Surface recombination and charged exciton in nanocrystal quantum dots on photonic crystals under two-photon excitation

Abstract

In this study, the two-photon excited fluorescence spectra from cadmium selenide quantum dots (QDs) on a silicon nitride photonic crystal (PhC) membrane under femtosecond laser irradiation were investigated. These spectra can be fit to a tri-Gaussian function in which one component is negative in amplitude, and in which the Gaussian components with positive amplitude are assigned to exciton emission and charged-exciton emission and that with negative amplitude is assigned to absorption from surface recombination. The photonic crystal enhance the charged-exciton emission and exciton emission and, at the same time, also the absorption from surface recombination. Both the charged-exciton emission and the surface recombination are related to Auger recombination; therefore, the photonic crystal controls both radiative recombination and non-radiative recombination. The asymmetries of the two-photon excited fluorescence spectra are due to not only the location of the resonant guide mode of the PhC slab but also the enhancement of the absorption from surface recombination by PhC.

Figure 1. Recorded and fitted two-photon excited fluorescence (TPF) spectra of QDs on a PhC with a lattice constant 560 nm.

(a), TPF spectrum fitted to a tri-Gaussian function with one negative amplitude under an excitation power of 4.64 mW. (b), The three fitted Gaussian components are shown separately; curves A₁ and A₂ correspond to the components with a positive amplitude, and curve A₃ corresponds to the component with a negative amplitude. The curves are shown together with their superposition (curve D) and the measured spectrum. (c), QD TPF spectrum fit to a tri-Gaussian function with one negative amplitude under an excitation power of 9.3 mW. (d), The three fitted Gaussian components in c are shown separately; curves A₁ and A₂ correspond to the components with a positive amplitude, and curve A₃ corresponds to the component with a negative amplitude. The spectra are shown together with their superposition (curve D) and the measured spectrum.

Figure 2. TPF spectrum and Gaussian fitting for QDs on a PhC with a lattice constant of 580 nm.

(a), TPF spectrum fitted to a tri-Gaussian function with one negative amplitude under an excitation power of 4.64 mW. (b), The three fitted Gaussian components are shown separately; curves A₁ and A₂ correspond to the components with a positive amplitude, and curve A₃ corresponds to the component with a negative amplitude. The spectra are shown together with their superposition (curve D) and the measured spectrum. (c), TPF spectrum fitted to a tri-Gaussian function with one negative amplitude under an excitation power of 9.3 mW. (d), The three fitted Gaussian components in (c) are shown separately; curves A₁ and A₂ correspond to the components with a positive amplitude, and curve A₃ corresponds to the component with a negative amplitude. The spectra are shown together with their superposition (curve D) and the measured spectrum. (e), Comparison of the fitted spectra to the tri-Gaussian under the excitation powers of 4.64 mW and 9.3 mW. (f–h), Fitting parameters for the tri-Gaussian function for different excitation intensities (subscripts ‘1', ‘2' and ‘3' indicate the long-, middle- and short-wavelength components, respectively): (f), centre wavelengths ω₁, ω₂ and ω₃; (g), spectral widths δω₁, δω₂ and δω₃; (h), ratios of components, A₁/(A₁ + A₂ + A₃), A₂/(A₁ + A₂ + A₃) and A₃/(A₁ + A₂ + A₃).

Figure 3. Fitting parameters for the tri-Gaussian function as a function of the PhC lattice constant.

(a), Centre wavelengths. (b), Ratios of the components.

Figure 4. Schematic diagram of the band structure of the QDs illustrating carrier trapping and optical transitions.

(a), exciton emission X⁰; (b), charged exciton (positive trion) emission X⁺; (c), negative trion emission X⁻; (d), Auger recombination; (e), photo-induced loss of the emission light due to surface recombination, in which the hole is driven to the surface state, and surface recombination takes place. The horizontal lines besides band gap represent the surface states, the horizontal dotted line represents the potential of QD boundary.