Shape control with atomic precision: anisotropic nanoclusters of noble metals

Abstract

When plasmonic metal nanoparticles become smaller and smaller, a new class of nanomaterials—metal nanoclusters of atomic precision—comes to light and has become an attractive research topic in recent years. These ultrasmall nanoparticles (or nanoclusters) are unique in that they are molecularly uniform and pure, often possess a quantized electronic structure, and can grow into single crystals as do protein molecules. Exciting achievements have been made by correlating their properties with the precise structures at the atomic level, which has provided a profound understanding of some mysteries that could not be elucidated in the studies on conventional nanoparticles, such as the critical size at which plasmons are emergent. While most of the reported nanoclusters are spherical or quasi-spherical owing to the reduced surface energies (and hence stability), some anisotropic nanoclusters of high stability have also been obtained. Compared to the anisotropic plasmonic nanoparticles, the nanocluster counterparts such as rod-shaped nanoclusters can provide insights into the growth mechanisms of plasmonic nanoparticles at the early stage (i.e., nucleation), reveal the evolution of properties (e.g., optical), and offer new opportunities in catalysis, assembly, and other themes. In this Review, we highlight the anisotropic nanoclusters of atomic precision obtained so far, primarily gold, silver, and bimetallic ones. We focus on several aspects, including how such nanoclusters can be achieved by kinetic control, and how the anisotropy gives rise to new properties over the isotropic ones. The anisotropic nanoclusters are categorized into three types, (i) dimeric, (ii) rod-shaped, and (iii) oblate-shaped nanoclusters. For future research, we expect that anisotropic nanoclusters will provide exciting opportunities for tailoring the physicochemical properties and thus lead to new developments in applications.

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Introduction

One of the fundamental goals of nanoscience is to design and synthesize size- and shape-controllable materials at the nanometer scale.^1,2 It has been widely accepted that the physicochemical properties of metal nanoparticles (NPs) are highly dependent on their size and shape.^3–7

Starting from Faraday's time and later developments by Turkevich, Frens, Schmid,^8–10et al., the preparation of colloidal gold NPs of monodispersity has long been of major interest to researchers.¹¹ Ordered monolayers of alkanethiol-stabilized gold NPs with tunable interparticle spacing (by different lengths of alkane chains) were reported by Mulvaney and co-wokers.¹² Gold NPs functionalized by thiol-terminated DNA led to major research in nanobiomedicine.¹³ A two-phase method was established by Brust et al., which stimulated interest in organic soluble Au NPs.¹⁴ The Murray group demonstrated that the mean size of Au core can be adjusted by Au : thiolate ratio as well as temperature and the reduction rate.¹⁵ In the meantime, Whetten et al. isolated a series of ultrasmall Au NPs with tight distributions and observed the molecular nature in the smaller cores of Au NPs.^16,17 In 2004, the Tsukuda group successfully separated different Au clusters (∼1 nm, ranging from Au₁₀ to Au₃₉) protected by glutathione via polyacrylamide gel electrophoresis, and their chemical compositions were precisely determined by electrospray ionization mass spectrometry (ESI-MS).^18,19 About three years later, crystallization was worked out and the total structures of Au₁₀₂(SR)₄₄ and [Au₂₅(SR)₁₈]⁻ (SR = thiolate) nanoclusters (NCs) were solved by X-ray crystallography,^20–22 thus, a new age began, so-called atomically precise nanochemistry.^23–26 Since then, the research thrust on Au-SR NCs by the Jin group and others^24,27–30 has led to the creation of a series of noble metal NCs of atomic precision.

Evolution from complexes to nanoclusters to nanoparticles

The metal (e.g., Au) NCs occupy a unique position in bridging the metal–organic complexes and regular metal NPs, revealing the important molecule-to-metal transition (Scheme 1). A model binuclear Au(I)–L complex—Au₂(S₂PH₂)₂—and its orbital interaction diagram for Au₂²⁺ with two S₂PH₂⁻ ligands (no d orbitals are included) are illustrated in Scheme 1A (simplified from the original work).³¹ Substantial s and p mixing into the d block gives aurophilic interactions,³⁴ which is responsible for the tendency of Au(I) ions to clustering. Note that there is no metal–metal (M–M) bond in these Au(I)–L complexes. As the Au(I)–L precursor is reduced, Au–Au bonds form and give rise to a Au(0) kernel, which is the nucleation process at the initial stage of NP growth. To the surprise of nanochemists, at such an early stage, NCs with tens to hundreds of metal atoms (ultrasmall NPs) can also be very stable due to the passivation by ligands which are strongly bonded to the metal surface, as well as their close-shelled geometric structures (e.g., magic numbers) and/or electronic structures (e.g., superatoms).^35,36 Taking the most thoroughly studied Au₂₅(SR)₁₈⁻ NC as example, it has an icosahedral (I_h) Au₁₃ kernel and six Au₂(SR)₃ staple-like motifs at the interface (Scheme 1B, bottom), and the metal core (Au_kernel + Au_motif) is then wrapped inside an organic ligand shell. Almost all of the Au–thiolate NCs have such a kernel-motif structure, but for NCs without any surface motif (e.g., Au–phosphine NCs), only a metal core is inside the ligand shell. Note that the oxidation state of the Au_kernel is not exactly 0, but between 0 and 1 with more Au(0) character. In addition, NCs have discrete energy levels with a distinct HOMO–LUMO gap (E_g > 0, HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) that are akin to molecules (Scheme 1B, top). A distinctive feature of NCs compared to complexes is that they have multiple free valence electrons to show unique optical properties with rich features (see Fig. 1).³² For example, isomeric Au₃₀(SR)₁₈ NCs (SR = adamantanethiolate) with face-centered-cubic (fcc) and hexagonal-close-packed (hcp) kernels have been obtained by the Jin group,^37,38 and their absorption spectra are very different (Scheme 1C); note that the discussion on the fcc and hcp kernel structures is given in the section of “Rod-shaped nanoclusters”. Remarkably, a series of Au-SR NCs protected by the same adamantanethiol ligand have been achieved, which exhibit an atom-by-atom addition evolution and vivid colors, including Au₁₆(SR)₁₂ (orange), Au₁₆Ag₁(SR)₁₃ (light green), Au₂₁(SR)₁₅ (red), Au₂₂(SR)₁₆ (light brown), Au₂₂Cd₁(SR)₁₆ (turquoise), Au₂₄(SR)₁₆ (greenish brown), Au₂₉(SR)₁₉ (brown), Au₃₀(SR)₁₈-fcc (green), Au₃₀(SR)₁₈-hcp (magenta), and Au₃₆Ag₂(SR)₁₈ (brown), although their sizes are very close (Scheme 1D). This phenomenon is different from the chalcogen-bridged silver or copper clusters (Ag(I) or Cu(I/II) sulfides/selenides) by Fenske et al., which often show less features in absorption,^39,40 or the chalcogenide quantum dots with tunable band gaps by changing the composition,^33,41,42 or the metal NPs with surface plasmon resonance which, for example, shifts from 520 to 570 nm as the diameter of spherical Au NPs increases from ∼10 to ∼100 nm (Scheme 1G).²⁴

Scheme 1 The size evolution from Au(I)–L complexes, to Au(0/I)–L NCs, then to Au(0)–L NPs. (A) Binuclear Au(I)–L complex Au₂(S₂PH₂)₂ and its orbital diagram.³¹ Redrawn by the authors. (B) The crystal structure of Au₂₅(SR)₁₈⁻ NC and its molecular-like electronic structure with HOMO–LUMO gap (E_g) > 0. Reproduced with permission.²¹ Copyright 2008, American Chemical Society. (C) The optical absorption spectra of isomeric Au₃₀(SR)₁₈-hcp and Au₃₀(SR)₁₈-fcc (the spectra are offset to avoid overlap). (D) The photographs of a series of adamantanethiol-protected NCs in CH₂Cl₂, from left to right: Au₁₆(SR)₁₂, Au₁₆Ag₁(SR)₁₃, Au₂₁(SR)₁₅, Au₂₂(SR)₁₆, Au₂₂Cd₁(SR)₁₆, Au₂₄(SR)₁₆, Au₂₉(SR)₁₉, Au₃₀(SR)₁₈-fcc, Au₃₀(SR)₁₈-hcp, and Au₃₆Ag₂(SR)₁₈. (C/D) Data collection performed by Y. L. (E) E_g of Au-SR NCs with I_hversus fcc and decahedral kernels as a function of the total number of Au atoms in NC. Reproduced with permission.³² Copyright 2021 Wiley-VCH GmbH. (F) The size dependence of the electronic properties in atomically precise gold nanoparticles with a sharp transition from non-metallic Au₂₄₆(SR)₈₀ (E_g > 0) to metallic Au₂₇₉(SR)₈₄ (E_g = 0). Reproduced with permission.³³ Copyright 2021 The Author(s). (G) Optical extinction spectra of gold colloids (3–100 nm diameters), inset: photograph of 10 nm Au colloid, and the continuous band electronic structure of metallic-state NPs (E_f = Fermi level). Reproduced with permission.²⁴ Copyright 2016 American Chemical Society.

Fig. 1 TEM images of single-crystalline Au nanospheres with controlled average diameters (A–D) 5, 8, 10, and 16 nm, by adding (A–D) 1000, 500, 50, and 20 μL Au seed solution, respectively. In Au seed solution preparation, the molar ratio of Au : ligand : reductant = 1 : 400 : 2.4. Reproduced with permission.⁶⁷ Copyright 2013, WILEY-VCH GmbH. UV-vis spectra, MALDI-MS spectra, and corresponding crystal structures or scanning-TEM image of (E) [Au₂₅(SR)₁₈]⁻; (F) Au₃₈(SR)₂₄; (G) Au₁₄₄(SR)₆₀; and (H) Au₃₃₃(SR)₇₉ NCs. (E and F) Data collected by Y. L.; (G) reproduced with permission.⁶⁹ Copyright 2022, The Authors, published by the Royal Society of Chemistry. (H) Reproduced with permission.^70,71 Copyright 2012/2019, The Authors, published by the National Academy of Sciences USA. Color codes: magenta/violet = Au, yellow = S, R groups are omitted for clarity.

Noble metal NCs show multiple absorption peaks and their energy gaps are very sensitive to and highly tunable by size (i.e., the number of metal atoms), composition (e.g., doping or alloying) and structure (e.g., shape).^24–29 In general, the energy gap of atomically precise NCs decreases when the size of the NCs increase (Scheme 1E).⁴³ As the Au(0) kernel grows larger, however, the number of Au atoms in the Au(I)–S motifs does not change much: there are 12 Au_motif atoms in Au₂₅(SR)₁₈, and still only 40 Au_motif atoms in the much larger Au₂₄₆(SR)₈₀, indicating the Au(0)/Au(I) ratio increases when the NC becomes larger. Moreover, with atomically precise nanochemistry, a sharp transition from semiconducting Au₂₄₆(SR)₈₀ to metallic-state Au₂₇₉(SR)₈₄ with only 33 Au atoms in the growth (Scheme 1F) has been observed.^33,44 The Au₂₇₉(SR)₈₄ NC shows a quasicontinuous band electronic structure as in metallic-state NPs. With the enlightenment by atomically precise NCs, we believe that the regular Au NPs should also have staple-like Au(I)–L surface structures although they cannot be fully resolved by transmission electron microscopy due to insufficient resolution and image contrast. But since the number of motif Au(I) atoms are quite small compared to the large Au(0) kernel, regular Au NPs are regarded as all Au(0) atoms of metallic state (E_g = 0, no energy gap between the conduction band and valence band), with tunable plasmon resonances as the size changes (Scheme 1G).

