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Tailoring silver nanoclusters via doping: advances and opportunities

Atomically precise noble metal nanoclusters (especially Au and Ag) have been pursued due to their fascinating molecule-like properties. In spite of the significant progress on Au nanoclusters (NCs), the structure and property evolution of Ag NCs is still in high demand. Doping is a useful strategy for improving the physicochemical performances of Ag NCs. Herein we summarize the recent advances in tailoring silver NC structures and properties via doping. First, we reviewed the recent studies on the synthesis of hetero metal atom doped silver bimetallic nanoclusters, which are classified by the dopants, including Au, Pt, Pd, Cu, Ni and Cd. Second, the doping effects on their properties were reviewed, including the locations of hetero metal atoms, the influence on their stability, and the charge state evolution. Moreover, we highlighted the doping-dependent improvement of the photo-luminescence (PL) performance and catalytic activity of Ag NCs.


Introduction
Atomically precise metal nanoclusters (NCs) composed of a discrete number of metal atoms and monodisperse coordination ligands have raised immense scientic interest. They are unique materials, which are stable at a smaller scale (<2 nm) compared with nanoparticles and still retain molecule-like characteristics, such as luminescence, molecular magnetism, chirality, etc. [1][2][3][4][5][6] Furthermore, their identied formulas and atomically precise structures provide atomic-level structureproperty correlation and modication.
As the two most outstanding species, gold and silver NCs showed different performances in stability. Improving the stability and enhancing their diverse properties is the major priority in studying Ag NCs. Adding hetero-metal atoms into the mono-metal kernel has been demonstrated to be an effective strategy to obtain stable and functional Ag NCs, where doping methods have been extensively carried out to tailor Ag NC properties in recent studies. Generally, doping with foreign atoms will tune the geometric and electronic structures of NCs and further inuence their optical properties and catalytic activity. Moreover, doping endows Ag NCs with better thermal stability. [7][8][9][10][11][12] Jie Yang is currently an Associate Professor of Materials Science and Engineering at Jiangsu University of Science and Technology. She received her PhD degree in MSE from the University of Science and Technology of China in 2015. Then she joined the MSE faculty of JUST in 2016, and she worked at Carnegie Mellon University (CMU, Pittsburgh, USA) in 2019. Her research interests include the synthesis of atomically precise metal nanoparticles and their alloys.
Runqiang Pang is currently a master's candidate in Materials Science and Engineering under the supervision of Prof. Jie Yang in Jiangsu University of Science and Technology. Her research interests focus on the syntheses and properties of silver nanoclusters and their alloy nanoclusters.

Synthesis of doped silver bimetallic nanoclusters
The concept of doping emerging in the elds of metal nanoclusters has been claried in previous reviews. [8][9][10] Herein, doped Ag NCs are identied as M x Ag nÀx (SR) m (M ¼ Au, Pt, Pd, Cu, etc.). The synthesis methods can be classied into four categories: (1) co-reduction of mixed metal-complexes, such as one-pot reduction of Ag-SR and M-SR mixed complexes, where the parent silver complexes and hetero-metal complexes are simultaneously reduced in one pot by the reductant; (2) doping homo-metal clusters with foreign metal salts or complexes, via galvanic exchange and anti-galvanic exchange, where heterometal salts or complexes react with Ag NCs via a redox process, and hetero-metals take the place of some Ag atoms, forming alloys; (3) inter-cluster reactions, where the reactions between Ag NCs and foreign metal NCs introduce foreign metals into Ag NCs, generating Ag bimetallic NCs; (4) the ligand-exchange method, where the structures of Ag bimetallic clusters can be converted by their reaction with introduced ligands. Atomically precise doping provides convenience for studying the composition and structure evolution. In addition, the number and nature of doping hetero-metal atoms and their doping locations (including centre, inner shell and outer shell) are also crucial factors, which need to be seriously considered. Different heteroatoms provide different electronic and geometric structures and diverse properties. This section summarizes the doping of different heteroatoms into silver nanoclusters, mainly including dopants like Au, Pt, and Pd, and non-noble metal atoms like Cu, Ni and Cd.
According to the advances in Ag NCs, there are several typical Ag NC templates for preparing doped Ag NCs: the most famous types are based on Ag 25 , Ag 29 and Ag 44 , and there a few studies that are also based on Ag 20 , Ag 21 and some other stable sizes. Here, we will summarize doped Ag NCs according to the doped foreign atoms. We have reviewed hetero-metal atom doped Ag NCs in Table 1

Au doping
Silver and gold have nearly identical atomic radii, which are 2.89 and 2.88Å, respectively. Gold is much more stable in the zero-valent state. Gold doped Ag NCs have been well studied and compared with their Ag NC analogue. The Au dopant can occupy both positions, i.e., the centre core and the kernel shell.
