Vapor-phase methods for synthesizing metal-organic framework thin films

Due to their unique structures and exceptional physical and chemical properties, metal-organic framework (MOF) materials have garnered extensive attention in various fields, including catalysis, separations, sensing, and optics. Compared with powders or bulk MOF materials, MOF thin films exhibit large vertical and horizontal dimensions, higher specific surface areas, and abundant active sites and undergo facile combination with other functional centers for adsorption/separation, catalysis, and photoelec-tronic device applications. Among the methods used in preparing MOF thin films, the vapor phase approach enables more effective growth of MOF films with controllable thicknesses, uniformity, and compatibility; thus, it has attracted significant interest. This extensive review presents four vapor-phase approaches for preparing MOF thin films: the steam-assisted conversion method, vapor-phase transformations of metal oxide templates, vapor-phase linker exchange, and the atomic layer deposition/molecular layer deposition method. We summarize the advantages and disadvantages of these different vapor-phase-based methods for thin-film preparation, aiming to promote their use in precise and controllable surface syntheses


INTRODUCTION
2][3] Due to their ultrahigh specific surface areas and pores with uniform sizes and versatile functionalities, MOFs have a wide range of applications in catalysis, [4][5][6][7] separation, [8][9][10] sense, [11][12][13] optics, [14][15][16][17] , etc.The high density of metal ions and ligands in the MOF channels effectively and selectively adsorb small molecules to achieve gas storage and separation.MOF films provide active sites to bond metals and catalyze reactions. 18,19Despite numerous fundamental studies, the practical uses of MOFs are limited, as they are typically obtained as polycrystalline micropowders with very low processabilities.In this respect, MOF thin films have the potential for unprecedented technological breakthroughs since they provide possibilities that are otherwise not feasible with traditional bulk powders.
In recent years, researchers have shown that two-dimensional thin-MOF films exhibit unique physical, chemical, electronic, and optical properties. 10,20,21ompared with powders or bulk materials, MOF thin films have higher diffusion efficiencies and impressive responsiveness to external fields, such as light, electrical, and magnetic, which is useful for conducting in-depth research on MOF films. 22,23MOF thin films can be compounded with other functional substrates to obtain special catalytic, optical, and sensing properties.However, the properties of most other substrates and MOF materials are quite different, and the quality and stability of the films formed on substrates are usually poor.8][29] Toxic organic molecules are often used as solvents in solvothermal processes, and residues often remain in the material pores, corroding the organic framework and affecting the performance of the material. 30In addition, the thin films prepared by the solvothermal method are typically formed by accumulating MOF particles, and uniform and dense nanofilms are difficult to obtain.Another problem affects the films obtained by the sacrificial template method.The metal oxide template films combine with the organic ligands to form a MOF thin film in a solvent used for the sacrificial template method.When the metal oxide film is thick, incomplete conversion occurs. 31ompared with the liquid-phase method, preparing MOF thin films via direct reactions between solid precursors and ligands is more challenging.
Researchers have endeavored to develop diverse gas-phase technologies for the growth of MOF thin films to control the film quality, thickness, and weak bonding strength while avoiding the utilization of toxic, polluting organic solvents.Compared to liquid phase approaches, the vapor phase methods employed in preparing MOF thin films minimized solvent usage and prevented film corrosion.Moreover, these methods enhance the bonding force, quality, and control accuracy of the MOF thin film on the substrate.This review will comprehensively summarize four distinct vapor-phase preparation technologies: steam-assisted conversion, vapor-phase conversion of metal oxide templates, vapor-phase cooperative exchange method, and atomic layer deposition (ALD)/molecular layer deposition (MLD).Their characteristics, advantages, applications, and limitations are systematically analyzed and summarized (Figure 1).Ultimately, our review is designed to provide a more systematic understanding of these innovative thin-film preparation technologies and stimulate research interest.

