On the benefit of the negative-spherical-aberration imaging technique for quantitative HRTEM

Employing an aberration corrector in a high-resolution transmission electron microscope, the spherical aberration C_S can be tuned to negative values, resulting in a novel imaging technique, which is called the negative C_S imaging (NCSI) technique. The image contrast obtained with the NCSI technique is compared quantitatively with the image contrast formed with the traditional positive C_S imaging (PCSI) technique. For the case of thin objects negative C_S images are superior to positive C_S images concerning the magnitude of the obtained contrast, which is due to constructive rather than destructive superposition of fundamental contrast contributions. As a consequence, the image signal obtained with a negative spherical aberration is significantly more robust against noise caused by amorphous surface layers, resulting in a measurement precision of atomic positions which is by a factor of 2–3 better at an identical noise level. The quantitative comparison of the two alternative C_S-corrected imaging modes shows that the NCSI mode yields significantly more precise results in quantitative high-resolution transmission electron microscopy of thin objects than the traditional PCSI mode.     

Fundamental problems of imaging subsurface structures in the backscattered electron mode in scanning electron microscopy

In‐depth imaging of subsurface structures in scanning electron microscopy (SEM) is usually obtained by detecting backscattered electrons (BSE). For a layer‐by‐layer imaging in BSE microtomography, it is preferable to use an energy filtering of BSE. A simple approach is used to estimate the contrast by using backscattering coefficients of bulk materials and the maximum escape depths of the BSE. The contrast obtained by BSE energy filtering is about twice that of the standard BSE method by varying the acceleration voltage. The contrast decreases with increasing information depth. The information depth is about four times smaller than the electron range. The transmission of the spectrometer influences the minimum current of the order of 10^?8 A that is needed to get a contrast of 1%, for example.     

Influence of UV irradiation in low frictional performance of CNx coatings

The influence of ultraviolet (UV) irradiation on low frictional performance of CN_x coatings with 100 nm thickness having nitrogen contents of 9%, 14% and 19% deposited on Si(100) substrate by ion beam mixing was investigated in N₂ atmosphere environment. Three UV lights of 254, 312 and 365 nm were used to irradiate the surface of CN_x / Si(100) for 60 min. The changes of N / C ratio and atomic binding energy in the coating were analysed using Auger electron spectroscopy and X‐ray photoelectron spectroscopy, respectively. The friction coefficient of Si₃N₄ ball sliding against CN_x was measured by a pin‐on‐disc tribometer, and wear tracks were analysed by the transmission electron microscope image. The results showed that UV irradiation on CN_x coating can decrease the critical frictional cycles for low friction coefficient and that the mechanism is due to the formation of graphite‐like structure in the topmost CN_x coating. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of two tungstates Ca0.5Cd0.5WO4 and Ca0.2Cd0.8WO4 by transmission electron microscopy

To better understand the role of crystal structures and local disorder in the photonic properties of the system (1 ? x)CaWO₄ ? xCdWO₄ with 0 < x < 1, two specific phases with compositions x = 0.5 (scheelite phase) and 0.8 (wolframite phase) have been studied by scanning and transmission electron microscopies. High‐resolution electron microscopy images and image simulations, associated with X‐ray diffraction data, allowed confirming the lattices and space groups I4₁/a and P2/c of the two scheelite and wolframite phases, at the local scale. The electron microscopy data show the existence of a high degree of crystallization associated with statistical distribution of Ca or Cd atoms on a Ca_1?xCd_x site in each lattice.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

An attachment to SEM for 3-D surface topography measurements of uniform materials

The paper reports on a SEM-based system for quantitative three-dimensional (3-D) surface topography measurements. In comparison with commercial line-width measuring systems, this metrology instrument is designed to measure quantitatively both critical dimensions in horizontal plane and features height while allowing for full cross section reconstruction from backscattered electrons (BSE) signal. The philosophy behind this technique has been described previously, (Aristov et al. 1988, 1991). Now the system is operational at our Institute, and its characteristics just as design criteria, and some examples of application are presented.     

