Publications by authors named "Peter D Nellist"

50 Publications

Significant Performance Improvement in n-Channel Organic Field-Effect Transistors with C :C Co-Crystals Induced by Poly(2-ethyl-2-oxazoline) Nanodots.

Adv Mater 2021 Aug 24;33(31):e2100421. Epub 2021 Jun 24.

Department of Physics, University of Oxford, Oxford, OX1 3PD, UK.

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C -I )[5,6]fullerene (C ) and (C -D )[5,6]fullerene (C ) blend films, with an increase from 0.1 to 2.1 cm V s . The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C :C co-crystal structure within the blend films.
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http://dx.doi.org/10.1002/adma.202100421DOI Listing
August 2021

Contrast transfer and noise considerations in focused-probe electron ptychography.

Ultramicroscopy 2021 Feb 17;221:113189. Epub 2020 Dec 17.

Department of Materials, University of Oxford, Parks Rd, Oxford OX13PH, United Kingdom.

Electron ptychography is a 4-D STEM phase-contrast imaging technique with applications to light-element and beam-sensitive materials. Although the electron dose (electrons incident per unit area on the sample) is the primary figure of merit for imaging beam-sensitive materials, it is also necessary to consider the contrast transfer properties of the imaging technique. Here, we explore the contrast transfer properties of electron ptychography. The contrast transfer of focused-probe, non-iterative electron ptychography using the single-side-band (SSB) method is demonstrated experimentally. The band-pass nature of the phase-contrast transfer function (PCTF) for SSB ptychography places strict limitations on the probe convergence semi-angles required to resolve specific sample features with high contrast. The PCTF of the extended ptychographic iterative engine (ePIE) is broader than that for SSB ptychography, although when both high and low spatial frequencies are transferred, band-pass filtering is required to remove image artefacts. Normalisation of the transfer function with respect to the noise level shows that the transfer window is increased while avoiding noise amplification. Avoiding algorithms containing deconvolution steps may also increase the dose-efficiency of ptychographic phase reconstructions.
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http://dx.doi.org/10.1016/j.ultramic.2020.113189DOI Listing
February 2021

Atomic-scale microstructure of metal halide perovskite.

Science 2020 10;370(6516)

Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, UK.

Hybrid organic-inorganic perovskites have high potential as materials for solar energy applications, but their microscopic properties are still not well understood. Atomic-resolution scanning transmission electron microscopy has provided invaluable insights for many crystalline solar cell materials, and we used this method to successfully image formamidinium lead triiodide [CH(NH)PbI] thin films with a low dose of electron irradiation. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI interfaces, with a striking absence of long-range disorder in the crystal. We found that beam-induced degradation of the perovskite leads to an initial loss of formamidinium [CH(NH) ] ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observed aligned point defects and climb-dissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead halide perovskites.
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http://dx.doi.org/10.1126/science.abb5940DOI Listing
October 2020

Strain effects in core-shell PtCo nanoparticles: a comparison of experimental observations and computational modelling.

Phys Chem Chem Phys 2020 Nov 27;22(42):24784-24795. Epub 2020 Oct 27.

Department of Chemistry, University of Southampton, Southampton, UK.

Strain in Pt nanoalloys induced by the secondary metal has long been suggested as a major contributor to the modification of catalytic properties. Here, we investigate strain in PtCo nanoparticles using a combination of computational modelling and microscopy experiments. We have used a combination of molecular dynamics (MD) and large-scale density functional theory (DFT) for our models, alongside experimental work using annular dark field scanning transmission electron microscopy (ADF-STEM). We have performed extensive validation of the interatomic potential against DFT using a PtCo nanoparticle. Modelling gives access to 3 dimensional structures that can be compared to the 2D ADF-STEM images, which we use to build an understanding of nanoparticle structure and composition. Strain has been measured for PtCo and pure Pt nanoparticles, with MD annealed models compared to ADF-STEM images. Our analysis was performed on a layer by layer basis, where distinct trends between the Pt and PtCo alloy nanoparticles are observed. To our knowledge, we show for the first time a way in which detailed atomistic simulations can be used to augment and help interpret the results of ADF-STEM strain mapping experiments, which will enhance their use in characterisation towards the development of improved catalysts.
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http://dx.doi.org/10.1039/d0cp04318dDOI Listing
November 2020

Low-dose phase retrieval of biological specimens using cryo-electron ptychography.

