Microscopic methods of characterization

STM is widely used in both industrial and fundamental research to obtain atomic scale images of metal surfaces. It provides a three dimensional profile of the surface, which is useful for characterizing surface roughness, observing surface defects and determining the size and conformation of molecules and aggregates on the surface.

It works on the principle of a record player. A sharp needle, called tip, is used to scan the surface and kept close to the surface. The voltage is applied between the metallic tip and the surface of the specimen typically between millivolts to a few volts. Current flow between metallic tip and the specimen is called tunneling current. The surface is scanned, keeping the distance between tip and specimen constant, so that the tunneling current is constant. This total set up is connected to a feed back loop which keeps the distance constant. Even a decrease in distance by 1 nm brings 10 times reduction in tunneling current. By perceiving all these changes, the feed back loop keeps the distance constant. When the tip encounters an elevation on the surface, instantly there is reduction in tunneling current; the feed back loop identifies this change and ensures the distance between tip and surface. This scanning pattern is drawn on computer screen, which looks like a contour map, which gives a 3-D view.

Instrumentation and detail working of atomic force microscope is very complex and beyond the scope of our present study and a schematic diagram of an STM is shown below.

Analysis of atomic arrangements.