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Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities Rongchao Jin,* Chenjie Zeng, Meng Zhou, and Yuxiang Chen

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Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1−3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the stability, metal−ligand interfacial bonding, ligand assembly on particle surfaces, aesthetic structural patterns, periodicities, and emergence of the metallic state) and to develop a range of potential applications such as in catalysis, biomedicine, sensing, imaging, optics, and energy conversion. Although most of the research activity currently focuses on thiolate-protected gold nanoclusters, important progress has also been achieved in other ligand-protected gold, silver, and bimetal (or alloy) nanoclusters. All of these types of unique nanoparticles will bring unprecedented opportunities, not only in understanding the fundamental questions of nanoparticles but also in opening up new horizons for scientific studies of nanoparticles.

CONTENTS 1. Introduction 1.1. What Is Known about Colloidal Nanoparticles 1.2. What Is Not Known 1.3. Why Atomically Precise Nanoparticles? 2. Thiolate-Protected Atomically Precise Gold Nanoclusters as a Paradigm System 2.1. Synthetic Methods 2.1.1. Size-Focusing Methodology 2.1.2. LEIST Methodology: From One Stable Size to Another 2.1.3. Carbon Monoxide Reduction Method 2.1.4. Ultracentrifugation and Chromatographic Isolation Methods 2.1.5. Summary of Aun(SR)m Sizes 2.2. Nanochemistry with Atomic Precision 2.3. X-ray Structures 2.3.1. Face-Centered Cubic (fcc) 2.3.2. Body-Centered Cubic (bcc) 2.3.3. Hexagonal Close-Packed (hcp) 2.3.4. Icosahedron 2.3.5. Decahedron 2.3.6. Summary of Kernel and Surface Structures 2.3.7. X-ray Absorption Spectroscopic Analysis 2.4. Growth Modes © 2016 American Chemical Society

2.4.1. Fusion 2.4.2. Interpenetration 2.4.3. Shell by Shell 2.4.4. Au4 Tetrahedron as a Building Block 2.4.5. Anisotropic Layer-by-Layer Growth 2.5. Optical Properties 2.5.1. Optical Absorption 2.5.2. Photoluminescence 2.5.3. Nonlinear Properties 2.5.4. Ultrafast Dynamics 2.6. Chirality in Nanoclusters 2.6.1. Chiral Arrangement of Surface Units 2.6.2. Chiral Kernel 2.6.3. Chiral Arrangement of Carbon Tails 2.6.4. Chiral Induction to Achiral Gold Core 2.7. Site-Specific Ligand Exchange 2.8. Chemical Reactivity 2.9. Magnetism in Nanoclusters 2.10. Electrochemical Properties 2.11. Thermal Stability 3. Gold Nanoclusters Protected by Other Types of Ligands 3.1. Phosphine-Protected Au Nanoclusters

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Special Issue: Nanoparticle Chemistry Received: December 1, 2015 Published: September 1, 2016 10346

DOI: 10.1021/acs.chemrev.5b00703 Chem. Rev. 2016, 116, 10346−10413

Chemical Reviews 3.2. Mixed Thiolate/Phosphine-Protected Au Nanoclusters 3.3. Selenolate-Protected Au Nanoclusters 3.4. Alkynyl-Protected Au Nanoclusters 4. Silver Nanoclusters 4.1. Synthesis 4.2. Crystal Structures 5. Bimetal Nanoclusters 5.1. Intermetallic Bimetal Nanoclusters 5.2. Au−M Nanoclusters with Thiolate Ligands 5.3. Au−M Nanoclusters with Phosphine Ligands 6. Applications 6.1. Catalysis 6.1.1. Catalytic Oxidation 6.1.2. Catalytic Hydrogenation 6.1.3. C−C Coupling Reactions 6.1.4. Electron-Transfer Catalysis 6.1.5. Electrocatalysis 6.1.6. Photocatalysis, Photoelectrochemical Water Splitting, and Photovoltaics 6.2. Chemical Sensing 6.3. Optical Imaging 6.4. Biological Labeling and Biomedicine 6.5. Light-Emitting Devices 7. Future Perspectives 7.1. Pushing the Limits of Crystallization of Atomically Precise Nanoclusters and Nanoparticles 7.2. Periodicity of Nanoclusters 7.3. Isomerization 7.4. Grand Evolution from the Semiconducting to the Metallic State and Implications 7.5. Ligand’s Role in Determining the Size and Structure of Nanoclusters 7.6. What Determines the Stability of Nanoclusters? 7.7. Origin of Photoluminescence 7.8. Biomedical Application of Nanoclusters 7.9. Catalysis 7.10. Nucleation of Aun(SR)m Nanoclusters 7.11. Doping and Alloying 7.12. Extension of Atomic Precision to Other Metals 7.13. Atomically Precise Colloidal CdSe and Magnetic Nanoclusters 7.14. Future Theoretical Work Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

realized in the case of ultrasmall gold nanoparticles with diameters ranging from subnanometer to ∼2.2 nm (equivalent to ∼10−300 atoms, often called nanoclusters).1,2 Based on such unprecedented nanoparticles, fundamentally important issues of colloids can now be glimpsed at the atomic level. In retrospect, scientific research on colloidal nanoparticles has a long history, especially for gold colloids, that dates back to Faraday’s time. In 1857, Faraday published a seminal work,3 in which he carried out systematic studies on gold colloid made by a two-phase reaction between HAuCl4(aq) and phosphorus (in ether)

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HAuCl4 + P → Au(sol)

The product was called “potable gold”. Faraday’s work laid down the foundation for later colloid science. In the early 20th century, systematic research on inorganic colloids of elements and their inorganic compounds was carried out (see an early textbook on colloid chemistry4). Among colloids, those of gold certainly received particular interest, largely owing to the vivid colors. In 1908, Gustav Mie solved Maxwell’s equations for spherical particles using the complex dielectric constants (ε1 + iε2) of metals and successfully modeled the optical extinction spectra of gold colloids5 for comparison with the experimental spectra measured by Steubing.6 The formation mechanism, physical and chemical properties, and practical applications of inorganic colloids (or sols) experienced systematic advances during the 1900−1930s and afterward. Many different processes for sol formation were developed, such as the reduction processes for gold and silver sols, the oxidation process for sulfur sol, and the thermal dissociation process for nickel sol, as shown in the following reactions

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HAuCl4 + H 2O2 → Au + HCl + O2

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Ag 2O + H 2 → Ag + H 2O

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H 2S + SO2 → S + H 2O

Ni(CO)4 → Ni + CO A quite popular method for gold hydrosol formation was the reduction of HAuCl4 with formaldehyde as the reducing agent, developed by Zsigmondy.7 Meanwhile, important concepts started to be developed during the early 20th century, including nucleation and growth (e.g., Von Weimarn’s theory);8 surface adsorption of ions;9 stabilization through charges;10 electrolyteinduced agglomeration of sols;4 protection of colloidal dispersions by gelatin, tannin, or other substances;7 electric double layer;10 and Derjaguin−Landau−Vervey−Overbeek (DLVO) theory.11−13 Prior to the invention of transmission electron microscopy (in the 1930s), the sizes of colloidal particles were primarily determined by the ultramicroscope method.7 It is worth noting that Scherrer reported that the size of gold particles could be estimated by X-ray diffraction;14 this method has since been widely used, even today. In addition, Svedberg and co-workers performed extensive work on ultracentrifugation to measure particle sizes using Stokes’ law of sedimentation;15 this method is still used to date. Following the invention of the transmission electron microscope by Ruska and Knoll in the 1930s and commercialization after World War II, Turkevich and coworkers carried out transmission electron microscopy (TEM) studies of gold sols.16−18 In the 1950s, the discovery of the

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1. INTRODUCTION The polydispersity of nanoparticles has long been a major issue in nanoscience research. Although relatively monodisperse nanoparticles have been made in some cases and thus the polydispersity issue has been partially alleviated, colloid chemists are still frustrated by the fact that no two nanoparticles are the same; thus, a major dream is to synthesize truly uniform nanoparticles at the ultimate atomic level, namely, atomically precise nanoparticles. This dream has now been partially 10347


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Figure 1. TEM images of Au and Ag nanoparticles of different shapes. (Images recorded by R.J.)

molecular structure of DNA gave birth to molecular biology, and gold sols attracted strong interest for use as biological labels in electron microscopy studies of cellular structures.19 In another line of research, the discovery of the enormous enhancement of Raman scattering signals by electrochemically roughened metal surfaces in the 1970s20−22 stimulated significant interest in the Raman enhancing properties of colloidal gold and silver nanoparticles.23,24 In the late 1990s, the era of nanotechnology came into being, especially with the establishment of the National Nanotechnology Initiative in 2000.25 Since then, research on metal nanoparticles, semiconductor nanocrystals, magnetic nanoparticles, nanowires, carbon nanostructures, and many other types of nanomaterials has achieved tremendous advances. The number of scientific reports on nanoscience and nanotechnology has experienced explosive growth. It is safe to say that, in the past two decades, almost every branch of natural sciences has been hugely impacted by nanoscience and nanotechnology.

Figure 2. Cartoon and high-resolution TEM image of a gold nanoparticle protected by ligands (e.g., thiolate). (Image recorded by R.J.)

scattering techniques are powerful in studying molecular compounds, but they are limited in characterizing nanoparticles because of the following major difficulties: (i) polydispersity and heterogeneity of nanoparticles and (ii) excess ligands or surfactants, residual reactants, and side products in colloids. Thus, it is very difficult to correlate the NMR/IR/Raman spectroscopic information with colloids. Therefore, for colloids, many simple questions are still difficult to answer even to date, such as the following: • What exactly protects the nanoparticle surface? • How are the surface stabilizers bonded to the inorganic core (e.g., at the vertex and edge sites)? • What determines the stability of nanoparticles? For example, taking the most well studied classical colloidal system, namely, citrate-protected gold nanoparticles, the precise composition of the surface adsorbates (i.e., stabilizers) and how they are adsorbed on the particle surface are not clear yet. There are also many other questions: Are citrate molecules the only ones on the particle surface? What about Cl− and OH−? Are the stabilizers between the adsorbed form and the free state in a dynamic equilibrium? In terms of stability, citrate-capped Au nanoparticles are well-known to be sensitive to salt (i.e., salt-induced aggregation), whereas gold colloid functionalized with thiol-terminated DNA is extremely stable against high concentrations of salt, heating, extreme pH, and chemical attack,32 so why is there such a huge difference for the same gold? How does the thiol impart extraordinary stability to gold nanoparticles? To address all of these basic questions, it is of paramount importance to reveal the interfacial bonding between stabilizers and the underlying inorganic core. Understanding the surfaces and interfaces of nanoparticles is also critically important for many applications of nanoparticles such as catalysis, which is almost all about the surface.

1.1. What Is Known about Colloidal Nanoparticles

After more than a century of scientific research on metal colloidal nanoparticles, chemists have achieved excellent control of particle size and shape in many cases.26−29 Figure 1 shows some examples of Au and Ag nanoparticles of different shapes. With modern TEM, particle size and shape as well as the crystalline structure can be readily determined.30 Based on the measured structural parameters, the optical properties of nanoparticles can now be accurately modeled and compared with experiment.30 Surface functionalization of nanoparticles is quite mature, which has opened up many new applications of nanoparticles in a wide range of fields.31 1.2. What Is Not Known

One might argue that current colloidal nanoparticles are sufficiently good and that essentially all aspects (e.g., size, shape, composition, and properties) are known, especially about gold colloids, which have been studied for more than 150 years. Unfortunately, that is not the case. First, the surface layer, including organic stabilizers and the inorganic−organic interface, still remains poorly understood, because TEM cannot directly image the organic stabilizers and the interface (Figure 2). The one or two layers of atoms at the interface are often said to be “amorphous” in TEM characterizations of nanoparticles. Although scanning probe microscopies (e.g., atomic force microscopy and scanning tunneling microscopy) can directly image the surface molecules with atomic resolution, unfortunately, the buried interface is not directly accessible. Spectroscopic tools such as nuclear magnetic resonance (NMR), infrared (IR) absorption, and Raman 10348


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Figure 3. (A) Optical reflectance spectrum of bulk gold. (B) Optical extinction spectra of gold colloids (3−100-nm diameters, see legend). Inset: Photograph of 10-nm Au colloid. Data collection performed by R.J.