[Ag 25 (SR) 18 17 ] 2+ , which contained 6 free valence electrons. 27 This work bridges the gap in the understanding of 6e Ag 24 M(SR) 18 NCs. As Ag 25 ts the super atom model, there are two reasonable charges of M@Au 24 : 0 or 2 charge states, which correspond to 6e and 8e, respectively. They synthesized this Au 1 Ag 24 via the co-reduction method, where Ag and Au ions were reduced in one pot. The formula has been identied as 1 Au and 24 Ag protected by 17 cyclohexyl mercaptan and 3 Dppm via ESI-MS measurements. The [Au 1 Ag 24 (Dppm) 3 (SR) 17 ] 2+ contains open icosahedral Au 1 Ag 12 surrounded by a Ag 12 (-Dppm) 3 (SR) 15 ring motif and two thiols, see Fig. 1A. In the Au 1 Ag 12 kernel, there is a distortion, which leads to longer Au- Co-reduction & galvanic reaction 28 Ag 29Àx Au x (BDT) 12 (TPP) 4 Co-reduction 29 [Au 4 Ag 23 (C^CBu t ) 10 (dppf) 4 Cl 7 ](PF 6 ) 2 Co-reduction 30 [Au 5 Ag 24 (C^CC 6 H 4 -p-Bu t ) 16 (dppf) 4 Cl 4 ](PF 6 ) 3 Co-reduction 30 Ag 44 [Ag 32 Au 12 (SR) 30 ] 4À 17 M 4 Au x Ag 44Àx (p-MBA) 30 Co-reduction 31  36 Co-reduction 54 Ag and Ag-Ag bond lengths that range from 2.730 to 3.068Å and from 2.851 to 2.982Å, respectively. There are more varieties of gold-doped Ag 29 NCs, and they contain different numbers of surface ligands. Groot's group tried to synthesize Au 1 Ag 28 (LA) 12 3À (LA ¼ lipoic acid) via the coreduction approach and galvanic reactions, and the products show a preference for mono-doped clusters. 28 In their work, they predicted the central location of the doped Au by X-ray absorption spectroscopy (XAS). Cui 29 and his co-workers prepared Ag 29Àx Au x (BDT) 12 (TPP) 4 (x ¼ 1-5), and they identi-ed the reasonable substitution site of x Au by using DFT and TD-DFT calculations. Of note, according to the calculations, the energy barrier of center doping is about 10 kcal mol À1 , which is less than that of the others, matching the experimental results that it is the most stable one. They further discussed the crucial inuences of the heteronuclear Au-Ag bonds on the electronic structures and optical performance. We will discuss the photoluminescence enhancement in the next section. In Wang's work, 30 DFT calculations of the isomers of Au 12 Ag 32 (SR) 30 4À conrm that the most stable isomer is the Au 12 @Ag 32 (E ¼ 0), where the Au occupies the centre positions of the kernel. These results also match those in the earlier studies. Apart from the Au doped Ag NCs, by using the common Ag NC templates, several other stable sizes of Ag alloys have been reported. Liu et al. synthesized Ag 20 and Ag 21 NCs, where S and Se were used as donor ligands, respectively. [18][19][20] In their studies, they also reported alloys based on these Ag 20  ] + by reaction with Ag + salt at low temperature. These studies give a prospect for further study of core-size controlled preparation of the doped Ag NCs.
Zang's 35 group synthesized 3-electron Ag 34 NCs and Au doped Ag 33 Au alloy NCs via the one-pot reduction method. Ag 33 Au has a similar framework to Ag 34 , where the doping Au atom takes the place of the core Ag atom. However, Ag 33 Au showed vigorous stability compared to Ag 34 . Furthermore, some larger sized Au doped Ag NCs have been reported, for instance, Au 24 Ag 46 (ref. 36) and Au x Ag 267Àx , 37 see Fig. 1C and D. The [Au 24 Ag 46 (SR) 32 ](BPh 4 ) 2 contains a Ag 2 @Au 18 @Ag 20 kernel which is protected by a chiral Ag 24 Au 6 (SR) 32 outer shell. 36 Unlike the other Ag nÀx Au x (SR) m alloy NCs we discussed above, the doped Au atoms didn't occupy the centre of the core of the Au 24 Ag 46 NCs. The spherical (AuAg) 267 (SR) 80 is generated with trigonal-prismatic (AuAg) 45 (2,4-SPhMe 2 ) 27 (ratio 1 : 1) via co-crystallization. The Ag 267Àx Au x (SR) 80 (ref. 37) contains a four concentric-shell icosahedral structure of Ag@M 12 @M 42 @M 92 @Ag 120 (SR) 80 (M ¼ Au or Ag) with I h symmetry. The centre atom of the core is Ag, the same as that in the [Ag 46 Au 24 (SR) 32 ](BPh 4 ) 2 .

