Size- and temperature-dependence of exciton lifetimes in CdSe quantum dots

Abstract

In this work we have investigated the temperature-dependence of the band-edge photoluminescence decay of efficiently luminescing organically capped CdSe quantum dots ͑QDs͒ with diameters ranging from 1.7 to 6.3 nm over a broad temperature range ͑1.3-300 K͒. The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime ͑below ϳ50 K͒ is characterized by purely radiative decay and can be modeled by a thermal distribution between a lower dark and a higher bright exciton state, with a size-dependent energy separation ͑viz., from 0.7 to 1.7 meV͒ and dark exciton lifetime ͑viz., from 0.3 to 1.4 s for QDs ranging from 6.3 nm to 1.7 nm in diameter͒. Nonradiative relaxation processes become increasingly important above ϳ50 K until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above ϳ200 K.

Exciton Storage by Mn2+ in Colloidal Mn2+-Doped CdSe Quantum Dots

“…This excitonic PL decay is multiexponential, suggesting a distribution in the rates of nonradiative processes, 16,17 but it can be fitted with two major components: τ exc,1 ) 39 ns (∼35%) and τ exc,2 ) 136 ns (∼65%), in reasonable agreement with 5 K PL decay times measured for undoped CdSe QDs of comparable diameter. 15 The comparison of Figure 2b,c shows that shifting the exciton to above the Mn 2+ 4 T 1 excited state quenches the 5 K excitonic PL via rapid energy transfer to Mn 2+ , whereas shifting the exciton to below the Mn 2+ 4 T 1 excited state leads to excitonic emission with excitonic PL decay times similar to those of the undoped analogues, even in cases of relatively high Mn 2+ concentrations such as the sample shown in Figure 2c. Overall, these results thus agree well with those found for MBE-grown Mn 2+ :CdSe QDs, 2,13 indicating that the colloidal QDs under consideration here are also representative of those grown by MBE.…”

“…In the latter, the longest exitonic PL decay times are τ exc ∼ 1.25 µs at T < ∼2.5 K, with τ exc only ∼15 ns at 185 K. 15,18 The gradual lengthening of τ exc below ∼100 K could be reproduced using a thermal equilibrium model similar to eq 1a, with dark-bright energy gaps of only ∼0.5 -2.0 meV and dark exciton lifetimes of only 0.3-1.4 µs, both size dependent. 15 Because the dark-bright energy gaps are relatively small, the action of the dark state as a reservoir feeding the bright state is not as clear as in the present Mn 2+ :CdSe QDs, where the reservoir is the 4 T 1 excited state of Mn 2+ and the relevant energy gap can be tuned over a much broader range by changing the CdSe QD diameter (Figure 1 inset). Additionally, whereas the dark-exciton lifetime is sensitive to nanocrystal shape anisotropy and size, the radiative lifetime of Mn 2+ in CdSe (∼273 µs, Figure 2b) is determined primarily by spin-orbit coupling, and secondarily by Mn 2+ -Mn 2+ exchange interactions (negligible at the present Mn 2+ concentrations), and it is therefore independent of nanocrystal shape and size.…”

“…The slower component derives from the subset of QDs containing no Mn 2+ ions and is similar in magnitude to the 5 K PL decay times measured for undoped CdSe QDs of comparable diameter. 15 At this low dopant concentration, the average number of Mn 2+ ions per nanocrystal is ∼0.4. Most QDs thus contain either 0 (67%) or 1 (28%) Mn 2+ ion.…”

Outermost tensile strain dominated exciton emission in bending CdSe nanowires

“…The later PL and CL spectra of the bending CdSe nanowires also demonstrate that the luminescence spectra are from exciton radiative recombination. The excitons of wurtzite CdSe crystal have long lifetime (~1 ns in bulk at 4 K [30,31] and ~1 μs in quantum dots below 10 K [32,33]) and strong exciton diffusion effect with diffusion length of about 1 μm at 10 K [34]. Therefore, during the lifetime before the exciton recombination, the bandgap gradient will drift the excitons generated in the bending region towards the outer edge, resulting in the aggregative flow of excitons in bending CdSe nanowires, as schematically shown in Fig.…”

Electronic structure of atomically coherent square semiconductor superlattices with dimensionality below two

“…In contrast, bulk PbSe has a small gap of 280 meV at 300 K 17 at the L point of the Brillouin zone, which makes it interesting for near-infrared optoelectronics applications. From a theoretical viewpoint, the fundamental properties of CdSe and CdTe NCs have been studied for several decades, [18][19][20][21][22][23] 18,24 ensuring transferability of the parameters for investigating the electronic structure of semiconductor nanostructures. 33 Presently used tight-binding parameters for PbSe are taken from Ref.…”