Analysis of Dynamical Peculiarities in Nanoalloys at Subsystems Level: Dynamical Degrees of Freedom, Temperature Differences, and the Chameleon Effect.

A novel analysis of the dynamical behavior of nanoalloy systems, as represented by model Ni/Al 13-atom clusters, over a broad range of energies that cover the stage-wise transition of the systems from their solid-like to liquid-like state is presented. Conceptually, the analysis is rooted in partitioning the systems into judiciously chosen subsystems and characterizing the latter in terms of subsystem-specific dynamical descriptors that include dynamical degrees of freedom, root-mean-square bond-length fluctuation, and element-specific subsystem temperature. The analysis reveals a host of intriguing new peculiarities in the dynamical behavior of the Ni/Al 13-mers, among which are what we call the chameleon effect and the difference in the temperatures of the Ni and Al subsystems at high energies, a difference that strongly depends on the cluster composition and also changes with energy. These do not have an analog in pure Ni13 and Al13 and are explained in terms of the coupled effects of the difference between the masses of the Ni and Al atoms (the mass effect) and of the difference in the anharmonicity of the overall interaction potential as experienced by the Ni and Al subsystems of the clusters (the potential effect).

Computational studies of structural, energetic, and electronic properties of pure Pt and Mo and mixed Pt/Mo clusters: Comparative analysis of characteristics and trends.

The added technological potential of bimetallic clusters and nanoparticles, as compared to their pure (i.e., one-component) counterparts, stems from the ability to further fine-tune their properties and, consequently, functionalities through a simultaneous use of the "knobs" of size and composition. The practical realization of this potential can be greatly advanced by the knowledge of the correlations and relationships between the various characteristics of bimetallic nanosystems on the one hand and those of their pure counterparts as well as pure constituent components on the other hand. Here, we present results of a density functional theory based study of pure Ptn and Mon clusters aimed at revisiting and exploring further their structural, electronic, and energetic properties. These are then used as a basis for analysis and characterization of the results of calculations on two-component Ptn-mMom clusters. The analysis also includes establishing relationships between the properties of the Ptn-mMom clusters and those of their Ptn-m and Mom components. One of the particularly intriguing findings suggested by the calculated data is a linear dependence of the average binding energy per atom in sets of Ptn-mMom clusters that have the same fixed number m of Mo atoms and different number n-m of Pt atoms on the fractional content (n-m)/n of Pt atoms. We derive an analytical model that establishes the fundamental basis for this linearity and expresses its parameters-the m-dependent slope and intercept-in terms of characteristic properties of the constituent components, such as the average binding energy per atom of Mom and the average per-atom adsorption energy of the Pt atoms on Mom. The conditions of validity and degree of robustness of this model and of the linear relationship predicted by it are discussed.

Electron Binding Energy Spectra of AlnPt- Clusters─A Combined Experimental and Computational Study.

Results of size-selected electron photo-detachment experiments and density functional theory calculations on anionic AlnPt-, n = 1-7, clusters are presented and analyzed. The measured and calculated spectra of electron binding energies are, overall, in excellent accord with each other. The analysis reveals the general importance of accounting for the multiplicity of structural forms of a given-size cluster that can contribute to its measured spectrum, especially when the clusters are fluxional and/or the conditions of the experiment allow for structural transitions. We show that for the systems studied here, the size-specific peculiarities of the measured spectra can be understood in terms of the combined contributions of corresponding different accessible stable equilibrium conformations, bona-fide transition-state configurations, and electronic-crossing structures that may play the role of effective barriers in electronically nonadiabatic dynamics.

Universality in size-driven evolution towards bulk polarizability of metals.

The properties and characteristics of materials on the subnano/nano scale are very different from those of their bulk counterparts. The evolution of materials properties with size is the holy grail of nanoscience. An intriguing question then is: Can one predict what type of material (metal, semiconductor or insulator) an unidentified element will be, when in bulk quantities, solely from the properties it exhibits over a limited range of the subnano/nano size-regime? We demonstrate here that for nominally metallic elements (i.e., elements that are metals in bulk quantities) the answer to this question is "yes", and the very identity of the element also can be established. Most importantly, we show that the phenomenon of size-induced transition to metallicity, as gauged by polarizability, is characterized by features and trends that are universal for all metals. Combining numerical simulation data with an analytical model we introduce a universal constant and derive equations that express the universality explicitly.

Si clusters are more metallic than bulk Si.

Dipole polarizabilities were computed using density functional theory for silicon clusters over a broad range of sizes up to N = 147 atoms. The calculated total effective polarizabilities, which include contributions from permanent dipole moments of the clusters, are in very good agreement with recently measured values. We show that the permanent dipole contributions are most important for clusters in the intermediate size range and that the measured polarizabilities can be used to distinguish between energetically nearly degenerate cluster isomers at these sizes. We decompose the computed total polarizabilities α into the so-called dipole and charge transfer contributions, αp and αq, using a site-specific analysis. When the per-atom values of these quantities are plotted against N-1/3, clear linear trends emerge that can be extrapolated to the large size limit (N-1/3→0), resulting in a value for αN of 30.5 bohrs3/atom that is significantly larger than the per-atom polarizability of semiconducting bulk Si, 25.04 bohrs3/atom. This indicates that Si clusters possess a higher degree of metallicity than bulk Si, a conclusion that is consistent with the strong electrostatic screening of the cluster interiors made evident by the analysis of the calculated atomic polarizabilities.

