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.     

On the visibility of small metallic particles on crystalline substrates

Dynamical diffraction theory was used to study the contrast of small metallic particles supported on crystalline substrates. The dependence of the particle visibility on the orientation and thickness is discussed. Experimental examples of changes in particle visibility for Pt/alumina and Pt/sapphire are presented.     

HREM and STEM of defects in multiply-twinned particles

The structure of defects in multiply-twinned particles has been studied in detail using high-resolution lattice imaging, dark field and microdiffraction techniques. Icosahedral particles with sizes greater than about 15 nm were found to contain defects, in the form of stacking fault loops parallel with the external surface, which were extremely difficult to detect by conventional amplitude contrast techniques. Microdiffraction mappings correlated with these results, showing large rotations of the face-centred cubic segments. Results for decahedral particles indicated the presence of stacking faults running adjacent to, and parallel with, the twin boundaries. Microdiffraction maps confirmed that the particle structure was face-centred cubic, and also verified that the apparent epitaxy of these particles was highly variable. Models for the defects are proposed and discussed, and the relative merits of HREM and STEM for elucidating structural details in small particles is briefly considered. Finally, the potential for direct imaging at surfaces, as demonstrated by some recent images, is discussed.     

Discrimination of particles by the scintillation pulse shape in the range of low energies (6–100 keV for electrons) using the linear filters

The use of the linear filters for particle discrimination by the scintillation pulse shape is considered. The particle separation process has been simulated by the Monte Carlo method, taking into account that the slow scintillation component decays according to the hyperbolic law and that its relative contribution is energy dependent. The best figures of merit of particle discrimination attainable with this technique have been obtained assuming that the PMT and electronic noises are zero. It is shown that, by contrast to the zero crossing method, pulse shape discrimination of particles using the linear filters can ensure rejection of the γ-ray background to a level of ∼10⁻⁴ at particle energies of up to 12 keV of the electron equivalent. For energies of <24 keV, it is expedient that the signal acquisition time be increased to a few microseconds.     

Characterization of polymer blends and block copolymers by conventional and low voltage SEM

Methods for characterizing block copolymers and polymer blends have been developed. Results from unstained and ruthenium tetroxide-stained samples obtained by scanning electron microscopy at various acceleration voltage and by transmission electron microscopy are presented. The contrast in secondary images between components in stained polymer blends, where one component is preferentially stained, is maximized at higher acceleration voltage (10–25 keV). For measurement of particle size and shape, this is the preferred operating condition. To obtain high-resolution images showing surface topography and fine structure, such as 20 nm domains in block copolymers, low-voltage (<5 keV) imaging is preferred. Observation of the 20 nm domain structure in hydrogenated styrenebutadiene-styrene shows that the spatial resolution now possible by scanning electron microscopy is comparable to that obtained by the traditional method of transmission electron microscopy.     

Analysis of Kikuchi band contrast reversal in electron backscatter diffraction patterns of silicon

We analyze the contrast reversal of Kikuchi bands that can be seen in electron backscatter diffraction (EBSD) patterns under specific experimental conditions. The observed effect can be reproduced using dynamical electron diffraction calculations. Two crucial contributions are identified to be at work: First, the incident beam creates a depth distribution of incoherently backscattered electrons which depends on the incidence angle of the beam. Second, the localized inelastic scattering in the outgoing path leads to pronounced anomalous absorption effects for electrons at grazing emission angles, as these electrons have to go through the largest amount of material. We use simple model depth distributions to account for the incident beam effect, and we assume an exit angle dependent effective crystal thickness in the dynamical electron diffraction calculations. Very good agreement is obtained with experimental observations for silicon at 20 keV primary beam energy.     

Optimized HREM imaging of objects supported by amorphous substrates using spherical aberration adjustment

High Resolution Electron Microscopy (HREM) is often used to characterize objects supported by amorphous substrates, usually amorphous carbon. HREM is currently undergoing step change in performance due to aberration correctors. This paper examines the aberration corrected imaging of objects supported by amorphous substrates. In particular, we show that a substantial increase in the ratio of the object contrast to the substrate contrast can be achieved by utilizing the strong variation of phase contrast with height, which is present when the spherical aberration has been adjusted to a small value. This variation is examined using the familiar Weak Phase Object Approximation model from which it is determined that the contrast ratio achieves a maximum at a small nonzero value of the spherical aberration. This result is confirmed by multislice modelling which allows for deviations from the Weak Phase Object Approximation and delocalization effects. One important practical result of this study is the need to place the object of interest on the correct side of the amorphous carbon substrate.     

