Showing papers by "Jannik C. Meyer published in 2018"
TL;DR: In this article, the authors demonstrated the direct manipulation of individual atoms in materials using scanning probe microscopy, allowing them to precisely move silicon impurities along an extended path, circulating a single hexagon or back and forth between the two graphene sublattices.
Abstract: The direct manipulation of individual atoms in materials using scanning probe microscopy has been a seminal achievement of nanotechnology. Recent advances in imaging resolution and sample stability have made scanning transmission electron microscopy a promising alternative for single-atom manipulation of covalently bound materials. Pioneering experiments using an atomically focused electron beam have demonstrated the directed movement of silicon atoms over a handful of sites within the graphene lattice. Here, we achieve a much greater degree of control, allowing us to precisely move silicon impurities along an extended path, circulating a single hexagon, or back and forth between the two graphene sublattices. Even with manual operation, our manipulation rate is already comparable to the state-of-the-art in any atomically precise technique. We further explore the influence of electron energy on the manipulation rate, supported by improved theoretical modeling taking into account the vibrations of atoms nea...
106 citations
TL;DR: This work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.
Abstract: We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS2) films. To this end, we deposit a model heterostructure of thin amorphous MoS2 films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS2 into crystalline MoS2 domains, thereby emulating widely used elevated temperature MoS2 synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS2 films in situ and at the atomic scale. Our observations suggest that during MoS2 processing, various MoS2 polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transi...
50 citations
TL;DR: In this paper, a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation.
Abstract: Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graph...
36 citations
TL;DR: D density functional theory simulations indicate that the QDs and QWs embedded in MoTe2 introduce new midgap states into the semiconducting material and may thus be used to control its electronic and optical properties.
Abstract: Studying the atomic structure of intrinsic defects in two-dimensional transition-metal dichalcogenides is difficult since they damage quickly under the intense electron irradiation in transmission electron microscopy (TEM). However, this can also lead to insights into the creation of defects and their atom-scale dynamics. We first show that MoTe2 monolayers without protection indeed quickly degrade during scanning TEM (STEM) imaging, and discuss the observed atomic-level dynamics, including a transformation from the 1H phase into 1T′, 3-fold rotationally symmetric defects, and the migration of line defects between two 1H grains with a 60° misorientation. We then analyze the atomic structure of MoTe2 encapsulated between two graphene sheets to mitigate damage, finding the as-prepared material to contain an unexpectedly large concentration of defects. These include similar point defects (or quantum dots, QDs) as those created in the nonencapsulated material and two different types of line defects (or quantu...
35 citations
TL;DR: This work has developed a software approach to correct for artifacts introduced by fast scans, making use of a scintillator and photomultiplier response that extends over several pixels.
22 citations
03 Aug 2018
TL;DR: In this paper, the shape of suspended 2D materials is measured from diffraction patterns recorded at different sample tilts while applying tensile strain on the sample carrier, allowing insight to the evolution of out-of-plane undulations in the graphene lattice.
Abstract: We demonstrate control over the three-dimensional (3D) structure of suspended 2D materials in a transmission electron microscope. The shape of our graphene samples is measured from the diffraction patterns recorded at different sample tilts while applying tensile strain on the sample carrier. The changes in the shape of the pattern and in individual diffraction spots allow us to analyze both corrugations and strain in the lattice. Due to the significant effect of ripples and strain on the properties of 2D materials, our results may lead to new ways for their engineering for applications. Corrugations in a free-standing graphene membrane can be measured in situ and tuned in an electron microscope chamber. A team led by Jani Kotakoski at the University of Vienna used transmission electron microscopy to control the 3D shape of suspended graphene samples. The diffraction patterns recorded from the samples were found to change when applying tensile strain, allowing insight to the evolution of out-of-plane undulations in the graphene lattice. Uniaxial pull was found to first flatten the corrugations in the direction parallel to the pulling force, after which the flattened graphene could be strained by continuous pulling. As the 3D ripples in graphene strongly affect its thermal and electronic properties, such direct control over its out-of-plane morphology may lead to graphene structures with tailored functionalities.
22 citations
TL;DR: This work presents an atomically resolved study on the dynamics of a monolayer CuPcCl16 crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy.
