Electronic bandgap and edge reconstruction in phosphorene materials

Optoelectronic and Optomechanical Properties of Few-Atomic-Layer Black Phosphorus Nanoflakes as Revealed by In Situ TEM

The optoelectronic signatures of free-standing few-atomic-layer black phosphorus nanoflakes are analyzed by in situ transmission electron microscopy (TEM). As compared to other 2D materials, the band gap of black phosphorus (BP) is related directly to multiple thicknesses and can be tuned by nanoflake thickness and strain. The photocurrent measurements with the TEM show a stable response to infrared light illumination and change of nanoflakes band gap with deformation while pressing them between two electrodes assembled in the microscope. The photocurrent spectra of an 8- and a 6-layer BP nanoflake samples are comparatively measured. Density functional theory (DFT) calculations are performed to identify the band structure changes of BP during deformations. The results should help to find the best pathways for BP smart band gap engineering via tuning the number of material atomic layers and programmed deformations to promote future optoelectronic applications.

Polarization-Independent Perfect Absorption in Monolayer Black Phosphorus Metasurfaces at Terahertz Frequencies via Critical Coupling

Two-dimensional (2D) materials, which possess rich underlying physical properties that can provide the potential for designing more efficient and compact optoelectronic devices, have attracted great interest among scientists. Due to the atomic-scale thickness and the anisotropy of in-plane conductivity, 2D black phosphorus (BP) exhibits a polarization-dependent absorption spectrum with low absorption, which limits its further development in polarization-independent applications such as light absorbers and sensors. In this paper, a polarization-independent perfect absorber in the terahertz band is proposed, which is composed of a patterned BP monolayer deposited on a lossless photonic crystal (PC) slab with a back reflection mirror. The absorption of the patterned BP monolayer can reach 100% at resonant frequencies through the critical coupling mechanism of guided resonance. Moreover, the absorber exhibits polarization-independent absorption characteristics for vertically incident light, which are attributed to the 4-fold rotational symmetry of the PC substrate and the patterned BP monolayer deposited on it. This work opens up the possibility of fabricating optically polarization-independent devices based on single-layer 2D anisotropic materials.

2D Amino-Functionalized Black Phosphorus: A New Approach to Improve Hydrogen Gas Detection Performance

In recent years, hydrogen has gained attention as a potential solution to replace fossil fuels, thus reducing greenhouse gas emissions. The development of ever improving hydrogen sensors is a topic that is constantly under study due to concerns about the inherent risk of leaks of this gas and potential explosions. In this work, a new, long-term, stable phosphorene-based sensor was developed for hydrogen detection. A simple functionalization of phosphorene using urea was employed to synthesize an air-stable material, subsequently used to prepare films for gas sensing applications, via the drop casting method. The material was deeply characterized by different techniques (scanning electron microscopy, X-ray diffraction, X-ray photoelectron, and Raman spectroscopy), and the stability of the material in a noninert atmosphere was evaluated. The phosphorene-based sensor exhibited high sensitivity (up to 700 ppm) and selectivity toward hydrogen at room temperature, as well as long-term stability over five months under ambient conditions. To gain further insight into the gas sensing mechanism over the surface, we employed a dedicated apparatus, namely operando diffuse reflectance infrared Fourier transform, by exposing the chemoresistive sensor to hydrogen gas under dry air conditions.

Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe2

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe2 have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe2 with a pentagonal fold structure. The thermal conductivity of PdSe2 flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04 W mk-1 for 60 nm PdSe2 to 34.51 W mk-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe2 becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.

Impact of quantum size effects to the band gap of catalytic materials: a computational perspective

The evolution of nanotechnology has facilitated the development of catalytic materials with controllable composition and size, reaching the sub-nanometer limit. Nowadays, a viable strategy for tailoring and optimizing the catalytic activity involves controlling the size of the catalyst. This strategy is underpinned by the fact that the properties and reactivity of objects with dimensions on the order of nanometers can differ from those of the corresponding bulk material, due to the emergence of quantum size effects. Quantum size effects have a deep influence on the band gap of semiconducting catalytic materials. Computational studies are valuable for predicting and estimating the impact of quantum size effects. This perspective emphasizes the crucial role of modeling quantum size effects when simulating nanostructured catalytic materials. It provides a comprehensive overview of the fundamental principles governing the physics of quantum confinement in various experimentally observable nanostructures. Furthermore, this work may serve as a tutorial for modeling the electronic gap of simple nanostructures, highlighting that when working at the nanoscale, the finite dimensions of the material lead to an increase of the band gap because of the emergence of quantum confinement. This aspect is sometimes overlooked in computational chemistry studies focused on surfaces and nanostructures.

Seeded growth of single-crystal black phosphorus nanoribbons

Two-dimensional materials have emerged as an important research frontier for overcoming the challenges in nanoelectronics and for exploring new physics. Among them, black phosphorus, with a combination of a tunable bandgap and high mobility, is one of the most promising systems. In particular, black phosphorus nanoribbons show excellent electrostatic gate control, which can mitigate short-channel effects in nanoscale transistors. Controlled synthesis of black phosphorus nanoribbons, however, has remained an outstanding problem. Here we report large-area growth of black phosphorus nanoribbons directly on insulating substrates. We seed the chemical vapour transport growth with black phosphorus nanoparticles and obtain uniform, single-crystal nanoribbons oriented exclusively along the [100] crystal direction. With comprehensive structural calculations, we discover that self-passivation at the zigzag edges holds the key to the preferential one-dimensional growth. Field-effect transistors based on individual nanoribbons exhibit on/off ratios up to ~104, confirming the good semiconducting behaviour of the nanoribbons. These results demonstrate the potential of black phosphorus nanoribbons for nanoelectronic devices and also provide a platform for investigating the exotic physics in black phosphorus.

