Stability , electronic structure , and optical properties of protected gold-doped silver Ag 29-x Au x ( x = 0-5 ) nanoclusters

In this work we used density functional theory (DFT) and linear response timedependent DFT (LR-TDDFT) to investigate the stability, electronic structure, and optical properties of Au-doped [Ag29-xAux(BDT)12(TPP)4] 3nanoclusters (BDT:1,3-benzenedithiol; TPP triphenylphosphine) with x=0-5. The aim is to shed light on the most favorable doped structures by comparing our results with previously published experimental data. The calculated relative energies, ranging between 0.8-10 meV/atom, indicate that several doped Ag29-xAux nanoclusters are likely to co-exist at room temperature. However, only the Au-doped [Ag29xAux(BDT)12(TPP)4] 3nanoclusters that have direct bonding between Au dopants and phosphines display an enhancement in the electronic transitions at ~450 nm. This agrees with the main spectral absorption features that have been experimentally reported for the mixture of Au-doped silver Ag29-xAux nanoclusters. In addition, the formation of Au-TPP bonding could prevent cluster degradation starting from the detachment of the phosphine molecules, since the Au-TPP Page 1 of 18 Physical Chemistry Chemical Physics P hy si ca lC he m is tr y C he m ic al P hy si cs A cc ep te d M an us cr ip t Pu bl is he d on 0 4 M ay 2 01 7. D ow nl oa de d by J yv as ky la n Y lio pi st o on 1 0/ 05 /2 01 7 09 :5 4: 31 . View Article Online DOI: 10.1039/C7CP01440F


INTRODUCTION
Several Ag-based nanoclusters stabilized by polymers 1 , dendrimers, 2 peptides, 3 proteins, 4 or stranded DNA, [5][6][7][8] display remarkable luminescence properties that render them promising for biolabeling, 9 sensing, 1,10 and bioimaging. 3,4,9,11,12 Nonetheless, the luminescence of such materials is strongly sensitive to the conformational structure and/or sequence of the protecting biomolecules. 7,13 On the other hand, the synthesis of stable silver nanoclusters protected with smaller molecules, such as thiols, has been challenging and only done fifteen years after the synthesis of the first thiolate-capped gold nanocluster. [14][15][16] Recently, the groups of Bakr and Zheng have implemented an interesting strategy to stabilize thiolate-protected silver nanoclusters based on the use of a mixture of thiols and phosphines as protecting agents. [17][18][19] With this approach, they have successfully synthesized silver nanoclusters with non-magic superatomic electronic configurations 17 and non-spherical shapes. [17][18][19] They found that in such materials the phosphines attach to the outermost layer forming an additional protecting shell. [17][18][19][20][21] The group of Bakr also reported that the incorporation of Au dopants to the thiolate-phosphine-protected Ag 29 nanocluster significantly enhance the cluster stability, and surprisingly, its photoluminescence quantum yield. 21 According to the mass spectrometry measurements, the Au-doped silver nanoclusters synthesized in the group of Bakr have molecular masses that correspond to [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3structures mainly with x=1-5. Furthermore, the P NMR analysis suggests that Au dopants bond to phosphines. 21 While the determination of the crystal structure of 3 [Ag 29 (BDT) 12 (TPP) 4 ] 3and [Ag 29 Au(BDT) 12 (TPP) 4 ] 3has been achieved, 21 the structure of the Au-doped nanoclusters with 2≤x≤5 remains unknown. As a consequence, the source of the enhancement of the photoluminescence quantum yield has still not been clarified.
The aim of this work is to unveil the molecular structure of the energetically-most favorable and likely Au-doped [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanoclusters present in the synthesized samples. 21 For that purpose, we calculate and compare the ground states energies of different [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanoclusters with x=0-5 by using DFT calculations. Additionally, we calculate the optical spectra of the pristine and doped nanoclusters and contrast them with the experimental data. 21 The calculated energy differences, ranging from 0.8-10 meV/atom, suggest that several Au-doped [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3isomers could to co-exist in the synthesized mixture of products. These energy differences per atom are of the same order of magnitude as difference between the energy of the fcc (face-centred cubic) Au and the metastable hcp (hexagonal close-packed) Au phases. [22][23][24][25][26] Moreover, the comparison between the calculated and the experimental optical spectra indicates that main absorption features, such as the broadening and the blue-shift of the characteristic Ag 29 peak (at 447 nm), of the experimental spectra result from the contributions of the diverse Au-doped isomers co-existing in the synthesized mixture.
Particularly, the blue-shifting is due to the presence of nanoclusters whose molecular structure contains direct Au-TPP bonding.  Bakr. 20,21 According to the total electron count formula introduced 4 by introduced by Häkkinen and collaborators 27 all the studied nanoclusters are 8-electrons closed-shell structures. Figure 1 illustrates the structure of a [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanocluster and its different shells (S0-S3) that can be occupied by the Au dopants. The experimental and built nanocluster structures were optimized in a cubic box with a grid spacing of 0.2 Å. All the calculations were carried out by using the real-space code-package GPAW (grid-based projector-augmented wave method). 28,29 The optimizations and electronic structure calculations were performed in the frame of density functional theory (DFT) by using the generalized gradient-corrected PBE 30 functional. The van der Waals vdW-DF-CX 31,32 functional was also used to compare the ground state energies and geometries with the ones obtained from the PBE calculations. The convergence criterion for the forces acting on individual atoms was set to a maximum value of 0.05 eV/Å.
The optical spectra were calculated by using Casida´s formulation of linear response time dependent DFT (LR-TDDFT). 33,34 The electronic transitions were analyzed based on the time-5 dependent density functional perturbation theory (TD-DFPT) 35 following the method reported earlier by Malola et al. 36 The Kohn-Sham wave functions were projected onto the spherical harmonic (Y lm ) basis by using the cluster and the atomic radial cutoffs of 11.5 Å, for the cluster, and 0.85 Å for C, 1.40 Å for Ag and Au, and 1.1 Å for P and S. The overlap integrals were broadened (with a Gaussian function of width = 0.01 eV) and plotted as projected local density of states (PLDOS) per cluster and per atom. The atomic charges were calculated by using the Bader charge analysis. 37

