Sin Clusters Research Articles

Theoretical exploration of the effects of alkali metal atoms on the structures and electronic properties of silicon clusters

The effects of alkali metals (Li, Na, K, Rb) on the geometric configurations and electronic properties of silicon clusters Sin− (n = 3 − 16) were studied through a genetic algorithm code coupled with density functional theory (DFT) calculations. Almost all of these AMSin− clusters share the same configurations and adopt the ground-state Sin− clusters as the structural motifs with the AM atoms adsorbed on the surface without severely distorting their structures. All Li, Na, K and Rb atoms act as electron donors in all these clusters as that of Cs atoms in CsSin− clusters. These AMSin− clusters possess similar electronic properties from the results of the on-site charges, vertical detachment energies (VDEs), HOMO-LUMO gaps, average binding energies, and secondary energy differences, especially for AM=K, Rb and Cs with closed values. Sizes n = 5, 10 and 13 for all these AMSin− and pure Sin− clusters are magic numbers.

From structure to surface tension of small silicon clusters by Quantum Monte Carlo simulations

We investigate the structural, surface and electronic properties of small silicon clusters Sin (for n=2 to 15) using HF, DFT and FN-DMC calculations. We analyze the atomic configurations, surface properties and electronic structures and show that the radius and average surface area of the clusters can be modeled by a Jellium-type model. We found that the surface tension σ of the clusters decreases with increasing cluster size in the range of the clusters under investigation. An estimate of the surface tension for bulk silicon yields σbulk=0.88(3) J/m2 in agreement with recent experiments. The average bond length of the clusters shows a non-monotonic behavior. Smaller clusters exhibit a high spin state influenced by electron correlation, especially during the 2D to 3D structural transition, which occurs at n=4 to n=5. We also find that the Si4, Si10 and Si12 clusters exhibit enhanced stability due to electron correlation. Our results are consistent with the experiments on bond length, binding energy and dissociation energy.

Dielectric Behavior and Prolate Growth Patterns of Silicon Clusters SiN with N = 12-30 by Cryogenic Electric Beam Deflection.

We present a comprehensive investigation of the dielectric behavior and geometric structures of cold neutral SiN clusters of intermediate size with N = 12-30 atoms. For this, cryogenic electric beam deflection experiments were carried out for the first time for Si clusters at nozzle temperatures below 30 K. In combination with quantum chemical calculations based on density functional theory and classical trajectory simulations of the rotational dynamics in the electric field, the geometric structures of the clusters are discriminated. Clusters with N < 15 favor a single-capped square antiprism as a nucleus for cluster growth, forming compact geometries in the molecular beam. Starting with 15 atoms, a prolate-like growth is observed. The prolate structures are based on stable building blocks which reappear for numerous sizes throughout the cluster growth. Finally, the transition from prolate to quasi-spherical shapes is shown to take place around Si29/Si30 as predicted theoretically by the literature. The influence of the exchange-correlation functional on the predicted structure and dielectric properties is discussed in detail for some clusters. Relaxation of the electric-dipole moment and therefore quenching of the observed electric response due to vibrational excitation and collisions with the background gas are also considered, which explains deviations between experiment and theory.

Pathways of cluster growth: infra-red multi-photon dissociation spectroscopy of a series of Re-Si clusters, [ReSin]+, n = 3-9.

Infra-red multiple-photon dissociation spectroscopy on Xe-tagged Re/Si clusters, [ReSin]+, n = 3-9, reveals intense absorption features around 400 cm-1, along with, in some cases, additional bands in the 250-350 cm-1 window. A survey of the potential energy surface using density functional theory in conjunction with particle swarm optimisation indicates a growth pattern based on a growing network of Si atoms wrapped around the Re centre: the Sin units can be viewed as fragments of a putative 16-vertex Frank-Kasper polyhedron. The structural evolution for the [ReSin]+ series differs significantly from the iso-electronic Mn series studied previously, where the metal ion is typically bound externally to the surface of a growing 3-dimensional Sin cluster, the differences reflecting the greater accessibility of 5d vs. 3d electron density.

Scaling of the permanent electric dipole moment in isolated silicon clusters with near-spherical shape.

