Photoluminescence Intensity Of Quantum Dots Research Articles

CsPb(Br/Cl)3-Sm3+ codoped glasses for optical temperature sensing in FIR and chromatic coordinate modes

We have fabricated Sm3+ doped glasses, CsPb(Br/Cl)3 quantum dots (QDs) glasses and CsPb(Br/Cl)3-Sm3+ codoped glasses by melt-quenching and subsequent heat treatment for optical temperature sensing. Photoluminescence (PL), photoluminescence excitation (PLE) spectra and PL decay have been measured. Two CsPb(Br/Cl)3-Sm3+ samples with different PL peaks of QDs at 486 nm and 503 nm have been studied for fluorescence intensity ratio (FIR) and chromatic coordinate sensing. The CsPb(Br/Cl)3 QDs and Sm3+ ions exhibit distinguished spectral components and they are dynamically independent. PL intensity of QDs rapidly decreases with temperature, whereas PL intensity of Sm3+ slowly decreases with temperature. PL intensity of Sm3+ ions and CsPb(Cl/Br)3 QDs have been independently fitted. Then, FIR sensing has been studied. The different variation in PL intensity with temperature for Sm3+ and CsPb(Cl/Br)3 leads to the change of luminescent color and chromatic coordinate. The chromatic coordinate sensing has also been studied. Absolute sensitivity (Sa) and relative sensitivity (Sr) have been analyzed for both FIR and chromatic coordinate sensing. Compared with the chromatic coordinate mode, FIR mode has the larger Sa and Sr. But the chromatic coordinate method is more convenient for application, which does not require separating two spectral components. Both FIR and chromatic coordinate sensing modes show outstanding temperature cycling characteristics.

Photoluminescence of dense arrays of InGaPAs/InGaAs quantum dots formed by substitution of group V elements

One drawback of self-organized quantum dots is their low optical gain. The development of new shaping methods that would allow higher gain, for example, due to a higher surface density of the QD array, remains an important task to date. In the present work, arrays of quantum dots have been formed by substituting phosphorous atoms with arsenic ones in a thin InGaP epilayer. Based on transmission electron microscopy data, the surface density was estimated to be about 1.3 × 1012 cm−2, which is one of the highest values for quantum dots on GaAs. The influence of an additional InGaAs epilayer (quantum well) on the luminescence properties of quantum dots was investigated in a wide temperature range and different optical pumping levels. It was found that placing the quantum well under the layer of quantum dots has little effect on the central emission wavelength, while quantum dots covered with the InGaAs demonstrate a strong red shift of the luminescence spectrum. A transition from a nonequilibrium to an equilibrium distribution of charge carriers over quantum-dot states, related to the lateral transport of charge carriers over quantum dot array, similar to that previously observed in In(Ga)As Stranski-Krastanow quantum dots, was revealed. This is manifested in a change in temperature dependencies of the linewidth and peak position of the photoluminescence band. For the structures under study, this transition occurs at a temperature of about 125 K. Rapid thermal annealing of the structures allows to increase the integrated photoluminescence intensity of quantum dots up to 4 times, while the maximum of the spectrum remains almost unchanged, provided that the annealing temperature does not exceed 620 °C.

High-entropy perovskite quantum dots of rare earth synergy with excellent luminescence stability via high-energy ball milling

The problem of low stability and high lead content limit the commercial development of halide perovskite. In this paper, the high-energy ball milling is used to rapidly and efficiently synthesize high-entropy Cs(Pb1/5Mn1/5Ni1/5Zn1/5Yb1/5)Cl3 quantum dots (QDs), the introduction of high-entropy and rare earth ions reduces lead content, increases mixing entropy, and synergistically enhances the photoluminescence (PL) intensity and stability of QDs with other ions. Because the increase of entropy enhances the stability, the QDs still have 70 % PL intensity after 60 days of placement. After the QDs are exposed to the ultraviolet lamp for 8 hours, the PL intensity maintains at 67.1 % due to the loss of ligand. Ionic bonds are easily broken in water, so after soaking in water for 8 hours, the PL intensity of the QDs keeps at 32 %. The thermal stability of QDs is maintained well because of physical and chemical stability of rare earth ions. Since ethanol dissolves the ligands on the surface of the QDs, the non-radiative recombination is enhanced, so the PL intensity of QDs remains at 59 % after soaking for 8 hours. This paper provides a new way that further enhances the stability and solves high lead content of all-inorganic perovskite materials.

