No abstract

The non-exponential room temperature luminescence decay of colloidal quantum dots is often well described by a stretched exponential function. However, the physical meaning of the parameters of the function is not clear in the majority of cases reported in the literature. In this work, the room temperature stretched exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms associated with the parameters of the function. Three classes of non-radiative transition processes between the excited and ground states of colloidal quantum dots are discussed: long-range resonance energy transfer, multiphonon relaxation, and contact quenching without diffusion. It is shown that multiphonon relaxation cannot explain a stretched exponential functional form of the luminescence decay while such dynamics of relaxation can be understood in terms of long-range resonance energy transfer to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) in the environment of the quantum dots acting as energy-donors or by contact quenching by acceptors (surface traps or molecules) distributed statistically on the surface of the quantum dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

While time-resolved luminescence spectroscopy is commonly used as a quantitative tool for the analysis of the dynamics of photoexcitation in colloidal semiconductor quantum dots, the interpretation of the virtually ubiquitous nonexponential decay profiles is frequently ambiguous, because the assumption of multiple discrete exponential components with distinct lifetimes for resolving the decays is often arbitrary. Here, an interpretation of the roomtemperature luminescence decay of CdSe/ZnS semiconductor quantum dots in colloidal solutions is presented based on the Kohlrausch relaxation function. It is proposed that the decay can be understood by using the concept of Förster resonance energy transfer (FRET) assuming that the role of acceptors of photoexcitation energy is played by highfrequency anharmonic molecular vibrations in the environment of the quantum dots. The term EVFRET (ElectronicVibrational Förster Resonance Energy Transfer) is introduced in order to unequivocally refer to this energy transfer process.Since the seminal works reporting a quantum size effect in semiconductor nanocrystals in 1981 [1] and luminescence from semiconductor colloidal solutions in 1982 [2,3] colloidal semiconductor quantum dots have become the subject of intensive experimental and theoretical research due to their wide range of size-tunable properties, notably optical properties [4-6] which make them potential candidates for applications in areas such as biolabeling, bioimaging, bio-and chemo-sensing, theranostics, lasing, photovoltaic energy conversion, photodetection, and light-emitting diodes among others [7][8][9][10][11][12][13][14][15].The understanding of the principles governing luminescence in colloidal quantum dot systems is necessary in order to envisage ways of controlling their properties. At the same time, the room-temperature luminescence decays of quantum dots are typically nonexponential, [7,[16][17][18] which may lead to ambiguity in their interpretation as the assumption of multiple discrete exponential components with distinct lifetimes for resolving the decays is often arbitrary. Indeed, pre-exponential factors and decay times in nonexponential luminescence decays have clear physical meaning only in the simplest cases [19]. On the other hand, while the intrinsic exciton dynamics of CdSe colloidal semiconductor quantum dots, by far the most extensively investigated quantum dot system, is generally understood [20,21], the same cannot be said about the effects of extrinsic factors, such as capping layer or colloidal solution environment, which are expected to play a prominent role in the photophysical processes in those systems [22].The Kohlrausch or stretched exponential relaxation function, exp [−(t/τ ) β ], where 0 < β ࣘ 1 and τ has the dimensions of time, has been in use for over 150 years to describe relaxation processes in electronic and molecular systems [23], including luminescence decay in condensed matter, most remarkably by Alfred Werner in 1907 and Theodor Förster in 1949 [24]. The stretching p...

The analysis of the steady-state and transient photoluminescence (PL) of PbS quantum dots (QDs) of diameter in the 3.2–6.9 nm range in porous matrixes at temperatures 77–300 K shows that QDs of different sizes possess entirely different temperature dependencies of their PL properties. The data indicates the presence of two emissive “in-gap” states in the low-energy electronic structure of the QDs with characteristic dependencies on QD size and temperature. The lowest energy state is associated with surface defect states while the higher energy state is “intrinsic” and arises due to size-dependent splitting of the lowest excitons.

Interfaces are crucial factors in shaping the properties of colloidal quantum dots (QDs), in particular the size-dependent optical properties that are a hallmark of these materials. However, the role played by the interfaces associated with QDs on the kinetics of photoluminescence (PL) decay of these nanocrystals is not fully understood even for the most extensively investigated II–VI QDs. In particular, interfaces are a hotbed of trapping sites over which control is essential for the efficient performance of QD-based devices because traps condition PL lifetimes and may be related to PL intermittency. In this work, we analyze the room-temperature PL decay of drop-cast films of CdSe/ZnS QDs varying a number of factors (casting solvent, capping ligands, core/shell interface character). We show how the use of a function that accounts for reversible trapping of photogenerated charge carriers with physically meaningful parameters (time constant, trapping and detrapping rate constants, and average number of traps per QD) can provide valuable information concerning the relevant interfaces, and therefore the nature of the trap states, involved in the recombination of those charge carriers. This approach should be applicable to QDs of a variety of compositions as well as materials beyond inorganic semiconductors.