Aluminum Clusters Research Articles

Distribution of Aluminum Clusters and their Ignition in Air during Dispersion of Aluminum Nanoparticles in a Shock Wave

Distribution of Aluminum Clusters and their Ignition in Air during Dispersion of Aluminum Nanoparticles in a Shock Wave

Room temperature single electron transistor based on a size-selected aluminium cluster.

Single electron transistors (SETs) are powerful devices to study the properties of nanoscale objects. However, the capabilities of placing a nano-object between electrical contacts under pristine conditions are lacking. Here, we developed a versatile two point contacting approach that tackles this challenge, which is demonstrated by constructing in situ a prototypical SET device consisting of a single aluminium cluster of 66 ± 5 atoms, deposited directly in a gold nanogap using an innovative cluster beam deposition technique. The gate driven conductance measurements demonstrate Coulomb blockade oscillations at room temperature correlating with an extracted charging energy of 0.14 eV, which is five times larger than kBT at 300 K. Our work provides a model SET device platform to probe the quantum features of nano-objects with high precision.

An Eight-Atom Iridium-Aluminum Oxide Cluster IrAlO6+ Catalytically Oxidizes Six CO Molecules.

Fundamental understanding regarding oxygen storage capacity involving how and why an active site can buffer a large number of oxygen atoms in redox processes is vital to the design of advanced oxygen storage materials, while it is challenging because of the complexity of heterogeneous catalysis. Herein, we identified that an eight-atom iridium-aluminum oxide cluster IrAlO6+ can transfer all the oxygen atoms to catalytically oxidize six CO molecules. This finding represents a breakthrough in cluster catalysis where at most three oxygen atoms from a heteronuclear metal oxide cluster can be catalytically involved in CO oxidation. We found that oxygen prefers to be stored on aluminum to form an O3-• radical in the energetically unfavorable IrAlO6+ isomer and generate the low-coordinated iridium that is pivotal to capturing CO and triggering the catalysis. The powerful electron cycling capability of iridium and the cooperative iridium-aluminum interplay are emphasized to drive the oxygen atom-transfer behavior.

What should the density of amorphous solids be?

A survey of published literature reveals a difference in the density of amorphous and crystalline solids (organic and inorganic) on the order of 10%-15%, whereas for metallic alloys, it is found to be typically less than 5%. Standard geometric models of atomic packing can account for the polymeric and inorganic glasses without requiring changes in interatomic separations (bond lengths). By contrast, the relatively small difference in density between crystalline and glassy metals (and metallic alloys) implies variations in interatomic separations due to merging orbitals giving rise to reduced atomic volumes. To test this hypothesis, quantum density functional theory computations were carried out on ordered and irregular clusters of aluminum. The results point to decreasing interatomic distances with decreasing coordination, from which one can deduce that the geometrical method of random hard sphere packing significantly underestimates the densities of amorphous metallic alloys.

First principles study on small ZrAln and HfAln clusters: Structural, stability, electronic states and CO2 adsorption

We report a first principles study based on density functional theory on the structural and electronic properties of transition metal Zr and Hf doped small aluminum clusters with 1–7 aluminum atoms. We have adopted B3PW91/LANL2DZ basis set to minimize the geometries of ZrAln and HfAln clusters. The stability analysis reveals that the ZrAl4 and HfAl4 structures with C2v symmetry and square pyramid geometry are the lowest energy structures. The most stable structures in ZrAl5 and HfAl5 are distorted tetrahedron type structure with symmetry C1. The binding energy per atom for transition metal-doped Aln clusters increases with the cluster size, while the second-order difference in total energy shows oscillatory behavior with even and odd cluster size. The HOMO – LUMO gap for ZrAln clusters is larger than the HfAln clusters except for n = 6 and 7. The HfAl4 clusters have more tendencies to accept or give away electrons. The negative charge exists on Zr and Hf indicating the electron transfers from Al atom to transition metal, Zr and Hf. The thermodynamical analysis suggests that the HfAl6 clusters with spin multiplicity one have the highest exothermicity compared to not only all considered Al clusters but also other transition metal-doped Al clusters as reported in J. Phys. Chem. C, 120, 10027 (2016). This reveals that the HfAl6 clusters with a spin multiplicity of one have more tendency towards the adsorption of CO2 compared to other clusters. The calculated adsorption energy of CO2 on ZrAln and HfAln nano clusters reveals the potential application for CO2 reduction.

