New electron microscopy techniques for determination of local structural features during plastic deformation

This talk will highlight recent advances in Transmission Electron Microscopy (TEM) techniques that provide insight into small-scale plasticity and the evolution of defect structures in materials. Through the development of fast direct electron detectors, it is now possible to acquire large multidimensional data sets of nanodiffraction patterns (4DSTEM) that can map local structural order and strain with nanometer precision, even during in situ nanomechanical testing. The method is widely applicable and examples will be given from systems such as organic semiconductor molecular thin films, structural alloys with local order such as Ti-Al and CrCoNi, and even to amorphous samples such as bulk metallic glass. This talk will describe our recent results utilizing fast direct electron detectors, energy filtered imaging and in situ TEM nanomechanical testing that provide insight into multiscale materials phenomena using these techniques

Knife-edge based measurement of the 4D transverse phase space of electron beams with picometer-scale emittance

Precise manipulation of high brightness electron beams requires detailed knowledge of the particle phase space shape and evolution. As ultrafast electron pulses become brighter, new operational regimes become accessible with emittance values in the picometer range, with enormous impact on potential scientific applications. Here we present a new characterization method for such beams and demonstrate experimentally its ability to reconstruct the 4D transverse beam matrix of strongly correlated electron beams with sub-nanometer emittance and sub-micrometer spot size, produced with the HiRES beamline at LBNL. Our work extends the reach of ultrafast electron accelerator diagnostics into picometer-range emittance values, opening the way to complex nanometer-scale electron beam manipulation techniques

Dislocation processes accompanying the Portevin-Le Chatelier effect in Al–Mg alloys

Aluminum alloys have great potential to replace steels in automotive structures and closure applications. However, formability limitations continue to remain an obstacle in their widespread usage. Solute strengthening in AA5000 Al–Mg alloys can be exploited to increase ductility and strength simultaneously, but dynamic strain ageing and negative strain rate sensitivity in these alloys lead to Portevin-Le Chatelier (PLC) instability and cause premature failure. PLC instability is associated with the diffusion of Mg atoms to dislocations and dislocations moving away from diffusing Mg atoms, resulting in plastic strain occurring in bursts. Macroscopically, the PLC effect has been well characterized. The underlying dislocation structures and atomistic mechanisms responsible for dynamic strain ageing have not been well understood and is the subject of this study. In this investigation, AA5754 sheets have been strained in situ in the scanning electron microscope and the transmission electron microscope (TEM). Combined electron backscatter diffraction and electron dispersive X-ray spectroscopy analyses show high local Mg concentration regions to correlate well with high dislocation densities. In situ TEM straining data show that glissile dislocations contribute to both nucleation and dissolution of small Mg clusters and precipitates. Electron tomography data show the dynamic nature of the dislocation network of glissile and sessile dislocations. The implications of these observations on the mechanism governing the PLC effect in Al–Mg alloys and the current theories of dynamic strain ageing in Al–Mg alloys will be discussed

Patterned probes for high precision 4D-STEM bragg measurements.

Nanoscale strain mapping by four-dimensional scanning transmission electron microscopy (4D-STEM) relies on determining the precise locations of Bragg-scattered electrons in a sequence of diffraction patterns, a task which is complicated by dynamical scattering, inelastic scattering, and shot noise. These features hinder accurate automated computational detection and position measurement of the diffracted disks, limiting the precision of measurements of local deformation. Here, we investigate the use of patterned probes to improve the precision of strain mapping. We imprint a "bullseye" pattern onto the probe, by using a binary mask in the probe-forming aperture, to improve the robustness of the peak finding algorithm to intensity modulations inside the diffracted disks. We show that this imprinting leads to substantially improved strain-mapping precision at the expense of a slight decrease in spatial resolution. In experiments on an unstrained silicon reference sample, we observe an improvement in strain measurement precision from 2.7% of the reciprocal lattice vectors with standard probes to 0.3% using bullseye probes for a thin sample, and an improvement from 4.7% to 0.8% for a thick sample. We also use multislice simulations to explore how sample thickness and electron dose limit the attainable accuracy and precision for 4D-STEM strain measurements

Selective Lanthanide Sensing with Gold Nanoparticles and Hydroxypyridinone Chelators.

The octadentate hydroxypyridinone chelator 3,4,3-LI(1,2-HOPO) is a promising therapeutic agent because of its high affinity for f-block elements and noncytotoxicity at medical dosages. The interaction between 3,4,3-LI(1,2-HOPO) and other biomedically relevant metals such as gold, however, has not been explored. Gold nanoparticles functionalized with chelators have demonstrated great potential in theranostics, yet thus far, no protocol that combines 3,4,3-LI(1,2-HOPO) and colloidal gold has been developed. Here, we characterize the solution thermodynamic properties of the complexes formed between 3,4,3-LI(1,2-HOPO) and Au3+ ions and demonstrate how under specific pH conditions the chelator promotes the growth of gold nanoparticles, acting as both reducing and stabilizing agent. 3,4,3-LI(1,2-HOPO) ligands on the nanoparticle surface remain active and selective toward f-block elements, as evidenced by gold nanoparticle selective aggregation. Finally, a new colorimetric assay capable of reaching the detection levels necessary for the quantification of lanthanides in waste from industrial processes is developed based on the inhibition of particle growth by lanthanides

In situ nanoindentation in a transmission electron microscope

This dissertation presents the development of the novel mechanical testing technique of in situ nanoindentation in a transmission electron microscope (TEM). This technique makes it possible to simultaneously observe and quantify the mechanical behavior of nano-scale volumes of solids

Multiple origins of extra electron diffractions in fcc metals.

Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article explores the many nonordering origins of commonly reported features using a combination of experimental electron microscopy and multislice diffraction simulations, which suggest that diffuse intensities largely represent thermal and static displacement scattering. A number of observations may reflect additional contributions from planar defects, surface terminations incommensurate with bulk periodicity, or weaker dynamical effects