Angus I. Kirkland

Dislocation-Driven Deformations in Graphene

Moving Dislocations The mechanical properties of crystalline materials are limited by the presence and motion of defects caused by extra or missing atoms in the crystal lattice. Plastic deformation of a material causes these defects, known as dislocations, to move and multiply. Much is known about the motion of dislocations in three dimensions but less so in two. Warner et al. (p. 209; see the Perspective by Bonilla and Carpio) used graphene as a model material to track dislocation dynamics in real time. The strain fields in the graphene sheet were mapped, which suggests that the dislocation motion is connected to the stretching, rotating, and breaking of individual carbon bonds. Two-dimensional dislocation dynamics and the resulting strain fields are studied at high resolution in graphene. The movement of dislocations in a crystal is the key mechanism for plastic deformation in all materials. Studies of dislocations have focused on three-dimensional materials, and there is little experimental evidence regarding the dynamics of dislocations and their impact at the atomic level on the lattice structure of graphene. We studied the dynamics of dislocation pairs in graphene, recorded with single-atom sensitivity. We examined stepwise dislocation movement along the zig-zag lattice direction mediated either by a single bond rotation or through the loss of two carbon atoms. The strain fields were determined, showing how dislocations deform graphene by elongation and compression of C-C bonds, shear, and lattice rotations.

Spatial control of defect creation in graphene at the nanoscale

Defects in graphene alter its electrical, chemical, magnetic and mechanical properties. The intentional creation of defects in graphene offers a means for engineering its properties. Techniques such as ion irradiation intentionally induce atomic defects in graphene, for example, divacancies, but these defects are randomly scattered over large distances. Control of defect formation with nanoscale precision remains a significant challenge. Here we show control over both the location and average complexity of defect formation in graphene by tailoring its exposure to a focussed electron beam. Divacancies and larger disordered structures are produced within a 10 × 10 nm(2) region of graphene and imaged after creation using an aberration-corrected transmission electron microscope. Some of the created defects were stable, whereas others relaxed to simpler structures through bond rotations and surface adatom incorporation. These results are important for the utilization of atomic defects in graphene-based research.

Nanogold: A Quantitative Phase Map

The development of the next generation of nanotechnologies requires precise control of the size, shape, and structure of individual components in a variety of chemical and engineering environments. This includes synthesis, storage, operational environments and, since these products will ultimately be discarded, their interaction with natural ecosystems. Much of the important information that determines these properties is contained within nanoscale phase diagrams, but quantitative phase maps that include surface effects and critical diameter (along with temperature and pressure) remain elusive. Here we present the first quantitative equilibrium phase map for gold nanoparticles together with experimental verification, based on relativistic ab initio thermodynamics and in situ high-resolution electron microscopy at elevated temperatures.

Structural studies of trigonal lamellar particles of gold and silver

The results of detailed structural studies of trigonal lamellar particles of both gold and silver are presented. The particles have been characterized both in sol by means of optical spectroscopy and powder X-ray diffraction and ex sol using high resolution electron microscopy in both plan view and profile imaging modes. The results of these studies have indicated that the particles have a trigonal outline and are shortened along a ≺111≻ direction to give a plate-like morphology. The presence of small numbers of parallel {111} twin planes has also been confirmed and used to explain the presence of the formally forbidden ⅓{422} reflections observed in plan view. The precise structural requirements for the observation of such reflections has also been confirmed using multislice calculations. Possible growth mechanisms for these particles are also discussed.

Characterisation of the signal and noise transfer of CCD cameras for electron detection.

Methods to characterise the performance of CCD cameras for electron detection are investigated with particular emphasis on the difference between the transfer of signal and noise. Similar to the Modulation Transfer Function MTF, which describes the spatial frequency dependent attenuation of contrast in the image, we introduce a Noise Transfer Function NTF that describes the transfer of the Poisson noise that is inevitably present in any electron image. A general model for signal and noise transfer by an image converter is provided. This allows the calculation of MTF and NTF from Monte‐Carlo simulations of the trajectories of electrons and photons in the scintillator and the optical coupling of the camera. Furthermore, accurate methods to measure the modulation and noise transfer functions experimentally are presented. The spatial‐frequency dependent Detection Quantum Efficiency DQE, an important figure of merit of the camera which has so far not been measured experimentally, can be obtained from the measured MTF and NTF. The experimental results are in good agreement with the simulations and show that the NTF at high spatial frequencies is in some cases by a factor of four higher than the MTF. This implies that the noise method, which is frequently used to measure the MTF, but in fact measures the NTF, gives over‐optimistic results. Furthermore, the spatial frequency dependent DQE is lower than previously assumed. Microsc. Res. Tech. 49:269–280, 2000.

The effects of electron and photon scattering on signal and noise transfer properties of scintillators in CCD cameras used for electron detection

Abstract The detection properties of scintillators used in charge-coupled device cameras suitable for electron microscopy are examined with particular emphasis on the statistics of electron scattering and photon generation in the scintillator. We show that the root of the power spectrum of an evenly illuminated white noise image is in general not equal to the modulation transfer function (MTF) of the scintillator, as the former corresponds to the statistical properties of the detectable light intensity caused by single incident electrons while the latter corresponds to the statistical properties of the detected intensity caused by many electrons incident on the same point. A difference between these statistical properties leads to over-optimistic estimations of the MTF when the noise method is used and furthermore deteriorates the detection quantum efficiency (DQE) at high spatial frequencies. Monte Carlo simulations are used to calculate the true MTF, the expected outcome of a noise method measurement (MTFnoise) and the spatial frequency dependent DQE for various scintillator thicknesses and acceleration voltages.

Atomic Imaging of Phase Transitions and Morphology Transformations in Nanocrystals

A newly developed SiN microhotplate allows specimens to be studied at temperatures up to 1000 K at a resolution of 100 picometer. Aberration-corrected transmission electron microscopy has become a commonplace tool to investigate stable crystals; however, imaging transient nanocrystals is much more demanding. Morphological transformations in gold nanoparticles and layer-by-layer sublimation of PbSe nanocrystals is imaged with atomic resolution.

Atomic Structure and Dynamics of Metal Dopant Pairs in Graphene

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

Gold–Palladium Core–Shell Nanocrystals with Size and Shape Control Optimized for Catalytic Performance

Right to the core: The design of nanocatalysts with maximized catalytic performance relies on control of size, shape, and composition. The shell thickness of nanocrystals with core–shell structures can be controlled, thus enabling control over the nanocrystal electronic structure and catalytic properties. Monodisperse faceted icosahedral Au–Pd core–shell nanocrystals (see picture) were synthesized, and optimized for the oxidation of benzyl alcohol to benzaldehyde.

Confocal operation of a transmission electron microscope with two aberration correctors

The authors demonstrate that confocal imaging trajectories can be established in a transmission electron microscope fitted with two spherical aberration correctors. An atomic-scale electron beam, focused by aberration-corrected illumination optics, is directly imaged by a second aberration-corrected system. The initial experiment described indicates how aberration-corrected scanning confocal electron microscopy will allow three-dimensional imaging and analysis of materials with atomic lateral resolution and with a depth resolution of a few nanometers. The depth resolution in the confocal mode is shown to be robust to the uncorrected chromatic aberration of the lenses, unlike depth sectioning using a single lens.