Role of Edge Reconstruction in the Synthesis of Few-Layer Black Phosphorene

Recent advancements in preparing few-layer black phosphorene (BP) are hindered by edge reconstruction challenges. Our previous studies have revealed the factors contributing to the difficulty of growing few-layer BP. In this study, we have successfully identified three reconstructed edges in bi- and multilayer BP through a combination of the crystal structure analysis by particle swarm optimization (CALYPSO) global structure search and density functional theory (DFT). Notably, the reconstruction between adjacent layers proves more beneficial than self-passivation or maintaining pristine edges. Among the reconstructed edges, the reconstructed ZZ edge is the most stable, regardless of the number of layers. Calculated electronic band structures reveal a significant transition in the electronic properties of black phosphorus nanoribbons (BPNRs), changing from metallic to semiconducting. This insight not only enhances the understanding of the fundamental properties of BP but also provides valuable theoretical guidance for the experimental growth of BPNRs or black phosphorus nanowires (BPNWs).

Transition from fractal-dendritic to compact islands for the 2D-ferroelectric SnSe on graphene/Ir(111)

Epitaxial growth is a versatile method to prepare two-dimensional van der Waals ferroelectrics like group IV monochalcogenides which have potential for novel electronic devices and sensors. We systematically study SnSe monolayer islands grown by molecular beam epitaxy, especially the effect of annealing temperature on shape and morphology of the edges. Characterization of the samples by scanning tunneling microscopy reveals that the shape of the islands changes from fractal-dendritic after deposition at room temperature to a compact rhombic shape through annealing, but ripening processes are absent up to the desorption temperature. A two-step growth process leads to large, epitaxially aligned rhombic islands bounded by well-defined110-edges (armchair-like), which we claim to be the equilibrium shape of the stoichiometric SnSe monolayer islands. The relaxation of the energetically favorable edges is detected in atomically resolved STM images. The experimental findings are supported by the results of our first-principles calculations, which provide insights into the energetics of the edges, their reconstructions, and yields the equilibrium shapes of the islands which are in good agreement with the experiment.

Heteroepitaxial Growth of Black Phosphorus on Tin Monosulfide

Black phosphorus (Black P), a layered semiconductor with a layer-dependent bandgap and high carrier mobility, is a promising candidate for next-generation electronics and optoelectronics. However, the synthesis of large-area, layer-precise, single crystalline Black P films remains a challenge due to their high nucleation energy. Here, we report the molecular beam heteroepitaxy of single crystalline Black P films on a tin monosulfide (SnS) buffer layer grown on Au(100). The layer-by-layer growth mode enables the preparation of monolayer to trilayer films, with band gaps that reflect layer-dependent quantum confinement.

Recent Advances in Phosphorene: Structure, Synthesis, and Properties

Phosphorene is a 2D phosphorus atomic layer arranged in a honeycomb lattice like graphene but with a buckled structure. Since its exfoliation from black phosphorus in 2014, phosphorene has attracted tremendous research interest both in terms of synthesis and fundamental research, as well as in potential applications. Recently, significant attention in phosphorene is motivated not only by research on its fundamental physical properties as a novel 2D semiconductor material, such as tunable bandgap, strong in-plane anisotropy, and high carrier mobility, but also by the study of its wide range of potential applications, such as electronic, optoelectronic, and spintronic devices, energy conversion and storage devices. However, a lot of avenues remain to be explored including the fundamental properties of phosphorene and its device applications. This review recalls the current state of the art of phosphorene and its derivatives, touching upon topics on structure, synthesis, characterization, properties, stability, and applications. The current needs and future opportunities for phosphorene are also discussed.

Enhance Hydrogen Evolution Reaction Performance via Double-Stacked Edges of Black Phosphorene

Based on the density functional theory (DFT) calculations, we explored the structures and HER catalytic properties of reconstructed and double-stacked black phosphorene (BP) edges. Ten bilayer BP edges were constructed by the double stacking of three typical monolayer edges, i.e., zigzag (ZZ) edge, armchair (AC) edge, skewed diagonal (SD) edge, and their reconstructed derivatives with their layer's configurations, edge deformations and thermodynamic stabilities were discussed. Based on these edges, five chemical sites on four bilayer BP edges were selected to be promising candidates for a HER catalyst, which present higher HER activities than that of Pt(111). Besides, among these four edges, two edges have even lower energetic barriers for the Tafel reaction. Compared with the monolayer edges, these selected bilayer BP edges confirm the remarkable enhancement of the HER catalytic properties, which can be attributed to their unique edge structures and the enhanced electronic densities after the hydrogen adsorptions. Finally, the thermostability of these edges at room temperature has also been proved by the DFT-MD simulations. This theoretic study deepens our fundamental understanding of the double-stacked edge structures of the BP and provides a new way for the rational design of highly efficient and noble-metal-free HER catalysts.

Diffusion and Coalescence of Phosphorene Monovacancies Studied Using High-Dimensional Neural Network Potentials

The properties of two-dimensional materials are strongly affected by defects that are often present in considerable numbers. In this study, we investigate the diffusion and coalescence of monovacancies in phosphorene using molecular dynamics (MD) simulations accelerated by high-dimensional neural network potentials. Trained and validated with reference data obtained with density functional theory (DFT), such surrogate models provide the accuracy of DFT at a much lower cost, enabling simulations on time scales that far exceed those of first-principles MD. Our microsecond long simulations reveal that monovacancies are highly mobile and move predominantly in the zigzag rather than armchair direction, consistent with the energy barriers of the underlying hopping mechanisms. In further simulations, we find that monovacancies merge into energetically more stable and less mobile divacancies following different routes that may involve metastable intermediates.

