Foreword
The modern method of analytical science is one of the most important and active fields in the treasure house of human knowledge. It is not only the object of research, but also an important means of observing and exploring the world, especially the microcosm. It is inseparable from all walks of life. . With the development of nanomaterial science and technology, it is required to improve and develop new analytical methods, new analytical techniques and new concepts, improve its sensitivity, accuracy and reliability, extract more information from it, and improve test quality, efficiency and economy.
Nanoscience and technology is a multidisciplinary high-tech that studies the properties and interactions of matter (including atoms, molecules) on a nanoscale (between 0.1 and 100 nm) and uses these properties. Nanotechnology is the foundation of future high technology, and instrumental analysis methods suitable for nanotechnology research are indispensable experimental tools in nanotechnology. Therefore, the analysis and characterization of nanomaterials have important significance and role in the development of nanomaterials and nanotechnology.
Nanotechnology and nanomaterials are a typical emerging high-tech field. Although many researchers have been involved in research in this field, there are still many researchers and practitioners in related industries who are not familiar with nanomaterials, especially how to analyze and characterize nanomaterials, and how to obtain some characteristic information of nanomaterials. In order to meet the needs of nanotechnology workers, some common analytical and characterization techniques for nanomaterials are briefly described from the aspects of composition analysis, morphology analysis, particle size analysis, structure analysis and surface interface analysis of nanomaterials.
1. Particle size analysis of nanomaterials
1.1 The concept of particle size analysis Most solid materials are constructed from particles of various shapes. Therefore, the shape and size of fine particle materials have an important influence on the structure and properties of materials. Especially for nanomaterials, the particle size and shape play a decisive role in the properties of the material. Therefore, it is important to characterize and control the particle size and shape of nanomaterials. Generally, the particle size of a solid material can be expressed by the concept of particle size.
For particle size analysis instruments with different principles, the measurement principle is different, and the particle characteristics are different. Only effective comparison can be made, and horizontal direct comparison cannot be performed. Since the particle shape of the powder material cannot be uniformly spherical and has various structures, in most cases, the particle size measured by the particle size analyzer is an equivalent particle size, and actual The particle size distribution will have a certain difference, so it has only a relatively comparative significance. In addition, the particle size and distribution data obtained by various particle size analysis methods may not be mutually validated, and absolute lateral comparisons cannot be made.
Since the particle size distribution of the powder material is wide, it can be from nanometer to millimeter. Therefore, when describing the particle size of the material, the particles can be divided into nanometer particles, ultrafine particles, fine particles, fine particles, coarse particles, etc. according to the size. . In recent years, with the rapid development of nanoscience and technology, the particle distribution and particle size of nanomaterials have become one of the important indicators of nanomaterial characterization. In the ordinary material particle size analysis, the particle size of the research is generally in the range of 100nm size. . For nanomaterials research, the particle size distribution range of the research is mainly between 1-500nm, especially 1-20nm is the size range most concerned by nanomaterial research.
In nanomaterial analysis and research, nanoparticles that are often encountered generally refer to ultrafine particles having a particle size on the order of nanometers (1-100 nm). Since the material has a particle size of the order of nanometers and has a small size effect, a quantum size effect, a surface effect, and a macroscopic quantum tunneling effect, it has many characteristics that are not possessed by conventional materials. Because the particle size, distribution, dispersion in the medium and the aggregation morphology of the secondary particles have an important influence on the performance of the nanomaterials, the analysis of the nanoparticle size is an important aspect of nanomaterials research. Also due to the nature and importance of nanomaterials, the development of methods and techniques for particle size analysis and characterization has been promoted. The analysis of nanomaterial particle size has evolved into an important area of ​​modern particle size analysis.
1.2. Types and Scope of Particle Size Analysis Although the analysis methods of particle size are various, they can be basically classified into the following methods. Conventional particle measurement methods include sieving, microscopy, sedimentation, and the like. Methods developed in recent years include laser diffraction, laser scattering, photon coherence spectroscopy, electron microscopy image analysis, particle size measurement based on Brownian motion, and mass spectrometry. Among them, laser scattering method and photon coherence spectroscopy are widely used due to their high speed, wide measurement range, reliable data, good repeatability, high degree of automation, and convenient online measurement.
1.2.1 Microscopy Microscopy is a common method for determining particle size. Depending on the material particles, either a general optical microscope or an electron microscope can be used. The optical microscope has a measurement range of 0.8 to 150 μm, and it is necessary to observe with an electron microscope. Scanning electron microscopy and transmission electron microscopy are often used to directly observe particles ranging in size from 1 nm to 5 μm, which is suitable for particle size and morphology analysis of nanomaterials. Image analysis technology is recognized as the best test technique for measuring the results consistent with the actual particle size distribution due to the randomness, statistical and intuitiveness of its measurement. The advantage is that the particle shape is directly observed, and it is possible to directly observe whether the particles are agglomerated. The disadvantage is that the sampling is poorly representative, the experimental repeatability is poor, and the measurement speed is slow.
In order to meet the needs of the development of nanotechnology, the analysis method of nanomaterial particle size has gradually become an important part of particle size analysis. At present, the methods suitable for particle size analysis of nanomaterials are mainly laser dynamic light scattering particle size analysis and photon correlation spectroscopy, which can measure the minimum particle size of particles up to 20 nm and 1 nm.
For the particle size analysis of nanomaterial systems, it is first necessary to distinguish whether the primary or secondary particle size of the particles is analyzed. The primary particle size analysis mainly uses the direct observation of electron microscopy. According to the needs and the particle size range of the sample, it can be observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunneling electron microscopy (STM) and atomic force microscopy (AFM). The original particle size and morphology of the individual particles. Since the electron microscopy method is an observation of a local region, when performing particle size distribution analysis, observation of a plurality of photographs is required, and statistical particle size distribution is obtained by software analysis. The results of primary particle size analysis obtained by electron microscopy are generally difficult to represent the distribution state of actual sample particles. The secondary particle size statistical analysis method of nano material particle system is divided into three advanced methods according to the principle: high-speed centrifugal sedimentation method, laser particle size analysis method and electro- ultrasonic particle size analysis method.
1.2.2. Electron microscopy observation of particle size analysis Electron microscopy of nanomaterials particle size analysis is also the most commonly used method for nanomaterials research. It can not only analyze the size of nanoparticles, but also analyze the particle size distribution, and also obtain the particle morphology. The data. The commonly used electron microscopes are scanning electron microscopy and transmission electron microscopy. The main principle of particle size distribution is to disperse the nano material samples on the sample stage by means of solution dispersion sampling, and then observe and photograph them by electron microscope. The particle size, particle size distribution, and shape data can be counted by a computer image analysis program.
The particle resolution of ordinary scanning electron microscope is generally about 6 nm, and the resolution of field emission scanning electron microscope can reach 0.5 nm. Scanning electron microscopy can be used for solution dispersion preparation of nano-powder samples. It can also be directly used for dry powder sample preparation. The requirements for sample preparation are relatively low. However, since electron microscopy requires certain conductivity properties for samples, it is required for non-conductive samples. The surface is vapor-deposited with a conductive layer such as gold plating on the surface, carbon deposition, or the like. Generally, samples below 10 nm cannot be steamed, because the particle size is about 8 nm, which may cause interference, and the carbonation method should be adopted. Scanning electron microscopy has a large scanning range. In principle, scanning electron microscopy can be used for particle size analysis from 1 nm to millimeter. For TEM, electron beam is required to pass through the sample. Therefore, the applicable particle size analysis range is 1-300 nm. between. For the electron microscopy particle size analysis, it can also be combined with other techniques of electron microscopy to realize the determination of the composition and crystal structure of the particles, which cannot be achieved by other particle size analysis methods.
2. Morphology analysis of nanomaterials
2.1 Importance of Morphology Analysis The morphology of materials, especially the morphology of nanomaterials, is an important part of material analysis. Many physical and chemical properties of materials are determined by their topographical features. For nanomaterials, their properties are not only related to the particle size of the material but also to the morphology of the material. Therefore, the morphology analysis of nanomaterials is an important research content of nanomaterials. The main content of the morphology analysis is to analyze the geometrical morphology of the material, the particle size of the material, the distribution of the particles, and the composition and phase structure of the morphology microdomain.
