Current-sensing scanning near-field optical microscopy using a metal probe for nanometre-scale observation of electrochromic films

A novel technique for scanning near‐field optical microscopy capable of point‐contact current‐sensing was developed in order to investigate the nanometre‐scale optical and electrical properties of electrochromic materials. An apertureless bent‐metal probe was fabricated in order to detect optical and current signals at a local point on the electrochromic films. The near‐field optical properties could be observed using the local field enhancement effect generated at the edge of the metal probe under p‐polarized laser illumination. With regard to electrical properties, current signal could be detected with the metal probe connected to a high‐sensitive current amplifier. Using the current‐sensing scanning near‐field optical microscopy, the surface topography, optical and current images of coloured WO₃ thin films were observed simultaneously. Furthermore, nanometre‐scale electrochromic modification of local bleaching could be performed using the current‐sensing scanning near‐field optical microscopy. The current‐sensing scanning near‐field optical microscopy has potential use in various fields of nanometre‐scale optoelectronics.     

The application of X-ray microscopy in materials science

X-ray microscopy is a form of high resolution radiography that uses low-energy X-rays (≤10 keV) to enhance the contrast between light elements such as hydrogen, carbon, nitrogen and oxygen. As performed on compact laboratory equipment the technique can achieve spatial resolutions of roughly 1 μm in virtually any material given that the specimen is sufficiently thin (typically 0·2–2 mm) to be adequately transparent to low-energy X-rays. Notwithstanding that the technique has lately found favour in biomedical radiology, its considerable potential in other fields, notably the materials sciences, remains largely unexploited. The scope and potential of laboratory X-ray microscopy in the materials sciences is demonstrated here by its application to ceramics, elastomers, coal-chars and reinforced composites. In all cases the technique provided valuable microstructural characterization often unobtainable by any other non-destructive method. These examples demonstrate that laboratory X-ray microscopy offers much to the materials sciences and deserves a wider application than is current.     

Surface studies in UHV SEM and STEM

Some recent advances in surface analyses with scanning electron microscopy techniques are reviewed. It is shown that secondary electron microscopy can image monatomic steps, emergent dislocations and the early growth of thin films and oxide nuclei. The chemical composition of inhomogeneous surfaces can, in some cases, be determined with nanometre resolution with Auger signals. Overlayer coverages can occasionally be monitored with the secondary electron signal if the change of work function with coverage has been determined previously. The interpretation of these secondary and Auger signals is discussed. The combination of angular electron spectroscopy with electron microscopy is potentially a powerful technique for determining atomic positions of surface atoms and one method for obtaining the experimental data is described. Several substrate/overlayer systems are used to illustrate the amount of information that can be obtained about surface processes with scanning electron microscopy.     

Triboelectrochemical investigation of the friction and wear behaviour of TiN coatings in a neutral solution

Triboelectrochemical techniques use an electrochemical set-up (mainly of the three-electrode type) for controlling the potential of the surface of a conducting material subjected to rubbing in a tribometer. In this way it is possible to carry out friction and wear tests in electrolytic solutions under well-defined chemical conditions determined by the applied electrode potential. In addition, triboelectrochemical techniques offer the possibility of following in-situ and in real time the kinetics of electrochemical oxidation reactions (corrosion) by the simple measure of an electrical current. In the present study triboelectrochemical experiments were carried out on sputter deposited TiN coatings sliding against alumina in a borate solution (pH 8.4). The surface of selected worn coatings was characterised by X-ray photoelectron spectroscopy (XPS) and the topography by scanning electron microscopy (SEM). Results show that the rate of wear critically depends on the prevailing (electro)chemical conditions which determine the chemical surface state of the TiN coating. The behaviour is attributed to the lubricating properties of surface oxide films having a thickness in the nanometre range.     