VAPOR-PHASE METHODS USED TO SYNTHESIZE MOF FILMS Vapor-assisted conversion (VAC)
Vapor-assisted conversion (VAC) separates the solvent and the precursors during the generation of the MOF.This method is based on converting the precursors in the solution layer into a continuous crystalline and porous film by exposing them at moderate temperatures to a vapor with a specific composition.Crystallization of the precursor is achieved through the interactions between the solvent vapor and the precursor.In 1990, the steamassisted conversion method was developed to synthesize the inorganic zeolite ZSM-5. 32In 2011, Jinxiang Dong et al. developed a water vaporassisted approach to synthesize ZIF-8 and ZIF-67 in a hydrothermal reactor. 33The inert holder isolated the precursors from the water solvent, and crystallization of the precursors occurred after H 2 O vaporization at a high temperature (Figure 2).However, solvent molecules were usually condensed into the pores during cooling after vapor-assisted conversion, and they affected the MOF performance.To solve this problem, Jinping Li et al. added a molecular sieve adsorbent to store a certain amount of solvent before the vapor-assisted conversion method. 34The adsorbed solvent was desorbed at high temperatures to promote MOF crystallization and then readsorbed at low temperatures.A relatively dry MOF was obtained without further washing and drying steps.The prepared MOFs showed good performance in NH 3 adsorption, high water absorption, and structural stability.
The steam-assisted reforming method is also suitable for exchanging the metal ions in MOFs.In 2019, a team 35 mixed PCN-6 particles with metal salts (CoCl 2 , FeCl 2 , MnCl 2 and ZnCl 2 ) and placed them on a holder in a hydrothermal reactor.Therein, the methyl formamide solvent in the bottom of the reactor was volatilized, and the metal ions were exchanged to form composite MOFs.The metal introduction ratio exceeded 20%.After introducing Fe ions into PCN-6, the BET surface area of PCN-6'(Fe) reached 1221.6 m 2 g −1 , and its CO 2 adsorption performance was better than that of PCN'(M) doped with Co, Mn, or Zn ions.
The VAC method converts a film of the precursor mixture into an MOF film.In 2018, Erika Virmani et al. 36 synthesized thin zirconium-based MOF films via vapor-assisted conversion, and the protocols were suitable for the growth of UiO-66, UiO-66(NH 2 ), UiO-67, and UiO-68(NH 2 ).In addition, porous, oriented continuous films of interpenetrated Zr-organic frameworks were established (Figure 3A).The thin films were investigated by X-ray diffraction (Figure 3B).The observed reflections indicated that the [111] axes are vertically aligned with the gold surface and underwent unrestricted rotation around these axes, demonstrating a high degree of crystal orientation.Therein, the precursor concentrations controlled the thickness of the oriented thin film to several microns.Acetic acid at the proper concentration and the diffusion rate determined the uniformity and crystallinity of the MOF film.Dana D. Medina et al. 37 introduced the synthesis of a unique nanoscale architecture with pillar-like Co-CAT-1 MOF crystallites (Figure 3C).The gold film was deposited on stainless steel meshes (SSMs) via physical vapor deposition (PVD), and the resulting Au@SSM was used as a substrate for the VAC synthesis.Briefly, a solution of water and 1-propanol was evaporated at 85 °C for 12 h to align the Co-CAT-1 crystals perpendicular to the surface preferentially.In 2022, Gregory J. Szulczewski's team 38 synthesized thin films of Co-MOF-74 and Ni-MOF-74 with a VAC method that precluded activation via postsynthetic solvent exchange.A series of studies demonstrated that the Co-MOF-74 and Ni-MOF-74 thin films grown by VAC maintained the intrinsic properties of the frameworks without surface barriers to adsorption.
