Metal-nanocluster science and technology: my personal history and outlook PCCP PERSPECTIVE

Metal nanoclusters (NCs) are one of the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for the past 60 years or so. I have been continuously studying metal NCs since 1995 while changing my own interests, and thereby research topics, according to development of the field and the global situation. Specifically, I started my research on gas-phase metal NCs, and then investigated systematic isolation of glutathionate-protected gold NCs. Thereafter, I worked on development of flexible and precise techniques for controlling ligand-protected metal NCs and creation of highly functional energy and environmental catalysts using ligand-protected metal NCs. This personal account summarizes my previous studies on metal NCs. Through this personal history, I would like to describe my motivation for each previous subject, what type of research I want to perform for each subject in the future, and how I consider the outlook of the field. HOMO–LUMO gap of the Au NCs. 127,128 the chemical composition, structure, and activity and (b) the key factors for the activity and therefore for creation of NCs suitable for activation. This cycle is repeated to create high-performance energy and environmental catalysts.


Introduction
Research on nanoscale materials is generally believed to have begun with a talk by Feynman in 1959 entitled ''There's Plenty of Room at the Bottom''. 1 In this talk, he stated that in the nanoscale region of a few atoms, properties that are completely different from the macroscopic world appear. He also claimed that it is very important for humanity to conduct research in this area, and he predicted that in the future we would be able to arrange atoms one by one as we wish to create desired materials. Later, high-resolution analysis devices, such as scanning tunneling microscopes 2 and atomic force microscopes, 3 were developed, which made it possible to directly observe atoms on a surface. In addition, instruments such as lasers and molecular beams 4,5 have been successively developed, and much knowledge has been gained about the stability and electronic/geometric structure of aggregates consisting of metal atoms in a vacuum, namely, gas-phase metal nanoclusters (NCs). Since 2000, many countries have adopted nanotechnology as a national policy, 6 which has led to dramatic advancement in the technology and knowledge related to these materials, as well as a dramatic increase in the number of researchers involved in research of nanoscale materials. Through this knowledge, it is now possible to synthesize metal NCs consisting of certain metal elements with atomic precision.  In addition, through these studies, it has been experimentally demonstrated that in the nanoscale region, novel electronic/ geometric structures and physicochemical properties different from those of ordinary metals actually emerge. In recent years, a number of studies on creation of new materials that take advantage of the unique properties of such nanoscale materials have been reported. Thus, with regard to nanoscale materials, both elucidation of their properties (science) and development of control techniques (technology) have made great progress in the past 60 years or so. [28][29][30][31][32][33][34][35][36][37][38] I have been continuously studying metal NCs since 1995 (Fig. 1). In this time, I have witnessed dramatic progress of the written several accounts [49][50][51][52][53][54][55][56][57] and reviews 58-66 on each research topic. Therefore, in this personal account, I will describe my personal history in metal NCs. Through this personal history, I will explain what has led me to perform each type of research, what type of research I would like to perform in the future, and what I consider to be the future prospects in the field.

Gas-phase research
Isolation of metal NCs in a vacuum where there is no interaction with the outside world and investigation of their formation distribution, structure, and physical properties can provide an essential and fundamental understanding of the origin of the stability and mechanism of the size-specific structures and properties of metal NCs. Investigation of isolated metal NCs and supported metal NCs in the gas phase has a great advantage in this respect, and it has therefore been carried out for many years, with various improvements in the equipment. [67][68][69][70][71][72][73][74][75][76][77][78][79][80] I conducted research on gas-phase NCs from 1995 to 2000 ( Fig. 1(a)) under the guidance of Professor Kaya and Associate Professor Nakajima. In this research, NCs were produced by laser vaporization 4,5 and their electronic structures were investigated by negative-ion photoelectron spectroscopy ( Fig. 2(a)). 81 This research revealed that in the bulk, group 14 elements are classified as semiconductors for silicon (Si) and germanium (Ge), semi-metals for tin (Sn), and metals for lead (Pb), whereas in the fine size range all NCs have highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps and overall similar electronic structures ( Fig. 2(b)). 40,82,83 In a study of Au-silver (Ag) alloy NCs, we experimentally demonstrated that not only controlling the number of constituent atoms, but also mixing the different elements is very effective for creating a new electronic structure (Fig. 2(c)). 84 Through this research, I confirmed that new materials can be created by controlling the number of constituent atoms and chemical composition of NCs with atomic precision.
However, through this research, I also strongly felt that it is essential to develop a large-scale synthesis method for NCs to create new materials that take advantage of their size-specific properties while understanding their fundamental properties. In addition, at this time, I got the chance to deposit germanium oxide (Ge n O m ) NCs generated in a vacuum on a support and investigate their luminescence properties in air. 85 Because the Ge n O m NCs deposited at that time were originally oxides, their electronic structure did not significantly change even when exposed to air, and they exhibited blue luminescence in air. Through this experiment, I was impressed that it was essential to develop NCs that are stable in air to create NCs that can be used as materials.

Isolation of glutathionate-protected Au NCs
While my interest was shifting from gas-phase experiments to materials chemistry research, in 2000, I got the chance to reported in this paper were incorrect. Therefore, the chemical compositions of the series of Au n (SG) m NCs were corrected to Au 10-12 (SG) [10][11][12] , Au 15 (SG) 13 , Au 18 (SG) 14 , Au 22 (SG) 16 , Au 22 (SG) 17 , Au 25 (SG) 18 , Au 29 (SG) 20 , Au 33 (SG) 22 , and Au 39 (SG) 24 in a paper published in 2005 (Fig. 3(c)). 102 Note that the chemical composition of the stable Au 25 (SR) 18 NC 46,105 was determined for the first time in this work. Determination of the chemical compositions of a series of Au NCs was also achieved for the Au NCs protected by mercaptosulfuric acid (S(SA)) or N-(2-mercaptopropionyl)glycine (S(SG)). 104 Through these studies, we succeeded in demonstrating that Au n (SR) m NCs can be treated as compounds with defined chemical compositions, similar to organic and complex compounds. Research of Au n (SR) m NCs has provided a deep understanding of the correlation between the chemical composition, stability, electronic structure, and luminescence properties of Au n (SR) m NCs. 43,45,[103][104][105][106] 4. Toward development of the ultimate nanotechnology and a deep understanding of ligand-protected metal NCs After reporting Au n (SG) m NCs, I got the opportunity to run my own laboratory at Tokyo University of Science in 2008. Therefore, after 2008, I began new projects. I had the feeling that atomic control of ligand-protected metal NCs would be the ultimate nanotechnology for metal-molecule complex systems. However, the NCs that we could control at my previous institute were limited to small Au n (SR) m NCs (n = 10-39) protected by certain SRs (SG, S(SA), and S(SG)). To develop the ultimate nanotechnology for metal-molecule complex systems and thereby gain a better understanding of the correlation between the chemical composition, structure, and physical properties for ligand-protected metal NCs, it was necessary to develop new precise synthesis methods for (1) Au NCs with a large number of constituent atoms, (2) alloy NCs composed of several metal elements, (3) metal NCs composed of other elements, and (4) metal NCs protected by other ligands. According to the report by Teo et al., 107 certain metal NCs could become the constituent units of ''clusters of clusters''. To create new materials using such connections in the future, it was necessary to also make further progress in (5) the connection techniques of ligand-protected metal NCs.

