Feb 6, 2014
Strong blue emission from tin dioxide quantum dots related to surface and deep-level defects
Wide-bandgap oxides have rich functionalities and a variety of technological applications. For example, tin dioxide (SnO2), is extensively applied in gas sensors, transparent conducting thin films, catalysis and solar cells as it has excellent optical, luminescence and electrical properties. Now, researchers at the University of the Free State, South Africa, are able to tune the emission intensity of SnO2 quantum dots (QDs) and improve these properties further.
SnO2, an n-type semiconductor with a wide bandgap of ~3.62 eV at 300 K, has novel chemical and physical properties. However bulk SnO2 crystals show a very low emission efficiency at room temperature which limits their application in optoelectronic devices. Reporting in Nanotechnology, the researchers have found that surface and deep level defects play very important roles in the emission of SnO2 QDs. Through better understanding the role of defects on the origin of emission in SnO2 QDs, and varying the defect concentration, they are able to tune the emission intensity.
The bandgap of QDs can be easily tuned by varying the size of the QDs, which in turn affects the emission from QDs. A strong blue emission has been observed, which is attributed to the formation of oxygen vacancy related defects. Whilst it is already recognised that these vacancies play an important role in the luminescence bands of SnO2, there is still controversy over the effects of oxygen vacancies on the blue emission.
Here, the reported maximum intensity of emission is due to the formation of singly occupied and double occupied oxygen vacancies oxygen vacancies (see diagram) which is attributed to the strong quantum confinement. As highlighted in the schematic diagram above, the low intensity is observed only when singly occupied vacancies are formed.
The surface trapped hole tunnels from potential well-1 to potential well-2 crossing the first potential barrier (ϕb1). Some holes get recombined with the electrons and emit blue light; however some of them cross the second potential barrier (ϕb2) by hopping and arrive at potential well-3. Here they participate in further recombination events with available electrons and give out blue emission. This result could widen the potential applications of SnO2 QDs in future blue light emitting devices.
The team now plans to synthesize different kinds of oxide QDs using a chemical route for use in oxide based light emitting devices. This will also help the group to understand the various aspects of light emission and its application in devices.
More information can be found in the journal Nanotechnology 25 135701.
About the author
Professor H.C. Swart is a South African Research Chairs Initiative (SARChI) research chair and group leader of the Phosphors research group at the Department of Physics, University of the Free State, Bloemfontein, South Africa. The group members involved in this work were Professor O.M. Ntwaeaborwa, Dr Vinod Kumar, Dr Vijay Kumar, and Dr Sudipta Som. The main focus of the group is on the fabrication of light emitting devices and solar cells based on nanophosphors.