All-phosphorus flexible devices with non-collinear electrodes: a first principles study

Effect of vacancy defects on transport in all-phosphorene nanoribbon devices from first principles

Defects in experimentally manufactured phosphorene nanoribbons (PNRs) occur unavoidably, affecting the functionality of PNR-based devices. In this work, we theoretically propose and investigate an all-PNR devices with single-vacancy (SV) defects and double-vacancy (DV) defects along the zigzag direction, accounting for both hydrogen passivation and non-passivation scenarios. We discovered that, in the case of hydrogen passivation, the DV defect can introduce in-gap states, whereas the SV defect can result in p-type doping. The unpassivated hydrogen nanoribbon exhibits an edge state with a considerable influence on the transport properties, which also masks the effect of defects on transport; furthermore, it demonstrates the phenomenon of negative differential resistance, whose occurrence and characteristics depend less on the presence or absence of defects.

Non-equilibrium spin-transport properties of Co/phosphorene/Co MTJ with non-collinear electrodes under mechanical bending

Monolayer phosphorene has outstanding mechanical flexibility, making it rather attractive in flexible spintronics that are based on 2D materials. Here, we report a first-principles study on non-equilibrium electronic-transport properties of the Co/phosphorene/Co magnetic tunnel junction (MTJ) with two α-Co electrodes. The magnetic moments of the two electrodes are considered in the parallel configuration (PC) and the anti-parallel configuration (APC). The tunneling current through the MTJ is investigated at a small bias from 0 to 40 mV when mechanical bending is applied on the MTJ with different central angle (θ) values. For both the PC and APC, the tunneling current increases evidently and monotonously with increasing mechanical bending for 25° < θ < 40°, as compared to that without bending, which is mainly due to the reduced tunnel barrier. In the PC, the spin-injection efficiency (SIE) of the current is largely increased at a small bias from 0 to 40 mV for 25° ≤ θ ≤ 30° with a maximum of 90%, while the SIE is overall increased under all mechanical bending angles for the APC. The tunnel magnetoresistance is decreased with an increasing bias voltage, which can be largely enhanced for θ ≥ 25°, especially at small bias. Our results indicate that the Co/phosphorene/Co MTJ has promising applications in flexible low-power spintronic devices.

Investigation of the half-metallicity, magnetism and spin transport properties of double half-Heusler alloys Mn2CoCrZ2 (Z = P, As)

A new subfamily of Heusler alloys, i.e. double half-Heusler alloys Mn₂CoCrZ₂ (Z = P, As), are investigated by employing density functional theory combined with the nonequilibrium Green's function. The calculations of their magnetic properties reveal that Mn₂CoCrZ₂ (Z = P, As) are half-metallic ferrimagnets. Mn₂CoCrP₂ possesses an indirect spin-down bandgap of 0.671 eV and maintains half-metallicity with the lattice constant ratio c/a ranging from 1.72 to 2.40, while Mn₂CoCrAs₂ owns a direct spin-down bandgap of 0.993 eV and maintains half-metallicity with c/a ranging from 1.5 to 2.5. By employing Mn₂CoCrZ₂ as the electrodes and GaAs as the tunnel barrier, two kinds of magnetic tunnel junctions (MTJs) are constructed. When two electrodes of MTJs are in parallel magnetic configuration, the spin-up electrons have strong transmission ability, while the transmission ability of spin-down electrons is severely suppressed. When two electrodes of MTJs are in antiparallel magnetic configuration, the transmission ability of both spin-channel electrons is suppressed. The calculated tunnel magnetoresistance ratios of Mn₂CoCrP₂/GaAs/Mn₂CoCrP₂ and Mn₂CoCrAs₂/GaAs/Mn₂CoCrAs₂ MTJs reach up to 7.96 × 10⁸ and 1.85 × 10⁸, respectively, indicating that they are promising candidates for high performance spintronic devices.

The study of bending properties of monolayer MoS2 in non-collinear electrodes using first principles theory

In this work, we report the theoretical maximum bending angle of MoS2 devices using the creative non-collinear electrodes method based on the first principles theory. The results show that the device with 1T-phase MoS2 electrodes sandwiching N-type MoS2 in a zigzag direction has a better conducting behavior as compared with P-type in an armchair direction. The conductance decreases less than 15% when the angle between the two electrodes is less than 45° in both the equilibrium state and non-equilibrium state because of the continuous resonant response between the two electrodes and the little deformed band structure. This work provides guidance and a physical mechanism for achieving flexible MoS2 transistors that are reliable at a sub-nm bending radius.

First principles modeling of pure black phosphorus devices under pressure

Black phosphorus (BP) has a pressure-dependent bandgap width and shows the potential for applications as a low-dimensional pressure sensor. We built two kinds of pure BP devices with zigzag or armchair conformation, and explored their pressure-dependent conductance in detail by using first principles calculations. The zigzag BP devices and the armchair BP devices exhibit different conductance-pressure relationships. For the zigzag BP devices conductance is robust against stress when the out-of-plane pressure ratio is less than 15%, and then increases rapidly until the conductive channels are fully opened. For the armchair pure BP devices conductance decreases at first by six orders of magnitude under increasing pressure and then increases quickly with further increase of pressure until the devices enter the on-state. This shows that the pure zigzag BP devices are more suitable for the application as flexible electronic devices with almost constant conductance under small pressure, while armchair BP devices can serve as bidirectional pressure sensors. Real-space distributions of band alignments were explored to understand the different pressure-related properties. We fitted a set of parameters based on the results from the empirical Wentzel-Kramers-Brillouin method, which provides an effortless approximation to quantitatively predict the pressure-related behaviors of large pure BP devices.

Modifying spin current filtering and magnetoresistance in a molecular spintronic device

The zigzag edged graphene nanoribbon (ZGNR) is excellent for spintronics devices, and many efforts have been made to investigate its properties such as spin filtering, rectification and magnetoresistance. Here we propose a molecular spintronic transport device based on two ZGNR electrodes connected with a dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3-i]phenazine (DDQP) molecule. By performing first-principles electron transport computations, we found an enhanced spin polarized current–voltage curve, giant spin filter efficiency, magnetoresistance and rectification ratio properties of the device compared to its all-carbon molecular analogue. Our systematic investigation suggests the vital role played in spin polarized electron transport by nitrogen atoms in DDQP, the ZGNR probe's width and terminal geometry, especially the increased spin filter efficiency with higher ZGNR width.

Three general factors of the molecule device were investigated to enhance its spin filtering efficiency.