Metal nanoclusters: from fundamental aspects to electronic properties and optical applications

Monolayer-protected noble metal clusters, also called nanoclusters, can be produced with the atomic precision and in large-scale quantity and are playing an increasingly important role in the field of nanoscience. To outline the origin and the perspectives of this new field, we overview the main results obtained on free metal clusters produced in gas phase including mainly electronic properties, the giant atom concept, the optical properties, briefly the role of the metal atom (alkali, divalent, noble metal) and finally the atomic structure of clusters. We also discuss the limitations of the free clusters. Then, we describe the field of monolayer-protected metal clusters, the main results, the new offered perspectives, the added complexity, and the role of the ligand beyond the superatom concept.

Anion-templated silver nanoclusters: precise synthesis and geometric structure

Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.

Recent progress in dichalcophosphate coinage metal clusters and superatoms

Atomically precise clusters of group 11 metals (Cu, Ag, and Au) attract considerable attention owing to their remarkable structure and fascinating properties. One of the unique subclasses of these clusters is based on dichalcophosphate ligands of [(RO)₂PE₂]^- type (E = S or Se, and R = alkyl). These ligands successfully stabilise the most diverse Cu, Ag, and Au clusters and superatoms, spanning from simple ones to amazing assemblies featuring unusual structural and bonding patterns. It is noteworthy that such complicated clusters are assembled directly from cheap and simple reagents, metal(I) salts and dichalcophosphate anions. This reaction, when performed in the presence of a hydride or other anion sources, or foreign metal ions, results in hydrido- or anion-templated homo- or heteronuclear structures. In this feature article, we survey the recent advances in this exciting field, highlighting the powerful synthetic capabilities of the system "a metal(I) salt - [(RO)₂PX₂]^- ligands - a templating anion or borohydride" as an inexhaustible platform for the creation of new atomically precise clusters, superatoms, and nanoalloys.

"Z"-Type Tilted Quasi-One-Dimensional Assembly of Actinide-Embedded Coinage Metal Near-Plane Superatoms and Their Optical Properties

In this work, a novel discovery that the coinage-metal near-plane superatoms (CM-NPSs) formed by embedding actinide elements into the coinage metal rings can realize the "Z"-type tilted quasi-one-dimensional (1D) direct assembly is reported. This success can be attributed to the strong bonding between the overlapping parts of adjacent superatomic motifs. First-principles calculations reveal that the motifs maintain their geometric and electronic structures robustly during the assembly process. With the accumulation of motifs, the intensity of the absorption peak increases continuously in the ultraviolet-visible (UV-Vis) absorption spectra range of 300-450 nm, resulting in the hyperchromic effect, which is closely related to the degree of the participation of Th atoms. Furthermore, the absorption spectra show a continuously tunable feature in the 450-900 nm range, as the interlayer stacking pattern leads to a pronounced redshift. More importantly, the valence 5f-shells of Th atoms have an increased contribution to the final orbitals of electronic transition, which demonstrates the advantages of the active high angular momentum electrons of actinide elements in spectral properties. These findings provide a valuable reference for the direct artificial assembly of near-plane superatoms and optical properties of superatomic assemblies embedded with rare elements.

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]- (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy

Au₃₈(PET)₂₄ (PET = SC₂H₄Ph) is known to have a bi-icosahedral Au₂₃ core consisting of two Au₁₃ icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au₂₃ core is similar to that in the F₂ molecule. The SVB model predicted that the electron configuration of the Au₂₃ core with 14 valence electrons is expressed as (1Σ)²(1Σ*)²(1Π)⁴(2Σ)²(1Π*)⁴ where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au₁₃ units. Therefore, the bi-icosahedral Au₂₃ can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M₁Au₃₇(PET)₂₄]^- (M = Pd and Pt), which are isoelectronic and isostructural with Au₃₈(PET)₂₄. To this end, the neutral precursors [M₁Au₃₇(PET)₂₄]⁰ were first synthesized by fusion reactions between hydride-doped clusters [HAu₉(PPh₃)₈]²⁺ and [M₁Au₂₄(PET)₁₈]^-. The formation of bi-icosahedral M₁Au₂₂ cores with open electronic structure in [M₁Au₃₇(PET)₂₄]⁰ was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M₁Au₃₇(PET)₂₄]^- were obtained by reducing [M₁Au₃₇(PET)₂₄]⁰ with NaBH₄, and isoelectronicity with [Au₃₈(PET)₂₄]⁰ was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M₁Au₃₇(PET)₂₄]^- observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

A Face-to-Face Dimer of Au3 Superatoms Supported by Interlocked Tridentate Scaffolds Formed in Au18 S2 (SR)12

