How are the imaging principles of scanning electron microscopy and transmission electron microscopy different
Scanning electron microscopy mainly involves secondary electron imaging after electron beam irradiation on the sample, while the bright field image of transmission electron microscopy is transmission electron imaging.
Electron microscope, abbreviated as electron microscope, has become an indispensable and important tool in modern science and technology after more than fifty years of development.
The electron microscope consists of three parts: a mirror tube, a vacuum device, and a power cabinet.
The lens barrel mainly consists of electronic sources, electronic lenses, sample racks, fluorescent screens, and detectors, which are usually assembled into a column from top to bottom.
Electronic lenses are used to focus electrons and are the most important component in the tube of an electron microscope. Magnetic lenses are generally used, and sometimes electrostatic lenses are also used. It uses a spatial electric or magnetic field symmetrical to the axis of the mirror tube to bend the electron trajectory towards the axis, forming a focus. Its function is the same as that of an optical lens (convex lens) in an optical microscope to focus the beam of light, so it is called an electron lens. The focus of an optical lens is fixed, while the focus of an electron lens can be adjusted, so an electron microscope does not have a movable lens system like an optical microscope. Most modern electron microscopes use electromagnetic lenses, which focus electrons through a strong magnetic field generated by a stable DC excitation current passing through a coil with pole shoes. The electron source is composed of a cathode that releases free electrons, a gate, and an anode that accelerates electrons in a circular pattern. The voltage difference between the cathode and anode must be very high, typically between thousands of volts and 3 million volts. It can emit and form electron beams with uniform velocity, so the stability of the acceleration voltage is required to be no less than one thousandth.
The sample can be stably placed on the sample rack, and there are often devices that can be used to change the sample (such as moving, rotating, heating, cooling, stretching, etc.).
Why use a fluorescent screen? Because the electron beam cannot be seen by the naked eye, it is necessary to use a fluorescent screen to turn the electron beam into a visible light source in order to form an image that can be seen by the eyes.
Detectors are used to collect electronic signals or secondary signals.
The electron beam of a scanning electron microscope does not pass through the sample, only focuses the electron beam as much as possible on a small area of the sample, and then scans the sample row by row. The incident electrons cause the sample surface to be excited with secondary electrons. The microscope observes the electrons scattered from each point. The scintillation crystal placed beside the sample receives these secondary electrons, and modulates the electron beam intensity of the picture tube after amplification, thus changing the brightness of the picture tube fluorescent screen. The image is a three-dimensional image that reflects the surface structure of the specimen. The deflection coil of the picture tube is synchronized with the electron beam on the sample surface for scanning, so that the fluorescent screen of the picture tube displays the morphology image of the sample surface, which is similar to the working principle of industrial television. Due to the fact that electrons in such a microscope do not need to transmit through the sample, the voltage at which electrons accelerate does not need to be very high.
The resolution of a scanning electron microscope mainly depends on the diameter of the electron beam on the surface of the sample. The magnification is the ratio of the scanning amplitude on the imaging tube to the scanning amplitude on the sample, which can continuously change from tens of times to hundreds of thousands of times. Scanning electron microscopy does not require very thin samples; Images have a strong sense of stereoscopy; It can analyze the composition of substances using information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction between electron beams and substances.
The manufacturing of scanning electron microscopy is based on the interaction between electrons and matter. When a high-energy human beam of electrons bombards the surface of a substance, the excited region will generate secondary electrons, Auger electrons, characteristic and continuous X-ray, backscattered electrons, transmitted electrons, and electromagnetic radiation in the visible, ultraviolet, and infrared regions. At the same time, electron hole pairs, lattice vibrations (phonons), and electron oscillations (plasma) can also be generated.
