High-symmetry Clusters Research Articles

Enhancing the Electronic and Optical Properties of Germanium Nanoclusters through Alkali Metal Doping: A Case Study on Na₂

Abstract In the present study, we have examined the key insight into structural, electronic, and optical properties of Na2 doped germanium nano-cluster using density functional theory. Analysis of binding energy (BE) indicates enhanced thermodynamical stability increases with cluster size, where Ge5Na2 and Ge10Na2 exhibiting highest BE. Second order energy suggest that cluster with n = 2, 5, 6, and 9 are most stable while thpse with n = 3, 4, and 7 shows lower stability. Embedding energy (EE) shows decreases curve with cluster size indicating large clusters are more favourable for Na encapsulation. Large HOMO-LUMO gap ranges from 1.4 to 3.2eV illustrate semiconductor nature of clusters. Further investigation of hyperpolarizability suggesting non-linear optical (NLO) activity for GenNa2 (n = 1-4), while lower NLO features shown in high symmetry clusters with n = 5 and 6. These results imply that GenNa2 clusters have potential uses in optoelectronic and semiconductor devices.

High-symmetry core-shell B12@Ca20C12: A nanocluster-based hydrogen storage material

Since the discovery of C60 fullerene, scientists devoted themselves to searching novel clusters. Among them, the high-symmetry clusters, especially clusters with Ih symmetry, has drawn increasing attention due to their special properties and stabilities. Here, a stable core-shell B12@Ca20C12 with Ih symmetry, which is featured as an icosahedral Ih-B12 centered on the dodecahedron Ca20C12, has been identified using first principles calculations. Its stability has been further confirmed by vibrational frequency analysis with the highest frequency of 834.9 cm−1 and the lowest frequency of 93.7 cm−1. Besides, molecular dynamics simulations indicate that its core-shell structure cannot be destroyed at 325 K under NVT ensembles with Nosé-Hoover chain thermostat. As for the electronic structure, 64 multi-center two-electron bonds were observed throughout the entire core-shell B12@Ca20C12 to keep the cluster maintain its core-shell configuration. More importantly, the core-shell B12@Ca20C12 can storage about 77 H2 molecules by closed-shell interactions with an average adsorption energy of −0.16 eV/H2, resulting in the gravimetric hydrogen density of 12.6 wt%. Based on these results we infer that core-shell B12@Ca20C12 is a potential high-capacity hydrogen storage material and might promote positive experimental efforts in designing excellent hydrogen storage materials.

Recreating the shear band evolution in nanoscale metallic glass by mimicking the atomistic rolling deformation: a molecular dynamics study.

Rolling processes are extensively used to induce network of shear bands (SBs) in the bulk metallic glasses, which in turn enhances the overall plasticity of the specimen. However, the atomic-level understanding of shear band formation/propagation mechanism during mechanical processing is still limited. In this perspective, we have developed a molecular dynamics (MD) simulation model to recreate the rolling deformation process and investigate the SB formation in Cu-Zr metallic glass (MG) specimen. Results have shown that dense and concentrated primary SBs along with secondary branching are formed during cryo-rolling, whereas a scattered and thicker SBs are formed during hot rolling process. Meanwhile, Voronoi cluster analysis revealed that the high five-fold symmetry clusters tend to decrease, while the crystalline-like cluster increases during the hot rolling process. These findings from the study are in good agreement with previous experimental studies substantiated in literature, which shows that the model correctly predicts the shear-banding phenomenon.

Pressure-induced maximum shear strength and transition from shear banding to uniform plasticity in metallic glass

