Magnetic characterization of microscopic particles by MO-SNOM

This paper reports on the development of a magneto‐optical scanning near‐field optical microscope and the experimental near‐field study of the domain structure for a model magnetic particle of 16 × 16 µm² of a Co_70.4Fe_4.6Si₁₅B₁₀ amorphous thin film, deposited on a silicon substrate. We present the topographic, optical and magneto‐optical differential susceptibility (MODS) images of the particle. Imaging by using the local MODS reveals the domain structure. These images are also used for positioning the tip in order to acquire local hysteresis loops, with submicrometre spatial resolution.     

Optical transition process in AgOx super-resolution near-field structure

A sandwiched 15 nm AgO_x thin film of the super‐resolution, near‐field optical disk was studied using a confocal Z‐scan system. Nonlinear optical properties of quartz glass/ZnS–SiO₂ (170 nm)/AgO_x (15 nm)/ZnS–SiO₂ (40 nm) were measured using a Q‐switch Nd : YAG pulse laser of wavelength 532 nm, pulse width 0.7 ns, and 15.79 kHz repetition rate. Transmittance and reflectance of the sandwiched AgO_x thin film show important optical responses at the focused position of Z‐scan. The dissociation processes of AgO_x, recombination of the silver and oxygen, and the resonance of the localized surface plasmon of the nano‐composites of the AgO_x thin film are correlated to transmittance and reflectance at the focused position of the Z‐scan for different input laser powers. An irreversible upper threshold intensity of 4.40 × 10⁶ mW cm^?2 at the focused position was found. A reversible working window of the focusing intensity between 1.86 × 10⁶ and 4.40 × 10⁶ mW cm^?2 was measured with sandwiched AgO_x thin film alone. The near‐field interactions of the AgO_x thin film and the recording layers of super‐resolution near‐field optical disk are also discussed.     

A method of normalizing cathodoluminescence images of electron transparent foils for thickness contrast applied to InGaN quantum wells

The uniformity of panchromatic cathodoluminescence (CL) from In_0.09Ga_0.91N/GaN quantum wells at 100 K was investigated using a combined transmission electron microscope–cathodoluminescence instrument. A technique for correcting CL images of electron‐transparent wedge specimens for thickness contrast artefacts is presented. The foil thickness was estimated using the dynamical formulation of the relationship between the thickness and the (experimentally observed) transmitted electron intensity. For a given thickness the CL intensity was calculated using the Everhart–Hoff depth‐dose function and also taking into account surface recombination losses. Experimental CL images were normalized by dividing by the calculated CL value at each point. The procedure was successful in calculating the underlying materials contrast in CL images of thin specimens of InGaN single quantum wells. Non‐uniformities in the CL emission on the scale of ～0.7 µm were observed.     

Nanoscale elemental sensitivity study of Nd2Fe14B using absorption correlation tomography

Transmission X‐ray microscopy (TXM) is a rapidly developing technique with the capability of nanoscale three dimensional (3D) real‐space imaging. Combined with the wide range in energy tunability from synchrotron sources, TXM enables the retrieval of 3D microstructural information with elemental/chemical sensitivity that would otherwise be inaccessible. The differential absorption contrast above and below absorption edges has been used to reconstruct the distributions of different elements, assuming the absorption edges of the interested elements are fairly well separated. Here we present an “Absorption Correlation Tomography” (ACT) method based on the correlation of the material absorption across multiple edges. ACT overcomes the significant limitation caused by overlapping absorption edges, significantly expands the capabilities of TXM, and makes it possible for fully quantitative nano‐scale 3D structural investigation with chemical/elemental sensitivity. The capability and robustness of this new methodology is demonstrated in a case study of an important type of rare earth magnet (Nd₂Fe₁₄B). Microsc. Res. Tech. 76:1112–1117, 2013. © 2013 Wiley Periodicals, Inc.     