Correlative light and backscattered electron microscopy of bone--part II: automated image analysis

Detailed studies of biological phenomena often involve multiple microscopy and imaging modes and media. For bone biology, various forms of light and electron microscopy are used to study the microscopic structure of bone. Integrating information from the different sources is necessary to understand how different aspects of the bone structure interact. To accomplish this, methods were developed to prepare and image thin sections for correlative light microscopy (LM) and backscattered electron imaging in the scanning electron microscope (BSE-SEM). Images of the same fields of view may then be analyzed for degrees of relationships between specimen features not observed by LM or SEM alone. These methods are applied here to study possible associations between the degree of bone mineralization and pattern of collagen fiber orientation in the mid-shaft of the human femur. The "relational images" obtained allow us to examine the relationship between these two variables, both objectively and quantitatively.     

Iodine vapor staining for atomic number contrast in backscattered electron and X‐ray imaging

Iodine imparts strong contrast to objects imaged with electrons and X‐rays due to its high atomic number (53), and is widely used in liquid form as a microscopic stain and clinical contrast agent. We have developed a simple technique which exploits elemental iodine's sublimation‐deposition state‐change equilibrium to vapor stain specimens with iodine gas. Specimens are enclosed in a gas‐tight container along with a small mass of solid I₂. The bottle is left at ambient laboratory conditions while staining proceeds until empirically determined completion (typically days to weeks). We demonstrate the utility of iodine vapor staining by applying it to resin‐embedded tissue blocks and whole locusts and imaging them with backscattered electron scanning electron microscopy (BSE SEM) or X‐ray microtomography (XMT). Contrast is comparable to that achieved with liquid staining but without the consequent tissue shrinkage, stain pooling, or uneven coverage artefacts associated with immersing the specimen in iodine solutions. Unmineralized tissue histology can be read in BSE SEM images with good discrimination between tissue components. Organs within the locust head are readily distinguished in XMT images with particularly useful contrast in the chitin exoskeleton, muscle and nerves. Here, we have used iodine vapor staining for two imaging modalities in frequent use in our laboratories and on the specimen types with which we work. It is likely to be equally convenient for a wide range of specimens, and for other modalities which generate contrast from electron‐ and photon‐sample interactions, such as transmission electron microscopy and light microscopy. Microsc. Res. Tech. 77:1044–1051, 2014. © 2014 The Authors. Microscopy Research Technique published by Wiley Periodocals, Inc.     

Non-monotonic material contrast in scanning ion and scanning electron images

30 keV Ga⁺ focused ion beam induced secondary electron (iSE) imaging was used to determine the relative contrast between several materials. The iSE signal compared from C, Si, Al, Ti, Cr, Ni, Cu, Mo, Ag, and W metal layers does not decrease with an increase in target atomic number Z₂, and shows a non-monotonic relationship between contrast and Z₂. The non-monotonic relationship is attributed to periodic fluctuations of the stopping power and sputter yield inherent to the ion–solid interactions. In addition, material contrast from electron-induced secondary electron (eSE) and backscattered electron (BSE) images using scanning electron microscopy (SEM) also shows non-monotonic contrast as a function of Z₂, following the periodic behavior of the stopping power for electron–solid interactions. A comparison of the iSE and eSE results shows similar relative contrast between the metal layers, and not complementary contrast as conventionally understood. These similarities in the contrast behavior can be attributed to similarities in the periodic and non-monotonic function defined by incident particle–solid interaction theory.     

Backscattered electron imaging and electron backscattered diffraction in the study of bacterial attachment to titanium alloy structure

The application of secondary electron (SE) imaging, backscattered electron imaging (BSE) and electron backscattered diffraction (EBSD) was investigated in this work to study the bacterial adhesion and proliferation on a commercially pure titanium (cp Ti) and a Ti6Al4V alloy (Ti 64) with respect to substrate microstructure and chemical composition. Adherence of Gram‐positive Staphylococcus epidermidis 11047 and Streptococcus sanguinis GW2, and Gram‐negative Serratia sp. NCIMB 40259 and Escherichia coli 10418 was compared on cp Ti, Ti 64, pure aluminium (Al) and vanadium (V). The substrate microstructure and the bacterial distribution on these metals were characterised using SE, BSE and EBSD imaging. It was observed that titanium alloy‐phase structure, grain boundaries and grain orientation did not influence bacterial adherence or proliferation at microscale. Adherence of all four strains was similar on cp Ti and Ti 64 surfaces whilst inhibited on pure Al. This work establishes a nondestructive and straight‐forward statistical method to analyse the relationship between microbial distribution and metal alloy structure.     

