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Medium-range order in molecular materials: Fluctuation electron microscopy for detecting fullerenes in disordered carbons
Abstract
During a fluctuation electron microscopy (FEM) study of disordered carbons, we found that samples containing C(60) exhibit a normalized variance peak at 7.1 nm(-1) that appears to be a unique indicator of tight curvature in layered materials. This peak is associated with the characteristic in-plane carbon-carbon bond distance of approximately 0.14 nm in graphene. Diffraction from this spacing is normally forbidden in planar graphene (and graphite), but becomes allowed when the layer structure is interrupted. Such interruptions arise at the edges of graphite fragments and also when 5-rings are incorporated into a layer. We show that the curvature induced by a high density of 5-rings, such as that in C(60), can dominate the variance peak at 7.1 nm(-1). FEM simulations reveal that the variance peak at approximately 7.1 nm(-1), which we label F(1), is one of several fullerene-signature peaks, with others occurring at Q values of 10.6 nm(-1) (F(2)) and 12.4 nm(-1) (F(3)). We conclude that FEM is a sensitive method for detecting dilute quantities of highly curved pentagon-rich fullerenes, such as C(60), when dispersed within disordered graphitic carbon.
... This refers to the structural correlations of atomic positions on length scales between 0.5 and 3 nm. It has been shown that many "amorphous" systems, such as clusters [7], [8], metallic glasses [9], and amorphous silicon [1] actually have significant MRO. However, probing MRO in materials has proved to be the most challenging of all these ranges of structural order, it is too irregular for bulk X-ray methods and too spacious for typical diffraction methods. ...
... Such analysis has also been applied to other amorphous materials, such as Al-RE (RE: rare earth element) films [196], [197] and phase-change alloy Ge2Sb2Te5 [198]. Some materials do show essentially no speckliness, such as amorphous silica and some amorphous carbon films [7], consistent with a continuous random network structure, but speckliness is more the norm than the exception. ...
Crystallography loves order, but many organic materials are disordered and only partially, if at all, crystalline. Nonetheless, these disordered materials are functionally complex and require characterisation. Their lack of crystallinity poses not only fundamental questions about how to best describe such structures, but also blunts the typically precise tools of crystallography in describing atomic order. Yet, disordered structures not only have structural characteristics, but also complex micro- and nanostructures, defects, and phase distributions. Much of this can be gleaned from diffraction spots, but even more of the information lies in-between the spots in a diffraction pattern. The diffracted intensity outside of the diffracted spots contains the necessary information to not only obtain structural information, but to also characterise the disorder present. Recent developments in transmission electron microscopy (TEM) have enabled the collection of numerous spatially separated diffraction patterns across a specimen, and when combined with computational tools opened a new space for the crystallographic analysis of disordered materials. In scanning electron diffraction (SED), a two-dimensional diffraction pattern is acquired at each probe position in a two-dimensional scan across a specimen. This four-dimensional (4D) data set can be extensively manipulated post-acquisition using computational tools, enabling the acquisition of multiple correlated conventional TEM experiments at once. Yet this is just the tip of the iceberg. Within such a 4D data set, any pixel can be correlated with another, even ones that may seem at first glance unphysical. In this work, such computational microscopy is applied to SED data to characterise the structure of disordered materials. In this work, the requisite computational methods are developed and applied to extract crystallographic information in metal-organic frameworks through pair distribution function analysis, in pharmaceutical cocrystals through nanoscale twist characterisation, and in polymers through semi-crystalline variance and correlation analyses. As all information is contained within a single scan, all of this analysis is done at doses low enough to avoid irradiation damage in the probed beam-sensitive samples.
... This is supposedly due to their higher electron beam sensitivity compared to metallic glasses and semiconductor materials, which have been studied extensively by FEM [15][16][17][18][19][20][21][22]. Rare FEM studies of materials other than semiconductors and metallic glasses include [23][24][25][26]. ...
We have used fluctuation electron microscopy (FEM) to investigate the nucleation stage of TiO2 crystal formation in binary TiO2-SiO2 glasses with heat treatment. It was found that spatial fluctuations of electron scattering in the glass with 13 wt% TiO2 increases with heat treatment above 800 °C but before any crystals precipitate, i.e. before crystals are detectable by electron diffraction. We have attributed this to TiO2 clustering and increasing medium-range order due to gradual ordering of TiO2 phase. Moreover, we have found that FEM is sensitive to structural changes at temperatures as low as 400 °C but the nature of the changes yet to be determined. This demonstrates that FEM can be sensitive to structural changes in oxide glasses occurring during thermal treatment but preceding detection of the first crystals.
... In particular, shungite shows unique electrochemical properties due to high resistance to acids. The most intriguing is the question of the presence of fullerenes in natural shungite [1][2][3][4][5]. The discussion on this question continues, but some researchers propose to use a shungite as a raw material for fullerene preparation [6,7]. ...
Using a scanning electron microscopy, elemental analysis, electron paramagnetic resonance, and Raman scattering methods, two types of the shungite materials (Sh-II from Zazhogino deposit and shungite from a commercial filter (ShF)), with different carbon content and porosity, are studied in this work. It was established by scanning electron microscopy data that the structure of the shungite samples is formed by a micron-size agglomeration of carbon and silicon dioxide clusters. It is found from the Raman data that carbon fraction is formed from sp2-hybridized clusters, size of which increases from 9 up to 12 nm after annealing of the samples. High conductivity of shungite is found to belong to the carbon nanoclusters of different sizes. Big clusters give the conduction electron spin resonance signal with a Dysonian line shape with variable g-factor and line width.
The careful search of the nature of two other narrow electron paramagnetic resonance signals in shungite, which used to be prescribed to fullerene-like molecules, is fulfilled. Here, it is shown that the oxygen-deficient E'γ centers are responsible for these signals. A strong correlation is revealed between the concentration of Е'γ centers and the line width of conduction electron spin resonance signal, which occurs under annealing process of the samples at T = 570 K. The correlation reasons are a spin-spin coupling between two spin subsystems and time dependent of the Е'γ concentration during annealing process.
PACS: 81.05.U-, 76.30.-v, 73.61.Ph.
... FEM [45] uses spatially resolved electron nanodiffraction data from a large nanoscale area (e.g. 30 · 30 nm 2 ) to probe the so-called medium-range (5-30 Å ) order (MRO) of the solid structure, well beyond that which can be explicitly elucidated by XRD or selected area electron diffraction (SAED). It has been successfully used to study MRO in a range of materials [45], including amorphous carbon films [46,47], and graphitic carbons [48]. EELS-SI uses spatially resolved EELS to determine maps of sp 2 carbon content across nanoscale areas comparable to that used in FEM. ...
