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Research Overview
The Aikens group is interested in the application and development of electronic structure theory and related methods in order to understand the fundamental interactions in materials. Three areas of interest in the group include the optical properties and self-assembly of nanoparticle superlattices, the interactions of biomolecules with nanoparticles, and the energetics and growth mechanisms of complex intermetallic systems.
Self-Assembly and Physical Properties of Nanoparticle Superlattices
Crystalline arrays of one or more types of metallic, magnetic, or semiconductor nanoparticles have potential revolutionary applications in fields such as optoelectronics, 3-D photonic band gap materials, high-density data storage, chemical and biological sensing, spintronics, and catalysis. The scientific and technological promise of nanocrystals depends on our ability to control factors such as size, shape, and organizational characteristics. In order to control these factors, individual interparticle interactions must be well understood.
Binary nanoparticle superlattices yield a wide variety of materials with precise chemical composition and ordered packing of the components. Recently, over 15 different structures have been formed with nanoparticle building blocks.[1] The same nanoparticle mixture can assemble into superlattices with very different stoichiometry and packing symmetry, but the reason for this is not fully understood. One important structure has a diamond-like morphology. Diamond lattice structures have the advantage of a noncentrosymmetric symmetry, which is an essential requirement for important physical properties such as second-order nonlinear optical activity, piezoelectricity, pyroelectricity, ferroelectricity, electrooptical activity, and photorefractivity.[2] A diamond-like superlattice structure with dimensions up to 3mm in each direction was recently formed during a titration of charged gold and silver nanoparticles capped with HS(CH2)10COOH (MUA) and HS(CH2)11NMe3+Cl- (TMA) ligands.[3] A theoretical examination of the interparticle forces could yield insight into the assembly of nanocrystalline architectures such as this diamond-like structure.
In this research, interparticle forces, structures, and resulting optical and chiroptical properties of nanoparticles and nanoparticle assemblies will be investigated. The interparticle potentials and preferred assembly modes of a series of metal nanoparticles will be examined. The dependence of properties on the alkyl ligand length and metal core size, shape, charge, and composition will be determined. Structures formed from the interaction of two different types of nanoparticles are especially of interest, since binary superlattices yield a wide variety of materials with tunable properties. The ab initio-based effective fragment potential method will be used to examine the interactions between nanoparticles, and the optical properties of these systems will be determined by time-dependent Hartree Fock and time-dependent density functional theory calculations.
Adhesion of Biomolecules on Nanoparticle Surfaces
In order to develop a sustainable world, our current dependence on fossil fuels must be diminished. Because many of the polymers we use are derived from fossil fuels, replacement of petroleum-based monomers with biorenewable monomers is one potential way to decrease our reliance on nonrenewable resources. Kansas State University is committed to the search for biorenewable materials, and researchers in the Center for Biobased Polymers by Design (CBPD) are involved in determining underlying principles that govern the structure and properties of biobased materials.
One potential way to modify the properties of a material is to incorporate an additive, and recent research in the CBPD has examined the effects of integrating nanoparticles in materials. The goal of the work in the Aikens group is to develop a detailed understanding of the interactions of bio-based molecules with nanoparticles. The preferred conformations and binding energies of molecules containing reactive functional groups (carboxylic acid, hydroxyl, esters, amino groups, etc.) on metal and metal oxide nanoparticle surfaces will be investigated using electronic structure methods in order to provide insight into the adhesion and adsorption properties of biopolymers on these surfaces. A combination of quantum mechanical/molecular mechanics (QM/MM) and periodic density functional theory (DFT) methods will be employed. Development of methods with a decreased computational cost will be pursued.