The VAC method was also utilized to synthesize other functional or composite MOF films.To overcome the incompatibility between the anode and the cathode chemistry for Mg batteries, Dunwei Wang et al. 39 synthesized a Mg-MOF-74 thin film on Au and porous anodized aluminum (AAO) and used VAC to separate the anolyte and the catholyte for low-resistance, selective transport/transport of Mg 2+ .The composite MOF film was synthesized after growing Mg-MOF-74 on the AAO film, and it exhibited high selectivity in transporting Mg 2+ and blocking solvent (propylene carbonate) transport.However, the location and penetration of Mg-MOF-74 into the pores of AAO were not easily controlled.In 2020, Zhi-Gang Gu and Jian Zhang et al. used VAC to deposit a pretreated precursor solution onto a quartz substrate to form a continuous porphyrin-based PCN-222 film. 40Furthermore, Ptdoped carbon nanodots were then loaded into the pores of the PCN-222 film to enhance its nonlinear optical limiting effect.In 2022, Norikazu Nishiyama Materials Figure 2. Formation processes of ZIFs Top: hydrothermal synthesis; Bottom: steam-assisted conversion (mim=2-methylimidazolate). 33 et al. reported the synthesis of glycine-modified UiO-66 with a VAC method. 41heir results demonstrated that the HCl-VAC method was the most suitable for synthesizing glycine-modified UiO-66 with high crystallinity and porosity.The modified glycine improved the adsorption capacity of CO 2 .
The VAC method has exhibited superior conversion efficiencies and operates under milder reaction conditions than the solvothermal phase method.In contrast to the solvothermal approach, VAC is achieved by separating the precursors and solvent and minimizing the influence of the solvent molecules on the MOF film.Highly crystalline or composite MOF films can be prepared by controlling the precursors and solvent ratio on a large scale.However, this technique requires a solvent and does not address the corrosion and chemical contamination issues.Furthermore, nonuniform crystallization and reactivity variations in the precursors also limited control of the MOF films with the VAC method.

Vapor phase transformation (VPT) of metal oxide films to MOF films
Vapor phase transformation of a metal oxide film offers a direct and attractive approach for preparing a MOF film.The morphology, thinness, and location of the MOF film depend on the metal oxide film precursor, which must match the oxide dissolution and MOF crystallization rates (Figure 4A).Unlike solvothermal methods, issues related to solvent corrosion are avoided by supplying the organic linker as a vapor instead of in solution.For the first time, Rob Ameloot et al. reported an all-gas-phase method (MOF-CVD) for preparing nanoscale MOF films (Figure 4B). 30They used MOF-CVD to prepare high-quality films of ZIF-8, a prototypical MOF material with a uniform and controlled thickness and high-aspect-ratio features.The MOF-CVD method consisted of a metal oxide deposition step and a vapor-solid reaction step.In lift-off patterning, zinc oxide was deposited by directional reactive sputtering instead of ALD, indicating the flexibility of the oxide deposition step in MOF-CVD.For comparison, a widely used liquid-phase method for depositing ZIF-8 led to the unwanted collapse of the ore pillar, resulting in the loss of original micropattern features.MOF-CVD was the first vapor-phase deposition method used for any solid microporous crystalline network, marking a milestone in processing these materials.This method was also utilized to prepare other MOF films (ZIF-61, ZIF-67, and ZIF-72) by altering the metals and linkers, and it considerably expedited the integration of the MOF materials into microelectronics.Additionally, Rob Ameloot et al. 42 reviewed the progress made in the vapor processing of MOFs (Figure 4C), summarized the underlying chemistry and principles, and highlighted promising directions for future research.MOF-CVD is a disruptive new direction for transforming vapordeposited metal oxides to MOFs.It could permit new opportunities for tuning film microstructures, chemical compositions (e.g., doping), and optical or electrical properties.In 2021, our team applied this approach to coat the surface of Pt/SiO 2 with a ZIF-8 nanofilm. 15Our findings confirmed that the ZIF-8 nanofilm covered the Pt nanoparticle and SiO 2 nanowire surfaces, thereby regulating their surface properties and environment and enhancing the selectivity of enanthate semihydrogenation.
