Increasing Cluster Size Research Articles

Global optimization of [formula omitted] clusters by deep neural network

Global optimization is performed on Tan (n = 9–13) clusters by deep neural network (DNN) combined with density functional theory (DFT) method. All of the previously known cluster isomers within relative energy of 1.5 eV (except 2.0 eV for Ta10) are confirmed by our calculations. Moreover, new cluster isomers within relative energy of 1.5 eV (except 2.0 eV for Ta10) are reported. More complicated high-dimensional PESs that correspond to larger-sized clusters can be better explored by the DNN method because more new low-lying energy isomer configurations are found with increasing cluster size.

Tailoring the Energy Band Structure and Interfacial Morphology of the ETL via Controllable Nanocluster Size Achieves High-Performance Planar Perovskite Solar Cells.

Planar-type perovskite solar cells (p-PSCs) based on SnO2 have garnered further attention due to their simple and low-temperature fabrication. Improving the critical properties of the electron transport layer (ETL) is an effective way to enhance the performance of p-PSC devices. Here, a brand-new method is developed to relieve the contact recombination caused by the rough fluorine-doped tin oxide (FTO) surface and further boosts the electrical concentration of the ETL. A SnO2-ethylene diamine tetraacetic acid (EDTA) acylamide compound (SEAC) with hydrogen bond-induced adjustable cluster size is reported for the first time. The rational choice of the SEAC cluster size is the key for obtaining the smooth interfacial morphology of the ETL on the rugged FTO substrate. In addition, the energy band gap decreases with the increasing cluster size, and consequently, results in improved electrical conductivity of the SEAC. The upshifted Fermi energy level leads to higher electron concentration, which is an important physical quantity of the ETL. The PSC devices based on the optimized SEAC achieve an improved power conversion efficiency of 21.29% with negligible J-V hysteresis due to significantly enhanced electron transport and reduced contact charge recombination at the ETL/perovskite interface. In general, this paper comes up with a unique strategy to improve the quality of the SnO2-based ETL.

Tuning the thermal properties of aqueous nanofluids by taking advantage of size-customized clusters of iron oxide nanoparticles

In this study, the thermal conductivity of aqueous nanofluids containing clusters of iron oxide (Fe3O4/γ-Fe2O3) nanoparticles has been investigated experimentally for the first time, with the aim of assessing the role of a controlled aggregation of nanoparticles in these final nanofluids. For that, clusters of iron oxide nanoparticles of different cluster size (46–240 nm diameter range) were synthesized by a solvothermal method and fully characterized by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The rheological behavior of the optimal nanofluids was also studied by rotational rheometry. The nanofluids were obtained by dispersing the clusters of iron oxide nanoparticles in water taking into account different solid volume fractions (from 0.50 to 1.5 wt%) and the experiments were conducted in the temperature range from 293.15 K to 313.15 K. The study reveals and quantifies enhancements in the thermal conductivity of nanofluid with increase of cluster size and temperature. Furthermore, a 0.50 wt% concentration of clusters of iron oxide nanoparticles within the whole range of proposed nanofluids offers great stability and improved thermal conductivity for heat transfer applications with an small dynamic viscosity increase. In addition, the larger the size of the clusters of iron oxide nanoparticles, the greater the increase in thermal conductivity for the designed Fe3O4/γ-Fe2O3 cluster-based nanofluids, with thermal conductivity values following a constant upward trend and reaching a maximum increase of 4.4% for the largest synthesized clusters (average size of 240 nm). These results open the door for the development of iron oxide-based nanofluids on which taking advantage of an optimized aggregation of nanoparticles by using size-customized clusters.

Computational Appraisal of Silver Nanocluster Evolution on Epitaxial Graphene: Implications for CO Sensing.

