Surface engineering of phosphorene nanoribbons by transition metal heteroatoms for spintronics

Gap engineering effects on transport and tunneling magnetoresistance properties in phosphorene ferromagnetic/normal/ferromagnetic junction

Tuning the band gap is of utmost importance for the practicality of two-dimensional materials in the semiconductor industry. In this study, we investigate the ballistic transport and the tunneling magnetoresistance (TMR) properties within a modulated gap in a ferromagnetic/normal/ferromagnetic (F/N/F) phosphorene junction. The theoretical framework is established on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-Büttiker formalism to characterize electron behavior and obtain transmittance, conductance and TMR. Our results reveal that a reduction in gap energy leads to an enhancement of conductance for both parallel and anti-parallel magnetization configurations. In contrast, a significant reduction and redshift in TMR have been observed. Notably, the application of an electrostatic field in a gapless phosphorene F/N/F junction induces a blueshift and a slight increase in TMR. Furthermore, we found that introducing an asymmetrically applied electrostatic field in this gapless junction results in a significant reduction and redshift in TMR. Additionally, intensifying the applied magnetic field leads to a substantial increase in TMR. These findings could be useful for designing and implementing practical applications that require precise control over the TMR properties of a phosphorene F/N/F junction with a modulated gap.

Recent Advances in Phosphorene: Structure, Synthesis, and Properties

Phosphorene is a 2D phosphorus atomic layer arranged in a honeycomb lattice like graphene but with a buckled structure. Since its exfoliation from black phosphorus in 2014, phosphorene has attracted tremendous research interest both in terms of synthesis and fundamental research, as well as in potential applications. Recently, significant attention in phosphorene is motivated not only by research on its fundamental physical properties as a novel 2D semiconductor material, such as tunable bandgap, strong in-plane anisotropy, and high carrier mobility, but also by the study of its wide range of potential applications, such as electronic, optoelectronic, and spintronic devices, energy conversion and storage devices. However, a lot of avenues remain to be explored including the fundamental properties of phosphorene and its device applications. This review recalls the current state of the art of phosphorene and its derivatives, touching upon topics on structure, synthesis, characterization, properties, stability, and applications. The current needs and future opportunities for phosphorene are also discussed.

Regulating the electronic properties of the WGe2N4 monolayer by adsorption of 4d transition metal atoms towards spintronic devices

We study the regulation of the electronic and spin transport properties of the WGe2N4 monolayer by adsorbing 4d transition metal atoms (Y-Cd) using density functional theory combined with non-equilibrium Green's function. It is found that the adsorption of transition metal atoms (except Pd, Ag and Cd atoms) can introduce a magnetic moment into the WGe2N4 monolayer. Among the transition metal atoms, the adsorption of Nb and Rh atoms transforms WGe2N4 from a semiconductor to a half-metal and a highly spin-polarized semiconductor, respectively. The half-metallic Nb-adsorbed WGe2N4 system is selected to investigate the spin transport properties, and a high magnetoresistance ratio of 107% is achieved. In both parallel and antiparallel magnetization configurations, the spin filtering efficiency reaches close to 100% in the whole bias range, and the antiparallel magnetization configuration exhibits a dual spin filtering effect with a rectification ratio of up to 104. Our study predicts that the adsorption of 4d transition metal heteroatoms is an effective method to regulate the electronic and magnetic properties of WGe2N4 towards high-performance spintronic devices.

Tuning the magnetoresistance properties of phosphorene with periodic magnetic modulation

Periodic superlattices constitute ideal structures to modulate the transport properties of two-dimensional materials. In this paper, we show that the tunneling magnetoresistance (TMR) in phosphorene can be tuned effectively through periodic magnetic modulation. Deltaic magnetic barriers are arranged periodically along the phosphorene armchair direction in parallel (PM) and anti-parallel magnetization (AM) fashion. The theoretical treatment is based on a low-energy effective Hamiltonian, the transfer matrix method and the Landauer-Büttiker formalism. We find that the periodic modulation gives rise to oscillating transport characteristics for both PM and AM configurations. More importantly, by adjusting the electrostatic potential appropriately we find Fermi energy regions for which the AM conductance is reduced significantly while the PM conductance keeps considerable values, resulting in an effective TMR that increases with the magnetic field strength. These findings could be useful in the design of magnetoresistive devices based on magnetic phosphorene superlattices.