Nonlinear Plasmonics in Nanostructured Phosphorene

Phosphorene has emerged as an atomically thin platform for optoelectronics and nanophotonics due to its excellent optical properties and the possibility of actively tuning light-matter interactions through electrical doping. While phosphorene is a two-dimensional semiconductor, plasmon resonances characterized by pronounced anisotropy and strong optical confinement are anticipated to emerge in highly doped samples. Here we show that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit high tunability in relation to both edge termination and doping charge polarity and can trigger an intense nonlinear optical response at moderate doping levels. Our explorations are based on a second-principles theoretical framework, employing maximally localized Wannier functions constructed from ab initio electronic structure calculations, which we introduce here to describe the linear and nonlinear optical response of PNRs on mesoscopic length scales. Atomistic simulations reveal the high tunability of plasmons in doped PNRs at near-infrared frequencies, which can facilitate the synergy between the electronic band structure and plasmonic field confinement to drive efficient high-harmonic generation. Our findings establish nanostructured phosphorene as a versatile atomically thin material candidate for nonlinear plasmonics.

Edge-Confined Excitons in Monolayer Black Phosphorus

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. We observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

Nonempirical Range-Separated Hybrid Functional with Spatially Dependent Screened Exchange

Electronic structure calculations based on density functional theory (DFT) have successfully predicted numerous ground-state properties of a variety of molecules and materials. However, exchange and correlation functionals currently used in the literature, including semilocal and hybrid functionals, are often inaccurate to describe the electronic properties of heterogeneous solids, especially systems composed of building blocks with large dielectric mismatch. Here, we present a dielectric-dependent range-separated hybrid functional, screened-exchange range-separated hybrid (SE-RSH), for the investigation of heterogeneous materials. We define a spatially dependent fraction of exact exchange inspired by the static Coulomb-hole and screened-exchange (COHSEX) approximation used in many-body perturbation theory, and we show that the proposed functional accurately predicts the electronic structure of several nonmetallic interfaces, three- and two-dimensional, pristine, and defective solids and nanoparticles.

Straintronics in phosphorene via tensile vs shear strains and their combinations for manipulating the band gap

We study the effects of the uniaxial tensile strain and shear deformation as well as their combinations on the electronic properties of single-layer black phosphorene. The evolutions of the strain-dependent band gap are obtained using the numerical calculations within the tight-binding (TB) model as well as the first-principles (DFT) simulations and compared with previous findings. The TB-model-based findings show that the band gap of the strain-free phosphorene agrees with the experimental value and linearly depends on both stretching and shearing: increases (decreases) as the stretching increases (decreases), whereas gradually decreases with increasing the shear. A linear dependence is less or more similar as compared to that obtained from the ab initio simulations for shear strain, however disagrees with a non-monotonic behaviour from the DFT-based calculations for tensile strain. Possible reasons for the discrepancy are discussed. In case of a combined deformation, when both strain types (tensile/compression + shear) are loaded simultaneously, their mutual influence extends the realizable band gap range: from zero up to the values respective to the wide-band-gap semiconductors. At a switched-on combined strain, the semiconductor-semimetal phase transition in the phosphorene is reachable at a weaker (strictly non-destructive) strain, which contributes to progress in fundamental and breakthroughs.

Black Phosphorus for Photonic Integrated Circuits

Black phosphorus gives several advantages and complementarities over other two-dimensional materials. It has drawn extensive interest owing to its relatively high carrier mobility, wide tunable bandgap, and in-plane anisotropy in recent years. This manuscript briefly reviews the structure and physical properties of black phosphorus and targets on black phosphorus for photonic integrated circuits. Some of the applications are discussed including photodetection, optical modulation, light emission, and polarization conversion. Corresponding recent progresses, associated challenges, and future potentials are covered.

Excitonic ground states in phosphorene nanoflakes

By using a configuration-interaction approach beyond the framework of independent multiexcitons, we predict that an excitonic ground state may exist in phosphorene nanoflakes when an in-plain electric field is applied. The ground state of the system is shown to undergo a transition from purely electronic to almost fully biexcitonic with the increasing strength of the electric field. As the field exceeds 0.25 V nm-1, a biexcitonic ground state is revealed to be energetically more favorable by a few hundred meV than the system without excitons. A similar transformation of the ground state is also found as the screening effect varies from strong to weak. The enhanced electron-hole correlation, mostly caused by the applied electric field as well as the lack of strong screening in low-dimensional nanostructures, is believed to account for such an extraordinary transition. Furthermore, the system with a biexcitonic ground state is found to exhibit an absorption spectrum where many transitions are polarized along the zigzag direction, which breaks the optical anisotropy well-known for bulk phosphorene.

