Doping of the Mn vacancy of Mn2B2 with a single different transition metal atom as the dual-function electrocatalyst.

The design of efficient electrocatalysts is essential to enhance the performance of rechargeable metal-air cells, renewable fuel cells and overall water splitting. Based on this, how to improve the catalytic activity of oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) becomes self-evident. Currently, single atom catalysts (SACs) are widely used as structural design models for the OER, ORR and HER because of the single active site and maximum metal atom utilization, but significant challenges remain. Herein, the catalytic properties of the OER, ORR and HER with a single metal atom as the active site are discussed through first-principles calculations by introducing a single metal atom in the Mn vacancy of Mn2B2 (TM@Mn2B2, TM = Au, Ag, Co, Cd, Cu, Ir, Pd, Ni, Rh, Ru and Pt). The results show that Ni@Mn2B2 is suitable as a dual-function electrocatalyst for the OER/ORR with overpotentials of 0.38 V and 0.37 V, which are lower than those of the OER overpotential of RuO2/IrO2 (0.42 V/0.56 V) and the ORR overpotential of Pt (0.45 V). Meanwhile, Pt@Mn2B2 is available as an OER/HER dual-function electrocatalyst for overall water splitting with a lower overpotential of OER (0.45 V) and lower |ΔGH| (-0.15eV) under 1/4 hydrogen coverage for the HER. This work proposes a practical strategy for developing single metal atom doped MBene as a dual-function electrocatalyst.

Exchange-dependent spin polarized transport and phase transition in a triple monomer molecule.

Molecular junctions contribute significantly to the fundamental understanding of the quantum information technologies in molecular spintronics. In this paper, with the aid of the state of the art numerical renormalization group method, we find a triple monomer molecule structure with strong electron-electron interactions could be a potential candidate for a multifunctional spin polarizer when an external magnetic field along the z axis is applied. It is demonstrated that the polarizing scenarios depend closely on the inter-orbital exchange couplings, and results in several kinds of spin polarizers, e.g., the unidirectional, the bidirectional, the dual, and the ternary spin polarizers. We show in detail the related phase diagram, and conclude the Zeeman effect and the charge switching for the bonding, anti-bonding and non-bonding orbitals are responsible for the spin polarizing transport. We stress even when the energy levels are chosen beyond the Kondo regime, the structure still shows a promising platform for molecular spintronics components.

High spin polarization in formamidinium transition metal iodides: first principles prediction of novel half-metals and spin gapless semiconductors.

The electronic structure and magnetic properties of ten formamidinium transition metal iodides in the ground state and under strain have been studied. These formamidinium transition metal iodides have a stable cubic perovskite structure. In the ground state, FAVI3 is a spin gapless semiconductor, and FAScI3, FATiI3, FACrI3, FAFeI3, FACoI3 and FANiI3 are ferromagnetic half-metals. They all have 100% spin polarization and integer total magnetic moment. Under the action of strain, the high spin polarization of some formamidinium transition metal iodides can still be well maintained, and several novel spin gapless semiconductors such as FATiI3, FAFeI3 and FACoI3 have been discovered. Magnetic studies show that these formamidinium transition metal iodides with half-metal, semiconductor and spin-gapless semiconductor properties have integral total magnetic moments under strain ranging from -10.0% to 10.0%. These newly discovered half-metallic ferromagnetic materials and spin gapless semiconductors have broad application prospects in the field of spintronics due to their high spin polarization.

Bidirectional spin filter in a triple orbital molecule junction by tuning the magnetic field along a single direction.

Metal-molecule-metal junction is considered the basing block and key element of molecular spintronic devices, within which to generate spin polarized currents is one of the most fundamental issues for quantum computation and quantum information. In this paper, by employing a parallel triple orbital molecule junction with large inter-orbital tunneling couplings, we propose theoretically a bidirectional spin filter where both spin-up and spin-down currents could be obtained by simply adjusting the external magnetic field to different regimes along a single direction, and the filtered efficiencies could reach almost 100%. The Zeeman effect and the occupancy switching for the bonding and anti-bonding states are found to be responsible for the spin selective transport. We demonstrate that our scheme is robust for large parameter spaces of the orbital energy level, except the particle-hole symmetric point, and is widely suitable for the strong-, weak-, and non-interacting cases. To implement these problems, we use the Wilson's numerical renormalization group technique to treat such systems.

Ferroelectric mechanism of croconic acid: a first-principles and Monte Carlo study.

The ferroelectric mechanism of croconic acid in terms of the electronic structure and the molecular structure was studied by first principles using the density functional theory with the generalized gradient approximation. The spontaneous polarization (Ps) was simulated by the Berry phase method. It is found that the large polarization originates from charge transfer due to the strong "push-pull" effect of electron-releasing and -withdrawing groups along the hydrogen bond. According to the characteristics of polarization of croconic acid, we constructed a one-dimensional ferroelectric Hamiltonian model to describe the ferroelectric properties of croconic acid. Based on the Hamiltonian model, the thermal properties of the ferroelectricity of croconic acid were studied by Monte Carlo method. The simulated Curie temperature is 756 K, and the spontaneous polarization keeps well temperature range stability up to 400 K. These results are in good agreement with the experimental data.

Stable antiferromagnetic property and tunable electronic structure of two-dimensional MnPX3(X = S and Se) from pristine structure to Janus phase.

