Non-thermal pulsed plasma activated water: environmentally friendly way for efficient surface modification of semiconductor nanoparticles

Semiconductor nanoparticles proved to be an important material in the composition of modern technologies. The most important properties which shape these nanoparticles include not only their atomic composition and size, but also surface chemistry. Here, we present an environmentally friendly method of their surface modification. The proposed method does not require any possibly toxic chemicals, it relies solely on the use of water and air and an inexpensive plasma-generating system. Water treated by discharge is known to produce the so-called plasma-activated water, containing a complex mixture of water- and air-originating reactive species. We show that water treated by transient spark discharge in a vessel with restricted air flow leads to a novel chemical composition of plasma activated water, beneficial for the modification of the surface of nanoparticles. After the treatment of silicon nanoparticles with our plasma activated water, their photoluminescence quantum yield increases several times and their dispersibility in water is significantly improved. The modification is stable at laboratory conditions for at least several weeks and when the liquid is dried out from the sample. We demonstrate that the modification is caused by surface incorporation of nitrate-water complexes, which subsequently leads to a change of surface-oxide-related strain affecting the charge carrier radiative rates. The general applicability of the method is confirmed using commercial MgO and ZnO nanoparticles. Thus, the proposed method enables chemicals-free surface modification of nanoparticles and opens the door for a wide range of variations based on different liquids and discharge parameters.