Carbon phosphide monolayers with superior carrier mobility

Carrier transport in bulk and two-dimensional Zn2(V,Nb,Ta)N3 ternary nitrides

Density functional theory-based simulations are applied to study the electronic structures, carrier masses, carrier mobility and carrier relaxation times in bulk and two-dimensional (2D) Zn2(V,Nb,Ta)N3 ternary nitrides. Bulk Zn2(V,Nb,Ta)N3 possess moderate band gap sizes of 2.17 eV, 3.11 eV, and 3.40 eV, respectively. Two-dimensional Zn2(V,Nb,Ta)N3 have slightly higher band gap sizes of 2.77 eV, 3.33 eV, and 3.23 eV, respectively. Carrier mass, carrier mobility and carrier relaxation time are found to be anisotropic in all the studied structures. Bulk and 2D samples show an order of magnitude higher electron mobility compared to hole mobility. The highest electron mobility in bulk Zn2NbN3 and Zn2TaN3 is about ∼103 cm2 V-1 s-1. Importantly, for 2D Zn2NbN3, an abnormally high electron mobility of 1.67 × 104 cm2 V-1 s-1 is observed, which is not inferior to the highest known electron mobility values in 2D materials. Such a high electron mobility in 2D Zn2NbN3 can be attributed to a strong delocalization of the conduction band minimum, which is responsible for electron transport. Therefore, this work opens up new materials for high performance nanodevices, such as tandem solar cells and field-effect transistors. This study also provides deep physical insights into the nature of carrier transport mechanisms in bulk and 2D Zn2(V,Nb,Ta)N3 ternary nitrides.

Electrostatic Gating of Phosphorene Polymorphs

The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.

A study of 2D GeI2/InTe van der Waals hetero bilayer as a photocatalyst material

The computational study of the van der Waals hetero (vdW) bilayer GeI2/InTe has been carried out in present study. The isolated monolayer GeI2and InTe have been studied first and the results were compared to the previous studies. The possible stackings are considered after the vdW interaction correction is applied in the structure relaxation. The vdW hetero bilayer stability has been checked from the phonon dispersion andab initioMolecular Dynamics calculations. The charge transfer from InTe to GeI2monolayer. Type-II indirect band gap (1.98, 2.01 eV) is verified by the projected band structure and band alignment calculations. The vdW hetero bilayer is a superior photocatalyst for the pH value up to pH = 0 to 11. The optical properties are calculated from the complex dielectric constant. The absorption coefficient shows the enhance absorption of light in the visible and ultraviolet regions. The vdW hetero bilayer has shown low reflectivity (37%) and a high refractive index (2.80) in the visible region. The enhanced optical properties have shown its possible applications in optoelectronic devices.

Instructive Synergistic Effect of Coordinating Phosphorus in Transition-Metal-Doped β-Phosphorus Carbide Guiding the Design of High-Performance CO2RR Electrocatalysts

Developing efficient electrocatalysts for the CO2 reduction reaction (CO2RR) is the key and difficult point to alleviate energy and climate issues. The synergistic catalytic effects between metal and nonmetal elements have gained attention for the design of the CO2RR electrocatalysts. The realization of this effect requires a suitable combination of metal and nonmetal elements, as well as the support of suitable substrates. Based on this, the transition-metal-doped β-phosphorus carbide (TM-PC) (TM = 4d and 5d transition metals except Tc) catalysts are designed, and their structures, electronic properties, and CO2RR catalytic performances are studied in depth via first-principle calculations. The strong bonding ability and high reactivity brought by the moderate electronegativity and abundant electrons and orbitals of phosphorus are the key to the excellent catalytic performance of TM-PCs. Coordinating phosphorus atoms improve the catalyst activity in two ways: (1) regulating the electron transfer of the TM active site, and (2) acting as the active site and changing the reaction mechanism. With the participation of coordinating P atoms, the "relay" of active sites reduces the limiting potential values for the reduction from CO2 to CH4 catalyzed by Cr-PC and Mo-PC by 0.27 and 0.23 V, respectively, compared with pathways where only the TM atom is the active site, reaching -0.55 and -0.63 V, respectively. Regarding the coordinating P atom as the second active site, Cr-PC and Mo-PC can catalyze the production of CH3CH2OH at limiting potential values of -0.54 and -0.67 V, respectively. This study demonstrates the dramatic enhancement of catalytic activity caused by suitable nonmetal coordinating atoms such as P and provides a reference for the design of high-performance CO2RR electrocatalysts based on metal-nonmetal coordinating active centers.

