Metal-organic framework-based 2D magnetic semiconductors

Luke Liu¹, Liang Li², T. David Harris²

¹School of Chemical and Physical Sciences, Victoria University of Wellington

²Northwestern Univeristy, USA

Controlling electrical conductivity and magnetic properties of materials has led to technology revolution. For instance, CPUs perform logic operations through manipulating electrical conductivity, whereas hard disks write information through switching magnetization directions of disk materials. The interplay between conductivity and magnetism has sparked the design of many functional devices. It is believed that if both properties are controllable from a material, such material might find used in multifunctional chips that combine both computing and storage functions. A major challenge towards this goal is to discover or synthesise materials that exhibit both switchable conductivity and magnetism. Here we present metal-organic frameworks (MOFs) as platform materials to tackle this challenge.

Thermal stability of metal-organic frameworks

Colm Healy¹, Nathan Harvey-Reid¹, Ben Howard¹, Paul Kruger¹

¹School of Physical and Chemical Sciences University of Canterbury

Metal-organic frameworks (MOFs) are materials constructed from inorganic nodes and organic linkers, resulting in porous structures with potential applications in the separation and purification of a number of industrially relevant gases, including CO₂ and acetylene. However, although these materials may be required to operate at high temperatures in practical industrial settings, very little attention has been paid to their high-temperature behaviour. Using the only widely available thermal data (thermogravimetric analysis, or TGA), we assessed the thermal stability of approximately 450 MOFs, identifying the underlying structure-property relationships which determine the high-temperature behaviour of these materials. On the basis of this study, we have proposed method for assessing thermal stability which analyses the node and linker components of the material separately, allowing overall thermal behaviour to be rationalised from structural principles. This framework also provides insight into “meltable” MOFs, a recently-discovered class of fascinating materials which transition to liquid-like and glassy states at high temperatures. Our study provides the first method for assessing which MOFs will melt and which will simply decompose at high temperatures.

Additionally, we have further investigated Hybrid Ultramicroporous Materials (HUMs), a subset of MOFs which show particularly promising gas sorption properties, but particularly poor thermal stability. We have experimentally identified the thermal decomposition products of these materials, which has in turn allowed us to identify the structural factors which limit the thermal stability in these materials. This is an important first step in designing more thermally robust HUM-type materials with a combination of both desirable gas sorption properties and high-temperature stability.

Polyethylene and zinc cyanide composite for non-expanding gasket applications

Savannah Egerton¹, Claudia Sim¹, Heon Park¹, Mark Staiger², Matthew Cowan¹

¹Chemical and Process Engineering, University of Canterbury

²Mechanical Engineering, University of Canterbury

Gaskets are used largely in industry to ensure sufficient sealing between two mating surfaces such as between pipes to prevent fluid leakage. Thermal cycles due to plant start up and shutdown can result in the gasket material expanding more than surrounding equipment, resulting in a breach of the seal. A composite material was therefore made from polyethylene, zinc cyanide and an ionic liquid to identify if a gasket with a reduced coefficient of thermal expansion (CTE) could be possible.

Zinc cyanide is a metal organic framework (MOF) with a negative thermal expansion (-16 x 10^-6 /K) which reduced the CTE of the composite overall. The addition of ionic liquid was investigated as it was hypothesised that it could fill the void spaces between zinc cyanide and polyethylene to ensure a more uniform CTE of the composite. This hypothesis was disproven, as the addition of ionic liquid did not affect the overall CTE.

Mechanical testing using a Universal Testing Machine (UTM) was used to identify if the addition of zinc cyanide affected the mechanical properties of the composite. The addition of zinc cyanide was found to significantly decrease the material toughness, where a 10wt% increase of zinc cyanide resulted in a 70% decrease in material toughness (13.4 MPa to 4.15 MPa). The addition of zinc cyanide also resulted in a material with decreased elasticity and a lower tensile strength but didn’t significantly the yield strength.