... Carbon nanotubes. anthropogenic amorphous and/or crystalline mixed complex nanominerals, and fullerenes have been detected in various geological materials in trace concentration (Buseck et al., 1992;Buseck, 2002;Jehlička et al., 2000Jehlička et al., , 2003Jehlička et al., , 2005Chen et al., 2004;Kovalevski et al., 2005;Hower et al., 2008;Silva et al., 2009;Vítek et al., 2009;Zhao et al., 2009;Silva et al., 2010a, b;. ...
A suite of high-As, high-C fly ashes from a university-based stoker-fired coal boiler were analyzed by a number of techniques, including high-resolution transmission electron microscopy (HR-TEM), time-of-flight secondary ion mass spectrometry (Tof-SIMS), X-ray photoelectron spectroscopy (XPS), and field-emission scanning electron microscopy (FE-SEM). The sooty carbon is in the form of nano balls with the major fullerenes at C60 + , C70 + , and C80 + , with species at C 2 increments from C 56 + to C 78 + .
... A number of coal properties, often interrelated, affect spontaneous combustion (e.g., Güney, 1968;Kaymakçi and Didari, 2002), including: 1) Moisture content and volatile matter content 2) Particle size and available surface area Carbon nanotubes, anthropogenic amorphous and/or crystalline mixed complex nanominerals, and fullerenes have been detected in various geological materials in trace concentration (Buseck, 2002;Jehlička et al., 2005;Hower et al., 2008;Silva et al., 2009;Vítek et al., 2009;Silva et al., 2010). These include hard coals, coal ashes, solid bitumen, clays from the Cretaceous-Tertiary boundary, and rocks from the Permian-Triassic boundary (Buseck et al., 1992;Jehlička et al., 2000;Jehlička et al., 2003;Chen et al., 2004;Kovalevski et al., 2005;Zhao et al., 2009;Silva and DaBoit;. ...
Mineral sublimates from the Ruth Mullins fire in abandoned underground and surface mines in the high volatile A bituminous Middle Pennsylvanian Hazard No. 7 coalbed, Perry County, Kentucky, were examined by optical mineralogy, X-ray diffraction (XRD), and high-resolution–transmission electron microscopy (HR-TEM). Optical examination revealed the presence of salammoniac and a fine, unidentified fibrous mineral. XRD also showed the presence of salammoniac, along with trace amounts of quartz, kaolinite, and, possibly, phengite. Both cubic and dendritic salammoniac forms were observed with HR-TEM. Gypsum, jarosite, with cubic pseudomorphs after pyrite, and Fe-minerals, including Cr-bearing hematite in association with jarosite, were observed with HR-TEM. Dehydration of jarosite can lead to the formation of less hydrous Fe-sulfates and hematite.
... Carbon nanotubes. anthropogenic amorphous and/or crystalline mixed complex nanominerals, and fullerenes have been detected in various geological materials in trace concentration (Buseck et al., 1992;Buseck, 2002;Jehlička et al., 2000Jehlička et al., , 2003Jehlička et al., , 2005Chen et al., 2004;Kovalevski et al., 2005;Hower et al., 2008;Silva et al., 2009;Vítek et al., 2009;Zhao et al., 2009;Silva et al., 2010a, b;. ...
Coal fires typically generate a variety of mineral and organic deposits associated with the venting emission gases. In addition to the tars typically found at the Ruth Mullins coal fire, Perry County, Kentucky, a sooty carbon, superficially similar to a carbon from a university-based stoker-fired power plant, was sampled in an August 2010 visit. Carbons in the soot include complex carbon particles, nanotubes encapsulating Hg, onion-like structures with polyhedral and quasi-spherical morphology with hollow centers, and metal-bearing multiwalled nanotubes. Mineral and amorphous inorganic phases included glassy Al–Si spheres with associated Pb and Se; nanopyrite grains with trace As and Se; nanohematite with V3+; salammoniac; quartz; Cr- and Pb-bearing jarosite; fibrous pickeringite with surficial natrojarosite; and Cd-, Co-, Mo-, Ni, V-, W-, and Zr-bearing nanospheres. The enrichment of 15N in the soot is associated with the fractionation of NH3 to NH4 in the formation of salammoniac. Selenium, Pb, and Zn are found in relatively high concentrations in the soot and Hg, with 5.68ppm, has a higher concentration than any Kentucky fly ash.
This review highlights the implications of the local crystal structure for phonon dynamics and explores various strategies for enhancing thermoelectric performance in crystalline materials through local structure engineering.
Structural disorder in molecular crystals is a fundamental limitation for achieving high charge carrier mobilities. Quantifying and uncovering the mechanistic origins of disorder are, however, extremely challenging. Here we use variable coherence transmission electron microscopy to analyze disorder in tri-isopropyl silane pentacene films, utilizing diffuse scattering that is present both as linear streaks and as a slowly varying, isotropic background. The former is due to thermal vibration of the pentacene molecules along their long axis, while the latter is due to static defects kinetically frozen during film deposition. The thermal vibrational amplitude is ∼0.4 Å, while the static displacement parameter in our simplified analysis is much larger (1.0 Å), because it represents the cumulative scattering of all defect configurations that are frozen in the film. Thin film fabrication therefore has an important effect on crystallinity; our technique can be readily used to compare samples prepared under different conditions.
In this paper, we show that low density nano-porous amorphous carbon (a-C) consists of interconnected regions of amorphous graphene (a-G). We include experimental information in producing models, while retaining the power and accuracy of \textit{ab initio} methods with no biasing assumptions. Our models are highly disordered with predominant bonding and ring connectivity mainly of sizes 5-8. The structural, dynamical and electronic signatures of our 3-D amorphous graphene is similar to the monolayer amorphous graphene. We predict an Extended X-ray Absorption Fine Structure (EXAFS) signature of amorphous graphene. Electronic density of states calculations for 3-D amorphous graphene reveal similarity to monolayer amorphous graphene and the system is not conducting.
