Probing and manipulating the optical properties of porous silicon: Films, nanoparticles, and microstructures

Visible light emission from porous silicon (PSi) has stimulated tremendous interest over the past several years due to its potential application in opto-electronic devices, lasers, and the ability to be integrated with current Si processing technology. Luminescence from porous silicon was observed over a decade ago, but scientists have struggled to develop a mechanism to describe its photophysical properties. Despite the lack of a consistent physical description, a host of test devices have been produced which make use of the tunable luminescence and high surface area of porous silicon. Some of these devices have included: molecular sensors, cavity lasers, light emitting diodes, optical switches, and photo-voltaic cells. Development of devices of this sort requires a detailed understanding of the luminescence properties and the underlying structure of the material.; Due to the high degree of structural inhomogeneity and parametric tunability of etched bulk samples, a direct correlation between chromophore size and emission wavelength has been difficult. Our approach to better describing the luminescence mechanism in PSi is two-fold. First, we develop a clear understanding of how the bulk morphologies correlate to the observed optical behavior. Secondly, we remove the effects of spatial averaging in order to better describe the luminescence behavior of this material. This is achieved by removing the Si chromophore from the bulk material and applying single molecule spectroscopy techniques. Using this method we have observed distributions of emission wavelengths, resolved vibronic structure, optical anisotropy, discrete jumps in intensity, luminescence intermittency, and irreversible photobleaching. In addition, we have determined the number of emitters per PSi particle.; From an application standpoint, we have been interested in creating active PSi structures for possible use in sensory devices, waveguides, and memory storage. The drive for designing PSi-based devices has pushed researchers to investigate ways of controllably patterning PSi surfaces. Our approach to directly patterning luminescent silicon single and multi-layered films involves dry removing microstructures of PSi from the Si substrate using nothing more than a bare elastomer stamp made from poly(dimethylsiloxane) (PDMS). These PSi structures can then be further transferred to a free-standing flexible polymer film.