HREM image contrast from supported small metal particles

HREM, image calculations and small probe diffraction/AEM have been used to characterize structure and contrast of supported small metal particles of ≤5 nm diameter. Such small particles are thought to be active species in industrial applications such as in heterogeneous catalysis where, in general, the particles employed as catalysts are supported. Image calculations (HREM and diffraction contrast) carried out at both 200 keV and 400 keV at various defoci and support thicknesses have shown that in HREM, particle images are obscured by the support contrast with the loss of edge definition and particles appear to be smaller than they actually are. The particle visibility is better at 400 keV. The calculations have also indicated that particle shape varies as a function of support thickness and defocus. The results have clear implications for identification and interpretation of surface structure of the supported small particles accurately by HREM if not performed under controlled conditions and for determining their size and shape.     

HREM image simulations for small particle catalysts on crystalline supports

The imaging conditions for electron microscope studies of supported ultrafine particle catalysts have been investigated by multislice simulations. Images of Pt and ReO₄ particles ranging from 0·4 to 2·3 nm in size were simulated in both plan view and profile view with a rutile (TiO₂) support. It was shown that particle visibility varied greatly with the objective lens defocus. Optimum defocus was not favourable for supported particles in plan view since the ultrafine supported particles were least visible at this defocus. Underfocusing, especially at defoci corresponding to half-spacing fringes in the TiO₂ support, led to improved visibility and resolution of the supported particles. Although the structure and shape of supported ultrafine particles should be resolved better with a 400-kV high-resolution electron microscope, their detectability is poorer than with a 200-kV instrument. An ReO₄ cluster should be detectable at 200 kV on TiO₂ supports up to 5 nm in thickness, whereas it is only likely to be detectable at 400 kV on supports up to 3 nm in thickness. The simulations confirmed that optimum defocus is most favourable for imaging supported particles in profile view. Atomic information for particles as small as a 13-atom Pt cuboctahedral cluster should be resolvable with a 400-kV instrument. The crystalline Ti monolayer observed on surfaces of Pt particles, which could explain the mechanism known as SMSI, was simulated as an example of profile imaging.     

Surface plasmon excitation for supported metal particles

Classical dielectric theory has been used to study the effects of a support on the surface plasmon excitation frequencies for small metal particles. Two experimentally accessible cases have been considered and the theoretical results have been compared with observations. For small spherical Al particles, coated with a thin oxide layer and half-buried in an infinite support of Al, four surface plasmon frequencies (ω₁ to ω₄) are predicted. Comparison with results for isolated particles and planar interfaces suggests the identification of ω₃ as the normal small particle surface plasmon; ω₁ and ω₂ are generated by the presence of the support and ω₄ is related to the oxide shell. The ω₂ and ω₃ frequencies are observed in accord with the predictions. For the second case considered, the Al support is replaced by a dielectric, AlF₃. The support then contributes a new peak at about 5 eV which has been observed. The observations were made with the EELS attachment to a conventional analytical electron microscope.     

A study of the formative mechanism of spherical wear particles

In this paper, spherical wear particles which are produced by sliding wear processes are carefully studied by means of a scanning electron microscope and an energy-dispersive X-ray analyser etc.Much direct and indirect evidence from the examination demonstrated spherical particles to be a product of the local wear surface which is melted, i.e. droplets of the melting metal solidify into spherical particles.     

Revealing local variations in nanoparticle size distributions in supported catalysts: a generic TEM specimen preparation method

The specimen preparation method is crucial for how much information can be gained from transmission electron microscopy (TEM) studies of supported nanoparticle catalysts. The aim of this work is to develop a method that allows for observation of size and location of nanoparticles deposited on a porous oxide support material. A bimetallic Pt‐Pd/Al₂O₃ catalyst in powder form was embedded in acrylic resin and lift‐out specimens were extracted using combined focused ion beam/scanning electron microscopy (FIB/SEM). These specimens allow for a cross‐section view across individual oxide support particles, including the unaltered near surface region of these particles. A site‐dependent size distribution of Pt‐Pd nanoparticles was revealed along the radial direction of the support particles by scanning transmission electron microscopy (STEM) imaging. The developed specimen preparation method enables obtaining information about the spatial distribution of nanoparticles in complex support structures which commonly is a challenge in heterogeneous catalysis.     

Three‐dimensional shapes and spatial distributions of Pt and PtCr catalyst nanoparticles on carbon black

High‐angle annular dark‐field scanning transmission electron microscopy tomography is applied to the study of Pt and PtCr nanoparticles supported on carbon black, which are used as heterogeneous catalysts in the electrodes of proton exchange membrane fuel cells. By using electron tomography, the three‐dimensional architecture of the heterogeneous catalyst system can be determined, providing high‐spatial‐resolution information about the shapes, faceting and crystallographies of 5–20 nm single and multiply twinned catalyst particles, as well as their positions with respect to the carbon support. Approaches that can be used to provide improved information about the distribution and orientation of the particles on their support are proposed and discussed. Our results show that electron tomography provides important information that is complementary to high‐resolution lattice imaging. Both techniques are required to understand fully the nature and role of the surfaces of faceted catalyst particles.     