Isolation of Au-based alloy NCs
Even if only one metal element is used as a constituent element, various electronic structures and physicochemical properties can be obtained by controlling the number of constituent atoms in the metal NCs. However, as mentioned in Section 2, if the size effect is combined with the synergy effect caused by mixing different elements, it is expected that a variety of different electronic structures and physicochemical properties can be increased. Therefore, since 2008, we have been working on isolation of alloy NCs and elucidation of their geometric/electronic structures and physicochemical properties. [110][111][112][113][114][115][116][117] When I started this research in 2008, it was not easy to control the number of constituent atoms, even for metal NCs consisting of a single metal element. Furthermore, when I started my laboratory at Tokyo University of Science, I only had undergraduate students in my laboratory. Therefore, I thought that it was necessary to investigate metal-NC systems in which I had some experience to create new alloy NCs. Accordingly, Au n (SR) m NCs, which I had experience in handling, were selected as the base metal NCs for alloying. 46,47  Specifically, we attempted to isolate alloy NCs in which some of the Au atoms in [Au 25 (SR) 18 ] À and [Au 38 (SR) 24 ] 0 (SR = SC12 or PET) [153][154][155][156][157] were replaced by Pd, Ag, or Cu atoms.
( Fig. 6(g)), 112,158 (5) heteroatom substitution with Pd, Ag, or Cu enhances the speed of ligand-exchange reaction of NCs, 52,159,160 and (6) Pd substitution improves the hydrogen evolution reaction (HER), oxygen evolution reaction, and oxygen reduction reaction (ORR) activities of the NCs. 48 In addition, studies on tri-metal 115 and tetra-metal 116 alloy NCs have revealed that simultaneous substitution with different elements at different substitution sites causes superimposed addition of each substitution effect on the base metal NCs. 116 In these early studies, the substitution sites were predicted based on DFT calculations by Nobusada and/or X-ray absorption fine structure (XAFS) analysis by Tsukuda and Yamazoe. However, since 2016, 128,161 we have also been able to determine the geometric structures of metal NCs by SC-XRD, and such studies have enabled us to (1) create [Au B20 MAg B4 (PET) 18 ] À (M = Pd or Pt), where both the number and position of Ag substitution are controlled ( Fig. 6(h)), 117 and (2) change the Cu substitution site by combining Cu and Pt substitution. 162

Isolation of Ag and Pt NCs
When I started my own laboratory, it was not easy to control the number of constituent atoms, even for Au n (SR) m NCs. Therefore, until 2010, the examples of isolated Ag n (SR) m NCs were limited to Ag 7 (DMSA) 4 (DMSA = meso-2,3-dimercaptosuccinic) reported by Jin and co-workers 163 and Ag 9 (S(SA)) 8 reported by Pradeep and coworkers. 164 Thus, we attempted to isolate larger Ag n (SR) m NCs.
Based on the report of Murray and co-workers, 165 it was expected that using 4-(tert-butyl)benzylmercaptan (SBB) as a ligand would result in synthesis of stable Ag n (SR) m NCs. Therefore, we first synthesized a mixture of Ag n (SR) m NCs and then applied the reaction with thiols, which had been frequently used in isolation of Au n (SR) m NCs, 105,166 to the mixture to convert the unstable species to stable species. As a result, Ag n (SBB) m NCs with an average particle size of 2.1 nm were successfully obtained in a size-selective manner. 118 The obtained stable Ag n (SBB) m NCs were assigned to Ag B280 (SBB) B120 based on the results of ESI-MS. However, in this experiment, which was conducted in 2010 (the paper was published in 2011), the chemical composition was determined by a home-made MS apparatus without high resolution. In recent years, many ESI-MS apparatuses with high sensitivity and resolution have been developed. It is expected that the chemical composition of the stable Ag n (SBB) m NCs obtained in that study will be determined more accurately using such ESI mass spectrometers in the future.
Ag n NCs are attractive because of their high quantum yield of luminescence 167 and high selectivity for carbon dioxide reduction. 168 Since we have also been working on catalytic application of metal NCs, we have continued to work on development of isolation techniques for Ag n NCs also after the report of Ag n (SBB) m NCs. As a result, for example, in 2013, we successfully isolated Ag 44 (SePh) 30 (SePh = phenylselenolate) in collaboration with Pradeep and co-workers ( Fig. 7(a)). 119 DFT calculations by Häkkinen and co-workers showed that Ag 44 (SePh) 30 has a geometric structure similar to that of [Ag 44 (SR) 30 ] 4À (SR = SPhF 2 or 4-mercaptobenzoic acid (p-MBA)), whose geometric structure was determined just before our reports by Zheng and co-workers 169 and Bigioni and coworkers. 170 In addition to this NC, we recently succeeded in isolating several new Ag NCs 171 using anions as templates (the paper is being prepared for submission).
Isolation of Pt n NCs has been reported by Chini and co-workers and Longoni and co-workers. 22,27 In their studies, CO was used as the main ligand, but in later studies on Pt n and its alloy NCs by Dahl, Slovokhotov, Zacchini, etc., PR 3 was also used as a ligand. 23,172,173 In these studies, the geometric structures of the metal NCs were determined by SC-XRD. For Au n (SR) m NCs, in 2007, Kornberg, Ackerson and co-workers reported the geometric structure of [Au 102 (p-MBA) 44 ] 0 , which was the first report of the geometric structure of Au n (SR) m NCs determined by SC-XRD. 174 The advancement of research of Pt n and its alloy NCs at that time was astonishing. However, synthesis and the subsequent handling of the reported Pt n NCs had to be carried out under CO or an inert-gas atmosphere, 23,172,173 which was not easy for those who were not used to using CO gas or Schlenk techniques. Therefore, we searched for Pt n NCs that could be synthesized by mixing reagents in air, similar to Au n (SR) m NCs. When a Pt salt is dissolved in polyol and the resulting solution is heated, the Pt salt is reduced to generate Pt atoms (polyol reduction). [175][176][177][178][179] The polyol generates CO, and the generated CO coordinates to the Pt surface. Addition of PR 3 to the solution of the resulting product is expected to give Pt n (PR 3 ) m (CO) l NCs without the need for a CO atmosphere. Indeed, we successfully isolated [Pt 17 (PPh 3 ) 8 (CO) 12 ] z+ (PPh 3 = triphenylphosphine, z = 1 or 2) with atomic precision by this method (Fig. 7(b)). 120 The obtained NC had a geometric structure similar to previously reported [Pt 17 (PEt 3 ) 8 (CO) 12 ] 0 (PEt 3 = triethylphosphine) 23 and [Pt 13 Au 4 (PPh 3 ) 8 (CO) 12 ] 0 . 172 Optical absorption spectroscopy revealed that [Pt 17 (PPh 3 ) 8 (CO) 12 ] z+ (z = 1 or 2) has a discrete electronic structure similar to a small Au n (SR) m NC ( Fig. 7(c)). 102 For Pt n NCs, we have also succeeded in obtaining a series of Pt n (PET) m (CO) l NCs (n = B35, B51, and B66) in high yields (B80% yield for Pt B66 ) by changing the ligand from PPh 3 to PET ( Fig. 7(d)). 121 Unfortunately, the chemical compositions of these Pt n (PET) m (CO) l NCs cannot be controlled with atomic precision at present. Therefore, the chemical compositions and geometric structures of these NCs have not been accurately determined. Thus, we are now attempting to apply various size-focusing methods 166,180 to Pt n (PET) m (CO) l NCs to isolate the Pt n (PET) m (CO) l NCs with a single chemical composition. If these efforts are successful and the chemical compositions and geometric structures of the isolated Pt n (PET) m (CO) l NCs are accurately determined, it is expected that a deeper understanding of the chemical compositions and geometric structures of the Pt n (PET) m (CO) l NCs that can be isolated will be obtained.

