Dec 9, 2009
Nanocrystals: size does matter
Silicon quantum dots are useful materials, be it as phosphors, fluorescence microscopy labels or light emission sources for LEDs. They are non-toxic, durable and made of the second most abundant material on earth. So far, studies have tended to concentrate on easier to fabricate spherical designs, but theory predicts that elongated versions – so called quantum rods – should exhibit more efficient light emission.
Researchers at the Royal Institute of Technology (KTH) in Stockholm, Sweden, and Charles University in Prague have teamed up to investigate the underlying physics of quantum dots using techniques such as nanolithography and single-dot spectroscopy.
By studying single nanocrystals, the team avoids the averaging effects of measuring ensembles and can strike a path to understanding the physical details of how light emission from such quantum dots actually works.
Transforming wire into rods
In an article published in the journal Nanotechnology, the team introduces a method for fabricating silicon quantum rods. Starting with electron-beam lithography, the researchers create silicon walls by plasma etching and then shrink these structures by oxidation. An effect called self-limiting oxidation helps to change the shape of the remaining silicon core within the walls so that it separates into an undulating silicon nanowire at the top and a bulk component at the bottom. Further oxidation then transforms the wire into a set of well separated quantum rods.
It is possible to see the light emission of such single dots using just an optical microscope and a CCD camera. These nanocrystals have a strikingly high polarization ratio when it comes to both absorption and emission, making them suitable for different applications such as polarization sensors or emitters of highly linearly polarized light.
Further research is now being conducted on these rods to test different theoretical predictions such as suppressed Auger recombination of excitons, increased quantum efficiency and stronger emission.
About the author
Benjamin Bruhn is a PhD student at the Royal Institute of Technology in Stockholm, Sweden. His project involves lithographic nanofabrication, optical spectroscopy and electron microscopy. Jan Valenta is docent at Charles University in Prague, Czech Republic, but he still finds some time to enjoy active research work across different projects in the lab. Jan Linnros is professor at the Royal Institute of Technology (Stockholm, Sweden) and supervises a variety of projects. He is also involved in a spin-off company that fabricates highly efficient structured X-ray detectors.