Nat Commun 2020 06 2;11(1):2773. Epub 2020 Jun 2.

National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory of Artificial Functional Materials, College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China.

Cryo-electron microscopy is an essential tool for high-resolution structural studies of biological systems. This method relies on the use of phase contrast imaging at high defocus to improve information transfer at low spatial frequencies at the expense of higher spatial frequencies. Here we demonstrate that electron ptychography can recover the phase of the specimen with continuous information transfer across a wide range of the spatial frequency spectrum, with improved transfer at lower spatial frequencies, and as such is more efficient for phase recovery than conventional phase contrast imaging. We further show that the method can be used to study frozen-hydrated specimens of rotavirus double-layered particles and HIV-1 virus-like particles under low-dose conditions (5.7 e/Å) and heterogeneous objects in an Adenovirus-infected cell over large fields of view (1.14 × 1.14 μm), thus making it suitable for studies of many biologically important structures.
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http://dx.doi.org/10.1038/s41467-020-16391-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7265480PMC
June 2020

Measuring Dynamic Structural Changes of Nanoparticles at the Atomic Scale Using Scanning Transmission Electron Microscopy.

Phys Rev Lett 2020 Mar;124(10):106105

EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.

We propose a new method to measure atomic scale dynamics of nanoparticles from experimental high-resolution annular dark field scanning transmission electron microscopy images. By using the so-called hidden Markov model, which explicitly models the possibility of structural changes, the number of atoms in each atomic column can be quantified over time. This newly proposed method outperforms the current atom-counting procedure and enables the determination of the probabilities and cross sections for surface diffusion. This method is therefore of great importance for revealing and quantifying the atomic structure when it evolves over time via adatom dynamics, surface diffusion, beam effects, or during in situ experiments.
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http://dx.doi.org/10.1103/PhysRevLett.124.106105DOI Listing
March 2020

Control of Knock-On Damage for 3D Atomic Scale Quantification of Nanostructures: Making Every Electron Count in Scanning Transmission Electron Microscopy.

Phys Rev Lett 2019 Feb;122(6):066101

Department of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, United Kingdom.

Understanding nanostructures down to the atomic level is the key to optimizing the design of advanced materials with revolutionary novel properties. This requires characterization methods capable of quantifying the three-dimensional (3D) atomic structure with the highest possible precision. A successful approach to reach this goal is to count the number of atoms in each atomic column from 2D annular dark field scanning transmission electron microscopy images. To count atoms with single atom sensitivity, a minimum electron dose has been shown to be necessary, while on the other hand beam damage, induced by the high energy electrons, puts a limit on the tolerable dose. An important challenge is therefore to develop experimental strategies to optimize the electron dose by balancing atom-counting fidelity vs the risk of knock-on damage. To achieve this goal, a statistical framework combined with physics-based modeling of the dose-dependent processes is here proposed and experimentally verified. This model enables an investigator to theoretically predict, in advance of an experimental measurement, the optimal electron dose resulting in an unambiguous quantification of nanostructures in their native state with the highest attainable precision.
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http://dx.doi.org/10.1103/PhysRevLett.122.066101DOI Listing
February 2019

Targeted T Magnetic Resonance Imaging Contrast Enhancement with Extraordinarily Small CoFeO Nanoparticles.

ACS Appl Mater Interfaces 2019 Feb 8;11(7):6724-6740. Epub 2019 Feb 8.

Materials Department , University of Oxford , Parks Road , Oxford OX1 3PH , England.

