Surface-sensitive Analytical Techniques Research Articles

Surface interactions and tribochemistry in boundary lubrication of hypereutectic aluminium—silicon alloys

The tribofilms formed on a hypereutectic Al—Si alloy after sliding against AISI 52100 steel pins in a reciprocating pin-on-plate tribometer with polyalpha olefin (PAO) base oil containing zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been studied. Surface-sensitive analytical techniques (X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), Raman spectroscopy, and nanoindentation) have been used for tribofilm chemical and mechanical characterization. The thickness of the tribofilms has been evaluated using depth profiling and focus ion beam (FIB) milling. This is the only reported work to analyse this kind of tribofilm on Al—Si alloys in this detail. From the analysis obtained from miniSIMS, differences in the composition of the top layer of the tribofilms formed from ZDDP and ZDDP+MoDTC lubricants have been observed. When using only ZDDP, this top layer is rich in Zn and Fe; with the addition of MoDTC the top layer contains phosphates, probably from the decomposition of the ZDDP, and MoS2. The effect of adding MoDTC can also be observed in the overall tribofilm thickness and coverage of the samples.

Quantitative static Time-of-Flight Secondary Ion Mass Spectrometry analysis of anionic minority species in microelectronic substrates

Reliability and yield of nano-electronic devices can be seriously affected by the presence of surface contamination, even at low concentration. The microelectronics industry is, thus, in need for a quantitative, highly sensitive surface analysis technique capable of detecting both elementary and molecular species present at the surface. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) provides a submicronic lateral resolution and excellent sensitivity with high secondary ion yields on a broad mass range but, nevertheless, remains a qualitative technique. To convert normalized ion intensities into concentrations and, thus, to provide reliable quantification, the so-called relative sensitivity factors (RSFs) need to be determined. In earlier studies, ToF-SIMS RSFs for trace metals were determined from the calibration of ToF-SIMS positive ion intensities against quantitative analysis techniques able to determine a surface coverage such as vapour phase decomposition inductively coupled plasma mass spectrometry (VPD-ICP-MS) or total reflection X-ray fluorescence (TXRF) results. Here, the aim is to quantify elementary anionic minority species (S, Cl, P, Br). Deliberately contaminated samples were prepared and analyzed with ToF-SIMS and several quantitative surface analytical techniques like TXRF, liquid phase extraction ionic chromatography (LPE-IC) or VPD-ICP-MS. None of these latter techniques can by itself successfully handle all the anionic species cases and ToF-SIMS turns out to be the more versatile and precise characterization technique in this context.

Tribological performance of an Al—Si alloy lubricated in the boundary regime with zinc dialkyldithiophosphate and molybdenum dithiocarbamate additives

The tribological performance of the cast iron/Al—Si alloy system lubricated in the boundary regime, using lubricants containing zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been investigated. The results obtained are discussed in relation to the performance of these additives in cast iron/steel (ferrous)-based material systems. Tribological tests were conducted on a Plint reciprocating test machine (TE 77) in the contact conditions comparable to the conditions in the piston ring/cylinder liner system of the internal combustion engine. Surface sensitive analytical techniques such as environmental scanning electron microscopy and energy dispersive X-ray analysis have been used to determine the nature of the wear occurring as well as the chemical nature of the tribofilms formed on the Al—Si material. It has been shown that the MoDTC additive, added into a ZDDP-containing lubricant, improves the effectiveness of friction reduction in a completely ferrous system and a cast iron/Al—Si system. In terms of wear, an increase in MoDTC concentration (from 0.22 to 1.2 wt%) in the ZDDP-containing lubricant resulted in an antagonistic effect on the anti-wear performance of ferrous materials but improved the effect on Al—Si alloys. The tribofilm formation and wear mechanisms are discussed in relation to recent literature and it is shown that additives strongly affect both the processes.

