Fabrication And The Turning Of Optical Properties Of MOFs-based Optical Thin Films

Metal-organic frameworks have attracted immense attention due to their outstanding features,such as ultrahigh porosity,large internal surface area,good thermal stability and high chemical tailorability.These excellent properties make MOFs have great potential to become excellent optical thin film materials,and play an important role in optical/optoelectronic devices.In this thesis,a series of nanoscale MOFs were prepared by hydrothermal method and used to fabricate optical films by spin coating.Then,the influence factors on the optical properties of MOF optical films were investigated.The optical properties of MOFs optical films were successfully tuned by post-modification.The following work were mainly carried out:?1?The MOFs of MIL-101 series,MIL-53 series and NH₂-MIL-88B?Fe?were prepared by hydrothermal method and characterized in detail.Also the optical thin films of these MOFs were fabricated by spin coating.It was found these MOFs in nanoscale were well crystallized and with rich pore structure.The MOFs optical films prepared by spin coating were uniform,smooth and with good thermal stability and chemical stability.?2?The effects of ligand,morphology and crystal type on the optical properties of MOFs optical films were investigated.The effective refractive index(n_eff)and extinction coefficient?k?of optical films were characterized by ellipsometry.The porosity of optical films was estimated by Image J software based on the SEM pictures.The intrinsic refractive index(n_dense)was calculated according to the formula of effective medium theory.It was found that the effective refractive index(n_eff)of MOFs thin films decreased with the increase of functional groups,and the extinction coefficient?k?increased with the increase of electron donating groups,and decreased with the increase of electron withdrawing groups.In addition,the n_dense of the dense MOF materials evaluated according to their porosity percent increases with the increase of the functional group which were attributed to the increase of atomic density of MOFs.The influence of the morphology on the MOFs optical films is only related to their porosity.The NH₂-MIL-53?Al?NRs and NH₂-MIL-53?Al?NPs optical films have almost the same n_dense.However,the n_eff of NH₂-MIL-53?Al?NRs optical film is smaller than that of NPs due to the richer pore structure from the stacking of nanorods.Optical films fabricated by MOFs in different crystal types also have different optical properties mainly due to the different porosity of the films.NH₂-MIL-101?Cr?optical film has the largest porosity that makes it a lower refraction index than NH₂-MIL-53?Al?and NH₂-MIL-88B?Fe?optical films.?3?The tuning of the optical properties of MOFs thin films were realized by post-modification.Three aldehydes with different lengths of carbon chains were chosen to modify the MOFs optical thin film to tune their optical properties.It was found that the refractive index and extinction coefficient changed the most after pentanaldehyde modification,followed by propionaldehyde,and there was almost no change after heptaldehyde modified.The is because the sizes of propionaldehyde and valeraldehyde were small which makes it easy to go into the channel of MOFs and react with amino group.Such high increases in n_eff of MOFs thin films were not only due to the increase in the alkyl chain in the inner pore of NH₂-MIL-53?Al?but also attributed to the change of the electronic structure induced by modifiers.And the heptaldehyde is unable to enter its pores due to its large molecular size,resulting in a low modification rate.