From atom-precise nanoclusters to superatom materials

Nanometer-sized atomic clusters constitute a unique phase of matter that lies between the atoms and their bulk. Early studies on free atomic clusters have revealed that their structures and properties do not resemble those of their corresponding bulk materials. In addition, the properties of nanoclusters vary discontinuously and sensitively with the size, shape, and composition.¹ These features have made the clusters promising and attractive building units for novel functional materials. A “superatom” concept has been developed based on the findings that clusters of certain elements, such as Na, Au, and Al, mimic the electronic structures of atoms in the Periodic Table.^2–5 This concept explains the high stability of magic clusters by the closure of the electronic shells and has motivated researchers to synthesize a new class of materials using superatomic clusters as the building blocks, just as a variety of molecules are formed using atoms as the building blocks.

The key requirement for the nanocluster synthesis is atomic precision because the addition or removal of a single atom can drastically change the structure and properties of the nanocluster. The challenge of synthesis has been accomplished with gold clusters protected by monolayers of ligands, such as phosphines, thiolates (RS), alkynyls, and N-heterocyclic carbenes (Scheme 1).^6–9 The superatomic concept can be also applied to the electronic structures of these monolayer-protected clusters (MPCs) of gold and silver.^10–12 Another important requirement for establishing the structure–property correlation is the determination of the geometric structures of the metallic core and ligand layer (Scheme 1).¹³ Single crystal x-ray diffraction (SCXRD) analysis has been successfully applied to Au/Ag MPCs. Over the past two decades, the number of MPCs whose structures have been elucidated with atomic-level precision has been rapidly increasing.^7,8 These studies revealed that the properties of MPCs can be controlled over a wide range by the size, composition, and atomic packing of the metal core as well as the nature of the ligands. The main purpose of this special issue is to provide a glimpse into the current status of development of novel nanoclusters and superatomic materials.

The following is a brief overview of the main topics covered in this special issue.

Atomically precise synthesis and structure determination by SCXRD provide the basis for investigating the property–structure correlations of the MPCs. Table I summarizes the chemical compositions and core structures of MPCs experimentally determined in this special issue.^14–20 It can be seen that icosahedral (Ih) M₁₃ is the ubiquitous building unit of the ultrasmall metal clusters. In addition, the number and position of the dopant atoms are defined by the nature of the element and the specific interaction with the ligand. Ni₈ adopts a unique folded sheet structure due to the ligand addition.²⁰ Mullins et al. demonstrated, via first-principles density functional theory (DFT) calculations, that the 60-atom hollow shell of chiral-icosahedral symmetry (I-Au₆₀) is robust and well retained in various bilayer structures, such as I-C₆₀@I-Au₆₀, I-Au₃₂@I-Au₆₀²⁺, and Au₆₀(MgCp)₁₂.²¹ Nelli et al. revealed by molecular dynamics (MD) and metadynamics the unexpected diffusion pathways of Au or Ag impurity atoms within icosahedral Co or Cu clusters: the displacement of the impurity is coupled with the creation of vacancies in the central part of the cluster due to the presence of nonhomogeneous compressive stress.²² 

The structural motif and doping remarkably affect the electronic structures of MPCs as evidenced by optical and photophysical measurements. Shichibu et al. demonstrated that a subtle deformation of the Au₁₃ core resulted in a notable difference in the optical absorption and photoluminescence properties.¹⁴ Miyamoto et al. theoretically demonstrated a substantial difference in the absorption onset energies between Au₁₃ clusters with Ih and face-centered cubic (fcc) structures and ascribed it to the difference in the natural charges of the central Au atoms.²³ Fagan et al. reproduced the high-resolution absorption spectra of [Au₉(PPh₃)₈]³⁺ and [Au₈(PPh₃)₇]²⁺ in the gas phase using time-dependent DFT calculations and particle-in-a-box analysis for an asymmetrical ellipsoidal superatomic core, respectively.²⁴ Wang et al. theoretically proposed a Au₂S network model to explain and predict the structural origin, evolution, and formation mechanism of Ag_n(SR)_m containing quasi-fcc cores.²⁵ Medves et al. proposed efficient algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, to predict the optical spectra of ligand-protected alloy clusters, such as Ag₂₄Au(DMBT)₁₈ (DMBT = 2,4-dimethylbenzenethiolate).²⁶ 

One of the attractive properties of MPCs is photoluminescence (PL), which originates from the quantized, molecular-like electronic structures. Much effort has been made to tune the emission wavelengths and enhance the quantum yields (QYs) through the precise control of the structural parameters. A fundamental understanding of the relaxation dynamics has been deepened by joint studies on time-resolved spectroscopy and theoretical calculations.