The correlation between morphology, local structure and magnetic properties of the different origin shungite material with nanocarbon content 25-40 wt. % was studied by SEM, EPR, and Raman scattering methods. It was established that structure of the shungite samples is formed by micron size agglomerations of carbon and silicon dioxide clusters with impregnations of pyrite (FeS2), iron oxide and aluminium oxide particles. It was found from the Raman data that nanocarbon fraction is formed from sp2-hybridized well ordered carbon clusters, size of which increases from 9 nm up to 12 nm after annealing of the samples. It was found for the first time that origin of L3 and LI EPR lines is due to oxy-deficiency centers in the silicon dioxide clusters.
IntroductionWhat Is Speckle?What Causes Speckle?Diffuse ScatteringFrom Bragg Reflections to SpeckleCoherenceFluctuation Electron MicroscopyVariance versus MeanSpeckle StatisticsPossible Future Directions for Electron Speckle AnalysisReferences
We compare experimental fluctuation electron microscopy (FEM) speckle data with electron diffraction simulations for thin amorphous carbon and silicon samples. We find that the experimental speckle intensity variance is generally more than an order of magnitude lower than kinematical scattering theory predicts for spatially coherent illumination.
We hypothesize that decoherence, which randomizes the phase relationship between scattered waves, is responsible for the anomaly. Specifically,
displacement decoherence
can contribute strongly to speckle suppression, particularly at higher beam energies. Displacement decoherence arises when the local structure is rearranged significantly by interactions with the beam during the exposure. Such motions cause diffraction speckle to twinkle, some of it at observable time scales.
We also find that the continuous random network model of amorphous silicon can explain the experimental variance data if displacement decoherence and multiple scattering is included in the modeling. This may resolve the longstanding discrepancy between X-ray and electron diffraction studies of radial distribution functions, and conclusions reached from previous FEM studies.
Decoherence likely affects all quantitative electron imaging and diffraction studies. It likely contributes to the so-called Stobbs factor, where high-resolution atomic-column image intensities are anomalously lower than predicted by a similar factor to that observed here.
Fluctuation electron microscopy is a technique for measuring nanoscale order in amorphous materials. Implementing fluctuation microscopy using electron nanodiffraction in a STEM has significant advantages, including improved coherence and a high degree of flexibility in the probe forming optics. Here we review the fluctuation microscopy technique in the STEM, including theory, practice, and example applications.
Fluctuation electron microscopy (FEM) is a nanodiffraction technique for measuring nanometer scale order in amorphous materials. Either dark-field transmission electron microscope imaging or coherent electron nanodiffraction in a scanning transmission electron microscope is used to measure the diffracted intensity from the sample with nanometer spatial resolution. Fluctuations in that intensity from place to place on the sample reveal the size, density, and internal atomic arrangements of nanometer-scale order. Nanometer scale order in glasses is also called medium-range order (MRO). Fluctuation microscopy is one of the few experimental techniques sensitive to this type of structure in amorphous materials.Keywords:fluctuation electron microscopy;amorphous materials;nanodiffraction;FEM simulation;medium-range order
Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.
The structures of four types of amorphous silicon are examined by an experimentally constrained structural relaxation method (ECSR). Experimental selected area electron diffraction data and fluctuation electron microscopy normalized diffraction variance data were used as constraints to guide a Monte Carlo relaxation procedure towards best fit models. A Tersoff potential was also used to further restrict the space of possible solutions. The materials examined were self-ion-implanted silicon and pressure-amorphized silicon, both in their as-prepared and thermally annealed states. In the fitted models for these materials regions containing two types of medium-range order were identified. One type involves formation of paracrystallites with cubic and hexagonal structures, where both short-range crystalline and medium-range order are present. The other type of medium-range order appears in the form of extended crystalline planes without associated short-range crystalline order. These two types can coexist. It is observed that the best fit models for both as-prepared samples contain approximately 10–15% paracrystalline ordered regions, reducing to about 5–10% in the annealed materials. None of the models are true continuous random networks. We conclude that, with long computational times and with a suitable potential function, the ECSR procedure provides a powerful, although at present semi-quantitative, tool for determining the structural form of medium-range order in thin amorphous materials.
We used reverse Monte Carlo (RMC) modeling to simulate the atomic structure of a Zr-based bulk metallic glass (BMG), incorporating short-range structural data from the electron diffraction total reduced density function G(r) and medium-range structural data from fluctuation electron microscopy (FEM). Including the FEM data created within the model loosely ordered planar atomic arrangements covering regions ∼1 nm in diameter without degrading the agreement with G(r). RMC refinement against only G(r) produced no agreement with FEM. Improved simulations are needed to create fully realistic BMG structures, but these results show that including FEM in RMC further constrains the structure compared with G(r) data alone and that the FEM signal in real materials is likely to arise from pseudo-planar arrangements of atoms.
A fluctuation electron microscopy study of amorphous zircons is reported. A natural metamict zircon sample was compared with amorphous samples prepared by irradiating a gem-quality zircon crystal with a 1.0 MeV beam of N2+ ions over a range of fluences up to 1.5 × 1017 ions/cm2. The two types of amorphous material exhibit similar selected-area diffraction patterns. However, the statistics of the scattering from 1 nm wide volumes indicates that they have significantly different normalised intensity variance plots. Modelling of the variance suggests that the natural metamict zircon consists of paracrystalline grains of zircon embedded within an amorphous ZrSiO4 matrix. The ion-irradiated samples are better modelled as segregated paracrystalline grains of ZrO2 composition within an amorphous SiO2-rich matrix. Although both samples have been rendered disordered by radiation damage, there are two important differences. First, the ion-irradiated materials experience significantly higher fluence rates compared with the metamict materials. Second, the metamict zircon has had geologic time to anneal. The experiments show that zircon, freshly-amorphised by ion-irradiation, exhibits different medium-range order to that in natural metamict zircon.
A nanocrystal/amorphous composite is an idealized model of the medium-range order structure in a variety of amorphous materials. We have investigated the fluctuation electron microscopy (FEM) signal from such a model analytically and with computer simulations. In the analytical modeling, we improved the previous model by Stratton and Voyles (Ultramicroscopy 108, 727 (2008)) by introducing the partial occupancy of nanocrystals in a column, the effect of the deviation parameter on the diffracted intensity, and a distribution of nanocrystal sizes. The improved model no longer has a maximum in the FEM signal as a function of the volume fraction of nanocrystals. In the computer modeling, we compared the variance calculated using kinematic scattering and dynamical scattering and investigated the effects of disorder and strain inside the nanocrystals on the variance. The variance based on dynamical scattering is about 15% smaller than that based on kinematic scattering. Disorder introduced in the nanocrystal reduces the variance peaks at all scattering vectors, with a much larger reduction at longer scattering vector.
