Tuning optical properties of MOF-based thin films by changing the ligands of MOFs

Abstract

The preparation and development of novel optical thin films are of great importance to functional optical and opto-electric components requiring a low refractive index. In this study, a typical metal-organic framework (MOF), MIL-101(Cr), is selected as the research model. The corresponding MOF nanoparticles are prepared by a hydrothermal method and the optical thin films are successfully prepared by spincoating. The optical properties of the corresponding MOF thin films are controlled by changing the type of functional groups on the benzene ring of the ligand (terephthalic acid) on MOFs. The functional groups are hydrogen atoms (H), electron donating groups (-NH 2, -OH) and electron withdrawing groups (-NO 2, -(NO 2 ) 2 or F 4 ), respectively. It is found that the effective refractive index (n eff ) of MOF thin films decreases along with the increasing voids among MOF nanoparticles. In addition, the extinction coefficient (k) increases with the addition of electron donating groups, and decreases with the addition of electron withdrawing groups. Among the MOFs used in this study, the n eff of NO 2 -MIL-101(Cr) containing electron withdrawing groups is as low as~1.2, and value of k is particularly low, which suggests its potential application in antireflective devices. In addition, the intrinsic refractive index (n dense ) of the dense MOF materials evaluated according to their porosity increases with the number of the functional groups, and the n dense of the two nitro-substituted MOFs is greater than that of the single nitro-substituted one, and the latter is bigger than that of hydroxyl-substituted one, which is close to that of amino-functionalized one. The diversity of ligands in MOFs makes them a promising new generation of optical materials.

Metal Organic Frameworks-Based Optical Thin Films

“…Our group [16,[22][23][24] successfully applied this method to construct many other MOF thin films, such as iron (III) terephthalate MIL-88B (MIL = Materials of Institut Lavoisier), aluminum terephthalate MIL-53, MIL-101(Cr), and their analogs. We [16] reported the first example of flexible MOF optical thin film fabricated by spin coating (Figure 1).…”

“…Such a low refractive index benefits from the cracks and voids between the UiO-66 nanocrystals. Assuming the intrinsic refractive index of UiO-66 was 1.512, the index of air is 1, the void percent in the film was estimated according to the effective medium theory [7,23,42]. The voids of OTF-SP, OTF-DP, and OTF-SA films are about 25, 30, and 23%, respectively.…”

“…The OTF-SP film has the best flatness, while the OTF-DG has the worst roughness (Ra = 40.3 nm). The roughness of MOF thin films made of octahedral UiO-66 nanocrystals is generally higher than that of spherical MOF nanoparticles-based thin films [23,24,27]. For the dip-coated films, the more the depositions, the larger the roughness due to accumulation.…”

“…Our group [23] reported the regulation of the optical properties of MOF optical films by changing linkers with different functional groups. Five different ligands were selected and synthesized with chromium ions to obtain five MOFs of the same MIL-101 configuration, which were respectively recorded as MIL-101(Cr), NH 2 -MIL-101(Cr), NO 2 -MIL-101(Cr), OH-MIL-101(Cr), and (NO 2 ) 2 -MIL-101(Cr), and the corresponding optical film was produced by spin coating.…”

Mass transport through metal organic framework membranes

“…Thus, MOFs demonstrate great potential applications in various fields. Owing to the decades' efforts of researchers, thousands of structures of MOFs have been discovered and applied in various fields, such as separation [31][32][33][34][35][36][37][38], adsorption [20,[39][40][41][42], optics [43][44][45][46][47][48][49][50], catalysis [19,[51][52][53][54][55][56], sensors [57][58][59][60][61][62][63], etc. However, most of these studies are based on MOFs powders.…”