The advantages of achieving NCs at the atomically precise level are obvious: (1) although transmission electron microscopy (TEM) can easily determine the size and shape of NPs, analyze their chemical compositions, crystalline structures, and valence states, insufficient image contrast limits the identification of organic monolayer on the particle surface. By contrast, the inter- or intra-particle interactions between ligands—which are especially important in elucidating the self-assembly in supercrystals—can be clearly determined by X-ray crystallography for atomically precise NCs;^38,45–48 (2) for both quantification of adsorption and understanding of the catalytic mechanism, revealing the nature and structure of the active site(s) is critical since the interactions of active sites with substrates (i.e. reactants) can only be understood when the active site structure is known;^30,49–52 (3) the organic–inorganic interface of the nanostructure which is crucial for resolving the surface science can be fully unveiled with the total structure (metal core + ligand shell) solved, showing that the intrinsic origin of chirality of metal NCs is the chiral arrangements of interface atoms and motifs;^53–56 (4) not only a sharp transition from non-metallic Au₂₄₆(SR)₈₀ to metallic Au₂₇₉(SR′)₈₄ is demonstrated,⁴⁴ but also the energy gaps of NCs are found to be related to different atomic packing modes (e.g. I_hvs. fcc or decahedral (D_h) kernels) due to the different distribution of electron wavefunctions in the NCs (Scheme 1E).^32,43

A brief introduction concerning the synthetic evolution from size controllable Au NPs to Au NCs is given. Note that there have been excellent reviews on noble metal NP syntheses,^57–62 thus in this Review, only selected works relating the size control of Au NPs are highlighted in order to introduce how atomically precise Au-SR NCs of different sizes are produced. For Au NPs larger than 5 nm, a digestive ripening process in excess thiol developed by Lin et al. resulted in greatly narrowed size distribution and long-range-ordered superlattices.⁶³ Based on the seed-growth procedure—which has been a popular technique for decades,^64,65 Jana and Murphy prepared Au NPs in the range of 5–40 nm, of which the size can be manipulated by varying the ratio of seed to metal salt.⁶⁶ The size of single crystalline Au nanospheres could also be controlled by altering the size or volume of the seeds.⁶⁷ As shown in Fig. 1A–D, the diameter of the Au nanospheres could be varied conveniently by changing the volume of the seed solution added to the reaction mixture. The precursor ratio to make size-controlled Au NPs⁹ gives an opportunity to develop an advanced kinetic control for making atomically precise NCs.²⁴ In the typical Brust–Schiffrin method for Au–thiolate NPs, the Au : ligand : reductant molar ratio was 1 : 1.1 : 0.1.¹⁴ By contrast, in the one-pot method to produce the atomically precise Au₂₅(SR)₁₈ NCs (SR = C₂H₄Ph), the molar ratio of Au : PhC₂H₄SH : reductant was 1 : 5 : 10.⁶⁸ The optical absorption spectrum, MALDI-mass spectrometry (MS) spectrum, and the corresponding single crystal structure of anionic Au₂₅(SR)₁₈ NC are shown in Fig. 1E.²¹

As indicated by the above comparison, the successful synthesis of atomically precise Au-SR NCs usually go through two steps: (1) excess thiol is used to convert Au^III into Au^I-SR complexes (or polymers), and (2) a great excess of NaBH₄ is used to reduce Au^I-SR to Au(0). Moreover, the size control of atomically precise NCs is no longer dependent on the seed size or its concentration since they are at the scale of seed size (i.e., ∼1–3 nm in diameter).^72,73 Then, how can one obtain NCs of different sizes in a controllable manner just like what was achieved in the plasmonic NPs? The Jin group provided an early strategy—size-focusing—to create a chemical environment for the formation of an exclusive size of Au-SR NC while suppressing other sizes.⁷⁴ Specifically, the mass range of the initial product (i.e., Au_x(SR)_y mixture) was first kinetically controlled by Au : thiol ratio, temperature, reducing rate, etc., and then the Au_x(SR)_y nanocluster mixture was etched at elevated temperatures and with excess thiol (i.e., thermodynamic selection for the most stable size), and finally a specific size such as Au₃₈(SR)₂₄ of molecular purity was achieved (Fig. 1F).^75,76 The Au₁₄₄(SR)₆₀ NCs were obtained by etching for 24 h to eliminate impurities of small sizes (Fig. 1G),⁷⁷ and the total structure of Au₁₄₄(SR)₆₀ was later solved by using HSCH₂Ph instead of HSC₂H₄Ph in the initial synthesis (Fig. 1G, inset).⁷⁸ When adjusting the Au : thiol molar ratio from 1 : 3 (for the preparation of Au₁₄₄(SR)₆₀) to 1 : 2, larger sized Au₃₃₃(SR)₇₉ was made (Fig. 1H).^70,71 Other methodologies, including ligand-exchange-induced size and/or structure transformation,^79–84 ligand substituent group hindrance induced size selection,^85–88 heteroatom doping induced size/structure transformation, metal exchange or anti-galvanic reduction,^89–92 have been developed to enrich the library of noble metal NCs during the past two decades.

The unique chemical and physical properties, especially the optical properties, of plasmonic NPs are not only determined by the size, but also by the shape of the core. Shape-controlled syntheses of NPs,^93–97 especially the anisotropic structures with gold nanorods as the most thoroughly studied one^98–102 were obtained in a convenient way around 2000. Different from the less tunable plasmon peak wavelength range of isotropic Au NPs of 4–200 nm, anisotropic Au nanorods permit one to drastically tune the longitudinal plasmon wavelength from the visible to near-infrared range by controlling the aspect ratio (AR) of rods. Over the past two decades, researchers have frequently taken advantages of such specific phenomena to enable sensing applications,^103–105 plasmon-enhanced spectroscopies,^93,106–109 bio-imaging,^110,111 and photothermal therapy.^112–117

The same trend has also been seen in the relatively short history of atomically precise metal–thiolate NCs, in which isotropic structures were obtained at the beginning of research since the spherical structures have less surface energy, that is, higher stability.^20,21,77,118 Fortunately, with the advances in synthetic methods, we have witnessed more and more cases on non-spherical NCs. Spheroidal shapes (Scheme 2) are found in large-sized NCs with more than 100 metal atoms. For example, Au₁₃₀(SR)₅₀ is the first reported NC to have multiple concentric shells with a barrel-shaped structure,⁸⁸ but only with a small aspect ratio (AR = 1.13). The Ag@Au₁₇@Ag₂₇@Au₁₇(C≡CR)₃₄ structure exhibits a spheroidal shape with AR = 1.26, and this Au₃₄Ag₂₈(C≡CR)₃₄ NC shows much better catalytic performance in its intact form than the partially or completely ligand-removed NCs.¹¹⁹ AR of 1.17 is found in the Au₁₅₆(C≡CR)₆₀ NC, which is in the transition region from molecular to metallic state.¹²⁰ These spheroidal shapes are at the borderline between isotropic and anisotropic structures, and will not be the focus in this review. Larger AR values are highly desirable from the shape effect point of view.

Scheme 2 Selected anisotropic Au and Au-based NCs of atomic precision and their corresponding properties. Dimeric structures include: (A) Au₃₈(SR)₂₄ and the circular dichroism spectra of its two enantiomers. Reproduced with permission.¹²¹ Copyright 2013 Wiley-VCH GmbH; (B) Au₁₈Ag₃Cd₁(SR)₁₅Br and its assembly in supercrystals.¹²² Redrawn by the authors; (C) kinetic traces of nanosecond transient absorption for Au₂₉(SR)₁₉ and Au₃₀(SR)₁₈, and spectra of absorbance, emission and excitation for Au₂₉(SR)₁₉. Reproduced with permission.³⁸ Copyright 2021 Springer Nature Limited; (D) Au₂₂(dppo)₆ and the CO oxidation of Au₂₂(dppo)₆/TiO₂ catalyst pretreated in O₂ at different temperatures. Reproduced with permission.¹²³ Copyright 2016 American Chemical Society; (E) Au₂₄H₃(NHC)₁₄Cl₂ and its electrochemical CO₂ reduction performance at different current densities. Reproduced with permission.¹²⁴ Copyright 2022 American Chemical Society; rod-shaped structures include: (F) the fcc series and absorption spectra. Reproduced with permission.¹²⁵ Copyright 2020 Wiley-VCH GmbH; (G) Au₂₄(SR)₂₀ and Au₂₄(SeR)₂₀ and the photographs of their solutions or solids under visible and UV light irradiation. Reproduced with permission.¹²⁶ Copyright 2016 Wiley-VCH GmbH; (H) Au₄₂(SR)₃₂ and its temperature changes upon irradiation compared to Au₃₈(SR)₂₄, Au₅₂(SR)₃₂, Au₂₄(SR)₂₀, and time-dependent images of Au₄₂(SR)₃₂ solution upon irradiation; temperature-dependent PL spectra of Au₄₂(SR)₃₂ solution. Reproduced with permission.^127,128 Copyright 2022 American Chemical Society; (I) Au₆(dppp)₄, Au₈(dppp)₄Cl₂, Au₁₁(dppe)₆ and their absorption spectra. Reproduced with permission.¹²⁹ Copyright 2013 American Chemical Society; (J) Au₃₇(SR)₁₀(PPh₃)₁₀Cl₂, Au₂₅(SR)₅(PPh₃)₁₀Cl₂, Au₂₄(SR)₅(PPh₃)₁₀Cl₂, Au₁₃(dppe)₅Cl₂, and their absorption spectra. Reproduced with permission.¹³⁰ Copyright 2017 National Academy of Science. Oblate-shaped structures include: (K) Au₂₂(C≡CR)₁₈ and its PL spectra and lifetimes with different ligands. Reproduced with permission.¹³¹ Copyright 2019 American Chemical Society; (L) Au₃₆Ag₂(SR)₁₈ and its HER electocatalytic activity compared to Au₃₈(SR)₂₄ and Au₂₅(SR)₁₈⁻. Reproduced with permission.¹³² Copyright 2021 American Chemical Society; (M) Au₅₆(SR)₂₄(PR₃)₆Br₂ and the kinetic traces of its transient absorption, and corresponding fits at selected wavelengths, and spectra at selected time-delays. Reproduced with permission.¹³³ Copyright 2022 The Author(s). Spheroidal structures include: Au₁₃₀(SR)₅₀ and Au₁₅₆(C≡CR)₆₀. The structures are redrawn from the cif in ref. 88 and 120.

Herein, we focus on the work on noble metal NCs of atomic precision with anisotropic structures in the past 15 years to highlight their distinctive chemical and physical properties. Some illustrations are collected in Scheme 2. We divide the anisotropic NCs into three categories—dimeric structures, rod shapes and oblate (or disk-shaped) structures. Homoleptic Au and Au-based NCs (ligands including thiol, phosphine, and alkynyl) will be our primary coverage, whereas multileptic NCs, and Ag- and Cu-based NCs will be selectively discussed as supplementary cases. Note that we eliminate the charges and counter ions on most of the NCs in the Review for the simplicity of the formulas, unless those specifics become necessary in the discussion of electronic properties of the anisotropic NCs.

Dimeric nanoclusters

Plasmonic Au NP dimers and trimers can be constructed and separated in high purity,¹³⁴ and such nanostructures have been used for surface-enhanced Raman spectroscopy.¹³⁵ In terms of structural control, such dimers and trimers are formed by NPs attracting each other without any bond (i.e., with a subnanometer or larger gap between NPs). By contrast, when dimerization or trimerization occurs in atomically precise NCs, monomeric units are linked by metal–metal bonds inside a single particle. Thus, NC dimers can be regarded as the simplest anisotropic structures.

Y. Zhu et al. found that two Au₂₅(SR)₁₈ (SR = SC₂H₄Ph) monomers could be linked by two Ag atoms to form a Au₅₀Ag₂(SR)₃₆ dimer (Fig. 2A, left).¹³⁶ Although the optical absorption spectrum does not show distinct differences when two monomers are bridged (Fig. 2A, right), normalized partial density of states diagrams indicate that Ag^I has an influence on the HOMO of the dimer, causing band broadening. Moreover, the Au₅₀Ag₂(SR)₃₆/CeO₂ catalyst exhibited enhanced activity for CO oxidation compared to Au₂₅(SR)₁₈/CeO₂, while the catalyst based on Au_25−xAg_x(SR)₁₈ NCs had much less activity (Fig. 2B).¹³⁶

Fig. 2 (A) Dimeric Au₅₀Ag₂(SR)₃₆ and its UV-vis spectrum compared to monomeric Au₂₅(SR)₁₈. (B) CO oxidation profiles of Au₅₀Ag₂(SR)₃₆, Au₂₅(SR)₁₈, and Au_25−xAg_x(SR)₁₈ catalysts supported on CeO₂. Reproduced with permission.¹³⁶ Copyright 2020, Wiley-VCH GmbH. (C) Au₂₄(SR)₅(PPh₃)₁₀Cl₂ and the CO₂ hydrogenation on Au₂₄/SiO₂ catalysts with respect to the reaction time. Reaction conditions: Au₂₄/SiO₂ (0.5% Au) 100 mg, 2 MPa reaction gas (CO₂ : H₂ = 1 : 3) with 5% N₂, H₂O 15 mL, 130 °C. Reproduced with permission.¹³⁷ Copyright 2020, American Chemical Society. (D) Au₂₄H₃(NHC)₁₄Cl₂ and its electrochemical CO₂ reduction performance compared to Au₁₃(NHC)₉Cl₃ NCs and Ag NPs at different current densities. Reproduced with permission.¹²⁴ Copyright 2022, American Chemical Society. (E) Au₂₂(dppo)₆ and the CO oxidation curves of the Au₂₂(dppo)₆/TiO₂ catalysts pretreated in O₂ at different temperatures. Reproduced with permission.¹²³ Copyright 2016, American Chemical Society. (F) Electrocatalytic reduction of CO₂ to CO based on Au₂₂H₃(dppe)₃(PPh₃)₈ catalyst compared to Au₁₁(dppe)₅, inset: the theoretical structure of Au₂₂H₃(dppe)₃(PPh₃)₈. Reproduced with permission.¹³⁸ Copyright 2022, American Chemical Society. (G) The structure and mass spectra of Au₂₀H₃(PPh₃)₁₂ and Au₂₀D₃(PPh₃)₁₂. Reproduced with permission.¹³⁹ Copyright 2020, The Royal Society of Chemistry. (H) Au₄₃Ag₃₈(C≡CR)₃₆Cl₁₂ and its CO₂ reduction performance compared to Au₂₄Ag₂₀(C≡CR)₂₄Cl₂. Reproduced with permission.¹⁴⁰ Copyright 2022, The Author(s). (I) Au₂Ag₄₈(SR)₂₀(dppm)₆Br₁₁. Color codes: magenta/violet = Au, light grey/light blue = Ag, light orange = P, yellow = S, green = Cl, brown = Br, light purple = N, and grey = C, R groups are omitted for clarity.