Abstract: Atomically resolved images of monolayer organic crystals have only been obtained with scanning probe methods so far. On the one hand, they are usually prepared on surfaces of bulk materials, which are not accessible by (scanning) transmission electron microscopy. On the other hand, the critical electron dose of a monolayer organic crystal is orders of magnitudes lower than the one for bulk crystals, making (scanning) transmission electron microscopy characterization very challenging. In this work we present an atomically resolved study on the dynamics of a monolayer CuPcCl16 crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy. The results show the dynamics and the radiation damage mechanisms in the 2D layer of this material, complementing what has been found for bulk crystals in earlier studies. Furthermore, being able to image the undamaged molecular crystal allows the characterization of new composites consisting of 2D materials and organic molecules.
20 citations
25 Sep 2018
TL;DR: In this article, the 3D structure of defects in graphene, in particular grain boundaries, is revealed through an optimization process where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found.
Abstract: We demonstrate insights into the three-dimensional (3D) structure of defects in graphene, in particular grain boundaries, obtained via a new approach using two transmission electron microscopy images recorded at different angles. The structure is revealed through an optimization process where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found. We first demonstrate that this method works using an embedded defect in graphene that allows direct comparison to the computationally predicted 3D shape. We then apply the method to a set of grain boundary structures with misorientation angles spanning nearly the whole available range (2.6°–29.8°). The measured height variations at the boundaries reveal a strong correlation with the misorientation angle with lower angles resulting in stronger corrugation and larger kink angles. Our results allow for the first time a direct comparison to theoretical predictions for the corrugation at grain boundaries, revealing the measured kink angles are significantly smaller than the largest predicted ones.
17 citations
TL;DR: In this article, the focused helium ion beam of a HIM is utilized to create nanopores with diameters down to 1.3 nm, which can be milled into silicon nitride, carbon nanomembranes (CNMs) and graphene with well-defined aspect ratio.
Abstract: The Helium Ion Microscope (HIM) has the capability to image small features with a resolution down to 0.35 nm due to its highly focused gas field ionization source and its small beam-sample interaction volume. In this work, the focused helium ion beam of a HIM is utilized to create nanopores with diameters down to 1.3 nm. It will be demonstrated that nanopores can be milled into silicon nitride, carbon nanomembranes (CNMs) and graphene with well-defined aspect ratio. To image and characterize the produced nanopores, helium ion microscopy and high resolution scanning transmission electron microscopy were used. The analysis of the nanopore's growth behavior, allows inferring on the profile of the helium ion beam.
17 citations
TL;DR: In this article, an atomically resolved study on the dynamics of a monolayer CuPcCl crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy was presented.
Abstract: Atomically resolved images of monolayer organic crystals have only been obtained with scanning probe methods so far On the one hand, they are usually prepared on surfaces of bulk materials, which are not accessible by (scanning) transmission electron microscopy On the other hand, the critical electron dose of a monolayer organic crystal is orders of magnitudes lower than the one for bulk crystals, making (scanning) transmission electron microscopy characterization very challenging In this work we present an atomically resolved study on the dynamics of a monolayer CuPcCl\textsubscript{16} crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy The results show the dynamics and the radiation damage mechanisms in the 2D layer of this material, complementing what has been found for bulk crystals in earlier studies Furthermore, being able to image the undamaged molecular crystal allows the characterization of new composites consisting of 2D materials and organic molecules
17 citations
TL;DR: In this paper, single-walled carbon nanotubes (SWCNTs) are suspended on graphene to create a model system for the study of a 1D-2D molecular interface through atomic-resolution scanning transmission electron microscopy observations.
Abstract: Molecular self-assembly due to chemical interactions is the basis of bottom-up nanofabrication, whereas weaker intermolecular forces dominate on the scale of macromolecules. Recent advances in synthesis and characterization have brought increasing attention to two- and mixed-dimensional heterostructures, and it has been recognized that van der Waals (vdW) forces within the structure may have a significant impact on their morphology. Here, we suspend single-walled carbon nanotubes (SWCNTs) on graphene to create a model system for the study of a 1D-2D molecular interface through atomic-resolution scanning transmission electron microscopy observations. When brought into contact, the radial deformation of SWCNTs and the emergence of long-range linear grooves in graphene revealed by the three-dimensional reconstruction of the heterostructure are observed. These topographic features are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order. Finally, despite the random deposition of the nanotubes, we show that the competition between strain and vdW forces results in aligned carbon-carbon interfaces spanning hundreds of nanometers.
16 Aug 2018
TL;DR: Employing characterization by advanced microscopy techniques, it is shown that AC and r-GO assemble into an interconnected network structure, resulting in a composite with high specific capacitance, excellent rate capability, and long cycling life stability.