Paracrystalline
Amorphous silicon has traditionally been represented by a continuous random network model in which there is no long-range ordering for the atoms, and some have less than fourfold coordination, which form dangling bonds—a type of defect. Treacy and Borisenko (p. 950 ; see the Perspective by Gibson ) used fluctuation electron microscopy to explain that models including regions of crystalline order are needed to fit the observed local variations in structure. Thus, on the 1- to 2-nanometer-length scale, this material should be thought of as having a paracrystalline structure containing localized crystalline regions.
We examine simulated electron microdiffraction patterns from models of thin polycrystalline silicon. The models are made by a Voronoi tessellation of random points in a box. The Voronoi domains are randomly selected to contain either a randomly-oriented cubic crystalline grain or a region of continuous random network material. The microdiffraction simulations from coherent probes of different widths are computed at the ideal kinematical limit, ignoring inelastic and multiple scattering. By examining the normalized intensity variance that is obtained in fluctuation electron microscopy experiments, we confirm that intensity fluctuations increase monotonically with the percentage of crystalline grains in the material. However, anomalously high variance is observed for models that have 100% crystalline grains with no imperfections. We confirm that the reduced normalized variance, V(k,R) - 1, that is associated with four-body correlations at scattering vector k, varies inversely with specimen thickness. Further, for probe sizes R larger than the mean grain size, we confirm that the reduced normalized variance obeys the predicted form given by Gibson et al. [Ultramicroscopy, 83, 169-178 (2000)] for the kinematical coherent scattering limit.
The structures of many disordered materials are not ideally random, but contain structural order on the scale of 1-3 nm. However, such nanoscale order, called medium-range order, cannot be detected by conventional diffraction methods in most cases. Fluctuation transmission electron microscopy (FTEM) has the capability to detect medium-range order in disordered materials based on statistical analysis of nanodiffraction patterns or dark-field images from TEM. FTEM has been successful in demonstrating the theoretically predicted development of nanoscale nuclei in amorphous chalcogenides, as well as in revealing the subtle effect of different preparation routes on the medium-range order in amorphous semiconductors and metals. The fluctuation principle can also be applied to study structural order on longer length scales in polymers and other disordered materials using X-rays or visible light. Further advances in theory and practice of FTEM will greatly increase our understanding of amorphous structures and nucleation phenomena.
The Lower Proterozoic rocks (2.0 to 2.1 Ga) of the Shunga district near Lake Onega, Karelia, Russia contain large amounts of elemental carbon - about 25 × 1010 tonnes in an area of roughly 9,000 square kilometers. The rocks occur in a horst-graben transition zone between the Baltic Shield and the Russian Platform. Biogenic, metasomatic, and volcanogenic origins have variously been proposed for the carbon in these rocks. Most rocks in a 1200- to 2000-m stratigraphic sequence contain at least several weight percent carbon, and localized areas contain up to 98 wt% glassy carbon, a most unusual natural form. The glassy carbon is deep black, has a pronounced conchoidal fracture, and a Mohs hardness of 3.5. Its high luster makes it look almost metallic; it has a low density (1.9-2.0 g/cm3) and high electrical conductivity (about 100 S/cm). The glassy bodies are relatively small (tens of meters in extent) and extremely brittle. The carbon shows diffuse X-ray spectra; high-resolution transmission electron microscopy images indicate that limited structure exists, primarily in the form of poorly organized graphite-like layers in roughly rounded units, but there is considerable heterogeneity. Analysis of carbon isotope ratios of samples from three localities yields δ13C values between -26.4 and -37.6‰ PDB. Values correlate to locality rather than to rock type, suggesting that the glassy carbon was locally remobilized from the surrounding country rocks. In one sample, clasts of almost pure glassy carbon have a value of -37.5‰ and occur in a matrix containing roughly 30 wt% carbon with a composition of -37.6‰. In samples from another locality, vein material of almost pure carbon (-26.7‰) cross-cuts rock of the same isotopic composition (-26.5‰), but also with only roughly 30 wt% carbon. The authors differ regarding the implications of the carbon-isotopic data. JWV and PRB interpret them as indicating a biogenic origin, either in situ or remobilized during low-grade metamorphism, whereas LPG interprets the field and isotopic data as indicating an abiogenic, volcanic origin.
An ideal atomistic model of a disordered material should contradict no experiments, and should also be consistent with accurate force fields (either ab initio or empirical). We make significant progress toward jointly satisfying both of these criteria using a hybrid reverse Monte Carlo approach in conjunction with approximate first-principles molecular dynamics. We illustrate the method by studying the complex binary glassy material g-GeSe2. By constraining the model to agree with partial structure factors and ab initio simulation, we obtain a 647-atom model in close agreement with experiment, including the first sharp diffraction peak in the static structure factor. We compute the electronic state densities and compare to photoelectron spectroscopies. The approach is general and flexible.
In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 °C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However, after high temperature annealing up to 1000 °C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 °C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during and subsequent to film growth, and this may play an important role in stress relaxation. © 2004 American Institute of Physics.
Electron energy-loss spectroscopy was used to monitor structural damage to solid C 60 as a function of electron exposure. The characteristic dose was found to be in the range 300–700 C/cm2 for incident energies in the range 100–200 keV and specimen temperatures between 100 and 300 K. The absolute value of this dose, and its energy and temperature dependence, suggest that the damage mechanism is predominantly electronic rather than knock-on displacement. © 1999 American Institute of Physics.
The shungites of Karelia (Russia) form a large, diverse group of black Precambrian rocks, all of which contain an intriguing type of poorly crystalline carbon. Wide differences of opinion exist about its structural state and its relation to carbon from other geological environments and origins. We used a variety of measurement techniques to determine the structural features of the carbon in shungite samples and to relate them to other natural sources of carbon. Although there is a wide range of types of shungite rocks, it appears as if the structure of their carbon is similar throughout in respect to high-resolution transmission electron microscopy (HRTEM) images, and electron and X-ray diffraction patterns. Other samples whose carbon is indistinguishable using these techniques include those from the Erickson gold mine (Canada), the Sovetskaya gold mine (Russia), and the Sudbury impact structure (Ontario). Carbon samples from different localities of the Shunga district are characterized by containing curved layers, similar to samples from natural and synthetic cokes. The HRTEM images and nanodiffraction patterns of shungites suggest that some 3-dimensional closed shells occur but, more commonly, there are fractions of such shells or regions of structure that are highly disordered into bent stacks of graphene layers.
