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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - VAPOMOF (Vapour phase deposition of metal-organic frameworks with luminescent guests for solid-state lighting and sensing)

Teaser

Summary

Metal-organic frameworks (MOFs) are solid compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. MOF materials possess unique advantages for luminescence-based applications primarily because of their capability of incorporating luminescent guests at both metal centers and organic ligands. This allows efficient modulation of luminescent properties. In addition, permanent porosity in these structures permits the accommodation of luminescent guests (LGs) within their frameworks, offering another degree of tunability in their emission properties. Because of the well-defined nano-environments in their pores, MOFs have high potential as encapsulation matrices to enhance both the efficiency and stability of LGs.
For many applications, especially in (micro)electronics, a key enabling step in leveraging the properties of MOFs will be the development of robust thin film deposition methods. However, all reported thin-film fabrication techniques is solution based, which is incompatible with microelectronic fabrication processes because of contamination and corrosion upon exposure to the MOF synthesis mixture and the environmental impact and cost related to solvent use.
The recently developed vapor phase deposition technique for MOF film deposition (MOF-CVD) has shown great advantages for the fabrication of uniform MOF thin films. However, it was only demonstrated as a proof-of-concept, for the MOF material ZIF-8. In the VAPOMOF project, we will broaden the scope of MOF-CVD and implement it for the fabrication of LG@MOF thin films for development of phosphor-converted white light-emitting diodes (PC-WLED) and volatile organic compounds (VOSs) sensors. This general objective translates in a set of specific partial objectives: 1) Broadening the scope of MOF-CVD; 2) Fabrication of LG@MOF films through MOF-CVD; 3) PC-WLED based on LG@MOF layers deposited though MOF-CVD; 4) Optical sensing of small VOCs via responsive LG@MOF films deposited through MOF-CVD.

Work performed

The implemented experimental details for targeting above-mentioned individual objectives are reported below.
The implementation of MOF-CVD technique for MOF film deposition was only demonstrated on ZIF-8, a prototypical zeolitic-imidazolate framework (ZIF). The first objective is to demonstrate the feasibility of this technique for other MOF films deposition. A variety of characterization techniques were employed to evaluate the film deposition, including grazing-incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), atomic layer deposition (AFM), ellipsometry and quartz crystal microbalance (QCM). As the ZIF family is big based on the large variety of imidazolate ligands, the extension of MOF-CVD method to other ZIFs was first investigated. Several deposition parameters were studied including thickness of precursor layer, temperature, reaction time and vapor additives. Some of the ZIFs such as ZIF-7-III and ZIF-72 can be successfully deposited by MOF-CVD. The possibility of this technique for other metal based MOFs were also investigated. The selected MOF is MOF-74 which consists of metal nodes connected by 2,5-dihydroxyterephtalic acid (DOBDC). The metal centers can vary from alkaline earth metal Mg to transition metals Zn, Fe, Ni, Co, Mn and Cu. Here, with the optimization of the reactor setup together with deposition parameters (e.g., temperature, time and additives), the deposition of MOF-74 (Zn, Co and Mg) through MOF-CVD technique was demonstrated.
As targeting the applications of LG@MOFs for solid-state light and chemical sensing, the first important task is to develop an efficient method to load LGs in the pores of MOFs. Therefore, we developed the MOF-CVD technique for the efficient loading of LGs inside MOFs with tunable loading amount. Several types of LGs were demonstrated to be able to be loaded in the MOF pores, such as polycyclic aromatic hydrocarbon and metal-organic complex. The successful loading were demonstrated by powder x-ray diffraction (PXRD), N2 physisorption, nuclear magnetic resonance spectroscopy (NMR) and thermal gravimetric analysis (TGA). Interestingly, different fluorescence colors can be achieved by just loading one LG with different loading amount. Importantly, as these organic LGs were encapsulated inside the MOF pores, the thermal stability are dramatically enhanced.
As the encapsulation of LGs inside MOFs can be readily achieved through MOF-CVD technique, the obtained LG@MOFs were introduced on the top of the UV emitter. The fluorescence emission color can be tuned from blue to orange. Importantly, carefully control the loading amount, white light emission can be achieved. Also, some LGs loaded MOFs show yellow fluorescence light via the blue light excitation, thus, have the potential for blue light converted WLED. Interestingly, when the LG@MOFs exposed to some VOCs, the photoluminescence properties changed, such as fluorescence intensity and spectrum shift, thus, showing high potential for VOC sensing.

Final results

At the point of the starting of the fellowship, MOV-CVD was only demonstrated for the deposition of one MOF (ZIF-8). The extension of this technique for other MOF films was expected, as the physical and chemical properties can arise from different MOFs. Up to now, a couple of MOF films can be deposited using MOF-CVD technique. Depending on the properties of MOFs such as conductivities, gas sorption properties, different types of MOFs are foreseen to be integrated into microelectronics devices.
The encapsulation of LGs in MOFs for solid-state lighting has been explored, and has shown great potential. However, the tunable loading of LGs in the pores are difficult to achieve, especially the high loading because of the solubility issue. By using the MOF-CVD method, this challenge can be overcame because of this solvent-free synthesis process. In this project, we achieved tunable emission by only one LG encapsulated in MOFs with different loading amount. Importantly, this achievement is based on common organic LGs based on polycyclic aromatic hydrocarbons which is inexpensive compared to rare earth based compounds. According to the economical consideration, these materials show high potential for new generation of LED. Also, the high thermal stability of these LG@MOFs show great advantages for the integration in LED lights.
The remaining porosity of LG@MOF materials was demonstrated and the influence of VOCs on the photoluminescence properties was observed. Therefore, the potential of these materials for chemical sensing is foreseen. Because of the well-defined pore geometry of MOFs, the selective sensing of VOCs are expected which are very important for further development of commercially available sensors.