Metal-free Janus α- and β-SiCP4: designing stable and efficient two-dimensional semiconductors for water splitting

Two-dimensional (2D) semiconductors exhibit exceptional potential in the field of photocatalytic water splitting due to their unique structural characteristics and photoelectric properties. In this study, based on first-principles density functional theory, we theoretically proposed two SiCP4 Janus 2D semiconductors with high stability, namely monolayer α- and β-SiCP4. By performing the calculation of HSE06 functionals, the band structures of monolayer α- and β-SiCP4 have been estimated, and the results show that both α- and β-SiCP4 are direct-band-gap semiconductors with band gaps of 1.64 eV and 1.91 eV, respectively. Meanwhile, the band edge levels of monolayer α- and β-SiCP4 meet the band structure requirements of photocatalysts in water splitting. Notably, because of the internal build-in electric fields and tiny band gaps, monolayer α- and β-SiCP4 exhibit separated photogenerated electron-hole pairs and high solar-to-hydrogen (STH) efficiency, reaching up to 33.68% and 23.72%, respectively. Additionally, we also investigate the impact of uniaxial strain on electronic, optical and photocatalytic properties of monolayer α- and β-SiCP4 considering pH values ranging from 0 to 14. Our results demonstrate that the maximum STH efficiency for α-SiCP4 is achieved under X-direction strain (η) of 2%, Y-direction strain (η) of 8%, and pH values between 2 and 4. Conversely, β-SiCP4 exhibits the highest STH efficiency under X-direction strain (η) of 8%, Y-direction strain (η) of 6%, and pH values between 2 and 4.

Metallic CrP2 monolayer: potential applications in energy storage and conversion

Phosphorus-rich compounds have emerged as a promising class of energy storage and conversion materials due to their interesting structures and electrochemical properties. Herein, we propose that a metallic CrP2 monolayer, isomorphic to 1H-phase MoS2, is a good prospect as an anode for K-ion batteries and a catalyst for hydrogen evolution through first-principles calculations. The CrP2 monolayer demonstrates not only a desirable high K storage capacity (940 mA h g-1) but also a low K-ion diffusion barrier (0.10 eV) and average open circuit voltage (0.40 V). On the other hand, its Gibbs free energy (0.02 eV)/active site density is superior/comparable to that of commercial Pt, resulting from the contribution of the lone pair electrons of the P atom. Its high structural stability and intrinsic metallicity can ensure high safety and performance during the cyclic process. These interesting properties make the CrP2 monolayer a promising multifunctional material for energy storage and conversion devices.

Strain engineering of hyperbolic plasmons in monolayer carbon phosphide: a first-principles study

Natural and tunable in-plane hyperbolic plasmons have so far been elusive, and hence few two-dimensional hyperbolic materials have been theoretically and experimentally discovered. Here, comprehensive first-principles calculations were conducted to study the electronic and plasmonic properties of biaxially strained monolayer carbon phosphide (β-CP). We found that (i) a compressed β-CP hosts strong anisotropic Dirac-shaped fermions with robust modulated Fermi velocity, (ii) for biaxial strain of -3% an unprecedented ultra-wide hyperbolic window is extended continuously from terahertz (9 THz) to mid-visible (blue light, 693 THz), (iii) the tunable optical Van Hove singularity as the origin of hyperbolic plasmons in deformed β-CP is disclosed, (iv) an elliptic to hyperbolic transition in the σ-near-zero regime is demonstrated in terahertz frequencies (9 THz), (v) the propagation angle of the concave wavefront can be actively tuned using biaxial strains, and (vi) hyperbolic dispersion reorientation from one principal axis to another orthogonal one under compressive strains larger than 8% is observed. This study sheds new light on the unique properties of hyperbolic two-dimensional (2D) materials having exotic optoelectronic characteristics which are promising candidates for anisotropic light control with ultimate dexterity in the flat optics.

Conductive C3NS Monolayer with Superior Properties for K Ion Batteries

K-ion batteries (KIBs) have been considered as appealing alternatives to Li ion batteries due to the high abundance of K, their high working voltages, and allowing the use of mature LIB technology. Thus far, anode materials that can meet the rigorous requirements of KIBs are still rather rare. Here, we have identified a desirable anode material, a metallic C₃NS monolayer with high stability, a high storage capacity of 980 mAh/g, a low diffusion barrier of 0.24 eV, and a low open-circuit voltage of 0.36 V, through first-principles calculations. Metallic C₃NSK_n (n = 1-3) can ensure a high electron conductivity during the charge/discharge process. Valence electrons of the N atom in a triangular bipyramid configuration favor the formation of a planar edge-sharing hexagonal C₄N₂ unit and delocalized π bonding with C 2p electrons. The lone pair electrons of the S atom induce strong interactions with K atoms, facilitating storage capacity. These interesting properties make the C₃NS monolayer a promising anode for KIBs.

A Novel Two-Dimensional ZnSiP2 Monolayer as an Anode Material for K-Ion Batteries and NO2 Gas Sensing

Using the crystal-structure search technique and first-principles calculation, we report a new two-dimensional semiconductor, ZnSiP₂, which was found to be stable by phonon, molecular-dynamic, and elastic-moduli simulations. ZnSiP₂ has an indirect band gap of 1.79 eV and exhibits an anisotropic character mechanically. Here, we investigated the ZnSiP₂ monolayer as an anode material for K-ion batteries and gas sensing for the adsorption of CO, CO₂, SO₂, NO, NO₂, and NH₃ gas molecules. Our calculations show that the ZnSiP₂ monolayer possesses a theoretical capacity of 517 mAh/g for K ions and an ultralow diffusion barrier of 0.12 eV. Importantly, the ZnSiP₂ monolayer exhibits metallic behavior after the adsorption of the K-atom layer, which provides better conductivity in a period of the battery cycle. In addition, the results show that the ZnSiP₂ monolayer is highly sensitive and selective to NO₂ gas molecules.