2.2 The main methods of morphology analysis The main methods of morphology analysis of nanomaterials are: scanning electron microscope (SEM), transmission electron microscope (TEM), scanning tunneling microscope (STM), atomic force microscope (AFM). Scanning electron microscopy and transmission electron microscopy analysis can not only analyze nano-powder materials, but also analyze the morphology of bulk materials. The information provided mainly includes the geometrical morphology of the material, the dispersion state of the powder, the size and distribution of the nanoparticles, the elemental composition of the specific morphology region, and the phase structure. Scanning electron microscopy analysis can provide a topographical image from a few nanometers to a millimeter. The observation is considered to be large, and its resolution is generally 6 nm. For field emission scanning electron microscopy, the spatial resolution can reach the order of 0.5 nm. Transmission electron microscopy has a high spatial resolution and is especially suitable for the analysis of powder materials. It is characterized by the small amount of sample used, not only the morphology, particle size and distribution of the sample, but also the elemental composition and phase structure information of a specific region. Transmission electron microscopy is more suitable for the morphology analysis of nano-powder samples, but the particle size should be less than 300 nm, otherwise the electron beam will not penetrate. For the analysis of bulk samples, TEM generally requires thinning of the sample. The scanning tunneling microscope is mainly used for the analysis of the morphology of some special conductive solid samples, which can reach the atomic order resolution. It is only suitable for the morphology analysis and surface atomic structure distribution analysis of conductive film materials, and can not analyze the nano powder materials. Scanning atomic force microscopy can analyze the morphology of nano-films with resolutions of several tens of nanometers, which is worse than scanning tunneling microscopes, but suitable for conductor and non-conductor samples, and is not suitable for the morphology analysis of nano-powders. In short, these four topographical analysis methods have their own characteristics, and electron microscopy analysis has more advantages, but scanning tunneling microscopy and atomic force microscopy have the characteristics of in-situ morphology analysis under atmosphere.
The resolution of the human eye is about 0.5 mm. The resolution of the microscope can be expressed by the formula d=0.61λ/(nsinα), so that the resolution of the microscope is proportional to the wavelength of the light. When the wavelength of the light is longer, the lower the resolution, the light with a shorter wavelength can be used to obtain a higher magnification. Waves shorter than the wavelength of visible light have ultraviolet rays, X-rays, and electron waves. The electron beam is used as a new light source for improving the resolution of the microscope, that is, an electron microscope. At present, the magnification of the electron microscope has reached 1.5 million times, which is impossible to achieve by optical microscopy.
The electronic lens not only has the characteristics of large resolution, but also has the characteristics of large depth of field and long focal length. The so-called depth of field refers to the distance that the sample can move along the mirror axis above and below the object plane while maintaining the sharpness of the object image. In other words, changes in the position of the sample do not affect the sharpness of the image within the depth of field. It can also be considered that the sample exceeds the allowable thickness of the object plane. The so-called focal depth refers to the distance that the image plane can move along the mirror axis, or the moving distance allowed by the viewing screen or the photo substrate along the mirror axis, while keeping the image of the object clear. In metallographic photography, as long as the depth of field allows, the surface of the sample can be sharply and clearly displayed on the photo.
Scanning electron microscopy (SEM) is a common widely used surface topography instrument. High-magnification photographs of the microscopic topography of the surface of the material are produced by electron-scanning the surface of the material with a high concentration of energy.
Scanning electron microscopy can magnify large multiples because the basic electron beam can focus on a very small area (<10 nm) and can be generated when a basic electron beam with less than 1 KeV energy is scanned over a surface area of ​​less than 5 nm. High sensitivity to microscopic morphology.
The principle of scanning electron microscopy is similar to that of optical imaging. An imaging method that uses an electron beam to switch visible light and an electromagnetic lens instead of an optical lens.
The imaging principle of a scanning electron microscope is different from that of an optical microscope, but it is not exactly the same as a transmission electron microscope.
The advantages of scanning electron microscopy are: high magnification, continuous adjustment between 20 times and 200,000 times; large depth of field, large field of view, imaging is rich in three-dimensional salt, can directly observe the uneven surface of various samples Fine structure; sample preparation is simple. The current scanning electron microscopes are equipped with X-ray energy spectrometer devices. This allows simultaneous observation of microstructure and micro-component analysis, so it is a very useful scientific research instrument like TEM.
Resolution is the main performance indicator of SEM. For micro-area component analysis, it refers to the smallest area that can be analyzed; for imaging, it refers to the minimum distance between two points, and the resolution is the same as the incident electron beam diameter and the adjustment signal type. Decide. The smaller the electron beam diameter, the higher the resolution. However, due to different imaging signals, such as secondary electrons and back-reflected electrons, the emission range on the surface of the sample is also different, which affects its resolution.
Like the TEM analysis of depth of field, the depth of field of a scanning electron microscope can also be expressed as Df>〉2Δr0/α, where α is the electron beam aperture angle. It can be seen that the electron beam aperture angle is the main factor determining the depth of field of the scanning electron microscope, and it depends on the pupil diameter and working distance of the final lens. The final lens of the scanning electron microscope uses a small aperture angle and a long focal length, so that a large depth of field can be obtained, which is 100-500 times larger than that of a general optical microscope and 10 times larger than a transmission electron microscope. Due to the large depth of field, the image of the scanning electron microscope is three-dimensional and strong, and the shape is realistic. For the fracture sample with rough surface, the optical microscope is powerless because of the small depth of field. The transmission electron microscope is very demanding on the sample, that is, the use of the replica sample is inevitable, and the depth of field is smaller than that of the scanning electron microscope. Therefore, observation by scanning electron microscopy Fracture specimens have advantages that are unmatched by other analytical instruments.
3. Composition analysis
3.1 Composition analysis methods and scope The properties of light, electricity, sound, heat and magnetism of nanomaterials are closely related to the chemical composition and structure of nanomaterials. Therefore, determining the elemental composition of nanomaterials and determining the type and concentration of impurities in nanomaterials is one of the important contents of nanomaterial analysis.
Nanomaterial composition analysis can be divided into two types according to the analysis object: micro sample analysis and trace component analysis. The micro sample analysis is in terms of the amount of sample. Trace component analysis is based on the content of the component to be tested in the nanomaterial. Because the content of impurities or doped components is very low, in the concentration range of one part per million or less, this type of analysis is called a trace. Quantitative component analysis.
The composition analysis method of nanomaterials is divided into bulk element composition analysis, surface component analysis and micro-area composition analysis according to the purpose of analysis.
Methods for analyzing the bulk elemental composition of nanomaterials and their impurity components include atomic absorption, atomic emission, ICP mass spectrometry, and X-ray fluorescence and diffraction analysis methods. The first three methods require the sample to be dissolved and then measured, so it is a destructive sample analysis method, and the X-ray fluorescence and diffraction analysis method can directly measure the solid sample, so it is also called non-destructive element analysis method.
3.2 X-ray fluorescence spectrometry (XFS) is a non-destructive analytical method that can directly measure solid samples, so it has great advantages in the analysis of nanomaterial composition. The energy or wavelength of X-ray fluorescence is characteristic and has a one-to-one correspondence with elements. Therefore, as long as the wavelength of the fluorescent X-ray is measured, the type of the element can be known, which is the basis of the qualitative analysis of the fluorescent X-ray. In addition, the intensity of the fluorescent X-rays has a certain relationship with the corresponding elemental content. According to this, quantitative analysis of elements can be performed.
The surface analysis methods for nanomaterials are currently the most commonly used X-ray photoelectron spectroscopy (XPS) analysis methods, Auger electron spectroscopy (AES) analysis methods, electron diffraction analysis methods and secondary ion mass spectrometry (SIMS) analysis methods. These methods can measure the chemical composition, distribution state and valence state of the nanomaterials, and the adsorption and diffusion reactions of the surface and interface. When the energy spectrum is combined with electron probe technology and scanning or transmission electron microscopy technology, The micro-area composition of nano-materials can be analyzed, so it is widely used in the composition analysis of nano-materials, especially the micro-area composition analysis of nano-films.