Structural analyses of Nd-doped CeO2 thin films deposited by pulsed laser deposition

CeO₂ thin films doped with neodymium oxides for application to gas sensors have been elaborated by the pulsed laser deposition technique. The films were deposited on orientated Si (100) substrates with variable deposition times (t = 90, 180 and 360 s) and molar fractions of Nd₂O₃ (0, 6.5, 15, 21.5 and 27 at.%). The resulting Nd–CeO₂ thin films were characterized by means of X‐ray diffraction analysis, scanning electron microscopy and transmission electron microscopy equipped with EDS (Energy Dispersive Spectrometer) microanalysis. From X‐ray diffraction analyses, it is clearly established that the texture is modified by Nd additions. The preferred (111) orientations of the CeO₂ crystals change into the (200) orientation. The morphology of the CeO₂ grains changes from triangles, for pure CeO₂ thin films, to spherical grains for Nd‐doped films. In addition, cell parameter analyses from X‐ray diffraction data show that a partial chemical substitution of Ce by Nd should occur in the face‐centred cubic lattice of ceria: this should give rise to Ce_1‐xNd_xO_2?z phases with oxygen non‐stoichiometry.     

Pecularities of nanocrystal formation in rapidly quenched (FeCo)MoCuB amorphous alloys

The effect of the substitution of Fe by Co on the enhancement of glass‐forming ability limits and subsequent nanocrystallization was studied in a rapidly quenched amorphous system (Fe_xCo_y)₇₉Mo₈Cu₁B₁₂ for y/x ranging from 0 to 1. The effect of Cu on nanocrystallization was investigated by comparison with Cu‐free amorphous Fe₈₀Mo₈B₁₂. Systems partially crystallized at the surface layer were prepared for y/x = 0 using different quenching conditions. The effect of heat treatment of master alloys used for ribbon casting was also assessed. The microstructure and surface/bulk crystallization effects were analysed using transmission electron microscopy and electron and X‐ray diffraction in relation to the expected enhancement of high‐temperature soft magnetic properties, drastically reduced grain sizes (～5 nm) and Co content. Unusual surface phenomena were observed, indicating the origin of possible nucleation sites for preferential crystallization in samples with low Co content.     

Transmission electron microscopy study of crystallization in Fe-Si–B–Cr–C amorphous alloy

The crystallization behaviour of Fe₇₂Si_9.1B_14.8Cr_2.2C_1.9 alloy was studied. Three forms of the examined alloy were studied: water‐atomized powder, as‐spun ribbon and fully crystallized ribbon. Transmission electron microscopy studies of the examined alloy in the form of powder revealed partial crystallization of Fe₂B. The as‐spun ribbon was found to be fully amorphous, and no evidence of any crystalline phases was detected. Formation of metastable phases in the fully devitrificated ribbon was observed. The examined alloy, heated to above its crystallization temperature, consisted of α‐Fe₃(Si), Fe₂₃(C,B)₆, Fe₃B and Fe₂B crystalline phases.     

Combining FIB milling and conventional Argon ion milling techniques to prepare high‐quality site‐specific TEM samples for quantitative EELS analysis of oxygen in molten iron

This paper reports a procedure to combine the focused ion beam micro‐sampling method with conventional Ar‐milling to prepare high‐quality site‐specific transmission electron microscopy cross‐section samples. The advantage is to enable chemical and structural evaluations of oxygen dissolved in a molten iron sample to be made after quenching and recovery from high‐pressure experiments in a laser‐heated diamond anvil cell. The evaluations were performed by using electron energy‐loss spectroscopy and high‐resolution transmission electron microscopy. The high signal to noise ratios of electron energy‐loss spectroscopy core‐loss spectra from the transmission electron microscopy thin foil, re‐thinned down to 40 nm in thickness by conventional Argon ion milling, provided us with oxygen quantitative analyses of the quenched molten iron phase. In addition, we could obtain lattice‐fringe images using high‐resolution transmission electron microscopy. The electron energy‐loss spectroscopy analysis of oxygen in Fe_0.94O has been carried out with a relative accuracy of 2%, using an analytical procedure proposed for foils thinner than 80 nm. Oxygen K‐edge energy‐loss near‐edge structure also allows us to identify the specific phase that results from quenching and its electronic structure by the technique of fingerprinting of the spectrum with reference spectra in the Fe‐O system.     

