Showing papers on "Cluster (physics) published in 2014"

Pre-nucleation clusters as solute precursors in crystallisation

Abstract: Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid–liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

Nonscalable oxidation catalysis of gold clusters.

Abstract: Small, negatively charged gold clusters isolated in vacuum can oxidize CO via electron-transfer-mediated activation of O2. This suggests that Au clusters can act as aerobic oxidation catalysts in the real world when their structure parameters satisfy given required conditions. However, there is a technical challenge for the development of Au cluster oxidation catalysts; the structural parameters of the Au clusters, such as size and composition, must be precisely controlled because the intrinsic chemical properties of the clusters are strongly dependent on these parameters. This Account describes our efforts to achieve precision synthesis of small (diameter <2 nm) Au clusters, stabilized by polymers and immobilized on supports, for a variety of catalytic applications. Since we aim to develop Au cluster catalysts by taking full advantage of their intrinsic, size-specific chemical nature, we chose chemically inert materials for the stabilizers and supports.We began by preparing small Au clusters weakly stabi...

Understanding boron through size-selected clusters: structure, chemical bonding, and fluxionality.

Abstract: ConspectusBoron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center–two-electron (2c–2e) σ bonds on the periphery and delocalized multicenter–two-electron (nc–2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron’s electron deficiency and leads to fluxional behavior, which has been observed in B13+ and B19–. A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structu...

The B35 Cluster with a Double-Hexagonal Vacancy: A New and More Flexible Structural Motif for Borophene

Abstract: Elemental boron is electron-deficient and cannot form graphene-like structures. Instead, triangular boron lattices with hexagonal vacancies have been predicted to be stable. A recent experimental and computational study showed that the B36 cluster has a planar C6v structure with a central hexagonal hole, providing the first experimental evidence for the viability of atom-thin boron sheets with hexagonal vacancies, dubbed borophene. Here we report a boron cluster with a double-hexagonal vacancy as a new and more flexible structural motif for borophene. Photoelectron spectrum of B35– displays a simple pattern with certain similarity to that of B36–. Global minimum searches find that both B35– and B35 possess planar hexagonal structures, similar to that of B36, except a missing interior B atom that creates a double-hexagonal vacancy. The closed-shell B35– is found to exhibit triple π aromaticity with 11 delocalized π bonds, analogous to benzo(g,h,i)perylene (C22H12). The B35 cluster can be used to build atom...

Special and general superatoms.

Abstract: ConspectusBridging the gap between atoms and macroscopic matter, clusters continue to be a subject of increasing research interest. Among the realm of cluster investigations, an exciting development is the realization that chosen stable clusters can mimic the chemical behavior of an atom or a group of the periodic table of elements. This major finding known as a superatom concept was originated experimentally from the study of aluminum cluster reactivity conducted in 1989 by noting a dramatic size dependence of the reactivity where cluster anions containing a certain number of Al atoms were unreactive toward oxygen while the other species were etched away. This observation was well interpreted by shell closings on the basis of the jellium model, and the related concept (originally termed “unified atom”) spawned a wide range of pioneering studies in the 1990s pertaining to the understanding of factors governing the properties of clusters.Under the inspiration of a superatom concept, advances in cluster sci...

Neutral molecular cluster formation of sulfuric acid-dimethylamine observed in real time under atmospheric conditions.

Abstract: For atmospheric sulfuric acid (SA) concentrations the presence of dimethylamine (DMA) at mixing ratios of several parts per trillion by volume can explain observed boundary layer new particle formation rates. However, the concentration and molecular composition of the neutral (uncharged) clusters have not been reported so far due to the lack of suitable instrumentation. Here we report on experiments from the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research revealing the formation of neutral particles containing up to 14 SA and 16 DMA molecules, corresponding to a mobility diameter of about 2 nm, under atmospherically relevant conditions. These measurements bridge the gap between the molecular and particle perspectives of nucleation, revealing the fundamental processes involved in particle formation and growth. The neutral clusters are found to form at or close to the kinetic limit where particle formation is limited only by the collision rate of SA molecules. Even though the neutral particles are stable against evaporation from the SA dimer onward, the formation rates of particles at 1.7-nm size, which contain about 10 SA molecules, are up to 4 orders of magnitude smaller compared with those of the dimer due to coagulation and wall loss of particles before they reach 1.7 nm in diameter. This demonstrates that neither the atmospheric particle formation rate nor its dependence on SA can simply be interpreted in terms of cluster evaporation or the molecular composition of a critical nucleus.

