Charging of Insulating Samples

If the sample under study is an insulator (e.g., an oxide-supported catalyst), the arrival of positively charged primary ions and the emission of electrons by either potential or kinetic emission leads to positive charging of the sample. This has two negative effects  

One can compensate for charging by using a so-called flood gun , which sprays low-energy electrons onto the sample. Charging can also be minimized by using a beam of atoms instead of ions as primary particles. In this case, the kinetic emission of electrons is the only source of charging, if we ignore the low yields of secondary ions. 

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A disadvantage of AES is that the intense electron beam easily causes damage to sensitive materials (polymers, insulators, adsorbate layers). Charging of insulating samples also causes serious problems. 

Charging of insulating samples under the electron beam may be a problem in SEM, although coating the sample with a thin film of gold or carbon alleviates the... 

The metal-matrix interface can also be at the top rather than the bottom of the sample. Thin metal layers can be sputter-coated with little difficulty. The MALDI benefits of a top layer may extend beyond reducing surface charging of insulating samples to a moderate... 

Veereecke, G. and Rouxhet, P. G. Surface charging of insulating samples in x-ray photoelectron spectroscopy. Surf. Int. Anal, 26,490 (1998). 

J. Cazaux, About the Charge Compensation of Insulating Samples in XPS, J. Electron Spectros. Relat. Phenom. 113, 15-33 (2000). 

It is tempting to extrapolate to the behavior of the corresponding solids, but the experimental data here have an uncertain energy scale and display time-dependent variations due to charging of the samples, which are insulators. Future work may involve studies of trimers and higher clusters whose abundance has been enhanced by supersonic expansion. 

Sample discoloration may occur and is usually due to ionisation, (whereas overheating causes physical deformation). It affects only sensitive specimens under intense bombardment. The sensitivity depends inversely on the beam diameter. Some effects must be taken into account, such as the charging of insulating components, and the sputter removal of some components, which prevents repeat investigations. Image contrast may also arise from chemical or topographic rather than isotopic differences. 

Conductive coatings are applied to prevent charging of insulator specimens. For organic samples the metal film plays an additional role in the stabilization of the structure and increases the heat conduction. For... 

The sample in ultra-high vacuum is bombarded with a beam of positively charged ions (e.g. Ar+, Oj, Ga" ", Cs" "). Material is sputtered from the surface, mostly as neutral species but with a few percent of charged species (positive and negative secondary ions). These are collected by a mass spectrometer and mass analysed. Charging of insulators is controlled by simultaneous flooding with relatively low-energy electrons. 

The sample is an insulator there will probably be positive charging of the sample. If XPS or SIMS analysis is to be carried out, then use an electron flood gun it may be better to use AES sputter profiling. 

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