Isolation of metal NCs protected by functional ligands
For ligand-protected metal NCs, it is possible to impart new functions by controlling not only the metal core, but also the functional groups of the ligands. The solvent solubility, redox potential, 181-184 luminescence properties, 185 optical activity, 186 and catalytic activity of Au n (SR) m NCs are remarkably dependent on the functional group structure of SR. If Au n (SR) m NCs with these characteristics are protected by SR, which changes its molecular structure in response to light irradiation, it is expected that the physicochemical properties of Au n (SR) m NCs can be reversibly changed by light irradiation.
Azobenzene is a typical photoresponsive molecule. 187,188 It changes from trans to cis by ultraviolet (UV)-light irradiation and from cis to trans by visible-light irradiation. When SR containing azobenzene is used as a protective ligand, Au n (SR) m NCs, whose physicochemical properties repeatedly change by light irradiation, could be created. Accordingly, we have synthesized Au 25 (S(Az)) 18 protected by an azobenzene derivative thiolate (S(Az)) ( Fig. 8(a)) to create such photoresponsive Au n (SR) m NCs. 122 When the obtained Au 25 (S(Az)) 18 was irradiated with light, S(Az) repeatedly underwent photoisomerization ( Fig. 8(b)). In addition, the redox potential of Au 25 (S(Az)) 18 reversibly changed with such structural changes ( Fig. 8(c)). These results demonstrated that the combination of azobenzene and Au 25 NCs can be used to create Au n (SR) m NCs with photoresponsive redox potentials.
Au 25 (SR) 18 has a compact and symmetric metal core ( Fig. 8(a)). 153,154 Therefore, the metal-core structure of Au 25 (S(Az)) 18 remains almost unchanged even after isomerization of the ligand by photoirradiation. The change in the redox potential is considered to be because of the change in the dipole moment of the S(Az)  molecule caused by isomerization of azobenzene, which in turn changes the orbital energy of Au 25 (S(Az)) 18 . 189,190 In 2016, Jin and co-workers reported that [Au 28 (SR) 20 ] 0 can change the metal-core structure depending on the functional group structure of the ligand. 191 Therefore, in the future, we will combine S(Az) with a Au n (SR) m NC whose metal core was easily deformable, 192 and thereby create ''intelligent'' Au n (SR) m NCs in which the metal-core structure changes by light irradiation, leading to significant changes in the redox potential, luminescence properties, optical activity, and catalytic activity.
In the above study of Au 25 (S(Az)) 18 , a new function was provided to the Au n NCs by including a function in a functional group that was not directly bound to Au. However, changing the functional group directly bound to Au (i.e., S) to Se is expected to change the binding mode between Au and the ligand, 193 thereby providing Au n NCs with new physical and chemical properties. With this expectation, we precisely synthesized [Au 25 (SeR) 18 ] À (SeR = octaneselenolate (SeC8) or dodecaneselenolate (SeC12)), 123,124 [Au 25 (PET) 18Ày (SePh) y ] À (y = 1-18), 128 [Au 25Àx Cu x (SeC8) 18 ] À (x = 1-9), 125 and [Au 38 (SeC12) 24 ] À 126 with selenolate (SeR) ligands. Studies on the obtained Au NCs revealed that changing the ligand from SR to SeR (1) changes the binding mode between Au and the ligand, thereby making the Au NCs more stable, 124 and (2) has the effect of narrowing the HOMO-LUMO gap of the Au NCs ( Fig. 8(d)). 124,126,128 We have also systematically synthesized [Au 25 (SC8) 18Ày (TePh) y ] À (TePh = phenyltellurolate, y = 1-18) containing a tellurolate (TeR) in the ligand layer. 127,128 We found that changing the ligand from SR to TeR further narrows the HOMO-LUMO gap of the Au NCs. 127,128 Based on a study of two-dimensional self-assembled monolayers, 194 changing the ligand from SR to SeR or TeR is expected to induce an increase in the conductivity between the Au core and the ligand. High conductivity is a desired feature in creation of single-molecule devices based on Au NCs, 195 and we hope to clarify this point in future studies by comparing the conductivities of Au n (SR) m , Au n (SeR) m , and Au n (TeR) m NCs.