The effect of pressure on the mechanical and physical properties of metallic glasses (MGs) has been widely reported. In experiments, it remains challenging to mechanically loading the materials under extreme hydrostatic pressures and carrying out the corresponding measurements. Here, we investigate the pressure dependence of shear response of a typical brittle FeP MG by using molecular dynamics (MD) simulations. We show that the plastic deformation of this MG loaded at a pressure below 10 GPa is highly localized in the form of a dominant shear band. The plastic flow stress during shear banding is measured to increase with the applied pressure, leading to a pressure-induced strengthening behavior at pressures in the range of 0–10 GPa. As the pressure is increased to a critical pressure of approximately 15 GPa, there occurs a transition in the deformation mechanism from localized shear banding to a delocalized shear deformation mechanism governed by the uniform activation of individual shearing events in the material. At this critical pressure, both the yield strength and the average flow stress are maximized. Further increasing the pressure lowers the strength of the material. The pressure effect on the evolution of different types of atomic clusters is analyzed, revealing that the pressure-sensitivity of atomic clusters in MGs is related to their structural symmetry. Under high hydrostatic pressures, low-symmetry clusters become sluggish due to their larger pressure sensitivity. In such circumstances, high-symmetry clusters with lower pressure sensitivity are relatively more active during shear deformation and evenly distributed in the whole sample, leading to the uniform plastic deformation at high pressures. Our findings calls for further experimental investigations of the effect of pressure on mechanical properties and deformation mechanisms of MGs.

Predicting the Photoelectron Spectra of Quasi Octahedral Al6Mo- Cluster.

We have recently developed a computational methodology to separate the effects of size, composition, symmetry and fluxionality in explaining the experimental photoelectron spectra of mixed‐metal clusters. This methodology was successfully applied first in explaining the observed differences between the spectra of Al13 ‐ and Al12Ni‐ and more recently to explain the measured spectra of AlnMo‐, n=3–5,7 clusters. The combination of our approach and new synthesis techniques can be used to prepare cluster‐based materials with tunable properties. In this work we use the methodology to predict the spectrum of Al6Mo‐. This system was chosen because its neutral counterpart is a perfect octahedron and it is distorted to a D3d symmetry and was not observed in the recent experiments. This high symmetry cluster bridges the less symmetric Al5Mo‐ and Al7Mo‐structures.The measured spectra of Al5Mo‐ has well defined peaks, while that of Al7Mo‐does not. This can be explained by the fluxionality of Al7Mo‐, as at least 6 different structures lie within the range that can be reached by thermal effects. We predict that Al6Mo‐ has well defined peaks, but some broadening is expected as there are two low‐lying isomers, one of D3d and the second of D3h symmetry that are only 0.052 eV apart.

Dynamic Assembly of Small Parts in Vortex-Vortex Traps Established within a Rotating Fluid.

Stable, purely fluidic particle traps established by vortex flows induced within a rotating fluid are described. The traps can manipulate various types of small parts, dynamically assembling them into high-symmetry clusters, cages, interlocked architectures, jammed colloidal monoliths, or colloidal formations on gas bubbles. The strength and the shape of the trapping region can be controlled by the strengths of one or both vortices and/or by the system's global angular velocity. The system exhibits a range of interesting dynamical behaviors including a Hopf-bifurcation transition between equilibrium-point trapping and the so-called limit cycle in which the particles are confined to circular orbits. Theoretical considerations indicate that these vortex-vortex traps can be further miniaturized to manipulate objects with sizes down to ≈10 µm.

Magic-number gold nanoclusters with diameters from 1 to 3.5 nm: relative stability and catalytic activity for CO oxidation.

Relative stability of geometric magic-number gold nanoclusters with high point-group symmetry ((Ih), D(5h), O(h)) and size up to 3.5 nm, as well as structures obtained by global optimization using an empirical potential, is investigated using density functional theory (DFT) calculations. Among high-symmetry nanoclusters, our calculations suggest that from Au(147) to Au(923), the stability follows the order Ih > D(5h) > Oh. However, at the largest size of Au(923), the computed cohesive energy differences among high-symmetry I(h), D(5h) and O(h) isomers are less than 4 meV/atom (at PBE level of theory), suggesting the larger high-symmetry clusters are similar in stability. This conclusion supports a recent experimental demonstration of controlling morphologies of high-symmetry Au(923) clusters ( Plant, S. R.; Cao, L.; Palmer, R. E. J. Am. Chem. Soc. 2014, 136, 7559). Moreover, at and beyond the size of Au(549), the face-centered cubic-(FCC)-based structure appears to be slightly more stable than the Ih structure with comparable size, consistent with experimental observations. Also, for the Au clusters with the size below or near Au(561), reconstructed icosahedral and decahedral clusters with lower symmetry are slightly more stable than the corresponding high-symmetry isomers. Catalytic activities of both high-symmetry and reconstructed I(h)-Au(147) and both Ih-Au(309) clusters are examined. CO adsorption on Au(309) exhibits less sensitivity on the edge and vertex sites compared to Au(147), whereas the CO/O2 coadsorption is still energetically favorable on both gold nanoclusters. Computed activation barriers for CO oxidation are typically around 0.2 eV, suggesting that the gold nanoclusters of ∼ 2 nm in size are highly effective catalysts for CO oxidation.