We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3ox), Axtell (CV3ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.
Using fluctuation electron microscopy, we have observed an increase in the mesoscopic spatial fluctuations in the diffracted intensity from vapor-deposited silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. We interpret this increase as an increase in paracrystalline medium-range order in the sample. A paracrystal consists of topologically crystalline grains in a disordered matrix; in this model the increase in ordering is caused by an increase in the grain size or density. Our observations are counter to the previous belief that the amorphous to polycrystalline transition is a discontinuous disorder-order phase transition.
An implementation of the Reverse Monte Carlo algorithm is presented for the study of amorphous tetrahedral semiconductors. By taking into account a number of constraints that describe the tetrahedral bonding geometry along with the radial distribution function, we construct a model of amorphous silicon using the reverse monte carlo technique. Starting from a completely random configuration, we generate a model of amorphous silicon containing 500 atoms closely reproducing the experimental static structure factor and bond angle distribution and in improved agreement with electronic properties. Comparison is made to existing Reverse Monte Carlo models, and the importance of suitable constraints beside experimental data is stressed. Comment: 6 pages, 4 PostScript figures
Graphite exists in both hexagonal and rhombohedral forms. Usually about 5% of well-crystallised graphite, natural or synthetic, is of the rhombohedral form. In previous HRTEM studies on carbonization and graphitization only the (0002) spacings (d = 3.36Å) were resolved. In order to study the relationship between the hexagonal and rhgmbohedral phases directly, it is necessary to resolve the (10 0) (d = 2.13Å) and the (10 1) (d = 2.03Å) spacings. This study is concerned with graphite of less than perfect crystallinity. Compared to perfectly crystallised graphite, these samples have a smaller crystallite size and more diffuse diffraction spots. The samples were studied at 400kV in a 4000EX TEM (top entry, ±15° double-tilt, C s = 1.0mm, structure resolution limit = 1.7Å).
Atomic structure on the 1-2 nm scale, often referred to as “medium-range order” (MRO), is of great importance for understanding the properties of disordered materials. It is, however, difficult to adequately characterize medium-range order. Fluctuation microscopy is a newly developed TEM technique that has successfully been employed to characterize MRO in amorphous semiconductors. The predominance of highly directional covalent bonds leads naturally to the presence of MRO in amorphous semiconductors. For metallic glasses, however, the bonding is primarily metallic and nondirectional. Thus, it is not readily apparent whether MRO would exist, or whether fluctuation microscopy is a useful tool for studying the structure of metallic glasses. in this work we report the use of fluctuation microscopy to identify differing degrees of MRO in Zr-based bulk metallic glasses; the MRO depends on the alloy content and the method of TEM specimen preparation.
Variable coherence microscopy shows that as-deposited amorphous germanium and silicon contain medium-range order. The ordering can be explained by a fine-grained paracrystallite material in which intergranular stress has been relaxed by deformation. On annealing, the paracrystallite structure transforms towards the lower-energy continuous random network (CRN). We present data on a variety of vacuum-evaporated samples, and on molecular dynamics simulations of candidate structures.
In a recent paper, Jungk et al. [Ultramicroscopy 104 (2005) 206–219] conclude that standard fluctuation electron microscopy speckle analysis, which examines the normalized variance of scattered intensity as a function of scattering angle, does not provide more information about disordered materials than does classical diffraction.In this letter, I point out flaws in both their experiments and their modeling that led them to this erroneous conclusion. In their experiments, they use resolutions that are excessively high relative to the length scale of the medium-range ordering in their samples, and consequently the diffraction fluctuations they seek are very weak. In their modeling, they attempt to correct this by applying low-pass image processing filters in order to broaden the effective point-spread function (i.e. to lower the image resolution after the experiment). Such filters, when applied to image intensities, do not emulate properly the evolution of coherent speckle as the point-spread function becomes wider. I affirm that fluctuation microscopy does indeed provide more information about disordered materials than classical diffraction when the experiments and modeling are conducted appropriately.
A broad review of the unusual one-dimensional properties of phonons in carbon nanotubes is presented, including phonons in isolated nanotubes and in crystalline arrays of nanotubes in nanotube bundles. The main technique for probing the phonon spectra has been Raman spectroscopy and the many unique and unusual features of the Raman spectra of carbon nanotubes are reviewed. Also included is a brief review of the thermal properties of carbon nanotubes in relation to their unusual phonon dispersion relations and density of states.
Shungites are highly carbonaceous, highly metamorphosed rocks associated with several stratigraphic levels of the Lower Proterozoic in Karelia. Being a complex material, shungite is very important also as oldest accumulations of concentrated organic matter. The nature of shungite is still unclear despite its more than a century-long study by Russian and western scientists.A chemical coal-petrographic study of shungite has been conducted using a specially designed technique: the etching of polished surfaces. It is found that the shungite bed (near Shunga village) is a coal seam with a typical set of megascopic features rather than an accumulation of bitumen. This bed consists not only of sapropelic but of humic organic matter as well.Unlike other biogenic structures from Precambrian coal, such as the Huron Series, Michigan Lake, the structures established in shungite near Shunga village have a more complex plant composition. Organic matter from various types of shungite-bearing rocks show different petrographic and chemical properties. Thus, organic matter of sapropelic nature from silicate rocks with traces of stromatolite texture, differs from the coal matter composing the Shunga bed. Shungites are the most metamorphosed coals among all those known at present and they are assigned here to metaanthracite rank. The results obtained show a new insight into the biogenic remains composing an ancient coal bed, and in terms of the present stratigraphic scale allow the beginning of coal accumulation to be placed in the Proterozoic, although the possibility of a non-Precambrian age cannot be ruled out.
Hollow-cone dark-field images from crystalline silicon were examined as a function of thickness and scattering angle. We find that both coherent scattering and multiple scattering can cause the intensity to depend non-linearly on thickness, even for high-angle scattering events. A simple method for estimating the effect of multiple elastic scattering in thick specimens is provided. We show theoretically, that a coherence volume exists around each scatterer whose dimensions depend inversely on scattering angle. The coherence volume has a narrow “cigar” shape, elongated along the optic axis. The narrowness ensures that inter-atomic-column interferences, such as zero-order Laue zone Bragg reflections, are suppressed, whereas the elongation permits intra-atomic-column interference (or columnar diffraction). In high-resolution “Z-contrast” images, this bias towards columnar diffraction acts to enhance the signal from atom columns.
