Synthesis and Characterization of PECVD-Grown, Silane-Terminated Silicon Quantum Dots

Semiconductor quantum dots (QDs) have been the subject of intense research interest due to novel experimentally observed properties, such as tunable bandgap, phonon bottleneck, and a variety of surface effects. The control of these properties makes quantum dots a candidate for revolutionizing a variety of fields, including photovoltaics. Because silicon is such a well characterized PV material in its bulk form, it would be a good choice for QD research for application in solar cells. In addition, there is recent theoretical evidence that its indirect gap may become more direct as size decreases, allowing for a fine-tuning of the absorption characteristics for photovoltaics. We present a method for grafting silanes onto lowtemperature- plasma synthesized silicon quantum dots. The resulting solution of dots is characterized with Fourier transform infrared spectroscopy and transmission electron microscopy, and determined to be a colloidal suspension. The silane is attached at a single point on the quantum dot surface to avoid cross-linking and multilayer formation, and photoluminescence spectroscopy shows the colloidal suspension of dots is stable for over two months in air. The hydroxyl-terminated surfaces required for silanization are created by wet chemical etch, which can be used to tune the luminescence of the silicon dots in the green- to redwavelength range. Unpassivated Si quantum dots show vastly different behaviors in electron paramagnetic resonance than wet-chemically oxidized, silane-functionalized particles. The dangling bond density of unpassivated Si quantum dots is large and changes over time, while the dangling bond density of the silanized dots is unchanged and undetectable. This suggests silanized dots will be better for solution-processed PV devices since transport will not be hindered by dangling bonds. Finally, we perform PL excitation (PLE) spectroscopy on both ensembles of dots, and discuss the way such spectra are represented in the literature, especially in comparison with absorption. This discussion is critical to the success of Si QDs in optoelectronic devices, since absorption and luminescence play critical roles.