Structure and Stability of Zn, Cd, and Hg Atom Doped Golden Fullerene (Au32)

Structures and properties of various complexes formed between the “golden fullerene”, Au32, and group IIB atoms such as Zn, Cd, and Hg have been investigated using density functional theory (DFT). Binding energy values indicate that the group IIB atoms can form stable clusters in most of the different isomeric forms of the Au32 cage. The HOMO–LUMO gap of the Au32 cage remains almost the same even after doping of Zn, Cd, and Hg atoms for high symmetry clusters, while it decreases for the low symmetry isomers. The highest stable isomer for the Hg-doped Au32 cluster is found to be associated with Ih symmetry with a large energy difference from the other low symmetry isomers, using generalized gradient approximation (GGA) type functionals. However, for the Zn and Cd encapsulated Au32 clusters, the highest stable structures are of Cs[1] and C5v symmetry, respectively, along with one low symmetry isomer for each of them, having energy very close to the respective most stable isomer. Nevertheless, depending on the energy density functional, the relative energy orderings for the various isomers are found to be modified strongly. In fact, the meta-GGA TPSS functional predicts low symmetry compact isomers to be more stable for all the metal atom doped Au32 clusters. Moreover, low symmetry compact isomers are found to be more stable with the dispersion-corrected GGA type PBE functional for the Zn- and Cd-doped cluster, in agreement with the TPSS results; however, the same dispersion correction fails to reproduce the TPSS results for the Hg-doped Au32 system. Structural data, energetic parameters, and spectral analysis point toward the possible experimental observation of group IIB atom doped golden fullerene, which in turn might help to understand the nature of interactions between the metal atom and the Au32 cage. Furthermore, experimental investigations would likely confirm the predictive ability of the different functionals used in this work.

Development and optimization of a novel genetic algorithm for identifying nanoclusters from scanning transmission electron microscopy images

Whilst technological advancements have allowed imaging at atomic resolution using scanning transmission electron microscopy (STEM), identification of nanocluster structures has proven difficult due to their low thermal stability, and often resultant low-symmetry. In this work, we look at a novel solution to this problem using a genetic algorithm (GA). GAs are search methods for the minimization of statistical problems based on natural evolution. We develop a STEM model first described by Curley et al. (2007) and, using high-symmetry cluster structures as test subjects, look at the effectiveness and efficiency of the GA at optimizing orientation parameters for a cluster when compared to a model solution. We find for a 309-atom icosahedron that a random minimizing search would prove more efficient than a GA; however, for a 309-atom decahedron the GA becomes more effective and efficient than a random search. We predict that as we continue to lower symmetry of our test cases, we will find the GA becomes even more efficient at optimizing this otherwise computationally expensive problem.

Magnetic properties of the Ag–In–rare-earth1/1 approximants

We have performed magnetic susceptibility and neutron scatteringmeasurements on polycrystalline Ag–In–RE (RE, rare-earth)1/1 approximants. In the magnetic susceptibility measurements, for most ofthe RE elements, inverse susceptibility shows linear behaviour in a widetemperature range, confirming well localized isotropic moments for theRE3 + ions. Exceptionally for the light RE elements, such as Ce and Pr, nonlinear behaviour wasobserved, possibly due to significant crystalline field splitting or valence fluctuation. ForRE = Tb, thesusceptibility measurement clearly shows a bifurcation of the field-cooled and zero-field-cooled susceptibilityat Tf = 3.7 K, suggesting a spin-glass-like freezing. On the other hand, neutron scatteringmeasurements detect significant development of short-range antiferromagnetic spincorrelations in the elastic channel, which is accompanied by a broad peak at meV in the inelastic scattering spectrum. These features have striking similarity to thosein the Zn–Mg–Tb quasicrystals, suggesting that the short-range spin freezingbehaviour is due to local high-symmetry clusters commonly seen in both systems.