We use variable coherence transmission electron microscopy to examine medium-range ordering in vacuum-deposited amorphous germanium. The method accesses higher-order atomic correlations through statistical analysis of hollow-cone dark-field image speckle. This yields greater sensitivity to medium-range order than the familiar pair correlation function obtained from diffraction. We find that thermal annealing of amorphous germanium reduces the degree of medium-range order, affirming the thermodynamic stability of the random network.
A series of hydrogenated diamondlike carbon films grown using plasma-enhanced chemical-vapor deposition is systematically studied as a function of source gas composition using transmission electron microscopy. The structure of the films is examined at three distinct length scales. Both plan-view and cross-sectional studies are undertaken to reveal any large-scale inhomogeneities or anisotropy in the films. The degree of medium-range order in the films is measured by performing fluctuation electron microscopy on the plan-view and cross-sectional specimens. Electron-energy-loss spectroscopy is employed to measure the mass density and sp²:sp³ carbon bonding ratios of the samples. Thus, inhomogeneity as a function of depth in the film is revealed by the measurements of the short- and medium-range orders in the two different sample geometries. Soft, low-density diamondlike carbon films with low coefficients of friction are found to be more homogeneous as a function of depth in the film and possess reduced medium-range order in the surface layer. We find that these properties are promoted by employing a high hydrogen content methane and hydrogen admixture as the growth ambient. In contrast, harder, denser films with higher coefficients of friction possess a distinct surface layer with a relatively elevated level of carbon sp³ bonding and a higher degree of medium-range order. The structure of the films is examined in the light of the energetics of the growth process. It appears that a high flux of penetrating hydrogen ions modifies the surface layer containing the remnant damage from the carbon ions, homogenizing it and contributing to a lowering of the coefficient of friction.
Plastic deformation in metallic glasses is governed by the initiation and propagation of shear bands. The successful use of bulk metallic glasses in structural applications will depend on controlling these processes to improve ductility and toughness. In Zr-Cu-Ni-Al metallic glasses, the addition of Ta can influence the structure of the material and hence the shear band behavior in two ways. At low Ta contents (<4 at.%), the material is amorphous but has enhanced order over length scales of 5-15 Angstrom Higher levels of Ta result in the precipitation of bcc Ta-rich solid solution particles in a metallic glass matrix. Under uniaxial compression, both of these materials show greater apparent plastic strain to failure than the glass without Ta. This appears to be the result of the influence of the structure on the initiation and propagation of shear bands in the amorphous matrix.
Fullerenes have been reported from diverse geologic environments since their discovery in shungite from Karelian Russia. Our investigation is prompted by the presence of onionskin-like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of Oklo, Gabon. The same series of extractions and the same instrumental techniques, laser desorption ionization and high-resolution mass spectroscopy (electron-impact mass spectroscopy), were employed to test for fullerenes in samples from three different localities: two sites containing putative fullerenes (Sudbury Basin and Russian Karelia) and one new location (Oklo, Gabon). We confirm the presence of fullerenes (C60 and C70) in the Black Tuff of the Onaping Formation impact breccia in the Sudbury Basin, but we find no evidence of fullerenes in shungite samples from various locations in Russian Karelia. Analysis of carbonaceous substances associated with the natural nuclear fission reactors of Oklo yields no definitive signals for fullerenes. If fullerenes were produced during sustained nuclear fission at Oklo, then they are present below the detection limit (˜100 fmol), or they have destabilized since formation. Contrary to some expectations, geologic occurrences of fullerenes are not commonplace.
Variable coherence microscopy, a tool for quantitative analysis of structural fluctuations in disordered materials, is introduced. The method involves transmission electron microscopy of thin films and uses hollow-cone illumination. Experiments were performed on annealed evaporated amorphous germanium. Although many aspects of the data agree with the continuous random network model, there is experimental evidence for additional medium-range structure on the 10-20 Angstrom scale.
A form of hydrogenated diamond-like-carbon, “near-frictionless carbon,” developed at Argonne National Laboratory has been studied by several spectroscopic techniques to determine the hydrogen content and carbon bonding within the film. The techniques used include hydrogen forward scattering, ultraviolet Raman spectroscopy, Fourier transform infrared spectroscopy, near-edge x-ray absorption fine structure, and fluctuation microscopy. These complementary techniques reveal the different types of carbon bonding, such as sp2 and sp3, the medium-range order in the film, and its composition. © 2004 American Institute of Physics.
We show, using variable coherence transmission electron microscopy, that light soaking of amorphous hydrogenated silicon thin films leads to structural changes. We speculate that the structural changes are associated with instability in the as-deposited material. We suggest that improved immunity to Staebler–Wronski degradation could be achieved by a less-ordered material which is closer to the ideal continuous random network. © 1998 American Institute of Physics.
X-ray and electron sources are extensively used to explore disordered structures. In the case of electron, small-angle diffraction can help to testify the argument about micro-crystallites in glassy states. Diffraction intensity has two types of variance in reciprocal space: radial and azimuthal. Previously, variance as a function of k was largely used to elucidate medium-range order in amorphous semiconductors. Here azimuthal variance is introduced. This variance reveals orientational order for possible crystallites. Furthermore, the oversampling method proposed here can change our view on amorphous structure. We find that a broad peak might not truly reflect one single crystallite. In fact, two reflections can be folded to yield one broad peak. In this paper, the issues are discussed with three examples: silica, silicon, and germanium. © 2004 American Institute of Physics.
Medium-range structural order consisting of polyhedral connections,
rings, and cluster structures in various covalent amorphous solids is
discussed. The experimental structural probes used to investigate the
structure of amorphous solids are described, as is the use of the first
sharp diffraction peak in diffraction data of such solids. The need for
new experimental techniques and theoretical descriptions of the
disordered state of matter is addressed.
We have developed a new electron microscopy technique called fluctuation microscopy which is sensitive to medium-range order in disordered materials. The technique relies on quantitative statistical analysis of low-resolution dark-field electron micrographs. Extracting useful information from such micrographs involves correcting for the effects of the imaging system, incoherent image contrast caused by large scale structure in the sample, and the effects of the foil thickness.
We outline recent advances in the fluctuation microscopy technique for probing medium-range structural correlations in disordered materials. The technique was originally developed for electron microscopy, but has now been extended to optical and x-ray microscopies.
We show that fluctuation microscopy can detect trace quantities of C60 in a disordered graphite matrix, even though the diffraction signature from the C60 is essentially undetectable. This result indicates that the technique can be used to discern dilute distributions of macromolecules in an otherwise disordered matrix.
