This project seeks to develop low-cost, high-resolution gamma scintillators based on new fracture-resistant alloys of cerium bromide. These alloys will eliminate limitations imposed by self-activity and fracture of currently available lanthanum-based scintillators.

MD simulation of dislocation migration under an electrical field.

Optical transparency is a critical but often overlooked property of MOFs considered for optical applications and luminescence sensing. Zr-1,4-NDC samples with various crystallite dimensions (35 nm to 100 μm) were prepared and their bulk optical transmittance measured. The nanocrystalline (35 nm) sample exhibited the highest optical transmittance, which boosts the luminescence signal for sensing applications by reducing scattering loss.

We present a carbon nanotube phototransistor using C60 as a sensitizer with responsivity exceeding 107 A/W (1010 V/W) under visible light. Carbon nanotube/C60 heterojunctions have previously been studied in photovoltaics. However, the responsivity of photovoltaics is limited by its lack of gain, with literature devices having responsivities less than 1 A/W. Our devices display significantly higher responsivity due to their phototransistor structure, with the C60 serving as the sensitizer to a carbon nanotube field-effect transistor. Excitons generated in the C60 diffuse to the heterointerface with the nanotube where they dissociate. Holes transfer to the nanotube and conduct away, leaving electrons bound on the nanotube surface which gate the transistor. The extreme sensitivity of the nanotube to local electric fields then results in a high responsivity. Devices are further improved by using an array of aligned nanotubes, which is significantly better in device current and responsivity compared to random network devices. The resulting devices are sensitive to less than 100 fW of incident light, offering a path forward to single photon detection.

Because of their extraordinary surface areas and tailorable porosity, metal-organic frameworks (MOFs) have the potential to be excellent sensors of gas-phase analytes. MOFs with open metal sites are particularly attractive for detecting Lewis basic atmospheric analytes, such as water. Here, we demonstrate that thin films of the MOF HKUST-1 can be used to quantitatively determine the relative humidity (RH) of air using a colorimetric approach. HKUST-1 thin films are spin-coated onto rigid or flexible substrates and are shown to quantitatively determine the RH within the range of 0.1-5% RH by either visual observation or a straightforward optical reflectivity measurement. At high humidity (>10% RH), a polymer/MOF bilayer is used to slow the transport of H2O to the MOF film, enabling quantitative determination of RH using time as the distinguishing metric. Finally, the sensor is combined with an inexpensive light-emitting diode light source and Si photodiode detector to demonstrate a quantitative humidity detector for low humidity environments.

Ultra large pore isostructural metal organic frameworks (MOFs) which exhibit both photoluminescence and scintillation properties, were synthesized from trans-4,4'-stilbenedicarboxylic acid (H2L) and trivalent lanthanide (Ln) metal salts under solvothermal conditions (Ln = Er3+ (1) and Tm3+ (2)). This new class of mesoporous materials is a non-interpenetrating network that features ultra-large diamond shaped pores of dimensions with approximate cross-sectional dimensions of 28 Å × 12 Å. The fully deprotonated ligand, L, is isolated and rigidified as it serves as the organic linker component of the MOF structure. Its low density unit cells possess asymmetric units with two crystallographically independent Ln3+ ions in seven coordinate arrangements. A distinct feature of the structure is the bis-bidentate carboxylate groups. They serve as a ligand that coordinates two Ln(iii) ions while each L connects four Ln(iii) ions yielding an exceptionally large diamond-shaped rectangular network. The structure exhibits ligand-based photoluminescence with increased lifetime compared to free stilbene molecules on exposure to UV radiation, and also exhibits strong scintillation characteristics, comprising of both prompt and delayed radioluminescence features, on exposure to ionizing radiation.

TlBr has the properties to become the leading radiation detection semiconductor. It has not yet been deployed due to a short lifetime of only hours to weeks. While the rapid structural deteriorations must come from ionic conduction under operating electrical fields, detailed aging mechanisms have not been understood. As a result, progress to extend lifetime has been limited despite extensive studies in the past. We have developed new atomistic simulation capabilities to enable study of ionic conduction under electrical fields. Our combined simulations and experiments indicate that dislocations in TlBr climb under electrical fields. This climb is the root cause for structural deterioration. Hence, we discovered new strengthening methods to reduce aging. Our new atomistic simulation approach can have broader impact on other Sandia programs including battery research. Our project results in 4 publications, a new invention, new LAMMPS capabilities, solution to mission relevant materials, and numerous presentations.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. This discovery can lead to new understanding of TlBr aging mechanisms under external fields.

