Matrix Sputtering Method: A Novel Physical Approach for

Article Cite This: Acc. Chem. Res. 2017, 50, 2986−2995

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Matrix Sputtering Method: A Novel Physical Approach for Photoluminescent Noble Metal Nanoclusters Yohei Ishida, Ryan D. Corpuz, and Tetsu Yonezawa* Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan

CONSPECTUS: Noble metal nanoclusters are believed to be the transition between single metal atoms, which show distinct optical properties, and metal nanoparticles, which show characteristic plasmon absorbance. The interesting properties of these materials emerge when the particle size is well below 2 nm, such as photoluminescence, which has potential application particularly in biomedical fields. These photoluminescent ultrasmall nanoclusters are typically produced by chemical reduction, which limits their practical application because of the inherent toxicity of the reagents used in this method. Thus, alternative strategies are sought, particularly in terms of physical approaches, which are known as “greener alternatives,” to produce highpurity materials at high yields. Thus, a new approach using the sputtering technique was developed. This method was initially used to produce thin films using solid substrates; now it can be applied even with liquid substrates such as ionic liquids or polyethylene glycol as long as these liquids have a low vapor pressure. This revolutionary development has opened up new areas of research, particularly for the synthesis of colloidal nanoparticles with dimensions below 10 nm. We are among the first to apply the sputtering technique to the physical synthesis of photoluminescent noble metal nanoclusters. Although typical sputtering systems have relied on the effect of surface composition and viscosity of the liquid matrix on controlling particle diameters, which only resulted in diameters ca. 3−10 nm, that were all plasmonic, our new approach introduced thiol molecules as stabilizers inspired from chemical methods. In the chemical syntheses of metal nanoparticles, controlling the concentration ratio between metal ions and stabilizing reagents is a possible means of systematic size control. However, it was not clear whether this would be applicable in a sputtering system. Our latest results showed that we were able to generically produce a variety of photoluminescent monometallic nanoclusters of Au, Ag, and Cu, all of which showed stable emission in both solution and solid form via our matrix sputtering method with the induction of cationic-, neutral-, and anionic-charged thiol ligands. We also succeeded in synthesizing photoluminescent bimetallic Au−Ag nanoclusters that showed tunable emission within the UV−NIR region by controlling the composition of the atomic ratio by a double-target sputtering technique. Most importantly, we have revealed the formation mechanism of these unique photoluminescent nanoclusters by sputtering, which had relatively larger diameters (ca. 1−3 nm) as determined using TEM and stronger emission quantum yield (max. 16.1%) as compared to typical photoluminescent nanoclusters prepared by chemical means. We believe the high tunability of sputtering systems presented here has significant advantages for creating novel photoluminescent nanoclusters as a complementary strategy to common chemical methods. This Account highlights our journey toward understanding the photophysical properties and formation mechanism of photoluminescent noble metal nanoclusters via the sputtering method, a novel strategy that will contribute widely to the body of scientific knowledge of metal nanoparticles and nanoclusters. show characteristic plasmon absorbance (Figure 1).3 The photoluminescence properties of this material are attributed first to “quantum size effects,”4 wherein the nanoclusters behave somewhat like a molecule and show tunable absorbance and

1. INTRODUCTION Photoluminescent noble metal nanoclusters can be defined as a new class of materials with dimensions less than 2 nm that show absorbance and emission in the blue to near-infrared region (NIR).1,2 These nanoclusters are considered to be transitional between single noble metal atoms, which show distinct optical properties, and noble metal nanoparticles, which © 2017 American Chemical Society

