Aluminum-Doped ZnO Nanopyramids to Nanoplatelets: A Defect-Engineered Route to Transparent Infrared Plasmonic Films

Plasmonic aluminum-doped zinc oxide (AZO) nanocrystals were synthesized via a colloidal hot-injection technique. The successful aliovalent substitution of Zn²⁺ by Al³⁺ in the ZnO lattice significantly enhanced free carrier concentration in the conduction band, addressing a key challenge in developing high-conductivity transparent conductive oxides (TCOs). The use of oleylamine as a polar capping agent enabled kinetic control over crystal growth, stabilizing thermodynamically unfavorable facets and yielding well-defined hexagonal pyramidal nanocrystals. With increasing Al content, the morphology gradually transformed into nearly hexagonal nanoplatelets. Owing to the inherently low defect formation energy in ZnO, multiple defect-related emission bands—including blue, green, and yellow—were observed. As defect formation energy is low for ZnO system, which results in a large amount of oxygen vacancies spontaneously. When aliovalent Al is put as dopant, an excess amount of electrons is generated abruptly with initial intuition of excess charge compensation but these vacancies actually trigger excess electrons to accumulate in the conduction band. Rather, the charge compensation mechanism helps to form other defects like zinc vacancies and interstitials. High-quality, continuous thin films fabricated from the colloidal ink exhibit excellent optical transparency and electrical conductivity in the visible range, offering a promising low-cost alternative to commercial indium tin oxide.