Connection of ligand-protected metal NCs
Among ligand-protected metal NCs, icosahedral NCs have been reported to become constituent units of ''clusters of clusters''. 196 For example, in the 1990s, Teo and co-workers reported synthesis of [(p-tol 3 P) 10 199 Further development of techniques for connecting metal NCs could lead to fabrication of ultra-thin metal nanowires. Therefore, we have also been working on development of techniques to connect metal NCs. The first connected structure that was successfully isolated was [Au 24 Pd(PPh 3 ) 10 (PET) 5 Cl 2 ] + ( Fig. 9(a)). 129 This connected structure was size-selectively synthesized under appropriate conditions. In this connected structure, only one central Au atom is substituted by a different atom, unlike most of the connected structures reported in previous papers. [200][201][202][203] DFT calculations by Jiang and co-workers showed that [Au 24 Pd(PPh 3 ) 10 (PET) 5 Cl 2 ] + NCs with these hetero bi-icosahedral cores have dipole moments in the NC. 129 Very recently, we also succeeded in synthesizing [Ag 23 Pd 2 (PPh 3 ) 10 Cl 7 ] 0 consisting of two Ag 12 Pd cores ( Fig. 9(b)). 131 Stability experiments and DFT calculations by Iwasa and co-workers indicated that it is very important to strengthen the framework of individual icosahedral cores and increase their stability to form the connected structures described by the chemical formula [Ag 23 M 2 (PPh 3 ) 10 Cl 7 ] z (M = Ag, Pd, or Pt, z = 2+ or 0). 131 In the above study, two metal NCs were connected. On the other hand, in the study on [Au 4 Pt 2 (SR) 8 ] 0 (SR = PET ( Fig. 10(a)) or 2-chlorobenzylmercaptane (SCH 2 PhCl)), we successfully connected them continuously in one dimension. 130 This was the second report of formation of a one-dimensional connected structure, following that reported by Maran and co-workers for [Au 24 M(SR) 18 ] 0 (M = Au, Hg, or Cd, SR = butanethiolate or   8 ] 0 to form one-dimensional connected structures and control their geometry ( Fig. 10(b)). 130 The obtained one-dimensional connected structures have a semiconductor band gap, and the band gap size can be modulated by the ligand structure. These findings are expected to provide clear design guidelines for creation of metal-NC connected structures with desired geometric structures and physicochemical properties.
We are now also working on connection of these metal NCs via linker molecules. 62,208-213

Development of high-resolution separation methods
Most of the ligand-protected metal NCs and their connected structures discussed in Sections 4.2-4.6 can be isolated by only mixing the reagents and washing the by-products in the ambient atmosphere. However, all NCs cannot be necessary isolated by such a simple process in the case of Au n (SC12) m NCs 109 described in Section 4.2. Therefore, it was necessary to develop new high-resolution separation techniques for ligand-protected metal NCs to isolate the metal NCs precisely and systematically, and thereby to understand the correlation between the chemical composition and the structure/properties. PAGE ( Fig. 3(a)), which is described in Section 3, is suitable for high-resolution separation of hydrophilic compounds, but it is not so suitable for high-resolution separation of hydrophobic compounds. On the other hand, HPLC ( Fig. 11(a)) is suitable for separation of both hydrophilic and hydrophobic compounds. Indeed, in 2003, Murray and co-workers succeeded in isolating   Au NCs, such as Au 38 , Au 140 , and Au 225 (the chemical composition is reported one in the papers), from a mixture of Au n (SC6) m NCs (SC6 = hexanethiolate) by RP-HPLC. 144,214,215 Inspired by the work of Murray and co-workers, we attempted to separate metal NCs, which were difficult to isolate by only washing the by-products, at the atomic and molecular levels by advanced use of RP-HPLC. 53 As a result, in addition to separation depending on the number of constituent atoms (Fig. 11(b)), 108,109 ligand-protected metal NCs were also successfully separated depending on the charge state (Fig. 11(c)), 110 alloy composition (Fig. 11(d)), 132,138 ligand combination (Fig. 11(e)), 133,134 and structural isomer (Fig. 11(f and g)). 132,137 The separation techniques described in Fig. 11(f and g) led to not only control of the structural isomers, but also elucidation of the reaction mechanisms of the metal-exchange 138 and ligand-exchange 137 reactions. Although these high-resolution separations were achieved for hydrophobic metal NCs, in later studies, we also succeeded in separating hydrophilic metal NCs depending on their constituent atoms with high resolution by ion-pair chromatography 135 or hydrophilic-interaction chromatography. 136 In the future, we would like to use two-dimensional HPLC 216 for such high-resolution separations. It allows the use of two different separation modes. We expect that this will make it possible, for example, to first separate the products generated by ligand exchange depending on the number of exchanged ligands, 134 and then to separate the products of the same chemical composition depending on the optical isomers 217 generated by the position of the exchanged ligands.

Summary of control of ligand-protected metal NCs
We have succeeded in developing several new techniques for (1) isolation of large Au NCs, (2) isolation of alloy NCs, (3) isolation of metal NCs consisting of Ag or Pt, (4) isolation of metal NCs protected by functional ligands, (5) connection of metal NCs, and (6) high-resolution separation of metal NCs. Other groups have also been simultaneously working in these areas, and they have developed several other isolation methods. 7,218 Through these studies, the control techniques for ligandprotected metal NCs, that is, ''bottom-up nanotechnology for complexes composed of metals and molecules'', has been dramatically enhanced in recent years. In the future, it is expected that the ''science of ligand-protected metal NCs'' will be further deepened by further development of these control techniques.

Toward creation of high-performance energy and environmental catalysts
When I started investigating gas-phase NCs in 1995, I was not concerned about the applications of metal NCs as materials. However, after I had deposited Ge n O m NCs on a support and observed their luminescence in air with my own eyes, I thought to contribute to society by applying metal NCs as materials in future studies. Because energy and environmental issues have become major global challenges, I decided to contribute to the field of energy and environmental materials.

