Cluster Symmetry Research Articles

Light-Assisted, Templated Self-Assembly Using a Photonic-Crystal Slab

We experimentally demonstrate the technique of light-assisted, templated self-assembly (LATS). We excite a guided-resonance mode of a photonic-crystal slab with 1.55 μm laser light to create an array of optical traps. We demonstrate assembly of a square lattice of 520 nm diameter polystyrene particles spaced by 860 nm. Our results demonstrate how LATS can be used to fabricate reconfigurable structures with symmetries different from traditional colloidal self-assembly, which is limited by free energetic constraints.

In-Medium phenomena in Low Density Nuclear Matter

In-medium phenomena of low density nuclear matter are studied using collisions of 47 A MeV 40 Ar and 64 Zn projectiles with 112Sn and 124Sn target nuclei. Low density equations of state are tested by confronting various models with the data. In-medium cluster binding energies and symmetry density are also studied at low densities. We find that models that account for in-medium effects in general describe the data better than models that do not.

Effects of B upon glass forming ability of Al87Y8Ni5 amorphous alloy

In this study, 15at.% of boron is added to increase the thermal stability and amorphous forming ability of Al87Y8Ni5 alloy ribbons by single roller melt-spinning process. Thermal properties including crystallization activation energy and the Avrami exponent of crystallization are investigated using non-isothermal and isothermal analyses. Only the (Al87Y8Ni5)85B15 amorphous alloy ribbon demonstrates a glass transition temperature (Tg) at 529K, and its ΔTx (=Tx−Tg) value is 24.6K. Crystallization kinetic study show that the 15at.% of boron increases the activation energy for crystallization from 159 to 228kJ/mol. The Avrami exponent n value of Al87Y8Ni5 amorphous alloy is 1.5∼2.1 indicating a decreasing nucleation rate with crystallization time, whereas the n value of (Al87Y8Ni5)85B15 amorphous alloy ribbon is 2.3∼3.1 or the nucleation rate increases with time. The addition of boron could affect the crystal symmetry in atomic clusters and thus the phase separation behavior in the amorphous alloy. Boron is shown to delay the nucleation of boron-containing Al nano-crystals in crystallization. The maximum hardness is obtained for both non-boron and boron added alloys after crystallization at 592–596K region due to formation of nano-crystallites. The highest hardness achieved is 575 and 595Hv for Al87Y8Ni5 and (Al87Y8Ni5)85B15 alloys, respectively. Both the Al nano-crystals and solute-rich amorphous phase formed by phase separation contribute to the hardness increase.

Characterization of [4Fe-4S] Cluster Vibrations and Structure in Nitrogenase Fe Protein at Three Oxidation Levels via Combined NRVS, EXAFS, and DFT Analyses

Azotobacter vinelandii nitrogenase Fe protein (Av2) provides a rare opportunity to investigate a [4Fe-4S] cluster at three oxidation levels in the same protein environment. Here, we report the structural and vibrational changes of this cluster upon reduction using a combination of NRVS and EXAFS spectroscopies and DFT calculations. Key to this work is the synergy between these three techniques as each generates highly complementary information and their analytical methodologies are interdependent. Importantly, the spectroscopic samples contained no glassing agents. NRVS and DFT reveal a systematic 10-30 cm(-1) decrease in Fe-S stretching frequencies with each added electron. The "oxidized" [4Fe-4S](2+) state spectrum is consistent with and extends previous resonance Raman spectra. For the "reduced" [4Fe-4S](1+) state in Fe protein, and for any "all-ferrous" [4Fe-4S](0) cluster, these NRVS spectra are the first available vibrational data. NRVS simulations also allow estimation of the vibrational disorder for Fe-S and Fe-Fe distances, constraining the EXAFS analysis and allowing structural disorder to be estimated. For oxidized Av2, EXAFS and DFT indicate nearly equal Fe-Fe distances, while addition of one electron decreases the cluster symmetry. However, addition of the second electron to form the all-ferrous state induces significant structural change. EXAFS data recorded to k = 21 Å(-1) indicates a 1:1 ratio of Fe-Fe interactions at 2.56 Å and 2.75 Å, a result consistent with DFT. Broken symmetry (BS) DFT rationalizes the interplay between redox state and the Fe-S and Fe-Fe distances as predominantly spin-dependent behavior inherent to the [4Fe-4S] cluster and perturbed by the Av2 protein environment.