Early stages of silver nucleation on a two-dimensional (2D) substrate, here, monolayer epitaxial graphene (MEG) on SiC, play a critical role in the formation of application-specific Ag nanostructures. Therefore, it is of both fundamental and practical importance to investigate the growth steps when Ag adatoms start to form a new phase. In this work, we exploit density functional theory to study the kinetics of early-stage nuclei Agn (n = 1–9) assembly of Ag nanoparticles on MEG. We find that the Ag1 monomer tends to occupy hollow site positions of MEG and interacts with the surface mainly through weak dispersion forces. The pseudoepitaxial growth regime is revealed to dominate the formation of the planar silver clusters. The adsorption and nucleation energies of Agn clusters exhibit evident odd–even oscillations with cluster size, pointing out the preferable adsorption and nucleation of odd-numbered clusters on MEG. The character of the interaction between a chemisorbed Ag3 cluster and MEG makes it possible to consider this trimer as the most stable nucleus for the subsequent growth of Ag nanoparticles. We reveal the general correlation between Ag/MEG interaction and Ag–Ag interaction: with increasing cluster size, the interaction between Ag adatoms increases, while the Ag/MEG interaction decreases. The general trend is also supported by the results of charge population analysis, according to which the average charge per Ag adatom in a Agn cluster demonstrates a drastic decrement with cluster size increase. 2D–3D structural transition in Agn clusters was investigated. We anticipate that the present investigation is beneficial by providing a better understanding of the early-stage nucleation of Ag nanoparticles on MEG at the atomic scale. Specific interaction between odd-numbered Ag clusters preadsorbed onto the MEG surface and carbon monoxide (CO) as well as clusters’ stability at 300 K is discussed in terms of sensing applications.

A Mechanistic Study of Clustering and Diffusion of Molybdenum and Rhenium Atoms in Liquid Sodium

Liquid Na is widely used as the heat transfer medium in high-temperature heat pipes based on Mo-Re alloys. In this study, ab initio molecular dynamics are employed in order to understand the interactions between the Na solvent and Mo or Re solute in the liquid phase. Both the temperature and concentration effects on the clustering and diffusion behaviors of solute atoms are investigated. It is found that Mo2 and Re2 dimers can be stabilized in liquid Na, and the higher temperature leads to a stronger binding force. Pure Re and Mo-Re mixed solutes can form tetramers at the highest concentration. However, for the pure Mo solute, Mo4 is not observed. The diffusivities of a single solute atom and clusters are calculated. It is found that the Mo species diffuse faster than the Re species, and the diffusivity decreases as the cluster size increases.

Identifying Communication Topologies on Twitter

Social networks are known for their decentralization and democracy. Each individual has a chance to participate and influence any discussion. Even with all the freedom, people’s behavior falls under patterns that are observed in numerous situations. In this paper, we propose a methodology that defines and searches for common communication patterns in topical networks on Twitter. We analyze clusters according to four traits: number of nodes the cluster has, their degree and betweenness centrality values, number of node types, and whether the cluster is open or closed. We find that cluster structures can be defined as (a) fixed, meaning that they are repeated across datasets/topics following uniform rules, or (b) variable if they follow an underlying rule regardless of their size. This approach allows us to classify 90% of all conversation clusters, with the number varying by topic. An increase in cluster size often results in difficulties finding topological shape rules; however, these types of clusters tend to exhibit rules regarding their node relationships in the form of centralization. Most individuals do not enter large-scale discussions on Twitter, meaning that the simplicity of communication clusters implies repetition. In general, power laws apply for the influencer connection distribution (degree centrality) even in topical networks.

Ab Initio Simulations of Poorly and Well Equilibrated (CH3CN)n- Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions.

Excess electrons in liquid acetonitrile are of particular interest because they exist in two different forms in equilibrium: they can be present as traditional solvated electrons in a cavity, and they can form some type of solvated molecular anion. Studies of small acetonitrile cluster anions in the gas phase show two isomers with distinct vertical detachment energies, and it is tempting to presume that the two gas-phase cluster anion isomers are precursors of the two excess electron species present in bulk solution. In this paper, we perform DFT-based ab initio molecular dynamics simulations of acetonitrile cluster anions to understand the electronic species that are present and why they have different binding energies. Using a long-range-corrected density functional that was optimally tuned to describe acetonitrile cluster anion structures, we have theoretically explored the chemistry of (CH3CN)n- cluster anions with sizes n = 5, 7, and 10. Because the temperature of the experimental cluster anions is not known, we performed two sets of simulations that investigated how the way in which the cluster anions are prepared affects the excess electron binding motif: one set of simulations simply attached excess electrons to neutral (CH3CN)n clusters, providing little opportunity for the clusters to relax in the presence of the excess electron, while the other set allowed the cluster anions to thermally equilibrate near room temperature. We find that both sets of simulations show three distinct electron binding motifs: electrons can attach to the surface of the cluster (dipole-bound) or be present either as solvated monomer anions, CH3CN-, or as solvated molecular dimer anions, (CH3CN)2-. All three species have higher binding energies at larger cluster sizes. Thermal equilibration strongly favors the formation of the valence-bound molecular anions relative to surface-bound excess electrons, and the dimer anion becomes more stable than the monomer anion and surface-bound species as the cluster size increases. The calculated photoelectron spectra from our simulations in which there was poor thermal equilibration are in good agreement with experiment, suggesting assignment of the two experimental cluster anion isomers as the surface-bound electron and the solvated molecular dimer anion. The simulations also suggest that the shoulder seen experimentally on the low-energy isomer's detachment peak is not part of a vibronic progression but instead results from molecular monomer anions. Nowhere in the size range that we explore do we see evidence for a nonvalence, cavity-bound interior-solvated electron, indicating that this species is likely only accessible at larger sizes with good thermal equilibration.

