(Invited) Facile Formation of Luminescent Colloidal Silicon Quantum Dots from Porous Silicon
Authors: Nakamura Toshihiro , Koshida Nobuyoshi 
Published: July 2021

Silicon quantum dot is nanometer sized single crystalline silicon particles. It shows exotic luminescence properties such as size tuneablilty of luminescence band and quasi-direct electron-hole recombination resulting in efficient visible luminescence at room temperature, due to quantum confinement effects. Because of its environmental friendliness and low cost, the silicon quantum dot is a promising material for future light-emitting applications. Recently, a free-standing form of silicon quantum dot dispersed in solution, i.e., colloidal silicon quantum dots are intensively investigated due to their potential usefulness in solution based light emitting technology. The luminescence properties and solution dispersibility in silicon quantum dots are known to depend on the surface properties such as the coverage of surface termination and the kinds of surface terminated species.
Various formation techniques for colloidal silicon quantum dot have been proposed, such as wet-chemical synthesis, plasma synthesis and high-temperature thermal processing. Pulsed-laser irradiation in liquid is one of the simplest top-down method for the formation of colloidal silicon quantum dots. By irradiating bulk crystalline silicon target with a high power pulsed-laser beam in liquid, silicon quantum dots are easily formed due to the ablation of silicon crystal and resultant condensation effect of ablated silicon at the liquid interface. An additional merit of this process is the easiness of surface termination control; because the nanocrystal surface formed by laser ablation actively reacts with molecules consisting of solution, one can control the surface termination by changing the kinds of solution. However, the control of the formed quantum dot size is still a challenging work and the size distribution of the quantum dots prepared by this process is usually wide.
To overcome these issues, we developed a novel top-down pulsed laser irradiation technique for the formation of colloidal silicon quantum dots. We used an interconnected wire-nanostructure assembly, i.e., porous silicon (PSi), as a target material for the laser irradiation process, on behalf of bulk crystalline silicon target. PSi has beneficial properties for laser irradiation target such as low thermal conductivity and large surface areas. Because of these properties, the pulsed-laser irradiation of PSi target in organic solution allows us to generate the silicon quantum dots with relatively high preparation yields, and with various emission bands by changing irradiation conditions and the porosity of PSi. Furthermore, in our developed process, the formed dots are brighter than the original PSi target due to the stable organic terminations.
Recently, we further improved our developed facile formation process of luminescent colloidal silicon quantum dots via porous silicon target. In the improved process, we employed electrochemically prepared PSi flakes, which purely consists of porous layer. The PSi flakes show extremely low thermal conductivity and heat capacity due to their intrinsic geometrical feature. Thus, even a low temperature thermal treatment as well as pulsed laser irradiation to PSi flakes in organic solution are efficiently converted porous silicon layer to colloidal silicon quantum dots. Furthermore, we succeeded in preparing silicon quantum dots dispersed in a polar and non-polar solvent depending on the ligands terminated on the dot surface by using different solutions. The present facile formation process of silicon quantum dots from PSi allows us to develop future top-down and low-cost mass production of luminescent colloidal silicon quantum dots.