Characterization of Polymers and Corrosion Products by Raman Spectroscopy

The Raman spectroscopy is a nondestructive lateral surface sensitive analytical technique aimed at characterizing newly developed coatings and examining the quality of corrosion resistance of surface modifications. Due to the small spatial detection volume of 1 μm3, the chemical microstructure of the surface and organic or inorganic layers can be identified. The crystallinity of coatings, the degree of molecular order, orientation and cross-linking, measurable by the stereo selectivity of the Raman spectroscopy was applied for the classification of Polyethylenetherephtalate (PET) coatings. For determining multilayer systems, the confocal measuring technology of the Raman microscope has been performed for depth profiling through transparent PET layers. Furthermore, this confocal measuring technique provided the best results for the local identification of corrosion products by monitoring the corrosion under transparent organic corrosion protection layers. Selected samples displayed the facilities of Raman spectroscopy to characterize the formation of organic surface coatings and inorganic contaminations by lateral and confocal detection properties. Therefore, Raman spectroscopy received special interest in coating and steel applications to analyse the degradation, processing history or interlacing of coatings and to reduce production sources of error.

Organic Chemistry on Solid Surfaces

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ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio

Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 °C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 °C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high- and low-friction species in the tribofilm.

Surface lignin and extractives on hardwood RDH kraft pulp chemically characterized by ToF-SIMS

Abstract The chemical characteristics of lignin and extractives on the outermost surface of hardwood rapid displacement heating (RDH) kraft pulp were studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Changes in these surface compounds induced by totally chlorine-free (TCF) bleaching were also revealed. The results suggest that the surface lignin remaining on unbleached RDH pulp was remnant from native wood lignin, which was never released from wood fibers, but had undergone structural changes during kraft cooking. Less than half of the surface lignin was removed during bleaching, with the oxygen stage and the final peroxide stage being the most effective. The main wood extractives on the hardwood RDH kraft pulp surface were sterols and steryl esters. TCF bleaching strongly reduced their abundance. Bleaching also removed fatty acids, but less effectively than neutral extractives. Fatty acid sodium salts were partly removed during TCF bleaching, in contrast to fatty acid calcium salts, which were very stable. ToF-SIMS has the unique capability to give valuable chemical information about the complex mixture of compounds on the pulp surfaces. Thus, it complements other surface-sensitive analytical techniques such as electron spectroscopy for chemical analysis (ESCA), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). ToF-SIMS can give detailed chemical information about the effects of process conditions on pulp surface characteristics.

Surface analyses of In–V oxide films aged electrochemically by Li insertion reactions

The surface of a Li-insertion In–V mixed-oxide electrode and the evolution of its composition and morphology upon prolonged intercalation/deintercalation cycles have been studied by a combination of the following bulk and surface-sensitive analytical techniques: Electrochemical analysis (cyclic voltammetry, impedance spectroscopy), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning photoelectron spectromicroscopy (SPEM). The In–V oxide film, obtained by rf sputtering as a thin film on conductive glass for use as a transparent electrode in electrochromic windows, is a Li-insertion material with good reversibility and high charge capacity, having a surface morphology characterised by a low initial roughness. XPS results showed that Li+-insertion brought about the reduction of V5+ to V4+ and V3+, with the onset of new structures in the valence band, related to the formation of Li2CO3 on the electrode surface. Such chemical changes were largely reversed upon Li-deinsertion and only a barely detectable effect on the surface morphology was seen after up to ten cycles. Extensive Li-charge–discharge cycling induced larger changes in the electrode morphology (increase in grain size and roughness), in the surface composition and microscopic aspect, with a partial passivation effect on the film electrochemical behaviour. The formation of surface deposits with a characteristic, elongated shape has been revealed to occur after 500 Li charge–discharge cycles. Submicron chemical analysis of such deposits onto aged oxide electrodes has been performed by SPEM, which enabled the location and assignment of the surface composition.

