WhenElectronic wave(with certain energyElectronics）Fall tocrystalUpward, in the crystalatomScattering, each scattered electron wave generates mutualinterferencePhenomenon.Each atom in the crystal scatters electrons to change their direction andwavelength。In the scattering process, some electrons exchange energy with atoms, and the wavelength of electrons changesInelastic scattering；If there is no energy exchange and the wavelength of the electron remains unchanged, then it is calledelastic scattering 。In the process of elastic scattering, due to the arrangement of atoms in the crystalPeriodicityThe electronic waves scattered by each atom interfere with each other when they are superposed,Scattered waveThe distribution of the total intensity of the scattering wave is not continuous in space. Except in a certain direction, the total intensity of the scattering wave is zero.
In 1927, C.JDavisson And LH.Germer The electron beam with energy of 100 electron volts was observed on the surface of nickel single crystalscatteringIt is found that the spatial distribution discontinuity of the scattered beam intensity isdiffractionPhenomenon.Almost at the same time, G.P. Thomson and A. Reed also observed the diffraction pattern when they used an electron beam with energy of 20000 electron volts to pass through the polycrystalline film.The discovery of electron diffraction confirmed that LV.De BroglieThe assumption that electrons are volatile constitutesquantum mechanicsThe experimental basis.
In 1943, German scientist K ö nig first reported the cubic ice structure through electron diffraction.[2]
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The simplest electron diffraction device.fromcathodeThe electrons emitted by K pass throughanodeAperture hole andlensL arrives at sample S and is diffracted by the sample to form an electron diffraction pattern on the fluorescent screen or photographic plate P.Because the substance (including air) has a strong absorption of electrons, the above parts are placed in vacuum.ElectronicAccelerating voltageIt is generally tens of thousands of volts to about 100000 volts, called high-energy electron diffraction.In order to study the surface structure, the electron acceleration voltage can also be as low as thousands or even tens of volts. This device is calledLow-energy electron diffractionDevices.
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Electron diffraction experiment
Electron diffraction can be used to study the film structure with thickness less than 0.2 μ m, or the surface structure of large sample.The former is called transmission electron diffraction, and the latter is called reflection electron diffraction.For reflected electron diffraction, the angle between the electron beam and the sample surface is very small, generally within 1 °～ 2 °Glancing angle。
Since the 1960s, commoditiestransmission electron microscopeAll have electron diffraction function (see electron microscope), and can use the lens behind the sample to select an area as small as 1 μ m for diffraction observation, calledSelected area electron diffractionAnd the case that no lens is used after the sample is called high-resolution electron diffraction.Transmission electron microscope with scanning device can select areas as small as thousands of angstroms or even hundreds of angstroms for electron diffraction observation, which is called microzone diffraction.The incident electron beam is generally focused on the photographic plate, but it can also be focused on the sample, which is called convergent beam electron diffraction.
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Like X-ray diffraction, electron diffraction also follows the Bragg formula 2dsin θ=λ (see X-ray diffraction).When the angle θ between the incident electron beam and the crystal plane cluster, the crystal plane spacing and the electron beam wavelength λ satisfy the Bragg formula, then there is a diffracted beam along the reflection direction of the crystal plane cluster to the incoming beam.Although electron diffraction has the same geometric principle as X-ray diffraction.But their physical contents are different.When interacting with the crystal, the X-ray is scattered by the electron cloud in the crystal, and the electron is scattered by the potential field formed by the atomic nucleus and its outer electrons.In addition to using Bragg formula or reciprocal lattice and reflection sphere to describe the diffraction geometry principle that produces electron diffraction, the strict electron diffraction theory is based onSchrodinger equationH ψ=E ψ, where ψ is electronwave function, E is the total energy of the electron, H isHamiltonian operatorIt includes the kinetic energy obtained by the electron from the external electric field and the potential energy in the crystal electrostatic field.If, when solving this equation, considering that its potential energy is far less than the kinetic energy, it is considered that the diffracted beam is far weaker than the incoming beam, ignoring the second-order small quantity in the equation, the solution obtained is called the kinematic solution, which is consistent with the above diffraction geometry principle.The electron diffraction theory based on the kinematics solution of Schrodinger equation is called the electron diffraction kinematics theory. The physical content of this theory is to ignore the interaction between diffracted waves and incident waves, as well as between diffracted waves.If a higher approximation is made when solving the equation, for example, it is considered that the diffracted beam is much weaker than the incoming beam except for one beam (or two beams, or three beams,..., or n-1 beams), then the solution obtained is called the dynamic solution of two beams (or three beams, or four beams,..., or n-beams).The electron diffraction theory based on the dynamic solution is called electronTheory of diffraction dynamics。
