Flexoelectric and local heating effects on CdSe nanocrystals in amorphous As₂Se₃ films

Ternary CdS1–xSexnanocrystals formed in Cd‐doped As–Se–S films due to photoenhanced diffusion during micro‐Raman measurement

“…Such assumption is quite reasonable because for amorphous As 2 Se 3 and As 2 S 3 films, it is well known that under illumination by a tightly focused laser beam at such power densities, the film viscosity drastically decreases and the film material moves away from the laser spot with a pit in its center being formed, clearly confirmed by AFM, scanning electron microscopy (SEM), and optical microscopy. [ 18–20,45 ] In our case, the formation of such a pit is confirmed by optical images as well as by the Raman peak from the silicon substrate (521 cm −1 ) emerging in the spectra at high P exc ≥ 420 kW/cm 2 because of a reduced As–Se–S:Cd film thickness (Fig. 3).…”

“…It is known from our earlier studies of As 2 Se 3 and As 2 S 3 films that the depth of the pit on the surface at such P exc values typically reaches 400–500 nm [ 18,20,45 ] and a rim as high as 80 nm is formed around the pit on the film surface extending by several micrometers. [ 19,45 ] The pit formation is caused not by photoinduced or thermoinduced ablation (evaporation of the film material) but by a non‐thermal effect—intense directional material transfer due to the strong gradient of illuminance. There can be no doubt that at the high‐ P exc Raman measurement, the illumination‐induced local photofluidity in the laser spot leads to a strong and rapid mixing of the As 2 (Se 0.5 S 0.5 ) 3 :Cd film material, which destroys the surface layer initially containing CdS 1– x Se x crystallites.…”

“…Hence, one may suppose that during the high‐ P exc measurement, an additional amount of CdS 1– x Se x crystallites is formed in the laser spot due to local photoenhanced diffusion, which facilitates aggregation of Cd, S, and S atoms in crystallites. Localized formation of binary II–VI nanocrystals under illumination is well known for Cd‐ or Zn‐doped amorphous As 2 Se 3 and As 2 S 3 , [ 18–21,45 ] and it is quite probable that, governed by a similar mechanism, CdS 1– x Se x crystallites are formed under high‐ P exc illumination in the As–Se–S:Cd films. It is essential to emphasize that the mechanism responsible for the II–VI nanocrystal formation in the As–Se–S:Cd films is not due to thermal annealing caused by local heating of the film surface by the laser beam.…”

“…It is essential to emphasize that the mechanism responsible for the II–VI nanocrystal formation in the As–Se–S:Cd films is not due to thermal annealing caused by local heating of the film surface by the laser beam. Our recent studies showed that under illumination by a laser beam of similar wavelength, power density, and duration the film surface temperature in the laser spot increases by about 100 K. [ 45 ] No melting or ablation can occur at such a temperature. This correlates with in situ studies of the transient component of photostimulated deformation of As 2 S 3 glass surface under 532 nm laser beam illumination where non‐thermal and thermal contributions into the illuminated film thickness increment were separated and local temperature increase was thereby evaluated.…”

“…[ 67 ] Our experiments with films annealed up to 433 K demonstrated that such temperature is not sufficient to result in structural changes that can be evidenced by Raman spectroscopy. [ 45 ] It is a non‐thermal effect of photosoftening or photofluidity, a drastic decrease of the film viscosity in the illuminated area. [ 4,68 ] Photofluidity in non‐crystalline chalcogenides is related to an increasing mobility of a considerable fraction of atoms in the course of exciton self‐trapping and at their recombination.…”