Luminescent Silicon Nanocrystals: From Single Quantum Dot to Light-harvesting Devices

Silicon (Si) serves as the basic material of the system-on-a-chip industry and photovoltaic panels nowadays. This is mostly thanks to its high abundance in the earth’s crust, thereby low cost, virtually non-toxicity, and superior stability. Nano-silicon, especially silicon quantum dots (Si QDs), is endowed by the quantum confinement effect with the ability to emit light efficiently under photoexcitation, different from the bulk counterpart. The bright photoluminescence (PL), first found in the 1990s, has paved the way for this nanomaterial to be applied for light conversions in the last decades, such as for biosensing/biolabeling, light emitting diodes and luminescent solar concentrators (LSCs). The latter is used to concentrate sunlight in the slab on the edge-attached solar cells by means of PL. This thesis, on the one hand, deepens the comprehension on the optical properties of Si QDs by single-dot spectroscopy; on the other hand, a low-cost mass synthesis of high-quality Si QDs is developed here, which favors high QD loading applications, demonstrated as large-area “quantum dot glass”. First, the photo-physics mechanism behind PL was studied by single-dot spectroscopy, excluding the QD size inhomogeneity in the ensemble measurements. A new method was developed to fabricate large-area (~mm 2 ) isolated oxide-passivated Si QDs on a silicon-on-insulator wafer. Linearly polarized PLs were observed on those single dots. System-limited PL linewidths, ~250 μeV, were measured at 10 K on QDs here, indicating a good quality of oxide shell endowed by high temperature annealing. Based on this method, it is possible to modify the ambient optical environment of QDs without tenuous alignments. With Si QDs residing on a metal membrane with an oxide spacer, the PL yields of single dots were enhanced ~10 times in average compared to those residing outside the membrane. Next, we have achieved, for the first time, direct observation on the temperature-dependent radiative lifetimes on single ligand-passivated Si QDs. Most importantly, these single-dot PL decays can be well-fitted mono-exponentially, indicating trap-free dynamics, as opposite to oxide-passivated counterparts. Secondly, a chemical synthesis method of ligand-passivated Si QDs by using triethoxysilane (TES) as precursors is introduced. The quantum yield of as-synthesized Si QDs is ~40% in solution and ~55% in Si QDs/polymer nanocomposites. Such QDs have near-unity internal quantum efficiency