Characterization of dentine structure in three dimensions using FIB-SEM

All biological tissues are three dimensional and contain structures that span a range of length scales from nanometres through to hundreds of millimetres. These are not ideally suited to current three-dimensional characterization techniques such as X-ray or transmission electron tomography. Such detailed morphological analysis is critical to understanding the structural features relevant to tissue function and designing therapeutic strategies intended to address structural deficiencies encountered in pathological states. We show that use of focused ion beam milling combined with scanning electron microscopy can provide three-dimensional information at nanometre resolution from biologically relevant volumes of material, in this case dentine.     

Morphology and protein adsorption characteristics of block copolymer surfaces

Biocompatible polymers are known to act as scaffolds for the regeneration and growth of bone. Block copolymers are of interest as scaffold materials because novel, structurally diverse polymers can be synthesized from biocompatible blocks. Block copolymer nanostructure and surface morphology is easily tunable with synthetic techniques and the diverse nanostructures can be used to affect cell and tissue behaviour. In this paper, we present atomic force microscopy studies on the morphology and corresponding protein adsorption behaviour of a novel class of methyl methacrylate and acrylic acid diblock and triblock copolymers. The topography, phase angle and adhesion maps were obtained to study the morphology. Atomic force microscopy imaging reveals that the diblock and triblock copolymers present distinct nanomorphologies, although their chemical composition is the same. This has implications on the role of nanomorphology in cell-polymer interactions independent of chemical composition. Protein adsorption on a biomaterial surface is critical to understanding its biocompatibility and bovine serum albumin was used to model that behaviour on the block copolymer surfaces. An increase in the adhesive force of the surface was observed to correlate with the adsorption of bovine serum albumin on the block copolymer surfaces investigated.     

A global investigation into in situ nanoindentation experiments on zirconia: from the sample geometry optimization to the stress nanolocalization using convergent beam electron diffraction

Nanoindentation experiments inside a transmission electron microscope are of much interest to characterize specific phenomena occuring in materials, like for instance dislocation movements or phase transformations. The key points of these experiments are (i) the sample preparation and the optimization of its geometry to obtain reliable results and (ii) the choice of the transmission electron microscope observation mode, which will condition the type of information which can be deduced from the experiment. In this paper, we will focus on these two key points in the case of nanoindentation of zirconia, which is a ceramic material well known to be sensitive to stress because it can undergo a phase transformation. In this case, the information sought is the stress localization at the nanometre scale and in real time. As far as the sample preparation is concerned, one major drawback of nanoindentation inside a transmission electron microscope is indeed a possible bending of the sample occurring during compression, which is detrimental to the experiment interpretation (the stress is not uniaxial anymore). In this paper, several sample preparation techniques have been used and compared to optimize the geometry of the sample to avoid bending. The results obtained on sample preparation can be useful for the preparation of ceramics samples but can also give interesting clues and experimental approaches to optimize the preparation of other kinds of materials. The second part of this paper is devoted to the second key point, which is the determination of the stress localization associated to the deformation phenomena observed by nanoindentation experiments. In this paper, the use of convergent beam electron diffraction has been investigated and this technique could have been successfully coupled to nanoindentation experiments. Coupled nanoindentation experiments and convergent beam electron diffraction analyses have finally been applied to characterize the phase transformation of zirconia.     

The study of fission track and other crystalline defects using confocal scanning laser microscopy

Confocal scanning laser microscopy (SLM) is a technique that offers geologists a new way of studying structures in minerals at the submicrometre level. As an example we show how the non-destructive nature of confocal SLM can be used to measure and count fission tracks (line defects formed by the spontaneous fission of²³⁸U) in the uranium-bearing mineral apatite, and to provide information about the geometry and crystallographic orientation of fluid inclusions trapped inside apatite grains during crystallization. The technique also provides a means of studying the internal geometry of chemical zonation in minerals. The digitized nature of the SLM images makes them amenable to a variety of image analysis techniques, and we show how image analysis can be used to measure fission tracks in mica sheets and provide crude estimates of track dip. Finally, using a chemically etched mica sheet we show how confocal SLM can be used to provide a detailed near-surface (1–5 μm) analysis of geological materials.     