Received: September 25, 2017 Published: November 30, 2017 2986

DOI: 10.1021/acs.accounts.7b00470 Acc. Chem. Res. 2017, 50, 2986−2995

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Accounts of Chemical Research

principles calculations (DFT) to understand the effect of these strong coordinating ligands on the observed photoluminescence.6 A common method of synthesizing photoluminescent noble metal nanoclusters is chemical reduction in the presence of stabilizing molecules using metal salt precursors and reducing agents such as NaBH4.7,8 Under this scheme, stabilizing templates/ligands such as DNA, proteins, dendrimers, phosphine, and thiol are commonly employed to cap, protect, stabilize, and control the growth of the nanoclusters.9−13 Among these strategies, thiol-based synthesis pioneered by Brust for the synthesis of plasmonic nanoparticles and extended by Whetten and Murray for the synthesis of (photoluminescent) monometallic2,4,11,13 or bimetallic12 metal nanoclusters is becoming popular and is known to produce nanoclusters with atomic precision. The potential application of chemically synthesized photoluminescent metal nanoclusters includes biomedical applications, and they are hypothesized to someday replace organic dyes and quantum dots, particularly for sensing, imaging, therapy, and targeted drug delivery.9,10 However, as promising as it may sound, the application of photoluminescent metal nanoclusters is still in its infancy. Intensive investigations are still needed, especially for in vitro and in vivo biomedical applications due to purity- and toxicity-related issues arising from these chemically synthesized photoluminescent nanoclusters.9 A “green synthetic” approach is a better alternative in this regard, leading some scientists (including us) to develop strategies based on a physical synthetic approach such as laser ablation techniques14 and sputtering techniques.15−19 This physical synthetic approach is a new emerging field for the synthesis of “greener” metal nanoclusters. Noble metal nanoclusters synthesized by physical means, which use sputtering techniques reported by our group,20−30 are quite rare. The subsequent sections summarize recent progress on the preparation of metal nanoparticles and photoluminescent

Figure 1. Nanoclusters are the missing link between isolated metal atoms and plasmonic metal nanoparticles.

emission depending on the number of atoms, and second to “surface−ligand effects”5 for emissive nanoclusters with strong coordinating ligands that deviate from the predictions based on nanocluster nuclearity. The tunability of emission colors within the blue to NIR region with respect to the number of atoms was first experimentally demonstrated by Dickson et al.1 for clusters encapsulated in a polyamideamine dendrimer. In their study, a scaling function was introduced, Eemission = EFermi/N1/3, which relates the emission energy with the nuclearity (N) of the nanocluster, wherein they hypothesized that there would be no emission in the visible region once the number of atoms exceeded 30, which was in good agreement with the experimentally observed values. However, this scaling function failed to predict emissions of photoluminescent nanoclusters with stabilizing ligands that contained functional groups that had strong affinities to the metal atoms, such as thiols and phosphines. This anomalous behavior opened up new avenues of research, especially in theoretical studies through first-

Figure 2. Schematic illustration of a difference between common chemical approach and our physical approach via a single-target sputtering system for the synthesis of noble metal nanoparticles/nanoclusters. 2987


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Accounts of Chemical Research noble metal nanoclusters using sputtering deposition over liquid matrices (Figure 2).

2. FUNDAMENTALS OF THE SPUTTERING PROCESS: FROM THIN FILM TO COLLOIDAL NANOPARTICLES Discovered by Grove31 in 1852, sputtering is a technique employed for the fabrication of thin films,32,33 and it is commonly executed under high vacuum and dry conditions. Typically, there are three ways in which it could be performed: DC-diode, RF-diode, and magnetron sputtering, wherein the basic idea is the physical ejection of surface atoms through the collision of ionized gas on a metal target and deposition of these ejected atoms/nanoclusters onto a solid substrate to obtain the thin film.15−19 With the advancement of understanding, however, sputtering systems historically employed on solid substrates under dry conditions are now available for liquid substrates and wet conditions, which in the past was unimaginable (Figure 2).15−19 Fundamentally, a common feature of these liquids is their low vapor pressure, which is a prerequisite to preventing vaporization under vacuum conditions. Torimoto et al.,34 for instance, pioneered the utilization of room-temperature ionic liquids in sputtering systems for the synthesis of Au nanoparticles less than 10 nm in size in an environmentally friendly way without the necessity of using toxic reducing reagents. Nowadays, it is a common knowledge that, in principle, any liquid that has a low vapor pressure could be utilized as a substrate in a sputtering system. Examples of effective liquid matrices employed are ionic liquids (ILs), silicone oils, glycerol, polyethylene glycol (PEG), and so forth.15−30 This revolutionary concept widened the application of the sputtering technique, which is a very promising means to synthesize high-purity materials at high yield. It also opens new avenues of research, particularly in the synthesis of colloidal nanoparticles and nanoclusters in liquid form. Following Torimoto’s seminal report,34 the sputtering technique has become a well-exploited method, not only for the synthesis of Au, Ag, and Cu plasmonic nanoparticles but also for other metal nanoparticles, including Pd, Pt, Rh, Ir, Ru, W, Mo, Nb, Ti, In, Sn, and Zr, and oxidized nanoparticles such as titanium oxide, copper oxide, and tantalum oxide.25−27,34−47 The popularity of the sputtering method had expanded not only for the synthesis of monometallic nanoparticles but also for the synthesis of bimetallic nanoparticles such as AuAg, AuCu, AuPt, and AuPd, which are produced by sequential sputtering35,36 using two different kinds of metal targets, by sputtering using metal alloy37−39 targets, or by simultaneous sputtering using two different metal targets in a double-head40,41,47 sputtering system (Figure 3).