Supported metal model catalyst surfaces examined by scanning tunnelling microscopy

Topographic and/or barrier-height images of ultrafine Pt and Au metal particles supported on a vacuum-deposited carbon film or titanium oxide thin films grown on titanium metal sheets were obtained. The topographic images of colloidal Au particles (5-nm diameter) adsorbed on a titanium oxide thin film showed a structure elongated in the direction normal to the x scan, indicating their weak interaction with the support surface. The topographic images of Pt vacuum-deposited on a carbon film showed c. 4-nm diameter particles, larger than those observed in electron microscopy. The problems inherent to the STM observation of such dispersed metal systems are identified. In the case of Pt particles vacuum-deposited on titanium oxide film, its barrier-height image gave better indication of different phases on the surface than its topographic image. The significance of obtaining barrier-height images along with topographic images for such sample systems is demonstrated.     

Analysis of networks based on styrene and divinylbenzene containing iron anchored using variable pressure scanning electron microscopy

There is great demand for the development of composite materials containing small metal or metal oxides particles, owing to their variable properties and wide application. However, microscopic evaluation of these materials using high‐vacuum scanning electron microscopy is difficult because the samples must undergo a series of preparation steps to reach a high image quality and to avoid becoming shrunk inside the microscope vacuum chamber. Thus, in this study, we used variable pressure scanning electron microscopy to evaluate the morphology and iron distribution on the surface of magnetic microspheres based on poly(styrene‐co‐divinylbenzene). These materials were obtained by suspension copolymerization of styrene and divinylbenzene in the presence of fine iron particles. Energy‐dispersive X‐rays were also used to analyse distribution of the iron particles. The results indicate that, under the conditions used, magnetic microspheres with a relatively narrow size distribution were formed. Moreover, the micrographs show that agglomerated iron particles appeared only on the microsphere surface.     

A specimen preparation technique for transmission electron microscopy of surface layers

A TEM specimen preparation method is described, with the aid of which electron transparent foils can be obtained across the external surface of a specimen. After careful pre-treatment, steel specimens have been electrolytically coated with nickel. Conventional thinning in a plane cutting the substrate-coating interface, gave thin foils displaying the internal structure as a function of depth under the initial free surface. The method has also been applied to minute metal particles, of dimensions too small to allow manipulating and foil preparation by conventional methods. Image examples are shown, and the applicability of the method is discussed.     

Image analysis of macroporous polystyrene sorbents using electron spectroscopic imaging to enhance mass contrast. The architectural relationship between polymer porosity and absorption capacity

We describe the microscopical analysis of two polystyrene divinylbenzene copolymers (PS-DVB) made using 20 and 55% (v/v) DVB cross-linker respectively, in which the hydrated structure of each polymeric material solvated in water is preserved by employing very rapid freezing and low-temperature freeze-drying techniques. Using the zero-loss mode of electron spectroscopic imaging to enhance the contrast of unstained polymer sections, we introduce some novel methods for the application of computer-assisted image analysis for routine examination of polymer pore structure. We also relate these parameters to the ability of the two polymers to absorb nanometre-sized particles—neutral colloidal gold particles and cationized ferritin. Both methods will be invaluable in the structural analysis of porous polymers, but particularly for comparative analyses of macroporous sorbents.     

Extremely thin layer plastification for focused‐ion beam scanning electron microscopy: an improved method to study cell surfaces and organelles of cultured cells

Since the recent boost in the usage of electron microscopy in life‐science research, there is a great need for new methods. Recently minimal resin embedding methods have been successfully introduced in the sample preparation for focused‐ion beam scanning electron microscopy (FIB‐SEM). In these methods several possibilities are given to remove as much resin as possible from the surface of cultured cells or multicellular organisms. Here we introduce an alternative way in the minimal resin embedding method to remove excess of resin from two widely different cell types by the use of Mascotte filter paper. Our goal in correlative light and electron microscopic studies of immunogold‐labelled breast cancer SKBR3 cells was to visualise gold‐labelled HER2 plasma membrane proteins as well as the intracellular structures of flat and round cells. We found a significant difference (p < 0.001) in the number of gold particles of selected cells per 0.6 m² cell surface: on average a flat cell contained 2.46 ± 1.98 gold particles, and a round cell 5.66 ± 2.92 gold particles. Moreover, there was a clear difference in the subcellular organisation of these two cells. The round SKBR3 cell contained many organelles, such as mitochondria, Golgi and endoplasmic reticulum, when compared with flat SKBR3 cells. Our next goal was to visualise crosswall associated organelles, septal pore caps, of Rhizoctonia solani fungal cells by the combined use of a heavy metal staining and our extremely thin layer plastification (ETLP) method. At low magnifications this resulted into easily finding septa which appeared as bright crosswalls in the back‐scattered electron mode in the scanning electron microscope. Then, a septum was selected for FIB‐SEM. Cross‐sectioned views clearly revealed the perforate septal pore cap of R. solani next to other structures, such as mitochondria, endoplasmic reticulum, lipid bodies, dolipore septum, and the pore channel. As the ETLP method was applied on two widely different cell types, the use of the ETLP method will be beneficial to correlative studies of other cell model systems and multicellular organisms.     

