Observations on crystal lattices and imperfections by transmission electron microscopy through thin films

Some striking advances have occurred in the use of high resolution electron microscopes in the field of solid state physics since the International Meeting of 1954. These have been concerned with the direct study by transmission of thin crystalline specimens in two distinct ways. In the first the aperture of the microscope is chosen so that some diffracted beams from the specimen pass through to the image and are recombined to form a periodic pattern, the form and spacing of which is closely related to the relative dispositions and spacings of the lattice planes in the crystal. Using this method, the basic periodicity of net planes of the lattice may be directly imaged and departures from perfect periodicity in the form of distortions and discontinuities arising from lattice imperfections such as dislocations may be studied. In the second method the aperture of the microscope objective is chosen so that all diffracted beams from the specimen are intercepted and contrast arises from changes in thickness and orientation and from lattice distortion of the crystal. In particular, the lattice disturbance associated with a dislocation line is sufficient to cause a large local change in the electron intensity scattered outside the objective aperture in the vicinity of the line, thereby making the line visible in the image. Both of these methods have required a complementary study of the diffraction pattern by the selected area technique. For this, the three stage design has been invaluable and the microscope has come into its own as an integrated research tool for the study of crystals and their imperfections. With the addition of a hot stage and means for applying stress to the specimen in situ very wide fields of investigation in physics, chemistry and metallurgy have been opened up.