Second, ultrasmall nanoparticles (e.g., 12 h of harsh reaction. The Au36(TBBT)24 yield was exceptionally high (>90%, Au atom basis), and the product was molecularly pure, as evidenced by the clean mass spectrum and thermogravimetry analysis (Figure 10). Of note, the previous Au36(SR)23 formula should be corrected.99 A few key conditions were found to affect the LEIST process: (i) The molar ratio of incoming TBBT to the original PET on the Au38 cluster was kept very high (∼160:1), much higher than in conventional ligand-exchange processes. (ii) Thermal conditions were used to overcome the energy barrier between stable sizes. The discovery of this elegant yet simple transformation chemistry greatly facilitated the crystallization of the Au36 nanocluster, and its atomic structure was determined to be an fcc structure with a tetrahedral shape (see section 2.3.1). The Au36(TBBT)24 structure was indeed significantly different from that of the starting biicosahedral Au38(PET)24 nanocluster, that is, structural transformation occurred, even though their sizes differ by only two gold atoms. Following the success of the Au38(PET)24-to-Au36(TBBT)24 conversion, Zeng et al. further performed ligand exchange on the Au25(PET)18 nanocluster at 80 °C with TBBT and indeed obtained a new stable size, formulated as Au28(TBBT)20, in high yield (>90%, Au atom basis), indicating the potential universality of LEIST.100 Furthermore, by carefully tuning the transformation kinetics [i.e., 40 °C, as opposed to 80 °C for the case of Au28(TBBT)20 synthesis], thermal reaction of Au25(PET)18 with excess TBBT (thiol) led to a new cluster, Au20(TBBT)16.101 Das et al. achieved the transformation of [Au23(S-c-C6H11)16]−TOA+ [where S-c-C6H11 = 1-cyclohexanethiolate and TOA = + N(n-C 8 H 17 ) 4 ] to a new Au24(SCH2Ph-tBu)20 nanocluster following LEIST reaction of [Au23(S-c-C6H11)16]− with excess HSCH2Ph-tBu thiol.102 All of these new products were crystallographically characterized. Finally, a major achievement is the transformation of Au144(PET)60 to Au133(TBBT)52.103 The new nanocluster was obtained through the LEIST reaction of molecularly pure Au144(PET)60 with excess TBBT (TBBT/PET ≈ 370:1) at 80 °C for a very long time (∼4 days). Significantly, Zeng et al. further succeeded in crystallization of this nanocluster, which is the largest structure so far.103 Compared to conventional ligand exchange, which is typically performed at room temperature and with low ratios of incoming thiol molecules relative to the bound ligands, LEIST requires a large excess of thiol and thermal conditions to overcome the energy barriers for size and structure transformation. The LEIST process (Scheme 4) is remarkable in that it resembles organic transformation chemistry, in which one type of molecules is transformed into a new type. This new methodology has largely expanded the “universe” of Aun(SR)m nanoclusters for the exploration of stable (n, m) combinations, structures, properties, and applications. It is worth noting that the conversions of Au25(SR)18 to Au28(SR′)20, Au38(SR)24 to Au36(SR′)24, and Au144(SR)60 to Au133(SR′)52 do not mean that the starting nanoclusters are not stable. For example, in the case of Au 38 (PET) 24 to Au36(TBBT)24, the “precursor” Au38(SR)24 exhibits high thermal and chemical stability (e.g., resistant to reduction and oxidation by common reagents). Here, what the thermal ligand exchange does is the activation of the nanocluster when many ligands on Au38 are exchanged by TBBT (see mechanistic

Scheme 3. Isomeric Methylbenzenethiols (MBTs) for SizeControlled Synthesis of Aun(SR)m Nanoclustersa

a

Reproduced with permission from ref 96. Copyright 2015 American Chemical Society.

Specifically, molecularly pure Au130(p-MET)50, Au104(mMBT)41, and Au40(o-MBT)24 nanoclusters were respectively obtained by harsh size-focusing conditions at 80−90 °C in the presence of excess thiol (Figure 9). The decreasing size trend is interestingly in line with the increasing hindrance of the methyl group with respect to the interfacial AuS bond. Given all of these examples, the size-focusing methodology is remarkable and represents a major step toward the rational synthesis of atomically precise Aun(SR)m nanoclusters of molecular purity. The essence of the size-focusing methodology lies in the effective control of the size range of the initially produced Aux(SR)y nanoclusters prior to size focusing. The size-focusing methodology is not simply the earlier-reported etching method,76 because the size-focusing method involves significant kinetic control that is missing from the etching method. Overall, the fundamental basis of the size-focusing methodology lies in the stability properties of Aux(SR)y nanoclusters, and their different stabilities lead to the survival of the most robust, reminiscent of natural selection: survival of the fittest. This leads to a central question in nanocluster research: What determines the stability of magic-sized nanoclusters? Although this issue is not yet completely clear, some insights have nevertheless been obtained in recent research, particular regarding the major role of the ligand.90,96,97 This is indeed reflected in the second synthetic methodology for the size- and structure-controlled synthesis of nanoclusters (see the next section), which involves the critical roles of ligands. A summary of the effects of ligand is provided in the discussion of future perspectives (section 7). 2.1.2. LEIST Methodology: From One Stable Size to Another. The Aun(SR)m nanoclusters that are made by size focusing offer an opportunity to create new magic sizes through a ligand-exchange-induced size/structure transformation (LEIST) process.70 Notable examples include the transformations of Au25(SR)18 to Au28(SR′)20, Au38(SR)24 to Au36(SR′)24, and Au144(SR)60 to Au133(SR′)52, where the prime indicates a thiolate with a different carbon tail than the original one protecting the starting Aun(SR)m clusters. This new methodology is built on the size-focusing methodology and allows one to access new magic sizes that are otherwise difficult to synthesize directly by size focusing. In this section, we illustrate the LEIST process of Au 38 (SC 2 H 4 Ph) 24 to Au36(SPh-tBu)24, where SPh-tBu represents 4-tert-butylbenzenethiolate (abbreviated as TBBT for both the thiolate and thiol forms).98 10354


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Figure 9. Characterization of molecularly pure (A) Au130(p-MET)50, (B) Au104(m-MBT)41, and (C) Au40(o-MBT)24 nanoclusters obtained using isomeric methylbenzenethiols (p-, m-, and o-MBT, respectively). Adapted with permission from ref 96. Copyright 2015 American Chemical Society.

NaBH4 reduction method for Au25(SR)18 synthesis, in which NaOH was used to tune the formation kinetics by decreasing the reducing capability of NaBH4 and enhancing the etching capability of thiol.106 They obtained Au25 clusters protected by two or three types of thiolates on the cluster surface with adjustable ratios. No segregation of ligands on the cluster surface was found based on NMR spectroscopic analysis. The Xie group also reported a protection−deprotection method for the synthesis of Au25(S-Cys)18.107 In this process, a surfactant (i.e., cetyltrimethylammonium bromide, CTAB) was used to protect Au(I)−SR complexes during the reduction and, thus, to control the formation rate of gold−thiolate nano-

discussion in section 2.2). An analogy from organic chemistry is, for example, the conversion of benzene to hexane through hydrogenation, but this does not mean that benzene is not stable. Molecules can be activated under appropriate conditions, and this is also true for Aun(SR)m nanoclusters. 2.1.3. Carbon Monoxide Reduction Method. Aside from the above two universal methods, Xie and co-workers devised a kinetically controlled route utilizing carbon monoxide (CO) as a mild reducing agent for the synthesis of Au25(SR)18 by slow growth, as well as the size-tunable syntheses of Au15(SR)13, Au18(SR)14, and so on (Scheme 5).104,105 In a recent work, Xie and co-workers reported a NaOH-mediated 10355


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Figure 10. Characterization of the Au36(SPh-tBu)24 nanocluster converted from Au38(SC2H4Ph)24 by a LEIST process. (a) ESI-MS spectrum (CsOAc added to form Cs+ adducts with the charge-neutral nanoclusters) and (b) thermogravimetric analysis (in N2 atmosphere, 10 °C/min). Reproduced with permission from ref 98. Copyright 2012 Wiley-VCH.

clusters. Subsequent removal of the protecting layer restored the functional group on the thiolate of the nanoclusters. 2.1.4. Ultracentrifugation and Chromatographic Isolation Methods. Regarding the isolation of mixed sizes, Bakr and co-workers demonstrated ultracentrifugation for Aun(SR)m nanoclusters and quantitative determination of the ligand-togold ratio.108 Ghosh et al. demonstrated thin-layer chromatography (TLC) isolation of Au25(SR)18 and Au144(SR)60.109 In addition, isolation of mixed-thiolate-protected Au25 clusters according to ligand composition was also achieved by TLC. Negishi et al. reported the high-resolution HPLC isolation of various sizes of Aun(SR)m clusters.110 In recent reviews, Negishi and co-workers summarized their works on HPLC isolation, and the interested reader is referred to these reviews.111,112 2.1.5. Summary of Aun(SR)m Sizes. Table 1 lists the wellcharacterized, definitive Aun(SR)m nanoclusters. In consideration of recent works on the important roles of ligands in stabilizing the nanocluster structures, the type of surface ligand for each nanocluster is also listed. We note that Table 1 does not include nanoclusters that were observed only in mixtures without isolation, and those less ubiquitous ones [such as Au39(SR)29 and Au40(SR)30] are also left out. Giant nanoclusters larger than Au144 are still very challenging for ESI-MS analysis, and their formulas critically require cross-checking by X-ray crystallographic determination.93,113,114 For these giant nanoclusters, future breatkthroughs in ESI-MS analysis115−118,126 are still needed.

Scheme 4. Creation of New Magic-Sized Nanoclusters by the LEIST Processa

a


Scheme 5. Size-Controlled Synthesis of Aqueous Nanoclusters through pH Controla

2.2. Nanochemistry with Atomic Precision

The molecularly pure nanoclusters have opened up new nanochemistry, and the reaction processes can be explicitly expressed with precise equations, resembling those for smallmolecule reactions. We illustrate such nanochemistry with the LEIST process transforming Au 3 8 (SC 2 H 4 Ph) 2 4 to Au36(SPh-tBu)24.148 Zeng et al.148 clearly mapped out the mechanism for this process based on time-dependent mass spectrometry and optical spectroscopy analyses and identified an interesting disproportionation mechanism. The LEIST process transforming Au38(PET)24 to Au36(TBBT)24 can be roughly divided into four stages (Scheme 6 and Figure 11). The first stage (0−5 min) involves ligand exchange without size or structure transformation, as evidenced by the fact that the optical spectra are identical to that of the starting

a Reproduced with permission from ref 105. Copyright 2013 American Chemical Society.

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Table 1. Selected Aun(SR)m Nanoclusters That Have Been Synthesized in High Purity and Adequately Characterized formula

thiolate ligand type (SR)

crystal structure

ref(s)

Au15(SR)13 Au18(SR)14 Au20(SR)16 Au22(SR)16,17,18 Au23(SR)16− Au24(SR)20 Au24(SR)16 Au25(SR)18q (q = −1, 0, +1)

glutathione (SG) glutathione (SG), S-c-C6H11 SC2H4Ph, SCH2Ph, SPh-tBu (TBBT) glutathione (SG) S-c-C6H11, S-tBu SC2H4Ph, SCH2Ph, SCH2Ph-tBu adamantanethiolate (SC10H15) glutathione (SG) SC2H4Ph SCnH2n+1, SPhNH2, captopril (Capt), and others SPh-tBu (TBBT), S-c-C6H11 S-tBu adamantanethiolate (SC10H15) SPh-tBu (TBBT), S-c-C5H9, SPh SC2H4Ph, SCnH2n+1 adamantanethiolate (SC10H15) SC2H4Ph, SPh-o-CH3 (o-MBT) SPh-tBu (TBBT) SPh-tBu (TBBT) SC2H4Ph S-c-C6H11 SC2H4Ph SPh, SPh-p-CH3 SPh-p-COOH SPh-m-CH3 SPh-p-CH3 (p-MBT), SCnH2n+1, SC2H4Ph dithioldurene/SC2H4Ph SPh-tBu (TBBT) SC2H4Ph SC2H4Ph, SCnH2n+1

− available available − available available available available

available available available available available available available available available − − − − available − available

66, 105, 119 66, 105, 119−122 101, 123, 124 66, 125−127 128, 129 102, 124, 130 131 66, 106, 132−134 71−73, 75, 135, 136 94, 137−141 100, 142 143−146 147 98, 148−150 77, 79−81, 151 152 96, 153−155 156, 157 155 158−160 90 161 91 162, 163 96 96, 114, 164, 165

available − −

103, 166 167 77, 86, 168

Au28(SR)20 Au30S(SR)18 Au30(SR)18 Au36(SR)24 Au38(SR)24 Au38S2(SR)20 Au40(SR)24 Au44(SR)28 Au52(SR)32 Au55(SR)31 Au64(SR)32 Au67(SR)35 Au99(SR)42 Au102(SR)44 Au104(SR)41 Au130(SR)50 Au133(SR)52 Au137(SR)56 Au144(SR)60

Scheme 6. Four-Stage Reaction Pathway for the Conversion of Au38(PET)24 to Au36(TBBT)24: (I) Ligand Exchange, (II) Structure Distortion, (III) Disproportionation, and (IV) Size Focusinga

a


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Au38*(TBBT)m (PET)24 − m(m < 12) + TBBT → Au38*(TBBT)m (PET)24 − m(m > 12) + PET

where Au38* denotes the distorted structure. The Au38 structural distortion preludes the subsequent size and structural transformation (stage III, 20−60 min). An interesting “disproportionation” reaction is identified, in which one Au38* cluster releases two gold atoms to form Au36(SR)24, and another Au38(SR)24 cluster captures the two released gold atoms and also two free TBBT ligands to form Au40(SR)26. This is manifested in the two new sets of mass peaks with comparable intensities on the lower- and higher-mass sides of the Au38 peak set (Figure 11A, III). The lower-mass set is Au36(TBBT)m(PET)24−m (m = 19−24), and the higher-mass set is Au40(TBBT)m+2(PET)24−m (m = 21−26). As the reaction continues, Au38 clusters are gradually converted to Au36 and Au40 until all of the Au38 clusters are converted. The reaction in this stage can be written as

Figure 11. Size/structure transformation from Au38(PET)24 to Au36(TBBT)24 monitored by (A) ESI-MS and (B) UV−vis spectroscopy. In panel A, the most abundant number of TBBT ligands is marked for each set of mass peaks. Reproduced with permission from ref 148. Copyright 2013 American Chemical Society.

