Plasmonic Enhancement or Energy Transfer? On the Luminescence of Gold-, Silver-, and Lanthanide-Doped Silicate Glasses and Its Potential for Light-Emitting Devices By Maik Eichelbaum and Klaus Rademann* 1. Introduction The surface plasmon resonance of gold and silver nanoparticles is a remarkable phenomenon that already has fostered many new powerful analytical techniques and applications. The collective oscillation of the noble metal valence electrons resonantly excited by visible light, the surface plasmon resonance (SPR), causes a tremendous enhancement of the electromagnetic near-field in the immediate vicinity of the particles. Char- acteristic enhancement factors of the electric field E are in the range of 10 2 . [1] This means that molecules adsorbed to these noble metal particles or that are at least not more than 10 nm apart from them feel up to a 100 times more intense field as opposed to the direct excitation in a plasmon-free environment. In this way, the Raman scattering of adjunct molecules as well as their luminescence can be enhanced enormously. These phenomena are commonly known as surface-enhanced Raman scattering (SERS) [1,2] and metal- enhanced fluorescence (MEF) or phosphor- escence (MEP) (or generally metal-enhanced luminescence, MEL), [3,4] respectively. However, MEL can be mistaken with a ‘classical’ luminescence energy transfer between the metal particle and the lumines- cent molecule. In a Fo ¨rster resonance energy transfer (FRET) process, the donor chromo- phor absorbs the excitation light first and then transfers the energy by a non-radiative multipole coupling mechanism to the accep- tor luminophor. For noble metal particles, the correct assignment of the different enhance- ment mechanisms becomes even more complicated, because the particles can also quench the luminescence of molecules by a non-radiative transfer of energy from the excited luminophor to the metal under excitation of plasmons that subsequently decay non-radiatively. [3,4] All these processes are a function of metal particle-luminophor distance, but are a function of particle size as well. Generally, gold and silver particles with diameters larger than 5 nm show a strong plasmon absorption, and therefore enhance the luminescence by a plasmonic near-field enhancement in a typical distance between 1 and 10 nm and/or quench the luminescence of atoms or molecules that are directly attached to them. In contrast, very small, sub-nanometer sized noble metal particles do not exhibit an SPR effect due to their low density of states. [1] Consequently, a plasmonic enhancement by these ‘molecule-like’ metal clusters is very unlikely. However, the fabrication of samples containing nearly monodisperse particles is especially for very small particles still not straightforward. And therefore, the differentiation between the different luminescence enhancement mechanisms has remained a challenge. FULL PAPER www.afm-journal.de [*] Prof. Dr. K. Rademann, Dr. M. Eichelbaum Institut fu¨r Chemie, Humboldt-Universita ¨t zu Berlin Brook-Taylor-Straße 2, 12479 Berlin (Germany) E-mail: klaus.rademann@chemie.hu-berlin.de Dr. M. Eichelbaum Department of Earth and Environmental Engineering (HKSM) Columbia University 500 West 120th Street New York, NY 10027 (USA) DOI: 10.1002/adfm.200801892 With the technique of synchrotron X-ray activation, molecule-like, non- plasmonic gold and silver particles in soda-lime silicate glasses can be generated. The luminescence energy transfer between these species and lanthanide(III) ions is studied. As a result, a significant lanthanide luminescence enhancement by a factor of up to 250 under non-resonant UV excitation is observed. The absence of a distinct gold and silver plasmon resonance absorption, respectively, the missing nanoparticle signals in previous SAXS and TEM experiments, the unaltered luminescence lifetime of the lanthanide ions compared to the non-enhanced case, and an excitation maximum at 300–350 nm (equivalent to the absorption range of small noble metal particles) indicate unambiguously that the observed enhancement is due to a classical energy transfer between small noble metal particles and lanthanide ions, and not to a plasmonic field enhancement effect. It is proposed that very small, molecule-like noble metal particles (such as dimers, trimers, and tetramers) first absorb the excitation light, undergo a singlet- triplet intersystem crossing, and finally transfer the energy to an excited multiplet state of adjacent lanthanide(III) ions. X-ray lithographic microstructuring and excitation with a commercial UV LED show the potential of the activated glass samples as bright light-emitting devices with tunable emission colors. Adv. Funct. Mater. 2009, 19, 2045–2052 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2045