4.1 Photoluminescence and Time-resolved Photoluminescence System
4.1.2 Result
Fig. 4-2 shows the normalized PL intensity of three different samples. The sample C and D are pure CdSe nanoparticles with water, respectively and prepared with different reaction time in synthesis. The peak position of sample C locates at a longer wavelength that that of sample D. Since the reaction time of sample C is longer than that of sample D, the particle size of sample C is supposed to be larger than that of sample D and the shifted PL peak position of sample C and D may be due to the different particle sizes. Because of Size Quantization Effect, the peak position of sample D is red-shifted. Sample J has the same composition with sample C, but adding PVA and in the form of film so that there is no water in this PVA film. Besides the main peak, samples C and D Fig. 4-2 Normalized PL intensity of sample C , D , J.
34
have another weak and broad peak at the longer wavelength sides. This broad peaks may be due to the defects existing on the surface of CdSe nanoparticles .[40] Sample J does not have apparent defect-related PL peak.
Next, we expose the sample C and J to UV light source, and compare the PL spectrums at different excitation times.
Fig. 4-3 PL spectrum of sample C with different excitation times.
35
In Fig. 4-3, we can find that the defect-related PL peak of sample C becomes smaller with the increase excitation time, while the main PL peak intensity first decreases and then increases as the exposure time increases. For sample J, however, the main peak intensity continuously decreases with the increase of exposure time and there was no significant defect-related peak. We refer this phenomena to “Photoactivation theory” [40,41] .
Fig. 4-4 PL spectrum of sample J with different excitation times.
36
The Photoactivation of water-soluble nanoparticles can be broadly divided into three steps.
1. By photo-oxidation, the surface thiol groups produce disulfide molecules, which are water-soluble and readily removed from the nanoparticles surface and dissolved into the aqueous solution. And then nanoparticles will stick together to form aggregates of nanoparticles. In this process, H2O or surfactant molecules have probability to be absorbed to the nanoparticles surface and passivate surface defects. As a result, PL intensity gradually increases.
Fig. 4-5 Schematic picture of the mechanism of the photoactivation reaction occurring on water-solube CdSe nanoparticles and changes on PL intensity observed during this pathway.
37
2. Photo-oxidation of the nanoparticles surface to form a SeO2 layer as a result of the transfer of absorbed energy to the surface of nanoparticles and following oxidation of surface Se.
CdSe + O
2Cd
2++ SeO
2 Eq. 4-1This oxide layer passivates the surface defects and leads to a large increase in photoluminescence.
3. When the excitation times become very long, oxidative dissolution of the nanoparticles occurs and SeO32- and Cd2+ ion are desorbed from the QD surface.
CdSe + H
2O + O
2Cd
2++ SeO
32-+ 2H
+ Eq. 4-2The SeO2 layer gradually disintegrates, and exposes to new surface defects.
As the nanoparticle surface is destroyed gradually, the PL intensity also decrease gradually.
While the similar process is observed for sample C in Fig. 4-2, in the case of sample J, the photoactivation process was not obviously observed in sample J in the form of film without water molecules.
Next, in order to understand the relation between photoactivation and CdSe/PVA ratio, we compare the PL spectrums of four samples with different CdSe/PVA ratios.
hν
38
Fig. 4-6 Total PL intensity change with excitation time of sample E.
Fig. 4-7 Total PL intensity change with excitation time of sample F.
39
Fig. 4-8 Total PL intensity change with excitation time of sample G.
Fig. 4-9 Total PL intensity change with excitation time of sample H.
40
Fig. 4-6~4-9 show the total PL intensity versus excitation time of samples E, F, G. and H. The samples E, F, G, and H have different CdSe/PVA ratio, 2/1, 1/3, 1/10, 1/30, respectively. The PL intensities of samples F, G, and H increase by photoactivation, while that of sample E monotonically decreases as the excitation time increases. The enhancement of PL intensity in sample F, G, and H are ×1.8, ×8.96, ×4.79 after light exposure of 45 min, 38 min, 5 min, respectively and the highest enhancement was observed for sample G (CdSe:
PVA = 1:10). (see in Fig. 4-7)
Meanwhile, the high CdSe ratio (sample E, and F) will lead to the quenching effect and reduce the PL intensity. In the case of low concentration of CdSe, although the quenching effect is not obvious, the particles are too far away from each other so that the enhancement is still difficulty.[42] Our results show that the sample G has the best ratio of CdSe/PVA to balance these problems and enhance the PL intensity. For sample G, the concentration of PVA is large enough and after photoactivation PVA can protect CdSe nanoparticles effectively and prevent disintegration of SeO2 layer. Therefore, the oxidative dissolution is less obvious and the PL intensity decays slowly. So, we propose a modified process for our case (see Fig. 4-10).
41
1. The CdSe nanoparticles are capped by PVA. By photo-oxidation, the surface thiol groups produce disulfide molecules, which are water-soluble and readily removed from the nanoparticles surface and dissolved into the aqueous solution. And then nanoparticles will stick together to form aggregates of nanoparticles. In this process, H2O, PVA or surfactant molecules have probability to be absorbed to the nanoparticles surface and passivate surface defects. As a result, PL intensity gradually increases.