2Au38*(TBBT)m (PET)24 − m + 2TBBT → Au36(TBBT)m (PET)24 − m + Au40(TBBT)m + 2 (PET)24 − m

During stage IV (120−300 min), the ligand exchange occurs to completion, and the Au40(TBBT)m+2(PET)24−m species start to convert to Au36 under the conditions of high temperature and excess thiol

nanoclusters (Figure 11, marked I). The reaction can be written as

Au40(SR)26 → Au36(SR)24 + 2Au(0) + 2Au(I)SR

and eventually molecularly pure Au36(SR)24 is obtained. The overall equation is

Au38(PET)24 + TBBT → Au38(TBBT)m (PET)24 − m + PET

(m < ca. 12)

2Au38(SR)24 → 2Au36(SR)24 + 2Au(0)

During stage II (10−15 min), ligand exchange continues, but the optical spectrum starts to change (Figure 11B, II) indicating that the large number of bulky TBBT ligands (m > 12) on the Au38 core starts to induce structural distortion of the Au38 cluster, even though the cluster formula is still [Au38(TBBT)m(PET)24−m] (m > 12). This intermediate gives rise to a new absorption band at 550 nm in the UV−vis spectrum (Figure 11B, top arrow-marked peak). The reaction for stage II can be written as

The above mechanism should give a theoretical yield of 94% for the final Au36(SR)24 product, and the experimental yield is 90%,148 consistent with the reaction route. It should be noted that the transformation occurs in the original cluster through internal reconstruction, as opposed to a complete disintegration and then reassembly of pieces. A question arises naturally from the LEIST process: What drives the transformation, the ligand bulkiness or the electronic

Scheme 7. Construction of a Cuboctahedron from the fcc Unit Cell

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Figure 12. X-ray structure of Au36(SPh-tBu)24. Adapted with permission from ref 98. Copyright 2012 Wiley-VCH.

observed and retains all of the fcc symmetry features. To illustrate the transformation of the noncentered Au14 fcc cage to the centered Au13 cuboctahedron, we first extend the fcc cell by a half-cell (Scheme 7) and then drop the left half-cell, resulting in a centered Au13 structure after the AuAu bonds have been connected. This structure is called a cuboctahedron and does not change any essential feature of the fcc structure; that is, the cuboctahedral structure still shows cubic-close-packing (ccp) characteristics of fcc structures, rotation axes (2-, 3-, and 4fold), and the 12-coordinate feature of fcc structure. Moreover, if one views the cuboctahedron along the C3 axis (Scheme 7), one recognizes the 3:7:3 atom layer sequence, that is, the a/b/c stacking sequence of fcc structures. Thus, the cuboctahedron is simply a fragment of the fcc structure. A cuboctahedron has eight triangular facets and six square facets (Scheme 7). The first observation of an fcc-structured Aun(SR)m cluster was Au36(SR)24.98 It was once thought that the the fcc structure was unstable in ultrasmall Aun(SR)m nanoclusters (e.g., less stable than the multiple-twinned structure such as icosahedron) and that the fcc packing mode would appear only when the size reached a certain threshold. This was partly reflected in the three earlier structures: the general trend observed from icosahedral Au 25 (SR) 18 and Au 38 (SR) 24 to decahedral Au102(SR)44 and to fcc structures in plasmonic nanoparticles and bulk gold.1 The structure of Au36(SPh-tBu)24 consists of a 28-atom (Au28) fcc kernel with 4 interpenetrating cuboctahedra, exhibiting a truncated tetrahedral shape (Figure 12). It exposes four {111} facets, each of which is protected by a Au2(SR)3 dimeric staple motif, and six {100} facets, each of which is protected by two simple bridging SR thiolates (i.e., one ligand per Au4 square). The simple bridging thiolate [i.e., without incorporating any gold atom as in Au(SR)2 and Au2(SR)3 staple motifs] was a new feature compared to the earlier reported Au25(SR)18, Au38(SR)24, and Au102(SR)44 clusters. The emergence of an fcc-structured Au28 kernel at such a small size indeed came as a surprise, and an important

conjugation effect of the aromatic TBBT ligand? In later work, Das et al.149 tested the reaction with cyclopentanethiol and obtained Au 36 (SC 5 H 9 ) 24 with the same structure as Au36(TBBT)24, thus ruling out the relevance of the aromatic properties of the ligand. Further insight into the effects of steric factors, ligand conformation and interactions, and other unknown factors remain to be uncovered in future work. 2.3. X-ray Structures

X-ray structures are the “holy grail” of nanocluster research. Unraveling the total structures of gold nanoclusters is of paramount importance for understanding their stability, AuS interfacial bonding, and physicochemical properties. The establishment of the size-focusing and LEIST synthetic methodologies has paved the way to the rational synthesis of atomically precise gold nanoclusters with molecular purity.69,70 Such high-quality nanoclusters have enabled the crystallization of nanoclusters and the determination of their total structures. Through the growth of single crystals, a number of Aun(SR)m total structures have been solved by X-ray crystallography, and many fundamental issues can now be understood, at least partially. The first reported structure was that of the decahedral Au102(SR)44 cluster by the Kornberg group in 2007,162 followed by those of icosahedral Au 25 (SR) 18 in 2008 72,73 and biicosahedral Au38(SR)24 in 2010.81 Since then, more structures have been achieved. In this section, we provide a summary of the reported Aun(SR)m structures. Of note, theoretical works are not included, so the interested reader is referred to other reviews169−172 and reports.173−201 2.3.1. Face-Centered Cubic (fcc). When the size of a gold nanoparticle shrinks, the first question is whether the fcc structure inherent to bulk gold can be maintained, say, down to the size of a single fcc unit cell (Scheme 7).202 The fcc unit cell comprises 8 vertices and 6 face centers and, hence, a total of 14 gold atoms (not to be confused with the net 4 atoms per unit cell). Although no such Au14 structure has been observed to date, its transformed structure (i.e., cuboctahedron) has been 10359


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Figure 13. X-ray structure of [Au23(S-c-C6H13)16]− [counterion: TOA+ = +N(C8H17)4]. Top: Total structure and bicapped Au15 kernel. Bottom: Protection of the kernel by (i) trimeric staples, (ii) monomeric staples, and (iii) simple bridging thiolate. Yellow, S; other colors, Au. Redrawn from ref 128.

discovery was that the ligand played a critical role in stabilizing the fcc-structured Au36 nanocluster. Later, the cuboctahedral structure was also observed in the [Au23(SR)16]− nanocluster.128 Its kernel comprises a single Au13 cuboctahedron plus two additional capping atoms along the C4 axis, forming a Au15 kernel (Figure 13). The Au15 kernel is protected by two trimeric Au3(SR)4 staples, two monomeric Au(SR)2 staples, and four simple bridging SR ligands (Figure 13). Among these surface-protecting motifs, the trimeric Au3(SR)4 staple was observed for the first time. Such a trimeric staple was predicted in earlier theoretical analyses of the experimentally identified Au20(SR)16,203,204 Au18(SR)14,180 and Au15(SR)13618 nanoclusters. The bulky adamantanethiolateprotected Au24(S-Adm)16 cluster (where S-Adm represents adamantanethiolate) was also found to exhibit a structure131 similar to that of the [Au23(SR)16]− cluster protected by cyclohexanethiolate. Other fcc structures include Au28(SR)20, Au30S(SR)18, Au40(SR)24, Au44(SR)28, and Au52(SR)32 (see section 2.4). 2.3.2. Body-Centered Cubic (bcc). Bulk gold is wellknown to adopt a fcc structure, but recently, Liu et al. discovered a bcc-structured Au38S2(S-Adm)20 nanocluster.152 The Au38S2(S-Adm)20 nanocluster was synthesized by the sizefocusing method. The starting polydisperse crude product was controlled in a size range of 5−13 kDa, and size focusing of the mixture in the presence of excess adamantanethiol at 90 °C for ∼24 h gave rise to the bcc nanocluster product. The inner core (Au30) shows the bcc arrangement of 30 gold atoms (Figure 14A), which can be carved out of a 3 × 3 × 2 multicell bcc lattice (see the connected balls within the lattice in Figure 14B). The central part of the experimental Au30 structure consists of two bcc cubes stacked vertically, forming a Au14 unit, and the remaining 16 Au atoms extend the Au14 inner core according to the bcc structure. The Au30 core is protected by four dimeric staples and eight simple bridging thiolatesw (Figure 14C), plus two sulfido atoms in a tripodal bonding manner (see the arrows in Figure 14C). Note that the bonding between the Au30 unit and the gold atoms of the dimeric staple motifs slightly pulls four gold atoms out of the ideal bcc positions (see arrows in Figure 14A). The two sulfido

Figure 14. X-ray structure of Au38S2(S-Adm)20. Adapted with permission from ref 152. Copyright 2015 Wiley-VCH.

atoms in the bcc-Au38 cluster were from the cleavage of adamantanethiol during the 90 °C harsh size-focusing reaction. Density functional theory (DFT) analysis revealed that the adamantanethiolate ligand and the two sulfido atoms play important roles in stabilizing the bcc structure.152 This bcc structure is in striking contrast to the fcc structure of gold. Scheme 8 illustrates the conversion of a cuboctahedron Scheme 8. Transformation from a Cuboctahedron (fcc) to a bcc Structure by Squeezing along the C4 Axis, Where the Four Atoms (Meshed) Become the Body Centers of the bcc Structure

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to a bcc unit by squeezing along the C4 axis so that the top and bottom layers of atoms approach more closely and, eventually, the rectangular unit (see the framework of dashed lines in Scheme 8) becomes a square. The resultant unit is a bcc structure. The squeezing force presumably comes from the ligands. The bcc-structured Au38 nanocluster exhibits a gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of ∼1.5 eV, much larger than the 0.9 eV energy gap of the same-sized biicosahedral Au38(SC2H4Ph)24 nanocluster; this implies the important effect of the gold atom arrangement on the HOMO−LUMO gap and electronic properties of the cluster. 2.3.3. Hexagonal Close-Packed (hcp). Whereas bulk gold commonly adopts an fcc structure, rare structures have been observed.205 In this section, we discuss the hexagonal closepacked (hcp) structure, which is rare for bulk gold even though local hcp structure is quite often found in conventional gold nanoparticles. In the case of nanoclusters, Teo et al.348 previously reported an hcp-structured Au39 nanocluster protected by phosphine (see section 3.1). For Aun(SR)m nanoclusters, the cyclohexanethiolate-protected Au18(SC6H11)14 nanocluster121,122 was found to have a small hcp kernel of Au9 with an a−b−a three-layered structure (Figure 15), each layer being a Au3 triangle. The Au9 kernel is