Extraordinarily small (2.4 nm) cobalt ferrite nanoparticles (ESCIoNs) were synthesized by a one-pot thermal decomposition approach to study their potential as magnetic resonance imaging (MRI) contrast agents. Fine size control was achieved using oleylamine alone, and annular dark-field scanning transmission electron microscopy revealed highly crystalline cubic spinel particles with atomic resolution. Ligand exchange with dimercaptosuccinic acid rendered the particles stable in physiological conditions with a hydrodynamic diameter of 12 nm. The particles displayed superparamagnetic properties and a low r/ r ratio suitable for a T contrast agent. The particles were functionalized with bile acid, which improved biocompatibility by significant reduction of reactive oxygen species generation and is a first step toward liver-targeted T MRI. Our study demonstrates the potential of ESCIoNs as T MRI contrast agents.
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http://dx.doi.org/10.1021/acsami.8b17162DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6385080PMC
February 2019

High dose efficiency atomic resolution imaging via electron ptychography.

Ultramicroscopy 2019 01 18;196:131-135. Epub 2018 Oct 18.

Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna 1090, Austria.

Radiation damage places a fundamental limitation on the ability of microscopy to resolve many types of materials at high resolution. Here we evaluate the dose efficiency of phase contrast imaging with electron ptychography. The method is found to be far more resilient to temporal incoherence than conventional and spherical aberration optimized phase contrast imaging, resulting in significantly greater clarity at a given dose. This robustness is explained by the presence of achromatic lines in the four dimensional ptychographic dataset.
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http://dx.doi.org/10.1016/j.ultramic.2018.10.005DOI Listing
January 2019

Low-Dose Aberration-Free Imaging of Li-Rich Cathode Materials at Various States of Charge Using Electron Ptychography.

Nano Lett 2018 11 4;18(11):6850-6855. Epub 2018 Oct 4.

Department of Materials , University of Oxford , Parks Road , OX1 3PH Oxford , United Kingdom.

Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy. Annular bright-field scanning transmission electron microscopy is often used to image elements with low atomic numbers, but due to its low efficiency and high sensitivity to precise imaging parameters it comes at the price of potentially significant beam damage. In this paper, we show that electron ptychography is a powerful technique to retrieve reconstructed phase images that provide the full structure of beam-sensitive materials containing light and heavy elements. Due to its much higher efficiency, we can reduce the beam currents used down to the subpicoampere range. Electron ptychography also allows residual lens aberrations to be corrected at the postprocessing stage, which avoids the need for fine-tuning of the probe that would result in further beam damage and provides aberration-free reconstructed phase images. We have used electron ptychography to obtain structural information from aberration-free reconstructed phase images in the technologically relevant lithium-rich transition metal oxides at different states of charge. We can unambiguously determine the position of the lithium and oxygen atomic columns while amorphization of the surface, formation of beam-induced surface reconstruction layers, or migration of transition metals to the alkali layers are drastically reduced.
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http://dx.doi.org/10.1021/acs.nanolett.8b02718DOI Listing
November 2018

Determining EDS and EELS partial cross-sections from multiple calibration standards to accurately quantify bi-metallic nanoparticles using STEM.

Micron 2018 10 28;113:69-82. Epub 2018 Jun 28.

Department of Materials, University of Oxford, Parks Road, OX1 3PH, UK. Electronic address:

Spectroscopic signals such as EDS and EELS provide an effective way of characterising multi-element samples such as Pt-Co nanoparticles in STEM. The advantage of spectroscopy over imaging is the ability to decouple composition and mass-thickness effects for thin samples, into the number of various types of atoms in a sample. This is currently not possible for multi element samples using conventional ADF quantification techniques alone. With recent developments in microscope hardware and software, it is now possible to acquire the ADF, EDS and EELS signals simultaneously and at high speed. However, the methods of quantifying the signals emitted from the sample vary greatly. Most approaches use pure-element standards in the form of needles, nanoparticles and wedges to quantify the spectroscopic signal into either partial scattering cross-sections, zeta-factors or k-factors. But self-consistency between the different methods has not been verified and the units of the quantification are not standardised. We present a robust approach for measuring and combining ADF, EDS and EELS signals using needle and nanoparticle standards in units of the partial scattering cross-section. The partial scattering cross-section allows an easy interpretation of the signals emitted from the sample and enables accurate atom-counting of the sample.
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http://dx.doi.org/10.1016/j.micron.2018.06.015DOI Listing
October 2018

Managing dose-, damage- and data-rates in multi-frame spectrum-imaging.