Abstract: Using reduced graphene oxide (r-GO) as a multifunctional conductive binder, a simple, cost-effective, and environmentally friendly approach is developed to fabricate activated carbon/reduced graphene oxide (AC/r-GO) composite electrodes for supercapacitors with outstanding performance. In such a composite, r-GO provides several much needed critical functions: r-GO not only serves as the binder material improving the AC particle/particle cohesion and electrode-film/substrate adhesion but also improves the electrical conductivity of the composite and provides additional surfaces for ion adsorption. Furthermore, during electrode fabrication, initial GO precursor functions as an effective dispersant for AC, resulting in a stable electrode material slurry. Employing characterization by advanced microscopy techniques, we show that AC and r-GO assemble into an interconnected network structure, resulting in a composite with high specific capacitance, excellent rate capability, and long cycling life stability. Such high-performance electrodes coupled with their relatively simple, scalable, and low-cost fabrication process thereby provide a clear pathway toward large-scale implementation of supercapacitors.
01 Jan 2018
TL;DR: It is shown that the competition between strain and vdW forces results in aligned carbon–carbon interfaces spanning hundreds of nanometers, which are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order.
Abstract: Molecular self-assembly due to chemical interactions is the basis of bottom-up nanofabrication, whereas weaker intermolecular forces dominate on the scale of macromolecules. Recent advances in synthesis and characterization have brought increasing attention to two- and mixed-dimensional heterostructures, and it has been recognized that van der Waals (vdW) forces within the structure may have a significant impact on their morphology. Here, we suspend single-walled carbon nanotubes (SWCNTs) on graphene to create a model system for the study of a 1D-2D molecular interface through atomic-resolution scanning transmission electron microscopy observations. When brought into contact, the radial deformation of SWCNTs and the emergence of long-range linear grooves in graphene revealed by the three-dimensional reconstruction of the heterostructure are observed. These topographic features are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order. Finally, despite the random deposition of the nanotubes, we show that the competition between strain and vdW forces results in aligned carbon-carbon interfaces spanning hundreds of nanometers.
TL;DR: In this article, the core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers.
Abstract: X-ray photoelectron spectroscopy of graphene is important both for its characterization and as a model for other carbon materials. Despite great recent interest, the intrinsic photoemission of its single layer has not been unambiguously measured, nor is the layer dependence in free-standing multilayers accurately determined. We combine scanning transmission electron microscopy and Raman spectroscopy with synchrotron-based scanning photoelectron microscopy to characterize the same areas of suspended graphene samples down to the atomic level. This allows us to assign spectral signals to regions of precisely known layer number and purity. The core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers. This trend is reproduced by density functional theory with or without explicit van der Waals interactions, indicating that intralayer charge rearrangement dominates, but in our model of static screening the magnitudes of the shifts are underestimated by half.
TL;DR: In this paper, the authors directly visualize the 3D geometry and dynamics of silicon impurities in graphene as well as their dynamics by aberration-corrected scanning transmission electron microscopy.
Abstract: We directly visualize the three-dimensional (3D) geometry and dynamics of silicon impurities in graphene as well as their dynamics by aberration-corrected scanning transmission electron microscopy. By acquiring images when the sample is tilted, we show that an asymmetry of the atomic position of the heteroatom in the projection reveals the non-planarity of the structure. From a sequence of images, we further demonstrate that the Si atom switches between up- and down- configurations with respect to the graphene plane, with an asymmetric cross-section. We further analyze the 3D structure and dynamics of a silicon tetramer in graphene. Our results clarify the out-of-plane structure of impurities in graphene by direct experimental observation and open a new route to study their dynamics in three dimensions.
TL;DR: In this article, the core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers.
Abstract: X-ray photoelectron spectroscopy of graphene is important both for its characterization and as a model for other carbon materials. Despite great recent interest, the intrinsic photoemission of its single layer has not been unambiguously measured, nor is the layer-dependence in free-standing multilayers accurately determined. We combine scanning transmission electron microscopy and Raman spectroscopy with synchrotron-based scanning photoelectron microscopy to characterize the same areas of suspended graphene samples down to the atomic level. This allows us to assign spectral signals to regions of precisely known layer number and purity. The core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers. This trend is reproduced by density functional theory with or without explicit van der Waals interactions, indicating that intralayer charge rearrangement dominates, but in our model of static screening the magnitudes of the shifts are underestimated by half.
TL;DR: Understanding of the interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation, which could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p. p.
Abstract: The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al2O3 to cubic close-packed (c.c.p.) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.
Posted Content•
TL;DR: In this paper, the authors demonstrate insights into the three-dimensional structure of defects in graphene, in particular grain boundaries, obtained via a new approach from two transmission electron microscopy images recorded at different angles.