Diffraction pattern
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Electron diffraction pattern
It can also be used as in the case of X-ray diffractionReciprocal latticeAnd reflection sphere to describe the conditions for electron diffraction, but the wavelength of the electron is far shorter than the X-ray, so the curvature of the reflection sphere is very small.According to Sommerfeld formula, the electron scattering intensity decreases rapidly with the increase of scattering angle.Therefore, the area of the effective reflection sphere is not
The reflection sphere can be regarded as a plane passing through the origin of reciprocal lattice and perpendicular to the incident electron beam.The electron diffraction pattern is the projection on the photographic plate of the reciprocal lattice plane that passes through the origin of the reciprocal lattice and is perpendicular to the incident electron beam when starting from the center of the reflecting sphere.In general, the electron diffraction pattern of single crystal shows regularly distributed spots, the electron diffraction pattern of polycrystal shows a series of concentric circles, and the electron diffraction pattern of amorphous material shows a series of dispersed concentric circles.The convergent beam electron diffraction pattern of a single crystal is a diffraction disk with regular distribution.
When the crystal is thick and complete, we can get aInelastic scatteringEffect.Because in the scattering process, some of the electrons passing through the upper crystal keep their wavelength unchanged, but slightly change their direction.For the lower crystal, the incident electrons are distributed in the cone with the original incident electron beam as the axis.At this time, the electron diffraction pattern is composed of many pairs of parallel black and white lines. This diffraction pattern, called Kikuchi diffraction pattern, can be used to accurately determine the crystal orientation.
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Two-dimensional crystal lattice
Electron diffraction tester
If we limit the crystal structure analysis to the surface atomic layer, it can be found that the rules of the surface atomic arrangement may not maintain the continuity of its internal three-dimensional lattice, that is, it may not be the same as the internal parallel atomic plane (see the crystal surface).To useElectron diffraction methodTo study the two-dimensional structure of this surface layer, the following two conditions must be met: ① the incoming beam wavelength is short enough, and according to the Bragg equation of two-dimensional lattice diffraction, the wavelength should be less than the lattice period; ②The penetration and escape depth of the electron beam is limited to a few atomic layers on the surface.The best way to meet the above requirements is to use low-energy (50~500eV) electron beam and high-energy (30~50keV) electron beam with grazing angle close to zero as microprobe for surface structure analysis, respectively calledLow-energy electron diffraction(lowenergyeletronidifraction) andReflection high-energy electron diffraction（reflectedhighenergyelectrondiffraction）。
Low-energy electron diffraction
electron diffractionA beam of low-energy electrons incident parallel to the sample surface, and about 1% of all backscattered electrons are elastic backscattered electrons (with the same energy as the incident electrons).Due to the lattice characteristics of surface atoms, this elastic coherent scattering of electrons will display regular spot patterns on the fluorescent screen of the receiving anode.In order to detect the weak signal of low-energy electrons, the so-called post acceleration technology is usually used. The electrons backscattered from the sample surface pass through the grid G1 at the same potential as the sample before being accelerated by the receiving anode at the high potential and hitting the fluorescent screen to produce diffraction spots for observation or recording.Grid G2 is slightly negative than the electron gun filament to blockInelastic scatteringThe electron passes through, reducing the background of the pattern.In order to study the real surface structure, the pollution caused by residual gas adsorption in the analysis room must be strictly controlled, and the ultra-high vacuum of 10-9~10-10 Torr (10-7~10-8 Pa) is generally required.
With the development of surface science,Low-energy electron diffractionIt has been widely used in the study of surface structure, surface defects, the formation of vapor deposited surface films (such as epitaxial growth), the structure of oxide films, gas adsorption and catalytic processes.Low energy electron diffraction is often combined with Auger electron spectrometer (AES), electron spectroscopy chemical analyzer (ESCA) to form a multi-functional surface analyzer, because they are relatively close in terms of ultra-high vacuum requirements and the energy range of the detected electronic information.