Nanometer size 3d–4d and 3d–5d substitutional clusters: Promising candidates for magnetic storageapplications

Spin-polarized density-functional calculations including spin–orbit coupling (SOC) have been performed for FemRhn and FemPtn clusters having N=m+n,N≤19 atoms. The spin magnetic moments, orbital magnetic moments, and the magnetic anisotropy energies have been determined. A significant enhancement of magnetic anisotropy energies is found by the substitutional nanoalloying of Fe with Rh and Pt atoms. We obtained a remarkable non-monotonous dependence of the MAE as a function of Fe content, i.e., upon going from pure Fe to pure Rh and Pt. The substitutional nanoalloying boost the magnetic anisotropy energies by creating significant cluster symmetry lowerings.

Optically isotropic responses induced by discrete rotational symmetry of nanoparticle clusters

Fostered by the recent progress of the fields of plasmonics and metamaterials, the seminal topic of light scattering by clusters of nanoparticles is attracting enormous renewed interest gaining more attention than ever before. Related studies have not only found various new applications in different branches of physics and chemistry, but also spread rapidly into other fields such as biology and medicine. Despite the significant achievements, there still exists unsolved but vitally important challenges of how to obtain robust polarisation-invariant responses of different types of scattering systems. In this paper, we demonstrate polarisation-independent responses of any scattering system with a rotational symmetry with respect to an axis parallel to the propagation direction of the incident wave. We demonstrate that the optical responses such as extinction, scattering, and absorption, can be made independent of the polarisation of the incident wave for all wavelengths. Such polarisation-independent responses are proven to be a robust and generic feature that is purely due to the rotational symmetry of the whole structure. We anticipate our finding will play a significant role in various applications involving light scattering such as sensing, nanoantennas, optical switches, and photovoltaic devices.

Theoretical study of neutral and charged Sc n≤2–(benzene) m≤3 clusters

Interactions of benzene molecules with scandium atoms, Scn≤2–(C6H6)m≤3, in the gas phase were studied by means of density functional theory. All-electron calculations were performed using the B3LYP hybrid functional in concert with 6-311+G(d,p) orbital basis sets for the Sc, C, and H atoms. Multiple-decker sandwich (MDS) structures are identified as the ground states for Scn≤2–(C6H6)m≤3, where the ligands are attached to the metal through Sc–C bonding, formed between the 3d electrons and the π-clouds of the benzene rings. Significant distortion is produced on the absorbed benzene molecules by the metal–ligand bonding. Rice ball structures also appeared, but they were found at higher energies, in such a way that essentially MDS isomers may emerge in the molecular beams. Even the low number of valence electrons (3d24s1) of the Sc atom; sextuple coordinations are formed, but they show different Sc–C bond lengths, diminishing the symmetry of neutral and charged clusters. The estimated ionization energies, in near agreement with experimental data, and electron affinities, suggest delocalization of the valence electrons through the network of 3d–π bonds of Sc1,2–(C6H6)m≤3. The binding energies decrease with the absorption of more benzene molecules, and in some cases increase as more metal atoms are added to the cluster.

Nucleation in Short-Range Attractive Colloids: Ordering and Symmetry of Clusters

Results from extensive Brownian dynamics simulations are presented for nucleation in a system of colloidal particles interacting via a short-range attractive potential. Our analysis shows that, even though the system is not in equilibrium, structures of small size clusters compare well with the theoretically predicted and experimentally observed ground state structures for short-range colloidal systems. In addition, the distribution of the symmetric structures in nucleation is comparable to the distribution seen in equilibrium. We also investigate how the shape and structure of fluctuating clusters in the prenucleation regime affect the formation of a stable nucleating cluster.

Search for global minimum geometries of medium sized CdnTen clusters (n = 15, 16, 20, 24 and 28)

Following our recent work which revealed the lowest-energy structures of CdnTen (n=1–14) clusters follow the hollow cage and the endohedral cage structural growth patterns, we extend the search for the most stable structures to some larger clusters, i.e., CdnTen (n=15, 16, 20, 24 and 28). Within the size range studied, the endohedral cages are more stable than the hollow cages. The endohedral atoms increase as the cluster size increases. The computed dipole moments and polarizabilities show a clear dependence on the cluster geometry and symmetry. The hollow cage isomers possess smaller dipole moments and larger polarizabilities than the endohedral ones.

