In the Laboratories for Hybrid Optoelectronics we combine the purity of inorganic semiconductors and the versatility of organic and biological materials in novel hybrid configurations and we explore the physical properties and possible applications of this amalgamation.

Hybrid nanostructures for light emitting and light harvesting devices

By complementing the advantages of different classes of materials to overcome their drawbacks, highly efficient hybrid optoelectronic devices can be achieved. Semiconductor colloidal quantum dots (QDs) offer the possibility of flexible, low cost, large area, and simple-processed optoelectronic devices. However, they have inherently low carrier transfer compared to, for example, III-V semiconductors. We design and fabricate hybrid colloidal QD/patterned pin GaAs heterostructures that utilise nonradiative energy transfer from highly absorbing colloidal QDs to a high carrier mobility patterned semiconductor slab. The investigated hybrid configuration releases the potential of colloidal QDs as luminophores with large oscillator strength in efficient light harvesting devices, while overcoming altogether the limitations imposed by low carrier transfer.

In the reverse configuration, we fabricate highly efficient colour-conversion light emitting diodes (LEDs) consisting of surface-textured blue emitters and colloidal QDs. Electrically injected carriers in the blue emitter are efficiently transferred to the QD overlayer via nonradiative energy transfer in addition to conventional radiative energy transfer. High colour-conversion efficiency compared to conventional colour conversion LEDs is demonstrated. This novel hybrid structure can be used in various applications including solid-state lighting, full-colour displays and lasers.

Hybrid DNA-dye molecular scaffolds for biological application

• Blog-style discussion of the DNA FRET project
• FRETlab example program for download

The structural and geometrical conformation of double stranded DNA has been studied by means of resonance energy transfer (RET) between two fluorescent dyes of appropriately chosen spectral overlap. This process is driven by the coupling of dipoles in the two adjacent dyes and was first studied by Förster (Förster, 1946) to describe the funnelling of energy in natural light harvesting complexes. In this work we study RET on donor-acceptor (Fluorescein-Texas Red) labelled DNA double strands with different separation distances between the donor and the acceptor with a focus on the separation and the change of the relative orientation of the dipoles due to the helical geometry. For the first time the von Mises-Fisher distribution function (Fisher, 1953) is used to model the fluctuation of the vectorial orientation of the dipoles around their equilibrium direction, resulting in the previously observed non-exponential characteristic shape of the fluorescence decay curve of an ensemble of RET pairs. This distribution function in conjunction with a geometrical model of the DNA (Clegg, Murchie, Zecher, & Lilley, 1993) is used to fit the characteristic shapes of the fluorescent decay curves obtained by time resolved spectroscopy. The plot of the average RET efficiency of an ensemble vs the separation distance obtained from enhanced acceptor fluorescence in time integrated measurements shows oscillations with the period of a half turn of the helix. The modelled curve is in good agreement with the experimental data, especially because the model accounts non-zero RET efficiencies for distances where the equilibrium orientations of the dyes are nearly perpendicular and omit any coupling. This is due to the contribution of dipoles which are deflected from the equilibrium orientation according to the von Mises-Fisher distribution. These results indicate that the characteristic shape of the fluorescent decay curve can be used to gain insight into the dynamics and conformation of the constituting dye-linker complexes. The observed modulation period of the RET efficiency proves that for certain dye-linker systems the often used assumption of fully rotational freedom does not hold and consequently leads to distorted distance and conformational information.