Cluster-assembled Material Research Articles

A silver cluster-assembled material as a matrix for enzyme immobilization towards a highly efficient biocatalyst.

Silver cluster-assembled materials (SCAMs) epitomize well-defined extended crystalline frameworks that combine the ingenious designability at the atomic/molecular level and high structural robustness. They have captivated the interest of the scientific fraternity because of their modular construction which enables to systematically tailor their functions, and their capacity to not only inherit the characteristics of component building units but also introduce their uniqueness in endowing the final material with extraordinary properties. Herein, we demonstrate the synthesis of a novel (3,6)-connected two-dimensional (2D) SCAM [Ag12(StBu)6(CF3COO)6(THIT)6]n (described as TUS 5, THIT = 2,4,6-tri(1H-imidazol-1-yl)-1,3,5-triazine) composed of Ag12 cluster nodes and tritopic imidazolyl linkers. We have leveraged, for the first time, this precisely architected extended SCAM structure as a support matrix for enzyme immobilization. The electrostatic attraction between the negatively charged amano lipase PS and positively charged TUS 5 as well as the surface hydrophobicity of TUS 5 catered to great binding of lipase onto the TUS 5 matrix, in addition to boosting the activity of lipase via interfacial activation. Capitalizing on the cooperative benefits of organic and inorganic support matrices wherein organic supports impart with cost-efficiency, biocompatibility, and improved enzyme stability and reusability and inorganic supports confer high thermal, mechanical and microbial resistance, we have utilized the immobilized lipase on TUS 5 SCAM (lipase@TUS 5) for the kinetic resolution of (R,S)-1-phenylethanol by transesterification reaction. Importantly, lipase@TUS 5 could attain appreciably higher conversion into (R)-1-phenylethyl acetate, besides featuring superior thermal stability, solvent tolerance and recyclability, over the native lipase.

Two-Dimensional Silver-Chalcogenolate-Based Cluster-Assembled Material: A p-type Semiconductor.

We have synthesized and characterized a new two-dimensional honeycomb architecture resembling a single-layer of atomically precise silver cluster-assembled material (CAM), [Ag12(StBu)6(CF3COO)6(4,4'-azopyridine)3] (Ag12-azo-bpy). The interlayer noncovalent van der Waals interactions within the single-crystals were successfully disrupted, leading to the creation of this unique structure. The optimized Ag12-azo-bpy CAM demonstrates a valence band that is localized on the Ag12 cluster node situated near the Fermi energy level. This localization induces electron injection from the linker to the cluster node, facilitating efficient charge transportation along the plane. Exploiting this single-layer structure as a distinctive platform for p-type channel material, it was employed in a field-effect transistor configuration. Remarkably, the transistor exhibits a high hole mobility of 1.215 cm2 V-1 s-1 and an impressive ON/OFF current ratio of ∼4500 at room-temperature.

Ligand-induced room-temperature dual emission in two-dimensional silver cluster-assembled material

Ligand-induced room-temperature dual emission in two-dimensional silver cluster-assembled material

A new two-dimensional luminescent Ag12 cluster-assembled material and its catalytic activity for reduction of hexacyanoferrate(III).

Silver cluster-assembled materials (SCAMs) have garnered significant interest as promising platforms for different functional explorations. Their atomically precise structures, intriguing chemical/physical properties, and remarkable luminescent capabilities make them highly appealing. However, the properties of these materials are primarily determined by their structural architecture, which is heavily influenced by the linker molecules used in their assembly. The choice of linker molecules plays a pivotal role in shaping the structural characteristics and ultimately determining the unique properties of SCAMs. To this end, the first SCAM with an intriguing (3,6)-connected kgd topology, [Ag12(StBu)6(CF3COO)6(TPBTC)6]n (termed TUS 3), TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide, has been synthesized by reticulating C6-symmetric Ag12 cluster cores with C3-symmetric tripodal pyridine linkers. Due to the structutural architecture of the linker molecule, TUS 3 posseses a luminescent porous framework structure where each two-dimensional (2D) layers are non-covalently linked with each other to form a three dimensional (3D) framework and ultimately offers uniaxial open channels. The compact mesoporous structural architecture not only gives the excellent surface area but also offers impressive stability of this material even in water medium. Taking advantage of these properties, TUS 3 shows brilliant catalytic activity in the reduction of hexacyanoferrate(III) using sodium borohydride in aqueous solutions.

