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Si nanostructures (Si-NSs) epitaxially grown or adsorbed on Si (111) surface, with various shapes including pit-like, bars, islands, hill-like, diamond-like and double cage, were studied theoretically using density-functional theory (DFT) calculations. The electronic and optical properties of these Si-NSs were calculated, showing that the designed Si-NSs modifications can enhance the optical absorption for Si surface.

Iron hydrides can be formed under high pressure and have special physical and chemical properties. Structures and stabilities of FeXSeHY (X = 1–2, Y = 1–6) hydrides under high pressure were studied using a genetic algorithm method combined with DFT calculations. Further, using triangular phase diagrams, we found that the C2/m-FeSeH is most stable among FeSeH1–6 at 150 GPa. Among Fe2SeH1–6, the enthalpy values of Amm2- and I4/mmm-Fe2SeH2 phases are the lowest at 150 and 200 GPa, respectively, and the Immm-Fe2SeH phase is the lowest in enthalpy at 250–300 GPa. The predicted superconducting transition temperatures (Tc) show that C2/m-FeSeH and Amm2-Fe2SeH2 have almost no superconductivity, while the predicted Tc values of I4/mmm-Fe2SeH2 and Immm-Fe2SeH at 150 GPa are 8.6 and 1.1 K, respectively, and that of Pm-FeSeH6 at 150 GPa is 34.4 K.

Stable structures of uranium hydrides at 0–550 GPa were studied using genetic algorithm (GA) combined with density functional theory calculations. We investigated two stoichiometries of U–H system, UH4 and UH10 at high pressure, and properties of UH4, UH8 and UH10. The study found that UH8 is the most stable phase at 100–550 GPa, UH4 is a stable phase at 100–550 GPa, and UH10 could become stable over 450 GPa. For UH10, the most phase is F m 3 ‾ m , which was found to be a superconductor with the transition temperature T c = 51 , 21 , 12 , 10 and 15 K at 100, 200, 300, 400 and 550 GPa, respectively.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

Using genetic algorithm (GA) method combined with first-principles calculations, the structures, dynamical and thermodynamic stabilities of GaH 5 were studied. The calculated results suggested that at the pressure range 150–400 GPa, the P2 1 /m phase of GaH 5 is the most favorable phase and dynamically stable, but thermodynamically it is unstable and can decompose into GaH 3 and H 2 . The superconducting property of GaH 5 was further calculated, and the predicted superconducting transformation temperature T c of GaH 5 P2 1 /m phase is about 35.63 K at 250 GPa. Besides, we compared the GaH 5 and GaH 3 superconducting properties, and found that GaH 3 -Pm-3n structure has a larger DOS near Fermi level than GaH 5 -P2 1 /m structure, which may be the main reason causing higher T c of GaH 3 than GaH 5 .

The optical absorption spectra of Si2–Si33 clusters were systematically studied by a time-dependent density functional theory approach. The calculations revealed that the absorption spectrum becomes significantly broad with increasing cluster size, stretching from ultraviolet to the infrared region. The absorption spectra are closely related to the structural motifs. With increasing cluster size, the absorption intensity of cage structures gradually increases, but the absorption curves of the prolate and the Y-shaped structures are very sensitive to cluster size. If the transition energy reaches ∼12 eV, it is noted that all the clusters have remarkable absorption in deep ultraviolet region of 100–200 nm, and the maximum absorption intensity is ∼100 times that in the visible region. Further, the optical responses to doping in the Si clusters were studied.

The structures and optical properties of silicon nanoclusters (Si NCs) have attracted continuous interest in the last few decades. However, it is a great challenge to determine the structures of Si NCs for accurate property calculation due to the complication and competition of various structural motifs. In this work, a Si172 NC with a size of about 1.8 nm was investigated using a genetic algorithm combined with tight-binding and DFT calculations. We found that a diamond crystalline core with 50 atoms (1.2 nm) was formed in the Si172 NC. It can be expected that at a size of about 172 atoms, a diamond crystalline structure can nucleate from the center of the Si NCs. The optical properties of the pure and hydrogenated Si172 NC structures also have been studied using the TDDFT method. Compared with the pure Si172 NC, the absorption peaks of the hydrogenated Si172 NC are obviously blue-shifted.

The optical absorption spectra of Si

Modified Si(111) surface with designed nanostructural modifications including grown pits, nanobars and nanoislands as well as deposited hill-, diamond- and cage-like nanoclusters were studied using density-functional theory (DFT) calculations. The thermal stabilities, electronic structures and optical properties of these various nanostructural modifications of the Si(111) surface were calculated and discussed. The results indicate that the optical absorption of the modified Si(111) surface can be enhanced by these surface modifications especially when depositing diamond-like nanoclusters on the surface.