Theoretical and experimental insights into the origin of the catalytic activity of subnanometric gold clusters: attempts to predict reactivity with clusters and nanoparticles of gold.

Abstract: Particle size is one of the key parameters determining the unexpected catalytic activity of gold, with reactivity improving as the particle gets smaller. While this is valid in the 1-5 nm range, chemists are now investigating the influence of particle size in the subnanometer regime. This is due to recent advances in both characterization techniques and synthetic routes capable of stabilizing these size-controlled gold clusters. Researchers reported in early studies that small clusters or aggregates of a few atoms can be extremely active in some reactions, while 1-2 nm nanoparticles are catalytically more efficient for other reactions. Furthermore, the possibility that small gold clusters generated in situ from gold salts or complexes could be the real active species in homogeneous gold-catalyzed organic reactions should be considered. In this Account, we address two questions. First, what is the origin of the enhanced reactivity of gold clusters on the subnanometer scale? And second, how can we predict the reactions where small clusters should work better than larger nanoparticles? Both geometric factors and electronic or quantum size effects become important in the subnanometer regime. Geometric reasons play a key role in hydrogenation reactions, where only accessible low coordinated neutral Au atoms are needed to dissociate H2. The quantum size effects of gold clusters are important as well, as clusters formed by only a few atoms have discrete molecule-like electronic states and their chemical reactivity is related to interactions between the cluster's frontier molecular orbitals and those of the reactant molecules. From first principles calculations, we predict an enhanced reactivity of small planar clusters for reactions involving activation of CC multiple bonds in alkenes and alkynes through Lewis acid-base interactions, and a better catalytic performance of 3D gold nanoparticles in redox reactions involving bond dissociation by oxidative addition and new bond formation by reductive elimination. In oxidation reactions with molecular O2, initial dissociation of O2 into basic oxygen atoms would be more effectively catalyzed by gold nanoparticles of ∼1 nm diameter. In contrast, small planar clusters should be more active for reactions following a radical pathway involving peroxo or hydroperoxo intermediates. We have experimentally confirmed these predictions for a series of Lewis acid and oxidation reactions catalyzed by gold clusters and nanoparticles either in solution or supported on solid carriers.

Super-Reduced Polyoxometalates: Excellent Molecular Cluster Battery Components and Semipermeable Molecular Capacitors

Abstract: Theoretical investigations are presented on the molecular and electronic structure changes that occur as α-Keggin-type polyoxometalate (POM3–) clusters [PM12O40]3– (M = Mo, W) are converted toward their super-reduced POM27– state during the discharging process in lithium-based molecular cluster batteries. Density functional theory was employed in geometry optimization, and first-principles molecular dynamics simulations were used to explore local minima on the potential energy surface of neutral POM clusters adorned with randomly placed Li atoms as electron donors around the cluster surface. On the basis of structural, electron density, and molecular orbital studies, we present evidence that the super-reduction is accompanied by metal–metal bond formation, beginning from the 12th to 14th excess electron transferred to the cluster. Afterward, the number of metal–metal bonds increases nearly linearly with the number of additionally transferred excess electrons. In α-Keggin-type POMs, metal triangles are a p...

