OLED Fabricated on Cantilever
Lauren I. Rugani
Scanning near-field optical microscopy conventionally has been an optically pumped technique. Several attempts at creating electrically pumped microscale light sources have been limited because of the unfavorable properties of III-V materials and fabrication difficulties. However, a team of researchers from the University of Michigan in Ann Arbor has employed a series of fabrication techniques to produce such a light source that is based on molecular organic LEDs and is designed for potential use in scanning near-field optical microscopy.
Researchers fabricated a tiny OLED on the end of an atomic force microscope cantilever.
Three 2-μm-thick silicon scanning probe cantilevers were connected in parallel, each containing an identical organic LED (OLED) and sharing a common anode and cathode. The anode, comprising a 100-nm-thick layer of aluminum under a 13-nm-thick layer of nickel, was thermally evaporated onto the cantilever and topped by an electrically insulating, 800-nm-thick layer of vapor-deposited parylene-C. The researchers then milled the insulator with a focused gallium-ion beam at 0.76 nA to form the active region on the cantilever.
Through energy-dispersive x-ray detection, they could monitor the milling process and control the well depth, removing a circular patch of nickel a few microns in diameter. After depositing the active organic layers (110 nm total thickness) and a semitransparent metallic cathode, they formed a ring-shaped region for hole injection into the active organic layers.
The emission pattern — imaged using an upright microscope with a high-numerical-aperture objective and a sensitive CCD camera — was in the shape of a thin ring whose rim thickness was on the order of 1 μm in the far field.
The researchers analyzed the current-voltage properties of the ring-shaped OLEDs on the cantilevers with an Agilent parameter analyzer and found the leakage current to be very small.
They suggest that this fabrication technique can easily be applied to other cantilever substrates and that the device may be used in a variety of probing methods such as transferring excitons to a sample through a cathode. The apparatus could be used to study exciton dynamics in photovoltaic devices and potentially could be applied to techniques like scanning near-field optical microscopy.
Applied Physics Letters, Sept. 11, 2006, 111117.
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