Related Publications

  • Building Structures Atom by Atom via Electron Beam Manipulation

    Building materials from the atom up is the pinnacle of materials fabrication. Until recently the only platform that offered single‐atom manipulation was scanning tunneling microscopy. Here controlled manipulation and assembly of a few atom structures are demonstrated by bringing together single atoms using a scanning transmission electron microscope. An atomically focused electron beam is used to introduce Si substitutional defects and defect clusters in graphene with spatial control of a few nanometers and enable controlled motion of Si atoms. The Si substitutional defects are then further...
  • Chemical nature of ferroelastic twin domains in CH3NH3PbI3 perovskite

    The extraordinary optoelectronic performance of hybrid organic–inorganic perovskites has resulted in extensive efforts to unravel their properties. Recently, observations of ferroic twin domains in methylammonium lead triiodide drew significant attention as a possible explanation for the current–voltage hysteretic behaviour in these materials. However, the properties of the twin domains, their local chemistry and the chemical impact on optoelectronic performance remain unclear. Here, using multimodal chemical and functional imaging methods, we unveil the mechanical origin of the twin domain...
  • Grazing-Angle Neutron Diffraction Study of the Water Distribution in Membrane Hemifusion: From the Lamellar to Rhombohedral Phase

    The water distribution between lipid bilayers is important in understanding the role of the hydration force at different steps of the membrane fusion pathway. In this study, we used grazing-angle neutron diffraction to map out the water distribution in lipid bilayers transiting from a lamellar structure to the hemifusion “stalk” structure in a rhombohedral phase. Under osmotic pressure exerted by different levels of relative humidity, the lipid membrane sample was maintained in equilibrium at different lattices suitable for neutron diffraction. The D2O used to hydrate the lipid membrane...
  • Application of Polynomial Chaos Expansion in Inverse Transport Problems with Neutron Multiplication Measurements and Multiple Unknowns

    The polynomial chaos expansion technique is used to build surrogate models of the dependences of gamma-ray fluxes and neutron multiplication to unknown physical parameters in radiological source/shield systems. These surrogate models are used with the DiffeRential Evolution Adaptive Metropolis (DREAM), a method to solve and quantify uncertainty in inverse transport problems. Measured data in the inverse problems includes both passive gamma rays and neutron multiplication. The polynomial chaos expansion approach is shown to increase the speed...
  • Organic agents offer innovation

    Capturing CO2 directly from the air could lead to negative emissions, but more efficient technologies are still required. Now, researchers use a multi-stage cycle based on amino acids and organic salts to capture CO2 from air, which can be released with concentrated solar power.
  • Direct air capture of CO2 via aqueous-phase absorption and crystalline-phase release using concentrated solar power

    Using negative emissions technologies for the net removal of greenhouse gases from the atmosphere could provide a pathway to limit global temperature rises. Direct air capture of carbon dioxide offers the prospect of permanently lowering the atmospheric CO2 concentration, providing that economical and energy-efficient technologies can be developed and deployed on a large scale. Here, we report an approach to direct air capture, at the laboratory scale, using mostly off-the-shelf materials and equipment. First, CO2 absorption is achieved with readily available and environmentally friendly...
  • A composite position independent monitor of reactor fuel irradiation using Pu, Cs, and Ba isotope ratios

    When post-irradiation materials from the nuclear fuel cycle are released to the environment, certain isotopes of actinides and fission products carry signatures of irradiation history that can potentially aid a nuclear forensic investigation into the material's provenance. In this study, combinations of Pu, Cs, and Ba isotope ratios that produce position (in the reactor core) independent monitors of irradiation history in spent light water reactor fuel are identified and explored. These position independent monitors (PIMs) are modeled for various irradiation...
  • Flexible classification with spatial quantile comparison and novel statistical features applied to spent nuclear fuel analysis

    Multivariate classification algorithms are a common tool in contemporary data science that have begun to be applied to nuclear forensic analyses. Using a multivariate generalization of quantile–quantile plots for comparing unknown statistical distributions, a new model-free multivariate classifier called the Quantile–Quantile Comparator has been developed and tested on the analysis of simulated irradiated nuclear fuel. A method to estimate the sample-specific probability of having made an incorrect classification decision by analyzing the distribution of quantile comparisons has been...
  • Reconstructing Reactor Operating Histories Using the INDEPTH Code

