Optics of Hybrid Nanostructures

A central focus of the project is to apply advanced optical microscopy and spectroscopy techniques to study and utilize processes occurring at the nanoscale in hybrid systems comprising of biological and inorganic nanostructures. In particular our intention is to develop ways to control the optical properties of protein-pigment complexes that take part in photosynthesis either through absorbing sunlight or performing charge separation. This can be achieved by conjugating light-harvesting complexes with inorganic nanostructures, such as metal nanoparticles and semiconductor nanocrystals. The core of the project based upon three PhD subprojects:

The first PhD project will concentrate on optical studies of hybrid nanostructures composed of a metal nanoparticles and a semiconductor nanocrystal. Although in recent years there has been many reports on this topic, dominant mechanisms responsible for either fluorescence quenching or enhancement are still not satisfactorily understood. In particular precise knowledge required for controlled design of the optical properties of metal nanoparticles is far from sufficient, which reduces potential applications of these nanostructures as, for instance, sensitive biosensors. Coherent description of plasmon interaction with a dipole moment associated with a semiconductor nanocrystal will be a central task of this project.

Optical experiments will include a thorough characterization of such hybrid nanostructures by means of ensemble spectroscopy in solution followed by single molecule spectroscopy of individual conjugated hybrids. High fluorescence quantum yields combined with extreme photostability render semiconductor nanocrystals ideal system for these measurements. The results obtained for this relatively simple hybrid system will serve as a reference for designing and understanding experiments on more complicated hybrid nanostructures containing light-harvesting complexes.

The second PhD project will focus on assembling and investigating hybrids composed of metal nanoparticles and light-harvesting complexes present in algae and purple bacteria. The latter include peridinin-chlorophyll-protein (PCP) complex as well as light-harvesting complex 2 (LH2). Importantly, these two complexes represent opposite cases as far as interaction between chlorophyll molecules and major absorbing pigment are concerned. In PCP the chlorophylls are weakly coupled and consequently can be easily distinguished in a single-molecule experiment. The major absorber of PCP is a carotenoid, peridinin. On the other hand, LH2 contains strongly interacting chlorophyll molecules which also are major absorbing pigments. These differences make both light-harvesting antennae very appealing for studying the impact of plasmon resonances on their optical properties.

A key question related to this project will concern possible ways of improving the performance of light-harvesting complexes by attaching them to metal nanoparticles. We anticipate that local electromagnetic field enhancement caused by plasmon resonances in metal nanoparticles will increase the absorption of carotenoids and simultaneously shorten the energy transfer time from carotenoids to chlorophylls. Moreover, the proximity of metal nanoparticle could improve the photostability of the complexes. All these effects are highly desirable for implementing natural light-harvesting systems into artificial light-harvesting.

The third PhD project will be devoted to studying the effects of large absorption of a semiconductor nanocrystal on the performance of a light-harvesting complex. Typically, light-harvesting complexes feature large absorption gaps in the region between 500 nm and 700 nm. In this regard, the ability to tune the ground state energy of the nanocrystal by changing its size offers an excellent method to extend the absorption range of resulting hybrid nanostructure. Important research goals include controlled assembly of the aforementioned structure as well as observation of efficient energy transfer between the nanocrystal and the light-harvesting system. The latter requires placing the two components at a distance of about 10 nm.

Assembling together a light-harvesting system and a semiconductor nanocrystal presents a novel concept as compared to analogous hybrid structure containing metal nanoparticles discussed before. Namely, by attaching semiconductor nanocrystals to light-harvesting complex we anticipate to dramatically expand the spectral range in which a hybrid nanostructure absorbs light, whereas the previous case we attempt to modify the properties of a light-harvesting complex via plasmon interaction without changing the overall absorption pattern. Unambiguous experimental demonstration of this effect requires fluorescence excitation measurement, where the emission of the light-harvesting complex is monitored when scanning the excitation wavelength. By studying hybrid nanostructures consisting of nanocrystals with different sizes we aim at optimizing the energy transfer condition from the nanocrystal to the antenna.