In many applications, QDs operate directly in open gas environments. Due to their small sizes, the ambient conditions could impose essential influence on their properties. In this section, the photoluminescence properties of the MPA-capped CdSe QDs in dry powder form are investigated in different gas environments. The as-prepared QDs (120 hours of synthesis) were first purified with the procedure described in section 4.2.2. Then the QD precipitation was left in air overnight to dry naturally. The obtained QD powder was transferred onto a silicon wafer and spread to a thin layer for full exposure to ambient gas. This sample is used for the subsequent photoluminescence measurements. The sample is placed in a vacuum chamber which can be evacuated to∼ 10−6 mbar and flushed with gas flow. A HeCd laser emitting at 325 nm is used as excitation source. A mild laser intensity (∼ 0.01 W/cm2) was used to avoid significant degradation of the QDs under irradiation.
Figure 5.7 shows the temporal evolution of the excitonic luminescence intensities of the QD powder measured in air, vacuum and nitrogen flow, respectively. The time zero is set at the start of the laser irradiation. All results have been normalized to their initial PL intensities for easy comparison. In air, the PL intensity decays slightly over time. It is reduced down to 0.9 of the initial value after 10 minutes. This is probably due to a little degradation of the QDs under laser irradiation. When the measurements are performed in vacuum, the PL intensity significantly decreases. In the first minute, the luminescence is reduced to half of the initial intensity. Then the decrease gradually slows down. After 10 minutes of irradiation, it decreases to 0.24 of the initial value. When atmospherical air is filled into the sample chamber, the PL intensity of the QD powder is observed to increase rapidly and return to the initial value within 10 minutes. The whole process is reproducible. Repeated evacuation-flooding processes yield similar results. In addition, it is interesting to notice that the initial PL intensity of the QDs recorded in vacuum is similar to that measured in air. This indicates that the vacuum pumping solely could not reduce the PL intensity. These observations indicate that the PL quenching of the QD powder in vacuum is a surface effect.
The PL measurements are further performed in a nitrogen flow. For this purpose, a constant dry nitrogen flow is introduced to pass through the sample chamber. Before the PL measurements, the chamber is flushed for 10 minutes with the gas to remove the inside air. A gas flow is used with the consideration that it can carry away the molecules that could be detached from the QD surfaces during the PL measurements. As shown in Fig. 5.7, the PL evolution of the QD powder exhibits a very similar behavior to that in vacuum. The final intensity in nitrogen flow is 0.3 times the initial value, which is slightly higher than that measured in vacuum. Filling of air into the chamber restores again the photoluminescence intensity of the QDs. These results demonstrate that nitrogen is not the component in air which controls the photoluminescence properties of the QD powder.
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5.5 Photoluminescence quenching in oxygen-deficient environments
Figure 5.7: Temporal evolution of the excitonic photoluminescence maxima of MPA stabilized CdSe QD powder measured in air, vacuum, and nitrogen flow, respectively. A HeCd laser operating at 325 nm is used as excitation source. All the measurements are performed with the same laser intensity. The measuring setup is presented in Chapter 4. The time zero is set at the start of laser irradiation. The results are normalized to the initial photoluminescence intensities for easy comparison.
In previous studies it has been found that under continuous photo-excitation, the photolumi-nescence of individual QDs randomly switches between emission states with high and low intensities, which are referred to as emission ON and OFF states, respectively [152–155]. This phenomenon is known as luminescence blinking, which is not only observed in semiconductor QDs but also in many other types of molecular emitters such as dyes [156], polymers [157], and carbon nanotubes and nanowires [158]. These ON and OFF states are conventionally attributed to a neutral QD and a charged QD model, respectively [155].
A schematic of the luminescence blinking phenomenon is shown in Fig. 5.8(a). In the bright ON state, the luminescence is dominated by radiative recombinations of excitons, which is characterized with a mono-exponential decay dynamics with a lifetime typically in the range of 15–30 ns. The occurrence of OFF states is induced by the generation of an additional charge in the QD core. In this case, excitons will recombine through a non-radiative Auger-type process, in which the exciton transfers its energy to the extra charge carrier rather than emitting a photon. The Auger recombination process further induces a significantly shortened lifetime of the band-edge excitons (a few nanoseconds or less [155], Fig. 5.8(b)). As a result, the photoluminescence intensity of the QD is largely reduced.
The mechanism for the QD charging during the luminescence OFF period is attributed to surface defects of the QDs such as unpassivated dangling bonds and structured defects [159], which could form energy levels in the forbidden gap that act as trap centers for the photoexcited carriers, as schematically shown in Fig. 5.8(c). When a charge carrier is captured by the trap centers, the QD core becomes charged, initiating the luminescence OFF period. During this period, the extra free carrier could be excited to higher energy states by absorbing the energy of excitons. This energy is, in some cases, high enough to excite the carrier into the outside capping surfactant ligands. This also hinders the neutralization of the defect states. Therefore,
5 Synthesis, Optical Properties, and Chemical Stability of Colloidal CdSe Quantum Dots
Figure 5.8: Schematic of the luminescence blinking of CdSe QDs and the effects of oxygen and water molecules. (a) Random switching between the ON and the OFF states in the blinking process. The bright ON state corresponds to radiative exciton recombination in a neutral QD.
The OFF state is related to non-radiative Auger processes in a charged QD. (b) Schematic of the PL decay during the ON and OFF period. The Auger recombination process is much quicker than the neutral exciton recombination, leading to a shorter radiative lifetime of the OFF state. (c) Left: surface defect level in QDs acts as trap centers for photoexcited holes, inducing a charged QD core. Right: oxygen molecule passivates a defect state, ensuring a neutral core and high-intensity emission. (d) Energy level alignment of CdSe QDs and the oxygen redox potential. The oxygen redox potential level is significantly broadened under the presence of water molecules.
a macroscopic time period (on the order of seconds) of the ON and OFF states are normally observed.
The luminescence blinking of QDs was found to be sensitive to the gas atmospheres [153,154].
In a study of single CdSe/ZnS core shell nanocrystal, when the measuring environment was changed from air to vacuum, the duration of the ON state was significantly shortened while the probability of the OFF event is remarkably increased [153]. As a result, the overall lumines-cence intensity was observed to decrease by a factor of 60 in vacuum [153]. Oxygen molecules were proposed to account for this phenomenon. It was suggested that oxygen molecules in air can effectively passivate the surface defects in QDs. A direct oxidation of the defects could occur by transferring their extra electrons to surface oxygen molecules, as illustrated in Fig. 5.8(c). This effect passivates the possible hole traps and prevents the QD core from charging during photoexcitation. In an oxygen-deficient environment, oxygen molecules are detached from the QD surface. As a result, the probability of the luminescence OFF state is significantly enhanced by the active surface traps.
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