Density functional theory study of ZnnInn (n = 2–10) alloy clusters

The geometrical structure, mean square displacement and electronic properties of ZnnInn(n = 2–10) alloy clusters have been systematically studied by means of density functional theory. The stability of zinc-indium clusters has been discussed. Ab-initio molecular dynamics were performed to exhaust search the global minimum structures of the clusters on the level of GGA-PBE by DMol3 package, Then, the structures were optimized and further calculated the mean square displacement, HOMO-LUMO gap and NBO on the level of B3LYP/LANL2DZ by using Gaussian program.The results show that as the size increases, the structure tends to be spherical, In atoms and Zn atoms are dispersed in the system, The binding force of In-In bond and Zn-In bond is stronger than that of Zn-Zn bond. Combined with our calculated results, it is found that with the increase of cluster size, the non-metallicity of clusters decreases while the metallicity of clusters increases, and even clusters are more stable than odd clusters. The interaction between zinc and indium can be analyzed at the atomic level to provide ideas for the separation and recovery of indium from zinc and indium in practical experiments.

Acid-Base Clusters during Atmospheric New Particle Formation in Urban Beijing.

Molecular clustering is the initial step of atmospheric new particle formation (NPF) that generates numerous secondary particles. Using two online mass spectrometers with and without a chemical ionization inlet, we characterized the neutral clusters and the naturally charged ion clusters during NPF periods in urban Beijing. In ion clusters, we observed pure sulfuric acid (SA) clusters, SA-amine clusters, SA-ammonia (NH3) clusters, and SA-amine-NH3 clusters. However, only SA clusters and SA-amine clusters were observed in the neutral form. Meanwhile, oxygenated organic molecule (OOM) clusters charged by a nitrate ion and a bisulfate ion were observed in ion clusters. Acid-base clusters correlate well with the occurrence of sub-3 nm particles, whereas OOM clusters do not. Moreover, with the increasing cluster size, amine fractions in ion acid-base clusters decrease, while NH3 fractions increase. This variation results from the reduced stability differences between SA-amine clusters and SA-NH3 clusters, which is supported by both quantum chemistry calculations and chamber experiments. The lower average number of dimethylamine (DMA) molecules in atmospheric ion clusters than the saturated value from controlled SA-DMA nucleation experiments suggests that there is insufficient DMA in urban Beijing to fully stabilize large SA clusters, and therefore, other basic molecules such as NH3 play an important role.

Increasing cluster size asymptotics for nested error regression models

This paper establishes asymptotic results for the maximum likelihood and restricted maximum likelihood (REML) estimators of the parameters in the nested error regression model for clustered data when both of the number of independent clusters and the cluster sizes (the number of observations in each cluster) go to infinity. Under very mild conditions, the estimators are shown to be asymptotically normal with an elegantly structured covariance matrix. There are no restrictions on the rate at which the cluster size tends to infinity but it turns out that we need to treat within cluster parameters (i.e. coefficients of unit-level covariates that vary within clusters and the within cluster variance) differently from between cluster parameters (i.e. coefficients of cluster-level covariates that are constant within clusters and the between cluster variance) because they require different normalisations and are asymptotically independent.

Binding of noble gas atoms by superhalogens.

Because of their closed shells, noble gas (Ng) atoms (Ng = Ne, Ar, Kr, and Xe) seldom take part in chemical reactions, yet finding such mechanisms not only is of scientific interest but also has practical significance. Following a recent work by Mayer et al. [Proc. Natl. Acad. Sci. U. S. A. 116, 8167-8172 (2019)] on the room temperature binding of Ar to a superelectrophilic boron site embedded in a negative ion complex, B12(CN)11 -, we have systematically studied the effect of cluster size and terminal ligands on the interaction of Ng by focusing on B12X11(Ng) (X = H, CN, and BO) and B12X10(Ng)2 (X = CN and BO) whose stabilities are governed by the Wade-Mingos rule and on C5BX5(Ng) (X = H, F, and CN) and C4B2(CN)4(Ng)2 whose stabilities are governed by the Huckel's aromaticity rule. Our conclusion, based on density functional theory, is that both the cluster size and the terminal ligands matter-the interaction between the cluster and the Ng atoms becomes stronger with increasing cluster size and the electron affinity of the terminal ligands. Our studies also led to a counter-intuitive finding-removing multiple terminal ligands can enable electrophilic centers to bind multiple Ng atoms simultaneously without compromising their binding strength.