Figure 3. (a) Colloidal bimetallic Au−Ag nanoparticles in Torimoto’s bimetallic target system. Reproduced with permission from ref 38. Copyright 2008 Royal Society of Chemistry. (b) Bimetallic Au−Ag nanoparticles prepared by our double-target sputtering system; values in the sample images denote the applied current (in mA) for each target. Reproduced with permission from ref 40. Copyright 2016 Elsevier.

particle-size distribution.15,20 These parameters in general result in controlling the number of target metal atoms ejected per unit of time and area of substrate, resulting in nanoparticles of different sizes obtained due to the varied deposition rates. In order to control the parameters related to the solution, the chemical structure and viscosity of the liquid matrix are the most important parameters.25,31,43 As a typical example, ILs with different chemical structures result in nanoparticles of different sizes.43 Varying the temperature of the liquid matrix from 0 to 80 °C controlled the viscosity and the size of the nanoparticles (Figure 4a). Moreover, the stirring of liquid matrix during sputtering deposition decreased the size and particle-size distribution of the nanoparticles (from 7.4 ± 2.1 nm to 3.7 ± 0.9 nm) because of the effective suppression of the coalescence of nanoparticles on the surface of the viscous liquid matrix by stirring (Figure 4b).20 These methods for controlling the sizes of nanoparticles have been widely attempted. However, only plasmonic particles with diameters in the range of ca. 3−10 nm were obtained. The synthesis of photoluminescent nanoclusters has not yet been achieved through the above-mentioned strategies. To this end, we recently developed a novel methodology to precisely control the size as well as the photophysical properties of metal nanoparticles/nanoclusters by the sputtering process. Although previous approaches have relied on the effect of the surface composition and viscosity of the liquid matrix on controlling the particle diameters, our new approach introduced thiol molecules as a stabilizer, inspired from chemical methods.

3. SIZE CONTROL OF NANOPARTICLES VIA SPUTTERING DEPOSITION Controlling the particle size and particle-size distribution is the primary aim in the synthesis of metal nanoparticles by both chemical and physical means. In the sputtering technique, this control could be done in two ways: either by manipulating the parameters related to the sputtering process or manipulating the parameters related to the solution.15 To control the parameters related to the sputtering process, parameters such as applied current, voltage, distance between the target and the substrate, chamber pressure, temperature, and sputtering time are adjusted to control the particle size and 2988


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Figure 5. Concept of matrix sputtering method: (a) with nonvolatile ligands dissolved in liquid matrix and (b) with volatile ligands evaporated in the gas phase. Subfigure (b) is reproduced with permission from ref 23. Copyright 2015 American Chemical Society.

