Preparation And Characterization Of ZnO Nanocrystals Stabilized By Ionic Liquid Molecules

Owing to quantum-confinement effects, semiconductor nanocrystals (NCs) usually exhibit unique properties different from their bulk materials. Especially for the optical properties, NCs have more predominance than the bulks in applications, such as optoelectronic conversion, photocatalysis, fluorescent labels, sensors and biomedical imaging et al. However, compared with the bulk ones, NCs tend to aggregate or undergo Ostwald ripening because of their high surface energy. To stabilize NCs, appropriate surface modifications are quite necessary.To prepare highly luminescent blue-emitting ZnO NCs with long-term stability, the functionalized ionic liquid (IL) molecule with long alkyl chains were chosen to be the surface modifier. In this approach the ZnO NCs are formed directly from an IL salt containing ZnII cations through a sol-gel method, and the nanocrystal size, and consequently the photoluminescent (PL) properties of the NCs, can be tuned by varying the reaction conditions. The nano-materials with size of 2 to 4 nm emit blue to yellow light (420 nm to 550 nm) efficiently. Furthermore their PL quantum yields reach as high as 45 % in solvent, and the PL spectra of the solvent-free ones are stable even when they are heated at 80 oC. Note that when either the three solvated or solvent-free samples of IL-ZnO-A, -B and -C are mixed together in a weight ratio of approximately 1:1:1, the resulting material emits bright white light. And the IL-ZnO composite clearly shows its liquid-like features after heating to 80 oC. This fluidity implies that the material should be highly processable upon mild thermal treatment. In addition IL-ZnO NCs are redispersible in many solvents such as chloroform, ether and dichloromethane to form stable colloids. It is really an easy strategy to get mass-production of IL-ZnO with long-term stability, which will be in great favor for us of understanding ZnO nanocrystal dots in their optical properties.Nano-ZnO has been known as the most important and promising semiconductor. Different from other high-temperature techniques such as CVD and hydrothermal methods, the ZnO nanocrystals prepared by sol-gel process at low temperatures are far from thermodynamic equilibrium and rich in defects and surface states, which will strongly affect the PL. So until now there are still a number of unresolved issues on which contradictory explanations, such as the nature of shallow donors as well as the orgin of the visible emission. Generally speaking, many defects and impurities can exist at one time in ZnO, such as interstitial zinc Zni, interstitial oxygen Oi, zinc vacancy VZn and oxygen vacancy VO (including singly charged ones, VO+ and doubly charged ones, V_O⁺⁺, as well as impurity atoms like Li,Na,Cu,C,N). However in spite of numerous studies on ZnO, many contradictions in terms of the positions of reported peaks in ESR and PL and their assignment can be found in the literature. So to make sense of the PL mechanism, we synthesized the other two samples, acetate and PEG modified ZnO NCs by the same sol-gel process in comparison with IL-ZnO NCs. Through the characterization of UV-vis, PL spectra, and ESR et al, it can be identified that the defect energy levels existing in ZnO NCs, such as Li⁺ and oxygen vacancies should be responsible for the visible light emission. In addition there is no exciton emission is detected from any of the samples, so we conclude that V_o⁺ plays the key role in producing visible luminescence.And also the as-prepared ZnO NCs can spontaneously form 3D hexagonal and face-centered cubic superlattices. Here we provide a sphere model composed of"hard"(ZnO nanocrystal, core) and"soft"(IL molecule, shell) components to describe the assembly of IL-ZnO NCs, suggesting that it is feasible to fabricate different 2D or 3D organizations just by controlling the NC particle size of the inorganic core. To get further proofs, ZnS and CdS NCs capped with mercapto-IL molecules are prepared, and as NC size increases, the"soft"/"hard"component ratio becomes smaller, leading to deviation of the NC stacking from a lamellar structure to an fcc superlattice successfully. The first-time supposed"soft"/"hard"sphere model will be of great importance in designing NCs with different super-structures.In conclusion, we have demonstrated a facile route for the stabilization of ZnO NCs that show tunable photoluminescence and self-assemblies. Moreover the surface capped IL molecules render the NCs very stable, highly luminescent and well ordered. The PL and ESR results indicate that V_o⁺ plays the key role in producing visible luminescence efficiently. The size-dependent luminescent and self-assembled properties of the obtained ZnO nanocrystals indicate their bright future for applications as new luminescent materials.