Studying the effect of CuInSe2 quantum dot size on the distribution of surface defects using positron annihilation spectroscopy

Abstract

In this work, chalcopyrite CuInSe₂ (CIS) quantum dots (QDs) with an average size of 3.3 and 7.6 nm as well as CIS nanoparticles (NPs) with an average size of 11.7 nm were synthesized using a hot injection method to study the relationship between particle size and surface defect density. X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and UV–Vis spectroscopy were used to characterize the synthesized samples. The UV–Vis absorption spectra reveal that by changing the size of the CIS particles, the band gap energy of the CIS particles can be precisely tuned. Furthermore, the dependence of surface defect density on particle size was explored using positron annihilation spectroscopy (PAS). The measured values of the lifetime component τ₂ related to positrons annihilating mainly at the surfaces of the 3.3 and 7.6 nm CIS QDs samples were found to be 428.3 ± 1.0 and 426.3 ± 1.8 ps, respectively. On the other hand, the τ₂ value for 11.7 nm CIS NPs is 376.6 ± 1.1 ps. These τ₂ values are higher than those of positrons annihilated at vacancy defects in CIS nanocrystals due to the relatively large contribution of positrons annihilated at surface defects of CIS QDs. The results of correlation studies reveal that the dependence of the size and concentration of surface defects on the size of CIS particles is inverse.