Based On The Magic Number Clusters (of Sio <sub> 2 </ Sub>) <sub> 4 </ Sub> The O <sub> 2 </ Sub> H <sub> 4 </ Sub> Au Adsorption Clusters Optical Nature Of The Study

Nano-materials are widely attractive and useful in many fields,such as semiconductors, biomedicine and optics,because of their unique properties compared with the bulk counterparts.Recent researches have found the gold nanoparticles had advantages in photodiagnostics and photothermal therapy for tumors because of their enhanced surface plasmon resonance,as well as their noncytotoxicity and biocompatibility.In particular,the silica-gold nanoshells exhibited useful effect in this application in experiments,because their absorption wavelengths are easily tuned to the NIR region by changing the thick ratio between the shell and the core.Considering the structures of nanoshells,two groups of shell-like clusters in molecular scale were designed,Au_n(SiO₂)₄O₂H_4-n(n=1-4) and Au_n(SiO₂)₄O₂(n=5-8), which were obtained by substituting Au atoms for the H ones in the magic number cluster(SiO₂)₄O₂H₄ with S₄ symmetry.The structures of these clusters were optimized by the Gaussian 03 program using DFT method and then the excitation energies were calculated using TD-DFT method.Finally the absorption spectra of these clusters were obtained and analysed.The results of all optimizations shown that trivial changes in the bond lengths and bond angles happened to those clusters and especially the core cluster(SiO₂)₄O₂ almost kept its original structure with T_d symmetry.Moreover,the structures of whole clusters were determined by the core structure.For the optimized structures,adsorption energies of Au atoms and Au₂ radicals were calculated to analyse the stabilities.It was found that the Au atoms in these shell-like clusters had lower adsorption energies than the H atoms.However,the values of the adsorption energies still met the adsorbed conditions for common experimental circumstances.Moreover,the stability of the core duster(SiO₂)₄O₂ suggests the use of it as a building block for exploring new cluster materials.The absorption spectra shown that great change happened to the optical properties of the clusters when the Au atoms were introduced,which was concluded by the reduction of optical gaps from the UV region to the visible and infrared regions.From the results of electronic orbital transitions,it was concluded that the weak electronic charge transfers on Au-O and Au-Au bonds induced the visible absorption at about 440 nm and the NIR absorption at 900～1200 nm,respectively.As a result,for the clusters of each group,the type of the adsorbates determined the absorption wavelength and the number of the adsorbates induced the linear variation of the absorption intensities.These features are advantages for the optical application of these clusters.More interestingly,the clusters of second group,especially Au₈(SiO₂)₄O₂,can absorb the NIR light strongly,which satisfies the demand of photodiagnostics and photothermal therapy for tumors.So it is promising to use these shell-like clusters for this application like the nanoshells.