Heteroligand-protected NCs composed of two connected monomeric units without staple motifs have been studied as well, and such NCs also show interesting catalytic properties. In the process of making rod-shaped Au₂₅(SR)₅(PPh₃)₁₀Cl₂ (SR = SC₂H₄Ph), adding excess PPh₃ resulted in Au₂₄(SR)₅(PPh₃)₁₀Cl₂ consisting of two Au₁₂ units (one vertex missing) joined together in an eclipsed manner (Fig. 2C, left).¹⁴¹ Further work indicated that the internal vacancy (i.e., the missing vertex) could provide Au₂₄(SR)₅(PPh₃)₁₀Cl₂ with more structural flexibility and mitigate the deactivation of the Au₂₄/SiO₂ catalysts which efficiently converted CO₂ to dimethyl ether (DME) (Fig. 2C, right).¹³⁷ When the open pentagonal faces of the two incomplete I_h Au₁₂ units rotated by ∼120° with respect to each other, an N-heterocyclic carbene (NHC) stabilized NC—Au₂₄H₃(NHC)₁₄Cl₂—was obtained by the Crudden group (Fig. 2D, left).¹²⁴ Three bridging hydrides were not resolved by X-ray analysis but were predicted to be located at the connection points by density-functional theory (DFT). Using a membrane electrode assembly electrochemical cell in which the cathode and anode were pressed against an anion exchange membrane, Au₂₄H₃(NHC)₁₄Cl₂ was tested for CO₂ electroreduction to CO, and the dimeric NC-based catalyst exhibited higher selectivity (i.e., faradaic efficiency) than the catalysts of monomeric Au₁₃(NHC)₉Cl₃ and benchmark Ag NPs at all tested current densities (Fig. 2D, right).¹²⁴

L.-S. Wang et al. reported a Au₂₂(dppo)₆ (dppo = Ph₂P(CH₂)₈PPh₂) NC with two Au₁₁ units clipped together by four dppo ligands (Fig. 2E, left).¹⁴² Low-temperature CO oxidation activity was observed for the Au₂₂(dppo)₆/TiO₂ catalyst pretreated in O₂ at different temperatures without ligand removal (Fig. 2E, right).¹²³ Recently, the group found that Au₂₂(dppo)₆ gradually transformed via H evolution from Au₂₂H₄(dppo)₆ of which the tetrahydrides were confirmed by electrospray ionization MS and DFT calculations.¹⁴³ In another work, three hydrides were determined on heteroleptic Au₂₂H₃(dppe)₃(PPh₃)₈ (dppe = Ph₂P(CH₂)₂PPh₂) by ESI-MS and ¹H-NMR, and the structure was proposed to have two Au₁₁ units connected at six Au sites and bridged by three H (Fig. 2F, inset). The six bridged Au atoms in the trihydrido-NC were found to be critical in catalyzing electrochemical CO₂ reduction to CO, and a 92.7% faradaic efficiency at −0.6 V vs. RHE and high activity (134 A/g_Au mass activity) were obtained (Fig. 2F).¹³⁸ Beside the homodimeric Au NCs, the Q.-M. Wang group demonstrated an Au₂₀H₃(PPh₃)₁₂ NC with a heterodimeric core, i.e., an Au₉ unit was connected to an Au₁₁ unit (Fig. 2G). The presence of trihydrides on Au was probed by ESI-MS and ¹H-NMR, and the hydrido-Au NC showed good stability for long-time storage.¹³⁹

When two hollow I_h Au₁₂ units are linked by one additional Au atom to form a dimeric kernel which is enclosed by a bimetallic shell, an Au₄₃Ag₃₈(C≡CR)₃₆Cl₁₂ NC is obtained (Fig. 2H, left). Compared to Au₂₄Ag₂₀(C≡CR)₂₄Cl₂ with a monomeric kernel and a superatomic electronic configuration, the Au₄₃Ag₃₈(C≡CR)₃₆Cl₁₂ NC shows less activity in catalytic reduction of CO₂ to CO (Fig. 2H, right).¹⁴⁰ This is in contrast to the observations in the hyrido-Au cases, i.e., dimeric Au₂₂H₃(dppe)₃(PPh₃)₈ and Au₂₄H₃(NHC)₁₄Cl₂ are better than their monomeric counterparts (Fig. 2D and F). We suppose that the hydrides and chlorides at the bridging sites are responsible for the catalytic difference. Besides, two Au@Au₁₂ superatomic units (1S²1P⁶) can also be combined into a dimeric kernel which is protected by different surface Ag motifs to form [Au₂Ag₄₂(SR)₂₇]⁺ and [Au₂Ag₄₈(SR)₂₀(dppm)₆Br₁₁]³⁺, respectively (dppm = Ph₂PCH₂PPh₂, Fig. 2I).¹⁴⁴ DFT indicates the valence electronic structures of both NCs are analogous to that of Ne₂.

Another category of dimeric NCs has kernels formed by fusion of two I_h monomeric units. Jin et al. first solved the total structure of Au₃₈(SR)₂₄ (SR = SC₂H₄Ph) with a dimeric kernel as another benchmark NC⁷⁶ (in addition to the Au₂₅(SR)₁₈ in earlier work). Since then, Au₃₈(SR)₂₄ remains actively studied to establish extensive structure–property correlations in atomically precise nanomaterials. The dimeric Au₂₃ kernel is composed of two I_h Au₁₃ units fused together via sharing a common Au₃ face, i.e., Au₁₃ + Au₁₃ – Au₃. The Au₂₃ kernel is protected by six Au₂(SR)₃ and three Au₃(SR)₄ staples in a chiral configuration (Fig. 3A),⁷⁶ consistent with the DFT calculations by Aikens et al.¹⁴⁵ Compared to the superatom model of (1S)²(1P)⁶(1D)⁶ for a 14e⁻ NC, a theoretical study based on supervalence bond (SVB) model does a better job in demonstrating that the electronic configuration of the Au₂₃ kernel is (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 two I_h Au₁₃ units.^146,147 In this way, the SVB model relates the Au₂₃(14e⁻) kernel to an isoelectronic analog of the F₂ molecule (Fig. 3B). Based on the structure of Au₃₈(SR)₂₄, a systematic ligand-exchange study revealed that ortho-substituted benzenethiols could preserve the Au₃₈(SR)₂₄ structure, while para- or non-substituted benzenethiols would cause the transformation of Au₃₈(SR)₂₄ into Au₃₆(SR′′)₂₄.¹⁴⁸ In Au₃₈(SC₂H₄Ph)₂₄, the six Au₂(SR)₃ motifs resemble two tri-blade fans at the top and bottom, being arranged in a staggered conformation by ∼60° to each other (Fig. 3C, left);⁷⁶ by contrast, when applying an ortho-substituted aromatic thiol, the crystal structure of Au₃₈(SR′)₂₄ (SR′ = SPh-2,4(CH₃)₂) shows that one fan rotates only by ∼45° relative to the other along the C₃ axis (Fig. 3C, right). As a result, each Au(SR′)₂ motif at the waist is parallel to the two adjacent Au₂(SR′)₃ motifs of the Au₃₈(SR′)₂₄ structure (Fig. 3C, indicated by blue lines).¹⁴⁸ The dimeric Au₃₈(SR)₂₄ NC shows a series of absorption peaks, with the first band (i.e., the lowest energy) centered at 1050 nm (see Fig. 1F),⁷⁵ which is significantly red-shifted from that of Au₂₅(SR)₁₈ with a mono-I_h Au₁₃ kernel (its first absorption band at 780 nm).¹⁴⁹

Fig. 3 (A) The face-fused Au₂@Au₂₁ kernel in Au₃₈(SR)₂₄. (B) Comparison of the Kohn–Sham orbital diagrams between Au₂₃⁽⁺⁹⁾ (left) and F₂ (right). Reproduced with permission.¹⁴⁶ Copyright 2013, The Royal Society of Chemistry. (C) Top and side views, and the schematic diagram of two tri-blade ‘fans’ of Au₃₈(SR)₂₄ (left) and Au₃₈(SR′)₂₄ (right). Reproduced with permission.¹⁴⁸ Copyright 2020, The Royal Society of Chemistry. (D) DFT-calculated electronic energy levels of [Pt₂Au₃₆(SCH₃)₂₄]²⁻, Au₃₈(SCH₃)₂₄, and [Pd₂Au₃₆(SCH₃)₂₄]⁰. Reproduced with permission.¹⁵⁴ Copyright 2018, American Chemical Society. (E) Circular dichroism spectra of two enantiomers and the racemic Au₃₈(SR)₂₄. Reproduced with permission.¹²¹ Copyright 2012, Macmillan Publishers Limited. (F) Circular dichroism spectra of two enantiomers of Pd₂Au₃₆(SR)₂₄. Reproduced with permission.¹⁵⁶ Copyright 2014, American Chemical Society. (G) Absorption spectra of Pd₂Au₃₆(SR)₂₄, PdPtAu₃₆(SR)₂₄, Pt₂Au₃₆(SR)₂₄ and Au₃₈(SR)₂₄ as a function of photon energy. Reproduced with permission.¹⁵⁷ Copyright 2020 Wiley-VCH GmbH. (H) Absorption spectra of Pd₁Au₃₇(SR)₂₄, Pt₁Au₃₇(SR)₂₄ and Au₃₈(SR)₂₄ as a function of photon energy. Reproduced with permission.¹⁵⁸ Copyright 2022, The Authors. (I) The 23-metal kernels with different dopants. (J) CO faradaic efficiency and (K) CO partial current density for Pt₁Au₃₇(SR)₂₄, Au₃₈(SR)₂₄, and Pt₂Au₃₆(SR)₂₄ catalysts at different applied potentials. Reproduced with permission.¹⁵⁹ Copyright 2022, Wiley-VCH GmbH. (L) The Au₂@Au₁Ag₂₀ kernel of Au₃Ag₃₈(SR)₂₄X₅.¹⁶³ (M) The Pt₂@Cu₁₈ kernel in Pt₂Cu₃₄(SR′)₂₂Cl₄, and the enhanced catalytic activity of Pt₂Cu₃₄@TiO₂ for silane oxidation to silanol. Reproduced with permission.¹⁶⁰ Copyright 2021, American Chemical Society. (N) The fusion of Au₁₂Ag₇ core in Au₁₂Ag₇(PR)₁₀(NO₃)₉. (O) The fusion of Au₂₀ core in Au₂₀(PR’)₁₀Cl₄. Color codes: magenta/violet = Au, dark cyan = Pd, dark blue = Pt, light grey/light blue = Ag, pink = Au/Ag, orange/brown = Cu, yellow = S, light orange = P, green = Cl, red = O, light purple = N, and grey = C. R groups are omitted or simplified for clarity.

In light of the successful synthesis and crystallization of Au₃₈(SR)₂₄, heteroatom doping was investigated subsequently. Co-reducing Au^I-SR and Ag^I-SR, followed by thermo-etching with excess thiol, Au_38−xAg_x(SR)₂₄ was resulted (x = 1–6, SR = SC₂H₄Ph) with the 1050 and 750 nm absorption bands blue-shifted and diminished at higher Ag dopants.¹⁵⁰ Crystallography shows that the doped Ag atoms are distributed in the kernel on the middle fusion plane and the two ends.¹⁵¹ By contrast, Au_38−xCu_x(SR′)₂₄ (x = 0–6, SR′ = SPh–2,4(CH₃)₂) obtained by a ligand exchange method demonstrates that all Cu atoms are selectively doped in the trimeric motifs, and the optical absorption spectrum remains the same as that of the homogold counterpart. This is reasonable since Cu dopants contribute little to the frontier orbitals.¹⁵²

Doping the group 10 elements (i.e., Pd, Pt) into Au₃₈(SR)₂₄ provides a more interesting story. In 2011, Negishi et al. used size exclusion chromatography to separate Pd_xAu_38−x(SR)₂₄ (x = 1, 2), which was then heated at 60 °C for 8 days to preferentially decompose the singly doped NC.¹⁵³ The resulted pure Pd₂Au₃₆(SR)₂₄ NC was found to be more stable than Au₃₈(SR)₂₄ against degradation.¹⁵³ Lee et al. used a similar method to produce Au₃₈(SR′′)₂₄, [Pd₂Au₃₆(SR′′)₂₄]⁰ and [Pt₂Au₃₆(SR′′)₂₄]²⁻ (SR′′ = SC₆H₁₃), and studied their optical and electronic properties.¹⁵⁴ X-ray photoelectron spectroscopy (XPS) shows the Pd⁰ and Pt⁰ state, indicating that both dopants are located at the central positions of the bi-I_h kernel, in contrast to the observation of Pd–S bonds in the K-edge extended X-ray absorption fine-structure data.¹⁵⁵ Moreover, the Pt-doped NC is dianionic as two associated cations are observed by NMR analysis.¹⁵⁴ Voltametric investigations show that the HOMO–LUMO gap of [Pt₂Au₃₆(SR′′)₂₄]²⁻ (14e⁻) is 0.95 eV, slightly larger than the 0.86 eV gap of Au₃₈(SR′′)₂₄ (14e⁻); whereas [Pd₂Au₃₆(SR′′)₂₄]⁰ shows a drastically decreased gap (0.26 eV) due to the Jahn–Teller distortion of the 12e⁻ NC (Fig. 3D).

