Cesium-Coated Halide Perovskites as a Photocathode Material: Modeling Insights.

Photocathodes emit electrons when illuminated, a process utilized across many technologies. Cutting-edge applications require a set of operating conditions that are not met with current photocathode materials. Meanwhile, halide perovskites have been studied extensively and have shown a lot of promise for a wide variety of optoelectronic applications. Well-documented halide perovskite properties such as inexpensive growth techniques, improved carrier mobility, low trap density, and tunable direct band gaps make them promising candidates for next-generation photocathode materials. Here, we use density functional theory to explore the possible application of pure inorganic perovskites (CsPbBr3 and CsPbI3) as photocathodes. It is determined that the addition of a Cs coating improved the performance by lowering the work function anywhere between 1.5 and 3 eV depending on the material, crystal surface, and surface coverage. A phenomenological model, modified from that developed by Gyftopoulos and Levine, is used to predict the reduction in work function with Cs coverage. The results of this work aim to guide the further experimental development of Cs-coated halide perovskites for photocathode materials.

A Commons for a Supply Chain in the Post-COVID-19 Era: The Case for a Reformed Strategic National Stockpile.

Policy Points Reflecting on current response deficiencies, we offer a model for a national contingency supply chain cell (NCSCC) construct to manage the medical materials supply chain in support of emergencies, such as COVID-19. We develop the following: a framework for governance and response to enable a globally independent supply chain; a flexible structure to accommodate the requirements of state and county health systems for receiving and distributing materials; and a national material "control tower" to improve transparency and real-time access to material status and location. CONTEXT: Much of the discussion about the failure of the COVID-19 supply chain has centered on personal protective equipment (PPE) and the degree of vulnerability of care. Prior research on supply chain risks have focused on mitigating the risk of disruptions of specific purchased materials within a bounded region or on the shifting status of cross-border export restrictions. But COVID-19 has impacted every purchase category, region, and border. This paper is responsive to the National Academies of Sciences, Engineering and Medicine recommendation to study and monitor disasters and to provide governments with course of action to satisfy legislative mandates. METHODS: Our analysis draws on our observations of the responses to COVID-19 in regard to acquisition and contracting problem-solving, our review of field discussions and interactions with experts, a critique of existing proposals for managing the strategic national stockpile in the United States a mapping of the responses to national contingency planning phases, and the identification of gaps in current national healthcare response policy frameworks and proposals. FINDINGS: Current proposals call for augmenting a system that has failed to deliver the needed response to COVID-19. These proposals do not address the key attributes for pandemic plan renewal: flexibility, traceability and transparency, persistence and responsiveness, global independence, and equitable access. We offer a commons-based framework for achieving the opportunities and risks which are responsive to a constellation of intelligence assets working in and across focal targets of global supply chain risk. CONCLUSIONS: The United States needs a "commons-based strategy" that is not simply a stockpile repository but instead is a network of repositories, fluid inventories, and analytic monitoring governed by the experts. We need a coordinated effort, a "commons" that will direct both conventional and new suppliers to meet demands and to eliminate hoarding and other behaviors.

Theoretical studies of the vibrational properties of octahedrane (C12H12): A polyhedral caged hydrocarbon molecule.

The vibrational properties of octahedrane (C12H12) are calculated using density-functional theory employing two different computational methods: an all-electron Gaussian orbital approach and a Naval Research Laboratory-tight-binding scheme (NRL-TB) coupled with molecular dynamics (NRL-TBMD). Both approaches yield vibrational densities of states for octahedrane that are in good general agreement with each other. NRL Molecular Orbital Library can also provide accurate infrared and Raman spectra which can be analyzed and compared with experimental results, while NRL-TBMD can be conveniently scaled up for larger finite-temperature simulations. This latter approach is used in our paper to produce a theoretical prediction for a stable room temperature structure of octahedrane.

A general framework for numerical simulation of improvised explosive device (IED)-detection scenarios using density functional theory (DFT) and terahertz (THz) spectra.

We have developed a general framework for numerical simulation of various types of scenarios that can occur for the detection of improvised explosive devices (IEDs) through the use of excitation using incident electromagnetic waves. A central component model of this framework is an S-matrix representation of a multilayered composite material system. Each layer of the system is characterized by an average thickness and an effective electric permittivity function. The outputs of this component are the reflectivity and the transmissivity as functions of frequency and angle of the incident electromagnetic wave. The input of the component is a parameterized analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated using density functional theory (DFT) and parameter adjustment according to any additional information that may be available, e.g., experimentally measured spectra or theory-based assumptions concerning spectral features. A prototype simulation is described that considers response characteristics for THz excitation of the high explosive ß-HMX. This prototype simulation includes a description of a procedure for calculating response spectra using DFT as input to the Smatrix model. For this purpose, the DFT software NRLMOL was adopted.