They can be visualized by electron microscopy or fluorescence microscopy. Dislocations can be observed using transmission electron microscopy, field ion microscopy and atom probe techniques. Electron microscopy and scanning probe microscopy are the dominant methods. These are the convergence of DNA hybridization, fluorescence microscopy, and solid surface DNA capture. Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Morphology is understood by Scanning electron microscopy (SEM). It is used extensively in fluorescence microscopy. Various techniques for in situ electron microscopy of gaseous samples have been developed as well. Many of these photonic mechanisms correspond to elaborate structures visible by electron microscopy. It can be used as a stain in scanning electron microscopy. Historically these have included histopathology, electron microscopy, and virus isolation. High voltage is used for generating electron beams for microscopy. Acrolein is sometimes used as a fixative in preparation of biological specimens for electron microscopy.