Pt doping
Platinum doping in Ag NCs has been demonstrated to be feasible. According to the reported studies, most of the Pt-Ag alloys are synthesized based on the Ag 25 18 ] 2À (M ¼ Pd, Pt) NC bears a M@Ag 12 core capped by six distorted dimeric -RS-Ag-SR-Ag-SRstaple motifs, see Fig. 2A and B. Basset and his co-workers have synthesized a rodlike Pt 2 Ag 23 Cl 7 (PPh 3 ) 10 via a one pot co-reduction method. 39 It contains two PtAg 12 icosahedra, which are self-assembled into a rod-shape by vertex-sharing, see Fig. 2C. Electronic structure calculations further illustrate the origin of Pt centre-doping-dependent enhancement of stability. Zhu and his co-workers obtained Pt 1 Ag 12 (dppm) 5 (SPhMe 2 ) 2 NCs by using the ligandexchange method with Pt 2 Ag 23 Cl 7 (PPh 3 ) 10 . 40 Aer rst etching with a HS-PhMe 2 ligand, Pt 2 Ag 23 Cl 7 (PPh 3 ) 10 transformed to PtAg 24 (SPhMe 2 ) 18 , where the rod-like PtAg 12 icosahedra split into one PtAg 12 icosahedron capped with six Ag 2 (SR) 3 dimeric motifs. Aer that, the PtAg 24 (SPhMe 2 ) 18 NC was etched into a smaller mono-icosahedral PtAg 12 nanocluster by a dppm ligand. However, PtAg 12 NCs are not as stable as Pt 2 Ag 23 and PtAg 24 , and it is difficult to obtain their single crystals. Wu's group reported a PtAg 26 (2-EBT) 18 (PPh 3 ) 6 alloy, in which a bipyramid Pt@Ag 14 kernel is surrounded by six dimeric (-SR-Ag-SR-Ag-PR 3 -) staple motifs. 41 Both PtAg 26 and PtAg 24 are 8e superatoms, and they share highly similar electronic structures.
There are several studies that reported the synthesis of PtAg 28 NCs. Unlike Au 1 Ag 28 NCs, PtAg 28 NCs were prepared via the ligand-exchange approach and more kernel structure types were formed, including icosahedral and FCC structures. Basset et al. synthesized [PtAg 28 (BDT) 12 (TPP) 4 ] 4À by the reaction of Pt 2 Ag 23 Cl 7 (TPP) 10 with a BDTH 2 (1,3-benzenedithiol) ligand. 42 The Pt dopant in PtAg 28 also occupies the center position of the icosahedron core. PtAg 28 possesses a closed-shell electronic conguration, showing enhanced stability, see Fig. 2D. Of note, they further doped Au atoms into these bimetallic NCs, and found out that the Au atom takes the place of the Ag atom, forming [PtAuAg 27 (BDT) 12 ] 4À trimetallic NCs. Kang et al. 43 prepared PtAg 28 (S-Adm) 18 (PPh 3 ) 4 NCs via etching Pt 1 Ag 24 (-SPhMe 2 ) 18 with the 1-adamantanethiolate and triphenyl phosphine ligands. The Pt atom in the PtAg 28 (S-Adm) 18 (PPh 3 ) 4 also occupies the center position. This NC bears a face-centered cubic (FCC) PtAg 12 kernel, which is different from that of the doped Ag NCs that we discussed above. PtAg 28 (S-Adm) 18 (PPh 3 ) 4 exhibited better thermal-stability and stronger luminescence than Pt 1 Ag 24 (SPhMe 2 ) 18 . In subsequent research, chemists tried to identify the relationship between the icosahedron and FCC structures. Bakr et al. 44 obtained [PtAg 28 (BDT) 12 (PPh 3 ) 4 ] 4À NCs with a FCC structure by the ligand exchange method, using dithiolate 1,3-benzenedithiolate to replace the bulky monothiolate 1-adamantanethiolate in the icosahedron [PtAg 28 (S-Adm) 18 (PPh 3 ) 4 ] 2+ . Zhu and his co-workers 45 reported the reversible transformation between the icosahedral and FCC structures: the 1-adamantanethiolate capped PtAg 28 with a FCC kernel was transformed to its isomer with an icosahedral kernel in the presence of cyclohexanethiolate. Interestingly, the reverse process was also observed by the addition of 1-adamantanethiolate. In the latest work of Huang's group, 46 they synthesized [PtAg 28 (S-Adm) 18 (PPh 3 ) 4 ] 2+ by a one-pot wet chemical method. It was a breakthrough in preparing PtAg 28 alloy NCs. Compared with the previous ligand-exchange method, they achieved high symmetry with the FCC kernel in high yield.