Scheme 9. Cuboctahedron-to-Icosahedron Transformation through Corrugation of the Middle Au6 Plane

icosahedron, which has exclusively triangular Au3 faces, namely, 12 (new) + 8 (initial) = 20 faces. The central atom still has the same 12-coordinate feature (i.e., 12 radial AuAu bonds), but the number of edges (i.e., peripheral AuAu bonds) is increased from 24 to 30, leading to an appreciable gain in energy due to the formation of more bonds and thus stabilizing the icosahedral structure. Both the radial and peripheral bonds are important for stabilizing the icosahedral structure because the volume and surface area are both contracted in comparison to those of the cuboctahedron (Scheme 9). Of note, Li et al.435 found experimentally that the [Au23(SC6H11)16]− cluster (with a bicapped cuboctahedral core) converts to icosahedral [Au25−xAgx(SC6H11)18]− after reaction with Ag(I)−SC6H11; thus, such a transformation can indeed occur under certain conditions. The Au25(SR) 18 nanocluster was found to have an icosahedral Au13 kernel (Figure 16).72,73 The kernel is

Figure 15. X-ray structure of Au18(S-c-C6H11)14. Magenta, Au; yellow, S. Adapted with permission from refs 121 and 122. Both copyright 2015 Wiley-VCH. Figure 16. X-ray structure of Au25(SC2H4Ph)18. The carbon tails ( SC2H4Ph) are omitted for clarity. Redrawn from refs 72 and 73.

protected by multiple staple motifs, including one Au4(SR)5 tetramer at the bottom, one Au2(SR)3 dimer at the top, and three Au(SR)2 monomers at the Au9 waist. Theoretical calculations revealed that the HOMO−LUMO transition undergoes charge transfer from the Au4(SR)5 staple to the Au9 kernel,122 a feature not found in other nanoclusters. The optical absorption spectrum of organic-soluble Au18(SC6H11)14 closely matches with that of water-soluble Au18(SG)14 reported previously by Negishi et al., 66 indicating that the Au18(SC6H11)14 and Au18(SG)14 clusters should share the same structure. Overall, the Au9 hcp kernel is certainly very small, but very recently, a larger hcp Au30(S-Adm)18 nanocluster was discovered by Higaki et al.147 2.3.4. Icosahedron. The icosahedron is perhaps the most widely observed structure in nanoclusters. Geometrically, this structure can be obtained from the cuboctahedron. A cuboctahdron has six square faces and eight triangular faces (Scheme 9). In terms of atomic layers, the structure contains a top Au3 layer, a middle Au6 (or Au7 if counting the central atom as well), and a bottom Au3 layer. Corrugation of the Au6 plane into the chair conformation of cyclohexane eliminates the 6 square faces and produces 12 new triangular faces, plus the original 8 triangular faces; the resultant structure is an

protected by six dimeric staples, the latter capping onto the kernel along the three mutually perpendicular C2 axes of the kernel. The entire Au25S18 framework (without R groups) exhibits a quasi-D2h symmetry (or more appropriately Th). Because the icosahedral kernel has 20 triangular faces and only 12 facets are capped by 12 exterior Au atoms, the exterior Au12 shell is apparently an incomplete shell (Au12), leaving 8 facets uncapped and hence forming pocket-like sites. Such surface features are potentially interesting in catalysis. A two-shell Au55 icosahedral structure was recently observed in the Au133(SR)52 nanocluster103 (see section 2.4.3). Dahl and co-workers also reported an icosahedron-based Pd55 structure41 and a four-shell 165-atom icosahedral Pd−Pt nanocluster.39 2.3.5. Decahedron. There are three types of decahedral structures, namely, the regular decahedron,206,207 the Ino decahedron, and the Marks decahedron; the latter two were discovered by Ino208 and Marks,209 respectively, in their early TEM studies of bare gold nanoparticle structures. Using a theoretical approach, Landman and co-workers studied the energetics of decahedral, icosahedral, and truncated octahedral 10361


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Figure 17. X-ray structure of Au102(SPh-p-COOH)44. Redrawn from ref 162.

Scheme 10. (A) Compilation of the Basic Kernel Structuresa and (B) Surface-Protecting Motifs Discovered so Far in Aun(SR)m Nanoclusters, Including the Simple Bridge Thiolate SR, Staple-like Aux(SR)x+1 Motifs of x-Mers, and Ring Motifs Such as Au8(SR)8b

a

Magenta, Au. bYellow, S; blue, Au. All R groups are omitted for clarity.

clusters.206,210 Their theory predicted the Marks decahedron (Dh) and the twinned truncated octahedron (t-TO) to be more stable than the icosahedron in the 100−1000-atom size range.210 A transition from the Dh structure to the t-TO at ∼200 gold atoms was also predicted.210 Au102(SR)44 was discovered to have a decahedral structure (Figure 17), which was the first solved structure of Aun(SR)m nanoclusters.162 The structure can be dissected as follows. First, a truncated Au49 kernel (Marks decahedron) can be identified (Figure 17).211 The Au49 kernel is further capped by a 15-atom unit at the top and a second 15-atom unit at the bottom, giving rise to a Au79 kernel. This Au79 kernel is then protected by five monomeric staples at the top and another five at the bottom, along with nine monomers and two dimers at the waist. The arrangement of 13 gold atoms (from nine monomers and two dimers, 9 + 2 × 2 = 13) at the waist, and the placement of the gold−thiolate staple units destroys the 5-fold symmetry of the entire Au102 cluster, and this arrangement induces chirality.

Recently, Chen et al.165 discovered a larger decahedral structure, namely, Au130(SR)50, with perfect 5-fold symmetry (see section 2.4.3). 2.3.6. Summary of Kernel and Surface Structures. The above structures illustrate that the kernel can be singlecrystalline (e.g., fcc, bcc, hcp) or multiple-twinned (e.g., icosahedron, decahedron). The observed basic polyhedronbased kernel units (except the bcc type) are summarized in Scheme 10A. Fusion of two Au7 decahedra through vertex sharing gives rise to the regular Au13 Ino decahedron (if the two Au7 decahedra are in an eclipsed conformation) or the Au13 icosahedron (if the two Au7 decahedra are in a staggered conformation). The Au13 icosahedron (Ih), cuboctahedron (fcc), and anticuboctahedron (hcp) can be mutually interconverted (Schemes 9 and 10). Interestingly, the transformation of an icosahedron to a cuboctahedron was experimentally observed in the conversion of biicosahedral Au38(SR)24 to fcc Au36(SR)2498 and also in the conversion of 10362


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Figure 18. (a) Typical XAS spectrum consisting of XANES and EXAFS spectra (inset: k-space EXAFS spectrum), (b) L3-edge XANES spectra of some 5d transition metals, (c) EXAFS spectrum of Pd foil with the best fit (fitted bond distances in blue) and structural model, (d) simulated EXAFS spectra with increased coordination number. Adapted with permission from ref 213. Copyright 2014 American Chemical Society.

icosahedral Au25(SR)18 to fcc Au28(SR)20.100 These are oneshelled kernels, whereas the Marks decahedron has to be at least two shells (e.g., Au49) to exhibit the reentrances (Figure 17). Of note, the two-shell Au55 icosahedral and Ino decahedral structures were discovered in the Au133(SR)52 and Au130(SR)50 nanoclusters, respectively (see section 2.4.3). All of these highly symmetric polyhedral structures are primarily responsible for the high stability of the nanoclusters. In addition to the kernel structure, the surface structure is also crucial for ligand-stabilized colloidal nanoclusters. Because of the high surface-to-volume ratio in nanoclusters (even higher than in regular nanoparticles), the surface structure sometimes indeed determines the stability, properties, and chemical reactivity of the particles. Atomically precise metal nanoclusters afford an exciting opportunity to decipher the surface structure at the atomic level. It is now known that the surfaces of Aun(SR)m nanoclusters are mainly protected by oligomeric Aux(SR)x+1 staple-like motifs (Scheme 10B).1,171,212 Since the discovery of monomeric staple motifs (SRAuSR) in Au102(SR)44,162 and dimeric staple motifs (SRAuSR AuSR) in Au25(SR)18 and Au38(SR)2472,73,81 elongated staple motifs have also been identified, such as the trimeric Au3(SR)4 staple in Au23(SR)16,128 the tetrameric Au4(SR)5 staple in Au18(SR)14121,122 and Au24(SR)20,102 and the pentameric Au5(SeR)6 staple in Au24(SeR)20.350 In the staple motifs, the Au(I) atoms are bonded to two SR groups in a linear fashion (i.e., the SAuS angle is ca.180°), and the S atom of the terminal SR group is bonded to a staple Au atom and a kernel Au atom, with the AuSAu angle being ∼100°. The Aux(SR)x+1 staple motifs accommodate the surface curvature of the kernel. Smaller nanoclusters typically have more curved surfaces and thus require longer staple motifs for accommodation, whereas in larger spherical nanoclusters, shorter staple motifs are found to be more common. For example, the surfaces of the giant Au130(SR)50 and Au133(SR)52 nanoclusters are protected exclusively by monomeric staple

motifs, forming aesthetic helical and ripple patterns.103,165 Increasing the surface curvature can result in a closed Aux(SR)x ring-like surface structure; this is indeed the case for the Au 20 (SR) 16 nanocluster, which has a very curved Au 7 bitetrahedral kernel and is found to be protected by a Au8(SR)8 ring-like macrocycle.101 Jiang175 suggested the concept of “staple fitness”, whereby the fittest combination of pairing up dots on a surface corresponds to the experimental structure, which was used to interpret the Au25(SR)18 and Au38(SR)24 structures and was also applied to predict other Aun(SR)m sizes, especially for the relatively small clusters, whereas large clusters are difficult to treat with this method because of the dramatic increase in the number of combinations. Future work in total structure determination should target particles larger than Au133(SR)52. Generally speaking, the surface structures of colloidal nanoparticles (e.g., ∼5-nm nanoparticles; see Figure 2) are more difficult to characterize than the kernel structures, the latter can be imaged by highresolution TEM, and thus the surface structure of regular-size nanoparticles will remain a mystery for a long time until atomically precise nanoparticles can be made and crystallization can be realized. 2.3.7. X-ray Absorption Spectroscopic Analysis. The structurally characterized Aun(SR)m nanoclusters have enabled atomic site-specific analysis of local structure and electronic character by X-ray absorption spectroscopy (XAS; Figure 18).213 Unlike optical absorption spectroscopy, which probes electronic transitions in valence orbitals, XAS involves electronic transitions from the core level to unoccupied valence states. The core-level energies are highly element-specific, and thus, XAS enables element-specific analysis of electronic properties of the absorbing atom in the near-edge region (i.e., X-ray absorption near-edge structure, XANES); for example, holes in the d state can be determined by monitoring the intensity of the first spectral feature (historically called the 10363


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“white line”) in XANES spectra (Figure 18b). Accurate information on the local structure of the X-ray-absorbing atom (e.g., bond length, coordination number) can be obtained from extended X-ray absorption fine structure (EXAFS) signals by fitting the EXAFS spectrum (Figure 18c,d). The Zhang group carried out a series of works using the XANES and EXAFS techniques, including studies of the effects of solvation and the temperature dependence of nanoclusters.213 In this section, we highlight some of the results. A more detailed account can be found in a recent review by Zhang.213 MacDonald et al.214 observed that, with decreasing temperature, the AuAu bond length in the Au13 core of Au25(SR)18 contracts, but the Au(kernel)Au(staple) bond length expands, implying metal-like behavior of the kernel and molecule-like behavior of the staple shell. Chevrier et al.215 performed a temperature-dependent EXAFS and XANES analysis of the fcc Au36(SR)24 nanocluster in comparison to Au38(SR)24 and found an interesting negative thermal expansion (opposite the behavior of larger metallic particles) of the two AuAu shells in Au36, implying that the electronic behavior of Au36 is more molecule-like than that of its neighbor Au38. They revealed that the molecule-like AuAu bonding in Au36 is largely determined by tightly bonded Au4 tetrahedral units within the Au28 kernel. This behavior in fcc nanoclusters is different from the behavior fcc bulk metals. The Tsukuda group216 recently investigated bond stiffness in Au25(SR)18, Au38(SR)24, and Au144(SR)60 by EXAFS spectroscopy and revealed the following hierarchy: The long AuAu bonds (those at the icosahedron-based gold core surface) are more flexible than those in the bulk metal; in contrast, the short AuAu bonds (in the radial direction of the core) are stiffer than those in the bulk metal and form a cyclic structural backbone with rigid AuSR oligomeric staple motifs. The Wei group217 reported an icosahedral-to-cuboctahedral structural transformation of Au nanoclusters driven by a changing chemical environment. Specifically, for the icosahedral Au13 clusters protected by binary ligands (dodecanethiolate and triphenylphosphine), a solvent change from ethanol to hexane caused the rapid selective desorption of the thiolates and then the conversion of the to a cuboctahedral structure. Jiang et al. also investigated the critical role of the solvent in the controlled synthesis of gold−thiolate nanoclusters using XAS.218 They found that increasing the solvent polarity leads to higher thiol coverage on the nanocluster surface and, accordingly, retards the growth of the particles. Overall, XAS techniques are helpful for probing the structures of nanoclusters, in particular the solution-phase or oxide-supported nanoclusters used in catalysis research, as well as the growth process under physical or chemical influences.