Transition-metal phosphorus trichalcogenides have been considered as very promising two-dimensional (2D) magnetic candidates up-to-date. We performed a systematical first-principles study on the electronic structures and magnetic properties of pristine MnPX3(X = S and Se) and Janus Mn2P2S3Se3monolayers. All monolayers behave as a direct-band-gap semiconductor in antiferromagnetic ground state which is caused by strong direct and indirect exchange interactions. It is found that the electronic structures and magnetic properties can be manipulated by Janus phase. The calculated band gap is 2.44 eV, 1.80 eV and 1.86 eV for MnPS3, MnPSe3and Mn2P2S3Se3with a valley polarization with consideration of spin-orbital coupling (SOC), respectively. In particular, significant energy-splittings emerge in the SOC-band structures of Janus Mn2P2S3Se3due to its broken-inversion-symmetry. Estimated by Monte Carlo simulations, the Néel temperature is 96 K, 71 K and 79 K based on Ising model while halved down to 41 K, 33 K and 36 K on the basis ofXYmodel for MnPS3, MnPSe3and Mn2P2S3Se3, respectively, indicating theXYmodel should be more reliable to describe the spin dynamics. Our research offers an insight into the magnetic mechanism and paves a feasible path to modulate the magnetism for 2D magnets in realistic applications on spintronics.

Investigations on structural, electronic and optical properties of ZnO in two-dimensional configurations by first-principles calculations.

The electronic structures and optical properties of two-dimensional (2D) ZnO monolayers in a series of configurations were systematically investigated by first-principles calculations with HubbardUevaluated by the linear response approach. Three types of 2D ZnO monolayers, as planer hexagonal-honeycomb (Plan), double-layer honeycomb (Dlhc), and corrugated tetragonal (Tile) structures, show a mechanical and dynamical stability, while the Dlhc-ZnO is the most energetically stable configuration and Plan-ZnO is the second one. Each 2D ZnO monolayer behaves as a semiconductor with that Plan-, Dlhc-ZnO have a direct band gap of 1.81 eV and 1.85 eV at theΓpoint, respectively, while Tile-ZnO has an indirect band gap of 2.03 eV. Interestingly, the 2D ZnO monolayers all show a typical near-free-electron character for the bottom conduction band with a small effective mass, leading to a tremendous optical absorption in the whole visible and ultraviolet window, and this origination was further confirmed by the transition dipole moment. Our investigations suggest a potential candidate in the photoelectric field and provide a theoretical guidance for the exploration of wide-band-gap 2D semiconductors.

Polypyrrole-MXene coated textile-based flexible energy storage device.

Recently, more and more researchers have devoted their efforts to developing flexible electrochemical energy storage devices to meet the development of portable and wearable electronics. Among them, supercapacitors (SCs) have been widely studied due to their high specific capacitance and power density. However, most flexible SCs often use traditional carbon materials and transition metal oxides as electrode materials. In this paper, we used an easy and low-cost way to fabricate a flexible supercapacitor based on a new type of two-dimensional material, transition metal carbides, nitrides, or carbonitrides (MXenes). By taking full advantage of the hydrophilicity and metal conductivity of MXene nanosheets, an extremely simple "dipping and drying" method was used to achieve conductive textile electrodes with a specific capacitance of 182.70 F g-1, which is higher than reported for carbon nanotubes (CNTs) and active carbon. To further improve the capacitive performance of the MXene-based electrode and avoid the poor oxygen oxidation of MXene, polypyrrole (PPy) was electrochemically deposited on the surface of MXene textiles, thus producing a PPy-MXene coated textile electrode with a specific capacitance of 343.20 F g-1. In addition, a symmetrical solid-state supercapacitor based on MXene-PPy textiles was assembled, which achieved an energy density of 1.30 mW h g-1 (power density = 41.1 mW g-1). This work introduces a new type of MXene-based textile SC, which provides a promising candidate for flexible and wearable energy storage devices.

3D Synergistical MXene/Reduced Graphene Oxide Aerogel for a Piezoresistive Sensor.

A piezoresistive sensor based on ultralight and superelastic aerogel is reported to fabricate MXene/reduced graphene oxide (MX/rGO) hybrid 3D structures and utilize their pressure-sensitive characteristics. The MX/rGO aerogel not only combines the rGO's large specific surface area and the MXene's (Ti3C2 T x) high conductivity but also exhibits rich porous structure, which leads to performance better than that of single-component rGO or MXene in terms of the pressure sensor. The large nanosheets of rGO can prevent the poor oxidization of MXene by wrapping MXene inside the aerogel. More importantly, the piezoresistive sensor based on the MX/rGO aerogel shows extremely high sensitivity (22.56 kPa-1), fast response time (<200 ms), and good stability over 10 000 cycles. The piezoresistive sensor based on the MX/rGO hybrid 3D aerogel can easily capture the signal below 10 Pa, thus clearly testing the pulse of an adult at random. Based on its superior performance, it also demonstrates potential applications in measuring pressure distribution, distinguishing subtle strain, and monitoring healthy activity.

A highly flexible and sensitive piezoresistive sensor based on MXene with greatly changed interlayer distances.

Since the successful synthesis of the first MXenes, application developments of this new family of two-dimensional materials on energy storage, electromagnetic interference shielding, transparent conductive electrodes and field-effect transistors, and other applications have been widely reported. However, no one has found or used the basic characteristics of greatly changed interlayer distances of MXene under an external pressure for a real application. Here we report a highly flexible and sensitive piezoresistive sensor based on this essential characteristics. An in situ transmission electron microscopy study directly illustrates the characteristics of greatly changed interlayer distances under an external pressure, supplying the basic working mechanism for the piezoresistive sensor. The resultant device also shows high sensitivity (Gauge Factor ~ 180.1), fast response (<30 ms) and extraordinarily reversible compressibility. The MXene-based piezoresistive sensor can detect human being's subtle bending-release activities and other weak pressure.