Microscopy (Oxf) 2018 Mar;67(suppl_1):i98-i113

Department of Materials, University of Oxford, Oxford, UK.

As an instrument, the scanning transmission electron microscope is unique in being able to simultaneously explore both local structural and chemical variations in materials at the atomic scale. This is made possible as both types of data are acquired serially, originating simultaneously from sample interactions with a sharply focused electron probe. Unfortunately, such scanned data can be distorted by environmental factors, though recently fast-scanned multi-frame imaging approaches have been shown to mitigate these effects. Here, we demonstrate the same approach but optimized for spectroscopic data; we offer some perspectives on the new potential of multi-frame spectrum-imaging (MFSI) and show how dose-sharing approaches can reduce sample damage, improve crystallographic fidelity, increase data signal-to-noise, or maximize usable field of view. Further, we discuss the potential issue of excessive data-rates in MFSI, and demonstrate a file-compression approach to significantly reduce data storage and transmission burdens.
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http://dx.doi.org/10.1093/jmicro/dfx125DOI Listing
March 2018

Electron ptychographic microscopy for three-dimensional imaging.

Nat Commun 2017 07 31;8(1):163. Epub 2017 Jul 31.

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK.

Knowing the three-dimensional structural information of materials at the nanometer scale is essential to understanding complex material properties. Electron tomography retrieves three-dimensional structural information using a tilt series of two-dimensional images. In this paper, we report an alternative combination of electron ptychography with the inverse multislice method. We demonstrate depth sectioning of a nanostructured material into slices with 0.34 nm lateral resolution and with a corresponding depth resolution of about 24-30 nm. This three-dimensional imaging method has potential applications for the three-dimensional structure determination of a range of objects, ranging from inorganic nanostructures to biological macromolecules.Three-dimensional ptychographic imaging with electrons has remained a challenge because, unlike X-rays, electrons are easily scattered by atoms. Here, Gao et al. extend multi-slice methods to electrons in the multiple scattering regime, paving the way to nanometer-scale 3D structure determination with electrons.
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http://dx.doi.org/10.1038/s41467-017-00150-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5537274PMC
July 2017

Predicting the Oxygen-Binding Properties of Platinum Nanoparticle Ensembles by Combining High-Precision Electron Microscopy and Density Functional Theory.

Nano Lett 2017 07 28;17(7):4003-4012. Epub 2017 Jun 28.

Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, United Kingdom.

Many studies of heterogeneous catalysis, both experimental and computational, make use of idealized structures such as extended surfaces or regular polyhedral nanoparticles. This simplification neglects the morphological diversity in real commercial oxygen reduction reaction (ORR) catalysts used in fuel-cell cathodes. Here we introduce an approach that combines 3D nanoparticle structures obtained from high-throughput high-precision electron microscopy with density functional theory. Discrepancies between experimental observations and cuboctahedral/truncated-octahedral particles are revealed and discussed using a range of widely used descriptors, such as electron-density, d-band centers, and generalized coordination numbers. We use this new approach to determine the optimum particle size for which both detrimental surface roughness and particle shape effects are minimized.
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http://dx.doi.org/10.1021/acs.nanolett.6b04799DOI Listing
July 2017

Electron ptychographic phase imaging of light elements in crystalline materials using Wigner distribution deconvolution.

Ultramicroscopy 2017 09 1;180:173-179. Epub 2017 Apr 1.