growth seems to occur in the conversion of Au11 to Au25 rod nanoclusters,345 as well as the synthesis of linear triicosahedral Au37 nanoclusters,347 with both being protected by mixed ligands (phosphine and thiolate). 2.4.1. Fusion. The Au38(SR)24 nanocluster has a Au23 kernel, which can be viewed as two Au13 icosahedra fused together by sharing a common Au3 face (Figure 19).81 The

Figure 19. Total structure of Au 38 (SC 2 H 4 Ph) 24 . (Top) Au 23 biicosahedral kernel and (bottom) positions of dimeric staple Au2(SR)3 and monomeric staple Au(SR)2 on the kernel, as well as the two enantiomers. Yellow, sulfur; other colors, Au. The carbon tails (SC2H4Ph) are omitted for clarity. Redrawn from ref 81.

fusion of the two icosahedra occurs along the C3 axis of the icosahedron. The resultant Au23 rod is structurally strengthened by three monomeric staples at the waist (Figure 19); then, the top icosahedron is further capped by three dimeric staples that are arranged in a rotary fashion, resembling a triblade “propeller”. Similarly, another three staples are arranged on the bottom icosahedron, but the bottom propeller is rotated by ∼60° relative to the top one, forming a staggered dual-propeller configuration. The entire cluster is chiral because of the rotary arrangement of the dimeric staples, even though the Au23 kernel is achiral. 2.4.2. Interpenetration. This mode was previously discovered in a number of phosphine-protected Pdn nanoclusters by Dahl and co-workers219,220 and is inherent to fcc nanoclusters larger than one cuboctahedron. In Aun(SR)m nanoclusters, the kernel of fcc Au36 nanocluster98 can be viewed as four interpenetrating cuboctahedra (see section 2.3.1). Another example of the interpenetrating mode is the Au28(SR)20 nanocluster.100 The Au20 kernel of Au28(SR)20 is composed of two interpenetrating cuboctahedra that share six common atoms (Figure 20a, highlighted in green); hence, the kernel has 13 + 13 − 6 = 20 gold atoms. Note that the cuboctahedra are slightly distorted because of surface bonding with the thiolate ligands. The rod-like Au20 kernel is apparently a fragment of the fcc structure, and layer-

2.4. Growth Modes

Among the basic structural units of the kernel, the Au13 cuboctahedron, icosahedron, and decahedron are quite often observed. An important question is the growth of such building blocks into larger structures. In the following subsections, we summarize a few modes that have been observed experimentally, including the fusion, interpenetration, shell-by-shell, layer-by-layer, and Au4 tetrahedron-based vertex-sharing growth modes. We note that the following discussions are based on structural aspects and that the growth modes might not necessarily reflect the real growth mechanisms of nanoclusters in the solution phase. Nevertheless, experimentally, fusion 10364


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Figure 21. Cuboctahedron interpenetration mode in the series of nanoclusters including Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 (R = Ph-tBu). Reproduced with permission from ref 157. Copyright 2016 American Chemistry Society.

different-sized (quasi)spherical nanoparticles. This growth mode allows the isotropic three-dimensional expansion of particle size and is expected to apply to the structures of giant nanoclusters. Indeed, this mode has been observed in Au133, Au130, and Au102 nanoclusters. Zeng et al. reported the largest crystallographic structure thus far, namely, Au133(SPh-tBu)52.103 Remarkably, the Au133(SR)52 nanocluster exhibits aesthetic orderings and patterns at multiple scales, from the gold kernel to the AuS interface and the carbon tails of the thiolates. The gold kernel follows a shell-by-shell growth pattern, starting with a 13-atom icosahedron (Figure 22), enclosed by a second shell

Figure 20. Total structures of Au28(SPh-tBu)20 and Au30S(S-tBu)18. (a) Au20 kernel formed by two interpenetrating cuboctahedra, (b) chiral Au28S20 framework, (c) addition of two more Au atoms to the Au20 kernel, (d) Au30S(S-tBu)18 structure (carbon tails omitted). Panels a−c adapted with permission from refs 100 and 145. Copyright 2013 American Chemistry Society and copyright 2014 Royal Society of Chemistry, respectively. Panel d redrawn from refs 144 and 145.

by-layer atomic planes can be identified. The Au20 rod exposes four trapezoid-shaped Au5 {111} facets (two in the front and two in the back, Figure 20) and two rectangle-shaped Au6 {100} facets (top and bottom). The Au20 kernel is protected by four dimeric staples (one on each {111} facet) and eight bridging thiolates (one on each Au4 square). An alternative view of Au28(SR)20 is a Au146+ kernel protected by two Au3(SR)4 trimers and four Au2(SR)3 dimers, and the stability was explained by the formation of a distorted eight-electron (8e) superatom.221 The Au28(SR)20 nanocluster is chiral, and a pair of enantiomers (Figure 20b) was found in the unit cell. The Au30S(S-tBu)18 structure144,145 reported by the Dass and Zheng groups has an fcc Au22 kernel (Figure 20c), similar to the Au20 kernel in the Au28(SPh-tBu)20 structure, but with different surface motifs, which include two trimers, two monomers, six bridging thiolates, and a sulfido ligand (bare S atom) in a μ3coordinating position, Figure 20d. The interpenetrating mode can also be extended to Au44(SR)28 and Au52(SR)32.157 These two clusters, together with Au28(SR)20 and Au36(SR)24, form a magic series with a uniform progression of Au8(SR)4 units. As shown in Figure 21, the nanocluster grows through sequential interpenetration of two additional cuboctahedral units at the bottom of the smaller kernel. As the nanocluster grows from Au28(SR)20 to Au36(SR)24, Au44(SR)28, and Au52(SR)32, the kernel evolves from two to four, six, and eight interpenetrating cuboctahedra. 2.4.3. Shell by Shell. The shell-by-shell mode is the most common growth mode for nanoparticles, as reflected in

Figure 22. Au107 kernel in the Au133(SR)52 nanocluster. (a) First shell, magenta; (b) second shell, gray; (c) third shell, cyan/blue; (d,e) slices showing the atom packing modes of three shells. Adapted with permission from ref 103. Copyright 2015 American Association for the Advancement of Science.

(42 atoms), giving rise to the famous 55-atom Mackay icosahedron (MI). The Au55 MI contains 20 tetrahedral units that are joined together at a common vertex (i.e., the innermost center). The gold atoms in each tetrahedral unit are assembled in a layered a−b−c manner (Figure 22d, green−pink−gray), that is, cubic close-packing. The third shell contains 52 gold atoms and is as a transition layer between the Au55 MI and the outermost protecting layer. This transition layer caps the 20 triangular (111) facets of the Au55 MI; among the facets, 16 are 10365


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Each staple stabilizes two gold atoms of the third shell through SAu bonds. Interestingly, the SAuS staple motifs self-organize into pentagonal “ripple-like” patterns (total of five, from the top to the bottom) on the Au105 rod kernel (Figure 25E). The overall Au130S50 framework resembles a barrel shape (diameter = 1.6 nm, height = 1.9 nm), and the five SAu S ripple stripes resemble the metal hoops that bind the wooden staves into the barrel shape (Figure 25E). The Au130S50 framework exhibits quasi-D5 symmetry and is chiral because of the different rotational arrangements of the SAuS motifs (Figure 25E). The harmony of the 5-fold symmetry in the Au130(p-MBT)50 imparts high stability to the nanocluster. 2.4.4. Au4 Tetrahedron as a Building Block. Unlike the shell-by-shell growth mode in larger nanoclusters, many smaller structures have been found to be assembled from Au4 tetrahedra; thus, the Au4 tetrahedron serves as a basic building block. In earlier theoretical works, Au4 was predicted in several nanocluster structures.170,171,180,186,204,221−223 Zhang and coworkers identified the interesting behavior of Au4 in XAS analyses of nanoclusters.215,224 The Au 20 (SR)16 structure (R = Ph- t Bu) exhibits a bitetrahedral Au7 kernel, which can be viewed as two Au4 building blocks fused together by vertex sharing (Figure 26).101 The structure also features an unprecedented “ring” motif, namely, Au8(SR)8, that protects the bitetrahedral Au7 kernel through strong Au(ring)Au(kernel) bonding, but without any involvement of SAu(kernel) bonding (Figure 26); this is in striking contrast to the common staple motifs in which the S Au(kernel) bonding is dominant but the Au(staple)Au(kernel) interaction is weak (i.e., aurophilic). Aside from the vertex sharing in Au20(SPh-tBu)16, the Au24(SCH2Ph-tBu)20 nanocluster exhibits a bitetrahedral Au8 kernel that involves face joining in an antiprismatic manner (Figure 27).102 The AuAu distances within each Au4 tetrahedron range from 2.70 to 2.75 Å, much shorter than the 2.88 Å AuAu distance in bulk gold. The Au8 kernel is protected by four tetrameric staple-like motifs (Figure 27). The tetrameric staple was observed for the first time in this nanocluster. It is also interesting to notice that Au20(TBBT)16, Au24(SR)20, and Au18(SR)14 nanoclusters form a 4e family (note that the free valence electrons are counted by the difference between the number of gold atoms and the number of ligands for charge-neutral nanoclusters). The recently solved Au40(SR)24 and Au52(SR)32 structures exhibit elegant patterns of Au4 tetrahedra.155 The segregation of tetrahedral Au4 units in both Au40 and Au52 clusters is clearly manifested in the differences in AuAu bond lengths according to the different positions of the Au atoms, and four “steps” were observed in the distribution of AuAu bond lengths.155 The Au40(o-MBT)24 structure exhibits an oblate shape resembling a hexagonal prism (Figure 28a,b), which is indeed for the first time observed in the Aun(SR)m nanocluster structures.155 The kernel comprises 25 gold atoms and is segregated into eight tetrahedral Au4 units based on the Au Au bond length analysis, with two units forming the central bitetrahedral antiprism (Figure 28c, green) and the remaining six tetrahedra forming a Kekulé-like external ring (Figure 28c, blue). The Kekulé ring is protected by six monomer staples (Figure 28d), whereas the central bitetrahedron is protected by three trimer staples (Figure 28e). The cluster is chiral. The Au52 nanocluster contains a Au32 kernel, which is segregated into 10 Au4 units (Figure 29).155 Surprisingly, the 10

each capped by Au3 (Figure 22d), and the remaining four are each capped by only one gold atom (Figure 22e). The resultant Au107 kernel is quasispherical and protected by 26 monomeric staples. Significantly, the gold−thiolate interface exhibits a helical “stripe” pattern, in which four such stripes are found, with each stripe having six monomeric staples stacked into a ladder on the curved surface (Figure 23). Furthermore, the

Figure 23. Chirality in the Au133(SPh-tBu)52 nanocluster imposed by chiral arrangements of SAuS staples into helical stripes (four) on the Au107 kernel. Yellow, sulfur; other colors, gold. Reproduced with permission from ref 103. Copyright 2015 American Association for the Advancement of Science.

carbon tails of the thiolates surprisingly do not follow the underlying stripe pattern, instead forming “swirls” (Figure 24). All of these patterns (including the Au107 kernel, stripes, and swirls) impart chirality to the cluster.

Figure 24. (a) Chirality in the Au133(SPh-tBu)52 nanocluster imposed by chiral arrangements of carbon tails into swirls. (b) Left and right rotary arrangements of four carbon tails. Yellow, S; gray/blue, C; light blue (large balls), Au atoms in the third shell. Reproduced with permission from ref 103. Copyright 2015 American Association for the Advancement of Science.

Au130(p-MBT)50 can also be dissected in a shell-by-shell manner, with a total of four shells.165 Starting with the 13-goldatom Ino decahedron (Figure 25A), the second shell contains 42 gold atoms (Figure 25B), and the third shell contains 50 gold atoms (Figure 25C). The third shell is constructed in the following way: Each of the 10 (111) facets of the Au55 Ino decahedron is capped by a Au3 triangle in a hexagonal closepacked (hcp) manner, and each of the five (100) facets is capped by a Au4 square, thus giving 10 × 3 + 5 × 4 = 50 atoms. The third shell is a transition layer between the inner Au55 decahedron and the exterior gold−thiolate surface. The fourth shell comprises 25 monomeric SAuS staple motifs. 10366


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Figure 25. X-ray structure of the Au130(p-MBT)50 nanocluster. (A) Central atom Au1 (green) and the first shell Au12 (magenta), (B) second shell Au42 (gray), (C) third shell Au50 (blue), (D) fourth shell Au25 (orange) and S50 (yellow), (E) five pentagon ripples and top/side views. Red/green/ blue, gold; yellow, sulfur. Reproduced with permission from ref 165. Copyright 2015 American Chemical Society.