RESULTS AND DISCUSSION
In tables 1 and S1 (see Supplementary Information, SI), we show the set of [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanoclusters studied in this work. The relative energies of each cluster are calculated with respect to its lowest-energy isomer. The results from both the PBE and van der Waals functionals demonstrate that nanoclusters doped at the inner shells, S0 and S1, are energetically more stable compared to structures in which Au atoms are at the S2 and/or S3 shells. Nevertheless, the energy difference between the calculated isomers range between 0.243-3.085 eV, that is 0.8-10 meV/atom (considering the 309 atoms constituting the nanoclusters).
The most extreme cases are the structures of type xA and xD. In the former one, all of the 1-5 Au atoms are at the innermost shells forming the energetically most stable structures, and in the latter one, 1-4 of the Au dopants are at the outermost shell having higher energy structures.
According to DFT-based calculations, the energy difference between fcc-hcp and fcc-bcc phases of gold and silver are between 0.6-3.4 meV/atom 22-25 and 19-23 meV/atom, 22 respectively. Moreover, the hcp Au phase, which is stable under ambient conditions, transform to fcc phase on exposure to a transmission electron microscope beam. 26 This might suggest that in the mixture of However, it is worth to point out that the increase of the stability is not exclusively caused by the Au-TPP bonding, but also by the Au doping at innermost shells as shown in Table 1. Besides, the formation of Au-SR bonding by doping at the S2 shell could also contribute to the enhancement of the cluster stability since the interaction between gold and thiols is known to be stronger than in the case of silver. 38 According to the results obtained from the Bader charge analysis (see Table S2), the atomic charge on each atom remains almost unaffected by the number and the location of Au atoms within the cluster. Nevertheless, the largest variation (0.212 a.u.) is observed in the atomic charge of P from the reference TPP molecule (with atomic charge 1.382 a.u. on P) to the 3D structure (where the atomic charge of P is 1.594 a.u.), which has three TPP molecules attached to the Au dopants. This suggest that there is a slightly more charge transfer from phosphorous to the rest of the molecule through Au-P bonding compared to the Ag-P one. This could also give insight into the shifting of the P NMR signal that has been observed for the mixture of Ag 29-x Au x nanoclusters. 21 In Figure 2, we show the optical spectra of the [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanoclusters, with x=0-5, for the structures of type xA and xD. The comparison between both sets of spectra reveals that for the structures with Au-TPP bonding, xD, a third absorption peak appears at ~450 nm which is not observed in the spectra of the corresponding isomers with structure xA. To have a more complete picture, we also calculated the absorption spectra of the 8 [Ag 26 Au 3 (BDT) 12 (TPP) 4 ] 3nanoclusters with structures 3B, 3C, 3F, and 3H (see Figure 3 and Figure S4).  to our results shown in figures 2,3, and S4, we suggest that the blue-shift of the main absorption peak that has been experimentally observed is mainly a consequence of the appearance of the absorption peak at ~450 nm in the spectra of the nanoclusters that contain Au-TPP bonding.
Moreover, the intensity of this peak increases with the number of Au-TPP bonds, e.g. from structure 3H (with one Au-TPP bond) to 3D (with three Au-TPP bonds), as shown in Figure S4.
According to PBE calculations, the relative energies of the structures 3H, 3F, and 4F, which also contain Au-TPP bonding, are significantly smaller than for their corresponding 3D and 4D isomers (c.f. tables 1 and S1). These results indicate that the Au-TPP bonding is not exclusive of the high energy xD structures, and that species with lower energies could contribute significantly to the observed blue-shifting of the main absorption peak. On the other hand, based on the small energy differences between the Au doped isomers we suggest that the broadening of the peak in the experimental absorption spectra of the Au-doped samples results from the contributions from all nanoclusters co-existing in the synthesized mixture of products. Thus, our results support the statement made by experimentalists 21 that among the possible synthesized nanoclusters there are species with chemical structure that contain direct Au-TPP bonding.
10 To identify the energy range of the electronic transitions associated to the main absorption peaks, we analyzed the spectral features of the 3A and 3D structures using transition contribution maps (TCMs) (see Figure 4 and Figure S2 in SI). We found that in both 3A and 3D structures the transitions related to the first two peaks at 543-555 nm and 472-488 nm are mainly originated from the occupied states at around -1.20 eV to the unoccupied states at ~1.20-1.40 eV (see Figure S2). However, at 450 nm the most intense electronic transitions occur between different states in the 3A and 3D structures. While, in the case of the 3A structure the most intense electronic transitions arise from the occupied states at around -1.60 eV to the unoccupied states 11 at 1.20 eV, in the 3D structure they occur from the states at -1.32 eV to the states at 1.39 eV (see

