Luminescence property enhancement of TiO2 NCs through oxygen vacancy control

Oxide-nanocrystals have been a key contributor to the technological advancement witnessed today owing to their quantum confinement effect depending on the size, shape, and composition. As a result, we have recently witnessed an increase in the fabrication of oxide nanocrystals and particularly titanium oxide. Generally, different methods such as sol-gel and hydrothermal synthesis are available for synthesizing oxide nanocrystals. However, the ability of the existing methods to produce nanocrystal materials with desirable properties has been questioned due to several disadvantages involved. Consequently, the introduction of capping ligands despite their effective role in controlling size has been limited by several side effects such as property deterioration. Also, the growing concerns regarding the disadvantages of the synthesis have necessitated the need to develop alternative ligand treatment methods.

To this end, a joint effort by researchers Dr. Wooje Han and Professor Hyung-Ho Park from Yonsei University in collaboration with Dr. Jiwan Kim at Kyonggi University developed a new ligand treatment method using ultraviolet-exposure. First, they identified densified titanium oxide nanocrystals with different states of surface ligands and evaluated their oxygen vacancy properties. Secondly, the nanocrystal surface capping ligands were exchanged with the newly introduced fluorous ligands and their feasibility in overcoming the aforementioned treatment challenges investigated. The effect of the fluorous ligand modification and residual ligands on the surface of nanocrystal using ultraviolet exposure treatment was investigated. The work is published in the journal, Applied Surface Science.

Analytical results of the surface, crystalline sizes, optical properties and ligand states of the titanium oxide nanocrystal were found and presented. The resulting nanocrystalline exhibited 10% less roughness and thickness as compared to oleylamine-titanium oxide. This was attributed to the high polarity of the fluorous ligand on the nanocrystal surface. The authors applauded the role of fluorous ligand modification in the removal of surface oxygen vacancies on the titanium oxide nanocrystal thin films through substitution of the non-polar ligands and passivation of the oxygen vacancies. Besides, it was worth noting that the ultraviolet exposure treatment decomposed the ligands on the nanocrystals under various atmospheres ranging from inert to reactive.

The nanocrystal films were densified with a record flatness improvement by 60% compared to the non-ultraviolet treated films. These results validated the contribution and importance of the fluorous ligands in overcoming the disadvantages involved in the synthesis of oxide nanocrystal materials. Specifically, fluorous ligands with shorter lengths were identified as a good inhibitor of ligand binding by inducing nanocrystal dispersion.

In summary, the authors ascertained the feasibility of using a combination of fluorous modification and ultraviolet-exposure treatment in controlling the recombination effects of oxygen vacancies. In a statement to Advances in Engineering, Professor Hyung-Ho Park, the lead author highlighted that the proposed new ultraviolet-exposure treatment for oxygen vacancy control will facilitate defects control in the fabrication of titanium oxide nanocrystal-based devices desirable for enhancing the device properties.