A Density Functional Study of the Structural and Electronic Properties of Silicon Monoxide Clusters

Identification of Ge≡O Triple Bond in Ge6O- Cluster: Anion Photoelectron Spectroscopy and Theoretical Calculations

Unlike C≡O, which is common in coordination chemistry and organometallic chemistry, little is known about Si≡O or Ge≡O compounds. Here we report a Ge₆O^- cluster featuring a Ge≡O triple bond. The structural and chemical bonding properties of Ge₆O^-/0 are investigated using anion photoelectron spectroscopy and theoretical calculations. Two nearly degenerate isomers have been found for Ge₆O^-. The lowest-energy structure (6A) can be viewed as an O atom bonding with a tetragonal bipyramidal Ge₆. The second one (6B) can be considered as an O atom interacting with a capped trigonal bipyramidal Ge₆. Chemical bonding analyses reveal that Ge₆O^- (6A) can be viewed as a Ge≡O unit interacting with a σ antiaromatic C_2v symmetric tetragonal pyramidal Ge₅^3- moiety. Comparisons of the chemical bonding in Ge₆O^- (6A) with that in Ge₅CO^- and Ge₅MnO^- indicate the similar behavior of Ge≡O to C≡O and Mn≡O in its bonding to the Ge₅^3- and Ge₅^4- moieties.

Method for Simultaneous Prediction of Atomic Structure and Stability of Nanoclusters in a Wide Area of Compositions

We present a universal method for the large-scale prediction of the atomic structure of clusters. Our algorithm performs the joint evolutionary search for all clusters in a given area of the compositional space and takes advantage of structural similarities frequently observed in clusters of close compositions. The resulting speedup is up to 50 times compared to current methods. This enables first-principles studies of multicomponent clusters with full coverage of a wide range of compositions. As an example, we report an unprecedented first-principles global optimization of 315 Si _nO _m clusters with n ≤ 15 and m ≤ 20. The obtained map of Si-O cluster stability shows the existence of both expected (SiO₂) _n and unexpected (e.g., Si₄O₁₈) stable (magic) clusters, which can be important for a variety of applications.

Under what conditions does (SiO)N nucleation occur? A bottom-up kinetic modelling evaluation

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)_N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable.

Super-oxidation of silicon nanoclusters: magnetism and reactive oxygen species at the surface

Oxidation of silicon nanoclusters depending on the temperature and oxygen pressure is explored from first principles using the evolutionary algorithm, and structural and thermodynamic analysis. From our calculations of 90 Si_nO_m clusters we found that under normal conditions oxidation does not stop at the stoichiometric SiO₂ composition, as it does in bulk silicon, but goes further placing extra oxygen atoms on the cluster surface. These extra atoms are responsible for light emission, relevant to reactive oxygen species and many of them are magnetic. We argue that the super-oxidation effect is size-independent and discuss its relevance to nanotechnology and miscellaneous applications, including biomedical ones.

The impact of electron correlations on the energetics and stability of silicon nanoclusters

The first-principles prediction of stable nanocluster structure is often hampered by the existence of many isomer configurations with energies close to the ground state. This fact attaches additional importance to many-electron effects beyond density functional theory (DFT), because their contributions can change a subtle energy order of competitive structures. To analyze this problem, we consider, as an example, the energetics of silicon nanoclusters passivated by hydrogen Si10H2n (0 ≤ n ≤ 11), where passivation changes the structure from compact to loosely packed and branched. Our calculations performed with DFT, hybrid functionals, and Hartree-Fock methods, as well as by the GW approximation, confirm a considerable sensitivity of isomer energy ordering to many-electron effects.

Structure, stability, and dissociation of small ionic silicon oxide clusters [SiO(n)+ (n = 3, 4)]: insight from density functional and topological exploration

The structures, energies, isomerization, and decomposition pathways of small ionic silicon oxide clusters, SiO(n)(+) (n = 3, 4), on doublet and quartet energy surfaces are investigated by density functional theory. New structural isomers of these ionic clusters have been obtained with this systematic study. The energy ordering of the isomeric cluster ions on doublet spin surface is found to follow the same general trend as that of the neutral ones, while it differs on the quartet surface. Our computational results reveal the energetically most preferred decomposition pathways of the ionic clusters on both spin surfaces. To comprehend the reaction mechanism, bonding evolution theory has also been employed using atoms in molecules formalism. The possible reasons behind the structural deformation of some isomers on quartet surface have also been addressed. Our results are expected to provide important insight into the decomposition mechanism and relative stability of the SiO(n)(+) clusters on both the energy surfaces.

Charge-transfer processes in the assembly of Si(n)O(m) neutral clusters

The chemical bond formation in oxygen-rich Si(n)O(m) clusters was investigated by sampling the potential energy surface of the model systems SiO + SiO(2) → Si(2)O(3) and (SiO)(2) + SiO(2) → Si(3)O(4) along a two-dimensional reaction coordinate, by density functional theory calculations. Evidence for crossing between the weakly bound neutral-neutral (SiO)(n) + SiO(2) and the highly attractive ion-pair (SiO)(n)(+) + SiO(2)(-) surfaces was found. Analysis of frontier molecular orbitals and charge distribution showed that surface crossing involves transfer of valence electron charge from (SiO)(2) to SiO(2). The sum of the natural atomic charges over the (SiO)(n) and (SiO(2)) groups of the Si(n)O(m) cluster products, gave a net positive charge on the (SiO)(n) "core" and a net negative charge on the (SiO(2)) groups. This is interpreted as the "ion-pair memory" left on the Si(n)O(m) products by the charge-transfer mechanism and may provide a way to assess the role of charge-transfer processes in the assembly of larger Si(n)O(m) neutral clusters.