As mentioned above, the chiral arrangement of Au₃(SR)₄ motifs on the surface of Au₂₃ kernel gives rise to the chirality of Au₃₈(SR)₂₄. Bürgi et al. first separated the two enantiomers by chiral high-performance liquid chromatography, showing mirror-image circular dichroism (CD) responses (Fig. 3E) of Au₃₈(SR)₂₄ with large anisotropy factors (up to 4 × 10⁻³).¹²¹ The enantiomers with two doped Pd atoms show significantly changed CD spectra compared to their parent NCs (Fig. 3F), although the anisotropy factors are of similar magnitude. However, the doped 38-metal NCs undergo racemization at a much lower temperature.¹⁵⁶

Syntheses other than the original size-focusing method have also been developed to understand the formation of dimeric Au₃₈(SR)₂₄ NCs. Maran et al. found that the charge-neutral Au₂₅(SR)₁₈ NCs could undergo a spontaneous bimolecular fusion to form the dimeric Au₃₈(SR)₂₄ NC during a period of two weeks.¹⁶¹ The Tuskuda group developed a hydride-mediated transformation approach by reacting equivalent amounts of [HM₁Au₈(PPh₃)₈]⁺ (8e) and [M₁Au₂₄(SR)₁₈]⁻ (M = Pd/Pt), resulting in homodimeric Pd₂Au₃₆(SR)₂₄ and Pt₂Au₃₆(SR)₂₄ as well as heterodimeric Pd₁Pt₁Au₃₆(SR)₂₄ with crystal structures solved (Fig. 3G), proving that Pd or Pt atoms are doped at the centers of the individual icosahedra. The lowest-energy absorption bands of M₂Au₃₆(SR)₂₄ (12e⁻) are determined to be ∼0.7 eV, and the electron configuration of (1Σ)²(1Σ*)²(1Π)⁴(2Σ)²(1Π*)² corresponds to the O₂ electron configuration.¹⁵⁷ Very recently, the authors modified the method by mixing equivalent [HAu₉(PPh₃)₈]²⁺ (8e) and [M₁Au₂₄(SR)₁₈]⁻ with excess Au(I)-SR oligomer, and after chromatography separation, heterodimeric M₁Au₃₇(SR)₂₄ (13e⁻) was obtained.¹⁵⁸ The optical absorption spectra of M₁Au₃₇(SR)₂₄ show an additional band below 1 eV compared to that of Au₃₈(SR)₂₄ (Fig. 3H), which can be assigned to the electronic transition to a singly occupied molecular orbital. The 23-metal-atom kernel structures with different dopant(s) are shown in Fig. 3I. In the meantime, Y. Zhu et al. independently reported the same 13e⁻ Pt₁Au₃₇(SR′′′)₂₄ NC (SR′′′ = SCH₂Ph^tBu) by traditional co-reduction and separation methods. More interestingly, Pt₁Au₃₇(SR′′′)₂₄ with an unpaired electron displays a higher catalytic activity than that of Pt₂Au₃₆(SR′′′)₂₄ in electroreduction of CO₂, whereas Au₃₈(SR′′′)₂₄ is in between (Fig. 3J and K).¹⁵⁹

Moreover, Au₃₈(SR)₂₄ has a structural isomer, denoted Au₃₈(SR)₂₄′ which possesses an Au₂₃ kernel composed of one I_h Au₁₃ and one Au₁₂ cap fused together by sharing two Au atoms.¹⁶² Although Au₃₈(SR)₂₄′ is thermally less stable than the homodimeric Au₃₈(SR)₂₄, it shows much higher catalytic activity in reducing 4-nitrophenol to 4-aminophenol at 0 °C.

In addition to the NCs with Au-based bi-I_h kernels, Au₃Ag₃₈(SR)₂₄X₅ (X = Cl or Br, SR = SCH₂Ph) with an Ag-based bi-I_h kernel was also reported.¹⁶³ The two icosahedrons centrally doped with Au atoms are face-fused into a dimeric Au₂@Au₁Ag₂₀ kernel (one Au is distributed at two sites (∼50%) on the surface of the kernel, Fig. 3L, left), which is further capped by two halogens at two ends and an outer Ag₁₈(SR)₂₄X₃ shell (Fig. 3L, right). The NC also has the same 14e⁻ configuration as the Au₃₈(SR)₂₄ system, and shows an emission peak at 827 nm and a shoulder band at 851 nm in solid state at room temperature. In another work, due to the distinct chemical reactivity of [Cu₃₂H₈(SR′)₂₄Cl₂]²⁻ (SR′ = SC₂H₄Ph), Hyeon et al. reacted it with Na₂PtCl₆·6H₂O under an Ar atmosphere to produce a [Pt₂Cu₃₄(SR′)₂₂Cl₄]²⁻ (10e⁻) NC. Two Pt₁Cu₁₂ units are fused at the Cu₆ hexagon, resulting in a rod-shaped Pt₂Cu₁₈ kernel inside a Cu₁₆(SR′)₂₂Cl₄ shell (Fig. 3M). The synergistic effect of the bimetallic NC was found to enhance the catalytic activity by ∼300 fold in silane-to-silanol conversion under mild conditions.¹⁶⁰

For phosphine-protected NCs, when two I_h Au₇Ag₆ units are fused by sharing an Au₂Ag₅ part, Au₁₂Ag₇(PR)₁₀(NO₃)₉ (PR = PPh(CH₃)₂) with a nearly staggered Au₅–Ag₅–Au₅ configuration is obtained (Fig. 3N).¹⁶⁴ Another Au₂₀(PR′)₁₀Cl₄ NC (PR′ = bis(2-pyridyl)phenyl-phosphine) has an Au₂₀ core generated from the fusion of two Au₁₁ units (incomplete icosahedron) via sharing two vertices (Fig. 3O).¹⁶⁵

The formation of dimeric structures is elusive due to their ultrasmall sizes. But we noticed that many of the dimeric I_h NCs also have their corresponding monomeric I_h structures that can be separately synthesized to have superatomic character. Thus, the formation mechanism of dimeric NCs could be referred to the model of diatomic molecules or fusion growth.

All the above dimeric structures are formed by two I_h or quasi-I_h units which are easy to be identified. By contrast, understanding the formation of fcc nanostructures at the atomic level remains a major task. The Jin group used an atom-tracing strategy by heteroatom doping into Au₃₀(SR)₁₈ (SR = S^tBu) to label the specific positions in it, which is consistent with the Ag-doping site in Au₂₀Ag₁(SR)₁₅, indicating that the 30-metal-atom NC is a homodimeric structure (Fig. 4A). Electronic orbital analysis by DFT also shows the intrinsic orbital localization at the two specific positions in M₃₀(SR)₁₈ (M = Au/Ag), regardless of Au or Ag occupancy. The mechanism is proposed as follows: first, Au₁₆Ag₁(SR)₁₁ intermediates (its structure proposed from Au₂₀Ag₁(SR)₁₅ by losing Au₄(SR)₄) are formed at the initial stage; then, two of them are fused in a face-to-face manner; after eliminating another Au₄(SR)₄ unit from the adduct, the dimeric Au₂₈Ag₂(SR)₁₈ is achieved (Fig. 4A).¹⁶⁶

Fig. 4 (A) Proposed formation of dimeric Au₂₈Ag₂(SR)₁₈. Reproduced with permission.¹⁶⁶ Copyright 2020, American Chemical Society. (B) Geometric and electronic structural evolutions achieved by the ‘one gold in, one thiolate out’ approach, from Au₂₈(SR)₂₀ to Au₂₉(SR)₁₉, then to Au₃₀(SR)₁₈. Color codes in (A/B): magenta/violet/dark pink/purple = Au, light grey = Ag, yellow = S, R groups are omitted for clarity. (C) Self-assembly of Au₂₉(SR)₁₉ NCs into a double-helical structure; (D) two enantiomers (E1 and E2) of Au₂₉(SR)₁₉ in the supercrystal; and (E) four types of motifs matching between neighboring enantiomers: E1C1–E2C3 (D type), E1C3–E2C1 (C type), E1L3–E2L1 (B type) and E1L1–E2L3 (A type). Color code in C/D/E: blue and cyan = Au in the different strands; yellow = S; grey = C. The four different staple motifs of Au₂₉(SR)₁₉ are marked with larger spheres of different colors. (F) Nanosecond transient absorption data and kinetic traces for Au₂₉(SR)₁₉, and Au₃₀(SR)₁₈. λ_ex = 400 nm. Reproduced with permission.³⁸ Copyright 2021 Springer Nature Limited.

The homodimeric structure of fcc Au NCs has long been overlooked until the report on heterodimeric fcc NCs. Before being experimentally proved, Pei et al. proposed in 2017 a “Au insertion, SR elimination” mechanism based on the structures of Au₃₀(SR)₁₈ and Au₂₈(SR)₂₀, and predicted the atomic structure of Au₂₉(SR)₁₉ with 10e⁻.¹⁶⁷ The crystal structure of heterodimeric Au₂₉(SR)₁₉ was finally obtained in 2021 by applying adamantanethiol (SR = SC₅H₁₀).³⁸ Indeed, half of the Au₂₉(SR)₁₉ structure is the same as that of Au₃₀(SR)₁₈, and the other half is identical to that of Au₂₈(SR′)₂₀ (SR′ = S-c-C₆H₁₁) reported before.¹⁶⁸ In addition to their geometric structure relationship, Au₂₉(SR)₁₉ is also heterodimeric in terms of its electronic structure (4e⁻ + 6e⁻), whereas the two parent homodimeric NCs show 6e⁻ + 6e⁻ for Au₃₀(SR)₁₈, and 4e⁻ + 4e⁻ for Au₂₈(SR′)₂₀ (Fig. 4B).³⁸ With respect to the NP assembly behavior, isotropic NPs lacking directional interactions usually adopt basic packing rules in their self-assembly, whereas remarkable physicochemical properties in metallic NPs are usually originated from the symmetry breaking in the particles (i.e., anisotropic NPs). Thus, we are also expecting some special properties resulted from the heterodimeric structure of the atomically precise Au₂₉(SR)₁₉ NC. Suprisingly, these heterodimeric Au₂₉(SR)₁₉ NCs self-assemble into double- and quadruple-helical superstructures, whereas the homodimeric Au₃₀(SR)₁₈ NCs only form layer-by-layer assemblies.³⁸ Each pitch of the double helix contains 16 NCs (pitch length 12.8 nm and width 3.6 nm), in which 8 NCs in each strand adopt different rotation angles in the assembly (Fig. 4C).³⁸ Moreover, the great advantage of atomically precise nanochemistry is that it reveals how such complex arrangements are formed, while this cannot be achieved in conventional NPs due to imperfections. With atomically precise NCs, two enantiomers (E1 and E2) are identified, and the heterodimeric Au₂₉(SR)₁₉ has four types of staple motifs: one monomeric Au(SR)₂ motif (denoted L1) and one trimeric Au₃(SR)₄ (denoted L3) inherited from Au₃₀(SR)₁₈; and one monomeric Au(SR)₂ (denoted C1) and one trimeric Au₃(SR)₄ (denoted C3) inherited from Au₂₈(SR)₂₀ (Fig. 4D). When assembling into a supercrystal, the two neighboring enantiomers approach each other by matching their C1 and C3 staple motifs, or their L1 and L3 staple motifs (Fig. 4E), resulting in four types of interactions: E1C1–E2C3 (denoted D), E1C3–E2C1 (denoted C), E1L3–E2L1 (denoted B) and E1L1–E2L3 (denoted A).³⁸ Specifically, when the C1 and C3 staple motifs of two enantiomers pair up (Fig. 4E, via D and C interactions), the ligands surrounding them are triply paired (resembling base pairing of C and G in DNA); when the L1 and L3 staple motifs of two enantiomers pair up (Fig. 4E, via B and A interactions), the ligands surrounding them are doubly paired (resembling base paring of A and T in DNA). It is noteworthy that the helical assembly is driven by van der Waals interactions through particle rotation and conformational matching, as opposed to hydrogen-bonding and π–π stacking in DNA. Furthermore, the heterodimeric Au₂₉(SR)₁₉ has a photoexcited carrier lifetime that is ∼65 times longer than that of the homodimeric Au₃₀(SR)₁₈ (Fig. 4F), resulting in the NIR photoluminescence in Au₂₉(SR)₁₉ (see Scheme 2C).³⁸ An Au₂₉(SAdm)₁₉ film in which nanoclusters are randomly packed enhances the photoluminescence by suppressing non-radiative decays. Supercrystals in which nanoclusters are self-assembled into double-helixes lead to further photoluminescence enhancement, arising from the collective properties of assembly and ordered anisotropic interactions.³⁸

Additionally, M. Zhu et al. reported a halogen-induced anisotropic growth of Ag₄₀(C₆H₅COO)₁₃(SR)₁₉(CH₃CN) into Ag₄₅(C₆H₅COO)₁₃(SR)₂₂Cl₂ (SR = SCH₂Ph^tBu), in which the adsorption of halogen ions might lead to the formation of defected intermediates for epitaxial growth at both ends of the kernel.¹⁶⁹ In another work, Ag₄₈Cl₁₄(SR′)₃₀ and Ag₅₀Cl₁₆(SR′)₂₈(dppp)₂ (SR′ = SC₅H₁₀, dppp = Ph₂P(CH₂)₃PPh₂) share the same framework containing a prolate Ag₈ kernel (an Ag₆ octahedron with two polar Ag atoms), except that the Ag₂(SR′)₃ active units at the two ends of Ag₄₈Cl₁₄(SR′)₃₀ are replaced with Ag₃(SR′)₂(dppp)Cl units in Ag₅₀Cl₁₆(SR′)₂₈(dppp)₂ with a more anisotropic shape.¹⁷⁰ In Au₇Cu₁₂(PR)₆(SR′′)₆Br₄ (PR = diphenylphosphinopyridine, SR′′ = SPh^tBu), the central Au is capped by an Au₃Cu₃ ring and further by a Cu₃ motif from either side.¹⁷¹ The bimetallic NC demonstrates strong emission with 13.2% quantum yield (QY), which can be further improved by exchanging the aromatic thiolates with non-aromatic ones to break the intramolecular π⋯π interactions.¹⁷¹

