Cluster-assembled Materials Research Articles

Second-Row Transition-Metal Doping of (ZniSi), i = 12, 16 Nanoclusters: Structural and Magnetic Properties

TM@ZniSi nanoclusters have been characterized by means of the Density Functional Theory, in which Transition Metal (TM) stands from Y to Cd, and i = 12 and 16. These two nanoclusters have been chosen owing to their highly spheroidal shape which allow for favored endohedral structures as compared to other nanoclusters. Doping with TM is chosen due to their magnetic properties. In similar cluster-assembled materials, these magnetic properties are related to the Transition Metal-Transition Metal (TM-TM) distances. At this point, endohedral doping presents a clear advantage over substitutional or exohedral doping, since in the cluster-assembled materials, these TM would occupy the well-fixed center of the cluster, providing in this way a better TM-TM distance control to experimentalists. In addition to endohedral compounds, surface structures and the TS’s connecting both isomers have been characterized. In this way the kinetic and thermal stability of endohedral nanoclusters is predicted. We anticipate that silver and cadmium endohedrally doped nanoclusters have the longest life-times. This is due to the weak interaction of these metals with the cage, in contrast to the remaining cases where the TM covalently bond to a region of the cage. The open-shell electronic structure of Ag provides magnetic properties to Ag@ZniSi clusters. Therefore, we have further characterized (Ag@Zn12S12)2 and (Ag@Zn16S16)2 dimers both in the ferromagnetic and antiferromagnetic state, in order to calculate the corresponding magnetic exchange coupling constant, J.

Cluster-assembled metallic glasses

A bottom-up approach to nanofabricate metallic glasses from metal clusters as building blocks is presented. Considering metallic glasses as a subclass of cluster-assembled materials, the relation between the two lively fields of metal clusters and metallic glasses is pointed out. Deposition of selected clusters or collections of them, generated by state-of-the-art cluster beam sources, could lead to the production of a well-defined amorphous material. In contrast to rapidly quenched glasses where only the composition of the glass can be controlled, in cluster-assembled glasses, one can precisely control the structural building blocks. Comparing properties of glasses with similar compositions but differing in building blocks and therefore different in structure will facilitate the study of structure–property correlation in metallic glasses. This bottom-up method provides a novel alternative path to the synthesis of glassy alloys and will contribute to improving fundamental understanding in the field of metallic glasses. It may even permit the production of glassy materials for alloys that cannot be quenched rapidly enough to circumvent crystallization. Additionally, gaining deeper insight into the parameters governing the structure–property relation in metallic glasses can have a great impact on understanding and design of other cluster-assembled materials.

A Density Functional Study of the Gold Cages MAu16 (M = Si, Ge, and Sn)

Relativistic density functional calculations are performed to explore the promise of MAu16(M=Si, Ge, and Sn) clusters as magic clusters and building blocks in developing cluster-assembled materials. C1 and Cs, two isomers of SiAu16, GeAu16 and SnAu16 with M (Ge or Sn) at the center of the cage, named, respectively, as SiAu16—C1, SiAu16—Cs, GeAu16-center, and SnAu16-center, are calculated to be the most stable. The Au—M bond should have both ionic and covalent characteristics. Their static linear polarizabilities and first-order hyperpolarizabilities are found to be sensitive to the delocalization of the valence electrons of the M atom, as well as their structures and shapes.

Endohedrally doping the gold cage [formula omitted] with an trivalent atom B, Al, Ga, and In: Density functional studies

The relativistic density functional calculations explore that M@Au16- (M=B, Al, Ga, and In) with M at the cage center, named as M@Au16--center, should be the most stable. The M@Au16--center (M=B, Al, Ga, and In) clusters have closed-shell electronic structures and large energy gaps. All of these properties are characteristic of a magic cluster and can be well understood by the jellium model. Therefore, we strongly suggest M@Au16--center (M=B, Al, Ga, and In) are magic clusters and promising as building blocks in developing cluster-assembled materials. The energy levels and the wavefunctions of frontier orbitals explore that the HOMOs of M@Au16--center should be partly occupied by the M atom, while the LUMOs of them should be mostly from the Au16- bodies. The difference charge densities and the natural bonding orbital charge analyses imply the Au–M bonds have both the ionic and covalent characters. Finally, the mean static linear polarizabilities and first-order hyperpolarizabilities of M@Au16--center are larger than those of Au16-, while their anisotropies of the polarizability tensors are smaller than those of Au16-. We rationalize the nonlinear properties by studying the low-energy optical absorption band obtained by employing time-dependent density functional theory.

