Excellent control over the elemental composition of ionically bonded semiconductor nanocrystals (for example, II-VI nanocrystals) has been well demonstrated, mainly owing to the relatively easy synthesis of freestanding nanocrystals by wet-chemistry approaches. In contrast, work on controlling the composition of covalently bonded group-IV nanocrystals is limited because freestanding nanocrystals cannot be produced efficiently in the liquid phase. However, group-IV nanocrystals are of particular interest due to the lack of toxicity, the abundance and the low cost of the materials.

Recently, Pi and Kortshagen, researchers at the University of Minnesota, have demonstrated the synthesis of boron- or phosphorus-doped freestanding silicon nanocrystals with non-thermal plasmas. Now they have extended this approach by demonstrating the capability of non-thermal plasmas to produce alloy group IV (Si1–xGex) nanocrystals. One of the challenges is the actual production of an alloy. The materials should be neither a mixture of two types of nanocrystals nor in a core-shell structure. Pi and Kortshagen used an oxidation-etching step combined with element analysis, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy to demonstrate that the nanocrystals have an alloy structure.

The scientists have shown that the composition of Si1–xGex nanocrystals can be seamlessly adjusted between pure silicon and pure germanium nanocrystals. Their development may extend the applicability of group-IV nanocrystals in thermoelectrics, microelectronics, optoelectronics and photovoltaics.

The researchers presented their work in Nanotechnology.