Su et al. demonstrated that photocurrent-generating and PL properties are significantly affected by the difference in the core structures using two Ag₆₀ clusters having different core–shell architectures (Ag₁₂@Ag₄₈ vs Ag₁₄@Ag₄₆).²⁷ Petty et al. reported that DNA-bound Ag₁₀⁶⁺ exhibited prompt visible emission and metastable near-infrared PL and demonstrated that the metastable states could be tuned by synthetically modifying the DNA.²⁸ Zhang et al. demonstrated that the fluorescent properties of Ag₁₀⁶⁺ are significantly affected by the host, a single-stranded DNA C₄AC₄TC₃XT₄ with guanosine and inosine.²⁹ These results suggested that the brightness of the DNA-bound Ag cluster can be modulated by a single-site mutation of the DNA. Bootharaju et al. self-assembled Ag₂Cl₂(dppe)₂ clusters [dppe = 1,2-bis(diphenylphosphino)ethane] through the ligand-exchange-induced transformation of [Pt₂Ag₂₃Cl₇(PPh₃)₁₀] and observed intense PL with a QY of ∼18%, which is attributed to the metallophilic interactions and rigidification of the ligand shell.³⁰ Havenridge and Aikens theoretically assigned dual PL from Au₁₄Cd(SAdm)₁₂ (SAdm = adamantanethiolate) to emissions from two minima on the first excited state.³¹ Yousefalizadeh and Stamplecoskie demonstrated diversity in terms of the excited state energy, relaxation dynamics, and PL properties using excitation emission matrix spectroscopy and pump–probe transient absorption spectroscopy on Au₂₅(SR)₁₈, Ag₂₅(SR)₁₈, and their alloys.³² Jeffries et al. resolved state-to-state energy relaxation dynamics in Au₃₈(SC₆H₁₃)₂₄ by employing femtosecond transient absorption and two-dimensional electronic spectroscopy.³³ 

Redox and electronic properties are other interesting properties of MPCs arising from nonmetallic (molecular-like) electronic structures. Cowan et al. introduced novel structure–property relationships that predict electronic properties, such as the ionization potential and electron affinity, of thiolate-protected AgAu nanoclusters based on physically relevant descriptors.³⁴ Chen et al. reported that [Ag₁₆Au₁₃L₂₄]³⁻ (L = alkynyl) with an icosahedral Au@Ag₁₂ core underwent one-way oxidation by losing three electrons.³⁵ Cesari et al. studied the redox properties of [Rh₁₂E(CO)₂₇]ⁿ⁻ (n = 4 for E = Ge or Sn; n = 3 for E = Sb or Bi) with E@Ag₁₂ icosahedral cores and showed that three clusters (all except E = Sn) act as electron reservoirs.³⁶ Fiedler et al. demonstrated the emergence of electronic properties characteristic of a charge interaction between superatoms within the solid state material [Au₉(PPh₃)₈](NO₃)_3−x(C₆₀)_x.³⁷ López-Estrada et al. investigated the electronic structure and magnetically induced currents of doped nanoclusters [M@Au₈(PPh₃)₈]^q (M = Pd, Pt, Ag, Au, Cd, Hg, Ir, and Rh; and q = 0, +1, +2) having 6–10 delocalized valence electrons and found that the aromaticity of eight-electron cubic clusters increased in the order of M = Au < Pt < Pd.³⁸ 

The catalytic application of metal nanoclusters could have a huge impact on the realization of a sustainable society by taking advantage of the high and selective reactivities. However, the development of cluster catalysts according to a rational design principle has been hampered by the challenges of the synthesis: size and compositions must be controlled with atomic precision. Typical approaches to this goal are as follows: (1) adsorption of MPCs with well-defined compositions on a solid support, (2) removal of ligands from MPCs with well-defined compositions adsorbed on the support, and (3) soft-landing of mass-selected cluster ions onto solid supports. Choi et al. demonstrated the production of syngas with controlled H₂/CO ratios by CO₂ electroreduction on specifically formulated mixtures of [Au₂₅(SC₆H₁₃)₁₈]⁻ and [PtAu₂₄(SC₆H₁₃)₁₈]⁰ in aqueous media.³⁹ Kawamura et al. found that Au₁₄₄(pMBA)₆₀ (pMBA = p-mercaptobenzoic acid) deposited on TiO₂ enhanced the sonocatalytic activity for OH production by suppressing the electron–hole recombination.⁴⁰ Yang et al. revealed that Au₃₈S₂(SAdm)₂₀ with a body-centered cubic (bcc) Au₁₄ core outperformed Au₃₀(SAdm)₁₈ and Au₂₁(SAdm)₁₅ with hexagonal close-packed (hcp) Au₁₇ and fcc Au₂₃ cores, respectively, in the CO₂ cycloaddition with epoxides toward cyclic carbonates.⁴¹ Chen et al. successfully removed the PET (phenylethanethiolate) ligands from Au₂₅(PET)₁₈ supported on sulfur-doped graphene by thermal annealing and electrochemical biasing and showed that the resulting Au₂₅ clusters catalyzed the electroreduction of CO₂ to CO.⁴² Alotabi et al. demonstrated that the thermal-induced agglomeration of [Au₉(PPh₃)₈]³⁺ on TiO₂ could be suppressed by the photodeposition of the Cr₂O₃ layer.⁴³ Valtera et al. found that Cu₄ on titania prepared by soft-landing showed the highest activity for oxidative dehydrogenation of cyclohexene to benzene among Cu_n (n = 1–7).⁴⁴ 

