wileyonlinelibrary.comfunctionality. Some optimized structures could provide tunable responsiveness and switching properties in response to external stimuli (such as temperature, pressure, electric potential, acoustic waves, and chemical environment), leading to the selectivity and sensitivity to detect such particular analytes. [5][6][7] Generally, in order to employ MOFs for chemical detection (except those based on luminescence quenching), [ 3 ] some external means of signal transduction such as optical, electrical, and mechanical schemes have to be applied. [6][7][8][9][10][11][12] Thus, integrating MOFs as thin fi lms on device surfaces is able to form a physical interface for signal transduction. As the change in oscillating frequency of quartz crystal microbalance (QCM) is directly related to the mass change on the surface, [ 13 ] it has shown its applicability in chemical detection coupling with MOF thin fi lms. [14][15][16][17][18] As a sub-class of MOFs, zeolitic-imidazolate frameworks (ZIFs) constructed by linking metal centers (e.g., Zn, Co, and Cd) with imidazolate ligands show high porosity, structure diversity, tunable surface functionality, structure fl exibility, and especially, good chemical and thermal stability which are essential for realistic applications. [19][20][21][22] These interesting properties make them good candidates as host sensing materials. [23][24][25][26] For example, Lu and Hupp [ 25 ] fabricated ZIF-8 fi lm on Fabry-Pérot device for gases and chemical vapors detection by monitoring the changes of refractive index upon adsorption of guest molecules. The repeated direct growth method they used for ZIF-8 fi lm growth offers several advantages including mild growth condition, rapid growth rate, and good control of thickness. However, the method has not been extended to fabricate other functional ZIF thin fi lms up to date. Integrating a range of different ZIFs into thin fi lm devices on various surfaces remains a challenge with respect to various applications such as membrane based gas/ liquid separation, electronic and opto-electronic devices. [8][9][10][11][12] Furthermore, the sensing performances not only depend on the quality of ZIF thin fi lms but also strongly rely on the properties of host ZIF materials such as limited pore diameter (LPD), surface functionality, and structure fl exibility. Thus, the systematic investigation of the effects of these properties on the performance of ZIF based sensors would promote the further development in sensing applications such as array-based sensing Thin fi lms of sodalite-type zeolitic-imidazolate frameworks (ZIFs, ZIF-7, 8, 9, 67, 90, and ZIF-65-Zn) with different metal centers and functional moieties are fabricated on SiO 2 coated quartz crystal microbalance (QCM) substrates using automatic program controlled repeated direct growth method. The repeated direct growth procedure manipulated here shows great applicability for rapid growth of uniform ZIF thin fi lms with controllable thickness. The fabricated ZIF/QCM devices are used to...

Control of localized metal-organic framework (MOF) thin fi lm formation is a challenge. Zeolitic imidazolate frameworks (ZIFs) are an important sub-class of MOFs based on transition metals and imidazolate linkers. Continuous coatings of intergrown ZIF crystals require high rates of heterogeneous nucleation. In this work, substrates coated with zinc oxide layers are used, obtained by atomic layer deposition (ALD) or by magnetron sputtering, to provide the Zn 2+ ions required for nucleation and localized growth of ZIF-8 fi lms ([Zn(mim) 2 ]; Hmim = 2-methylimidazolate). The obtained ZIF-8 fi lms reveal the expected microporosity, as deduced from methanol adsorption studies using an environmentally controlled quartz crystal microbalance (QCM) and comparison with bulk ZIF-8 reference data. The concept is transferable to other MOFs, and is applied to the formation of [Al(OH)(1,4-ndc)] n (ndc = naphtalenedicarboxylate) thin fi lms derived from Al 2 O 3 nanolayers.

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The pillared-layered metal−organic framework compounds M 2 (BME-bdc) 2 (dabco) (M 2+ = Zn 2+ , Co 2+ , Ni 2+ , Cu 2+ ; BME-bdc 2− = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M 2+ . This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO 2 pressure reveal different mechanisms for the np to lp phase transition during CO 2 adsorption.

Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.

The moisture‐tolerant metal‐organic frameworks (MOFs) of formula [Zn4O(L)3]n (L = di‐substituted carboxypyrazolate derivatives) are fabricated as thin films on self‐assembled monolayer (SAM) functionalized gold substrates by employing the step‐by‐step liquid phase epitaxial (LPE) deposition method in a continuous operation mode. The in situ monitoring of the deposition by quartz crystal microbalance (QCM) and grazing incidence X‐ray diffraction reveal different growth regimes and crystallinities of the obtained thin films in dependence of the chosen alkyl side chain functionality at the carboxypyrazolate linkers, L. To overcome the relatively poor crystallinity and low porosity of a particular homostructured metal‐organic framework type B film, the step‐by‐step heteroepitaxial growth of this MOF B on top of the crystallite surfaces of a well‐grown and lattice‐matched MOF type A is applied. This approach enables the fabrication of oriented, core‐shell‐like MOF B@A surface mounted heterocrystals as an intergrown homogeneous coating for the selective adsorption of volatile organic compounds. The accessible pore volumes of the individual components and the heterostructured films are characterized by performing adsorption measurements of different organic probe molecules using an environmentally controlled QCM instrument. The results show good adsorption capacity, excellent size exclusion selectivity for alcohols, and a high degree of moisture‐tolerance of the heteroepitaxial MOF films.

Size selective adsorption of alcohols and molecular recognition of methanol over water are observed in the heterostructured Zn4O(3,5-dialkylcarboxypyrazolate)3 films.

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