Session K32: Focused Session: Computational Nanoscience IV

Trends in the structure of neutral, cationic, and anionic Ag and Au clusters from DFT calculations

The physical and chemical properties of small coinage metal clusters, with atomic structures $nd^{10}(n+1)s^{1}$ ($n$= 3,4,5 for Cu, Ag and Au, respectively), are of great interest since, for example, can be viewed as a bridge between those existing in the ``simple'' $s$-only alkali metal and the more complicated transition metal clusters. Several theoretical calculations and indirect experimental evidence have shown that Au$_N$ clusters prefer planar structures up to sizes around $N$=13, cage-like structures for the $T_d$ Au$_{20}$ and $I_h$ Au$_{32}$ and low- symmetry (amorphous) configurations for Au$_{55}$. In this work, we present a theoretical study of the structural and electronic properties of Ag and Au neutral, cationic and anionic clusters with up to $N$ =55 atoms, using density functional theory in the generalized gradient approximation. A different behavior is found in the physical properties of Ag and Au clusters, mainly due to the stronger relativistic effects present in the Au clusters. Another trend indicates that not only the size (number of atoms) and shape (isomer geometry) of a given cluster determine its properties, but also the charge present in the system may affect the cluster behavior even in the larger sizes with N=55. A discussion of the structural motifs existing in neutral, cationic and anionic Ag and Au clusters with up to N=55 atoms and their connection with recent experimental data will be presented.   

Enantiospecific adsorption study at chiral gold nanoclusters from DFT calculations

Chiral structures have been found as the lowest energy isomers of bare and passivated gold nanclusters from density functional theory (DFT) studies. Recently, experimental evidence on the existence of chiral gold nanoclusters has been published. In this work, DFT calculations are performed to study the adsorption of chiral molecules (like cysteine) on chiral and achiral (icosahedral) Au$_{55}$ clusters. Different locations on the metal cluster surfaces are used to identify the most favorable adsorption sites of the chiral molecule. Enantioselectivity is investigated through the comparison of the binding energies of the (S) and (R)-cysteine molecule adsorbed on the different Au$_{55}$ cluster enantiomers.   

Structural and electronic properties of sodium nanoclusters

Recent advances on mass selection of sodium nanoclusters and their characterization by photoemission electron spectroscopy [1] have given useful data for a variety of clusters sizes. These data may lead to assignments of the relevant structures by comparing the measured photoelectron spectra (PES) with the electronic density of states (DOS) obtained from DFT calculations. In this work, the lowest energy structures modeled by the many-body Gupta potential, are obtained by using molecular dynamics simulations for Na$_{n}$ (n= 178, 204, 271, 298-300, 309). DFT calculations were then performed for neutral, positively- and negatively-charged Na$_{n}$ clusters. A comparison between the DOS of clusters of the same size but different charge will be presented, as well as between the available experimental PES and the theoretical obtained DOS.\newline \newline [1] H. Haberland, T. Hippler, J. Donges, O. Kostko, M. Schmidt, B. von Issendorff, Phys. Rev. Lett. 94, 035701 (2005).   

Electron-Stimulated Emission of Na Atoms from NaCl Nanocubes

Emission of neutral cations/anions from solid akali halides can in principle be provoked by donating/subtracting electrons to the surface of alkali halide crystals, but generally constitutes a very endo-energetic process. The amount of energy required is expected to decrease for atoms located in less favorable positions, such as at surface steps and kinks, where the local coordination is smaller. The most favorable case of all should be represented by corner atoms of alkali halides cubes, where ionic binding is the weakest. We carried out first principles density functional calculations and simulations of magic-size neutral and charged NaCl nanocubes, to ascertain the stability and extractibility of neutralized corner ions. We find that detachment of neutral Na atoms upon donation of an excess electron is possible for a very modest energetic cost of the order of 0.6 eV, in line with early experimental observations. The atomic and electronic structure of the resulting Na$^+$ vacancy is analyzed in detail.   