Fe@B6H6 aggregates: from simple building blocks to graphene analogue

We suggest the possibility to build graphene analogue with the planar hexacoordinate wheel-type Fe@B6H6 cluster as the building block through studying theoretically the geometry, stability, and electron structure of its dimer and trimer as well as the dimerization of the two trimers. Employing the dehydrogenation route to polymerization, we can obtain the hexagonal boron sheet that are partly and uniformly filled by Fe atoms in the center of the holes, achieving uniform chemical doping and a very large hexagonal-hole density. Thus, we may offer a novel cluster-assembled material for experimental chemists to construct graphene analogue.

Switchable photon and phonon emission properties of an atomically precise Ag14core-based two-dimensional silver cluster-assembled material

Changes in structural architectureviainterlayer C–H⋯F interactions triggered by solvent molecules influence the dual (photon and phonon) emission mechanism of a two-dimensional cluster-assembled material, [Ag14(StBu)10(CF3COO)4(4,4′-azopyridine)2].

Ambient Chemical Fixation of CO2 Using a Robust Ag27 Cluster-Based Two-Dimensional Metal-Organic Framework.

Unprecedented double S2- templated Ag27 clusters have been stabilized by 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP-H2 ) ligands to afford a robust 2D metal-organic framework (Ag27-MOF). This silver cluster-assembled material serves as a highly efficient heterogeneous catalyst for the cyclization of both terminal and internal propargylamines with CO2 under atmospheric pressure. Density functional theory (DFT) calculations illustrate that the high catalytic activity and broad substrate scope are attributable to the saddle-shaped metallic node in Ag27-MOF, which features an accessible platform with high-density silver atoms as π-Lewis acid sites for activating C≡C triple bonds. As a result, different sterically hindered alkyne substrates can be effectively activated through π-interactions with these cationic silver centers.

High-performance primary explosives derived from copper thiolate cluster-assembled materials for micro-initiating device

As an important primary explosive, copper azide is attractive for its outstanding detonation power, but it is limited by high electrostatic sensitivity and inferior ignition ability for use in micro-initiating device. In this work, we report the first copper thiolate cluster-assembled material (Cu12bpy, bpy = 4,4′-bipyridine) and successfully employ it as a precursor to prepare copper azide-heteroatoms doped porous carbon hybrids (denoted as CA-HPCH). Given the dense copper content, abundant heteroatoms and periodic structure of Cu12bpy, as-obtained CA-HPCH exhibits high copper azide content (89.43%), excellent flame ignition capability (H50 ≥ 60 cm) and reduced electrostatic sensitivity (E50 = 1.1 mJ). The performance is superior to pristine copper azide and surpasses that of most reported primary explosives. Furthermore, the micro-initiating device fabricated by CA-HPCH displays relatively short ignition time (7.0 µs) and low input ignition energy (0.106 mJ). These findings provides new sight to develop high-performance primary explosives for use in micro-initiators.

2-D organization of silica nanoparticles on gold surfaces: CO2 marker detection and storage.

A single layer of silica nanoparticles with an average size of ∼200 nm was deposited over the surface of pristine gold wafers, aided by (3-mercaptopropyl)trimethoxysilane. The nanoparticle immobilization was driven by covalent bonding rather than a self-assembly process, leading to a cluster-assembled material which has CO2 sensing features. Here, we show how this device can be used for CO2 physisorption and chemisorption. We analyse the device, both spectroscopically and morphologically, before and after exposure to an atmosphere of 7 mbar of CO2, inside a planetary atmospheres and surfaces simulation chamber, (PASC) mimiking Martian atmospheric conditions. Our studies demonstrate that these clusters are suitable for CO2 detection and storage, under well controlled experimental Martian conditions. Their high sensitivity at a very low concentration of CO2, 12.4 ppm, makes them ideal candidates in the nanosensor field.