Mass separated particle flux from a laser-ablation metal cluster source

AbstractFlux waveforms of aluminum cluster beams supplied from a laser-ablation cluster source were precisely investigated under various source conditions such as background pressure, ablation laser intensity, and nozzle structure. A time-of-flight mass spectroscopy revealed that aluminum clusters with sizes up to 200 were generated and the amount of the clusters could be maximized by choosing a proper background pressure (~2 MPa) and an ablation laser fluence (~40 mJ/cm2). Flux waveforms of clusters having specific sizes were carefully reconstructed from the observed mass spectra. It is found that the pulse widths of the aluminum cluster beams were typically about 100 µs and much smaller than that of the monoatomic aluminum beam, indicating that the cluster formation was limited in a relatively small volume in the laser-ablated vapor. Introducing a conical nozzle having a large open angle was also found to enhance the cluster beam velocity and reduce its pulse width. A velocity measurement of particles in the cluster beam was conducted to examine the velocity spread of the supplied clusters. We found that the aluminum clusters were continuously released from the source for about 100 µs and this release time mainly determined the pulse width of the cluster beam, suggesting that controlling the behavior of an ablated vapor plume in the waiting room of the cluster source holds the key to drastically improving the cluster beam flux.

Al Valence Controls the Coordination and Stability of Cationic Aluminum-Oxygen Clusters in Reactions of Aln+ with Oxygen.

The reactivity of cationic aluminum clusters with oxygen is studied via a customized time-of-flight mass spectrometer. Unlike the etching effect for anionic aluminum clusters exposed to oxygen, here, the cationic Aln+ clusters react and produce a range of small AlnOm+ clusters. Relatively large-mass abundances are found for Al3O4+, Al4O5+, and Al5O7+ at lower O2 reactivity, while at higher O2 concentration, oxygen addition leads to Al2O7+, Al3O6,8-10+, and Al4O7,9+, showing relatively high abundance, and Al5O7+ remains as a stable species dominating the Al5Om+ distribution. To understand these results, we have investigated the structures and stabilities of the AlnOm+ clusters. First-principles theoretical investigations reveal the structures, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, fragmentation energies, ionization energies, and Hirshfield charge of the AlnOm+ clusters (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). Energetically, Al3O4+, Al4O5+, and Al5O7+ are calculated to be most stable with high fragmentation energies; however, they still allow for the chemisorption of additional O2 with large binding energies leading to clusters with higher O/Al ratios. The stability of the species is consistent with Al possessing three valence electrons, while O typically accepts two, leading to the expectation that Al3O4+, Al5O7+, and Al7O10+ are reasonably stable. In addition to this, Al3O+, Al5O3+, and Al7O5+ are found to exhibit large HOMO-LUMO gaps associated with the different oxidation states of Al. The oxygen-rich species such as Al2O7+, Al3O10+, and Al4O9+ all display superoxide structures providing further insights into the oxidation of aluminum clusters.

Curvature distribution and autocorrelations in elliptic cylinders and cones

Not all micro-vessels (MV) are traditionally circular and there are examples of elliptic cylindrical MVs in life sciences, particularly if projected with a slant. Similarly, certain biological structures, ferroelectric liquid crystals, aluminum oxide clusters and witherite crystallites’ cross-section appear to be elliptical cones. We studied the mean curvature (H) distribution of these elliptic morphological structures with geometric parameter such as eccentricity; e (ratio of semi-minor to semi-major axes) and a measure of how much diagonal section deviates from circularity and height (h) in case of cones. By means of topographical cues, we defined the curvature-curvature autocorrelation function (gk) and applied this notion to mean curvature (H) of circular and elliptical cylinders and cones. The Fourier transform of correlation function, i.e. “curvature factor” is analogous to “structure factor (or Patterson function)” in X-ray and neutron scattering intensity. It elucidates critically important information related to surface curvature fluctuation relevant to shape (geometry), network and phase transformation. The latter is induced by cells under mechanical stress, occurring in many soft systems (polymeric liquid crystals, foams, bubbles) and biological tissues, particularly cell walls of primary and branched vessels bed in microvasculature that distributes blood within tissue during hypertension and migraines. This perspective is useful in a sustained release of angiogenic/vasculogenic factors and relevant for precision medicine and engineered microvessels and tissues in vitro and in vivo extended cellular processes. The quantitative analysis carried out in this work facilitates our understanding of the mechanical mechanisms associated with thrombosis during surgery that typically occur in bent or stretched MVs due to microenvironment such as localized shear stresses and biochemical factors.