Comparison of TDLDA and GW-Bethe-Salpeter Methods for Optical Excitations in Noble Metal Clusters

We perform a comparative analysis of first principles results for optical spectra of Ag$_n$ and Au$_n$ ($n\le 8$) clusters calculated within the time-dependent local density approximation (TDLDA) and the GW-Bethe-Salpeter (GW-BSE) technique. Both the TDLDA and GW-BSE calculations are performed entirely in real space based on the higher-order finite difference {\em ab initio} pseudopotential method. In the GW-BSE method, the screened Coulomb potential is calculated from the TDLDA polarizability. The convergence of the results with respect to various computational parameters is carefully examined. In general, the TDLDA and GW-BSE methods are in reasonable agreement with respect to peak positions at low energies, while high-energy excitations and oscillator strengths can be different. The GW-BSE results are found to be in a somewhat better agreement with the available experimental data.   

{\it Ab initio} calculations for the photoelectron spectra of iron clusters

We studied negatively charged iron clusters Fe$^-_n$ ($n=3-6$). We use a real-space pseudopotential approach based on the density-functional theory within local spin density approximation. The real space approach assumes no explicit basis. Wave functions are evaluated on a uniform grid; only one parameter, the grid spacing, is used to control the convergence of the electronic properties. Charged states are easily handled in real space, in contrast to method based on supercells where electrostatic divergences require special handling. For each cluster, we determined the ground state structure. To compare with the experimental photoelectron spectra, we calculated the electron binding energies, which include the final state relaxation effects. Our results for the photoelectron spectra, are able to reproduce the main features of the experiments. Our results for the magnetic moment of each cluster agree well with the other theories.   

Size Dependence of the Static Polarizabilities and Absorption Spectra of Gold Clusters

We present results for static polarizabilities and absorption spectra for ground state structures of Au$_n$, $n=2-14, 20$, clusters calculated within static and time-dependent density functional theory. The static polarizabilities of the clusters with less than 14 atoms exhibit even-odd oscillations. The polarizabilities of Au$_{14}$ and Au$_{20}$ are noticeably lower. This change in the behavior of static polarizability is correlated with the transition from two-dimensional to three- dimensional structures at $n=14$. The $d$ electrons have a large effect on the optical spectra as they quench the oscillator strengths significantly and are heavily involved in low-energy excitations. The calculated spectra are compared with available experimental data and spectra obtained for Ag$_n$ clusters.\footnote{J. C. Idrobo, S. Ogut, and J. Jellinek, Phys. Rev. B {\bf 72}, 085445 (2005)}   

Interactions of water and oxygen molecules with gold clusters

The interaction of water and oxygen molecules with neutral gold clusters, Au$_{n}$ for n=2-8, is investigated using the first- principles calculations based on the density functional theory with generalized gradient correction. The O$_{2}$ binding energy, the intramolecular bond-length of O$_{2}$, and the excess charge on the adsorbed O$_{2}$ show even-odd alternations as a function of the number of gold atoms, correlating with the vertical electron detachment energy of Au$_ {n}$. The influence of pre-adsorbed H$_{2}$O on the O$_{2}$ adsorption is pronounced when the O$_{2}$ is adsorbed with an Au-O bond with the gold cluster and an O-H bond with pre- adsorbed H$_{2}$O, while the influence is relatively small for O$_{2}$ adsorbed with double Au-O bonds. Compared to the O$_{2} $ adsorption in the absence of H$_{2}$O, the binding energy of O$_{2}$ are increased by 0.4 eV, and the intramolecular bond- length are stretched by up to 0.03 \AA.   

Density-Functional Study of Au$_{n}$$^{-}$ ($n=16-24$): Atomic and Electronic Structures and Interaction with O$_{2}$

Anionic gold clusters with 16 to 24 atoms are studied using the Born-Oppenheimer local-spin-density molecular dynamics method. The structures of the ground-state clusters and energetically lowest-lying isomers are 3-dimensional, while the ground-state structures of smaller Au$_{n}$$^{-}$ with up to 14 atoms were reported to be planar (H\"akkinen, et al., J. Chem. Phys. 117, 6982 (2002)). The calculated vertical electron detachment energies ($vDE$) are in good agreement with the experimental results (Taylor, et al., J. Chem. Phys. 98, 3319 (1992)); $vDE$'s are smaller for even $n$'s and larger for odd $n$'s, with the exception of $n=16$. Compared to the other even-numbered clusters, Au$_{16}$$^{-}$ exhibits relatively large $vDE$, $vDE$(Au$_{16}$$^{-}$)=4.03 eV. The smallest $vDE$ is measured for $n=20$, $vDE$(Au$_{20}$$^{-} $)=2.71 eV, The adsorption of O$_{2}$ to Au$_{n}$$^{-}$ is also sensitive to the cluster size; the O$_{2}$ adsorption is relatively strong for the even-numbered clusters with the exeption of Au$_{16}$$^{-}$. The O$_{2}$ binding energy, the intramolecular bond-length of O$_{2}$, and the excess charge on O$_2$ correlate strongly with the vertical electron detachment energy of Au$_{n}$$^{-}$.   