Jin et al. also introduced a Cd–Br bond into Au NCs, resulting in bimetallic Au₂₂Cd₁(SR)₁₅Br (see Scheme 2B), and trimetallic Au_22−xAg_xCd₁(SR)₁₅Br (avg. x = 1.87, SR = SC₅H₁₀).¹²² Half of the I_h Au₁₃ or Au_13−xAg_x kernel is capped by three Au₃(SR)₄ motifs, whereas the other half has only a (SR)₃CdBr motif on it.¹²² Due to the presence of the special Cd–Br on the surface, the NC with metal core radius of 5 Å shows a giant dipole (∼18 D), falling in the experimental trend of II–VI quantum dots. The strong dipole–dipole interactions lead to a “head-to-tail” alignment of the Au_22−xAg_xCd₁(SR)₁₅Br NCs in the crystalline state.¹²² In another case resulted from surface reconstruction due to doping, the cuboctahedral Au₁₃ kernel is capped by two Cd₃[Au(SR′)₂]₃ (SR′ = S-c-C₆H₁₁) motifs on each side, resulting in a Au₁₉Cd₂(SR′)₁₆ NC.¹⁷² Compared to Au₂₃(SR′)₁₆ prior to surface reconstruction, the Cd-substituted NC greatly enhances the CO₂ electroreduction selectivity to 90–95% at applied potential between −0.5 to −0.9 V vs. RHE.¹⁷³

Rod-shaped nanoclusters

The Konishi group has done extensive works on diphosphine coordinated [core + exo]-type Au NCs (nuclearity = 6, 7, 8, and 11) featuring a polyhedral core with exo one or two gold atom(s),^174–176 resembling ultrasmall Au nanorods. The Au₆(dppp)₄, Au₈(dppp)₄Cl₂, and Au₁₁(dppe)₆ solutions (dppp = Ph₂P(CH₂)₃PPh₂, dppe = Ph₂P(CH₂)₂PPh₂) show vivid colors corresponding to the intense single band for each NC in the visible region, well-separated from the absorption at higher energy (Fig. 5A).¹²⁹ By contrast, the core-only isomers of the same nuclearity show overlapped bands from UV to visible region. DFT calculations revealed that the exo Au atoms are highly involved in the frontier molecular orbitals and HOMO–LUMO transition, giving rise to the isolated single absorption band of the [core + exo]-type Au NCs.¹⁷⁷ By replacing dppp with dppmb (dppmb = Ph₂P-Ph-PPh₂), Au₆(dppmb)₄ has its Au₆ framework proximate to the phenylene bridges, and hydrogen bonding interaction can be identified and a large red shift (45 nm) of the visible absorption band is observed by spectroscopy.¹⁷⁸

Fig. 5 (A) Absorption spectra of Au₆(dppp)₄, Au₈(dppp)₄Cl₂, Au₁₁(dppe)₆, and corresponding structures. Reproduced with permission.¹²⁹ Copyright 2013, American Chemical Society. (B) Absorption spectra of Au₃₇(SR)₁₀(PPh₃)₁₀Cl₂, Au₂₅(SR)₅(PPh₃)₁₀Cl₂, Au₂₄(SR)₅(PPh₃)₁₀Cl₂, Au₁₃(dppe)₅Cl₂, and corresponding structures. Reproduced with permission.¹³⁰ Copyright 2017, The Authors, published by National Academy of Sciences. (C) Absorption spectra of Au_25−xAg_x(SR)₅(PPh₃)₁₀Cl₂ (x = 0, 1–12, or 1–13), and corresponding structure. Inset, absorption and excitation spectra (left), and emission spectrum (right) at different excitation wavelengths. Reproduced with permission.¹⁷⁹ Copyright 2014, Wiley-VCH GmbH. (D) Absorption spectra of Au₂₅(NHC)₁₀Br₇, and corresponding structure. Reproduced with permission.¹⁸⁰ Copyright 2019, Wiley-VCH GmbH. (E) Optical absorption spectra of Ag₆₁(dpa)₂₇ and Ag₂₁(dpa)₁₂ NCs, and corresponding total structures and I_h cores. Reproduced with permission.¹⁸¹ Copyright 2021, American Chemical Society. (F) Optical absorption spectra of Pt₁Ag₂₀(dtp)₁₂, Pt₂Ag₃₃(dtp)₁₇, Pt₃Ag₄₄(dtp)₂₂, and corresponding total structures and I_h cores. Reproduced with permission.¹⁸² Copyright 2019, American Chemical Society. Color labels: magenta/violet = Au, pink = Au/Ag, light gray/light blue = Ag, blue = Pt, yellow = S, light orange = P, green = Cl, brown = Br, purple = N, red = O, grey = C. R groups are omitted or simplified for clarity.

Using bimetallic I_h building blocks to form cluster of clusters was reported by the Teo group in the 1980s, and the M₁₃ (M = Au/Ag) units are connected by vertex sharing.^183–185 In 2005, when thiolate was introduced to the solution of Au₁₁(PPh₃)₈Cl₂, Au₂₅(SR)₅(PPh₃)₁₀Cl₂ (SR = SC_nH_2n+1, n = 2–18) NCs with two vertex-sharing I_h Au₁₃ units (aspect ratio of 2.3) was produced.^186,187 Later, the Jin group reported another bi-I_h Au₂₄(SR)₅(PPh₃)₁₀Cl₂ NC (Fig. 5B, purple line) with the shared vertex eliminated by adding excess PPh₃, which is in contrast to the 25-metal-atom rod counterpart (Fig. 5B, green line). DFT calculations indicate that the broad band at ∼560 nm arises from the HOMO−1 to LUMO+2 transition.¹⁴¹ Excitingly, a tri-I_h Au₃₇(SR)₁₀(PPh₃)₁₀X₂ (X = Cl/Br) with AR = 3.5 was prepared by reducing Au^I(PPh₃)X to clusters, then etching the clusters with excess thiol (Fig. 5B, red line).¹⁸⁸ Along with the Au₁₃(dppe)₅Cl₂ NC (Fig. 5B, blue line) with a mono-I_h core reported by Konishi and coworkers in 2010,¹⁸⁹ the first rod-shaped series of Au NCs is achieved, and the mono-I_h, bi-I_h, and tri-I_h NCs possess 8e⁻, 16e⁻, and 24e⁻, respectively, according to Mingo's electron-counting rule.¹⁹⁰ Moreover, the series shows an interesting trend in the optical absorption spectra (Fig. 5B): the high-energy peaks are similar for the series of NCs since the electronic transitions occur within the individual Au₁₃;^130,191 whereas the features of dimerization or trimerization due to the interaction between neighboring Au₁₃ units are reflected in the red-shifted peaks in the visible range.³² It is noteworthy that the peak significantly shifts to 1230 nm for tri-I_h Au₃₇ due to the trimeric interaction (Fig. 5B, red line). Moreover, an excited-state exciton localization was observed in the Au₃₇ NC which exhibits strong coupling between Au₁₃ units after photoexcitation, and kinetic traces indicate the existence of both radial and axial vibration modes.¹³⁰ Femtosecond spectroscopic investigations on Au₂₅(SR)₅(PPh₃)₁₀Cl₂ show overlapped excited state absorption and ground state bleach signals. The 0.8 ps component of the lifetime is attributed to the fast internal conversion process from LUMO+n to LUMO, whereas the 2.4 μs long component is due to electron relaxation to the ground state.¹⁹²

The interesting stories of rod-shaped Au₂₅ are also highlighted by heteroatom doping. Wang et al. continued to illustrate that substituting Au with Ag not only changed the absorption profile (Fig. 5C), but also drastically enhanced the photoluminescence (PL) to as high as QY = 40.1% for Au_25−xAg_x(SR)₅(PPh₃)₁₀Cl₂ (x = 1–13, Fig. 5C, inset),¹⁷⁹ in striking contrast to the weakly luminescent species (x = 1–12, QY = 0.21%) and the homogold counterpart.¹⁹³ In other words, the 13th Ag atom is critical in enhancing PL. Based on the highly photoluminescent rod-shaped Au_25−xAg_x(SR)₅(PPh₃)₁₀Cl₂, (x = 13) the self-annihilation electrogenerated chemiluminescence (ECL) was found to be 10 times higher than the standard tris(bipyridine)–ruthenium(ii) complex, and co-reactant (tripropylamine) ECL was about 400 times stronger.¹⁹⁴

By replacing the 10 phosphine ligands with N-heterocyclic carbene ligands and the 5 thiolates with halides, a rod-shaped Au₂₅(NHC)₁₀Br₇ NC (NHC = 1,3-diisopropylbenzimidazolin-2-ylidene) was resulted,¹⁸⁰ showing the first absorption peak at ∼650 nm, similar to the case of Au₂₅(SR)₅(PPh₃)₁₀Cl₂ (Fig. 5D). The high thermal and air stabilities of this NC are ascribed to the stronger Au–carbene bond than the Au–phosphine bond (difference up to 0.9 eV) as indicated by DFT. The NC also shows high catalytic activity in homogenous cycloisomerization of alkynyl amines to indoles compared to the trace activity of Au₂₅(SR)₅(PPh₃)₁₀Cl₂.¹⁸⁰ With the presence of Pd²⁺, the assembly of two I_h Pd@Au₁₂ units into rod-shaped Pd₂Au₂₃(PPh₃)₁₀Br₇ was also achieved.¹⁹⁵ Hetero-bi-I_h Pd₁Au₂₄(SR)₅(PPh₃)₁₀Cl₂, i.e., only one I_h unit is central-doped with Pd, was synthesized by etching the crude product (made by co-reducing AuCl(PPh₃) and Pd(PPh₃)₄) with thiol in refluxing chloroform.¹⁹⁶ Compared to the rod-shaped Au₂₅ NC, obtaining the rod-shaped Ag₂₅ NC (i.e., the silver counterpart) of the same structure is challenging. However, Pt or Pd doping at the centers of the two I_h units could improve the stability of the NCs, resulting in M₂Ag₂₃(PPh₃)₁₀Cl₇ (M = Pt/Pd).^197,198

Very recently, the first supercluster with four I_h Ag₁₃ units linearly connected was reported by Q.-M. Wang and Xu.¹⁸¹ The resulted Ag₆₁(dpa)₂₇ (dpa = dipyridylamine) was produced by reducing Ag^I-dpa in the presence of bidentate phosphine, AgSbF₆, and CH₃ONa in dichloromethane. The 30e⁻ of Ag₆₁ makes it isolobal to the linear tetraiodide anion I₄²⁻.¹⁹⁹ Compared with Ag₂₁(dpa)₁₂ of which the large energy gap is mainly decided by the superatomic Ag₁₃⁵⁺ (8e⁻) unit,²⁰⁰ Ag₆₁(dpa)₂₇ with AR = 4.2 shows an intense absorption peak extending into NIR (819 nm) and the first absorption band at 1170 nm (Fig. 5E).¹⁸¹ The authors attributed the loss of the optical features of mono-I_h Ag₂₁ in the spectrum of tetra-I_h Ag₆₁ to the stronger electron couplings between the I_h Ag₁₃ units, in contrast to the case of Au₁₃vs. Au₃₇ (Fig. 5B). Besides, with the help of Pt doping, Liu et al. reported another series of NCs containing one, two or three I_h Pt₁@Ag₁₂ units (Fig. 5F) by co-reducing Ag^I and Pt^II with LiBH₄ and chromatography separation. Similar to the Au series, the lowest-energy absorption bands of the Ag series display a red-shift from 412 nm in mono-I_h Pt₁Ag₂₀(dtp)₁₂ to 710 nm in bi-I_h Pt₂Ag₃₃(dtp)₁₇, then to 956 nm in tri-I_h Pt₃Ag₄₄(dtp)₂₂ (dtp: dipropyl dithiophosphate). Although the first two alloy NCs follow the superatomic electron counts of 8e⁻ and 16e⁻, respectively, the tri-I_h NC bears only 22e⁻, representing the first case in NCs that is isoelectric to linear triiodide anion I₃⁻.¹⁸² Note that in heteroleptic rod-shaped 25-metal or 37-metal NCs,^{82,187,188,196–198} the two or three M₁₃ (M = Au/Ag) units are aligned in an eclipsed conformation; whereas in the homoleptic rod-shaped Pt₂Ag₃₃, Pt₃Ag₄₄ as well as Ag₆₁ NCs,^181,182 a gauche rotational conformation is adopted in the connection of multiple I_h units.

