Jordan Pierce

Efficient linear phase contrast in scanning transmission electron microscopy with matched illumination and detector interferometry

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

Efficient diffractive phase optics for electrons

Electron diffraction gratings can be used to imprint well-defined phase structure onto an electron beam. For example, diffraction gratings have been used to prepare electron beams with unique phase dislocations, such as electron vortex beams, which hold promise for the development of new imaging and spectroscopy techniques for the study of materials. However, beam intensity loss associated with absorption, scattering, and diffraction by a binary transmission grating drastically reduces the current in the beam, and thus the possible detected signal strength it may generate. Here we describe electron-transparent phase gratings that efficiently diffract transmitted electrons. These phase gratings produce electron beams with the high current necessary to generate detectable signal upon interaction with a material. The phase grating design detailed here allows for fabrication of much more complex grating structures with extremely fine features. The diffracted beams produced by these gratings are widely separated and carry the designed phase structure with high fidelity. In this work, we outline a fabrication method for high-efficiency electron diffraction gratings and present measurements of the performance of a set of simple prototypical gratings in a transmission electron microscope. We present a model for electron diffraction gratings that can be used to optimize the performance of diffractive electron optics. We also present several new holograms that utilize manipulation of phase to produce new types of highly efficient electron beams.

Origins and demonstrations of electrons with orbital angular momentum

The surprising message of Allen et al. (Allen et al. 1992 Phys. Rev. A 45, 8185 (doi:10.1103/PhysRevA.45.8185)) was that photons could possess orbital angular momentum in free space, which subsequently launched advancements in optical manipulation, microscopy, quantum optics, communications, many more fields. It has recently been shown that this result also applies to quantum mechanical wave functions describing massive particles (matter waves). This article discusses how electron wave functions can be imprinted with quantized phase vortices in analogous ways to twisted light, demonstrating that charged particles with non-zero rest mass can possess orbital angular momentum in free space. With Allen et al. as a bridge, connections are made between this recent work in electron vortex wave functions and much earlier works, extending a 175 year old tradition in matter wave vortices. This article is part of the themed issue ‘Optical orbital angular momentum’.

Observation of nanoscale magnetic fields using twisted electron beams

Electron waves give an unprecedented enhancement to the field of microscopy by providing higher resolving power compared to their optical counterpart. Further information about a specimen, such as electric and magnetic features, can be revealed in electron microscopy because electrons possess both a magnetic moment and charge. In-plane magnetic structures in materials can be studied experimentally using the effect of the Lorentz force. On the other hand, full mapping of the magnetic field has hitherto remained challenging. Here we measure a nanoscale out-of-plane magnetic field by interfering a highly twisted electron vortex beam with a reference wave. We implement a recently developed holographic technique to manipulate the electron wavefunction, which gives free electrons an additional unbounded quantized magnetic moment along their propagation direction. Our finding demonstrates that full reconstruction of all three components of nanoscale magnetic fields is possible without tilting the specimen.Beyond high resolving power, electron microscopy can be used to study both the electronic and magnetic properties of a sample. Here, Grillo et al. combine electron vortex beams with holographic detection to measure out-of-plane nanoscale magnetic fields.

Development of STEM-Holography

Low-atomic number materials play a crucial role in life sciences, medicine, and the carbon energy cycle. However, our ability to image these materials at the atomic length scale is limited because they do not scatter electrons at high-angles in the same way a crystalline or high atomic number material does. Additionally, these materials are easily damaged under electron beam illumination. To get around these issues, bold efforts have been made in the fields of electron holography [2] and ptychography [3, 4], leading to myriad techniques that can potentially achieve sub-nanometer resolution. Additionally, offaxis electron holography has been developed and applied in many research groups [5 8], pushing the boundaries of electron microscopy with unprecedented feats such as the atomic resolution electrostatic potential mapping of graphene sheets [9].