Detection of Interfacial Structures of Poly(ethylene glycol), Poly(propylene glycol) and Their Copolymers at Liquid/Solid Interfaces Using Sum Frequency Generation Vibrational Spectroscopy

ABSTRACTWe have applied sum frequency generation (SFG) vibrational spectroscopy, a submonolayer surface sensitive analytical technique, to study interfacial structures of poly (ethylene glycol) (PEG), poly (propylene glycol) (PPG), their solutions, their copolymers, and copolymer solutions while contacting different media including air, fused silica, and various polymers.For the first time, conformations of polyether materials at various interfaces have been shown at the molecular level in situ. Depending on the hydrophobicity of the solid contacting media, the liquid polymers PEG and PPG show different conformations at different interfaces, which can be correlated to molecular interactions at these interfaces. The favorable interaction between hydrophobic media and the hydrophobic segments, methylene or methyl groups, of polyethers causes an ordered conformation with these groups standing up at the interface. The unfavorable interaction between hydrophilic media and hydrophobic segments of the polyethers induces interfacial methylene or methyl groups to have a random structure or to lie down at the interfaces, indicated by the weakening or even absence of SFG signals. For comparison, interfaces between aqueous PEG or PPG solutions and air, polystyrene (PS), poly(methyl methacrylate) (PMMA), and fused silica have also been investigated. In addition, we have studied conformations of pluronics at various interfaces, showing that different blocks have different interface activities. The interfaces are always dominated by the PPG segments. The studies on molecular level interfacial structures of PEG, PPG and pluronics will help us to understand and control the interfacial behavior of liquids at interfaces.

Speciation of surface gold in pressure oxidized carbonaceous gold ores by TOF-SIMS and TOF-LIMS

To the best of our knowledge, this is the first attempt ever to speciate gold preg-robbed by carbonaceous matter using a surface sensitive microbeam technique. This approach enables the direct determination of gold species sorbed on carbonaceous particulates thus providing a new tool in understanding the chemistry of gold sorption on carbon. The reasoning behind this effort was to study the detrimental effect chloride ions have on gold recovery by pressure oxidation of carbonaceous sulfide ores, a technology largely used by the mining industry. The characterization of the sorbed gold species involved three surface sensitive microbeam analytical techniques (TOF-SIMS, TOF-LIMS and XPS) providing confirmatory results for better accuracy. Optimum conditions for detection of gold compounds with minimum fragmentation by TOF-SIMS and TOF-LIMS mass spectrometers have been determined. A reference library of 16 major gold complexes with halogen, thiosulfate, cyanide and thiocyanate groups relevant to the gold recovery processes has been established. The most suitable of the microbeam techniques tested was found to be negative (−ve) ion TOF-LIMS, offering best sensitivity and a small analytical spot size.

Comparative investigation of the surface properties of commercial titanium dental implants. Part I: chemical composition.

The surfaces of five commercially available titanium implants (Brånemark Nobel Biocare, 3i ICE, 3i OSSEOTITE, ITI-TPS, and ITI-SLA) were compared by scanning electron microscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy. All five implant types were screw-shaped and fabricated from commercially pure (cp) titanium, but their surface properties differed both as regards surface morphology and surface chemical composition. The macro- and microstructure of the implant surfaces were investigated by scanning electron microscopy. The surfaces chemical composition was determined using the surface-sensitive analytical techniques of X-ray photoelectron spectroscopy and time-of-flight secondary ion spectrometry. Surface topographies were found to reflect the type of mechanical/chemical fabrication procedures applied by the manufacturers. The titanium oxide (passive) layer thickness was similar (5-6 nm) and typical for oxide films grown at or near room temperature. A variety of elements and chemical compounds not related to the metal composition were found on some implant types. They ranged from inorganic material such as sodium chloride to specific organic compounds believed to be due to contamination during fabrication or storage. The experimental findings are believed to make a contribution to a better understanding of the interplay between industrial fabrication procedure and physico-chemical implant surface properties.

