Non-Hydrogen Peroxide Plasma Activated Water—An Efficient Way of Surface Modification of Semiconductor Nanostructures

The development and application of nanomaterials is one of the most impacted trends in the current science and also industry. The reason of its attractiveness is the possibility to significantly tune the properties of bulk materials by limiting their dimensions and using various surface termination or encapsulation strategies. Silicon nanocrystals (SiNCs) are one group of materials which gain significant interest. Even though Si is an indirect semiconductor, SiNCs show relatively high light emission efficiency with size tunable spectral position, good stress resistance and also surface reactivity. A wider application of SiNCs is, however, limited by the absence of a cheap and simple method for their surface termination and modification together with significantly low water dispersibility.<br/><br/>We have recently shown that non-thermal plasma can be effectively used for tuning of the SiNCs surface chemistry through plasma activated liquids (PAL).¹ The liquid activation is realized through treating a liquid by an electric discharge, resulting in the generation of reactive species in the liquid based on the chemical composition of the liquid, surrounding atmosphere and also properties of the used discharge. The most common PAL is plasma activated water (PAW), which is a well-known agent in medicine, agriculture and related field, because of its antibacterial, antifungal and disinfection properties. The limiting parameter of common PAW for nano-semiconductor surface treatment is the presence of a high concentration of hydrogen peroxide, which not only damages the nanocrystals, but also blocks any other surface termination except for oxidation. To overcome this effect, we have developed a technique for the preparation of PAW very low in hydrogen peroxide and containing a high concentration of nitrogen radicals (HiN:PAW). The technique is based on the application of transient spark discharge and semi-closed atmosphere at ambient conditions. Using a specific ratio between the electric pulse period and the peak current, the discharge is able to generate plasma containing a high concentration of nitrogen atoms in air, resulting in the suppression of peroxide generation in favor of nitrites and nitrates in HiN:PAW. The treatment of SiNCs by HiN:PAW for about 30 minutes resulted in nitrogen-based surface termination, manifested by a significant increase in their photoluminescence quantum yield (more than 10 times) and an increase in their dispersibility in water. The modification is stable for more than a month. While the first effect increases the prospective utilization of SiNCs in light conversion and generation and imagining technology, the latter makes them interesting for further applications in hybrid technologies, where water-based chemical processes are involved, such as the preparation of electrodes for lithium batteries. Analogical treatment can also be effectively applied to other kinds of semiconductor nanoparticles, which makes HiN:PAW a novel, cheap, easy and effective way of surface termination of semiconductor nanoparticles.<br/><br/>¹Galar, P., Khun, J. et al. <i>Green. Chem.</i> 2021, 23, 898 - 911. 10.1039/D0GC02619K2