2. Photo-oxidation of the nanoparticles surface to form an SeO2 layer as a result of the transfer of absorbed energy to the surface of nanoparticles and following oxidation of surface Se.
Fig. 4-10 Modified photoactivation reaction.
42
CdSe + O
2Cd
2++ SeO
2 Eq. 4-3This oxide layer passivates the surface defects and leads to a large increase in photoluminescence.
3. When the excitation times become very long, oxidative dissolution of the nanoparticles occurs and SeO32- and Cd2+ ion are desorbed from the QD surface.
CdSe + H
2O + O
2Cd
2++ SeO
32-+ 2H
+ Eq. 4-4The SeO2 layer gradually disintegrate, and exposes to new surface defects.
As the nanoparticles surface is destroyed gradually, the PL intensity also decrease gradually.
4. If we expose the CdSe nanoparticles to UV light source in a very long time.
The “wall” form of PVA between CdSe nanoparticles will be broken by the laser pulses. And then nanoparticles will stick together to form aggregates of nanoparticles.
hν
43
Fig. 4-11 PL spectrum in different excitation times of sample E.
Fig. 4-12 PL spectrum in different excitation times of sample F.
44
Fig. 4-13 PL spectrum in different excitation times of sample G.
45
Fig. 4-14 PL spectrum in different excitation times of sample H.
46
In order to confirm that photoactivation effect can indeed reduce the surface defect, we compared the PL spectrum at different excitation times of each samples. In Fig. 4-12, 4-13, and 4-14, the defect-induced PL peak has decreased with the increase of excitation time.
Furthermore, we also used time-resolved PL (TR-PL) of sample G and H to measure the time constant and understand the recombination process of carriers and plotted in Fig. 4-15. The decay time constants of TR-PL signals are obtained from the fitting to the experimental data with the double exponential functions.
47
Fig. 4-15 Time-resolved photoluminescence curve of sample G,H.
48
In Table. 4-1, we show the time constant of sample G and H obtained at two different excitation times corresponding to the time at the highest PL peak.
Sample G and H have one thing in common, which is when the PL peak is the highest, PL signal has only one time constant. This may be due to that after the completion of the photoactivation, the surface defects of CdSe nanoparticles have been replaced by SeO2 layer and the carriers have only one recombination channel. This can confirm that the photoactivation actually can reduce the surface defect and enhance the PL intensity.
Moreover, Ref. 40 showed that mention the wavelength of laser and photon energy plays an important role in photoactivation. So we change the wavelength of laser from 365 nm to 400 nm, and repeated the same experiment to investigate the relation of wavelength and photoactivation.
Excitation time
49
Fig. 4-16 Total PL intensity change with excitation time of sample G at 400 nm.
Fig. 4-17 Total PL intensity change with excitation time of sample H at 400 nm.
50
Fig. 4-18 Time-resolved photoluminescence curve of sample G,H.
51
Figure 4-16 and 4-17 show that when the wavelength of laser is changed from 365 nm to 400 nm, the photoactivation still exists and it reduces the surface defect. Compared to the results measured with 365 nm excitation laser pulses, the enhancement of PL intensity of the sample G and H is smaller and it takes longer time to reach the highest PL peak. It may be due to that the photon energy of 400 nm is smaller so that the absorbed energy transferred to the surface of nanoparticles becomes smaller. Then the production of SeO2 layer becomes more difficult and results in the reduction of PL intensity enhancement and longer excitation time to the highest PL peak.
Excitation time
Sample G 0 min 5.43195 0.80586
60 min 4.77284
Sample H 0 min 8.57092 0.97587
8 min 5.8104
Table. 4-2 Time constant of sample G,H
52
4.3 Zscan System
4.3.1 Experiment Setup
1. Laser : i. Ti:sapphire laser, wavelength: 800nm, max power:
400 mW, pulse width: 120 fs, repetition rate: 82 MHz.
ii. amplified, Ti:sapphire laser, wavelength: 800 nm,
max power: 1.5 W, pulse width: 160 fs, repetition rate: 1 kHz.
2. M : silver mirror.
3. ND : ND Filter, ND1 is used to adjust the incident laser power, ND2 is used to prevent the saturation of detector.
4. C : chopper.
5. L : focusing lens.
6. P : pinhole.
7. F : edgepass filter, only let 800nm laser pass, block off the 520nm light emitted by CdSe.
Fig. 4-19 Z-scan system setup
53
The setup of the Z-scan system is shown in Fig. 4-19. A Ti:sapphire laser or an amplified Ti:sapphire laser is used to drive this system. A motorized translation stage is used to move sample from +Z to –Z with the spatial resolution of 10 μm. In order to increase the signal to noise ratio, an optical chopper and lock-in amplifier are used. In order to keep the thickness proper to measure Z-scan, a glass cell with the gap of 1 mm was prepared to hold the liquid state of CdSe samples. (see Fig. 4-20) We chose the glass cell which is amorphous to prevent the occurrence of optical nonlinear effects by the focused intense laser pulses.
