Scanning Tunneling Microscope (STM)

The conventional electrochemical techniques like cyclic voltammetry and chronoamperometry are based on the measurement of currents and potentials at the interfaces of liquid-solid. While their results can be very precise, they give no direct information on the microscopic structure of the interface.(Schmickler 1996) In 1981, the invention of the scanning tunneling microscope(STM) by G. Binnig and H. Rohrer, who were awarded the Nobel prize in physics for their invention, provided new and easy path for an excellent topographic information of an surface with atomic resolution.
An STM(Figure) consists of a metal tip with atomically sharp point, piezoelectric crystals which control the movement in all three directions of space, and a sample substrate with an electrical conductivity. When the tip is brought within about 10 Å of the sample, the electron from a sample begins to tunnel through about 10 Å layer of air or solution into the tip or vice versa, depending upon the sign of the applied potential difference between the tip and sample. The tunneling current is an exponential function of distance between STM tip and substrate as eq (I.1). This exponential dependence gives STM remarkable sensitivity of vertical resolution. (Song 2002)



STM has many powerful advantages to study not only the silicon surface but also various surfaces as described below.
Firstly, STM can image the surfaces with sub-angstrom precision and atomic resolution. Especially, in the atomic studies of the surfaces, STM is unrivaled by any microscope because of its 0.1 Å precision of vertical resolution.
Secondly, STM can operate in various environmental conditions; in the air, UHV, aqueous solutions, and organic solvents. Therefore, STM materializes the in situ studies of the silicon etching reaction in the fluoride solutions.
Thirdly, STM can not give sample surface any damage during measurements. During the etching reactions of the silicon, the STM images can be obtained in situ without any interference on the reaction.
Fourthly, STM can combine directly with some instrumentations (for example, potentiostat, impedance analyzer, FT-IR, and so fourth). These combinations can simultaneously give additional informations with the STM images about the surface reactions. For example, the combination with FT-IR gives the informations about the morphology and the identification of the surfaces, which can satisfy the only STM fault not to identify the species on the surfaces.