    The Inverse Depletion Theory (INDEPTH) code is used to reconstruct design and operating history parameters of a nuclear reactor from measurements of spent nuclear fuel. Typical parameters of interest are the initial enrichment of the fuel, the burnup, and the cooling time. These reconstructed values can be used to verify safeguards declarations, confirm that environmental samples are consistent with declared nuclear activities, or recover surveillance on spent fuel for which the history has been lost. INDEPTH uses the Oak Ridge Isotope Generation (ORIGEN) code to perform forward depletion and...
  • Verifying Safeguards Declarations with INDEPTH: A Sensitivity Study

    A series of ORIGEN calculations were used to simulate the irradiation and decay of a number of spent fuel assemblies. These simulations focused on variations in the irradiation history that achieved the same terminal burnup through a different set of cycle histories. Simulated NDA measurements were generated for each test case from the ORIGEN data. These simulated measurement types included relative gammas, absolute gammas, absolute gammas plus neutrons, and concentrations of a set of six isotopes commonly measured by NDA. The INDEPTH code was used to reconstruct the initial enrichment,...
  • Sensitivity Study of INDEPTH for Verification of Facility Spent Nuclear Fuel Declarations

    The Inverse Depletion Theory (INDEPTH) code is one of the tools being used to analyze the traditional nondestructive assay (NDA) measurements and verify the initial enrichment, burnup, and cooling time values of spent nuclear fuel (SNF) declared by facilities. The INDEPTH code attempts to reconstruct the initial enrichment and operating history by using the Oak Ridge Isotope Generation (ORIGEN) code to simulate irradiation and cooling of the fuel. This work examined the sensitivity of INDEPTH results to variations in irradiation conditions. Three types of measured data were simulated to...
  • Vorticity generation by the instantaneous release of energy near a reflective boundary

    The instantaneous release of energy in a localized area of a gas results in the formation of a low-density region and a series of shock and expansion waves. If this process occurs near a boundary, the shock reflections can interact with the density inhomogeneity, leading to the baroclinic generation of vorticity and the subsequent organization of the flow into several structures, including a vortex ring. By means of numerical simulations we illustrate the qualitative changes that occur in the pressure wave patterns and vorticity distribution as the distance from the area of energy release to...
  • Application of the ORIGEN Fallout Analysis Tool and the DELFIC Fallout Planning Tool to National Technical Nuclear Forensics

    The objective of this project was to provide a robust fallout analysis and planning tool for the National Technical Nuclear Forensics interagency ground sample collection team. Their application called for a fast-running, portable mission-planning tool for use in response to emerging improvised nuclear device (IND) post-detonation situations. The project met those goals by research and development of models to predict the physical, chemical, and radiological properties of fallout debris. ORNL has developed new graphical user interfaces for two existing codes, the Oak Ridge Isotope Generation...
  • In situ atomistic insight into the growth mechanisms of single layer 2D transition metal carbides

    Developing strategies for atomic-scale controlled synthesis of new two-dimensional (2D) functional materials will directly impact their applications. Here, using in situ aberration-corrected scanning transmission electron microscopy, we obtain direct insight into the homoepitaxial Frank–van der Merwe atomic layer growth mechanism of TiC single adlayers synthesized on surfaces of Ti3C2 MXene substrates with the substrate being the source material. Activated by thermal exposure and electron-beam irradiation, hexagonal TiC single adlayers form on defunctionalized surfaces of Ti3C2 MXene at...
  • Toward cities without slums: Topology and the spatial evolution of neighborhoods

    The world is urbanizing quickly with nearly 4 billion people presently living in urban areas, about 1 billion of them in slums. Achieving sustainable development from rapid urbanization relies critically on creating cities without slums. We show that it is possible to diagnose systematically the central physical problem of slums—the lack of spatial accesses and related services—using a topological analysis of neighborhood maps and resolved by finding solutions to a sequence of constrained optimization problems. We set up the problem by showing that the built environment of any city can be...
  • Directed Atom-by-Atom Assembly of Dopants in Silicon

    The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant...
  • A language and hardware independent approach to quantum–classical computing

