Fracture behavior of MOF monoliths revealed by nanoindentation and nanoscratch

Monolithic metal-organic frameworks (MOFs) represent a promising solution for the industrial implementation of this emerging class of multifunctional materials, due to their structural stability. When compared to MOF powders, monoliths exhibit other intriguing properties like hierarchical porosity, that significantly improves volumetric adsorption capacity. The mechanical characterization of MOF monoliths plays a pivotal role in their industrial expansion, but so far, several key aspects remain unclear. In particular, the fracture behavior of MOF monoliths has not been explored. In this work, we studied the initiation and propagation of cracks in four prototypical MOF monoliths, namely ZIF-8, HKUST-1, MIL-68 and MOF-808. We observed that shear faults inside the contact area represent the main failure mechanism of MOF monoliths and are the source of radial cracks. MIL-68 and MOF-808 showed a remarkably high resistance to cracking, which can be ascribed to their consolidated nanostructure.Comment: 15 pages, 5 figures, Supporting Informatio

Optochemically Responsive 2D Nanosheets of a 3D Metal-Organic Framework Material

Outstanding functional tunability underpinning metal-organic framework (MOF) confers a versatile platform to contrive next-generation chemical sensors, optoelectronics, energy harvesters and converters. We report a rare exemplar of a porous 2D nanosheet material, constructed from an extended 3D MOF structure. We develop a rapid supramolecular self-assembly methodology at ambient conditions, to synthesize readily-exfoliatable MOF nanosheets, functionalized in situ by adopting the Guest@MOF (Host) strategy. Nanoscale confinement of light-emitting molecules (as functional guest) inside the MOF pores generates unusual combination of optical, electronic, and chemical properties, arising from the strong host-guest coupling effects. We show highly promising photonics based chemical sensing opened up by the new Guest@MOF composite systems. By harnessing host-guest optochemical interactions of functionalized MOF nanosheets, we have accomplished detection of an extensive range of volatile organic compounds (VOCs) and small molecules important for many practical applications.Comment: 21 pages, 5 figures, 1 schem

Accelerating Extreme Learning Machine on FPGA by Hardware Implementation of Given Rotation - QRD

Currently, Extreme Learning Machine (ELM) is one of the research trends in the machine learning field due to its remarkable performances in terms of complexity and computational speed. However, the big data era and the limitations of general-purpose processor cause the increasing of interest in hardware implementation of ELM in order to reduce the computational time. Hence, this work presents the hardware-software co-design of ELM to improve the overall performances. In the co-design paradigm, one of the important components of ELM, namely Given Rotation-QRD (GR-QRD) is developed as a hardware core. Field Programmable Gate Array (FPGA) is chosen as the platform for ELM implementation due to its reconfigurable capability and high parallelism. Moreover, the learning accuracy and computational time would be used to evaluate the performances of the proposed ELM design. Our experiment has shown that GR-QRD accelerator helps to reduce the computational time of ELM training by 41.75% while maintaining the same training accuracy in comparison to pure software of ELM

Stress–strain relationships and yielding of metal-organic framework monoliths

Metal-organic frameworks (MOFs) have emerged as a versatile material platform for a wide range of applications. However, the development of practical devices is constrained by their inherently low mechanical stability. The synthesis of MOFs in a monolithic morphology represents a viable way for the transition of these materials from laboratory research to real-world applications. For the design of MOF-based devices, the mechanical characterization of such materials cannot be overlooked. In this regard, stress-strain relationships represent the most valuable tool for assessing the mechanical response of materials. Here, we use flat punch nanoindentation, micropillar compression and Raman microspectroscopy to investigate the stress-strain behaviour of MOF monoliths. A pseudo-plastic flow is observed under indentation, where the confining pressure prevents unstable crack propagation. Material flow is accommodated by grain boundary sliding, with occasional stepwise cracking to accommodate excessive stress building up. Micropillar compression reveals a brittle failure of ZIF-8, while plastic flow is observed for MIL-68

Guest-Tunable Dielectric Sensing Using a Single Crystal of HKUST-1

There is rising interest on low-k dielectric materials based on porous metal-organic frameworks (MOFs) for improved electrical insulation in microelectronics. Herein, we demonstrate the concept of MOF dielectric sensor built from a single crystal of HKUST-1. We study guest encapsulation effects of polar and non-polar molecules, by monitoring the transient dielectric response and AC conductivity of the crystal exposed to different vapors (water, I2, methanol, ethanol). The dielectric properties were measured along the crystal direction in the frequency range of 100 Hz to 2 MHz. The dielectric data show the efficacy of MOF dielectric sensor for discriminating the guest analytes. The time-dependent transient response reveals dynamics of the molecular inclusion and exclusion processes in the nanoscale pores. Since dielectric response is ubiquitous to all MOF materials (unlike DC conductivity and fluorescence), our results demonstrate the potential of dielectric MOF sensors compared to resistive sensors and luminescence-based approaches.Comment: 6 pages, 5 figure

Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework

We show clear experimental evidence of co-operative terahertz (THz) dynamics observed below 3 THz (~100 cm-1), for a low-symmetry Zr-based metal-organic framework (MOF) 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 (DFT) calculations. For completeness, we obtained Raman spectra and characterized the alterations to the complex pore architecture caused by the THz rotations. We discovered an array of soft modes with trampoline-like motions, which could potentially be the source of anomalous mechanical phenomena, such as negative linear compressibility and negative thermal expansion. Our results also demonstrate coordinated shear dynamics (~2.5 THz), a mechanism which we have shown to destabilize MOF crystals, in the exact crystallographic direction of the minimum shear modulus (Gmin).Comment: 10 pages, 6 figure

Tracking Thermal-Induced Amorphization of a Zeolitic Imidazolate Framework via Synchrotron In Situ Far-Infrared Spectroscopy

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.Comment: 5 pages, 3 figures, 2 table