Bethany M. Hudak

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

Direct observation of Li diffusion in Li-doped ZnO nanowires

The direct observation of Li diffusion in Li-doped zinc oxide nanowires (NWs) was realized by using in situ heating in the scanning transmission electron microscope (STEM). A continuous increase of low atomic mass regions within a single NW was observed between 200 °C and 600 °C when heated in vacuum, which was explained by the conversion of interstitial to substitutional Li in the ZnO NW host lattice. A kick-out mechanism is introduced to explain the migration and conversion of the interstitial Li (Lii) to Zn-site substitutional Li (LiZn), and this mechanism is verified with low-temperature (11 K) photoluminescence measurements on as-grown and annealed Li-doped zinc oxide NWs, as well as the observation of an increase of NW surface roughing with applied bias.

Atomic Manipulation on a Scanning Transmission Electron Microscope Platform using Real-Time Image Processing and Feedback

Fabrication of atomic scale structures remains the ultimate goal of nanotechnology. The reigning paradigms have been scanning probe microscopy (SPM) and synthesis. SPM assembly dates to seminal experiments by Don Eigler, who demonstrated single atom manipulation. However, stability and throughput remain issues. Discuss here are research activity towards the next paradigm — the use of the atomically focused beam of a scanning transmission electron microscope (STEM) to control and direct matter on atomic scales. Traditionally, STEM’s are perceived only as imaging tools and beam induced modifications as undesirable beam damage. Our team and several groups worldwide have demonstrated that beam induced modifications can be more precise and controllable. We have demonstrated ordering of oxygen vacancies, single defect formation in 2D materials including adding and moving dopants within a lattice [1-4], and beam induced migration of single interstitials in diamond like materials. What is remarkable is that these changes often involve one atom or small group of atoms and can be monitored real-time with atomic resolution.

Shell-Induced Ostwald Ripening: Simultaneous Structure, Composition, and Morphology Transformations during the Creation of Hollow Iron Oxide Nanocapsules

The creation of nanomaterials requires simultaneous control of not only crystalline structure and composition but also crystal shape and size, or morphology, which can pose a significant synthetic challenge. Approaches to address this challenge include creating nanocrystals whose morphologies echo their underlying crystal structures, such as the growth of platelets of two-dimensional layered crystal structures, or conversely attempting to decouple the morphology from structure by converting a structure or composition after first creating crystals with a desired morphology. A particularly elegant example of this latter approach involves the topotactic conversion of a nanoparticle from one structure and composition to another, since the orientation relationship between the initial and final product allows the crystallinity and orientation to be maintained throughout the process. Here we report a mechanism for creating hollow nanostructures, illustrated via the decomposition of β-FeOOH nanorods to nanocapsules of α-Fe2O3, γ-Fe2O3, Fe3O4, and FeO, depending on the reaction conditions, while retaining single-crystallinity and the outer nanorod morphology. Using in situ TEM, we demonstrate that the nanostructured morphology of the starting material allows kinetic trapping of metastable phases with a topotactic relationship to the final thermodynamically stable phase.

Movement and Imaging of Single-Atom Dopants in Silicon

Theoretical predictions show that quantum computers should be able to perform beyond the capabilities of the most powerful current supercomputers, making the realization of quantum computing of interest to both civilian and government institutions. One quantum computing architecture that is particularly appealing consists of individual atoms doped into a semiconductor, where the spin states of these dopant atoms provide a method to encode the qubits [1]. The widespread application of silicon in electronics makes it an ideal material for quantum computing due to existing infrastructure for the production, study, and use of Si-based devices. Group V elements are promising candidates for use as single-atom qubit dopants in Si [1,2]. Atoms with a similar atomic number (Z) to Si, such as phosphorous, are difficult to observe through conventional single-atom imaging techniques such as scanning transmission electron microscopy (STEM), making approaches to accurate single atom positioning challenging. Pnictogens other than phosphorous, particularly the heavier Group V elements, are potential candidates to function as qubits and overcome these obstacles. Bismuth – with large spin-orbit coupling, strong clock transitions [3], and a greater potential to be imaged by STEM – is very promising. Bi in Si has been shown theoretically to allow gates operating in the MHz regime [4]. It has also been suggested that Bi may allow qubit operation at liquid nitrogen temperature, an important aspect of practical device operation.

Direct Observation of Hafnia Structural Phase Transformations

1. Department of Chemistry, University of Kentucky, Lexington, Kentucky, USA 2. Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY, USA 3. Department of Chemistry, Texas A&M University, College Station, TX, USA 4. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA 5. Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, TN, USA

Understanding nanomaterial synthesis with in situ transmission electron microscopy

The vapor-liquid-solid (VLS) nanowire growth technique is a synthesis method widely used to grow high-quality, single-crystalline semiconductor nanowires [1-3]. First introduced in 1964 by Wagner and Ellis to grow silicon nanowires, this method has evolved to utilize many different metal catalyst materials to grow a wide variety of inorganic nanowires with facile control of diameter, length, and dopant concentration [1,4]. These inorganic nanowires have many applications, such as for Li-ion battery electrodes, gas sensors, and solar cell components [5-7]. While VLS is a ubiquitous growth method, understanding of the growth kinetics is limited, especially for binary and ternary crystal systems. Theoretical predictions suggest that the growth of such nanowires is governed by steady-state kinetics, and that the crystal chemistry of the reverse process may be different from that which governs the nanowire growth [8]. The use of in situ techniques has advanced the understanding of the VLS process and the kinetics of VLS growth [9]. By use of heating in a transmission electron microscope (TEM), we have developed a method to observe the Au-catalyzed VLS growth of metal oxide nanowires occurring in reverse; this nanowire dissolution is dubbed the solid-liquid-vapor (SLV) process.