Contrast in the transmission mode of a low-voltage scanning electron microscope

The contrast thicknesses (x_k) of thin carbon and platinum films have been measured in the transmission mode of a low-voltage scanning electron microscope for apertures of 40 and 100 mrad and electron energies (E) between 1 and 30 keV. The measured values overlap with those previously measured for E (≥ 17keV) in a transmission electron microscope. Differences in the decrease of x_k with decreasing E between carbon and platinum agree with Wentzel-Kramer-Brillouin calculations of the elastic cross-sections. Knowing the value of x_k allows the exponential decrease ∝ exp(—x/x_k) in transmission with increasing mass-thickness (x = ρt) of the specimen and the increasing gain of contrast for stained biological sections with decreasing electron energy to be calculated for brightfield and darkfield modes.     

Sample preparation and microscopical investigation techniques for metal and ceramics containing graphite and graphene‐like layered particles

Scanning electron microscopy (SEM) techniques are widely used in microstructural investigations of materials since it can provide surface morphology, topography, and chemical information. However, it is important to use correct imaging and sample preparation techniques to reveal the microstructures of materials composed of components with different polishing characteristics such as grey cast iron, graphene platelets (GPLs)‐added SiAlON composite, SiC and B₄C ceramics containing graphite or graphene‐like layered particles. In this study, all microstructural details of gray cast iron were successfully revealed by using argon ion beam milling as an alternative to the standard sample preparation method for cast irons, that is, mechanical polishing followed by chemical etching. The in‐lens secondary electron (I‐L‐SE) image was clearly displayed on the surface details of the graphites that could not be revealed by backscattered electron (BSE) and Everhart–Thornley secondary electron (E‐T SE) images. Mechanical polishing leads to pull‐out of GPLs from SiAlON surface, whereas argon ion beam milling preserved the GPLs and resulted in smooth surface. Grain and grain boundaries of polycrystalline SiC and B₄C were easily revealed by using I‐L SE image in the SEM after only mechanical polishing without any etching process. While the BSE and E‐T SE images did not clearly show the residual graphites in the microstructure, their distribution in the B₄C matrix was fully revealed in the I‐L SE image.     

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.     

Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (～0.5 nm) interfacial SiO_x layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO₂‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.     

Quantification of the modulated structures in TiPdCr alloys

Ti₅₀Pd_(50-x)Cr_xβ₂ alloys (B2, CsCl-type crystal structure) have been investigated as part of an overall study of the systematics of phase transformation behaviour and also for the commercial applicability of the shape memory effect at elevated temperatures. The phase transformations are unique in the sense that electron diffraction and high-resolution transmission electron microscopy of Ti₅₀Pd_(50-x)Cr_x alloys with x between 6 and 10 at.% reveal {110}_B2 modulated structures with periodicities between 3 and 4·5 planes that vary linearly with composition. Satellites along certain <110>* lying at incommensurate positions are observed in electron diffraction patterns and are consistent with transverse lattice displacements. High-resolution transmission electron microscopy (HRTEM) imaging usually reveals sinusoidal displacements; however, a few images appear ‘banded’ at three- and four-plane intervals suggesting the possibility that three- and four-plane commensurate packets combine to give the overall incommensurate satellite spacing. With the use of a newly developed technique for quantifying atomic displacements from HRTEM images, modulated phases in Ti₅₀Pd₄₃Cr₇ and Ti₅₀Pd₄₂Cr₈ alloys, where atoms are shifted from the cubic lattice sites by a transverse lattice displacement wave, have been studied. The real space displacements of the modulated structures were determined and compared to truly incommensurate and varying three- and four-plane wave packet combinations by examination of the goodness of fit as assessed by the correlation coefficient for a least-squares fit. In the cases of the Ti₅₀Pd₄₃Cr₇ and Ti₅₀Pd₄₂Cr₈ alloys, the incommensurate sine wave exhibited much better matching than the commensurate packets, indicating that these modulations are probably incommensurate.     

