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20 Apr 2009
Andor says that replacing a standard CCD with electron multiplying technology can deliver big gains in the speed and sensitivity of a low-light spectroscopy technique.
Scanning tunnelling microscopes have become one of the most powerful tools for probing surfaces at the atomic level, and in recent years researchers have even started to exploit the light emitted during the scanning tunnelling process to uncover more information about the material surface.
Spectroscopic studies of these emitted photons can reveal, for example, extra details about the surface topography and structure, as well as the chemistry of molecules adsorbed on the surface.
The problem is that this so-called scanning tunnelling luminescence (STL) – which is produced by the inelastic scattering of tunnelling electrons – generates only very small amounts of light. Although high-sensitivity CCD detectors can measure such low photon fluxes, data must typically be collected for several minutes to gather enough information for spectroscopic studies. That in turn limits the accuracy of the analysis, while the sample surface can also be damaged by prolonged exposure to the tunnelling current.
Electron multiplying CCD (EMCCD) cameras, which were originally pioneered by Andor in 2000, could offer a solution. These cameras are designed to amplify the response from very low light signals, and allow high sensitivity to be maintained at higher read-out speeds.
"The enhanced sensitivity and higher acquisition speeds of our EMCCD cameras means that spectral data can now be acquired far faster and with less background noise," commented Gerald Cairns, spectroscopy application specialist at Andor. "This will help to broaden the utility of STL and help increase our understanding of nano- and micro-scale optoelectronic and photonic devices."
As an example, STL emission spectra were recorded with both a standard CCD and an EMCCD under the same exposure and setup conditions (see figure). It's clear that the EMCCD data suffers from much less background noise, which suggests that EM technology should allow meaningful STL spectroscopic data to be collected in much shorter exposure times.
Based on these results, Andor believes that replacing conventional CCD cameras with EM technology will make STL spectroscopy a much more useful technique. As well as studying photonic devices and structures, the enhanced experimental setup could be used for applications such as ultra-sensitive chemical analysis, single-molecule optical spectroscopy, and luminescence studies on quantum structures, polymers and biomolecules.
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