Carrier induced magnetic coupling transitions in phthalocyanine-based organometallic sheet

Enhanced Curie temperature in partially decorated CrSnSe3 monolayer with alkali metals (Li, Na, and K)

A two-dimensional ferromagnetic layer with a large Curie temperature is highly desired for spintronics applications. Herein, we investigated the effect of the partial decoration of the CrSnSe₃ monolayer with alkali metals Li, Na, and K on the structure, electronic and magnetic properties. The calculated formation energy, phonon dispersion curves, and ab initio molecular dynamics indicated that the decorated CrSnSe₃ layers are stable and can be fabricated. The Li, Na, and K decorated systems display semiconducting band features, with bandgaps of 0.53, 0.55, and 0.55 eV, respectively, with the HSE06 hybrid functional. We found a ferromagnetic ground state and an in-plane magnetic anisotropy of -2.12, -2.42, and -2.39 meV per cell in the Li, Na, and K-decorated systems, respectively. Based on Monte Carlo Simulations, we obtained largely enhanced Curie temperatures of 241, 256, and 265 K in the Li, Na, and K decorated systems, respectively. Our findings suggest that the decorated layers could be used as potential candidates for spintronics applications.

Exploring promising gas sensing and highly active catalysts for CO oxidation: transition-metal (Fe, Co and Ni) adsorbed Janus MoSSe monolayers

From first-principles calculations, the transition-metal (TM) atom (Fe, Co and Ni) adsorbed Janus MoSSe monolayer, toxic gas molecules (CO, NH3 and H2S) adsorbed on the Ni-MoSSe monolayer and CO catalytic oxidation on the Fe-MoSSe monolayer are systematically investigated. An increasing order (Fe-MoSSe < Co-MoSSe < Ni-MoSSe) is found for the stability and band gap of the TM atom adsorbed Janus MoSSe monolayer. These toxic gas molecules are found to be weakly physisorbed and strongly chemisorbed on the pristine and Ni-MoSSe monolayers, respectively. The electronic structure and gas molecular adsorption properties of the Janus MoSSe monolayer can be modulated by adsorbing different TM atoms and gas molecules. Particularly, the CO catalytic oxidation can be realized on the Fe-MoSSe monolayer in light of the more preferable Eley-Rideal (ER) mechanism with the two-step route (CO + O2 → OOCO → CO2 + Oads, CO + Oads → CO2) with highly exothermic processes in each step. The adsorption of TM atoms which may greatly enhance gas sensing performance and catalytic performance of CO oxidation based on the Janus MoSSe monolayer is further discussed.

High Curie temperature and strain-induced semiconductor-metal transition with spin reorientation transition in 2D CrPbTe3 monolayer

One of the major obstacles for Cr-based 2D materials such as CrI₃, CrSiTe₃ and CrGeTe₃ for spintronics applications is their low Curie temperature. Herein, we investigated the strain-induced magnetic properties of 2D CrPbTe₃ (CPT) monolayer belonging to members of the Cr-based 2D family. We explored the possibility of the fabrication of 2D layer through the mechanical stability, dynamical stability, formation energy, cohesive energy and thermal stability calculations. We found ferromagnetic ground state and the pristine CrPbTe₃ monolayer had an indirect band gap of 0.25 eV with an in-plane magnetic anisotropy of -1.37 meV cell^-1. The Curie temperature was 110 K and this is much larger than that of CrI₃, CrSiTe₃ and CrGeTe₃. Under 4% tensile strain, the band gap was increased to 0.45 eV and the Curie temperature was increased to 150 K. We found strain-induced semiconductor-metal transition at 3% compressive strain and also spin reorientation transition from in-plane to perpendicular magnetic anisotropy at 4% compressive strain, and the perpendicular magnetic anisotropy energy was almost three times larger than that of the CrGeTe₃ layer. Our finding may suggest that the CrPbTe₃ system can be utilized for spintronics and straintronics applications.