In silicon clusters a structural transition from prolate to near-spherical structures takes place at a size of about 25-30 atoms. While some of the prolate clusters are very polar, there has been no experimental evidence of the presence of dipole moments in larger silicon clusters with near-spherical shape. By means of electric molecular beam deflection experiments at cryogenic temperatures, it was possible to prove for the first time that SiN clusters with more than N = 30 atoms are also polar. Interestingly, the dipole moment per atom for clusters in the range between 30 and 80 or 90 atoms is almost constant and amounts to 0.02 D. This unusual behaviour manifests in effective polarizabilities increasing linearly with cluster size. The dipolar contribution to the polarizability means that SiN clusters with N = 80 atoms can be polarized more than twice as well as a correspondingly small sphere with the dielectric properties of bulk α-Si. This finding is analysed with quantum chemical calculations concerning the geometric structure as well as the charge distribution and is related to the dielectric behaviour of polar semiconductor nanocrystals.

Probing the structural and electronic properties of anionic europium-doped silicon clusters by density functional theory and comparison of experimental photoelectron spectroscopy

Density functional theory (DFT) calculations combined with “stochastic kicking” (SK) global search technique were performed to investigate the structural properties of EuSin-(1≤n≤13). It was found that the Eu atom in EuSin- preferred to occupy the surface position up to a size of 11Si atoms. However, starting fromn = 12, the endohedral structures were found, as depicted by the Eu atom inserting into the center of Si frame. The global minimum structures for each size was determined by comparing with the experimental photoelectron spectroscopy (PES). Natural population analysis (NPA) coupled with spin density isosurfaces were utilized to further investigate the electronic and magnetic properties of EuSin-. The observed magnetic moments significantly concentrates among the Eu atom, ascribed to the Eu 4f-electrons remaining largely as before encapsulating the Sin clusters. Meanwhile, the representative caged cluster EuSi12- was subjected to density of states (DOS) and HOMO-LUMO analysis, and the results showed a significant spin polarization.

Structures, stabilities and electronic properties of the bimetal V2-doped Sin (n = 1–10) clusters: a density functional investigation

Structures, stabilities and electronic properties of mixed silicon/vanadium clusters (V2Sin; n = 1–10) have been investigated systematically using the CALYPSO structural searching approach and density functional theory calculations. Results indicate two vanadium atoms tend to form V2 bond encapsulated gradually into silicon cages with the increasing number of silicon atoms. Analyses of stabilities reveal that V2Si6 has the highest stability and the doping of two vanadium atoms makes the stabilities of silicon clusters decrease. At last, charge transfer, Mayer bond order, electron localization function, IR and Raman spectrum are operative for characterizing and rationalizing the electronic properties of doped clusters.

The stabilities and geometries of Re-encapsulated Sin(n=16, 20, 24, 28, 32, 36, and 40) clusters: A computational investigation

Abstract Geometry optimization of the mixed SinRe (n=12, 16, 20, 24, 28, 32, 36, and 40) cages with doublet, quartet, and sextet spin configurations is carried out systematically at the UHF/LanL2DZ level. Equilibrium structures, total energies, and stabilities of Re@Sin cages are presented and discussed. The calculated results show that all Re@Sin cages of highest symmetry undergo slight distortion into much more stable structures of lower symmetry. The Re atom in the Re@Sin (n=12, 16, 20, 24, 28, 32, 36, and 40) cages deviates from the cage center site of Sin fullerenes. Charge-transfer between Re and Si atoms makes a contribution to the stability of the Sin fullerenes; In addition, the relative stability is discussed, the most stable geometry is assigned.

First-principles molecular dynamics study on ultrafast potassium ion transport in silicon anode

Si is widely known as a high-capacity alloying anode material for lithium-ion batteries. However, Si is electrochemically inactive in potassium-ion batteries that attract considerable interest as cost-effective alternatives to lithium-ion batteries. Herein, we report that, unlike crystalline Si showing an inert nature toward K ions, amorphous Si is available as an alloying anode material for potassium-ion batteries. Amorphous Si can store 1.1 K ions per Si atom while offering a high capacity of 1049 mA h g−1. The K ions in amorphous Si can diffuse very rapidly. Their diffusivity is almost two orders of magnitude higher than that for Na ions in amorphous Si and is even three times higher than that for Li ions in amorphous Si. The fast K ion transport in amorphous Si is ascribed to the weak electrostatic KSi attraction, fairly high carrier ion concentration, and the formation of isolated Sin (n = 3–5) clusters during the potassiation process. This work suggests that, despite alloying with large K ions, amorphous Si anodes can show high performance in both capacity and rate capability.

