Nonthermal Plasma Synthesis and Optical Properties of Colloidal Group IV Nanocrystals

Abstract

Silicon is a popular material for various applications due to its earth abundance. Interestingly, at extremely small sizes, silicon exhibits size-tunable light emission (silicon quantum dots) or scattering (silicon Mie particles). In the form of colloidal dispersion, silicon nanocrystals (Si NCs) exhibit versatility in terms of biological applications, integration with other materials, and metafluids. To obtain colloidal stability, the surface of Si NCs are modified. Surface modification is an important process to effectively utilize Si NCs’ optical properties, because it enhances not only the optical performance but also colloidal stability. This study focuses on the synthesis of colloidal Si NCs using nonthermal plasma and the optical characterization of the colloidal Si NCs. For colloidal Si quantum dots, two types of Si NCs produced by one-step synthesis using nonthermal plasma were demonstrated. One of them has decent water-dispersibility, which is generally challenging to achieve due to the hydrophobic nature of Si NC surfaces. Their water-dispersibility and red emission with a high quantum yield (~30%) indicate promise for biological applications. The other is alloyed with germanium (Ge) to expand the photoluminescence (PL) in the NIR range of Si NCs, passivated by chlorine atoms that provide dispersibility in certain solvents such as benzonitrile. NIR emission at ~1040 nm, which is longer than the reported PL of Si NCs, was found after heterostructuring with perovskite. Since the synthesis routes do not need any post treatment for surface passivation, NC liberation, or purification, they are energy- and time-saving methods. Si Mie scatterers in sizes of 60 to 200 nm were synthesized by nonthermal plasma. It was shown that the nonthermal plasma synthesis is advantageous to produce Si Mie scatterers with narrow size and shape distributions which are desirable for their optical performances. The Si Mie scatterers formed native oxide shell under ambient air to generate colloidal dispersion with water. From the colloidal Si Mie scatterers, light scattering across the visible range was observed under white light irradiation. It indicates the feasibility of nonthermal plasma synthesized Si Mie scatterers to act as metafluid, which will be versatile for manufacturing using metamaterials.