Properties of Pt$_{37}$ and Pt$_{6}$Ru$_{31}$ Clusters on Carbon: comparison of theory and experiment

Using DFT calculations, we analyze the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on a carbon support and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metallo-organic Pt or PtRu5 complexes and annealed at 670 K in hydrogen atmosphere, the Pt and Pt-Ru based clusters were observed to form fcc(111)-stacked cuboctahedral geometry and essentially bulk-like metal-metal bond lengths, even for the 10-40 atom nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic clusters segregates to the ambient surface. We explain these observations and characterize the cluster structures with graphite as a model for the support. Our study reveals the origin of the observed bulk-like metal-metal bond lengths and bond-length disorder, and demonstrates the profound consequences that result from the cluster/support interactions and hydrogen passivation on their structural and electronic properties.   

Structure and functionality of a prospective (Pt, Ru)/MgO nanocatalyst

The atomic structure of metal nanoparticles in the prospective catalyst (Pt, Ru)/MgO was imaged by Z-contrast scanning transmission electron microscopy. On the MgO (110) surface we find regular monolayer-like particles; all atoms show uniform intensity in the image and are likely to be Pt because of the large atomic number. Density functional theory calculations have been used to determine the structure of possible particles. We find that regular monolayer-like structures can be formed, with Pt bonded to surface oxygen. The Pt particles are loosely connected to the substrate in agreement with microscopic observations. The influence of surface defects on cluster structure and mobility will be analyzed, and the implications for the catalytic activity of the (Pt, Ru)/MgO system will be discussed.   

Vibrational Dynamics and Thermodynamics of AgCu nanoparticles

We present results of a systematic study of the structure, vibrational dynamics and thermodynamics of Ag$_{n}$Cu$_{34-n }$nanoparticles including. The starting structure were generated using a structural optimization using a genetic algorithm [1]. Using the embedded atom method potentials, we have calculated the vibrational densities of states for all stoichiometries and the corresponding vibrational free energies, in the harmonic approximations. At 300K, the vibrational free energy is found to behave linearly with the increasing number of Ag atoms in the nanoparticles. The vibrational contributions to the free energy increase from 5.5{\%} for Ag$_{0}$Cu$_{34}$ to 8.3{\%} Ag$_{34}$Cu$_{0}$. Selected force constants for several nanoparticles were calculated using density functional theory (DFT) and were found to be very close to those determined using EAM potentials. [1] G. Rossi, A. Rapallo, C. Mottet, A. Fortunelli, F. Baletto and R. Ferrando Phys. Rev. Lett, \textbf{93}, 105503 (2004)   

Selective modification of the surface structure of oxide nano-particles using sub-bandgap photons

Controlled manipulation of the physical and chemical properties of nano-structured materials requires correlating their spectroscopic properties and reactivity with specific surface sites. We have demonstrated experimentally that laser excitation of MgO nano-crystalline films and nanocube samples with 4.66 eV photons desorbs neutral O and Mg atoms with hyper-thermal kinetic energies in the range of 0.1--0.4 eV. Using an ab initio embedded cluster approach we developed the mechanisms for the hyper-thermal emission of atomic species. We demonstrate that the desorption of O atoms from 3-coordinated (3C) surface sites occurs via the following steps: i). an exciton is excited at the 3C site; ii) the exciton is ionized, while the remaining electronic hole is trapped at the 3C O site converting it to an O$^{-}$ radical; iii) absorption of another 4.66 eV photon in the vicinity of the 3C site forms an excited state, which relaxes with desorption of a neutral O atom. Similar process was identified for Mg-terminated 3C site. The proposed general mechanism can be used to control atomic scale modification of insulating surfaces. [1] P.E. Trevisanutto et al., \textit{Surf. Sci}., \textbf{593}, 210 (2005); W.P. Hess et al, \textit{J. Phys. Chem. B}, \textbf{109}, 19563 (2005).