Controlling the Band Gap Energy of Cluster-Assembled Materials

Cluster-assembled materials combine the nanoscale size and composition-dependent properties of clusters, which have highly tunable magnetic and electronic properties useful for a great variety of potential technologies. To understand the emergent properties as clusters are assembled into hierarchical materials, we have synthesized 23 cluster-assembled materials composed of As7(3-)-based motifs and different countercations and measured their band gap energies. We found that the band gap energy varies from 1.09 to 2.21 eV. In addition, we have carried out first principles electronic structure studies to identify the physical mechanisms that enable control of the band gap edges of the cluster assemblies. The choice of counterion has a profound effect on the band gap energy in ionic cluster assemblies. The top of the valence band is localized on the arsenic cluster, while the conduction band edge is located on the alkali metal counterions. Changing the counterion changes the position of the conduction band edge, enabling control of the band gap energy. We can also vary the architecture of the ionic solid by incorporating cryptates as counterions, which provide charge but are separated from the clusters by bulky ligands. Higher dimensionality typically decreases the band gap energy through band broadening; however band gap energies increased upon moving from zero-dimensional (0D) to two-dimensional (2D) assemblies. This is because internal electric fields generated by the counterion preferentially stabilize the adjacent lone pair orbitals that mark the top of the valence band. Thus, the choice of the counterion can control the position of the conduction band edge of ionic cluster assemblies. In addition, the dimensionality of the solid via internal electric fields can control the valence band edge. Through covalently linking arsenic clusters into composite building blocks, we have also been able to tune the band gap energy. We used a theoretical description based on cluster orbital theory to provide microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy. Also, we have shown how dimeric linkers can be used to control the band gap energy. Lastly, we also investigated the effects of charge transfer complexes of M(CO)3 on the band gap energy.

Density functional studies of magic clusters Ga13M(M = Li, Na, K, and Rb)

Density functional studies are performed on the structures, stabilities, electronic properties, and polarizabilities of the Ga13M(M=Li, Na, K, and Rb) clusters, which have closed electronic shell structures, enhanced stabilities, large HOMO–LUMO energy gaps, high vertical ionization potentials, and low vertical electron affinities. All of these properties are characteristic of a magic cluster and can be well understood by the jellium model. Therefore, we strongly suggest Ga13M(M=Li, Na, K, and Rb) are magic clusters and promising as building blocks in developing cluster-assembled materials. The calculated vertical electron affinity of Ga13 is close to that of bromine, implying it would be like a halogen atom in the Ga13M molecule. The static linear polarizabilities and first-order hyperpolarizabilities of Ga13M clusters are significantly larger than those of Ga13 because of their lower symmetries and higher delocalization of p electrons.

MSi20H20 Aggregates: From Simple Building Blocks to Highly Magnetic Functionalized Materials

Density-functional theory based global geometry optimization is used to scrutinize the possibility of using endohedrally-doped hydrogenated Si clusters as building blocks for constructing highly magnetic materials. In contrast to the known clathrate-type facet-sharing, the clusters exhibit a predisposition to aggregation through double Si-Si bridge bonds. For the prototypical CrSi$_{20}$H$_{20}$ cluster we show that reducing the degree of hydrogenation may be used to control the number of reactive sites to which other cages can be attached, while still preserving the structural integrity of the building block itself. This leads to a toolbox of CrSi$_{20}$H$_{20-2n}$ monomers with different number of double "docking sites", that allows building network architectures of any morphology. For (CrSi$_{20}$H$_{18}$)$_{2}$ dimer and [CrSi$_{20}$H$_{16}$](CrSi$_{20}$H$_{18}$)$_{2}$ trimer structures we illustrate that such aggregates conserve the high spin moments of the dopant atoms and are therefore most attractive candidates for cluster-assembled materials with unique magnetic properties. The study suggests that the structural completion of the individual endohedral cages within the doubly-bridge bonded structures and the high thermodynamic stability of the obtained aggregates are crucial for potential synthetic polymerization routes $via$ controlled dehydrogenation.

Low-density nanoporous phases of group-III nitrides built from sodalite cage clusters

We report a new family of M12N12 (M = Al and Ga) cluster-assembled low-density materials with distinguished structures and properties based on state-of-the-art first-principles calculations. Specifically, the thermodynamic stability of the sodalite cage M12N12, with Th symmetry and a large HOMO-LUMO gap, is firstly proved using a first-principles molecular dynamics (FPMD) study. We consider this novel structure as a building block to construct new cluster-assembled materials. On the basis of the interaction of the cages with each other, eight new low-density nanoporous phases have been characterized, some of which with high stability are even more stable than experimentally synthesized MN phases. The intrinsic higher flexibilities (lower bulk moduli) and porous characteristics (the pore size: from 0.360 to 0.952 nm for AlN, 0.381 to 0.982 nm for GaN) of these phases should make them extremely promising for molecular sieving, gas storage, and particularly, atomic transport, control and purification applications. Furthermore, these new materials can not only retain the structural characteristics of the building block, but also preserve its electronic properties of wide-energy gap, with an indirect or a direct band gap of 1.038-2.640 eV. Our results may be feasible for extending the range of properties and applications of the corresponding MN compound.