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK. Electronic address:

Recent development in fast pixelated detector technology has allowed a two dimensional diffraction pattern to be recorded at every probe position of a two dimensional raster scan in a scanning transmission electron microscope (STEM), forming an information-rich four dimensional (4D) dataset. Electron ptychography has been shown to enable efficient coherent phase imaging of weakly scattering objects from a 4D dataset recorded using a focused electron probe, which is optimised for simultaneous incoherent Z-contrast imaging and spectroscopy in STEM. Therefore coherent phase contrast and incoherent Z-contrast imaging modes can be efficiently combined to provide a good sensitivity of both light and heavy elements at atomic resolution. In this work, we explore the application of electron ptychography for atomic resolution imaging of strongly scattering crystalline specimens, and present experiments on imaging crystalline specimens including samples containing defects, under dynamical channelling conditions using an aberration corrected microscope. A ptychographic reconstruction method called Wigner distribution deconvolution (WDD) was implemented. Experimental results and simulation results suggest that ptychography provides a readily interpretable phase image and great sensitivity for imaging light elements at atomic resolution in relatively thin crystalline materials.
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http://dx.doi.org/10.1016/j.ultramic.2017.02.006DOI Listing
September 2017

Optimising multi-frame ADF-STEM for high-precision atomic-resolution strain mapping.

Ultramicroscopy 2017 08 15;179:57-62. Epub 2017 Apr 15.

Department of Materials, University of Oxford, Oxford, OX13PH, UK.

Annular dark-field scanning transmission electron microscopy is a powerful tool to study crystal defects at the atomic scale but historically single slow-scanned frames have been plagued by low-frequency scanning-distortions prohibiting accurate strain mapping at atomic resolution. Recently, multi-frame acquisition approaches combined with post-processing have demonstrated significant improvements in strain precision, but the optimum number of frames to record has not been explored. Here we use a non-rigid image registration procedure before applying established strain mapping methods. We determine how, for a fixed total electron-budget, the available dose should be fractionated for maximum strain mapping precision. We find that reductions in scanning-artefacts of more than 70% are achievable with image series of 20-30 frames in length. For our setup, series longer than 30 frames showed little further improvement. As an application, the strain field around an aluminium alloy precipitate was studied, from which our optimised approach yields data whos strain accuracy is verified using density functional theory.
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http://dx.doi.org/10.1016/j.ultramic.2017.04.007DOI Listing
August 2017

Hybrid statistics-simulations based method for atom-counting from ADF STEM images.

Ultramicroscopy 2017 06 25;177:69-77. Epub 2017 Jan 25.

Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium. Electronic address:

A hybrid statistics-simulations based method for atom-counting from annular dark field scanning transmission electron microscopy (ADF STEM) images of monotype crystalline nanostructures is presented. Different atom-counting methods already exist for model-like systems. However, the increasing relevance of radiation damage in the study of nanostructures demands a method that allows atom-counting from low dose images with a low signal-to-noise ratio. Therefore, the hybrid method directly includes prior knowledge from image simulations into the existing statistics-based method for atom-counting, and accounts in this manner for possible discrepancies between actual and simulated experimental conditions. It is shown by means of simulations and experiments that this hybrid method outperforms the statistics-based method, especially for low electron doses and small nanoparticles. The analysis of a simulated low dose image of a small nanoparticle suggests that this method allows for far more reliable quantitative analysis of beam-sensitive materials.
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http://dx.doi.org/10.1016/j.ultramic.2017.01.010DOI Listing
June 2017

3D elemental mapping with nanometer scale depth resolution via electron optical sectioning.

Ultramicroscopy 2017 03 5;174:27-34. Epub 2016 Dec 5.

EPSRC SuperSTEM Facility, Daresbury Laboratory, Warrington WA4 4AD, UK; Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.

Electron energy loss spectroscopy in the scanning transmission electron microscope has long been used to perform elemental mapping but has not previously exhibited depth sensitivity. The key to depth resolution with optical sectioning is the transfer of sufficiently high lateral spatial frequencies. By performing spectrum imaging with atomic resolution we achieve nanometer scale depth resolution, enabling us to optically section an oxide heterostructure spectroscopically. Such 3D elemental mapping is sensitive to atomic scale changes in structure and composition and is more interpretable than Z-contrast imaging alone.
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http://dx.doi.org/10.1016/j.ultramic.2016.12.002DOI Listing
March 2017

Enhanced phase contrast transfer using ptychography combined with a pre-specimen phase plate in a scanning transmission electron microscope.

