Eradicating negative-Set behavior of TiO2-based devices by inserting an oxygen vacancy rich zirconium oxide layer for data storage applications.

Memristors, with low energy consumption, long data storage and fast switching speed, are considered to be promising for applications such as terabit data storage memory and hardware based neurocomputation applications. However, unexpected negative-Set behavior is a serious issue that causes deterioration of reliability and uniformity of switching parameters. In this work, negative-Set behavior of TiO2-based RRAM is successfully eradicated by inserting a thin oxygen vacancy rich ZrO2-x layer. In addition, oxygen vacancy rich ZrO2-x layer is also responsible for the enhancement of resistive switching characteristics in terms of excellent endurance performance (2000 DC cycles), good data retention upto 104 s and uniformity in Set/Reset voltages. Experimental results and density functional theory (DFT) analysis confirm that an interface layer TiOx has formed between highly reactive electrode (Ti) and ZrO2 interlayer. This interface layer is serving as a low series resistance layer and oxygen ion reservoir in Set-process and oxygen ions supplier in Reset-process to generate/refill the oxygen vacancies in the formation and rupture of conductive filaments. Comparing with the single layer Ti/TiO2/Pt device, it is noteworthy that the switching process in the bilayer (BL) Ti/ZrO2-x/TiO2/Pt memristor device is not affected even at high Reset-voltages, but the negative-Set behavior has been eradicated effectively. This work demonstrates that the insertion of a thin oxygen vacancy rich ZrO2-x interlayer into TiO2-based devices is a feasible approach to solve unpredicted negative-Set behavior of RRAM devices.

Superconductivity in two-dimensional ferromagnetic MnB.

Using the universal structure predictor algorithm, we proposed that two-dimensional MnB structures with p4mmm (α-MnB) and pmma (ß-MnB) symmetries could be synthesized. This finding was verified by calculating the dynamical stability, molecular dynamics, and mechanical properties. The α-MnB had an in-plane stiffness Y x (=Y y ) around 100 N/m while the ß-Mn displayed an asymmetric mechanical stiffness of Y x = 186 N/m and Y y = 139 N/m. Both systems displayed a ferromagnetic ground state with metallic band structures. The calculated magnetic moments were 2.14 and 2.34 µB per Mn-B pair in the α-MnB and ß-MnB. Furthermore, we investigated the potential superconductivity. In the α-MnB, we found the unique feature of Kohn anomaly at q~2kF in the diagonal direction of the Brillouin zone. The ß-MnB phonon spectra showed a valley of degenerated localized softening vibration modes at the edge of the Brillouin zone. The ZA and LA phonon branches in this valley induced the largest contribution to electron-phonon coupling strength. The calculated total electron-phonon coupling parameters were 1.20 and 0.89 in α-MnB and ß-MnB systems. Overall, we predict that the α-MnB and ß-MnB systems can display 2D ferromagnetic superconducting states with the estimated critical temperatures of Tc ≈ 10-13 K.

Two-Dimensional Magnetic Semiconductor in Feroxyhyte.

A few years ago, it was claimed that the two-dimensional (2D) feroxyhyte (δ-FeOOH) layer could possess a net magnetic moment and it could be applied for potential spintronics applications because it showed a band gap. However, the exact crystal structure is still unknown. Hereby, we investigate the crystal structure, electronic band structure, and magnetic and optical properties of 2D δ-FeOOH using density functional calculations. On the basis of the experimental observation and dynamical stability calculations, we propose that the 2D δ-FeOOH originates from bulk Fe(OH)2 via oxidation. A perfect antiferromagnetic ground state was observed in the monolayer structure with an indirect band gap of 2.4 eV. On the other hand, the bilayer structure displayed a direct band gap of 0.87 eV, and we obtained a ferrimagnetic state. The net magnetic moment in the bilayer was 1.49 µB per cell. The interlayer distance and film thickness in bilayer δ-FeOOH were 1.68 and 7.37 Å, respectively. This interlayer distance was suppressed to 1.47 Å in a trilayer system, and the band gap of 1.6 eV was found. The trilayer δ-FeOOH had a film thickness of 11.57 Å, and this is comparable to the experimental thickness of 12 Å. To compare with the experimental band gap of 2.2 eV obtained from a UV-visible optical spectrum measurement, we also calculated the absorption spectra, and the onset of the absorption peak in the monolayer, bilayer, and trilayer appeared at 3.2, 2.8, and 2.2 eV, respectively. Overall, considering the magnetic state, optical absorption, and film thickness, we propose that the trilayer structure agrees with the experimentally synthesized structure.