A large number of the isomers of neutral and ionic Sn n (n=2–15) clusters have been designed and optimized at the level of MP2/LANL2DZ with the aid of the Gaussian 09 software package. The most stable geometric structures of ionic(cationic and anionic) Sn clusters have been compared with the corresponding neutral geometries. Most structures of Sn n + and Sn n are similar to each other except for the structures of those with n=5, 11 and 15; while the anions Sn n − are almost different from Sn n clusters with n=9–15 except for Sn12 − and Sn14 −. The electronic properties have been calculated, including binding energy, second difference in energy, HOMO-LUMO gaps, ionization potentials, and electron affinities.

Snn (n = 34–44) clusters were studied using a combined method of the genetic algorithm/tight-binding search and the DFT–PBEPBE calculations. Snn (n = 34–44) clusters are shown to favor an assembly motif composed of small stable Sn9 and Sn10 subunits which are connected by cohesive linkages of smaller clusters (Sn1–5). Snn (n = 34–44) clusters were found to transform from a prolate pattern for Sn34,35 to a ‘Y’ pattern for Sn36–39 to a plate motif for Sn40–44. Obvious structural transitions would easily occur at the cross points of binding energy curves when isomers become competitive.

First-principles calculations were performed to study the structure and stability of the Al 46 cluster. The results suggest two competing structural motifs, the spherical endohedral cage and layer-stacking structures. It is shown that although medium-sized Al clusters tend to form layered structures (i.e., fcc fragments with a stacking fault), a spherical endohedral cage configuration is also an important structural motif for the magic Al 46 cluster. Besides, the cluster ions were also studied, and the calculated PES of the Al 46 - isomer with the minimum of free energy is in better agreement with the experimental PES data.

A genetic algorithm (GA) coupled with a tight-binding (TB) interatomic potential was used to search for the low-energy structures of the medium-sized Nin (n = 20−30) clusters. The low-energy candidate structures from the GA/TB search were further optimized by using the density functional theory calculations with the Perdew, Burke, and Ernzerhof exchange-correlation energy functional. The obtained lowest-energy structures of the medium-sized Nin (n = 20−30) clusters are shown to exhibit double icosahedron-based motif. The properties of the nickel clusters including binding energies, second differences in energy, and especially magnetic properties have also been studied. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

The carbon-rich structures of C9Sin (n = 1–5) clusters were studied by first-principles density functional calculations using the B3LYP hybrid exchange–correlation energy functional and 6-311++G(2df) basis set. By systematic investigation of the structures and energies, we found that in the structures of the carbon-rich clusters C9Sin (n = 1–5), the C atoms were found to form linear (n = 2), or single-ring (n = 1 and 3) or double-rings (n = 4 and 5) while the Si atoms prefer to attach to the carbon rings in the form of C2Si units. Based on the lowest-energy structures obtained from our calculations, the properties of the clusters such as binding energy, second difference in energy, HOMO–LUMO gap, adiabatic ionization potential (AIP), adiabatic electron affinity (AEA), vibrational frequency, bond orders and NBO charge transfer have been calculated and analyzed.

We have performed global structural optimizations for neutral lead clusters Pb(n) (n = 2-20) by using a genetic algorithm (GA) coupled with a tight-binding (TB) potential. The low-energy structures identified from a GA/TB search were further optimized at the DFT-PBE level. The calculated results show that the Pb(n) (14n/= 20) clusters favor compact spherical structures with hexagon and pentagon rings. These structures are different from those of Si(n), Ge(n), and Sn(n) clusters which favor prolates in the same size range. The binding energies, second differences in energy, and fragmentation behaviors of the Pb(n) clusters were also discussed. Pb(n) (n = 4, 7, 10, 13, 15, and 17) clusters are found to be special stable clusters, which is in good agreement with the experimental results.

The lowest-energy structures of Ge{sub 2}-Ge{sub 33} have been optimized using all-electron DFT method. The properties of the germanium clusters including binding energies, second order differences in energy, HOMO-LOMO gaps, and especially fragmentation energies and fragmentation behavior have been studied. Our calculation shows that the fragmentation products of Ge{sub 6}, Ge{sub 7}, and Ge{sub 10} clusters are abundant and appear frequently in fragmentation processes, which is consistent with the experimental observations.