A BC2N/blue phosphorene heterostructure as an anode material for high-performance sodium-ion batteries: first principles insights

Blue phosphorene (Blu-Pn) is a new phosphorene allotrope capable of hosting a substantial amount of sodium (Na) atoms. However, it has been reported to exhibit low electrical conductivity, chemical sensitivity, and structural stability, thus limiting its utility as an anode material for Na-ion batteries (NIBs). In this work, we introduce BC₂N as a protective layer for Blu-Pn. Based on van der Waals (vdW) corrected density functional theory (DFT), we conduct a comprehensive first-principles study to explore the main electrochemical properties of the BC₂N/Blu-Pn vdW heterostructure. The BC₂N/Blu-Pn system exhibits a small band-gap of 0.03 eV that fades away and indicates metallic behavior upon Na adsorption. Furthermore, the binding energy of Na incorporated into the inter-layer of the BC₂N/Blu-Pn system is lower (-2.03 eV) compared with those of free-standing BC₂N (-1.25 eV) and Blu-Pn monolayer (-1.52 eV). Therefore, the growth of Na dendrites can be avoided. Furthermore, the migration energy barrier for the BC₂N/Blu-Pn system is about 0.11 eV, indicating fast Na diffusion and excellent rate performance. Moreover, the theoretical storage capacity is 763 mA h g^-1. Finally, we show that the intercalation of Na in the BC₂N/Blu-Pn system has the advantage of a small average voltage of approximately 0.24 V. Besides these properties, the proposed heterostructure is based on chemical elements that are widely available and technologically established and have low atomic mass, which are all advantages for Na-ion battery applications.

Ultra-Narrow Phosphorene Nanoribbons Produced by Facile Electrochemical Process

Phosphorene nanoribbons (PNRs) have inspired strong research interests to explore their exciting properties that are associated with the unique two-dimensional (2D) structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several important properties of PNRs, the production of these structures with narrow widths is still a great challenge. Here, a facile and straightforward approach to synthesize PNRs via an electrochemical process that utilize the anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction, is reported. The produced PNRs display widths of good uniformity (10.3 ± 3.8 nm) observed by high-resolution transmission electron microscopy, and the suppressed B2g vibrational mode from Raman spectroscopy results. More interestingly, when used in field-effect transistors, synthesized bundles exhibit the n-type behavior, which is dramatically different from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined widths, paving the way toward the development of phosphorene and other highly anisotropic nanoribbon materials for high-quality electronic applications.

Electric-Field Control in Phosphorene-Based Heterostructures

Phosphorene is a graphene-like material with an intermediate band gap, in contrast to zero-gap graphene and large-gap dichalcogenides or hexagonal boron nitride (hBN), which makes it more suitable for nanoelectronic devices. However, inducing band-gap modulation in freestanding phosphorene nanoribbons (PNRs) is problematic, as high in-plane electric fields are necessary to close the gap. We perform here a detailed investigation concerning the substrate influence on the electric-field control exerted by an external gate, using the density functional theory-non-equilibrium Green's functions (DFT-NEGF) framework. It is established that the interaction with a hexagonal boron nitride supporting layer significantly enhances the gap modulation. Furthermore, we address the issue of contacting the PNRs, by using conducting graphene nanoribbons embedded in the support hBN layer. Within this setup, a measurable spin polarization is achieved owing to the anti-ferromagnetic coupling between the edges of the graphene nanoribbons.