Molecular structures of short-lived species by gas electron diffraction and computational methods

Pitambar Poudel¹, Sarah Masters¹

¹School of Physical and Chemical Sciences, University of Canterbury

Reactive intermediates are useful in organic synthesis, industrial chemistry and many biological processes. Knowledge of their structure and energetic behaviour enables the mechanisms of reactions to be further understood. This project seeks to determine the gaseous molecular structure of various short-lived species and reactive intermediates using a new combined gas electron diffraction / mass spectrometry (GED/MS) apparatus. The proof-of-concept is focussed on ketene (ethenone) which can be generated cleanly from pyrolysis of diketene (4-methylideneoxetan-2-one). This will enable the apparatus to be tested fully before moving to more complicated systems.

Computational modelling of the structures and reaction pathways for the generation of ketene from diketene was undertaken using the Gaussian09 and NW-Chem suite of programs, utilizing the New Zealand e-Science Infrastructure (NeSI). The parent diketene can break down to give either ketene or allene and CO₂. Both of these pathways have been studied in detail and we are able to demonstrate that the reaction pathway for the formation of ketene has a lower activation barrier than the alternative pathway (formation of allene/CO₂) using CBS-QB3 calculations. The predicted entropy, enthalpy and free energy of pyrolysis dissociation of diketene at the CCSD(T)/CBS level indicates that the ketene formation pathway is spontaneous at elevated reaction temperature.

The second reactive species to be discussed is methyleneketene. Pyrolysis of diazotetranoic acid [3-diazofuran-2, 4(3H, 5H)-dione] could give either methyleneketene/CO₂ or carbon sub-oxide/formaldehyde with liberation of nitrogen gas in the both cases. Both pathways have been studied computationally. The methyleneketene/CO₂ formation pathway was found to be exergonic with a lower energy barrier than the carbon sub-oxide/formaldehyde formation pathway by CBS-QB3 calculations. The thermochemical parameters have also been studied using CCSD(T)/CBS calculations.

This presentation will discuss for computational investigation of both ketene and methylketene, and our progress towards integrating the MS with the Canterbury GED apparatus.

Applications of speckle-based 3D dark-field tomography

Samantha Alloo¹, Konstantin Pavlov¹, David Paganin¹

¹School of Physical and Chemical Sciences, University of Canterbury

Dark-Field (DF) imaging gives access to structural information which is typically not provided by traditional attenuation-based and phase-contrast X-Ray imaging. DF images are constructed by considering small-angle X-Ray scattering (SAXS), namely scattering of X-Rays on features smaller than the detector pixels. Such features are typically unresolvable using standard projection type imaging. DF images can be obtained using many techniques, including grating-based phase contrast (PC), edge-based PC, and analyser-based PC techniques. These approaches require multiple or long exposures and therefore give a large radiation dose to the sample. Due to the damaging effects of radiation, long exposure times are not ideal in medical applications, making the current DF imaging techniques unviable in application. Within this research work we have computed low-dose tomographic reconstructions of wood using Multimodal Intrinsic Speckle Tracking (MIST) [1]. These are the first dark-field tomographic reconstruction using any form of X-ray speckle-tracking. By using a speckle-based imaging (SBI) set-up, we can obtain dark-field tomographic reconstructions at a 500% to 1000% reduction in patient exposure, compared to alternative techniques currently in the field. In medical applications it will provide additional diagnostic and therapeutic information. This is currently the only work being done utilizing SBI to obtain DF images. The imaging technique can be used for samples with small differences in absorption between composite materials in the sample, or for samples with low absorption. In such cases, the use of refraction information gives advantage in acquiring additional information. Anticipated applications include biology, airport security, industrial quality control and material science.

[1] Pavlov, K., Paganin, D., Heyang, L., Berujon, S., Rougé-Labriet, H., & Brun, E. X-ray Multimodal Intrinsic-Speckle-Tracking (MIST). Journal of Optics, in press. https://arxiv.org/abs/1911.06814 (2019).