A quantitative tool to establish magic number clusters, ε3, applied in small silicon clusters, Si2-11.

The present work focuses on establishing a function to rank the stability of small silicon clusters to characterize their magic numbers. This function is composed by a thermodynamic descriptor, the atomization Gibbs free energy, and indirect kinetic descriptors, the highest occupied molecular orbital energy and the lowest excitation energy of each system. The silicon clusters geometries were optimized using density functional theory within a hybrid meta-GGA approximation (M06), while the electronic energy was corrected by single-point calculation using CASPT2 level of theory to obtain the molecular properties. Both methodologies were combined with polarized diffused triple zeta, 6-311++G(3df,3pd), basis set for all atoms. Some molecular properties and their combinations were considered to create the aforementioned function to represent the clusters chemical stability and their magic numbers. The chosen stability ranking function, called ε3, presents results in agreement with the previous mass spectrometry experimental data identifying 4, 6, 7 and 10 as magic numbers for small silicon clusters. We believe this stability ranking function can be useful to study other intramolecular atomic and molecular clusters. Graphical abstract Stability ranking function,ε31, applied on Sin (n = 2 - 11) clusters showing Fukui's functions for the Sin (n = 2 - 11) obtained by the electronic density difference through CASPT2//M06/6-311++G(3df,3pd) with an isosurface value equal to 0.003.

Role of NO in highly selective SiN/SiO2 and SiN/Si etching with NF3/O2 remote plasma: Experiment and simulation

This paper describes the study of mechanisms of highly selective silicon nitride etching, in particular, the role of NO in silicon nitride etching by atomic fluorine. This paper presents experimental and simulation data about SiN, SiO2, and Si etching with NF3/O2 remote plasma. Quantum chemistry simulations show that NO reacts with the F–N bond in a SiN cluster with lower activation energy than the F-atom. Thus, NO increases the rate of fluorine migration on the silicon nitride surface from the nitrogen atom to the silicon atom during the etching process. In the absence of NO, such migration proceeds with relatively high activation energy, which limits the etching rate. The analytical model based on the SiN etching mechanism taking into account the fluorine migration is proposed. The results of calculations with the analytical model show a good agreement with the experimental SiN, SiO2, and Si etch rates.

A Fukui function-guided genetic algorithm. Assessment on structural prediction of Sin (n = 12-20) clusters.

Theoretical studies are essential for the structural characterization of clusters, when it comes to rationalize their unique size-dependent properties and composition. However, the rapid growth of local minima on the potential energy surface (PES), with respect to cluster size, makes the candidate identification a challenging undertaking. In this article, we introduce a hybrid strategy to explore the PES of clusters. This proposal involves the use of a biased initial population of a genetic algorithm procedure. Each individual in this population is built by assembling small fragments, according to the best matching of the Fukui function. The performance of a genetic algorithm procedure. The performance of the method is assessed on the PES exploration of medium-sized Sin clusters (n = 12-20). The most relevant results are: (a) the method converges at almost half of the time used by the canonical version of the GA and, (b) in all the studied cases, with the exception of Si13 and Si16 , the method allowed to identify the global minimum (GM) and other important low-lying structures. Additionally, the apparent deficiency of the proposal to identify the GM was corrected when a Si atom, or other low-lying isomers, were considered to build the clusters. © 2017 Wiley Periodicals, Inc.

Revisit of Sin (n = 21–29) Clusters by Ab Initio Global Search

Using our improved genetic algorithm combined with density functional theory calculations, we perform unbiased global search for the most stable structures of Sin clusters with n = 21–29. A new principal direction crossover operation is introduced to search the prolate structures on the potential energy surface. We confirmed previous results for most sized Sin clusters and find more stable structure at n = 23 and 26. In particular, we find a new endohedral cage structure at Si23, which is 0.4 eV lower in energy than previously reported prolate structure and exhibit a distinct peak on the second order difference of total cluster energy. We discuss the competition between prolate and spherical motifs in the medium-sized Sin clusters in light of binding energy and HOMO–LUMO gap. The transition of structural motif from prolate to near spherical in Sin clusters occurs at n = 26. The outer cages are composed of pentagon, hexagon, quadrangle and heptagon, with one to three interior atoms filled inside the cage. These results establish a more complete picture for the structural evolution of the medium-sized silicon clusters. Average bond lengths and Mayer bond order are also discussed.

