Current Research Activities

Novel Directions and Potential Transformative Impacts
Our research facilitates new, transformative directions in chemistry and nanoscale science by extending the capabilities of molecular simulations on the 1 to 1000 nm scale to biomedical and materials applications and enabling improvements in accuracy up to two orders of magnitude compared to earlier models. Since 2001, we developed the basic theory and tools to employ one single energy expression for biomolecules and inorganic structures, and we introduced a method to derive chemically realistic atomic charges for classical simulations. The parameterization protocol covers individual compounds across the periodic table and enables access to a multitude of combinatorial hybrid materials for simulations in at least one order of magnitude higher accuracy than previously feasible (the INTERFACE force field, Langmuir Feature Article 2013, 29, 1754).

Earlier simulation engines for biomolecules (e.g. the CHARMM engine and force field by Martin Karplus) could not handle oxides, metals, or related biological interfaces, thus excluding trillions of undiscovered, inaccessible new hybrid materials. Similarly, simulation engines for metals and oxides (EAM models, Buckingham potentials, bond order potentials) could not include water, organics, biopolymers, or related interfaces, equally excluding an unlimited number of composite materials. Moreover, parameters for inorganic compounds were previously either not available, incompatible, or unreliable. We made drastic changes in the way force field parameters are assigned: from empirical “ab-initio” parameterizations to full-scale physical-chemical interpretation of all parameters in the context of known reactivity, leaving essentially no arbitrary, “adjustable” parameters. The most critical steps are thereby the elucidation of the precise balance between ionic and covalent contributions to chemical bonding, the validation of surface and interface energies, and close interconnection of the models with available experimental data. This approach yields improvements in accuracy by orders of magnitude over earlier force fields and was shown to introduce thermodynamic consistency among parameters for different materials classes, not requiring additional parameters for interfacial interactions (e.g. minerals and water, polymers and solid surfaces etc) unless chemcial reactions are involved.

The new level of accuracy and integration of different material types makes force fields fit for predictions of molecular recognition and self-assembly of many potential new hybrid materials. Specifically, (1) the accuracy of crystal geometries increases from ~5% to ~0.5% relative to experiment, mobility of all atoms is guaranteed as opposed to the need for fixed atoms (equal to T = 0 K) in earlier models, and realistic temperatures are feasible. (2) Computed surfaces energies improve from up to 500% deviation to <5% deviation from experiment, and likewise the computed interfacial properties of inorganic compounds with water, biopolymers, and organics reach quantitative accuracy without adding extra adjustable parameters for interfacial interactions (CHARMM-INTERFACE, PCFF-INTERFACE). (3) Computed mechanical and thermal properties improve, e.g., for metals from >100% to <20% deviation, often <10% relative to experiment (especially with new polarizable models). The overall performance in interfacial properties even exceeds that of DFT methods due to chemical understanding and validation against experiment. While the simulation of chemical reactions remains challenging using classical models, extensions for chemical reactions are easier to make than previously and have been demonstrated. Promising multiscale approaches including DFT, chemical knowledge, as well as reactive extensions of force fields are being further developed.

Overall improvements in the accuracy of computational models by up to two orders of magnitude allow to tackle fundamentally new chemical and materials challenges. The feasibility of the improvements is rooted in advanced knowledge of chemical bonding, chemical reactions, substance properties, and its translation into models. Developments are driven by understanding of the true electronic structure, deliberately restraining the use of electronic structure calculations with preset complex density functionals where underlying assumptions are often unseen and can produce widely scattered results.

Current Projects
The expertise in our team is interdisciplinary and involves collaborative efforts on phase transformations in nanoalloys, biomaterials, catalysis, aqueous corrosion, mineralization, hydrogels, and polymer materials. Many core and collaborative projects employ multiscale simulation techniques including DFT, molecular dynamics, Monte Carlo methods, coarse grain models, as well as intensive coordination with experimental partners and techniques.

