Volume effect

When the size of nano particles is equal to or smaller than the de Broglie wave of conducting electrons, the periodic boundary conditions will be destroyed, and the magnetism, internal pressure, light absorption, thermal resistance, chemical activity, catalysis and melting point have changed greatly compared with ordinary particles, which is the volume effect of nano particles. The following effects of nanoparticles and their various applications are based on their volume effects. For example, the melting point of nano particles can be far lower than the bulk, which provides a new process for the powder metallurgy industry; Using the property that the plasma resonance frequency shift changes with the particle size, the particle size can be changed, the absorption displacement can be controlled, and a microwave absorbing nano material with a bandwidth can be manufactured for electromagnetic shielding, stealth aircraft, etc

Surface effect

Surface effect refers to the change in properties caused by the sharp increase of the ratio of surface atoms to total atoms of nanoparticles with the decrease of particle size. Table 9-2 shows the relationship between the size of nanoparticles and the number of surface atoms

Table 1 The relationship between the size of nanoparticles and the number of surface atoms

It can be seen from the table that the number of surface atoms increases rapidly with the decrease of particle size. In addition, with the decrease of particle size, the surface area and surface energy of nanoparticles increase rapidly. This is mainly because the smaller the particle size, the more atoms on the surface. The crystal field environment and binding energy of surface atoms are different from those of internal atoms. The surface atoms lack adjacent atoms, have many dangling bonds, have unsaturated properties, and are easy for other atoms to combine and stabilize, thus showing great chemical and catalytic activity

Quantum size

When the particle size drops to a certain value, the electronic energy level close to the Fermi level changes from a quasi continuous level to a discrete level, which is called the quantum size effect. Kubo used an electron model to calculate the energy level spacing of metal ultrafine particles as 4Ef/3N

Where Ef is Fermi potential energy and N is the number of atoms in the particle. The N of macroscopic objects tends to be infinite, so the energy level spacing tends to be zero. Because the number of atoms of nanoparticles is limited and the N value is small, there is a certain value, that is, the energy level spacing splits. The electronic state of semiconductor nano particles transits from the continuous energy band of bulk material to the energy level with discrete structure as the size decreases, which is shown in the absorption spectrum, that is, from the wide absorption band without structure to the absorption characteristics with structure. The fluctuation of electrons in discrete quantized energy levels in nanoparticles brings a series of characteristics of nanoparticles, such as high optical nonlinearity, specific catalytic and photocatalytic properties

Quantum tunnel

The ability of microscopic particles to penetrate the barrier is called tunneling effect. It is found that some macroscopic quantities, such as the magnetization of micro particles, the magnetic flux of quantum coherent devices and the charge, also have tunneling effects. They can pass through the potential barrier of the macroscopic system to produce changes, so they are called macroscopic quantum tunneling effects. This concept can be used to qualitatively explain the super paramagnetism of ultrafine nickel particles at low temperatures

Dielectric limit

The dielectric confinement effect of nanoparticles is seldom noticed. In actual samples, particles are surrounded by media such as air, polymer, glass and solvent, and the refractive index of these media is usually lower than that of inorganic semiconductor. When irradiated by light, due to the different refractive index, an interface is generated, and the field strength of the area adjacent to the surface of the nano semiconductor, the surface of the nano semiconductor, and even the interior of the nano particle is increased compared with the light intensity of the radiation light. This local field strength effect has a direct impact on the photophysical and nonlinear optical properties of semiconductor nanoparticles. For inorganic organic hybrid materials and photocatalytic materials used in heterogeneous reaction systems, the dielectric confinement effect has an important influence on the reaction process and kinetics

The above-mentioned small size effect, surface effect, quantum size effect, macro quantum tunneling effect and dielectric confinement should all be the basic characteristics of nano particles and nano solids. This series of effects have led to nano materials showing special physical and chemical properties in many physical and chemical aspects, such as melting point, vapor pressure, optical properties, chemical reactivity, magnetism, superconductivity and plastic deformation. It makes nano particles and nano solids show many strange physical and chemical properties