Thermally Activated Photoluminescence in Lead Selenide Colloidal Quantum Dots

Abstract

The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground-state exciton emission are discussed. Activation of a dark exciton occurs at 1.4-7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100-200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark-bright-state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be approximately 6-12 micros at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.

The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum Dots

“…From a single exponential fit to the decay curves of a 4.6 nm PbSe QD sample (see Figure 4a), we obtain an experimental lifetime 0 of 0.969 Ϯ 0.002 s, 0.979 Ϯ 0.002 s, and 1.067 Ϯ 0.002 s respectively at a wavelength of 1375, 1425, and 1500 nm. These results agree with the 1.04 s measured by Kigel et al 15 for 4.7…”

Temperature-Dependent Optical Properties of Colloidal IV-VI Quantum Dots, Composed of Core/Shell Heterostructures with Alloy Components

“…The cw-PL spectra of the smallest PbSe cores CQDs are dominated by a single exciton band over the entire temperature range, similar to the observation found in Ref. [21]. However, the emission spectra of the larger PbSe cores occasionally exhibit a split band at elevated temperatures.…”

“…39 However, the trend is interrupted in two distinct points: (i) occurrence of unusual climax in the intensity profile at a temperature between 150 K to 250 K, appearing at higher temperature in c/sh and c/a-sh, compared with that of the primary cores, but with a very small effect in a-c/a-sh CQDs. This abnormal climax was previously explained 21 as a thermal activation between dark and bright states, with activation energy (E a ) close to the LO phonon energy (LO (PbSe) = 16.8 meV, LO (PbS) = 26 meV). The values of E a of the investigated samples are listed in Table 3, spanning a range that is in close agreement to the suggested theoretical values of the dark-bright splitting in pure PbSe cores; 27 (ii) unexplained a minor decrease of the intensity <10 K with an activation energy ~ 0.4 meV, way below the acoustic phonon energy (LA, TA ~ 4-6 meV).…”

“…We take this evidence as proof that the dual band is an intrinsic property of the electronic structure of the samples. 27,30 and exsperimentally 21,23,29,[31][32][33] . These studies have suggested that splitting within the exciton fine structure near the band gap may be responsible 23,27,29 .…”

PbSe/CdSe Thin-Shell Colloidal Quantum Dots

“…Inorganic core/shell heterostructures were formed either by epitaxial coating a semiconductor core by another semiconductor shell (e.g., CdSe/CdS and PbSe/PbS CQDs) or by a cation-exchange process, involving diffusion of foreign cations from the outer to the inner CQD layers (e.g., PbSe/CdSe and PbS/CdS CQDs) [29][30][31][32][33][34]. Our studies demonstrated the advantage of PbSe/PbSe x S 1−x [29,[35][36][37][38][39][40][41][42] and CdTe/CdTe x Se 1−x [43,44] alloyed heterostructures, where the alloyed interface reduced the crystallographic and dielectric mismatch between the core and the shell and also created a smooth boundary potential, which suppressed the Auger non-radiative relaxation processes in the CQDs.…”