Detection of wood cell wall porosity using small carbohydrate molecules and confocal fluorescence microscopy

A novel approach to nanoscale detection of cell wall porosity using confocal fluorescence microscopy is described. Infiltration of cell walls with a range of nitrophenyl‐substituted carbohydrates of different molecular weights was assessed by measuring changes in the intensity of lignin fluorescence, in response to the quenching effect of the 4‐nitrophenyl group. The following carbohydrates were used in order of increasing molecular weight; 4‐nitrophenyl β‐D‐glucopyrano‐side (monosaccharide), 4‐nitrophenyl β‐D‐lactopyranoside (disaccharide), 2‐chloro‐4‐nitrophenyl β‐D‐maltotrioside (trisaccharide), and 4‐nitrophenyl α‐D‐maltopentaoside (pentasaccharide). This technique was used to compare cell wall porosity in wood which had been dewatered to 40% moisture content using supercritical CO_2, where cell walls remain fully hydrated, with kiln dried wood equilibrated to 12% moisture content. Infiltration of cell walls as measured by fluorescence quenching, was found to decrease with increasing molecular weight, with the pentasaccharide being significantly excluded compared to the monosaccharide. Porosity experiments were performed on blocks and sections to assess differences in cell wall accessibility. Dewatered and kiln dried wood infiltrated as blocks showed similar results, but greater infiltration was achieved by using sections, indicating that not all pores were easily accessible by infiltration from the lumen surface. In wood blocks infiltrated with 4‐nitrophenyl α‐D‐maltopentaoside, quenching of the secondary wall was quite variable, especially in kiln dried wood, indicating limited connectivity of pores accessible from the lumen surface.     

TEM and AES investigations of the natural surface nano‐oxide layer of an AISI 316L stainless steel microfibre

The chemical composition, nanostructure and electronic structure of nanosized oxide scales naturally formed on the surface of AISI 316L stainless steel microfibres used for strengthening of composite materials have been characterised using a combination of scanning and transmission electron microscopy with energy‐dispersive X‐ray, electron energy loss and Auger spectroscopy. The analysis reveals the presence of three sublayers within the total surface oxide scale of 5.0–6.7 nm thick: an outer oxide layer rich in a mixture of FeO.Fe₂O₃, an intermediate layer rich in Cr₂O₃ with a mixture of FeO.Fe₂O₃ and an inner oxide layer rich in nickel.     

Wavelength- and energy-dispersive Electron Probe Microanalysis (EPMA) measurements with non-perpendicular incidence of the electron beam

A database of 416 wavelength‐ and energy‐dispersive EPMA measurements on tilted specimens of NiAl, TiO₂ and Ti₃Al is presented. The analyses were performed between 10 and 30 kV and the tilt angles were varied between 0° and 60° in seven steps. The necessary hardware modifications for the specimen holder are discussed, as well as the various focusing techniques used in the measurements. A comparison between the experimental data, the calculations of our proza 96t program and the results of Monte Carlo simulations shows that up to 50° tilt the predictions of our software are more than satisfactory. At larger tilt angles some deviations become noticeable. The Monte Carlo simulations appear to produce deviations at a somewhat earlier stage already, for reasons as yet unknown.     

Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces

The electron energy‐loss near‐edge structure (ELNES) of Mo/SrTiO₃ interfaces has been studied using high spatial resolution electron energy‐loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)‐orientated SrTiO₃ surfaces by molecular beam epitaxy at 600 °C. High‐resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)_Mo plane parallel to the substrate surface. Ti‐L_2,3 (～460 eV), O‐K (～530 eV), Sr‐L_2,3 (～1950 eV) and Mo‐L_2,3 (～2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface‐specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti‐L_2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO₃, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO₆ octahedra was disturbed. No interfacial Sr‐L_2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface‐specific O‐K edge was found, which differs from that of the bulk in both position (0.8 ± 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 ± 0.3 eV) occurred for the interfacial Mo‐L_2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO₃ was terminated with TiO₂. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.     