Social problems to be solved by chemistry
In the 21st century, there are serious issues regarding energy and the environment, such as depletion of fossil fuels and destruction of the environment on a global scale. Therefore, chemistry in the 21st century is expected to address these issues, and thereby build a clean and sustainable society. The energy conversion system shown in Fig. 12 is one of the systems Fig. 12 Schematic of the energy-conversion system for constructing a sustainable society. Note that in addition to solar power, sunlight also produces wind, biomass, and hydro power. Reproduced with permission from ref. 56 necessary to build such a society. In this system, hydrogen (H 2 ) is produced by photocatalysts or electrocatalysts, and the obtained H 2 is converted to electricity by fuel cells. Using such a system, it is possible to obtain energy from only water and sunlight, which are almost unlimited on earth. In addition, this system allows the energy medium (H 2 ) to be circulated, thus preventing the issues of energy depletion. Furthermore, this system does not generate carbon dioxide, which leads to global warming. In this system, the properties of the photocatalyst, 219 electrocatalyst, 63,220-232 and fuel cell 60,224,[233][234][235][236][237][238] are highly dependent on the chemistry. Therefore, modern chemistry is expected to improve the functions of these components. Considering the fact that the shift to H 2 society cannot be realized immediately, it is necessary to improve the current system at the same time. In other words, automotive exhaust-gas purification catalysts, 239 which inhibit emission of harmful gases, need to be further improved. Therefore, we have been working on development of highly functionalized water-splitting photocatalysts, fuel-cell electrocatalysts, and automotive exhaust-gas purification catalysts ( Fig. 1(d and e)) using precise metal NCs, in addition to developing flexible and precise control techniques for metal NCs.

Our strategy for activation of energy and environmental catalysts
For preparation of water-splitting photocatalysts, fuel-cell electrocatalysts, and automotive exhaust-gas purification catalysts, metal nanoparticles (NPs) are generally loaded on the support by impregnation (Fig. 13(a)) 240 or photodeposition method ( Fig. 13(b)). 241 In the impregnation method, metal NPs are immobilized on the support by stirring the support and metal salt in solution followed by calcination ( Fig. 13(a)). In the photodeposition method, the support is dispersed in a solution containing the precursor metal salts of the metal NPs, which is then irradiated with light. The electrons and holes generated by photoexcitation are used to reduce or oxidize the precursor metal salts of the metal NPs, thereby immobilizing the metal NPs on the surface of the support (Fig. 13(b)). These loading methods are widely used because of their simplicity. However, the size distributions of the metal NPs obtained by these methods are wide because the metal atoms are aggregated on the support. Therefore, it is extremely difficult to load precisely controlled metal NPs on the support by these methods. On the other hand it is possible to load metal NCs with a controlled particle size on the support by adsorbing ligandprotected metal NCs synthesized in the liquid phase onto the support and then removing the ligand by calcination (Fig. 13(c)). [242][243][244] If the chemical composition of the loaded metal NCs can be controlled with atomic precision, it could be possible to create high-performance heterogeneous catalysts based on selective loading of highly active metal NCs. 66 For atomically controlled loaded metal NCs, a deep understanding of the electronic/geometric structure of the loaded metal NCs and the electronic/geometric interaction between the loaded-metal NCs and the catalyst surface can be obtained by various high-resolution analyses and theoretical calculations ( Fig. 14(a)). 11,[245][246][247][248][249][250][251][252][253][254][255] It is expected that this will provide a deep understanding of the important factors for high activation and clear design guidelines for further activation (Fig. 14(b)). Design and creation of appropriate metal NCs based on the obtained information are expected to lead to further functionalization of water-splitting photocatalysts, fuel-cell electrocatalysts, and automotive exhaust-gas purifying catalysts (Fig. 14). However, when we started this work, there were no examples of atomically precise control of loaded metal NCs on these catalysts using ligand-protected metal NCs as precursors.

Activation of water-splitting photocatalysts
Using photocatalytic reactions (Fig. 12), [256][257][258][259][260][261][262][263][264][265][266][267] it is possible to produce H 2 directly from water and sunlight. Therefore, watersplitting photocatalysis is considered to be one of the cleanest energy production reactions. However, at present, the energy conversion efficiency from sunlight to H 2 (STH) is only about 1.1%. 268 To realize practical use of water-splitting photocatalysts, it is essential to improve the STH to about 5-10% in the future. 264 In many cases, metal/metal-oxide NPs/NCs (cocatalysts) should be loaded on water-splitting photocatalysts as the active sites (Fig. 12). Controlling their particle sizes is very effective for improving the water-splitting activity. 64,258,269 Therefore, we have focused on (1) precisely controlling the loaded metal NCs on the photocatalyst using precise ligand-protected metal NCs as precursors, (2) elucidating the details of how control of the cocatalyst affects the water-splitting activity, and (3) actually improving the functionality of the water-splitting photocatalyst.
The first step was to precisely load Au n NCs onto BaLa 4 -Ti 4 O 15 (an UV-light-driven water-splitting photocatalyst), 270 which is one of the most advanced photocatalysts developed by our collaborator, Kudo and co-workers. 256 We successfully determined the appropriate precursor NCs for precise loading ( Fig. 15(a)), the appropriate adsorption conditions, and the appropriate calcination conditions. 271,272 Activity measurement ( Fig. 15(b)) of the obtained Au 25 NC-loaded BaLa 4 Ti 4 O 15 (Au 25 / BaLa 4 Ti 4 O 15 ) revealed that the small Au cocatalyst accelerates not only the HER, but also the oxygen (O 2 )-photoreduction reaction, which is one of the reverse reactions. 273 Therefore, we next attempted to form a layer to prevent the reverse reaction on the Au 25 cocatalyst. Maeda, Domen and co-workers reported that formation of a chromium oxide (Cr 2 O 3 ) layer is extremely effective for preventing the reverse reaction. 244,[274][275][276][277][278][279][280][281][282][283] However, using their formation method, it was difficult to form the Cr 2 O 3 layer on the Au 25 cocatalyst while maintaining the number of constituent atoms of the cocatalyst. Therefore, we developed a new method for formation of the Cr 2 O 3 layer. 273 18 and embed Au 25 into the Cr 2 O 3 layer (Fig. 15(c)). The obtained Cr 2 O 3 / Au 25 /BaLa 4 Ti 4 O 15 photocatalyst showed high water-splitting activity. Thus, we succeeded in creating a highly active watersplitting photocatalyst with the advantage of high HER activity of fine Au NCs by combining miniaturization of the Au cocatalyst with formation of the reverse-reaction prevention layer (Fig. 15(c)). In a later study using Au 25 as the cocatalyst, it was found that even higher activity can be obtained by substituting one Pt atom for one Au atom in the Au 25 cocatalyst. 284 24 Pt is more negatively charged than Au in Au 25 (Fig. 15(d)).
In the above study, Au was used as the metal element for the cocatalyst. However, based on the volcano plot 285 for H 2 adsorption and desorption, rhodium (Rh) is predicted to have higher catalytic activity than Au for the HER. Therefore, it is expected that highly active water-splitting photocatalysts can be created by loading a fine cocatalyst consisting of rhodium and chromium oxides (Rh 2Àx Cr x O 3 ) 286 (Fig. 16(a  and b)). 290 The obtained photocatalyst showed the highest apparent quantum yield of 16% for BaLa 4 Ti 4 O 15 (excitation wavelength = 270 nm, Fig. 16(c)). The loading method of Rh 2Àx Cr x O 3 NCs developed in this study can in principle be applied to other photocatalysts. In addition, Rh 2Àx Cr x O 3 NCs are useful cocatalysts for many water-splitting photocatalysts. [286][287][288][289]291,292 In the future, it is expected that high quantum yields will be obtained for many other water-splitting photocatalysts using this technique.
For practical use of a water-splitting photocatalyst, it is essential that the photocatalyst causes water splitting by visible light (visible-light-driven water-splitting photocatalyst), which accounts for about 40% of solar energy. Currently, there are only a few semiconductor photocatalysts that enable complete water splitting in one step using visible light. 264 However, overall water splitting by visible light can also be achieved by photocatalytic systems using a two-step reaction called the Z-scheme, which mimics plant photosynthesis. 257,260 We are currently working on improving the functionality of both of these visible-light-driven water-splitting photocatalysts based on the knowledge obtained in research of BaLa 4 Ti 4 O 15 .