Figure 27. Structure of Au24(SCH2Ph-tBu)20 consisting of a bitetrahedral Au8 kernel protected by four tetramers. Magenta, kernel Au atoms; blue, staple Au atoms; yellow, S. Carbon tails are omitted for clarity. Adapted with permission from ref 102. Copyright 2014 Royal Society of Chemistry. Figure 26. Anatomy of the structure of the Au 20 (SPh-tBu) 16 nanocluster. The Au7 kernel is protected by an octameric ring motif and then by monomers (front and back of the kernel) and one trimer (top of the kernel). Carbon groups are omitted for clarity. Reproduced with permission from ref 101. Copyright 2014 American Chemical Society.

This double helix is protected by eight Au2(SR)3 dimer staples at the top and bottom and four Au(SR)2 monomer staples at the waist. This cluster is also chiral. 2.4.5. Anisotropic Layer-by-Layer Growth. Whereas shell-by-shell growth is three-dimensional and leads to isotropic expansion of the structure, layer-by-layer growth is onedimensional and thus leads to anisotropic growth. In an alternative view, the Au40(SR)24 and Au52(SR)32 structures show anisotropic layer-by-layer growth.155 For Au40(o-MBT)24, the structure can be viewed as three layers that are stacked

tetrahedra are assembled into a double-helical superstructure, resembling double-stranded DNA. Within each helix, five tetrahedra are connected through vertex sharing (Figure 29c). 10367


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Figure 28. X-ray structure of the Au40(o-MBT)24 nanocluster. (a) Unit cell comprising two enantiomers. (b) Mirror symmetry of the enantiomers. (c) Snowflake-like Au25 kernel with tetrahedral units coiled up into a Kekulé-like superstructure. (d) Six monomeric staples protecting the Kekulé ring. (e) Three trimeric staples protecting the central Au7 bitetrahedron. (f) Overall Au40S24 framework. Blue/green, Au atoms in the kernel; orange, Au atoms in the staples; yellow, sulfur; gray, carbon; pink, hydrogen. Reproduced with permission from ref 155. Copyright 2015 American Association for the Advancement of Science.

damped, and eventually disappears below ∼2 nm.33,72,593 Small nanoclusters were believed to have discrete energy levels (as opposed to continuous band in the metallic state), but the exact picture of the discrete energy levels and the nature of quantum effects in ultrasmall gold nanoparticles remained elusive before crystal structures were reported. For the treatment of ultrasmall Au nanoclusters by quantum mechanics (e.g., density functional theory, DFT), atomic structures were critically needed. In 2008, the first DFT analysis based on the crystal structure of Au25(SR)18 was performed by Aikens and co-workers.72 Herein, we discuss the DFT-computed optical spectra and electronic transitions of two sizes of nanoclusters as examples. 2.5.1. Optical Absorption. After the crystal structure of [Au25(SR)18]− [counterion: N+(n-C8H17)4] was obtained, Aikens and co-workers carried out time-dependent density functional theory (TD-DFT) calculations and reproduced the experimental spectrum.72 Both the sp and d bands were found to be quantized. The experimental peak at 670 nm (peak a in Figure 31) was revealed to be the HOMO-to-LUMO transition, which is essentially an intraband (sp ← sp) transition. The HOMO set (3-fold quasidegenerate, above the d band) is

along the [111] direction in an a−b−c manner, forming a hexagonal prism (Figure 30a−d). For Au52(TBBT)32, 48 of the 52 gold atoms can be fit into the fcc lattice, but the six layers are assembled along the [100] direction, forming a tetragonal rod enclosed by {100} facets (Figure 30e−h). In this layer-by-layer view, both the Au40 and Au52 cores are protected by simple bridging thiolates. These two structures demonstrate an anisotropic layer-by-layer construction mode, similar to the anisotropic growth of “2D” nanoprisms and “1D” nanorods in shape-controlled plasmonic nanocrystals. It also provides atomic-scale insight into the effects of surface passivation in tailoring the shapes of clusters. 2.5. Optical Properties

The elegant optical properties of gold nanoparticles have attracted scientists since Faraday’s time. Plasmonic Au nanospheres exhibit a single SPR band at 520 nm for 5−20nm particles, and classical electrodynamics (e.g., Maxwell’s equations) is sufficient for the theoretical treatment of such nanoparticles. As the size of the particles shrinks, the metallic state fades, and the SPR peak gradually blue shifts, becomes 10368


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Figure 29. Total structure of the Au52(TBBT)32 cluster. (a) Unit cell comprising two enantiomers. (b) Mirror symmetry of the enantiomers. (c) Two helical pentatetrahedral strands forming the double helical kernel. (d) Four monomeric staples protecting the waist of the kernel. (e) Four dimeric staples protecting the top and another four protecting the bottom of the kernel. (f) Overall Au52S32 framework. Reproduced with permission from ref 155. Copyright 2015 American Association for the Advancement of Science.

count as well.128 DFT simulations by Nobusada and coworkers reproduced the optical spectral profile (Figure 32). The experimental peak at ∼570 nm involves both the HOMO-toLUMO (529 nm, intense) and HOMO-to-LUMO + 1 (507 nm, less intense) electronic transitions. The simulated HOMO cloud exhibits two large lobes (Figure 32C, blue) that involve contributions from both the Au13 cuboctahedron and the two extra surface Au atoms that serve as the “hubs” (see section 2.3.1). The third lobe (red) of the HOMO is located within the cuboctahedron (Figure 32C). In the LUMO diagram (Figure 32D), two blue and two red lobes are seen, and the surface hub Au atoms are also involved. These MO clouds indicate that the two hub atoms are indeed part of the kernel, namely, an overall Au15 kernel, as opposed to Au13. The lowest-lying peak (experiment, 570 nm; theory, 530 nm) in the optical spectrum uniquely arises from the Au15 kernel. Finally, it should be clarified that the HOMO is not a d-like orbital; instead, it is a hybrid orbital constructed from atomic orbitals of Au. Thus, the [Au23(c-C6)16]− nanocluster is a nonsuperatom, even though the nominal valence electron count is 8e, the same as that of superatomic [Au25(SR)18]−. This demonstrates the importance of the atom-packing structures of nanoclusters in dictating their electronic structures.

essentially of s character; thus, transitions arising from the other occupied HOMO − n orbitals (belonging to the d band) are interband (sp ← d) transitions, such as peak c, whereas peak b is of mixed type. In terms of atomic contributions, the HOMO and LUMO are composed, almost exclusively, of atomic orbital contributions from the 13 Au atoms in the icosahedral core rather than the 12 exterior Au atoms. Thus, the first peak at 670 nm in the absorption spectrum can be viewed as a transition that is due entirely to the electronic and geometric structure of the Au13 core. Interestingly, the HOMO exhibits a two-lobe distribution resembling the atomic p orbital (see section 2.9), and the LUMO exhibits d-like character. Thus, in some sense, [Au25(SR)18]− can be viewed as a superatom,74 and the formal count of valence electrons is 8e for the anionic cluster. In a subsequent work, Aikens and co-workers carried out more detailed analysis of the Au25(SR)18 system.225 Recent work by Jiang et al.226 reported that the spin−orbit interaction is important, which can cause splitting of the superatomic 1P HOMO set and thus give rise to the tail band at ∼800 nm in the experimental spectrum (Figure 31B), which was missing from previous scalar relativistic TD-DFT simulations.72 Another interesting case is [Au23(SR)16]−, which has the same 1− charge state as [Au25(SR)18]− and the same 8e formal 10369


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Figure 30. Anisotropic growth of the gold fcc lattice into a hexagonal prism in Au40(o-MBT)24 and a tetragonal rod in Au52(TBBT)32. (a−c) Model of a 43-gold-atom hexagonal prism composed of three layers (green, orange, blue) stacked along the [111] direction in an a−b−c manner (the three arrows indicate the three missing gold atoms in the real Au40 cluster). (d) Au40(o-MBT)24 as a hexagonal prism. (e−g) Model of a 48-gold-atom tetragonal rod composed of six layers stacked along the [100] direction. (h) Au52(TBBT)32 as a tetragonal rod, with the four gold atoms not included in the lattice indicated by arrows. Reproduced with permission from ref 155. Copyright 2015 American Association for the Advancement of Science.

Figure 32. (A) Experimental UV−vis spectrum of [Au23(S-cC6H13)16]− and (B) theoretically simulated spectrum of the [Au23(SCH3)16]− model cluster. (C,D) Schematic diagrams of the (C) HOMO of [Au23(SCH3)16]− and (D) LUMO of the cluster. Blue and red represent the phases of the orbital functions. Reproduced with permission from ref 128. Copyright 2013 American Chemical Society.

Theoretical analyses on the optical absorption spectra have also been carried out for other sizes, such as Au28(SR)20,221 Au36(SR)24,148 and Au38(SR)24.82,83 Discrete transitions and molecular-like electronic structures were identified. Ramakrishna and co-workers227 investigated the low-temperature optical absorption behavior of Au25 and Au38 nanoclusters and observed that the peaks became stronger, sharper, and blue-shifted with decreasing temperature (Figure 33). Using a theoretical model of electron−phonon interactions, an average

Figure 31. (A) Kohn−Sham orbital level diagram for [Au25(SR)18]−. (B) Peak assignment of the absorption spectrum of [Au25(SR)18]− clusters. Adapted with permission from ref 72. Copyright 2008 American Chemical Society.

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Figure 33. Optical absorption spectra of (A) Au25(SC6H11)18 and (B) Au38(SC2H4Ph)24 clusters dissolved in a methyl cyclohexane/methyl cyclopentane mixture at two different temperatures. Reproduced with permission from ref 227. Copyright 2011 American Chemical Society.

Figure 34. (Top left) Solvent-induced AIE properties of oligomeric Au(I)−thiolate complexes and (bottom left) photograph of Au(I)−thiolate complexes in mixed ethanol and water with different volume percentages of ethanol under UV light. (Top right) AIE-guided synthesis of luminescent nanoclusters and (bottom right) corresponding images. Reproduced with permission from ref 251. Copyright 2012 American Chemical Society.

phonon energy of ∼400 cm−1 was determined and attributed to the mode of surface staples.227 The kernels’ phonon modes have not been experimentally investigated, which is of interest to pursue in future work. 2.5.2. Photoluminescence. The photoluminescence (PL) of ligand-protected gold nanoclusters228−230 has attracted wide research interest because of their promising applications in cell labeling, biosensing, and phototherapy, among other areas.231−237 In early work, the reported quantum yields (QYs) of nanoclusters were very low.66,228−230 Using dithiols, the Wang group recently improved the QY.238−240 Watersoluble thiolate-protected gold nanoclusters often exhibit relatively stronger luminescence than their organic-soluble counterparts.249 For both types of nanoclusters, the PL is stronger in the near-infrared (NIR) region, whereas the visible luminescence is weak at room temperature.66,230 Time-resolved fluorescence investigations indicate that the visible and NIR luminescence of gold nanoclusters with a core−shell structure originates from the metal core state and from the surface states of the SRAuSRAuSR staples, respectively.241 However, the details remain unclear to a large extent. Whereas most Aun(SR)m nanoclusters exhibit low fluorescence quantum yields (200 °C). (b) Isothermal stability analysis of Au25(SR)18 nanoclusters (maintained at 150 °C in air for 60 min). No discernible loss of ligands was observed. The inset shows an enlargement of the TGA curve at constant 150 °C. Reproduced with permission from ref 335. Copyright 2013 Elsevier B.V.