Tilted Laue analyser-based imaging: ultra small angle scattering tomographic reconstruction using multiple image radiography.

Jayan Gunasekera¹, Marcus Kitchen², Kentaro Uesugi³, Greg Falzon^4,5, Peter Quin^5,6, and Konstantin Pavlov¹

¹School of Physical and Chemical Sciences, University of Canterbury

²Monash University, Australia

³Japan Synchrotron Radiation Research Institute, Japan

⁴Flinders University, Australia

⁵University of New England, Australia

⁶University of Tasmania, Australia

We aim to reconstruct the Ultra Small Angle Scattering (USAXS) 3D image of a high-scatter sample (finely sieved soil) using Tilted Laue Analyser-Based Imaging (TLABI). TLABI enables the extraction of 2D information about the phase gradient [1] and USAXS. USAXS or dark-field imaging can provide micrometre or sub-micrometre length scale information of a sample structure. Usually, the size of features producing USAXS is smaller than the pixel size of the detector. Therefore, these features, causing USAXS, cannot be resolved by the detector’s pixels. ABI [2] is an extremely sensitive imaging modality, which can produce high contrast images of weakly absorbing materials. This sensitivity is predominantly associated with the analyser crystal, which is used to render the phase gradients visible. The highest diffracted intensity is recorded at the detector when the analyser crystal is at the Bragg position with respect to the incident radiation. As the crystal is rotated around the Bragg position, the diffracted intensity sharply decreases, creating a bell-shaped curve (Rocking curve). USAXS information can be extracted by measuring the Full Width at Half Maximum of this Rocking Curve with and without the sample. Curve fitting was done on a pixel-by-pixel basis by stacking 261 RC images without the sample. Gaussian [3], Voigtian [4], Pearson VII (P7) [5] and Pseudo-Voigtian [4] models have primarily been used to estimate the RC without the sample. χ² analysis has shown that Pseudo-Voigtian curves best estimate the RC without the sample, giving a mean χ² value (with standard deviation) of (4.5±0.5)x10². For the same image stack, Gaussian, Voigtian and P7 gave means (with standard deviation) of (8.8±0.9)x10³, (7.9±1.7)x10² and (7.9±2.2)x10², respectively. Rocking curve fitting with the sample shows that P7 achieves the best fit, followed by the Lorentzian model. Due to computational complexities with P7, we have chosen the Lorentzian model to fit the image sets with the sample.

[1] M. C. Chalmers, M. J. Kitchen, K. Uesugi, G. Falzon, P. Quin, and K. M. Pavlov, Tomographic Reconstruction using Tilted Laue Analyser Based X-ray Phase-Contrast Imaging, J. Synchr. Rad (in press) (arXiv:2007.10520), 2020.

[2] E. Forster, K. Goetz, and P. Zaumseil, Double crystal diffractometry for the characterization of targets for laser fusion experiments, Kristall und Technik, vol. 15, no. 8, pp. 937-945, Jan. 1980, doi: 10.1002/crat.19800150812.

[3] Ya. I. Nesterets, P. Coan, T. E. Gureyev, A. Bravin, P. Cloetens, and S. W. Wilkins, On qualitative and quantitative analysis in analyser-based imaging, Acta Crystallogr A Found Crystallogr, vol. 62, no. 4, pp. 296-308, Jul. 2006, doi: 10.1107/S0108767306017843.

[4] H. Suhonen, M. Fernández, A. Bravin, J. Keyriläinen, and P. Suortti, Refraction and scattering of X-rays in analyser-based imaging, Journal of Synchrotron Radiation, vol. 14, no. 6, pp. 512-521, 2007, doi: 10.1107/S0909049507044664.

[5] M. J. Kitchen et al., X-ray phase, absorption and scatter retrieval using two or more phase contrast images, Opt. Express, vol. 18, no. 19, p. 19994, Sep. 2010, doi: 10.1364/OE.18.019994.