In system 1, nonvolatile thiol molecules are dissolved in a liquid matrix such as PEG, silicone oils, or diglycerol (DG) to stabilize the nanoclusters on the surface of liquid matrix. In system 2, volatile thiol molecules are separately added in the sputtering chamber in order to passivate the nanoclusters in the gas phase. In both systems, the thiol molecules dissolve in liquid matrix or evaporate in the gas phase to prevent the aggregation and growth of nanoclusters by the effective coordination according to their concentrations, resulting in systematic size control to a single nanometer order. To understand the effect of the functional groups of the added molecules, we carried out a systematic investigation using ligands with thiol, amine, and carboxyl groups with similar carbon chain lengths [6-mercapt-1-hexanol (6-MH), 6amine-1-hexanol (6-AH), and 1-heptanoic acid (HA)] in a DG matrix (Figure 6).25 Plasmon absorption was observable for 6AH and HA; however, plasmon absorption disappeared for 6MH because of the higher affinity of thiol for Au. The TEM diameters of the nanoparticles were 2.1, 3.0, 4.6, and 6.7 nm for 6-MH in DG, 6-AH in DG, HA in DG, and pure DG,

Figure 4. (a) Size dependence of Ag nanoparticles varied by the viscosity of the liquid matrix. Inset and right images denote the nanoparticle colloidal suspension and metal film prepared under different viscosities, respectively. A metal thin film was obtained at higher viscosity (over the horizontal line inside the graph. (b, c) TEM images of Ag nanoparticles prepared without or with stirring during sputtering deposition. Reproduced with permission from ref 46. Copyright Elsevier 2016. And adopted from ref 20 (CC-BY 4.0) 2016 Nature Publishing Group.

In the chemical synthesis of metal nanoparticles (NPs), controlling the concentration ratio between metal ions and the stabilizing reagents is a possible means of systematic size control. Would this be applicable in a sputtering system? Our latest results20 showed that it was. The use of thiol molecules as a stabilizing reagent due to their strong affinity with noble metal atoms for controlling the particle diameters via sputtering deposition and the formation of photoluminescent nanoclusters is described in the next section.

4. MATRIX SPUTTERING METHOD: TOWARD PHOTOLUMINESCENT NANOCLUSTERS In 2010, we developed a simple matrix sputtering technique using (6-mercaptohexyl)trimethylammonium bromide (6MTAB) as a molten thiol liquid matrix for the first synthesis of NIR photoluminescent Au nanoclusters with large Stoke shifts.21 The particle size of these synthesized nanoclusters was 1.3 ± 0.3 nm, which is nonplasmonic, quite monodispersed, and water-soluble because of the cationic-charged ligand used. In a subsequent report, we utilized pentaerythritol ethoxylate (PEEL) and pentaerythritol tetrakis(3-mercaptopropionate) (PEMP) as liquid matrices for the synthesis of Au nanoparticle/urethane and Au nanoparticle/thiourethane hybrid resins, respectively. We found a nonphotoluminescent material in the former but an NIR photoluminescent hybrid material for the latter because of the strong coordinating thiol group in PEMP with Au atoms (as compared to with hydroxyl groups in PEEL), which effectively prevented the coalescence and growth of particles.22 Inspired by chemical methods where the metal/ligand ratio is a crucial factor for systematic size control, we then constructed two types of strategies for matrix sputtering systems (Figure 5).

Figure 6. TEM images Au nanoparticles/nanoclusters prepared by sputtering over (a) DG, (b) HA in DG, (c) 6-AH in DG, and (d) 6MH in DG. Reproduced with permission from ref 25 with a slight modification. Copyright 2016 The Chemical Society of Japan. 2989


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Figure 7. (a) Extinction spectra, (b) TEM diameters vs thiol concentration (inset: enlarged below 0.008 M), and (c) photoluminescence spectra of Au nanoclusters prepared by the sputtering deposition over PEG with various MUA concentrations. Reproduced from ref 20 (CC-BY 4.0). Copyright 2016 Nature Publishing Group.

