Scanning photocurrent microscopy (SPCM) is a powerful way to image nanoelectronic devices such as carbon nanotubes, graphene or inorganic nanowire-based field-effect transistors. In this technique, a focused laser beam is raster scanned across a sample and the photocurrent simultaneously recorded. The new method, dubbed antenna-enhanced photocurrent microscopy, co-developed by Achim Hartschuh's group at the LMU, Ralph Krupke and colleagues at the KIT and Phaedon Avouris' team at IBM, works on the same principle except that the set-up comprises a sharp gold tip centred in the laser focus, held only a few nanometres from the sample surface.

The gold nanotip effectively acts as a "nanoantenna" – a device that collects and focuses at optical wavelengths (as opposed to a conventional antenna that works at radio frequencies). "The nanoantenna can be used to generate localized surface charge density oscillations at the surface of the gold tip that are sources of locally enhanced electric fields," explained Hartschuh. "Since these surface charge densities are bound to the tip surface, the resulting fields are spatially confined on the length scale of the diameter of the tip apex and thus can have dimensions far smaller than half the wavelength of the incident light."

30 nm resolution

Because light enhancement only takes place underneath the gold tip, the spatial resolution of the images obtained can be improved by more than a factor of 10, he adds. "This means that we can image with a resolution of 30 nm, compared with 300 nm using a conventional SPCM."

In the case of photocurrent measurements, it is the light absorption that is enhanced in the tiny space around the tip, something that leads to a larger electrically detected current signal. For optical processes such as Raman scattering, both optical excitation and emission can be enhanced by the tip, which leads to an even higher spatial resolution of about 20 nm. "We demonstrated this in experiments by simultaneously measuring Raman scattering and photocurrent signals from the same nanotube," said Hartschuh.

Antenna-enhanced photocurrent microscopy could be a new tool to characterize nanoscale devices in more detail than ever before, he adds. And it is not just restricted to carbon nanotubes, but could in principle, be applied to any other type of 1D structure such as inorganic nanowires. "Concerning nanotubes, the technique could allow us to study the all-important carbon nanotube-metal interface, and potential modulations along the CNTS device could be detected and resolved," he told "This is not possible with conventional confocal techniques. Being able to map the potential modulations in CNT-based and other optoelectronic devices on the nanoscale could help us to understand and improve their performance."

The work is detailed in ACS Nano.