The VPT method does not involve the use of solvents and does not induce corrosion in the film structure.It effectively preserves the original topographic structure of the metal oxide film and enables conformal deposition on materials with diverse shapes.Shunsuke Tanaka et al. 43 demonstrated the mechanism for gas phase conversion of zinc oxide nanowire arrays to MOF films, elucidating how different crystal faces of zinc oxide influenced the film formation speed and mechanism.They found that the MOF film was generated by a reaction between dimethylimidazole and zinc on zinc oxide, and the crystal planes and reaction temperature of the zinc oxide affected the film formation speed and mechanism.
Oxide films prepared by various methods can also be transformed into MOF films with the VPT strategy.For instance, Christine Young et al. 44 used a carbon cloth as a carrier to fabricate zinc oxide arrays and cobalt hydrate arrays (Figure 5A).The array structures remained unchanged after gas phase crystallization with dimethylimidazole, while the surface experienced over-  36 (C) Schematic illustration of Co-CAT-1@Au@SSM preparation via VAC. 37rowth.Shohreh Fatemi et al. 45 employed electrophoretic deposition to deposit zinc oxide on alumina tubes and vapor phase transfer technology to prepare ZIF-8 thin films (Figure 5B).The uniform zinc oxide predeposition layer obtained by electrophoretic deposition was the zinc source that provided local nucleation sites.The VPT method was carried out to seed the surface with ZnO nanoparticles and produce a more uniform layer, which led to stronger adhesion between the film and substrate than conventional methods.Additionally, this approach prevented detachment during conversion into a ZIF-8 film.Building upon this work, Rob Ameloot et al. 46 deposited copper or copper oxide precursor films through physical vapor deposition followed by phase transformations using 1,4-phthalic acid (H 2 BDC) or trans-1,4-cyclohexane alkane dicarboxylic acid (H 2 CDC) vapors to obtain CuCDC or CuBDC thin films, respectively.Since direct conversion of the oxide precursor thin film exhibited excellent shape retention, VPT technology could be used to preserve the specific pattern of the thin film precursor.
During vapor phase transformations, inward diffusion of ligand molecules is typically restricted by the micropores of the newly formed MOF film on the surface, thereby inhibiting further transformation of the internal oxide film.This diffusion limitation results in incomplete conversion of the thickened metal oxide film.Rob Ameloot et al. 47 adopted the chemical vapor-phase deposition method to prepare zeolitic imidazolate frameworks (ZIFs) and address this issue, as proposed in Figure 6A.The proposed strategy was validated with thin films of the zeolitic imidazolate frameworks ZIF-8 and ZIF-67, which were formed in 2-methylimidazole vapor from ALD of ZnO and native CoO x , respectively.This method provided uniform filling of the MOF in trenches with pitches of 45 nm and depths of 90 nm.For the first time, this team 48 realized the reaction of diethylimidazole vapor with a zinc oxide film to deposit an MAF-6 crystalline film with nanopores (Figure 6B).Sungmin Han et al. 49 utilized a combination of physical vapor deposition and chemical vapor deposition to prepare HKUST-1 thin films (Figure 6C).Physical vapor deposition was initially employed to deposit an ultrathin copper film on a SiO 2 /Si substrate, followed by the introduction of trimellitic acid (H 3 BTC) to initiate a chemical vapor deposition reaction.Through this alternating process of physical deposition and chemical vapor deposition, layer-by-layer growth of the MOF film was controlled, resulting in the preparation of MOF films with thicknesses ranging from 20 nm to 200 nm.By utilizing appropriate ligands for the gas phase reactions, it is possible to finely adjust the pore sizes of the film, leading to the synthesis of a highly nanoporous framework within the MOF film.Following a bottom-up growth approach under high vacuum conditions, H 3 BTC and Cu were directly deposited on the substrate with precise control over the layer-by-layer (LBL) growth process resulting in well-oriented HKUST-1 thin films.
