Scanning electron microscopy noise

The main inconvenience of the ERDs construction is the lack of reproducibility. Due to the tiny electrode surfaces, small variations imply big changes. The sealing between the electrode surface and the insulator material is very crucial for obtaining a well-defined electrode surface and low noise. Their characterization can be achieved by different techniques [17]. Scanning electron microscopy (SEM) is suitable for UMEs but not for smaller ERDs. Information about ERD dimensions can be obtained from the experimental (by chronoamperometry or cyclic voltammetry) and theoretical response in well-defined electrochemical systems [5]. Moreover, this electrochemical characterization shows several limitations when ERDs approach the low nanometric scale [8,14,36]. 

Finally, the issue of polymer detection limits was addressed by Pastor et al. Using PEG 2000 and 6000 standards, they determined a lower detection limit of 40 femtomoles (from 4 laser shots) and produced a spectrum with a signal-to-noise ratio of at least 5 1. Part of the added reproducibility of the MALDl events came from the use of an aerospray sample deposition technique that could produce highly uniform sample surfaces. Scanning electron microscopy was used to quantitate the laser spot sizes. 

The last part of the chapter refers to the use of techniques that are relatively new. They consist of scanning tunneling microscopy and atomic force microscopy by which one can get better than 10 A resolution in looking at surfaces. A laser beam, in collaboration with a quadrupole mass spectrometer, can be used to detect local hydrogen with a resolution of about 0.1-1 pm. Finally, ideas of electronic noise are applied to the random fluctuations of the potential of an electrode and can give a value of the corrosion current without any influence of the IR drop. This has made the method particularly useful for nonaqueous solutions. 

Three factors may limit the resolution of an image the diffraction limit, lens aberrations and noise. Noise can be a problem in scanning microscopy. A very bright source is required if rapid, TV scan rates are to be used at high resolution. In CTEM noise is a problem for radiation sensitive polymers (Section 3.4.4) because a limited number of electrons can be used to form the image. 

---