A silicon-based light source represents a new path towards integrated, compact and mass-producible microsystems for advanced computing, networking and sensing. Unfortunately, silicon is an indirect bandgap material and therefore is an inefficient light emitter. To address the problem, researchers at the University of Texas at Austin, US, have used quantum dots (QDs) as the light emitter in an inorganic light-emitting device with silicon as the hole-transporting layer. QDs such as CdSe:ZnS (core shell configuration) are size tunable and hence wavelength tunable. Advantages such as narrow bandwidth and high luminescence quantum yield make them suitable for LED applications.

Patterned deposition of QD monolayers was demonstrated on a silicon substrate using an efficient and silicon compatible micro-contact printing (or "stamping") technique. Then sputtering of ZnO:SnO2 created an amorphous electron transport film on top of the QD layer. A thin transparent metal cathode was deposited using e-beam evaporation to observe light emission. A look-up table was created in the computer to correlate the fluorescence with the atomic force microscopy measurement of the thickness of the nanoparticle thin films. Transmission electron micrographs were taken to determine the uniformity of the particles on stamping. Electrical injection into the inorganic transport layers resulted in light emission. Multicolor LEDs were demonstrated by loading the patterned PDMS stamp with particles of different sizes (and therefore different emission wavelengths).

This research is likely to open up many exciting opportunities in novel optoelectronic applications including near-field microscopy beyond the diffraction limit, MEMS-based medical endoscopes for sub-cellular imaging, and compact light-on-chip biosensors and biochips.