Reappraising the Luminescence Lifetime Distributions in Silicon Nanocrystals

Matthias Jakob,William Morrish,Alkiviathes Meldrum,Amira Aissiou,Aras Kartouzian,Muhammad Islam,Frank Marsiglio

doi:10.1186/s11671-018-2785-x

Matthias Jakob, William Morrish + Show 5 more

Open Access

https://doi.org/10.1186/s11671-018-2785-x

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Abstract

The luminescence dynamics in ensembles of nanocrystals are complicated by a variety of processes, including the size-dependence of the radiative and non-radiative rates in inhomogeneous broadened samples and interparticle interactions. This results in a non-exponential decay, which for the specific case of silicon nanocrystals (SiNCs) has been widely modeled with a Kohlrausch or “stretched exponential” (SE) function. We first derive the population decay function for a luminescence decay following exp[− (t/τ)β]. We then compare the distributions and mean times calculated by assuming that either the luminescence decay or the population decay follows this function and show that the results are significantly different for β much below 1. We then apply these two types of SE functions as well as other models to the luminescence decay data from two thermally grown SiNC samples with different mean sizes. The mean lifetimes are strongly dependent on the experimental setup and the chosen fitting model, none of which appears to adequately describe the ensemble decay dynamics. Frequency-resolved spectroscopy (FRS) techniques are then applied to SiNCs in order to extract the lifetime distribution directly. The rate distribution has a half width of ~ 0.5 decades and mainly resembles a somewhat high-frequency-skewed lognormal function. The combination of TRS and FRS methods appear best suited to uncovering the luminescence dynamics of NC materials having a broad emission spectrum.

Highlights

Colloidal nanoparticles can be used in a manifold of applications including catalysis, medical treatments, and optoelectronic applications [1–4]
The sizes were further checked by high-resolution imaging of selected NCs (Fig. 2b, e), where the lattice fringes could be used as another way to identify the NCs and estimate their diameters
The Fourier transform infrared (FTIR) spectroscopy and XPS data showed that the prepared Silicon nanocrystals (SiNCs) were successfully functionalized with dodecene; the small SiNCs are more oxidized than the large ones and show a smaller degree of functionalization (Additional file 1: Figures S1 and S2)

Summary

Introduction

Colloidal nanoparticles can be used in a manifold of applications including catalysis, medical treatments, and optoelectronic applications [1–4]. Semiconductor nanoparticles are of particular interest for light emission, photovoltaic, and photocatalytic applications [5, 6]. Silicon nanocrystals (SiNCs) are a focus of current attention owing to the tunable emission properties [7] as well as the abundance and biocompatibility of silicon [8]. The origin of the SE luminescence decay in silicon and other semiconductor quantum dots has been heavily debated in the last two decades, and the debate has continued recently [10]. Various explanations have been proposed for the appearance of the SE in the decay dynamics, including carrier tunneling and trapping in closely spaced ensembles of nanocrystals [11], the inhomogeneously broadened size distribution [12], size-dependent electron-phonon coupling [10], and a distribution of barrier heights for non-radiative recombination [13], the latter being similar to a previous suggestion for porous

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