Activation of ORR electrode catalysts
The polymer electrolyte fuel cell (PEFC, Fig. 12) is already in practical use. However, a significant amount of Pt is used in current PEFCs, which makes the current PEFCs extremely expensive. This has impeded widespread use of PEFCs, and thereby transition to a society using H 2 as an energy source (Fig. 12) has stagnated. Even if the cost of PEFCs can be lowered by improving the parts other than the Pt catalyst, we will inevitably encounter a shortage of Pt if the current Pt catalyst is used. Therefore, it is essential to reduce the amount of Pt used in PEFC electrocatalysts to build a clean and sustainable society.
In PEFCs, the ORR at the cathode is the rate-limiting reaction. Pt NPs with particle sizes of 2-3 nm loaded on carbon black (CB) (Pt NPs/CB) are widely used ORR cathodes. Recently, the following facts were reported by Yamamoto and coworkers, 293,294 Nakajima and co-workers, 295 and Fischer and co-workers 296 for smaller B1 nm particle size Pt n NCs: (1) the Pt n NCs have higher mass activity than the currently used Pt NPs and (2) the mass activity of the Pt n NCs dramatically varies depending on the number of constituent atoms. Therefore, it is expected that if highly active B1 nm Pt n NCs can be sizeselectively synthesized and then loaded on CB, highly active ORR catalysts can be created, which will lead to cost reduction of PEFCs.
As described in Section 4.4, we have already developed a method to isolate [Pt 17 (PPh 3 ) 8 (CO) 12 ] z+ (z = 1 or 2) with atomic precision by simple manipulation in air. 120 In addition, by further modification of this method, we successfully synthesized a series of Pt n (PET) m (CO) l NCs (n = B35, B51, and B66) in a size-selective manner with a narrow particle-size distribution. 121 Pt n /CB catalysts were then prepared by adsorbing these Pt n NCs on CB and partially removing the ligands at appropriate temperatures ( Fig. 17(a and b)). We found that the mass activities of Pt n /CB (n = B35, B51, and B66) were higher than that of commercial Pt NPs/CB (Fig. 17(c)). 121 Among the Pt n /CB catalysts, Pt B51 /CB showed 2.1 times higher mass activity than the commercial Pt NPs/CB catalyst. On the basis of estimation of the electrochemical activity area and specific activity, we concluded that Pt n /CB shows higher ORR mass activity than commercial Pt NPs/CB because both the percentage of surface Pt atoms and specific activity of each surface Pt atom are higher for Pt n /CB than for commercial Pt NPs/CB. Regarding the ORR activity of Pt NCs, it has been reported that the presence of Pt atoms in the generalized coordination number (GCN) region of 7.7-8.3 on the surface of the Pt NCs induces high activity. 297,298 The GCN is a coordination number that takes into account the second nearest neighbor atoms in addition to the nearest neighbor atoms. When surface atoms are removed and concave cavities are created on the Pt-NC surface, Pt atoms with such GCNs are generated on the surface of the Pt NC. Unfortunately, in our previous studies, we did not obtain detailed information about the geometric structures of Pt n /CB (n = B35, B51, and B66). Therefore, we are currently working on elucidating the details of their geometric structures by obtaining high-resolution high-angle scattering dark-field scanning transmission electron microscopy (HAADF-STEM) images and analyzing the GCNs in the obtained geometric structures by DFT calculations. This is expected to provide a deep understanding of the factors that lead to high activity and clear design guidelines for further enhancement of the activity.

