Condensed
Matter & Surface Sciences
COLLOQUIUM
Thomas A. Klar
Ludwig-Maximilians-Universität
München
Nano-optical
functions of
solid
state nanoparticle hybrid systems
Metal- as well as semiconductor nanocrystals show electromagnetic responses that are decisively determined by their nanoscopic dimensions. In the case of II-IV semiconductor nanocrystals the absorption and fluorescence spectra can be tuned by the size of the nanoparticles due to the quantum confinement effect. Differently, the absorption and scattering spectra of noble metal nanoparticles are dominated by the nanoparticle plasmon resonance which is a collective oscillation of the conduction band electrons. Hybrid systems comprising these solid state nanocrystals as well as chromophores or biomolecules show interesting spectroscopic features due to mutual electrodynamic interactions on a nanoscopic length scale.
For example, noble metal nanoparticles absorb energy from surface bound fluorescent molecules but also change their radiative lifetime. Both effects lead to a very efficient quenching of fluorescence [1,2] that can be used in biophysical applications. Furthermore, a change of the refractive index in the metal particles’ nanoenvironment shifts the scattering spectrum of the nanoparticles. This opens the way to use single gold nanoparticles as a sensitive assay for the detection of proteins.[3,4]
Semiconductor nanoparticles of several different sizes can be used to construct an energy gap gradient normal to multiple layers of nanocrystals applying the layer-by-layer technique. The diameters of the nanocrystals are monotonically increased or decreased in subsequent layers. In such devices we observe a highly efficient funnelling of excitation energy from layers comprising smaller nanocrystals towards the layer with the largest nanocrystals. Most important, not only excitations in radiative states are transferred but also excitations from trapped states, usually lost for luminescence, can be effectively recycled, hence increasing the overall luminescence yield.[5,6]
[1] E. Dulkeith et al. Phys. Rev. Lett. 89, 203002 (2002); [2] E. Dulkeith
et al., Nano Letters, 5, 585 (2005); [3] G. Raschke et al. Nano Letters, 3,
935 (2003); [4] G. Raschke et al. Nano Letters, 4, 1853
(2004); [5] T. Franzl et al. Nano Letters, 4, 1599 (2004); [6] T. A. Klar et
al. Advanced Materials, 17, 769 (2005)