3.3 Electron spectroscopy mainly includes X-ray photoelectron spectroscopy and Auger electron spectroscopy. The common feature of the two methods is the method of analyzing the surface elements of the material based on the characteristic energy distribution (energy spectrum) of the electrons excited by the surface of the material. The main difference between the two is that the laser source used is different. X-ray photoelectron spectroscopy uses X-ray as the excitation source, while Auger electron spectroscopy uses electron beam as the excitation source.
As a typical surface analysis method, X-ray photoelectron spectroscopy (XPS) can provide the element content and morphology of the sample surface with an information depth of about 3-5 nm. Auger electron spectroscopy is a method of analyzing the surface of a material by Auger electrons ejected by an electron beam emitted by an electron gun, and is a highly sensitive analysis method with an information depth of 1.0--3.0 nm and absolute sensitivity. A 10-3 monoatomic layer is a useful analytical method.
3.4 Transmission electron microscopy and scanning electron microscopy have been widely used in the morphology analysis of nanomaterials. When people are interested in the elemental composition of a microdomain observed after imaging nanomaterials, they can be combined with electron microscopy and energy spectrum. The method collectively analyzes the situation of a certain micro-area. In addition, micro-analysis can be used to study microscopic phenomena such as inclusions, precipitates, and grain boundary segregation, which is very useful.
4. Structural analysis of nanomaterials
4.1. Structural characteristics of nanomaterials According to the structure of nanomaterials, nanomaterials can be roughly classified into four types, namely: nanostructured crystals or three-dimensional nanostructures (such as equiaxed microcrystals); two-dimensional nanometer structures (such as Nanofilms; one-dimensional nanostructures (such as nanotubes); and zero-dimensional clusters or clusters (such as nanoparticles with a particle size of no more than 2 nm). Nanomaterials include crystals, germanium crystals, amorphous metals, ceramics, and compounds. Nanomaterials have the following properties.
4.1.1. Small size effect When the size of the nanoparticle is equivalent to the wavelength of the light wave, the de Broglie wavelength of the conduction electron, and the physical feature size such as the coherence length or penetration depth of the superconducting state, the periodic boundary condition of the crystal will be Destruction, sound, light, force, electricity, heat, magnetism, internal pressure, chemical activity, etc. have a great change compared with ordinary particles, which is the small size effect (also called volume effect) of nanoparticles.
4.1.2. Surface and Interfacial Effects Due to its small size, large surface area, high surface energy, and the atoms on the surface are in a serious vacancy state, the nanoparticles are extremely active and unstable, and are quickly combined when encountering other atoms. This activity is the surface effect.
4.1.3. Quantum size effect When the crystallite size is equivalent to the de Broglie wavelength, the electron motion in the particle is limited in all three directions, and the continuous energy band of the electron is split to near the molecular orbital level, and the nanoparticle The acoustic, optical, electrical, magnetic, thermal, and superconductivity are very different from the macroscopic characteristics, called the quantum size effect.
4.1.4. Macroscopic Quantum Tunneling Tunneling refers to the ability of microscopic particles to penetrate the barrier. Later, it was discovered that some macroscopic quantities, such as magnetization and magnetic flux in quantum interference devices, also have tunneling effects, called macroscopic quantum tunneling effects. . Nanoparticles have tunneling effects similar to microscopic particles that can penetrate the barrier. This effect is also known as macroscopic quantum tunneling.
The nature of the material is closely related to the microstructure. There must be a close relationship between these special properties of nanomaterials and their structure. Therefore, it is very meaningful to study the microstructure of nanomaterials. Nanomaterials are mainly composed of nano-grain and grain interface, which have an important influence on the properties of nanomaterials. Studies on the microstructure of nano-grains show that the microstructure of nano-materials is basically consistent with the traditional crystal structure, but since each unit cell contains only a limited unit cell, the lattice lattice will inevitably have a certain degree of elasticity. Deformation.
4.2 Submicroscopic Characteristics of Phase Structure It has been known that not only the composition and morphology of nanomaterials have an important influence on their properties, but also the phase structure and crystal structure of nanomaterials have an important influence on the properties of materials. Therefore, the analysis of the phase structure of nanomaterials is also an important part of material analysis. The purpose of phase structure analysis is to accurately characterize the following submicroscopic features:
3. Composition analysis
3.1 Composition analysis methods and scope The properties of light, electricity, sound, heat and magnetism of nanomaterials are closely related to the chemical composition and structure of nanomaterials. Therefore, determining the elemental composition of nanomaterials and determining the type and concentration of impurities in nanomaterials is one of the important contents of nanomaterial analysis.
Nanomaterial composition analysis can be divided into two types according to the analysis object: micro sample analysis and trace component analysis. The micro sample analysis is in terms of the amount of sample. Trace component analysis is based on the content of the component to be tested in the nanomaterial. Because the content of impurities or doped components is very low, in the concentration range of one part per million or less, this type of analysis is called a trace. Quantitative component analysis.
The composition analysis method of nanomaterials is divided into bulk element composition analysis, surface component analysis and micro-area composition analysis according to the purpose of analysis.
Methods for analyzing the bulk elemental composition of nanomaterials and their impurity components include atomic absorption, atomic emission, ICP mass spectrometry, and X-ray fluorescence and diffraction analysis methods. The first three methods require the sample to be dissolved and then measured, so it is a destructive sample analysis method, and the X-ray fluorescence and diffraction analysis method can directly measure the solid sample, so it is also called non-destructive element analysis method.
3.2 X-ray fluorescence spectrometry (XFS) is a non-destructive analytical method that can directly measure solid samples, so it has great advantages in the analysis of nanomaterial composition. The energy or wavelength of X-ray fluorescence is characteristic and has a one-to-one correspondence with elements. Therefore, as long as the wavelength of the fluorescent X-ray is measured, the type of the element can be known, which is the basis of the qualitative analysis of the fluorescent X-ray. In addition, the intensity of the fluorescent X-rays has a certain relationship with the corresponding elemental content. According to this, quantitative analysis of elements can be performed.
The surface analysis methods for nanomaterials are currently the most commonly used X-ray photoelectron spectroscopy (XPS) analysis methods, Auger electron spectroscopy (AES) analysis methods, electron diffraction analysis methods and secondary ion mass spectrometry (SIMS) analysis methods. These methods can measure the chemical composition, distribution state and valence state of the nanomaterials, and the adsorption and diffusion reactions of the surface and interface. When the energy spectrum is combined with electron probe technology and scanning or transmission electron microscopy technology, The micro-area composition of nano-materials can be analyzed, so it is widely used in the composition analysis of nano-materials, especially the micro-area composition analysis of nano-films.
3.3 Electron spectroscopy mainly includes X-ray photoelectron spectroscopy and Auger electron spectroscopy. The common feature of the two methods is the method of analyzing the surface elements of the material based on the characteristic energy distribution (energy spectrum) of the electrons excited by the surface of the material. The main difference between the two is that the laser source used is different. X-ray photoelectron spectroscopy uses X-ray as the excitation source, while Auger electron spectroscopy uses electron beam as the excitation source.
As a typical surface analysis method, X-ray photoelectron spectroscopy (XPS) can provide the element content and morphology of the sample surface with an information depth of about 3-5 nm. Auger electron spectroscopy is a method of analyzing the surface of a material by Auger electrons ejected by an electron beam emitted by an electron gun, and is a highly sensitive analysis method with an information depth of 1.0--3.0 nm and absolute sensitivity. A 10-3 monoatomic layer is a useful analytical method.
3.4 Transmission electron microscopy and scanning electron microscopy have been widely used in the morphology analysis of nanomaterials. When people are interested in the elemental composition of a microdomain observed after imaging nanomaterials, they can be combined with electron microscopy and energy spectrum. The method collectively analyzes the situation of a certain micro-area. In addition, micro-analysis can be used to study microscopic phenomena such as inclusions, precipitates, and grain boundary segregation, which is very useful.