Anharmonic densities of states: A general dynamics-based solution.

Density of states is a fundamental physical characteristic that lies at the foundation of statistical mechanics and theoretical constructs that derive from them (e.g., kinetic rate theories, phase diagrams, and others). Even though most real physical systems are anharmonic, the vibrational density of states is customarily treated within the harmonic approximation, or with some partial, often limited, account for anharmonicity. The reason for this is that the problem of anharmonic densities of states stubbornly resisted a general and exact, yet convenient and straightforward in applications, solution. Here we formulate such a solution within both classical and quantum mechanics. It is based on actual dynamical behavior of systems as a function of energy and as observed, or monitored, on a chosen time scale, short or long. As a consequence, the resulting anharmonic densities of states are fully dynamically informed and, in general, time-dependent. As such, they lay the ground for formulation of new statistical mechanical frameworks that incorporate time and are ergodic, by construction, with respect to actual dynamical behavior of systems.

Reactive scattering calculations for (87)Rb+(87)RbHe→Rb2((3)Σ(u)(+),v)+He from ultralow to intermediate energies.

We investigate atom-diatom reactive collisions, as a preliminary step,in order to assess the possibility of forming Rb(2) molecules in their lowest triplet electronic state by cold collisions of rubidium atoms on the surface of helium nanodroplets [corrected]. A simple model related to the well-known Rosen treatment of linear triatomic molecules [N. Rosen, J. Chem. Phys. 1, 319 (1933)] in relative coordinates is used, allowing to estimate reactive probabilities for different values of the total angular momentum. The best available full dimensional potential energy surface [Guillon et al., J. Chem. Phys. 136, 174307 (2012)] is employed through the calculations. Noticeable values of the probabilities in the ultracold regime, which numerically fulfill the Wigner threshold law, support the feasibility of the process. The rubidium dimer is mainly produced at high vibrational states, and the reactivity is more efficient for a bosonic helium partner than when the fermion species is considered.

H2 saturation on palladium clusters.

The interaction of PdN clusters (N = 2, 3, 4, 7, and 13) with multiple H2 adsorbate molecules is investigated using density functional theory with the hybrid PBE0 functional. The optimal structure for each PdNH2(L) complex is determined systematically via a sequential addition of H2 units. The adsorption energy for each successive H2 addition is computed to determine the maximum number of molecules that can be stably added to a PdN at T = 0 K. The Gibbs free energy is then used to determine the saturation coverage at finite temperature. For N = 2, 3, and 4, a single H2 is found to dissociate, and up to two additional molecular H2 units per Pd atom can bind stably to the clusters at 0 K. At 300 K, one H2 unit dissociates, and only one additional H2 molecular unit per Pd atom is stably bound. For N = 7 and T = 0 K, two H2 units dissociate, and 11 additional H2 units bind molecularly. At 300 K, two units dissociate, and eight are bound molecularly. For N = 3, 4, and 7, we find that an additional H2 unit may dissociate if the underlying cluster structure rearranges. Eight H2 units dissociate on Pd13 at 0 K. At least one additional H2 binds molecularly at 0 K, but none bind at 300 K. This suggests that only dissociated H2 units will stably bind to larger Pd particles at room temperature. The influence of molecularly adsorbed H2 units on the migration of dissociated H atoms is investigated in a preliminary way. Both barrier heights and the relative stability of local minima of Pd4H2(L) are found to be affected by the degree of molecular H2 coverage.

Characterizing metal coordination environments in porous organic polymers: a joint density functional theory and experimental infrared spectroscopy study.

Very POP right now! DFT computational analysis on the structural, energetic, and IR spectroscopic characteristics of a porous organic polymer support, [Ta(NMe2 )5 ] as a molecular precursor, and the catalytic material synthesized from these two components are presented and analyzed against recorded IR spectra of these systems. The analysis leads to unambiguous identification of the atomic structure of the POP-supported Ta-amide reaction center synthesized in the experiment.

H2 reactions on palladium clusters.

Adsorption of an H2 molecule on Pd(N) clusters (N = 2-4, 7, 13, 19, and 55) is investigated using density functional theory with the hybrid PBE0 functional. Low-energy Pd(N) isomers, taken from a large pool of candidate structures for all cluster sizes (except N = 55), are used in systematic searches for the most stable Pd(N)H2 (molecular) and Pd(N)2H (dissociative) adsorption complexes. Molecular adsorption of H2 is found to occur strictly at atop sites, with the strongest binding typically occurring at the site with the smallest coordination. Binding of dissociated H atoms occurs preferentially on 3-fold faces and on certain favorable edge sites, while binding at atop sites is unstable. Dissociative adsorption is energetically preferred to molecular adsorption for all cluster sizes. The dissociative adsorption energy decreases with cluster size, with pronounced variations due to cluster size effects for the smallest clusters. Adsorption reaction pathways are computed for cluster sizes up to N = 13. Molecular adsorption is found to be barrierless in all cases. Dissociative adsorption occurs without a barrier for the pathways studied for N = 7 and 13 and with a small barrier on the smaller clusters. Finally, lowest-energy pathways for the migration of a dissociated hydrogen atom between local minima on a cluster surface are computed for the Pd4, Pd7, and Pd13 clusters. Calculated migration barriers range from 0.05 to 0.25 eV.

