Graphene band structure with its two valleys shown in blue and gold.
The exploration of low-dimensional materials is vital in our search for advanced materials that are light, strong, adaptable, and offer new functionality for the Navy. 2D materials are particularly interesting, as their planar geometry makes them both flexible and suitable for use in chemical, electronic, and optoelectronic applications benefitting from large surface areas. New functionality can also be achieved by either stacking layers with different properties or forming nanoscale structures.
The Section is currently exploring the influence of defects and interfaces on the properties of 2D materials. Recently, we predicted that randomly distributed sulfur vacancies in monolayer MoS2 dramatically increase the optical absorption below the band gap, which could be advantageous for solar cell applications that benefit from a large adsorption in the 1.0–2.5 eV range that dominates the solar irradiance spectrum.
Sarin molecule bound to a reconstructed anatase TiO2 surface.
Heterogeneous catalysis is a critical component of a wide range of industrial chemical processes. Air scrubbing, water purification, and sunlight-to-chemical-fuel conversion are some areas that could benefit from new catalyst materials. Each chemical process however requires tailored catalyst materials, whose development and design requires tremendous experimental investment.
Theoretical chemistry calculations are a proven tool to accelerate this search. In collaboration with experimentalists in the Chemistry Division, the Section has shown that low-cost, commercially available anatase TiO2 and Zr(OH)4 can bind and catalyze the decomposition of nerve agents and their chemical simulants. This includes an addition-elimination mechanism that decomposes the chemical simulant DMMP on a Zr(OH)4 tetramer eliminating methanol.
A metal-organic framework [Zr6O4(OH)4(bpydc)6].
The theoretical exploration of electrode materials is central to the development of new energy storage devices with both high energy and power density that operate under a wide range of environmental conditions. These devices require novel electrodes materials capable of storing and transferring large amounts of charge. This capability is dependent on how well both electrons and ions conduct through the electrode surface.
To better understand these electrochemical processes, the Section is using a combination of DFT and newly developed perturbative mean-field techniques. DFT gives insight into direct chemical interactions between the electrolyte and electrode and the dynamics of small numbers of molecules, while classical mean-field theory can describe ion concentration profiles on a larger scale. We are currently working on using perturbation theory to bridge the gap between these two length scales.
Snapshot of ab initio MD simulation.
The Navy is exploring advanced manufacturing technologies to enhance the energy density of explosives and solid rocket propellants, while maintaining insensitivity to accidental or adverse insults. Tailoring microstructural properties of energetic materials for example, can both enhance the rates at which materials react and improve resilience against unintentional initiation. Better understanding of molecular-level chemistry responsible for reaction initiation and energy release is needed to relate the effect of microstructural heterogeneities on the performance and sensitivity of advanced energetic materials.
The Section employs ab initio MD simulations and reaction path optimizations to determine mechanisms and rate constants of condensed-phase reactions triggered in explosives under extreme temperatures above 1000 K and pressures up to tens of GPa. These first-principles predictions are then combined with known kinetic data to construct reduced-order reaction rate models that describe reaction initiation and energy release in energetic materials. We recently transitioned a rate model for RDX, a commonly used energetic material, to the Navy Surface Warfare Center Indian Head and Army Research Laboratory to support their modeling and simulations efforts to design advanced energetic materials.
An LC molecule (5CB) on graphene.
Liquid crystals (LCs) offer a unique combination of properties for Navy-relevant electro-optic applications. LCs that consist of anisotropic molecules that form anisotropic alignment structures exhibit double refraction or birefringence. LCs are also sensitive to electric fields, which can be used to align the LC molecules, and thus control the LCs’ optical properties. Furthermore, the LC molecules are affected by nearby materials, and because of this connection, LCs can be used for surface imaging and characterization. As the interfaces impact properties often critical to applications such as light scattering and waveguide attenuation, we need a better understanding of the interactions between the LC molecules and material surfaces.
Our Section is particularly interested in the interactions between LCs and 2D materials. Through DFT calculations, we found a new adsorption structure for 5CB on graphene between the two expected structures in which the LC molecule is aligned with the crystallographic directions of graphene. This finding explains the unexpected halved azimuthal period observed in experiment. This effort is conducted in collaboration with the Center for Bio/Molecular Science & Engineering.
The Al50Cp* metalloid cluster.
Metalloid clusters comprise a new set of materials that hold promise as highly energetic materials and materials capable of storing large amounts of charge and magnetic moments. Because the clusters are weakly coupled, they are electrically insulating and might even in some cases be transparent. Synthesizing large metalloid clusters however, is difficult and similar to C60, is currently driven by self-assembly.
The Section uses DFT to better understand the synthesis as well as the properties of metalloid clusters. In addition to standard electronic structure techniques, we are developing new methods for studying Jahn-Teller distortions that exploit the near symmetry of these molecules. The use of symmetry makes electronic structure calculations dramatically faster, allowing for the exploration of larger, more complex systems.
Hydrogen fluoride energy from Hartree-Fock and full configuration QC simulations.
Complete simulations of large quantum mechanical systems on classical computers is exponentially or factorially hard. Consider a system with M spin orbitals that are either occupied or unoccupied. The number of configurations is then 2M, although this number reduces to M choose N for N identical particles. Either way, the scaling makes the calculations impossible in all but the smallest systems. As a result, we generally resort to approximations, which can mask underlying physics and chemistry, in particular in strongly correlated systems. One solution to this roadblock is to instead perform the simulations on quantum computers capable of treating the configurations in parallel.
The Section leads an NRL effort on quantum computing (QC) simulations performed in collaboration with the Materials Division and the Center for Computational Science. Because of current hardware limitations, the initial focus will be on small molecular systems, including hydroxide and water. This will provide better understanding of the chemical reactions responsible for the anomalously large hydroxide diffusion rate in water. This diffusion is one of the critical components affecting corrosion, which costs the DoN billions every year.
MD simulation of H2O in a chitosan membrane.
Protonics (or bioprotonics) is a research area that seeks to develop a new class of biologically compatible devices that control the flow of protons so as to enhance communications between biological systems and electronic devices, which could lead to enhanced prosthetics for our wounded veterans. Critical to realizing this goal is the development of efficient proton conducting membranes.
To better understand the atomistic scale processes that influence proton transport, the Section has employed DFT-based MD simulations of aqueous proton transport through chitosan-based polymer membranes in the presence of applied electric fields. These simulations have explicitly shown that the intrinsic conductivity of chitosan membranes is due to the basic amine groups of the polymer deprotonating water molecules to create free charge carriers. In the future, we plan to explore how the proton transport properties are affected by different functionalization of the polymer.