Week 14 Nanostructures Characterization Techniques (Scanning Electron Microscopy)

Chapter#4  Nanostructures Characterization Techniques

Scanning Electron Microscopy

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a  raster scan pattern, and the beam's position is combined with the detected signal to produce an image. SEM can achieve resolution  better than 1 nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet conditions (in environmental SEM), and at a wide range of cryogenic or elevated temperatures.

The types of signals produced by a SEM include secondary electrons (SE), backscattered Electrons (BSE), characteristic Xrays, and light (cathodoluminescence) (CL), but it is rare that a single machine would have detectors for all possible signals.

The most common SEM mode is detection of secondary electrons emitted by atoms excited by the electron beam The number of secondary electrons depends on the angle at which beam meets surface of specimen, i.e. on specimen topography. By scanning the sample and collecting the secondary electrons with a special detector, an image displaying the topography of the surface is created.

The spatial resolution of the SEM depends on the size of the electron spot, which in turn depends on both the wavelength of the electrons and the electronoptical system that produces the scanning beam. The resolution is also limited by the size of the interaction volume, the volume of specimen material that interacts with the electron beam. The spot size and the interaction volume are both large compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, as is possible transmission electron microscope (TEM). The SEM has compensating advantages, though, including the ability to image a comparatively large area of the specimen; the ability to image bulk materials (not just thin films or foils); and the variety of analytical modes available for measuring the composition and properties of the specimen. Depending on the instrument, the resolution can fall somewhere between less than 1 nm and 20 nm.

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