Concocal microscope records an image (200 × 200 pixel) and nanoparticles appear as a bright spot on those images (Figure 5.1).
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A MATLAB code is written to extract confocal images information from binary format to digital format and the code is shown in Appendix A.3. QDs are seen to be appear as a bright spot in the confocal image (Figure 5.1 A) and some of the bright spots are encircled (Figure 5.1 B) for confirmation.
Confocal microscope can record time trace of fluorophore apart from recording image. The detector (APD) can be parked on a bright spot under continuous illumination and fluorescence can be recorded over long time period. This time trace shows blinking behavior of a single particle under continuous illumination and it is shown in Figure 5.2. Blinking behavior of undoped and indium doped CdSe quantum dots are shown in part A and D of the Figure 5.2 respectively.
Figure 5.2 Time trace of undoped CdSe quantum dots (A) and indium doped CdSe quantum dots (D). Power law behavior of on time (B) and off time (C) for both undoped and indium doped QDs respectively.
Time transient data is recorded for 1000 seconds with a time resolution of 20 ms. Emitted fluorescence is represented in counts and they are plotted against time as shown in Figure 5.2 A and D. A blue line is shown on both time trace of undoped and indium doped QDs and it is known as threshold line. The definition of threshold line is it is the square root of background
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emission. It is important to draw threshold line because it is believed that any emission above and below the threshold line is defined as “on state” and “off state” respectively.
The most important data extracted from time transient measurement is the probabilities of the QDs to stay in “on state” or “off state” continuously. A calculation is carried out to determine the probability of QDs to stay in “on state” or “off state” and it is shown in Figure 5.2 B and C respectively. Power law behavior of undoped and indium doped quantum dots are expressed by black hollow squares and filled red circles respectively. Y and X axis of the two figures (5.2 B and C) are expressed in logarithmic form and in seconds, respectively. The probability distribution (P(τ)) can be fitted by power law equation which states that power law is proportional to the exponent of “on” or “off” time (αon/off) and it is expressed in eq (1). The
proportionality constant is expressed by A.
(1)
The average trend of power law distribution is expressed by black and red line for undoped and indium doped QDs respectively (Figure 5.2 B and C). The values of exponent for undoped and indium doped QDs are expressed in Table 5.1.
Sample αon αoff
Undoped CdSe QDs 1.833±0.004 1.974±0.007
Indium doped CdSe QDs 1.871±0.003 1.928±0.004
Table 5.1 Time exponent on undoped and indium doped CdSe QDs.
It is observed that the trend of power laws of undoped and indium doped QDs do not show significant difference from each other (Figure 5.2 B and C). A possible reason for this could be erroneous sampling of spots to record data. Since doping is completely random and concentration of dopants is low (only 5%), it is highly possible that many QDs are not containing any dopants. If by any chance, the sampling region is full of such undoped QDs, then no difference of blinking could be observed. In other words recorded data will resemble that of undoped CdSe QDs. A possible solution to this problem would be to introduce a marker to different particle. If it is possible to design an experimental technique where doped quantum dots can be marked by SEM and those marked points can be observed under optical microscope, new set of data would reveal the behavior of undoped and doped quantum dots.
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5.4 Conclusions
Fluorescence intermittency of undoped and indium doped CdSe quantum dots are recorded. The data reveals completely random nature of the “on” and “off” time of both systems. Power law behavior of both undoped and indium doped QDs are found to be very similar to each other. A possible explanation is that indium dopant does not change the blinking dynamics at room temperature due to the lack of ionization of defects site.
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Fluorescence Intermittency of CdSe Nanorods with
Chapter 6 -
Changing Polarization of Incident Light
6.1 Introduction
CdSe nanocrystals have drawn significant attention in recent past due to their wide range of optical and electrical properties. CdSe quantum dots (QDs) have been studied as a model system and applied to different fields such as lasers,3 LEDs,1,2 and bio-labeling4. The emission from QDs is found to be unpolarized,37 which is believed to be caused by their spherical structure. Unlike to the QDs, CdSe nanorods (NRs) emit polarized light.71 NRs can be considered as an elongated QD in crystallographic z-direction. The elongated axis in NRs gives rise to different crystal structure as compared to that of QDs. The crystal structure of CdSe NRs is known to be Wurtzite structure as opposed to Zinc-blend structure of CdSe QDs.
It is interesting to study anisotropic emission of CdSe NRs in the presence of polarized light and the effect of P3HT on it. Polarized light emission from single CdSe NRs is important as it provides information about the orientation of NRs and the relaxation process that lead to depolarization of the exciton. A single nanorod with a fixed dipole moment emits linearly polarized light upon excitation with plane polarized light. Probability of transition is the square of absolute value of the dot product between the dipole moment and polarized light vectors. Transition probability indicates cosine dependence of the angle between the two vectors. Thus the direction of atomic dipole can be determined from the polarized emission if the direction of polarized excitation is known.
Any real system is always a collection of atoms. The emission from a material depends on the dipole moment orientation of the ensemble of atoms which is also known as molecular dipole moment. Probability of transition is the absolute value of dot product between molecular dipole moment and polarized light vectors. It is experimentally determined that CdSe NRs emit37 and have a large dipole moment along the crystallographic z-direction.30 Intensity of polarized emission depends on the degree of alignment of individual dipoles. Fluorescence anisotropy study requires recording of the polarized emissions. The emissions are recorded both in parallel and perpendicular direction to that of incident polarization of light. Thus any observed anisotropy indicates the preference of emission in a certain direction. This research is expected to provide a clear picture of direction of polarized emission and their dependence with the
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environment. With the acquired knowledge of polarization, this research can be directly extended to align the NRs in polymer matrix by some external force such as electricity. A completely polarized light emission can be achieved once the NRs are perfectly parallel to each other.