Compared to the rod-shaped NCs composed of more commonly observed I_h units, anisotropic growth of fcc structures into nanorods (bulk Au also adopts fcc packing) was also reported for Au NCs of atomic precision. It was surprising when an fcc kernel was first discovered in Au₃₆(SR)₂₄ (SR = SPh^tBu) in 2012.⁷⁹ The close-packed atomic planes show an a–b–c–a stacking sequence (Fig. 6A), resulting in the cubic and cuboid shapes of NCs of fcc structures. The numbers (Fig. 6A) indicate how many metal atoms are in each (001) layer. After continuous successes in synthesizing and characterizing Au₂₈(SR)₂₀,⁸⁰ and Au₅₂(SR)₃₂,⁸⁷ the Jin group completed a magic series with a unified formular of Au_8n+4(SR)_4n+8 (n = 3–6) in 2016 thanks for the structural determination of Au₄₄(SR)₂₈.²⁰¹ The progression of NCs from small to large is neatly shown in the mass spectra of the four NCs (Fig. 6B). In a concise layer-by-layer mode, all six surfaces of the series of NCs are exclusively terminated by {100} facets, and the anisotropic growth of the magic series can be viewed as subsequently adding an additional 8-gold-atom (001) layer along the z direction, with n representing the number of (001) layers (Fig. 6A). That is, n = 3 for Au₂₈(SR)₂₀, n = 4 for Au₃₆(SR)₂₄, n = 5 for Au₄₄(SR)₂₈, and n = 6 for Au₅₂(SR)₃₂.²⁰¹ In 2019, Wu et al. reported that a half (001) layer composed of 4 Au atoms could also be stacked on the full 8-gold-atom (001) layer, resulting in Au₅₆(SR)₃₄.¹²⁵ Both works show the evolution of optical properties, which is indicated by the red-shift of the lowest energy peak (assigned to sp → sp transition) as the kernel grows, and their optical energy gaps are estimated to be 1.76, 1.75, 1.50, 1.39, and 1.30 eV, respectively (Fig. 6C, from Au₂₈ to Au₅₆). Based on the observation of a top half layer, the theoretical structures of Au₃₂(SH)₂₂, Au₄₀(SH)₂₆, and Au₄₈(SH)₃₀ are predicted as well.²⁰² Additionally, when the (001) layers composed of 6 Au atoms and 8 Au atoms are stacked alternatively (total 6 layers), another Au₄₂(SR)₂₆ NC can be obtained (Fig. 6A).²⁰³

Fig. 6 (A) The anisotropic growth of the fcc lattice of Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, Au₄₄(SR)₂₈, Au₅₂(SR)₃₂, and Au₅₆(SR)₃₄ NCs with top views of the Au_8n+4(SR)_4n+8 magic series compared to Au₅₆(SR)₃₄, and the structure of Au₄₂(SR)₂₆. (B) ESI-MS spectra of the Au_8n+4(SR)_4n+8 series, n = 3–6. The m/z = [Au_n(SR)_m + Cs]⁺. Reproduced with permission.²⁰¹ Copyright 2016, American Chemical Society. (C) Absorption spectra of the fcc series. Reproduced with permission.¹²⁵ Copyright 2020, Wiley-VCH GmbH. (D) Powder XRD patterns of Au₇₆(SR)₄₄ before (green) and after (blue) amidation reaction, and bulk gold (black). (E) UV-vis-NIR absorption spectrum of Au₇₆(SR)₄₄ solution. Reproduced with permission.²⁰⁴ Copyright 2015, American Chemical Society. (F) The optimized theoretical structures of Au₆₀(SR)₃₆, Au₆₈(SR)₄₀, and Au₇₆(SR)₄₄. Reproduced with permission.²⁰⁵ Copyright 2016, The Royal Society of Chemistry. (G) Top and side views of Au₃₆(SR)₈Cl₂₀. (H) Top and side views of Ag₂₃(SR)₁₈(PPh₃)₈. (I) Experimental and calculated absorption spectra of Ag₃₈(SR)₂₆(PR)₈, and corresponding top and side views of the structure. Reproduced with permission.²¹⁰ Copyright 2016, American Chemical Society. (J) Absorption spectra of Ag₆₇(SR)₃₂(PPh₃)₈, and corresponding top and side views of the structure. Reproduced with permission.²¹¹ Copyright 2016, American Chemical Society. Color codes: magenta/purple/pink/violet = Au, light grey/light blue = Ag, yellow = S, green = Cl, light orange = P. R groups are omitted for clarity.

In the meantime that the fcc magic series was achieved, the Tsukuda group also synthesized a water soluble Au₇₆(SR)₄₄ NC (SR = SC₂H₄PhCOOH) by slow reduction,²⁰⁴ and the powder X-ray diffraction indicated a fcc kernel structure elongated along the {100} direction (Fig. 6D). Interestingly, the proposed rod-shaped fcc Au₇₆ NC exhibited a strong NIR absorption band centered at 1340 nm with molar absorption coefficient ε of 3 × 10⁵ M⁻¹ cm⁻¹ (Fig. 6E), much larger than that of the coefficient of HOMO–LUMO transition in most of other gold–thiolate NCs (∼1 × 10⁴ M⁻¹ cm⁻¹).¹⁹ The large value of ε was tentatively ascribed to the large electronic–transition moment arising from the one-dimensional structure.²⁰⁴ Since Au₇₆(SR)₄₄ could be fitted in the Au_8n+4(SR)_4n+8 series with n = 8, theoretical calculations were conducted to predict its structure. Zeng et al. found highly negative values of the nucleus-independent chemical shift in the tetrahedral Au₄ units, explaining the overall stability of the series of fcc NCs, and the structures of Au₆₀(SR)₃₆, Au₆₈(SR)₄₀, and Au₇₆(SR)₄₄ were simulated (Fig. 6E inset and Fig. 6F).²⁰⁵ Infinite extending of the lattice showed a band gap of 0.78 eV for the gold–thiolate nanowire.²⁰⁵ At the same time, the Pei group also obtained the same simulated structure of Au₇₆(SR)₄₄ and assigned its NIR absorption to the anisotropic Au kernel which enhanced the longitudinal orbital transition and the ligand passivation, resulting in the red shift of the absorption peak.²⁰⁶ In another fcc NC, Au–Cl–Au motif was first observed in Au₃₆(SR)₈Cl₂₀ (SR = SCH₂Ph^tBu), and the total structure can be regarded as four stacking layers in a–b–c–a sequence (Fig. 6G).²⁰⁷

In 2013, Zheng et al. reported a cubic fcc Ag building block —Ag₁₄(SR)₁₂(PPh₃)₈ (SR = SPhF₂) NC.²⁰⁸ Later, by stacking multiple of such building blocks together, a series of cuboid fcc Ag NCs were achieved with six faces and twelve edges capped by thiolates and eight corners terminated by phosphine ligands. In the Ag₂₃(SR′)₁₈(PPh₃)₈ (SR′ = SC₂H₄Ph), its structure is a combination of two cubic units with some twist along the C₂ axis, endowing chirality to the NC (Fig. 6H).²⁰⁹ Ag₃₈(SR)₂₆(PR′)₈ (PR′ = tributylphosphine) is composed of four of such cubes, and its optical spectrum shows three peaks at 413, 507, 563 nm, with some weak absorption bands as well (Fig. 6I),²¹⁰ while the Ag₁₄ building block NC only shows two shoulder bands.²⁰⁸ Linear-response time-dependent DFT indicates that the Ag₃₈ is rather electronically stable, with an energy gap of 0.67 eV. A larger cubic Ag₆₃(SR)₃₈(PR′)₈ of 2 × 2 × 2 size was also reported in the same work and a theoretical 3 × 3 × 3 Ag₁₇₂(SR)₇₂(PR′)₈ was predicted.²¹⁰ Additionally, the Bakr group revealed an Ag NC built up with 12 units, i.e., Ag₆₇(SR′′)₃₂(PPh₃)₈ (SR′′ = SPh(CH₃)₂). Its UV-vis spectrum shows similar strong absorption between 400 to 600 nm as Ag₃₈(SR)₂₆(PR′)₈, but with an additional peak at ∼680 nm (Fig. 6J).²¹¹

So far, NCs with I_h, fcc, or decahedral kernels constitute the majority of atomically precise noble metal NCs.^{32,48,212–214} By comparison, Au NCs with hexagonal-close-packed (hcp) kernels are very rare, and no hcp structure is ever found in Ag NCs. We start from Au₂₄(SR)₂₀, although it does not have an hcp kernel. Based on the formular of Au₂₄(SR)₂₀ obtained in experiment,²¹⁵ two isomeric structures with low energies were predicted by Pei et al. in 2011,²¹⁶ and both of them were proved by crystallography later. The total structure of Au₂₄(SR)₂₀ was first solved by applying SR = SCH₂Ph^tBu.⁸² Two tetrahedral Au₄ units are joined at their triangle faces by 60° rotation to form a bi-tetrahedral Au₈ kernel. One pair of Au₄(SR)₅ staples is assembled onto the two ends of the Au₈ kernel via bi-dentate bonding; and a second pair of the tetrameric staples is further attached to the Au₈ kernel by rotating ∼90° relative to the first pair (Fig. 7A, inset). Other thiols were also used to prepare the same 24-gold-atom NC, and the fluorescence QY decreased from 2% for Au₂₄(SCH₂Ph^tBu)₂₀, to 1.5% for Au₂₄(SCH₂Ph)₂₀, then to 0.3% for Au₂₄(SC₂H₄Ph)₂₀, with the emission peak slightly blue-shifted (Fig. 7B and C), correspondence to the electron-donor strength of the ligand.¹²⁶ By contrast, selenolate-protected Au₂₄(SePh)₂₀ shows an isomeric structure in which two tetrahedral Au₄ units cross-join together at the edges (rather than the faces), and the new Au₈ kernel is protected by a pair of Au₅(SePh)₆ motifs as well as a pair of Au₃(SePh)₄ motifs (Fig. 7D, left).²¹⁷ The Au₂₄(SePh)₂₀ NC is not luminescent in the detected range (Fig. 7B and C).¹²⁶ In another case, the Au₉ kernel of Au₁₈(SR)₁₄ (SR = S-c-C₆H₁₁) can be viewed as three Au₃ layers arranged in an a–b–a manner (Fig. 7D, middle),^218,219 and the middle Au₃ layer can be replaced by an Ag₃ layer without changing the structure.²²⁰ It is until the report of Au₃₀(SR)₁₈ (SR = adamantanethiolate, SC₅H₁₀) that a complete hcp kernel was identified since four layers of Au₃–Au₆–Au₆–Au₃ are packed in an a–b–a–b manner, and the total structure has an unprecedentedly high S₆ symmetry (Fig. 7D, right), resulting in the special solubility of the NC, i.e., only soluble in benzene.²²¹

Fig. 7 (A) Absorption spectrum (photon energy scale) of Au₂₄(SR)₂₀ with corresponding structure shown in the inset. (B) Fluorescence spectra of Au₂₄(SR)₂₀ NCs with different thiolate ligands and Au₂₄(SeR)₂₀ NCs in CH₂Cl₂, and (C) digital photographs of samples in solution or solid state under visible and 365 nm UV light irradiation. Reproduced with permission.¹²⁶ Copyright 2016, Wiley-VCH GmbH. (D) Structures of Au₂₄(SeR)₂₀, Au₁₈(SR)₁₄ and Au₃₀(SR)₁₈. (E) Absorption spectra (photon energy scale) of Au₄₂(SR)₃₂ with corresponding structure shown in the inset. (F) Temperature changes of Au₄₂(SR)₃₂, Au₃₈(SR)₂₄, Au₅₂(SR)₃₂, and Au₂₄(SR)₂₀ in toluene (50 μg mL⁻¹) upon irradiation at 808 nm, 1 W cm⁻², inset: the relationship between ΔT and molar absorption coefficients of NC solutions at irradiation wavelength. (G) Time-dependent images of Au₄₂(SR)₃₂ solution upon irradiation. (H) ns-TA data map of Au₂₄(SR)₂₀ and its TA kinetic traces and fitting (probed at 580 nm). (I) ns-TA data map of Au₄₂(SR)₃₂ and its TA kinetic traces and fitting (probed at 813 nm). λ_ex = 400 nm. (J) Experimental absorption spectrum (blue) and TDDFT-simulated absorption spectrum (red) of (J) Au₄₂(SR)₃₂ and (K) Au₂₄(SR)₂₀, insets, schematic illustrations of α and β electronic transitions. (A and E–K) Reproduced with permission.¹²⁷ Copyright 2022, American Chemical Society. Temperature-dependent PL spectra of (L) Au₄₂(SR′)₃₂ in 2-methyltetrahydrofuran, and (M) Au₄₂(SR′)₃₂/PS film. Reproduced with permission.¹²⁸ Copyright 2022, American Chemical Society. Color codes: magenta/purple = kernel Au, violet/pink = motif Au, yellow = S, light orange = Se. Organic ligands are omitted for clarity.