Phase Contrast Imaging of Weakly-Scattering Samples with Matched Illumination and Detector Interferometry–Scanning Transmission Electron Microscopy (MIDI–STEM)

Aberration-corrected scanning transmission electron microscopy (STEM) has made an enormous impact on materials science, with recent examples including observations of nanometer-scale polar vortices in oxide superlattices [1], atomic-scale chemical imaging [2], atomic-resolution 3D tomography [3], and many others. However, the majority of these studies image chemical species with intermediate or high atomic numbers. Samples composed primarily of low atomic number species are more difficult to study in STEM due to their poor scattering efficiency. This is why the vast majority of biological research in electron microscopy (EM) uses phase contrast plane-wave methods, including CTF-corrected cryo-EM [4] phase-plate HRTEM [5], etc. Phase contrast methods can also be used in STEM to improve contrast in weakly-scattering samples, such as ptychography [6, 7]. These methods improve contrast, but require significantly more involved processing of the experimental data.

Structured Electron Beam Illumination: A New Control Over the Electron Probe Weird Probes and New Experiments

1. CNR-Istituto Nanoscienze, Centro S3, Via G. Campi 213/a, I-41125 Modena, Italy 2. CNR-IMEM Parco Area delle Scienze 37/A, I-43124 Parma, Italy 3. Department of Physics, University of Oregon, Eugene, 97403-1274 Oregon, USA 4. Department of Physics, University of Ottawa, 25 Templeton, Ottawa, Ontario, K1N 6N5 Canada 5. CNR-IMM Bologna, Via P. Gobetti 101, 40129 Bologna, Italy 6. Dipartimento FIM, Universitá di Modena e Reggio Emilia, Via G. Campi 213/a, I-41125 Modena, Italy 7. Institute of Optics, University of Rochester, Rochester, New York 14627, USA

Electron holograms encoding amplitude and phase for the generation of arbitrary wavefunctions

1. CNR-Istituto Nanoscienze, Centro S3, Via G. Campi 213/a, I-41125 Modena, Italy 2. CNR-IMEM Parco Area delle Scienze 37/A, I-43124 Parma, Italy 3. Department of Physics, University of Oregon, Eugene, 97403-1274 Oregon, USA 4. Department of Physics, University of Ottawa, 25 Templeton, Ottawa, Ontario, K1N 6N5 Canada 5. CNR-IMM Bologna, Via P. Gobetti 101, 40129 Bologna, Italy 6. Dipartimento FIM, Universitá di Modena e Reggio Emilia, Via G. Campi 213/a, I-41125 Modena, Italy 7. Institute of Optics, University of Rochester, Rochester, New York 14627, USA

Aberration-Corrected STEM by Means of Diffraction Gratings

In the past 15 years, the advent of aberration correction technology in electron microscopy has enabled materials analysis on the atomic scale. This is made possible by precise arrangements of multipole electrodes and magnetic solenoids to compensate the aberrations inherent to any focusing element of an electron microscope. Here, we describe an alternative method to correct for the spherical aberration of the objective lens in scanning transmission electron microscopy (STEM) using a passive, nanofabricated diffractive optical element. This holographic device is installed in the probe forming aperture of a conventional electron microscope and can be designed to remove arbitrarily complex aberrations from the electrons wave front. In this work, we show a proof-of-principle experiment that demonstrates successful correction of the spherical aberration in STEM by means of such a grating corrector (GCOR). Our GCOR enables us to record aberration-corrected high-resolution high-angle annular dark field (HAADF-) STEM images, although yet without advancement in probe current and resolution. Improvements in this technology could provide an economical solution for aberration-corrected high-resolution STEM in certain use scenarios.

Characterization of Electron Orbital Angular Momentum Transfer to Nanoparticle Plasmon Modes

We observed the decay of an electron vortex beam from a state with orbital angular momentum l = 1ħ to l = 0 by interaction with gold nanoparticle surface plasmon modes. Several optical studies have induced plasmon vortices using optical vortices and circularly polarized light and suggested their use in nanophotonic and plasmonic devices [1,2]. Direct observation of angular momentum transfer from electron vortices allows for unique identification of the orbital angular momentum associated with localized plasmon excitations down to the nanometer scale.