4. Structural analysis of nanomaterials
4.1. Structural characteristics of nanomaterials According to the structure of nanomaterials, nanomaterials can be roughly classified into four types, namely: nanostructured crystals or three-dimensional nanostructures (such as equiaxed microcrystals); two-dimensional nanometer structures (such as Nanofilms; one-dimensional nanostructures (such as nanotubes); and zero-dimensional clusters or clusters (such as nanoparticles with a particle size of no more than 2 nm). Nanomaterials include crystals, germanium crystals, amorphous metals, ceramics, and compounds. Nanomaterials have the following properties.
4.1.1. Small size effect When the size of the nanoparticle is equivalent to the wavelength of the light wave, the de Broglie wavelength of the conduction electron, and the physical feature size such as the coherence length or penetration depth of the superconducting state, the periodic boundary condition of the crystal will be Destruction, sound, light, force, electricity, heat, magnetism, internal pressure, chemical activity, etc. have a great change compared with ordinary particles, which is the small size effect (also called volume effect) of nanoparticles.
4.1.2. Surface and Interfacial Effects Due to its small size, large surface area, high surface energy, and the atoms on the surface are in a serious vacancy state, the nanoparticles are extremely active and unstable, and are quickly combined when encountering other atoms. This activity is the surface effect.
4.1.3. Quantum size effect When the crystallite size is equivalent to the de Broglie wavelength, the electron motion in the particle is limited in all three directions, and the continuous energy band of the electron is split to near the molecular orbital level, and the nanoparticle The acoustic, optical, electrical, magnetic, thermal, and superconductivity are very different from the macroscopic characteristics, called the quantum size effect.
4.1.4. Macroscopic Quantum Tunneling Tunneling refers to the ability of microscopic particles to penetrate the barrier. Later, it was discovered that some macroscopic quantities, such as magnetization and magnetic flux in quantum interference devices, also have tunneling effects, called macroscopic quantum tunneling effects. . Nanoparticles have tunneling effects similar to microscopic particles that can penetrate the barrier. This effect is also known as macroscopic quantum tunneling.
The nature of the material is closely related to the microstructure. There must be a close relationship between these special properties of nanomaterials and their structure. Therefore, it is very meaningful to study the microstructure of nanomaterials. Nanomaterials are mainly composed of nano-grain and grain interface, which have an important influence on the properties of nanomaterials. Studies on the microstructure of nano-grains show that the microstructure of nano-materials is basically consistent with the traditional crystal structure, but since each unit cell contains only a limited unit cell, the lattice lattice will inevitably have a certain degree of elasticity. Deformation.
4.2 Submicroscopic Characteristics of Phase Structure It has been known that not only the composition and morphology of nanomaterials have an important influence on their properties, but also the phase structure and crystal structure of nanomaterials have an important influence on the properties of materials. Therefore, the analysis of the phase structure of nanomaterials is also an important part of material analysis. The purpose of phase structure analysis is to accurately characterize the following submicroscopic features:
4.2.1 the size, distribution and morphology of the grains;
4.2.2. The nature of the grain boundary and phase interface;
4.2.3 crystal integrity and intercrystalline defects;
4.2.4 Composition and distribution across grain and intergranular boundaries;
4.2.5 Analysis of impurities in crystallites and grain boundaries.
In addition, the purpose of the analysis is to determine the structural properties of nanomaterials, providing an experimental basis for explaining the relationship between material structure and properties. At present, commonly used phase analysis methods include X-ray diffraction analysis, laser Raman analysis, and micro-region electron diffraction analysis.
4.3. New Progress in Structural Analysis of Nanomaterials There are quite a number of methods for structural characterization of materials. The instruments suitable for structural analysis of nanomaterials are not limited to the above, and new characterizations are constantly emerging. For example, high-resolution electron microscopy has been able to display atomic alignment and chemical composition at atomic resolution, tunnel scanning microscopy can determine atomic arrangement and electronic structure on the surface and near surface of materials, and low-energy electron microscopy can be used to display surface defect structures.
With the continuous development of analytical instruments and technologies, nanomaterial structure research institutes can use more and more test instruments, including high-resolution transmission electron microscopy (HRTEM), scanning probe microscopy (SPM), scanning tunneling microscopy (STM), and atomic force. Microscope (AFM), field ion microscope (FIM), X-ray diffractometer (XRD), extended X-ray absorption fine structure analyzer (EXAFS), Mössbauer spectrometer (MS), Raman scattering instrument (RS), etc. Wait. It can be considered that the research methods of nanostructures have almost all involved instruments for structural analysis of substances.
5. Nanomaterial surface and interface analysis
5.1 Nanomaterial surface and interface analysis methods The surface and interface analysis of solid materials has been developed into an important part of nanofilm material research, especially for elemental chemical state analysis, elemental three-dimensional distribution analysis and micro-area analysis of solid materials. At present, commonly used surface and interface analysis methods are: X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SIMS) and ion scattering spectroscopy (ISS). Among them, XPS accounts for 50% of the total surface composition analysis, AES accounts for 40%, and SIMS accounts for 8%. Among these surface and interface analysis methods, XPS has the widest application range and can be applied to the analysis of various materials, especially for the analysis of chemical state of materials, and is more suitable for research involving chemical information.
At present, commercial surface analysis spectrometers have a high vacuum of about 10-10 Torr. Ultra-high vacuum systems must be used in X-ray photoelectron spectroscopy and Auger electron spectroscopy, mainly for two reasons. First, XPS and AES are surface analysis techniques, if the vacuum in the analysis chamber is very high. Poor, cleaning the surface in a short time may be covered by residual gases in the vacuum. It is impossible to obtain true surface composition information without ultra-high vacuum conditions.
5. 2 XPS (X-ray Photoelectron Spectroscopy) analysis method
5.2.1 Qualitative analysis of surface elements This is the most common analytical method, generally using the wide scanning procedure of the XPS spectrometer.
5.2.2 Semi-quantitative analysis of surface elements First of all, it should be clear that XPS is not a good quantitative analysis method. It gives only a semi-quantitative result, ie relative content rather than absolute content.
5.2.3 Chemical valence analysis of surface elements The chemical valence analysis of surface elements is the most important analytical function of XPS, and it is also the most difficult part of XPS spectrum analysis, which is more prone to errors.
5.3 Auger electron spectroscopy Auger electron spectroscopy is one of the most widely used analytical methods. Its advantages are: high sensitivity in chemical analysis in the range of 0.5-2 years near the surface; fast data analysis; Auger Electron spectroscopy can analyze all elements except hydroquinone. It has become an important means of qualitative, semi-quantitative analysis, elemental depth distribution analysis and micro-area analysis of surface elements. The new Auger electron spectrometer has strong micro-analysis capability and three-dimensional analysis capability, and its micro-analysis diameter can be as small as 6 nm, greatly improving the analysis capabilities in microelectronics and nanotechnology. In addition, the Auger electron spectrometer has a strong chemical valence analysis capability, which can not only carry out elemental chemical composition analysis, but also elemental chemical valence analysis. Auger electron spectroscopy is one of the most important and commonly used methods of surface analysis and interface analysis. Due to its high spatial resolution (6 nm) and surface analysis capability (0.5-2 nm), Auger electron spectroscopy is especially suitable for surface and interface analysis of nanomaterials. Auger electron spectrometer has broad application prospects in the research of nanomaterials, especially nanodevices.
