Week 15 Nanostructures Characterization Techniques (Scanning Tunneling Microscopy)

Scanning Tunneling Microscopy

The scanning tunneling microscope (STM) is widely used in both industrial and fundamental research to obtain atomic scale images of metal surfaces.  For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. It provides a three dimensional profile of the surface which is very useful for characterizing surface roughness, observing surface defects, and determining the size and conformation of molecules and aggregates on the surface.

The STM can be used not only in ultrahigh vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986.

The electron cloud associated with metal atoms at a surface extends a very small distance above the surface. When a very sharp tip in practice, a needle which has been treated so that a single atom projects from its end is brought sufficiently close to such a surface, there is a strong interaction between the electron cloud on the surface and that of the tip atom, and an electric tunneling current flows when a small voltage is applied.  The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample . At a separation of a few atomic diameters, the tunneling current rapidly increases as the distance between the tip and the surface decreases. The information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form.

STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration control, and sophisticated electronics.

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