With a metal or metal oxide film as the precursor, direct conversion of a metal oxide film into a MOF film can be achieved with vapor phase conversion technology, offering excellent shape retention and controllability.The solvent-free nature of this method shows significant potential for use with  30 (C) SEM top view of ZIF-8 film and replicated hexagonal pattern. 42lectronic devices by producing thin films.Most recent reports employed metal oxides as templates, and the shape and thickness of the precursor film determined the morphology of the resulting MOF film.However, during the conversion from oxide to MOF films, substantial deformation occurs with increases in thickness typically ranging from 10-20 times, leading to significant internal stress on the film and poor mechanical stability.Although layerby-layer growth of MOF films has been achieved via physical deposition combined with vapor deposition, these methods require relatively demanding preparation conditions and present challenges that limit their versatility.

Vapor-phase linker exchange (VPLE)
The gas-phase joint exchange method refers to a technique wherein an organic ligand vapor is reacted with existing inorganic-organic hybrid films or MOFs to fabricate MOF films. 50,51Compared to other vapor-phase methods, this approach exhibits minimal film deformation during the conversion process due to the slight change in the thickness of the film after the reaction.Based on the precursor types, this method can be categorized into two groups: conversion of inorganic-organic hybrid films into MOF films and conversion of MOF films into functionalized MOF films.
The inorganic-organic hybrid film is converted into a MOF film by using the hybrid film as a template, and the vaporized organic ligand is substituted with the ligand present in the inorganic-organic hybrid film at a high temperature to obtain the MOF film.Ge'rard Fe'rey et al. 28,52 dissolved iron acetate in an ethanol solution to obtain a transparent iron-based colloid, which was then reacted with muconic acid, immersed in monocrystalline silicon, heated, and dried to obtain MIL-89.Building upon the conversion of liquid-phase inorganic-organic membranes into MOFs, Wanbin Li's team 53 reported a gas exchange organic joint method for preparing nano-MOF films in 2017 (Figure S1).By adjusting both the sol concentration and the immersion time of the hollow fibers within the sol, control over the sol thickness coated on fiber surfaces was achieved, along with regulation of the ZIF-8 film thickness.The hybrid film underwent a reaction at 150 °C for 2 h, resulting in highly efficient and complete film conversion.This approach enabled continuous modulation of the ZIF-8 film thickness ranging from 87 nm to 757 nm while exhibiting minimal transformational deformation and surface roughnesses below 10 nm for the obtained films.
Conversion of the MOF film to a functionalized MOF film was achieved by employing organic ligand exchange on an existing MOF film, enabling surface modification.Wanbin Li's team 51 used a gas-phase exchange organic bone framework to build MOFs via coordination reactions between the metal ions and organic compounds.They utilized 4-bromoimidazole, 4-iodoimidazole, or 2-chloromethylbenzimidazole as ligands to replace the dimethylimidazole in the ZIF-8 framework and modified ZIF-8 in the gas phase (Figure 7A).Gener-Figure 5. Conversion of oxide film to MOF film by the VPT strategy (A) The synthetic process and TEM images of ZnO@ZIF-8 and Co(CO 3 ) 0.5 (OH)• 0.11 H 2 O@ZIF-67. 44(B) SEM images and schematic diagram of the ZIF-8 film on the outer surface of a porous α-alumina tube. 45lly, the replacement rate of the MOF film can be controlled by adjusting the reaction time, and it was observed that the replacement rate increased over time.No crystallinity losses or morphology changes were detected during the exchange process.Furthermore, gas-phase organic framework exchange significantly enhanced the MOF and polymer substrate compatibility.The ZIF-8/I and ZIF-8/Br prepared by VPLE exhibited uniform distribution within polysulfone, while filler aggregation and interface defects were observed in the ZIF-8/polysulfone composite material.Additionally, carboxylated MOFs were synthesized in 2021 through simple exposure to vaporized acyl chloride followed by immersion in water (Figure 7B).The modified MOFs retained their crystalline structures and porosities while demonstrating substantially improved fluoride removal performance. 