Enhancement of the alloyed CdZnSeS/ZnS quantum dots photoluminescence by thiol ligands capping

Most of the existing hydrophilization techniques for the semiconductor quantum dots (QDs) lead to degradation of photoluminescence (PL). Therefore, for the applying of QDs in the areas requiring their stability in water media (e.g. bio- and chemical analysis, labeling and sensing), it is often necessary to find a balance between high stability, good optical properties, and difficulty of acquiring. This article describes simple, fast, and easy scalable methods for alloyed CdZnSeS/ZnS QDs ligand exchange using 2-mercaptoethanol, thioglycolic, 3-mercaptorpopionic and dihydrolipoic acids. The proposed ligand exchange techniques provide the enhancement of the QDs photoluminescence intensity in contrast to most of other ligand exchange procedures on the QDs surface. In optimized conditions the twofold photoluminescence quantum yield increase compared to the initial hydrophobic CdZnSeS/ZnS QDs was achieved. Hydrophilized nanoparticles maintained colloidal stability for a period more than 13 months. The suggested mechanism of the PL increase is the passivation of the QDs surface with molecules containing a thiol group. The combination of the proposed effective CdZnSeS/ZnS QDs synthesis and modification methods shows the prospect of their implementation as basic luminescent material for a wide range of applications.

Enhanced photoluminescence of a microporous quantum dot color conversion layer by inkjet printing

Owing to their high color purity, tunable bandgap, and high efficiency, quantum dots (QDs) have gained significant attention as color conversion materials for high-end display applications. Moreover, inkjet-printed QD pixels show great potential for realizing full-color mini/micro-light emitting diode (micro-LED)-based displays. As a color conversion layer, the photoluminescence intensity of QDs is limited by the insufficient absorptance of the excitation light due to the lack of scattering. Conventional scatterers, such as titanium dioxide microparticles, have been applied after additional surface engineering for sufficient dispersity to prevent nozzle clogging in inkjet printing process. In our work, as an alternative approach, we use inkjet printing for depositing a phase separating polymer ink based on polystyrene (PS) and polyethylene glycol (PEG). QD/polymer composite pixels with scattering micropores are realized. The morphology of the micropores can be tailored by the weight ratio between PS and PEG which enables the manipulation of scattering capability. With the presence of the microporous structure, the photoluminescence intensity of the QD film is enhanced by 110% in drop-cast films and by 35.3% in inkjet-printed QD pixel arrays compared to the reference samples.

Emission behaviour of CdTe/SiO2 core/shell quantum dots in external electric field

Optical properties of CdTe/SiO2 core/shell colloidal quantum dots were investigated in external electric field in the range of 0–140 kV/cm. Maximum of photoluminescence intensity was centered at 2.39 eV with the full width at half maximum of 0.26 eV. It was found that the photoluminescence intensity of quantum dots tends to decrease by 22% and the integrated luminescence intensity tends to decrease by 23% with increasing electric field to the maximum value. However, an increase in the integrated luminescence intensity in the region of 60 kV/cm was observed. We show that both interband and trap-state luminescence is affected by an external electric field with no Stark shift observed. Based on these results, we propose that relaxation of the excited states in CdTe/SiO2 core/shell colloidal quantum dots exposed to an external electric field is affected by two field-dependent mechanisms, i.e. field-induced quenching of luminescence and blocking of charge carrier trapping. The obtained results can be useful to gain better understanding of the electric field effect on the optical properties of CdTe/SiO2 colloidal semiconductor quantum dots and, in particular, on the operation of hybrid organic-inorganic LEDs with emitting layers based on the studied nanocrystals.

