Template-assisted synthesis of nanomaterials

Abstract

Fabricating nanostructures and tailoring their properties on the nanometer scales by conventional lithographic approaches are challenging. In particular, the creation of nanostructures with good alignment and uniform size, to enable and enhance desirable collective behaviors, is even more difficult. Such difficulties have compelled us to seek alternative approaches such as utilization of template methods to produce uniform nanostructures on the nanometer scale, and to form self-organized highly ordered arrays. Anodic aluminum oxide (AAO) template has a hexagonal array of pores which diameter can be readily controlled in the range of 25-200 nm with pore spacing from 50 to 400 nm. The surface structure, anodizing voltage, anodizing time all have significant impact on the structure of AAO and these influential factors were carefully discussed. An AAO template typically has a thick barrier layer at the bottom of its pores. If we electrodeposit nanostructures into the pores of AAO, high potentials are required for the tunneling of the electrons through the barrier layer. Several techniques were developed to solve this barrier layer problem including chemical etching methods and adjusting the anodizing voltage to reduce the barrier thickness. Utilizing an AC electrodeposition technique, metal, such as gold, nanoparticle arrays with diameters of 25 nm and 50 nm have been directly electrodeposited into the AAO templates. Frequency of the current was found to have strong influences on the pore-filled ratio. Metal nanowires have been electrodeposited into the AAO channels by a DC electrodepostion technique. Gold patterned AAO templates have been used to prepare ZnS nanowires via a hydrogen-assisted thermal evaporation approach. The nanowires grow aligned perpendicularly to the template, and their diameters resemble the pore size of the AAO template. Photoluminescence results show that these ZnS nanowires are able to support lasing action. We have identified the threshold for amplified emission and the presence of resonant modes within a narrow (~2.2 nm) PL emission centered at ~ 338 nm (3.67 eV) that is attributed to free exciton emission. It was found that the 50 nm diameter nanowires have inferior PL characteristics compared to the 25 nm ZnS nanowires as evidenced by a higher (~160 kW/cm2 vs. ~130 kW/cm2) threshold for stimulated amplification and a broader (~12.2 nm vs. 10.2 nm) FWHM of the PL below threshold. In this thesis, fabrication of p-type doping ZnS nanowires, synthesis of CdSe quantum dots, and exploring work for nanodevice are also briefly introduced.

Date of Award	14 Jul 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)