Although most of the studies on Pt doped Ag NCs are based on templates of Ag 25 or Ag 29 , some other sized Pt-Ag alloy NCs were investigated as well. Kang et al. 47 prepared Pt 1 Ag 31 (S-Adm) 16 (-DPPM) 3 Cl 3 via the reaction between Pt 1 Ag 28 (SAdm) 18 (PPh 3 ) 4 and Ag 2 (DPPM)Cl 2 complexes. The Ag 2 (DPPM)Cl 2 substitutes the Ag-PPh 3 units, leading to the size growth of the NCs. Although it maintains the same composition as the Pt 1 Ag 12 kernel, the FCC conguration in the Pt 1 Ag 28 changes to an icosahedral conguration. The Pt 1 Ag 12 kernel is protected by a three Ag 6 (SR) 6 (DPPM) Cl assembly via sharing Ag 2 (SR) 3 edges. Pt 1 Ag 31 (S-Adm) 16 (-DPPM) 3 Cl 3 exhibited a different UV absorption compared to Pt 1 -Ag 28 (SAdm) 18 (PPh 3 ) 4 , so it is easy to distinguish them.
[Pt 3 Ag 33 (PPh 3 ) 12 Cl 8 ] + with three icosahedral PtAg 12 building blocks, where adjacent PtAg 12 units share a vertex with silver atoms in a cyclic manner, was reported by Zhu's group. 48 It can be classied as the same homologous series with rod-like Ag 23 Pt 2 -Cl 7 (PPh 3 ) 10 and Pt 1 Ag 12 (dppm) 5 (SPhMe 2 ) 2 NCs. It exhibits growing evolution from single icosahedral PtAg 12 to rod-like biicosahedral PtAg 12 , and then to a cyclic tri-icosahedral PtAg 12 . Comparing the UV absorption and PL excitation of Pt 3 Ag 33 with those of Pt 1 Ag 12 and Pt 2 Ag 23 , it was found that there is a continuous red shi with the assembly of more PtAg 12 units. Furthermore, Liu and his co-workers 49 obtained three bimetallic Pt doped Ag NCs: PtAg 20 (dtp) 12 , Pt 2 Ag 33 (dtp) 17 , and Pt 3 Ag 44 (dtp) 22 (dtp: dipropyl dithiophosphate) via the one pot co-reduction approach, which are 8e, 16e and 22e super molecules, respectively. Similar to Pt 1 Ag 12 , Pt 2 Ag 23 and Pt 3 Ag 33 , these three homoleptic PtAg 20 (dtp) 12 , Pt 2 Ag 33 (dtp) 17 , and Pt 3 Ag 44 (dtp) 22 contain one, two and three centered icosahedral Pt@Ag 12 units, respectively. Pt 2 Ag 33 (dtp) 17 with 16 electrons contains two noninteracting 8-electron vertexsharing icosahedral super molecules, and its electronic structure is isolobal to Ne 2 . The Pt@Ag 12 units in Pt 3 Ag 44 (dtp) 22 are assembled linearly by vertex-sharing, and its electronic structure is isolobal to the triiodide ion. These inter-icosahedral bonding interactions provide evidence and chances for deeper understanding and further investigation of super molecular alloys.

Pd doping
Similar to Pt doped Ag NCs, palladium doped Ag NCs are usually prepared based on Ag 20 , Ag 25 and Ag 29 NCs, and the Pd dopant also prefers center doping. As we discussed above, Zheng's group 25 used the one pot method to prepare [PdAg 24 (SR) 18 ] 2À . It has a centred icosahedral Pd@Ag 12 core stabilized by six distorted dimeric staple -RS-Ag-SR-Ag-SRmotifs, which is similar to Au 25 (SR) 18 . They carefully compared the structures of [PdAg 24 (SR) 18 ] 2À and Au 25 (SR) 18 , and found that there were several distortions in PdAg 24 . Moreover, the distorted dimeric staple motifs change the surface chemical environment. Due to the interactions between neighbouring staple units on the surface, they bear shorter Ag.S distances (3.006Å in average) than Au 25 (SR) 18 . Therefore, the structure of [PdAg 24 (SR) 18  ] has a sulde-centered Pd 6 Ag 2 rhombohedron core capped by twelve silver atoms with S 6 symmetry, and it displays a smaller electrochemical HOMO-LUMO gap. Compared with the common icosahedron or FCC core, this octahedral hexa-palladium(0) kernel embedded within a silver(I) cluster is a novel structure type in alloy NCs.