Atomic Scale Energetics and Growth Mechanisms for Complex Intermetallic Systems
Complex intermetallic materials have many potential applications due to their wide variety of electrical, magnetic, and structural properties. These systems have recently been investigated as hydrogen storage materials,[4] catalysts for steam reforming of methanol,[5] and photonic materials.[6] Some quasicrystalline icosahedral alloys including i-AlCuFe and i-AlPdMn have a high electrical resistivity, low surface energy, a low friction coefficient, high hardness, and good corrosion resistance, which combine to give many technologically interesting applications as coatings and composites.[7]
The icosahedral materials have a structure that consists of a series of clusters rather than a traditional unit cell; since these systems do not have three-dimensional periodic translational order, few quantitatively accurate calculations have been undertaken for these systems until now. One question that is not well understood is the growth mechanism of these materials. Exceptional stability of certain clusters could influence whether the material grows in an atom-by-atom fashion or by the addition of clusters. In the first stage of this project, the structures, binding energies, and other properties of relatively small clusters containing Al, Pd, and Mn will be investigated in order to ascertain whether electronic or geometric effects drive the creation of “magic clusters” observed during laser vaporization of complex intermetallic alloys. Ab initio approaches including coupled cluster theory, second order perturbation theory, and multireference methods will be employed to calculate the thermodynamic stability and properties of these clusters. Information on the electronic structure and energetics of the system will provide a basis for the second part of the project, in which the atom-by-atom growth mechanism of the icosahedral AlPdMn surface is examined.
References
1. Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O'Brien, S.; Murray, C. B., Structural diversity in binary nanoparticle superlattices. Nature 2006, 439, 55.
2. Zhang, H.; Wang, X.; Zhang, K.; Teo, B. K., Functional Crystals: Search Criteria and Design Principles. Journal of Solid State Chemistry 2000, 152, 191.
3. Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A., Electrostatic Self-Assembly of Binary Nanoparticle Crystals with a Diamond-Like Lattice. Science 2006, 312, 420.
4. Kim, J. Y.; Hennig, R.; Huett, V. T.; Gibbons, P. C.; Kelton, K. F., Hydrogen absorption in Ti-Zr-Ni quasicrystals and 1/1 approximants. Journal of Alloys and Compounds 2005, 404-406, 388.
5. Tanabe, T.; Kameoka, S.; Tsai, A. P., A novel catalyst fabricated from Al-Cu-Fe quasicrystal for steam reforming of methanol. Catalysis Today 2006, 111, 153.
6. Lee, T. D. M.; Parker, G. J.; Zoorob, M. E.; Cox, S. J.; Charlton, M. D. B., Design and simulation of highly symmetric photonic quasi-crystals. Nanotechnology 2005, 16, 2703.
7. Cai, T.; Shi, F.; Shen, Z.; Gierer, M.; Goldman, A. I.; Kramer, M. J.; Jenks, C. J.; Lograsso, T. A.; Delaney, D. W.; Thiel, P. A.; Van Hove, M. A., Surface Science 2001, 495, 19.
Selected Publications
Time-Dependent Density Functional Theory Examination of the Effects of Ligand Adsorption on Metal Nanoparticles. C. M. Aikens, G. C. Schatz, ACS Symposium Series, submitted.
TDDFT Studies of Absorption and SERS Spectra of Pyridine Interacting with Au20. C. M. Aikens, G. C. Schatz, J. Phys. Chem. A, 2006, 110, 13317-13324.
Incremental Solvation of Nonionized and Zwitterionic Glycine. C. M. Aikens, M. S. Gordon, J. Am. Chem. Soc., 2006, 128, 12835-12850.
Scalable Implementation of Analytic Gradients for Second-Order Z-Averaged Perturbation Theory Using the Distributed Data Interface. C. M. Aikens, G. D. Fletcher, M. W. Schmidt, M. S. Gordon, J. Chem. Phys., 2006, 124, 014107 (14 pp.).
Feature Article: A Derivation of the Frozen-Orbital Unrestricted Open-Shell and Restricted Closed-Shell Second-Order Perturbation Theory Analytic Gradient Expressions. C. M. Aikens, S. P. Webb, R. L. Bell, G. D. Fletcher, M. W. Schmidt, M. S. Gordon, Theor. Chem. Acc., 2003, 110, 233-253.
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