Synthesis of Styryl-Terminated Silicon Quantum Dots: Reconsidering the Use of Methanol

Abstract

Quantum dot solids, in which individual quantum dots (QDs) such as CdSe, PbSe, and InP QDs are assembled into 3 dimensions, have been paid much research interest due to their potential applicability to diverse optoelectronic fields. Silicon QDs whose electric transitions are directlike due to the relaxation of k conservation rule are hopeful to replace these QDs owing to its non-toxicity, abundance, and easy integrating into the widely developed silicon industries. Solution processes are preferred for the generation of these solids due to the low cost and large area capability. A post ligand exchange and/or thermal annealing is necessary to enhance physical interactions and electronic communications among the individual QDs. However, in the cases of Si QDs, the post ligand exchange is inapplicable due to the strong covalent bonds between the dot and the capping molecules. Therefore, capping Si QDs with organic molecules has to be logically designed during the synthetic course not only to prevent the Si QDs from unexpected oxidation and give high solubility but also to ensure sufficient QD-QD physical interactions for the solid assemblies. Recently, Tilley group has reported the use of miniemulsion approach for the generation of hydrogen-terminated Si QDs (H-Si QDs), which are easily post-functionalized with 1-alkene using Pt catalyzed hydrosilylation. We initially adopted this process to synthesize styryl-terminated Si QDs (PheAc-Si QDs), as illustrated in Scheme 1. In the first step, SiCl4 confined inside the inverse micelles of TOAB (tetraotylammonium bromide) was reduced by LiAlH4 (3 molar equivalent) to generate H-Si QDs. The remaining LiAlH4 was quenched by methanol (MeOH) before phenylacetylene (PheAc) and H2PtCl6 catalyst were added to start the hydrosilylation reaction in the capping step. Finally, multiple-step purification was carried out to obtain TOAB-free Si QD solution in hexane. However, the resultant Si QD contains very few styryl groups as confirmed by low intensity of aromatic C-H stretching peaking in the 3000-3200 cm range in its IR spectrum, curve (c) in Figure 1. At the same time, a broad band from 1000 to 1100 cm originated from oxidized components, such as Si-O-Si or Si-O-R is intensive. Its intensity is about 60% when compared with oxidized components of a control Si QD sample, which was obtained without capping step, curve (d) of Figure 1.