Cu doping
Cu(0) nanoclusters are rarely reported due to their easy oxidation. There is only a little preliminary progress on copper doped Ag NCs. Zheng's group reported the rst Ag-Cu bimetallic alloy, [Ag 28 Cu 12 (SR) 24 ] 4À . 52 It has a chiral metal framework with T symmetry, and its kernel is arranged in a three-concentric shell structure composed of Ag 4 @Ag 24 @Cu 12 (SR) 24 , where a two-shell Ag 4 @Ag 24 core is capped by four nearly planar Cu 3 (SR) 6 staple motifs, see Fig. 3A. The Cu atoms take the place of the Ag atoms in the outer shell, and these 28 Ag atoms are arranged in a distorted FCC pattern, which results in a distorted HCP Ag-Cu interface. Its chiral separation and asymmetric synthesis was conducted by ion pairing with chiral quaternary ammonium salts. In the synthesis process, they used achiral counter cations ( n Bu 4 N + ), and hence the as-prepared cluster was a racemic mixture. Furthermore, this racemic mixture can be separated into enantiomers by using chiral quaternary ammonium salts, such as N-benzylcinchonidinium chloride (BCDC) and N-benzylcinchoninium chloride (BCNC), thereby obtaining optically pure enantiomers separately. This asymmetric synthesis and ion paring chiral enantio-separation provides effective routes for preparing chiral metal NCs.
Zhu's 53 group reported kernel-doping of Cu atoms in Ag NCs, and the preparation of a giant bimetal NC [Ag 61 Cu 30 (SAdm) 38 S 3 ] BPh 4 which possesses a rare valence of 46 free electrons. Ag 61 Cu 30 (Ag 13 @Cu 30 @Ag 48 (SAdm) 38 S 3 ) also has a three-shell framework, where the Cu 30 is in the middle layer. The icosahedral Ag 13 kernel is enclosed by a Cu 30 icosidodecahedron, where each Ag atom connects ve pentagonal Cu atoms, and is further capped by an Ag 48 (SAdm) 38 S 3 outer shell. Doping active metal atoms in the kernel of metal NCs offers new insights into the atomically controlled synthesis of metal alloy NCs.

Other hetero-metal doping
Apart from the above-mentioned Ag alloy nanoclusters doped with Au, Pt, Pd and Cu, several other metal doped Ag alloy NCs, such as Ni and Cd doped Ag alloy NCs, were also reported.
Hyeon and Zheng et al. 54 reported a Cd doped Ag nanocluster Cd 12 Ag 32 (SePh) 36 by a phosphine-assisted process, see Fig. 3B. Cd occupied the outer shell positions, and formed an asymmetric two-shell Ag 4 @Ag 24 @Cd 3 Ag(SePh) 9 framework. The Ag 4 @Ag 24 kernel contains a Ag 4 tetrahedron, which is encapsulated by four Ag 6 facets. This Ag 28 kernel in Cd 12 Ag 32 (SePh) 36 is similar to the core in [Cu 12 Ag 28 (SR) 24 ] 4À . 52 In the outer layer of Cd 3 Ag(SePh) 9 , there are three CdSe 4 tetrahedra, where three Se bind with one Ag to form a AgSe 3 cap-like structure. According to the electronic structure analysis and DFT calculations, it is a 20e superatom, which exhibits a chiral optical response.
Ni doped Ag NCs were synthesized based on Ag 25 and Ag 29 . The synthesis of NiAg 24 (ref. 14) was carried out by the co-reduction method while the NiAg 28   [NiAg 24 (DMBT) 18 ] 2À . However, these Ni-Ag alloy NCs were less stable than their parent Ag NCs. Further work focusing on the preparation of stable Ni doped Ag NCs and understanding their geometric and electronic structures is needed.