Ultramicroscopy 2016 12 14;171:117-125. Epub 2016 Sep 14.

Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Electronic address:

The ability to image light elements in both crystalline and noncrystalline materials at near atomic resolution with an enhanced contrast is highly advantageous to understand the structure and properties of a wide range of beam sensitive materials including biological specimens and molecular hetero-structures. This requires the imaging system to have an efficient phase contrast transfer at both low and high spatial frequencies. In this work we introduce a new phase contrast imaging method in a scanning transmission electron microscope (STEM) using a pre-specimen phase plate in the probe forming aperture, combined with a fast pixelated detector to record diffraction patterns at every probe position, and phase reconstruction using ptychography. The phase plate significantly enhances the contrast transfer of low spatial frequency information, and ptychography maximizes the extraction of the phase information at all spatial frequencies. In addition, the STEM probe with the presence of the phase plate retains its atomic resolution, allowing simultaneous incoherent Z-contrast imaging to be obtained along with the ptychographic phase image. An experimental image of Au nanoparticles on a carbon support shows high contrast for both materials. Multislice image simulations of a DNA molecule shows the capability of imaging soft matter at low dose conditions, which implies potential applications of low dose imaging of a wide range of beam sensitive materials.
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http://dx.doi.org/10.1016/j.ultramic.2016.09.002DOI Listing
December 2016

Unscrambling Mixed Elements using High Angle Annular Dark Field Scanning Transmission Electron Microscopy.

Phys Rev Lett 2016 Jun 17;116(24):246101. Epub 2016 Jun 17.

EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.

The development of new nanocrystals with outstanding physicochemical properties requires a full three-dimensional (3D) characterization at the atomic scale. For homogeneous nanocrystals, counting the number of atoms in each atomic column from high angle annular dark field scanning transmission electron microscopy images has been shown to be a successful technique to get access to this 3D information. However, technologically important nanostructures often consist of more than one chemical element. In order to extend atom counting to heterogeneous materials, a new atomic lensing model is presented. This model takes dynamical electron diffraction into account and opens up new possibilities for unraveling the 3D composition at the atomic scale. Here, the method is applied to determine the 3D structure of [email protected] core-shell nanorods, but it is applicable to a wide range of heterogeneous complex nanostructures.
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http://dx.doi.org/10.1103/PhysRevLett.116.246101DOI Listing
June 2016

Quantitative Energy-Dispersive X-Ray Analysis of Catalyst Nanoparticles Using a Partial Cross Section Approach.

Microsc Microanal 2016 Feb 12;22(1):71-81. Epub 2016 Jan 12.

1Department of Materials,University of Oxford,Parks Road,Oxford OX1 3PH,UK.

The new generation of energy-dispersive X-ray (EDX) detectors with higher count rates than ever before, paves the way for a new approach to quantitative elemental analysis in the scanning transmission electron microscope. Here we demonstrate a method of calculating partial cross sections for use in quantifying EDX data, beneficial especially because of the simplicity of its implementation. Applying this approach to acid-leached PtCo catalyst nanoparticles leads to quantitative determination of the Pt surface enrichment.
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http://dx.doi.org/10.1017/S1431927615015494DOI Listing
February 2016

Crystal Structure of the ZrO Phase at Zirconium/Zirconium Oxide Interfaces.

Adv Eng Mater 2015 Feb 27;17(2):211-215. Epub 2014 Jun 27.

Dr. R. J. Nicholls, Dr. S. Lozano-Perez, A. London, Prof. P. D. Nellist, Prof. C. R. M. Grovenor, Dr. J. R. Yates, Department of Materials, University of Oxford Parks Road, Oxford, OX1 3PH, UK.

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal-oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.
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http://dx.doi.org/10.1002/adem.201400133DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4393322PMC
February 2015

Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy.