New nanomaterials based on In12As12cages: an ab initio bottom-up study

We report the results of density functional calculations on nanostructures of InAs, including the small InnAsn (n = 1–15) clusters, the stable hollow cage In12As12 and its derivative nanomaterials (In12As12-based dimer, trimer, nanowire, sheet, and new nanoporous crystal phase). Our results show that among the small InnAsn (n = 1–15) clusters, the In12As12 cage structure with Th symmetry and large HOMO–LUMO gap is proved to be the most stable one, indicating that it would be an ideal building block for the synthesis of cluster-assembled materials. Via the coalescence of In12As12 building blocks, we find that the In12As12 cages can bind into various nanomaterials with different dimensionality by the most stable bonding via In6As6 hexagonal faces forming intercluster hexagonal prism. All of these constructed nanostructures are found to be more stable than the individual In12As12 cage and rock salt (RS) phase of InAs. The band analysis reveals that the periodic In12As12-assembled nanomaterials have relative band gap magnitudes of 2.00–4.89 (Eg/Eg,ZB) with respect to the zinc blende (ZB) phase of InAs, and the new nanoporous polymorph holds an indirect gap of ∼0.86 eV, which may extend the range of properties and applications of the InAs compound.

Density functional investigation on the structures and properties of Li atom doped Au20 cluster

Structures and properties of an Au20 cluster doped with two Li atoms, Au18Li2, have been investigated using relativistic density functional theory within the framework of the zeroth-order regular approximation. Various initial structures have been generated and employed for geometry optimization followed by vibration analysis to check the stability of the final optimized structures. We have calculated various properties like binding energy, ionization potential, electron affinity and the HOMO–LUMO gap of these structures. It has been found that two dopant Li atoms favour occupying two different surface positions of the pyramidal Au20 cluster. The binding energy of the surface-doped Au18Li2 cluster is 1.017 eV higher than that of the pure Au20 cluster and the HOMO–LUMO gap (1.742 eV) is as high as a pure Au20 cluster (1.786 eV). Interestingly, we observe that the HOMO–LUMO gap as well as the binding energy can be increased beyond those of the Au18Li2 cluster with the help of further Li atom doping. In fact, a doped tetrahedral Au16Li4 cluster, where all the dopants are at the surface sites, possesses a very high HOMO–LUMO gap of 2.117 eV. Geometric and energetic parameters indicate that the Au16Li4 cluster might be considered as a possible ‘superatom’ in the design of novel cluster-assembled materials.

Measurement of cluster–cluster interaction in liquids by deposition and AFM of silicon clusters onto HOPG surfaces

We have investigated the interaction and aggregation of novel fluorescent silicon nanoclusters in liquids by measuring the size distribution of dried clusters on graphite. The clusters were produced by gas aggregation and co-deposition with a beam of water vapour. Drops of the solutions were placed on freshly cleaved highly oriented pyrolitic graphite, subsequently vacuum dried and investigated by atomic force microscopy (AFM) in ultra high vacuum. The AFM images show single clusters and agglomerates. The height distributions are Gaussian-shaped with average heights of 1 nm and widths of 1 nm. The heights never exceed 3 nm. In some regions a second cluster layer is observed. In all samples the separation between first and second layers is larger than the separation between the first layer and the graphite substrate, which we attribute to a stronger interaction between clusters and surface than the cluster self-interaction. We conclude that the separation between first and second layer represents a much better fingerprint of the original size distribution of the clusters in solution than the height of the first layer. The observation of a second cluster layer is important for using silicon clusters as building blocks for cluster-assembled materials.

Palladium in the Gap: Cluster Assemblies with Band Edges Localized on Linkers

Cluster-assembled materials composed of [Pd2As14]4– polyvalent anions, where a palladium dimer links two As7 clusters, have been synthesized in zero- and two-dimensional architectures. Their dimensionality was controlled through the selection of the counterions, and their structures were characterized by single-crystal X-ray diffraction. Reflectance spectroscopy measurements indicate that the zero- and two-dimensional assemblies have band gaps of 1.35 and 1.15 eV, respectively, and are smaller than the 1.80 eV gap of the As73– cluster. Theoretical investigations reveal that the frontier orbitals of the cluster building blocks are located primarily on the Pd dimer. Further, these orbitals are embedded inside the intrinsic band gap of the As73– clusters, reducing the band gap energy with edge states that are localized on the Pd sites. It is suggested that such a periodic array of narrow localized states may lead to interesting transport and optical properties as the carriers may become localized at these sites.