Fluorescence lifetime imaging of lignin autofluorescence in normal and compression wood

Wood cell walls fluoresce as a result of UV and visible light excitation due to the presence of lignin. Fluorescence spectroscopy has revealed characteristic spectral differences in various wood types, notably normal and compression wood. In order to extend this method of characterising cell walls we examined the fluorescence lifetime of wood cell walls using TCSPC (Time‐Correlated Single Photon Counting) as a method of potentially detecting differences in lignin composition and measuring the molecular environment within cell walls. The fluorescence decay curves of both normal and compression wood from pine contain three exponential decay components with a mean lifetime of τ_m = 473 ps in normal wood and 418 ps in compression wood. Lifetimes are spatially resolved to different cell wall layers or cell types where individual lifetimes are shown to have a log‐normal distribution. The differences in fluorescence lifetime observed in pine compression wood compared to normal wood, are associated with known differences in cell wall composition such as increased p‐hydroxyphenyl content in lignin as well as novel deposition of β(1,4)‐Galactan. Our results indicate increased deposition of lignin fluorophores with shorter lifetimes in the outer secondary wall of compression wood. We have demonstrated the usefulness of fluorescence lifetime imaging for characterising wood cell walls, offering some advantages over conventional fluorescence imaging/spectroscopy. For example, we have measured significant changes in fluorescence lifetime resulting from changes to lignin composition as a result of compression wood formation that complement similar changes in fluorescence intensity.     

Wear studies on boronized mild steel

Unlubricated dry sliding wear tests were carried out using a pin-on-disc machine on boronized mild steel pins containing FeB, Fe₂B and both phases in the surface layers. The discs were made of hardened tool steel. Singlephase boride layers exhibited better wear resistance than two-phase boride layers. The wear debris contained essentially Fe₂B, α-Fe, Fe₃O₄ and Fe₂O₃. The untreated mild steel substrates and the FeB layers undergo oxidation and mechanical abrasion during wear tests. The two-phase borides show adhesive transfer and oxidation. The Fe₂B layers fail by fatigue.     

Improving the quality of electron backscatter diffraction (EBSD) patterns from nanoparticles

In this study, we investigated the relative contributions of atomic number (Z) and density (ρ) to the degradation of the electron backscatter diffraction (EBSD) pattern quality for nanoparticles < 500 nm in diameter. This was accomplished by minimizing the diffuse scattering from the conventional thick mounting substrate through the design of a sample holder that can accommodate particles mounted on thin‐film TEM substrates. With this design, the contributions of incoherently scattered electrons that result in the diffuse background are minimized. Qualitative and quantitative comparisons were made of the EBSD pattern quality obtained from Al₂O₃ particles approximately 200 nm in diameter mounted on both thick‐ and thin‐film C substrates. For the quantitative comparison we developed a ‘quality’ factor for EBSD patterns that is based on the ratio of two Hough transforms derived from a given EBSD pattern image. The calculated quality factor is directly proportional to the signal‐to‐noise ratio for the EBSD pattern. In addition to the comparison of the thick and thin mounting substrates, we also estimated the effects of Z and ρ by comparing the EBSD pattern quality from the Al₂O₃ particles mounted on thin‐film substrates with the quality of patterns obtained from Fe–Co nanoparticles approximately 120 nm in diameter. The results indicate that the increased background generated in EBSD patterns by the electrons escaping through the bottom of the small particles is the dominant reason for the poor EBSD pattern quality from nanoparticles < 500 nm in size. This was supported by the fact that we were able to obtain usable EBSD patterns from Al₂O₃ particles as small as 130 nm using the thin‐film mounting method.     