Sci Rep 2014 Dec 22;4:7555. Epub 2014 Dec 22.

1] SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Warrington WA4 4AD, United Kingdom [2] Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom.

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and directly observe the atomic scale dynamics behind the transformation of Mn3O4 into MnO. The results demonstrate we now have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions.
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http://dx.doi.org/10.1038/srep07555DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4273600PMC
December 2014

Morphology--composition correlations in carbon nanotubes synthesised with nitrogen and phosphorus containing precursors.

Phys Chem Chem Phys 2015 Jan 8;17(3):2137-42. Epub 2014 Dec 8.

Department of Materials, University of Oxford, Oxford OX1 3PH, UK.

We have correlated the elemental composition with the structure of multi-wall carbon nanotubes synthesised with nitrogen and phosphorus containing precursors and identified two chemically distinct dominant morphologies. The first type are cone-structured tubes and the second are nanotubes with fewer walls which can accommodate N2 gas along their inner channel and contain up to ten times more nitrogen than the cone-structured nanotubes. Phosphorus was present in the catalyst particles but was not detected within the walls of either type of nanotube. Elemental analysis combined with in situ electrical measurements has allowed us to monitor the evolution of the doped nanotubes when current is passed. The N2 gas becomes bonded immediately when current flows and the gas-containing nanotubes restructure more easily than the cone-structured ones. Since the inclusion of heteroatoms in multi-wall carbon nanotubes is generally inhomogeneous, understanding the distribution of elements across the sample is an important step towards the optimization of devices including gas sensors and components in electrical applications.
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http://dx.doi.org/10.1039/c4cp04272gDOI Listing
January 2015

Efficient phase contrast imaging in STEM using a pixelated detector. Part II: optimisation of imaging conditions.

Ultramicroscopy 2015 Apr 5;151:232-239. Epub 2014 Nov 5.

University of Oxford, Department of Materials. Parks Rd, Oxford OX1 3PH, UK; EPSRC SuperSTEM Facility, Daresbury Laboratory, WA4 4AD, UK.

In Part I of this series of two papers, we demonstrated the formation of a high efficiency phase-contrast image at atomic resolution using a pixelated detector in the scanning transmission electron microscope (STEM) with ptychography. In this paper we explore the technique more quantitatively using theory and simulations. Compared to other STEM phase contrast modes including annular bright field (ABF) and differential phase contrast (DPC), we show that the ptychographic phase reconstruction method using pixelated detectors offers the highest contrast transfer efficiency and superior low dose performance. Applying the ptychographic reconstruction method to DPC segmented detectors also improves the detector contrast transfer and results in less noisy images than DPC images formed using difference signals. We also find that using a minimum array of 16×16 pixels is sufficient to provide the highest signal-to-noise ratio (SNR) for imaging beam sensitive weak phase objects. Finally, the convergence angle can be adjusted to enhance the contrast transfer based on the spatial frequencies of the specimen under study.
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http://dx.doi.org/10.1016/j.ultramic.2014.10.013DOI Listing
April 2015

Efficient phase contrast imaging in STEM using a pixelated detector. Part 1: experimental demonstration at atomic resolution.

Ultramicroscopy 2015 Apr 15;151:160-167. Epub 2014 Oct 15.

EPSRC SuperSTEM Facility, Daresbury Laboratory, Warrington WA4 4AD, UK; Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.

We demonstrate a method to achieve high efficiency phase contrast imaging in aberration corrected scanning transmission electron microscopy (STEM) with a pixelated detector. The pixelated detector is used to record the Ronchigram as a function of probe position which is then analyzed with ptychography. Ptychography has previously been used to provide super-resolution beyond the diffraction limit of the optics, alongside numerically correcting for spherical aberration. Here we rely on a hardware aberration corrector to eliminate aberrations, but use the pixelated detector data set to utilize the largest possible volume of Fourier space to create high efficiency phase contrast images. The use of ptychography to diagnose the effects of chromatic aberration is also demonstrated. Finally, the four dimensional dataset is used to compare different bright field detector configurations from the same scan for a sample of bilayer graphene. Our method of high efficiency ptychography produces the clearest images, while annular bright field produces almost no contrast for an in-focus aberration-corrected probe.
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http://dx.doi.org/10.1016/j.ultramic.2014.09.013DOI Listing
April 2015

Rapid estimation of catalyst nanoparticle morphology and atomic-coordination by high-resolution Z-contrast electron microscopy.

