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

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

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

Porous Boron Nitride with Tunable Pore Size

On the basis of a global structural search and first-principles calculations, we predict two types of porous boron-nitride (BN) networks that can be built up with zigzag BN nanoribbons (BNNRs). The BNNRs are either directly connected with puckered B (N) atoms at the edge (type I) or connected with sp³-bonded BN chains (type II). Besides mechanical stability, these materials are predicted to be thermally stable at 1000 K. The porous BN materials entail large surface areas, ranging from 2800 to 4800 m²/g. In particular, type-II BN material with relatively large pores is highly favorable for hydrogen storage because the computed hydrogen adsorption energy (−0.18 eV) is very close to the optimal adsorption energy (−0.15 eV) suggested for reversible hydrogen storage at room temperature. Moreover, the type-II materials are semiconductors with width-dependent direct bandgaps, rendering the type-II BN materials promising not only for hydrogen storage but also for optoelectronic and photonic applications

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

Bi(Sb)NCa₃: Expansion of Perovskite Photovoltaics into All-Inorganic Anti-Perovskite Materials

Perovskite photovoltaics (PVs) have attracted intense interest largely because of their high power conversion efficiency and low cost. The chemical structures of perovskite materials can be generally described by the formula of ABX3, where cations occupy “A” and “B” sites and anions occupy “X” sites. Herein, we present a comprehensive theoretical study of two inorganic anti-perovskite materials, namely, BiNCa3 and SbNCa3, for perovskite PVs. Note that in anti-perovskites, anions occupy “A” and “B” sites, whereas cations occupy “X” sites. Specifically, for both materials, we investigate their thermodynamic stability, dynamic stability, optoelectronic properties and defect properties through ab initio calculations. Our computation suggests that both BiNCa3 and SbNCa3 possess direct band gaps of 0.65 and 1.14 eV, respectively. Notably, both materials are predicted to be thermodynamically stable, as demonstrated by their relatively large stable region based on the phase stability analysis. Dynamic and thermal stabilities are also suggested via the computed phonon spectra and ab initio molecular dynamics simulation. Furthermore, both materials possess desired optical absorption coefficients in the visible light region, comparable to that of the prevailing organic–inorganic hybrid perovskite, MAPbI3. Both exhibit enhanced optical absorption in the infrared region and have good defect tolerance. Lastly, good n-type and p-type conductivity may be realized by controlling the growth condition. The combined desirable properties render both BiNCa3 and SbNCa3 as promising all-inorganic and lead-free optical absorbers for PV application

Edge-Modified Phosphorene Nanoflake Heterojunctions as Highly Efficient Solar Cells

We propose to use edge-modified phosphorene nanoflakes (PNFs) as donor and acceptor materials for heterojunction solar cells. By using density functional theory based calculations, we show that heterojunctions consisting of hydrogen- and fluorine-passivated PNFs have a number of desired optoelectronic properties that are suitable for use in a solar cell. We explain why these properties hold for these types of heterojunctions. Our calculations also predict that the maximum energy conversion efficiency of these type of heterojunctions, which can be easily fabricated, can be as high as 20%, making them extremely competitive with other types of two-dimensional heterojunctions

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)

Ultrafast Solid-State Transformation Pathway from New-Phased Goethite VOOH to Paramontroseite VO₂ to Rutile VO₂(R)

Monoclinic vanadium dioxides VO2(M) is prototype material for interpreting correlation effects in solids, and its fully reversible metal−insulator transition (MIT) also brings the great interest in construction of intelligent devices such as temperature sensors and energy-efficient smart windows. The solid-state transformation started from vanadium precursors has been long-term regarded as the classic effective route to rutile VO2(R), while the conventional vanadium precursors usually requires indispensable atomic lattice rearrangement and reshuffling to realize rutile VO2(R) phase, leading to strict experimental conditions, high cost, and long conversion time (even more than one day) during the VO2(R) formation process. Herein, under the theoretical guidance of atomically structural analysis, a new structure-conversion pathway from goethite VOOH to paramontroseite VO2 to rutile VO2(R) realized an alternative ultrafast transformation into desired monoclinic VO2(M), of which each two steps only requires within 60 s. Thanks to the discovered new-phased goethite VOOH, the well-crystalline synthetic paramontroseite VO2 was realized from the chemically synthetic way, and in effect the paramontroseite structure plays the decisive role in achieving the desired monoclinic VO2(M) from the structural viewpoint, which would further promote this expensive material into the realm of conventional laboratory synthesis. The realized monoclinic VO2(M) exhibits the smart switching properties in regulating thermal, magnetic, and near IR light behaviors, and more importantly the metal−insulator transition (MIT) parameters such as the MIT temperature and the width of heating−cooling hysteresis are now precisely controlled. These intriguing findings may pave new way for designing other functional solid materials with correlation effects and then providing the material guarantee for constructing the intelligent devices in future