Glutathione-Capped ZnS Quantum Dots-Urease Conjugate as a Highly Sensitive Urea Probe

Quantum dots (QDs) possess characteristic chemical and optical features. In this light, ZnS QDs capped with glutathione (GSH) were synthesized via an easy aqueous co-precipitation technique. Fabricated QDs were characterized in terms of X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), Fourier transform infrared (FTIR) and Zeta potential analyses. Optical properties were examined using photoluminescence (PL) and ultraviolet–visible (UV–visible) spectroscopies. Moreover, GSH-capped ZnS QDs were evaluated as an optical probe for non-enzymatic detection of urea depending on the quenching of PL intensity of ZnS QDs in the presence of urea from concentration range of 0.5–5 mM with a correlation coefficient (R2) of 0.995, sensitivity of 0.0875 mM−1 and LOD of 0.426 mM. Furthermore, GSH-capped ZnS QDs-urease conjugate was utilized as an optical probe for enzymatic detection of urea in the range from 1.0 µM to 5.0 mM. Interestingly, it was observed that urea has a good affinity towards ZnS QDs-urease conjugate with a linear relationship between the change of PL intensity and urea concentration. It was found that R2 is 0.997 with a sensitivity of 0.042 mM−1 for mM concentration (0.5–5 mM) and LOD of 0.401 mM. In case of µM concentration range (1–100 µM), R2 was 0.971 with a sensitivity of 0.0024 µM−1 and LOD of 0.687 µM. These data suggest that enzyme conjugation to capped QDs might improve their sensitivity and applicability.Graphical

The effect of loading modes on the strain-dependent energy gap of CdTe quantum dots: A first-principles study

The strain-dependent photoluminescence (PL) properties of quantum dots (QDs) make them potential stress/strain sensing materials (SSM), especially for high-level stress states with nanometer and nanosecond resolution. However, the PL intensity and wavelength of QDs in experiments show nonlinear and non-monotonic dependence on applied strain/stress under some loading conditions, and the underlying mechanism needs microscopic investigations. In the work, first-principles calculations are performed on CdTe QDs of different sizes under three loading modes: hydrostatic compression (HC), shock compression (SC) and uniaxial compression (UC). Results show that the relationship between energy gap and applied strain is significantly dependent on the size of QD and the loading mode. Under the HC mode, the energy gap changes of CdTe QDs increase linearly with strain and the relationship is size-independent which is suitable for stress sensing. Under the SC mode, the energy gap also increases with strain, but the relationship is size-dependent. Under the UC mode, the relationship is negatively correlated for most cases and also shows significant size-dependence. LUMO/HOMO energies and electron cloud distributions are further investigated to find the key factor that controls the variations of energy gaps with strain under different loading modes. The findings help to understand the experimental phenomena of QDs under different loading modes, and also provide information for developing QDs-based SSMs.

ELECTROLYTIC AGGREGATION IN SOLUTIONS WITH QUANTUM DOTS AND GOLD NANOPARTICLES MODIFIED WITH OLIGONUCLEOTIDES

Aim. To investigate electrolytic aggregation of different nano-objects in solutions with quantum dots (QDs) and Au nanoparticles (NPs) modified by oligonucleotides as well as the effect of aggregates on the photoluminescence (PL) of QDs. Methods. Au NPs and AgInS2/ZnS QDs were modified by oligonucleotides. Two types of QDs that differ in size and stabilizing ligand were used. PL and optical absorption of nano-objects in water and SSC buffer solutions were studied. Results. The transfer of modified by oligonucleotides QDs from water to a buffer solution and the addition of Au NP modified by oligonucleotides to the solution caused quenching of the QD PL intensity. The PL quenching was observed for the QDs of two types and increased during the incubation of solutions, but didn’t depend on its multiplicity. An aggregation of Au-DP occurred only in buffer solutions with QDs of one type and increased with multiplicity of the buffer solution. Conclusion. It is found that the electrolytic aggregation of Au NPs modified by oligonucleotides in buffer solutions with QDs depends on the QD type and didn’t affect the quenching of the PL intensity of the QDs.

CdSe/CdS/ZnS core/multi-shell QDs: new microwave synthesis and applications for dye photodegradations

CdS and ZnS shells were grown around CdSe quantum dots (QDs) by a new, simple and ultrafast microwave synthesis. XRD result confirmed formation of CdSe/CdS/ZnS core/multi-shell structures and synthesized QDs had zinc blend structure. PL (photoluminescence) results showed that formation of CdS and ZnS shells enhanced PL intensity of CdSe QDs. TEM results showed that synthesized QDs were spherical. These QDs were characterized by using absorption, EDX, Raman, and FT-IR spectrums. Photocatalyst performance of CdSe/CdS/ZnS QDs was investigated for methylene blue, methylene orange and Rhodamine B as pollutants under both sun and UV illuminations. The obtained results showed that the best photodegradation among all samples belonged to the CdSe/CdS/ZnS QDs with MB under UV irradiation. Its dye degradation percentage and the linear kinetic rate constant (k) were 88.9% and 24.9 × 10−3 min−1. Radical scavengers were used to discover the main reactive species in the degradation of methylene blue with CdSe/CdS/ZnS QDs under UV irradiation; electrons play the key role in the degradation process.

