In 1959, the famous physicist and Nobel laureate Richard Feynman predicted in a speech titled “There is Plenty of Room at the Bottom” that in the future human beings can Using miniaturized instruments to manipulate and measure the properties of materials with nanostructures will eventually turn into arranging atoms one by one and making products according to human will. This is the earliest dream of human beings about nanotechnology. Today, nanotechnology has involved many fields such as physics, chemistry, materials, biology and medicine. In the face of various “nano products”, nanomaterials are quietly changing human life and guiding human beings into a new “nano product”. era”. As the famous Chinese scientist Qian Xuesen once predicted: “Nano will bring about a technological revolution, which will lead to another industrial revolution in the 21st century”.
In a narrow sense, nanomaterials refer to ultrafine solid materials composed of nanoparticles with a particle size of less than 100 nm; in a broad sense, nanomaterials refer to materials with at least one dimension in three-dimensional space less than 100 nm in size or use them as the basic structure. The material the unit is made of.
Nanomaterials can be divided into zero-dimensional nanomaterials, one-dimensional nanomaterials and two-dimensional nanomaterials. Zero-dimensional nanomaterials refer to materials that are below 100 nm in all three dimensions of space, that is, nanoparticles, atomic clusters, etc. One-dimensional nanomaterials refer to materials below 100 nm in both dimensions, namely nanowires, nanorods, nanowires, nanoribbons, and nanotubes. Two-dimensional nanomaterials refer to materials that have one dimension in the nanometer size, such as ultra-thin films, graphene, etc.
The properties of nanomaterials are closely related to the property that at least one dimension is smaller than 100nm. To put it figuratively, the length of 1nm is equivalent to one ten-thousandth of the diameter of a human hair. This scale is located at the junction of the microscopic world represented by atoms and molecules and the macroscopic system represented by human activities, and is the intermediate link connecting these two important parts. Therefore, nanomaterials exhibit many unique properties and new laws, such as quantum size effect, small size effect, surface effect, macroscopic quantum tunneling effect and dielectric effect.
(1) Quantum size effect
As the particle size decreases, photogenerated electrons and holes are subject to spatial confinement effects, and these carriers occupy orbitals with higher energies than those of bulk materials. When the particle size decreases to a certain value, the electron energy near the Fermi level of the metal changes from quasi-continuous to discrete, and the semiconductor continuous energy band (valence band and conduction band) becomes discrete energy level structure and band gap change. This phenomenon is known as the quantum size effect. When the energy level spacing is larger than thermal energy, photon energy, magnetic energy, electrostatic energy or superconducting state condensation energy, the magnetic, optical, acoustic, thermal, electrical and superconductivity of nanoparticles will be significantly different from that of bulk materials.
(2) Small size effect
When the particle size is comparable to or smaller than the wavelength of the light wave, the de Broglie wavelength, and the physical properties such as the coherence length or transmission depth of the superconducting state, the crystalline periodic boundary condition will be destroyed; near the particle surface layer of amorphous nanoparticles Atomic density is reduced, leading to new small-size effects in acoustic, optical, electrical, magnetic, thermal, and mechanical properties. Nanoparticles are small in size and large in specific surface area, and have undergone great changes in melting point, magnetic properties, electrical properties and optical properties.
(3) Surface effect
When the size of the particle is reduced to the nanometer scale, the number of surface particles, surface area and surface energy will increase greatly. Due to the increase in the number of atoms on the surface of the nanoparticle, insufficient atomic coordination and high surface energy, the nanoparticle has a large amount of Surface defects. These defects will introduce many surface states into the energy-forbidden band gap, which will become electron or hole traps, and seriously affect the optical, photochemical, electrochemical and nonlinear optical properties of the particles. Due to the large specific surface area and a large number of surface defects, these atoms are stabilized by combining with other atoms, so they have high chemical activity.
(4) Macroscopic quantum tunneling effect
Tunneling refers to the ability of microscopic particles to penetrate a potential barrier. In recent years, it has been found that some macroscopic physical quantities, such as the magnetization of microparticles and the magnetic flux in quantum coherent devices, all exhibit tunneling effects, which are called macroscopic quantum tunneling effects.
(5) Coulomb blocking effect and quantum tunneling
When the system scale enters the nanometer, the system is “quantized” in charge, that is, charging and discharging are discontinuous, and the energy required to charge an electron is e2/2C (C is the system capacitance), which leads to a small system charge. During the discharge process, electrons cannot be transported collectively, but are transported one by one, which is called the Coulomb blockage effect. Only when the voltage V>e/C applied to a quantum dot can a single electron on one quantum dot pass through the energy barrier to another quantum dot, this behavior is called quantum tunneling. Due to the existence of the Coulomb blockage effect, on the I-V curve, the current presents a sawtooth-like step with the rise of the voltage. This special electrical property of nanoparticles becomes the basis of microelectronics.
(6) Mechanical properties effect
Due to the refinement of nanoparticles, the number of grain boundaries or phase boundaries is greatly increased, which can greatly improve the strength, toughness and superplasticity of the material. The response of its structured particles to light, mechanical stress and electricity is completely different from that of micron or millimeter-scale structured particles, which makes nanomaterials show many wonderful properties at the macroscopic level. Adding nanoparticles to polymer materials can greatly improve their strength and toughness. Nanoparticles (such as SiO2, CaCO3, etc.) have small size, large surface area, large contact area between filler and matrix, and many surface active centers of nanoparticles, which can be closely combined with the matrix and have good compatibility. The role of nano-scale particles in the matrix is to maximize the potential of the matrix itself. When the composite is subjected to external force, the particles are not easily separated from the matrix, and because of the interaction of the stress field, a lot of micro-deformation occurs in the matrix. It absorbs a lot of energy, which determines that it can not only transmit the external stress, but also cause the matrix to yield, consume a large impact energy, and achieve the effect of strengthening and toughening at the same time.