Jul 1, 2009
US-China team tunes composition of group IV nanocrystals
For many years, nanocrystal science has focused on tailoring crystal properties through the size of the nanocrystal. Recently the focus has shifted to nanocrystal composition as an additional means of broadening the spectrum of semiconductor nanocrystal properties. For example, intentional doping of nanocrystals with impurities only slightly changes their composition, yet the electrical, optical and magnetic properties of the nanocrystals may be significantly improved. Alloying is another critical technique for tuning the composition of semiconductor nanocrystals.
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.
About the author
Dr Xiaodong Pi worked as a research assistant professor with Prof. Uwe Kortshagen at the University of Minnesota, US. Most of the studies were performed at the University of Minnesota and part of the work was carried out at Zhejiang University, China, where Dr Pi is now an associated professor. Prof. Kortshagen's main research areas are basic plasma studies and plasma nanotechnology. His work focuses on the plasma synthesis of group-IV nanocrystals and their integration into photovoltaic, electroluminescent and thermoelectric devices. Dr Pi works on harnessing group-IV nanomaterials for energy.