Observation of Distinct Two-Photon Transition Channels in CdTe Quantum Dots in a Regime of Very Strong Confinement.

Marcelo Gonçalves Vivas,Leonardo De Boni,José Leandro De Sousa,Cleber Mendonca,Marco Schiavon

doi:10.3390/ma10040363

Abstract

We report here on the direct observation of distinct two-photon transition channels in glutathione-capped (GSH) CdTe quantum dots (QDs) in a very strong confinement regime. CdTe-GSH QDs with different average diameters (2.5, 3.0, and 3.3 nm) were synthesized through the one-pot method and their two-photon absorption (2PA) spectrum determined by a femtosecond wavelength-tunable Z-scan. Our results show that the two lower-energy one-photon-allowed excitonic transitions, 1S3/2(h) → 1S(e) and 2S3/2(h) → 1S(e), are also accessed via 2PA. These results were ascribed to the relaxation of the parity selection rules due to the noncentrosymmetric structure of the CdTe QDs (zinc-blended structure), whose magnitude are determined by surface defects and structural irregularities present in CdTe-GSH QDs, in the strong confinement regime.

Highlights

Semiconductor quantum dots (QDs) are nanomaterials represented by a three-dimensionally confined electron-hole system
This work reports on the femtosecond 2PA cross-section spectra of water soluble colloidal CdTe-GSH QDs in the very strong confinement regime, emphasizing the influence of the noncentrosymmetric 2PA transition channel
It is important to mention that the QDs samples exhibit spherical shapes and size dispersion of approximately 25 %, as determined by Transmission Electron

Summary

Introduction

Semiconductor quantum dots (QDs) are nanomaterials represented by a three-dimensionally confined electron-hole system. Qu et al [25] reported a spherical eight-band Pidgen and Brown model that considers the mixing between the conduction and the valence bands, as well as the complex structure of the valence bands This model exhibits only 2PA transitions that are governed by the electric-dipole selection rules; it does not consider the “forbidden” 2PA transition, such as the 1S3/2 (h) → 1S(e) lowest-energy excitonic transition. In this context, this work reports on the femtosecond 2PA cross-section spectra of water soluble colloidal CdTe-GSH QDs in the very strong confinement regime, emphasizing the influence of the noncentrosymmetric 2PA transition channel. We reported the broadband 2PA cross-section spectra from 600 nm to 1220 nm, in 10 nm intervals, providing high spectral resolution to the nonlinear measurements

Results and Discussion

Materials and Methods

Final Remarks