Study on the structures and properties of praseodymium-doped silicon clusters PrSi n (n = 3–9) and their anions with density functional schemes

The equilibrium geometries and properties such as adiabatic electron affinities (AEAs), simulated photoelectron spectra (PES), dissociation energies, relative stabilities, HOMO–LUMO gaps, charges transfer, and magnetic moments of PrSin (n = 3–9) and their anions have been made a detailed study by means of the ABCluster global search technique combined with density functional methods. The structure optimization is carried out with three exchange correlation functionals (B3LYP, PBE0, and mPW2PLYP). The ground state structures predicted by mPW2PLYP are thought to be trustworthy. The experimental PES of PrSi4− is reassigned in light of the theoretical results, and the experimental AEAs of 2.0 ± 0.1 eV are obtained. The mPW2PLYP AEAs of PrSin are in excellent agreement with the experimental values. The average absolute deviations from experiment are only 0.05 eV, and the maximum deviations are 0.10 eV. The accordance between the experimental PES and the theoretical simulations indicates that the ground state structures of PrSin− (n = 4–9) are trustworthy. Doping Pr atom to Sin (n = 3–9) clusters raises the photochemical sensitivity. A large proportion of the total magnetic moments for all of these species are contributed by Pr atom.

Si-rich W silicide films composed of W-atom-encapsulated Si clusters deposited using gas-phase reactions of WF6 with SiH4.

We formed Si-rich W silicide films composed of Sin clusters, each of which encapsulates a W atom (WSi(n) clusters with 8 < n ≤ ∼ 12), by using a gas-phase reaction between WF6 and SiH4 in a hot-wall reactor. The hydrogenated WSi(n)H(x) clusters with reduced F concentration were synthesized in a heated gas phase and subsequently deposited on a substrate heated to 350-420 °C, where they dehydrogenated and coalesced into the film. Under a gas pressure of SiH4 high enough for the WSi(n)H(x) reactant to collide a sufficient number of times with SiH4 molecules before reaching the substrate, the resulting film was composed of WSi(n) clusters with a uniform n, which was determined by the gas temperature. The formed films were amorphous semiconductors with an optical gap of ∼0.8-1.5 eV and an electrical mobility gap of ∼0.05-0.12 eV, both of which increased as n increased from 8 to 12. We attribute this dependence to the reduction of randomness in the Si network as n increased, which decreased the densities of band tail states and localized states.

Samarium doped silicon clusters SmSin (n = 3–10) and their anions: Structures, thermochemistry, electron affinities, and magnetic moments

The structures, adiabatic electron affinities (AEAs), dissociation energies (DEs), hardness, and magnetic moments of SmSin (n=3–10) and their anions were examined by means of B2PLYP, TPSSh, PBE0 and B3LYP methods. Basis sets adopted are of segmented (SEG) Gaussian valence basis sets and relativistic small-core effective potentials (ECP) with additional diffuse 2dfg functions, denoted aug-SEG/ECP for Sm atoms and aug-cc-pVTZ for Si atoms. The most stable structures can be viewed as replacing a Si of the ground-state structure of Sin+1 with a Sm atom. The theoretical AEAs are in excellent agreement with experimental data, especially the AEAs at B2PLYP and TPSSh levels. The average absolute errors from experiment are by 0.04, 0.06, 0.10 and 0.11eV at the B2PLYP, the TPSSh, the PBE0 and the B3LYP levels, respectively. The DEs of Sm from SmSin (n=3–10) and their anions were estimated to examine their relative stabilities. Natural population analysis (NPA) reveals that the Sm atom in all the neutral and anionic clusters acts as an electron donor. Hardness analysis reveals that doping Sm atom to Sin (n=3–10) clusters raises the photochemical sensitivity. Analysis of magnetic moments reveals that the total magnetic moments are contributed by Samarium atom.