Fig. 4-20 Glass cell. 1
54
Prior to measure the CdSe sample, we measured the nonlinearity of ZnTe which has a large nonlinear coefficient and its value is already well known.
Figure 4-21 shows the typical Z-scan trace of ZnTe and the nonlinear coefficients measured by our system. In our case, the two-photon absorption coefficient β we measured is 24.7 cm/GW, which is consistent with that of 16 cm/GW in the literature.[43]
Fig. 4-21 Normalized transmittance with S=1 of ZnTe.
55
4.3.2 Result
Fig. 4-22 Normalized transmittance with S=1 of sample A.
Fig. 4-23 Normalized transmittance with S=1 of sample B.
56
Fig. 4-24 Normalized transmittance with S=1 of sample F.
Fig. 4-25 Normalized transmittance with S=1 of sample I.
57
Figures 4-22 and 4-23 show the normalized transmittance signals from sample A and B, Cdse nanoparticles with tris-HCl and PBS as buffer, respectively, measured with the open aperture (S=1). Figure 4-24 and 4-25 are the Z-scan traces of sample F and I , CdSe nanoparticles with PVA with tris-HCl and PBS as buffer, respectively. Unamplified laser (Tsunami) was used as the light source for the measurement of sample A and B. Meanwhile the measurement of sample F and I was done with the amplified laser system (Spitfire) in order to get the measurable signal-to-noise ratio.
In these experiments, we estimated their two-photon absorption coeffcient β by Eq. 2-18.
First, we measure the absorption of sample A and B by UV/Vis absorption spectrometer. In the Fig. 4-26, the absorption of samples with Tris or PBS as buffer at 800 nm is 0. So, the Leff is equal to the thickness of cell 0.1 cm.
58
Tsunami Spitfire amplifier
Power of
Laser Sample A Sample B Power of
Laser Sample F Sample I
50 mW 15.9302 12.116 400 μW 0.049 0.0328
40 mW 16.0967 12.7239 300 μW 0.047 0.0334 30 mW 16.1667 12.7293 200 μW 0.0469 0.0315 20 mW 15.8594 12.3866 100 μW 0.0488 0.0357 10 mW 16.5155 12.4694 10 μW 0.0464 0.0293
Average 16.1137 12.485 0.04762 0.03254
Table. 4-3 β (cm/GW) of each samples.
Fig. 4-26 Absorption of each samples.
59
In Table. 4-3, we list β of each sample. Sample A and sample B are pure CdSe nanoparticles in different buffers without PVA. On the other hand, samples F and I are CdSe nanoparticles in different buffers with PVA. The CdSe nanoparticles are capped by PVA and form a PVA cluster. (see Fig. 4-28, 4-29 &
4-30)
In the first part, The table shows that β reduce dramatically with the different buffers respectively and it may be due to the ions of Tris and PBS have different electric charge. The ions of Tris are NH3
+ and they have positive charge.
Relatively, the ions of PBS are PO4- and they have negative charge. However, the CdSe nanoparticles with ligand COO- have negative charge, so they will more close in the Tris than in PBS. (see Fig. 4-27)
So in the unit area, the number of CdSe nanoparticles in sample B is less than that of CdSe nanoparticles in sample A. When the laser is focused on sample, fewer CdSe nanoparticles in sample B are hit by the laser pulses than in Fig. 4-27 Different electric charges in Tris and PBS.
60
sample A. Therefore, the probability of two photon absorption becomes smaller in sample B and results in the smaller two photon absorption coefficient β. In the second part, the two photon absorption coefficients β of sample F and sample I are close. Because when we add PVA, the CdSe nanoparticles are capped by PVA to form a PVA cluster. They have similar structure, so the two photon absorption coefficients β are close.
Fig. 4-29 FESEM imaging of sample F
Fig. 4-28 The CdSe nanoparticles/PVA cluster.
61
Fig. 4-30 FESEM imaging of sample I
62
Chapter5 Conclusion
In this thesis, we have investigated the photoluminescence and optical nonlinearity of water-soluble PVA-capped CdSe nanoparticles. The experiment could be summarized to two parts: 1. photoactivation 2. two photon absorption coefficient β.
(1) Photoactivation
We found the photoactivation occurs during the exposure by ultrafast laser.
Upon irradiation, the PL intensity linearly increases with the increase of the excitation time and the surface defect can be reduced. The reaction involving the PL activation is dependent on the wavelength of the irradiated light. Larger photon energy excitation induces a relatively good photoactivation effect. In addition, the CdSe/PVA ratio also can influence the photoactivation. Sample G with the CdSe/PVA ratio of 1/10 shows the highest PL intensity enhancement and photoactivation effect.
(2) Two photon absorption coefficient
We measured the two photon absorption coefficient β of CdSe nanoparticle with different buffers. We found that number of CdSe nanoparticles in a unit area influence β. When the number of nanoparticles in a unit area becomes small, accordingly the probability of light absorption is reduced and reduces the β.
63
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