The Auger electron spectroscopy qualitative analysis mainly uses the characteristic energy of Auger electrons to determine the elemental composition of the solid surface. Since the energy of the Auger electron is only related to the orbital energy level of the atom itself, it is independent of the energy of the incident electron, that is, independent of the excitation source. For a particular element and a specific Auger transition process, the energy of Auger electrons is characteristic. Thus, the type of material elements on the surface of the sample can be qualitatively analyzed according to the kinetic energy of the Auger electron.由于æ¯ä¸ªå…ƒç´ 会有多个俄æ‡å³°ï¼Œå®šæ€§åˆ†æžçš„å‡†ç¡®æ€§å¾ˆé«˜ã€‚å› æ¤ï¼ŒAESæŠ€æœ¯æ˜¯é€‚ç”¨å¯¹æ‰€æœ‰å…ƒç´ è¿›è¡Œä¸€æ¬¡å…¨åˆ†æžçš„有效定性分æžæ–¹æ³•ï¼Œè¿™å¯¹äºŽæœªçŸ¥æ ·å“的定性鉴定是éžå¸¸æœ‰æ•ˆçš„。
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The modern method of analytical science is one of the most important and active fields in the treasure house of human knowledge. It is not only the object of research, but also an important means of observing and exploring the world, especially the microcosm. It is inseparable from all walks of life. . With the development of nanomaterial science and technology, it is required to improve and develop new analytical methods, new analytical techniques and new concepts, improve its sensitivity, accuracy and reliability, extract more information from it, and improve test quality, efficiency and economy.
Nanoscience and technology is a multidisciplinary high-tech that studies the properties and interactions of matter (including atoms, molecules) on a nanoscale (between 0.1 and 100 nm) and uses these properties. Nanotechnology is the foundation of future high technology, and instrumental analysis methods suitable for nanotechnology research are indispensable experimental tools in nanotechnology. Therefore, the analysis and characterization of nanomaterials have important significance and role in the development of nanomaterials and nanotechnology.
Nanotechnology and nanomaterials are a typical emerging high-tech field. Although many researchers have been involved in research in this field, there are still many researchers and practitioners in related industries who are not familiar with nanomaterials, especially how to analyze and characterize nanomaterials, and how to obtain some characteristic information of nanomaterials. In order to meet the needs of nanotechnology workers, some common analytical and characterization techniques for nanomaterials are briefly described from the aspects of composition analysis, morphology analysis, particle size analysis, structure analysis and surface interface analysis of nanomaterials.
1. Particle size analysis of nanomaterials
1.1 The concept of particle size analysis Most solid materials are constructed from particles of various shapes. Therefore, the shape and size of fine particle materials have an important influence on the structure and properties of materials. Especially for nanomaterials, the particle size and shape play a decisive role in the properties of the material. Therefore, it is important to characterize and control the particle size and shape of nanomaterials. Generally, the particle size of a solid material can be expressed by the concept of particle size.
For particle size analysis instruments with different principles, the measurement principle is different, and the particle characteristics are different. Only effective comparison can be made, and horizontal direct comparison cannot be performed. Since the particle shape of the powder material cannot be uniformly spherical and has various structures, in most cases, the particle size measured by the particle size analyzer is an equivalent particle size, and actual The particle size distribution will have a certain difference, so it has only a relatively comparative significance. In addition, the particle size and distribution data obtained by various particle size analysis methods may not be mutually validated, and absolute lateral comparisons cannot be made.
Since the particle size distribution of the powder material is wide, it can be from nanometer to millimeter. Therefore, when describing the particle size of the material, the particles can be divided into nanometer particles, ultrafine particles, fine particles, fine particles, coarse particles, etc. according to the size. . In recent years, with the rapid development of nanoscience and technology, the particle distribution and particle size of nanomaterials have become one of the important indicators of nanomaterial characterization. In the ordinary material particle size analysis, the particle size of the research is generally in the range of 100nm size. . For nanomaterials research, the particle size distribution range of the research is mainly between 1-500nm, especially 1-20nm is the size range most concerned by nanomaterial research.
In nanomaterial analysis and research, nanoparticles that are often encountered generally refer to ultrafine particles having a particle size on the order of nanometers (1-100 nm). Since the material has a particle size of the order of nanometers and has a small size effect, a quantum size effect, a surface effect, and a macroscopic quantum tunneling effect, it has many characteristics that are not possessed by conventional materials. Because the particle size, distribution, dispersion in the medium and the aggregation morphology of the secondary particles have an important influence on the performance of the nanomaterials, the analysis of the nanoparticle size is an important aspect of nanomaterials research. Also due to the nature and importance of nanomaterials, the development of methods and techniques for particle size analysis and characterization has been promoted. The analysis of nanomaterial particle size has evolved into an important area of ​​modern particle size analysis.
1.2. Types and Scope of Particle Size Analysis Although the analysis methods of particle size are various, they can be basically classified into the following methods. Conventional particle measurement methods include sieving, microscopy, sedimentation, and the like. Methods developed in recent years include laser diffraction, laser scattering, photon coherence spectroscopy, electron microscopy image analysis, particle size measurement based on Brownian motion, and mass spectrometry. Among them, laser scattering method and photon coherence spectroscopy are widely used due to their high speed, wide measurement range, reliable data, good repeatability, high degree of automation, and convenient online measurement.
1.2.1 Microscopy Microscopy is a common method for determining particle size. Depending on the material particles, either a general optical microscope or an electron microscope can be used. The optical microscope has a measurement range of 0.8 to 150 μm, and it is necessary to observe with an electron microscope. Scanning electron microscopy and transmission electron microscopy are often used to directly observe particles ranging in size from 1 nm to 5 μm, which is suitable for particle size and morphology analysis of nanomaterials. Image analysis technology is recognized as the best test technique for measuring the results consistent with the actual particle size distribution due to the randomness, statistical and intuitiveness of its measurement. The advantage is that the particle shape is directly observed, and it is possible to directly observe whether the particles are agglomerated. The disadvantage is that the sampling is poorly representative, the experimental repeatability is poor, and the measurement speed is slow.
In order to meet the needs of the development of nanotechnology, the analysis method of nanomaterial particle size has gradually become an important part of particle size analysis. At present, the methods suitable for particle size analysis of nanomaterials are mainly laser dynamic light scattering particle size analysis and photon correlation spectroscopy, which can measure the minimum particle size of particles up to 20 nm and 1 nm.
For the particle size analysis of nanomaterial systems, it is first necessary to distinguish whether the primary or secondary particle size of the particles is analyzed. The primary particle size analysis mainly uses the direct observation of electron microscopy. According to the needs and the particle size range of the sample, it can be observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunneling electron microscopy (STM) and atomic force microscopy (AFM). The original particle size and morphology of the individual particles. Since the electron microscopy method is an observation of a local region, when performing particle size distribution analysis, observation of a plurality of photographs is required, and statistical particle size distribution is obtained by software analysis. The results of primary particle size analysis obtained by electron microscopy are generally difficult to represent the distribution state of actual sample particles. The secondary particle size statistical analysis method of nano material particle system is divided into three advanced methods according to the principle: high-speed centrifugal sedimentation method, laser particle size analysis method and electro- ultrasonic particle size analysis method.
1.2.2. Electron microscopy observation of particle size analysis Electron microscopy of nanomaterials particle size analysis is also the most commonly used method for nanomaterials research. It can not only analyze the size of nanoparticles, but also analyze the particle size distribution, and also obtain the particle morphology. The data. The commonly used electron microscopes are scanning electron microscopy and transmission electron microscopy. The main principle of particle size distribution is to disperse the nano material samples on the sample stage by means of solution dispersion sampling, and then observe and photograph them by electron microscope. The particle size, particle size distribution, and shape data can be counted by a computer image analysis program.
The particle resolution of ordinary scanning electron microscope is generally about 6 nm, and the resolution of field emission scanning electron microscope can reach 0.5 nm. Scanning electron microscopy can be used for solution dispersion preparation of nano-powder samples. It can also be directly used for dry powder sample preparation. The requirements for sample preparation are relatively low. However, since electron microscopy requires certain conductivity properties for samples, it is required for non-conductive samples. The surface is vapor-deposited with a conductive layer such as gold plating on the surface, carbon deposition, or the like. Generally, samples below 10 nm cannot be steamed, because the particle size is about 8 nm, which may cause interference, and the carbonation method should be adopted. Scanning electron microscopy has a large scanning range. In principle, scanning electron microscopy can be used for particle size analysis from 1 nm to millimeter. For TEM, electron beam is required to pass through the sample. Therefore, the applicable particle size analysis range is 1-300 nm. between. For the electron microscopy particle size analysis, it can also be combined with other techniques of electron microscopy to realize the determination of the composition and crystal structure of the particles, which cannot be achieved by other particle size analysis methods.
