The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of 1 nm radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye's radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model. DOI: 10.1103/PhysRevLett.89.203002 PACS numbers: 33.50.-j, 81.07.Pr Resonant energy transfer (RET) systems consisting of organic dye molecules and noble metal nanoparticles have recently gained considerable interest in biophotonics [1][2][3][4] as well as in materials science [5,6]. Closely related are donor-acceptor pairs of organic dye molecules forming Förster resonant energy transfer (FRET) systems. They have been theoretically modeled [7] and applied in biophysics extensively during the past decade (see, e.g., [8]). Yet these classical purely dye-based systems show disadvantages regarding quenching efficiency [4] and photostability [9].If the donor molecule is placed in the vicinity of a metal surface instead of an organic acceptor, not only resonant energy transfer takes place but also the radiative lifetime of the donor molecule changes. For metal films this has been investigated extensively [10 -13]. Much less is known about donor molecules in the vicinity of metal nanoparticles. Theoretical treatments of the moleculenanoparticle problem [14 -17] predict energy transfer rates and radiative decay rates that deviate substantially from what has been found for dye molecules in front of a metal film. Both radiative and nonradiative rates are expected to depend critically on size and shape of the nanoparticle, the distance between the dye molecule and the nanoparticle, the orientation of the molecular dipole with respect to the dye-nanoparticle axis, and the overlap of the molecule's emission with the nanoparticle's absorption spectrum. Recent experimental investigations deal with metal island films or rough surfaces only (see [18,19] and references in [20]), where the above mentioned parameters are undefined.Here we report results of time-resolved fluorescence experiments on a donor-acceptor system composed of lissamine molecules (donor) chemically attached to a gold nanoparticle (acceptor). The distance between the lissamine molecule and the surface of the nanoparticle is kept constant at 1 nm, whereas the nanoparticle radius is varied between 1 and 30 nm. We find time constants for the energy transfer on a picosecond time scale which turn out to decrease with increasing nanoparticle size. In addition, the dye's radiative rate is reduced by more than an order of magnitude. Both effects are responsible for the drastic quenching of the fluorescence yield as predicted by the so-called...

Light emission at the particle plasmon frequency is observed in optically excited spherical gold nanoparticles. We find a photoluminescence efficiency of 10 −6 , which is essentially independent of particle size and four orders of magnitude higher than the efficiencies determined from metal films. Our experimental findings are explained with a process in which excited d-band holes recombine nonradiatively with sp electrons, emitting particle plasmons. These plasmons subsequently radiate, giving rise to the photoluminescence observed in the experiment. We determine the quantum efficiencies involved in this process.

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres-chemical species whose oxidation number, and hence electronic structure, can be changed reversibly-support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.

4‐dimethylaminopyridine (DMAP) is the answer to the quest for an efficient transfer of metallic nanoparticles from organic to aqueous solutions. The picture shows the transfer of gold nanoparticles from toluene to water by the addition of DMAP (0.1 M, pH 10.5). This method enables the generation of high concentrations of nanoparticles with better monodispersity than those commonly prepared in water.