Tunneling transport of 2D anisotropic XC (X = P, As, Sb, Bi) with a direct band gap and high mobility: a DFT coupled with NEGF study

Direct bandgap and significant anisotropic properties are crucial and beneficial for nanoelectronic applications. In this work, through first-principles calculations, we investigate novel two-dimensional (2D) α-XC (X = P, As, Sb, Bi) materials, which possess a direct bandgap of 0.73 to 1.40 eV with remarkable anisotropic electronic properties. Intriguingly, 2D α-XC presents the highest electron mobility near 8 × 10³ cm² V^-1 s^-1 along the Γ-X direction. Moreover, the transfer characteristics of the 2D α-XC TFETs are thoroughly assessed through NEGF methods. AsC TFETs demonstrate an on-state current larger than 2.2 × 10³ μA μm^-1, which can satisfy the International Technology Roadmap for Semiconductors (ITRS) for high-performance requirements. In particular, the minimum value of subthreshold swing of devices is as low as 15 mV dec^-1, indicating excellent device switching characteristics. The relevant calculation results show that 2D α-XC monolayers could be a promising candidate in next-generation high-performance device applications.

Metallic B2C3P Monolayer as Li-Ion Battery Materials: A First-Principles Study

The search for and design of high-performance electrode materials is always an important topic in rechargeable batteries. Using a global structure prediction method together with first-principles calculations, a free-standing two-dimensional B2C3P monolayer with honeycomb structure was identified. The stability of the B2C3P monolayer was confirmed by cohesive energy, phonon curves, and ab initio molecular dynamics calculations. Of note, the B2C3P monolayer was demonstrated to be metallic, which shows excellent performance for Li-ion batteries. For example, the B2C3P monolayer also exhibited a metallic characteristic after Li adsorption, therefore the ability to keep good electrical conductivity during battery operation. Furthermore, when a B2C3P monolayer is used as a lithium-ion battery anode, it shows an ultra-high theoretical capacity of 3024 mAh/g, and a comparatively low diffusion barrier of 0.33 eV. All calculated results showed that the B2C3P monolayer is an appealing anode material, and has great potential in energy storage devices.

Infrared Light Emission Devices Based on Two-Dimensional Materials

Two-dimensional (2D) materials have garnered considerable attention due to their advantageous properties, including tunable bandgap, prominent carrier mobility, tunable response and absorption spectral band, and so forth. The above-mentioned properties ensure that 2D materials hold great promise for various high-performance infrared (IR) applications, such as night vision, remote sensing, surveillance, target acquisition, optical communication, etc. Thus, it is of great significance to acquire better insight into IR applications based on 2D materials. In this review, we summarize the recent progress of 2D materials in IR light emission device applications. First, we introduce the background and motivation of the review, then the 2D materials suitable for IR light emission are presented, followed by a comprehensive review of 2D-material-based spontaneous emission and laser applications. Finally, further development directions and challenges are summarized. We believe that milestone investigations of 2D-material-based IR light emission applications will emerge soon, which are beneficial for 2D-material-based nano-device commercialization.

Janus Ga2SSe nanotubes as efficient photocatalyst for overall water splitting

Using sunlight to decompose water into hydrogen and oxygen is one of the most important ways to solve the current global environmental issues and energy problems. In this paper, we use density functional theory to predict the photocatalytic performance of Janus Ga₂SSe nanotubes (JGSSe NTs) for the first time. The result shows that the small formation energy and strain energy ensure the stability of the nanotubes. Compared with monolayers, the visible light absorption range of JGSSe NTs is wider, and the large radius (>26.60 A) nanotubes all meet the hydrolysis potential. Surprisingly, the hole mobility of JGSSe NT was estimated to be as high as 2.89 × 10⁴cm²V^-1S^-1. In conclusion, JGSSe nanotubes are expected to be an excellent photocatalyst due to their low electron-hole recombination rate, high hole mobility, solar absorption in the visible light range, and good oxidation capacity. In addition, the nanotube band gap can be effectively regulated by applying strain. It is hoped that our research will provide meaningful progress in the development of novel and efficient photocatalysts. We hope that our research will provide a possible way to develop novel and efficient photocatalysts.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

Doping effects on the antibonding states and carriers of two-dimensional PC6

The absence of a bandgap in pristine graphene severely restricts its application, and there is high demand for other novel two-dimensional (2D) materials. PC₆ has recently emerged as a promising 2D material with a direct band gap and ultrahigh carrier mobility. In light of the remarkable properties of an intrinsic PC₆ monolayer, it would be intriguing to find out whether a doped PC₆ monolayer displays properties superior to the pure system. In this study, we have performed density functional theory calculations to understand the doping effects of both P-site and C-site substitution in PC₆ and, for the first time, we discovered doping-related impurity-level anomalies in this system. We successfully explained why no donor or acceptor defect states exist in the band structures of X_P-PC₆ (X = C, Ge, Sn, O, S, Se, or Te). In group-IV-substituted systems, these dopant states hybridize with host states near the Fermi level rather than act as acceptors, which is deemed to be a potential way to tune the mobility of PC₆. In the case of group-VI substitution, the underlying mechanism relating to doping anomalies arises from excess electrons occupying antibonding states.

