Unlocking the Limits of the Plasma-Activated Liquids and Plasma Synthesis of Silicon Nanoparticles

In the past years, the topic of nanoparticles of a broad variety of materials, mainly semiconductors, led to significant advances in science and technology. To make these materials more applicable, the search for variable methods of synthesis and surface modification is ongoing. Two groups of nanoparticles are of special interest. The first group are the sub-10-nm particles, which in case of several materials, e.g., silicon nanocrystals (SiNCs), exhibit efficient light generation (photoluminescence – PL). The properties of the generated light can be set up and modulated by the proper surface modification. The second group is represented by the particles with sizes around 100 nm, which can be employed in the production of batteries. Importantly, these larger particles with also benefit from surface modification, which will lead to better dispersibility in various liquids and thus simplify the fabrication of the battery anodes. In this study, we demonstrate that non-thermal plasma (NTP) is a powerful environment for both the synthesis of size-tunable SiNCs and the surface chemistry modification, considering its speed, simplicity and high reactivity. We overcome the limits of the NTP systems, showing the applicability of the NTP systems to synthesize the light emitting SiNCs, as well as the large ca. 80 nm SiNCs, and to provide a novel method of generating highly reactive liquid environment suitable for the SiNCs’ surface chemistry modification.<br/><br/>The main complications with respect to the potential of synthetizing larger nanoparticles in the NTP low pressure synthesis of SiNCs are the charging of the surface and the depletion of the precursor. We solved this issue by employing a two-stage process. In the first stage, the crystalline sub-10-nm core is formed with the use of planar electrodes. In the second stage, the long plasma beam, generated by helix electrodes, with additional silane gas, are utilized to synthetize SiNCs ca. 80-nm in diameter. In combination with high yields and the speed of the NTP method, we achieve a broadly variable synthetic system. We can provide crystalline sub-10-nm cores, whose effective PL is tunable by the size from 700 nm to 950 nm, as well as the large particles.<br/><br/>Surface chemistry is another key factor for the properties of silicon nanoparticles. Recently, we have shown that with the proper settings of the NTP system using two needle electrodes (one submerged) and generating the transient spark discharge, the high in nitrogen species plasma-activated water (HiN:PAW) can be produced.¹ This HiN:PAW is suitable for the nitrogen enrichment of the SiNCs’ surface, leading to enhancement and shifting of the PL, as well as improvement of dispersibility in water.² However, the generalization of this system to other liquids, i.e. the NTP activation of different organic liquids (PAOL) is not yet well-established, due to several drawbacks of these liquids, e.g., volatility or flammability. Here, we present a successful PAOL system, which solves the problematic behavior of organic liquid, mainly with the use of a novel designed reactor box. This system was applied on five different organic liquids with diverse structural (e.g., aromatic structure, aliphatic chains) and physical (e.g., polarity of molecule) properties, leading to a successful change of surface chemistry, accompanied by the tunability of the PL properties (position and intensity) and an improvement of dispersibility in different media. Therefore, with the use of the NTP activation of liquids, we achieved a novel adjustable system, which generates reactive environment suitable for tailoring of the SiNCs’ surface chemistry, leading to broad applicability of light emitting SiNCs in a plethora of environments (e.g., water, polar, non-polar).<br/><br/>¹Matejka, F., et al. 2023 Phys. Scr. 98 045619 10.1088/1402-4896/acc48e<br/>²Galar, P., et al. Green. Chem. 2021, 23, 898 - 911. 10.1039/D0GC02619K2