respectively. NIR photoluminescent Au nanoclusters could only be produced using 6-MH, as expected from the lack of plasmon absorption. We then explored α-thioglycerol (α-TG), a volatile stabilizer to synthesize NIR photoluminescent Au nanoclusters (system 2, Figure 5b). The evaporation of α-TG inside the chamber enables to coordinate to the very small “nucleation-stage” Au nanoclusters in the gas phase. By changing the sputtering parameters, we obtained varying diameters (ca. 1.2−3.3 nm) of Au nanoclusters, wherein we found a red-shift in emission as the particle size increased.23 At the optimum condition, the synthesized photoluminescent Au nanoclusters showed a 16.1% emission quantum yield, which is significantly higher than the commonly reported quantum yield for thiolated Au nanoparticles or nanoclusters synthesized by chemical means (generally ∼1%). Recent report achieved a higher emission quantum yield (25%) from the aggregate of Au nanoclusters larger than 100 nm by employing the aggregation induced emission.48 We then investigated a systematic thiol-concentration effect on the produced nanoclusters using PEG as the liquid matrix and 11-mercaptoundecanoic acid (MUA) as the stabilizing ligand (system 1, Figure 5a).20 Figure 7a shows the extinction spectra of Au nanoparticles/nanoclusters prepared in PEG with various concentrations of MUA (5.2 × 10−3 to 5.2 × 10−1 M) and those of Au nanoparticles/nanoclusters prepared directly in molten MUA without PEG (corresponding to 3.9 × 101 M). The plasmon absorption became featureless as the thiol concentration increased, and it almost disappeared when the concentration exceeded 5.2 × 10−2 M. The change in average particle diameter as a function of MUA concentration is shown in Figure 7b. The higher MUA concentration resulted in the formation of smaller Au nanoclusters. Moreover, the difference in their particle sizes reflects the change in plasmon absorption in Figure 7a. Thus, this result clearly indicates that the size of Au nanoparticles can be systematically controlled by controlling the thiol concentration. We also observed the size dependence of emission (sample picture in Figure 8), which showed a redshift as the nanocluster size increased with decreasing thiol concentration, as verified by TEM (Figure 7c). We also tried to synthesize Au nanoclusters in various combinations of liquid matrix and thiol ligands, as summarized in Table 1. Other than Au nanoclusters, we also tried the PEG−MUA sputtering system for the synthesis of Ag nanoclusters, wherein we likewise found that at higher concentrations of ligand, the

Figure 8. Representative sample images of produced nanoclusters under sunlight (left) and UV light (right): (a) Au nanocluster dispersion prepared in PEG−α-TG system, (b) Cu nanocluster dispersion prepared in PEG−MUA system, (c) purified Au nanocluster powder prepared in PEG−MUA system, and (d) polymerized thiourethane resin containing Ag nanoclusters prepared in PEMP matrix. Reproduced with permission from (a) ref 23, copyright 2015 American Chemical Society; (b) ref 30 (CC-BY 3.0) published 2016 by The Royal Society of Chemistry; (c) ref 20 (CC-BY 4.0) published 2016 by from Nature Publishing Group.

particle size decreased and the plasmon absorbance of Ag disappeared, a phenomenon similar to that of photoluminescent Au nanoclusters.27 In another study, we were able to produce water-soluble photoluminescent Ag nanoclusters with cationic or anionic thiol ligands using 11-mercaptoundecylN,N,N-trimethylammonium bromide (MUTAB) or sodium 3mercaptopropionate (SMP).44,45 In addition to Ag nanoclusters, we were able to synthesize photoluminescent Cu nanoclusters, which was quite challenging because Cu is much more prone to oxidation than Ag and Au.29 With the new sputtering technique, however, we 2990


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matrix focused on thiol-based synthesis using varieties of neutral, anionic, and cationic charge states with either short- or long-carbon−chain thiol molecules (Table 1). Controlling the particle size and particle-size distribution in this system was done simply by controlling the structure and concentration of thiol molecules in the liquid matrix.

