Synthesis and Surface Reactions of Hydride Terminated Silicon Nanocrystals with Strong Quantum Size Effect

Among the variety of semiconductor quantum dots targeted for applications in optoelectronics, biological imaging and chemical sensing, silicon nanocrystals (ncSi) are drawing increasing attention because of their earth-abundance, low cost, and purported lack of toxicity. To synthesize freestanding ncSi, we employed several methods that could all yield hydride-terminated ncSi and which allow further surface functionalization and chemical reactions. Among these methods, a batch synthesis based upon the solid-state disproportionation of SiO powder has not previously drawn much attention in chemistry. We describe some remarkable effects observed in the diffraction, microscopy and spectroscopy of SiO powder upon thermal processing in the temperature range 850-1100 ℃, pinpointing that the most significant disproportionation transformations happened in the range between 900 and 950 ℃. With this know-how a high yield synthesis was developed that produced polydispersions of decyl-capped, hexane-soluble ncSi predominantly with NIR photoluminescence. The hydrosilylation reaction was carried out via both the conventional heating and the unconventional microwave heating methods, and the kinetics were studied for the first time. Using size-selective precipitation, these polydispersions were separated into monodisperse fractions, which allowed their photoluminescence absolute quantum yield to be studied as a function of ncSi size. This investigation yielded volcano-shaped plots confirming the most efficient photoluminescence wavelength for ncSi to be located at 820-830 nm, which corresponded to a size of 3.5-4.0 nm. Finally, we describe the unprecedented observation that ncSi:H can function as a single component heterogeneous reducing agent for converting gaseous CO2 selectively to CO, at an impressive rate of hundreds of micromol/h∙g. The large surface area, broadband light harvesting and reducing power of SiH surface sites together play key roles in the conversion. This thesis work provides opportunities for applications of size-selected near infrared emitting ncSi in biomedical imaging, photothermal therapy, and CO2 reduction for the production of chemicals and fuels.Ph.D