Electrochemistry of P-C Bonds in Phosphorus-Carbon Based Anode Materials

Phosphorus-carbon anode materials for alkali-metal ion storage in rechargeable batteries can simultaneously achieve high-energy density and fast charging. The P-C-bonded structure in the phosphorus-carbon materials has been observed and acknowledged to be a critical structural feature that renders improved cycling stability and rate performance. However, the underlying mechanisms, especially the role played by P-C bonds, remain elusive. By combining computational simulations and spectroscopic characterizations, we reveal that the stability of P-C bonds is critical to the electrochemical performance. In the discharge process, P-P bonds are fragile, while the bonding state of the P-C bonds is almost unchanged since electrons were mainly received by the P atoms to form lone pairs. The preserved P-C clusters can effectively serve as a reunion center for the recovery of P-P bonds in the recharging process, leading to a moderate energy change and improved cycling reversibility and structural stability of the phosphorous for electrochemical energy storage.

Two-dimensional semiconductor materials with high stability and electron mobility in group-11 chalcogenide compounds: MNX (M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N)

It is still an urgent task to find new two-dimensional (2D) semiconductor materials with a suitable band gap, high stability and high mobility for the applications of next generation electronic devices. Based on first-principles calculations, we report a new class of 2D group-11-chalcogenide trielement monolayers (MNX, where M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N) with a wide band gap, excellent stability (dynamic stability, thermodynamic stability and environmental stability) and high mobility. At the mixed density functional level, the energy band gap extends from 0.61 eV to 2.65 eV, covering the ultraviolet-A and visible light regions, which is critical for a broadband optical response. For δ-MNX monolayers, the carrier mobility is as high as 10⁴ cm² V^-1 s^-1 at room temperature. In particular, the mobility of δ-AgAuS is as high as 6.94 × 10⁴ cm² V^-1 s^-1, which is of great research significance for the application of electronic devices in the future. Based on the above advantages, group-11 chalcogenide MNX monomolecular films have broad prospects in the field of nanoelectronics and optoelectronics in the future.

Monolayer PC3: A promising material for environmentally toxic nitrogen-containing multi gases

Carbon and its analogous nanomaterials are beneficial for toxic gas sensors since they are used to increase the electrochemically active surface region and improve the transmission of electrons. The present article addresses a detailed investigation on the potential of the monolayer PC₃ compound as a possible sensor material for environmentally toxic nitrogen-containing gases (NCGs), namely NH₃, NO, and NO₂. The entire work is carried out under the frameworks of density functional theory, ab-initio molecular dynamics simulations, and non-equilibrium Green's function approaches. The monolayer-gas interactions are studied with the van der Waals dispersion correction. The stability of pristine monolayer PC₃ is confirmed through dynamical, mechanical, and thermal analyses. The mobility and relaxation time of 2D PC₃ sensor material with NCGs are obtained in the range of 10¹-10⁴ cm² V^-1 s^-1 and 10¹-10³ fs for armchair and zigzag directions, respectively. Out of six possible adsorption sites for toxic gases on the PC₃ surface, the most prominent site is identified with the highest adsorption energy for all the NCGs. Considering the most stable configuration site of the NCGs, we have obtained relevant electronic properties by utilizing the band unfolding technique. The considerable adsorption energies are obtained for NO and NO₂ compared to NH₃. Although physisorption is observed for all the NCGs on the PC₃ surface, NO₂ is found to convert into NO and O at 5.05 ps (at 300 K) under molecular dynamics simulation. The maximum charge transfer (0.31e) and work function (5.17 eV) are observed for the NO₂ gas molecule in the series. Along with the considerable adsorption energies for NO and NO₂ gas molecules, their shorter recovery time (0.071 s and 0.037 s, respectively) from the PC₃ surface also identifies 2D PC₃ as a promising sensor material for those environmentally toxic gases. The experimental viability and actual implications for PC₃ monolayer as NCGs sensor material are also confirmed by examining the humidity effect and transport properties with modeled sensor devices. The transport properties (I-V characteristics) reflect the significant sensitivity of PC₃ monolayer toward NO and NO₂ molecules. These results certainly confirm PC₃ monolayer as a promising sensor material for NO and NO₂ NCG molecules.

SiCP4 Monolayer with a Direct Band Gap and High Carrier Mobility for Photocatalytic Water Splitting

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP₂ and SiCP₄ monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP₄ monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼10⁴ cm^-1) and high carrier mobility (∼10⁵ cm² V^-1 s^-1) of SiCP₂ and SiCP₄ facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP₂ and SiCP₄ monolayers promising candidates for applications in photocatalytic water splitting.