Renaissance of elemental phosphorus materials: properties, synthesis, and applications in sustainable energy and environment

The polymorphism of phosphorus-based materials has garnered much research interest, and the variable chemical bonding structures give rise to a variety of micro and nanostructures. Among the different types of materials containing phosphorus, elemental phosphorus materials (EPMs) constitute the foundation for the synthesis of related compounds. EPMs are experiencing a renaissance in the post-graphene era, thanks to recent advancements in the scaling-down of black phosphorus, amorphous red phosphorus, violet phosphorus, and fibrous phosphorus and consequently, diverse classes of low-dimensional sheets, ribbons, and dots of EPMs with intriguing properties have been produced. The nanostructured EPMs featuring tunable bandgaps, moderate carrier mobility, and excellent optical absorption have shown great potential in energy conversion, energy storage, and environmental remediation. It is thus important to have a good understanding of the differences and interrelationships among diverse EPMs, their intrinsic physical and chemical properties, the synthesis of specific structures, and the selection of suitable nanostructures of EPMs for particular applications. In this comprehensive review, we aim to provide an in-depth analysis and discussion of the fundamental physicochemical properties, synthesis, and applications of EPMs in the areas of energy conversion, energy storage, and environmental remediation. Our evaluations are based on recent literature on well-established phosphorus allotropes and theoretical predictions of new EPMs. The objective of this review is to enhance our comprehension of the characteristics of EPMs, keep abreast of recent advances, and provide guidance for future research of EPMs in the fields of chemistry and materials science.

Effects of spin-orbit coupling on transmission and absorption of electromagnetic waves in strained armchair phosphorene nanoribbons

We compute the optical conductivity, both the imaginary and real parts of the dielectric constant, and the optical coefficients of armchair phosphorene nanoribbons under application of biaxial and uniaxial strains. The Kane-Mele model Hamiltonian has been applied to obtain the electronic band structure of phosphorene nanoribbons in the presence of a magnetic field. The effects of uniaxial and biaxial in-plane strain on the frequency behavior of the optical dielectric constant, and the frequency behavior of the optical absorption and refractive index of phosphorene nanoribbons have been studied, in terms of magnetic field, spin-orbit coupling and strain effects. Linear response theory and the Green's function approach have been exploited to obtain the frequency behavior of the optical properties of the structure. Moreover, the transmissivity and reflectivity of electromagnetic waves between two media separated by a phosphorene-nanoribbon layer are determined. Our numerical results indicate that the frequency dependence of the optical absorption includes a peak due to applying a magnetic field. Moreover, the effects of both in-plane uniaxial and biaxial strains on the refractive index of single-layer phosphorene have been addressed. Also, the frequency dependence of the transmissivity and reflectivity of electromagnetic waves between two media separated by armchair phosphorene nanoribbons for normal incidence has been investigated in terms of the effects of magnetic field and strain parameters. Both compressive and tensile strain have been considered for the armchair phosphorene nanoribbons in order to study the optical properties of the structure. In particular, the control of the optical properties of phosphorene nanoribbons could lead to extensive applications of phosphorene in the optoelectronics industry. Also, such a study of the optical properties of phosphorene nanoribbons has further applications in light sensors. Meanwhile, the effects of spin-orbit coupling on the optical absorption and transmissivity of electromagnetic waves in phosphorene nanoribbons could be a novel topic in condensed-matter physics.

Strong bulk photovoltaic effect in engineered edge-embedded van der Waals structures

Bulk photovoltaic effect (BPVE), a second-order nonlinear optical effect governed by the quantum geometric properties of materials, offers a promising approach to overcome the Shockley-Quiesser limit of traditional photovoltaic effect and further improve the efficiency of energy harvesting. Here, we propose an effective platform, the nano edges embedded in assembled van der Waals (vdW) homo- or hetero-structures with strong symmetry breaking, low dimensionality and abundant species, for BPVE investigations. The BPVE-induced photocurrents strongly depend on the orientation of edge-embedded structures and polarization of incident light. Reversed photocurrent polarity can be observed at left and right edge-embedded structures. Our work not only visualizes the unique optoelectronic effect in vdW nano edges, but also provides an effective strategy for achieving BPVE in engineered vdW structures.

Quantum Spin Hall States and Topological Phase Transition in Germanene

We present the first experimental evidence of a topological phase transition in a monoelemental quantum spin Hall insulator. Particularly, we show that low-buckled epitaxial germanene is a quantum spin Hall insulator with a large bulk gap and robust metallic edges. Applying a critical perpendicular electric field closes the topological gap and makes germanene a Dirac semimetal. Increasing the electric field further results in the opening of a trivial gap and disappearance of the metallic edge states. This electric field-induced switching of the topological state and the sizable gap make germanene suitable for room-temperature topological field-effect transistors, which could revolutionize low-energy electronics.

Band Edges Engineering of 2D/2D Heterostructures: The C3 N4 /Phosphorene Interface

We investigate the interface between carbon nitride (C₃ N₄ ) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃ N₄ /P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers' separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers' migration as a function of the P-ene thickness.

Highly Specific Antibiotic Detection on Water-Stable Black Phosphorus Field-Effect Transistors

Two-dimensional (2D) black phosphorus (BP) has been reported to have appealing semiconducting properties as the sensing channel in field-effect transistor (FET) sensors. However, the intrinsic instability of BP in water greatly hinders its application, and little is known about its sensing performance and mechanism in aqueous medium. Herein, a water-stable BP FET sensor for antibiotic detection is reported. A novel surface engineering strategy with Ag⁺ coordination and melamine cyanurate (MC) supramolecular passivation is utilized to enhance the stability and transistor performance of BP. With molecularly imprinted polymers (MIPs) as the detection probe for tetracycline, the BP_Ag(+)/MC/MIPs sensor shows high sensitivity to tetracycline with a detection limit of 7.94 nM and a quick response within 6 s as well as high selectivity against other antibiotics with similar molecular structures. A new sensing mechanism relying on the conjugation effect of the probe structure is proposed, and new knowledge about alkalinity-enhanced and ionic strength-related response from the electrostatic gating effect is given based on the solution chemistry impact study. This work offers an efficient surface engineering strategy to enable the application of 2D BP for antibiotic detection in aqueous medium and presents a new sensing mechanism in chemical analysis by FET sensors.

