Implementação de técnicas experimentais para a caracterização das respostas ópticas lineares e não lineares em nanomateriais semicondutores

Abstract

Semiconductor nanocrystals (NCs) are zero-dimensional materials that have called great attention in recent years due to their exceptional linear and nonlinear tunable optical properties. For example, cadmium-based NCs have high fluorescence quantum efficiency (> 0.8), high one-photon absorption cross section (1PA, 10-15 cm2) and two photons (2PA, 106 Goeppert-Mayer) and excited state (10-15 cm2), and a band gap that covers the entire visible spectrum (quantum confinement effect). Thus, semiconductor NCs are excellent candidates for numerous applications, such as: solar panels, electronic devices, biological mappers, to name a few. However, quantifying the linear and nonlinear optical properties of NCs is not an easy task, because it is very difficult to accurately obtain the concentration of NCs in a solution, for example. As a consequence, in the last two decades there has been a great conflict of data reported in the literature for the magnitude of linear and nonlinear optical responses of nanocrystals (for the same NC). Thus, this dissertation aims to implement experimental optical techniques to quantify: (i) the concentration of NCs in solution (measure the 1PA cross section); (ii) excited state absorption; (iii) the two-photon absorption cross section; (iv) to study the fluorescence photobleaching mechanism of NCs when excited by nanosecond laser pulses. As a proof of concept, we made use of CdTe NCs due to the huge volume of data available in the literature for this material. In this way, it was initially developed from the PL saturation technique, which consists of a simple optical setup and with accurate results to measure the one-photon cross section of semiconductor NCs of different sizes. The results show 1PA cross section are on the order of 10-16 cm2 for NCs of CdTe with diameters between 2.2 and 3.5 nm and the concentrations are around 1015 NCs / cm3. Regarding the suppression of the photoluminescence of CdTe NCs when exposed to pulses of high irradiance laser light, our results suggest that the 532 nm laser light pulse generates a CdS shell around the CdTe nucleus via two different mechanisms that depend on the size of the nanocrystal. For the nonlinear measurements, the Z scan technique was developed to quantify the cross section of the excited state of the NCs. Our results show that for the excitation at 532 nm, the cross section of the excited state is about 20% smaller than that of the ground state, resulting the saturated absorption effect. Finally, using the fluorescence technique induced by two-photon absorption, we obtained the magnitude of the 2PA cross section for CdTe NCs and results that are in good agreement with results reported in the literature using femtosecond laser pulses.