Nanoscale Light Sources Could Benefit Near-Field Scanning Optical Microscopy

Gary Boas

The probe tip traditionally used with near-field scanning optical microscopes comprises an optically pumped device that transmits light from a far-field source to the sample through either a hollow pyramid or an optical fiber, possibly leading to low optical transmission efficiency, undesirable heating of the tip or insufficient reproducibility with respect to probe fabrication.

Scientists at the University of Michigan in Ann Arbor, however, have overcome these limitations by incorporating an electrically pumped nanoscale light source on the tip of an atomic force microscopy (AFM) probe.

Researchers have described an electrically pumped nanoscale light source for use in near-field scanning optical microscopy, realized on the tip of an atomic force microscopy probe. As shown in the scanning electron micrograph above, they coated the tip with ultrathin films comprising an organic LED. As illustrated in inset (a), bipolar current injection leads to radiative charge recombination inside the organic layers on the tip. Insets (b) and (c) show optical micrographs of the probe in the “off” and “on” states, respectively.

Other investigators have reported electrically pumped light sources on AFM probes, typically using group III-V semiconductors, but the inorganic material that makes up the semiconductors is poorly compatible with the silicon-based microelectromechanical systems used as scanning probe substrates.

For this reason, the Michigan researchers, led by assistant professors Max Shtein and Kevin P. Pipe, began to explore the use of organic light-emitting diodes (OLEDs). After describing in 2006 the assembly of a circular OLED on a flat silicon cantilever, they found that positioning the light source within nanometers of the sample was difficult because of the planar geometry of the probe and because of the distance between the light-emitting region and the surface of the probe. Further complicating the matter, the probe fabrication included steps that necessitated additional tooling and sample transfers.

More recently, the researchers reported an OLED-based light source created directly on the tip of an AFM probe, offering simpler fabrication and a significantly smaller emissive region. They used a silicon nitride pyramidal AFM probe made by Veeco Probes of Camarillo, Calif., as a substrate and deposited the OLED on the probe’s tip using vacuum thermal evaporation, vapor-phase deposition and ion-beam lithography.

They showed that, using the nonplanar substrate geometry, they could achieve localization of the charge injection, the charge recombination and the light emission. Also, by measuring and comparing the current-voltage characteristic in devices with and without the tip intact, and by acquiring electroluminescence and optical images with an optical microscope made by Mitutoyo America Corp. of Aurora, Ill., and a 12-bit CCD camera made by Opteon Corp. of Cambridge, Mass., they demonstrated that they could turn on the device at the tip only.

The researchers are in the process of applying this probe to the imaging of nanostructures and biological systems.

Nano Letters, ASAP Edition, Nov. 3, 2007.