Anne L. Fischer
Semiconductor nanocrystals have received attention for their potential in LEDs. Researchers have explored a variety of novel approaches for the assembly of the nanomaterials into a device structure. Now a group from Samsung Advanced Institute of Technology in Yongin-Si, South Korea, has described the patterning of a nanocrystal-based LED using a photolithographic method.
The team exposed a film of coated CdSeS nanocrystals to 300- to 400- nm radiation, causing oleic acid ligands on the surface of the nanocrystals to form an insoluble cross-linked network. Oleic acid has a photosensitive double bond in the middle of the carbon chain, so it is photopolymerized by exposure to UV radiation.
Using a shadow mask, the investigators created unexposed areas on the device structure that remained soluble to toluene. Rinsing the device with the solvent thus selectively removed the undesired areas. Feature sizes of approximately 2 μm were produced in this manner.
Following the rinse, they thermally deposited layers of a triazole, aluminum tris(8-hydroxyquinoline), LiF and Al. An applied current density of 65 mA/cm2 produced 100-cd/m2 light at a wavelength of approximately 550 or 620 nm, depending on the bandgap of the nanocrystals used. The nanocrystals have resisted photobleaching for more than a year under daylight.
The researchers plan to optimize the fabrication process to try to increase the efficiency from the relatively low 0.15 cd/A. The patterning technique may be useful for the spatial positioning of nanocrystals in photonic and electronic devices.
Langmuir, online Feb. 7, 2006, doi:10.1021/la051756k.