Two-dimensional honeycomb hafnene monolayer: stability and magnetism by structural transition.

A few years ago, it was claimed that the two-dimensional ferromagnetic planar Hf monolayer could be synthesized on Ir(111). However, several questions remained unanswered. Herein, we unravel the structural stability of the HF monolayer and its influence on magnetism using first principles calculations. Despite the ferromagnetic state in the planar free-standing Hf layer, extensive systematic calculations with phonon spectra reveal that the planar free-standing Hf layer is unstable and it has a non-magnetic high-buckled structure in the ground state. We also find a structural transition from buckled to flat honeycomb geometry on the Ir(111) substrate. Nonetheless, 2D hafnene has no magnetic state due to strong hybridization with the Ir(111) surface. The evolution from the non-magnetic to the ferromagnetic state combined with structural transition is observed by adding BN as a spacer layer on the Ir(111) substrate (BN/Ir(111)). In addition, we find that 2D Hf on BN/Ir(111) has a giant perpendicular magnetic anisotropy of 3.41 meV.

Ferromagnetism controlled by electric field in tilted phosphorene nanoribbon.

Study on phosphorene nanoribbon was mostly focused on zigzag and armchair structures and no ferromagnetic ground state was observed in these systems. Here, we investigated the magnetic property of tilted black phosphorene nanoribbons (TPNRs) affected by an external electric field. We also studied the edge passivation effect on the magnetism and thermal stability of the nanoribbons. The pure TPNR displayed an edge magnetic state, but it disappeared in the edge reconstructed TPNR due to the self-passivation. In addition, we found that the bare TPNR was mechanically unstable because an imaginary vibration mode was obtained. However, the imaginary vibration mode disappeared in the edge passivated TPNRs. No edge magnetism was observed in hydrogen and fluorine passivated TPRNs. In contrast, the oxygen passivated TPNR was more stable than the pure TPNR and the edge-to-edge antiferromagntic (AFM) ground state was obtained. We found that the magnetic ground state could be tuned by the electric field from antiferromagnetic (AFM) to ferromagnetic (FM) ground state. Interestingly, the oxygen passivated TPNR displayed a half-metallic state at a proper electric field in both FM and AFM states. This finding may provoke an intriguing issue for potential spintronics application using the phosphorene nanoribbons.

Long-Range Magnetic Ordering and Switching of Magnetic State by Electric Field in Porous Phosphorene.

We explored the possibility of long-range magnetic ordering in two-dimensional porous phosphorene (PP) layer by means of ab-initio calculations. The self-passivated pore geometry showed a nonmagnetic state while the pore geometry with dangling bond at two zigzag edges with a distance of 7.7 Å preferred an antiferromagnetic ordering (AFM). Pore to pore magnetic interaction with a distance of 13.5 Å between two pores was found to be remarkably long ranged, and this emerges from the interactions between the magnetic tails of the edge states in the armchair direction. The AFM state was persisted by the oxidation of the edge. Interestingly, the long-range AFM ordering changed to long-range ferromagnetic (FM) ordering by external electric field. The results are noteworthy in the interplay between electric field and electronic spin degree of freedom in phosphorene studies and may also open a promising way to explore phosphorene-based spintronics devices.

Geometry, electronic structures and optical properties of phosphorus nanotubes.

Using a first principles approach, we investigated the geometry, electronic structures, and optical properties of phosphorus nanotubes (PNTs). Two possible 1D configurations, the so-called α-PNTs and ß-PNTs, are proposed, which are structurally related to blue and black phosphorus monolayers, respectively. Hereby, we predict that both armchair and zigzag geometries can be synthesized in α-PNTs, but the zigzag form of ß-PNT is highly unfavorable because of large strain and conformation energies. The band gap of α-PNTs is expected to be ∼2.67 eV, and this is insensitive to the chirality when the tube's inner diameter is larger than 1.3 nm, while the armchair ß-PNTs have a much smaller band gap. Interestingly, we find nearly flat band structures in the zigzag α-PNT system. This may indicate that an excited particle-hole pair has a huge effective mass. We also find asymmetric optical properties with respect to the polarization direction. The armchair α-PNT for parallel polarization shows a large refractive index of 2.6 near the ultraviolet wavelength, and also we find that the refractive index can be even smaller than 1 in certain frequency ranges. The zigzag tubes show very weak reflectivity for parallel polarization, while the armchair tube displays high reflectivity.

