Scanning Tunneling Electron Microscope working mode
Although scanning tunnelling electron microscopes have different configurations, they all consist of the following three main parts: a mechanical system (mirror) that drives the probe in a three-dimensional motion relative to the surface of the conductive specimen, an electronic system that controls and monitors the distance between the probe and the specimen, and a display system that converts the measured data into an image. It has two modes of operation: constant current mode and constant height mode.
Constant Current Mode
The tunnel current is controlled using a set of electronic feedback circuits to keep it constant. A computer system controls the scanning of the tip of the needle on the surface of the sample, i.e., the tip moves in two dimensions along the x and y directions. As the tunneling current is controlled to be constant, the local height between the tip and the sample surface is also kept constant, and thus the tip will make the same undulating motion with the height of the sample surface, and the height information will be reflected as a result. This means that the scanning tunnelling electron microscope obtains three-dimensional information about the sample surface. This mode of operation is widely used because of its comprehensive image information and high microscopic image quality.
Constant Height Mode
In the scanning process of the sample to maintain the absolute height of the tip of the needle constant; so the tip of the needle and the sample surface of the local distance will change, the size of the tunneling current I also changed; through the computer to record the changes in the tunneling current, and converted into image signals displayed, that is, to get the scanning tunnelling electron microscope microscope picture. This way of working is only applicable to the sample surface is relatively flat, and the composition of a single component.
