| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Nanomaterials | Wednesday Sessions |
| Session NM-WeE |
| Session: | Nanocomposites |
| Presenter: | Chih-Yi Cheng, National Taiwan University, Taiwan, Republic of China |
| Authors: | C.-Y. Cheng, National Taiwan University, Taiwan, Republic of China C.-H. Chen, National Taiwan University, Taiwan, Republic of China M.-H. Mao, National Taiwan University, Taiwan, Republic of China |
| Correspondent: | Click to Email |
Colloidal quantum dots (QDs) have many advantages, such as the tunability of emission wavelength during synthesis and easy integration.1 However, they are very sensitive to environment conditions because of high surface to volume ratio. Photo-oxidation of QDs is one of the critical problems. It creates defect states and decreases the quantum yield.2 Strong photo-degradation was observed even in the core/shell structure under laser excitation.3 Recently we reported the method of passivating QDs in Al2O3 using atomic layer deposition (ALD) and systematically studied the time-resolved photoluminescence (PL) of QDs.4 In this work, we took a further step to investigate the impact of laser excitation on QD PL intensity by applying the ON/OFF cycles (5 min each). One unpassivated and one passivated QDs samples were prepared with details described elsewhere.4 The STEM image shows a clear single QDs layer uniformly passivated by an Al2O3 layer. Both samples were studied under a 532 nm continuous wave laser excitation with a power density of 407 kW/cm2. The unpassivated QDs showed rapid PL degradation due to strong photo-oxidation. Moreover, the PL intensity did not recovery after OFF periods indicating an irreversible process. In contrast to the sever photo-degradation of the unpassivated QDs, the passivated QDs were found to remain stable. It is interesting that both samples show small PL recovery at the beginning of each ON period. This can be explained by photo-ionization of QDs. During the ON period, the electrons are ejected from QDs and form an electrostatic barrier which leads to PL intensity reduction due to Coulomb blockade.5 The ionized QDs can be neutralized during the OFF period and then the PL intensity can be recovered to some extent. However, the PL recovery of unpassivated QDs is gradually reduced after several cycles because more and more QDs photo-oxidized. Since the passivated QDs didn’t suffer from photo-oxidation, it shows similar amount of PL recovery in every ON period. In conclusion, we demonstrated the photo-stability enhancement of QDs by Al2O3 passivation using ALD technique. This method can provide good encapsulation of QDs and the application of this technique for other types of QDs, such as PbS, will be also presented in this study.
Reference:
1J. Y. Kim, et al., Adv. Mater. 25, 4986 (2013).
2 W. G. J. H. M. van Sark, et al., J. Phys. Chem. B 105, 8281 (2001).
3H. Zhang, Y. Liu, X. Ye, and Y. Chen, J. Appl. Phys. 114, 244308 (2013).
4C.-Y. Cheng and M.-H. Mao, J. Appl. Phys. 120, 083103 (2016).
5K. Patty, et al., J. Appl. Phys. 116, 114301 (2014).