Super‐atom properties of 13 atom clusters of group 13 elements

AbstractWe report first principles calculations of the geometry and electronic structure of 13 atom clusters of boron, aluminium, gallium and indium. These density functional theory calculations support the jellium model in the energy levels and molecular orbitals of the cluster and enable us to discuss the relevance of the superatom concept. We go on to examine a number of cluster symmetries in detail as a function of charge and comment on the successes and limitations of the jellium and superatom models in describing these clusters. In particular we find that the monovalent anionic cluster is the most stable and has the most symmetric structure. As charge changes the symmetry of the clusters decreases in a way that is dependent on symmetry and charge, but not atomic species.

EPR and 57Fe ENDOR investigation of 2Fe ferredoxins from Aquifex aeolicus

We have employed EPR and a set of recently developed electron nuclear double resonance (ENDOR) spectroscopies to characterize a suite of [2Fe-2S] ferredoxin clusters from Aquifex aeolicus (Aae Fd1, Fd4, and Fd5). Antiferromagnetic coupling between the Fe(II), S = 2, and Fe(III), S = 5/2, sites of the [2Fe-2S](+) cluster in these proteins creates an S = 1/2 ground state. A complete discussion of the spin-Hamiltonian contributions to g includes new symmetry arguments along with references to related FeS model compounds and their symmetry and EPR properties. Complete (57)Fe hyperfine coupling (hfc) tensors for each iron, with respective orientations relative to g, have been determined by the use of "stochastic" continuous wave and/or "random hopped" pulsed ENDOR, with the relative utility of the two approaches being emphasized. The reported hyperfine tensors include absolute signs determined by a modified pulsed ENDOR saturation and recovery (PESTRE) technique, RD-PESTRE-a post-processing protocol of the "raw data" that comprises an ENDOR spectrum. The (57)Fe hyperfine tensor components found by ENDOR are nicely consistent with those previously found by Mössbauer spectroscopy, while accurate tensor orientations are unique to the ENDOR approach. These measurements demonstrate the capabilities of the newly developed methods. The high-precision hfc tensors serve as a benchmark for this class of FeS proteins, while the variation in the (57)Fe hfc tensors as a function of symmetry in these small FeS clusters provides a reference for higher-nuclearity FeS clusters, such as those found in nitrogenase.

Tuning the electronic and optical properties of hydrogen-terminated Si nanocluster by uniaxial compression

The structural, electronic, and optical properties of hydrogen-terminated Si nanocluster (Si66H64) with a diameter of 1.3 nm under uniaxial compression have been investigated by means of density functional theory calculations. The structural deformation of silicon nanoparticle under axial strain manifests as reduction of cluster symmetry, contraction of bond length, and broadening of bond angle distribution. Such strain-induced distortion modifies the highest occupied molecular orbital (HOMO) the lowest unoccupied molecular orbital (LUMO) eigenvalues, HOMO–LUMO gap, and isosurfaces of HOMO and LUMO wavefunctions, that is, the HOMO–LUMO gap diminishes as strain increases and isosurface of HOMO and LUMO wavefunctions redistributes along the strain orientation. Moreover, uniaxial compression has a strong influence on the optical absorption spectra of the Si66H64 cluster. With increasing strain, the onset of absorption spectra red shifts. Interestingly, the strain-tunable photoluminescence in Si nanoparticle (Si66H64) can cover a broad spectrum (i.e., from visible light to ultraviolet), implying an exciting possibility for optical devices.