Giant Density of States Enhancement Driven by a Zero-Mode Landau Level in Semimetallic Black Phosphorus under Pressure

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0 T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0 T reaches more than 20 times larger than that at 2.0 T. The increase in 1/T_{1}T above 6.5 T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100 K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.

Recent advances of two-dimensional material additives in hybrid perovskite solar cells

Perovskite solar cells (PSCs) have become one of the state-of-the-art photovoltaic technologies due to their facile solution-based fabrication processes combined with extremely high photovoltaic performance originating from excellent optoelectronic properties such as strong light absorption, high charge mobility, long free charge carrier diffusion length, and tunable direct bandgap. However, the poor intrinsic stability of hybrid perovskites under environmental stresses including light, heat, and moisture, which is often associated with high defect density in the perovskite, has limited the large-scale commercialization and deployment of PSCs. The use of process additives, which can be included in various subcomponent layers in the PSC, has been identified as one of the effective approaches that can address these issues and improve the photovoltaic performance. Among various additives that have been explored, two-dimensional (2D) materials have emerged recently due to their unique structures and properties that can enhance the photovoltaic performance and device stability by improving perovskite crystallization, defect passivation, and charge transport. Here, we provide a review of the recent progresses in 2D material additives for improving the PSC performance based on key representative 2D material systems, including graphene and its derivatives, transitional metal dichalcogenides, and black phosphorous, providing a useful guideline for further exploiting unique nanomaterial additives for more efficient and stable PSCs in the near future.

Quantum manifestations in electronic properties of bilayer phosphorene nanoribbons

Recently, a new edge structure named ZZ(U) has been evidenced as the lowest-energy structure for bilayer phosphorene nanoribbons (PNRs). Owing to strong quantum confinement effects and edge states, width and edge are the two most important factors that influence the properties of PNRs in nanosized microelectronics. In this study, we systematically investigated the evolution of the electronic properties of bilayer PNRs with different edge configurations as the widths vary. The four types of edges explored include ZZ(Pristine), ZZ(Klein), ZZ(Tube), and newly found ZZ(U). As the widths change from 14 to 40 Å, the ZZ(Pristine) are always metallic with edge states penetrating the Fermi level, while the others are semiconductors. The edge states in ZZ(Klein) are located in the two lowest conduction bands. However, in ZZ(U), the edge states are nearly hidden in the bulk band structure, and its carrier transportation exhibits almost perfect 2D layers, nearly eliminating the U-edge influence. Our results pave the way for phosphorene's utilization in electronics and optoelectronics.

Mixed finite element numerical mode matching method for designing infrared broadband polarization-independent metamaterial absorbers

Conventional numerical methods have found widespread applications in the design of metamaterial structures, but their computational costs can be high due to complex three-dimensional discretization needed for large complex problems. In this work, we apply a recently developed numerical mode matching (NMM) method to design a black phosphorus (BP) absorber. NMM transforms a complex three-dimensional (3D) problem into 2D numerical eigenvalue problems plus a 1-D analytical propagation solution, thus it can save a lot of computational costs. BP is treated as a 2D surface and represented by the anisotropic surface conductance. With a realistic simulation study, we show that our method is more accurate and efficient than the standard finite element method (FEM). Our designed absorber can achieve an average absorption of 97.4% in the wavelength range of 15 to 23 μm under normal incidence. Then, we investigate the physical mechanism of the absorber, tuning the geometric parameters and electron doping to optimize the performance. In addition, the absorption spectra under oblique incidence and arbitrary polarization are studied. The results confirm that our absorber is polarization-independent and has high absorption at large incident angles. Our work validates the superiority of NMM and provides a new simulation platform for emerging metamaterial device design.

Graphitic carbon nitride nanosheets as promising candidates for the detection of hazardous contaminants of environmental and biological concern in aqueous matrices

Monitoring of pollutant and toxic substances is essential for cleaner environment and healthy life. Sensing of various environmental contaminants and biomolecules such as heavy metals, pharmaceutics, toxic gases, volatile organic compounds, food toxins, and pathogens is of high importance to guaranty the good health and sustainable environment to community. In recent years, graphitic carbon nitride (g-CN) has drawn a significant amount of interest as a sensor due to its large surface area and unique electrochemical properties, low bandgap energy, high thermal and chemical stability, facile synthesis, nontoxicity, and electron rich property. Furthermore, the binary and ternary nanocomposites of graphitic carbon nitride further enhance their performance as a sensor making it a cost effective, fast, and reliable gadget for the purpose, and opens a wide area of research. Numerous reviews addressing a variety of applications including photocatalytic energy conversion, photoelectrochemical detection, and hydrogen evolution of graphitic carbon nitride have been documented to date. But a lesser attention has been devoted to the mechanistic approaches towards sensing of variety of pollutants concerned with environmental and biological aspects. Herein, we present the sensing features of graphitic carbon nitride towards the detection of various analytes including toxic heavy metals, pharmaceuticals, phenolic compounds, nitroaromatic compounds, volatile organic molecules, toxic gases, and foodborne pathogens. This review will undoubtedly provide future insights for researchers working in the field of sensors, allowing them to investigate the intriguing graphitic carbon nitride material as a sensing platform that is comparable to several other nanomaterials documented in the literature. Therefore, we hope that this study could reveal some intriguing sensing properties of graphitic carbon nitride, which may help researchers better understand how it interacts with contaminants of environmental and biological concern. Graphitic carbon nitride Nanosheets as Promising Analytical Tool for Environmental and Biological Monitoring of Hazardous Substances.

