Date of Award

8-2013

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Michael D. Mason

Second Committee Member

Douglas W. Bousfield

Third Committee Member

David J. Neivandt

Abstract

In the production of some devices, novel micro fabrications techniques are important, where the goal is compatibility with the system, convenience, and cost rather than high resolution. Electron beam and photolithography techniques are common fabrication techniques used in the semiconductor industry. Even though these techniques are common, they are high cost and not compatible with many substrates. This dissertation work explores a versatile and cost effective novel technique to fabricate fluorescent active micron scale Laser Induced Structures (LIS). The optimization of the instrumental parameters and the mechanisms involved in structure formation are given, the photo-physical characterization of the LIS and a photoactive film used in solar cells is reported.

Here silver nanoparticles were synthesized through a wet chemical process, concentrated and drop cast as films onto different substrates. The drop cast films were air dried and subjected to 532nm continuous wave laser irradiation to generate structures in the film. A silver nanoparticle-polymer coating formulation was also explored for structure formation and critical instrumental parameters were studied. A mechanism is proven, similar to Liquid Melt Ejection (LME), to explain the structure formation. Thermo gravimetric analysis was performed to characterize the thermal decomposition of silver nanoparticle-citrate films. In a related study, photoactive film (made out of SrAl2O4: Eu, Dy phosphor material) for solar cells was photo-physically characterized to understand its emission mechanism and was investigated against the backdrop of its effect on photovoltaic cell efficiency.

A number of novel results were obtained in this work. One unexpected result relates to the structure being fluorescent active at the edges: a physical and chemical change of the material near the peak laser intensity occurred. This was attributed to light absorption and heating of the silver nanoparticles. LIS were generated with 14.3 mW laser power on silver-polymer coating based coating. Data described here suggests that the mechanism for structure formation is likely a result of: 1) absorption of radiation by the nanoparticles to generate heat, 2) decomposition of material near the particles creating water vapor, 3) the movement of material (mainly Na-citrate and Ag) by the vapor away from the laser focus. Sodium citrate dihydrate was found to be responsible for the fluorescent nature of the LIS. As sodium citrate dihydrate move away from the laser focus during heating and LIS formation, sodium citrate dihydrate close to the center of the beam is decomposed to an extent of loosing there fluorescent activity and forming sodium carbonate. But few microns away from the center of the laser beam the accumulated citrate molecule still have its fluorescent nature intact, which forms the edge of the LIS.

In the last section of this dissertation, the photoactive film (SrAl2O4: Eu, Dy phosphor material) coatings demonstrated a measured Photovoltaic (PV) response. However, the scattering losses due to the addition of nanoparticulates reduced the amount of "solar" radiation hitting the PV device surface. While it was evident that, under constant illumination, the photoactive films do add back some additional reemissions, via fluorescence upconversion or downconversion, radiation in the direction of the PV active element, the net losses are greater than the net gains.

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