Real‐space observation of strong metal‐support interaction: state‐of‐the‐art and what's the next

The real‐space resolving of the encapsulated overlayer in the well‐known model and industry catalysts, ascribed to the advent of dedicated transmission electron microscopy, enables us to probe novel nano/micro architecture chemistry for better application, revisiting our understanding of this key issue in heterogeneous catalysis. In this review, we summarize the latest progress of real‐space observation of SMSI in several well‐known systems mainly covered from the metal catalysts (mostly Pt) supported by the TiO₂, CeO₂ and Fe₃O₄. As a comparison with the model catalyst Pt/Fe₃O₄, the industrial catalyst Cu/ZnO is also listed, followed with the suggested ongoing directions in the field.     

Dislocations in nanostructured two‐phase Fe30Ni20Mn20Al30

In a previous study, the dislocations in Fe₃₀Ni₂₀Mn₂₅Al₂₅ (at. %), which consist of 50 nm wide alternating b.c.c. and B2 phases, were shown to have a/2<111> Burgers vectors after room temperature deformation. The dislocations were found to glide in pairs on both {110} and {112} slip planes and were relatively widely separated in the b.c.c. phase, where the dislocations were uncoupled, and closely spaced in the B2 phase, where the dislocations were connected by an anti‐phase boundary. In this article, we analyze the dislocations in the two ～5 nm‐wide B2 phases in a related two‐phase alloy Fe₃₀Ni₂₀Mn₂₀Al₃₀, with compositions Fe‐23Ni‐21Mn‐24Al and Fe‐39Ni‐12Mn‐34Al, compressed to ～3% strain at a strain rate 5 × 10^?4 s^?1 at 873 K (the lowest temperature at which substantial plastic flow was observed). It is shown that slip occursby the glide of a<100> dislocations. A review of the literature suggests that the differences in the observed slip vector between these B2 phases could be due to the differences in composition, differences in deformation temperature, or possibly both. Microsc. Res. Tech. 76:263–267, 2013. © 2013 Wiley Periodicals, Inc.     

New pathways for improved quantification of energy‐dispersive X‐ray spectra of semiconductors with multiple X‐ray lines from thin foils investigated in transmission electron microscopy

Theoretical approaches to quantify the chemical composition of bulk and thin‐layer specimens using energy‐dispersive X‐ray spectroscopy in a transmission electron microscope are compared to experiments investigating (In)GaAs and Si(Ge) semiconductors. Absorption correctors can be improved by varying the take‐off angle to determine the depth of features within the foil or the samples thickness, or by definition of effective k‐factors that can be obtained from plots of k‐factors versus foil thickness or, preferably, versus the K/L intensity ratio for a suitable element. The latter procedure yields plots of self‐consistent absorption corrections that can be used to determine the chemical composition, iteratively for SiGe using a set of calibration curves or directly from a single calibration curve for InGaAs, for single X‐ray spectra without knowledge of sample thickness, density or mass absorption coefficients.     

Evolution of rapidly solidified NiAlCu(B) alloy microstructure

This study concerned phase transformations observed after rapid solidification and annealing at 500, 700 and 800 °C in 56.3 Ni‐39.9 Al‐3.8 Cu‐0.06 B (E1) and 59.8 Ni‐36.0 Al‐4.3 Cu‐0.06 B (E2) alloys (composition in at.%). Injection casting led to a homogeneous structure of very small, one‐phase grains (2–4 µm in size). In both alloys, the phase observed at room temperature was martensite of L1₀ structure. The process of the formation of the Ni₅Al₃ phase by atomic reordering proceeded at 285–394 °C in the case of E1 alloy and 450–550 °C in the case of E2 alloy. Further decomposition into NiAl (β) and Ni₃Al (γ′) phases, the microstructure and crystallography of the phases depended on the path of transformations, proceeding in the investigated case through the transformation of martensite crystallographic variants. This preserved precise crystallographic orientation between the subsequent phases, very stable plate‐like morphology and very small β + γ′ grains after annealing at 800 °C.