Doping effects on properties
As we discussed in our previous studies, Ag NCs exhibit relatively poor stability compared to Au NCs, which makes their further study and applications challenging. Doping methods have been demonstrated to be efficient in tuning the properties of Ag NCs. The doping of Ag NCs with hetero atoms brings changes in constituent structure and electronic structure, which further inuence their properties including PL, catalytic activity, chirality, etc.  18 ] À undergoes minimal degradation in ve days under ambient conditions, while pure Ag 25 suffers obvious degradation aer 24 h. 56 Beneting from the better stability, many Ag alloy NCs are reported with a determined crystal structure. The crystal structures of these doped Ag alloy NCs were studied using a single crystal X-ray diffractometer (SC-XRD). Different dopants prefer to occupy different locations in the Ag kernel due to their unique nature, see Fig. 4. Studying the reported structures of these Ag alloy NCs, we nd some rules in doping locations. The Pt, Pd and Ni heteroatoms prefer to occupy the centre core position, while Au sometimes occupies the outer shell positions in giant Ag alloy NCs. For instance, the Au doped Ag alloy NCs Ag 46 Au 24 (SR) 32 ](BPh 4 ) 2 36 contain a Ag 2 @Au 18 @Ag 20 kernel. And the centre position is also occupied by a Ag atom in the Ag 267Àx Au x (SR) 80 NC kernel. Unlike the relatively stable noble metals, the Cu dopant is very active, and centre doping is full of challenges. Accordingly, Cu prefers to occupy the outer shell positions. In the case of [Ag 28 Cu 12 (SR) 24 ] 4À , 52 its kernel is arranged in a three-concentric shell framework composed of Ag 4 @Ag 24 @Cu 12 (SR) 24 , and the Cu atoms occupy the outermost shell to protect the two Ag 4 @Ag 24 kernels. For [Ag 61 Cu 30 (-SAdm) 38 S 3 ]BPh 4 , 53 Cu 30 is arranged in the middle layer of the kernel, which is connected with the icosahedral Ag 13 kernel and outer shell Ag 48 (SAdm) 38 S 3 .
Density functional theory (DFT) calculations and the TD-DFT method were used to study the geometric and electronic structures and further theoretically validate the optimal location of the dopants in the Ag alloy nanocluster. Dolg and Cui et al. 29 studied the structures of Ag 29Àx Au x (x ¼ 1-5) NCs with possible isomers by performing DFT calculations. In their work, they claim that the Au atom prefers to occupy the central position. For instance, in the case of x ¼ 1, the most stable isomer is the one in which Au is located in the central Ag 13 icosahedral core. And when x ¼ 2, the most stable isomer is the one in which two Au dopants occupy the central and vertical positions. Furthermore, they found that the Ag 29Àx Au x (x ¼ 1-5) NCs exhibit increased stability with the increase of Au-Ag bonds. In the work of Basset et al., 42 they used TDDFT UV-vis spectra to analyze the optimal location of Pt in the [PtAg 28 (BDT) 12 (-P(CH 3 ) 3 ) 4 ] 4À NC, see Fig. 5A. They compared the simulated curves of PtAg 28 with different Pt locations based on the experimental spectrum. Accordingly, the Pt center doped curve is well matched with the experimental spectrum as they have similar main peaks and no low-energy features. The center doped [PtAg 28 (BDT) 12 (P(CH 3 ) 3 ) 4 ] 4À NC shows higher thermodynamic stability. In Kappes's work, they monitored the reaction process via high-resolution trapped ion mobility mass In addition to the approach we mentioned above, the extended X-ray absorption ne structure (EXAFS) and the Xray absorption near edge structure (XANES) methods were used in combination with electronic structure calculations to conrm the location of the Au dopants in the Au-doped Ag 29 clusters. 28 And NMR measurements were also used to predict the Au location in the Ag 29 (BDT) 12 (TPP) 4 cluster, 56 see Fig. 5C. Overall, the SC-XRD, TIMS, EXAFS, XANES, DFT and TD-DFT methods were used to analyze the crystal structures of the Ag alloy NCs, which provide experimental and theoretical evidence to understanding the doping process. Although several structures of the Ag alloy NCs have been reported, further studies still need to focus on the preparation of Ag alloy NCs with more stable sizes and new types of dopants.

Charge state
Metal atoms show peculiarity in their outer electron arrangements and activities. Take 55 In the Ag 44 (SR) 30 template, the charge state remains the same. The charge states of Ag 20 and Ag 21 are illustrated in Scheme 1. Doping reduced charge stripping will bring more opportunities in their reaction activities. What we need to consider is how to understand the mechanisms and further explain the charge stripping.