Improved Characteristics of CdSe/CdS/ZnS Core-Shell Quantum Dots Using an Oleylamine-Modified Process.

CdSe/CdS with ZnS/ZnO shell quantum dots (QDs) are synthesized by a one-pot method with various oleylamine (OLA) contents. The crystal structures of the QDs were analyzed by X-ray diffractometry, which showed ZnS diffraction peaks. It was represented that the ZnS shell was formed on the surface of the CdSe/CdS core. Interestingly, QDs with a high OLA concentration exhibit diffraction peaks of ZnS/ZnO. As a result, the thermal stability of QDs with ZnS/ZnO shells exhibits better performance than those with ZnS shells. In addition, the photoluminescence intensity of QDs with ZnS/ZnO shells shows a relatively slow decay of 7.1% compared with ZnS shells at 85 °C/85% relative humidity aging test for 500 h. These indicate that QDs with different OLA modifications can form ZnS/ZnO shells and have good stability in a harsh environment. The emission wavelength of QDs can be tuned from 505 to 610 nm, suitable for micro-LED display applications.

Dynamic regulation of photoluminescence based on mechanochromic photonic elastomers

Photonic crystals (PCs) are emerging advanced functional materials with a unique capability of regulating light propagation and have been demonstrated promising in tuning photoluminescence (PL) of fluorophores for sensing and displaying devices. However, it remains highly desired and challenging to develop robust PC platforms for facile dynamic regulation of PL of fluorophores. Herein, we demonstrate dynamic regulation of the PL properties of fluorophores by using photonic elastomers (PEs) with a tunable photonic bandgap (PBG) via a facile stretching process. When being combined with fluorophores (e.g., quantum dots (QDs), Rhodamine B (RhB), and rare earth complexes), shifting of the PBG can realize dynamic regulation of PL properties of fluorophores. We show that during stretching, when the PBG matched well with the PL peak, ~1.8-fold enhancement of PL intensity of QDs on the surface of the PEs and ~40% inhibition of PL intensity of RhB incorporated within the PEs. We further showed that the composite PEs with embedded RhB can be used as a multi-mode anticounterfeiting tape, which exhibited mechanochromic properties in both structural color and PL under natural and UV light. This study presents a new platform for the dynamic regulation of PL and expands the practical application prospect of PCs.

The electrochemical reaction controlled optical response of cholestrol oxidase (COx) conjugated CdSe/ZnS quantum dots

This paper reports the enhanced performance of cholesterol oxidase (COx) conjugated CdSe/ZnS quantum dots (QDs) by using water-soluble mercaptoacitic acid (MAA) as linker. The functionalized MAA-CdSe/ZnS QDs conjugated in four different dilutions of cholesterol oxidase significantly affected QDs photoluminescence intensities, which affected the process of charge transfer from QDs to MAA. The conjugation of COx to MAA-QDs in increased dilutions resulted in the regain of PL intensities, which were attributed to the passivation of MAA HOMO/LUMO states. The electrochemical impedance spectroscopy and cyclic voltammetry of the conjugated QDs were performed to get study the charge transfer mechanism. The 1:1000 diluted COx conjugated MAA-CdSe/ZnS QDs was found to have the lowest charge transfer resistance of 228 Ω, the highest diffusion (~ 1.39 × 10–13 cm2/s) and charge transfer rates (~ 4.5 × 10–6 s−1) between the electrode and the redox species. The current study demonstrated the sensitivity of electrochemical and optical based detection on the alkaline.