Nano Lett 2014 Nov 27;14(11):6336-41. Epub 2014 Oct 27.

Department of Materials, University of Oxford , OX13PH Oxford, United Kingdom.

Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.
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http://dx.doi.org/10.1021/nl502762mDOI Listing
November 2014

WS₂ 2D nanosheets in 3D nanoflowers.

Chem Commun (Camb) 2014 Oct;50(82):12360-2

Department of Materials, University of Oxford, Parks Road, OX1 3PH, UK.

In this work it has been established that 3D nanoflowers of WS2 synthesised by chemical vapour deposition are composed of few layer WS2 along the edges of the petals. An experimental study in order to understand the evolution of these nanostructures shows the nucleation and growth along with the compositional changes they undergo.
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http://dx.doi.org/10.1039/c4cc04218bDOI Listing
October 2014

The development of a 200 kV monochromated field emission electron source.

Ultramicroscopy 2014 May 12;140:37-43. Epub 2014 Mar 12.

University of Oxford, Department of Materials, Parks Road, Oxford, OX1 3PH, UK.

We report the development of a monochromator for an intermediate-voltage aberration-corrected electron microscope suitable for operation in both STEM and TEM imaging modes. The monochromator consists of two Wien filters with a variable energy selecting slit located between them and is located prior to the accelerator. The second filter cancels the energy dispersion produced by the first filter and after energy selection forms a round monochromated, achromatic probe at the specimen plane. The ultimate achievable energy resolution has been measured as 36 meV at 200 kV and 26 meV at 80 kV. High-resolution Annular Dark Field STEM images recorded using a monochromated probe resolve Si-Si spacings of 135.8 pm using energy spreads of 218 meV at 200 kV and 217 meV at 80 kV respectively. In TEM mode an improvement in non-linear spatial resolution to 64 pm due to the reduction in the effects of partial temporal coherence has been demonstrated using broad beam illumination with an energy spread of 134 meV at 200 kV.
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http://dx.doi.org/10.1016/j.ultramic.2014.02.004DOI Listing
May 2014

Unusual stacking variations in liquid-phase exfoliated transition metal dichalcogenides.

ACS Nano 2014 Apr 5;8(4):3690-9. Epub 2014 Mar 5.

Department of Materials, University of Oxford , Parks Road, OX1 3PH Oxford, United Kingdom.

Liquid-phase exfoliation of layered materials offers a large-scale approach toward the synthesis of 2D nanostructures. Structural properties of materials can however change during transition from bulk to the 2D state. Any such changes must be examined and understood for successful implementation of 2D nanostructures. In this work, we demonstrate nonbulk stacking sequences in the few-layer MoS2 and WS2 nanoflakes produced by liquid-phase exfoliation. Our analysis shows that nonbulk stacking sequences can be derived from its bulk counterparts by translational shifts of the layers. No structural changes within the layers were observed. Twenty-seven MoS2 and five WS2 nanoflakes were imaged and analyzed. Nine MoS2 and four WS2 nanoflakes displayed nonbulk stacking. Such dominance of the nonbulk stacking suggests high possibility of unusual stacking sequences in other 2D nanostructures. Notably, the electronic structure of some non bulk stacked bilayers presents characteristics which are uncommon to either the bulk phase or the single monolayer, for instance, a spin-split conduction band bottom. Our main characterization technique was annular dark-field scanning transmission electron microscopy, which offers direct and reliable imaging of atomic columns. The stacking characterization approach employed here can be readily applied toward other few-layer transition metal chalcogenides and oxides.
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http://dx.doi.org/10.1021/nn5003387DOI Listing
April 2014
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