Figure 4).
To distinguish between the atomic orbitals involved in the electronic transitions at ~450 nm, we analyzed the PLDOS per atom (and per phenyl ring) for the 3A, 3C, and 3D structures (see In Figure 5, we also show the shape of the molecular orbitals of the 3D nanocluster that appear at -1.32 eV and 1.30-1.39 eV, and that correspond to HOMO -15 and LUMO + 18, LUMO + 19, and LUMO + 20. Note that for simplicity we draw the three unoccupied states in a single picture.
The presence unoccupied states surrounding the nanocluster edges, where the Au-TPP bonds are located, also points out the importance of the Au-TPP bonding to determine electronic 13 transitions, and therefore, the optical properties of the thiolate-phosphine-protected Ag 29-x Au x materials.

CONCLUSIONS:
In this work, we show the importance of the bonding interaction between Au dopants and phosphine molecules in thiolate-phosphine-protected Ag nanoclusters. Particularly, we demonstrate that in the recently synthesized mixture of Au-doped [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3nanoclusters the interaction between Au-TPP play a crucial role in determining the cluster stability and electronic transitions. We show that the formation of Au-TPP bonds in the [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3structures could prevent the degradation of the cluster starting from the detachment of the phosphine molecules, since the Au-TPP bond is ~0.4 eV stronger than the analogous Ag-TPP one. Moreover, nanoclusters that have Au-TPP bonding exhibit an enhancement of the electronic transitions at ~450 nm, which is not observed in nanoclusters with pure Ag-TPP bonds. These electronic transitions, which involve the local p, d and s states of gold, explain the blue-shifting and broadening of the main absorption peak that has been experimentally observed in the synthesized mixture of products. We, moreover, suggest that the formation of Au-TPP bonds in the Au-doped nanoclusters could play a role in the enhancement of the photoluminescence quantum yield that has been experimentally observed. On the other hand, the small energy differences between the Au-doped isomers point out the importance of developing new strategies to produce (or isolate) the nanoclusters in which the Au dopants are strategically bonded to phosphine molecules.