| dc.description.abstract | Nanostructured and/or nanoparticles based thin film are of paramount importance for very diverse fields of science and technology such as fuel cells, thermoelectric and optoelectronic devices. Two innovative methods of light controlled material fabrication developed and im- plemented during this PhD research work are outlined in this thesis. First is the pulsed laser deposition (PLD) of thin films with the plume dynamics controlled by noble gases to generate nanoparticles. Second one is the kinetically controlled phase transition of material driven by scanning pulse laser annealing.
As a demonstrative applications of the first method, PLD in a gas atmosphere was used to grow nanoporous thin films composed of stacked nanoparticles of platinum on gas diffusion layer for application to proton-exchange membrane fuel cells. The 10 ns pulsed laser radi- ation of 532 nm wavelength was used to ablate Pt target in Ar filled chamber. Argon pressure was varied to simultaneously optimize both the particle size and dispersion of nanoparticles on the substrate to raise the electrochemical surface area of the platinum nanoparticles, which in turn resulted in the enhanced power destiny of PEM fuel cell. The grown catalyst layer was then assembled into a membrane electrode assembly and it’s single cell performance were measured. Initially the PLD grown catalyst was only used on the anode side of the fuel cell. In this case, with a Pt loading of 17 μg cm?2 the fuel-cell current density at 0.6 V reaches 1.08 A cm?2, which was close to that of a cell with the anode made by using E-TEK Pt/C of 200 μg cm?2 Pt loading. Thus the usage of Pt was decreased by 12-fold.
Next, PLD was used to grow catalyst for the cathode side of the fuel cell. Since the oxygen reduction reaction (ORR) at the cathode is very sluggish in nature compared to hydrogen oxidation reaction (HOR) at the anode, the cathode requires a much larger amount of catalyst compared to the anode. Therefore, generally the ORR at the cathode is the bottleneck and the reduction of Pt usage at the cathode holds the key to lowering the overall cost of the fuel cell. When PLD grown catalyst was used on cathode side of the fuel cell, the current density of a single cell reached 1.2 A cm?2 at 0.6 V, at the Pt loading of 100 μg cm?2, which was close to that of a single cell using E-TEK Pt/C electrode with a cathode Pt loading of 100 μg cm?2. Further- more, for a PEM fuel cell with both electrodes prepared by PLD and a total anode and cathode Pt loading of 117 μg cm?2, the overall Pt mass-specific power density at 0.6 V reached 7.43 kW g?1, which was about 5 times higehr than that of a fuel cell with E-TEK Pt/C elec- trodes. Finally, the electrochemical durability of the catalyst/support was evaluated by using accelerated degradation test based on CV. It was found that the pulsed laser deposition sample retains 60% of its initial electrochemical surface area after 5000 potential cycles, much higher than that with E-TEK Pt/C, which retains only 7% of its initial electrochemical surface area.
The second method of light controlled material fabrication devised during this study was used to produce semiconductor quantum dots on a substrates for application to infrared photodetector and ther- moelectric materials. A scanning pulsed laser beam (λ = 532 nm, pulse width = 10 ns) with a line profile was implemented for kinet- ically controlled induction of self-assembly of quantum dots (Ge/Si) via photo-thermal strain so as to produce a homogeneous quantum dot layer. it was demonstrated with suitable setting, Ge/Si quantum dots with a mean height of 2.9 nm, a mean diameter of 25 nm, and a dot density of 6×1010 cm?2 could be formed over an area larger than 4 mm2. Based on the observed dependence of the characteristics of QDs on the laser parameters, a model consisting of laser-induced strain, surface diffusion, and Ostwald ripening was proposed for the mechanism responsible the formation of Ge/Si QDs.
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