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Hart, James L.

Publications and source records attributed to Hart, James L..

25 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

RapidEELS: machine learning for denoising and classification in rapid acquisition electron energy loss spectroscopy

Recent advances in detectors for imaging and spectroscopy have afforded in situ, rapid acquisition of hyperspectral data. While electron energy loss spectroscopy (EELS) data acquisition speeds with electron counting are regularly reaching 400 frames per second with near-zero read noise, signal to noise ratio (SNR) remains a challenge owing to fundamental counting statistics. In order to advance understanding of transient materials phenomena during rapid acquisition EELS, trustworthy analysis of noisy spectra must be demonstrated. In this study, we applied machine learning techniques to denoise high frame rate spectra, benchmarking with slower frame rate “ground truths”. The results provide a foundation for reliable use of low SNR data acquired in rapid, in-situ spectroscopy experiments. Such a tool-set is a first step toward both automation in microscopy as well as use of these methods to interrogate otherwise poorly understood transformations.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of growth substrate on the nucleation of monolayer MoTe 2

The discovery and characterization of two-dimensional (2D) materials beyond graphene has increased dramatically over the past decade with increasingly fine control over the growth dynamics of these materials. MoTe 2 represents a model material for studying phase change properties of 2D materials due to small energy differences between its 2H semiconducting phase and 1T' semimetallic phase. Although some substrates are demonstrated to be better than others for the wafer-scale growth of high quality MoTe 2 films, substrate effects on the nucleation and growth of MoTe 2 are still not well understood. Here, we grow monolayer MoTe 2 by converting MoO x thin films deposited on three different substrates: sapphire Al 2 O 3 (0001), amorphous SiO 2 , and amorphous AlO x , and examine the early stages of the conversion reaction to elucidate the substrate effects on the nucleation of MoTe 2 . We observe that the chemical composition of the substrate is more important than the surface topography and crystallinity of the substrate, with high quality monolayer 2H MoTe 2 formed on both Al 2 O 3 (0001) and AlO x in contrast to mixed phase 2H/1T' MoTe 2 formed on SiO 2 , as determined by Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A percolation theory for designing corrosion-resistant alloys

Iron-chromium and nickel-chromium binary alloys containing sufficient quantities of chromium serve as the prototypical corrosion-resistant metals owing to the presence of a nanometre-thick protective passive oxide film. Should this film be compromised by a scratch or abrasive wear, it reforms with little accompanying metal dissolution, a key criterion for good passive behaviour. This is a principal reason that stainless steels and other chromium-containing alloys are used in critical applications ranging from biomedical implants to nuclear reactor components. Unravelling the compositional dependence of this electrochemical behaviour is a long-standing unanswered question in corrosion science. Herein, we develop a percolation theory of alloy passivation based on two-dimensional to three-dimensional crossover effects that accounts for selective dissolution and the quantity of metal dissolved during the initial stage of passive film formation. We validate this theory both experimentally and by kinetic Monte Carlo simulation. Our results reveal a path forward for the design of corrosion-resistant metallic alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis of Narrow SnTe Nanowires Using Alloy Nanoparticles

Topological crystalline insulator tin telluride (SnTe) provides a rich playground to examine interactions of correlated electronic states, such as ferroelectricity, topological surface states, and superconductivity. The study of SnTe nanowires may lead to even richer physics owing to the one-dimensional (1D) confinement effect and an increased contribution from the topological surface states. Thus, for transport measurements, SnTe nanowires must be synthesized with reduced diameters and high crystalline quality to ensure 1D confinement and phase coherence of the topological surface electrons. Here, we report a facile growth method to produce narrow SnTe nanowires with a high yield using alloy nanoparticles as growth catalysts. The average diameter of the SnTe nanowires grown using alloy nanoparticles is 85 nm, nearly a factor of three reduction compared to the average diameter of 240 nm when using gold nanoparticles as growth catalysts. Transport measurements reveal the effect of the nanowire diameter on the residual resistance ratio and magnetoresistance. Particularly, the ferroelectric transition temperature for SnTe evolves systematically with the nanowire diameter. In situ cryogenic cooling of narrow SnTe nanowires in a transmission electron microscope directly reveals the cubic to rhombohedral structural transition, which is associated with the ferroelectric transition. Thus, these narrow SnTe nanowires represent a model system to study electronic states arising from 1D confinement, such as 1D topological superconductivity and potential multiband superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Evidence of a magnetic transition in atomically thin Cr 2 TiC 2 T x MXene

Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr 2 TiC 2 T x MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr 2 TiC 2 T x at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr 2 TiAlC 2 MAX phase). Overall, this work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Structural transition and recovery of Ge implanted β -Ga 2 O 3

Ion implantation-induced effects were studied in Ge implanted β-Ga 2 O 3 with the fluence and energy of 3 × 10 13 cm -2 /60 keV, 5 × 10 13 cm -2 /100 keV, and 7 × 10 13 cm -2 /200 keV using analytical electron microscopy via scanning/transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction via TopSpin. Imaging shows an isolated band of damage after Ge implantation, which extends ~130 nm from the sample surface and corresponds to the projected range of the ions. Electron diffraction demonstrates that the entirety of the damage band is the κ phase, indicating an implantation-induced phase transition from β to κ-Ga 2 O 3 . Post-implantation annealing at 1150 °C for 60 s under the O 2 atmosphere led to a back transformation of κ to β; however, an ~17 nm damage zone remained at the sample surface. Despite the back transformation from κ to β with annealing, O K-edge spectra show changes in the fine structure between the pristine, implanted, and implanted-annealed samples, and topspin strain analysis shows a change in strain between the two samples. These data indicate differences in the electronic/chemical structure, where the change of the oxygen environment extended beyond the implantation zone (~130 nm) due to the diffusion of Ge into the bulk material, which, in turn, causes a tensile strain of 0.5%. This work provides a foundation for understanding of the effects of ion implantation on defect/phase evolution in β-Ga 2 O 3 and the related recovery mechanism, opening a window toward building a reliable device for targeted applications.

36 MATERIALS SCIENCE↗