Table 1. Summary of Matrix Sputtering Systems Examined metal target Au Au Au Au Au Au Au Au Au Au Ag Ag Ag Ag Cu Cu Cu Au−Ag

liquid matrix

ligand

6-MTAB PEMP PEG α-TG silicone oil 1-octadecanethiol diglycerol 6-MH diglycerol 6-AH diglycerol HA diglycerol thiocholine chloride PEG MUA PEG MUTAB PEMP PEG SMP PEG MUA PEG MUTAB PEMP PEG MUA PEG MUTAB PEG MUTAB

emission NIR NIR NIR NIR NIR

NIR NIR NIR NIR NIR blue blue blue blue blue−NIR

ref 21 22 23 24 25 25 25 26 20 45 28 44 27 45 29 30 45 47

5. FORMATION MECHANISM OF THE PHOTOLUMINESCENT NANOCLUSTERS VIA MATRIX SPUTTERING METHOD Although we successfully obtained photoluminescent noble metal nanoclusters via matrix sputtering, the origin of their photoluminescence was unknown. Typically, Au NPs with diameters of 1.6 nm or above prepared by a chemical reduction method do not fluoresce. It is well-known that the emission wavelength of such small Au nanoclusters depends on the number of Au atoms contained in the nanocluster (quantum size effect). However, our Au nanoclusters prepared by sputtering with diameters of ca. 1.0−3.3 nm showed NIR photoluminescence with a small spectral change (from 657 nm to approximately 750 nm). Some of these diameters (especially those >2 nm) were significantly larger than those reported previously for photoluminescent Au nanoclusters synthesized by chemical methods, and the range should be large enough to exhibit more drastic spectral changes in their emission. For understanding the formation mechanism and the origin of the unique photoluminescence of the Au nanoclusters, a purified sample was analyzed with X-ray photoelectron spectroscopy (XPS), wherein we found that the maximum peaks were shifted at high binding energies with respect to the binding energy of the bulk gold at the Au4f region and that they coincided with the binding energies of Au nanoclusters with Au10(SR)10, Au11(SR)11, Au12(SR)12, and Au15(SR)13. The composition ratio (Au/SR) of the nanocluster was then determined using thermogravimetric analysis (TGA), and we found that in the case of Au nanoclusters, the Au/SR ratio was approximately 0.96, which is close to the hypothesized structure [Au12(SR)12 or smaller nanoclusters] by XPS. The particle sizes of the purified nanoclusters were 1.6 nm, which was in good agreement with the particle size of Au144(SR)60. However, we found this result somewhat contradictory to the results of UV− vis extinction and the photoluminescence measurement because nanoclusters of this size and structure are typically nonphotoluminescent. These findings led us to hypothesize that the nanoclusters most probably contained a mixture of very small nanoclusters or multinuclear complexes in their aggregated form and not as single crystals.

successfully synthesized blue-emitting Cu nanoclusters with small particle sizes and narrow particle-size distribution (1.1 ± 0.2 nm). These blue-emitting Cu nanoclusters could transform into red-emitting Cu2S nanoclusters either by storage for several days or immediately with the aid of UV irradiation.29 Furthermore, a stable blue photoluminescent Cu nanocluster was achieved using MUA as the ligand in a PEG matrix. The intensity of the photoluminescence increased as the amount of ligand increased.30 Only recently, using the same PEG matrix and cationic MUTAB as the ligand, we finally obtained very stable photoluminescent Cu nanoclusters that showed similar blue emission in both solution and solid states.45 We recently extended our matrix sputtering system to synthesize the first known bimetallic photoluminescent Au−Ag nanoclusters by means of a double-target sputtering technique (Figures 3b and 9).47 These new photoluminescent bimetallic nanoclusters showed emission tunability from blue to NIR regions with respect to the Ag/Au composition ratio, which was easily manipulated by applying varying current for each target. Moreover, the high-angle annular dark-field−scanning transmission electron microscopy (HAADF-STEM) image and energy dispersive X-ray spectroscopy (EDX) mapping directly evidenced the formation of bimetallic alloy nanoclusters (Figure 9). To summarize this section, most of our studies to produce photoluminescent metal nanoclusters on a liquid polymer

Figure 9. HAADF-STEM representative image and EDX mapping of Au−Ag bimetallic nanocluster produced by double-target sputtering technique. Reproduced with permission from ref 47. Copyright 2017 American Chemical Society. 2991


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Accounts of Chemical Research We also reported time-dependent secondary aggregation of sputtered Ag NPs in PEMP.28 The average TEM diameter increased with increasing time under dark conditions for 40 days from 2.5 to 13.7 nm. However, the NIR emission remained and plasmonic absorption was not observable in such large nanoparticles (Figure 10). These results clearly

