Oblique angle physical vapor deposition technique has led to the evolution of new class of nanostructured thin films. These films posses' novel anisotropic electrical, magnetic, optical, properties which could be potentially engineered based on the growth conditions and the deposition parameters. The technique is based on the atomistic level self-shadowing principle. In the oblique angle deposition (OAD) technique, the substrate is held at an oblique angle with respect to the incoming vapor flux. As the vapor atoms condense and nucleate on the substrate, the shadowed regions behind each site stop receiving the subsequent vapor atoms. Instead, they land on the previously formed droplets, resulting in the evolution of a columnar morphology.
In this dissertation research, three aspects of nanostructured thin films grown using this method are investigated. When grown at room temperatures, soft metals such as silver (Ag), copper (Cu), gold (Au) etc; generally produce low aspect ratio collapsed columnar structures compared to high melting temperature metals such as titanium (Ti), chromium (Cr), nickel (Ni), etc. Using e-beam evaporation and a custom built cryogenic substrate cooling apparatus; we investigate the growth of nanostructured thin films made from soft metals at near 100 K and 300 K side-by-side. Growth of these films at cryogenic substrate temperatures has resulted in high aspect ratio nanocolumns thin film morphology.
Ag nanorods (AgNRs) thin films are known to have surface enhanced Raman scattering (SERS) response. In this study, AgNRs thin films are incubated in liquid and vapor phase with SERS test probe molecules. The SERS response of AgNRs thin films grown at room and cryogenic substrate temperature is compared. The hypothesized improved SERS response of cryogenically grown AgNRs is attributed to the morphological differences and higher surface of AgNRs at this growth conditions. Rigorous SERS enhancement factor (SEF) calculations are discussed by estimating the number of molecules absorbed on the surface of AgNRs through the use of fluorescence spectrophotometer measurements.
The third aspect of this research is to investigate the effect of liquids exposure and lithographic processing of these films. It is known that exposure of nanorods thin films to liquids and solvents, permanently changes the physical structure of these films. This prevents conventional lithographic patterning of such thin films since it involves wet processing. In this study, we show the use of CO2 based critical point drying (CPD) technique to mitigate the structural collapse of nanorods thin films after liquids exposure. Further, we discuss dry lift-off based lithographic process to pattern these thin films.