Carbon phosphide nanoribbons with spatial inversion symmetry: robust generators of pure spin current with photogalvanic effect

Our recent work has demonstrated that spin-dependent photogalvanic effect (PGE) is an ideal way to induce pure spin current in certain centrosymmetric systems (Phys. Rev. B 102, 081402 (2020)), and...

Structural and compositional properties of 2D CH3NH3PbI3 hybrid halide perovskite: a DFT study

Two-dimensional (2D) hybrid halide perovskites have been scrutinized as candidate materials for solar cells because of their tunable structural and compositional properties. Results based on density functional theory demonstrate its thickness-dependent stability. We have observed that the bandgap decreases from the mono- to quad-layer because of the transformation from 2D towards 3D. Due to the transformation, the carrier mobility is lowered with the corresponding smaller effective mass. On the other hand, the multilayer structures have good optical properties with an absorption coefficient of about 10⁵ cm⁻¹. The calculated absorption spectra lie between 248 nm and 496 nm, leading to optical activity of the 2D multilayer CH₃NH₃PbI₃ systems in the visible and ultraviolet regions. The strength of the optical absorption increases with an increase in thickness. Overall results from this theoretical study suggest that this 2D multilayer CH₃NH₃PbI₃ is a good candidate for photovoltaic and optoelectronic device applications.

We have studied 2D CH₃NH₃PbI₃ and its multilayer halide perovskites. These systems have a high formation energy. The optical properties absorption spectra lie between 248 nm to 496 nm with an absorption coefficient of about 10⁵ cm⁻¹.

Gas sensing detection in carbon phosphide monolayer: Improving COx sensitivity through B-doping

A 2D materials engineering challenge is searching for a nanodevice capable to detect and distinguish gas molecules through electrical identification. Herein, the B-doped carbon phosphide monolayer (B-doped γ-CP) was explored...

First-principles study on the electronic structures and contact properties of graphene/XC (X = P, As, Sb, and Bi) van der Waals heterostructures

The electrical contacts at the van der Waals (vdW) interface between two-dimensional (2D) semiconductors and metal electrodes could dramatically affect the device performance. Herein, we construct a series of graphene (Gr)/XC (X = P, As, Sb, and Bi) vdW heterostructures, in which XC monolayers have aroused considerable attention recently as an emerging class of 2D semiconductors. The electronic structures and contact properties of Gr/XC vdW heterostructures are investigated systematically using first-principles calculations. The band structures indicate that both Gr/PC and Gr/AsC heterostructures form n-type Schottky contacts with Schottky barrier heights (SBHs) of 0.01 eV and 0.43 eV, respectively, while both Gr/SbC and Gr/BiC heterostructures preferably form Ohmic contacts. The different X atoms result in different work functions, electron flows, charge distributions and orientations of the dipole moment in Gr/XC heterostructures. Moreover, the tunneling probabilities increase with the increasing atom radius of X from P to Bi, indicating the most improved current and smaller contact resistance at the interfaces of Gr/BiC compared to Gr/PC, Gr/AsC and Gr/SbC heterostructures. Our work could provide meaningful information for designing high-performance nanoelectronic devices based on Gr/XC heterostructures.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P₃S and P₂S monolayers. The P₃S monolayer, consisting of n-bicyclo-P₆ units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10⁴ cm² V^-1 s^-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10⁵ cm^-1, respectively. Additionally, the P₃S monolayer has a high structural stability and resistance to air oxidation.

Anisotropic and High-Mobility C3S Monolayer as a Photocatalyst for Water Splitting

Taking into account the high conductivity and stability of carbon materials, such as graphene, and the strong polar covalent bonding character of main-group compounds, we explore potential 2D materials in the C-S binary system through first-principles structure search calculations. Herein, a hitherto unknown semiconducting C₃S monolayer is identified, consisting of well-known n-biphenyl and S atom linked benzenes, exhibiting an obvious direction-dependent atomic arrangement. Thus, it exhibits anisotropic mechanical properties and carrier mobility. Its electron mobility reaches 2.14 × 10⁴ cm² V^-1 s^-1 in the b direction, along which n-biphenyl units are arranged, and is much higher than that in the well-used MoS₂ monolayer and black phosphorus. Meanwhile, the C₃S monolayer has high optical absorption coefficients (10⁵ cm^-1), high thermal and dynamical stabilities, and a moderate ability to split water. All these desirable properties make the C₃S monolayer a promising candidate for applications in novel optoelectronic devices.