Photoelectric and deep level study of metamorphic InAs/InGaAs quantum dots with GaAs confining barriers for photoluminescence enhancement

Metamorphic InAs/InGaAs quantum dots (QDs) have been proposed as active elements for optoelectronic light-emitting devices operating in the infrared range. However, advanced structure design to allow efficient and stable enhancement of quantum yield at room temperature are still needed. Here, we compare a metamorphic InAs/In0.15Ga0.85As QD heterostructure with and without GaAs confining barriers, to investigate the effect of introducing GaAs barriers on the photo- and thermo-electrical properties. GaAs confining barriers allow to enhance the QD photoluminescence intensity at 1.3 µm, i.e. second telecommunication window, by more than two orders of magnitude at room temperature and at 80 K. We also discuss the effect of GaAs barriers on the carrier transport and on defect-related levels detected by means of photocurrent and deep level thermally stimulated current spectroscopies. GaAs confining barriers decrease the thermal escape rate of electrons confined in QD and wetting layer, thereby highly increasing the radiative recombination and also quenching the photocurrent. At low temperatures, the barriers also reduce the capture of electrons generated in InGaAs by the QD layer and, on the other hand, prevent the trapping of electrons outside the QD layer, decreasing carrier lifetimes. The deep levels identified as point and extended defects have been detected in the InGaAs layers. There are no new types of defects introduced in the structure by the addition of the barriers, but this causes a weakly increased density of traps near QDs. Our results show that InAs/InGaAs QDs with GaAs confining barriers can be efficient light emitters with only a slight increase of defects in the structure. Hence, such advanced design for metamorphic QDs can be of relevant interest for applications in energy-efficient QD lasers, optical amplifiers and single-photon emitters operating at 1.3–1.55 µm.

Enhancement of spontaneous emission of semiconductor quantum dots inside one-dimensional porous silicon photonic crystals.

Controlling spontaneous emission by modifying the local electromagnetic environment is of great interest for applications in optoelectronics, biosensing and energy harvesting. Although the development of devices based on one-dimensional porous silicon photonic crystals with embedded luminophores is a promising approach for applications, the efficiency of the embedded luminophores remains a key challenge because of the strong quenching of the emission due to the contact of the luminophores with the surface of porous silicon preventing the observation of interesting light-matter coupling effects. Here, we experimentally demonstrate an increase in the quantum dot (QD) spontaneous emission rate inside a porous silicon microcavity and almost an order of magnitude enhancement of QD photoluminescence intensity in the weak light-matter coupling regime. Furthermore, we have demonstrated drastic alteration of the QD spontaneous emission at the edge of the photonic band gap in porous silicon distributed Bragg reflectors and proved its dependence on the change in the density of photonic states.

Probing reversible photoluminescence alteration in CH3NH3PbBr3 colloidal quantum dots for luminescence-based gas sensing application

Methylammonium lead bromide (CH3NH3PbBr3) colloidal quantum dots (QDs) exhibit strong green photoluminescence (PL) with high photoluminescence quantum yield (PLQY) making it valuable for various optoelectronic applications. Under the influence of polar gaseous molecules, hybrid halide perovskites show changes in its structural and electrical properties. We, for the first time, have investigated the influence of NH3 gas molecules on the optical properties of CH3NH3PbBr3 colloidal QDs. The investigations carried out under a controlled environment reveal that even the presence of 37 ppm of ammonia (NH3) gas molecules causes a significant reduction in the PL intensity of CH3NH3PbBr3 colloidal QDs. The reduction rate of PL intensity can be tuned with the concentration of NH3 gas molecules. We propose that the decrease in PL intensity is because of the formation of a non-luminescent NH4PbBr3 phase under the presence of NH3 gas molecules. Further, the non-luminescent NH4PbBr3 retransformed into luminescent CH3NH3PbBr3 on the introduction of methylamine (CH3NH2) gas molecules. This reversible alternation in PL properties enables us to demonstrate its application for (NH3) gas sensing. The advantage of using CH3NH3PbBr3 colloidal QDs for luminescence-based sensing is that its green emission is visible with the naked eye even under daylight, which is easy to detect.

Magnetically controlled exciton transfer in hybrid quantum-dot–quantum-well nanostructures

A magnetophotoluminescence study of the carrier transfer with hybrid InAs/GaAs quantum dot(QD)-InGaAs quantum well (QW) structures is carried out where we observe an unsual dependence of the photoluminescence (PL) on the GaAs barrier thickness at strong magnetic field and excitation density. For the case of a thin barrier the QW PL intensity is observed to increase at the expense of a decrease in the QD PL intensity. This is attributed to changes in the interplane carrier dynamics in the QW and the wetting layer (WL) resulting from increasing the magnetic field along with changes in the coupling between QD excited states and exciton states in the QW and the WL.