Formation and ionization energies of small chlorine‐doped lithium clusters by thermal ionization mass spectrometry

Theoretical calculations have shown that the first ionization energy of clusters of the type Li(n) Cl (n ≥2), with more than eight valent electrons, is lower than that of alkali metal atoms; hence they are named superalkali. Superalkali clusters can mimic the chemical behavior of alkali metals and may be used as building blocks of new cluster-assembled materials. There is currently no reliable experimental proof of this kind of clusters and such proof is required. The Li(n) Cl (n = 2-6) clusters were produced by a thermal ionization source of modified design, and mass selected by a magnetic-sector mass spectrometer. The modification pertains to the replacement of the side filaments by a cylinder in the triple-filament thermal ionization source. The sample, which is LiCl salt, was pressed into a ring and placed on the inner wall of the cylinder. It was observed that the ions of clusters with an even number of lithium atoms (Li(2) Cl(+) , Li(4) Cl(+) , Li(6) Cl(+) ) are more stable than the odd-numbered ones (Li(5) Cl(+) , Li(3) Cl(+) ). The ionization energies were determined to be 3.98 ± 0.25 eV for Li(2) Cl, 4.10 ± 0.25 eV for Li(3) Cl, 3.90 ± 0.25 eV for Li(4) Cl, 4.01 ± 0.25 eV for Li(5) Cl, and 4.09 ± 0.25 eV for Li(6) Cl. The presence of a halogen atom reduces the ionization energy of the metal clusters. The thermal ionization source of modified design presents a suitable simple way to obtaining and measuring the ionization energies of very small lithium monochloride clusters. Clusters Li(n) Cl, n = 4 to 6, were detected for the first time.

Metal-substituted Ti8C12 metallocarbohedrynes: toward less reactive clusters as building blocks of cluster-assembled materials

To form cluster-assembled materials, the clusters should have low reactivity and be characterized by a closed-shell electronic configuration with a large gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO). Using spin-polarized density functional theory calculations, we investigate the M-substituted Ti(8)C(12) metallocarbohedrynes to search for less reactive clusters as building blocks for cluster-assembled materials (M = Be, Mg, Ca, Sr, Ba and Sc, Y). The selected atoms in the correct stoichiometry would produce a metallocarbohedryne that is isoelectronic with the Ti(8)C(12)(2+), which has a closed-shell electronic configuration and an enhanced HOMO-LUMO gap of 1.735 eV. According to our results, the HOMO-LUMO gaps of the M-substituted Ti(8)C(12) metallocarbohedrynes are in the range of 0.715-0.979 eV for the case of Be, Mg, Ca, Sr and Ba and in the range of 0.865-1.294 eV for the case of Sc and Y. Among all the M-substituted metallocarbohedrynes we consider here, one of the isomers of Ti(6)Sc(2)C(12) is not only energetically more favorable but also exhibits a larger HOMO-LUMO gap of 1.294 eV. This result indicates that the Ti(6)Sc(2)C(12)(4) metallocarbohedryne should be less reactive than the Ti(8)C(12) metallocarbohedryne which has a narrow HOMO-LUMO gap of 0.146 eV. Moreover, we show that the intercluster interaction between two individual Ti(6)Sc(2)C(12)(4) metallocarbohedrynes is relatively weak compared to the Ti(8)C(12) dimer.

Synthesis, structure and band gap energy of covalently linked cluster-assembled materials

We have synthesized a series of cluster assembled materials in which the building blocks are As(7)(3-) clusters linked by group 12 metals, Zn, Cd and Hg, to investigate the effect of covalent linkers on the band gap energy. The synthesized assemblies include zero dimensional assemblies of [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-), [Hg(2)(As(7))(2)](4-), and [HgAsAs(14)](3-) in which the clusters are separated by cryptated counterions, and assemblies in which [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-) are linked by free alkali atoms into unusual three-dimensional structures. These covalently linked cluster-assembled materials have been characterized by elemental analysis, EDX and single-crystal X-ray diffraction. The crystal structure analysis revealed that in the case of Zn and Cd, the two As(7)(3-) units are linked by the metal ion, while in the case of Hg, two As(7)(3-) units are linked by either Hg-Hg or Hg-As dimers. Optical measurements indicate that the band gap energy ranges from 1.62 eV to 2.21 eV. A theoretical description based on cluster orbital theory is used to provide a microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy.