Detection and speciation of brominated flame retardants in high-impact polystyrene (HIPS) polymers

Polymeric materials have been suggested as possible environmental sources of persistent organic pollutants such as flame retardants. In situ, micrometre-scale characterization techniques for polymer matrix containing flame retardants may provide some insight into the dominant environmental transfer mechanism(s) of these brominated compounds. In this work, we demonstrate that micro X-ray fluorescence spectroscopy (μXRF), focused ion beam scanning electron microscopy (FIB-SEM) combined with energy dispersive X-ray spectroscopy (EDS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are promising techniques for the elemental and chemical identification of brominated fire retardant compounds (such as the deca-congener of polybrominated diphenyl ether, BDE-209) within polymeric materials (e.g. high-impact polystyrene or HIPS). Data from μXRF demonstrated that bromine (Br) inclusions were evenly distributed throughout the HIPS samples, whereas FIB SEM-EDS analysis revealed that small antimony (Sb) and Br inclusions are present, and regionally higher concentrations of Br surround the Sb inclusions (compared to the bulk material). Four prominent mass-to-charge ratio peaks (m/z 485, 487, 489 and 491) that correspond to BDE-209 were identified by ToF-SIMS and can be used to chemically distinguish this molecule on the surface of polymeric materials with respect to other brominated organic molecules. These techniques can be important in any study that investigates the route of entry to the environmental surroundings of BDE-containing materials.     

Fuzzy logic approaches to the analysis of HREM images of III–V compounds

It is known that high-resolution electron microscopy (HREM) can provide quantitative information on the properties of crystalline materials. The HREM patterns of layered structures of III–V semiconductors vary with the chemical composition of the latter within the sublattices, which is also influenced by interdiffusion. Local variations of the crystal cell similarity are recorded for image analysis and compared with templates of known material composition.
Of the advanced theories of data interpretation, the now well-established fuzzy logic is highly suited for corresponding image processing techniques. Combining neighbouring image cell similarities, the underlying chemical composition is evaluated by applying fuzzy logic criteria of inference to masks of about 1 nm × 1 nm in size.
The new approach can be used to localize regions of significant changes in composition, i.e. edge detection, and to determine the composition across the interface region. The methods introduced prove successfully applicable to simulated as well as to experimental images of AlAs/Al _x Ga_{1− x} As. Similarity/composition relations of nonlinear as well as nonmonotonic characteristics are studied to establish an alternative fuzzy logic approach.     

Simple technique for high‐throughput marking of distinguishable micro‐areas for microscopy

Today's (nano)‐functional materials, usually exhibiting complex physical properties require local investigation with different microscopy techniques covering different physical aspects such as dipolar and magnetic structure. However, often these must be employed on the very same sample position to be able to truly correlate those different information and corresponding properties. This can be very challenging if not impossible especially when samples lack prominent features for orientation. Here, we present a simple but effective method to mark hundreds of approximately 15×15 μm sample areas at one time by using a commercial transmission electron microscopy grid as shadow mask in combination with thin‐film deposition. Areas can be easily distinguished when using a reference or finder grid structure as shadow mask. We show that the method is suitable to combine many techniques such as light microscopy, scanning probe microscopy and scanning electron microscopy. Furthermore, we find that best results are achieved when depositing aluminium on a flat sample surface using electron‐beam evaporation which ensures good line‐of‐sight deposition. This inexpensive high‐throughput method has several advantageous over other marking techniques such as focused ion‐beam processing especially when batch processing or marking of many areas is required. Nevertheless, the technique could be particularly valuable, when used in junction with, for example focused ion‐beam sectioning to obtain a thin lamellar of a particular pre‐selected area.     

