Introduction: Photoelectrochemical Cells and Donor / Acceptor Hybrids Right: Photoelectrochemical cells isolate charge carrier generation and transport, unlike conventional solid state doped silicon devices. – Dye sensitized solar cells (DSSC) can reach overall efficiencies up to 11%, at a fraction of cost of conventional Si devices. – Typically, a nanocrystalline TiO 2 is used as a semiconducting layer on ITO, covered with a ruthenium based dye, which donates photoexcited electron into TiO 2 where charge separation takes place. The porous structure TiO 2 allows for large amount of dye molecules per unit area, but can have recombination due to poor diffusion of excited electrons. At the foundation is the interactions of donor / acceptor hybrids, where a molecular donor with ideal optical properties donates excited electron to an acceptor, slowing charge recombination and transporting charge to electrodes.
Carbon Nanotubes in Donor / Acceptor Hybrids Carbon Nanotubes (CNT) have been significantly explored as acceptors in D/A hybrids. – High capacity for accepting electrons – Semi-ballistic conductivity. – High surface to area ratio with extended electron system. Covalent functionalization: – Typically rely on acid treatment to break C-C bonds, exposing carboxyl groups for covalent interactions. – Disrupts pristine lattice, distorts optical / electronic properties. Non-covalent functionalization: – Relies on - bonding of SP2 carbon lattice with other molecules with strong systems – e.g. molecules with benzene rings. – Can preserves sidewall and electronic structure while rendering water- soluble. – Electrostatic interactions between a SWNT-pyrene and porphyrin molecular donor have demonstrated monochromatic internal photoconversion efficiency of up to 8.5%. Limations have been due to recombination of excited electron-hole pairs.
Proposed Design Oligonucleotide DNA used as surfactant in aqueous solution. DNA sequence custom designed: – solubilizing (adsorption) regime, using sequences shown to have strong affinity to sidewall. – (Inert) Spacing Regime – Molecular Donor-Binding Regime, using structure demonstrated to bind with donor molecule. The proposed design relies on demonstrated specific binding between the G-quadruplex conformation of DNA and porphyrin, a molecular donor. Benefit of design: potentially tunable intermolecular distance, can be used to optimize charge transfer kinetics, a key factor in charge recombination. Graphene: Similar scheme possible with graphene. Possible applications include tuning of electronic properties of graphene via charge transfer. SC graphene nanoribbons may serve same function of SWNTs in charge separation. G-Quadruplex DNA. 3 Stacks.
Intermediary Investigations DNA nucleobase sequence design and verification of DNA-CNT conjugation with preservation of secondary DNA structure for binding. Tuning of redox potentials (driving force of charge transfer) between porphyrin and nanotube. Demonstrate capability of tuning intermolecular spacing between donor and acceptor based on sequence.
Possibly Useful Characterization Schemes 1) Optical Absorption / photoluminescence spectroscopy. – Transduction of Absorption / PL signatures of CNT and porphyrin signatures observed. – Confirms CNT dispersion, interactions between porphyrin and DNA on SWNT hybrid by characteristic shifts. – PL quenching of porphyrin demonstrates charge transfer / FRET of excited electron. 2) Spectroelectrochemistry – Place sample in photoelectrochemical cell. Study interactions under illumination from different wavelengths. Observe effects on current. – Expect current peaks at resonances with porphyrin. 3) Transient absorption spectroscopy – Picosecond scale observation of kinetics of excited porphyrin electrons. 4) (Resonance) Raman – Vibrational signatures within hybrid, further confirmation of conjugation. 5) Circular dichroism – Method for measuring symmetry of molecules, studying difference between right and left handed polarized light. Used to confirm formation of secondary G- quadruplex structure on and off nanotubes. 6) AFM – Can be used to generate statistics of height profile of nanohybrids of varying length of oligonucleotide spacing regime. Preliminary results: PL quenching of porphyrin upon addition of DNA coated carbon nanotubes.
References Guldi, DM; Rahman, GMA; Zerbetto, F, et al. Carbon nanotubes in electron donor-acceptor nanocomposites. Accounts of Chemical Research , p November /Bases.jpg