Light from silicon is possible after all
The entire world of microelectronics is based on silicon, a common element abundant on earth. One thing silicon is not good at, however, is emitting light. That's why lasers and LEDs are made from rarer and thus more expensive elements, and why there are no well-functioning silicon optoelectronic applications. That might change now due to FOM-sponsored research by scientists at the University of Amsterdam and at the Ioffe Physical-Technical Institute in St. Petersburg, Russia. They have observed a variant of light emission in silicon nanocrystals that, until now, the experts thought impossible. The researchers published their findings in a Nature Nanotechnology Advance Online Publication that appeared on 28 November 2010. The newly-discovered form of light emission from silicon nanocrystals is 1000 times greater than that of silicon itself. A light source based on silicon could therefore still be possible, according to the researchers.
Silicon is a semiconductor made popular by the invention of the silicon transistor. Moreover, the element is exceedingly abundant on earth, making it very attractive for large-scale applications. Unfortunately, silicon is less suitable as a light source, in other words for the release (emission) of light particles (photons). Silicon's characteristic material properties ensure that the vast majority of energy that could be used for the transmission of photons is converted into heat. That is one of the reasons why silicon solar cells are inefficient. For the production of efficient light sources based on silicon this is, of course, a huge drawback.
Manipulating silicon
There are various ways to manipulate silicon so that it still emits light. A popular approach is to process silicon into nanocrystals, also called quantum dots (they consist of just 1000 to 10,000 atoms). At those small sizes, all kinds of quantum effects start to play an important role. One of the main consequences is that you can change the wavelength (or colour) of the light that silicon emits. Normally, silicon produces light at the infra-red end of the spectrum, but as you make the nanocrystal smaller, the wavelength of the emitted light shifts towards blue. Another important consequence is that photons are emitted with increasing efficiency: so more energy is turned into light instead of heat.
Various nanocrystal emissions
Although scientists have been able to make nanocrystals for about 20 years, many of their properties have still not been identified or are poorly understood. That is because materials at the nanoscale often present difficult complications. For example, it was discovered that the material in which the nanocrystals must be incorporated (you cannot just mix them together) exerts a major influence. That "matrix material" can significantly influence the "extent" of quantum effects and can even emit light itself. You want to avoid the latter, of course, because otherwise you won't know how much light originates from the nanocrystals nor the wavelength at which they are propagated. This is still a topic of much discussion.
The same applies to the nanocrystals that the researchers from Amsterdam used. Their crystals are in a matrix of glass. This is a highly popular combination because its quality and properties are very stable, and it is relatively easy to manufacture. Since the 1980s, it has been known that this configuration shifts the colour of the emitted light from infra-red wavelengths to wavelengths just within the visible spectrum (red) as the nanocrystals get smaller. In addition to the "usual" nanocrystal-related wavelengths, the researchers also saw other wavelengths being emitted by the matrix.
Ultrafast visible emission from nanocrystals
First author and STW researcher Wieteke de Boer, a member of Tom Gregorkiewicz's group from the University of Amsterdam, now claims that a specific emission that she and her colleagues detected can, indeed, be attributed to the nanocrystals. Up until now, it was generally assumed that this specific emission came from the glass matrix; no systematic investigation was performed in the past. De Boer's colleagues have now done that - and with a surprising outcome.
In their research, they found that the observed emission has characteristic properties due to quantum effects, an important argument for this emission not being related to the matrix, but to the nanocrystals themselves. After all, if this emission were propagated by the matrix, it would hardly be affected by the size of the nanocrystals. To confirm this hypothesis about the emission, the researchers produced theoretical models of the material properties of silicon nanocrystals. These demonstrated that quantum effects allow more emissions than are possible in bulk silicon, with exactly the same properties as those found in the experiments: smaller nanocrystals exhibit a red shift in wavelength, not a blue shift as with "conventional" emissions from a silicon nanocrystal. In bulk silicon, the colour of this emission would lie in the ultraviolet, but with nanocrystals, this shifts to the visible spectrum. Moreover, the experiments show that the efficiency of this "new" emission is 1000 times higher than that of bulk silicon. According to the researchers, we can view these extra emissions as energy that is converted not into heat, as is the case in ordinary silicon, but into extra light (in the visible spectrum). So visible emission is possible in silicon nanocrystals after all!
Reference
Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals, Wieteke de Boer1, Dolf Timmerman1, Katerina Dohnalová1, Irina Yassievich2, Hong Zhang3, Wybren Jan Buma3 and Tom Gregorkiewicz1, Advance Online Publication, Nature Nanotechnology, 28 November 2010.
1 Van der Waals-Zeeman Institute, University of Amsterdam
2 Ioffe Physical-Technical Institute, St. Petersburg, Russia
3 Van 't Hoff Institute for Molecular Sciences, University of Amsterdam
This study was partly funded by Technology Foundation STW, the Foundation for Fundamental Research on Matter (FOM) and the Dutch Organisation for Scientific Research (NWO). Wieteke de Boer is working on an STW project, Dolf Timmerman has a Top Talent grant from NWO. FOM funds postdoc Katerina Dohnalová and funded guest researcher Irina Yassievich.
Information
Further information:
Wieteke de Boer, Prof. Tom Gregorkiewicz, +31 (0)20 525 56 43.
The illustrations are available in high resolution. They can be requested through pr@stw.nl