Electronic factors determining the methane bond breaking process on small aluminum clusters

AbstractIn order to understand the catalytic activity of small metal clusters as a function of their size, we have studied the interaction of CH4 with Al4 and Al5 neutral and charged clusters, as well as neutral thermally expanded clusters in the two lowest lying spin states, using density functional theory. These calculations, via extended search, are used to determine the stable positions of H and CH3 near the cluster, and the transition state to break the H─CH3 bond. In order to understand the factors underlying the reactivity of the clusters, we have analyzed the electronic structure at the transition state. By an analysis of the change of the electronic density of states close to the transition state, we identify the orbitals involved in the bond breaking process. In conjunction with our previous studies of Al2 and Al3 clusters, we find that the small Al clusters, except for Al5, lower the CH3─H dissociation barrier with respect to the gas‐phase value, although Al lacks occupied d‐orbitals. Still, Al5 does not catalyze methane bond breaking, which is attributed to the required interaction with low‐lying Al sp‐states. Furthermore, in all cases where stable methyl‐aluminum‐hydrides are possible, the recombinative desorption of methane is studied by vibrational analysis and application of transition state theory.

Theoretical Study of the Structure and Stability of Layerwise Hydrogenated Aluminum Clusters Al44Hn and Al89Hm

Structural parameters, energies, and spectroscopic characteristics of two series of layerwise hydrogenated aluminum clusters Al44Hn (n = 27–44) and Al89Hm (m = 15, 24, 39, and 63) have been calculated by the density functional theory method. It has been shown that increasing number of H atoms in both series entails rapid enhancement of structural distortions up to cooperative rearrangements accompanied by a change in the shape and composition of the surface layer and internal core of the cluster. At the end of the first series Al44Hn, several surface atoms migrate to the outer sphere of the cage to form valence-unsaturated “outer-surface” AlHn and Al2Hn moieties, which can be active sites at the stages of deeper hydrogenation. Simultaneously, the inner core [Al]5 disintegrates, and its atoms are introduced into the surface layer. A family of “inverted” Al42H42 isomers with the hollow [Al42] cage has been localized; the isomers contain the endohedral AlH4 group and “inner” Al-H bonds with their hydrogen end directed to the center of the inner cavity. At the end of the second series, five alanate groups AlH4 and two Al3H2 fragments bonded to the surface through hydrogen bridges are formed in the outer sphere of the $$\rm{Al}_{89}H_{63}^-$$ cluster. The results are of interest for DFT modeling of hydrogenation of nanosized aluminum clusters at the molecular level.

Multiple-Valence Aluminum and the Electronic and Geometric Structure of AlnOm Clusters

Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of Al nO m (2 ≤ n ≤ 7; 1 ≤ m ≤ 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al2 nO3 n clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al4O4, Al5O3, Al6O5, and Al6O6. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.

Glass transition in disordered clusters

Using molecular dynamics simulations, the melting behavior of disordered aluminum clusters with 30∼100 atoms has been systematically studied. We find that, all the disordered clusters undergo a glass transition before completely melting. We identify two characteristic temperatures, i.e.Ts and Tg (the glass transition temperature). When the temperature is higher than Ts, the cluster shape starts to change, accordingly the configurational entropy emerges. For temperatures between Ts and Tg, the disordered clusters show a remarkable cooperative diffusion, more importantly, the activation energy of cooperative diffusions below Tg is always lower than that of the liquid state. We have tried to understand such behavior from potential landscape and configurational entropies.

Hydrogen Chemisorption on Doubly Vanadium Doped Aluminum Clusters

Abstract The interaction of hydrogen with doubly vanadium doped aluminum clusters, Al n V2 + (n = 1–12), is studied experimentally by time-of-flight mass spectrometry and infrared multiple photon dissociation spectroscopy. The hydrogen binding geometry is inferred from comparison with infrared spectra predicted by density functional theory and shows that for the more reactive clusters the hydrogen adsorbs dissociatively. Three sizes, n = 4, 5 and 7, are remarkably unreactive compared to the other clusters. For larger sizes the reactivity decreases, a behavior that is similar to that of singly vanadium doped aluminum clusters, and that might be attributed to geometric and/or electronic shielding of the dopants. By examining the electronic structure of Al6V2 + and Al7V2 +, interactions between the frontier orbitals of the clusters and those of H2 that explain the size-dependent reactivity are identified.