UPMASK: unsupervised photometric membership assignment in stellar clusters

Abstract: Aims. We develop a method for membership assignment in stellar clusters using only photometry and positions. The method is aimed to be unsupervised, data driven, model free, and to rely on as few assumptions as possible.Methods. The approach followed in this work for membership assessment is based on an iterative process, principal component analysis, clustering algorithm, and kernel density estimations. The method, UPMASK, is able to take into account arbitrary error models. An implementation in R was tested on simulated clusters that covered a broad range of ages, masses, distances, reddenings, and also on real data of cluster fields.Results. Running UPMASK on simulations showed that the method effectively separates cluster and field populations. The overall spatial structure and distribution of cluster member stars in the colour-magnitude diagram were recovered under a broad variety of conditions. For a set of 360 simulations, the resulting true positive rates (a measurement of purity) and member recovery rates (a measurement of completeness) at the 90% membership probability level reached high values for a range of open cluster ages (107.1 − 109.5 yr), initial masses (0.5 − 10 × 103 M ⊙ ) and heliocentric distances (0.5 − 4.0 kpc). UPMASK was also tested on real data from the fields of open cluster Haffner 16 and of the closely projected clusters Haffner 10 and Czernik 29. These tests showed that even for moderate variable extinction and cluster superposition, the method yielded useful cluster membership probabilities and provided some insight into their stellar contents. The UPMASK implementation will be available at the CRAN archive.

Au₂₅(SEt)₁₈, a nearly naked thiolate-protected Au₂₅ cluster: structural analysis by single crystal X-ray crystallography and electron nuclear double resonance.

Abstract: X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV-vis absorption spectroscopy features of similar Au25 clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au25(SEt)18 were fully characterized by (1)H and (13)C NMR spectroscopy, which confirmed the monolayer's properties and the paramagnetism of neutral Au25(SEt)18(0). X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure's approximations or omissions.

Weighing Galaxy Clusters with Gas. II. On the Origin of Hydrostatic Mass Bias in ΛCDM Galaxy Clusters

Abstract: The use of galaxy clusters as cosmological probes hinges on our ability to measure their masses accurately and with high precision. Hydrostatic mass is one of the most common methods for estimating the masses of individual galaxy clusters, which suffer from biases due to departures from hydrostatic equilibrium. Using a large, mass-limited sample of massive galaxy clusters from a high-resolution hydrodynamical cosmological simulation, in this work we show that in addition to turbulent and bulk gas velocities, acceleration of gas introduces biases in the hydrostatic mass estimate of galaxy clusters. In unrelaxed clusters, the acceleration bias is comparable to the bias due to non-thermal pressure associated with merger-induced turbulent and bulk gas motions. In relaxed clusters, the mean mass bias due to acceleration is small ( 3%), but the scatter in the mass bias can be reduced by accounting for gas acceleration. Additionally, this acceleration bias is greater in the outskirts of higher redshift clusters where mergers are more frequent and clusters are accreting more rapidly. Since gas acceleration cannot be observed directly, it introduces an irreducible bias for hydrostatic mass estimates. This acceleration bias places limits on how well we can recover cluster masses from future X-ray and microwave observations. We discuss implications for cluster mass estimates based on X-ray, Sunyaev-Zel'dovich effect, and gravitational lensing observations and their impact on cluster cosmology.

[B₃₀]⁻: a quasiplanar chiral boron cluster.

Abstract: Chirality is vital in chemistry. Its importance in atomic clusters has been recognized since the discovery of the first chiral fullerene, the D2 symmetric C76. A number of gold clusters have been found to be chiral, raising the possibility to use them as asymmetric catalysts. The discovery of clusters with enantiomeric structures is essential to design new chiral materials with tailored chemical and physical properties. Herein we report the first inherently chiral boron cluster of [B30](-) in a joint photoelectron spectroscopy and theoretical study. The most stable structure of [B30](-) is found to be quasiplanar with a hexagonal hole. Interestingly, a pair of enantiomers arising from different positions of the hexagonal hole are found to be degenerate in our global minimum searches and both should co-exist experimentally because they have identical electronic structures and give rise to identical simulated photoelectron spectra.