54In 2023, Wei Du's team 55 developed a novel MOF using phthalic acid as the exchange ligand (Figure 7C), and the MOF exhibited dual stability in water and pure oleic acid environments.Lipase immobilized on LeZIF-8-PA 0.5 showed a higher specific activity for methanolysis during biodiesel production.Rob Ameloot's team 56 reported progress in preparing MOFs via the gas phase joint exchange method.A ZIF-8 thin film was obtained by chemical vapor deposition, followed by gas-phase joint exchange to obtain the composite MOF thin film.The gas-phase joint exchange method included three parts: 1) adsorption of incoming joints; 2) proton exchange between diformaldehyde imidazole and dimethyl imidazole joints adsorbed in the ZIF-8 frame; and 3) removal of the protonated dimethylimidazole (Figure 8A-B).Their experimental results showed that the gas phase exchange of the organic ligands ensured that the entering ligands coordinated the Zn 2+ cations while maintaining the sodium-salt topology of the MOF.This process was also applied to the preparation of MOF films.In addition, ligands with similar structures and different structures replaced the dimethylimidazole in the ZIF-8 skeleton to form a range of composite ZIF structures.In 2020, Hee Cheul Choi's team 57 used chemical vapor deposition (CVD) to prepare large, smooth Mo(CO) 4 (2,2'-bipyridyl) films (Figure 8C).This approach exploited the geometric properties of the thin films and provided great opportunities for fundamental research on conventional organometallic complexes and their applications.
The gas-phase method enables the exchange of organic ligands in the inorganic-organic hybrid film or MOF film, resulting in minimal changes in the precursor film thickness and MOF film conversion without inducing high stress and distortion, thereby facilitating the production of a more uniform film.Moreover, the method allows modification of the MOF properties and facilitates the synthesis of multifunctional composite MOFs by adjusting the mixing ratio.However, further investigation is needed to understand the mechanism governing the hybridization degree in the composite MOF.

Atomic layer deposition (ALD)/molecular layer deposition (MLD)
2][63][64] As depicted in Figure 9A, one precursor saturates the top surface of the substrate, forming a monolayer, followed by the introduction of another precursor on top; this initiates chemical reactions Materials Figure 6.The preparation of ZIF via the chemical vapor-phase deposition method (A) Schematic representation of the MOF-CVD process to convert ZnO and CoO x into ZIF-8 and ZIF-67. 47(B) Large-pore MAF-6 from the reaction between ZnO and 2-ethylimidazole vapor. 48(C) Schematic diagram of HKUST-1 film growth. 49etween these two layers.ALD and MLD technologies offer precise control over film compositions and thicknesses at the atomic and molecular levels, providing excellent shape retention and repeatability (Figure 9B). 65However, most of the materials deposited by ALD/MLD have amorphous structures, and achieving high crystallinity remains challenging for direct MOF film deposition via ALD/MLD.
To address the poor crystallinity of MOF films prepared by ALD, researchers have employed postprocessing techniques to adjust the crystallinities of the films after ALD/MLD.Leo D. Salmi et al. 66 alternatingly pulsed zinc acetate and 1,4-phthalic acid precursors on the substrate surface during an ALD reaction, resulting in an amorphous inorganic-organic hybrid film.Subsequently, the modified film was recrystallized in dimethylformamide to obtain a MOF-5 nanofilm.The experimental results demonstrated that precise control over the reaction parameters, such as the temperature, pulse time, and purge time, effectively regulated the growth rate.Additionally, an amorphous film was deposited via the ALD method with zinc acetate and 2,6naphthalenedicarboxylic acid as the precursors 67 .Under 70% relative humidity, this deposited amorphous film crystallized into an unknown crystalline phase with a large unit cell before being recrystallized from dimethylformamide solution to eventually yield an IRMOF-8 thin film.These findings confirmed that utilizing ALD for MOF film preparation resolved the issues related to discontinuity and poor adhesion between the film and substrate while highlighting the significance of selective adsorption through self-limiting ALD reactions on the substrate surface.Karena W. Chapman et al. 68 demonstrated that ALD exhibited regioselectivity, with a preference for depositing oxy-Zn(II) species within the small pores of NU-1000.This method enabled uniform coating of the surface of a complex 3D structure, significantly enhancing the functionality and application potential of the MOF film.
