Dec 2, 2009
OLEDs miniaturized for nanoscale duties
Nanoscale light sources are required for applications including bio-sensing, high-density data storage, near-field optical lithography and nanophotonics. Unfortunately, most inorganic light-emitting semiconductors are not suitable for miniaturization down to the nanoscale due to fabrication challenges and quantum confinement related changes in band structure. The good news is that organic light emitting diodes (OLEDs) present a possible alternative as they are not bound by quantum confinement due to the amorphous structure of the active organic materials.
Recently, there have been attempts to make nanoscale OLEDs by isolating active materials in-between insulating materials such as silicon dioxide. These studies show the viability of patterning active OLED materials in an insulating layer to produce light from sub-micron-scale pixels. Unfortunately, the fabrication routes to making such devices typically involve processes that ultimately lead to charge spreading, resulting in much larger emissive areas than intended.
Isolating the emission
Researchers at the University of Texas at Dallas, US, are using nano-imprint lithography (NIL) to prepare electronically isolated nanoscale OLEDs. The fabrication process, which is described in the journal Nanotechnology, involves directionally evaporating the small molecule materials NPB and Alq3 into nano-imprinted SU-8 nanochannels with vertical sidewalls to avoid charge spreading. A cross-sectional electron micrograph shows physically isolated active materials deposited in the SU-8 channels.
The team compared its nanochannel design with micro-scale devices and found that device performance is retained as the dimensions are reduced to the nanoscale. Furthermore, the scientists applied surface potential microscopy (SPM) to reveal a periodicity in potential on the surfaces of devices without metal electrodes (see image), which indicates electrical separation of active materials and, therefore, isolated light emission from the nanoscale confined OLEDs.
About the author
Krutarth Trivedi is a PhD student in the electical engineering department at the University of Texas at Dallas (UTD). His research interests include both organic and inorganic nanoelectronic devices for optical and biomedical applications. Unnat S Bhansali is a PhD student in the materials science and engineering department at UTD and his research focuses on using metal-organic complexes for white OLEDs, supported by the US Department of Energy. Dr Bruce Gnade is a professor of materials science and engineering at UTD. His research focuses on electronic materials that are compatible with flexible electronics. Dr Walter Hu is an assistant professor of electrical engineering at UTD. His group is doing research in nanolithography, nanostructured materials and devices for organic electronics, biosensing and nanomedicine.