Spin-dependent phenomena mediated by silicon nanocrystal assemblies

To encourage chemical reactions to proceed, and to exercise control over them, a promising strategy is to add energy to the molecules involved in the reaction, putting them in a more reactive state. A convenient way to add this energy would be via a beam of light. Unfortunately, it is often impossible to pump energy directly into a molecule using light because of obstacles due to fundamental quantum mechanics; the molecule may simply not be able to absorb the light.

A way of avoiding this problem is to bring together a nanoparticle (termed a donor) that is able to absorb the light and then transfer that energy to the molecule in question. This proposal is based on our discovery that silicon (Si) nanostructures are ideal candidates to donors for energy transfer to, e.g, O2 and a variety of organic molecules. It is also supported by our recent advances in the development of freestanding spherical Si nanocrystals.

The ideal donor nanoparticle should have many key characteristics and nanosilicon satisfies all of them. It has an extremely long indirect exciton lifetime (thus storing the energy effectively), tunable energy of excitons (1.1-2.5 eV) and a large surface area (facilitating the transfer process). This unique combination of factors means that, for instance, the efficiency of excitation of O2 molecules to singlet states is found to be ~ 90 % even at room temperature. This process is accompanied by a spin-flip excitation of energy-accepting molecules (via a direct electron exchange mechanism) and should result in a variety of spin allowed photo-chemical processes. Therefore photoexcited systems containing Si nanocrystals can be viewed as universal spin-flip activators for molecules and clusters having singlet-triplet splitting energies below 2.5 eV.

For energy transfer, the mutual spin orientation of interacting species is crucial. We propose to investigate this through control of the participating spin states by magnetic field and microwave experiments. Remarkably, a very small magnetic energy (~1 meV) should efficiently control the energy exchange processes at the scale of eV by aligning the spins of the interacting species. Energy transfer can also be affected via variation of spacing between Si nanocrystals and accepting substances, or by variation of the surface potential barrier height and width. Studies of these will allow us to achieve full control over energy transfer process.
One key property of porous Si is that its pores can be almost completely be filled by energy accepting molecules. Optical excitation followed by the spin-exchange process thus significantly modifies the magnetic state of composite nanosilicon materials (from para- to diamagnetic or vice versa). An applied magnetic field should result in spin alignment and thus modify the magnetic state of the material. This will produce Faraday rotation and magneto-optic Kerr effects, which we will study.
Chemical reactions between singlet organic molecules and triplet O2 molecules forming new singlet organic molecules are forbidden by spin selection rules. Thus, the triplet multiplicity of O2 molecules is the reason why most reactions do not occur between O2 and organic substances at room temperature. Si nanocrystals can mediate either the excitation of O2 molecules into singlet states or of organic molecules into triplet states. Thus spin selection rules for oxidation reactions can be overcome if the spin state of only one substance is changed. The light-induced chemical reactivity of excited O2 or organic molecules will be studied using photo-bleaching experiments and infrared absorption spectroscopy to monitor the oxidation of the organic molecules and to identify reactants and products. Because scalable production of Si nanocrystal assemblies is feasible, these nanosilicon-based composites systems have real potential for eventual "green chemistry" industrial development.

One of the strategic objectives of the nanotechnology concept is the development of new materials having nanometer sizes that have entirely new physical properties and, therefore, new functionality. This project is based on our finding Si at the nanoscale has completely new functionality: it facilitates efficient energy transfer to oxygen and to key organic materials. This project combines different physical and chemical aspects of Si-based nanotechnology which are relevant for physics, material science, photochemistry, biology and medicine. The project is application-oriented and belongs to the EPSRC priority research area: Nanoscience through Engineering to Application and Healthcare Technologies.

The aim of this project is to obtain a full understanding of this remarkable behaviour, which is fundamental to the physics of excitons, molecular physics, magnetooptics, photophysics and photochemistry. The potential benefits of a full understanding and full control of these photoexcited processes are immense, ranging from the promotion of photochemical reactions to cancer therapy. Recently, together with our partners from the University of Leuven, using cancer cells we have demonstrated the photodynamic activity of one of modification of nanosilicon, namely porous Si. For biomedical applications, the freestanding spherical Si nanocrystal systems recently developed by our group are obviously superior to porous Si. Their bio-functionalization requires a proper choice of specific surface termination, which will affect the efficiency of singlet oxygen generation and, therefore, the photodynamic activity of Si nanocrystals. Because the study of the effect of surface termination on singlet oxygen generation is one of the main objectives of the project it will help our partners from the University of Leuven in collaboration with our group to develop new nanosilicon-based compounds which can be used for photodynamic cancer therapy (support letter is attached).

We also aim to reach the point where we can fully exploit this functional material for possible industrial applications in the physical, chemical and life science. As an example, the knowledge gained will be very important for the development of photochemical reactors based on Si nanocrystals since they can be constructed in a predictive manner. Because scalable production of Si nanocrystal assemblies is feasible, new nanosilicon-based composite systems developed in the framework of the project have real potential for eventual "green chemistry" industrial development.

Large part of our nanosilicon research is application-oriented and we have experience of close collaboration with industrial partners.
Our finding that nanosilicon-based compounds can exhibit enormous explosive properties resulted in the investment of 3.6 Mio EUR equally by the TRW Automotive (Germany) and German Ministry of the Research and Technology (BMBF) in the joint R&D project devoted to the development of a new generation of porous Si-based airbag initiators. Together with Vesta Ceramics (USA) we developed a route for the scalable production of porous Si from metallurgical quality Si micropowders and this product is on the market. At the moment among customers of this company are NASA and the USA Army. After the graduation of our postgraduate student previously supported by Vesta Ceramics, they intend to continue support for our research in the form of a new research studentship to work on this project (support letter is attached).

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