Silver atomic energy levels

Abstract Silver clusters, composed of only a few silver atoms, have remarkable optical properties based on electronic transitions between quantized energy levels. They have large absorption coefficients and fluorescence quantum yields, in common with conventional fluorescent markers. But importantly, silver clusters have an attractive set of features, including subnanometer size, nontoxicity and photostability, which makes them competitive as fluorescent markers compared with organic dye molecules and semiconductor quantum dots. In this chapter, we review the synthesis and properties of fluorescent silver clusters, and their application as bio-labels and molecular sensors. Silver clusters may have a bright future as luminescent probes for labeling and sensing applications. 

Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous band of energy levels, the limited number of atoms in metal clusters results in discrete energy levels, allowing interaction with light by electronic transitions between energy levels. Metal clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not yet firmly established...

Ultraviolet photoemission spectroscopy (UPS) has been used to measure the energy levels of small silver particles in a model system. We find that small particles have properties quite different from those of the bulk and that their electron-donating ability increases with size. The UPS difference spectrum of halogen chemisorbed to silver particles changes up to a size of 40 silver atoms, indicating the size range over which chemical properties are size dependent. 

Figure 7. Energy levels of the silver atom and the silver ion in aqueous solution in the presence and absence of cyanide3 ...

Figure 11. Comparison of the experimental valence band spectrum for 2.Sxl0l atoms/cm silver on PPS and the calculated discrete orbital energy levels deteimined from the molecular orbital calculation on the PPS fragment bonded to silver in the trigonal pyramidal and trigonal planar sulfur bonding configurations.

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

---