Fabrication of Quantum Dots@Metal–Organic Frameworks Nanocomposites By Direct Surface Modification

Abstract

Semiconductor quantum dots (QDs), have been attracting much attention as the next generation of light-emitting materials due to their unique properties, such as their broad absorption range and narrow band-edge photoluminescence (PL) with high emission quantum yield. Since, in particular, precise controllability of luminescent color by changing the QD size is quite attractive and useful, they have been practically used in devices such as back light films of liquid crystal display. Although highly dense QDs are required for color filters and on chip light sources with high efficiency, Förster resonance energy transfer (FRET) to non-fluorescent QDs is inevitable when each QDs adjacently exists, resulting in large decline in quantum yields (QY). One of the reasons to generate non-fluorescent QDs is the accumulation of damage over time to the organic ligands attached to the surface as a passivation layer. To overcome the QD degradation derived from organic ligands desorption, the development of robust and easily grown surface protective materials is important.Metal–organic frameworks (MOFs), which are a new class of microporous crystalline materials formed by the self-assembly of metal ions and organic linkers, are promising candidates as new surface modification materials for QDs. Owing to their versatile architecture and adjustable functionality derived from the combination of their components, MOFs possess high variety of applications, such as gas storage, catalysts, and semiconductors. Among them, the combinations of MOFs with other categories of nanomaterials, such as metal nanoparticles, have attracted much attention lately to give further functionalities to MOFs. Therefore, if MOFs are utilizable as new classes of surface modifiers for fluorescent QDs, not only the stability improvement but also new functionality appearance of QDs are expected.In this study, the aforementioned possibility of MOFs toward fluorescent QDs was investigated. CdSe/CdS core/shell QDs were attempted to be covered by zeolitic imidazolate framework-8 (ZIF-8) through a direct link between the two without using any other bonding materials like polymers. The QDs were observed to be fully incorporated into the ZIF-8 crystal and were photoluminescent with a solid-state PL QY as high as 40%. Narrow band-edge emission unique to the QDs was maintained during and after the modification with ZIF-8. In addition, we visually confirmed the structure of ZIF-8 crystals gradually transformed from a roughly spherical shape to rhombic dodecahedra (Fig. 1). The incorporation of a single QD into a ZIF-8 crystal was also successfully accomplished by changing the ligands on the starting QDs. To evaluate the potential of ZIF-8 as a protecting shell for QDs, 1,4-benzoquinone, one of the efficient electron acceptors that can quench the PL from CdSe/CdS core/shell QDs, was added to the solution of pyridine-capped CdSe/CdS core/shell QDs as well as a suspension of CdSe/CdS QD@ZIF-8 composite. As shown in Fig. 2, the addition of 1,4-benzoquinone hardly affected the PL of the QDs after ZIF-8 modification, indicating complete incorporation of the QDs into ZIF-8 crystals. Figure 1