Ligand bonding is the key to the stability of MPCs. Matsuyama et al. revealed by thermogravimetry, differential thermal analysis, and mass spectrometric analysis that the ligand desorption temperature of [MAu₈(PPh₃)₈]²⁺ (M = Pd, Pt) is higher than that of [Au₉(PPh₃)₈]³⁺ because of the formation of a large bonding index of M–Au and a change in the Au–PPh₃ bonding energy by the doping.⁴⁵ Wang and Gao theoretically investigated the desorption energies of -SR-(Au-SR)_n- and the resulting effects on the geometries and electronic structures of the remaining Au clusters.⁴⁶ They found that the detachment of longer motifs normally required more energy and resulted in an upshift of the HOMO and a reduction in the HOMO–LUMO gap. Liu and Jiang predicted by DFT calculations that carboxylate groups could be promising ligands to support atomically precise metal clusters, especially for Cu and Ag.⁴⁷ 

The conventional synthesis of MPCs is based on the chemical reduction of the relevant metal–organic compounds. New approaches have been developed to synthesize novel species that cannot be obtained by the conventional chemical reduction. Truttmann et al. reported the selective synthesis of Au₁₆(PET)₁₄ by the ligand exchange process of Au₁₅(SG)₁₃ (SG = glutathione).⁴⁸ Emori et al. found that the irradiation of atmospheric pressure plasma to [Au₉(PPh₃)₈]³⁺ in methanol yielded novel CN-containing clusters [Au₉(PPh₃)₇(CN)₁]²⁺ and [Au₁₀(PPh₃)₇(CN)₂]²⁺ as the main products via the sequential addition of AuCN unit(s) formed in situ.⁴⁹ Dong et al. synthesized a highly stable gold hydride nanocluster, [Au₂₂H₃(dppee)₇]³⁺ (dppee = bis(2-diphenylphosphino) ethyl ether), and proposed that the Au₂₂ core is composed of two Au₁₁ units bonded via two triangular faces, creating six uncoordinated Au sites that are bridged by three H atoms.⁵⁰ 

Daly et al. produced the novel argentate aggregates, [Ag_nPh_n+1]⁻, by transmetalation reactions between phenyl lithium and silver cyanide.⁵¹ Mass-selected UV photodissociation spectroscopy and theoretical calculations suggested novel organometallic characteristics built from Ag₂Ph subunits. Fang et al. theoretically predicted that the binding energy of the noble gas (Ng = Ne, Ar, Kr, and Xe) atoms becomes stronger with an increase in the cluster size and the electron affinity of the terminal ligands X in B₁₂X₁₁(Ng) (X = H, CN, and BO) and B₁₂X₁₀(Ng)₂ (X = CN and BO).⁵² Nguyen et al. examined the structures and absorption spectra for CdS nanoplatelets (NPLs) with thicknesses of two and three monolayers (2 MLs and 3 MLs) and passivated by formate and acetate ligands.⁵³ The spectral redshift for 3-ML CdS NPLs is attributed to the electron delocalization due to the expansion of the nanoplatelets in the lateral and vertical directions. Park et al. controlled the stoichiometry of both anions (P, As, S, and Se) and cations (In and Zn) at the interface of InP/ZnSe quantum dots (QDs) and correlated these changes with the resultant steady-state and time-resolved optical properties of the nanocrystals.⁵⁴ The results show that the dynamics are not dramatically impacted at the ns timescale while the PL QYs are highly sensitive to the interfacial composition. Sikorska and Gaston theoretically predicted that the bandgap of a perovskite CsPbBr₃ is narrowed by replacing the Cs⁺ cation with bimetallic superalkalis (LiMg, NaMg, LiCa, and NaCa).⁵⁵ 

This special issue highlights recent advances in the emerging field of atomically precise nanoclusters and cluster-assembled materials. We introduced the background and motivation of the research and provided a brief overview on how to control the structures and properties with a focus on the monolayer-protected metal clusters. We hope that the papers in this special issue will convince the readers of the bright future and opportunities in the materials science of nanoclusters and superatoms. We believe that new experimental and theoretical methods as well as interdisciplinary approaches will accelerate the exploration of new materials and deepen the understanding of the origin of their unique properties.

We thank all the authors and reviewers who contributed to this issue, the journal editors Professor Lai-Sheng Wang and Professor Xiaoyang Zhu, and the editorial staff who assisted throughout its preparation.

The authors have no conflicts to disclose.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.