Studies of thin film growth and oxidation processes for conductive Ti–Al diffusion barrier layers via in situ surface sensitive analytical techniques

Conducting diffusion barrier layers play a critical role in high-density memory integration. We recently demonstrated that Ti–Al can be used as a diffusion barrier layer for the integration of ferroelectric capacitors with complementary metal–oxide semiconductor devices for the fabrication of nonvolatile ferroelectric random access memories (NVFRAMs). Here, we discuss results from systematic studies designed to understand Ti–Al film growth and oxidation processes using in situ ion beam sputter deposition in conjunction with complementary in situ atomic layer-resolution mass spectroscopy of recoil ion (MSRI) and surface sensitive x-ray photoelectron spectroscopy (XPS). The concurrent MSRI/XPS analysis revealed that amorphous Ti–Al layers produced by tailored sputter-deposition methods are resistant to oxidation to at least 600 °C, and that oxidation occurs only when the a-Ti–Al layers are exposed to oxygen at >600 °C, via the segregation of Ti species to the surface and TiO2 formation. The a-Ti–Al layers discussed in this letter could be used in the double functionality of a bottom electrode/diffusion barrier for the integration of ferroelectric capacitors with Si substrates for the fabrication of NVFRAMs and other devices.

An investigation into the early stages of oxide growth on gallium nitride

The early stages of thermal oxidation of gallium nitride epilayers in dry O 2 have been studied using surface sensitive analytical techniques and transmission electron microscopy. Deconvolution of the Ga 3d core level spectra from X-ray photoelectron spectroscopy revealed peak positions representing gallium nitride, gallium oxide, and an intermediate phase. Transmission electron microscopy revealed an overlayer approximately 1.5–3.0 nm thick with registry to the (0001) GaN after dry oxidation at 800°C for 1 h. Based on the data from X-ray photoelectron spectroscopy, this layer is believed to be a Ga ( x+2) N 3 x O (3−3 x) compound. Atomic force microscopy and transmission electron microscopy revealed the formation of discrete oxide crystallites on top of the oxynitride layer. The crystallites become more numerous and grow with continued oxidation.

Low-energy grazing-angle argon-ion irradiation of silicon: A viable option for cleaning?

In recent publications, it has been suggested that atomically clean, flat, crystalline silicon surfaces can be obtained by low-energy (0.1–1 keV) oblique-angle (⩾45° off-normal) argon-ion bombardment at mildly elevated target temperatures (∼500 °C). Here, this procedure has been applied to a multiple boron delta-doped Si structure. It leads to a massive relocation of subsurface doping atoms because of the accompanying injection of point defects into the bulk. This greatly affects the usefulness of the proposed cleaning method and shows that it is hazardous to base claims of quality solely on results obtained with surface-sensitive (∼1 nm) analytical techniques.

Microstructural characterization and analysis of inclusions in C-Mn steel and weld metals

Ferritic steels used for the construction of welded pressure vessels may contain trace concentrations of impurity elements that can influence their overall mechanical properties. Often, the C-Mn ferritic steels and weld metals used for welded nuclear pressure vessels contain trace concentrations of boron (<10 ppm), and the role of this impurity element could be significantly different depending upon whether it is present as the free atomic species or incorporated into specific microstructural features, such as inclusions or precipitates, of these materials. In this article, the results of work designed to characterize the microstructure of C-Mn steels and weld metals used for the construction of Magnox nuclear pressure vessels are described. In particular, the type, size, distribution, and chemical composition of inclusions present are considered. A range of techniques are used to characterize the microstructure, but, in particular, two surface sensitive analytical techniques, namely, Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), are used to detect and quantitatively analyze impurity boron. The results are discussed with respect to the relationship of the boron to the stable silicate inclusions and the potential influence this may have on mechanical properties of these materials.

Surface structure of human mucin using X-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) is a surface sensitive analytical technique that measures the binding energy of electrons in atoms and molecules on the surface of a material. XPS was used to determine the distribution of the oligosaccharide side chains in the glycoprotein, MUC1 mucin. Low-resolution XPS spectra provided elemental composition of MUC1 mucin (fully glycosylated), mucin polypeptide (nonglycosylated), and carbohydrates found in mucin. The nitrogen content of MUC1 mucin was determined to be intermediate between the mucin polypeptide and the carbohydrates. Assuming a uniform distribution of carbohydrate on MUC1 mucin, the average thickness of the carbohydrate layer was calculated to be 4.9 nm using the low-resolution N 1s signals. High-resolution XPS spectra give detailed information about the chemical bonding of the surface molecules. Calculations based on the high-resolution O 1s spectra showed a carbohydrate thickness of 6.6 nm. These experimentally determined values agree reasonably well with an estimated 5 nm of carbohydrate thickness from a simple model which assume that the core protein is a rodlike molecule approximately 5 nm in diameter. Although the carbohydrate coating on the MUC1 mucin appears to be thick enough to cover the core protein entirely, fully glycosylated breast milk MUC1 mucin is susceptible to proteolytic digestion without removal of any oligosaccharide side chain, suggesting areas of exposed core protein. A possible explanation is that the oligosaccharide side chains may form patches of carbohydrate along the core protein with regions of exposed core protein.

