Surface plasmon enhanced optical properties & Förster resonant energy transfer between Colloidal Quantum Dots
Colloidal semiconductor quantum dots (QDs) (see Figure 1) have recently attracted a lot of interest in application oriented as well as fundamental research. Due to their unique optical properties including tunable emission wavelength, broad absorption, high quantum yield and high photostability as well as the possibility to functionalize their surface, they offer a great base unit / scaffold for numerous applications such as (bio-)sensors, light emitting structures or photovoltaic devices ( and references within).
Figure 1.CdTe QDs in water excited by a UV-lamp.
As mentioned above QD exhibit unique optical properties, including tunable emission wavelength and high quantum yields, as such QDs represent valuable energy donors and acceptors for FRET.
Nano-assemblies in solution
Recently there has been an increased interest in composite nano-assemblies for use in sensing and light harvesting
applications [4, 5]. These structures employ FRET to generate energy flow.
We are investigating the formation of electrostatically-bonded nanostructures created from oppositely charged
colloidal QDs and examining the optical properties of these nano-assemblies for a range of concentration ratios.
The oppositely charged QDs are of two different sizes and have been selected on account of their spectral properties which
should allow for resonant energy transfer. Energy transfer processes are studied by means of steady-state and time-resolved
photoluminescence spectroscopy. The clustering mechanism by which the nano-assemblies form is monitored via TEM and is the
subject of further investigation through dynamic light scattering experiments and fluorescence correlation spectroscopy.
Most FRET theories have been developed based on molecules, representing the donor and acceptor as point dipoles. However, it is not clear how well these theories can be applied to QDs as, in comparison to molecules, they have a larger size (comparable to the Förster radius) and show an inhomogeneous broadening. Therefore it is important to investigate FRET in QD structures in more detail and to find out how these two properties influence the QD – QD FRET. A Layer-by-Layer deposition technique is used to prepare (multi-)layer structures composed of the negatively charged CdTe QD donors and acceptors. Structures with a mixed monolayer containing both types of QDs as well as structures with separated donor and acceptor QD layers are investigated by steady-state absorption and photoluminescence spectroscopy as well as time-resolved photoluminescence measurements. For mixed QD monolayers FRET efficiencies of 90% have been achieved at high acceptor concentrations. The change of the donor decay in the presence of the acceptors in the monolayer as well as the dependence of the FRET efficiency on the acceptor concentration (see Figure 2) can be well explained within a theory of FRET in two dimensions - if the size of the QDs is taken into account in form of an exclusion zone around the donors . With the help of this theory all important FRET parameters (such as Förster radius, FRET rate, FRET efficiency and the exclusion zone radius) can be extracted from the time-resolved donor decays alone.
Figure 2. FRET efficiency in a mixed QD monolayer as a function of the acceptor QD concentration.
Figure 3. Distance dependence of the FRET efficiency in a separated donor – acceptor QD layer structure.
Surface Plasmon enhancement
Surface Plasmons are oscillations of free electrons in metal. Surface plasmon polaritons (metal films) can be excited at a metal / dielectric interface and localized surface plasmon resonances (LSPR) are supported by metal particles. A strong electromagnetic field is created at the metal surface that decays exponentially in air. This plasmon field can be used for enhanced Raman scattering as well as to enhance the photoluminescence of fluorophores such as dyes and QDs .
Current research:Monodisperse QD monolayers have been deposited by the layer-by-layer process on top of colloidal gold nanoparticle layers, separated by a polymer spacer. By varying the spacer thickness the typical distance dependence of the plasmon interaction can be observed. In close proximity to the gold particles the luminescence is quenched due to energy transfer to the gold particles. At a short distance away from the metal particles (typically a few nm) the PL intensity is enhanced to a maximum and then decreases down to the normal value (of a structure without metal nanoparticles) as the interaction with the metal particles decreases. The strongest enhancement has been observed for QDs with emissions red-shifted relative to the maximum in the metal NP absorption. A detailed analysis of the SP enhancement mechanism of the QD photoluminescence is currently one of our aims and the suitability of different metal particles (prepared by nanosphere lithography or pulsed laser deposition and colloidal nanoparticles) is under investigation.
References:1.F. Caruso, Colloids and colloid assemblies: synthesis, modification, organization and utilization of colloid particles, Wiley-VCH ; (2004)
2.T. Forster, "Zwischenmolekulare Energiewanderung und Fluoreszenz", Ann. Phys.-Berlin 2 55 (1948)
3.J. R. Lakowicz, "Principles of fluorescence spectroscopy", Kluwer Academic/Plenum (1999)
4.Medintz, I. L. et al., "Self-assembled nanoscale biosensors based on quantum dot FRET donors", Nat. Mater. 2 630 (2003)
5.Franzl, T. et al., "Exciton recycling in graded gap nanocrystal structures", Nano Lett. 4, 1599 (2004)
6.P. K. Wolber and B. S. Hudson, "Analytic solution to the Forster energy-transfer problem in 2 dimensions", Biophys. J. 28, 197 (1979)
7.C. D. Geddes and J. R. Lakowicz, "Topics of fluorescence spectroscopy", Vol. 8, Springer Science (2005)