Session F3: Materials/Nanomaterials Science

Atomic and electronic structures of GaN:ZnO Alloys

GaN:ZnO is a new class of alloy which currently holds the record for the efficiency of water photo-splitting. The mechanism of the large band gap bowing of this alloy and its detailed atomic structure, which are essential to understand the remarkable performance, however, are still not clear. We developed a model Hamiltonian describing the ab initio energies of different alloy atomic configurations and used it in Monte Carlo simulations to study the atomic structures of systems containing thousands of atoms. The equilibrium atomic structures from the MC simulations at different temperatures are then used to calculate their electronic structures. We found that at the experimental synthesis temperature of 1100 K, uniform alloy can be formed, albeit with a strong short range ordering. Consequently, their electronic structure is very different from the completely random alloy. Based on our calculation, we also predict that higher synthesis temperature can yield even lower energy band gap.   

GW study of the half metallic band gap of zinc blende CrAs

We determined the semiconducting gap of zinc blende (ZB) CrAs within the $GW$ approximation ($GW$A). This is the first $GW$ calculation of a half-metal. Previous calculations using density functional theory within the generalized gradient approximation (GGA) determined a gap of 1.8 eV, but the GGA is known to give too small of a value for this quantity in semiconductors. Additionally, since ZB CrAs is a half metal, one of its spin channels behaves like a metal and changes the quasiparticle screening compared to the insulating case. Due to the local field effect, we only included the $\Gamma$-point term in the metallic channel calculation of the polarizability while keeping the full set of terms in the insulating channel $GW$ calculation. Preliminary results suggest these terms from the polarizability produce little change in the value of the semiconducting gap when compared to the ``full'' $GW$A calculation.   

Creating wide-band negative-index-of-refraction metamaterials with fractal-based geometry

A burgeoning topic of modern research in electrodynamics and antenna design is the design and fabrication of ``left-handed'' metamaterials. This ``left-handedness'' is often created through use of an array of conductive structures with geometry appropriate for coupling on the wavelength scale with incident radiation to produce a phase-shifted reflected wave that cancels out incoming radiation and prevents transmission. This property has been demonstrated in several papers published in the last decade. In every instance, though the ``left-handed'' response is only exhibited in a small bandwidth centered about a specific frequency (bandwidth typically less that 0.1 GHz). I will show that through use of tessellated, fractal-based structures, one can create a repeatable geometry that exhibits a negative index of refraction (NIR) for multiple frequency bands, limited only by fabrication precision, with the ultimate goal being a wide-band absorptive response.   

Influence of Nanostructuring and Heterogeneous Nucleation on the Thermoelectric Figure of Merit in AgSbTe2

Thermoelectric materials directly interconvert heat and electricity in the solid state. In some cases, nanoscale microstructures improve thermoelectric efficiency, but this phenomenon has rarely been studied systematically for precipitates in bulk materials. We quantified the influence of nanostructuring on the thermoelectric figure of merit (zT) by embedding Sb2Te3 inclusions, from nanometer to micron sizes, in an Sb-rich AgSbTe2 matrix through solid-state precipitation. Nucleation/growth and coarsening regimes of precipitate formation had a clear effect on transport properties, which could be understood using the effective medium theory of a two-phase composite. The majority of precipitates nucleated heterogeneously at grain boundaries and at planar defects found in the matrix phase, forming a complex interconnected network. This heterogeneous nucleation causes the precipitate/matrix system to follow effective medium theory even at small precipitate sizes, thus lowering zT. Therefore, heterogeneous nucleation is a major obstacle to zT improvement using nanoscale precipitates in bulk thermoelectrics.   

Electro-optical properties of quantum dots dispersed in chiral nematic liquid crystal

The electro-optical properties of quantum dots can be significantly altered if they are assembled in close proximity to each other. The partial ordering of liquid crystal molecules can be utilized to form directed quantum dot assemblies.~ Typically, this results in a red shift in the emission spectrum of the dots as the induced order leads to enhanced dipolar interactions, resulting in electronically coupled states. Spherical cadmium selenide quantum dots of different diameters are dispersed in various concentrations in a chiral nematic liquid crystal phase.~ The quantum dots are seen to aggregate, the sizes of the aggregates depend on the size and concentration of the dots as well as the mixing time. Optimum mixing times and quantum dot concentrations are determined for dots of different sizes to achieve a uniform quantum dot dispersion. Quantum dots with emission peaks ranging from 490 nm to 640 nm were studied using polarized optical microscopy and scanning microscopy photoluminescence measurements.   