Cluster-Assembled Nanocomposites: Functional Properties by Design.

The unique functional properties of nanocomposites meet many of the material requirements sought after in numerous applications of today's high-tech industry. This, in turn, inspires material scientists to devise new methods that can further expand the palette of available nanocomposites. Precise control over the chemistry, morphology, and microstructure of nanocomposites' constituents promises the eventual ability to design any composite material for any specific requirement. However, today's synthesis methods still lack the ability to simultaneously control all chemical, morphological, and microstructural features of nanocomposites in a one-step process. Here, an alternative approach to fabricate fully tailorable nanocomposites under well-defined conditions is described. In particular, this progress report focuses on the combination of cluster ion beam and thin-film deposition technologies to fabricate cluster-assembled nanocomposites via codeposition of cluster ions and matrix materials. Emphasis is given to the state-of-the-art cluster deposition system, designed and built by our research group, as well as to its unique abilities. Moreover, case studies on two cluster-assembled nanocomposite material systems (Fe/Agm and Fe/Crm ) prepared with this method are presented. Finally, an outlook on research directions for cluster-assembled nanocomposites is discussed.

Designing a tunable magnet using cluster-assembled iron

The design of a cluster-assembled three dimensional crystal is performed through first principle calculations. In particular, Fe6 and Fe5A (A = B, C, N, F, or Cl) clusters are treated as a single atom and periodically placed in a body center cubic (BCC) unit cell. Calculations reveal that van der Waals forces play a major role between the clusters where exothermic binding energy is observed in Fe6 and Fe5A (A = B, C, N, or Cl) cluster-assembled BCC crystal. The magnetic moment of the designed cluster-assembled crystal is much higher than that of bulk iron where the magnetic moment is tunable upon the introduction of the dopants. In addition, the bandgap of cluster-assembled crystal is different from bulk Fe where Fe6, Fe5B, and Fe5C cluster-assembled BCC crystal have a large bandgap. One can thereby consider that the ability to tune the magnetic moment and bandgap is achievable in principle through the design of the cluster-assembled material as well as through the introduction of dopants. Thus, cluster-assembled crystal expands upon how a material can be designed and properties that are tailorable through controlling the structure and through the composition of the clusters.

Oxidation of ligand-protected aluminum clusters: An ab initio molecular dynamics study

We report Car-Parrinello molecular dynamics simulations of the oxidation of ligand-protected aluminum clusters that form a prototypical cluster-assembled material. These clusters contain a small aluminum core surrounded by a monolayer of organic ligand. The aromatic cyclopentadienyl ligands form a strong bond with surface Al atoms, giving rise to an organometallic cluster that crystallizes into a low-symmetry solid and is briefly stable in air before oxidizing. Our calculations of isolated aluminum/cyclopentadienyl clusters reacting with oxygen show minimal reaction between the ligand and O2 molecules at simulation temperatures of 500 and 1000 K. In all cases, the reaction pathway involves O2 diffusing through the ligand barrier, splitting into atomic oxygen upon contact with the aluminum, and forming an oxide cluster with aluminum/ligand bonds still largely intact. Loss of individual aluminum-ligand units, as expected from unimolecular decomposition calculations, is not observed except following significant oxidation. These calculations highlight the role of the ligand in providing a steric barrier against oxidizers and in maintaining the large aluminum surface area of the solid-state cluster material.

Chair like NiAu6: Clusters assemblies and CO oxidation study by ab initio methods

Based on the first-principles approach, this Letter reports that the ground state geometry of the Ni@Au6 cluster forms chair-like conformation, which is distinctly different than the Pd@Au6 and Pt@Au6 clusters, where hexagonal planar structure is favored over non-planar isomers. The higher stability of the chair-like conformation has been verified through various complementary basis sets and methodologies. Further investigations were carried out to show the stability of a cluster assembled material (CAM) based on these small clusters and the CO oxidation reaction was carried out to establish their uses as an industrial catalyst.