Computer-controlled spot-scan imaging of crotoxin complex crystals with 400 keV electrons at near-atomic resolution.

400 keV electrons yield a better relative image contrast than 100 keV electrons for a beam-sensitive organic crystal when spot-scan imaging is used [J. Brink and W. Chiu, J. Microscopy 161 (1991) 279]. A FORTRAN 77 program has been written to operate the spot-scan imaging system on a computer workstation under the VMS operating system which is interfaced serially to the JEOL4000 electron microscope. We demonstrate the application of this implementation by imaging crotoxin complex crystals embedded in either vitreous ice or glucose to 2.5 A resolution. The intensity strength of the structure factors of this protein crystal are different at low (> 10 A) resolution but similar at high resolution (< 10 A) for the two embedding media as expected from their scattering contrast difference. Based on our experience as judged from the electron diffraction patterns of highly tilted crystals, flat crystals embedded in glucose can be readily obtained. Furthermore, our spot-scan imaging system also has the option of correcting the focus gradient that is present in images of tilted specimens.     

Direct versus iterative structure retrieval for a Cu/Ti misfit dislocation: a comparison of various 1Å HREM technologies

Using an interface between Cu/Ti as an example, two HREM-based image analysis techniques, strain mapping and iterative digital image matching, are compared. The validity limit of these techniques is discussed as a function of specimen thickness and microscope technology. Two criteria are used to assess the limits: (i) the difference between two geometric phase maps, one calculated in image plane and one in object plane, and (ii) a difference image from two HREM simulations of two structure models differing in one atomic column. The latter displays the overall delocalisation of information by the microscope due to diffraction and imaging. It is outlined how far images and strain maps, obtained for high-voltage microscopy at 1250 kV and C _S correction at 200 kV, are identical. Both techniques exhibit a significantly increased regime of applicability of strain mapping near defect cores. Simulations for a 400 kV HREM and a 300 kV FEGTEM with and without focal series reconstruction complement the study.     

High-resolution electron microscopy and its applications

A review of research on high-resolution electron microscopy (HREM) carried out at the Institute of Physics, the Chinese Academy of Sciences, is presented. Apart from the direct observation of crystal and quasicrystal defects for some alloys, oxides, minerals, etc., and the structure determination for some minute crystals, an approximate image-contrast theory named pseudo-weak-phase object approximation (PWPOA), which shows the image contrast change with crystal thickness, is described. Within the framework of PWPOA, the image contrast of lithium ions in the crystal of R-Li₂Ti₃O₇ has been observed. The usefulness of diffraction analysis techniques such as the direct method and Patterson method in HREM is discussed. Image deconvolution and resolution enhancement for weak-phase objects by use of the direct method are illustrated. In addition, preliminary results of image restoration for thick crystals are given.     

High resolution electron microscopy of crystalline polymer wedges

High resolution electron micrographs (HREM) of wedge-shaped crystalline samples of the polydiacetylene 1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD) are compared to quantitative predictions of image contrast obtained from dynamical electron scattering theory. Multislice calculations using experimentally determined instrument operating parameters make it possible to interpret the variation in HREM image contrast as a function of crystal thickness. Pendell?sung plots of the intensity of the main beam and the scattered beams as a function of thickness corroborate characteristic features in the lattice images including extinctions and half-spacings. The position of these contrast features with respect to the edge of the polymer crystal wedge are compared to the theoretical calculations and used to estimate the height profile. This profile is then compared to wedge height profiles measured with scanning probe microscopy (SPM). The two approaches give similar results for DCHD crystal thicknesses below 50 nm.     

采用三点定位原理的反射镜支撑结构设计

在选择空间反射镜的支撑形式时,大口径反射镜一般采用柔性支撑结构加背板的支撑结构;小尺寸反射镜采用周边支撑的结构,但支撑结构质量都较大。为了降低反射镜支撑结构质量,通过对三点定位原理的自由度分析,设计了3个柔性铰链结构,通过柔性铰链的结构参数分析及有限元分析计算,设计出参数为R=1mm,t=2.5mm的三点柔性铰链,实现400mm口径反射镜的周边支撑,并使支撑反射镜的三个铰链直接与相机机身连接,省略了常规反射镜支撑的背板及边框,大大降低支撑结构质量;且该支撑结构具有较好的动态刚度和力、热环境适应能力。     

