Huo et al. Lu et al. Redel et al. Although there have been a few successful cases to prepare MOF thin films, there is still a lack of systematic exploration on the performance and potential application of the thin films from the perspective of optical films.
Moreover, for the same MOF system, there is a lack of detailed comparison on these preparation methods. In addition, the stability of optical thin films is vital for practical application.
If UiO can be successfully prepared into a high quality optical film, it will be possible to apply it to actual optical or optoelectronic devices in the future.
In this study, UiO was chosen as a model. The UiO thin films were fabricated by five methods Scheme 1 b—f , i. These optical thin films were compared from the aspects of appearance, reflectivity, micro-morphology, surface roughness, and optical properties.
The OTF-SP film has a uniform color, greater reflectivity, flat microstructure, and minimal roughness. The OTF-DG film has the highest roughness, which cannot meet the requirement for the use in optical films.
The OTF-LBL film has the brightest color, a dense and smooth film surface, low surface roughness, and the maximum effective refractive index.
Figures S6 and S7. All reagents were used as received without further purification. After thorough rinsing with deionized water, the wafers were dried under air flow and stored in ethanol.
For spin-coating, dip-coating, self-assembly deposition, and direct growth, the substrates were dried under nitrogen flow before use.
For layer-by-layer growth, the substrates were functionalized with H 2 BDC by incubating in a N , N -dimethyl formamide DMF solution containing 33 mM H 2 BDC at room temperature for 3 h, and then cleaned with ethanol and dried with a stream of nitrogen gas before use.
The OTF-SA films were prepared according to a previous report [ 9 ], in brief, the polyvinylpyrrolidone PVP -functionalized UiO nanocrystals self-assembled on the water-air interface, and then the formed close-packed monolayer of UiO nanocrystals was transferred onto the silicon substrate through the simple dip coating process Scheme 1 d.
The experimental details were shown in Supplementary Materials. The N 2 adsorption-desorption isotherm of the samples at liquid nitrogen temperature 78 K and gas saturation vapor tension range was measured by a BEL Mini sorption instrument Bel Japan Inc.
The samples were sputtered with a thin layer of Au prior to imaging. As shown in Scheme 1 a, in perfect crystal structure of UiO, each Zr-O metal center [Zr 6 O 4 OH 4 octahedron] is fully coordinated by 12 terephthalate linkers to form a highly connected framework.
The UiO nanocrystals exhibit octahedron morphology, that has a side length of approximately nm Figure S1. The isotherm of UiO Figure S4a shows typical absorption of a microporous material.
Accordingly, the total pore volume was 0. Based on the desorption isotherms, the pore size of UiO was 0. All these results confirm the success of the preparation of UiO nanocrystals.
In general, octahedron-shaped nanoparticles are more difficult to form homogenous and flat films than spherical nanoparticles. Here, octahedral UiO nanocrystal-based thin films were fabricated by spin-coating, dip-coating, and the self-assembly method.
Figure 1 a shows the optical thin film fabricated by spin-coating, which displays a uniform pink color. The reflectance spectra of them are very similar Figure S9 , which agrees with their colors.
The OTF-SA shows varied color for different deposition times, from light yellow once , dark yellow twice , to yellowish green trice Figure 1 e—g.
The corresponding reflection spectra have reflection peaks at nm, nm, and nm, respectively Figure S Besides, the MOF thin films were also constructed by hydrothermal direct growth, the obtained OTF-DG shows dark green and looks lackluster Figure 1 h , which is in agreement with its reflection spectrum which has no apparent reflections between nm and nm Figure S The smooth spectrum means there is less light scattering by the film because of the absent of obvious aggregations or cracks.
Photographs of MOF optical thin films. Surface micro-morphologies of the optical thin films were observed by SEM, as shown in Figure 2.
When the film is dried, UiO colloidal particles are more likely to attract each other and accumulate due to the capillary force, and induced the cracks between the domains.