Single Domain Metallothioneins: Supermetalation of Human MT 1a

Metallothioneins are a family of small, cysteine rich proteins that have been implicated in a range of roles including toxic metal detoxification, protection against oxidative stress, and as metallochaperones involved in the homeostasis of both essential zinc and copper. We report that human metallothionein 1a, well-known to coordinate 7 Zn(2+) or Cd(2+) ions with 20 cysteinyl thiols, will bind 8 structurally significant Cd(2+) ions, leading to the formation of the supermetalated Cd(8)-βα-rhMT 1a species, for which the structure is a novel single domain. ESI-mass spectrometry was used to determine the exact metalation status of the βα-rhMT. The derivative-shaped CD envelope of Cd(7)-βα-rhMT [peak extrema (+) 260 and (-) 239 nm] changed drastically upon formation of the Cd(8)-βα-rhMT with the appearance of a sharp monophasic CD band centered on 252 nm, a feature indicative of the loss of cluster symmetry. The structural significance of the eighth Cd(2+) ion was determined from a combination of direct and indirect (113)Cd nuclear magnetic resonance (NMR) spectra. In the case of Cd(8)-βα-rhMT, only four peaks were observed in the direct (113)Cd NMR spectrum. Significantly, while both of the isolated domains can be supermetalated forming Cd(4)-β-rhMT and Cd(5)-α-rhMT, Cd(8)-βα-rhMT and not Cd(9)-βα-rhMT was observed following addition of excess Cd(2+). We propose that both domains act in concert to coordinate the eighth Cd(2+) atom, and furthermore that this interaction results in a coalescence of the two domains leading to collapse of the two-domain structure. This is the first report of a possible single-"superdomain" metallothionein structure for Zn(2+) and Cd(2+) binding mammalian proteins. A computational model of a possible single-domain structure of Cd(8)-βα-rhMT is described.

Automated Dot Mapping: How to Dot the Dot Map

Dot mapping is a cartographic representation method to visualize quantitative absolute values, e.g. population figures, and spatial distribution patterns. In the course of the development of computer-assisted cartography many cartographic methods have already been implemented in computer algorithms, but for dot mapping only a few solutions with some considerable constraints have been found until now. The most important constraint is the lack of repeatable results because these solutions work with random dot placement. This may result in different maps, where the only real difference is the date of production, not the data that is mapped. Another constraint is the overlap of dots which may interfere with the user's intention to count the dots. Although, this last constraint may not be considered important by all, the first constraint is clearly problematic for the cartographer. This paper deals with the automated production of dot distribution maps which will not suffer from the above named constraints. The paper focuses on the placement of dots. It presents a method of placing dots in a centred manner as dot clusters grouping around one central point, e.g. the centroid of an enumeration area. The geometric basis is a spiral whirl. Besides the regular basis, the dot clusters should not be perceived as strict geometric figures and a pseudo-random element is therefore introduced to blur the symmetry of the dot clusters. Although, this blurring is apparently random, it is repeatable. The new approach is a first step to produce dot maps automatically by providing a mathematical structure to connect the area needed for placing the dots with the number and size of the dots. The method is meant to support cartographic laypersons as well as professional cartographers in the process of map design.

Efficient parallel algorithm for pixel classification in remote sensing imagery

An important approach for image classification is the clustering of pixels in the spectral domain. Fast detection of different land cover regions or clusters of arbitrarily varying shapes and sizes in satellite images presents a challenging task. In this article, an efficient scalable parallel clustering technique of multi-spectral remote sensing imagery using a recently developed point symmetry-based distance norm is proposed. The proposed distributed computing time efficient point symmetry based K-Means technique is able to correctly identify presence of overlapping clusters of any arbitrary shape and size, whether they are intra-symmetrical or inter-symmetrical in nature. A Kd-tree based approximate nearest neighbor searching technique is used as a speedup strategy for computing the point symmetry based distance. Superiority of this new parallel implementation with the novel two-phase speedup strategy over existing parallel K-Means clustering algorithm, is demonstrated both quantitatively and in computing time, on two SPOT and Indian Remote Sensing satellite images, as even K-Means algorithm fails to detect the symmetry in clusters. Different land cover regions, classified by the algorithms for both images, are also compared with the available ground truth information. The statistical analysis is also performed to establish its significance to classify both satellite images and numeric remote sensing data sets, described in terms of feature vectors.