Figure 11. Correlation between TEM core diameters and emission spectral change (Δλem/nm). Red squares denote the size-fractionated Au nanoclusters by SEC-HPLC, and the blue circles denote the differently sized photoluminescent Au nanoclusters synthesized by varying the concentration of the thiolate ligand (MUA). Reproduced from ref 20 (CC-BY 4.0). Published 2016 by Nature Publishing Group.

sputtering over a liquid matrix was plasmonic nanoparticles, which were probably formed by the aggregation of atoms/small nanoclusters and coalescence on the liquid surface and inside the liquid matrix because there is no strong coordinating group in PEG (or ILs) for suppressing the coalescence and growth (Figure 12a). However, photoluminescent nanoclusters were

Figure 10. (a) Time-dependent photoluminescence spectra excited at 360 nm and TEM images (b) immediately after preparation and (c) after 40 days. Schematic illustrations of (d) primary aggregation and (e) secondary aggregation of Ag nanoclusters in PEMP matrix obtained via sputtering technique. Reproduced with permission from ref 28 with modifications−Copyright 2016 Elsevier.

demonstrated that the secondary aggregation mainly contributed to the increase of the particle diameters, with a minor contribution by primary aggregation that induced an increase in the core diameter, resulted in the emission spectral shift (or generation of plasmon absorption if aggregated further). This result also supports our hypothesis. To further clarify this issue, we isolated the nanoclusters by means of a size exclusion chromatography−high-performance liquid chromatography (SEC-HPLC) system. The initial Au nanoclusters (1.6 ± 0.3 nm) were isolated into several fractions from 1.2 ± 0.6 to 1.9 ± 0.3 nm. Interestingly, we did not observe any emission spectral change in these fractions according to their core diameters, although the difference in the core diameters of the fractions was certainly large enough to exhibit drastic spectral changes in their emission. When we synthesized Au nanoclusters at various concentration of MUA in PEG (Figure 8), the average diameters of the photoluminescent Au nanoclusters could be systematically controlled from 1.6 to 2.7 nm, and the nanoclusters showed obvious changes in their photoluminescence maxima [Δλem, defined as the change of photoluminescence maximum (in nm) from the smallest core diameter] (blue circles in Figure 11). However, Au nanoclusters with different core diameters fractionated from one synthetic batch did not show such a tendency (red squares in Figure 11). These results clearly suggest that the TEM core diameters do not determine the photoluminescent properties; it is the synthetic protocol during sputtering deposition that determines the photophysical properties of the resultant nanoclusters. We now discuss the formation mechanism of photoluminescent nanoclusters via the matrix sputtering system. In the absence of thiol ligands, what we produced in direct

Figure 12. Plausible mechanism for the formation of photoluminescent Au nanoclusters via the matrix sputtering method: (a) plasmonic Au nanoparticles in the absence of MUA and (b) photoluminescent Au nanoclusters in the presence of thiols in PEG. Reproduced from ref 20 (CC-BY 4.0). Published 2016 by Nature Publishing Group.

obtained in the presence of thiol ligands. SEC-HPLC studies revealed that the TEM core diameters did not determine the photoluminescence properties; rather, the synthetic protocol during sputtering deposition determined the photoluminescence properties of the resultant NPs. These observations clearly suggest that the obtained photoluminescent Au nanoclusters were not single crystals but were rather composed of aggregates of the smaller photoluminescent nanocluster/ complex components. Thus, the formation process of nanoclusters in the presence of thiol ligands can be depicted as shown in Figure 12b. The nanoclusters are quickly stabilized by thiol ligands on the surface of the liquid matrix, resulting into the formation of quasi-stable nanoclusters. These quasi-stable 2992


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Accounts of Chemical Research components further coalesce into larger Au nanoclusters inside the liquid matrix and finally result in the formation of photoluminescent nanoclusters with TEM diameters of 1.6− 2.7 nm (in the case of MUA-PEG). Because the coalescence in the liquid matrix is a diffusion-limited process, the obtained Au nanoclusters have a distribution in their diameters. However, the emission wavelengths of these differently sized Au nanoclusters prepared at one MUA concentration were constant (red squares in Figure 11) because they were formed by the coalescence of smaller photoluminescent nanoclusters. This is the most plausible mechanism for the formation of photoluminescent Au nanoclusters via sputtering deposition.

