2D rare-earth metal carbides (MXenes) are attractive due to their novel electronic and magnetic properties and their potential as scalable 2D magnets. In this study, we used density functional theory with the Hubbard U correction to characterize the structure, termination, and magnetism in an out-of-plane ordered rare-earth containing M 3 C 2 T x MXene, Mo 2 NdC 2 T 2 (T = O or OH). We investigated the effect of the U parameter on the stability and magnetism of two possible termination sites: the hollow sites aligned with the inner Nd atoms (Nd-hollow sites) and those aligned with the closest C atoms (C-hollow sites). We found that increasing U Mo stabilized the Nd hollow sites, which minimized electrostatic repulsion between C and O atoms. Using U Mo = 3.0 eV and U Nd = 5.6 eV, obtained via the linear response method, we found that the energetically preferred termination site was C-hollow in Mo 2 NdC 2 O 2 and Nd-hollow in Mo 2 NdC 2 (OH) 2 . Regardless of termination and the Hubbard U value, we found Mo 2 NdC 2 O 2 and Mo 2 NdC 2 (OH) 2 to be magnetic. The C-hollow termination resulted in ferromagnetic states for all Hubbard U tested with no magnetic moment in Mo. In the case of Nd-hollow, Mo became magnetic for U Mo ≥ 4 eV. The difference of Mo magnetism in Nd-hollow and C-hollow was explained by crystal field splitting of the Mo d orbital caused by a distorted ligand.
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High-temperature 2D ferromagnetism in conjugated microporous porphyrin-type polymers
The need for magnetic 2D materials that are stable to the enviroment and have high Curie temperatures is very important for various electronic and spintronic applications. We have found that two-dimensional porphyrin-type aza-conjugated microporous polymer crystals are such a material (Fe-aza-CMPs). Fe-aza-CMPs are stable to CO, CO 2 , and O 2 atmospheres and show unusual adsorption, electronic, and magnetic properties. Indeed, they are semiconductors with small energy band gaps ranging from 0.27 eV to 0.626 eV. CO, CO 2 , and O 2 molecules can be attached in three different ways where single, double, or triple molecules are bound to iron atoms in Fe-aza-CMPs. For different attachment configurations we find that for CO and CO 2 a uniform distribution of the molecules is most energetically favorable while for O 2 molecules aggregation is most energetically preferable. The magnetic moments decrease from 4 to 2 to 0 for singly, doubly, triply occupied configurations for all gasses respectively. The most interesting magnetic properties are found for O 2 molecules attached to the Fe-aza-CMP. For a single attachment configuration we find that an antiferromagnetic state is favorable. When two O 2 molecules are attached, the calculations show the highest exchange integral with a value of J = 1071 μeV. This value has been verified by two independent methods where in the first method J is calculated by the energy difference between ferromagnetic and anitferromagnetic configurations. The second method is based on the frozen magnon approach where the magnon dispersion curve has been fitted by the Ising model. For the second method J has been estimated at J = 1100 μeV in excellent agreement with the first method.
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- Award ID(s):
- 1710512
- NSF-PAR ID:
- 10280753
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 26
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 14480 to 14488
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized d z2 bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ -point and for larger k -wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3–8 eV. The xy -component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5–5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μ B . The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case J nn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that J nn = 19.5 μeV and J nnn = −3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference.more » « less
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Abstract Dissociation of CO2on iron clusters was studied by using semilocal density functional theory and basis sets of triple‐zeta quality. Fe2, Fe4, and Fe16clusters were selected as the representative host clusters. When searching for isomers of Fe
n CO2,n =2, 4 and 16 corresponding to carbon dioxide attachment to the host clusters, its reduction to O and CO, and to the complete dissociation, it was found that the total spin magnetic moments of the lowest energy states of the isomers are often quenched with respect to those of initial reagents Fen +CO2. Dissociation pathways of the Fe2+CO2, Fe4+CO2, and Fe16+CO2reactions contain several transition states separated by the local minima states; therefore, a natural question is where do the spin flips occur? Since lifetimes of magnetically excited states were shown to be of the order of 100 fs, the search for the CO2dissociation pathways was performed under the assumption that magnetic deexcitation may occur at the intermediate local minima. Two dissociation pathways were obtained for each Fen +CO2reaction using the gradient‐based methods. It was found that the Fe2+CO2reaction is endothermic with respect to both reduction and complete dissociation of CO2, whereas the Fe4+CO2and Fe16+CO2reactions are exothermic to both reduction and complete dissociation of carbon dioxide. The CO2reduction was found to be more favorable than its complete dissociation in the Fe4case. -
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Abstract One route to address climate change is converting carbon dioxide to synthetic carbon‐neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo‐ and heterogeneous catalysis, deoxygenative coupling of CO to products with C−C bonds—as in liquid fuels—remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2(CO)2
L (2 ), whereL 2−is a bis(β‐diketiminate) cyclophane, gives [K(THF)5][Fe2(CO)2L ] (3 ), which undergoes silylation to Fe2(CO)(COSiMe3)L (4 ). Subsequent C‐OSiMe3bond cleavage and C=C bond formation occurs upon reduction of4 , yielding Fe2(μ‐CCO)L . CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallestJ value, with changes to local metal ion spin states and coupling schemes (ferro‐ vs. antiferromagnetism) based on DFT calculations, Mössbauer and EPR spectroscopy. Finally, reaction of5 with KEt3BH or methanol releases the C2O2−ligand with retention of the diiron core -
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Abstract One route to address climate change is converting carbon dioxide to synthetic carbon‐neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo‐ and heterogeneous catalysis, deoxygenative coupling of CO to products with C−C bonds—as in liquid fuels—remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2(CO)2
L (2 ), whereL 2−is a bis(β‐diketiminate) cyclophane, gives [K(THF)5][Fe2(CO)2L ] (3 ), which undergoes silylation to Fe2(CO)(COSiMe3)L (4 ). Subsequent C‐OSiMe3bond cleavage and C=C bond formation occurs upon reduction of4 , yielding Fe2(μ‐CCO)L . CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallestJ value, with changes to local metal ion spin states and coupling schemes (ferro‐ vs. antiferromagnetism) based on DFT calculations, Mössbauer and EPR spectroscopy. Finally, reaction of5 with KEt3BH or methanol releases the C2O2−ligand with retention of the diiron core
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP02312D
