Hybrid density functional study of small Rh-n (n=2-15) clusters

The physical properties of small rhodium clusters, Rh-n, have been in debate due to the shortcomings of density functional theory (DFT). To help in the solution of those problems, we obtained a set of putative lowest energy structures for small Rh-n (n = 2-15) clusters employing hybrid-DFT and the generalized gradient approximation (GGA). For n = 2-6, both hybrid and GGA functionals yield similar ground-state structures (compact), however, hybrid favors compact structures for n = 7-15, while GGA favors open structures based on simple cubic motifs. Thus, experimental results are crucial to indicate the correct ground-state structures, however, we found that a unique set of structures (compact or open) is unable to explain all available experimental data. For example, the GGA structures (open) yield total magnetic moments in excellent agreement with experimental data, while hybrid structures (compact) have larger magnetic moments compared with experiments due to the increased localization of the 4d states. Thus, we would conclude that GGA provides a better description of the Rh-n clusters, however, a recent experimental-theoretical study [ Harding et al., J. Chem. Phys. 133, 214304 (2010)] found that only compact structures are able to explain experimental vibrational data, while open structures cannot. Therefore, it indicates that the study of Rh-n clusters is a challenging problem and further experimental studies are required to help in the solution of this conundrum, as well as a better description of the exchange and correlation effects on the Rh n clusters using theoretical methods such as the quantum Monte Carlo method.

Site localization of Cd impurities in sapphire

By combining first-principles electronic structure calculations and existing time-differential gamma-gamma perturbed-angular-correlation experiments we studied the site localization, the local environment, and the electronic structure of Cd impurities in sapphire (alpha-Al2O3) single crystals in different charged states. The ab initio calculations were performed with the full-potential augmented plane wave plus local orbitals method and the projector augmented wave method. Comparing the calculated electric-field-gradient tensor at the Cd nuclei in the alpha-Al2O3 host lattice and the corresponding available experimental values, we have seen that it is equally possible for Cd to replace an Al atom (in a negative charge state) or to be placed in an interstitial site (in a neutral charge state). To finally address the issue of the Cd impurity localization, we performed formation energy calculations. These results have shown that Cd placed in the substitutional Al site, in the negatively charged state, is the most probable configuration.

Quantum Monte Carlo study of small aluminum clusters Al-n (n=2-13)

Using fixed node diffusion quantum Monte Carlo (FN-DMC) simulations and density functional theory (DFT) within the generalized gradient approximations, we calculate the total energies of the relaxed and unrelaxed neutral, cationic, and anionic aluminum clusters, Al-n (n = 1-13). From the obtained total energies, we extract the ionization potential and electron detachment energy and compare with previous theoretical and experimental results. Our results for the electronic properties from both the FN-DMC and DFT calculations are in reasonably good agreement with the available experimental data. A comparison between the FN-DMC and DFT results reveals that their differences are a few tenths of electron volt for both the ionization potential and the electron detachment energy. We also observe two distinct behaviors in the electron correlation contribution to the total energies from smaller to larger clusters, which could be assigned to the structural transition of the clusters from planar to three-dimensional occurring at n = 4 to 5.

Encapsulation of small magnetic clusters in fullerene cages: A density functional theory investigation within van der Waals corrections

The encapsulation of magnetic transition-metal (TM) clusters inside carbon cages (fullerenes, nanotubes) has been of great interest due to the wide range of applications, which spread from medical sensors in magnetic resonance imaging to photonic crystals. Several theoretical studies have been reported; however, our atomistic understanding of the physical properties of encapsulated magnetic TM 3d clusters is far from satisfactory. In this work, we will report general trends, derived from density functional theory within the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE), for the encapsulation properties of the TMm@C-n (TM = Fe, Co, Ni; m = 2-6, n = 60,70,80,90) systems. Furthermore, to understand the role of the van der Waals corrections to the physical properties, we employed the empirical Grimme's correction (PBE + D2). We found that both PBE and PBE + D2 functionals yield almost the same geometric parameters, magnetic and electronic properties, however, PBE + D2 strongly enhances the encapsulation energy. We found that the center of mass of the TMm clusters is displaced towards the inside C-n surfaces, except for large TMm clusters (m = 5 and 6). For few cases, e. g., Co-4 and Fe-4, the encapsulation changes the putative lowest-energy structure compared to the isolated TMm clusters. We identified few physical parameters that play an important role in the sign and magnitude of the encapsulation energy, namely, cluster size, fullerene equatorial diameter, shape, curvature of the inside C-n surface, number of TM atoms that bind directly to the inside C-n surface, and the van der Waals correction. The total magnetic moment of encapsulated TMm clusters decreases compared with the isolated TMm clusters, which is expected due to the hybridization of the d-p states, and strongly depends on the size and shape of the fullerene cages.

