Hongyan Guo

We perform a comprehensive first-principles study of the electronic properties of phosphorene nanoribbons, phosphorus nanotubes, multilayer phosphorene sheets, and heterobilayers of phosphorene and two-dimensional (2D) transition-metal dichalcogenide (TMDC) monolayer. The tensile strain and electric-field effects on electronic properties of low-dimensional phosphorene nanostructures are also investigated. Our calculations show that the bare zigzag phosphorene nanoribbons (z-PNRs) are metals regardless of the ribbon width, whereas the bare armchair phosphorene nanoribbons (a-PNRs) are semiconductors with indirect bandgaps and the bandgaps decrease with increasing ribbon width. We find that compressive (or tensile) strains can reduce (or enlarge) the bandgap of the bare a-PNRs while an in-plane electric field can significantly reduce the bandgap of the bare a-PNRs, leading to the semiconductor-to-metal transition beyond certain electric field. For edge-passivated PNR by hydrogen, z-PNRs become semiconductor with nearly direct bandgaps and a-PNRs are still semiconductor but with direct bandgaps. The response to tensile strain and electric field for the edge-passivated PNRs is similar to that for the edge-unpassivated (bare) a-PNRs. For single-walled phosphorus nanotubes, both armchair and zigzag nanotubes are semiconductors with direct bandgaps. With either tensile strains or transverse electric field, behavior of bandgap modulation similar to that for a-PNRs can arise. It is known that multilayer phosphorene sheets are semiconductors whose bandgaps decrease with an increase in the number of multilayers. In the presence of a vertical electric field, the bandgaps of multilayer phosphorene sheets decrease with increasing electric field and the bandgap modulation is more significant with more layers. Lastly, heterobilayers of phosphorene (p-type) with an n-type TMDC (MoS2 or WS2) monolayer are still semiconductors while their bandgaps can be reduced by applying a vertical electric field as well. We also show that the combined phosphorene/MoS2 heterolayers can be an effective solar cell material. Our estimated power conversion efficiency for the phosphorene/MoS2 heterobilayer has a theoretical maximum value of 17.5%.

We have performed a comprehensive first-principles study of the electronic and magnetic properties of two-dimensional (2D) transition-metal dichalcogenide (TMD) heterobilayers MX2/MoS2 (M = Mo, Cr, W, Fe, V; X = S, Se). For M = Mo, Cr, W; X = S, Se, all heterobilayers show semiconducting characteristics with an indirect bandgap with the exception of the WSe2/MoS2 heterobilayer which retains the direct-bandgap character of the constituent monolayer. For M = Fe, V; X = S, Se, the MX2/MoS2 heterobilayers exhibit metallic characters. Particular attention of this study has been focused on engineering the bandgap of the TMD heterobilayer materials via application of either a tensile strain or an external electric field. We find that with increasing either the biaxial or uniaxial tensile strain, the MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can undergo a semiconductor-to-metal transition. For the WSe2/MoS2 heterobilayer, a direct-to-indirect bandgap transition may occur beyond a critical biaxial or uniaxial strain. For M (=Fe, V) and X (=S, Se), the magnetic moments of both metal and chalcogen atoms are enhanced when the MX2/MoS2 heterobilayers are under a biaxial tensile strain. Moreover, the bandgap of MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can be reduced by the vertical electric field. For two heterobilayers MSe2/MoS2 (M = Mo, Cr), PBE calculations suggest that the indirect-to-direct bandgap transition may occur under an external electric field. The transition is attributed to the enhanced spontaneous polarization. The tunable bandgaps in general and possible indirect-direct bandgap transitions due to tensile strain or external electric field make the TMD heterobilayer materials a viable candidate for optoelectronic applications.

Developing highly efficient sorbent materials for CO2 separation and capture from gas mixture is most important for reducing impact of CO2 on the environment. On the basis of density functional theory calculations with dispersion correction, we show that hexagonal boron nitride sheet (h-BN), when under an external electric field, can become an effective sorbent for CO2 capture. In the absent of the electric field, CO2 molecules are physisorbed on the h-BN sheet. Under the external electric field, the adsorption of CO2 molecules on h-BN monolayer can be strongly strengthened. Compared to CO2, the adsorption of H2, N2, CH4, CO, or H2O on h-BN sheet is notably weaker, indicating that the capture of CO2 on h-BN sheet under the electric field is highly preferred over other gas molecules. The calculated ratio of adsorption rate constant of CO2 to other gas molecule can be as high as 105. Moreover, the capture of CO2 molecule on h-BN sheet is reversible; that is, the adsorbed CO2 can be released by shutting down the applied electric field. This study suggests potential application of h-BN sheet not only for CO2 capture but also as a gas-storage material with high selectivity. The degree of selectivity can be controlled by an applied external electric field.

We have studied structural, electronic, and magnetic properties of the graphene-like ZnO monolayer doped with nonmetal species using the first-principles calculations. Particular attention has been placed on the ZnO monolayer with one or two oxygen atoms per supercell substituted by carbon, boron, or nitrogen atoms. We find that the ZnO monolayer with one oxygen atom per supercell substituted by a carbon or boron atom is ferromagnetic (FM) half metal (HM), while that with a nitrogen atom per supercell is a FM semiconductor. Upon the ZnO monolayer with two oxygen atoms per supercell substituted by carbon or boron, the magnetic properties vary, depending on the distance between two impurities. Two neighboring carbon or boron atoms in the ZnO monolayer form dimer pairs, which convert the ZnO monolayer into an n-type semiconductor with a nonmagnetic (NM) ground state. As the distance between two carbon or boron atoms increases, the doped ZnO monolayer undergoes both NM–AFM–FM and semiconductor–HM transitions. However, the ZnO monolayer with two N atoms per supercell is a p-type semiconductor with the antiferromagnetic (AFM) ground state, regardless of the distance between N atoms. The negligible energy difference between AFM and FM states of the N-doped ZnO monolayer implies it exhibits paramagnetic behavior at room temperature. Our study demonstrates that nonmetal-doped ZnO monolayers possess tunable magnetic and electronic properties, suitable for applications in electronics and spintronics at nanoscale.

Two-dimensional (2D) semiconductors with direct and modest band gap and ultrahigh carrier mobility are highly desired functional materials for nanoelectronic applications. Herein, we predict that monolayer CaP3 is a new 2D functional material that possesses not only a direct band gap of 1.15 eV (based on HSE06 computation) but also a very high electron mobility up to 19 930 cm2 V–1 s–1, comparable to that of monolayer phosphorene. More remarkably, contrary to bilayer phosphorene which possesses dramatically reduced carrier mobility compared to its monolayer counterpart, CaP3 bilayer possesses even higher electron mobility (22 380 cm2 V–1 s–1) than its monolayer counterpart. The band gap of 2D CaP3 can be tuned over a wide range from 1.15 to 0.37 eV (HSE06 values) through controlling the number of stacked CaP3 layers. Besides novel electronic properties, 2D CaP3 also exhibits optical absorption over the entire visible-light range. The combined novel electronic, charge mobility, and optical properties render 2D CaP3 an exciting functional material for future nanoelectronic and optoelectronic applications.