Investigation of C3S hydration by environmental scanning electron microscope

Tricalciumsilicate (C₃S, Alite) is the major component of the Portland cement clinker, The hydration of the Alite is decisive for the properties of the resulting material due to the high content in cement. The mechanism of the hydration of C₃S is very complicated and not yet fully understood. There are some models that describe the hydration of C₃S in various ways. The Environmental Scanning Electron Microscopy (ESEM) working in gaseous atmosphere enables high‐resolution dynamic observations of structure of materials, from micrometre to nanometre scale. This provides a new perspective in material research. ESEM significantly allows imaging of specimen in their natural state without the need for special preparation (coating, drying, etc.) that can alter the physical properties. This paper presents the results of our experimental studies of hydration of C₃S using ESEM. The ESEM turned out to be an important extension of the conventional scanning microscopy. The purpose of these investigations is to gain insight of hydration mechanism to determine which hydration products are formed and to analyze if there are any differences in the composition of the hydration products.     

The assessment of microscopic charging effects induced by focused electron and ion beam irradiation of dielectrics

Energetic beams of electrons and ions are widely used to probe the microscopic properties of materials. Irradiation with charged beams in scanning electron microscopes (SEM) and focused ion beam (FIB) systems may result in the trapping of charge at irradiation induced or pre-existing defects within the implanted microvolume of the dielectric material. The significant perturbing influence on dielectric materials of both electron and (Ga(+)) ion beam irradiation is assessed using scanning probe microscopy (SPM) techniques. Kelvin Probe Microscopy (KPM) is an advanced SPM technique in which long-range Coulomb forces between a conductive atomic force probe and the silicon dioxide specimen enable the potential at the specimen surface to be characterized with high spatial resolution. KPM reveals characteristic significant localized potentials in both electron and ion implanted dielectrics. The potentials are observed despite charge mitigation strategies including prior coating of the dielectric specimen with a layer of thin grounded conductive material. Both electron- and ion-induced charging effects are influenced by a delicate balance of a number of different dynamic processes including charge-trapping and secondary electron emission. In the case of ion beam induced charging, the additional influence of ion implantation and nonstoichiometric sputtering from compounds is also important. The presence of a localized potential will result in the electromigration of mobile charged defect species within the irradiated volume of the dielectric specimen. This electromigration may result in local modification of the chemical composition of the irradiated dielectric. The implications of charging induced effects must be considered during the microanalysis and processing of dielectric materials using electron and ion beam techniques.     

High-resolution X-ray CT for 3D petrography of ferruginous sandstone for an investigation of building stone decay

Diestian ferruginous sandstone has been used as the dominant building stone for monuments in the Hageland, a natural landscape in east-central Belgium. Like all rocks, this stone type is sensitive to weathering. Case hardening was observed in combination with blackening of the exterior parts of the dressed stones. To determine the 3D petrography and to identify the structural differences between the exterior and interior parts, X-ray computed tomography was used in combination with more traditional research techniques like optical microscopy and scanning electron microscopy. The 3D characterization of the ferruginous sandstone was performed with a high-resolution X-ray CT scanner (www.ugct.ugent.be) in combination with the flexible 3D analysis software Morpho+, which provides the necessary petrophysical parameters of the scanned samples in 3D. Besides providing the required 3D parameters like porosity, pore-size distribution, grain size, grain orientation, and surface analysis, the results of the 3D analysis can also be visualized, which enables to understand and interpret the analysis results in a straightforward way. The complementarities between high-quality X-ray CT images and flexible 3D software and its relation with the more traditional microscopical research techniques are opening up new gateways in the study of weathering processes of natural building stones.     

