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The core idea of reducing the band gap of a crystal is to control its electronic structure by introducing impurities, defects, or changing the lattice environment, thereby narrowing the energy gap between the valence band top and the conduction band bottom. Common methods include:
Elemental doping
Cation doping: Replace the lattice cations with metal ions of similar radius and different oxidation states, such as doping W⁶⁺ and Nb⁵⁺ in TiO₂ to introduce impurity levels; or use cations of narrow-band-gap semiconductors for doping, such as doping Cd²⁺ in ZnO to utilize the narrow band gap of CdS to lower the overall band gap.
Anion doping: Replace the lattice anions with non-metal ions, such as doping N³⁻ and S²⁻ in TiO₂ to form impurity levels in the band gap and reduce the band gap value.
Constructing heterojunctions / solid solutions
Form solid solutions between the target crystal and narrow-band-gap semiconductors, such as TiO₂ and SnO₂ forming Ti₁₋ₓSnₓO₂ solid solution, where lattice distortion enables the band gap to be continuously adjustable;
Constructing Type-II heterojunctions, where the energy bands of the two materials are interleaved, effectively reducing the energy level difference required for light response.
Defect engineering control
Introduce intrinsic defects such as oxygen vacancies and nitrogen vacancies, for example, reducing TiO₂ through reduction to generate oxygen vacancies, forming defect levels in the band gap and narrowing the band gap;
Control the crystal growth conditions (such as low oxygen pressure sintering), increase the concentration of lattice defects, and change the electron transition path.
Stress control
Apply stress to the crystal through methods such as epitaxial growth or high-pressure treatment, causing the lattice to undergo stretching or compression distortion, resulting in the movement of valence and conduction band energies, and thereby reducing the band gap. For example, applying tensile stress to ZnO films can slightly reduce the band gap.