Two-Dimensional Planar BGe Monolayer as an Anode Material for Sodium-Ion Batteries

Using first-principles swarm intelligence structure prediction computations, we explore a fully planar BGe monolayer with unique mechanical and electrical properties. Theoretical calculations reveal that a free-standing BGe monolayer has excellent stability, which is confirmed by the cohesive energy (compared to experimentally synthetic borophene and germanene monolayers), phonon modes (no imaginary frequencies appeared in the phonon spectrum), ab initio molecular dynamics (AIMD) simulations (no broken bonds and geometric reconstructions), and mechanical stability criteria. The metallic feature of the BGe monolayer can be maintained after absorbing different numbers of Na atoms, ensuring good electronic conductivity during the charge/discharge process. The calculated migration energy barrier, open-circuit voltage, and theoretical specific capacity of the BGe monolayer are much better than those of some other two-dimensional (2D) materials. These findings render the BGe monolayer a potential candidate for reversible Na-ion battery anode materials with desirable performance.

First-Principles Prediction of Two-Dimensional B3C2P3 and B2C4P2: Structural Stability, Fundamental Properties, and Renewable Energy Applications

The existence of two novel hybrid two-dimensional (2D) monolayers, 2D B₃C₂P₃ and 2D B₂C₄P₂, has been predicted based on the density functional theory calculations. It has been shown that these materials possess structural and thermodynamic stability. 2D B₃C₂P₃ is a moderate band gap semiconductor, while 2D B₂C₄P₂ is a zero band gap semiconductor. It has also been shown that 2D B₃C₂P₃ has a highly tunable band gap under the effect of strain and substrate engineering. Moreover, 2D B₃C₂P₃ produces low barriers for dissociation of water and hydrogen molecules on its surface, and shows fast recovery after desorption of the molecules. The novel materials can be fabricated by carbon doping of boron phosphide and directly by arc discharge and laser ablation and vaporization. Applications of 2D B₃C₂P₃ in renewable energy and straintronic nanodevices have been proposed.

Highly tunable electronic structure and linear dichroism in 90° twisted α-phosphorus carbide bilayer: a first-principles calculation

α-Phosphorus carbide (α-PC) shares a similar puckered structure with black phosphorus and has a high carrier mobility, showing great application potential in the future nano-electronic devices. Based on first-principles calculations, we reveal that an interlayer twist angle of 90° results in a symmetric band dispersion and spatial separation of electronic states in the α-PC bilayer, leading to isotropic electrical transport. Nevertheless, the anisotropic electronic states can be rebooted by introducing an out-of-plane electrostatic potential or an in-plane deformation potential, both of which can break the energy degeneracy and continuously modulate the bandgap of the 90° twisted bilayer. This highly tunable band structure can also induce a directionally exchangeable optical linear dichroism by flipping the voltage sign or changing the strain mode. These results indicate that the combination of the interlayer twist and gating/strain technique provides great flexibility to control the anisotropic behaviors in 2D puckered materials.

Point Defects in Two-Dimensional γ-Phosphorus Carbide

Defects are inevitably present in two-dimensional (2D) materials and usually govern their various properties. Here, a comprehensive density functional theory-based investigation of seven kinds of point defects in a recently produced γ allotrope of 2D phosphorus carbide (γ-PC) is conducted. The defects, such as antisites, single C or P, and double C and P and C and C vacancies, are found to be stable in γ-PC, while the Stone-Wales defect is not presented in γ-PC due to its transition-metal dichalcogenides-like structure. The formation energies, stability, and surface density of the considered defect species as well as their influence on the electronic structure of γ-PC is systematically identified. The formation of point defects in γ-PC is found to be less energetically favorable than in graphene, phosphorene, and MoS₂. Meanwhile, defects can significantly modulate the electronic structure of γ-PC by inducing hole/electron doping. The predicted scanning tunneling microscopy images suggest that most of the point defects are easy to distinguish from each other and that they can be easily recognized in experiments.

Two-dimensional Ga2O2 monolayer with tunable band gap and high hole mobility

By means of density functional theory and unbiased structure search computations, we systematically investigated the stability and electronic properties of a new Ga2O2 monolayer. The phonon spectra and ab initio molecular dynamics simulations show that the Ga2O2 monolayer is dynamically and thermally stable. Moreover, it also shows superior open-air stability. In particular, the Ga2O2 monolayer is an indirect semiconductor with a wide band gap of 2.752 eV and high hole mobility of 4720 cm2 V-1 s-1. Its band gap can be tuned flexibly in a large range by applied strain and layer control. It exhibits high absorption coefficients (>105 cm-1) in the ultraviolet region. The combined novel electronic properties of the Ga2O2 monolayer imply that it is a highly promising material for future applications in electronics and optoelectronics.

A Theoretical Analysis on the Oxidation and Water Dissociation Resistance on Group-IV Phosphide Monolayers

Group-IV phosphide monolayers (MP, M=C, Si, Ge and Sn) provide a versatile platform for photocatalysts, as well as optoelectronic and nanoelectronic devices. Herein, comprehensive first-principles calculations and ab initio molecular dynamics (AIMD) simulations were performed to explore their stabilities in the air. We identified that the MP monolayers have excellent mechanical properties and their carrier mobilities are higher than that of phosphorene. The MP monolayers were predicted to possess superior oxidation resistance than the boron phosphide (BP) monolayer based on the proposed donation-backdonation theory. It was observed that the dissociation and chemisorption of a water molecule on the monolayers are kinetically difficult both in the water and in oxygen-water environments involving energy barriers of 1.28-3.48 eV. We also performed AIMD simulations at 300, 1000, 1200 and 1500 K. It is noteworthy that only the carbon phosphide (CP) monolayer can retain an intact structure at 1500 K, while the other three monolayers can just sustain to 1200 K. These results provide a guidance for their practical application and experimental fabrication.