Information on quantum states pervades the visible spectrum of the ubiquitous Au144(SR)60 gold nanocluster

Abstract: Absorption spectra of very small metal clusters exhibit individual peaks that reflect the discreteness of their localized electronic states. With increasing size, these states develop into bands and the discrete absorption peaks give way to smooth spectra with, at most, a broad localized surface-plasmon resonance band. The widely accepted view over the last decades has been that clusters of more than a few dozen atoms are large enough to have necessarily smooth spectra. Here we show through theory and experiment that for the ubiquitous thiolate cluster compound Au144(SR)60 this view has to be revised: clearly visible individual peaks pervade the full near-IR, VIS and near-UV ranges of low-temperature spectra, conveying information on quantum states in the cluster. The peaks develop well reproducibly with decreasing temperature, thereby highlighting the importance of temperature effects. Calculations using time-dependent density-functional theory indicate the contributions of different parts of the cluster-ligand compound to the spectra.

Gold nanowired: a linear (Au25)(n) polymer from Au25 molecular clusters.

Abstract: Au25(SR)18 has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au25(SR)18 displays molecular behavior. We prepared a Au25 cluster capped by n-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallography, and studied its properties both experimentally and theoretically. Whereas in solution Au25(SBu)180 is a paramagnetic molecule, in the crystal it becomes a linear polymer of Au25 clusters connected via single Au–Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temperature, [Au25(SBu)180]n has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of molecular gold nanowires.

Generalized phase behavior of cluster formation in colloidal dispersions with competing interactions

Abstract: Colloidal liquids interacting with short range attraction and long range repulsion, such as proposed for some protein solutions, have been found to exhibit novel states consisting of equilibrium particle clusters. Monte Carlo simulations are performed for two physically meaningful inter-particle potentials across a broad range of interaction parameters, temperatures and volume fractions to locate the conditions where clustered states are found. A corresponding states phase behavior is identified when normalized by the critical point of an appropriately selected reference attractive fluid. Clustered fluid states and cluster percolated states are found exclusively within the two phase region of the state diagram for a reference attractive fluid, confirming the underlying intrinsic relation between clustered states and bulk phase separation. Clustered and cluster percolated states consistently exhibit an intermediate range order peak in their structure factors with a magnitude above 2.7, leading to a semi-empirical rule for identifying clustered fluids in scattering experiments.

Formation of [4Fe-4S] Clusters in the Mitochondrial Iron–Sulfur Cluster Assembly Machinery

Abstract: The generation of [4Fe-4S] clusters in mitochondria critically depends, in both yeast and human cells, on two A-type ISC proteins (in mammals named ISCA1 and ISCA2), which perform a nonredundant functional role forming in vivo a heterocomplex. The molecular function of ISCA1 and ISCA2 proteins, i.e., how these proteins help in generating [4Fe-4S] clusters, is still unknown. In this work we have structurally characterized the Fe/S cluster binding properties of human ISCA2 and investigated in vitro whether and how a [4Fe-4S] cluster is assembled when human ISCA1 and ISCA2 interact with the physiological [2Fe-2S](2+) cluster-donor human GRX5. We found that (i) ISCA2 binds either [2Fe-2S] or [4Fe-4S] cluster in a dimeric state, and (ii) two molecules of [2Fe-2S](2+) GRX5 donate their cluster to a heterodimeric ISCA1/ISCA2 complex. This complex acts as an "assembler" of [4Fe-4S] clusters; i.e., the two GRX5-donated [2Fe-2S](2+) clusters generate a [4Fe-4S](2+) cluster. The formation of the same [4Fe-4S](2+) cluster-bound heterodimeric species is also observed by having first one [2Fe-2S](2+) cluster transferred from GRX5 to each individual ISCA1 and ISCA2 proteins to form [2Fe-2S](2+) ISCA2 and [2Fe-2S](2+) ISCA1, and then mixing them together. These findings imply that such heterodimeric complex is the functional unit in mitochondria receiving [2Fe-2S] clusters from hGRX5 and assembling [4Fe-4S] clusters before their transfer to the final target apo proteins.

