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2008, 112, 19797–19800 Published on Web 11/21/2008
Origin of Discrete Optical Absorption Spectra of M25(SH)18- Nanoparticles (M ) Au, Ag) Christine M. Aikens Department of Chemistry, Kansas State UniVersity, Manhattan, Kansas 66502 ReceiVed: October 14, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008
Time-dependent density functional theory calculations at the SAOP/TZP level of theory are employed to study the optical absorption spectra of pure metal nanoparticles (Au25(SH)18- and Ag25(SH)18-) and mixed metal “core-shell” systems (Au13Ag12(SH)18- and Ag13Au12(SH)18-). All four systems exhibit discrete absorption spectra. The splitting between the LUMO and LUMO+1 and between the HOMO and HOMO-1 varies markedly with the metal used in the core and in the oligomeric thiolate ligands. The optical absorption spectra are not separable into core and ligand contributions; geometric and electronic interactions between the two fragments are responsible for complex absorption spectra. Monolayer-protected nanoparticles are of increasing interest because of their unique optical, biological, catalytic, and electronic applications.1-4 Whereas larger gold and silver metal nanoparticles display strong surface plasmon absorption that results in a sharp peak in the visible region of the optical spectrum, thiolated gold nanoclusters with diameters less than 2 nm exhibit a series of discrete absorption features that scale with size.5 Of these, the Au25(SR)18 nanoparticle exhibits unique properties in addition to remarkable stability. The optical absorption spectrum and highest occupied molecular orbitallowest unoccupied molecular orbital (HOMO-LUMO) gap of the nanoparticle appear to be fairly insensitive to the nature of the thiolate ligand R.6,7 Au25(SR)18 shows unusually strong circular dichroism (R ) glutathione),8 visible to infrared luminescence (R ) glutathione),2 and efficient two-photon absorption (R ) hexanethiolate).9 It also exhibits a high resistance to decomposition10 and core etching11 and is stable regardless of the charge state of -1, 0, or +1.7 Recent total structure determination of Au25(SR)18 (R ) phenylethylthiolate) in the anionic12-14 and neutral15 charge states shows that the structure consists of a nearly icosahedral Au13 core surrounded by six V-shaped -S-Au-S-Au-Smotifs in an approximately octahedral arrangement (Figure 1). These oligomeric Au(I)-SR motifs are also evident in the crystal structure of Au102(p-SPhCOOH)44.16 Time-dependent density functional theory (TDDFT) has been effectively employed to assign multiple transitions observed in the experimental UV-vis spectrum of the Au25(SCH2CH2Ph)18- nanoparticle using a model Au25(SH)18- system.14 This prior investigation yielded a semiquantitative correlation between the experimental and theoretical optical absorption spectra. Electronically, the Au25(SR)18- nanoparticle may be viewed as an Au13+5 core surrounded by six anionic Au2(SR)3- motifs, which leads to a formal count of eight electrons in the core.13,14 The HOMO is approximately triply degenerate,13,14 and the density of states projected onto spherical harmonics shows that these three orbitals possess mainly P-character, in agreement with the known 8-electron S2P6 shell closing in the superatom picture 10.1021/jp8090914 CCC: $40.75
for metals with a single valence s electron.13 This superatom picture is also applicable to larger gold nanoparticles.17 Recent experimental observation of discrete optical absorption spectra for silver nanoparticles with diameters