Kristian Blindheim Lausund and Ola Nilsen 69 reported the development of a complete vapor deposition method for preparing porous, crystalline UiO-66 (Figure 10A).They initially employed MLD to obtain an organic-inorganic hybrid film, which was then crystallized into the UiO-66 structured film through treatment with acetic acid vapor.The thin film prepared with acetic acid was subjected to high-pressure sterilization in a sealed container with 0.1 mL of acetic acid and heated at 160 ℃ for 24 hours.As observed from the cross-sectional SEM image in the inset, the thickness of the as-deposited UiO-66 film had increased from 229 nm to 500 nm.This method demonstrated exceptional control of the film thickness and facilitated uniform coating on irregular substrates, effectively addressing the challenges associated with difficult crystallization of ALD/MLD films.Consequently, this advancement has expanded the use of the full vapor process in depositing various MOF materials.Based on the design ability of the ALD/MLD method, in 2020, the team reported an all-gas-phase synthesis of MOF films with dual aromatic linkages. 24They used dimethyl 2,6-naphthalate (2,6-NDC) and biphenyl-4,4'-dicarboxylate (BP-4,4'-DC) as the organic linkers and ZrCl 4 as the metal precursor.The prepared amorphous film was crystallized under acetic acid vapor to obtain a crystalline MOF film.They controlled the pore sizes of the film with different organic ligands, and their study provided concepts for the syntheses of MOF films with larger pore size ranges.As shown in Figure 10B, in 2023, Rob Ameloot et al. 70 studied vapor-phase layerby-layer deposition of zeolitic imidazolate framework 8 (ZIF-8) via consecutive, self-saturating reactions of diethyl zinc, water, and 2-methylimidazole on a substrate.For the two-step ZIF-8 MLD, crystallization occurred during the  vapor deposition posttreatment.The deposition time was shortened, the quality of the MOF was improved, and thus, the crystallinity and absorptivity of the probe molecules were increased.
Compared to the indirect gas phase method, direct deposition of MOF films through MLD is more challenging yet appealing.Finnish scientist M. Karppinen and his team 71 proposed using MLD to synthesize copper terephthalate (Cu-TPA) MOF films directly (Figure S2A).Their method involved alternate deposition of 1,4-phthalic acid and copper-2,2,6,6-tetramethyl-3,5heptanedione precursors on the substrate surface to precisely control the MOF film structure at the atomic and molecular levels.Their results showed that the deposition temperature influenced the MOF film formation rate and surface roughness.Building upon this work, the team also synthesized a range of manganese-based and cobalt-based MOF films by reacting tris(2,2,6,6-tetramethyl-3,5-heptanedione)manganese (Mn(thd) 3 ), cobalt acetylacetonate or bis(2,2,6,6-tetramethyl-3,5-heptanedione)cobalt (i.e., Co(acac) 3 or Co(thd) 2 ) with terephthalic acid (Figure S2B). 72Additionally, M. Karppinen's team 73 prepared iron terephthalate coordination network films by using ALD/MLD techniques and emphasized that careful selection of the iron precursors was critical for achieving crystallization of these films.When FeCl 3 was selected as the reaction precursor, it induced and controlled the orientations of the organic molecules, and crystallization of the film occurred during the deposition process to provide a crystalline film.However, a larger volume of iron acetylacetonate restricted the molecular orientations and provided an amorphous film.