More specifically, ongoing and planned work aims at understanding corrosion in 4D, novel alloy and metal catalysts, rational design of carbon nanofiber composites, screening of osteoporosis drugs on apatite nanocrystals, understanding tissue calcification in atomic resolution, probing the interaction of various nanoparticles with common proteins and understanding toxicity, as well as modeling of the assembly and function of layered materials and cementitious materials. A common central task is the further development of our simulation platform, the INTERFACE force field (see feature article) for new compounds. This effort includes programming of an easy-to-use graphical user interface with a surface model builder for minerals that allows to specify facets, specific surface chemistry, pH dependence, and a model builder for associated multi-component interfaces. The computational tools aim at lowering the entrance barrier to simulations of inorganic-(bio)organic interfaces at the 1 to 100 nm scale for students and researchers to accelerate the design of functional biomaterials and nanomaterials. Specific projects are described in the following.

1. Understanding and designing corrosion-resistant alloys in 4D

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Figure 1. Toolkit to understand oxidation and corrosion in spatial and temporal multi-scale dimensions.

Corrosion processes are investigated using multiple computational techniques on the nano, micro, macro, and temporal scale in collaboration with experimental and theoretical partners at several institutions (Northwestern-lead, UCLA, UVa, U Wisconsin). The project is funded as an ONR-MURI from 2014 to 2019. The specific target is to understand in detail the early-stage oxidation and aqueous corrosion in three selected model systems. A comprehensive experimental and theoretical attack on the details will enable us to understand what matters, what does not, and lay the basis for a paradigm shift in improvements of corrosion-resistant materials. Specific systems include NiCrAl alloys, a classic two-phase high-temperature alloy with alumina or chrome protection, MoSiB alloys, a new class of alloys for higher-temperature applications with a self-forming glass protective coating, and aqueous corrosion resistant materials with dopants effecting oxide growth that are used for marine applications. Our team’s effort concentrates on molecular simulations, nucleation and growth theory, kinetic Monte Carlo, reactivity prediction (in conjunction with DFT and experiment), and development of predictive reactive force fields. Technical highlights include electron tomography in 3D atomic resolution using the world’s largest electron microscope (TEAM1@NCEM), pulse oxidation, electrochemical and oxide breakdown measurements in aqueous solution, as well the construction and computation of Pourbaix diagrams. Thorough chemical testing and characterization in a feedback loop with simulation aims at major advances in modeling, theory, and alloy design, including the refinement of computational tools for alloy-oxide-aqueous interfaces.

2. Design and testing of nanoalloy and metal catalysts in 3D atomic resolution

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Figure 2. A fuel cell and some Pt-M alloy designs for the electrode material to improve the efficiency of the oxygen reduction reaction.

We explore metal nanoparticles with specific surface features as well as alloy nanocrystals that have an exponentially larger design space in comparison to pure metals for catalysis applications. The project involves tuning of biomimetic synthesis controls, 3D atomic characterization by electron tomography, and performance testing in fuel cells in collaboration with experimental partners. Multiscale DFT/MD and kinetic MC simulations will guide at all stages to examine particle growth, atomic order, and elucidation of predictive structure-performance rules for the catalytic activity. Applications of newly designed alloy nanostructures in the ORR in fuel cells and in graphitic electrode materials will be extensively tested. New polarizable and reactive force force fields will be tested.

3. Rational Design of Carbon Nanofibers

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Figure 3. Tensile modulus of current carbon nanofibers as a function of diameter and anticipated improvements.

This project aims at understanding structural, thermal, mechanical, and processing properties of low-density polyacrylonitrile-carbon nanotube composites and carbon nanofibers for ultrahigh performance in aerospace and ground vehicles. Molecular and multi-scale simulations will be performed to understand the nature of polyacrylonitrile (PAN)-carbon nanotube (CNT) interactions in solution and the formation of highly ordered graphitic nanofibers upon carbonization between 1000 and 2000 °C. The computational studies are closely aligned with laboratory experiments and a DARPA-funded production facility at Georgia Tech. Specific computational tasks involve the determination of the effect of SWNTs versus DWNTs and MWCNTs on the composite fiber structure and properties near room temperature, such as molecular ordering, conformation, glass transition temperature, and interfacial strength (a “slipperiness” factor) between polymer chains and the CNTs. Further, the effects of solvents, co-monomers, and chain length on polymer templating and solvent evaporation will be investigated. A main aspect is thereby understanding the molecular mechanisms that account for the increase in Tg with CNT loading in the as-spun and drawn fibers, reflecting the influence of lower versus higher degree of polymer orientation, crystallinity, and crystallite size. For the chosen systems, dynamic mechanical properties, degree or polymer/CNT orientation, and crystallization of composites during stabilization below carbonization temperature will be analyzed. QM/MM simulations will also be engaged to follow the carbonization reaction at high temperature, predict the likely structure and ultimate mechanical properties of the carbonized fibers at different stages of the reaction. Ultimate goals are how to maximize the tensile properties (>8 GPa strength and >325 GPa tensile modulus desired) while minimizing the carbonization temperature. Concepts of time-temperature equivalence will be utilized in multi-scale simulations to enhance conformation sampling and analysis of time-dependent mechanical properties. Effective protocols to simulate solvent evaporation and suitable approximations for the reaction stages during carbonization of the nanofibers are being developed.

