Arqum Hashmi

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.

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.

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 β-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 β-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 tubes inner diameter is larger than 1.3 nm, while the armchair β-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.

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.

Ultra-high capacity hydrogen storage in a Li decorated two-dimensional C2N layer

Owing to naturally existing uniform periodic pores in two-dimensional (2D) C2N layers, they can be an ideal candidate for hydrogen storage materials among other 2D materials. Here, we explored the potential application of ultra-high capacity hydrogen storage using the first principles method. Remarkably, Li was strongly bonded with the C2N layer via a Kubas-type interaction with a large binding energy of 3–5 eV. This unique interaction does not exist in graphene or other 2D materials, and it rules out the possibility of Li alkali metal cluster formations. We found that the Li-decorated C2N could show a very high theoretical gravimetric density of 13 weight percentage (wt%). Very interestingly, this gravimetric density is not only 40% and 30% higher than those found in MgH2 and C60 but also significantly higher than the values obtained in alkali metal decorated graphene, MoS2 and phosphorene. Irrespective of the theoretical capacity, the most important physical quantity is the practical capacity (the difference in the number of adsorbed and desorbed hydrogen molecules) under ambient conditions of pressure and temperature. Our thermodynamic analysis showed that 75% of the adsorbed hydrogen molecules could be released under practical conditions of temperature and pressure and the practical capacity is about 10 wt%. Our findings suggest that the Li decorated C2N can be a very promising material for room-temperature hydrogen storage under realistic conditions.

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.

Magnetization reversal and spintronics of Ni/Graphene/Co induced by doped graphene

We have investigated the magnetization reversal induced by carrier doped graphene (Be, B, N, O, or Cl doping) in Ni/Graphene/Co. In undoped case, the magnetic layers have an antiferromagnetic (AFM) coupling and this is still preserved from Be to O doping. We find magnetization reversal from AFM to ferromagnetic interaction induced by Cl doped graphene. In addition, the Ni and the Co layers show the opposite spin asymmetry near the Fermi level and this implies that each layer will generate completely different in-plane spin current in the same direction if an external electric field is applied.

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.

Transition from half metal to semiconductor in Li doped g-C4N3

We have investigated the structural and magnetic properties of Li doped graphitic carbon nitride (g-C4N3) using the van der Waals density functional theory. A free standing g-C4N3 was known to show a half metallic state with buckling geometry, but this feature completely disappears in the presence of Li doping. Besides this structural modification, very interestingly, we have obtained that the Li doped g-C4N3 shows dramatic change in its electronic structure. Both ferromagnetic and nonmagnetic states are almost degenerated in one Li atom doped system. However, the transition from half metallic state to semiconductor is observed with further increase of Li concentration and the calculated energy gap is 1.97 eV. We found that Li impurity plays as a donor element and charge transfer from the Li atom to neighboring N atoms induces a band gap. Overall, we have observed that the electronic and magnetic properties of g-C4N3 are substantially modified by Li doping.