Ground-State Structures of Hydrated Calcium Ion Clusters From Comprehensive Genetic Algorithm Search.

We searched the lowest-energy structures of hydrated calcium ion clusters Ca2+(H2O)n (n = 10–18) in the whole potential energy surface by the comprehensive genetic algorithm (CGA). The lowest-energy structures of Ca2+(H2O)10–12 clusters show that Ca2+ is always surrounded by six H2O molecules in the first shell. The number of first-shell water molecules changes from six to eight at n = 12. In the range of n = 12–18, the number of first-shell water molecules fluctuates between seven and eight, meaning that the cluster could pack the water molecules in the outer shell even though the inner shell is not full. Meanwhile, the number of water molecules in the second shell and the total hydrogen bonds increase with an increase in the cluster size. The distance between Ca2+ and the adjacent water molecules increases, while the average adjacent O-O distance decreases as the cluster size increases, indicating that the interaction between Ca2+ and the adjacent water molecules becomes weaker and the interaction between water molecules becomes stronger. The interaction energy and natural bond orbital results show that the interaction between Ca2+ and the water molecules is mainly derived from the interaction between Ca2+ and the adjacent water molecules. The charge transfer from the lone pair electron orbital of adjacent oxygen atoms to the empty orbital of Ca2+ plays a leading role in the interaction between Ca2+ and water molecules.

Size-Resolved Electron Solvation in Neutral Water Clusters.

Cluster-size-resolved ultrafast dynamics of the solvated electron in neutral water clusters with n = 3 to ∼200 molecules are studied with pump–probe time-of-flight mass spectrometry after below band gap excitation. For the smallest clusters, no longer-lived (>100–200 fs) hydrated electrons were detected, indicating a minimum size of n ∼ 14 for being able to sustain hydrated electrons. Larger clusters show a systematic increase of the number of hydrated electrons per molecule on the femtosecond to picosecond time scale. We propose that with increasing cluster size the underlying dynamics is governed by more effective electron formation processes combined with less effective electron loss processes, such as ultrafast hydrogen ejection and recombination. It appears unlikely that any size dependence of the solvent relaxation dynamics would be reflected in the observed time-resolved ion yields.

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol-(Acetylene)n (∼16 ≤ n ≤ ∼30) and (Acetylene)n+ (10 ≤ n ≤ 70) Clusters.

The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with π-electrons, which shows a clear hydrogen bond property, is called activated CH/π interaction. In this study, cooperative enhancement of the activated CH/π interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)n (∼16 ≤ n ≤ ∼30) and (acetylene)n+ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/π interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20-30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/π interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger.

On the growth behavior, structures, energy, and magnetic properties of bimetallic $$\hbox {M}_{{n}}\hbox {Pd}_{{n}}$$ (M = Co, Ni; n = 1–10) clusters

In this work, the growth behavior, structures, energetic and magnetic properties of $$\hbox {Co}_{{n}}\hbox {Pd}_{{n}}$$ and $$\hbox {Ni}_{{n}}\hbox {Pd}_{{n}}$$ (n = 1–10) clusters were investigated employing auxiliary density functional theory (ADFT). Initial geometries for successive optimization were extracted from Born–Oppenheimer molecular dynamics (BOMD) trajectories. It is demonstrated that when the cluster size increases, the Co and Ni atoms became shrouded by Pd atoms, leading to the initial formation of M@Pd (M = Co and Ni) core–shell structures. The spin multiplicities of the $$\hbox {Co}_{{n}}\hbox {Pd}_{{n}}$$ and $$\hbox {Ni}_{{n}}\hbox {Pd}_{{n}}$$ (n = 1–10) clusters increase with cluster size. The CoPd clusters exhibit higher spin multiplicity and are characterized by higher spin magnetic moments than the NiPd counterparts. This study reveals that the spin density distributions are located on the Co and Ni atoms in the respective clusters. As the cluster size increases both systems tend to donate more easily electrons and the binding energies per atom grows monotonically.

Comparison of the Microsolvation of CaX2 (X = F, Cl, Br, I) in Water: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations.