4. Understanding the interaction of proteins and cells with engineered nanomaterials

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Figure 4. Models of (a) example proteins and (b) representative metal and oxide nanoparticles (from XRD data) for which binding affinities and structural changes upon adsorption under realistic solution conditions will be computed and visualized. Preferred nanocrystal shape and size as well as potential variations in protein sequence will be considered to explore the sensitivity to specific binding events and correlate with laboratory measurements.

Molecular simulations, in-vitro characterization, and in-vivo testing will be carried out to quantify nanoparticle interactions with specific proteins used in vaccines. Preferences in recognition on cell surfaces, translocation through membranes, and accumulation in specific cell types and organs will be tested to evaluate toxicity. Simulation and imaging will provide clues regarding the underlying mechanisms and biological response. Computations will employ all-atom and specific coarse-grain models as a function of pH and ionic strength.

5. Biomineral formation and dissolution

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Figure 5. Differences in surface structure of hydroxyapatite according to {hkl} facet and pH result in major changes in the binding mechanism of proteins and peptides, shown for the peptide SVSVGGK as an example (adsorbed residues and binding free energies are indicated).

We apply the pH resolved surface models and force fields developed in our laboratory to study specific binding of peptides, proteins, and drugs to surfaces of silica and apatites, as well as to understand the specific formation of nanocrystals in the presence of biopolymer templates. The propensity for the formation of various nanostructures and their stability will be assessed using a combination of experimental measurements and molecular dynamics simulation (see more details on Research Highlights Page).

6. Properties of Layered Materials

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Figure 6. Reversible swelling of layered titanates in the presence of amines for applications in photosensitizers, actuators, and photonic crystals.

This project aims at the explanation of reversible swelling of layered titanates in the presence of amine modifiers as a function of pH with applications in photonic crystals and actuators. Swelling, biological recognition, and electrical properties of layered nanomaterials for sensor, actuator, and electronics applications at the 1 to 100 nm scale are also of interest for graphitic and MoS2 layered systems in contact with various peptides. Molecular dynamics and multi-scale simulations focus on understanding the selective assembly as a function of concentration, layer morphology and defects, as well as the resulting amplification of electrical signals in combination with experimental characterization.

7. Nanoscale design of building materials

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Figure 7. Initial hydration of tricalcium aluminate (C3A) in the presence of water and organic modifiers.

An area of ongoing research efforts since 2004 is also the development of environmentally more sustainable building materials such as cement and gypsum wall-board. Global challenges faced by the industry include the reduction of CO2 footprint and energy consumption during cement production related to an annual concrete production over 2 billion tons worldwide. To make informed changes in composition and processing, it is necessary to understand nanoscale and microscale processes in this complex blend of reactive cement minerals. One key ingredient are polymers (plasticizers) that are used as modifiers for setting time, porosity, crystallite size, and strength. The INTERFACE force field features the most comprehensive and most accurate coverage of cement minerals to date for realistic simulations at the 1 to 100 nm scale. Collaborative efforts to understand particle grinding, hydration mechanisms, the action of plasticizers, are in progress using DFT, classical MD, as well as particle grinding simulations. Mineral phases of interest include hydrated tricalcium silicate (C3S), calcium-silicate-hydrate (C-S-H), tobermorites, and calcium sulfates. Specifically, models of the nanostructure of hydrated C3S and C-S-H phases will be prepared for various C/S ratios, pH values, and ionic strengths to gain mechanistic insight into the formation mechanism of C-S-H gel in close comparison to XRD, SANS, AFM, and spectroscopic data. The interaction of these phases with polycarboxylate esters (PCE’s), carbohydrates, and other common polymers will be examined to explain the role as rheology modifiers and retarding agents in cement hydration.

Prospective new group members please also see the Opportunities Page.

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