Optical properties of the PDDA-QDs with photoactivation

Previous investigations indicated that the capping molecular or photoactivation affected the characterization of photoluminescence of QDs [20-22]. Additionally, traditional water-soluble QDs synthetic methods were almost through the thio group surfactant substituted from their primary ligand (TOPO or HDA). Unfortunately, low luminescence QY is often observed in as-prepared nanoparticles, mainly due to the non-radiative recombination of the electron-hole pair at surface site and surface trap competing with band-edge emission. This displeasing property is particularly found in the water-soluble QDs capping with thiol group [5-6]. Very recently, the process of photoactivation was shown to enhance the photoluminescence (PL) of QDs [8]. Additionally, the

polymer molecular capped also eliminate the defects and traps on the QDs surface to increase the recombination of electron and hole [16].

Consequently, our investigation confirmed these finding and design the high luminescence water-soluble PDDA-QDs. Previous authors provided that different capping agents would affect the luminescience properties of QDs [20]. Nevertheless, the PDDA-QDs would be through repleasing the hydrophobic capping agent (HDA) with MSA and absorbing PDDA under photoactivation on the QDs. Figure 4.1 shows that the optical characterization of different capping molecular on QDs with or without photoactivation. Narrow emission bands centered at 562 nm (MSA-QDs) and 557nm (HDA-QDs) were observed when the QDs were excited with a 365 nm He/Ne source. The significant effects on photoluminescence as well as the low QE and red-shift (~5 nm) features of the MSA-QDs are attributed to the trap-states, which normally involves, at least, two reasons. First, the hole is trapped on a thiol molecule; radiative recombination of the exciton is not possible, resulting in a strongly reduced QE. Second, the red-shift observed when thiols adsorb may be due to an increase in the delocalization of the hole wave function due to the availability of new accessible energy states on the adsorbed chalcogenide atoms [7][20].

Hence, these particularly characteristics in the synthetic water-soluble QDs through thio groups capped are easily to production and not useful in biological. However, in this investigation, using the PDDA, absorbed on the MSA-QDs surface under light-trement, could improve the shortcomings (only thio capped QDs), such as the luminescence; Quantum yield; traps on the surface. Figure 4.1 shows

the narrow emission band of the PDDA-QDs centered at 533 nm (excited at 365 nm). Significantly, the results represent that the PL intensity and the emission spectrum were clearly enhancement (200 fold) and blue-shifted (~29 nm) comparing with MSA-QDs and PDDA-QDs, respectively. Upon the light irradiation in ambient environment, the PDDA-QDs has high QY (~48%) and showed significantly blue-shifted from 562 to 533 nm in their emission peak wavelength, indicating that the size of QDs were getting smaller as result of the photo-etching [8].

Figure 4.1 The optical characterization of different capping molecular on QDs, MSA capped (。。。); HDA capped (─); PDDA capped (●●●) under 365 nm excited.

These noticeable phenomenons in the PDDA-QDs with photoactivation can be ascribed in the QDs surface structure [16]. In accordance with the protocols of PDDA-QDs, the PDDA supplied the positive charge and were readily able to absorb on the MSA-QDs surface by van der Waals. Subsequently, the surface defects were passivated by the PDDA and the nonradiative recombination was eliminated. At the

same time, the light irradiated on the PDDA-QDs could enhance the PL and reduce the trap-state on the surfaces to improve the Q.Y from 0.2% to 48%. In order to compare the efficiency of polymer capped, a negative polyelectrolyte PSS (data not shown) was used to cape on the MSA-QDs with photoactivation. The effect of the PSS on the photoluminescence intensity and distribution of the MSA-QDs solution were very weak and aggregation easily after photoactivation. The PSS could not support well shielding and oxidizing easy on the QDs surface. Besides, Figure 4.2 depicts the time evolution of the emission intensity for PDDA-QDs and demonstrates that photoetching or photooxidation of the PDDA-QDs.

In Figure 4.2 (a), the emission peak of PDDA-QDs showed blue-shifted and enhancement with photoactivation under 0 to 35 hours, and the intensity of emission peak under 35 hours photoactivation is the same with 26 hours treament. Additionally, Figure 4.2 (b) represented the experiments of the maximum emission peak and the PL as a function of photoactivation time on the PDDA-QDs. The datas were directly derived from Figure 4.2 (a). The results pointed out the characteristics of PL that could be enhanced (~ 200 folds) and blue-shifted (~29 nm) during the photoactivated through 0 to 35 hours. Therefore, after light irradiation, the probability of the electrons and holes being present on the surface as result of photooxidation is increased in the QDs with QY.

This phenomenon was further verified by the surface-related emission [23]. In fact, this increasingly probability of the carriers being present on the QDs surface with high QY is a directly consequence of the optimal surface reconstruction in the sample growth process, which efficiently removes the carrier quenching defects from the surface [8,16].

Figure 4.2 The time evolution of the emission intensity for PDDA-QDs and demonstrates that photoetching or photooxidation of the PDDA-QDs;

(a) the emission peak of PDDA-QDs; (b) the experiments of the maximum emission peak and the PL as a function of photoactivation time on the PDDA-QDs.

Hence, using photoactivation on the PDDA-QDs not only improve the characterization of Q.Y and PL, but also prepare different size QDs

with various emission samples. Previous investigation reported using photoactivation, by irradiation times; control the size in the QDs. But this method cannot get almost Q.Y and photoluminescence on the different size QDs. By the new approach, it can fabricate the multicolor QDs with high PL, Q.Y, and well distribution. Figure 4.3 shows the photoluminescence and images (inset) with the multicolor of the PDDA-QDs, which emission peaks centered at 535 nm (Green), 555nm (Yellow) and 580 nm (Red), when these were excited by 365 nm.

Figure 4.3 The photoluminescence and images (inset) with the multicolor of the PDDA-QDs, which emission peaks centered at 535 nm (Green), 555nm (Yellow) and 580 nm (Red), when these were excited by 365 nm.

The result indicated the multicolor PDDA-QDs have narrow full width at half maximum (FWHM) and high luminescence in virtue of the photoactivation and polymer capping rearranging the QDs surface. The similar Q.Y and PL intensity of multicolor PDDA-QDs were made by MSA-QDs with different emission ranges. Although controlling the irradiation times could make the multicolor PDDA-QDs, it cannot improve the similar Q.Y and PL intensity. It’s because of the irradiation

times would affect the ratio of rearranging QDs surface and reversed the nonradiation to radiation recombination, which cause from trap electron or hole to nature band gap. Therefore, by irradiation times to fabricate the multicolor QDs is difficult to make the similar efficiency on the PDDA-QDs.

4.3.2 Surface and Size Characterization of the PDDA-QDs with