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- Nanoporous metal organic framework materials for smart applications
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This review is concerned with the recent advances in metal organic framework (MOF) materials. We highlight the unique combination of physicochemical and thermomechanical characteristics associated with MOF-type materials and illustrate emergent applications in three challenging technological sectors: energy, environmental remediation and biomedicine. MOFs represent an exciting new class of nanoporous crystalline solids constituting metal ions/clusters and multifunctional organic linkages, which self-assemble at molecular level to generate a plethora of ordered 3D framework materials. The most intriguing feature of a MOF lies in its exceptionally large surface area, far surpassing those of the best activated carbons and zeolites. Next generation multifunctional materials encompassing MOF based thin films, coatings, membranes and nanocomposites have potential for exploitation in an immense array of unconventional applications and smart devices. We pinpoint the key technological challenges and basic scientific questions to be addressed, so as to fulfil the translational potential for bringing MOFs from the laboratory into commercial applications.
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- Dielectric Properties of Metal-Organic Frameworks Probed via Synchrotron Infrared Reflectivity
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We present the frequency-dependant (dynamic) dielectric response of a group of topical polycrystalline zeolitic imidazolate-based metal-organic framework (MOF) materials in the extended infrared spectral region. Using synchrotron-based FTIR spectroscopy in specular reflectance, in conjunction with density functional theory (DFT) calculations, we have revealed detailed structure-property trends linking the THz region dielectric response to framework porosity and structural density. The work demonstrates that MOFs are promising candidate materials not only for low-k electronics applications but could also be pioneering for terahertz (THz) applications, such as next-generation broadband communications technologies.
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- Tracking thermal-induced amorphization of a zeolitic imidazolate framework via synchrotron in situ far-infrared spectroscopy
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We present the first use of in situ far-infrared spectroscopy to analyze the thermal amorphization of a zeolitic imidazolate framework material. We explain the nature of vibrational motion changes during the amorphization process and reveal new insights into the effect that temperature has on the Zn-N tetrahedra.
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- Explaining the mechanical mechanisms of zeolitic metal-organic frameworks: revealing auxeticity and anomalous elasticity
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The comprehensive elastic properties of Zeolitic Imidazolate Frameworks (ZIF-1 to ZIF-4) have been computed using density functional theory (DFT). We employed the periodic CRYSTAL14 code to calculate the single-crystal elastic coefficients (C-ij) at the B3LYP level of theory. While the chemical compositions of ZIFs-1 to -4 are the same, each structure features a distinct network topology, crystal symmetry and porosity configuration, which translate into differential structure-function mechanical correlations. We elucidate the anisotropic mechanical response with respect to the directionally dependent Young's and shear moduli properties. Our theoretical results suggest that ZIF-3 adopting a dft topology has an extremely low shear resistance (G(min) = 0.1 GPa), which is also underpinning the flexible mechanism responsible for its negative Poisson's ratio (auxetic v(min) = -0.43). Interestingly, we identified that ZIF-1, ZIF-2, and ZIF-4 could exhibit a nearly zero Poisson's ratio for certain crystal orientations, which is reminiscent of a rare "cork-like" phenomenon where there is practically no lateral deformation corresponding to an applied axial strain. Furthermore, we determined the bulk moduli and linear compressibilities, alongside the averaged elastic properties of the ZIF polycrystals.
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- Identifying the Role of Terahertz Vibrations in Metal-Organic Frameworks: From Gate-Opening Phenomenon to Shear-Driven Structural Destabilization
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We present an unambiguous identification of low-frequency terahertz vibrations in the archetypal imidazole-based metal-organic framework (MOF) materials: ZIF-4, ZIF-7, and ZIF-8, all of which adopt a zeolite-like nanoporous structure. Using inelastic neutron scattering and synchrotron radiation far-infrared absorption spectroscopy, in conjunction with density functional theory (DFT), we have pinpointed all major sources of vibrational modes. Ab initio DFT calculations revealed the complex nature of the collective THz modes, which enable us to establish detailed correlations with experiments. We discover that low-energy conformational dynamics offers multiple pathways to elucidate novel physical phenomena observed in MOFs. New evidence demonstrates that THz modes are intrinsically linked, not only to anomalous elasticity underpinning gate-opening and pore-breathing mechanisms, but also to shear-induced phase transitions and the onset of structural instability.
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- Discovering connections between terahertz vibrations and elasticity underpinning the collective dynamics of the HKUST-1 metal-organic framework
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We employed a combination of theoretical and experimental techniques to study the metal-organic framework (MOF)-mechanics central to the paddle-wheel Cu-3(BTC)(2) porous structure, commonly designated as HKUST-1. Lattice dynamics of the hybrid framework at below 18 THz were measured by means of Raman and synchrotron far-infrared spectroscopy, and systematically correlated to collective vibrational modes computed from ab initio density functional theory (DFT). We have identified a number of intriguing low-energy framework vibration mechanisms, reminiscent of the 'trampoline-like' deformations and new oscillatory motions associated with Cu paddle-wheel 'molecular rotors'. The three independent single-crystal elastic constants of the HKUST-1 (i.e. C-11, C-12 and C-44) were calculated using the DFT approach, taking into account the effects of dispersion corrections. We established the full elasticity solutions accompanying detailed deformation mechanisms that control its anisotropic mechanical properties, ranging from the Young's and shear moduli to linear compressibility and Poisson's ratio. Our results support the notion that the coexistence of soft modes and intrinsic shear distortions connected to the THz lattice dynamics dictate a range of anomalous elastic phenomena, for example: negative Poisson's ratio (auxeticity), negative thermal expansion (NTE), and exceedingly low shear moduli properties.
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- Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework
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We show clear experimental evidence of cooperative terahertz (THz) dynamics observed below 3 THz (similar to 100 cm(-1)), for a low-symmetry Zr-based metal-organic framework structure, termed MIL-140A [ZrO(O2C-C6H4-CO2)]. Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory calculations. The complex pore architecture caused by the THz rotations has been characterized. We discovered an array of soft modes with trampoline like motions, which could potentially be the source of anomalous mechanical phenomena such as negative thermal expansion. Our results demonstrate coordinated shear dynamics (2.47 THz), a mechanism which we have shown to destabilize the framework structure, in the exact crystallographic direction of the minimum shear modulus (G(min)).
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- Isoreticular zirconium-based metal-organic frameworks: discovering mechanical trends and elastic anomalies controlling chemical structure stability
- Description:
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Understanding the mechanical properties of metal-organic frameworks (MOFs) is crucial not only to yield robust practical applications, but also to advance fundamental research underpinning the flexibility of a myriad of open-framework chemical compounds. Herein we present one of the most comprehensive structural analyses yet on MOF-mechanics: elucidating the complex elastic response of an isoreticular series of topical Zr-based MOFs, explaining all the important mechanical properties, and identifying major trends arising from systematic organic linker exchange. Ab initio density functional theory (DFT) was employed to establish the single-crystal elastic constants of the nanoporous MIL-140(A-D) structures, generating a complete 3-D representation of the principal mechanical properties, encompassing the Young's modulus, shear modulus, linear compressibility and Poisson's ratio. Of particular interest, we discovered significantly high values of both positive and negative linear compressibility and Poisson's ratio, whose framework molecular mechanisms responsible for such elastic anomalies have been fully revealed. In addition to pinpointing large elastic anisotropy and unusual physical properties, we analyzed the bulk modulus of isoreticular Zr-MOF compounds to understand the framework structural resistance against the hydrostatic pressure, and determined the averaged mechanical behaviour of bulk (polycrystalline) MOF materials important for the design of emergent applications.
