G. Malcolm Stocks

CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010) Surface

Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al2O3), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir-Hinshelwood scheme (L-H scheme) which requires at least one Pt-Pt bond, we carried out a first-principles density functional theoretical study of a proposed pathway which is a variation on the conventional L-H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO2, leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism suggests that the single Pt atoms will get covered with CO3 unless the temperature is raised to eliminate CO2. We find evidence for CO3 coverage at room temperature supporting the proposed mechanism in an in situ diffuse reflectance infrared study of CO adsorption on the catalysts supported single atoms. Thus, our results clearly show that supported Pt single atoms are catalytically active and that this catalytic activity can occur without involving the substrate. Characterization by electron microscopy and X-ray absorption studies of the monodisperse Pt/θ-Al2O3 are also presented.

Half-Heusler Compounds as a New Class of Three-Dimensional Topological Insulators

Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits a distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the [001] direction, which opens a band gap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

Influence of chemical disorder on energy dissipation and defect evolution in concentrated solid solution alloys

A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiation-tolerant structural alloys for energy applications.

Melting and premelting of carbon nanotubes

We report the results of molecular dynamics simulations of melting and premelting of single-walled carbon nanotubes (SWNTs). We found that the traditional critical Lindemann parameter for the melting of bulk crystals is not valid for SWNTs. Using the much smaller critical Lindemann parameter for the melting of nanoparticles as a criterion, we show that the melting temperature of perfect SWNTs is around 4800 K. We further show that Stone–Wales defects in a SWNT significantly reduce the melting temperature of atoms around the defects, resulting in the premelting of SWNTs at 2600 K.

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2 and Ni0.8Cr0.2

Using ab initio calculations and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in Ni-based solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}. As defect formation energies depend sensitively on elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and local short-range order. We find that Fe has the biggest alloy effects for Ni among these four elements. Our results show that the distribution of migration energies for vacancies and interstitial have a region of overlap, which will facilitate the recombination between them.

Transition-metal interactions in aluminum-rich intermetallics

The extension of the first-principles generalized pseudopotential theory ~GPT! to transition-metal ~TM! aluminides produces pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal overbinding that creates an unrealistic TM-TM attraction at short separations in the absence of balancing many-body contributions. Even with this limitation, the GPT pair potentials have been used effectively in total-energy calculations for Al-TM systems with TM atoms at separations greater than 4 A. An additional potential term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials, resulting in structure-dependent TM-TM potentials. Towards this end, we have performed numerical ab initio total-energy calculations using the Vienna ab initio simulation package for an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a(r 0 /r) b to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions.

First-principles study of the doping effects in bilayer graphene

We used first-principles calculations to study the doping effects in bilayer graphene, focusing on Au substitute doping in the upper layer of graphene. We found that Au doping in the upper layer maintains the lattice structure of the lower graphene layer. Our study on binding energy shows that the Au-doped bilayer structure is stable with Au atom tightly confined in a small region between the upper and lower layers. Charge density analysis indicates that charge is transferred from the Au donor to the carbon atoms in the lower layer, increasing the carrier density in the lower graphene.

On the magnetic structure of γ-FeMn alloys

The alloy γ-FeMn is a rare example of a fcc antiferromagnet. It has become a prototype for pinning layer studies in magnetoelectronic devices. Here we report the results of first principles calculations of the magnetic structure of γ-FeMn based on large cell models of the disordered alloy. The calculations are based on the constrained local moment model and use of first principles spin dynamics to obtain the ground state orientational configuration. In agreement with previous layer KKR-CPA studies, we find the 3Q-state to be lowest of the three prototype structures studied (1Q,2Q,3Q). However, the constraining fields introduced into the theory to maintain a specific orientational configuration are not zero indicating that even the 3Q-structure is not the ground state. Subsequent optimization of the magnetic configuration using first principles spin dynamics yields a state that is lower in energy by 2.5 meV/atom.

Quantum critical behavior in a concentrated ternary solid solution

The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, Tc, goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess –TlnT (or power law) contribution to the low temperature heat capacity, and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x ≈ 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in this simple fcc material thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.

Surface reconstruction and core distortion of silicon and germanium nanowires

We report the results of molecular dynamics simulations for structures of pristine silicon nanowires and germanium nanowires with bulk cores oriented along the [110] direction and bounded by the (100) and (110) surfaces in the lateral direction. We found that the (100) surfaces for both silicon and germanium nanowires undergo 2 ? 1 dimerization while their (110) surfaces do not show reconstruction. The direction of the dimer rows is either parallel or perpendicular to the wire axis depending on the orientation of the surface dangling bonds. The dimer length for Si is in good agreement with the result obtained by first-principles calculations. However, the geometry of Si dimers belongs to the symmetrical 2 ? 1 reconstruction rather than the asymmetrical buckled dimers. We also show that surface reconstruction of a small nanowire induces significant change in the lattice spacing for the atoms not on the (100) surface, resulting in severe structural distortion of the core of the nanowire.