FRET-Modulated Multihybrid Nanoparticles for Brightness-Equalized Single-Wavelength Barcoding.

Semiconductor quantum dots (QDs) are the most versatile fluorophores for Förster resonance energy transfer (FRET) because they can function as both donors and acceptors for a multitude of fluorophores. However, a complete understanding of multidonor-multiacceptor FRET networks on QDs and their full employment into advanced fluorescence sensing and imaging have not been accomplished. Here, we provide a holistic photophysical analysis of such multidonor-QD-multiacceptor FRET systems using time-resolved and steady-state photoluminescence (PL) spectroscopy and Monte Carlo simulations. Multiple terbium complex (Tb) donors (1-191 units) and Cy5.5 dye acceptors (1-60 units) were attached to a central QD, and the entire range of combinations of FRET pathways was investigated by Tb, QD, and Cy5.5 PL. Experimental and simulation results were in excellent agreement and could disentangle the distinct contributions of hetero-FRET, homo-FRET, and dye dimerization. The FRET efficiency was independent of the number of Tb donors and dependent on the number of Cy5.5 acceptors, which could be used to independently adapt the PL intensity by the number of Tb donors and the PL lifetime by the number of Cy5.5 acceptors. We used this unique tuning capability to prepare Tb-QD-Cy5.5 conjugates with distinct QD PL lifetimes but similar QD PL intensities. These brightness-equalized multihybrid FRET nanoparticles were applied to optical barcoding via three time-gated PL intensity detection windows, which resulted in simple RGB ratios. Direct applicability was demonstrated by an efficient RGB distinction of different nanoparticle-encoded microbeads within the same field of view with both single-wavelength excitation and detection on a standard fluorescence microscope.

Photoluminescence enhancement of colloidal CdSe/ZnS quantum dots embedded in polyvinyl alcohol after 2 MeV proton irradiation: crucial role of the embedding medium

The growing interest on the use of colloidal Quantum Dots (QDs) in scintillation and dosimetry applications in comparison with organic fluorophores relies in their narrower and tunable photoluminescence (PL) emission, easier chemical processability and higher cross-section for ionizing radiation. In this context, a deep understanding of the role of the embedding medium on the QD optical properties in unirradiated and irradiated samples represents an important issue. In this paper, we present the optical characterization of colloidal core-shell CdSe/ZnS QDs embedded in polyvinyl alcohol (PVOH) and irradiated with 2 MeV protons at different fluences. The characterization of the samples, performed by both steady-state and time-resolved PL measurements, indicates a damage of the QDs at the lowest fluence (1013 H+cm−2), demonstrated by a quenching of the QD PL intensity and by a shortening of the QD lifetime. At the middle fluence (5 × 1013 H+cm−2) a partial recovering of the QD optical properties are observed, due to energy transfer phenomena between radiation-induced PL defects in the PVOH, acting as donors, and the QDs, acting as acceptors. The higher concentration of PVOH PL defects in the most irradiated sample (1014 H+cm−2) led to an enhancement of the QD PL intensity in comparison with the unirradiated sample, highlighting the crucial role of the embedding medium in the post-irradiation QD optical response.

Correlation of Intermittency of Quantum Dot Photoluminescence Intensity, Decay Time, and Energy

Intermittency of photoluminescence (PL) intensity is a common feature of semiconductor quantum dots (QDs). Here, we show for ZnS capped CdSe QDs that the intermittency (blinking) is closely correlated to the intermittency of PL decay times. Furthermore, we find that fluctuations of PL emission energies are also correlated with intensity and decay time blinking. The origin of energy fluctuations is a combination of switching either between energies of electronic states intrinsic to differently charged QDs or correspondingly Stark shifted states caused by slow fluctuations of the QD interface. By the method of change point analysis we assign the distribution of PL intensities intermediate between on/off intensities to distinct intermittent PL decay times and PL energies. Intermittency occurs on slow time scales between sub‐ms and minutes and depends strongly on the embedding matrix. Highly specific experiments allow for identification of a distribution of charged and non‐charged states near to the excitonic band edge. Most of the blinking phenomena are observed on the depopulation pathways of the band edge states. However, also population pathways are intermittent, but on a much shorter time scale. We assign this mechanism to creation of hot electrons followed by charge trapping and release at the QD interface.