Environmental Attack on Adhesive Joints. Part II: Locus of Failure.

The environmental failure of double-torsion(DT) mild steel adhesive joints was investigated with particular emphasis on the behaviour of the metal oxide/adhesive interface. The joints had previously been subjected to static loading in moist conditions using specifically built apparatus. Modern surface-sensitive analytical techniques, such as scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS), were employed to examine the resulting failure surfaces and to assess the effect of the environment.

A review of recent APS experimental results on the electron scattering investigation

Appearance potential spectroscopy (APS) was introduced in the early seventies as a relatively simple surface sensitive analytical technique. Later, various processes involved in APS signal formation have been investigated, making use of sophisticated experimental set-ups. In this way, APS became a possible tool for studying electron transport processes in the surface region of solids. In the experimental part of the submitted work that represents a considerable extension of the previously obtained results, ultrathin layers were deposited by in situ evaporation on polycrystalline substrates. Ti, Cr, Mn and Ni were chosen as substrates and deposits in a series of overlayer experiments. The L 3, L 2 and L 1 (if possible) subshell signal intensities have been measured as a function of the overlayer thickness. Auger spectra have also been taken to monitor the cleanliness of the substrates and the overlayers. The discrete model of the APS signal formation according to Eckertová, instead of the common exponential model, offered the possibility of fitting the experimental results. Its parameters represent the probabilities of the basic types of electron interactions related to one atomic layer. Approximate values of these quantities have been calculated.

Adhesion and Surface Analysis

Abstract In the last 25 years, surface sensitive analytical techniques have made a major contribution to our understanding of adhesion phenomena and problems. There are several areas where these techniques have provided important information including the identification of failure modes, the chemistry of a substrate before and after pretreatments, the stability of surfaces and interfaces, the identification of surface contaminants, the interaction across an interface and the nature of interphases. X-ray photoelectron spectroscopy (XPS or ESCA), Auger electron spectroscopy (AES) and static secondary ion mass spectrometry (SSIMS) have proved to be especially useful. Many examples of the usefulness of these techniques are given.

A Study of the Thermal Reactions of Methyl Iodide Coadsorbed with Hydrogen on Ni(111) Surfaces: Hydrogenation of Methyl Species to Methane

Chemistry on solid surfaces is central to many areas of practical interest such as heterogeneous catalysis, tribology, electrochemistry, and materials processing. With the development of many surface-sensitive analytical techniques in the past decades, great advances have been possible in our understanding of such surface chemistry at the molecular level. Earlier studies with model systems, single crystals in particular, have provided rich information about the adsorption and reaction kinetics of simple inorganic molecules. More recently, the same approach has been expanded to the study of the surface chemistry of relatively complex organic molecules, in large measure in connection with the selective synthesis of fine chemicals and pharmaceuticals. In this report, the chemical reactions of organic molecules and fragments on solid surfaces, mainly on single crystals of metals but also on crystals of metal oxides, carbides, nitrides, phosphides, sulfides and semiconductors as well as on more complex models such as bimetallics, alloys, and supported particles, are reviewed. A scheme borrowed from the organometallic and organic chemistry literature is followed in which key examples of representative reactions are cited first, and general reactivity trends in terms of both the reactants and the nature of the surface are then identified to highlight important mechanistic details. An attempt has been made to emphasize recent advances, but key earlier examples are cited as needed. Finally, correlations between surface and organometallic and organic chemistry, the relevance of surface reactions to applied catalysis and materials functionalization, and some promising future directions in this area are briefly discussed.