size being larger than the sheet thickness. The mechanism controlling the <100> orientation obtained is associated with the fact that the {100} faces are elastically compliant so that the <100> texture can develop in a manner consistent with minimization of strain energy. However, in commercial steels, although so many texture analyses have been conducted, the cube-on-face texture has been rarely observed. According to thermodynamic analysis, surface oxidation in commercial steels appears to be responsible for the deterioration of the <100> texture. This phenomenon can be explained in terms of the modification of the inherent elastic anisotropy of metal surface by the surface oxidation.

Hydridosilane Modification of Metals: An Exploratory Study

The interaction of hydridosilanes with oxide-free metal substrates was evaluated in order to determine their potential for surface modification analogous to alkoxysilanes with metal oxide substrates. Under mild conditions, trihydridosilanes interact with a variety of clean, hydrogenated and fresh metal and metalloid surfaces, including titanium, silicon and gold. In contrast, monohydridosilanes appear to have minimal interaction. All classes of hydridosilanes have minimal interaction with anhydrous oxide surfaces. Preliminary results suggest that surface modification with trihydridosilanes may provide a route for generating self-assembled monolayers on metal substrates. The synthesis of new trihydridosilanes is described. Contact angle, FTIR and XPS data for modified surfaces are provided.

C–H bond activation of methane on clean and oxygen pre-covered metals: A systematic theoretical study

Density functional theory calculations are presented for adsorption and dissociation of CH 4 on clean and oxygen atom pre-adsorbed metal surfaces (Cu, Ag, Au, Ni, Pd, Pt, Ru, Rh, Os, Ir, and Mo). The total energy change and the activation barrier have been calculated for the direct and the oxygen-assisted cleavage of the C–H bonds. Our results indicate that pre-adsorbed oxygen promotes the CH 4 dissociation process on IB group metal surfaces, but inhibits the dissociation process on transition metal surfaces. A good Brønsted–Evans–Polanyi correlation for CH 4 dissociation on clean and atomic oxygen pre-adsorbed metal surfaces is found, which is helpful to reveal the nature of CH 4 dissociation. From the analysis of activation barrier, we expect our work can provide a clear understanding of the nature of CH 4 dissociation.

Hydridosilane Modification of Metals: An Exploratory Study

Under mild conditions, trihydridosilanes interact with a variety of clean, hydrogenated and fresh metal and metalloid surfaces, including titanium, silicon and gold. In contrast, monohydridosilanes appear to have minimal interaction. All classes of hydridosilanes have minimal interaction with anhydrous oxide surfaces. Preliminary results suggest that surface modification with trihydridosilanes may provide a route for generating self-assembled monolayers on metal substrates. The synthesis of new trihydridosilanes is described. Contact angle, FTIR and XPS data for modified surfaces are provided.

The effect of Ti sputter target oxidation level on reactive High Power Impulse Magnetron Sputtering process behaviour

This paper investigates the effect of sputter target oxidation level on reactive process behaviour during High Power Impulse Magnetron Sputtering of transition metal target (Ti) in Ar/O2 atmosphere. It was found by this study that the sputter target state depends on the history of its use in reactive HIPIMS mode. The effect appears to be pronounced strongly due to the significance of ion irradiation induced effects leading to enhanced sputter target oxidation. The target sputter cleaning times (e.g. after a reactive deposition run) are long (compared to DC sputtering) due to the same reason. The abovementioned effect has serious implications for the hysteresis behaviour and reactive deposition process window. It is shown in this paper that the hysteresis loop can be artificially suppressed or even eliminated and the reactive deposition process window can be reduced a number of times if the starting target surface is not a clean metal surface but a partially oxidised metal surface. Such target surface condition is readily obtainable for the targets that have a history of prolonged use in reactive HIPIMS mode. Processing parameters, such as pulse frequency and peak voltage pulse values are shown to have influence on the degree to what the abovementioned effects occur.

Molecular dynamics characterization of the contact between clean metallic surfaces with nanoscale asperities

We use molecular dynamics (MD) simulations to characterize the tensile strength of contacts formed between various clean platinum surfaces with nanoscale asperities. Both commensurate contacts between (001) and (111) surfaces and incommensurate (001) ones are considered over a wide range of asperity sizes. In cyclic closing and opening, fresh asperities that form contacts for the first time show significant plastic deformation; this leads to a reduction in the effective contact area during the first few cycles, after which steady state is achieved both in terms of contact size and the pull-out force necessary to open the contacts. As is the case for commensurate surfaces [H. Kim and A. Strachan, Phys. Rev. Lett. 104, 215504 (2010)], the strength of the metallic bridges that form in incommensurate contacts exhibit strong size effects; their strength increases with decreasing size until a length of \ensuremath{\sim}5 nm, below which weakening is observed. Commensurate contacts lead to stronger bridges than incommensurate ones but only during the initial closing events; after steady state is achieved, commensurate and incommensurate (001) surfaces lead to bridges of similar strengths.

