[en] ZnO together with TiO₂ is a main photocatalyst for various redox reactions to convert light energy into a chemical one and to purify the environment. Intrinsic surface defects in ZnO—the vacancies in anionic and cationic sublattices (F-type and V-type centers)—allow creation of long-lived (up to 10³ s) photocatalysis centers and, therefore, tenfold increase in quantum yield of reactions. Slow surface states—the photocatalysis centers—appear via diffusion of electrons and holes generated during the interband transitions in the bulk of a photoactivated sample. The transfer efficiency, however, decreases sharply because of recombination of charge carriers and losses during overcoming the surface Schottky barrier. Neutral energy carriers—excitons—were used in this work to decrease these losses during the energy transfer to a surface. High exciton binding energy in ZnO (60 meV) allows it to move at room temperature without decay. The exciton energy loss for radiation is effectively decreased in our experiments via formation of a 2D surface structure. The results confirm high efficiency of exciton channel to form surface long-lived photocatalysis F-centers and V‑centers during the photoadsorption and photodesorption processes of oxygen, which simulate full cycle of a redox photocatalytic reaction.

[en] Highlights: • ZnO nanoparticles were obtained by laser breakdown nearby a surface of zinc. • Self-organized nanoparticles were produced in forms of tubes and others. • They exhibited low threshold random lasing under intense photo excitation. • The lasing is a result of electron scattering on electron-hole pairs correlated. • Self-organization assists in greater light scattering and guiding. -- Abstract: A simple laser-assisted method to transform metallic Zn to a ZnO-based random lasing medium is reported. The method uses Nd⁺³ laser-induced air breakdown to treat the surface of a Zn target in oxygen atmosphere, and thus produce self-organized nanoparticles (tubes, needles etc.) over nearby area. Room temperature photoluminescence spectra of the nanoparticles exhibit the emission band at 395 nm with semi-regular narrow spikes (<1 nm) with low threshold and non-linear growth with power greater 2 at photo excitation increase, which is typical for multi-mode lasing. Estimation of electron-hole pair concentration and temperature dependence of the average lasing energy prove that the lasing is originated from a scattering of free electrons on electron-hole pairs correlated. It was shown that self-organized nanoparticles exhibited better conditions for the lasing through stronger light scattering and guiding.

[en] The effect of intense UV irradiation on the photoluminescence (PL) spectra of ZnO powders and nanocrystalline films obtained by atomic layer deposition (ALD) was investigated. At room temperature, the behavior of the spectra under continuous UV irradiation in multiple vacuum-atmosphere cycles was studied. The changes in the intensities of exciton radiation and radiation in the “green” band region, associated with the phenomena of oxygen photodesorption and photoadsorption, are discussed. In the temperature range of 5–300 K, the effect of strong UV irradiation on the near-edge luminescence spectrum of ZnO films was studied. The nature of a new line arising in the photoluminescence spectra of an irradiated film in the region of emission of bound excitons is discussed.

[en] The exciton photoluminescence of zinc oxide is studied in the temperature range 5–300 K. The formation of the exciton emission band profile in the spectra of undoped crystals, high-purity powder, and thin ALD films 4–450 nm in thickness at 300 K and at excitation by a He–Cd laser is analyzed. The effects of the photodesorption and photoadsorption of atmospheric oxygen on the exciton emission spectrum of a ZnO powder in sequential vacuum–air cycles have been studied at room temperature during continuous excitation by a He–Cd laser.

[en] The kinetics of near-edge photoluminescence (PL) in ZnO nanofilms prepared by the atomic layer deposition has been investigated. It is established that the kinetics of near-edge PL in 4-nm films is determined to a great extent by surface 2D-exciton (SX) and biexciton (SXX) complexes. The contribution from surface biexcitons is estimated based on a photostimulated change in the surface potential in ZnO films with different thicknesses. Ultrafast dynamics of surface biexcitons in thin films are revealed. It is shown that biexcitons localized near the surface have the shortest radiative lifetime (less than 100 ps) among all bound exciton complexes, which is explained by the large oscillator strength.