Silicon nanocrystal doped polymer nanowire arrays

Abstract

In this thesis, we successfully produced silicon nanocrystal embedded polymer micro and nanowire arrays by using a new top—to—bottom nanofabrication approach. Silicon nanocrystal (Si-Nc) quantum dots are photoluminescent materials that give bright optical illumination under UV light excitation. Si-Ncs were used to fabricate large area luminescent thin polymer films before production of the fibers. Among many of Si—Nc fabrication methods that are available, we chose a chemical route which takes the advantage of high product yield and ease of production steps, although the resultant size distribution is not uniform as other methods such as electrochemical treatment of Si wafers. Dopant Si—Ncs in polymer sheets shows some improved properties compared to free standing silicon nanocrystals, like longer luminescent life time in normal atmospheric conditions and in high temperatures as high as 300 °C. With utilizing these properties, thermal drawing of Si—Nc doped polymer fibers is possible without harming the luminescence properties. Hence, throughout the work, different types of films were investigated and polycarbonate films were chosen for both their thermal and optical properties such as durable luminescence at high temperatures and low absorption at visible wavelengths. Consequently, with combining these properties with our iterative thermal size reduction method, we successfully produced silicon nanocrystal doped polymer micro and nanowire arrays. In literature, there are similar works treating the same idea of producing luminescent fibers, which were realized with different techniques and material sets, like dye/QD doped nanofibers or fibers produced with conjugated polymers. However, the methods used to produce these type of geometries lacks in some aspects such as limited length, uniformity, alignment, reproducibility, etc. On the other hand, our iterative thermal drawing method is very successful for producing indefinitely long, uniform and easily aligned fibers. Our production steps can be summed in five steps which are: Si—Nc synthesis, film preparation, filmrolling, consolidation, and two consecutive fiber drawing. Keeping the track of characterization of the product in each step is important. Hence, for silicon nanocrystals, we took photoluminescence (PL) intensity measurements, SEM/TEM images and temperature dependent PL measurements. Also for doped films, we performed temperature dependent PL measurements and for the resultant fibers we carried out cross—section SEM and PL characterizations. Silicon nanocrystal embedded micro and nanowires can be utilized as fiber gain medium, single photon source, directional emitter, light emitting diodes and optical sensing elements. Also, they increases light extraction efficiencies with guiding advantages and this can result to fluorescence enhancement for luminescent active material dopants.