Protein-Associated Water and Secondary Structure Effect Removal of Blood Proteins from Metallic Substrates

Removing adsorbed protein from metals has significant health and industrial consequences. There are numerous protein-adsorption studies using model self-assembled monolayers or polymeric substrates but hardly any high-resolution measurements of adsorption and removal of proteins on industrially relevant transition metals. Surgeons and ship owners desire clean metal surfaces to reduce transmission of disease via surgical instruments and minimize surface fouling (to reduce friction and corrosion), respectively. A major finding of this work is that, besides hydrophobic interaction adhesion energy, water content in an adsorbed protein layer and secondary structure of proteins determined the access and hence ability to remove adsorbed proteins from metal surfaces with a strong alkaline-surfactant solution (NaOH and 5 mg/mL SDS in PBS at pH 11). This is demonstrated with three blood proteins (bovine serum albumin, immunoglobulin, and fibrinogen) and four transition metal substrates and stainless steel (platinum (Pt), gold (Au), tungsten (W), titanium (Ti), and 316 grade stainless steel (SS)). All the metallic substrates were checked for chemical contaminations like carbon and sulfur and were characterized using X-ray photoelectron spectroscopy (XPS). While Pt and Au surfaces were oxide-free (fairly inert elements), W, Ti, and SS substrates were associated with native oxide. Difference measurements between a quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance spectroscopy (SPR) provided a measure of the water content in the protein-adsorbed layers. Hydrophobic adhesion forces, obtained with atomic force microscopy, between the proteins and the metals correlated with the amount of the adsorbed protein-water complex. Thus, the amount of protein adsorbed decreased with Pt, Au, W, Ti and SS, in this order. Neither sessile contact angle nor surface roughness of the metal substrates was useful as predictors here. All three globular proteins behaved similarly on addition of the alkaline-surfactant cleaning solution, in that platinum and gold exhibited an increase, while tungsten, titanium, and stainless steel showed a decrease in weight. According to dissipation measurements with the QCM-D, the adsorbed layer for platinum and gold was rigid, while that for the tungsten, titanium, and stainless steel was much more flexible. The removal efficiency of adsorbed-protein by alkaline solution of SDS depended on the water content of the adsorbed layers for W, Ti, and SS, while for Pt and Au, it depended on secondary structural content. When protein adsorption was high (Pt, Au), protein-protein interactions and protein-surface interactions were dominant and the removal of protein layers was limited. Water content of the adsorbed protein layer was the determining factor for how efficiently the layer was removed by alkaline SDS when protein adsorption was low. Hence, protein-protein and protein-surface interactions were minimal and protein structure was less perturbed in comparison with those for high protein adsorption. Secondary structural content determined the efficient removal of adsorbed protein for high adsorbed amount.

Laser ultrasonic inspection, a new inspection technique and its effects on the integrity and surface properties of the metallic surfaces

Laser ultrasonic inspection (LUI) is the name given to the techniques in which a laser beam interacts with the surface of a test sample and replaces piezoelectric transducers for launching and probing elastic sound waves. When this wave returns to the surface, a separate laser interferometer detects the small resulting displacement. This technique is strictly non-contact and is therefore suitable for in-process inspection of parts while at high temperature or moving at high speed. In the field of non-destructive techniques (NDT), the association of laser generation with optical detection provides a completely remote inspection system. As this process involves hitting of high power laser beam on to the surface to be inspected and ablation of the superficial layer of the surface, one of the objections from the conventional oil and gas industry to LUI system is apprehension of potential damage or adverse effect on the surface property and integrity. This possible problem is seen as severe on pressurized pipes to the extent of being dangerous to the system. This work attempts to present a qualification methodology which was experimentally applied to known metallic samples and with standard LUI testing conditions. Binary image processing techniques have been used with hybrid filtering and statistical measurements to quantify the findings. A relational approach is used within the digital image processing domain to qualify the extent of metal loss that occurs during a prolonged exposure of the clean metallic surface to the laser beam. A comparative study has been done on Duplex steel (UNS S31803) and Carbon steel. Samples were studied for relative metal loss, possible changes in hardness and micro-structural anomalies (if any). A new qualification method has been developed using digital image processing to approximate the damage sustained by the exposure. The results are quite promising and alleviate many doubts that conventional NDT experts may have in connection with the LUI system, especially in the oil and gas scenarios.

Self-assembling of Z-α-pyridylcinnamic acid molecules over polycrystalline Ag and Au surfaces followed by FT-IR and atomic force microscopies

Self-assembling layers of Z-2(3′-pyridyl)-3-phenyl propenoic acid molecules were built on silver and gold films prepared by pulsed laser deposition. The dip-coating technique was used for depositing the organic material onto the metals. The clean metal surfaces were studied by scanning electron microscopy, and the organic structures formed over the metal surfaces were investigated by FT-IR microspectroscopy and atomic force microscopy. Various surface structures were identified and their rationalisation was attempted.

Simple model explaining and predicting coverage-dependent atomic adsorption energies on transition metal surfaces

The adsorption energies of simple atomic adsorbates are coverage dependent. We use density-functional theory to show that the coverage dependence is due to an adsorbate-induced modification of the surface $d$-band structure. We developed a simple model for predicting the $d$-band widths of clean and adsorbate-covered metallic surfaces using a tight-binding formalism. The new model can be used in conjunction with the $d$-band adsorption model to estimate adsorption energies as a function of coverage.

Interfacial ultrafine-grained structures on aluminum alloy 6061 joint and copper alloy 110 joint fabricated by magnetic pulse welding

Magnetic pulse welding is a solid state impact welding process, similar to explosive welding, which produces metallurgical bond by oblique high-speed impact between two metal bodies. This violent impact removes the metal surface oxide layers and then joins the two atomic level clean metal surfaces together by the incidental compression pressure. The impact velocity is at 200–400 m/s and the being welded metal surface undergoes severe plastic deformation with strain rate in the order of 106–107 s−1. The ultrafine-grained structure was observed on the welded interface. This article studied two types of similar material lap joint interfaces and the base metals were aluminum alloy 6061 and copper alloy 110. Nano-indentation testing shows that the welded interfaces have significantly greater hardness than the base metals. The interface microstructure was studied by optical microscopy, electron backscatter diffraction microscopy, and transmission electron microscopy. The welded aluminum alloy 6061 interface exhibits extremely fine grains and an extremely high dislocation density. The impact welded copper alloy 110 interface presents nano-scale lamellar band structure and deformation twins. The interface hardness increasing was attributed to this impact-induced microstructural refinement.