2. Morphology analysis of nanomaterials
2.1 Importance of Morphology Analysis The morphology of materials, especially the morphology of nanomaterials, is an important part of material analysis. Many physical and chemical properties of materials are determined by their topographical features. For nanomaterials, their properties are not only related to the particle size of the material but also to the morphology of the material. Therefore, the morphology analysis of nanomaterials is an important research content of nanomaterials. The main content of the morphology analysis is to analyze the geometrical morphology of the material, the particle size of the material, the distribution of the particles, and the composition and phase structure of the morphology microdomain.
2.2 The main methods of morphology analysis The main methods of morphology analysis of nanomaterials are: scanning electron microscope (SEM), transmission electron microscope (TEM), scanning tunneling microscope (STM), atomic force microscope (AFM). Scanning electron microscopy and transmission electron microscopy analysis can not only analyze nano-powder materials, but also analyze the morphology of bulk materials. The information provided mainly includes the geometrical morphology of the material, the dispersion state of the powder, the size and distribution of the nanoparticles, the elemental composition of the specific morphology region, and the phase structure. Scanning electron microscopy analysis can provide a topographical image from a few nanometers to a millimeter. The observation is considered to be large, and its resolution is generally 6 nm. For field emission scanning electron microscopy, the spatial resolution can reach the order of 0.5 nm. Transmission electron microscopy has a high spatial resolution and is especially suitable for the analysis of powder materials. It is characterized by the small amount of sample used, not only the morphology, particle size and distribution of the sample, but also the elemental composition and phase structure information of a specific region. Transmission electron microscopy is more suitable for the morphology analysis of nano-powder samples, but the particle size should be less than 300 nm, otherwise the electron beam will not penetrate. For the analysis of bulk samples, TEM generally requires thinning of the sample. The scanning tunneling microscope is mainly used for the analysis of the morphology of some special conductive solid samples, which can reach the atomic order resolution. It is only suitable for the morphology analysis and surface atomic structure distribution analysis of conductive film materials, and can not analyze the nano powder materials. Scanning atomic force microscopy can analyze the morphology of nano-films with resolutions of several tens of nanometers, which is worse than scanning tunneling microscopes, but suitable for conductor and non-conductor samples, and is not suitable for the morphology analysis of nano-powders. In short, these four topographical analysis methods have their own characteristics, and electron microscopy analysis has more advantages, but scanning tunneling microscopy and atomic force microscopy have the characteristics of in-situ morphology analysis under atmosphere.
The resolution of the human eye is about 0.5 mm. The resolution of the microscope can be expressed by the formula d=0.61λ/(nsinα), so that the resolution of the microscope is proportional to the wavelength of the light. When the wavelength of the light is longer, the lower the resolution, the light with a shorter wavelength can be used to obtain a higher magnification. Waves shorter than the wavelength of visible light have ultraviolet rays, X-rays, and electron waves. The electron beam is used as a new light source for improving the resolution of the microscope, that is, an electron microscope. At present, the magnification of the electron microscope has reached 1.5 million times, which is impossible to achieve by optical microscopy.
The electronic lens not only has the characteristics of large resolution, but also has the characteristics of large depth of field and long focal length. The so-called depth of field refers to the distance that the sample can move along the mirror axis above and below the object plane while maintaining the sharpness of the object image. In other words, changes in the position of the sample do not affect the sharpness of the image within the depth of field. It can also be considered that the sample exceeds the allowable thickness of the object plane. The so-called focal depth refers to the distance that the image plane can move along the mirror axis, or the moving distance allowed by the viewing screen or the photo substrate along the mirror axis, while keeping the image of the object clear. In metallographic photography, as long as the depth of field allows, the surface of the sample can be sharply and clearly displayed on the photo.
Scanning electron microscopy (SEM) is a common widely used surface topography instrument. High-magnification photographs of the microscopic topography of the surface of the material are produced by electron-scanning the surface of the material with a high concentration of energy.
Scanning electron microscopy can magnify large multiples because the basic electron beam can focus on a very small area (<10 nm) and can be generated when a basic electron beam with less than 1 KeV energy is scanned over a surface area of ​​less than 5 nm. High sensitivity to microscopic morphology.
The principle of scanning electron microscopy is similar to that of optical imaging. An imaging method that uses an electron beam to switch visible light and an electromagnetic lens instead of an optical lens.
The imaging principle of a scanning electron microscope is different from that of an optical microscope, but it is not exactly the same as a transmission electron microscope.
The advantages of scanning electron microscopy are: high magnification, continuous adjustment between 20 times and 200,000 times; large depth of field, large field of view, imaging is rich in three-dimensional salt, can directly observe the uneven surface of various samples Fine structure; sample preparation is simple. The current scanning electron microscopes are equipped with X-ray energy spectrometer devices. This allows simultaneous observation of microstructure and micro-component analysis, so it is a very useful scientific research instrument like TEM.
Resolution is the main performance indicator of SEM. For micro-area component analysis, it refers to the smallest area that can be analyzed; for imaging, it refers to the minimum distance between two points, and the resolution is the same as the incident electron beam diameter and the adjustment signal type. Decide. The smaller the electron beam diameter, the higher the resolution. However, due to different imaging signals, such as secondary electrons and back-reflected electrons, the emission range on the surface of the sample is also different, which affects its resolution.
Like the TEM analysis of depth of field, the depth of field of a scanning electron microscope can also be expressed as Df>〉2Δr0/α, where α is the electron beam aperture angle. It can be seen that the electron beam aperture angle is the main factor determining the depth of field of the scanning electron microscope, and it depends on the pupil diameter and working distance of the final lens. The final lens of the scanning electron microscope uses a small aperture angle and a long focal length, so that a large depth of field can be obtained, which is 100-500 times larger than that of a general optical microscope and 10 times larger than a transmission electron microscope. Due to the large depth of field, the image of the scanning electron microscope is three-dimensional and strong, and the shape is realistic. For the fracture sample with rough surface, the optical microscope is powerless because of the small depth of field. The transmission electron microscope is very demanding on the sample, that is, the use of the replica sample is inevitable, and the depth of field is smaller than that of the scanning electron microscope. Therefore, observation by scanning electron microscopy Fracture specimens have advantages that are unmatched by other analytical instruments.
3. Composition analysis
3.1 Composition analysis methods and scope The properties of light, electricity, sound, heat and magnetism of nanomaterials are closely related to the chemical composition and structure of nanomaterials. Therefore, determining the elemental composition of nanomaterials and determining the type and concentration of impurities in nanomaterials is one of the important contents of nanomaterial analysis.
Nanomaterial composition analysis can be divided into two types according to the analysis object: micro sample analysis and trace component analysis. The micro sample analysis is in terms of the amount of sample. Trace component analysis is based on the content of the component to be tested in the nanomaterial. Because the content of impurities or doped components is very low, in the concentration range of one part per million or less, this type of analysis is called a trace. Quantitative component analysis.
The composition analysis method of nanomaterials is divided into bulk element composition analysis, surface component analysis and micro-area composition analysis according to the purpose of analysis.
Methods for analyzing the bulk elemental composition of nanomaterials and their impurity components include atomic absorption, atomic emission, ICP mass spectrometry, and X-ray fluorescence and diffraction analysis methods. The first three methods require the sample to be dissolved and then measured, so it is a destructive sample analysis method, and the X-ray fluorescence and diffraction analysis method can directly measure the solid sample, so it is also called non-destructive element analysis method.
3.2 X-ray fluorescence spectrometry (XFS) is a non-destructive analytical method that can directly measure solid samples, so it has great advantages in the analysis of nanomaterial composition. The energy or wavelength of X-ray fluorescence is characteristic and has a one-to-one correspondence with elements. Therefore, as long as the wavelength of the fluorescent X-ray is measured, the type of the element can be known, which is the basis of the qualitative analysis of the fluorescent X-ray. In addition, the intensity of the fluorescent X-rays has a certain relationship with the corresponding elemental content. According to this, quantitative analysis of elements can be performed.