A new sulfur-containing gold cluster, Au₁₈ S₂ (STipb)₁₂ , was serendipitously obtained using the bulky thiol, 2,4,6-triisopropylbenzyl mercaptan (TipbSH), as protecting ligands. Single-crystal X-ray diffraction analysis revealed that Au₁₈ S₂ (STipb)₁₂ has a deformed octahedral Au₆ core clutched by two tridentate S[Au₂ (STipb)₂ ]₃ units in an interlocked manner. Based on density functional theory calculations, we propose that the Au₆ core with two electrons is better viewed as a face-to-face dimer of Au₃ (1e) superatoms rather than an electronically closed Au₆ (2e) superatom. In situ formation of the sulfide anions (S^2- ) via C-S bond breakage is ascribed to the steric repulsion between the TipbS ligands.

Constructing the bonding interactions between endohedral metallofullerene superatoms by embedded atomic regulation

We present a possible principle that controls intercluster bonding through embedding different kinds of actinide atoms into the centre of fullerenes, thereby exhibiting different bonding forms. Moreover, these superatoms maintain the robustness of electronic structures.

Modern cluster design based on experiment and theory

For decades, chemists have explored cluster compounds according to theoretical models that have proved too simplistic to accurately predict cluster properties, stabilities and functions. By incorporating molecular symmetry into existing cluster models, we can better study real polyatomic molecules and have new guidelines for their design. This symmetry-adapted cluster model allows us to discover substances that shatter the conventional notion of clusters. Theoretical predictors will point to the viability of new clusters, whose syntheses can be realized with parallel advances in experimental methods. This Perspective describes these modern experimental and theoretical strategies for cluster design and how they may give rise to new fields in cluster chemistry.

Controlled Dimerization and Bonding Scheme of Icosahedral M@Au12 (M=Pd, Pt) Superatoms

Targeted syntheses of MM'Au₃₆ (PET)₂₄ (M, M'=Pd, Pt; PET=SC₂ H₄ Ph) were achieved by hydride-mediated fusion reactions between [MAu₈ (PPh₃ )₈ ]²⁺ and [M'Au₂₄ (PET)₁₈ ]^- . Single-crystal X-ray diffraction analysis indicated that the products have bi-icosahedral MM'Au₂₁ cores composed of M@Au₁₂ and M'@Au₁₂ superatoms. Although the MM'Au₂₁ superatomic molecules correspond to O₂ in terms of the number of valence electrons (12 e), the distances between the icosahedrons were larger than that in the bi-icosahedral Au₂₃ core of Au₃₈ (PET)₂₄ corresponding to F₂ and the spin state was singlet. These counterintuitive results were explained by a "bent bonding model" based on tilted (non-orthogonal) bonding interaction between the 1P superatomic orbitals of M@Au₁₂ and M'@Au₁₂ superatoms.

On the Interaction between Superatom Al12Be and DNA Nucleobases/Base Pairs: Bonding Nature and Potential Applications in O2 Activation and CO Oxidation

The interaction between quasi-chalcogen superatom Al₁₂Be and DNA nucleobases/base pairs has been explored by searching for the most stable Al₁₂Be-X (X = DNA bases and base pairs) complexes. Our results reveal that Al₁₂Be prefers to combine with guanine by two Al-O and Al-N bonds rather than the other DNA bases, no matter in free state or base pair. The formed Al-N and Al-O bonds between Al₁₂Be and DNA bases proved to be strong polar covalent bonds by the Wiberg bond index, nature bond orbitals, atoms in molecules theory, localized molecular orbitals, and electron localization functions analyses. More importantly, it is found that the formed global minimum of Al₁₂Be-G has the ability to activate an oxygen molecule into a peroxide dianion ¹O₂ ^2-, which can further catalyze the CO oxidation via the Eley-Rideal mechanism with a small energy barrier of 7.78 kcal/mol. We hope that this study could not only provide an in-depth understanding on the intermolecular interaction between metallic superatoms and DNA at the molecular level but also attract more interest in designing and synthesizing superatom-based heterogeneous catalysts with DNA/nucleobases as basic building blocks.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.

Bonding properties of a superatom system with high-Z elements: insights from energy decomposition analysis

Superatoms with high-Z elements have novel physicochemical properties, and a comprehensive and thorough view of their bonding properties plays a crucial role in the design of superatoms. Now, energy decomposition analysis shows increasingly prominent performance for understanding inter- and intra-molecular interactions, so the bonding properties of typical superatoms with high-Z elements, Th@Au₁₄, have been investigated here. It is found that under different electron occupation types of the fragments, the electrostatic interaction energy, polarization, and exchange repulsion energy change significantly in their intramolecular interaction energy components, resulting in quantitative or even qualitative differences in their main interaction energy. This indicates that the bonding properties of fragments are related to their electronic structures, and even has extraordinary reference value for the future regulation and control of interactions in superatoms with high-Z elements, which has great significance for superatom development.

