Question

1) What experimental techniques would one use to gain the following information about a sample surface? (a). conducting or insulating regions; (c) magnetic domains; (d) presence of dislocations.

2) Compare and contrast STM with AFM. How does each operate? What are the advantages and disadvantages of each method?

Answer 1

1. a) Scanning Tunelling Microscopy (STM) for conductive surfaces Atomic force Microscopy (AFM) for conductors and insulators.

c) Magnetic Domains: Magnetic Force Microscopy (MEM) , Scanning electron Microscopy, Lorentz Transmission Microscopy

d) Presence of Dislocations: Scanning Tunelling Microscopy , Transmission electron Microscopy

2. Scanning Tunneling Microscopy (STM):

STM allows to measure the topography of the metals, the position of their atoms and the electron characteristics of the metal. STM capitalizes on quantum Tunelling to generate an atom scale, electron density of the image.

Atomic Force Microscopy (AFM)

It is also allows imaging of the surfaces at sub- nanometre resolution. It is used to measure the topography and the roughness of the material surface.

Operation of STM and AFM

STM: A metal tip is there terminating in a single atom is bought with 5-10 angstrom of an electrically conductive surface. At these distances, the electron cloud of the atom at the "Tip of the Tip" will significantly overlap the electron cloud of an atom to the surface.The potential is applied between the tip of the surface, a Tunneling current will be established and to image a surface, this quantum tunneling current is used operates either by constant current or current height mode driven by piezo.

AFM: This also uses piezodriven mechanism. Here , instead of recording tunneling current, the deflection of the tip recording mounted on a flexible cantilever arm due to vanderwall forces and electrostatic repulsion/attraction between the atom at the tip and the atom on the surface is measured. Here the imaging modes are contact mode and the non-contact mode (tapping mode).

Advantages and disadvantages of STM:

Measures electric current and therefore is well suited for conductive and non-conductive surfaces. Biomolecules on conductive surface are amenable to imaging. Disadvantages include very stable and clean surface are required, vibration control and sharp tips, STM can be used by not easily oxidised and we'll conductive surfaces only.

Advantages and Disadvantages of AFM:

Allows imaging of surfaces at the sub- nanometre resolution. AFM is more widely used than STM because electrically conductive surfaces are not needed with the AFM and quantitative force measurements can be made. Imaging can be undertaken in a no of different environments.

Disadvantages include distortion of soft samples in the contact mode and lower scan speed in the tapping mode.