We also report preliminary studies of interferometric fluctuation microscopy using cross-correlations in diffraction between coherent double probes. This is a form of holography where the diffraction patterns from two neighboring regions are allowed to overlap and interfere. Young's fringes appear wherever both regions scatter strongly. The cross-correlation can be examined as a function of probe separation to estimate a structure correlation length. This method holds much promise for studying medium-range order, since it isolates the essential four-body terms underpinning the fluctuation microscopy technique.
Fluctuation microscopy is a hybrid diffraction-imaging technique that detects medium range order in amorphous materials by examining spatial fluctuations in coherent scattering. These fluctuations appear as speckle in images and diffraction patterns. The volume of material contributing to the speckle is determined by the point-spread function (the resolution) of the imaging optics and the sample thickness. The spatial periodicities being probed are related to the diffraction vector. Statistical analysis of the speckle allows the random and non-random (ordered) contributions to be discriminated. The image resolution that gives the maximum speckle contrast, as determined by the normalized variance of the image intensity, is determined by the characteristic length scale of the ordering. Because medium range ordering length scales can extend out to about the tenth coordination shell, fluctuation microscopy tends to be a low image resolution technique.This review presents the kinematical scattering theory underpinning fluctuation microscopy and a description of fluctuation electron microscopy as it has been employed in the transmission electron microscope for studying amorphous materials. Recent results using soft x-rays for studying nanoscale materials are also presented. We summarize outstanding issues and point to possible future directions for fluctuation microscopy as a technique.
Using fluctuation electron microscopy, we have measured the medium-range order of magnetron sputtered silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. We find a smooth increase in the medium-range order of the samples, which we interpret in the context of the paracrystalline structural model as an increase in the size of and/or volume fraction occupied by the paracrystalline grains. These data are counter to the long-standing belief that there is a sharp transition between amorphous and polycrystalline structures as a function of substrate temperature.
We have measured aluminum-like medium range order (MRO) in amorphous Al92Sm8 using fluctuation electron microscopy (FEM). Here we show similar but not identical medium-range structure in amorphous Al88Y7Fe5. Both compositions begin to crystallize in less than 7 min under a 120 kV TEM electron beam. Beam-induced heating of these samples is minimal, so we suggest that crystallization is caused by beam-induced atomic displacements (knock-on) that introduces free volume or localized mixing, both of which will induce crystallization. If this is the case, the higher beam energies used for high-resolution TEM will exacerbate, not reduce, the problem. Studying these materials in the TEM therefore requires low-dose FEM. Since FEM is an inherently statistical technique, the goal of which is to characterize the entire sample structure, we can reduce the dose to any one area of the sample by spreading the total dose across more material. The only experimental limit is the noise of the electron detector, which makes FEM experiments possible even on metastable materials that suffer from beam damage.
We report on the occurrence of fullerenes in Proterozoic shungite (∼2 Ga) from the shungite mine, Kondopoga, Karelia, Russia (62.12°N 34.17°E). The presence of fullerenes has been confirmed by mass spectrometry, with peaks at 360 and 720 amu (atomic mass unit), powder X-ray diffraction showing ten diffraction peaks corresponding to the fullerite structure with a = 1.4201(5) nm, and 13C nuclear magnetic resonance (NMR) spectroscopic studies, showing a peak at 143.2 ppm. In the Kondopoga shungite mine, fullerenes occur in silty shales that have experienced greenshist facies metamorphism.
We propose an extension to the technique of fluctuation electron microscopy that quantitatively measures a medium-range order correlation length in amorphous materials. In both simulated images from computer-generated paracrystalline amorphous silicon models and experimental images of amorphous silicon, we find that the spatial autocorrelation function of dark-field transmission electron micrographs of amorphous materials exhibits a simple exponential decay. The decay length measures a nanometre-scale structural correlation length in the sample, although it also depends on the microscope resolution. We also propose a new interpretation of the fluctuation microscopy image variance in terms of fluctuations in local atomic pair distribution functions.
Abstract— The C K edge of Orgueil nanodiamonds (Cδ diamonds) was acquired by electron energy-loss spectroscopy (EELS), with an energy resolution of 300 meV. The spectra show peaks at 282.5, 284.7, and 286.4 eV, which occur in the band gap below the main diamond edge and are absent from the bulk diamond spectrum. These peaks are attributed to transitions from C 1s surface core levels to unoccupied surface states, and arise from single and π-bonded dangling bonds and C-H bonds. A shoulder to the main absorption edge at 287.8 eV may correspond to hydrocarbon adsorbates. These results can be used to further our understanding of Cδ diamond structure and may reveal the presence of a fullerene-like surface. The unique surface electronic states of the Cδ diamond surfaces are expected to affect their optical properties, which are dependent on features such as extent of H coverage, particle size, and surface structure.
Nano-diamonds isolated from acid dissolution residues of primitive carbonaceous meteorites (Allende and Murchison) were studied using high-resolution transmission electron microscopy. To discriminate among their most likely formation mechanisms, high-pressure shock-induced metamorphism or low-pressure vapor condensation, the microstructures of presolar diamond crystallites were compared to those of (terrestrial) synthesized nano-diamonds. The synthesized diamonds used for comparison in this study were produced by high-pressure shock waves generated in controlled detonations and by direct nucleation and homoepitaxial growth from the vapor phase in low-pressure chemical vapor deposition (CVD)-type processes. Microstructural features were identified that appear unique to shock metamorphism and to nucleation from the vapor phase, respectively. A comparison of these features to the microstructures found in presolar diamonds indicates that the predominant mechanism for presolar diamond formation is a vapor deposition process, suggesting a circumstellar condensation origin. A new presolar grain component has also been identified in the meteoritic residues, the (2H) hexagonal polytype of diamond (lonsdaleite).
Diffraction methods for determining structure in non-crystalline materials often rely solely on the determination of pair correlation functions, extracted from measurements of the diffracted intensity. A dark field image of a non-crystalline solid taken with a conventional transmission electron microscope contains phase information lost in the measurement of the diffracted intensity which can be accessed by evaluating a variance function. This variance function is defined in terms of spherical averages of the diffracted intensity and the mean square of the diffracted intensity. The latter contains higher order correlation information derived from correlations between two pairs of atoms. We examine the sensitivity of the variance function, to subtle atomic structural differences between carbon network structures. The structures have similar pair correlations, but different levels of diamond like bonding. The variance function is shown to give improved discrimination between the networks.