Surface barriers and field direction dependent vortex phase diagrams of YBa2Cu3−xAlxOδ for H c

We report the observation of an unusual field direction dependent second magnetisation peak (SMP) anomaly at very low fields, close to the field of full penetration Hp, in an aluminium-doped single crystal of YBa2Cu3−xAlxOδ, for H c. Such an anomaly is observed at different field values in the field ramp-up direction (H) compared to the field-down direction (H), and it is also found to persist to different temperatures. For the field-up direction, it is observed in the range of 8 K to 30 K (upper range extendible to 40 K depending on the crystal), while for the field-down direction, it is observed between 8 K to 65 K (upper range extendible to 70 K depending on the crystal). The temperature dependence of the fields of full penetration Hp, the onset field of the SMP anomaly, as well as the central magnetisation peak, seem to be affected by surface barriers at low temperatures, where they are seen to rise anomalously. An exponential fit to the low temperature penetration field and central magnetisation peak width, as well as an asymmetric magnetisation towards the irreversibility field, illustrate the effect of surface barriers at low temperatures. At higher temperatures, the onset field of the SMP anomaly is described well by a dislocation mediated order–disorder transition. The temperature dependence of the peak field of the second magnetisation anomaly, however, does not seem to be affected by the surface barrier and is well described by a power law. An oxygen deficient aluminium cluster model is used to explain the differences in the SMP anomalies in the two quadrants. Two vortex phase diagrams, one for each field direction, have been made describing the various phases.

DFT modeling of successive hydrogenated subnano-size aluminum clusters

Energies, structural and vibrational characteristics of isomers of step-wise hydrogenated aluminum clusters Al13Hn, Al44Hn and Al89Hn were calculated using the DFT B3LYP method and the evolution of the clusters with increasing number n of H atoms was followed. It is found that the geometry distortions of the clusters grow with rising n, becoming ultimate at the end of the series and resulting in complicated cooperative rearrangements. An increasing number of the surface atoms are shifted to the oversurface area and form external valence-unsaturated fragments like AlHn, Al2Hn, etc., together with a gradual disintegration of the subsurface layer and embedment of the subsurface atoms into the surface shell. The surface shells acquire 5- or 6-membered “windows” which can serve as “gates” for the migration of external H atoms into the inner cavity and the formation of nontrivial structures with internal H-bridging bonds.

Theoretical study on the aggregation of aluminium clusters on a silicone oil surface

By using Amsterdam Density Functional (ADF) program based on first-principles, we have in detail investigated the formation mechanism of the aluminium (Al) nanoparticles (NPs) on a silicone oil surface. Our investigation shows that two moving Al clusters on liquid substrate meet to form a random initial structure. Collisions and interactions may make it become a more compact, more stable structure. As the deposition density increases, a discshaped morphology would be formed because of isotropy on the silicone oil surfaces. The structures built on the Face-Centered Cubic (FCC) units are not stable. Both atomic displacement and adsorption can almost destroy them. It is difficult to form Al crystals on the silicone oil surfaces.

Experimental and theoretical 2p core-level spectra of size-selected gas-phase aluminum and silicon cluster cations: chemical shifts, geometric structure, and coordination-dependent screening.

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al12+, Si15+, Si16+, and Si19+ are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.

Structure, stability, absorption spectra and aromaticity of the singly and doubly silicon doped aluminum clusters Al n Si m 0/+ with n = 3-16 and m = 1, 2.

Structures of the binary AlnSim clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Aln cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al4Si2, Al12Si and Al11Si2+ as “magic” clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al13−, Al12Si, and Al11Si2+ that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the “magic” 40 shell electrons.

Stabilization of cationic aluminum hydroxide clusters in high pH environments with a CaCl2/l-arginine matrix.

We present a way of stabilizing cationic partially hydrolyzed aluminum clusters in a non-acidic environment, through Ca2+ and l-Arginine doping. The Keggin Al13-mer (ε-AlO4Al12(OH)24(H2O)127+) aluminum cluster can be stabilized with CaCl2 and l-arginine in a way to preserve the metal clusters. We use size-exclusion chromatography (SEC) and 27Al nuclear magnetic resonance (NMR) spectroscopy to demonstrate that positively-charged Keggin structures are preserved and that the conversion to Al(OH)3 materials is halted even at alkaline pH. The system serves to stabilize acidic Al clusters in alkaline or neutral conditions, while preserving their inherent cationic behavior.

Geometric Structures and Electronic Properties of AlnV0/– (n = 5–14) Clusters: Photoelectron Spectroscopy and Theoretical Calculations

Aluminum and aluminum-doped clusters have gained much attention in cluster science due to their potential applications in nanotechnology. Here, we report a combined photoelectron spectroscopy and d...