Carbon phosphide nanosheets and nanoribbons: insights on modulating their electronic properties by first principles calculations

A carbon phosphide (CP) monolayer, a 2D structure derived from the same 3-fold coordination found both in graphene and phosphorene, has been successfully synthesized in an experiment recently. In this paper, we investigated the modulation of electronic structures and transport characteristics of 2D nanosheets and quasi-1D nanoribbons of CP nanomaterials in the α-phase by using first-principles density functional theory simulation. The calculated band structures show that the band gap of 2D CP nanosheets progressively increases as the uniform biaxial strain changes from compression to stretching. However, the biaxial strain cannot change the indirect band gap behavior of the original 2D CP nanosheet. In addition, the band structures of quasi-1D nanoribbons with different styles of H-passivated zigzag edges have also been studied. The results show that the H-passivated zigzag PC ribbons with two P edges are semiconductors with indirect band gaps, and the gaps decrease with increasing width of ribbons. However, the H-passivated CP nanoribbons with one P-atom terminated edge in combination with one P-atom edge, and H-passivated CC nanoribbons with two C-atom terminated edges display metallic behaviors. The semi-conductive or metallic behaviors of zigzag CP nanoribbons can be explained by presenting the wave function of their energy band around the Fermi level. Finally, the electronic transport properties of different CP nanoribbon based nanojunctions are studied in which arise the interesting negative differential resistance or rectification effects in their current-voltage characteristic curves.

Monolayer PC5/PC6: promising anode materials for lithium-ion batteries

Employing two-dimensional (2D) materials as anodes for lithium-ion batteries (LIBs) is believed to be an effective approach to meet the growing demands of high-capacity next-generation LIBs. In this work, the first-principles density functional theory (DFT) calculations are employed to evaluate the potential application of two-dimensional phosphorus carbide (2D PC_x, x = 2, 5, and 6) monolayers as anode materials for lithium-ion batteries. The 2D PC_x systems are predicted to show outstanding structural stability and electronic properties. From the nudged elastic band calculations, the single Li atom shows extreme high diffusivities on the PC_x monolayer with low energy barriers of 0.18 eV for PC₂, 0.47 eV for PC₅, and 0.44 eV for PC₆. We further demonstrate that the theoretical specific capacity of monolayer PC₅ and PC₆ can reach up to 1251.7 and 1235.9 mA h g^-1, respectively, several times that of a graphite anode used in commercial LIBs. These results suggest that both PC₅ and PC₆ monolayers are promising anode materials for LIBs. Our work opens a new avenue to explore novel 2D materials in energy applications, where phosphorus carbides could be used as high-performance anode in LIBs.

2D materials beyond graphene toward Si integrated infrared optoelectronic devices

Since the discovery of graphene in 2004, it has become a worldwide hot topic due to its fascinating properties. However, the zero band gap and weak light absorption of graphene strictly restrict its applications in optoelectronic devices. In this regard, semiconducting two-dimensional (2D) materials are thought to be promising candidates for next-generation optoelectronic devices. Infrared (IR) light has gained intensive attention due to its vast applications, including night vision, remote sensing, target acquisition, optical communication, etc. Consequently, the generation, modulation, and detection of IR light are crucial for practical applications. Due to the van der Waals interaction between 2D materials and Si, the lattice mismatch of 2D materials and Si can be neglected; consequently, the integration process can be achieved easily. Herein, we review the recent progress of semiconducting 2D materials in IR optoelectronic devices. Firstly, we introduce the background and motivation of the review. Then, the suitable materials for IR applications are presented, followed by a comprehensive review of the applications of 2D materials in light emitting devices, optical modulators, and photodetectors. Finally, the problems encountered and further developments are summarized. We believe that milestone investigations of IR optoelectronics based on 2D materials beyond graphene will emerge soon, which will bring about great industrial revelations in 2D material-based integrated nanodevice commercialization.

The interaction of two-dimensional α- and β-phosphorus carbide with environmental molecules: a DFT study

The recently fabricated two-dimensional phosphorus carbide (PC) has been proposed for application in different nanodevices such as nanoantennas and field-effect transistors. However, the effect of ambient molecules on the properties of PC and, hence, the productivity of PC-based devices is still unknown. Herein a first-principles investigation is performed to study the most structurally stable α- and β-PC allotropes upon their interaction with environmental molecules, including NH₃, NO, NO₂, H₂O, and O₂. It is predicted that NH₃, H₂O, and O₂ are physisorbed on α- and β-PC while NO and NO₂ may easily form a covalent bond with the PC. Importantly, NO and NO₂ possess low adsorption energies on PC which compared to these on graphene and phosphorene. Moreover, both molecules are strong acceptors to PC with a giant charge transfer of ∼1 e per molecule. For all the considered molecules PC is found to be more sensitive compared to graphene and phosphorene. The present work provides useful insight into the effects of environmental molecules on the structure and electronic properties of α- and β-PC, which may be important for their manufacturing, storage, and application in gas sensors and electronic devices.

