Nanoscale Organic LEDs Demonstrated
Daniel S. Burgess
Scientists at the US Naval Research Laboratory in Washington have reported the fabrication of electrically driven organic LEDs in which the active material is incorporated into holes as small as 60 nm in diameter. They suggest that it may be possible to produce such emitters using 10- to 15-nm-diameter holes, paving the way for the use of the devices in chemical and biological sensing, direct-writing photolithography and optical interconnects.
Using standard lithographic techniques, investigators have fabricated nanoscale organic LEDs featuring holes filled with the light-emitting polymer MEH-PPV. The false-color image, produced using a far-field optical microscope, reveals the emission from a single 60-nm-diameter LED. The inset shows one of the holes, viewed under a scanning electron microscope. Courtesy of the US Naval Research Laboratory.
The researchers employ standard techniques, using electron-beam lithography of a photoresist and dry etching to produce cylindrical holes in the 100-nm-thick Si3N4. They spin-cast a solution of the conjugated light-emitting polymer MEH-PPV onto the patterned surface, filling the holes. Semitransparent gold and LiF/Al layers serve as the hole- and electron-injecting electrodes.
Tests of devices featuring 60- and 100-nm-diameter holes revealed that their current density vs. electric field characteristics and turn-on electric fields were similar to those of a 2 × 2-mm single-layer MEH-PPV organic LED that they used as a reference. The researchers note that the turn-on electric field in the nanoscale organic LEDs could be reduced by half or better with the incorporation of a hole-injection layer to the device structure. Further optimization of the structure will enable better device performance.
A comparison of the photoluminescence spectrum of MEH-PPV and the electroluminescence spectra of the nanoscale emitters revealed a broadening of the output, which was centered at 590 nm. The scientists plan to investigate the cause of the phenomenon.
Zakya H. Kafafi, head of the organic optoelectronics section in the laboratory’s optical sciences division, said that the researchers also will seek to address fundamental questions related to the generation of light on the nanoscale.
She asked “how small can we make these light sources?” Can we make single-molecule light emitters? Can we make single-nanocrystal light emitters? Can we develop a uniform array of nano light emitters?”
Nano Letters, online Nov. 23, 2005, doi:10.1021/nl051811+.
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