As a whole, the film is still relatively flat. For the film deposited only once by dip-coating Figure 2 c , the surface coverage is only The discrete UiO nanocrystals are distributed across the surface of the substrate.
Therefore, under our experimental conditions, more than five depositions are required to obtain a more uniform film. For the self-assembly method, a single-layer film of MOFs can be obtained by the self-assembly once Figure 2 i.
When self-assembling twice, the nanocrystals of the second layer are randomly deposited into the gap between the first layer of nanocrystals to improve surface coverage of the film Figure 2 k.
During self-assembly deposition, the number of nanocrystal layers of the film corresponds to the number of assemblies.
Unlike the solution-based deposition methods of MOF nanoparticles, the film obtained by the direct growth method consists of a single layer of UiO crystals with varied sizes, as shown in Figure 2 o.
The large grains are about one micron, while the small grains are less than nm. The surface is very rough, and the uneven thickness of the film can also be seen from the side-view SEM image Figure 2 p.
Generally, the film is very uniform. Furthermore, the qualities of the films were quantitatively analyzed using surface roughness.
The surface roughness was determined using a profilometer with a diamond stylus. The sampling length and evaluating length were 0.
The total evaluating length was 10 mm for evaluating five times. The arithmetic average roughness, R a is the arithmetic average value of filtered roughness profile and the root mean squared roughness, R q is root mean squared value of filtered roughness profile.
The calculated R a and R q are listed in Table 1. The roughness of MOF thin films made of octahedral UiO nanocrystals is generally higher than that of spherical MOF nanoparticles-based thin films [ 19 , 22 , 23 ].
For the dip-coated films, the more depositions, the larger the roughness due to the accumulation, which is consistent with the SEM observation.
For the self-assembled films, the trend is just the opposite. Their roughness decreases with the increase of depositions, thanks to the interleaved filling of following nanocrystals, which is in agreement with the micro-morphology.
The optical constants of the MOFs-based thin films were explored by spectroscopic ellipsometry.
A Cauchy model was chosen to fit the experimental data. The experimental and generated ellipsometric parameters of the films are presented in Figures S22—S The fitting of all MOF thin films present sufficient agreement with the experimental data over the entire measured spectral range, except the film produced by direct growth Figure S26 , suggesting that OTF-DG films cannot meet the requirement of the optical films due to their high roughness.
The variation tendency of the thickness agrees well with the SEM observation. The roughness of the thin films led the ellipsometry method to overestimate their thicknesses because of the high domain on the surface of thin film would be recognized as the surface level.
The obtained optical constants for the MOF thin film as a function of wavelength are shown in Figure 3 and Figure S The n of OTF-LBL has the highest value, which can be considered as the intrinsic refractive index of UiO due to the compactness and integrity of the film.
For convenience of discussion, the average index values from nm to nm and that at the wavelength of nm the longest wavelength in the measured range for all samples are listed in Table 2.
The nanocrystal-based films can achieve a refractive index lower than 1. Such low refractive index benefits from the cracks and voids between the UiO nanocrystals.
Assuming the intrinsic refractive index of UiO was 1. The results are shown in Table 2. Average efficient refractive index n ave , specific efficient refractive index at nm n , and calculated voids of MOF optical thin films.
In summary, the UiObased thin films have been successfully prepared using five different methods.
When the deposition times are few, the surface coverage is low, and when increasing the times of deposition, a more homogenous film can be obtained.
The OTF-SA film has a uniform color, and the number of MOF layers is in direct proportion to the times of assembly, while the roughness decreases as the number of assembly increases, thanks to the interleaved filling of following nanocrystals.
The surface of the OTF-DG film is dark in color and has the highest roughness, which does not meet the requirements for use of optical films.
The OTF-LBL has the brightest color, a flowing reflection spectrum curve small scattering , dense and smooth film surface, low surface roughness, maximum effective refractive index, but small thickness, and a growth efficiency of about 3.
Zr, cyan; C, gray; O, red; H, omitted. Conceptualization, C. National Center for Biotechnology Information , U.
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