Ethanol and Water Adsorption on Close-Packed 3d, 4d, and 5d Transition-Metal Surfaces: A Density Functional Theory Investigation with van der Waals Correction

Nowadays, there is a great interest in the economic success of direct ethanol fuel cells; however, our atomistic understanding of the designing of stable and low-cost catalysts for the steam reforming of ethanol is still far from satisfactory, in particular due to the large number of undesirable intermediates. In this study, we will report a first-principles investigation of the adsorption properties of ethanol and water at low coverage on close-packed transition-metal (TM) surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), employing density functional theory (DFT) calculations. We employed the generalized gradient approximation with the formulation proposed by Perdew, Burke, and Erzenholf (PBE) to the exchange correlation functional and the empirical correction proposed by S. Grimme (DFT+D3) for the van der Waals correction. We found that both adsorbates binds preferentially near or on the on top sites of the TM surfaces through the 0 atoms. The PBE adsorption energies of ethanol and water decreases almost linearly with the increased occupation of the 4d and 5d d-band, while there is a deviation for the 3d systems. The van der Waals correction affects the linear behavior and increases the adsorption energy for both adsorbates, which is expected as the van der Waals energy due to the correlation effects is strongly underestimated by DFT-PBE for weak interacting systems. The geometric parameters for water/TM are not affected by the van der Waals correction, i.e., both DFT and DFT+D3 yield an almost parallel orientation for water on the TM surfaces; however, DFT+D3 changes drastically the ethanol orientation. For example, DFT yields an almost perpendicular orientation of the C-C bond to the TM surface, while the C-C bond is almost parallel to the surface using DFT +D3 for all systems, except for ethanol/Fe(110). Thus, the van der Waals correction decreases the distance of the C atoms to the TM surfaces, which might contribute to break the C-C bond. The work function decreases upon the adsorption of ethanol and water, and both follow the same trends, however, with different magnitude (larger for ethanol/TM) due to the weak binding of water to the surface. The electron density increases mainly in the region between the topmost layer and the adsorbates, which explains the reduction of the substrate work function.

Adsorption of NO on the Rh-13, Pd-13, Ir-13, and Pt-13 Clusters: A Density Functional Theory Investigation

The adsorption of NO on transition-metal (TM) surfaces has been widely studied by experimental and theoretical techniques; however, our atomistic understanding of the interaction of nitrogen monoxide (NO) with small TM clusters is far from satisfactory, which compromises a deep understanding of real catalyst devices. In this study, we report a density functional theory study of the adsorption properties of NO on the TM13 (TM = Rh, Pd, Ir, Pt) clusters employing the projected augmented wave method. We found that the interaction of NO with TM13 is much more complex than that for NO/TM(111). In particular, for low symmetry TM13 clusters, there is a strong rearrangement of the electronic charge density upon NO adsorption and, as a consequence, the adsorption energy shows a very complex dependence even for adsorption sites with the same local effective coordination. We found a strong enhancement of the binding energy of NO to the TM13 clusters compared with the TM(111) surfaces, as the antibonding NO states are not occupied for NO/TM13, and the general relationship based on the d-band model between adsorption energy and the center of gravity of the occupied d-states does not hold for the studied TM13 clusters, in particular, for clusters with low symmetry. In contrast with the adsorption energy trends, the geometric NO/TM13 parameters and the vibrational N-O frequencies for different coordination sites follow the same trend as for the respective TM(111) surfaces, while the changes in the frequencies between different surfaces and TM13 clusters reflect the strong NO-TM13 interaction.

Sulfur Radicals as Tethers for the Adsorption of Aromatic Molecules on Silicon Surface

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of benzenethiol and diphenyl disulfide with the silicon (001) surface. A direct comparison of different adsorption structures with Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) allow us to identify that benzenethiol and diphenyl disulfide dissociatively adsorb on the silicon surface. In addition, theoretically obtained data suggests that the C6H5SH:Si(001) presents a higher Schottky barrier height contact when compared to other similar aromatic molecules.