Morphological, nanomechanical and cellular structural characterization of human hair and conditioner distribution using torsional resonance mode with an atomic force microscope

Characterization of the cellular structure and chemical and physical properties of hair are essential to develop better cosmetic products and advance the biological and cosmetic sciences. Although the morphology of the fine cellular structure of human hair has traditionally been investigated using scanning electron microscopy and transmission electron microscopy, atomic force microscopy can be used for characterization in ambient conditions without requiring specific sample preparations and surface treatment. In this study, the tapping and torsional resonance modes in an atomic force microscope are compared for measurements of stiffness and viscoelastic properties. The materials were mapped using amplitude and phase angle imaging. The torsional resonance mode showed advantages in resolving the in-plane (lateral) heterogeneity of materials. This mode was used for investigating and characterizing the fine cellular structure of human hair. Various cellular structures (such as the cortex and the cuticle) of human hair and fine sublamellar structures of the cuticle, such as the A-layer, the exocuticle, the endocuticle and the cell membrane complex were easily identified. The distribution and thickness of conditioner on the treated hair surface affects the tribological properties of hair. The thickness of the conditioner was estimated using force distance measurements with an atomic force microscope.     

Notes on contributors

Various metallic pairs were tested under conditions of unlubricated solid contact. Experiments were conducted for repetitive impulsive and continuous sliding contact. Wide ranges of materials and conditions (nominal contact stress and relative transverse sliding velocity) and a variety of loading modes (pure normal impact at various frequencies, compound impact at various sliding velocities, and pure sliding under various stress levels) were explored.Particular attention was focused on the establishment of subsurface material zones developed in the tests, in situ. These zones exhibit dependences on velocity, stress, material, test duration and loading mode. The experimental findings, based on several analysis techniques, serve to characterize subsurface zone composition and morphology. Both surface and subsurface features were examined by optical and electron microscopy and analyzed by energy-dispersive X-ray techniques to allow interpretations concerning the role of external parameters, material transport and debris formation, as well as insight into operative mechanisms which act on specific materials under prescribed conditions to cause wear.     

Scanning near-field optical microscopy (SNOM)

SNOM is a non-contact stylus microscopy analogous to STM. Optical near-field interaction is used to sense approach and optical properties on the nanometre scale (?1 nm normal, 20–50 nm lateral). SNOM was demonstrated in transmission and reflection, in a topographic mode, and with amplitude as well as phase objects. The excitation of plasmons in the SNOM ‘tip’, a very recent development, greatly enhances sensitivity and permits intriguing new optical experiments. Overcoming the limit of diffraction, SNOM turns a long-held dream of optical microscopists into reality.     

Fluctuation X-ray microscopy: a novel approach for the structural study of disordered materials

Measuring medium‐range order is a challenging and important problem in the structural study of disordered materials. We have developed a new technique, fluctuation x‐ray microscopy, that offers quantitative insight into medium‐range correlations in disordered materials at nanometre and larger length scales.In this technique, which requires a spatially coherent x‐ray beam, a series of speckle patterns are measured at a large number of locations in a sample using various illumination sizes. Examination of the speckle variance as a function of the illumination spot size allows the structural correlation length to be measured. To demonstrate this technique we have studied polystyrene latex spheres, which serve as a model for a dense random‐packed glass, and for the first time have measured the correlation length in a disordered system by fluctuation X‐ray microscopy. We discuss data analysis and procedures to correct for shot noise and detector noise. This approach could be used to explore medium‐range order and subtle spatial structural changes in a wide range of disordered materials, from soft matter to nanowire arrays, semiconductor quantum dot arrays and magnetic materials.     

偏振光散射无标记活体＂显微＂成像技术研究进展

偏振光测量是一种用来探究物质结构和形态的有效方法。偏振光散射测量可以通过介质的偏振特性得到微观结构和形态信息,并可以提高样品浅层成像分辨率,从而减小光学成像受散射的影响。偏振光散射测量具有光学方法的非侵入、无损伤、快速测量等优势,在生物医学以及材料、海洋、大气等领域都有很好的应用前景,特别是在癌变检测方面的潜力尤其引人注目。本文通过介绍偏振光散射测量的相关方法及研究进展来分析目前偏振光学成像在生物医学等领域的发展状况,以及未来发展的前景。