Scanning Tunneling Microscopy was invented by Gerd Binnig and Heinrich Rohrer at IBM Zurich in 1981. With their work they won the Nobel Prize in 1986.

STM is an instrument which allows imaging surfaces at atomic scale by a extremely sharp conducting tip. It is based on quantum tunneling of injected electrons from the tip to surface or vice versa. Electron tunneling is provided by bringing that conducting tip near to the surface up to order of nanometer scales and applying bias difference between the conducting tip and the surface. Consequently, tunneled electrons generate tunneling current, which is the function of the distance and applied voltage between tip and surface. In order to obtain good resolution from the surface, tunneling junction parameters must be set according to local density of states of the surface.

Two types scanning can be performed which are constant height mode and constant tunneling current mode. In constant height mode, chosen distance between tip and the sample take the information from the surface depending on tunneling current changes. In constant current mode, feedback electronics adjust the distance between surface and tip by applying voltage to piezoelectric distance control mechanism.

Ambient Nanosurf easy scan2^TM setup in our lab. (Left) The STM head in a home made acoustic isolation box, (Right) Control electronics with a signal box and a lockin amplifier attached to the setup.

Scanning Tunneling Spectrocopy (STS) is a method that gives the electronic information of the chosen location of the sample. There are several modes of STS such as I/Z  and I/V. However main concept of STS is figuring out local density of states as a function of applied bias voltage. After disabling the feedback mechanism, sweeping bias voltage on the chosen location provides and I/V curve, which can be used to derive the local density of states as a function of energy (by d(lnI)/d(lnV) – V plots). Apart from these, by customizing STM with a lock-in amplifier, differantial conductivity map of a surface can be obtained.

(left) Atomic resolution of HOPG which was acquired at V_b=-50mV, I_t=400 pA of tunneling junction parameters. Green point indicates where the STS was performed. (right) I/V curve of HOPG between -1 V and 1 V.