Kayla X. Nguyen

High Dynamic Range Pixel Array Detector for Scanning Transmission Electron Microscopy

We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80-200 keV electron beams.

Enhancement of the anti-damping spin torque efficacy of platinum by interface modification

We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of ≈0.5 nm layer of Hf between a Pt film and a thin, ≤2 nm, Fe60Co20B20 ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnets absorption of the SHE generated spin current.

A single-molecule approach to ZnO defect studies: Single photons and single defects

Investigations that probe defects one at a time offer a unique opportunity to observe properties and dynamics that are washed out of ensemble measurements. Here, we present confocal fluorescence measurements of individual defects in ZnO nanoparticles and sputtered films that are excited with sub-bandgap energy light. Photon correlation measurements yield both antibunching and bunching, indicative of single-photon emission from isolated defects that possess a metastable shelving state. The single-photon emission is in the range of ∼560–720 nm and typically exhibits two broad spectral peaks separated by ∼150 meV. The excited state lifetimes range from 1 to 13 ns, consistent with the finite-size and surface effects of nanoparticles and small grains. We also observe discrete jumps in the fluorescence intensity between a bright state and a dark state. The dwell times in each state are exponentially distributed and the average dwell time in the bright (dark) state does (may) depend on the power of the exciting ...

Characterization of Sulfur and Nanostructured Sulfur Battery Cathodes in Electron Microscopy Without Sublimation Artifacts

Lithium sulfur (Li-S) batteries have the potential to provide higher energy storage density at lower cost than conventional lithium ion batteries. A key challenge for Li-S batteries is the loss of sulfur to the electrolyte during cycling. This loss can be mitigated by sequestering the sulfur in nanostructured carbon-sulfur composites. The nanoscale characterization of the sulfur distribution within these complex nanostructured electrodes is normally performed by electron microscopy, but sulfur sublimates and redistributes in the high-vacuum conditions of conventional electron microscopes. The resulting sublimation artifacts render characterization of sulfur in conventional electron microscopes problematic and unreliable. Here, we demonstrate two techniques, cryogenic transmission electron microscopy (cryo-TEM) and scanning electron microscopy in air (airSEM), that enable the reliable characterization of sulfur across multiple length scales by suppressing sulfur sublimation. We use cryo-TEM and airSEM to examine carbon-sulfur composites synthesized for use as Li-S battery cathodes, noting several cases where the commonly employed sulfur melt infusion method is highly inefficient at infiltrating sulfur into porous carbon hosts.

AirSEM: Electron Microscopy in Air, without a Specimen Chamber

A new generation of atmospheric scanning electron microscopes (ASEMs) allow samples to be imaged in liquid or at atmospheric pressure through an electron-transparent window that separates the column of the microscope from the sample [1, 2]. One approach to dealing with the short required working distance has been to directly image into the liquid with an inverted SEM column below a silicon nitride window held by a petri dish [1]. Here, we explore an alternative design for a general-purpose field-emission AirSEM from b-Nano [2]. This is an upright geometry where the sample is mechanically positioned 50-200 microns below electron-transparent window after a computer-controlled alignment with an optical microscope (Fig. 1a). This decouples the sample from the window, allowing for its reuse. The accessibility of the sample, without the need for vacuum feedthroughs makes it very simple to add imaging modes, including secondary ion detector, x-ray mapping, and cathodoluminescence.

Strain Mapping of Two-Dimensional Heterostructures with Subpicometer Precision

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector-the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures.

Theory and practice of electron diffraction from single atoms and extended objects using an EMPAD

What does the diffraction pattern from a single atom look like? How does it differ from the scattering from long-range potential? With the development of new high-dynamic range pixel array detectors to measure the complete momentum distribution, these questions have immediate relevance for designing and understanding momentum-resolved imaging modes. We explore the asymptotic limits of long-range and short-range potentials. We use a simple quantum mechanical model to explain the general and asymptotic limits for the probability distribution in both real and reciprocal space. Features in the scattering potential much larger than the probe size cause the bright field (BF) disk to deflect uniformly, while features much smaller than the probe size, instead of a deflection, cause a redistribution of intensity within the BF disk. Because long-range and short-range features are encoded differently in the diffraction pattern, it is possible to separate their contributions in differential phase-contrast (DPC) or center-of-mass (CoM) imaging. The shape profiles for atomic resolution CoM imaging are dominated by the shape of the probe gradient and not the highly singular atomic potentials or their local fields. Instead, only the peak height shows an atomic number sensitivity, whose precise dependence is determined by the convergence angle. At lower convergence angles, the contrast oscillates with increasing atomic number, similar to BF imaging. The range of collection angles impacts DPC and CoM imaging differently, with CoM being more sensitive to the upper cutoff limit, while DPC is more sensitive to the lower cutoff.

Dose-Efficient Cryo-STEM Imaging of Whole Cells Using the Electron Microscope Pixel Array Detector

For specimens thicker than the inelastic mean free path (~300 nm at 300 kV), such as whole cells, STEM imaging has shown better performance than TEM because it lacks chromatic blurring caused by postspecimen imaging optics in the TEM. This advantage is particularly useful for electron tomography of both embedded [1] and cryo-preserved cells [2]. However, typical STEM detector geometries—either an annular detector collecting electrons scattered to high angles or an on-axis bright field detector collecting the forward-scattered beam—make use of only a small fraction of the incident electron dose.

Reconstruction of Polarization Vortices by Diffraction Mapping of Ferroelectric PbTiO3 / SrTiO3 Superlattice Using a High Dynamic Range Pixelated Detector

Kayla X. Nguyen, Prafull Purohit, Ajay Yadav, Mark W. Tate, Celesta S. Chang, Ramamoorthy Ramesh, Sol M. Gruner, David A. Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Department of Physics, Cornell University, Ithaca, NY, USA Department of Material Science and Engineering, University of California, Berkeley Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY USA

Electron Diffraction from a Single Atom and Optimal Signal Detection

Imaging single atoms against a supporting background, made of many more atoms, is a common challenge for many areas of materials characterization from detecting dopants in semiconductors [1,2] to understanding the behavior of catalysts [3]. Electron energy loss spectroscopy and x-ray analysis can help to suppress the background support. While single atom detection is possible [2], their low cross sections imply they are not dose efficient or well-suited to radiation-sensitive materials. The elastic signal is the largest atomic-resolution signal available, but efficient discrimination of the target atom against the support atoms can be system specific. In the best case of single atoms on a graphene or uniform 2D support, the problem is well posed, and the optimal single-channel STEM detector is a low-angle annular dark field (ADF) detector [4]. The inclusion of a thick support considerably complicates matters, but detection of heavy atoms against a light matrix is possible using high-angle ADF imaging [1-3]. The question is can we do better by recording the full scattering distribution?