Study on structure and stability of AlnC and AlnC+(n=1—8)

Comparing the results of AlC cluster, computed by 7 methods of density functional theory (DFT), with experimental data, we choose B3lyp/6-311G(d) to optimize the structures and analyse the frequencies of AlnC and AlnC+(n=1—8) clusters. All ground states and metastable states of AlnC and AlnC+ clusters are obtained. Our calculations reveal that there exists a transition from planar to spacial structures with the number of Al atoms increasing. Planar structures is mostly triangle and spacial structure is primarily triangular prism cage structure. Of the high symmetry clusters, only one of neutral and cation has a stable structure. Compared with various kinds of AlnC and AlAlnC+(n=1—8) clusters under study, Al2C and Al5C are stable.

Cu 6Sc + and Cu 5Sc: Stable, high symmetry and aromatic scandium-doped coinage metal clusters

The Cu 6Sc + cation was detected experimentally as a stable bimetallic cluster [N. Veldeman, T. Höltzl, S. Neukermans, T. Veszprémi, M.T. Nguyen, Phys. Rev. A. 76 (2007) 011201]. DFT calculations using the BP86 functional with various basis sets yield a D 3h tricapped tetrahedron shaped structure as the most stable isomer. This cluster has a S 2P 6 electron configuration with a closed electronic shell, and its electrons tend to localize around Sc thereby fully occupying electron shell orbitals. Stability, high symmetry, and the NICS show that this cluster is aromatic. The neutral Cu 5Sc cluster has the same electron configuration as Cu 6Sc +. In agreement with qualitative predictions, DFT computations show that the C 4v octahedron-shaped Cu 5Sc cluster features a closed electronic structure and an aromatic character.

Gas hydrates and tetrahedrally coordinated structures determined by constructions of algebraic geometry

Graphs of 20-vertex polyhedra-cavities in gas hydrates and particular high-symmetry clusters of tetrahedrally coordinated structures are determined as incidence graphs of Desargues subconfigurations 103, 10 3 3 , and 10 3 4 . It is shown that symmetrically possible mutual transformations of rods of gas hydrates and tetrahedrally coordinated diamond-like structures correspond to conjugate subconfigurations of these configurations. It is established that the relations obtained determine on the symmetry level the mechanisms of incorporation of guest molecules into polyhedra-cavities of gas hydrates and their escape from these cavities. The corresponding structural models of mutual transformations of gas hydrates and tetrahedrally coordinated structures are implemented by the transfer of a minimum number of hydrogen bonds.

Atom ordering in cuboctahedral Ni–Al nanoalloys

Energy calculations have been carried out on high-symmetry cuboctahedral Ni–Al nanoalloy clusters, of varying composition, with the interatomic interactions modelled by the Gupta many-body potential. Relaxations of cuboctahedral fragments cut from the bulk lattice of Ni 3Al, with 13–561 atoms, were undertaken, as were relaxations of high symmetry clusters with 55 and 147 atoms. The lowest energy isomers were found to be dominated by three factors: the tendency toward mixing due to the favourable energy of mixing, Δ mix E; the size difference between nickel and aluminium; and the higher cohesive and surface energy of nickel compared to aluminium. The latter two factors favour Al-segregation to the surface. The most stable Ni:Al composition approaches 3:1 for larger clusters.

Magic polyicosahedral core-shell clusters.

A new family of magic cluster structures is found by genetic global optimization, whose results are confirmed by density functional calculations. These clusters are Ag-Ni and Ag-Cu nanoparticles with an inner Ni or Cu core and an Ag external shell, as experimentally observed for Ag-Ni, and present a polyicosahedral character. The interplay of the core-shell chemical ordering with the polyicosahedral structural arrangement gives high-symmetry clusters of remarkable structural, thermodynamic, and electronic stability, which can have high melting points (they melt higher than pure clusters of the same size), large energy gaps, and (in the case of Ag-Ni) nonzero magnetic moments.

Small Cluster Models of the Surface Electronic Structure and Bonding Properties of Titanium Carbide, Vanadium Carbide, and Titanium Nitride

AbstractFor Abstract see ChemInform Abstract in Full Text.

Small cluster models of the surface electronic structure and bonding properties of titanium carbide, vanadium carbide, and titanium nitride.