Despite occasional experimental hints, medium-range structural order in covalently bonded amorphous semiconductors had largely escaped detection until the advent of fluctuation electron microscopy (FEM) in 1996. Using FEM, we find that every sample of amorphous silicon and germanium we have investigated, regardless of deposition method or hydrogen content, is rich in medium-range order. The paracrystalline structural model, which consists of small, topologically ordered grains in an amorphous matrix, is consistent with the FEM data, and is rendered diffraction amorphous by strain effects. We present measurements on hydrogenated amorphous silicon deposited by different methods, some of which are reported to have greater stability against the Staebler–Wronski effect. The matrix material of these samples is relatively similar, but the order changes in different ways upon both light soaking and thermal annealing. Some materials are inhomogeneous, with either nanocrystalline inclusions or large area-to-area variation in the medium-range order. We discuss the implications of and future directions for understanding medium-range order.
Fullerenes have been reported from numerous terrestrial and meteoritic sources, but only at low concentrations. The occurrences are associated with localized energetic events such as lightning, impacts of extraterrestrial objects, and wildfires. Great interest has also been generated by a series of papers that report noble gases with anomalous isotopic compositions encapsulated within fullerenes. In spite of all of the published work, many unresolved questions remain about the identity and formation of fullerenes in geological environments.
Algorithms for the symmetry-adapted energy minimisation of solids using analytical first and second derivatives have been devised and implemented in a new computer program GULP. These new methods are found to lead to an improvement in computational efficiency of up to an order of magnitude over the standard algorithm, which takes no account of symmetry, the largest improvement being obtained from the use of symmetry in the generation of the hessian. Accelerated convergence techniques for the dispersion energy are found to be beneficial in improving the precision at little extra computational cost, particularly when a one centre decomposition is possible or the Ewald sum weighting towards real-space is increased.
An electronic device manipulates the primary beam in the conventional transmission microscope to produce a hollow cone of illumination with its apex located at the specimen. The device uses the existing tilt coils of the microscope, and modulates the D.C. signals to both x and y tilt directions simultaneously with various waveforms to produce Lissajous figures in the back-focal plane of the objective lens. Electron diffraction patterns can be recorded which reflect the manner in which the direct beam is tilted during exposure of a micrograph. In the bright-field imaging mode the device provides a microscope transfer function without zeros in all spatial directions and has been used to obtain high resolution images which are also free from the effect of chromatic aberration. A standard second condenser aperture is employed and the width of the cone annulus is readily controlled by defocusing the second condenser lens. The cone azimuthal angle is also controlled electronically; hence the device can also be used in the dark-field imaging mode. This device has been applied to imaging both amorphous and crystalline materials including biomolecular specimens.
Damaging fullerene C60 upon energetic irradiation has been modeled with molecular-dynamics simulations. The angular dependence of the threshold energy of the primary knock-on atom (PKA) escaping from the cage is investigated in the case of the initial PKA direction within a plane through the cage center and 6-6 ring fusions. The average threshold value is estimated to be about 29 eV. The simulations provided a detailed picture of the damaging processes, in which three mechanisms were revealed. Those included C60-C→C59, C60→CC59, and C60-2C→C58. The interactions between carbon atoms are described with the Tersoff mode modified to match a screened Coulomb potential at short range.
Fluctuation electron microscopy is a technique for studying medium-range order in disordered materials. We present an implementation of fluctuation microscopy using nanodiffraction in a scanning transmission electron microscope (STEM) at a spatial resolution varying from 0.8 to 5.0 nm. Compared to conventional TEM (CTEM), the STEM-based technique offers a denser scattering vector sampling at a reduced sample dose and easier access to variable resolution information. We have reproduced results on amorphous silicon previously obtained by CTEM-based fluctuation microscopy, and report initial variable-resolution measurements on amorphous germanium.
We have used fluctuation microscopy to reveal the presence of structural order on length scales of 1-2 nm in metallic glasses. We compare results of fluctuation microscopy measurements with high resolution transmission electron microscopy and electron diffraction observations on a series of metallic glass samples with differing degrees of structural order. The agreement between the fluctuation microscopy results and those of the other techniques is good. In particular, we show that the technique used to make thin specimens for electron microscopy affects the structure of the metallic glass, with ion thinning inducing more structural order than electro-polishing. We also show that relatively minor changes in the composition of the alloy can have a significant effect on the medium-range order; this increased order is correlated with changes in mechanical behavior.
The technique of fluctuation electron microscopy (FEM) is applied to thin films of amorphous germanium and of polycrystalline gold in a transmission electron microscope. Even though the method was introduced as a tool for quantitative analysis of structural fluctuations in amorphous materials, the basic principles are applicable to any disordered specimen independent of the dimension of disorder. Hence, we extended the technique of FEM to a well-known specimen, gold, whereby it was possible to reinterpret the results of the measurements on amorphous germanium. The hollow-cone dark field images, the statistical analysis of which is the basis of FEM, are examined with respect to the effects of frequency filtering, and are compared to electron diffraction. We find that the angular dependence of the normalised variance, as measured by FEM, yields information similar to the average intensity of hollow-cone dark field image series. Both plots are basically identical to a scan through a selected-area diffraction pattern convoluted by the corresponding angular resolution function. Hence, it appears questionable whether standard FEM analysis provides more information than the classical pair distribution function, which is experimentally limited to short-range order. Frequency selective analysis of the normalised variance, however, gives supplementary information on preferred inter-atomic distances related to the medium-range order of the specimen.
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.
This article is a technical comment and author response to the the discovery of naturally occuring fullerenes in Shunga rock samples. The commenting researchers did not find detectable amounts of fullerenes in a variety of carbon-rich rocks. They hypothesize that the fullerenes found in the Shunga sample were probably a localized event. The original researchers, Buseck and Tsipursky, provide a rebuttal with further comments including parallels to other minerological occurences. All agreed that more data are needed. 10 refs.
By means of high-resoluton transmission electron microscopy, both C(60) and C(70) fullerenes have been found in a, carbon-rich Precambrian rock from Russia The fullerenes were confirmed by Fourier transform mass spectrometry with both laser desorption and thermal desorption/electron-capture methods to verify that the fullerenes were indeed present in the geological sample and were not generated by the laser ionization event. The mass spectra were measured under conditions sufficient to resolve the (13)C/(12)C isotopic ratios for C(60) and C(70) and indicate that these ratios correspond to the normal range of isotopic values.