Chemical Structure of Nitrogen-Doped Graphene with Single Platinum Atoms and Atomic Clusters as a Platform for the PEMFC Electrode

Abstract: A platform for producing stabilized Pt atoms and clusters through the combination of an N-doped graphene support and atomic layer deposition (ALD) for the Pt catalysts was investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was determined, using imaging and spectroscopy techniques, that a wide range of N-dopant types entered the graphene lattice through covalent bonds without largely damaging its structure. Additionally and most notably, Pt atoms and atomic clusters formed in the absence of nanoparticles. This work provides a new strategy for experimentally producing stable atomic and subnanometer cluster catalysts, which can greatly assist the proton exchange membrane fuel cell (PEMFC) development by producing the ultimate surface area to volume ratio catalyst.

Non-linear optical properties of gold quantum clusters. The smaller the better.

Abstract: By developing a new method for synthesizing atomically monodisperse Au15 nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au25 nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au15, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au15 is ∼65 700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10−7 at 475 nm for Au15. The first hyperpolarizability β for Au15 clusters (509 × 10−30 esu), as compared to Au25 clusters (128 × 10−30 esu), is larger considering the difference in the number of gold atoms. Also, 1030 β per atom values reported for Au15 and Au25 clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10–50 nm, outlining the quantum cluster regime.

Cluster polylogarithms for scattering amplitudes

Abstract: Motivated by the cluster structure of two-loop scattering amplitudes in $\mathcal{N}=4$ Yang-Mills theory we define cluster polylogarithm functions. We find that all such functions of weight four are made up of a single simple building block associated with the A(2) cluster algebra. Adding the requirement of locality on generalized Stasheff polytopes, we find that these A(2) building blocks arrange themselves to form a unique function associated with the A(3) cluster algebra. This A(3) function manifests all of the cluster algebraic structure of the two-loop n-particle MHV amplitudes for all n, and we use it to provide an explicit representation for the most complicated part of the n = 7 amplitude as an example.This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to ‘Cluster algebras in mathematical physics’.

Cluster Linker Approach: Preparation of a Luminescent Porous Framework with NbO Topology by Linking Silver Ions with Gold(I) Clusters†

Abstract: A cluster-based luminescent porous metal-organic framework has been constructed through a "cluster linker" approach. The luminescent gold(I) cluster, prefunctionalized with pyrazinyl groups, was used as a cluster linker, similar to an organic linker, to connect silver ions in order to form a 3D framework. 1D channels with 1.1 nm diameter were observed in the framework. The cluster with its intrinsic luminescence was incorporated into a porous framework to give a luminescent bifunctional NbO net. This MOF shows solvatochromic behavior, and the interactions between solvent molecules and silver ions inside the channels account for the changes in absorption and emission spectra.

Combined quantum chemical and modeling study of CO hydrogenation on water ice

Abstract: Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most e cient mechanisms in interstellar environments for the formation of H2CO and CH3OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores. Aims. We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters. Methods. The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules. Results. Our main result is that the interaction of CO and H2CO with the water cluster surfaces through H-bonds with the O atoms increases the C O polarization, thus weakening the C O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H2CO and CH3OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.

The Role of Charge States in the Atomic Structure of Cun and Ptn (n = 2–14 atoms) Clusters: A DFT Investigation

Abstract: In general, because of the high computational demand, most theoretical studies addressing cationic and anionic clusters assume structural relaxation from the ground state neutral geometries. Such approach has its limits as some clusters could undergo a drastic structural deformation upon gaining or losing one electron. By engaging symmetry-unrestricted density functional calculations with an extensive search among various structures for each size and state of charge, we addressed the investigation of the technologically relevant Cun and Ptn clusters for n = 2–14 atoms in the cationic, neutral, and anionic states to analyze the behavior of the structural, electronic, and energetic properties as a function of size and charge state. Moreover, we considered potentially high-energy isomers allowing foresight comparison with experimental results. Considering fixed cluster sizes, we found that distinct charge states lead to different structural geometries, revealing a clear tendency of decreasing average coordin...

