title: Metal core bonding motifs of monodisperse icosahedral Au13 and larger Au monolayer-protected clusters as revealed by X-ray absorption spectroscopy and transmission electron microscopy.
abstract: The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.
authors.affiliation: School of Chemical Sciences and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Laurent D
authors.initials: LD
authors.lastname: Menard
authors.forename: Huiping
authors.initials: H
authors.lastname: Xu
authors.forename: Shang-Peng
authors.initials: SP
authors.lastname: Gao
authors.forename: Ray D
authors.initials: RD
authors.lastname: Twesten
authors.forename: Amanda S
authors.initials: AS
authors.lastname: Harper
authors.forename: Yang
authors.initials: Y
authors.lastname: Song
authors.forename: Gangli
authors.initials: G
authors.lastname: Wang
authors.forename: Alicia D
authors.initials: AD
authors.lastname: Douglas
authors.forename: Judith C
authors.initials: JC
authors.lastname: Yang
authors.forename: Anatoly I
authors.initials: AI
authors.lastname: Frenkel
authors.forename: Royce W
authors.initials: RW
authors.lastname: Murray
authors.forename: Ralph G
authors.initials: RG
authors.lastname: Nuzzo