Synthesis And Emission Mechanism Of Luminescent Ag Nanoclusters

Luminescent metal nanoclusters (LMNC) are new materials which have been rapidly developed in recent years. Very recently, scientific researchers have found that chemically synthesized Au, Ag, Cu, and Pt nanostructures may be strong luminescent when their diameters below a typical size (normally?2 nm), thus called them luminescent metal nanoclusters. LMNCs have attracted tremendous research interests due to their wide applications in biosensors, cell imaging, and chemical catalysis, etc.But until now, most of the researches on LMNCs are focused on developing new synthesis procedure of novel LMNC materials and their applications. There are rare studies on their emission mechanisms. The existing theory cannot totally explain the emission phenomena of LMNCs. In order to deeply study the emission mechanism of LMNCs, we have synthesized Ag nanoclusters (Ag NCs) by UV photo-reduction strategy. The Ag NCs have diameters between 2 and 5 nm, and emission wavelength at about 650 nm. By adopting this Ag NC as a model, we have studied the emission mechanism of Ag NCs.Importantly, when we changed the synthesis conditions, such as, the molar ratio of COO-/Ag+ and the pH value, the relative absorption intensity at 435 nm and 505 nm can be tuned but the position of the bands remained unchanged. Thus we conclude that the absorption bands were not decided by the number of atoms inside the Ag(0) core. We suggest that the two adsorption bands come from the plasmonic resonance (435nm) and LMMCT in Ag NCs (505nm), respectively. As the emission band centered at 650 nm did not change with the reaction conditions, we believe that the number of Ag atoms in NCs has no effect on the emission band.The type of surface ligands was another critical synthesis parameter which impact the photoluminescence of Ag NCs. When the surface anchoring ligands were replaced by carboxylate-free poly(sodium-4-styrenesulfonate) (PSSS), the Ag NCs did not show any photoluminescence even though the sizes are similar. However, when the poly (methyl vinyl ether-alt-maleic acid) (PMVEM, MW: 80000?216000) with the same anchoring ligand as PMMA (MW= 9000) was used as the surface anchoring ligand, the Ag NCs exhibited similar optical properties as Ag NCs synthesized using PMMA. These solid experimental evidences confirmed that the photoemission in Ag NCs highly depends on the ligand-to-metal-metal charge transfer (LMMCT) from Ag(?)-carboxylate complexes to the Ag atoms.We have firstly proposed a new Ag(0)NCs@Ag(?)-carboxylate complex core-shell structural model to explain the emission mechanism of Ag NCs. The emission from the Ag-carboxylate NCs could be attributed to ligand-to-metal-metal charge transfer (LMMCT) from Ag(?)-carboxylate complexes (the oxygen atom in the carboxylate ligands to the Ag(?) ions) to the Ag atoms and subsequent radiative relaxation.Moreover, we found that the emission wavelength of the Ag NCs depends on the excitation wavelength. A strong coupling model between surface plasmon and Ag NCs emitter was proposed to explain this phenomenon. The theoretical calculation agreed well with the experimental data.