3. GOLD NANOCLUSTERS PROTECTED BY OTHER TYPES OF LIGANDS Although much work in the field of nanocluster research focuses on gold nanoclusters protected by thiolate, other types of ligands have also been reported, including phosphine, diphosphine, mixed phosphine/thiolate, selenolate, and alkynyl. Some selected gold nanoclusters protected by such ligands are listed in Table 3. In the following subsections, we briefly discuss some structures with a focus on the relatively larger ones (e.g., the number of gold atoms, n > 13). Konishi336 wrote a quite comprehensive review on gold−phosphine clusters, and in particular, detailed discussions on those smaller clusters (n < 13) can be found there. 3.1. Phosphine-Protected Au Nanoclusters

Research on this topic can be traced back to the late 1960s.337 The Au11 structure, which adopts an incomplete icosahedral structure, was first reported in 1969. Mingos et al. then predicted the complete icosahedral Au13 structure and later achieved it experimentally.338,358 Schmid et al. reported the Au55 cluster, but its insufficient stability precluded crystallization and mass spectrometry confirmation of the Au55(PPh3)12Cl6 formula,359 although the experimental characterization suggested a two-shell icosahedral structure, namely, a Au13 icosahedron surrounded by a second full shell of 42 gold atoms (a two-shell Mackay icosahedron), but no crystal structure has been attained to date. 10380


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Table 3. Gold Nanoclusters Protected by Various Types of Ligandsa formula

ligand(s)

counter ion

ref(s)

SCN− PF6− − Cl− Cl− Cl− − SbF6− − Cl−

337 338 339 340 341, 342 343 344 345, 346 347 348

− − − − SbF6−

349 350 351 352 353

− SbF6− SbF6− SbF6−

354 355 356 357

Phosphine [Au11(PR)10]3+ [Au13(PR)10Cl2]3+ [Au14(PR)8(NO3)4]0 [Au20(PR)10Cl4]2+ [Au20(PP3)4]4+ [Au22(PPR)10]2+ [Au24(PR)10(SR)5Cl2]+ [Au25(PR)10(SR)5Cl2]2+ [Au37(PR)10(SR)10Cl2]+ [Au39(PR)14Cl6]2+ [Au18(SeR)14]0 [Au24(SeR)20]0 [Au25(SeR)18]− [Au38(SeR)24]0 [Au60Se2(PR)10(SeR)15]+ [Au8(CR)2(PPR)4]2+ [Au19(CR)9(PPR)3]2+ [Au23(CR)9(PR)6]2+ [Au24(CR)14(PR)4]2+ a

PPh3 PPh(CH3)2 PPh3 bis(2-pyridyl)phenylphosphine tris(2-(diphenylphosphino)ethyl)phosphine 1,8-bis(diphenylphosphino)octane PPh3, SC2H4Ph PPh3, SC2H5, SC2H4Ph PPh3, SC2H4Ph PPh3 Selenolate SePh SePh SePh, SeCnH2n+1 SeCnH2n+1 PPh3, SePh Alkynyl CCPh, 1,3-bis(diphenylphosphanyl)propane CCPh, N,N-bis(dipheylphosphino)amine CCPh, triphenylphosphine CCPh, triphenylphosphine

PR, phosphine; PPR, diphosphine; SR, thiolate; SeR, selenolate; CR, alkynyl.

Figure 48. X-ray structures of selected nanoclusters of gold−phosphine and gold−mixed ligands. Reproduced with permission from ref 336. Copyright 2014 Springer.

Konishi and co-workers,336,354,360−364 Hutchison and coworkers,368,369 and Hudgens and Pettibone365−367 have carried out extensive work on gold−phosphine clusters. Mass spectrometry analysis of gold−phosphine clusters and reactivities have also been carried out.370,371 The Konishi group determined the diphosphine-protected Au13 icosahedral structure.360 Recently, the Au14 cluster structure was also revealed.339 A chiral Au20 structure with tetradentate phosphine ligands was reported by two groups, and the structure was found to consist of an icosahedral Au13 motif (achiral) and a

helical Y-shaped Au7 motif (chiral).341,342 Chen et al.343 reported a Au22 nanocluster protected by six bidentate diphosphine ligands [1,8-bis(diphenylphosphino) octane (dppo)]. This Au22 cluster consists of two Au11 units clipped together by four dppo ligands, with the remaining two ligands coordinating to the two Au11 units in a bidentate fashion. An interesting feature is that eight gold atoms at the interface of the two Au11 units are not coordinated by any ligands and are thus possible active sites in catalysis.343 Teo et al. previously reported the [Au39(PPh3)14Cl6]Cl2 structure, which exhibited 10381


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an hcp layered structure (1:9:9:1:9:9:1).348 Figure 48 depicts some relatively large (n > 13) structures of nanoclusters of gold−phosphine and gold−mixed ligands.336 It is worth noting that the LEIST process was also observed in gold−phosphine nanoclusters. Konishi and co-workers reported the transformation of [Au9(PPh3)8](NO3)3 into [Au8(dppp)4](NO3)2 [where dppp = Ph2P(CH2)3PPh2, a bidentate phosphine ligand] through reaction with excess dppp at room temperature, along with the changes in the structure of the cluster from toroidal Au9 to edge-shared tritetrahedral Au8.361 The Au7, Au8, and Au9 series demonstrates atom-by-atom growth, and the optical spectra of these materials also show systematic red shifting from Au7 to Au9.360 Robinson et al. found the (CH2)x spacer length of diphosphine ligand affects the cluster size and monodispersity.370 Pettibone and Hudgens reported size-selective growth of small Aun clusters and mechanistic insights were revealed.365−367

from the [Au25(PPh3)10(SC2H4Ph)5X2]2+ nanocluster in terms of optical properties and electronic structure, such as the disappearance of the HOMO−LUMO transition in Au24 and the smaller HOMO−LUMO gap (1.35 eV for Au24 versus 1.74 eV for Au25). In a recent effort, the linear growth of three icosahedral units was achieved, giving rise to a Au37 triicosahedral rod structure (Figure 50).347 The [Au37(PPh3)10(SC2H4Ph)10X2]+ (where X

3.2. Mixed Thiolate/Phosphine-Protected Au Nanoclusters

The Tsukuda group obtained [Au25(PPh3)10(SC2H5)5Cl2]2+ through the thiol etching of phosphine-protected Au11 and revealed the structure to be a biicosahedral rod, with the five thiolate ligands joining the two icosahedra (Figure 48).345 It is worth noting that there are two compositions of ligands for Au11, namely, [Au11(PPh3)7Cl3]0 and [Au11(PPh3)8Cl2]+,372 and only the neutral one was found to exclusively convert to the Au25 biicosahedral rod cluster, whereas the cationic one led only to a mixture of different sizes (unidentified) after thiol etching.372,373 With phenylethanethiol, Qian et al. also obtained [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ by a different route, namely, the reaction of HSC2H4Ph with polydisperse PPh3-capped Au nanoparticles (as opposed to Au11 clusters), and the structure was found to the same as that of the ethanethiolate counterpart (Figure 48).346 Das et al. obtained a hollow Au24 nanocluster by treatment of the Au25 rod cluster with excess phosphine.344 Specifically, the central gold atom was lost (Figure 48), but the ligand composition was not changed compared with that of the Au25 rod cluster. Although this Au24 nanocluster can be viewed as two incomplete icosahedra joined together, another view is that the Au24 structure contains a hollow Au12 Ino decahedron with its top and bottom capped by Au5 units (Figure 49). This picture is reflected in the DFT-simulated HOMO distribution,344 which concentrates in the Au12 Ino decahedron. Although there is only a one-atom difference between Au24 and Au25 rods, the Au24 structure shows interesting differences

Figure 50. Structure of the [Au37(PPh3)10(SC2H4Ph)10X2]+ nanocluster. Reproduced with permission from ref 347. Copyright 2015 American Chemical Society.

= Cl/Br, counterion = Cl− or Br−) nanocluster was achieved by a kinetically controlled approach.347 This cluster was first predicted theoretically by Nobusada and Iwasa in 2007,173 but since then, there was no success in the synthesis and structure determination. The successful synthesis of this new nanocluster allows one to gain insight into the size, structure, and property evolution of gold nanoclusters from the monoicosahedral [Au13(PPh3)10X2]3+ to the biicosahedral [Au25(PPh3)10(SR)5X2]2+ and the triicosahedral [Au37(PPh3)10(SR)10X2]+. This growth is reminiscent of the “cluster of clusters” reported previously by Teo and coworkers.411 Figure 51 shows the optical property evolution, where the HOMO−LUMO gap systematically shrinks with increasing size. The possibility of achieving even longer rod nanoclusters based on the assembly of icosahedral building blocks remains to be seen. Jiang et al. computationally investigated linear chains of icosahedra formed by vertex sharing and face sharing and found that a vertex-sharing chain can be either semiconducting or metallic depending on the charge, whereas a face-sharing chain is always metallic.374 3.3. Selenolate-Protected Au Nanoclusters

The Negishi group reported a method for the direct synthesis of Au25(SeC8H17)18.351 The Zhu group demonstrated ligand exchange of Au 25 (SR) 18 with selenol and obtained Au25(SePh)18 and Au18(SePh)14.349 The structure of a Au24(SePh)20 nanocluster has been reported to consist of a prolate Au8 kernel.350 This kernel can be viewed as two Au4 tetrahedra cross-joined together that are then protected by two trimeric staple-like motifs as well as two pentameric staple motifs (Figure 52A). It is interesting to compare the structure of Au24(SePh) 20 with that of its thiolate counterpart, Au24(SCH2Ph-tBu)20 (discussed in section 2.4.4); the latter structure contains a different Au8 kernel and, accordingly, different surface motifs (i.e., exclusively tetramers).102 Whether the difference is caused by the ligand anchoring atom (Se versus S) or the steric effect of the carbon tails is not yet

Figure 49. Anatomy of the [Au24(PPh3)10(SC2H4Ph)5X2]+ structure. (Left) Au12 noncentered bicapped pentagonal prism, (right) capping by two roof-like Au6 units. Reproduced with permission from ref 344. Copyright 2012 American Chemical Society. 10382


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energies in the two structures and identified the high capacity of the Au24(SCH2Ph-tBu)20 structure to accommodate the perturbation of the large -CH2Ph-tBu group to the metal core and therefore the structural stability.376 In a theoretical work on Au24(SePh)20, Takagi et al.377 pointed out that the Au8 kernel coordinates with the selenolate staple motifs more strongly than with the thiolate staples and that the aurophilic interactions between the staples themselves and between the Au8 core and the staples play an important role in stabilizing the cluster. Of note, the experimental structure of Au24(SC2H4Ph)20 has not been determined,124,130 albeit theoretical prediction of the structure was reported.212 Song et al. recently reported the X-ray structure of Au25(SePh)18, which shares the same framework as its thiolate counterpart.378 In a temperature-dependent X-ray absorption spectroscopy analysis of Au25(SeR)18, Chevrier et al.379 found a significant thermal contraction of the AuAu framework in Au25(SeR)18, whereas such an effect is absent in the thiolate counterpart, indicating the sensitivity of AuSeAu bonds to changes in temperature. Song et al. also synthesized [Au60Se2(PR3)10(SeR)15]+ and solved its structure.353 Interestingly, this cluster contains five icosahedral Au13 building blocks, forming a closed ring with AuSeAu linkages (Figure 52B). Two bare Se atoms are located in the center and stabilize the cluster through Se(Au)5 pentacoordinate bonding. 3.4. Alkynyl-Protected Au Nanoclusters

The Tsukuda group introduced alkynes as ligands to protect gold nanoclusters.380,381 They identified several well-defined nanoclusters, such as Au54(CCR)26. The Wang group recently crystallized the three nanoclusters [Au19L9(PPR)3]2+, [Au23L9(PR)6]2+, and [Au24L14(PR)4]2+ (where L represents PhCC) (Figure 53A−C, respectively).355−357 The Au19 structure (Figure 53A) contains a Au13 icosahedral kernel, which is further protected by three V-shaped PhCCAuCC(Ph)AuCCPh motifs (Au2L3, Figure 53D).355 This motif resembles the RSAuSR AuSR dimeric staples. Theoretical analysis revealed that the PhCC groups participate in the frontier orbitals of the cluster.355 The Au19 is an 8e cluster assuming that each alkynide ligand localizes one electron, as does the thiolate ligand, while the phoshpine ligand does not. But the Au23 cluster contains 12e and contains a Au17 kernel protected by three Au2L3 dimeric staples and six PPh3 ligands (Figure 53B). A superatom picture was invoked to explain the stability of Au23.356 Regarding the Au24 cluster, its Au22 rod-like kernel is constructed by the sharing of a common square face of two Au13 cuboctahedra (Figure 53C) and is then protected by two PhCCAuCCPh staples and other units; the cluster exhibits near-infrared luminescence centered at 925 nm.357 The observation of some common sizes (including Au19, Au23, and Au24) between alkynide and thiolate ligand systems is interesting. It remains to explore further the similarities and differences in gold nanocluster sizes between different ligand systems in future work.