Density functional theory (DFT) calculations on stoichiometric, high-symmetry clusters have been performed to model the (100) and (111) surface electronic structure and bonding properties of titanium carbide (TiC), vanadium carbide (VC), and titanium nitride (TiN). The interactions of ideal surface sites on these clusters with three adsorbates, carbon monoxide, ammonia, and the oxygen atom, have been pursued theoretically to compare with experimental studies. New experimental results using valence band photoemission of the interaction of O(2) with TiC and VC are presented, and comparisons to previously published experimental studies of CO and NH(3) chemistry are provided. In general, we find that the electronic structure of the bare clusters is entirely consistent with published valence band photoemission work and with straightforward molecular orbital theory. Specifically, V(9)C(9) and Ti(9)N(9) clusters used to model the nonpolar (100) surface possess nine electrons in virtually pure metal 3d orbitals, while Ti(9)C(9) has no occupation of similar orbitals. The covalent mixing of the valence bonding levels for both VC and TiC is very high, containing virtually 50% carbon and 50% metal character. As expected, the predicted mixing for the Ti(9)N(9) cluster is somewhat less. The Ti(8)C(8) and Ti(13)C(13) clusters used to model the TiC(111) surface accurately predict the presence of Ti 3d-based surface states in the region of the highest occupied levels. The bonding of the adsorbate species depends critically on the unique electronic structure features present in the three different materials. CO bonds more strongly with the V(9)C(9) and Ti(9)N(9) clusters than with Ti(9)C(9) as the added metal electron density enables an important pi-back-bonding interaction, as has been observed experimentally. NH(3) bonding with Ti(9)N(9) is predicted to be somewhat enhanced relative to VC and TiC due to greater Coulombic interactions on the nitride. Finally, the interaction with oxygen is predicted to be stronger with the carbon atom of Ti(9)C(9) and with the metal atom for both V(9)C(9) and Ti(9)N(9). In sum, these results are consistent with labeling TiC(100) as effectively having a d(0) electron configuration, while VC- and TiN(100) can be considered to be d(1) species to explain surface chemical properties.

Charge and orbital states in spinless Hubbard clusters

The charge and orbital states of the two-orbital spinless Hubbard model with U → ∞in two-dimensional small clusters are investigated. This is a simplified model forcubic manganite in the ferromagnetic phase. The modified Lanczos method isused to calculate the ground state energy and wavefunction. Cluster sizes of 8,10 and 16 are studied with different boundary conditions and electronconcentrations. We find a tendency toward the charge uniform state for all cases.The orbital state is highly correlated with the cluster symmetry. There isno strong evidence for orbital ordering at low electron concentrationsfor high-symmetry clusters. However, there is orbital ordering at highelectron concentrations. It has been found that the system develops strongcharge and orbital correlations without long-range ordering at all electronconcentrations. This demonstrates the possibility of orbital order in theferromagnetic metal phase of the manganites which is electronically induced.

Stacking faults in close-packed clusters

Ground state geometries of small hard sphere clusters were studied using two different type of contact interaction, a pair-potential and a many-atom interaction. Monte Carlo method in an FCC lattice with all possible (111) stacking faults was used to obtain the minimum energy geometries for clusters up to 59 atoms. Due to the surface energy, FCC packing is generally favoured as opposite to the HCP structure. However, in most cluster sizes the ground state obtained with the many-atom interaction has one or more stacking faults. The most symmetric geometry is usually not the ground state. Clusters with 59 and 100 atoms were studied due the possibility of a high symmetry cluster with stacking faults in all four directions. The size dependence of the total energy has similarities with that of the average moment of inertia.

Structural properties and glass transition inAlnclusters

We have studied the structural and dynamical properties of several Al n clusters by the molecular-dynamics method combined with simulated annealing. The well-fitted glue potential is used to describe the interatomic interaction. The obtained atomic structures for n513, 55, and 147 are in agreement with results from ab initio calculations. Our results have demonstrated that the disordered cluster Al 43 can be considered as a glass cluster. The obtained thermal properties of glass cluster Al 43 are clearly different from the results for highsymmetry clusters, its melting behavior has properties similar to those of a glass solid. The present studies also show that the surface melting behavior does not exist in the studied Al n clusters. @S0163-1829~98!08808-0#