Low energy electron diffraction (LEED) is to incident monochromatic electrons with energy ranging from 5 to 500 eV on the sample surface. Through the interaction of electrons and crystals, some electrons are reflected into the vacuum in the form of coherent scattering, and the diffraction beam formed enters a movable receiver for intensity measurement, or is accelerated to fluorescence
Reflective high energy
Reflection high-energy electron diffraction
If 30~50kV electron gun acceleration voltage is used, the electron wavelength range is 0.00698~0.00536nm, and the parallel electron beam with such energy is used to incident on the sample surface at a grazing angle of less than 1 °, that isReflection high-energy electron diffraction。RHEED can also detect the surface structure with the same sensitivity as LEED.
Reflective high-energy electron diffraction (REED) is an effective analytical method for studying the epitaxial growth of crystals, accurately measuring the surface crystalline state, and surface oxidation and reduction processes.Due to the improvement of the receiving system, both RHEED and LEED can be carried out in the multi-functional surface analyzer, which makes the study of surface structure more convenient.
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Electron diffraction is the diffraction of matter waves, because electron is a kind of particle, which can be considered as a kind of physical object. Although it is very small, it is a real matter that can be seen by people after all, which is a kind of particle property
X-ray is an electromagnetic wave, not a physical objectSo it shows more wave characteristics
Waves can reflect and diffract, which is the basic property of waves. It was proved that light is an electromagnetic wave, which is to prove that it has the properties of reflection and diffraction, so it is not surprising that X-ray can diffract
But it is strange that matter (electron) can be diffracted, and we have to start from the past
EinsteinIt is believed that waves have particle characteristics（Wave particle duality）De Broglie boldly conjectured that matter also has the nature of waves, and called itMatter wave... Several years later, several young scientists succeeded in grating diffraction with electrons (a kind of particle matter), proving that his conjecture was correct
After the electron beam passes through the sample, it will produce a transmission beam and a diffractive beam of different angles, which will form a central spot (transmission beam) and multiple diffractive spots on the rear focal plane of the objective lens.These spots are then used as new light sources to image on the image plane of the objective lens.By adjusting the current of the intermediate mirror so that the object plane of the intermediate mirror coincides with the rear focal plane of the objective lens, an amplified diffraction spot image can be formed on the image plane of the intermediate mirror. After further amplification by the projection mirror, the diffraction spot image can be observed on the fluorescent screen of the electron microscope.So inelectron microscopeThe electron beam diffraction image of the crystal can be easily obtained by adjusting the current of the intermediate mirror.
2. Determination of camera constant
To oneelectron microscopeFor example, if the acceleration voltage is constant, the electron wavelength λ is constant. Since the distance L from the sample to the fluorescent screen is basically constant, L λ in the formula (Rd=L λ) is a constant, which is called the camera constant of the electron microscope.In electron diffraction analysis, it is necessary to determine the camera constant.The measurement method is simple. Under the same acceleration voltage condition, take the electron diffraction image of the standard sample with known crystal lattice, measure the R value of each diffraction point on the standard sample diffraction image, multiply it with the corresponding d value of the standard sample crystal, and calculate the L λ value according to the formula (Rd=L λ).The commonly used standard samples are polycrystalline films made of gold or aluminum.
stayelectron microscopeWhen observing the sample, there are often more grains in one image, and when analyzing, it is usually only necessary to analyze the crystal structure of one or one of the observed grains. Therefore, the electron microscope is equipped with selected area electron diffraction, which can conduct electron diffraction analysis of local areas in the microstructure.The diffraction area is limited by inserting an aperture adjustable aperture in the object image plane, which is called selective electron diffraction.
4. High resolution electron diffraction
When selecting the area for electron diffraction, because the middle mirror and projection mirror magnify the electron diffraction pattern formed on the rear focal plane of the objective lens, the camera constant and spot size are magnified by Mi · Mp times (Mi is the magnification of the middle mirror, Mp is the magnification of the projection mirror), the resolution of electron diffraction is not high.The high-resolution diffraction device places the sample near the projection mirror. The lens above the sample is involved in the illumination system to provide a fine focused parallel electron beam. The lens below the sample is closed. At this time, the camera constant is independent of the current, just like an ordinary electron diffractometer.If the high pressure stability is improved and the λ value is measured accurately, the crystal plane spacing value with a relative error of 10-4 can be obtained, which is equivalent to the accuracy of X-ray diffraction.[1]