To understand the microsolvation of alkaline-earth dihalides in water and provide information about the dependence of solvation processes on different halides, we investigated CaBr2(H2O)n-, CaI2(H2O)n-, and CaF2(H2O)n- (n = 0-6) clusters using size-selected anion photoelectron spectroscopy and conducted theoretical calculations on these clusters and their neutrals. The results are compared with those of CaCl2(H2O)n-/0 clusters reported previously. It is found that the vertical detachment energies (VDEs) of CaCl2(H2O)n-, CaBr2(H2O)n-, and CaI2(H2O)n- show a similar trend with increasing cluster size, while the VDEs of CaF2(H2O)n- show a different trend. The VDEs of CaF2(H2O)n- are much lower than those of CaCl2(H2O)n-, CaBr2(H2O)n-, and CaI2(H2O)n-. A detailed probing of the structures shows that a significant increase of the Ca-X distance (separation of Ca2+-X- ion pair) in CaCl2(H2O)n-/0, CaBr2(H2O)n-/0, and CaI2(H2O)n-/0 clusters occurred at about n = 5. However, for CaF2(H2O)n-/0, no abrupt change of the Ca-F distance with the increasing cluster size has been observed. In CaCl2(H2O)6-/0, CaBr2(H2O)6-/0, and CaI2(H2O)6-/0, the Ca atom coordinates directly with 5 H2O molecules. However, in CaF2(H2O)n-/0, the Ca atom coordinates directly with only 2 or 3 H2O molecules. The similarity or differences in the structures and coordination numbers are consistent with the fact that CaCl2, CaBr2, and CaI2 have similar solubility, while CaF2 has much lower solubility.

Non-local corrections to the typical medium theory of Anderson localization

We use the recently developed finite cluster typical medium approach to study the Anderson localization transition in three dimensions. Applying our method to the box and binary alloy disorder distributions, we find a fast convergence with the cluster size. We demonstrate the importance of the typical medium environment and the non-local spatial correlations for the proper characterization of the localization transition. As the cluster size increases, our typical medium cluster method recovers the correct critical disorder strength for the transition. Our findings highlight the importance of the non-local cluster corrections for capturing the localization behavior of the mobility edge trajectories. Our results demonstrate that the typical medium cluster approach developed here provides a consistent and systematic description of the Anderson localization transition in the framework of the effective medium embedding schemes.

Theoretical Investigation of the Size Effect on the Oxygen Adsorption Energy of Coinage Metal Nanoparticles

This study evaluates the finite size effect on the oxygen adsorption energy of coinage metal (Cu, Ag and Au) cuboctahedral nanoparticles in the size range of 13 to 1415 atoms (0.7–3.5 nm in diameter). Trends in particle size effects are well described with single point calculations, in which the metal atoms are frozen in their bulk position and the oxygen atom is added in a location determined from periodic surface calculations. This is shown explicitly for Cu nanoparticles, for which full geometry optimization only leads to a constant offset between relaxed and unrelaxed adsorption energies that is independent of particle size. With increasing cluster size, the adsorption energy converges systematically to the limit of the (211) extended surface. The 55-atomic cluster is an outlier for all of the coinage metals and all three materials show similar behavior with respect to particle size.Graphic

Competing nucleation of single- and double-layer Guinier\u2013Preston zones in Al\u2013Cu alloys

Solid-state precipitation is a key heat-treatment strategy for strengthening engineering alloys. Therefore, predicting the precipitation process of localized solute-rich clusters, such as Guinier–Preston (GP) zones, is necessary. We quantitatively evaluated the critical nucleus size and nucleation barrier of GP zones in Al–Cu alloys, illustrating the precipitation preferences of single-layer (GP1) and double-layer (GP2) GP zones. Based on classical nucleation theory using an effective multi-body potential for dilute Al–Cu systems, our model predicted GP1 and GP2 precipitation sequences at various temperatures and Cu concentrations in a manner consistent with experimental observations. The crossover between formation enthalpy curves of GP1 and GP2 with increasing cluster size determines the critical conditions under which GP2 zones can nucleate without prior formation of GP1 zones. This relationship reflects competing interactions within and between clusters. The results illustrate the underlying mechanisms of competing nucleation between zones, and provide guidance for tailoring aging conditions to achieve desired mechanical properties for specific applications.

Simultaneous double ionization of C60 molecules in single-cycle pulses

We theoretically investigate simultaneous double ionization of C60 Buckminsterfullerene clusters within the strong field approximation, taking into account two-body effects like Coulomb blocking. Our analysis suggests that for infrared single-cycle pulses, simultaneous double ionization becomes comparable in magnitude to sequential double ionization. Additionally, estimates show that Coulomb blocking weakens with increasing cluster size and field strength.