Computational screening of transition metal-doped phthalocyanine monolayers for oxygen evolution and reduction

Rationally designing efficient, low-cost and stable catalysts toward the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is of significant importance to the development of renewable energy technologies. In this work, we have systematically investigated a series of potentially efficient and stable single late transition metal atom doped phthalocyanines (TM@Pcs, TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt) as single-atom catalysts (SACs) for applications toward the OER and ORR through a computational screening approach. Our calculations indicate that TM atoms can tightly bind with Pc monolayers with high diffusion energy barriers to prevent the isolated atoms from clustering. The interaction strength between intermediates and TM@Pc governs the catalytic activities for the OER and ORR. Among all the considered TM@Pc catalysts, Ir@Pc and Rh@Pc were found to be efficient OER electrocatalysts with overpotentials η ^OER of 0.41 and 0.44 V, respectively, and for the ORR, Rh@Pc exhibits the lowest overpotential η ^ORR of 0.44 V followed by Ir@Pc (0.55 V), suggesting that Rh@Pc is an efficient bifunctional catalyst for both the OER and ORR. Moreover, it is worth noting that the Rh@Pc catalyst can remain stable against dissolution under the pH = 0 condition. Ab initio molecular dynamic calculations suggest that Rh@Pc could remain stable at 300 K. Our findings highlight a novel family of two-dimensional (2D) materials as efficient and stable SACs and offer a new strategy for catalyst design.

Electron delocalization in single-layer phthalocyanine-based covalent organic frameworks: a first principle study

In this work, we first investigate the localized electronic states in the band structures of three single-layer COFs based on typical building units of COFs chemistry. Our results confirm that the polar nature of strong bonds in these building units is a hindrance to a fully delocalized structure and disfavors the band-like mechanism of transport. We then show that a rational design of the building units can lead to dispersive band states in the electronic structure and results in conducting single-layer COFs. We demonstrate this strategy by investigating the charge carrier transport in a series of single-layer Ni-phthalocyanine (NiPc) covalent organic frameworks (COFs), namely, NiPc-P, NiPc-2P, and NiPc-3P. Three proposed COFs exhibit semiconducting band gaps ranging from 0.55 to 0.91 eV. Their room-temperature intrinsic mobility is predicted to be in range of 200–600 cm² V⁻¹ s⁻¹ and 20 000–60 000 cm² V⁻¹ s⁻¹ for electrons and holes, respectively, which are comparable to those of phosphorene and higher than those of the trigonal prismatic molybdenum disulfide. NiPc are dynamically and mechanically stable and can be synthesized via the co-evaporation between Ni and corresponding tetracyano linkers. Importantly, we demonstrate that the properties of the single-layer COFs can be tuned by engineering the organic building blocks. Our theoretical study not only provides insight into the design principles for semiconducting single-layer COFs but also highlights the significance of reticular chemistry in the development of a new generation of two-dimensional materials for optoelectronic applications.

A series of single-layer phthalocyanine-based covalent-organic frameworks (COFs) are shown to possess tunable delocalized electronic states which are attractive for optoelectronic applications.

High Curie-temperature intrinsic ferromagnetism and hole doping-induced half-metallicity in two-dimensional scandium chlorine monolayers

Two-dimensional (2D) ferromagnetic materials provide new platforms for spintronic applications, but most of the reported 2D magnetic orderings can only be maintained at low temperature. The search for high Curie temperature intrinsic ferromagnetism is still a current hotspot. Herein through comprehensive first-principles calculations and Monte Carlo simulation, we predict a promising 2D scandium chlorine (ScCl) monolayer with intrinsic ferromagnetism and a Curie temperature of 185 K, which is much higher than that of the reported CrI₃ monolayer (45 K) and the boiling point of liquid nitrogen (77 K). Moreover, a small amount of hole doping can induce a transition from a ferromagnetic metal to a half-metal. Furthermore, the ScCl monolayer possesses excellent thermal and dynamical stabilities as well as feasibility of experimental exfoliation from its layered bulk. These intriguing electronic and magnetic properties make the ScCl monolayer a promising candidate for spintronic applications.