Superatoms with high-Z elements have novel physicochemical properties, and a comprehensive and thorough view of their bonding properties plays a crucial role in the design of superatoms.

Hydride Doping of Chemically Modified Gold-Based Superatoms

Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-to-nonmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu₂₅(SR)₁₈]⁰ (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au₉(PPh₃)₈]³⁺ and [PdAu₈(PPh₃)₈]²⁺ formally described as (Au₉)³⁺ and (PdAu₈)²⁺, respectively, and (2) Au₃₄ cluster stabilized by poly( N-vinyl-2-pyrrolidone) (PVP). The (Au₉)³⁺ and (PdAu₈)²⁺ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au₃₄ cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H^-) was selectively doped to (Au₉)³⁺ and (PdAu₈)²⁺ upon reactions with BH₄^- to form (HAu₉)²⁺ and (HPdAu₈)⁺, respectively. Density functional theory (DFT) calculations showed that (HAu₉)²⁺ and (HPdAu₈)⁺ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu₉)²⁺ was selectively converted to the well-known (Au₁₁)³⁺ by electrophilic addition of two Au(I) units whereas (HPdAu₈)⁺ was converted to a new hydride-doped (HPdAu₁₀)³⁺. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au(I) units. The selective formation of Au₃₄ superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au₃₄ superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH₄^- due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au₃₄ to dissolved O₂, but reappeared by reaction with BH₄^-. Catalysis for hydrogenation of C═C bonds was generated by doping a single Pd or Rh atom to Au₃₄. The results reported here demonstrate that the hydride doped to chemically modified Au superatoms mimics Au^- in terms of electron count. The hydride-mediated growth processes observed will contribute to the development of an atomically precise, bottom-up method of synthesizing new artificial elements in a periodic table for nanoscale materials. The interaction of hydride(s) with Au superatoms will find application in hydrogenation catalysis and hydrogen sensing.

Binding for endohedral-metallofullerene superatoms induced by magnetic coupling

To design new materials based on artificial superatoms, clarifying their involved interaction is particularly important. In this study, we discuss first-principle calculations to show that the interaction between endohedral metallofullerenes (EMFs) of U@C28 can lead to different chemical and physical adsorption structures. Especially, these structures are derived from different magnetic coupling resonances, and they can transform by changing the distance between U@C28 superatoms. These findings will promote the future development for bottom-up assembling of new functional materials and even devices.

Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom

A hydride (H^-)-doped bimetallic superatom (HPdAu₈)⁺ was produced by reacting BH₄^- with an oblate (PdAu₈)²⁺ superatom protected by PPh₃. The H atom in (HPdAu₈)⁺ survived during the sequential addition of Au(I)Cl to form an (HPdAu₁₀)³⁺ superatom, in sharp contrast to the proton release from a H^--doped pure gold superatom (HAu₉)²⁺ in the growth process to (Au₁₁)³⁺. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu₁₀)³⁺ showed that the interstitially doped H atom induced a notable deformation of the core.

Gold Fusion: From Au25(SR)18 to Au38(SR)24, the Most Unexpected Transformation of a Very Stable Nanocluster

The study of the molecular cluster Au₂₅(SR)₁₈ has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au₂₅(SR)₁₈⁰ undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au₃₈(SR)₂₄. We tested this reaction with a series of Au₂₅ clusters. The fusion was confirmed and characterized by UV-vis absorption spectroscopy, ESI mass spectrometry, ¹H and ¹³C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au₂₅(SR)₁₈⁰ appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au₁₃ building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters.

Doping the cage. Re@Au11Pt and Ta@Au11Hg, as novel 18-ve trimetallic superatoms displaying a doped icosahedral golden cage

Trimetallic superatomic clusters. Theoretical proposal and evaluation of the 18-ve Ta@Au₁₁Hg and Re@Au₁₁Pt clusters.

The electrooxidation-induced structural changes of gold di-superatomic molecules: Au23vs. Au25

Face-sharing bi-icosahedral Au₂₃ core of Au₃₈(SC₂H₄Ph)₂₄ retained its structure, whereas vertex-sharing bi-icosahedral Au₂₅ core of [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ underwent irreversible structural change upon electrooxidation.

[TM13@Bi20]− clusters in three-shell icosahedral matryoshka structure: being as superatoms

Using the density functional theory (DFT) method, three-shell icosahedral matryoshka [TM₁₃@Bi₂₀]⁻ clusters (TM = 3d, 4d) have been systematically examined to explore the possibility of clusters being as superatoms.

Optical properties of nanoalloys

Optical absorption spectra of bare (left) and monolayer-protected (right) metal nanoalloys.