Institute of Technology (MSU-IIT) and his M.S. in Materials Science and Engineering from University of the Philippines-Diliman (UPDiliman). He obtained his Ph.D. degree in 2017 from Hokkaido University under the supervision of Prof. Tetsu Yonezawa. His research topic is cationically charged photoluminescent noble metal nanocluster synthesis and characterization. Tetsu Yonezawa (born in 1965, Japan) received his B. Eng., M. Eng., and Dr. Eng. degrees in 1988, 1990, and 1994, respectively, from The University of Tokyo, Japan. After his visiting studies at Ecole Polytechnique de Federale de Lausanne (Switzerland) and Institut sur la Catalyse, Villeurbanne (France), he joined Professor Toyoki Kunitake’s group at Kyushu University as an assistant professor in 1996. He worked at Nagoya University (2001−2002) and The University of Tokyo (2002−2009) as an associate professor. Then, he was appointed a full professor at Hokkaido University in 2009. He received the Hitachi Chemical Award 2011 from the Society for Polymer Science, Japan, a Xingda Lectureship from Peking University in 2015, and other awards. He has been a fellow of the Royal Society of Chemistry since 2016. His current major research interest is to develop functional metal nanoclusters and fine particles for various applications.

6. SUMMARY AND PERSPECTIVE Sputtering over a liquid matrix to synthesize high-purity photoluminescent metal nanoclusters without reductants is quite a new field of endeavor, and it opens up possibilities for future exploration and experimentation because of the high tunability of the system. The applied current for double (or more) target sputtering systems makes it possible to combine any composition ratio of metal atoms by varying the applied current for each metal target. We believe this strategy will be a novel methodology to create new series of multimetallic nanoparticles49/nanoclusters,47 especially for combining metal atoms with large differences in their reduction potentials that are usually difficult to prepare by chemical means. As for photoluminescent nanoclusters, the easy control of the metal composition ratio in alloy nanoclusters is highly beneficial for tuning of the emission wavelength and chemical properties. Moreover, owing to the unique coalescence process via sputtering deposition, which is different from chemical processes, novel metal combinations for alloy nanoparticles/ nanoclusters that are hindered according to the phase diagram could also be expected. We believe the high tunability of the sputtering systems presented here has significant advantages for creating novel photoluminescent nanoclusters as a complementary strategy for common chemical methods and that it will therefore widely contribute to the body of scientific knowledge of metal nanoparticles and nanoclusters.

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ACKNOWLEDGMENTS The authors are grateful to their outstanding and motivated students whose names appear in the references for their invaluable intellectual and experimental contributions and to their collaborators, whose names also appear in the references, for their willingness to share their knowledge and expertise with the authors. The authors acknowledge the financial support from Building of Consortia for the Development of Human Resources in Science and Technology, Ministry of Education, Culture, Sports, Science and Technology, Japan (to Y.I.), and Mutara Foundation (to T.Y.).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yohei Ishida: 0000-0001-8541-2714 Tetsu Yonezawa: 0000-0001-7371-204X Notes

The authors declare no competing financial interest. Biographies Yohei Ishida (born in 1986, Japan) received his B.S. and Ph.D. degrees in 2009 and 2013, respectively, from Tokyo Metropolitan University, Japan. After his visiting research at University of Miami, Max Planck Institute of Colloids and Interfaces, and National University of Singapore, he joined Hokkaido University in 2014 as an assistant professor. He also had his scientific training at The University of Texas at San Antonio and Universite Paris-Saclay. His current research interests are in the areas of supramolecular photochemistry, 2D layered materials, and inorganic nanoparticles. Ryan D. Corpuz (born in 1985, Philippines) received his B.S. degree in Ceramics Engineering from Mindanao State University-Iligan 2993


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Matrix Sputtering Method: A Novel Physical Approach for

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