Searching guidelines for scalable and controllable design of multifunctional materials and hybrid interfaces: Status and perspective

The urgent need to address the global sustainability issues that modern society is currently facing requires the development of micro and nanotechnologies, which rely largely on functional materials. Beyond studies focused solely on low-dimensional materials, broader research related to multifunctionality has shown that the major efforts to meet these criteria for new electronic, photonic, and optoelectronic concepts, particularly to achieve high-performance devices, are still challenging. By exploiting their unique properties, a comprehensive understanding of the implications of research for the synthesis and discovery of novel materials is obtained. The present article encompasses innovation research as an alternative optimization and design for sustainable energy development, bridging the scaling gap in atomically controlled growth in terms of surface heterogeneity and interfacial engineering. In addition, the corresponding research topics are widely regarded as a scientometric analysis and visualization for the evaluation of scientific contributions into the early 20 years of the 21st century. In this perspective, a brief overview of the global trends and current challenges toward high-throughput fabrication followed by a scenario-based future for hybrid integration and emerging structural standards of scalable control design and growth profiles are emphasized. Finally, these opportunities are unprecedented to overcome current limitations, creating numerous combinations and triggering new functionalities and unparalleled properties for disruptive innovations of Frontier technologies.

Atomically Sharp, Closed Bilayer Phosphorene Edges by Self-Passivation

Two-dimensional crystals' edge structures not only influence their overall properties but also dictate their formation due to edge-mediated synthesis and etching processes. Edges must be carefully examined because they often display complex, unexpected features at the atomic scale, such as reconstruction, functionalization, and uncontrolled contamination. Here, we examine atomic-scale edge structures and uncover reconstruction behavior in bilayer phosphorene. We use in situ transmission electron microscopy (TEM) of phosphorene/graphene specimens at elevated temperatures to minimize surface contamination and reduce e-beam damage, allowing us to observe intrinsic edge configurations. The bilayer zigzag (ZZ) edge was found to be the most stable edge configuration under e-beam irradiation. Through first-principles calculations and TEM image analysis under various tilting and defocus conditions, we find that bilayer ZZ edges undergo edge reconstruction and so acquire closed, self-passivated edge configurations. The extremely low formation energy of the closed bilayer ZZ edge and its high stability against e-beam irradiation are confirmed by first-principles calculations. Moreover, we fabricate bilayer phosphorene nanoribbons with atomically sharp closed ZZ edges. The identified bilayer ZZ edges will aid in the fundamental understanding of the synthesis, degradation, reconstruction, and applications of phosphorene and related structures.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Tunneling Magnetoresistance Transition and Highly Sensitive Pressure Sensors Based on Magnetic Tunnel Junctions with a Black Phosphorus Barrier

Black phosphorus is a promising material to serve as a barrier for magnetic tunnel junctions (MTJs) due to weak van der Waals interlayer interactions. In particular, the special band features of black phosphorus may endow intriguing physical characteristics. Here we study theoretically the effect of band gap tunability of black phosphorus on the MTJs with the black phosphorus barrier. It is found that the tunneling magnetoresistance (TMR) may transition from a finite value to infinity owing to the variation in the band gap of black phosphorus. Combined with the latest experimental results of the pressure-induced band gap tunability, we further investigate the pressure effect of TMR in the MTJs with a black phosphorus barrier. The calculations show that the pressure sensitivity can be quite high under appropriate parameters. Physically, the high sensitivity originates from the TMR transition phenomenon. To take advantage of the high pressure sensitivity, we propose and design a detailed structure of highly sensitive pressure sensors based on MTJs with a black phosphorus barrier, whose working mechanism is basically different from that of convential pressure sensors. The present pressure sensors possess four advantages and benifits: (1) high sensitivity, (2) good anti-interference, (3) high spatial resolution, and (4) fast response speed. Our study may advance new research areas for both the MTJs and pressure sensors.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Visible Out-of-plane Polarized Luminescence and Electronic Resonance in Black Phosphorus

Black phosphorus (BP) is unique among layered materials because of its homonuclear lattice and strong structural anisotropy. While recent investigations on few-layer BP have extensively explored the in-plane (a, c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. An unexpected b-polarized luminescence emission is detected in the visible, far above the fundamental gap. PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman with the enhancement of the A_g phonon modes scattering efficiency. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

Precise and Prompt Analyte Detection via Ordered Orientation of Receptor in WSe2-Based Field Effect Transistor

Field-effect transistors (FET) composed of transition metal dichalcogenide (TMDC) materials have gained huge importance as biosensors due to their added advantage of high sensitivity and moderate bandgap. However, the true potential of these biosensors highly depends upon the quality of TMDC material, as well as the orientation of receptors on their surfaces. The uncontrolled orientation of receptors and screening issues due to crossing the Debye screening length while functionalizing TMDC materials is a big challenge in this field. To address these issues, we introduce a combination of high-quality monolayer WSe₂ with our designed Pyrene-based receptor moiety for its ordered orientation onto the WSe₂ FET biosensor. A monolayer WSe₂ sheet is utilized to fabricate an ideal FET for biosensing applications, which is characterized via Raman spectroscopy, atomic force microscopy, and electrical prob station. Our construct can sensitively detect our target protein (streptavidin) with 1 pM limit of detection within a short span of 2 min, through a one-step functionalizing process. In addition to having this ultra-fast response and high sensitivity, our biosensor can be a reliable platform for point-of-care-based diagnosis.

