Definition
Transmission microscopy is a microscopic analytical instrument that uses an electron beam to penetrate a sample and image it through an electromagnetic lens. It obtains a magnified image of the microstructure inside the sample by detecting the transmitted signal after the electrons interact with the sample, with a resolution of up to the sub-nanometer level. The instrument plays a fundamental role in the fields of materials science, biology, nanotechnology, and industrial quality inspection.
Principle
The basic principle of transmission microscopy is based on the interaction of a high-energy electron beam with a thin sample. The electrons emitted by the electron gun are accelerated by an accelerating voltage (usually 60-300 kV) to form a high-energy electron beam. As the electron beam passes through the sample, some of the electrons are absorbed or scattered, and the rest pass through the sample. Transmitted electrons are focused and magnified by multi-stage electromagnetic lenses such as objectives, intermediate lenses, and projection mirrors, and finally form an image on a phosphor screen or detector. The image contrast is mainly derived from the mass thickness contrast, diffraction contrast and phase contrast.
Among them, the focusing ability of the lens is controlled by the magnetic field, and the focal length formula can be expressed as:
f = (k * V) / (I * N)
where f is the focal length, V is the acceleration voltage, I is the lens current, N is the number of coil turns, and k is the constant. The resolution is limited by the electron wavelength λ, and the relationship can be approximately expressed as:
λ = h / √(2mE)
h is Planck's constant, m is the mass of the electron, and E is the energy of the electron.
Measurement method
Conventional measurement methods for transmission microscopy include brightfield imaging, darkfield imaging, and high-resolution imaging. In brightfield imaging, only direct electrons are allowed to participate in the imaging, which is suitable for observing changes in sample morphology and thickness. Darkfield imaging utilizes scattered electron imaging, which can be used to analyze crystal defects or specific diffraction conditions. High-resolution imaging is based on the principle of phase contrast, which can directly display crystal lattice stripes or atomic arrangements. In addition, the combination of energy spectroscopy can be used for elemental composition analysis, and electron energy loss spectroscopy can be used to study the chemical state and energy band structure of elements.
Transmission factor
The imaging quality of a transmission microscope is influenced by several factors. Sample preparation is a critical part, and the sample thickness is usually less than 100 nanometers, which can lead to excessive electron scattering or absorption. The choice of accelerating voltage affects penetration and radiation damage, and higher voltages can reduce chromatic aberration but may exacerbate sample damage. Lens aberrations, particularly spherical and chromatic aberrations, can reduce image resolution and contrast. Environmental vibration, stray magnetic fields, and power supply stability can also introduce noise. Operating conditions such as focusing accuracy, diaphragm size, and alignment state need to be optimally controlled.
Applications
Transmission microscopy is widely used in a variety of fields. In materials science, it is used to observe the crystal structure, dislocations, grain boundaries and precipitated phases of metals, ceramics, semiconductors and other materials. In the field of nanotechnology, it can characterize the morphology and size distribution of nanoparticles, quantum dots and two-dimensional materials. In biological research, it is often used to observe cell ultrastructure and biological macromolecular structure, but it needs to be combined with freezing sample preparation technology. In terms of industrial testing, it is suitable for failure analysis, coating evaluation and composite interface research. It can be used in geology to analyze the microstructure and composition of minerals.
Selection considerations
When selecting, the technical parameters should be evaluated according to specific needs. Resolution is a core metric that usually needs to be considered in combination with point resolution and lattice resolution. The acceleration voltage range needs to match the nature of the sample, high voltage is suitable for dense materials, and low voltage is conducive to reducing beam damage. Equipped with an energy spectrometer or electron energy loss spectroscopy accessory to expand compositional analysis capabilities. The degree of automation and software-friendliness affect the efficiency of operations. The sample stage type (e.g., tilt, heated, or cooled) needs to meet the experimental conditions. Maintenance costs, space requirements, and training support should also be included in the decision.