Two-dimensional few-layered PC3 as a promising photocatalyst for overall water splitting

Recently, 2D carbon phosphides (PCs) have attracted much attention due to their superior electronic and photovoltaic properties suitable for potential applications in field effect transistors and photodetectors. In this work, we systematically investigate the stability, electronic properties, optical absorption and photocatalytic water splitting performance of few-layered PC3 by using the first principles calculation method. Numerical results indicate that both monolayered and bilayered PC3 can serve as efficient photocatalysts for overall water splitting due to their high stability, moderate band gaps, suitable band edge positions, anisotropic high carrier mobilities and strong capacity of solar absorption. Compared with monolayered PC3, bilayered PC3 displays higher carrier mobilities (2500-23 000 cm2 V-1 s-1) and a wider optical absorption spectrum. Moreover, by applying an in-plane biaxial strain, the utilization of solar energy and the pH range suitable for overall water splitting can be improved effectively for both monolayered and bilayered PC3. Our work reliably expands the potential application of 2D few-layered PC3 in the field of nano-electronics and nano-optoelectronics.

2D PC3 as a promising thermoelectric material

In the present article, we report the thermoelectric properties of monolayer PC3 for the first time. The structural, vibrational, electronic and thermoelectric properties of PC3 are investigated in detail using a combination of density functional and Boltzman transport theory, and are compared to the carbon (graphene) and phosphorous (phosphorene) analogues. The results show that the PC3 monolayer is dynamically stable and robust upon oxygen contact as well. Also, PC3 is found to be an indirect band gap semiconductor in comparison to the zero gap carbon (graphene) and direct gap phosphorous (phosphorene) analogues. The effect of axial strains is also investigated on the electronic and thermoelectric properties of PC3. The present work reveals monolayer PC3 to be an excellent thermoelectric material with significant thermoelectric performance (ZT ∼ 1) for a large scale operating temperature range of 200-1200 K.

Anisotropic protein diffusion on nanosurface

The unique puckered structure of α-phase phosphorene carbide (α-PC) results in anisotropic electronic and thermal transporting properties. In the present work, the interactions between a model protein, villin headpiece sub-domain (HP35), and the surface of α-PC and monolayer black phosphorus (MBP, another puckered nanostructure) were explored by molecular dynamic (MD) simulations. It is found that HP35 diffuses quickly only along the zigzag direction of the α-PC surface. On the MBP surface, HP35 migrates mainly along the zigzag direction but can also easily stride over the ridges and grooves along the armchair direction. Moreover, the mild binding strength between α-PC and HP35 does not cause distortion in the protein structure. The intrinsic biocompatibility of α-PC, which is distinct from several other widely studied nanomaterials, such as carbon nanotubes, graphene and MoS2, makes it a promising candidate in functional biomedical applications.

Mild lipid extraction and anisotropic cell membrane penetration of α-phase phosphorene carbide nanoribbons by molecular dynamics simulation studies

α-PC penetrates the interior of membrane efficiently only along its zigzag direction rather than its armchair direction.

Carbon-phosphide monolayer with high carrier mobility and perceptibleI–Vresponse for superior gas sensing

We introduce the first-principle theoretical calculations to understand the adsorption mechanism of different gas molecules on monolayered carbon phosphide with semi-metallic electrical conductivity and graphene-like Dirac cone response.

Exploration of the strain and thermoelectric properties of hexagonal SiX (X = N, P, As, Sb, and Bi) monolayers

Si based group V binary compounds have shown better thermoelectric performance at room temperature in addition with ultrahigh carrier mobilities.

The first-principles study of the adsorption of NH3, NO, and NO2 gas molecules on InSe-like phosphorus carbide

Novel γ-PC is a promising reversible material for room-temperature gas sensors.

Two-Dimensional Gold Sulfide Monolayers with Direct Band Gap and Ultrahigh Electron Mobility

It remains a pressing task to search for new two-dimensional (2D) semiconducting materials for future-generation electronic applications. By using density functional theory computations and global structure prediction methods, we demonstrate two new gold sulfide monolayers (2D Au₂S and AuS), both exhibiting excellent electronic properties and high stabilities. All the gold sulfide monolayers are semiconductors with band gaps in the range 1.0-3.6 eV. In particular, the α-Au₂S monolayer is predicted to possess a direct band gap of 1.0 eV and extremely high electron and hole mobilities of 8.45 × 10⁴ and 0.40 × 10⁴ cm² V^-1 S^-1, respectively. The phonon dispersion calculations and ab initio molecular dynamics simulations indicate that the gold sulfide monolayers exhibit robust dynamical and thermal stabilities. