Bilayer Phosphorene: Effect of Stacking Order on Bandgap and Its Potential Applications in Thin-Film Solar Cells

Phosphorene, a monolayer of black phosphorus, is promising for nanoelectronic applications not only because it is a natural p-type semiconductor but also because it possesses a layer-number-dependent direct bandgap (in the range of 0.3 to 1.5 eV). On basis of the density functional theory calculations, we investigate electronic properties of the bilayer phosphorene with different stacking orders. We find that the direct bandgap of the bilayers can vary from 0.78 to 1.04 eV with three different stacking orders. In addition, a vertical electric field can further reduce the bandgap to 0.56 eV (at the field strength 0.5 V/Å). More importantly, we find that when a monolayer of MoS₂ is superimposed with the p-type AA- or AB-stacked bilayer phosphorene, the combined trilayer can be an effective solar-cell material with type-II heterojunction alignment. The power conversion efficiency is predicted to be ∼18 or 16% with AA- or AB-stacked bilayer phosphorene, higher than reported efficiencies of the state-of-the-art trilayer graphene/transition metal dichalcogenide solar cells

Electron-Transport Properties of Few-Layer Black Phosphorus

We perform the first-principles computational study of the effect of number of stacking layers and stacking style of the few-layer black phosphorus (BPs) on the electronic properties, including transport gap, current–voltage (<i>i</i>–<i>v</i>) relation, and differential conductance. Our computation is based on the nonequilibrium Green’s function approach combined with density functional theory calculations. Specifically, we compute electron-transport properties of monolayer BP, bilayer BP, and trilayer BP as well as bilayer BPs with AB-, AA-, or AC-stacking. We find that the stacking number has greater influence on the transport gap than the stacking type. Conversely, the stacking type has greater influence on <i>i</i>–<i>v</i> curve and differential conductance than on the transport gap. This study offers useful guidance for determining the number of stacking layers and the stacking style of few-layer BP sheets in future experimental measurements and for potential applications in nanoelectronic devices

Al₂C Monolayer Sheet and Nanoribbons with Unique Direction-Dependent Acoustic-Phonon-Limited Carrier Mobility and Carrier Polarity

The intrinsic acoustic-phonon-limited carrier mobility (μ) of Al₂C monolayer sheet and nanoribbons are investigated using ab initio computation and deformation potential theory. It is found that the polarity of the room-temperature carrier mobility of the Al₂C monolayer is direction-dependent, with μ of electron (<i>e</i>) and hole (<i>h</i>) being 2348 and 40.77 cm²/V/s, respectively, in the armchair direction and 59.95 (<i>e</i>) and 705.8 (<i>h</i>) in the zigzag direction. More interestingly, one-dimensional Al₂C nanoribbons not only can retain the direction-dependent polarity but also may entail even higher mobility, in contrast to either the graphene nanoribbons which tend to exhibit lower μ compared to the two-dimensional graphene or the MoS₂ nanoribbons which have reversed polarity compared to the MoS₂ sheet. As an example, the Al-terminated zigzag nanoribbon with a width of 4.1 nm exhibits μ of 212.6 (<i>e</i>) and 2087 (<i>h</i>) cm²/V/s, while the C-terminated armchair nanoribbon with a width 2.6 nm exhibits μ of 1090 (<i>e</i>) and 673.9 (<i>h</i>) cm²/V/s; the C-terminated zigzag nanoribbon with a width 3.7 nm exhibits μ of 177.6 (<i>e</i>) and 1889 (<i>h</i>) cm²/V/s, and the Al-terminated armchair nanoribbon with a width 2.4 nm exhibits μ of 6695 (<i>e</i>) and 518.4 (<i>h</i>) cm²/V/s. The high carrier mobility, μ, coupled with polarity and direction dependence endows the Al₂C sheet and nanoribbons with unique transport properties that can be exploited for special applications in nanoelectronics

Fluorescence of A100 MOF and Adsorption of Water, Indole, and Naphthalene on A100 by the Spectroscopic, Kinetic, and DFT Studies

Metal–organic frameworks (MOFs) are promising materials for adsorption and separations. It is important to understand the details of chemical bonding between the adsorbate and structural units in the MOFs. In A100 MOF, the near-UV–visible fluorescence is found to be the intralinker fluorescence. Naphthalene and indole form the stoichiometric “host-guest” π–π adsorption complexes with A100 that contain one adsorbate molecule per two BDC linkers, and adsorption of indole causes a strong quenching of the intralinker fluorescence. The excitation wavelength dependent steady-state fluorescence spectra, the nanosecond time-resolved fluorescence spectra, and DFT calculations indicate the strong π–π interactions between adsorbed indole and naphthalene and aromatic ring of the BDC linker, as well as hydrogen bonding between adsorbed indole and COO group of the linker. Activated A100 adsorbs up to four water molecules per BDC linker. Kinetic study of adsorption of naphthalene and indole from <i>n</i>-alkane on hydrated A100 yields the preferential adsorption of indole as determined by the in-situ time-dependent fluorescence spectroscopy and complementary ex-situ UV–vis absorption spectroscopy

Capillary Isoelectric Focusing-Mass Spectrometry Method for the Separation and Online Characterization of Intact Monoclonal Antibody Charge Variants

We report a new online capillary isoelectric focusing-mass spectrometry (CIEF-MS) method for monoclonal antibody (mAb) charge variant analysis using an electrokinetically pumped sheath-flow nanospray ion source and a time-of-flight MS with pressure-assisted chemical mobilization. To develop a successful, reliable CIEF-MS method for mAb, we have selected and optimized many critical, interrelating reagents and parameters that include (1) MS-friendly anolyte and catholyte; (2) a glycerol enhanced sample mixture that reduced non-CIEF electrophoretic mobility and band broadening; (3) ampholyte selected for balancing resolution and MS sensitivity; (4) sheath liquid composition optimized for efficient focusing, mobilization, and electrospray ionization; (5) judiciously selected CIEF running parameters including injection amount, field strength, and applied pressure. The fundamental premise of CIEF was well maintained as verified by the linear correlation (<i>R</i>² = 0.99) between p<i>I</i> values and migration time using a mixture of p<i>I</i> markers. In addition, the charge variant profiles of trastuzumab, bevacizumab, infliximab, and cetuximab, obtained using this CIEF-MS method, were corroborated by imaged CIEF-UV (iCIEF-UV) analyses. The relative standard deviations (RSD) of absolute migration time of p<i>I</i> markers were all less than 5% (<i>n</i> = 4). Triplicate analyses of bevacizumab showed RSD less than 1% for relative migration time to an internal standard and RSD of 7% for absolute MS peak area. Moreover, the antibody charge variants were characterized using the online intact MS data. To the best of our knowledge, this is the first time that direct online MS detection and characterization were achieved for mAb charge variants resolved by CIEF as indicated by a well-established linear pH gradient and correlated CIEF-UV charge variant profiles

Efficient Visible-Light-Driven Photocatalytic Degradation with Bi₂O₃ Coupling Silica Doped TiO₂

A new TiO₂-based visible light photocatalyst (Bi₂O₃/Si–TiO₂) was synthesized by both Bi₂O₃ coupling and Si doping via a two-step method. The structural, morphological, light absorption, and photocatalytic properties of as-prepared samples were studied using various spectroscopic and analytical techniques. The results showed that Bi₂O₃/Si–TiO₂ catalysts held an anatase phase and possessed high thermal stability. The doped Si was woven into the lattice of TiO₂, and its content had a significant effect on the surface area and the crystal size of Bi₂O₃/Si–TiO₂. The introduced Bi species mainly existed as oxides on the surface of TiO₂ particles, and the Bi₂O₃ photosensitization extended the light absorption into the visible region. Bi₂O₃ coupling also favored the separation and transfer of photoinduced charge carriers to inhibit their recombination and Si doping enlarged the surface area of photocatalysts. Compared to bare TiO₂, Bi₂O₃/TiO₂, and Si–TiO₂, Bi₂O₃/Si–TiO₂ samples showed better activities for the degradation of methyl orange (MO) and bisphenol A (BPA) under visible light irradiation (λ > 420 nm). The highest activity was observed for 1.0% Bi₂O₃/15% Si–TiO₂ calcined at 500 °C. The superior performance was ascribed to the high surface area, the ability to absorb visible light, and the efficient charge separation associated with the synergetic effects of appropriate amounts of Si and Bi in the prepared samples. The adsorbed hydroxyl radicals (^•OH) were also found to be the most reactive species in the photocatalytic degradation

