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44219-G10
Bridge-Mediated Interparticle Electron Transfer in Self-Assembled Hybrid Semiconductor Nanomaterials

David F. Watson, State University of New York at Buffalo

Research overview. The objective of the research supported by this ACS PRF-G grant is to characterize the photoinduced interfacial electron transfer reactivity of coupled semiconductor nanomaterials. During the first year of the project, we have focused primarily on materials fabrication and characterization. Specifically, we have developed a method for tethering CdS and CdSe quantum dots (spherical particles with diameters of 2-6 nm) to the surfaces of nanocrystalline TiO₂ films through bifunctional organic linkers. The research has led to the publication of two manuscripts in Langmuir, the ACS journal of colloids and surface chemistry.

In our materials fabrication strategy, quantum dots are attached to the terminal thiol groups of TiO₂-adsorbed mercaptoalkanoic acids (MAAs). Quantum dot synthesis and surface attachment occur in two separate steps; therefore, the particle size and optical properties of surface-adsorbed quantum dots are tunable. We have used vibrational spectroscopy to characterize the adsorption of MAAs to TiO₂ surfaces from organic solvents. MAAs adsorb as deprotonated carboxylates with surface adduct formation constants (K_ad) of 10³-10⁴ M^-1. We have used UV/visible absorption spectroscopy and energy dispersive X-ray analysis to characterize the adsorption of CdS and CdSe quantum dots to thiolated TiO₂ surfaces. K_ad values of approximately 10⁴ M^-1 were measured. Quantum dots do not adhere to unmodified TiO₂ films. Equilibrium binding experiments have revealed that quantum dots bind more strongly to TiO₂ surfaces functionalized with mixed monolayers of alkanoic acids and MAAs than to pure MAA monolayers. We have shown that the decreased affinity of quantum dots for fully thiolated surfaces is caused by the formation of intermolecular disulfide bonds between surface-adsorbed MAA molecules.

Our research currently focuses on the spectroscopic characterization of photoinduced electron transfer reactions occurring within coupled semiconductor nanomaterials. Time-resolved absorption and emission measurements are used to probe the efficiency and dynamics of electron transfer processes. Preliminary spectroscopic data indicate that electrons are efficiently transferred from photoexcited quantum dots into TiO₂ acceptor states, generating long-lived charge-separated states. We are currently investigating the influence of materials composition and interconnectivity on the electron transfer reactivity.

Significance of research. Our materials assembly method is broadly applicable to the fabrication of nanoparticle-functionalized surfaces, which may have applications in photonic, electronic, magnetic, and sensing devices. The surface adsorption studies described above lend basic insight into the factors controlling materials assembly. Most importantly, we have shown that intermolecular bonding interactions within surfactant monolayers must be minimized for high surface loading of nanoparticles.

Impact of PRF funding. To date, the PRF funding has been used for the acquisition of supplies for materials fabrication and spectroscopic experiments. The results from these experiments provided the basis for a proposal which was funded by the National Science Foundation's CAREER program. In the coming year, PRF funds will be used to support a postdoctdoral fellow for six months and two graduate students over the summer. Thus, the funding will continue to be instrumental in supporting our research team.

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