Comparative analysis of the hydrogen-vacancy interaction in Mg and Al based on density functional theory

The interactions of vacancies (V) with atomic hydrogen (H) in the bulk of the metal are expected to play an important role in H-storage as well as H-embrittlement. Using density functional theory we have studied the H-V interactions in hcp-Mg and fcc-Al, two prototypic systems for H storage. We show that a single V can in principle host up to 9 H atoms in Mg and 10 in Al. In going beyond previous theoretical studies we further evaluate the concentration of the H-V complexes for different H loading conditions -- ranging from low pressures to high pressures of H2 gas. We find significant differences between Mg and Al. In the case of Al, up to 15 {\%} of H atoms are trapped in single vacancies even for very low H pressures, which strongly slows down the diffusion of H atoms. In the case of Mg, these trapping effects are negligible for low H pressures. However, vacancies containing multiple H atoms and H-induced superabundant vacancy formation are predicted to occur in Mg at much lower H loading pressures (about 1 GPa) than in Al (about 10 GPa).   

Strain-induced isosymmetric phase transition in multiferroic BiFeO$_3$

We examine the effect of large epitaxial strain on multiferroic bismuth ferrite, BiFeO$_3$, using density functional calculations. We investigate a previously unidentified phase transition induced by experimentally accessible values of compressive strain. The transition occurs between phases that are isosymmetric yet have dramatically different structures and properties, the most notable of which is a strong enhancement and rotation of the electric polarization. This presents the opportunity to shift the transition boundary with an applied electric field, similar to a morphotropic phase boundary. Our work contributes to the limited body of knowledge about isosymmetric transitions and explains recent experimental reports of morphotropic phase boundary-like behavior in highly strained films of BiFeO$_3$ (Zeches {\em et al.}, {\em to appear} Science (2009)).   

Superparamagnetic Magnetite Nanoparticles for Optical Modulation/Chopping

We demonstrate proof of concept operation of superparamagnetic magnetite nanoparticles and magnetite-TiO$_{2}$ peapod-superstructures for laser intensity optical modulation and chopping. The frequency of the modulation is shown to be twice that of the driving signal and a function of the size of the particles. Specifically, optical modulation with round nanoparticles of sizes 80, 130, 200 nm is compared with optical modulation with magnetite-TiO$_{2}$ peapod-superstructures of sizes of around 1 $\mu$m. The former gave rise to modulations of up to 2 kHz of frequency--a number comparable to that of the commercial optical choppers--, the latter up to 100 Hz. We also show that particle shape asymmetry and anisotropy enhance optical modulation.   

Phonon Transport in Graphene: Umklapp Quenching and Heat Conduction

Since its exfoliation, graphene attracted tremendous attention of the research community. Graphene, which consists of a single atomic plane of carbon atoms, revealed many unique properties including extremely high electron mobility. In this talk I will show that unusual properties of graphene are not limited to electrons alone. Phonons also behave differently in two-dimensional (2D) system such as graphene. We have recently discovered experimentally that thermal conductivity of suspended graphene layers is extremely high and exceeds that of diamond or graphite [2-3]. We explained our results theoretically by considering the Umklapp and edge scattering of phonons in graphene [3]. Unlike in bulk graphite, the phonon transport in graphene is pure 2D for all phonon energies. As a result, the thermal conductivity of graphene can become extremely high. The extraordinary high thermal conductivity of graphene can be used for thermal management of nanoscale electronic devices. This work was supported by SRC-DARPA Functional Engineered Nano Architectonics (FENA) center and Interconnect Focus Center (IFC). [1] A.A. Balandin, et al. Nano Letters, 8, 902 (2008); S. Ghosh, et al., Appl. Phys. Lett., 92, 151911 (2008). [2] D.L. Nika, et al., Phys. Rev. B, 79, 155413 (2009); D.L. Nika et al., Appl. Phys. Lett., 94, 203103 (2009)   

Computational Approach for Quantifying Structural Disorder in Biomolecular Lattices

We have developed a novel computational approach for quantifying structural disorder in biomolecular lattices with nonlinear susceptibility based on analysis of polarization-modulated second harmonic signal. Transient, regional disorder at the level of molecular organization is identified using a novel signal-processing algorithms sufficiently compact for near real-time analysis with a desktop computer. Global disorder and regional disorder within the biostructure are assessed and scored using a multiple methodologies. Experimental results suggest our signal processing method represents a robust, scalable tool that allows us to detect both regional and global alterations in signal characteristics of biostructures with a high degree of discrimination.   

