Pranav K. Suri

Communication: Effects of thermionic-gun parameters on operating modes in ultrafast electron microscopy

Ultrafast electron microscopes with thermionic guns and LaB6 sources can be operated in both the nanosecond, single-shot and femtosecond, single-electron modes. This has been demonstrated with conventional Wehnelt electrodes and absent any applied bias. Here, by conducting simulations using the General Particle Tracer code, we define the electron-gun parameter space within which various modes may be optimized. The properties of interest include electron collection efficiency, temporal and energy spreads, and effects of laser-pulse duration incident on the LaB6 source. We find that collection efficiencies can reach 100% for all modes, despite there being no bias applied to the electrode.

Defect Characterization in Irradiated Nanocrystalline Materials via Automated Crystal Orientation Mapping

Nanocrystalline metals and alloys are potential candidates for next generation nuclear reactors due to a high volume fraction of grain boundaries, which can act as efficient sinks for irradiation induced defects [1,2]. Present research is focused on understanding the annihilation and evolution of defects adjacent to grain boundaries and interfaces in irradiated materials [3,4]. Traditionally, transmission electron microscopy (TEM) has played a critical role in the evaluation of irradiation induced vacancy and interstitial dislocation loop size and density [5]. There are conventional methods of imaging defects in crystalline materials via TEM such as two-beam dynamical condition, down-zone imaging, and weak-beam dark-field microscopy which requires tilting of the specimen along desired crystal orientations [5]. However, specimen tilting along a definite crystal orientation is not easily achievable in nanocrystalline metals and alloys with grain sizes smaller than 100 nm. With the advent of automated crystal orientation mapping (ACOM) combined with the routine use of stable field-emission gun electron microscopes, it is now possible to map crystal orientations down to a nanometer spatial resolution without specimen tilting [6]. Here, we describe our efforts to develop new methods of defect characterization in nanocrystalline materials combining the use of ACOM with the conventional TEM defect imaging methods without employing specimen tilting. We explore the various defect imaging techniques and access the suitability of each technique for imaging defects in nanocrystalline materials. We use JEOL 2100F TEM operated at 200 kV having NanoMegas ASTAR precession diffraction system installed on it for all our experiments. We employ irradiated coarse and nanocrystalline grain Ni-5wt%Cr as our face-centered cubic model alloy system. Coarse and nanocrystalline grain specimens were irradiated at 20 MeV and 1.2 MeV, respectively, with Ni ions in Ion Beam Laboratory at Sandia National Laboratories.

Dynamical Effects on Atomic-Resolution Imaging and Diffraction of the Tetragonal and Orthorhombic Phases of LaFeAsO

The discovery of high-Tc superconductivity in the lightly-doped iron pnictides has led to an explosion in theoretical, synthetic, and characterization work [1,2]. Doping in these materials suppresses the temperatures at which the crystallographic phase transition and antiferromagnetic (AF) ordering occur relative to the undoped parent compounds. At sufficient doping levels, emergence of the superconducting dome is observed with the suppressed AF and so-called nematic states bordering this transition [3]. Of particular interest are the 1111-type compounds, which have been found to exhibit Tc > 50 K. Consequently, efforts have focused on elucidating the origin of such emergent properties in these and other strongly-electron correlated materials. Results suggest that there is a complex interplay between structural transformations and spin/charge/orbital ordering, the precise nature of which remains unknown with respect to the superconducting state [4].

Effects of Electron-Gun and Laser Parameters on Collection Efficiency and Packet Duration in Ultrafast Electron Microscopy

Real-space imaging of full-morphological, angstrom-scale structural dynamics occurring on the femtosecond (fs) timescale is possible with ultrafast electron microscopy (UEM) via the stroboscopic pump-probe methodology enabled by interfacing an otherwise standard TEM with a short-pulsed laser system [1-3]. The technology has matured to the point that commercial systems, such as the FEI Tecnai Femto, are now available [4]. The base platform for the Tecnai Femto is FEI’s Tecnai G2 20 200 kV instrument with a thermionic electron gun. In UEM mode, the LaB6 is operated cold such that electron emission occurs only during photo-illumination. Despite this, nanosecond single-shot imaging and diffraction – wherein >10 7 electrons per packet reach the specimen – are possible with a Tecnai-based UEM equipped with a conventional thermionic electron gun [5,6]. While this illustrates that sufficient photoelectrons for such experiments are generated in an otherwise unmodified Tecnai TEM, the parameter space for application-specific optimal electron-gun and photoelectron-generating laser properties remains ill-defined. Quantitatively and systematically mapping such parameter space is challenging due to the large number of variables affecting photoelectron packet properties [7-9].