Direct Observation of Orbital Driven Strong Interlayer Coupling in Puckered Two-Dimensional PdSe2

Interlayer coupling between individual unit layers is known to be critical in manipulating the layer-dependent properties of two-dimensional (2D) materials. While recent studies have revealed that several 2D materials with significant degrees of interlayer interaction (such as black phosphorus) show strongly layer-dependent properties, the origin based on the electronic structure is drawing intensive attention along with 2D materials exploration. Here, the direct observation of a highly dispersive single electronic band along the interlayer direction in puckered 2D PdSe₂ as an experimental hallmark of strong interlayer couplings is reported. Remarkably large band dispersion along the k_z -direction near Fermi level, which is even wider than the in-plane one, is observed by the angle-resolved photoemission spectroscopy measurement. Employing X-ray absorption spectroscopy and density functional theory calculations, it is revealed that the strong interlayer coupling in 2D PdSe₂ originates from the unique directional bonding of Pd d orbitals associated with unexpected Pd 4d⁹ configuration, which consequently plays a decisive role for the strong layer-dependency of the band gap.

Spin-orbit coupling in buckled monolayer nitrogene

Buckled monolayer nitrogene has been recently predicted to be stable above the room temperature. The low atomic number of nitrogen atom suggests, that spin–orbit coupling in nitrogene is weak, similar to graphene or silicene. We employ first principles calculations and perform a systematic study of the intrinsic and extrinsic spin–orbit coupling in this material. We calculate the spin mixing parameter \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b^2$$\end{document}b2, reflecting the strength of the intrinsic spin–orbit coupling and find, that \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b^2$$\end{document}b2 is relatively small, on the order of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10^{-6}$$\end{document}10-6. It also displays a weak anisotropy, opposite for electrons and holes. To study extrinsic effects of spin–orbit coupling we apply a transverse electric field enabling spin–orbit fields \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega$$\end{document}Ω. We find, that \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega$$\end{document}Ω are on the order of a single \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu$$\end{document}μeV in the valence band, and tens to a hundred of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu$$\end{document}μeV in the conduction band, depending on the applied electric field. Similar to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b^2$$\end{document}b2, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega$$\end{document}Ω is also anisotropic, in particular for the conduction electrons.

Eliminating Edge Electronic and Phonon States of Phosphorene Nanoribbon by Unique Edge Reconstruction

Edge termination plays a vital role in determining the properties of 2D materials. By performing compelling ab initio simulations, a lowest-energy U-edge [ZZ(U)] reconstruction is revealed in the bilayer phosphorene. Such reconstruction reduces 60% edge energy compared with the pristine one and occurs almost without an energy barrier, implying it should be the dominating edge in reality. The electronic band structure of phosphorene nanoribbon with such reconstruction resembles that of an intrinsic 2D layer, exhibiting nearly edgeless band characteristics. Although ZZ(U) changes the topology of phosphorene nanoribbons, simulated transmission electron microscope, scanning transmission electron microscope and scanning tunneling microscope images indicate it is very hard to be identified. One possible identified method is infrared/Raman analyses because the ZZ(U) edge alters vibrational modes dramatically. In addition, it also increases the thermal conductivity of PNR 1.4 and 2.3 times than the pristine and Klein edges.

Prediction of Atomically Thin Two-Dimensional Single Monolayer SnGe with High Carrier Mobility : A DFT Study

Using first principles plane-wave calculations within the framework of density functional theory (DFT), we propose a new two-dimensional (2D) honeycomb structure of SnGe. The dynamical stability of the structure SnGe...

Electron transport properties of PtSe2 nanoribbons with distinct edge reconstructions

Edge reconstructions of two-dimensional (2D) materials play a central role in determining the electronic transport properties of nanodevices. However, it is not feasible to study the relationship between edge reconstruction and electronic properties using experimental methods because of the complexity of the experimental environment and the diversity of edge reconstruction. Herein, we have combined density functional theory (DFT) calculations and the nonequilibrium Green's function (NEGF) method to investigate the inner physical mechanism of platinum diselenide (PtSe₂) nanoribbons, revealing distinctive negative differential resistance (NDR) behaviors in different nanoribbons with various edge reconstructions. The armchair PtSe₂ nanoribbons with different edge reconstructions are all metallic, while the zigzag PtSe₂ nanoribbons are semiconducting when the ratio of Pt to Se atoms at the edge is 1 : 2. These results reveal the internal source of the difference in the electron transport properties of PtSe₂ nanoribbons with different edge reconstructions, providing new ideas for the design of novel multifunctional PtSe₂ semiconducting and conducting electronic nanodevices with NDR properties.

Edge reconstructions of two-dimensional (2D) materials play a central role in determining the electronic transport properties of nanodevices.

Bandstructure and Size-Scaling Effects in the Performance of Monolayer Black Phosphorus Nanodevices

Nanodevices based on monolayer black phosphorus or phosphorene are promising for future electron devices in high density integrated circuits. We investigate bandstructure and size-scaling effects in the electronic and transport properties of phosphorene nanoribbons (PNRs) and the performance of ultra-scaled PNR field-effect transistors (FETs) using advanced theoretical and computational approaches. Material and device properties are obtained by non-equilibrium Green’s function (NEGF) formalism combined with a novel tight-binding (TB) model fitted on ab initio density-functional theory (DFT) calculations. We report significant changes in the dispersion, number, and configuration of electronic subbands, density of states, and transmission of PNRs with nanoribbon width (W) downscaling. In addition, the performance of PNR FETs with 15 nm-long channels are self-consistently assessed by exploring the behavior of charge density, quantum capacitance, and average charge velocity in the channel. The dominant consequence of W downscaling is the decrease of charge velocity, which in turn deteriorates the ON-state current in PNR FETs with narrower nanoribbon channels. Nevertheless, we find optimum nanodevices with W > 1.4 nm that meet the requirements set by the semiconductor industry for the “3 nm” technology generation, which illustrates the importance of properly accounting bandstructure effects that occur in sub-5 nm-wide PNRs.