Unusual Metallic Microporous Boron Nitride Networks

Two metallic zeolite-like microporous BN crystals with all-sp² bonding networks are predicted from an unbiased structure search based on the particle-swarm optimization (PSO) algorithm in combination with first-principles density functional theory (DFT) calculations. The stabilities of both microporous structures are confirmed via the phonon spectrum analysis and Born–Oppenheimer molecular dynamics simulations with temperature control at 1000 K. The unusual metallicity for the microporous BN allotropes stems from the delocalized p electrons along the axial direction of the micropores. Both microporous BN structures entail large surface areas, ranging from 3200 to 3400 m²/g. Moreover, the microporous BN structures show a preference toward organic molecule adsorption (e.g., the computed adsorption energy for CH₃CH₂OH is much more negative than that of H₂O). This preferential adsorption can be exploited for water cleaning, as demonstrated recently using porous boron BN nanosheets (Nat. Commun. 2013, 4, 1777)

Al_<i>x</i>C Monolayer Sheets: Two-Dimensional Networks with Planar Tetracoordinate Carbon and Potential Applications as Donor Materials in Solar Cell

We perform a global search of the most stable structures of 2D stoichiometric Al_<i>x</i>C (<i>x</i> = 1/3, 1, 2, and 3) monolayer sheets. In the most stable 2D planar AlC network, every carbon atom is tetracoordinated. In addition to the structure of AlC, structures of the most stable Al₂C and Al₃C monolayer sheets are also predicted for the first time. AlC and Al₂C monolayers are semiconducting, while Al₃C monolayer is metallic. In particular, Al₂C monolayer possesses a bandgap of 1.05 eV (based on HSE06 calculation), a value suitable for photovoltaic applications. Moreover, three Al₂C/WSe₂, Al₂C/MoTe₂, and AlC/ZnO van der Waals heterobilayers are investigated, and their power conversion efficiencies are estimated to be in the range of 12–18%. The near-perfect match in lattice constants between the Al₂C monolayer and PdO (100) surface suggests strong likelihood of experimental realization of the Al₂C monolayer on the PdO (100) substrate

Unusual Metallic Microporous Boron Nitride Networks

Two metallic zeolite-like microporous BN crystals with all-sp² bonding networks are predicted from an unbiased structure search based on the particle-swarm optimization (PSO) algorithm in combination with first-principles density functional theory (DFT) calculations. The stabilities of both microporous structures are confirmed via the phonon spectrum analysis and Born–Oppenheimer molecular dynamics simulations with temperature control at 1000 K. The unusual metallicity for the microporous BN allotropes stems from the delocalized p electrons along the axial direction of the micropores. Both microporous BN structures entail large surface areas, ranging from 3200 to 3400 m²/g. Moreover, the microporous BN structures show a preference toward organic molecule adsorption (e.g., the computed adsorption energy for CH₃CH₂OH is much more negative than that of H₂O). This preferential adsorption can be exploited for water cleaning, as demonstrated recently using porous boron BN nanosheets (Nat. Commun. 2013, 4, 1777)

Unusual Metallic Microporous Boron Nitride Networks

Two metallic zeolite-like microporous BN crystals with all-sp² bonding networks are predicted from an unbiased structure search based on the particle-swarm optimization (PSO) algorithm in combination with first-principles density functional theory (DFT) calculations. The stabilities of both microporous structures are confirmed via the phonon spectrum analysis and Born–Oppenheimer molecular dynamics simulations with temperature control at 1000 K. The unusual metallicity for the microporous BN allotropes stems from the delocalized p electrons along the axial direction of the micropores. Both microporous BN structures entail large surface areas, ranging from 3200 to 3400 m²/g. Moreover, the microporous BN structures show a preference toward organic molecule adsorption (e.g., the computed adsorption energy for CH₃CH₂OH is much more negative than that of H₂O). This preferential adsorption can be exploited for water cleaning, as demonstrated recently using porous boron BN nanosheets (Nat. Commun. 2013, 4, 1777)