Sep 13, 2012
Two-step plasma synthesis gives environmentally stable silicon nanocrystals
Highly luminescent nanocrystals (NCs) that retain their efficiency after three years of exposure to light and air have been prepared by researchers based at the University of Minnesota, US. Quantum-confined nanocrystals have a range of uses in electronic devices, non-toxic biological tags, optical devices and photovoltaics, but UV radiation present in sunlight, environmental oxygen and water vapour, as well as elevated temperatures, can degrade the performance of the material. Photoluminescence in NCs depends on a passivation of surface states through chemical bonding. The degradation has generally been attributed to ligand desorption.
The silicon nanocrystals (SiNCs) were prepared using a plasma synthesis technique. Untreated SiNCs, which have hydrogen on the surface, oxidize when exposed to air. These NCs have a poor photoluminescent quantum yield (PLQY). They degrade further upon environmental exposure. Previously, the group has demonstrated SiNCs passivated with 1-dodecene that exhibit PLQY exceeding 60%. However, these materials degrade sharply on exposure to air and UV.
The scientists added a second stage to the plasma system, which allowed the surface of the SiNC to be treated in situ in a low-power SF6 plasma. These F-terminated SiNCs oxidize quickly in air. In a surprising development, the team found that SiNCs covered with a fluorinated silicon oxide shell have remarkable environmental stability with respect to exposure to air, UV irradiation and heat, while retaining PL quantum yields of up to ~50%.
These NCs also have much lower trap densities, as measured by electron spin resonance, compared with hydrogen-terminated NCs, both as-produced and after native oxide growth. The results suggest that the inclusion of fluorine into the silicon-oxide shell produces a more relaxed silicon-oxide network, which plays a critical role in improving the photostability.
Additional information can be found in journal Nanotechnology.
About the author
The study was conducted by research teams led by Prof. Stephen Campbell (Electrical and Computer Engineering) and Prof. Uwe Kortshagen (Mechanical Engineering) at the University of Minnesota. Prof. Kortshagen specializes in the synthesis, passivation and application of semiconductor nanoparticles. Prof. Campbell works on the characterization and application of semiconductor nanoparticles. Dr Richard Liptak, a postdoctoral fellow who works with both groups did the majority of the work. Dr Liptak completed his PhD in Minnesota where he developed the fluorine termination technique, then completed a postdoc at the University of California Santa Barbara where he worked on III-V epitaxial growth, before returning to Minnesota in 2011. Jihua Yang, a postdoctoral researcher in Prof. Kortshagen’s group, assisted with sample preparation. Nicholas Kramer, who is pursuing a PhDME degree in the Kortshagen group, assisted with the ESR measurements. The two groups are part of a Materials Science and Engineering Center funded by the National Science Foundation (NSF).