James L. Hart

Direct Detection Electron Energy-Loss Spectroscopy: A Method to Push the Limits of Resolution and Sensitivity

In many cases, electron counting with direct detection sensors offers improved resolution, lower noise, and higher pixel density compared to conventional, indirect detection sensors for electron microscopy applications. Direct detection technology has previously been utilized, with great success, for imaging and diffraction, but potential advantages for spectroscopy remain unexplored. Here we compare the performance of a direct detection sensor operated in counting mode and an indirect detection sensor (scintillator/fiber-optic/CCD) for electron energy-loss spectroscopy. Clear improvements in measured detective quantum efficiency and combined energy resolution/energy field-of-view are offered by counting mode direct detection, showing promise for efficient spectrum imaging, low-dose mapping of beam-sensitive specimens, trace element analysis, and time-resolved spectroscopy. Despite the limited counting rate imposed by the readout electronics, we show that both core-loss and low-loss spectral acquisition are practical. These developments will benefit biologists, chemists, physicists, and materials scientists alike.

Real-Time Observation of Local Strain Effects on Nonvolatile Ferroelectric Memory Storage Mechanisms

We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics toward a Nucleation Limited Switching model.

Tracking the evolution of intergranular corrosion through twin-related domains in grain boundary networks

Tailoring the grain boundary network is desired to improve grain boundary-dependent phenomena such as intergranular corrosion. An important grain boundary network descriptor in heavily twinned microstructures is the twin-related domain, a cluster of twin-related grains. We indicate the advantages of using twin-related domains and subsequent statistics to provide new insight into how a grain boundary networks respond to intergranular corrosion in a heavily twinned grain boundary engineered 316L stainless steel. The results highlight that intergranular corrosion is typically arrested inside twin-related domains at coherent twins or low-angle grain boundaries. Isolated scenarios exist, however, where intergranular corrosion propagation persists in the grain boundary network through higher-order twin-related boundaries.Steels: clustered coherent twins stop corrosionClustered twin grain boundaries in stainless steel can stop intergranular corrosion, but only if they are coherent. A team led by Mitra Taheri at Drexel University in the USA analyzed microstructural regions in a 316 stainless steel where all grain boundaries were twinned and found that, when the twins in these clusters were coherent or had a low misorientation angle, they arrested interganular corrosion. They emphasized this effect by engineering more coherent and low-angle grain boundaries with thermomechanical processing, leading to larger twin-related domains. In contrast, twinned clusters with high-angle grain boundaries consistently failed at resisting corrosion, a similar manner to the rest of steel. Twin-related domains may therefore be a good predictor of intergranular corrosion and may help us mitigate metal damage.

Performance of a Direct Electron Detector for the Application of Electron Energy-Loss Spectroscopy

Transmission electron microscopes (TEMs) conventionally employ indirect detection cameras (IDC) for electron imaging. Such IDCs consist of a scintillator and a digital imaging device with a lens or fiber optic network coupling photons from the scintillator to the camera. Alternatively direct detection cameras (DDC) directly image electrons. Compared to IDCs, DDCs offer an improved point spread function (PSF), lower read-out noise, and potential for higher frame rates [1,2]. DDCs have been successfully utilized by the cryo-TEM community [3] and, more recently, for in-situ TEM applications [4]. Here we evaluate a DDC for the application of electron energy-loss spectroscopy (EELS). We compared the performance of a Gatan K2 Summit (DDC) with a Gatan US1000FTXP (IDC). Both detectors were mounted to a Gatan GIF Quantum energy filter. Our results show that the narrow PSF of the DDC improves measured resolution given a fixed beam energy spread and spectrometer dispersion. Additionally, the low read-out noise of the DDC increases spectrum signal to noise (SNR) for short acquisition times. These results indicate DDCs will enable efficient acquisition of low-noise spectra for applications ranging from in-situ EELS to low-dose chemical mapping.