Studies of singly doping of Me and Fe in Si to deduce simple guidelines in selecting transition metal elements for Si-based spintronic materials

Single dopings of Mn and Fe in Si are investigated using 8-, 64-, and 216-atom supercells and a first-principles method based on density functional theory. Atomic sizes play an essential role in determining the contraction or the expansion of neighboring atoms around the transition metal element at a substitutional site. At a tetrahedral interstitial site, there is only expansion. Magnetic moments/transition- metal-element at the two sites are calculated. Physical reasons for these properties are given. Some guidelines for selecting transition metal elements doped in Si for future Si-based effective spintronic materials are proposed.   

Faraday Effect in Magnetic and Non-Magnetic Colloidal Nanoparticles in Water

We have investigated Faraday Effect in a variety of nanoparticle solutions. Verdet constant of superparamagnetic nanocrystal clusters of magnetite (Fe3O4), diluted in water, is measured as a function of particle size. Particle sizes ranging from 3 to 210 nm, resulted in a nonlinear size dependence in Verdet constant. The relationship between Verdet constant and particle size is possibly due to variation in magnetic domain sizes within the particles. Domain size evolution investigations are underway using X-ray diffraction. Non-magnetic nanoparticle solutions investigated consisted of silver, silver oxide, magnesium oxide, nickel oxide, and carbon nanotubes. Solutions demonstrated diamagnetic and paramagnetic properties, as expected. We believe that Faraday Effect is an efficient method of investigating magnetic properties of nanoparticles.   

Computational study of the adsorption of methanol, formic acid, and formaldehyde on the $\beta $-SiC(100)-3x2 surface

The absorption of methanol, formic acid, and formaldehyde on the Si-rich $\beta $-SiC(100)-(3x2) surface has been studied using density functional theory (DFT) computational methods and small clusters to model the surface reactivity. A single cluster dimer model is used to calculate energies after the interaction of adsorbates on the surface. The dissociative adsorption of methanol on the SiC(100)-3x2 surface is predicted to take place facilely, giving rise to Si-OCH$_{3}$ and Si-H surface species and followed a path similar to that predicted for Si(100)-2x1 surface. The reaction is highly exothermic and predicted to occur with essentially no barrier. Formaldehyde is also predicted to adsorb with essentially no barrier on the SiC(100)-3x2 surface with formation of a 4-member ring on the surface. This adsorption is also exothermic and similar to the corresponding Si(100)-2x1 surface. This result shows that the carbonyl group can undergo cycloaddition onto the SiC(100) surface. Formic acid is also predicted to undergo dissociative chemisorption on the SiC(100) surface with the formation of Si-OCOH and Si-H surface species. This process is also highly exothermic (-283.1 kJ/mol) and essentially barrierless.   

Band-gap bowing, band offsets, and electron affinities for AlN, GaN, InN and InGaN: A DFT study

AlN, GaN, and InN and their alloys are successfully being used in optical, electronic, and photovoltaic devices; a novel application is for photochemical water splitting. In order to further improve nitride-based devices a detailed understanding of the materials properties as a function of alloy composition is needed. To obtain such insight we have investigated the band gap and absolute band positions of AlN, GaN, InN and InGaN using density functional theory. The HSE exchange correlation functional has been used in order to accurately calculate the electronic band structure [1]. Detailed surface calculations have been performed that, combined with bulk calculations for alloys, yield information about the positions of valence and conduction bands on an absolute energy scale. We will discuss bowing effects, band offsets, and electron affinities in light of the application for photochemical hydrogen production. \\[4pt] [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)   

Strain effect in group-III nitride semiconductors and their alloys

Strain plays a crucial role in group-III nitride semiconductor based devices since it affects the band structure near the valence- and conduction-band edges and thus the optical properties and the device characteristics. However, the deformation potentials that describe the change in band structure under strain have not yet been reliably determined. We present a systematic study of the strain effects in AlN, GaN and InN in the wurtzite phase. We apply density functional theory and hybrid functionals to address the band-gap problem. We observe nonlinearities of transition energies under realistic strain condition that may, in part, explain the appreciable scatter in previous theoretical work on deformation potentials of group-III-nitrides. For the linear regime around the experimental lattice parameters, we present a complete set of deformation potentials. Applying our deformation potentials, we study strain effects in InGaN alloys (including c-, m-, and semi-polar planes) grown on GaN substrates. We make predictions for the transition energies in these systems and their dependence on In composition.