A new breakthrough was achieved when the total structure of a rod-shaped Au₄₂(SR)₃₂ (SR = SCH₂Ph) NC with a long hcp kernel was resolved very recently.¹²⁷ When comparing the optical absorption spectra, the lowest energy absorption peak of Au₂₄(SR)₂₀ is at 2.5 eV (Fig. 7A), whereas that of Au₄₂(SR)₃₂ red-shifts to 1.5 eV (Fig. 7E) with 6.5 times stronger intensity. The hcp Au₂₀ kernel of Au₄₂(SR)₃₂ comprises six Au₃ layers with two capping Au atoms on opposite ends of the rod, i.e., Au₁–Au₃–Au₃–Au₃–Au₃–Au₃–Au₃–Au₁, compared to the Au₈ kernel (Au₁–Au₃–Au₃–Au₁) in Au₂₄(SR)₂₀. Two pairs of Au₄(SR)₅ staples are attached to the two ends of the Au₂₀ kernel, similar to what is observed in Au₂₄(SR)₂₀, but the much longer kernel further requires six short Au(SR)₂ motifs to protect its body (Fig. 7E, inset). The aspect ratio of the long hcp Au₂₀ kernel is as large as 6.2, providing a great opportunity to study new properties, since the confinement of excitons within the long but narrow kernel leads to strong photothermal conversion upon photoexcitation. The molar absorption coefficient ε of Au₄₂(SR)₃₂ is 1.4 × 10⁵ M⁻¹ cm⁻¹ at 815 nm, and upon 808 nm excitation (the most commonly used laser for photothermal therapy with minimum extinction by human tissues),^222,223 very rapid heating was observed as the temperature change (ΔT) reached ∼27 °C within only ∼5 min (Fig. 7F, green stars/Fig. 7G).¹²⁷ By comparison, other anisotropic Au-SR NCs, including Au₃₈(SR)₂₄ with dimeric Au₂₃ kernel (see Fig. 3A), and Au₅₂(SR)₃₂ with a layer-by-layer fcc Au₄₈ kernel (see Fig. 6A), increased only by ∼10 and ∼3 °C, respectively (Fig. 7F, red circles and magenta squares). ΔT is almost linear with respect to the exponential scale of ε at the irradiation wavelength. In transient absorption (TA) analysis, although almost the same fast decay was observed in both Au₄₂(SR)₃₂ and Au₂₄(SR)₂₀ (long and short rods), the nanosecond TA showed the decay lifetimes (τ) of Au₂₄(SR)₂₀ was 120 ns (Fig. 7H), much shorter than the 2400 ns of Au₄₂(SR)₃₂ (Fig. 7I). Moreover, TDDFT calculations (Fig. 7J) found that the transition dipole moment of peak α (corresponding to HOMO to LUMO transition) of Au₄₂(SCH₃)₃₂ is ∼10 times stronger along the longitudinal transition than along the transverse transition, indicating the strong NIR absorption is a longitudinal transition, reminiscent of the NIR band of plasmonic Au nanorods. It is a surprise to see that longitudinal excitation can be inherited by NCs of molecular state. For Au₂₄(SCH₃)₂₀ with a much shorter kernel, the longitudinal transition for the first peak is not as strong as the case of Au₄₂(SCH₃)₃₂, but still stronger than the transverse transitions (Fig. 7K).¹²⁷

In addition to the unprecedented photothermal conversion properties, a photoluminescence study on the same Au₄₂(SR′)₃₂ NC (SR′ = SC₂H₄Ph) found a dual NIR emission at 875 and 1040 nm, corresponding to fluorescence and phosphorescence, respectively (Fig. 7L).¹²⁸ The QY of Au₄₂(SR′)₃₂ was measured to be 11.9% in ambient CH₂Cl₂ solution at room temperature. Furthermore, when the NCs were embedded in polystyrene (PS) films (solid state), the fluorescence was dramatically suppressed while the phosphorescence was significantly enhanced (Fig. 7M), which was ascribed to the largely enhanced intersystem crossing from singlet to triplet excited state induced by dipolar interaction.¹²⁸ On a note, Wu et al. independently reported the same Au₄₂(SR)₃₂ NC obtained by a thermal and ligand exchange induced fcc-to-hcp structural transformation, and time-dependent DFT was performed to interpret the Au₄₂ dual emission.²²⁴

Finally, some clues can be drawn from the synthesis to understand how the ultrasmall Au₄₂(SR)₃₂ nanorod was achieved. In the synthesis reported by the Jin group,¹²⁷ Au₄₂(SR)₃₂ was separated from Au₂₄(SR)₂₀ by thin layer chromatography, and the as-synthesized mixture of NCs also contains Au₂₅(SR)₁₈ and Au₃₈(SR)₂₄. Since Au₃₈(SR)₂₄ is a dimeric structure of two Au₂₅(SR)₁₈ NCs through fusion (see Fig. 3 and discussion therein), Au₄₂(SR)₃₂ might also be formed from the anisotropic growth of Au₂₄(SR)₂₀. In the synthesis by the Wu group, the Au₄₂(SR)₃₂ nanocluster was obtained by a thermally induced ligand-exchange reaction from Au₂₈(SPh^tBu)₂₀, and then purified by thin layer chromatography,²²⁴ indicating that the anisotropic growth occurs simultaneously with fcc-to-hcp structural transformation.

Among the bimetallic NCs, Au₁₀Ag₂(C≡CR)₃(dppy)₆ (C≡CR = 2-pyridylethynyl, dppy = 2-pyridyldiphenylphosphine) has a trigonal bipyramidal Au₁₀Ag₂ core with Ag atoms at the two ends. The dual emission of the NC in solution includes (i) the visible emission originating from metal-to-ligand charge transfer from Ag atoms to phosphine ligands and (ii) the intense NIR emission which is associated with the participation of 2-pyridylethynyl in the frontier orbitals of the NC.²²⁵ The Pt₁Ag₂₆(SR)₁₈(PPh₃)₆ (SR = SPh(CH₃)₂) NC has an I_h Pt@Ag₁₂ kernel with two more Ag atoms at the two ends. With the Ag₁₂(SR)₁₈(PPh₃)₆ shell wrapping on the surface, the bimetallic NC shows an overall rod shape. Compared to Pt₁Ag₂₄(SR)₁₈ with a Pt@Ag₁₂ kernel, the optical absorption spectrum of Pt₁Ag₂₆(SR)₁₈ is almost the same, but their electrochemical gaps are quite different (1.48 V vs. 1.89 V) due to the charge influence.²²⁶ When replacing the central Pt with Au, the authors co-crystallized [Au₁Ag₂₆(SR′)₁₈(PPh₃)₆]⁺ and [Au₁Ag₂₄(SR′)₁₈]⁻ (SR′ = SPhC₂H₅) into alternating stacking array along the [001] direction via weak C–H⋯π and π⋯π interactions and strong electrostatic interactions.²²⁷

Additionally, Sun et al. prepared quite many polyoxometalate-templated Ag NCs under solvothermal conditions, and some of the NCs possess rod-shaped structures. Selected works include Ag₇₂ (a long Ag shell wrapping two linearly aligned [EuW₁₀O₃₆]⁹⁻ anions);²²⁸ Ag₇₆ (two Mo₆O₂₂⁸⁻ templates encaged in an Ag₇₂ shell);²²⁹ Ag₈₀ (the inside Ag₁₀ kernel is locked by a pair of Mo₇O₂₆¹⁰⁻ anions and further encapsulated by an outer Ag₇₀ shell);²³⁰ and Ag₉₀ (the dumbbell-shaped Ag₉₀ cage covers two smaller SO₄²⁻ and two larger W₅O₁₉⁸⁻ anions inside).²³¹ Compared to the linear Au coordination to two S atoms, a complete cage usually forms due to higher Ag–S coordination,⁵⁵ which is more flexible to accommodate the inner kernel shape. Note that these Ag^I NCs have no free valent electron as we discussed above for Au NCs.

Oblate nanoclusters

Heteroleptic Au₁₉(C≡CPh)₉(dppa)₃ (dppa = N,N-bis-(diphenyl-phosphino)amine) has an I_h Au₁₃ kernel, and three V-shaped Au₂(C≡CPh)₃ motifs along the C₃ axis.²³² The alkynyl ligands are anchored on surface Au atoms via both σ and π bondings, which is different from the bonding mode of thiolate on Au. The NC shows two major absorption peaks at 1.25 and 2.25 eV, and orbital analysis indicates that the PhC≡C– groups actively participate in the frontier orbitals of the whole NC (Fig. 8A). Although not mentioned in the original work, we here point out that the metal part of the NC resembles a tiny triangular disk despite that no (111) facet is found on the surface. Another alkynyl-protected Au₂₂(C≡CR)₁₈ NC (R = ^tBu) also has a triangular shape in which a flattened cuboctahedral Au₁₃ kernel is surrounded by three Au₃(C≡CR)₄ motifs.²³³ The Au₂₂(C≡CR)₁₈ NC required a relatively weak reduction environment in the synthesis. Although the QY of the NC solution (in CH₂Cl₂) was only 0.4%, the solid-state emission yield of Au₂₂(C≡CR)₁₈ increased to 15% with a predominant lifetime of 1.37 μs.²³⁴ The same Au₂₂ NC could also be prepared using triethylamine as a mild reductant, and different C≡CR′, including 3-ethynylthiophene (ETP-H), phenylacetylene (PA-H), 3-ethynyltoluene (ET-H), and 3-ethynylanisole (EA-H) were adopted.¹³¹ The PL QY was found to be dependent on the R group, and the highest QY = 4.6% (solution) was obtained when R = Ph (Fig. 8C).¹³¹

Fig. 8 (A) Top and side views of Au₁₉(C≡CR)₉(dppa)₃, and its experimental and simulated optical absorption spectra. Reproduced with permission.²³² Copyright 2014, American Chemical Society. (B) Top and side views of Au₂₂(C≡CR)₁₈. (C) UV-vis spectra, PL spectra and lifetimes of Au₂₂(C≡CR′)₁₈ with different ligands. Reproduced with permission.¹³¹ Copyright 2019, American Chemical Society. (D) Top and side views of Pt₃Ag₃₃(PPh₃)₁₂Cl₈, and UV-vis spectra of Pt₁Ag₁₂(dppm)₅(SR)₂, Pt₂Ag₂₃(PPh₃)₁₀Cl₇, and Pt₃Ag₃₃(PPh₃)₁₂Cl₈. Reproduced with permission.²³⁵ Copyright 2018, The Royal Society of Chemistry. (E) Top and side views of Au₉Ag₃₆(SR)₂₇(PPh₃)₆. (F) Top and side views of Ag₇₈(SR)₄₂(dppp)₆, and the CD spectra and corresponding anisotropy factors of R- and S-Ag₇₈ enantiomers. Reproduced with permission.²³⁷ Copyright 2017, American Chemical Society. (G) Top and side views of Au₆₀Se₂(SeR)₁₀(PPh₃)₁₅, and its UV-Vis spectrum compared to Au₂₅(SR)₅(PPh₃)₁₀Cl₂ and Au₁₃(dppe)₅Cl₂. Reproduced with permission.²³⁸ Copyright 2015, Wiley-VCH GmbH. (H) Top and side views of Au₅₂Cu₇₂(SR)₅₅, its Au₅Cu₂@Au₄₇ decahedral kernel, and the interactions among four nearest pentagonal NCs. Reproduced with permission.²³⁹ Copyright 2020, The author(s). (I) Top and side views of Au₄₀(SR)₂₄. (J) Top and side views of Au₅₆(SR)₂₄(PR₃)₆Br₂, the kinetic traces of transient absorption and corresponding fits at selected wavelengths, and spectra at selected time-delays. Reproduced with permission.¹³³ Copyright 2022, The author(s). (K) Top and side views of Au₃₆Ag₂(SR)₁₈, and the Lewis structure of the [Au₃₀Ag₂]¹²⁺ kernel compared to O₃²⁻, SA = superatom. (L) HER voltammograms of Au₃₆Ag₂(SR)₁₈, Au₃₈(SR)₂₄ and Au₂₅(SR)₁₈⁻ catalysts, and the calculated Gibbs free energy of H₂(g) formation on the catalysts. Reproduced with permission.¹³² Copyright 2021, American Chemical Society. (M) Top and side views of Cu₈₁H₃₂(SR)₄₆(^tBuNH₂)₁₀. Color codes: magenta/violet/purple/pink = Au, light grey = Ag, orange/green = Cu, yellow = S, light orange = P, light green = Se, light purple = N. R groups are omitted or simplified for clarity.

Not only the rod-shaped structure can be achieved by connecting three I_h units via vertex sharing,¹⁸⁸ a cyclic structure (i.e., an oblate shape) is also realized in bimetallic Pt₃Ag₃₃(PPh₃)₁₂Cl₈ (Fig. 8D, left) and trimetallic Pt₃Au₁₂Ag₂₁(PPh₃)₁₂Cl₈ NCs with three Pt atoms doped at the centers of three I_h M₁₃ (M = Ag/Au) units.²³⁵ Since the cyclic structure is composed of three I_h units, the whole shape can be regarded as a truncated triangular disk. A clear red-shift of the lowest energy peak was observed from 428 nm for mono-I_h Pt₁Ag₁₂(dppm)₅(SR)₂ (dppm = Ph₂PCH₂PPh₂, SR = SPh(CH₃)₂) to 557 nm for bi-I_h Pt₂Ag₂₃(PPh₃)₁₀Cl₇, then to 750 nm for cyclic tri-I_h Pt₃Ag₃₃(PPh₃)₁₂Cl₈ (Fig. 8D, right).²³⁵

Zheng et al. reported that Au₉Ag₃₆(SR)₂₇(PPh₃)₆ (SR = SPhCl₂) has an Au₉ kernel encaged in a trigonal prismatic Ag₃₆(SR)₂₇(PPh₃)₆ shell.²³⁶ This bimetallic NC can be viewed to have two dented (111) facets and three distorted (100) facets of fcc arrangement (Fig. 8E). The same group also revealed an Ag₇₈(SR)₄₂(dppp)₆ NC (SR = SPhCF₃, dppp = Ph₂P(CH₂)₃PPh₂) with a triangular prismatic structure conformed to ideal D₃ symmetry (Fig. 8F, left). When replacing the achiral dppp with chiral diphosphine, i.e., (2s,4s) or (2r,4r)-2,4-bis(diphenylphosphino) pentane, the flexible C–C–C of diphoshpine restricts the relative orientation of the four Ag–thiolate–phosphine moieties at the three side edges, resulting in enantioselectivity of the metal core (R- and S-Ag₇₈), with the maximum anisotropy factor up to 2 × 10⁻³ at 678 nm (Fig. 8F, right).²³⁷

Five I_h Au₁₃ building blocks can even form a closed ring by vertex sharing in the Au₆₀Se₂(SePh)₁₅(PPh₃)₁₀ structure, which shows a pentagonal oblate shape.²³⁸ The two neighboring Au₁₃ units are connected by selenolate linkages, and two Se atoms are capped at the top/bottom centers of the NC. The Au₆₀ NC not only maintains the optical property of the Au₁₃ unit, the interactions between neighboring Au₁₃ units also lead to further red-shift of the low-energy absorption peak to 835 nm compared to the 670 nm peak of bi-I_h Au₂₅(SR)₅(PPh₃)₁₀Cl₂ (Fig. 8G).²³⁸