Imaging colloidal gold labels in LVSEM

On biological samples, the topographic imaging capabilities of the new generation of scanning electron microscopes (SEM) (those having both field-emission guns and low aberration lenses) rival those of the replica techniques. In addition, they permit the localization of specific molecules on the sample surface using one of several labeling techniques utilizing heavy metal colloids. Normally, colloidal gold can be detected in the SEM both by the secondary electron signal (shape) and by the backscattered electron signal (BSE, Z-contrast). The new instruments seem to produce their best topographic images using low-beam voltage (1–5 kV) where topographic contrast is higher and the required thickness of the metal coating is less (Haggis and Pawley 1988, Ris and Pawley 1988). Although the detection of backscattered electrons is more difficult at low-beam voltage, we are able to show here that the secondary electron (SE) signal produced with a 2–5-kV beam permits the unambiguous detection of gold particles as small as 5 nm on carbon-coated specimens while a 1-kV beam produces a high-quality topographic image of the same sample.     

Towards sub-A atomic resolution electron diffraction imaging of metallic nanoclusters: a simulation study of experimental parameters and reconstruction algorithms

A simulation study is carried out to elucidate the effects of dynamical scattering, electron beam convergence angle and detection noise on atomic resolution diffraction imaging of small particles and to develop effective reconstruction procedures. Au nanoclusters are used as model because of their strong scattering. The results show that the dynamical effects of electron diffraction place a limit on the size of Au nanoclusters that can be reconstructed from the diffraction intensities with sufficient accuracy. For smaller Au nanoclusters, the simulations show that diffraction patterns recorded under the experimental conditions can be reconstructed using a combination of phase retrieval algorithms. The use of a low-resolution image is shown to be effective for reconstructing diffraction patterns without the central beam. A new algorithm for estimating the object support is proposed.     

Gradation of oxidational wear of metals

This paper considers mild-oxidational wear of metals by studying their behavior under friction with different loads. Low carbon, steel and copper are chosen as the model materials. We show that tribo-oxidation and the structure of surface layers of materials, both formed in the process of plastic deformation during friction, provide the boundary conditions of mild and severe wear. Oxidational wear is predominant when structural changes are minimal. As the load increases, oxidational wear is at first accompanied by metallic wear and afterwards the oxidational wear accompanies the metallic wear. The structure of the metal surface layers changes gradually during these processes, so that the strengthening of the metal is high enough to withstand friction forces. When the magnitude of frictional forces becomes higher than the maximal strength of the plastically deformed metal, the transition to severe wear occurs.The composition of different types of oxides and the fineness of wear particles varies with the friction conditions. Under light load friction conditions, fine wear particles are formed. These particles contain oxides of high oxygen content. As the friction conditions become tougher, in particular when the load increases, large-sized wear particles are formed. These particles contain oxides of a higher metal content. Phase composition and fineness of wear particles are used for gradation of mild wear.Analyses of phase composition of oxides and estimation of the fineness of wear particles are suggested as a method of wear character diagnostics. The electron diffraction method of the study of wear particles is used for this analysis in order to evaluate and choose appropriate friction and wear conditions.     

Chemical factors in carbon brush wear

Chemical factors involved in the wear of carbon brushes on metal slip rings are discussed. Under conditions of elevated temperature operation in an oxidizing atmosphere, a correlation is found between the oxidation characteristics and the wear rates of carbon brushes. It is demonstrated that temperatures in the oxidation range can readily be generated by current converging through high resistance constrictions in the contact zone. Copper and other metals present in the slip ring can act as catalysts for the oxidation, the metal oxide particles migrating into the interior of the brush and causing enhanced gasification rates and increased porosity. Treatment of the brush with oxidation inhibitors, such as phosphorus oxychloride, and surface alloying of the ring with non-catalytic materials, such as zinc, can reduce the wear rate at elevated temperatures under favorable conditions. Graphite surfaces are also attacked by reactive gaseous species such as atomic nitrogen and oxygen, ozone and nitrogen oxides which may form near the brush surface during arcing.     

