title: The superstable 25 kDa monolayer protected silver nanoparticle: measurements and interpretation as an icosahedral Ag152(SCH2CH2Ph)60 cluster.
abstract: A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH(2)CH(2)Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a well-defined metal core of ~2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster-Ag(152)(SCH(2)CH(2)Ph)(60)-evolves into a stable inorganic core-Ag(152)S(60). Exploiting combined insights from investigations of clusters and surface science, a core-shell structure model was developed, with a 92-atom silver core having icosahedral-dodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ~0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.
authors.affiliation: DST Unit of Nanoscience (DST UNS), Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India.
authors.forename: Indranath
authors.initials: I
authors.lastname: Chakraborty
authors.forename: Anuradha
authors.initials: A
authors.lastname: Govindarajan
authors.forename: Jayanthi
authors.initials: J
authors.lastname: Erusappan
authors.forename: Atanu
authors.initials: A
authors.lastname: Ghosh
authors.forename: T
authors.initials: T
authors.lastname: Pradeep
authors.forename: Bokwon
authors.initials: B
authors.lastname: Yoon
authors.forename: Robert L
authors.initials: RL
authors.lastname: Whetten
authors.forename: Uzi
authors.initials: U
authors.lastname: Landman