Elucidation of Insulin Assembly at Acidic and Neutral pH: Characterization of Low Molecular Weight Oligomers

Deficiency in insulin secretion and function that characterize type 2 diabetes often requires administration of extraneous insulin, leading to injection‐site amyloidosis. Insulin aggregation at neutral pH is not well understood. Although oligomer formation is believed to play an important role, insulin oligomers have not been fully characterized yet. Here, we elucidate similarities and differences between in vitro insulin aggregation at acidic and neutral pH for a range of insulin concentrations (2.5–100 μM) by using kinetic thioflavin T fluorescence, circular dichroism, atomic force and electron microscopy imaging. Importantly, we characterize the size distribution of insulin oligomers at different assembly stages by the application of covalent cross‐linking and gel electrophoresis. Our results show that at the earliest assembly stage, oligomers comprise up to 40% and 70% of soluble insulin at acidic and neutral pH, respectively. While the highest oligomer order increases with insulin concentration at acidic pH, the opposite tendency is observed at neutral pH, where oligomers up to heptamers are formed in 10 μM insulin. These findings suggest that oligomers may be on‐ and off‐pathway assemblies for insulin at acidic and neutral pH, respectively. Agitation, which is required to induce insulin aggregation at neutral pH, is shown to increase fibril formation rate and fibrillar mass both by an order of magnitude. Insulin incubated under agitated conditions at neutral pH rapidly aggregates into large micrometer‐sized aggregates, which may be of physiological relevance and provides insight into injection‐site amyloidosis and toxic pulmonary aggregates induced by administration of extraneous insulin.

Control of hidden ground-state order in NdNiO3 superlattices

The fascinating behavior of transition metal oxides can change dramatically when they are scaled down to atomic-size dimensions; however, understanding the emergent properties is a major challenge. In this paper, the authors observe the evolution of multiple phase transitions as the thickness is reduced from bulk to the atomic layer limit in NdNiO

Application of Electron Counting to Electron Energy-loss Spectroscopy and Implications for Low-Dose Characterization

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I2 basal stacking fault as a degradation mechanism in reverse gate-biased AlGaN/GaN HEMTs

superlattices. Their measurements demonstrate a separation of the insulating phase from magnetic and charge-ordered phases, which coexist in the bulk, and the emergence of a hidden, unordered insulating phase for a single atomic layer. Modeling shows that the phase manipulation uniquely takes advantage of the effects of 2D confinement and symmetry-breaking at the interface.

Electron Beam Induced Domain Motion in Ferroelectric RKTP Observed By Transmission Electron Microscopy

Electron counting with direct detection (DD) sensors offers large improvements in resolution and signal to noise ratio (SNR) compared to conventional indirect detection (ID) sensors. The benefits offered by DD sensors have yielded remarkable results for low-dose imaging [1]; however, it is unclear how electron counting would affect electron energy-loss spectroscopy (EELS). Here we quantify the performance of electron counting for EELS by comparing the Gatan K2 summit (DD sensor operated in counting mode) and the Gatan US1000FTXP (ID sensor with scintillator/fiber-optic/CCD design). The results indicate DD EELS will offer major advantages for low-dose spectroscopy.

Toward Deterministic Switching in Ferroelectric Systems: Insight Gained from In Situ TEM

Here, we present the observation of a bias-induced, degradation-enhancing defect process in plasma-assisted molecular beam epitaxy grown reverse gate-biased AlGaN/GaN high electron mobility transistors (HEMTs), which is compatible with the current theoretical framework of HEMT degradation. Specifically, we utilize both conventional transmission electron microscopy and aberration-corrected transmission electron microscopy to analyze microstructural changes in not only high strained regions in degraded AlGaN/GaN HEMTs but also the extended gate-drain access region. We find a complex defect structure containing an I2 basal stacking fault and offer a potential mechanism for device degradation based on this defect structure. This work supports the reality of multiple failure mechanisms during device operation and identifies a defect potentially involved with device degradation.