The well defined light from a light emitting diode (LED) can be tuned by altering the size of the semiconductor crystals from which it is constructed according to new work from scientists in Europe. The collaboration by teams from Austria and Germany, suggests a straightforward and economical method that lends itself to industrial-scale production.
Unlike incandescent lighting, LEDs can produce a relatively narrow band of color from the infrared to the ultraviolet by way of visible light. The precise wavelength range at which they emit is largely determined by the chemical composition of the semiconductor used and as such has been the focus of many years of work to develop different colored LEDs. It is also possible to adjust the emission wavelength by changing the size of those semiconductor crystals. When the crystals are on the nanometer scale, however, quantum mechanical effects come into play and these can be exploited for different applications.
Scientists at the Ludwig-Maximilians-University München, Germany and the University of Linz, Austria, have devised a simple method for producing semi-conducting nanocrystals with well-defined size from inexpensive mineral oxides known as perovskites. The nanocrystals are extremely stable which means high color fidelity. Also of practical relevance is that the materials can be printed on to a suitable substrate, which broadens their potential for displays that need not necessarily be flat like conventional LED displays.
The team explains that crucial element in their new approach is a "waffle-like" thin wafer just a few nanometers thick. The depressions in the "waffle" act as nanoscopic reaction vessels and it is the shape and volume that ultimately determines the form the nanocrystals will take. The team carried out measurements on their crystals using a fine beam of high-energy X-radiation at the Deutsche Elektronen-Synchrotron (DESY) in Hamburg. [S Demchyshyn et al, Sci Adv (2017) 3(8), e1700738; DOI: 10.1126/sciadv.1700738]
The fabrication process itself is an economical electrochemical process that allows the nanocrystals to be fashioned directly into LEDs. "Our nanostructure oxide layers also prevent contact between the semiconductor crystals and deleterious environmental factors such as free oxygen and water, which would otherwise limit the working lifetime of the LEDs," explains Martin Kaltenbrunner of the Johannes Kepler University, Linz explains. The team will next optimize their materials and the LEDs to boost efficiency still further and then begin to explore their potential for use in other applications, such as flexible displays.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.