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Atomic Force Microscopy (AFM)
Atomic force microscope (AFM) is a powerful tool that is used to study materials by scanning over the surface with a very sharp tip. When a sharp tip approaches a surface then first Van der Waals attractive forces apply that pull the tip closer to the surface and therefore also bend the cantilever. When the tip is close enough then electrostatic repulsive forces apply. The bending of the cantilever is monitored with a lazer beam that is focused on the cantilever and reflected into the detector.This method allows to obtain greatly magnified high resolution 3D images of the studied substrates (even atomic resolution is possible). The main working modes are contact, non-contact and tapping mode. In then case of contact mode the tip is in direct contact with the material. This is suitable for studying hard surfaces. In the case of non-contact mode the tip is vibrating close to the surface. This mode is used for studying sticky and soft surfaces. In the tapping mode the tip vibrates with a greater amplitude and briefly touches the sample at its lowest point of the trajectory. This method is useful for obtaining a “real” image of the studied surface as the tip penetrates the thin film of water that is always present on substrates when measuring in open air. There are also other types of scanning probe microscopes where different information can be obtained from the sample. For example a if one uses a thermocouple as a sharp tip for scanning then it is possible to study heat distribution on microscopic electronic devices in order to detect possible spots where oveheating occurs. In the case of scanning tunnel microscopy an electric potential is applied between the tip and the sample, which causes the movement of electrons from one to other. By measuring the tunneling current, it is possible to obtain valuable information about the state of the surface. Even atomic resolution is possible in STM and therefore can be used to study novel materials such as graphene. It is also possible to use magnetic needles or thin optical cables as tip for scanning over the studied surface.