Raman Spectroscopy - in situ Characterization of Growth and Surface Processes

Abstract

The goal of this thesis is to expand on the usefulness of Raman spectroscopy as an in situ probe to aid in the growth and implementation of electronic, optical, and biodetection materials. We accomplish this goal by citing two diverse optical characterization projects. In the first project, an autoclave similar to those used in solvothermal growth which has been outfitted with an optical window is used to collect vibrational spectra of solvents and mineralizes commonly used in the ammonothermal growth of gallium nitride. Secondly, novel silver nanowires created by ferroelectric lithography are evaluated by surface enhanced micro-Raman spectroscopy for use as surface enhanced substrates for low detection limit or single molecule bio-detectors.

Raman spectroscopy is already a widely accepted method to characterize and identify a wide variety of materials. Vibrational spectra can yield much information on the presence of chemical species as well as information regarding the phase and interactive properties. Because Raman spectroscopy is a generally non-intrusive technique it is ideal for analysis of hazardous or far-from-ambient liquids, gases, or solids. This technique is used in situ to characterize crystal growth and surface enhanced photochemistry.

The phenomenon of Surface Enhanced Raman Spectroscopy (SERS) has been observed in many systems but some fundamental understanding is still lacking and the technique has been slow to transition from the laboratory to the industry. Aggregated colloids and lithographically created islands have shown the best success as reproducible substrates for SERS detection. These techniques, however, lack control over shape, size, and position of the metal nanoparticles which leave them reliant on hotspots. Because of the potential for control of the position of aggregates, ferroelectric lithographically created silver nanowires are evaluated as a potential SERS substrate using pyridine, benzoic acid, and Rhodamine 6g.

There is a current need for homoepitaxial substrates for gallium nitride devices including light emitting diodes, transistors, and laser diodes. Ammonothermal growth is a promising technique for creating bulk single crystalline GaN, but questions remain concerning the intermediates of reactions in supercritical Ammonia. Neat ammonia and water are monitored by Raman spectroscopy from room temperature to 500°C and 20 kpsi with both UV and visible excitation. Binary solutions of sodium azide and ammonia are investigated to temperatures which allow observation of the breakdown of the azides. The rate of decrease of the Raman signal of the azide increases as the reaction proceeds suggesting that the reaction rate is proportional to the pressure.