Interpretando o espectro de absorção de dois fótons em nanocristais semicondutores de CuInS2

Abstract

Nanocrystals (NCs) semiconductors (or quantum dots) are described as nanomaterials that present interesting optical properties due to the strong quantum confinement effect that generates features dependent on the size of the nanocrystal. Properties such as absorption and fluorescence induced by one or more photons are observed in these materials. Due to these unique characteristics, semiconductor nanocrystals generate great interest in the scientific community and are now studied due to their diverse applications, such as solar and / or photovoltaic cells, monitors and electronic devices, biomarkers, photonic devices, among others. Thus, this work aims to study the nonlinear optical response of the semiconductor nanocrystal of CuInS2 focusing on the two-photon absorption. By using the femtosecond-Z-Scan technique and the theoretical model based on parabolic effective-mass approximation. The size of the nanocrystals was estimated 4.7nm. with confinement regime of the material considering between intermediate to strong. In addition, the 1PA spectrum was analyzed, obtaining absorption of the first exciton band at 428 nm and emission at 532. This result was compared with other results from the literature for CuInS2 NCs with diameters of 4.7 nm. The results showed a great divergence. In order to elucidate this difference, measurements of energy dispersive X-ray spectroscopy were carried out, and it was verified that this difference in optical properties was related to the stoichiometry of the sample synthesized in this work (). Thus, in the theoretical model of effective-mass was used a phenomenological correction, which proved appropriate for the determination of the absorption spectrum of the material. In this work a comparison was also made with other works on A2F in CuInS2 with the intention of understanding how the structural properties interfere in the nonlinear optical response. With respect to the A2F spectrum, our results will show two 2PA allowed bands between 550 nm and 800 nm and a region of 2PA resonance enhancement effect. The 2PA cross-section obtained for this material was (6.8 ± 1.0) x 102 GM and (4.5 ± 0.7) x 102 GM for the bands at 715 nm and 620 nm, respectively. These results were corroborated by the effective-mass model of four parabolic bands. For this, in the theoretical model of 2PA was introduced a phenomenological correction (analogous to the linear effect) due to different stoichiometry obtained for the CuInS2 sample investigated in this work.