The surface analysis methods for nanomaterials are currently the most commonly used X-ray photoelectron spectroscopy (XPS) analysis methods, Auger electron spectroscopy (AES) analysis methods, electron diffraction analysis methods and secondary ion mass spectrometry (SIMS) analysis methods. These methods can measure the chemical composition, distribution state and valence state of the nanomaterials, and the adsorption and diffusion reactions of the surface and interface. When the energy spectrum is combined with electron probe technology and scanning or transmission electron microscopy technology, The micro-area composition of nano-materials can be analyzed, so it is widely used in the composition analysis of nano-materials, especially the micro-area composition analysis of nano-films.
3.3 Electron spectroscopy mainly includes X-ray photoelectron spectroscopy and Auger electron spectroscopy. The common feature of the two methods is the method of analyzing the surface elements of the material based on the characteristic energy distribution (energy spectrum) of the electrons excited by the surface of the material. The main difference between the two is that the laser source used is different. X-ray photoelectron spectroscopy uses X-ray as the excitation source, while Auger electron spectroscopy uses electron beam as the excitation source.
As a typical surface analysis method, X-ray photoelectron spectroscopy (XPS) can provide the element content and morphology of the sample surface with an information depth of about 3-5 nm. Auger electron spectroscopy is a method of analyzing the surface of a material by Auger electrons ejected by an electron beam emitted by an electron gun, and is a highly sensitive analysis method with an information depth of 1.0--3.0 nm and absolute sensitivity. A 10-3 monoatomic layer is a useful analytical method.
3.4 Transmission electron microscopy and scanning electron microscopy have been widely used in the morphology analysis of nanomaterials. When people are interested in the elemental composition of a microdomain observed after imaging nanomaterials, they can be combined with electron microscopy and energy spectrum. The method collectively analyzes the situation of a certain micro-area. In addition, micro-analysis can be used to study microscopic phenomena such as inclusions, precipitates, and grain boundary segregation, which is very useful.
4. Structural analysis of nanomaterials
4.1. Structural characteristics of nanomaterials According to the structure of nanomaterials, nanomaterials can be roughly classified into four types, namely: nanostructured crystals or three-dimensional nanostructures (such as equiaxed microcrystals); two-dimensional nanometer structures (such as Nanofilms; one-dimensional nanostructures (such as nanotubes); and zero-dimensional clusters or clusters (such as nanoparticles with a particle size of no more than 2 nm). Nanomaterials include crystals, germanium crystals, amorphous metals, ceramics, and compounds. Nanomaterials have the following properties.
4.1.1. Small size effect When the size of the nanoparticle is equivalent to the wavelength of the light wave, the de Broglie wavelength of the conduction electron, and the physical feature size such as the coherence length or penetration depth of the superconducting state, the periodic boundary condition of the crystal will be Destruction, sound, light, force, electricity, heat, magnetism, internal pressure, chemical activity, etc. have a great change compared with ordinary particles, which is the small size effect (also called volume effect) of nanoparticles.
4.1.2. Surface and Interfacial Effects Due to its small size, large surface area, high surface energy, and the atoms on the surface are in a serious vacancy state, the nanoparticles are extremely active and unstable, and are quickly combined when encountering other atoms. This activity is the surface effect.
4.1.3. Quantum size effect When the crystallite size is equivalent to the de Broglie wavelength, the electron motion in the particle is limited in all three directions, and the continuous energy band of the electron is split to near the molecular orbital level, and the nanoparticle The acoustic, optical, electrical, magnetic, thermal, and superconductivity are very different from the macroscopic characteristics, called the quantum size effect.
4.1.4. Macroscopic Quantum Tunneling Tunneling refers to the ability of microscopic particles to penetrate the barrier. Later, it was discovered that some macroscopic quantities, such as magnetization and magnetic flux in quantum interference devices, also have tunneling effects, called macroscopic quantum tunneling effects. . Nanoparticles have tunneling effects similar to microscopic particles that can penetrate the barrier. This effect is also known as macroscopic quantum tunneling.
The nature of the material is closely related to the microstructure. There must be a close relationship between these special properties of nanomaterials and their structure. Therefore, it is very meaningful to study the microstructure of nanomaterials. Nanomaterials are mainly composed of nano-grain and grain interface, which have an important influence on the properties of nanomaterials. Studies on the microstructure of nano-grains show that the microstructure of nano-materials is basically consistent with the traditional crystal structure, but since each unit cell contains only a limited unit cell, the lattice lattice will inevitably have a certain degree of elasticity. Deformation.
4.2 Submicroscopic Characteristics of Phase Structure It has been known that not only the composition and morphology of nanomaterials have an important influence on their properties, but also the phase structure and crystal structure of nanomaterials have an important influence on the properties of materials. Therefore, the analysis of the phase structure of nanomaterials is also an important part of material analysis. The purpose of phase structure analysis is to accurately characterize the following submicroscopic features:
3. Composition analysis
3.1 Composition analysis methods and scope The properties of light, electricity, sound, heat and magnetism of nanomaterials are closely related to the chemical composition and structure of nanomaterials. Therefore, determining the elemental composition of nanomaterials and determining the type and concentration of impurities in nanomaterials is one of the important contents of nanomaterial analysis.
Nanomaterial composition analysis can be divided into two types according to the analysis object: micro sample analysis and trace component analysis. The micro sample analysis is in terms of the amount of sample. Trace component analysis is based on the content of the component to be tested in the nanomaterial. Because the content of impurities or doped components is very low, in the concentration range of one part per million or less, this type of analysis is called a trace. Quantitative component analysis.
The composition analysis method of nanomaterials is divided into bulk element composition analysis, surface component analysis and micro-area composition analysis according to the purpose of analysis.
Methods for analyzing the bulk elemental composition of nanomaterials and their impurity components include atomic absorption, atomic emission, ICP mass spectrometry, and X-ray fluorescence and diffraction analysis methods. The first three methods require the sample to be dissolved and then measured, so it is a destructive sample analysis method, and the X-ray fluorescence and diffraction analysis method can directly measure the solid sample, so it is also called non-destructive element analysis method.
3.2 X-ray fluorescence spectrometry (XFS) is a non-destructive analytical method that can directly measure solid samples, so it has great advantages in the analysis of nanomaterial composition. The energy or wavelength of X-ray fluorescence is characteristic and has a one-to-one correspondence with elements. Therefore, as long as the wavelength of the fluorescent X-ray is measured, the type of the element can be known, which is the basis of the qualitative analysis of the fluorescent X-ray. In addition, the intensity of the fluorescent X-rays has a certain relationship with the corresponding elemental content. According to this, quantitative analysis of elements can be performed.
The surface analysis methods for nanomaterials are currently the most commonly used X-ray photoelectron spectroscopy (XPS) analysis methods, Auger electron spectroscopy (AES) analysis methods, electron diffraction analysis methods and secondary ion mass spectrometry (SIMS) analysis methods. These methods can measure the chemical composition, distribution state and valence state of the nanomaterials, and the adsorption and diffusion reactions of the surface and interface. When the energy spectrum is combined with electron probe technology and scanning or transmission electron microscopy technology, The micro-area composition of nano-materials can be analyzed, so it is widely used in the composition analysis of nano-materials, especially the micro-area composition analysis of nano-films.
3.3 Electron spectroscopy mainly includes X-ray photoelectron spectroscopy and Auger electron spectroscopy. The common feature of the two methods is the method of analyzing the surface elements of the material based on the characteristic energy distribution (energy spectrum) of the electrons excited by the surface of the material. The main difference between the two is that the laser source used is different. X-ray photoelectron spectroscopy uses X-ray as the excitation source, while Auger electron spectroscopy uses electron beam as the excitation source.