Enhancement in the Stability of 36-Atom Fullerene through Encapsulation of a Uranium Atom

With an objective to rationalize the experimentally observed intense U@C36 peak in the mass spectrum of U@C2n metallofullerene, structural, stability, and spectroscopic aspects of the uranium doped C36 fullerene have been studied in a unified and systematic way using density functional theory (DFT) and its time-dependent variant. Relativistic effects have been taken into account within the framework of zeroth-order regular approximation using scalar and spin–orbit-based approaches. Among all of the 15 possible classical isomers reported for the C36 fullerene cage system, singlet D2d and triplet D6h structures are found to be isoenergetic and most stable. Encapsulation of uranium atom into various C36 cages leads to 15 distinct isomers with considerable energy differences. It has also been shown that this encapsulation process results in significant gain in thermodynamic stability. The most stable U@C36 isomer is found to be associated with C6v symmetry and closed-shell electronic configuration, derived from the open-shell D6h structure of C36. The next stable isomer is associated with Cs symmetry and obtained from the corresponding singlet D2d structure of the C36 cage. Distinct changes have also been found in the calculated vibrational and UV–visible spectra of the U@C36 cluster as compared to the corresponding bare C36 cage. All of the calculated quantities reported here suggest that the stability of the U@C36 cluster is high enough for possible formation of cluster-assembled material leading to synthesis of this metallofullerene experimentally.

Size-Selected Monodisperse Nanoclusters on Supported Graphene: Bonding, Isomerism, and Mobility

Soft-landing of size-selected Pd(n) (n ≤ 20) nanoclusters on a Moiré-patterned surface of graphene adsorbed on Ru(0001) leads to controlled formation of a truly monodisperse cluster-assembled material. Combined scanning tunneling microscopy and first-principles calculations allow identification of selective adsorption sites, characterization of size-dependent cluster isomers, and exploration of interconversion processes between isomeric forms that manifestly influence cluster surface mobility. Surface-assembled cluster superstructures may be employed in nanocatalytic applications, as well as in fundamental investigations of physical factors controlling bonding, structure, isomerism, and surface mobilities of surface-supported clusters.

Stability and magnetic properties ofT2Sin(T=Cr,Mn,1≤n≤8)clusters

First-principles studies on the geometry, electronic structure, and magnetic properties of neutral and anionic ${\text{Cr}}_{2}{\text{Si}}_{n}$ and ${\text{Mn}}_{2}{\text{Si}}_{n}$ $(1\ensuremath{\le}n\ensuremath{\le}8)$ have been carried out within a gradient corrected density-functional framework. We find that ${\text{Cr}}_{2}{\text{Si}}_{n}$ clusters containing up to five Si atoms and ${\text{Mn}}_{2}{\text{Si}}_{n}$ clusters containing up to eight Si atoms are all marked by finite local spin moments at the transition-metal site that order ferromagnetically or antiferromagnetically depending on the size and the charged state. Our studies of the variation in the binding energy upon addition of successive Si atoms and the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital indicate ${\text{Mn}}_{2}{\text{Si}}_{4}$ to be a potential motif for generating magnetic cluster-assembled material. While the individual ${\text{Mn}}_{2}{\text{Si}}_{4}$ motif has a ground state with antiferromagnetically coupled local moments, the studies on the assembly of two basic motifs show that it favors a ferromagnetic state. It is hoped that the present work will motivate examination of such assemblies in the recently developed cluster beam deposition experiments.