Strong isotopic effect in the electron-mediated nuclear-nuclear interaction in solids

We studied the nuclear-nuclear spin interaction mediated by an unpaired electron spin, focusing on an isotopic effect by electron nuclear double resonance (ENDOR) spectroscopy. We investigated a linear cluster Ga-Ti3+-Ga in titanium-doped gallium oxyde β − Ga2O3, whereby the unpaired electron spin density of Ti3+ is equally delocalized on the nuclear spins I = 3/2 of nearest-neighboring 69Ga and 71Ga nuclei. The linear geometry of the spin arrangement allowed us to easily identify the ENDOR spectra for the three possible isotopic configurations: 69Ga-Ti-71Ga 69Ga-Ti-69Ga and 71Ga-Ti-71Ga. Despite the magnetic moments of 71Ga and 69Ga nuclei differing by only by 27%, the experimental effect of the electron-mediated nuclear-nuclear interaction (pseudodipolar interaction) on the ENDOR spectra is one order of magnitude larger ( 1 MHz) for the symmetrical clusters (69Ga-Ti-69Ga and 71Ga-Ti-71Ga, respectively) than for the asymmetrical cluster 69Ga-Ti-71Ga (<0.1 MHz). This important isotopic effect in the internuclear interaction is a consequence of the cluster symmetry with a local inversion center for the symmetrical configurations, which is lacking in the asymmetrical configuration. These symmetrical clusters thus combine a resolved nuclear-nuclear spin interaction, a nuclear spin monitoring by an unpaired electron, and a large nuclear spin quantum register, which make them attractive for quantum information processing whereby nuclear qubits can be monitored by short selective radiofrequency pulses.

Low‐Symmetry Uranyl Pyrophosphate Cage Clusters

Is cluster symmetry so important? The combination of uranyl, peroxide, and pyrophosphate in aqueous solution results in the self-assembly of highly complex clusters (see figure) containing 45 uranyl bipyramids and 23 pyrophosphate groups. These clusters are built from three basic structural units, and are highly unusual because they completely lack symmetry. They provide a window into the complex chemistry of evolving cluster topologies under the conditions of their formation.

THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. X. NEW DETERMINATIONS OF CENTERS FOR 65 CLUSTERS

We present new measurements of the centers for 65 Milky Way globular clusters. Centers were determined by fitting ellipses to the density distribution within the inner $2\arcmin$ of the cluster center, and averaging the centers of these ellipses. The symmetry of clusters was also analyzed by comparing cumulative radial distributions on opposite sides of the cluster across a grid of trial centers. All of the determinations were done with stellar positions derived from a combination of two single-orbit ACS images of the core of the cluster in $F606W$ and $F814W$. We find that the ellipse-fitting method provides remarkable accuracy over a wide range of core sizes and density distributions, while the symmetry method is difficult to use on clusters with very large cores, or low density. The symmetry method requires a larger field, or a very sharply peaked density distribution.

Isotopic dipole moments in water clusters

AbstractDeuteration can break the symmetry of water clusters either by replacing a H atom with a D atom in monomers or by replacing H2O monomers with D2O ones in a cluster, or by both. These procedures generates isotopic dipole moments in cases the regular ones are forbidden by point‐group symmetry (tetramer, cyclic hexamer, octamer) or vanish because of dynamic symmetry (trimer, pentamer). We report, for the first time, calculations of these dipole moments in small water clusters. Though having small magnitudes, they result large enough to be observable and can be made progressively large with crescent cluster size, for specific isotopic configurations. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Strain effects on the stability and structure of vacancy clusters in Si: A first-principles study

Using first-principles density-functional theory calculations, we investigate the influence of both biaxial and uniaxial strain $(\ensuremath{-}4\mathrm{%}\ensuremath{\le}\ensuremath{\epsilon}\ensuremath{\le}4\mathrm{%})$ on the stability and structure of small, neutral vacancy clusters $({V}_{n},\text{ }n\ensuremath{\le}12)$ on Si (100). A thorough understanding of vacancy clusters under strain is an important step toward elucidation of the evolutionary life cycle of native defects, especially during semiconductor manufacturing. Fourfold-coordinated (FC) structures are more favorable than ``partial hexagonal ring'' (PHR) structures in the size regime of our study under strain-free conditions; however, FC structures are also more rigid and consequently more sensitive to strain. Our calculation results indicate that PHR structures can be thermodynamically more favorable than FC structures in the presence of specific strain conditions. In addition, we identify orientation effects in which the cluster symmetry and its alignment within the strain field dictate cluster stability; in consequence, both configuration and orientation are essential factors in the identification of minimum-energy vacancy structures in strained Si. Furthermore, highlights of our simulation results suggest that minimum-energy cluster configurations formed under strain are often different than minimum-energy cluster configurations formed in the absence of strain.