Phase-contrast imaging

Phase-contrast imaging, or – more casually – High Resolution (HR) imaging, is a method of imaging in Transmission Electron Microscopy (TEM). The ability to image using phase contrast is one of the key elements which differentiates Transmission Electron Microscopy from conventional optical (light) microscopy. This ability arises from the fact that the atoms in a material diffract electrons as the electrons pass through them (the relative phases of the electrons change upon transmission through the sample), causing diffraction contrast in addition to the already present contrast in the transmitted beam. Phase-contrast imaging is the highest resolution imaging technique ever developed, and can allow for resolutions of less than one angstrom (less than 0.1 nanometres). It thus enables the direct viewing of columns of atoms in a crystalline material.

The interpretation of phase-contrast images is not a straightforward task. Deconvoluting the contrast seen in an HR image to determine which features are due to which atoms in the material can rarely, if ever, be done with the naked eye. Instead, because the combination of contrasts due to multiple diffracting elements and planes and the transmitted beam is complex, computer simulations are used to determine what sort of contrast different structures may produce in a phase-contrast image. Thus, a reasonable amount of information about the sample needs to be understood before a phase contrast image can be properly interpreted, such as a conjecture as to what crystal structure the material has.

Phase-contrast images are formed by removing the objective aperture entirely or by using a very large objective aperture. This ensures that not only the transmitted beam, but also the diffracted ones are allowed to contribute to the image. Instruments that are specifically designed for phase-contrast imaging are often called HRTEMs (high resolution transmission electron microscopes), and differ from analytical TEMs mainly in the design of the electron beam column. Whereas analytical TEMs employ additional detectors attached to the column for spectroscopic measurements, HRTEMs have little or no additional attachments so as to ensure a uniform electromagnetic environment all the way down the column for each beam leaving the sample (transmitted and diffracted). Because phase-contrast imaging relies on differences in phase between electrons leaving the sample, any additional phase shifts that occur between the sample and the viewing screen will make the image impossible to interpret. Thus, a very low degree of lens aberration is also a requirement for HRTEMs, and advances in spherical aberration (Cs) correction have enabled a new generation of HRTEMs to reach resolutions once thought impossible.