Abstract: We demonstrate insights into the three-dimensional structure of defects in graphene, in particular grain boundaries, obtained via a new approach from two transmission electron microscopy images recorded at different angles. The structure is obtained through an optimization process where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found. We first demonstrate that this method works using an embedded defect in graphene that allows direct comparison to the computationally predicted three-dimensional shape. We then applied the method to a set of grain boundary structures with misorientation angles nearly spanning the whole available range (2.6-29.8°). The measured height variations at the boundaries reveal a strong correlation with the misorientation angle with lower angles resulting in stronger corrugation and larger kink angles. Our results allow for the first time a direct comparison with theoretical predictions for the corrugation at grain boundaries and we show that the measured kink angles are significantly smaller than the largest predicted ones.
TL;DR: In this article, the stage mechanics of the Nion UltraSTEM, combined with an intelligent algorithm to move the sample, allows the automated acquisition of atomically resolved images from micron-sized areas of a graphene substrate.
Abstract: Beam damage is a major limitation in electron microscopy that becomes increasingly severe at higher resolution. One possible route to circumvent radiation damage, which forms the basis for single-particle electron microscopy and related techniques, is to distribute the dose over many identical copies of an object. For the acquisition of low-dose data, ideally no dose should be applied to the region of interest prior to the acquisition of data. We present an automated approach that can collect large amounts of data efficiently by acquiring images in an user-defined area-of-interest with atomic resolution. We demonstrate that the stage mechanics of the Nion UltraSTEM, combined with an intelligent algorithm to move the sample, allows the automated acquisition of atomically resolved images from micron-sized areas of a graphene substrate. Moving the sample stage automatically in a regular pattern over the area-of-interest enables the collection of data from pristine sample regions without exposing them to the electron beam before recording an image. Therefore, it is possible to obtain data with minimal dose (no prior exposure from focusing), which is only limited by the minimum signal needed for data processing. This enables us to prevent beam induced damage in the sample and to acquire large datasets within a reasonable amount of time.
TL;DR: The structure of the platelet defect in diamond has been determined by transmission electron microscopy, distinguishing the best-matched atomic model that settles a long-standing debate.
Abstract: The structure of the platelet defect in diamond has been determined by transmission electron microscopy, distinguishing the best-matched atomic model that settles a long-standing debate.
TL;DR: In this paper, the authors used aberration-corrected scanning transmission electron microscopy (STEEM) to characterize the atomic structure of the Al2O3/MgAl2O4 interfaces at different growth stages, and observed that progression of the MgAl 2O4 interface into Al 2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations.
Abstract: The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (hcp) stacking in Al2O3 to cubic close-packed (ccp) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent {\Sigma}3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth-selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar hcp/ccp transition.
Posted Content•
14 Apr 2018
TL;DR: In this article, the authors used aberration-corrected scanning transmission electron microscopy (STEEM) to characterize the atomic structure of the Al2O3/MgAl2O4 interfaces at different growth stages, and observed that progression of the MgAl 2O4 interface into Al 2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations.
Abstract: The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (hcp) stacking in Al2O3 to cubic close-packed (ccp) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent {\Sigma}3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth-selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar hcp/ccp transition.
TL;DR: In this article, the authors proposed a method to enhance the resolution of a single particle image by averaging the images in the same orientation to improve the signal-to-noise ratio.
Abstract: For many applications in both the biological and materials sciences, finite dose tolerance and noise severely restrict the resolving power of microscopy. Without sufficient signal to overcome noise, microscopy cannot provide images at the full resolution of the optics. This has led to the rise of techniques such as cryo-electron microscopy (cryo-EM) and single particle analysis. By freezing samples, cryo-EM enhances their robustness to the electron beam. However many specimens remain extremely fragile in cryo-EM, and structure determination is only possible via single particle analysis, in which large numbers of particles with the same structure are imaged. Images of the structure in the same orientation are identified and averaged to enhance the signal to noise. From these images models are built of the 3D structure of the particles and optimized to match the experimental data. The technique has been considered revolutionary, as it has enabled high resolution imaging of molecules that resist the crystallization that is required for X-ray crystallography [1]. However the quality of the reconstruction still depends on the clarity of the raw images. For instance, the quality of the alignment of the images of the individual particles will increase as they themselves increase in clarity. Thus methods that provide enhanced clarity at low doses will help to push the limits of cryo-EM, and provide enhanced resolution.