Lattice measurement and alloy compositions in metal and bimetallic nanoparticles

A new reliable method for determining the lattice spacings of metallic and bimetallic nanoparticles in phase contrast high resolution electron microscopy (HREM) images was developed. In this study, we discuss problems in applying HREM techniques to single metal (Pt and Au) and bimetallic (AuPd) nanoparticles of unknown shapes and random orientations. Errors arising from particle tilt and edge effects are discussed and analysis criteria are presented to reduce these errors in measuring the lattice parameters of nanoparticles. The accuracy of an individual particle lattice measurement is limited by an effective standard deviation which depends on the size of the individual nanoparticle. For example, the standard deviation for 20-30 A Pt or Au nanoparticles is about 1.5%. To increase the accuracy in determining the lattice spacings of nanoparticles, statistical methods have to be used to obtain the average lattice spacing of an ensemble of nanoparticles. We measured approximately 100 nanoparticles with sizes in the range of 20-30 A and found that the mean lattice spacing can be determined to within 0.2%. By applying Vegard's law to the AuPd bimetallic systems we successfully detected the presence of alloying. For 30 A nanoparticles, the estimated ultimate error in determining the composition of the AuPd alloy is about 3% provided that at least 100 particles are measured. Finally, the challenges in determining the presence of more than one alloy phases in bimetallic nanoparticle systems were also discussed.     

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy-loss spectroscopy (EELS), and, to a lesser extent, by high-resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal-ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near-edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al-terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O₃M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion-bonded Nb/Al₂O₃ interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.     

In situ observation of oxygen deficiency occurring during electron irradiation in high Tc superconductor YBa2Cu3Oy

The decreasing process of oxygen in YBa₂Cu₃O _y is investigated through high resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED). Measurements of the axial length in HREM images show that oxygen content y decreases faster near a twin boundary than at the inner part of a twin lamella. The transformation from an orthorhombic to a tetragonal phase starts at a twin boundary and the transformed region propagates to an inner region of lamella. Lattice strains are observed near boundaries between transformed and non-transformed regions. The transformation is almost complete within 30 s during observation of HREM images at 400 kV and at room temperature. A value of y was quantitatively measured by analysing observed intensities of energy-filtered CBED patterns with the dynamical theory. The value of y decreases from 6.9 to 6.5 when 200 kV electrons are irradiated for 160 s in a microscope at 108 K. More precise analysis of the intensities provides information on charge distribution along the c -axis as well as local oxygen content at a spatial resolution of several nanometres.     

Microdiffraction and analysis

The intensity distribution in the microdiffraction pattern depends strongly on the coherence of the illuminated source. The coherence width at the specimen level is considered as a parameter to check the coherence of an electron microscope. Two application examples are illustrated. A small interphase precipitate of MgNi₂ in an Ni superalloy has been identified by a combination of microdiffraction and energy dispersive spectrum (EDS) techniques, though it can not be found by conventional transmission electron microscopy (TEM) and selected area diffraction (SAD). By means of microdiffraction as well as high-resolution electron microscopy (HREM) imaging a defect structure has been determined. Such a defect makes the lattice extinction spots appear with various intensities in the selected area diffraction and microdiffraction patterns.     

Observation of volatile cloud formation during the early stages of pulverized coal combustion

During the early stages of combustion when bituminous coal particles are in a nearly single particle combustion environment, a surrounding mantle of volatile products is observed. Initially, the cloud is of spherical shape and almost concentric with the particle. As the reaction progresses, more material is ejected in the shape of jet-like tails. This rapid mass release was observed through in-situ high speed photographs of particles in combustion environment, and was also confirmed by examining cross sections of the quenched samples of coal particles. The inner structure became porous, while base material softened and some portion of it was ejected due to high pressure build up inside. Direct sampling of the partly burned coal particles revealed that the material in the volatile cloud contained tarry substances and small sized particle fragments in addition to gaseous volatiles. These solid and condensible substances were seen as a luminous envelope which implied that they participated in the radiative transfer process.     

Excitation of the supported metal particle surface plasmon with external electron beam

Classical energy loss theory is used to study the surface plasmon excitations of a supported spherical particle under the non-retardation approximation. The predicted results for the case in which the metal sphere (Al) is half-embedded in dielectric support (AlF₃) are in good agreement with the experimental results. Theeffects of the surface oxide layer and of the particle size on the electron energy loss spectrum shape are presented. It is shown that the dipole approximation is not valid in the study of the supported particle structure. The applicabilities of both the results predicted by coherent-state theory and the results of the classical energy loss theory given in this paper for the study of the supported particle are discussed. It is suggested that use of the coherent-state theory for calculating the excitation probabilities of the surface plasmons is in reasonable agreement with the EELS experiment on the supported particle system for thin supports. The classical theoretical results used in this paper are applicable only for the relatively thick supports. The agreement of the experimental EELS results with either of these two theoretical predictions depends on the relative coupling strength of the particle with the support.     

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.