Menghao Wu

Nine new phosphorene polymorphs with non-honeycomb structures: A much extended family

We predict a new class of monolayer phosphorus allotropes, namely, ε-P, ζ-P, η-P, and θ-P. Distinctly different from the monolayer α-P (black) and previously predicted β-P (Phys. Rev. Lett. 2014, 112, 176802), γ-P, and δ-P (Phys. Rev. Lett. 2014, 113, 046804) with buckled honeycomb lattice, the new allotropes are composed of P4 square or P5 pentagon units that favor tricoordination for P atoms. The new four polymorphs, together with five additional hybrid polymorphs, greatly enrich the phosphorene structures, and their stabilities are confirmed by first-principles calculations. In particular, the θ-P is shown to be equally stable as the α-P (black) and more stable than all previously reported phosphorene polymorphs. Prediction of nonvolatile ferroelastic switching and structural transformation among different polymorphs under strains points out their potential applications via strain engineering.

Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues

Phosphorene and phosphorene analogues such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calculation evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temperature nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelectric switching readily readable via electrical, thermal, optical, mechanical, or even spintronic detection upon the swapping of the zigzag and armchair direction. In addition, it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temperature, suggesting high potential for practical device application.

Materials design of half-metallic graphene and graphene nanoribbons

Through patterned chemical modification, we show that both graphene sheets and zigzag-edged graphene nanoribbons (ZGNRs) can be converted to half-metals as long as the unmodified carbon strip (or width of ZGNRs) is sufficiently wide. Periodically functionalized graphene can mimic electronic behavior of edge-modified ZGNRs as the edge-modified zigzag carbon chains effectively divide a graphene sheet into a series of identical ZGNRs.

Planar Tetracoordinate Carbon Strips in Edge Decorated Graphene Nanoribbon

We predicted highly stable Cu-decorated zigzag graphene nanoribbons (ZGNRs) containing planar tetracoordinate carbon (ptC) strip(s). The computed electronic band structures suggest that the Cu-decorated ZGNRs are semiconducting. The Hirshfeld spin analysis suggests significant spin distribution at the edge ptC atoms. The two ptC strips are antiferromagnetically coupled, even though the edge Cu atoms do not exhibit significant magnetism. The stability of the multicenter ptC strip stems from both the highly delocalized pi orbital of ZGNRs and nearly perfect match between Cu-Cu bonding geometries and carbon atoms at the ZGNR edges.

Edge-decorated graphene nanoribbons by scandium as hydrogen storage media

On the basis of density functional theory calculations, we show that edge-decorated graphene nanoribbons (GNRs) by scandium can bind multiple hydrogen molecules in a quasi-molecular fashion. The average adsorption energy of H(2) on Sc ranges from 0.17 to 0.23 eV, ideally suited to hydrogen storage. For the narrowest GNR with either armchair or zigzag edges, the predicted weight percentage of H(2) is >9 wt%, exceeding the gravimetric target value set by the Department of Energy (DOE). The bonding energy between Sc and the GNR is significantly greater than the cohesive energy of bulk Sc so that clustering of Sc will not occur once Sc is bonded with carbon atoms at the edge of GNRs. Moreover, the adsorption energy of H(2) can be modestly tuned (either enhanced or reduced) by applying an external electric field.

Three-dimensional network model of carbon containing only sp2-carbon bonds and boron nitride analogues

We design two network models of carbon and boron nitride crystalline networks, namely, the honeycomb and triangular foams. The triangular carbon foam is a unique 3D network model of carbon which contains only sp(2)-carbon bonds. Also, it has large internal surface area per unit volume and bulk modulus.

Charge-injection induced magnetism and half metallicity in single-layer hexagonal group III/V (BN, BP, AlN, AlP) systems

Based on the first-principles calculations, we predict that strong ferromagnetism and half metallicity can be induced via charge injection in single-layer hexagonal boron nitride (BN) and BN nanoribbons. This phenomenon can be understood based on the Stoner criterion and the relationship between induced magnetic moment and charge density. Other group-III/V two-dimensional honeycomb systems such as boron phosphide (BP), aluminum nitride (AlN), and aluminum phosphide (AIP) exhibit similar ferromagnetic behavior upon charge injection. Like BN, the single-layer hexagonal AlN can be converted to a half metal at certain positive charge states.

Multiferroic materials based on organic transition-metal molecular nanowires

We report on the density functional theory aided design of a variety of organic ferroelectric and multiferroic materials by functionalizing crystallized transition-metal molecular sandwich nanowires with chemical groups such as -F, -Cl, -CN, -NO(2), ═O, and -OH. Such functionalized polar wires exhibit molecular reorientation in response to an electric field. Ferroelectric polarizations as large as 23.0 μC/cm(2) are predicted in crystals based on fully hydroxylized sandwich nanowires. Furthermore, we find that organic nanowires formed by sandwiching transition-metal atoms in croconic and rhodizonic acids, dihydroxybenzoquinone, dichloro-dihydroxy-p-benzoquinone, or benzene decorated by -COOH groups exhibit ordered magnetic moments, leading to a multiferroic organometallic crystal. When crystallized through hydrogen bonds, the microscopic molecular reorientation translates into a switchable polarization through proton transfer. A giant interface magnetoelectric response that is orders of magnitude greater than previously reported for conventional oxide heterostructure interfaces is predicted.

Bismuth Oxychalcogenides: A New Class of Ferroelectric/Ferroelastic Materials with Ultra High Mobility

Atomically thin Bi2O2Se has been recently synthesized, and it possesses ultrahigh mobility (Nat. Nanotechnol. 2017, 12, 530; Nano Lett. 2017, 17, 3021). Herein, we show first-principles evidence that Bi2O2Se and a related class of bismuth oxychalcogenides, such as Bi2O2S and Bi2O2Te, not only are novel semiconductors with ultrahigh mobility but also possess previously unreported ferroelectricity/ferroelasticity. Such a unique combination of semiconducting with ferroelectric/ferroelastic properties enables bismuth oxychalcogenides to potentially meet a great challenge, that is, integration of room-temperature functional nonvolatile memories into future nanocircuits. Specifically, we predict that bulk Bi2O2S is both ferroelastic and antiferroelectric and that a thin film with odd number of layers can even be multiferroic with nonzero in-plane polarization, and this polarization can be switchable via ferroelasticity. Moreover, Bi2O2Te possesses intrinsic out-of-plane ferroelectricity, while Bi2O2Se possesses piezoelectricity and ferroelectricity upon an in-plane strain. The in-plane strain on Bi2O2Se can induce giant polarizations (56.1 μC/cm2 upon 4.1% strain) with the piezoelectric coefficient being about 35 times higher than that of MoS2 monolayer. The in-plane strain can also enhance the bandgap or even convert indirect to direct bandgap beyond a critical value. The good match among the lattice constants of bismuth oxychalcogenides is also desirable, rendering the epitaxial growth of heterostructure devices free of fabrication issues related to lattice mismatch, thereby allowing high-quality bismuth oxychalcogenide heterostructures tailored by design for a variety of applications.

The integrated spintronic functionalities of an individual high-spin state spin-crossover molecule between graphene nanoribbon electrodes

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Greens-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.