Ferric Oxide-Supported Pt Subnano Clusters for Preferential Oxidation of CO in H2-Rich Gas at Room Temperature

Abstract: Pt single atoms and small clusters were dispersed on iron oxides by a facile coprecipitation method. These catalysts, with or without calcination at elevated temperatures, show excellent activity a...

Numerical study of collisional particle dynamics in cluster-induced turbulence

Abstract: We present a computational study of cluster-induced turbulence (CIT), where the production of fluid-phase kinetic energy results entirely from momentum coupling with finite-size inertial particles. A separation of length scales must be established when evaluating the particle dynamics in order to distinguish between the continuous mesoscopic velocity field and the uncorrelated particle motion. To accomplish this, an adaptive spatial filter is employed on the Lagrangian data with an averaging volume that varies with the local particle-phase volume fraction. This filtering approach ensures sufficient particle sample sizes in order to obtain meaningful statistics while remaining small enough to avoid capturing variations in the mesoscopic particle field. Two-point spatial correlations are computed to assess the validity of the filter in extracting meaningful statistics. The method is used to investigate, for the first time, the properties of a statistically stationary gravity-driven particle-laden flow, where particle‐particle and fluid‐particle interactions control the multiphase dynamics. Results from fully developed CIT show a strong correlation between the local volume fraction and the granular temperature, with maximum values located at the upstream boundary of clusters (i.e. where maximum compressibility of the particle velocity field exists), while negligible particle agitation is observed within clusters.

Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and Unique Optical Properties/Responses

Abstract: Synthetic techniques, geometrical structures, and electronic absorption spectra of phosphine-coordinated pure-gold molecular clusters (PGCs) accumulated over 40 years are comprehensively collected especially for those with unambiguous X-ray crystal structures available. Inspection of the electronic absorption spectra from geometrical aspects reveals that their optical properties are highly dependent on the cluster geometries rather than the nuclearity. Recent examples of unusual clusters that show unique color/photoluminescence properties and their utilization for stimuli-responsive modules are also presented.

Supramolecular functionalization and concomitant enhancement in properties of Au(25) clusters.

Abstract: We present a versatile approach for tuning the surface functionality of an atomically precise 25 atom gold cluster using specific host–guest interactions between β-cyclodextrin (CD) and the ligand anchored on the cluster. The supramolecular interaction between the Au25 cluster protected by 4-(t-butyl)benzyl mercaptan, labeled Au25SBB18, and CD yielding Au25SBB18∩CDn (n = 1, 2, 3, and 4) has been probed experimentally using various spectroscopic techniques and was further analyzed by density functional theory calculations and molecular modeling. The viability of our method in modifying the properties of differently functionalized Au25 clusters is demonstrated. Besides modifying their optoelectronic properties, the CD moieties present on the cluster surface provide enhanced stability and optical responses which are crucial in view of the potential applications of these systems. Here, the CD molecules act as an umbrella which protects the fragile cluster core from the direct interaction with many destabilizi...

Controlled Self-Assembly of Periodic and Aperiodic Cluster Crystals

Abstract: Soft particles are known to overlap and form stable clusters that self-assemble into periodic crystalline phases with density-independent lattice constants. We use molecular dynamics simulations in two dimensions to demonstrate that, through a judicious design of an isotropic pair potential, one can control the ordering of the clusters and generate a variety of phases, including decagonal and dodecagonal quasicrystals. Our results confirm analytical predictions based on a mean-field approximation, providing insight into the stabilization of quasicrystals in soft macromolecular systems, and suggesting a practical approach for their controlled self-assembly in laboratory realizations using synthesized soft-matter particles.