Lecture 6: Microscopy II PHYS 430/603 material Laszlo Takacs UMBC Department of Physics

Light microscopy The principle and a commercial scope Useful sites:

Optical microscope (OM) and transmission electron microscope (TEM)

TEM modes: Composite image, bright field image, dark field image, selected area diffraction Transmitted and diffracted beamsDiffraction patternFirst image Bright fieldBright field, transmitted electrons only Dark fieldSelected area diffraction pattern

Comparing imaging and diffraction in a TEM.

Electron diffraction from a monocrystalpolycrystalglass

How do scanning microscopies work? Image = table of numbers Measurement generates a value for every location: Reflectivity of light (scanner) Ejected electrons (SEM) Current between tip and surface (STM) Force between tip and surface (AFM) Any quantity of interest Interpret numbers as intensities for display or printer. Digital image processing.

The principle of STM What exactly is measured? Tunneling current ~ probability of an electron tunneling out from the surface ~ electron density. Largest where the atoms are, thus we “see” the atoms - kind of.

48 Fe atoms forming a “quantum collar” on a Cu (1 1 1) surface. The inner rings are not from atoms at the location.

STM of Si (1 1 1) 7x7 reconstructed surface

Surface structure Relaxation: change of distance between the first few lattice planes. Typically, ~10% change in the first, ~1% in the second distance. Reconstruction: change of the arrangement/ symmetry of the arrangement of atoms. Reactive, usually picks up other atoms, e.g. O.

Notice that smaller circles represent atoms farther from the top layer.

Low Energy Electron Diffraction Cu (1 0 0) ZnO ( )

Typical surface features after cleaving close to a lattice plane or building up a crystal from the gas phase

FeSi 2 on a Si (111) surface Notice: Reconstruction of Si Relationship between the orientations of substrate and coating Steps and kinks in the FeSi 2 layers

Contact-mode AFM: It is a miniature profilometer, directly measures height. Best for larger-scale (at least a few nm) features.

The principle of field ion microscopy

Images taken by field ion microscope Ir (1 1 0)Pt (0 1 2)

E. W. Müller, Platinum Metal Rev. 9 (1965) VacanciesDislocations on platinum tips