PL properties
The most anticipated property of silver nanoclusters is their photoluminescence (PL) property. However, considering their further application in a wide range of elds, there is an urgent need for the synthesis of NCs with high quantum yield (QY) and long life. We have discussed the strategies for enhancing the luminescence of Ag NCs in our latest review; these properties are governed by their constitutions and structures. 24 There are two ways to improve the PL performance: modifying the metal kernel or tailoring the surface ligands. Doping is one of the most proven methods to improve the PL performance of Ag NCs. Bakr et al. 56 synthesized Ag 29Àx Au x (BDT) 12 (TPP) 4 (x ¼ 1-5) NCs by doping Au into Ag 29 (BDT) 12 (TPP) 4 clusters. The Au doped Ag 29 clusters achieved 26-fold PL QY enhancement, from 0.9% (Ag 29 ) to 19% (30% Au dopants) and 24% (40% Au dopants). Cui et al. 29 employed fs-ns TA spectroscopy to explore the electronic origins of PL enhancement. According to the TA spectra, there is an obvious shi in the GSB position at 435 nm for the Au doped Ag 29 clusters. Moreover, the ESA curve displayed a change in the range of 485-800 nm aer Au doping. Unlike the parent Ag 29 NCs, aer Au doping, the alloy shows improved time constants, where doping brings a longer lifetime, up to 612.65 ns (20% Au dopants) and 890.71 ns (40% Au dopants). The PL enhancement of the clusters depends on the nature of the excited state (S 1 ), which is highly affected by the doping amount of the Au atoms, see Fig. 6A. They used DFT and TD-DFT methods to explore the origin of the PL enhancement of Au doped Ag 29Àx Au x (x ¼ 1-5) clusters, in the lowest S 1 and S 0 electronic states. Generally, when the doping number of Au is 1 or 2, the PL performance of the Ag 29Àx Au x clusters is very close to that of the undoped Ag 29 cluster. When it is highly doped, with the doping number of Au going up to more than 3, there is an obvious increase in the PL intensity. This reveals that the crucial effects on the PL enhancement originate from Au dopants. Furthermore, they gured out that the Au-Ag bond played an important role in increasing the stability and governing the PL intensity. However, the Au amount effects cannot be generalized. In Mattoussi's work, the PL intensity does not simply increase with the increase of Au amount. 57 Pt doped Ag NCs also exhibit a noticeable PL enhancement. Basset et al. reported an $2.3 fold PL intensity increase in Pt doped Ag 29 NCs, [PtAg 28 (BDT) 12 (TPP) 4 ] 4À . 42 Moreover, an $80 nm red shi of the PL peak with broadening appeared aer doping, where the broadening was attributed to the partial replacement of TPP ligands with solvent DMF molecules. They highlighted the solvent-effect on PL enhancement of the PtAg 28 alloy, whose surface was exposed to the solvent. In Zhu's work, 43 they obtained Pt 1 Ag 28 (SR) 18 (PPh 3 ) 4 by the ligand exchange method based on Pt 1 Ag 24 (SPhMe 2 ) 18 . They compared the energy scales and PL of these two alloy NCs, see Fig. 6B and C. The energy gaps of Pt 1 Ag 24 and Pt 1 Ag 28 are 1.72 eV and 1.86 eV, respectively. Moreover, the solution color of Pt 1 Ag 24 is green whereas that of Pt 1 Ag 28 is orange, and the PL of Pt 1 Ag 28 can be observed by the naked eye under UV light in the dark. In addition, the QYs of these two Pt doped Ag NCs are very different: the QY of Pt 1 Ag 24 is only 0.1% while that of Pt 1 Ag 28 is up to 4.9% (about a 50-fold enhancement). The PL peak of Pt 1 Ag 24 is at 728 nm, and it shis to 672 nm when the Pt 1 Ag 28 is formed. They also compared the energy gaps and the PL spectra of Pt 1 Ag 12 (dppm) 5 (SPhMe 2 ) 2 , Pt 1 Ag 24 (SPhMe 2 ) 18 and Pt 2 Ag 23 (-PPh 3 ) 10 Cl 7 . 40 The PL emission of Pt 2 Ag 23 is at 675 nm, while it shied to 730 nm and 613 nm when it was converted to Pt 1 Ag 24 and Pt 1 Ag 12 , respectively, see Fig. 6D. Blue-shis appeared in Pt 2 Ag 23 and Pt 1 Ag 24 when they were converted to Pt 1 Ag 12 , which can be assigned to the increase of the optical energy gap. However, the red shi of the PL emission on the conversion of Pt 2 Ag 23 to Pt 1 Ag 24 points out that the energy gap cannot fully represent the energy of PL due to the dramatic electronic perturbation and high vibration loss. Moreover, the PLQY displays a 28 times increase from 0.2 to 5.5% by re-assembling from Pt 2 Ag 23 to Pt 1 Ag 12 . The PL spectra of Pt 1 Ag 28 with different kernel structures (FCC and icosahedron) are very similar. 45 The PL QY of Pt 1 Ag 28 (icosahedron) is only 2.7% at room temperature, while that of Pt 1 Ag 28 (FCC) is 4.9%. However, the PL of Pt 1 Ag 28 (icosahedron) is temperature-dependent. It emits bright-red light at low temperature, and the QY reaches 100% at 98 K or lower temperature. Moreover, the PL intensity increases dramatically at low temperature.
Wu and his co-workers 58 studied a series of MAg 24 (SR) 18 NCs, which were doped with different hetero atoms: Au, Pt and Pd. They compared the PL spectra of these Ag alloy NCs, see Fig. 6E. The Au doped MAg 24 (SR) 18 showed the highest PL intensity.
