The electron microscope overcame the limits of optical microscopes. Electron microscopes observe objects by using an electron beam, which has a much shorter wavelength than light. There are two main types of electron microscope: the scanning electron microscope (SEM), which observes secondary electrons produced when the electron beam hits the specimen; and the transmission electron microscopes (TEM), which observes a "shadow image" of the specimen created by transmitting the electron beam through the specimen. TEMs are capable of very high magnifications (theoretically up to about x 500,000), enabling the observation of atom-sized objects.

In terms of resolution, TEMs can be used for observing the nano world, but nanomanufacturing requires a 3D view of the object, something that TEMs cannot provide. (When the objects are conductive (insensitive to the e-beam irradiation), SEMs can produce 3D views, but their resolution is insufficient to observe nano-sized objects. ) Moreover, the objects being observed by electron microscopes must be kept in a vacuum to ensure stability of the electron stream, and have to be exposed to the high energy of the electron beam, posing the risk of degradation of the specimen. For such reasons, electron microscopes are far from ideal instruments for viewing the nano world.

♦ Electron Beam