Activation of automobile exhaust-gas purification catalysts
It is expected that electric vehicles and fuel-cell vehicles will be widely used in the future. However, conventional vehicles (gasoline and diesel vehicles) are also estimated to account for 30-50% of the total market until 2030. Pt is widely used as an exhaust-gas purification catalyst, along with Pd and Rh, because of its catalytic activity to purify the harmful substances contained in exhaust gases. 239 However, reduction of Pt use is a critical issue at present because Pt is a rare and expensive precious metal. In addition, with the widespread use of automobiles, such as hybrid vehicles, where the engine is repeatedly stopped and restarted, high activation at low temperatures is also an issue that needs to be overcome. 239 Studies on Pt-free catalysts are now being performed to overcome the above issues. However, the activity and durability of Pt are overwhelmingly superior to those of non-metallic materials. Reducing the particle size of the Pt catalyst is an extremely effective way to both reduce the amount of Pt used and improve the functionality of the Pt catalyst. Indeed, Anderson and co-workers demonstrated that Pt n NCs (n = 14-18) loaded on alumina (Pt n /Al 2 O 3 ) have high catalytic activity for CO oxidation, one of the automotive exhaust-gas purification reactions, using a vacuum apparatus. 70 We have prepared Pt 17 /g-Al 2 O 3 catalysts by adsorbing [Pt 17 (PPh 3 ) 8 (CO) 12 ] z+ (z = 1 or 2) 120 on g-Al 2 O 3 and then partially removing the ligands at appropriate temperatures. 299 HAADF-STEM images and XAFS measurements showed that Pt does not become oxidized 300 in Pt 17 /g-Al 2 O 3 , and it maintains the skeletal structure of Pt 17 NC (Fig. 18(a-c)). To evaluate the catalytic performance of the obtained catalysts under conditions as close as possible to those of actual vehicles, Pt 17 /g-Al 2 O 3 was coated on a honeycomb substrate and the catalytic performance for CO or propylene (C 3 H 6 ) oxidation was evaluated under these conditions (Fig. 18(d)). The measurements of the HAADF-STEM images and catalytic activity were performed in collaboration with Nagaoka of Johnson Matthey Japan. 299 We found that Pt 17 / g-Al 2 O 3 has higher CO oxidation activity at each temperature than Pt NPs/g-Al 2 O 3 loaded with Pt NPs with average particle size of 3.1 AE 3.1 nm by the impregnation method, and, therefore, it is capable of purifying CO at lower temperature than Pt NPs/ g-Al 2 O 3 (Fig. 18(e)). Previous studies have suggested that in oxidation of CO, CO and O 2 are activated at Pt on the terrace and Pt in the step, respectively. 72,301,302 In the structures shown in Fig. 18(b and c), most of the terrace Pt atoms are located near the Pt step. This seems to be one of the factors that leads to effective reaction of CO and O 2 , that is, oxidation of CO, over Pt 17 /g-Al 2 O 3 . High oxidation capacity of Pt 17 /g-Al 2 O 3 at low temperatures was also observed for oxidation of C 3 H 6 . These results indicate that Pt 17 /g-Al 2 O 3 is a promising catalyst for reducing the amount of Pt used and improving the catalytic performance at low temperatures.
To use Pt 17 /g-Al 2 O 3 as a practical catalyst, it is necessary to investigate the catalytic activity and durability of the catalyst with the loading amount in the actual vehicle and at the exhaust-gas mixture ratio 302-305 during actual operation. We are currently performing measurements under such conditions in collaboration with industry.

Elucidation of the ligand-removal mechanism
In our catalyst preparation process (Fig. 13(c)), (1) metal NCs are precisely synthesized using ligands and then (2) the resulting NCs are adsorbed on a support. The presence of the ligand on the surface of the metal NCs generally leads to a decrease in the activity 306,307 because it prevents the approach of the reactants to the metal-NC surface and induces modulation of the electronic structure of the metal NCs. Therefore, (3) some or all of the ligands are removed by calcination to produce high-activity metal NCs. However, removal of the ligands simultaneously induces aggregation of the metal NCs and a resulting decrease in the catalytic activity. Thus, in calcination, it is extremely important to remove only the ligands while maintaining the number of constituent atoms of the metal NCs ( Fig. 19(a)).
Regarding removal of the ligands, in the studies reported in Section 5.3-5.5, the calcination conditions were determined by confirming the presence or absence of ligands in the catalysts after calcination by X-ray photoelectron spectroscopy (XPS) and XAFS. However, through these studies, we felt that it was essential to understand the reaction mechanisms occurring during calcination to perform calcination under appropriate conditions and thereby create high-performance heterogeneous catalysts. Therefore, we recently worked on elucidation of the reaction mechanism during ligand calcination. 308 Specifically, the ligand desorption process in Au 25 (PET) B9 (p-MBA) B9 /BaLa 4 Ti 4 O 15 during calcination was followed by five techniques: direct insertion probe-mass spectrometry, XAFS, Fourier-transform infrared spectroscopy, XPS, and TEM observation. As a result, we succeeded in elucidating that the ligand desorption process consists of dissociation of the ligand on the metal-NC surface, adsorption of the generated compound on the support, and desorption of the compound from the support, and at what temperatures these processes occur ( Fig. 19(b)). Based on the obtained knowledge, we also succeeded in developing a method to form the Cr 2 O 3 shell on the surface of Au NCs while preventing   their aggregation, and thereby obtained a highly active and stable water-splitting photocatalyst. 308 The desorption temperature of SR from the Au-NC surface is strongly related to the interactions between the ligands on the Au-NC surface, as well as the strengths of the Au-S and S-C bonds. 308 In addition, the temperature for desorption from the support is related to the strength of the compound-support interaction. 309 Furthermore, the easiness of dissociation/ desorption of the ligands and the resulting aggregation of the Au NCs slightly varies depending on the calcination atmosphere. 309,310 However, previous calcinations with different SR functional groups, supports, and atmospheres have often suggested occurrence of the phenomena in Fig. 19(b). 243,271,309,[311][312][313][314] Therefore, it is assumed that a phenomenon similar to Fig. 19(b) also occurs in calcination of the other Au n (SR) m NCs/metal oxides, although the temperature at which it occurs is different. There had not been a unified view on the behavior in Au n (SR) m NCs/ metal oxides during calcination before this study. 312,315 We succeeded in elucidating the details of the phenomena occurring during calcination of Au 25 (PET) B9 (p-MBA) B9 /metal-oxide catalysts by combining five measurements. The knowledge gained in this study is expected to provide clear design guidelines for creation of high-performance heterogeneous catalysts.

Summary of activation of energy and environmental catalysts using controlled metal NCs
We succeeded in precisely controlling the metal NCs on the support with atomic precision using precise ligand-protected metal NCs as precursors. Studies of the obtained energy and environmental catalysts revealed the correlation between the number of constituent atoms/chemical composition of the loaded metal NCs and the material function of each catalyst. Through these studies, each catalyst has been successfully highly functionalized. Specifically, we have achieved producing the highest water-splitting activity for BaLa 4 Ti 4 O 15 , 290 one of the most advanced water-splitting photocatalyst, creating electrode catalysts with higher ORR activity than the Pt catalysts currently used in fuel cells 121 and improving the functionality of automotive exhaust-gas purification catalysts. 299 These results strongly suggest that the bottom-up nanotechnology established in the metal-NC field can contribute to construction of a clean and sustainable society, that is, provide solutions for the issues faced by modern society.
For these catalytic applications, in the future, we will work on alloying 316 with various metal elements for the metal NC contained in each catalyst. In addition, the geometric and electronic structures of the loaded metal NCs during the reaction 317 will be revealed to gain a better understanding of the structure-property relationship of the loaded metal NCs and provide design guidelines for further enhancement of their activity. We believe that these catalysts could be further enhanced by repeating the cycle shown in Fig. 14 while incorporating the obtained knowledge. Regarding these applied studies, we would also like to continue our study while deepening both the science and technology of metal NCs.