Influence of topography and specimen preparation on backscattered electron images of bone

Backscattered electron (BSE) images of bone exhibit graylevel contrast between adjacent lamellae. Mathematical models suggest that interlamellar contrast in BSE images is an artifact due to topographic irregularities. However, little experimental evidence has been published to support these models, and it is not clear whether submicron topographical features will alter BSE graylevels. The goal of this study was to determine the effects of topography on BSE image mean graylevels and graylevel histogram widths using conventional specimen preparation techniques. White-light interferometry and quantitative BSE imaging were used to investigate the relationship between the BSE signal and specimen roughness. Backscattered electron image graylevel histogram widths correlated highly with surface roughness in rough preparations of homogeneous materials. The relationship between BSE histogram width and surface roughness was specimen dependent. Specimen topography coincided with the lamellar patterns within the bone tissue. Diamond micromilling reduced average surface roughness when compared with manual polishing techniques but did not significantly affect BSE graylevel histogram width. The study suggests that topography is a confounding factor in quantitative BSE analysis of bone. However, there is little quantitative difference between low-to-moderate magnification BSE images of bone specimens prepared by conventional polishing or diamond micromilling.     

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.     

Advantages of stereo imaging of metallic surfaces with low voltage backscattered electrons in a field emission scanning electron microscope

High emission current backscattered electron (HC-BSE) stereo imaging at low accelerating voltages (≤ 5 keV) using a field emission scanning electron microscope was used to display surface structure detail. Samples of titanium with high degrees of surface roughness, for potential medical implant applications, were imaged using the HC-BSE technique at two stage tilts of + 3° and − 3° out of the initial position. A digital stereo image was produced and qualitative height, depth and orientation information on the surface structures was observed. HC-BSE and secondary electron (SE) images were collected over a range of accelerating voltages. The low voltage SE and HC-BSE stereo images exhibited enhanced surface detail and contrast in comparison to high voltage (> 10 keV) BSE or SE stereo images. The low voltage HC-BSE stereo images displayed similar surface detail to the low voltage SE images, although they showed more contrast and directional sensitivity on surface structures. At or below 5 keV, only structures a very short distance into the metallic surface were observed. At higher accelerating voltages a greater appearance of depth could be seen but there was less information on the fine surface detail and its angular orientation. The combined technique of HC-BSE imaging and stereo imaging should be useful for detailed studies on material surfaces and for biological samples with greater contrast and directional sensitivity than can be obtained with current SE or BSE detection modes.     

Nanoscale characterization of gold nanoparticles created by in situ reduction at a polymeric surface

Transmission Electron Microscopy is used as a quantitative method to measure the shapes, sizes and volumes of gold nanoparticles created at a polymeric surface by three different in situ synthesis methods. The atomic number contrast (Z‐contrast) imaging technique reveals nanoparticles which are formed on the surface of the polymer. However, with certain reducing agents, the gold nanoparticles are additionally found up to 20 nm below the polymer surface. In addition, plan‐view high‐angle annular dark‐field scanning transmission electron microscopy images were statistically analyzed on one sample to measure the volume, height and effective diameter of the gold nanoparticles and their size distributions. Depth analysis from high‐angle annular dark‐field scanning transmission electron microscopy micrographs also gives information on the dominant shape of the nanoparticles.     

SEM analyses of cathodoluminescence in MgO,CdS and GaAs/GaxAl1–xAs crystals

The method of analysis developed for the study of cathodoluminescence in the scanning electron microscope is briefly outlined. Current studies of cathodoluminescence in three types of crystal are presented. These are: (1) deformed and annealed MgO, (2) CdS into which Ga has been diffused, and (3) double heterostructure GaAs/Ga_xAl_1–xAs laser material.