Pressure-induced metallization of Li+-doped hydrogen clusters.

Endohedrally encapsulated hydrogen clusters doped with inert helium (H24He) and ionic lithium (H24Li(+)) are investigated. The confinement model is a nanoscopic analogue of the experimental compression of solid hydrogen. The structural and electronic properties of the doped hydrogen clusters are determined under the effects of pressure. The results are compared with these of the isoelectronic (pure) hydrogen counterpart H26 under similar physical conditions. Pressure increase rates with respect to H26 of approximately 1.1 are observed with the insertion of helium or lithium. The changes of geometrical structures and HOMO-LUMO gap energies with the pressure point out the pressure-induced metallization of the Li(+)-doped cluster. The computations are done using density functional theory in the form implemented for molecules; they include zero-point energy effects and, to our best knowledge, are the first of their kind.

Capping ligands as selectivity switchers in hydrogenation reactions.

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ~99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.

Theoretical investigation of adsorption of molecular oxygen on small copper clusters.

Adsorption of molecular oxygen on Cu(N) (N = 2-10) clusters is investigated using density functional theory under the generalized gradient approximation of Perdew-Burke-Ernzerhof. An extensive structure search is performed to identify low-energy conformations of Cu(N)O(2) complexes. Optimal adsorption sites are assigned for low-energy isomers of the clusters. Among these are some new arrangements unidentified heretofore. Distinct size dependences are noted for the ground state Cu(N)O(2) complexes in stability, adsorption energy, Cu-O(2) bond strength, and other characteristic quantities. Cu(N)O(2) with odd-N tend to have larger adsorption energies than their even-N neighbors, with the exception of Cu(6)O(2), which has a relatively large adsorption energy resulting from the adsorption-induced 2D-to-3D structural transition in Cu(6). The energetically preferred spin-multiplicity of all the odd-N Cu(N)O(2) complexes is doublet; it is triplet for N = 2 and 4 and singlet for N = 6, 8, and 10.

Thermal behavior of a 13-molecule hydrogen cluster under pressure.

The thermal behavior of a 13-molecule hydrogen cluster is studied as a function of pressure and temperature using a combination of trajectory and density functional theory simulations. The analysis is performed in terms of characteristic descriptors such as caloric curve, root-mean-square bond length fluctuation, pair correlation function, velocity autocorrelation function, volume thermal expansion, and diffusion coefficients. The discussion addresses on the peculiarities of the transition from the ordered-to-disordered state as exhibited by the cluster under different pressures and temperatures.

Space-time properties of Gram-Schmidt vectors in classical Hamiltonian evolution.

Not all tangent space directions play equivalent roles in the local chaotic motions of classical Hamiltonian many-body systems. These directions are numerically represented by basis sets of mutually orthogonal Gram-Schmidt vectors, whose statistical properties may depend on the chosen phase space-time domain of a trajectory. We examine the degree of stability and localization of Gram-Schmidt vector sets simulated with trajectories of a model three-atom Lennard-Jones cluster. Distributions of finite-time Lyapunov exponent and inverse participation ratio spectra formed from short-time histories reveal that ergodicity begins to emerge on different time scales for trajectories spanning different phase-space regions, in a narrow range of total energy and history length. Over a range of history lengths, the most localized directions were typically the most unstable and corresponded to atomic configurations near potential landscape saddles.

Nanoalloys: tuning properties and characteristics through size and composition.

A brief sketch of the history of metals and alloys is followed by examples illustrating the current status of the field of nanoalloys and a discussion of our results on the characterization of structural and dynamical (thermal) properties of Ni-Al bimetallic clusters.

Pressure and size effects in endohedrally confined hydrogen clusters.

Density functional theory is used to carry out a systematic study of zero-temperature structural and energy properties of endohedrally confined hydrogen clusters as a function of pressure and the cluster size. At low pressures, the most stable structural forms of (H(2))(n) possess rotational symmetry that changes from C(4) through C(5) to C(6) as the cluster grows in size from n=8 through n=12 to n=15. The equilibrium configurational energy of the clusters increases with an increase of the pressure. The rate of this increase, however, as gauged on the per atom basis is different for different clusters sizes. As a consequence, the size dependencies of the configurational energies per atom at different fixed values of pressure are nonmonotonic functions. At high pressures, the molecular (H(2))(n) clusters gradually become atomic or dominantly atomic. The pressure-induced changes in the HOMO-LUMO gap of the clusters indicate a finite-size analog of the pressure-driven metallization of the bulk hydrogen. The ionization potentials of the clusters decrease with the increase of pressure on them.