Conclusions
Nanoscale materials have the following interesting and important features: (1) they possess great potential and value that cannot be found in bulk materials and (2) the nanoscale is the level of emergence of material functions, and thus if materials can be controlled at the nanoscale, we can control the properties of the larger materials. In this paper, I have described the contents of and motivation for my own studies in this field. The knowledge and skills required to study metal NCs are likely to change in the future, depending on the stage of the study and the situation in the world. However, the importance of understanding and controlling metal NCs will not change because they are deeply related to understanding and controlling various materials in the fields of energy, environment, and materials. I hope that the science and technology in the metal-NC field will be further advanced in the future through collaboration among researchers in many fields across physics, chemistry, and biology.

Outlook
The studies that we will perform in the future or are currently being performed have already been described at the end of each section. Therefore, in this section, I will describe the studies to be performed in the future for further development of the field of metal NCs (Fig. 20).

Elucidation of the synthesis mechanism
Recent work by Xie and co-workers has revealed the details of the reaction mechanism for synthesis of hydrophilic SR-protected metal NCs. 318,319 In addition, recent studies by Pradeep and co-workers, [320][321][322] Bürgi and co-workers, 323 and Pei and co-workers 324 have advanced our understanding of the reaction mechanism for synthesis of ligand-protected alloy NCs through metal exchange. However, the mechanism for synthesis of hydrophobic ligand-protected metal NCs in a two-phase system, which was first reported by Brust and co-workers, 95 has yet to be elucidated at the atomic and molecular levels. Similarly, ligandexchange-induced structure transformation 325 is not yet understood at the atomic or molecular level with respect to the chemical composition that induces the structural transformation. In the future, it is expected that high-resolution separation, mass spectrometry, ion-mobility experiments, and theoretical calculations will be used to clarify the details of reactions whose mechanisms are currently unknown (Fig. 20(a)), which will make it possible to further control synthesis of metal NCs.

Preparation of a textbook for students
For ligand-protected metal NCs, there was not a good textbook until 2014. 10 However, taking into account the number of syntheses, reactions, and applications reported so far, it seems that metal-NC chemistry has reached a stage where many textbooks for undergraduate and graduate students should be published, like for organic chemistry 326 and coordination chemistry. 327 Publication of such a textbook would dramatically increase the number of students and researchers working on metal-NC research, thereby further deepening and expanding the field of metal NCs. Publication of a textbook that includes the latest information on metal NCs is an urgent issue for creation of new materials with size-specific properties and thereby solving many issues in the world (Fig. 20(b)).

Development of super-atomic chemistry
Metal NCs, which consist of metal atoms such as Au and Ag, become stable when the total number of valence electrons fills a closed-shell electronic structure. Because this phenomenon is similar to that for atoms, metal NCs can be considered to be super-atoms. 328 Super-atoms are expected to be the constituent units of a variety of functional materials, like conventional atoms. The use of super-atoms as constituent units is expected to lead to creation of materials that are difficult to create with conventional atoms. Several researchers have been actively working on super-atomic metal NCs, in the fields of both gasphase metal NCs 67,329 and ligand-protected metal NCs. 330,331 However, more researchers need to be involved in this field to enrich the library of super-atomic metal NCs, just like not all conventional atoms were discovered by only a few researchers. It is expected that the number of researchers working on superatomic metal NCs will increase in the future, which will lead to completion of a three-dimensional periodic table (Fig. 20(c)). 330

Control of connected structures
Although studies of applications of ligand-protected metals NCs have been performed, at present, their main applications are limited to catalysts and chemical sensors in the dispersed state. If ligand-protected metal NCs could be connected in a regular pattern, they could be applied to designable electronic and luminescent devices and reaction-selective highly active catalysts. Connection techniques of ligand-protected metal NCs would also be indispensable for creation of materials with super-atomic metal NCs (Section 7.3) as a constituent unit. However, at present, there are few examples of direct connection of ligand-protected metal NCs through metal-metal bonds. Since 2017, connection of metal NCs through linker molecules has also been vigorously studied by several groups, including Zang and co-workers. 208 However, there have been few reports on connected structures, except for Ag NCs. In the future, it is expected that techniques to connect various ligand-protected metal NCs (synthesis of connected structures) will be developed, which will lead to creation of more diverse functions and expansion of the application fields ( Fig. 20(d)).
7.5. Control of supported metal NCs without calcination at high temperature As described in Section 5, it has recently become possible to control the number of constituent atoms and chemical composition of the loaded metal NCs on the support using ligand-protected metal NCs as a precursor. 332 However, the method involves removal of the ligands by calcination at high temperature, which wastes energy. This waste of energy might become a hindrance to industrial application of metal NCs. Therefore, it is expected that a new method to control the number of constituent atoms and chemical composition of the loaded metal NCs on the support will be  developed in the future (Fig. 20(e)). I expect that such control can be realized if a size-convergence method can be found for loaded metal NCs, similar to the case of ligand-protected metal NCs dispersed in solution. In other words, it is expected that the number of constituent atoms and chemical composition of the loaded metal NCs can be controlled by combining the conventional loading method and size-convergence method. If such a method can be developed, it might also be possible to control the geometric structure of the loaded metal NCs. Although control of ligandprotected metal NCs dispersed in solution was once thought to be extremely difficult to achieve with atomic precision, many control methods have been developed. 7 I hope that control of the number of constituent atoms and chemical composition of the loaded metal NCs on the support by an easier method will also be achieved through the enthusiasm of the readers of this paper.

Collaboration between academia and industry
The researchers in the field of metal NCs have discovered potential applications of metal NCs in various fields, as well as new facts hidden in nanoscale materials. For example, the applications described in Section 5 are such examples. Collaboration between industry and academia seems to be essential for successfully connecting the scientific discoveries found in these academic fields to development of industry. It is not necessary for all researchers in the academic field to collaborate with industry because the challenges that need to be overcome in the academic field and in industry are probably different. Thus, I would like to encourage young researchers to continue to make great effort to explore the new science hidden in the metal-NC field. However, the field of metal NCs is important in terms of both science and technology, and the techniques established in the field of metal NCs also have the potential to contribute to construction of the next-generation society (Section 5). For this reason, I urge researchers who already have the seeds of technology to also engage in industryuniversity collaborations so that the knowledge and techniques discovered in the field of metal NCs can be successfully linked to development of industry and society ( Fig. 20(f)).

Conflicts of interest
There are no conflicts to declare.