The T-2 phase in the Nb-Si-B system studied by ab initio calculations and synchrotron X-ray diffraction

The solid solution based on Nb5Si3 (Cr5B3 structure type, D8(l), tl32, 14/mcm, No140, a=6.5767 angstrom, c=11.8967 angstrom) in the Nb-Si-B system was studied from the structural and thermodynamic point of view both experimentally and by ab initio calculations. Rietveld refinement of powder X-ray synchrotron data allowed to determine the boron to silicon substitution mechanism and the structural parameters. Ab initio calculations of different ordered compounds and selected disordered alloys allowed to obtain in addition to the enthalpy of formation of the solution, substitution mechanism and structural parameters which are in excellent agreement with the experimental data. The stability of the phase is discussed. (C) 2012 Elsevier Inc. All rights reserved.

Platinum-Based Nanoalloys PtnTM55-n (TM=Co, Rh, Au): A Density Functional Theory Investigation

Structural and electronic properties of the PtnTM55-n (TM = Co, Rh, Au) nanoalloys are investigated using density functional theory within the generalized gradient approximation and employing the all-electron projected augmented wave method. For TM = Co and Rh, the excess energy, which measures the relative energy stability of the nanoalloys, is negative for all Pt compositions. We found that the excess energy has similar values for a wide range of Pt compositions, i.e., n = 20-42 and n = 28-42 for Co and Rh, respectively, with the core shell icosahedron-like configuration (n = 42) being slightly more stable for both Co and Rh systems because of the larger release of the strain energy due to the smaller atomic size of the Co and Rh atoms. For TM = Au, the excess energy is positive for all compositions, except for n = 13, which is energetically favorable due to the formation of the core-shell structure (Pt in the core and Au atoms at the surface). Thus, our calculations confirm that the formation of core-shell structures plays an important role to increase the stability of nanoalloys. The center of gravity of the occupied d-states changes almost linearly as a function of the Pt composition, and hence, based on the d-band model, the magnitude of the adsorption energy of an adsorbate can be tuned by changing the Pt composition. The magnetic moments of PtnCo55-n decrease almost linearly as a function of the Pt composition; however, the same does not hold for PtRh and PtAu. We found an enhancement of the magnetic moments of PtRh by a few times by increasing Pt composition, which we explain by the compression effects induced by the large size of the Pt atoms compared with the Rh atoms.

High-throughput non-destructive nuclear magnetic resonance method to measure intramuscular fat content in beef

High intake of saturated fat from meats has been associated with cardiovascular disease, cancer, diabetes, and others diseases. In this paper, we are introducing a simple, high-throughput, and non-destructive low-resolution nuclear magnetic resonance method that has the potential to analyze the intramuscular fat content (IMF) in more than 1,000 beef portions per hour. The results can be used in nutritional fact labels, replacing the currently used average value. The method is based on longitudinal (T(1)) and transverse (T(2)) relaxation time information obtained by a continuous wave-free precession (CWFP) sequence. CWFP yields a higher correlation coefficient (r=0.9) than the conventional Carr-Purcell-Meiboom- Gill (CPMG) method (r=-0.25) for IMF in beef and is just as fast and a simpler pulse sequence than CPMG. The method can also be applied to other meat products.

Resolution of isomeric multi-ruthenated porphyrins by travelling wave ion mobility mass spectrometry

The ability of travelling wave ion mobility mass spectrometry (TWIM-MS) to resolve cationic meta/para and cis/trans isomers of mono-, di-, tri- and tetra-ruthenated supramolecular porphyrins was investigated. All meta isomers were found to be more compact than the para isomers and therefore mixtures of all isomeric pairs could be properly resolved with baseline or close to baseline peak-to-peak resolution (Rp-p). Di-substituted cis/trans isomers were found, however, to present very similar drift times and could not be resolved. N-2 and CO2 were tested as the drift gas, and similar a but considerably better values of R-p and Rp-p were always observed for CO2. Copyright (C) 2012 John Wiley & Sons, Ltd.

Controlling chaos in wave-particle interactions

We analyze the behavior of a relativistic particle moving under the influence of a uniform magnetic field and a stationary electrostatic wave. We work with a set of pulsed waves that allows us to obtain an exact map for the system. We also use a method of control for near-integrable Hamiltonians that consists of the addition of a small and simple control term to the system. This control term creates invariant tori in phase space that prevent chaos from spreading to large regions, making the controlled dynamics more regular. We show numerically that the control term just slightly modifies the system but is able to drastically reduce chaos with a low additional cost of energy. Moreover, we discuss how the control of chaos and the consequent recovery of regular trajectories in phase space are useful to improve regular particle acceleration.