Magnetism and perfect spin filtering in pristine MgCl2 nanoribbons modulated by edge modification

The search for new materials, with ideal electronic and magnetic properties for potential applications in nanoelectronics, has been extremely successful so far, and has paved the way for us to reimagine all technological devices. In the present work, we study the design of MgCl₂ nanoribbons for applications in nanoelectronics and spintronics, by employing first-principles calculations based on density functional theory (DFT) and non-equilibrium Greens function techniques. Our results show that the properties of MgCl₂ nanoribbons depend strongly not only on their geometrical form (armchair or zigzag) but also on the atoms at their edges. The armchair MgCl₂ nanoribbon is a semiconductor and the zigzag nanoribbons vary from semiconducting, to metallic, to ferromagnetic, and to half-metallic, depending on the edge terminations. All these nanoribbons are very stable, with a relatively low cohesive energy per atom, and their attributes are not affected by the width of the nanoribbon. From transport calculations, we observed partial spin filtering in the ferromagnetic nanoribbon and perfect spin filtering in the two half-metallic nanoribbons. Moreover, we show how the current versus voltage curves can be fully understood by analysing the alignment of the energy levels of the electrodes. Our results corroborate the promising use of single-layer MgCl₂ for the development of spintronics devices.

Surface decoration of phosphorene nanoribbons with 4d transition metal atoms for spintronics

The recent production of phosphorene nanoribbons provides a platform for designing phosphorene-based high-speed electronic devices. Introducing a magnetic moment to phosphorene nanoribbons for spintronics application is attractive. Based on density functional theory combined with the non-equilibrium Green's function method, the electronic, magnetic and spin-polarized transport properties of phosphorene nanoribbons modified by adsorption and substitutional doping of 4d transition metal atoms (Y, Zr, Nb and Mo) are investigated systematically. The results show that both the adsorption and the doping of 4d transition metal atoms can introduce a magnetic moment into phosphorene nanoribbons, except the Y- and Nb-doping cases. The adsorption shows superior performance in terms of modulating the electronic and magnetic properties of phosphorene nanoribbons compared to substitutional doping, exhibiting higher spin polarization near the Fermi level with a narrower band gap. This discrepancy originates from the different electronic redistribution in the adsorption and doping situations. Furthermore, the nanoribbons with adsorbed 4d transition metal atoms exhibit excellent spin-polarized transport properties: a giant magnetoresistance ratio of the Mo-adsorbed nanoribbon reaches over 108 under low bias; the Y-Mo-adsorbed nanoribbons with parallel spin configurations show a spin filtering effect of about 100% with the bias larger than 0.1 V, and those with antiparallel spin configurations exhibit a dual spin filtering effect in an applied bias range of (-0.2 V, 0.2 V). Our results demonstrate that 4d-transition-metal-atom adsorption is a favourable approach to modify the electronic, magnetic and transport properties of phosphorene nanoribbons, thus providing a reference for the rational design of spintronic devices based on phosphorene nanoribbons.

Pervasive Ohmic contacts of monolayer 4-hT2 graphdiyne transistors

Monolayer (ML) graphdiyne, a two-dimensional semiconductor with appropriate band gap and high carrier mobility, is a promising candidate for channel material in field effect transistors (FETs). Using density functional theory combined with non-equilibrium Green's function method, we systematically investigate the contact and transport properties of graphdiyne FETs with various electrodes, including metals (Cu, Au, Ni, Al and Ag) and MXenes (Cr₂C, Ta₂C and V₂C). Strong interaction can be found between ML graphdiyne and the Cu, Ni and MXenes electrodes with indistinguishable band structure of ML graphdiyne, while weak or medium interaction exists in the contacts of ML graphdiyne and the Au, Al and Ag electrodes where the band structure of ML graphdiyne remains intact. Despite the different contact interactions, Ohmic contacts are generated with all considered electrode materials owing to the weak Fermi level pinning of graphdiyne. The linear I-V characteristic curve verifies the Ohmic contact between Au electrode and graphdiyne ultimately. The theoretically calculated Schottky barrier heights of graphdiyne with Cu electrode are consistent with the available experimental data. Our calculation suggests that graphdiyne is an excellent channel material of FETs forming desired Ohmic contacts with wide-ranging electrodes and thus is promising to fabricate high performance FETs.

Modulating the electronic structures of blue phosphorene towards spintronics

Modulation of the electronic and magnetic structure of blue phosphorene nanoribbons to explore the potential application in spintronics is appealing.