To sum up, heteroatoms play a positive role in tuning the PL properties of Ag NCs. Doping leads to an obvious enhancement in the PL intensity and QY. It would provide various opportunities to their potential applications in the elds of biolabelling, biomedicine, ion probing and so on. To synthesize and modify these functional alloy Ag NCs at the atomically precise level, we need to gure out the PL mechanism and the origin of PL enhancement. In previous studies, the origin of the PL enhancement was investigated via TA spectroscopy and DFT and TD-DFT methods. To date, the PL mechanism still needs deeper investigation. As we discussed above, many features can inuence the PL performance, such as the dopant species, the doping amount, the alloy kernel structures, the solvent, the temperature and so on. However, the effects cannot be explained with a unied mechanism. Further study needs to focus on the fundamental PL mechanisms of these alloy NCs.

Catalytic properties
As we discussed above, doping tunes the electronic structure of Ag NCs and inuences their catalytic properties. Zhu's group studied the catalytic performance of CNT-supported metal nanoclusters Au 24 Ag 46 NC. 36 Compared with undoped NCs, this bimetallic Au 24 Ag 46 NC takes advantage of both silver (high selectivity for benzaldehyde) and gold (high conversion) and showed higher conversion and selectivity for epoxides.
Zhu et al. 59 carried out centre doping of Ag 25 NCs with different foreign atoms (Au, Pt and Pd), and further studied their inuence on the catalytic performance in the carboxylation reaction of terminal alkynes with CO 2 , see Fig. 7. The doping atoms indeed contribute to the catalytic properties, and the enhancement factors depend on the nature of the foreign atoms. The elegant work from Zhu and co-workers revealed the crucial structure of the Au@Ag kernel of the Au 1 Ag 24 (SR) 18 NC, which showed high activity and stability in catalyzing CF 3ketone alkynylation. Their further studies demonstrated that the Au 12 Ag 32 (SR) 30 NCs, bearing a Au@Ag kernel, also showed high-activity, which conrmed their conclusion. 61 In Qin's work, 60 CeO 2 supported Au 13 Ag 16 L 24 NCs showed high catalytic activity and selectivity to A 3 -coupling reactions. In all, the doped Ag NCs show great potential for catalytic application, where the dopants enhance the catalytic activity and selectivity. DFT calculations help to analyse the effect of the dopants on the catalytic process and understand the improvement factors.

Conclusions and outlook
In conclusion, we have summarized the recent studies on tailoring Ag NCs via doping with hetero metal atoms. Doping is based on Ag NC templates, and classied by the dopants: Au, Pt, Pd, Cu and other hetero atoms. The synthetic methods and the structures of bimetallic Ag NCs are highlighted. Furthermore, the doping effects on the properties of Ag NCs have been concluded, including the location of dopants, charge states, PL properties and catalytic properties. Doping improves or diver-sies the properties of Ag NCs, providing versatile opportunities for their applications.
For future perspectives, we outline the following aspects: (1) To date, only a few sizes of Ag bimetallic NCs have been reported, and these reports are mainly concentrated upon the templates of Ag 25 and Ag 29 . Studies on trimetallic Ag NCs are scarcer. The synthesis methods for preparing hetero atom doped Ag NCs are limited, and most of them are carried out by the ligand exchange approach. Exploring facile synthesis methods for preparing new sizes of doped Ag NCs is required.
(2) Although the locations of doping atoms have been analysed via SCXRD measurements and DFT calculations and the diffusion process has been monitored by using in situ UV-vis absorption spectroscopy in combination with ESI mass and tandem mass spectrometry, 62 the fundamental physical mechanisms are not well understood. To control the doping location and design doped Ag NCs with atomic precision, the occupation mechanism needs to be further studied.
(3) As we discussed in our latest review, understanding the PL mechanism of Ag NCs is a big challenge for researchers. Although obvious PL intensity enhancement, PLQY increment and longer lifetime performance by doping Ag NCs were observed and the TA method or DFT calculations were applied to explain the experimental results, the universal mechanism of the PL properties is not clear. More studies on the PL mechanism of doped Ag NCs are needed.
(4) Doping improves the properties of Ag NCs, bringing opportunities for practical applications. For bio-application, we need to seriously consider the toxicity of these doped Ag NCs. As we discussed above, the addition of some dopants would lead to an open shell in the kernel, and expose the metals to the solution. This may increase the risk of toxicity. Therefore, the biological properties such as biocompatibility and toxicity of doped Ag NCs should be further explored before their application.

Conflicts of interest
There are no conicts to declare.