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Micro/Nano Lithography

Polymers provide nanoscale LEDs

Nanoscale light-emitters have been made as small as 60nm in diameter and will play an important role in future nanophotonic applications.
23 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0123

Nanophotonic applications will ultimately rely on nanoscale light sources. Nanoscale electroluminescent (EL) devices therefore could find use in areas as diverse as direct-writing nano-photolithography and quantum-communication schemes that depend on single-photon emitters. Other potential applications include optical interconnects, chemical and biological sensors, and sources for nanoplasmonic circuits. While it is not difficult to excite an isolated nanoscale emitter (such as a quantum dot) by optical means, electroluminescent devices pose challenges in confining either the electrical current, or the light source, or both.

Inorganic semiconductors provide one approach to nanoscale EL, but require architectures that overcome the micron-scale diffusion lengths often exhibited by current carriers and the light-emitting excitons they form.1 A promising alternative builds on the processability of organic materials developed for conventional organic LEDs targeted for flat-panel displays, solid-state lighting, and lasers.2 Although comprising only several hundred polymer molecules, these nanoscale organic light-emitting diodes (nanOLEDs) performed similarly to millimeter-scale devices.

The organic materials can easily be spun or evaporated into predefined lithographic nanostructures. Prior work has employed a variety of approaches to forming nanoscale light sources, including use of templates, such as formed in nanopore filters3 or by nanosphere lithography,4 and use of electron-beam (e-beam) lithography,5 which allows controlled placement of the nanOLED. In our efforts to produce even smaller devices, we have used conventional e-beam lithography and a single polymeric emissive layer (see Figure 1) to achieve devices as small as 60nm in diameter (see Figure 2).6

Devices were fabricated as sketched in Figure 1. First, a semi-transparent gold anode was patterned on glass via photolithography. Then we deposited an insulating 100nm-thick film of silicon nitride by chemical-vapor deposition. Next we etched nanoscale holes into the film, down to the anode at locations defined by electron-beam lithography: see Figure 2(a) and (b). The etching of the holes was followed by spin-casting the light-emitting polymer MEH-PPV poly[2-methoxy- 5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] over the entire device. The dynamics of the spinning process led to dried films of MEH-PPV that maintained a 50nm thickness inside the holes. To further ensure that light emission was confined to the nano-holes, we adopted a single-layer architecture, and so evaporated the cathode materials (LiF followed by Al) immediately on top of the MEH-PPV film. This single-layer approach departs from the typical multilayer architecture, in which additional organic layers are optimized to improve efficiency. It is a challenge to keep multiple layers from overfilling the depth of the hole, which can lead to spreading the current—and attendant light emission—over a larger area than desired.5

Figure 1. The device architecture for a nanoscale organic LED includes electrical contacts as well as holes that confine the light vertically. The nanoscopic hole on the right emits light (hν) down through the substrate. The hole is partially filled with the light-emitting polymer MEH-PPV.

Light emission from the individual nanOLEDs was recorded through the semitransparent gold anode by using an optical microscope (with a numerical aperture of 0.6), which was equipped with an electron-multiplying charge-coupled-device (CCD) camera for high sensitivity as well as an imaging spectrometer. Consequently, we could record both images and spectra from operating devices with the same CCD camera. Figure 2(c) shows EL images from two arrays of nanOLEDs with 100nm and 60nm diameters patterned on the same anode. The EL spectra were found to be entirely consistent with MEH-PPV, as determined from the emission band (∼550–700nm) and phonon progression.

Figure 2. (a) A scanning electron micrograph of 3 × 3 arrays of 100nm- and 60nm-diameter holes patterned in a 100nm-thick film of silicon nitride. (b) A close-up of a 60nm hole. (c) The electroluminescent image of nanOLEDs corresponding to the hole pattern in (a).

We determined the electrical characteristics by including organic LEDs as large as 1 μm in diameter on the same anode as the nanOLEDs, which increased the current to conveniently measured values. Plots of the current density versus the applied electric field show that the device arrays behave much like a large, 2 × 2mm2, reference organic LED.6 At the same time, the EL emission integrated over the 100nm and 60nm devices also turns on at the same electric field as the current. Taken together, the data show that the nanoscale devices behave much like a standard single-layer organic LED.

The ability to fabricate working 60nm nanOLEDs using electron-beam lithography suggests that devices several times smaller should be possible at the lithographic limit. The use of standard nanofabrication techniques, and the well-behaved operating characteristics of the nanOLEDs described here, hold great promise for future nanophotonic applications.

James Long, Hiromichi Yamamoto, John Wilkinson, Konrad Bussmann, Joseph Christodoulides, Zakya Kafafi
Naval Research Laboratory
Washington, DC
Dr. James Long is a Research Physicist in the Chemistry Division at the NRL. His research interests focus on the linear and nonlinear optical properties of nanostructured materials, especially the characteristics of individual nanoscale emitters such as quantum dots, semiconducting nanowires, plasmonic architectures, and nanOLEDs.
Dr. Zakya Kafafi is the Head of the Organic Optoelectronics Section at NRL and is an SPIE Fellow. She has organized and chaired numerous international and national meetings, including editing the proceedings of more than 20 SPIE conferences. She introduced the peer review system to the SPIE proceedings and has authored over 30 SPIE manuscripts. She has been chair of the SPIE Track Program on “Organic Photonics and Electronics” for the last three years.