TL;DR: In this paper, two different aspects of (scanning) transmission electron microscopy were studied for extended defects in graphene and hybrids of graphene with other materials, including the extraction of 3D structural information from (in first approximation) projections of the structure.
Abstract: This presentation contains two different aspects of (scanning) transmission electron microscopy which were studied for extended defects in graphene and hybrids of graphene with other materials. The first topic is the extraction of 3D structural information from (in first approximation) projections of the structure. The second topic is radiation damage in organic molecular structures, where we obtained direct insight into the bond scission at low electron doses from atomic resolution imaging of a monolayer of chlorinated copper phtalocyanine (ClCuPc) molecules on graphene.
Patent•
14 Feb 2018
TL;DR: In this paper, a method of determining an isotope concentration in a sample (20) such as a graphene sample, is disclosed and the method comprises imaging a section of the sample with a microscope (10) using particle irradiation over a period of time, identifying a change in the image contrast due to lost atom(s) in at least one of the series of images and calculating an accumulated irradiation dose until identification of the change in contrast.
Abstract: A method of determining an isotope concentration in a sample (20), such as a graphene sample, is disclosed and the method comprises imaging (130) a section of the sample (20) with a microscope (10) using particle irradiation over a period of time, identifying (135) a change in the image contrast due to lost atom(s) in at least one of the series of images and calculating (140) an accumulated irradiation dose until identification of the change in contrast. The isotope concentration at the section of the sample (20) can be determined (160) from a comparison to a theoretical model or experimental results. The microscope (10) is a scanning transmission electron microscope in one aspect of the invention.
TL;DR: In this article, the authors compare the dose efficiency of ptychography and high-resolution transmission electron microscopy (HRTEM) images of graphene simulated at a finite dose of 20,000 e/Å at an accelerating voltage of 80 kV, assuming both perfect coherence and realistic partial coherence including 3.2 nm defocus spread.
Abstract: Electron ptychography provides a means of efficient phase contrast imaging at atomic resolution in the scanning transmission electron microscope (STEM) [1]. Here we compare the dose efficiency of ptychography and high-resolution transmission electron microscopy (HRTEM). Figure 1 shows HRTEM and ptychographic STEM images of graphene simulated at a finite dose of 20,000 e/Å at an accelerating voltage of 80 kV, assuming both perfect coherence and realistic partial coherence including a 3.2 nm defocus spread. For the ptychographic images we employed the single-side band (SSB) method [1] with aberration free data and a 35 mrad aperture semi-angle. For HRTEM, using Cs=20μm with -9 nm defocus optimizes the graphene contrast in the partially coherent case, and corresponds to the values used in previous HRTEM experiments [2]. Although the HRTEM images simulated with these conditions used the same 35 mrad aperture size as the ptychographic images, the ptychographic images have clear advantages. The most obvious is that partial temporal coherence makes very little difference to the clarity of the ptychographic image compared to HRTEM. A second advantage is that ptychography does not produce the spurious atom-like contrast seen at the centers of the carbon hexagons that appears in the coherent HRTEM image under these conditions. Such spurious contrast can hamper interpretability and occurs because of the oscillations in the contrast transfer function (CTF). It is because of its single signed bandpass CTF [3] that ptychography does not produce such spurious contrast. Of course, bandpass conditions can also be created in HRTEM by adjusting Cs to push the first CTF crossover out to correspond with the aperture limit, cutting off the oscillations at higher frequencies. This is illustrated for HRTEM with 35 mrad and 70 mrad apertures in Figure 1. The ptychographic CTF extends to an upper frequency of 2α/λ, corresponding to 0.6 Å under the present conditions. As this is double the upper frequency limit for HRTEM, a 70 mrad aperture is needed to mimic the CTF of ptychography with HRTEM. These passband conditions remove the spurious contrast from the HRTEM images. However when a realistic level of partial coherence is imposed the contrast remains far poorer than in the ptychographic image.
TL;DR: In this paper, the inital defects formed by ejecting carbon atoms under electron irradiation were captured by imaging with very low doses and subsequent reconstruction of the frequently occuring defects via a maximum likelihood algorithm.
Abstract: Freestanding graphene displays an outstanding resilience to electron irradiation at low electron energies. Point defects in graphene are, however, subject to beam driven dynamics. This means that high resolution micrographs of point defects, which usually require a high electron irradiation dose might not represent the intrinsic defect population. Here, we capture the inital defects formed by ejecting carbon atoms under electron irradiation, by imaging with very low doses and subsequent reconstruction of the frequently occuring defects via a maximum likelihood algorithm.