Publications

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After more than 50 years of molecular simulations, accurate empirical models are still the bottleneck in the wide adoption of simulation techniques. Addressing this issue with a fresh paradigm is the need of the day. In this study, we outline a new genetic-programming based method to develop empirical models for a system purely from its energy and/or forces. While the approach was initially developed for the development of classical force-fields from ab-initio calculations, we also discuss its application to the molecular coarse-graining of methanol. Two models, one representing methanol by a single site and the other via two sites will be developed using this method. They will be validated against existing coarse-grained potentials for methanol by comparing thermophysical properties.

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In the past decade, organic optoelectronic devices have made much advances that they become viable technologies. These organic optoelectronic devices involve integration of organics with highly dissimilar materials, e.g. metals, semiconductors, and oxides, with critical device actions taking places at the organic-inorganic interfaces. For examples, in organic photovoltaics, exciton dissociation and carrier separation occur at the donor-acceptor heterojunctions; careful design of junction area and band alignment is critical for optimizing device performance. In this talk, I will show two examples of modifying organic-inorganic interfaces with alkanethiol self-assembled monolayers (SAMs) to improve device performance. Alkanethiols are large band gap molecules that are expected to act as a transport barrier. When the Au cathode in polyfluorene OLEDs is modified with alkanethiol SAMs, the current is found to be lower while the output luminescent intensity higher, leading to higher external quantum efficiency at a given current density. In ZnO-polythiophene hybrid solar cells, increasing alkanethiol SAM length surprisingly leads to higher photocurrent, despite the SAM reduces electron transfer. I will discuss the mechanisms behind these unexpected improvements.

This document is the final report for the Sandia National Laboratory funded Student Fellowship position at New Mexico State University (NMSU) from 2008 to 2010. Ivan Mecimore, the PhD student in Electrical Engineering at NMSU, was conducting research into image and video processing techniques to identify features and correlations within images without requiring the decoding of the data compression. Such an analysis technique would operate on the encoded bit stream, potentially saving considerable processing time when operating on a platform that has limited computational resources. Unfortunately, the student has elected in mid-year not to continue with his research or the fellowship position. The student is unavailable to provide any details of his research for inclusion in this final report. As such, this final report serves solely to document the information provided in the previous end of year summary.

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Structural phase transformations between ferroelectric (FE), antiferroelectric (AFE), and paraelectric (FE) phases are frequently observed in the zirconia-rich phase region on the lead zirconate-titanate (PZT) phase diagram. Since the free energy difference among these phases is small, phase transformation can be easily induced by temperature, pressure and electric field. These induced transformation characteristics have been used for many practical applications. This study focuses on a hydrostatic pressure induced FE-to-AFE phase transformation in a tin modified PZT ceramic (PSZT). The relative phase stability between FE and AFE phases is determined by the dielectric permittivity measurement as a function of temperature from -60 C to 125 C. A pressure-temperature phase diagram for the PSZT system will be presented.

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Material control and accounting (MC&A) safeguards operations that track and account for critical assets at nuclear facilities provide a key protection approach for defeating insider adversaries. These activities, however, have been difficult to characterize in ways that are compatible with the probabilistic path analysis methods that are used to systematically evaluate the effectiveness of a site's physical protection (security) system (PPS). MC&A activities have many similar characteristics to operator procedures performed in a nuclear power plant (NPP) to check for anomalous conditions. This work applies human reliability analysis (HRA) methods and models for human performance of NPP operations to develop detection probabilities for MC&A activities. This has enabled the development of an extended probabilistic path analysis methodology in which MC&A protections can be combined with traditional sensor data in the calculation of PPS effectiveness. The extended path analysis methodology provides an integrated evaluation of a safeguards and security system that addresses its effectiveness for attacks by both outside and inside adversaries.

Historically, U.S. arms control policy and the U.S. nuclear weapons enterprise have been reactive to each other, rather than interdependent and mutually reinforcing. One element of the divergence has been the long timescale necessary to plan and create substantive changes in the infrastructure vs. the inherent unpredictability of arms control outcomes. We explore several examples that illustrate this tension, some of the costs and implications associated with this reactive paradigm, and illustrate that, while the nuclear weapons enterprise has long considered the implications of arms control in sizing capacity of its missions, it has not substantively considered arms control in construction requirement for capabilities and products. Since previous arms control agreements have limited numbers and types of deployed systems, with delivery systems as the object of verification, this disconnect has not been forefront. However, as future agreements unfold, the warhead itself may become the treaty limited item and the object of verification. Such a scenario might offer both the need and the opportunity to integrate nuclear weapons and arms control requirements in unprecedented ways. This paper seeks to inspire new thinking on how such integration could be fostered and the extent to which it can facilitate significant reduction in nuclear stockpiles.

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We present a new fabrication technique called Membrane Projection Lithography for the production of three-dimensional metamaterials at infrared wavelengths. Using this technique, multilayer infrared metamaterials that include both in-plane and out-of-plane resonators can be fabricated.

With increasing concern over the ability to detect and characterize special nuclear materials, the need for computer codes that can successfully predict the response of detector systems to various measurement scenarios is extremely important. These computer algorithms need to be benchmarked against a variety of experimental configurations to ensure their accuracy and understand their limitations. The Monte Carlo code MCNP-PoliMi is a modified version of the MCNP-4c code. Recently these modifications have been ported into the new MCNPX 2.6.0 code, which gives the new MCNPX-PoliMi a wider variety of options and abilities, taking advantage of the improvements made to MCNPX. To verify the ability of the MCNPX-PoliMi code to simulate the response of a neutron multiplicity detector simulated results were compared to experimental data. The experiment consisted of a 4.5-kg sphere of alpha-phase plutonium that was moderated with various thicknesses of polyethylene. The results showed that our code system can simulate the multiplicity distributions with relatively good agreement with measured data. The enhancements made to MCNP since the release of MCNP-4c have had little to no effect on the ability of the MCNP-PoliMi to resolve the discrepancies observed in the simulated neutron multiplicity distributions when compared experimental data.

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A two-dimensional phononic crystal (PnC) that can operate in the GHz range is created in a freestanding silicon substrate using NanoFIBrication (using a focused ion beam (FIB) to fabricate nanostructures). First, a simple cubic 6.75 x 6.75 ?m array of vias with 150 nm spacing is generated. After patterning the vias, they are backfilled with void-free tungsten scatterers. Each via has a diameter of 48 nm. Numerical calculations predict this 2D PnC will generate a band gap near 22 GHz. A protective layer of chromium on top of the thin (100 nm) silicon membrane confines the surface damage to the chromium, which can be removed at a later time. Inspection of the underside of the membrane shows the vias flaring out at the exit, which we are dubbing the 'trumpet effect'. The trumpet effect is explained by modeling the lateral damage in a freestanding membrane.