CO2 reduction to CH4 on Cu-doped phosphorene: a first-principles study

Optimizing the electrochemical carbon dioxide reduction reaction (CRR) to fuels is one of the most significant challenges in materials science and chemistry. Recently, single metal atom catalysts based on 2D materials have shown promise to improve the electroreduction performance of pristine 2D materials in the CRR. The physical origins of such performance enhancements are still poorly understood. Herein, we report the potential of a single Cu atom doped phosphorene catalyst for CO₂ electroreduction based on density functional theory (DFT) calculations. The doping sites (hollow, bridge, and on-top) of Cu on phosphorene are investigated first. Phosphorene with a Cu atom anchored on the hollow site is chosen for further study. The pathways for different CRR products, including HCOOH, CO, CH₃OH, and CH₄, are examined via constructing free energy diagrams and via comparing the limiting potentials. CH₄ is the most likely product after analysis of the adsorption energies and free energy pathways. Cu-Doped phosphorene in general shows improved CRR performance with lower limiting potential values. Cu doping leads to a decrease in the band gap value (about 0.2 eV), which is likely to be the physical origin of the CRR performance enhancement. Our study provides a novel promising CRR candidate catalyst based on phosphorene.

A multifunctional 2D black phosphorene-based platform for improved photovoltaics

As one of the latest additions to the 2D nanomaterials family, black phosphorene (BP, monolayer or few-layer black phosphorus) has gained much attention in various forms of solar cells. This is due largely to its intriguing semiconducting properties such as tunable direct bandgap (from 0.3 eV in the bulk to 2.0 eV in the monolayer), extremely high ambipolar carrier mobility, broad visible to infrared light absorption, etc. These appealing optoelectronic attributes make BP a multifunctional nanomaterial for use in solar cells via tailoring carrier dynamics, band energy alignment, and light harvesting, thereby promoting the rapid development of third-generation solar cells. Notably, in sharp contrast to the copious work on revealing the fundamental properties of BP, investigation into the utility of BP is comparatively less, particularly in the area of photovoltaics. Herein, we first identify and summarize an array of unique characteristics of BP that underpin its application in photovoltaics, aiming at providing inspiration to develop new designs and device architectures of photovoltaics. Subsequently, state-of-the-art synthetic routes (i.e., top-down and bottom-up) to scalable BP production that facilitates its applications in optoelectronic materials and devices are outlined. Afterward, recent advances in a diverse set of BP-incorporated solar cells, where BP may impart electron and/or hole extraction and transport, function as a light absorber, provide dielectric screening for enhancing exciton dissociation, and modify the morphology of photoabsorbers, are discussed, including organic solar cells, dye-sensitized solar cells, heterojunction solar cells and perovskite solar cells. Finally, the challenges and opportunities in this rapidly evolving field are presented.

Quasiparticle energies and significant exciton effects of monolayered blue arsenic phosphorus conformers

Low-dimensional systems have strong multi-body interactions and fewer geometric constraints due to the screening effect of the Coulomb interaction. We use the single-shot GW-Bethe Salpeter equation (G₀W₀-BSE) to calculate the electronic and optical properties of six-blue arsenic phosphorus (β-AsP) conformers. The results show significant anisotropic exciton effects of covering visible regions, which apparently changed the light absorption. The maximum exciton binding energy is up to 0.99 eV, which is more extensive than the black phosphorus monolayer (0.9 eV). We predict that the different orbital contributions to valence bands may cause the anisotropic exciton effect difference. Our results indicate that β-AsP monolayers with the large binding energies of exciton hold a great promise for applications in optoelectronic devices.

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal (M@C) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of M@C type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.

Robust charge spatial separation and linearly tunable band gap of low-energy tube-edge phosphorene nanoribbon

Versatile applications have been proposed for phosphorene nanoribbons (PNRs), whose properties depend strongly on the edge structures. Recently, a unique tube-reconstruction at the zigzag edge (ZZ[Tube]) of PNRs was discovered to be the lowest configuration. Therefore, studies on PNRs should be reconsidered. In this paper, we systemically explore the width and strain effects on zigzag PNRs with different edge structures, including ZZ[Tube], ZZ and ZZ[ad] edges. ZZ PNRs always have small band gaps which are nearly independent of both width and strain. A remarkable band gap exists in ZZ[ad] PNRs which increases with a decrease in the ribbon width but is not sensitive to strain. In contrast, the band gaps of ZZ[Tube] PNRs change from 1.08 to 0.70 eV as the width increases from 12 to 65 Å. In addition, the band gaps of ZZ[Tube] PNRs show a linear response under a certain range of strain. In addition, the carrier effective masses (0.50 m 0 for electrons and 0.94 m 0 for holes) of ZZ[Tube] PNRs are much lower than for ZZ[ad], and the VBM and CBM charges are robustly spatially separated even under strains ranging from -5% to 5%. Their ease of formation, lowest energy, light effective mass, linear band gap response to strain and robust charge spatial separation provide ZZ[Tube] PNRs with potentially excellent performance in microelectronic and opto-electric applications.