NANO LETTERS
Nanoscale Organic Light-Emitting Diodes
2005 Vol. 5, No. 12 2485-2488
Hiromichi Yamamoto, John Wilkinson, James P. Long, Konrad Bussman, Joseph A. Christodoulides, and Zakya H. Kafafi* NaVal Research Laboratory, 4555 OVerlook AVenue SW, Washington, DC 20375-5320 Received September 8, 2005; Revised Manuscript Received November 3, 2005
ABSTRACT This study reports the fabrication and characterization of nanoscale organic light-emitting diodes (nano-OLEDs) based on poly[2-methoxy5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH−PPV). The nano-OLEDs were fabricated by spin casting MEH−PPV into cylindrical nanoholes lithographically patterned into silicon nitride. The electroluminescence (EL) spectrum of MEH−PPV was similar to its photoluminescence spectrum, confirming radiative decay from the same excited state. Device characteristics in the form of current density and EL versus applied electric field are presented and compared with those of a large-scale OLED.
Various approaches have been reported for the development of nanoscale light sources, motivated by potential applications in quantum communication,1,2 near-field scanning optical microscopy, and nanoscale photopatterning.3,4 For example, scanning tunneling microscope (STM) stimulated light emission has been observed from single molecules5,6 and quantum dots (QDs).7,8 Although this approach is important from a fundamental scientific point of view, it is not easily implemented in practical devices. More practical approaches include that of electroluminescence (EL) of single InAs QDs embedded in the intrinsic region of a p-i-n junction of GaAs,1 which can suffer from an increase in the active emissive region by several micrometers due to current spreading and a large exciton diffusion length.2 Organic materials offer an alternative, as they can be readily processed into lithographically defined architectures, where the emissive area is confined by the lithographic structure. This approach has been employed in organic light-emitting diodes (OLEDs) larger (g0.1 µm) than those reported here. These include neat films of the conjugated polymers polyfluorene,3 poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV),4 and poly[3-(4-octylphenyl)-2,2′-bithiophene],9 as well as the guest-host molecular system aluminum tris(8-hydroxyquinolate)-N,N′-di-isoamyl quinacridone.10 Here we report EL from nano-OLEDs, in which MEH-PPV was incorporated into nanoholes fabricated with e-beam lithography into silicon nitride. We have chosen to use a semitransparent Au anode and a LiF/Al cathode. To the best of our knowledge, this is the first report of EL from nanoOLEDs as small as 60 nm diameter based on MEH-PPV. Figure 1 shows a schematic diagram of the nano-OLED and the molecular structure of the light-emitting polymer * Corresponding author: tel, 1-202-767-9529; e-mail,
[email protected].
MEH-PPV used in this study. A 10 µm wide by 10 nm thick Au anode (including a 5 nm thick Ti adhesion layer) was first fabricated on glass by vacuum deposition and liftoff. This was followed by deposition of a 100 nm thick silicon nitride film using plasma-enhanced chemical vapor deposition. An array of nanoholes (∼60 to 1000 nm diameter) was patterned into the silicon nitride film using e-beam lithography as follows. A 130 nm thick poly(methylmethacrylate) (PMMA) photoresist film (950 PMMA C2, MicroChem) was spin-cast onto the silicon nitride layer and heated to 180 °C for 5 min. E-beam writing was performed using a Raith150 with an acceleration voltage of 10 kV and a beam aperture of 7.5 µm. E-beam exposure was 160 µC/ cm2 for large holes (g100 nm) and 210-290 µC/cm2 (depending on humidity) for small holes (