Oblate decahedral NCs have been reported when the kernel diameter reaches 2 nm. The thiolate (SR = SPh^tBu)-stabilized NC containing 374 Ag atoms was reported to become metallic with the emergence of surface plasmon resonance.²⁴⁰ From the structural perspective, both Ag₁₃₆ and Ag₃₇₄ NCs are regarded as miniatures of five-fold twinned NPs. Ag₁₃₆ has a pentagonal bipyramidal Ag₅₄ kernel with ten (111) facets, whereas Ag₃₇₄ has an elongated pentagonal bipyramidal Ag₂₀₇ kernel with five (100) faces at the five sides and five (111) facets at each end.²⁴⁰ A decahedral Ag₅₁ kernel with D_5h symmetry was also found in Ag₁₄₆(SR′)₈₀Br₂ (SR′ = SPhCH(CH₃)₂) with an overall pentagonal oblate shape.²⁴¹ Interestingly, the Ag₁₄₆(SR′)₈₀Br₂ NC shows an optical bandgap and power-independent electron dynamics, indicating its molecular-like nature, as opposed to the plasmonic Ag₁₃₆ NC with a smaller size. The bimetallic Au_130−xAg_x(SR)₅₅ (avg. x = 98) also has a decahedral M₅₄ kernel (M = Au/Ag) inside the Ag-SR shell.²⁴² In the M₅₄ kernel, all ~32 Au atoms are localized in the truncated M₄₉ Marks decahedron, whereas the five corners of the regular M₅₄ decahedron are all Ag.²⁴² An interesting question is why homogold NCs of similar size can only have the truncated Au₄₉ kernel,^20,85 while Ag or Au/Ag alloy NCs show M₅₄ full decahedrons inside the NCs. The case of Au₅₂Cu₇₂(SR′′)₅₅ (SR′′ = SPhCH₃) gives insights into the Marks decahedron truncation mechanism (Fig. 8H). When comparing the Au₅Cu₂@Au₄₇ kernel (Fig. 8H, top right) in the Au-Cu bimetallic NC and the Au₇@Au₄₂ kernel in Au₁₀₂(SR)₄₄ or Au₁₀₃S₂(SR)₄₁, it was found that the axial Au–Cu–Cu–Au length is contracted by 7% due to Cu doping, and the angle (θ) also expends, giving enough space for an Au atom to fit in (total five corners), completing a decahedral M₅₄ core. The same phenomenon was also observed in Ag or Ag–Au bimetallic NCs, so that M₅₄ full decahedrons are formed. Moreover, the Cu₇₀(SR′′)₅₅ exterior cage of Au₅₂Cu₇₂(SR′′)₅₅ resembles 3D Penrose tiling, and interparticle interactions in the supercrystal give rise to a “quadruple-gear-like” interlocking pattern (Fig. 8H, bottom right).²³⁹

Tetrahedral Au₄ units coil up into a Kekulé-like ring along two vertex-sharing Au₄ units to constitute the fcc kernel in Au₄₀(SR)₂₄ (SR = SPhCH₃).⁸⁷ Since the surface motifs are distributed along the C₃ axis, the whole NC exhibits a hexagonal oblate structure (Fig. 8I).⁸⁷ In the seed-sized Au₅₆(SR′)₂₄(PR₃)₆Br₂ (SR′ = SPh^tBu, R = PhCF₃, PhCl, or PhF) nanoprism, the side Au{100} facets are covered by bridging thiolates, whereas the top or bottom {111} facet is capped by phosphine ligands at three corners and Br⁻ at the center (Fig. 8I, left).¹³³ The bromide is key to effectively stabilize the Au{111} to fulfill a complete fcc core, while the halide-free gold precursor results in Au₅₅(SR′′)₂₄(PPh₃)₆ (SR′′ = SPhCH₃) with one missing gold on one of the two (111) facets.²⁴³ Moreover, the femtosecond transient absorption spectrum of Au₅₆ (Fig. 8J, right) is similar to that of larger-sized gold NCs with n > 100, which is ascribed to the completeness of the prismatic fcc core.¹³³

Beyond the Au₃ face-fusion of two I_h Au₁₃ units to form the dimeric kernel of homogold and I_h-center-doped Au₃₈(SR)₂₄ NCs (see Fig. 3A, C, D, G and H), the Jin group recently reported a Au₃₆Ag₂(SR)₁₈ (SR = SC₅H₁₀) NC in which three I_h units face-fused together in a cyclic way.¹³² The series of NCs, i.e., mono-I_h Au₂₅(SR′)₁₈⁻, di-I_h Au₃₈(SR′)₂₄ (SR′ = SC₂H₄Ph) and tri-I_h Au₃₆Ag₂(SR)₁₈ (Fig. 8K) is achieved with free valence electron number increasing from 8e⁻ to 14e⁻ to 20e⁻, in contrast to the vertex-sharing I_h NC series with 8e⁻ to 16e⁻ to 24e⁻ electron configurations (Fig. 8D). The two Ag atoms at the centers of the NC are found to be critical in achieving the trimeric face-fused structure, since the I_h central Au would take more electron density than the shell atoms²⁴⁴ and the much less electronegative Ag atoms (χ_Ag = 1.93 vs. χ_Au = 2.54) would be preferred at the two positions that are shared by all three I_h units. Since the [Au₃₀Ag₂]¹²⁺ kernel can be viewed as a union of three face-fused superatoms, Cheng et al. performed chemical bonding analysis suggesting a three-superatom-center two-electron bond for the octet rule of each superatom, which mimics the bonding framework of the D_3h O₃²⁻ molecular anion (Fig. 8K, right).²⁴⁵ Moreover, the new series of NCs was used in a comparative study in hydrogen evolution reaction (HER).²⁴⁵ The tri-I_h NC exhibited high catalytic activity for HER due to its low ligand-to-metal ratio, low-coordinated Au atoms and unfilled superatomic orbitals, providing a new strategy for constructing highly active catalysts from HER-inert metals (e.g. Au) via atomically precise nanochemistry. The current density of Au₃₆Ag₂(SR)₁₈ at −0.3 V vs. RHE was 3.8 and 5.1 times that of Au₂₅(SR′)₁₈⁻ and Au₃₈(SR′)₂₄, respectively. DFT calculations also revealed lower hydrogen binding energy and higher electron affinity of Au₃₆Ag₂(SR)₁₈ (Fig. 8L).¹³²

Bakr et al. reported a Cu₈₁H₃₂(SPh)₄₆(^tBuNH₂)₁₀ NC with an unprecedented planar Cu₁₇ kernel inside a hemispherical shell comprised of a curved surface layer and a planar surface layer (Fig. 8M). This copper NC was synthesized with a mild reducing agent (borane tert-butylamine complex).²⁴⁶ However, this NC does not have any free valence electron.

Challenges and perspectives

The optical and chemical properties of NPs are strongly dependent on the particle size and shape. As the size of NPs shrinks to the ultrasmall regime (<3 nm in diameter), distinct optical properties with successive multiple absorption bands appear because these ultrasmall NPs start to show quantized or molecular states.

One of the challenges is to design anisotropic NCs of atomic precision in hope that their optical properties can be tailored via anistropic growth. Indeed, as NCs grow from isotropic or spherical structures to the rod shapes, one can see that the lowest-energy absorption band red-shifts to longer wavelengths (from visible to NIR region). Note that only NCs with free valence electrons show tunable absorption bands, but not metal complexes. The NIR absorption of atomically precise NCs shows great importance due to the potential applications including photothermal conversion and NIR emmision which are very useful in medical therapy and bioimaging, especially because the NCs are smaller than the 5.5 nm—the renal clearance limit—and thus can undergo rapid excretion by kidney.^247–249 But before we can effectively apply these atomically precise anisotropic NCs to real applications, some challenges should be addressed.

The first challenge pertains to the methodology for the synthesis of anisotropic NCs, which is still not well developed. However, some clues for future work are provided herein. Dimeric homogold and Pt/Pd doped Au₃₈(SR)₂₄ can be prepared by either direct reduction,⁷⁵ or the recently developed method of fusion-mediated synthesis in which monomeric units are used as the precursor.¹⁵⁸ Both rod-shaped Au₂₅(SR)₅(PPh₃)₁₀Cl₂ and Au₃₇(SR)₁₀(PPh₃)₁₀Cl₂ NCs are synthesized by reducing Au^I(PPh₃)Cl with NaBH₄ to cluster-sized seeds, which further react with excess thiol upon moderate heating.^187,188 Whether the growth mechanism of rod-shaped NCs is similar to that of plasmonic nanorods²⁵⁰ or not remains to be found out. Moreover, for rod-shaped Au NCs with fcc or hcp kernels, slow reduction is required to introduce anisotropic growth. Ultrathin Au nanorods with a constant diameter of ∼2 nm (within the NC size regime) and tunable length (5–20 nm) were obtained by slow reducing Au^I in the presence of oleylamine, then ligand exchanging with thiolates.²⁵¹ The ultrathin Au-SR nanorods show an intense band in the NIR and are not plamonic.²⁵¹ The phenomenon is later proved in the resolved Au₄₂(SR)₃₂ nanorods of atomic precision.¹²⁷ We expect that even longer close-packed kernels can be achieved by delicate kinetic control, however, the crystal growth of NCs with larger aspect ratios would be very challenging. In such cases, high-angle annular dark field scanning TEM with ultra-low dose would be necessary to characterize the shape of the NCs.²⁵²

No matter how exciting the atomcially precise structure is, it is required to convert the organic soluble NCs into aqua phase for broader biological applications. In the case of rod-shaped Au_25−xAg_x(SR)₅(PPh₃)₁₀Cl₂ (x = 1–13), the organic soluble thiolates are replaced by SC₂H₄OH so that the NCs can be used in fluorescence confocal imaging for living cancer cells.¹⁷⁹ Au₂₂(SG)₁₈ (SG = glutathione) NC can be used as biocompatible light absorber to overcome the slow kinetics of electron transfer, enabling photosynthesis of acetic acid from CO₂ when translocating into non-photosynthetic bacteria.²⁵³ The Zhang group investigated the peptide ligands stablized Au₂₅ NCs with NIR-II emission (1100–1350 nm).²⁵⁴ The bright luminescence enhanced by Cu or Zn doping could penetrate deeper into the tissue, and be applied in in vivo brain vessel imaging and tumor metastasis.²⁵⁴ So far, very few biocompatable Au NCs of atomic precision have been synthesized using water-soluble ligands since it is very difficult for crystallization.^18,255,256 Therefore, looking for an effective way to transfer the organo-soluble NCs into aqua solution is highly desirable in future work.

The second challenge is to design NCs with regioselective surface functionalization. Anisotropic NPs are usually associated with distinctive self-assembly behavior,^257,258 making them quite attractive in fundamental research and various applications. In the NC regime, we have indeed observed the outstanding self-assemblies achieved by heterodimeric Au₂₉(SR)₁₉ NCs.³⁸ But many of the properties of this NC have not been fully understood, e.g., its much longer excited-state lifetime compared to that of the homodimeric counterpart, and its NIR emission, either. When coupling a pair of nonequivalent NPs to form a heterodimer, the out-of-phase plasmon mode is no longer silent, whereas the in-phase mode is only allowed for homodimers.²⁵⁹ Thus, more experimental and theoretical works are needed on heterodimeric NCs to reveal the mechanisms in a more profound way. Moreover, noble metal (e.g., Ag) NPs can combine with magnetic (e.g., Fe₃O₄) NPs to form a bifunctional heterodimer for both fluorescence imaging and magnetic manipulation.²⁶⁰ It has been shown that a single (SR)₃CdBr motif can be attached to the Au NC in atomic precision,¹²² which can be regarded as a simplified heterodimeric combination. It is worth trying to attach heteroatoms, e.g., Zn, Ni, Rh, on the surface of group-10 metal NCs to achieve a regioselective surface with specific functionalization.

The third challenge is how to grow oblate NCs into large sizes. The anisotropy of triangular metal nanoprisms show strong quadrupole plasmon excitation.^261–263 Introducing iodide ion leads to triangular Au nanoprisms,²⁶⁴ and gold nanoprism growth could be controlled by slow reduction of Au ions onto the surface of seeding nanoprisms with the quadrupole plasmon resonance λ_max shifting with the nanoprism edge length.²⁶⁵ Since the longitudinal transition has been confirmed in atomically precise NCs of rod-shape, we expect that quadrupole transitions might also be present in triangular oblate NCs. A delicate control over the aspect ratio with different ligands is still necessary.

In conclusion, atomically precise metal NCs with anisotropic structures reported so far have been summarized in this review, and their advantages over isotropic counterparts have been highlighted through comparison. While the synthesis of anisotropic NCs still remains a challenge, generally speaking, the presence of isotropic NC seeds in the precursor could be a way to prepare dimeric and rod-shaped NCs, and a very mild reducing environment is often required to prepare anisotropic nanostructures. With the longitudinal transition appeared in rod-shaped NCs and its tunability with the aspect ratio of the anisotropic structure, new physicochemical properties and applications have been observed and worth further studies in future research. The 2D or disk-shaped NCs also remain to be explored in terms of their unique properties and applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

R. J. acknowledges the financial support from the NSF (DMR-1808675).

Notes and references

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Footnote

† Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.

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