Molecular spectroscopy and reaction dynamics Group III Sigtryggur Bjarki Sigtryggsson Styrmir Svavarsson Úlfar Þór Björnsson Árdal

STM Scanning tunneling microscope

History The STM was invented 1981 by Gerd Binnig and Heinrich Rohrer. They received the Nobel prize in physics in 1986 for the STM The STM was first used in showing its atomic scale resolution in a image of silicon 7x7 restructed (111) surface.

Theory STM works on the principles of tunneling Electrons can not transfer between the tip of the STM and the sample because there is an energy barrier. When a voltage is imposed between the two, the shape of the energy barrier changes and electrons can move from the sample to the tip wich results in a small current if the distance is sufficiently small

The probability distribution is given by: Ψ 2 𝑧 = 𝑒 −2𝜅𝑧 The current is proportional to: 𝐼∝ Ψ 2

Where z is the distance and к is given by: 𝜅= 2𝑚 (𝑈−𝐸) 1 2 ℎ Where U is the potential energy, h is the planck‘s constant and E is the energy of the electron .

The tip used is usually made out of tungsten or a palladium-iridium alloy where at the very end of the tip there is only one atom. The positioning of the tip is changed using array of piezoelectrics. Piezoelectric materials change shape when a electric field is applied on them.

The STM can be operated in two different ways: Constant current mode or constant voltage mode. In constant current mode the voltage is kept unchanged and the vertical position of the tip is changed to maintain a constant current while scanning over the sample. In constant voltage mode the vertical position of the tip is kept constant while the current is changed by changing the voltage bias (difference)

The constant voltage mode is faster. The constant current mode shows contrast directrly related to electron charge density profiles.

AFM Atomic force microscope

Contrary to the STM wich can only scan elictricly conducting surfaces the AFM can scan all surfaces. The AFM measures short distance forces using a cantilever wich is monitored with laser beam pointed at the tip of the cantilever as the schematic picture shows.

https://www.youtube.com/watch?v=4VW767Qh6sQ