Structural, Electronic And Optical Properties Of Silver-Copper Bimetallic Clusters

Bimetallic clusters have a potential application prospect in the field of photovoltaic,catalysis, and magnetic material because of their unique physical and chemical properties.However, so far the studies on the relationships between compositions, structures andproperties of bimetallic clusters (such as Ag-Cu bimetallic clusters) are not sufficient topromote their practical applications. In this paper, by virtue of computational materialscience and computer, the relationships between structure, composition, and size andperformances of AgmCun(m+n=2-55) bimetallic clusters are theoretically predicated, andtheir optical properties applied in solar cells are experimentally verified.Firstly, we systematically investigated structural, electronic, magnetic, optical spectra,Raman spectra, and vibrational properties of Ag-Cu clusters using density functionaltheory (DFT). The lowest-energy stable motifs of Ag-Cu are primarily related to thequantity of Cu-Cu bonds, Ag-Cu bonds and Ag-Ag bonds, the surface energies of Ag andCu, the maxmium charge transfers between Ag and Cu, and the strain relaxation. TheAg5Cu2with two Cu atoms lie at the axis (2-I), the Ag4Cu4with tetracapped tetrahedron(TcTd), and the Ag2Cu11, Ag7Cu6and Ag11Cu2clusters are the lowest energy clusters in thefamily of the7,8, and13-atom Ag-Cu nanoclusters, respectively. The Ag5Cu22-I andAg7Cu6clusters with mixed motifs indicate that silver and copper may be miscible on thenanoscale but not in bulk. The magnetic properties of the13and55-atom Ag-Cu clustersindicate that silver and copper may have magnetism on the nanoscale but not in bulk. AllRaman and vibrational spectra of Ag-Cu clusters exhibit many significant vibration modes,the differences between Ag-Cu clusters in Raman and vibrational spectra are obviouslyobserved. The calculated vibrational and Raman spectroscopy of Ag-Cu clusters may behelpful in determining the size, composition and structure of the experimental cluster.Secondly, optical absorption spectra of silver-copper clusters were calculated usingtime-dependent DFT (TD-DFT) and symmetry adapted cluster theory with configurationinteraction (SAC-CI), and multiple exciton generation (MEG) behavior is observed for thefirst time in metal clusters. When the sizes, compositions, structures, and charges ofAg-Cu clusters alter, their optical absorptions exhibit redshifts or buleshifts. Thecalculated optical spectroscopy of Ag-Cu clusters also may be helpful in determining thesize, composition and structure of the experimental cluster. On the whole, the opticalspectra of Ag-Cu clusters obtained using TD-DFT and SAC–CI are in excellent agreement with experiment spectra. Both TcTd and monocapped pentagonal bipyramid (McPb) ofpure Ag8and Cu8clusters contribute to their experimental absorption spectra. It is inferredthat both TcTd and McPb of AgnCu8-n(n=1-7) clusters contribute to their experimentalabsorption spectra. The Ag chains doped with transition metal (TM) atom create anadditional peak with a local plasmon resonance mode which is related to chargeredistribution in the chain around TM atom. Multiple excitons appear in the visible andnear ultraviolet light ranges. Single excitations are mainly contribution for the opticalspectra, while the multiple excitons merely contribute for some peaks at the higherenergies. However, occurrence of MEG enhances the optical absorption in Ag–Cu clusters.Thirdly, within the framework of DFT with different kinds of exchange-correctionfunctions, as well as DFT-based semi-core pseudopotential (Dspp) and all-electron (AE),we investigated optical, Raman and vibrational properties of Ag–Cu clusters.Exchange-correction functions have little effect on the optical, Raman and vibrationalspectra of Ag–Cu clusters. For the8-atom Ag-Cu clusters, comparing these spectra usinggeneral gradient approximation (GGA) with that using local density approximation (LDA),position energies of corresponding peaks are lower, in particular, which obviously showsin the rich Cu atoms cluster. However, pseudopotentials have some effects on them. Theintensities of absorption peaks for Ag–Cu clusters using Dspp are weaker than that usingAE, in particular, this effect of rich Ag atom clusters is shown obviously. The intensities ofRaman peaks for Ag–Cu clusters using Dspp are larger than that using AE. An additionalpeak appears in the vibrational spectra of TcTd using Dspp but not in ones using AE forAgnCu8-n(n=2-8) clusters. The long-range correction applied to PBE (LC-ωPBE, ω=0.40)likely leads to enengy of absorption peak lowering and the number of excited statesreducing.Finally, we investigate bimetallic Ag-Au and Ag-Cu clusters effects on their opticalabsorptions and performances of solar cells. Compared with pure metal clusters, theoptical spectrum of Ag-Au and Ag-Cu clusters redshifts and broadens. The Ag-Au andAg-Cu clusters modified TiO2electrode improves short-circuit current density (Jsc) andmaximum power conversion efficiencies (η) of solar cell and keeps photocurrent stabilityof solar cell compared with that of pure metal clusters modified TiO2electrodes. TheAg-Au and Ag-Cu clusters can function as a new photosensitizes for solar cell. However,the costs of bimetallic clusters are cheaper than that of pure metal clusters.