Andrew C. Lang

Evidence for Bulk Ripplocations in Layered Solids.

Plastically anisotropic/layered solids are ubiquitous in nature and understanding how they deform is crucial in geology, nuclear engineering, microelectronics, among other fields. Recently, a new defect termed a ripplocation–best described as an atomic scale ripple–was proposed to explain deformation in two-dimensional solids. Herein, we leverage atomistic simulations of graphite to extend the ripplocation idea to bulk layered solids, and confirm that it is essentially a buckling phenomenon. In contrast to dislocations, bulk ripplocations have no Burgers vector and no polarity. In graphite, ripplocations are attracted to other ripplocations, both within the same, and on adjacent layers, the latter resulting in kink boundaries. Furthermore, we present transmission electron microscopy evidence consistent with the existence of bulk ripplocations in Ti3SiC2. Ripplocations are a topological imperative, as they allow atomic layers to glide relative to each other without breaking the in-plane bonds. A more complete understanding of their mechanics and behavior is critically important, and could profoundly influence our current understanding of how graphite, layered silicates, the MAX phases, and many other plastically anisotropic/layered solids, deform and accommodate strain.

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.

MgB2 ultrathin films fabricated by hybrid physical chemical vapor deposition and ion milling

In this letter, we report on the structural and transport measurements of ultrathin MgB2 films grown by hybrid physical-chemical vapor deposition followed by low incident angle Ar ion milling. The ultrathin films as thin as 1.8 nm, or 6 unit cells, exhibit excellent superconducting properties such as high critical temperature (Tc) and high critical current density (Jc). The results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit.

Atomic-Scale Characterization of Oxide Thin Films Gated by Ionic Liquid

Ionic liquids (ILs) have received considerable interest for use in electrostatic gating in complex oxide systems. Understanding the ionic liquid/oxide interface, and any bias-induced electrochemical degradation, is critical for the interpretation of transport phenomena. The integrity of the interface between ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate and La1/3Sr2/3FeO3 under various biasing conditions was examined by analytical transmission electron microscopy, and we report film degradation in the form of an irreversible chemical reaction regardless of the applied bias. This results in an intermixing region of 4-6 nm at the IL/oxide interface. Electron energy loss spectroscopy shows La and Fe migration into the ionic liquid, resulting in secondary phase formation under negative bias. Our approach can be extended to other ionic liquid/oxide systems in order to better understand the electrochemical stability window of these device structures.

Effects of cation stoichiometry on electronic and structural properties of LaNiO3

LaNiO3 films with varying La:Ni ratios were deposited onto SrTiO3 (001) substrates via molecular beam epitaxy to elucidate the effects of cation off-stoichiometry. The physical properties of La-deficient films are found to differ substantially from those of Ni-deficient films, with La-deficient films exhibiting lower electrical resistivities and smaller c-axis parameters than Ni-deficient films. No evidence of secondary phases is observed; however, transmission electron microscopy reveals an abundance of defects, the nature of which differs in lanthanum- and nickel-deficient films. This work illustrates the nontrivial role that cation stoichiometry can play on the functional properties of complex oxides.

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.

Enhancement of lower critical field by reducing the thickness of epitaxial and polycrystalline MgB2 thin films

For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2films, including the thickness dependence of the lower critical fieldHc1. MgB2thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxialfilms) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, Hc1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxialfilms and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.

On the interactions of Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC with palladium at 900 °C

Abstract Herein we report on the reactivity between palladium, Pd, and the MAX phases, Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC. Diffusion couples of Pd/MAX were heated to 900 °C under uniaxial stress of ∼20 MPa for 2, 4, and 10 h in a vacuum (

Advantages of Direct Detection and Electron Counting for Electron Energy Loss Spectroscopy Data Acquisition and the Quest of Extremely High-Energy Edges Using Eels

Transmission electron microscopes primarily employ indirect cameras (IDC) for electron detection in imaging, diffraction and EELS modes. Such cameras convert incident electrons to photons which, through a fiber optic network or lens, are coupled to a light sensitive camera. This indirect detection method typically has a negative impact on the point spread function (PSF) and detective quantum efficiency (DQE) of the camera. Over the last decade, radiation tolerant CMOS active pixel sensors, which directly detect high-energy incident electrons and have the speed to count individual electrons events, have been developed. These detectors result in greatly improved PSF and DQE in comparison to conventional IDCs. Such direct detection cameras (DDCs) have revolutionized the cryo-TEM field as well as have strong advantages for in-situ TEM in both imaging and diffraction applications. EELS applications can benefit from the improved PSF and the ability to count electrons. The improved PSF allows spectra to be acquired over larger energy ranges while maintaining sharp features and greatly reduced spectral tails. The ability to count electrons nearly eliminates the noise associated with detector readout and greatly reduces the proportional noise associated with detector gain variations. This effectively leaves the shot noise as the limiting noise source present. The implication for EELS acquisition is that fine structure analysis becomes more straightforward for typical conditions and even possible for the case of low signal levels.

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

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.