    Heterogeneous high-performance computing (HPC) systems offer novel architectures which accelerate specific workloads through judicious use of specialized coprocessors. A promising architectural approach for future scientific computations is provided by heterogeneous HPC systems integrating quantum processing units (QPUs). To this end, we present XACC (eX treme-scale ACC elerator) — a programming model and software framework that enables quantum acceleration within standard or HPC software workflows. XACC follows a coprocessor machine model that is independent of the underlying quantum...
  • Development and thermal performance verification of composite insulation boards containing foam-encapsulated vacuum insulation panels

    High-performance thermal insulation is a critical need for buildings. This article presents the development and thermal characterization of composite foam insulation boards containing low-cost vacuum insulation cores. The composite foam-vacuum insulation boards were created in a semi-automatic operation in a foam insulation manufacturing plant. The low-cost vacuum insulation is a new technology called modified atmosphere insulation. The production process of modified atmosphere insulation is much simpler than traditional vacuum insulation manufacturing, and it has the potential for...
  • Robust Mercury Methylation across Diverse Methanogenic Archaea

    Methylmercury (MeHg) production was compared among nine cultured methanogenic archaea that contain hgcAB, a gene pair that codes for mercury (Hg) methylation. The methanogens tested produced MeHg at inherently different rates, even when normalized to growth rate and Hg availability. Eight of the nine tested were capable of MeHg production greater than that of spent- and uninoculated-medium controls during batch culture growth. Methanococcoides methylutens, an hgcAB+ strain with a fused gene pair, was unable to produce more MeHg than controls. Maximal conversion of Hg to MeHg through a full...
  • Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials

    Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy...
  • Measuring oxygen reduction/evolution reactions on the nanoscale

    The efficiency of fuel cells and metal–air batteries is significantly limited by the activation of oxygen reduction and evolution reactions. Despite the well-recognized role of oxygen reaction kinetics on the viability of energy technologies, the governing mechanisms remain elusive and until now have been addressable only by macroscopic studies. This lack of nanoscale understanding precludes optimization of material architecture. Here, we report direct measurements of oxygen reduction/evolution reactions and oxygen vacancy diffusion on oxygen-ion conductive solid surfaces with sub-10 nm...
  • Nanoscale mapping of ion diffusion in a lithium-ion battery cathode

    The movement of lithium ions into and out of electrodes is central to the operation of lithium-ion batteries. Although this process has been extensively studied at the device level, it remains insufficiently characterized at the nanoscale level of grain clusters, single grains and defects. Here, we probe the spatial variation of lithium-ion diffusion times in the battery-cathode material LiCoO2 at a resolution of ∼100 nm by using an atomic force microscope to both redistribute lithium ions and measure the resulting cathode deformation. The relationship between diffusion and single grains...
  • Learning surface molecular structures via machine vision

    Recent advances in high resolution scanning transmission electron and scanning probe microscopies have allowed researchers to perform measurements of materials structural parameters and functional properties in real space with a picometre precision. In many technologically relevant atomic and/or molecular systems, however, the information of interest is distributed spatially in a non-uniform manner and may have a complex multi-dimensional nature. One of the critical issues, therefore, lies in being able to accurately identify (‘read out’) all the individual building blocks in different atomic...
  • Machine learning–enabled identification of material phase transitions based on experimental data: Exploring collective dynamics in ferroelectric relaxors

    Exploration of phase transitions and construction of associated phase diagrams are of fundamental importance for condensed matter physics and materials science alike, and remain the focus of extensive research for both theoretical and experimental studies. For the latter, comprehensive studies involving scattering, thermodynamics, and modeling are typically required. We present a new approach to data mining multiple realizations of collective dynamics, measured through piezoelectric relaxation studies, to identify the onset of a structural phase transition in nanometer-scale volumes, that is...
  • Knowledge Extraction from Atomically Resolved Images

    Tremendous strides in experimental capabilities of scanning transmission electron microscopy and scanning tunneling microscopy (STM) over the past 30 years made atomically resolved imaging routine. However, consistent integration and use of atomically resolved data with generative models is unavailable, so information on local thermodynamics and other microscopic driving forces encoded in the observed atomic configurations remains hidden. Here, we present a framework based on statistical distance minimization to consistently utilize the information available from atomic configurations...