In this paper we present results on a novel tin-loaded plastic scintillator. We will show that this particular plastic scintillator has a light output similar to that of BGO, a fast scintillation decay (< 10 ns), exhibits good neutron/gamma PSD with a Figure-of-Merit of 1.3 at 2.5 MeVee cut-off energy, and excellent energy resolution of about 12% (FWHM) at 662 keV. Under X-ray excitation, the radioluminescence spectrum exhibits a broad band between 350 and 500 nm peaking at 420 nm which is well-matched to bialkali photomultiplier tubes and UV-enhanced photodiodes.

Defects in multilayered films have long been a performance-limiting problem for the semiconductor industry. For instance, CdTe/CdS solar cell efficiencies have had significant improvement in the past 15 years or more without addressing the problem of high misfit dislocation densities. Overcoming this stagnation requires a fundamental understanding of interfacial defect formation. Herein, we demonstrate a new first principles-based CdTe bond-order approach that enables efficient molecular dynamics to approach the fidelity of density functional theory. Stringent quantum-mechanical verification and experimental validation tests reveal that our new approach provides an accurate prediction of defects that earlier methods cannot. Using this new capability, we show that misfit dislocations in CdTe/CdS multilayers can be significantly reduced via nano-patterning and composition grading and more importantly, dislocation-free multilayers naturally arise when the pattern dimension is reduced below 90 nm. Our predictive methods are generally applicable to other materials, highlighting a rational approach towards low-defect semiconductor films. Copyright (C) 2015 John Wiley & Sons, Ltd.

An aliovalently calcium-doped cerium tribromide (CeBr3:Ca2+) crystal was prepared with a gamma-energy resolution (FWHM) of 3.2% at the 137Cs 662 keV gamma energy. We completed a crystal assessment and calculated the predictive performance and physical characteristics using density functional theory (DFT) formalism. Detector performance, characteristics, calcium doping concentration, and crystal strength are reported. The structural, electronic, and optical properties of CeBr3 crystals were investigated using the DFT within generalized gradient approximation. Specifically, we see excellent linearity of photons per unit energy with the aliovalent CeBr3:Ca2+ crystal. Proportionality of light yield is one area of performance in which Ce-doped and Ce-based lanthanide halides excel. Maintaining proportionality is the key to producing a strong, high-performance scintillator. Relative light yield proportionality was measured for both doped and undoped samples of CeBr3 to ensure no loss in performance was incurred by doping. The light output and proportionality for doped CeBr3, however, appears to be similar to that of undoped CeBr3. The new crystal was subjected to additional testing and evaluation, including a benchmark spectroscopy assessment. Results, which present energy resolution as a function of energy, are summarized. Typical spectroscopy results using a 137Cs radiation source are shown for our crystallites with diameters <1 cm. We obtain energy resolution of 3.2% before packing the crystallite in a sealed detector container and 4.5% after packing. Spectra were also obtained for 241Am, 60Co, 228Th, and background to illustrate the spectrosocopic quality of CeBr3:Ca2+ over a broader energy range.

TlBr is promising for g- and x- radiation detection, but suffers from rapid performance degradation under the operating external electric fields. To enable molecular dynamics (MD) studies of this degradation, we have developed a Stillinger-Weber type of TlBr interatomic potential. During this process, we have also addressed two problems of wider interests. First, the conventional Stillinger-Weber potential format is only applicable for tetrahedral structures (e.g., diamond-cubic, zinc-blende, or wurtzite). Here we have modified the analytical functions of the Stillinger-Weber potential so that it can now be used for other crystal structures. Second, past modifications of interatomic potentials cannot always be applied by a broad community because any new analytical functions of the potential would require corresponding changes in the molecular dynamics codes. Here we have developed a polymorphic potential model that simultaneously incorporates Stillinger-Weber, Tersoff, embedded-atom method, and any variations (i.e., modified functions) of these potentials. We have implemented this polymorphic model in MD code LAMMPS, and demonstrated that our TlBr potential enables stable MD simulations under external electric fields.