The structures and optical properties of silicon nanoclusters (Si NCs) have attracted continuous interest in the last few decades. However, it is a great challenge to determine the structures of Si NCs for accurate property calculation due to the complication and competition of various structural motifs. In this work, a Si172 NC with a size of about 1.8 nm was investigated using a genetic algorithm combined with tight-binding and DFT calculations. We found that a diamond crystalline core with 50 atoms (1.2 nm) was formed in the Si172 NC. It can be expected that at a size of about 172 atoms, a diamond crystalline structure can nucleate from the center of the Si NCs. The optical properties of the pure and hydrogenated Si172 NC structures also have been studied using the TDDFT method. Compared with the pure Si172 NC, the absorption peaks of the hydrogenated Si172 NC are obviously blue-shifted.

The title aza-Wittig reaction has been studied theoretically by means of ab initio calculations. Geometries were optimized at the HF/6-31G ∗∗ level, and single-point energy calculations were performed at the MP2/6-31G ∗∗ level using the HF/6-31G ∗∗ structures. Calculated results show that two transition states exist on the aza-Wittig reaction surface and that both the first and second forward barriers decrease in the order Cl>H>CH 3 . It can be seen that the substituent effect makes the aza-Wittig reaction for X=CH 3 and H more favorable than for X=Cl.

The structures of Ge(n) (n=40-44) clusters were searched by genetic algorithm combined with a tight-binding method. First-principles calculations based on density functional theory were performed to further optimize the isomer structures. The calculated results show that Ge(n) (n=40-44) clusters favor platelike structures, consisted of four small magic clusters (Ge(9) or Ge(10)), and a Ge(4) core. The Ge(4) core along with the parts of the four linked small clusters forms a diamond segment. The cluster mobilities of the most stable structures are in good agreement with the experimental data.

Neutral aluminum clusters Al(n) (n=31-40) were studied using a genetic algorithm (GA)/tight-binding (TB) search combined with DFT-PBE calculations. It is found that the medium-sized aluminum clusters Al(31) to Al(40) exhibit a bulklike stacking pattern. Anion clusters were also studied.

The lowest-energy structures of neutral and cationic GenM (n = 9, 10; M = Si, Li, Mg, Al, Fe, Mn, Pb, Au, Ag, Yb, Pm and Dy) clusters were studied by genetic algorithm (GA) and first-principles calculations. The calculation results show that doping of the metal atoms and Si into Ge9 and Ge10 clusters is energetically favorable. Most of the metal-doped Ge cluster structures can be viewed as adding or substituting metal atom on the surface of the corresponding ground-state Gen clusters. However, the neutral and cationic FeGe9,10,MnGe9,10 and Ge10Al are cage-like with the metal atom encapsulated inside. Such cage-like transition metal doped Gen clusters are shown to have higher adsorption energy and thermal stability. Our calculation results suggest that Ge9,10Fe and Ge9Si would be used as building blocks in cluster-assembled nanomaterials because of their high stabilities.

Binary C(7)Si(n) (n = 1-7) clusters are studied using density functional calculations at the level of B3LYP/6-311G(d). Lowest-energy structures have been determined theoretically and their properties such as binding energies, second differences in energy and highest-occupied and lowest-unoccupied molecular orbital gaps have been analyzed. It is found that the lowest-energy structures of the C(7)Si(n) (n = 1-7) clusters change from linear to planar when n ≥ 3, and in the planar structures C atoms prefer to form five- and six-membered rings surrounded by extra Si atoms in the form of the C(2)Si units.

The structures of Ge(n) (n=34-39) clusters were searched by a genetic algorithm using a tight-binding interatomic potential. First-principles calculations based on density functional theory were performed to further identify the lowest-energy structures. The calculated results show that Ge(n) (n=34-39) clusters favor prolate or Y-shaped three-arm structures consisting of two or three small stable clusters (Ge(6), Ge(7), Ge(9), or Ge(10)) linked by a Ge(6) or Ge(9) bulk unit. The calculated results suggest the transition point from prolate to Y-shaped three-arm structures appears at Ge(35) or Ge(36).

The structures of Si(n) (n = 2-33) were confirmed by genetic algorithm (GA)/tight binding (TB) search and ab initio calculations at the B3LYP/6- 311++G(2d) and PW91/6-311++G(2d) level, respectively. The fragmentation energies, binding energies, second differences in energy, and highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps in the size range 2≤n≤33 were calculated and analyzed systematically. We extended the cluster size involved in the fragmentation analyses up to Si(33), and studied the multi-step fragmentations of Si(n). The calculated result is similar to the fragmentation behavior of small silicon clusters studied previously, showing that Si(6), Si(7), and Si(10) have relatively larger stabilities and appear more frequently in the fragmentation products of large silicon clusters, which is in good agreement with the experimental observations.