Self-assembling endohedrally doped CdS nanoclusters: new porous solid phases of CdS

Hollow CdS nanoclusters were predicted to trap alkali metals and halogen atoms inside their cavity. Furthermore, electron affinities (EA) of endohedrally halogen doped clusters and ionization potentials (IE) of endohedrally alkali doped clusters were predicted to be very similar. This makes them suitable to build cluster-assembled materials, in the same vein as do related ZnO, ZnS and MgO nanoclusters, which yield porous solid materials. With this aim in mind, we have focused on the assembly of bare Cd(i)S(i) and endohedral K@Cd(i)S(i)-X@Cd(i)S(i) (i = 12, 16, X = Cl, Br) clusters in order to obtain solids with tailored semiconducting and structural properties. Since these hollow nanoclusters possess square and hexagonal faces, three different orientations have to be considered, namely, edge-to-edge (E-E), square-to-square (S-S) and hexagon-to-hexagon (H-H). These three orientations lead to distinct zeolite-like nanoporous bulk CdS solid phases denoted as SOD, LTA and FAU. These solids are low-density crystalline nanoporous materials that might be useful in a wide range of applications ranging from molecular sieves for heterogeneous catalysis to gas storage templates.

As7M(CO)3]3– M = Cr, Mo, W: Bonding and Electronic Structure of Cluster Assemblies with Metal Carbonyls

Understanding the factors controlling the band gap energy of cluster-assembled materials is an important step toward nanoassemblies with tailored properties. To this end, we have investigated the band gap energies of cluster assemblies involving arsenic clusters bound to carbonyl charge-transfer complexes, M(CO)3, M = Cr, Mo, W. The binding of a single charge-transfer complex is shown to have a small effect on the band gap energy because the arsenic lone pair orbital and metal carbonyl orbitals are closely aligned in energy, resulting in a gap similar to the original cluster. The band gap energy is also found to be insensitive to the architecture of the assembled material. In the case where two charge-transfer complexes are bound to the cluster, the bottom of the conduction band is shown to be localized on a solvent molecule bound to the metal carbonyl.

On the stability of “non-magic” endohedrally doped Si clusters: A first-principles sampling study of M${\rm Si}_{16}^+$Si16+ (M = Ti,V,Cr)

Density-functional theory is used to study the geometric and electronic structure of cationic Si(16)(+) clusters with a Ti, V, or Cr dopant atom. Through unbiased global geometry optimization based on the basin-hopping approach, we confirm that a Frank-Kasper polyhedron, with the metal atom at the center, represents the ground-state isomer for all three systems. The endohedral cage geometry is thus stabilized even though only VSi(16)(+) achieves electronic shell closure within the prevalent spherical potential model. Our analysis of the electronic structure traces this diminished role of shell closure for the stabilization back to the adaptive capability of the metal-Si bonding, which is more the result of a complex hybridization than the originally proposed mere formal charge transfer. The resulting flexibility of the metal-Si bond can also help to stabilize "non-magic" cage-dopant combinations, which suggests that a wider range of materials may eventually be cast into this useful geometry for cluster-assembled materials.

Role of Hydrogen in Dimerizaton of Aluminum Clusters: A Theoretical Study

We have used density functional theory to investigate how Al(13) cluster dimers can be formed with or without a bridging hydrogen. We have identified several stable dimers in which 0, 1, or 2 hydrogen atoms link two bare clusters together. Each of these structures can adsorb further H atoms in atop sites on the surface of the dimer. Additional dimers were identified with 3 and 4 H atoms linking the clusters but these are only stable in the multihydrogenated form. Reaction profiles for the formation of these dimers from a range of cluster and H atom combinations indicate that the dimer structures are energetically favored over the isolated clusters. This observation may have significant implications for the design of cluster-assembled materials.

Soft-Landing Deposition of Al17− on a Hydroxyl-Terminated Self-Assembled Monolayer

Small aluminum clusters (<30 atoms) have been the subject of extensive study, demonstrating markedly different properties as even a single atom is added or removed. Successfully depositing and characterizing mass-selected clusters are the next steps in producing precise surfaces or cluster-assembled materials that demonstrate properties that differ from those of bulk materials. This proves to be difficult due to the reactivity of these all-metal clusters and their susceptibility to agglomerate in bulk form. In the present study, we have successfully deposited Al17− onto self-assembled monolayers using reactivity previously characterized in the gas phase.