Surface Termination of Plasma Synthesized Silicon Nanocrystals using Plasma-Activated Liquids

In the last decades, nanoparticles of a broad variety of materials, mainly semiconductors, made a breakthrough impact on science and industrial development. The main glamor of nanostructured materials is due to the alteration of their original bulk properties. Nano-scaled materials report a higher surface-to-volume ratio, leading to a crucial impact on the surface effects. Moreover, with sizes less than a dozen of nanometres, the quantum confinement of the wavefunctions is manifested. In the case silicon in the form of nanocrystals (SiNCs), these effects lead to the lighting up of the normally dull bulk material. The relatively high light emission efficiency of SiNCs is accompanied by wide spectral tunability from visible to near-infrared (IR)spectral regions. The peak emission and intensity of photoluminescence (PL) is dependent on the size, defects, and surface termination. However, the limitations of further applications of the SiNCs are their methods of synthesis and methods of surface termination. So far, especially the existing termination methods are low yield, expensive, complex, and not environmentally friendly.<br/><br/>In this work, we apply non-thermal plasma (NTP) for both the synthesis and the subsequent surface modification. In the case of the synthesis, the NTP low-pressure method generates SiNCs with a highly reactive hydrogen-terminated surface; due to the synthesis from silane gas, with a low number of defects, leading to initial high PL intensity. Additionally, the mean size of the NCs and therefore their PL peak emission is finely tuneable from 700 nm to 950 nm. The yield of this method is high, in micrograms per minute.<br/>The reactive hydrogen surface can be modified by several methods. The most common method for carbon termination creating C:SiNCs is hydrosilylation, which is costly on time, (reaction time up to 12 hours) and on energy, due to the long reaction time and high boiling points of organic liquids. Hence, we introduce an innovative approach of the NTP treatment of liquids, the so-called plasma activation (PAL). The NTP treatment provides a gentle interaction with the liquid, using reactive species generated by the discharge from the surrounding atmosphere (can be applied in air, or in a closed-space inert atmosphere), using by two needle electrodes. The reactive species then react with the molecules of the liquid, creating a reactive environment for the SiNCs with enough energy to exchange the surface bonds. In our study, we focused on organic liquids (PAOL), whose molecules report dipoles, leading to a better interaction of the discharge with the liquid phase (e.g., Ethanol, Acetic Acid, Octanol, etc.). For the PAOL treatment, the closed chamber reactor constantly flushed with Ar was designed to prevent the the interaction with humid air, suppress flammability, and establish stable discharge conditions. Despite that, the overall PAOL process can be done in less than an hour of reaction time. The PAOL treatment produced a stable (for more than 72 days) enhancement of the quantum yield (QY) and PL properties (intensity increment more than 25 times and slight blue-shift) and better dispersibility (proven by DLS measurements). The surface bonding was studied by IR spectroscopy and PL measurements.<br/>An analogical PAL treatment can be also applied to inorganic liquids. In our recent work, we showed that plasma activated water (PAW) can successfully enrich the surface termination of SiNCs with nitrogen-based species. In this on-air process, the creation of the intermediary phase above water is needed, therefore the settings of the shape of the reactor, volume of water, and discharge are crucial to generate PAW without hydrogen peroxide, which kills the SiNCs PL, simultaneously exhibiting a high concentration of nitrogen species. This treatment leads to stable enhancement of QY (up to 10 times) and water dispersibility of SiNCs, making them more attractive for further applications.