Fitting of the fluorescence lifetimes for fluorophores can be achieved using various techniques. Problems arise when the fluorophore sample itself is heterogeneous and the fluorescence decay is far from exponential, as a single sample may have a distribution of lifetimes. We study the fitting of fluorescence lifetimes for two systems, a homogeneous and near-exponential fluorescein dye and a highly heterogeneous system of graphene quantum dots. We fit a variety of different models to experimental time-correlated single photon counting fluorescence data from each system, evaluating the validity of each model for the two systems. We find that, for the near-exponential system, there is little difference between the fit of each model. However, for strongly nonexponential behavior, the models give quite different estimates of mean and standard deviation, indicating that appropriate model choice and assessment is crucial in obtaining meaningful lifetimes from fluorescence lifetime data. Moreover, we perform simu...

Fluorescence Lifetime Analysis of Graphene Quantum Dots

Most of the highly efficient luminescent silicon nanocrystals (SiNCs) reported to date consists of organically capped silicon cores. Here, we report a method of obtaining Si/SiO2 core/shell nanoparticles emitting at a peak energy of 1.5 eV with very high quantum yields (53-61%). The same method led to quantum yields of ~ 30% for porous silicon powder emitting at 1.9 eV. The SiNCs were very stable under continuous excitation for several hours. The lifetime at 1.5 eV was over 232 μs, the longest ever reported for SiNCs, consistent with the very high luminescence efficiency. The SiNCs were first fabricated by non-thermal plasma synthesis or anodization in the case of porous silicon. Then, a thin oxide shell (~ 1 nm) was grown using high-pressure water vapor annealing. This oxidation process allows for the growth of very good quality oxide with low defect concentration and low stress, resulting in very good surface passivation, which explains the very high quantum yields obtained.

/pdf/si-sio2-core-shell-luminescent-silicon-nanocrystals-and-4xrl3h7xln.pdf

Si/SiO2 Core/Shell Luminescent Silicon Nanocrystals and Porous Silicon Powders With High Quantum Yield, Long Lifetime, and Good Stability

Silicon nanocrystals (Si-nc) present several plus points as advanced fluorescent biomarkers but suffer from difficulties met in controlling their intrinsic photoluminescence (PL). Here, we first consider the reasons for this difficulty, showing results that support an interface defect-related origin of the PL. Attainment of a controlled PL emission would then require tuning of defects in the capping oxide, a hard and yet unaddressed task. Alternatively, we demonstrate the possible use of Si-nc as antennas, or sensitizers, of a luminescent rare-earth ion in an engineered fluorophore. In this approach the relatively high and broadband optical absorption of Si-nc was exploited, keeping the advantages of a near-infrared inorganic light emitter. Another fundamental part of the assessment of Si-nc for bioimaging is their biocompatibility. Here, we report toxicity tests based on the lactate dehydrogenase release and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays on epithelial cells and fibroblasts, confirming that Si-nc in concentration suitable for luminescent labeling do not affect significantly the cells viability.

An outlook on the potential of Si nanocrystals as luminescent probes for bioimaging

Nanocrystals are widely studied for their tunable optical properties, most importantly increased luminescence efficiency and emission energy. Quantum confinement effects are found for many different types of nanocrystals, and these introduce a relation between the emission wavelength and the size of nanocrystals. When ensembles of nanocrystals with a distribution of sizes are studied, this can have profound effects on their luminescence spectra. Here, we show how photoluminescence spectra of ensembles of silicon nanocrystals can shift under different excitation conditions, resulting from differences in absorption cross-section of the individual nanocrystal sizes. This effect, together with the fact that after a pulsed excitation a silicon nanocrystal can only emit a single photon, determines how the distribution of excited nanocrystals changes and leads to the spectral shift for different excitation powers. Next to this effect, the influence of different radiative rates in such ensembles is also addressed. These notions are important for the interpretation of photoluminescence data for silicon nanocrystals but can be extended to any nanoparticle system comprising size-distributed ensembles.

/pdf/power-dependent-spectral-shift-of-photoluminescence-from-44e0fstqrq.pdf

Power-dependent spectral shift of photoluminescence from ensembles of silicon nanocrystals.

A first-principles, excited-state analysis is carried out to identify ways of producing silicon quantum dots with low excitonic reorganization energy. These focus on the general strategy of either reducing or constraining exciton-phonon coupling, and four approaches are explored. The results can be implemented in quantum dot solids to mitigate polaronic effects and increase the lifetime of coherent excitonic superpositions. It is demonstrated that such designs can also be used to alter the shape of the spectral density for reorganization so as to reduce the rates of both decoherence and dissipation. The results suggest that it may be possible to design quantum dot solids that support partially coherent exciton transport.

Designing small silicon quantum dots with low reorganization energy

Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three discrete time components are apparent in the rise and decay data, which we associate with different classes of nanocrystals. The values of decay time are remarkably constant with emission energy, but the relative contributions of the three components vary strongly across the luminescence band. In keeping with the quantum confinement model for luminescence, we assign emission at high energies to small nanocrystals and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band, it is possible to study the migration of excitation from smaller (luminescence donor) to larger (luminescence acceptor) nanocrystals. We propose a model of diffusion of excitation between neighboring nanocrystals, with long lifetime emission being from the largest nanocrystal in the local neighborhood. Our data al...

/pdf/probing-energy-transfer-in-an-ensemble-of-silicon-3g6mqa6tma.pdf

Probing energy transfer in an ensemble of silicon nanocrystals

Time-correlated single-photon counting study of multiple photoluminescence lifetime components of silicon nanoclusters

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Probing energy transfer in an ensemble of silicon nanocrystals

Time-correlated single-photon counting study of multiple photoluminescence lifetime components of silicon nanoclusters