Dynamics of confined plumes during short and ultrashort pulsed laser ablation of graphite

The optical emission from electronically excited C species in the ablation plume following the short (ns) and ultrashort (fs) UV pulsed laser ablation of graphite is studied. Wavelength, time and spatially resolved imaging of the plume, in background pressures of inert gases such as argon and helium, is performed. Analysis of images of optical emission from ${\mathrm{C}}^{+*}$ ions and ${\mathrm{C}}_{2}^{*}$ radicals, yielded estimates of the apparent velocity of emitting species, which appear to arise both from the initial ablation event and, in the presence of background gas, mainly from impact excitation. At elevated background pressures of argon $({P}_{\mathrm{Ar}})$, the formation and propagation of a shock wave is observed for ns pulses, whereas for fs pulses, the propagation of two shock waves is observed. During fs ablation, the first shock wave we associate with an initial burst of highly energetic/electronically excited ablated components, indicative of an enhanced fraction of non-thermal ejection mechanisms when compared with ns ablation. The second shock wave we associate with subsequently ejected, slower moving, material. Concurrent with the plume dynamics investigations, nanostructured amorphous carbon materials were deposited by collecting the ablated material. By varying ${P}_{\mathrm{Ar}}$ from 5 to 340 mTorr, the film morphology could be changed from mirror smooth, through a rough nanostructured phase and, at the highest background pressures for ns pulses, to a low density cluster-assembled material. The evident correlations between the film structure, the mean velocities of the emitting C species, and their respective dependences upon ${P}_{\mathrm{Ar}}$ are discussed for both pulse durations. In addition, we comment on the effect of observed initial plume dynamics on the subsequent C cluster formation in the expanding plume.

Influence of cluster-assembly parameters on the field emission properties of nanostructured carbon films

Supersonic cluster beam deposition has been used to produce films with different nanostructures by controlling the deposition parameters such as the film thickness, substrate temperature and cluster mass distribution. The field emission properties of cluster-assembled carbon films have been characterized and correlated to the evolution of the film nanostructure. Threshold fields ranging between 4 and 10 V/μm and saturation current densities as high as 0.7 mA have been measured for samples heated during deposition. A series of voltage ramps, i.e., a conditioning process, was found to initiate more stable and reproducible emission. It was found that the presence of graphitic particles (onions, nanotube embryos) in the films substantially enhances the field emission performance. Films patterned on a micrometer scale have been conditioned spot by spot by a ball-tip anode, showing that a relatively high emission site density can be achieved from the cluster-assembled material.

In Situ Laser-Furnace TOF Mass Spectrometry of C36 and the Large-Scale Production by Arc-Discharge

A greatly improved synthesis of C36, obtained as its hydrides C36H4 and C36H6 and oxyhydrides C36H4O and C36H6O, via a high- temperature laser vaporization (laser-furnace) method and the conventional DC arc-discharge method is reported. We have found that doping metal catalysts such as Ni/Co or Ni/Y into the laser target graphite rod or the DC arc-discharge electrode, respectively, significantly enhanced the production of C36 species; the methods have been known for efficient production methods of single-wall carbon nanotubes (SWNTs). The results suggest that the catalytic formation of C36 species and SWNTs is closely related with each other in their early stage of formation. Furthermore, a molecular C36H6 gives C18H3 (m/z = 219) as a magic fragment upon desorption/ionization. By using a high-performance liquid chromatography (HPLC), C36H6 was purified (99%) in milligram quantity for the first time. The purified C36H6 exists as a cluster-assembled material such as oligomers or clusters in the solid state,...

ELECTRONIC STRUCTURE OF ASSEMBLED CLUSTERS WITHIN THE STABILIZED JELLIUM MODEL

The electronic structure of assembled closed shell clusters have been analyzed using a spherical, stabilized jellium description of the clusters. The effects of including stabilization terms to the jellium model were investigated by calculating the properties of cluster dimers, (Na20)2 and ( Cu 20)2. In addition, the electronic properties of a material consisting of closed shell Al 12X clusters, where X is C or Si, were investigated using a model of six clusters. The large gap at the Fermi level present for the building blocks was also found to appear for the cluster-assembled material. The binding energy of the clusters in the cluster-assembled material was compared with that of an atomic Al lattice.