Color generation mechanism of semiconductors and insulators
The essential difference between conductors and semiconductors and insulators is the absence of a band gap. In a conductor, the valence band and the conduction band overlap. There is no gap between them. Electrons move easily. However, in semiconductors and insulators, there is a gap between their valence band and guide band. The gap width of different materials is different, and the gap width of insulators is much wider than that of semiconductors. In a semiconductor, valence band electrons absorb enough energy to easily pump the conduction to the conduction band. In an insulator, valence band electrons cannot obtain energy beyond the gap width from thermal excitation and jump to the conduction band. Therefore, there are few electrons in the conduction band of an insulator, so there is almost no conduction.
Semiconductors and insulators found in nature usually include ceramics and polymers, which have band gaps. The wavelength range of visible light is 380-760nm, and the photon energy of the most energetic violet light is about 3.2ev. When the band gap of the insulator exceeds 3.2eV, the insulator will not absorb any visible light, which can be transmitted completely.At this point, the color of the insulator material will be mainly affected by its microstructure. For example, monocrystalline alumina is transparent; Polycrystalline alumina, due to the different refractive index of different grains, makes the incident light scattering in the process of internal propagation of the crystal, which reduces the transparency of the material; Polycrystalline or porous alumina is completely opaque due to the greater scattering of incident light. The material is white.
Typical ionic compounds can only absorb the light in the ultraviolet region with higher frequency, but not the light in the optic region. Therefore, typical ionic compounds are usually colorless or white solids. When the metal cation and anion of the compound polarize with each other, the electron cloud overlaps to a certain extent and shows a certain covalency. When the covalency of the compound reaches a certain degree, it absorbs part of the colored light, making the compound appear a certain color. With the increase of the covalency of the compound, the absorption range of visible light increases, and the color of the compound gradually deepens. Its covalency depends on the polarization and deformability of metal cations and anions. If the polarization and deformation of ions are large, the compound has strong covalency and dark color.
In general, unpaired electrons are more likely to absorb energy and jump than paired electrons, so most colored materials we encounter contain unpaired electrons, such as Fe3+, Cu2+, etc. And we notice that the transition metal ions have rich colors because they meet both of the above conditions:
(1) contain unpaired electrons;
(2) The energy difference between orbitals after the splitting of electron orbital energy level falls within the range of visible light energy.
Common transition metal ions often exist in the form of complexes, such as hydrates, ammonia complexes, cyanide complexes, etc. They are often accompanied by the splitting of d orbital energy levels in the process of forming complexes, which is related to the configuration of complexes and the properties of ligands themselves. The d electron originally had five equal energy orbitals: dxy dyz DZX dx2-y2 dz2. The first three of them are at an Angle of 45 degrees with the coordinate axis. Dx2-y2 is on the x-y plane and in the same direction with the coordinate axis. Dz2 is dumbbell shaped with ring and extends along the z axis.
These five kinds of orbitals are affected and restricted by ligands to varying degrees in the process of forming complexes. For example, considering the octahedral coordination configuration hydrate with a coordination number of 6, since the ligand is just located in the three axis directions of the central ion, that is, in conflict with the extension direction of dx2-y2dz2, then the two d orbitals of the central ion are repulsive by the negative charge of the ligand and the energy increases significantly.The other three d orbitals stagger with the ligand, and the energy change is much smaller than that of dx2-y2dz2. The d orbital of the central ion is then split into two groups: the relatively high energy dx2-y2dz2 and the relatively low energy dxy dyz DZX, whose energy difference (between 1.99· 10-19j and 5.96· 10-19j) can partially fall within the visible light range (5.5·10-19 and 3.0·10-19). D electrons easily jump between these two sets of orbitals, producing the color of light that the human eye can perceive.
Organic polymer materials, however, are mostly a class of compounds that are covalent bonds. Saturated organic molecules composed entirely of bonds are relatively solid in structure and require higher energy to excite electrons. Therefore, the absorbed light wave is in the far ultraviolet region with higher frequency, which determines that saturated organic compounds formed by bonds are colorless.
2.3 the color nature of blocks and powders
In short, the essence of the color of bulk matter, whether conductor, semiconductor or insulator, is the selective absorption of visible light.
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