MRT Letter: High resolution SEM imaging of nano‐architecture of cured urea‐formaldehyde resin using plasma coating of osmium

Nanoarchitecture of cured urea‐formaldehyde (UF) resins was examined with a field‐emission scanning electron microscope (FE‐SEM) after coating samples with osmium, which is considered to produce particles of considerably smaller size compared to other metal coatings used in SEM studies. This method enabled comparison of the nanoarchitecture of UF resins of low (1.0) and high (1.6) formaldehyde/urea (F/U) mole ratios to be made, based on imaging of extremely small size particles as part of UF resin architecture, not described before. Imaging revealed presence of relatively large globular particles (148.084–703.983 nm size range) as well as smaller substructures (28.004–39.604 nm size range) as part of the architecture of 1.0‐mole UF resin. Globular particles were also present in 1.6 mole UF resin, but of considerably smaller size (14.760–50.269 nm). The work presented demonstrates usefulness of osmium coating in unraveling the intricacies of the nanostructural organization of cured UF resins, prompting wider application of this immensely useful but grossly underutilized metal coating type in high resolution SEM examination of biological and materials samples. Microsc. Res. Tech. 76:1108–1111, 2013. © 2013 Wiley Periodicals, Inc.     

Material transport in metallic surface layers during mechanical treatment in specific environments

The changes in the structure and chemical composition of the subsurface layer of a metal which occur during a mechanochemical attack, especially in a liquid environment, have been investigated. The environment was an aqueous solution of metal salts. The treatment consisted of impacting small non-metallic particles against a sample surface. The impacting particles were activated by vibrators. A tracer method was used to determine penetration of the electrolyte into the metal samples exposed to the treatment. The penetration profiles which depend on the treatment parameters and the sample potential are discussed.     

磨料粒度对表面微织构纯钛干摩擦性能的影响

为了提高钛及钛合金钻具在超深钻探、深海钻探和外太空钻探工程中的减摩抗磨性能。利用激光表面加工技术在工业纯钛（TA2）表面制备了不同参数的点阵微织构。采用MS-T3000摩擦磨损试验机测试了微织构钛合金在不同粒度模拟月壤作用下的摩擦学性能。利用扫描电子显微镜和能谱分析仪分析磨痕形貌及元素含量。研究结果表明：当磨料粒度小于微织构点阵的直径时,磨料压入微织构点阵里,磨料具有滚动和滑动两种运动方式。当粒度大于微织构点阵的直径时,磨料不能完全压入微织构点阵里,磨料对微织构TA2表面产生了滑动犁削作用。由于两种磨料磨损的作用机理不同,同等条件下,小粒度的磨料作用下的微织构TA2的摩擦因数和磨损率较大粒度磨粒作用下的最大减少量分别为50%和53%。考虑磨料粒度与微结构的匹配性,可以大大降低摩擦减少磨损。     

Study of particle erosion damage in Haynes Stellite 6B I: Scanning electron microscopy of eroded surfaces

The erosion behavior of metals and alloys by solid particles entrained in relatively slow moving gases is of current interest as a result of ongoing efforts in coal conversion and the consequent production of dust-laden gases. Haynes Stellite 6B represents a typical alloy used for erosive wear resistance in such situations and also provides an appropriate alloy for the study of the mechanisms of erosion because it comprises essentially large brittle carbide phases in a ductile matrix. A scanning electron microscope study of the surface of Stellite 6B after erosion by alumina particles is described, and the types of erosion damage incurred by the ductile metal matrix and the brittle carbides are characterized. The only mechanism of material loss of the ductile metal for which positive evidence was found was cutting, with the possibility that fracture on a very fine scale may also be involved. The mechanism of material removal from the carbides appeared to be by surface crack interlinkage. Under the conditions studied, corners of the eroding alumina particles were found to break off and to adhere to the alloy or carbide surface; at the highest impact velocity studied an extensive layer of embedded alumina fragments was built up on the alloy surface and probably modified its erosion behavior.     

The STEM approach to the imaging of surfaces and small particles

The characteristics of scanning transmission electron microsopy, which make it a particularly powerful tool for the study of surfaces and small crystals, include the serial nature of the image signal which makes it amenable to recording and image processing procedures, the possibility of correlating the image data with microdiffraction patterns and microanalysis of chosen small regions and the possibilities for combining several different image signals obtained simultaneously. Images obtained from surfaces using reflected diffraction beams may be very sensitive to variations of surface structure but the resolution, depth of focus and contrast of the images are strongly dependent on the nature of the surface and the aperture sizes used in the microsope. Microdiffraction patterns may be obtained from small surface features. Electron energy loss analyses of the diffracted beams provides sensitive indications on surface electron excitations. Microdiffraction patterns obtained in transmission from regions of 1–5 nm diameter provide valuable information on the structure and defects of small crystals, such as those of platinum and gold which are of interest in relation to the study of catalysts.