Figure 51. UV−vis−NIR spectral evolution of the Au37, Au25, and Au13 nanoclusters. Reproduced with permission from ref 347. Copyright 2015 American Chemical Society.

Figure 52. Crystal structures of (A) Au24(SePh)20 and (B) [Au60Se2(PR3)10(SeR)15]+. The left panels show the kernels, and the right panels display the total structure. Redrawn from refs 350 and 353.

4. SILVER NANOCLUSTERS 4.1. Synthesis

In recent years, research on Ag nanoclusters has advanced. The Kitaev group earlier reported a silver nanocluster with distinct optical absorption features and chiroptical properties.400,401 However, mass spectrometry analysis was not successful at that time. Bakr et al. reported a silver species with multiband

completely clear. Theoretical work by Pei and co-workers indicated that thiolate- and selenolate-protected Au nanoclusters have similar electronic structures.375 Jiang and coworkers quantitatively compared the DFT and van der Waals 10383


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Figure 53. (A−C) Crystal structures of gold-alkynyl nanoclusters. (D) Observed monomeric PhCCAuCCPh and dimeric PhCC AuCC(Ph)AuCCPh staples. Redrawn from refs 355−357.

absorption features 402 that was later formulated as [Ag44(SR)30]4− based on ESI-MS analysis.395 Bigioni et al. isolated Ag32(SR)19 by gel electrophoresis.390,391 The small Ag clusters Ag7, Ag8, and Ag9 have also been obtained with various ligands.382−384 The Pradeep group reported an interesting solid-state method for the synthesis of nanoclusters, and this method works well for silver−thiolate nanoclusters.403 Briefly, the Ag(I) salt precursor and thiol (solid) are mixed and ground in a mortar. The solid mixture is reduced with NaBH4 by grinding and then dissolved in water, after which it is purified to give rise to pure Ag9 nanoclusters.383 Chakraborty et al.404 also applied the solid-state route to selenolate-protected [Ag 44 (SePh) 30 ] 4− {the counterpart of the thiolated [Ag44(SR)30]4−}. Ghosh and Pradeep405 reported various clusters with masses of 8.0, 13.4, 22.8, 29.2, and 34.4 kDa, but the precise formulas remain to be determined in future efforts. The synthesis of Ag nanoclusters has been summarized in some recent reviews.403,406 Antoine and coworkers474 synthesized Ag29(DHLA)12 (where, DHLA = dihydrolipoic

acid) and studied its photoluminescence and nonlinear optical properties. It is noteworthy that the LEIST process (see section 2.1.2) has also been observed in the synthesis of silver nanoclusters; for example, the Bakr group392 reported that the Ag35(SG)18 nanocluster was converted to Ag44(4-FTP)30 (where 4-FTP = 4-fluorothiophenol, SG = glutathione) at room temperature, and the process was reversible, but the reverse process proceeded through some intermediate cluster sizes much more slowly than the forward process. Overall, the synthesis and mass spectrometric identification of precise formulas of silver nanoclusters are still nontrivial because of the insufficient stability of Agn(SR)m nanoclusters and the significantly broadened mass peaks caused by silver isotopes. Table 4 lists some selected Ag nanoclusters protected by thiolates or mixed ligands (e.g., thiolate plus phosphine). 4.2. Crystal Structures

The first reported crystal structure of Agn(SR)m was that of [Ag44(SR)30]4−,393,394 which was soon after the report of the 10384


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Table 4. Selected Silver Nanoclusters formula

ligand

counter ion

ref(s)

− − − Na+ PPh4+ − − − − Na+ PPh4+

382−384 385 386 387 388 474 389 386 390, 391 392 393 394, 395

− − PF6− − PPh4+

396 397 398 399 397

Thiolate Ag99 Ag > Cu (least stable), and (ii) the interfacial bonding strength, for example, a typical order is as follows: covalent bond in gold−thiolate > dative bond in gold− phosphine (or amine) > weak adsorption in gold−citrate (least stable). For a specific system such as Aun(SR)m nanoclusters, geometric and electronic shell closings are often invoked to explain the particular stability of the magic-sized nanoclusters, but more details remain to be elucidated. The currently available results imply that the stability of nanoclusters involves multiple factors, rather than any single factor; however, if one had to pick a single factor, the decisive factor must be the atompacking structure. Highly symmetric structures lead to higher binding energies and, hence, high stabilities. Whereas theoreticians consider energy (i.e., thermodynamic stability), experimentalists often focus more on reactivity (with O2, H2O, etc.) and kinetic stability. With the gold−thiolate system, we believe the following factors to be critical: (i) metal-atom packing structure;612 (ii) SR ligand’s carbon tail;70,165 and (iii) number of valence electrons, which is determined as n − m − q, where thiolate is assumed to immobilize one 6s electron of gold.633 It appears that the metal core [e.g., the 13-atom core of Au25(SR)18] has a flexibility in the number of electrons between six and eight,1,416,417,440 although eight is more frequent. The bottom line is that the number of valence electrons should be sufficient to counteract the nuclear repulsion within the cluster. In terms of valence electron counting, Ciabatti et al. discussed a different method,50 for example, by viewing the [Au25(SR)18]− as an Au135+ core decorated by 6 dimeric staples (each as a 4e donor), the electron count is: 13 × 11e (d10s1) − 5e (core charge) + 6 × 4e (dimertic staple) = 162e. The 162e count is indeed the same as that of the icosahedral [Pt13(CO)12]8− cluster.50 Future work is expected to reveal more insight into the stability issue.

Strongly emissive nanoclusters are highly desirable. Strategies for increasing the PL quantum yield and tuning the PL colors of nanoclusters are to be devised. Although some strategies such as aggregation-induced fluorescence,251,447 silver doping,247,407 and ligand-shell rigidifying252 have been demonstrated, the origins of PL in nanoclusters are still not fully understood.241,634 Geometric and electronic structures are critically important in this respect. The extent to which the surface and kernel are coupled and how this coupling modulates the emission properties remain to be determined. Electron dynamics studies will help unravel the fundamental origins and devise new strategies for boosting the QY for practical applications. 7.8. Biomedical Application of Nanoclusters

Gold nanomaterials have received enormous interest in biomedical applications.28,31,635−640 Ultrasmall nanoclusters could find new opportunities (see, e.g., refs 448, 494, 510, 599, 603, and 639). The Xie group641 recently demonstrated the promise of Aun(SG)m nanoclusters as a new class of radiosensitizers for cancer radiotherapy that do not damage normal tissues, and ultrasmall nanoclusters were found to be efficiently cleared by the kidney. Future work is expected to further explore the biomedical applications of this unique class of nanoclusters based on their characteristics of high luminescence, atomic precision, site-specific functionality, and high permeability. 7.9. Catalysis

Despite exploratory catalytic studies with the Aun(SR)m nanoclusters (both ligand-on and ligand-off nanoclusters), a firm mechanistic understanding has not been achieved. Thus, future research should focus on the active site and mechanistic studies. Atomic-level mechanisms remain to be determined. Monitoring of the nanocluster structure during a catalytic 10395


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efficiency, electron traps on surfaces, and the relationship between the size-dependent electronic structure and optical and optoelectronic properties.644−647 There has also been research activity toward atomically precisely magnetic nanoclusters.631,631 Atomically precise doping in chalcogenide nanoclusters was reported by Feng and co-workers.642

reaction will be important for understanding the mechanism. In this regard, XAS is perhaps the most useful approach. Operando spectroscopy will find its power in mechanistic studies. 7.10. Nucleation of Aun(SR)m Nanoclusters

Nanoclusters serve as a link between the complexes and larger nanoparticles. It is not yet clear how Aun(SR)m nanoclusters nucleate from Au(I)−SR complexes.194,617−619 An understanding of the nucleation issue necessitates the structural elucidation of nanoclusters with decreasing size. Thus far, Au15(SR)13 constitutes the smallest Aun(SR)m nanocluster (n > m), and its crystal structure has not been elucidated,66,105,314 although theoretical predictions have been made.170,186,618 It also remains to be seen whether other sizes exist between Au10−12(SR)10−12 and Au15(SR)13. Such information is of importance for mapping out the growth mechanism of nanoclusters and for better controlling their size and structure.619

7.14. Future Theoretical Work

Whereas DFT is very powerful and is currently the primary method,169,171,172,606 the simulation of electronic structures and optical absorption spectra is still not sufficiently accurate, and computational demands rise sharply with large Aun(SR)m clusters. It is highly desirable to develop efficient methods for computing large nanoclusters such as Au130(SR)50 and Au133(SR)52. In addition, recent experimental discoveries of the major roles of ligands in stabilizing magic sizes and structures also call for theoretical input. In summary, the era of atomically precise nanoparticle research has arrived. Future research on controlling nanoparticles at the single-atom and single-electron levels is expected to significantly advance the fundamental science of nanoscale materials (refs 1, 280, 532, 533, and 648−657), especially in achieving atomically precise structure−property relationships, which will open up new horizons in nanoscience and nanotechnology.

7.11. Doping and Alloying

Determining the doping sites of heterometal atoms will provide fundamental insight into the alloying and intermetallic properties of gold-based nanoclusters. New synthetic routes are being devised.247,435,643 Theoretical work implies the compatibility of magnetic elements such as Fe, Co, and Ni in the cluster structures.610,611 In the bulk state, gold−cobalt can indeed be made, but attempts to alloy nanoclusters have not been successful. New synthetic strategies should be devised. Magnetic doping would be interesting for the investigation of magnetism and evolution, as well as spin coupling.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

7.12. Extension of Atomic Precision to Other Metals

Notes

Gold and silver are particularly appealing in terms of their optical properties; other metals (such as Pt, Pd, Cu, and Ni) are more attractive for catalysis, but stable nanoclusters of the latter metals (e.g., protected by thiolates) remain to be explored.620−622 Some preliminary work on Cu nanocluster synthesis has been reported by Pradeep and co-workers; for example, they obtained a 5.9 kDa species close to Cu38(PET)25.622 This species gave PL centered at 615 nm. Recently, Nguyen et al. reported a Cu25 nanocluster protected by ligands623 with a kernel that is a Cu13 icosahedron and features partial Cu(0) character. Nickel−thiolate nanoclusters have also been explored.624 Extension to carbene ligands also holds great promise for tailoring the functionality of metal nanoclusters.625

The authors declare no competing financial interest. Biographies Rongchao Jin is Professor of Chemistry at Carnegie Mellon University. He received his B.S. in chemical physics from University of Science and Technology of China (USTC, Hefei, China) in 1995; his M.S. in physical chemistry/catalysis in 1998 from Dalian Institute of Chemical Physics, Chinese Academy of Sciences; and his Ph.D. in chemistry from Northwestern University (Evanston, IL) in 2003. He then performed postdoctoral research at the University of Chicago. He joined the chemistry faculty of Carnegie Mellon University in 2006 and was promoted to Associate Professor in 2012 and Full Professor in 2015. His current research interests include atomically precise nanoparticles, optics of nanoparticles, and catalysis.

7.13. Atomically Precise Colloidal CdSe and Magnetic Nanoclusters

Chenjie Zeng is a Ph.D. candidate in chemistry at Carnegie Mellon University. She obtained her B.S. in chemistry from Nankai University (Tianjin, China) in 2011. She works under the supervision of Prof. Jin, and her research is focused on the structure and property evolution of gold nanoclusters.

The fundamentally important issues of colloidal metal nanoparticles (except the emergence of metallic state) discussed in section 1.2 are equally applicable to semiconductor nanocrystals and magnetic nanoparticles. To address such issues, researchers have started to pursue atomically precise CdSe semiconductor nanoclusters, which will allow for the understanding of fundamental issues such as fluorescence blinking, scaling laws, and many other questions.626−629 The properties of quantum dots (QDs) are extremely sensitive to size because of quantum confinement effects. However, the currently available highestquality QDs still have size distributions of ∼5% and exhibit heterogeneous broadening in PL. The pursuit of atomically precise QDs and solving their total structures will lead to deep insight into a number of major issues, such as surface-related PL blinking and spectral jumping, enhancement of photocatalytic

Meng Zhou is a Postdoctoral Research Associate in Prof. Jin’s group. He received his B.S. in physics in 2010 from Northeast Normal University (Changchun, China) and his Ph.D. in physical chemistry in 2015 from the Institute of Chemistry, Chinese Academy of Sciences, Beijing. His research interests include the optical properties and electron dynamics of nanomaterials. Yuxiang Chen is a Ph.D. candidate in Prof. Jin’s group at Carnegie Mellon University. He received his B.S. in chemistry from Nankai University (Tianjin, China). His research efforts focus on the synthesis and catalytic applications of precious metal nanoclusters. 10396


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