Anisotropic bias dependent transport property of defective phosphorene layer.

Phosphorene is receiving great research interests because of its peculiar physical properties. Nonetheless, no systematic studies on the transport properties modified due to defects have been performed. Here, we present the electronic band structure, defect formation energy and bias dependent transport property of various defective systems. We found that the defect formation energy is much less than that in graphene. The defect configuration strongly affects the electronic structure. The band gap vanishes in single vacancy layers, but the band gap reappears in divacancy layers. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property. Unlike the common belief, we observe that the vacancy defect can contribute to greatly increasing the current. Along the zigzag direction, the current in the most stable single vacancy structure was significantly increased as compared with that found in the pristine layer. In addition, the current along the armchair direction was always greater than along the zigzag direction and we observed a strong anisotropic current ratio of armchair to zigzag direction.

Manipulation of Magnetic State in Armchair Black Phosphorene Nanoribbon by Charge Doping.

Using first-principles studies, we investigated the width-dependent magnetic properties of armchair black phosphorene nanoribbons (APNRs) by controlling the electron charge doping. In the unrelaxed APNRs the antiferromagnetic coupling between two phosphorus atoms in the same edge was found. However, the edge magnetic moment vanished after structure relaxation, and all of the APNRs showed a semiconducting feature. Interestingly, the charge doping substantially altered the band structures of the APNRs because the metallic states reappeared in the charge-doped APNRs. Besides this, the magnetic moment was found in the charge-doped systems. We found that the Stoner condition could nicely explain the magnetic moment at the edge atoms. Moreover, we propose that the edge-to-edge magnetic coupling can be manipulated by charge doping because the transition from the antiferromagnetic to ferromagnetic state was achieved. Our findings may bring interesting issues for spintronics applications.

Transparent half metallic g-C4N3 nanotubes: potential multifunctional applications for spintronics and optical devices.

Multifunctional material brings many interesting issues because of various potential device applications. Using first principles calculations, we predict that the graphitic carbon nitride (g-C4N3) nanotubes can display multifunctional properties for both spintronics and optical device applications. Very interestingly, armchair tubes (n, n) with n = 2, 3, 4, 5, 6 and (5, 0) zigzag tubes are found to be half metallic, while zigzag tubes (n, 0) with n = 4, 6 show an antiferromagnetic ground state with band gaps. However, larger zigzag tubes of (7, 0), (8, 0), and (10, 0) are turned out to be half metallic. Along with the half metallic behavior of the tubes, those tubes seem to be optically transparent in the visible range. Due to these magnetic and optical properties, we suggest that the g-C4N3 nanotubes (CNNTs) can be used for both ideal spintronics and transparent electrode materials. We also explored the stability of magnetic state and nanotube structure using ab initio molecular dynamics. The CNNTs were found to be thermally stable and the magnetic moment was robust against the structural deformation at 300 K. Overall, our theoretical prediction in one dimensional CNNTs may provide a new physics in spintronics and also in other device applications because of potential multifunctional properties.

Metal free half metallicity in 2D system: structural and magnetic properties of g-C4N3 on BN.

Synthesis of a half metallic material on a substrate is highly desirable for diverse applications. Herein, we have investigated structural, adsorptive, and magnetic properties of metal free graphitic carbon nitride (g-C4N3) layer on hexagonal BN layer (h-BN) using the optB88-vdW van der Waals density functional theory. It is found that g-C4N3 layer can be adsorbed on BN layer due to the change of lattice constant of the hybridized system. The newly found lattice constant of g-C4N3 was 9.89 Å, which is approximately 2% lower and larger than to those of free standing BN and g-C4N3, respectively. Also, 2 × 2 surface reconstruction geometry predicted in free standing g-C4N3 layer disappears on the BN layer. Interestingly, we have found that metal free half metallic behavior in g-C4N3 can be preserved even on BN layer and the characters of spin polarized planar orbitals suggest that our theoretical prediction can be verified using normal incidence of K-edge X-ray magnetic circular dichroism (XMCD) measurement.