Ytterbium doped silicon clusters YbSin (n = 4–10) and their anions: Structures, thermochemistry, and electron affinities

The structures, electron affinities, dissociation energies, hardness, and dipole moments of YbSin (n=4–10) and their anions were examined using B3LYP, TPSSh, PBE and wB97X methods. The lowest-energy structures can be regarded as replacing a Si of the ground-state structure of Sin+1 with a Yb atom. The theoretical adiabatic electron affinities (AEAs) of YbSin are in excellent agreement with experimental data. The average absolute errors from experiment are by 0.08, 0.07, 0.05 and 0.08eV at the B3LYP, the TPSSh, the PBE and the wB97X levels, respectively. Theoretical AEAs of 2.33±0.05eV for YbSi9 are more reliable than the experimental value of 2.60±0.05eV. The hardness analysis reveals that doping Yb atom to Sin (n=4–10) clusters raises the photochemical sensitivity. The dissociation energies of Yb atom from YbSin and their anions were calculated to examine relative stabilities.

Geometries, stabilities and electronic properties of small-sized Pd2-doped Sin (n = 1–11) clusters

The geometries, growth patterns, relative stabilities and electronic properties of small-sized Pd2Sin and Sin+2 (n = 1–11) clusters are systematically studied using the hybrid density functional theory method B3LYP. The optimised structures revealed that the lowest energy Pd2Sin clusters are not similar to those of pure Sin clusters. When n = 9, one Pd atom in Pd2Si9 completely falls into the centre of the Si outer frame, forming metal-encapsulated Si cages. On the basis of the optimised structures, the averaged binding energy, fragmentation energy, second-order energy difference and highest occupied–lowest unoccupied molecular orbital energy gap are calculated. It is found that the Pd2Si5 and Pd2Si7 clusters have stronger relative stabilities among the Pd2Sin clusters. Additionally, the stabilities of Sin+2 clusters have been reduced by the doping of Pd impurity. The natural population and natural electronic configuration analysis indicated that the Pd atoms possess negative charges for n = 1–11 and there exist the spd hybridisation in the Pd atom. Finally, the chemical hardness, chemical potential, electrostatic potential and polarisability are discussed.

First-principle study of silicon cluster doped with rhodium: Rh2Sin (n = 1–11) clusters

The geometries, stabilities and electronic properties of rhodium-doped silicon clusters Rh2Sin (n = 1–11) have been systematically studied by using density functional calculations at the B3LYP/GENECP level. The optimized results show that the lowest-energy isomers of Rh2Sin clusters favor three-dimensional structures for n = 2–11. Based on the averaged binding energy, fragmentation energy, second-order energy difference and HOMO-LUMO energy gap, the stabilities of Rh2Sin (n = 1–11) clusters have been analyzed. The calculated results suggest that the Rh2Si6 cluster has the strongest relative stability and the doping with rhodium atoms can reduce the chemical stabilities of Sin clusters. The natural population and natural electron configuration analysis indicate that there is charge transfer from the Si atoms and 5s orbital of the Rh atoms to the 4d and 5p orbitals of Rh atoms. The analysis of electron localization function reveal that the Si–Si bonds are mainly covalent bonds and the Si–Rh bonds are almost ionic bonds. Moreover, the vertical ionization potential, vertical electron affinity, chemical hardness, chemical potential, vibrational spectrum and polarizability are also discussed.

Theoretical investigation on the geometries and electronic properties of cesium–silicon CsSi n (n = 2–12) clusters

The structures, relative stabilities, and electronic properties of pure Sin and Cs-doped silicon clusters (n = 2–12) are systematically investigated using the density functional theory at the B3LYP level. The optimized structures indicated that the lowest-energy structures of CsSin are similar to those of pure Sin clusters and prefer the 3-dimensional configuration for n = 3–12. The relative stabilities of CsSin clusters are analyzed based on the averaged binding energy, fragmentation energy, second-order energy difference, and HOMO–LUMO energy gap. It is found that CsSi6 and CsSi9 are the magic clusters, and the doping of Cs atom reduces the chemical stabilities of Sin frame. The Mulliken population analysis pointed out that the charges in the corresponding CsSin clusters always transfer from Cs atom to Sin host in the range of 0.80–0.91 electron. In addition, the partial density of states, infrared, and Raman spectra is discussed.