As a typical surface analysis method, X-ray photoelectron spectroscopy (XPS) can provide the element content and morphology of the sample surface with an information depth of about 3-5 nm. Auger electron spectroscopy is a method of analyzing the surface of a material by Auger electrons ejected by an electron beam emitted by an electron gun, and is a highly sensitive analysis method with an information depth of 1.0--3.0 nm and absolute sensitivity. A 10-3 monoatomic layer is a useful analytical method.
3.4 Transmission electron microscopy and scanning electron microscopy have been widely used in the morphology analysis of nanomaterials. When people are interested in the elemental composition of a microdomain observed after imaging nanomaterials, they can be combined with electron microscopy and energy spectrum. The method collectively analyzes the situation of a certain micro-area. In addition, micro-analysis can be used to study microscopic phenomena such as inclusions, precipitates, and grain boundary segregation, which is very useful.
4. Structural analysis of nanomaterials
4.1. Structural characteristics of nanomaterials According to the structure of nanomaterials, nanomaterials can be roughly classified into four types, namely: nanostructured crystals or three-dimensional nanostructures (such as equiaxed microcrystals); two-dimensional nanometer structures (such as Nanofilms; one-dimensional nanostructures (such as nanotubes); and zero-dimensional clusters or clusters (such as nanoparticles with a particle size of no more than 2 nm). Nanomaterials include crystals, germanium crystals, amorphous metals, ceramics, and compounds. Nanomaterials have the following properties.
4.1.1. Small size effect When the size of the nanoparticle is equivalent to the wavelength of the light wave, the de Broglie wavelength of the conduction electron, and the physical feature size such as the coherence length or penetration depth of the superconducting state, the periodic boundary condition of the crystal will be Destruction, sound, light, force, electricity, heat, magnetism, internal pressure, chemical activity, etc. have a great change compared with ordinary particles, which is the small size effect (also called volume effect) of nanoparticles.
4.1.2. Surface and Interfacial Effects Due to its small size, large surface area, high surface energy, and the atoms on the surface are in a serious vacancy state, the nanoparticles are extremely active and unstable, and are quickly combined when encountering other atoms. This activity is the surface effect.
4.1.3. Quantum size effect When the crystallite size is equivalent to the de Broglie wavelength, the electron motion in the particle is limited in all three directions, and the continuous energy band of the electron is split to near the molecular orbital level, and the nanoparticle The acoustic, optical, electrical, magnetic, thermal, and superconductivity are very different from the macroscopic characteristics, called the quantum size effect.
4.1.4. Macroscopic Quantum Tunneling Tunneling refers to the ability of microscopic particles to penetrate the barrier. Later, it was discovered that some macroscopic quantities, such as magnetization and magnetic flux in quantum interference devices, also have tunneling effects, called macroscopic quantum tunneling effects. . Nanoparticles have tunneling effects similar to microscopic particles that can penetrate the barrier. This effect is also known as macroscopic quantum tunneling.
The nature of the material is closely related to the microstructure. There must be a close relationship between these special properties of nanomaterials and their structure. Therefore, it is very meaningful to study the microstructure of nanomaterials. Nanomaterials are mainly composed of nano-grain and grain interface, which have an important influence on the properties of nanomaterials. Studies on the microstructure of nano-grains show that the microstructure of nano-materials is basically consistent with the traditional crystal structure, but since each unit cell contains only a limited unit cell, the lattice lattice will inevitably have a certain degree of elasticity. Deformation.
4.2 Submicroscopic Characteristics of Phase Structure It has been known that not only the composition and morphology of nanomaterials have an important influence on their properties, but also the phase structure and crystal structure of nanomaterials have an important influence on the properties of materials. Therefore, the analysis of the phase structure of nanomaterials is also an important part of material analysis. The purpose of phase structure analysis is to accurately characterize the following submicroscopic features:
4.2.1 the size, distribution and morphology of the grains;
4.2.2. The nature of the grain boundary and phase interface;
4.2.3 crystal integrity and intercrystalline defects;
4.2.4 Composition and distribution across grain and intergranular boundaries;
4.2.5 Analysis of impurities in crystallites and grain boundaries.
In addition, the purpose of the analysis is to determine the structural properties of nanomaterials, providing an experimental basis for explaining the relationship between material structure and properties. At present, commonly used phase analysis methods include X-ray diffraction analysis, laser Raman analysis, and micro-region electron diffraction analysis.
4.3. New Progress in Structural Analysis of Nanomaterials There are quite a number of methods for structural characterization of materials. The instruments suitable for structural analysis of nanomaterials are not limited to the above, and new characterizations are constantly emerging. For example, high-resolution electron microscopy has been able to display atomic alignment and chemical composition at atomic resolution, tunnel scanning microscopy can determine atomic arrangement and electronic structure on the surface and near surface of materials, and low-energy electron microscopy can be used to display surface defect structures.
With the continuous development of analytical instruments and technologies, nanomaterial structure research institutes can use more and more test instruments, including high-resolution transmission electron microscopy (HRTEM), scanning probe microscopy (SPM), scanning tunneling microscopy (STM), and atomic force. Microscope (AFM), field ion microscope (FIM), X-ray diffractometer (XRD), extended X-ray absorption fine structure analyzer (EXAFS), Mössbauer spectrometer (MS), Raman scattering instrument (RS), etc. Wait. It can be considered that the research methods of nanostructures have almost all involved instruments for structural analysis of substances.
5. Nanomaterial surface and interface analysis
5.1 Nanomaterial surface and interface analysis methods The surface and interface analysis of solid materials has been developed into an important part of nanofilm material research, especially for elemental chemical state analysis, elemental three-dimensional distribution analysis and micro-area analysis of solid materials. At present, commonly used surface and interface analysis methods are: X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SIMS) and ion scattering spectroscopy (ISS). Among them, XPS accounts for 50% of the total surface composition analysis, AES accounts for 40%, and SIMS accounts for 8%. Among these surface and interface analysis methods, XPS has the widest application range and can be applied to the analysis of various materials, especially for the analysis of chemical state of materials, and is more suitable for research involving chemical information.
At present, commercial surface analysis spectrometers have a high vacuum of about 10-10 Torr. Ultra-high vacuum systems must be used in X-ray photoelectron spectroscopy and Auger electron spectroscopy, mainly for two reasons. First, XPS and AES are surface analysis techniques, if the vacuum in the analysis chamber is very high. Poor, cleaning the surface in a short time may be covered by residual gases in the vacuum. It is impossible to obtain true surface composition information without ultra-high vacuum conditions.
5. 2 XPS (X-ray Photoelectron Spectroscopy) analysis method
5.2.1 Qualitative analysis of surface elements This is the most common analytical method, generally using the wide scanning procedure of the XPS spectrometer.
5.2.2 Semi-quantitative analysis of surface elements First of all, it should be clear that XPS is not a good quantitative analysis method. It gives only a semi-quantitative result, ie relative content rather than absolute content.
5.2.3 Chemical valence analysis of surface elements The chemical valence analysis of surface elements is the most important analytical function of XPS, and it is also the most difficult part of XPS spectrum analysis, which is more prone to errors.
5.3 Auger electron spectroscopy Auger electron spectroscopy is one of the most widely used analytical methods. Its advantages are: high sensitivity in chemical analysis in the range of 0.5-2 years near the surface; fast data analysis; Auger Electron spectroscopy can analyze all elements except hydroquinone. It has become an important means of qualitative, semi-quantitative analysis, elemental depth distribution analysis and micro-area analysis of surface elements. The new Auger electron spectrometer has strong micro-analysis capability and three-dimensional analysis capability, and its micro-analysis diameter can be as small as 6 nm, greatly improving the analysis capabilities in microelectronics and nanotechnology. In addition, the Auger electron spectrometer has a strong chemical valence analysis capability, which can not only carry out elemental chemical composition analysis, but also elemental chemical valence analysis. Auger electron spectroscopy is one of the most important and commonly used methods of surface analysis and interface analysis. Due to its high spatial resolution (6 nm) and surface analysis capability (0.5-2 nm), Auger electron spectroscopy is especially suitable for surface and interface analysis of nanomaterials. Auger electron spectrometer has broad application prospects in the research of nanomaterials, especially nanodevices.
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