Prediction of Intrinsic Ferromagnetic Ferroelectricity in a Transition-Metal Halide Monolayer

The realization of multiferroics in nanostructures, combined with a large electric dipole and ferromagnetic ordering, could lead to new applications, such as high-density multistate data storage. Although multiferroics have been broadly studied for decades, ferromagnetic ferroelectricity is rarely explored, especially in two-dimensional (2D) systems. Here we report the discovery of 2D ferromagnetic ferroelectricity in layered transition-metal halide systems. On the basis of first-principles calculations, we reveal that a charged CrBr_{3} monolayer exhibits in-plane multiferroicity, which is ensured by the combination of orbital and charge ordering as realized by the asymmetric Jahn-Teller distortions of octahedral Cr─Br_{6} units. As an example, we further show that (CrBr_{3})_{2}Li is a ferromagnetic ferroelectric multiferroic. The explored phenomena and mechanism of multiferroics in this 2D system not only are useful for fundamental research in multiferroics but also enable a wide range of applications in nanodevices.

Magnetic two-dimensional C3N2 carbonitrides: semiconductors, metals and half-metals

Using density-functional theory (DFT) calculations we probe the spin polarization of functionalized two-dimensional (2D) phthalo-carbonitrides (pc-C₃N₂), i.e., 2D polymers of tetra-cyanoethylene.

Exfoliating biocompatible ferromagnetic Cr-trihalide monolayers

Cr-trihalide monolayers CrX₃ (X = Cl, Br, I) can be exfoliated from their layered bulk phase, exhibiting high in-plane stiffness, tunable Curie temperatures and biocompatibility.

Two-dimensional iron-phthalocyanine (Fe-Pc) monolayer as a promising single-atom-catalyst for oxygen reduction reaction: a computational study

Searching for low-cost non-Pt catalysts for oxygen reduction reaction (ORR) has been a key scientific issue in the development of fuel cells. In this work, the potential of utilizing the experimentally available two-dimensional (2D) Fe-phthalocyanine (Fe-Pc) monolayer with precisely-controlled distribution of Fe atoms as a catalyst of ORR was systematically explored by means of comprehensive density functional theory computations. The computations revealed that O2 molecules can be sufficiently activated on the surface of the Fe-Pc monolayer, and the subsequent ORR steps prefer to proceed on the Fe-Pc monolayer through a more efficient 4e pathway with a considerable limiting potential of 0.68 V. Especially, the Fe-Pc monolayer is more stable than the Fe-Pc molecule in acidic medium, and can present good catalytic performance for ORR on the addition of axial ligands. Therefore, the Fe-Pc monolayer is quite a promising single-atom-catalyst with high efficiency for ORR in fuel cells.

Giant magnetocrystalline anisotropy of 5d transition metal-based phthalocyanine sheet

Giant magnetocrystalline anisotropy energy can be achieved under electric field or biaxial strain of 5d transition metal-based phthalocyanine sheet.

Robust ferromagnetism in monolayer chromium nitride

Design and synthesis of two-dimensional (2D) materials with robust ferromagnetism and biocompatibility is highly desirable due to their potential applications in spintronics and biodevices. However, the hotly pursued 2D sheets including pristine graphene, monolayer BN, and layered transition metal dichalcogenides are nonmagnetic or weakly magnetic. Using biomimetic particle swarm optimization (PSO) technique combined with ab initio calculations we predict the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible. Its dynamic, thermal and magnetic stabilities are confirmed by carrying out a variety of state-of-the-art theoretical calculations. Analyses of its band structure and density of states reveal that this material is half-metallic, and the origin of the ferromagnetism is due to p-d exchange interaction between the Cr and N atoms. We demonstrate that the displayed ferromagnetism is robust against thermal and mechanical perturbations. The corresponding Curie temperature is about 675 K which is higher than that of most previously studied 2D monolayers.

The superior catalytic CO oxidation capacity of a Cr-phthalocyanine porous sheet

Two-dimensional organometallic sheets containing regularly and separately distributed transition atoms (TMs) have received tremendous attentions due to their flexibility in synthesis, well-defined geometry and the promising applications in hydrogen storage, electronic circuits, quantum Hall effect, and spintronics. Here for the first time we present a study on the superior catalytic CO oxidation capacity of a Cr-phthalocyanine porous sheet proceeding first via Langmuir-Hinshelwood (LH) mechanism and then via Eley-Rideal (ER) mechanism. Compared to the noble metal based catalysts or graphene supported catalysts, our studied system has following unique features: without poisoning effect and clustering problem, having comparable reaction energy barrier for low-temperature oxidation, and low cost for large-scale catalytic CO oxidation in industry.