Near-field imaging technique, scanning tunnelling

The first and best known near-field technique to measure electrical properties in the nanoscale is of course Scanning Tunnelling Microscopy (STM). Since its invention by Binnig et al., STM has been used to explore the mechanisms of lots of phenomena on surfaces [289-294], ranging from experiments concerning the local work function to the use of an STM-tip to induce electropolymerisation [295]. Most of all, STM provides us with atomically resolved images of the surface structure. 

The inventions of the scanning tunnelling microscope (STM) [16] and the atomic force microscope (AFM) [17] have allowed sub-micrometre and, at times, atomic-scale spatially-resolved imaging of surfaces. Spatially-resolved temperature measurements using optical systems are diffraction limited by the wavelength of the radiation involved, which is about 5-10 pm for infrared thermography and about 0.5 pm for visible light [18]. The spatial resolution of near-field techniques (such as AFM) is only limited by the active area of the sensor (which in the case of STM may be only a few atoms at the end of a metal wire). 

The scanning tunnelling microscope (STM) is the most suited, and the most developed of the various SPMs, to perform local spectroscopic measurements. Discussion of STM techniques will constitute the bulk of this article. It also has the most restricted range of accessible substrates in terms of conductivity and roughness. The atomic force microscope (AFM) has limited spectroscopic capabilities but can image a wider range of samples. The near-field scanning optical microscope (NSOM) has excellent spectroscopic, but limited spatial resolution. These latter two SPMs are discussed at the end of this article. 

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