During ALD/MLD preparations of MOF films, precise and direct control over the film thickness was achieved by adjusting parameters, such as the precursors, temperature, and the number of cycles.The self-limiting nature of ALD/MLD ensured excellent uniformity and controllability of the resulting film, making it suitable for conformal coatings on complex surfaces in systems susceptible to structure and thickness issues.Furthermore, the chemical interactions between the ALD/MLD film and the substrate led to strong bonding with the substrate, a characteristic rarely observed in heteroepitaxialgrown films.Compared with the other gas phase methods described above for MOF thin film preparation, the ALD/MLD process offers simplicity, repeatability, and controllability and represents a bottom-up approach.However, certain challenges persist with this method, including improving the crystallinity and expanding the range of deposited MOF films.Additionally, due to the reduced deposition speed and the enhanced control of film growth, there is an associated increase in the time needed.

CONCLUSIONS AND PERSPECTIVES
MOF thin films exhibit significant potential in diverse domains, including electronic manufacturing, gas separation, sensing, catalysis, etc.These films possess different molecular network frameworks, pore structures, and adjustable microenvironments compared to conventional polymer membranes or molecular sieves.This review presents four vapor-phase methods for preparing MOF films: vaporassisted conversion, metal oxide template vapor-phase transformation, vapor-phase joint exchange, and atomic layer deposition/molecular layer deposition.The vapor-assisted method was developed from the conventional solvothermal approach and enabled efficient syntheses of MOF films with high crystallinities and large sizes.However, it exhibits limited control precision and incomplete removal of residues and solvents.Nanoscale widths of the MOF films can be achieved through vapor phase transformations of precisely deposited metal oxide film templates.Nevertheless, gas phase transformations involving metal oxides or elemental films reacted with organic ligands and led to structural dependence on the initial properties of the film on the substrate and in the crystallization reaction.Unfortunately, significant deformation (> 10 times) occurs in oxides or metals during this process, resulting in poor stability and low control precision of the film due to the accumulated internal stress.To address these issues associated with gas-phase transformations with metal oxide templates, the gas-phase joint exchange method offers a solution by enabling ligand exchange to convert the inorganic-organic hybrid film into a MOF while also allowing modulation to obtain new composite MOF films.However, challenges related to quality and efficiency arise due to competitive reactions among different ligands or organics.In contrast to indirect methods, the atomic layer/molecular layer deposition technique enables the direct fabrication of MOF films through self-limiting chemical reactions on a substrate surface.This method offers superior process control accuracy, optimal uniformity, and thickness control of the obtained film.It has been found that conformal coating of complex surfaces is particularly suited for systems requiring precise structure and thickness control.Further advancements in direct MOF deposition methods are crucial to address challenges related to the crystallinity and efficiency of thin film fabrication.
The gas phase method effectively preserves the pore structures of MOF films, facilitating integration with other technologies to design intricate structures.MOF films play pivotal roles in electronics, catalysis, sensing, separation, etc.However, further enhancement is needed for the preparation of MOF films with diverse gas-phase methods and inherent advantages.Moreover, the stabilities of MOFs remain relatively low, so numerous challenges must be overcome before large-scale application becomes feasible.Nevertheless, MOF films exhibit structural diversity, minimal pore defects, distinctive physical and chemical properties, and specialized microenvironments within the pores.These attributes confer significant potential for use in multiple fields.
Figure 10.Preparation of polycrystalline MOF thin film with vapor deposition method (A) Experimental setup for heat treatments of the films in acetic acid vapor and cross-section SEM images. 69

Figure 1 .
Figure 1.Vapor phase preparation technology and advantages of the MOF thin films.

Figure 4 .
Figure 4. Vapor phase transformation of a metal oxide film to MOF (A) Description of the basic strategies for MOF vapor deposition.(B) MOF integration routes enabled by the MOF-CVD process: lift-off patterning and coating of fragile features.30(C) SEM top view of ZIF-8 film and replicated hexagonal pattern.42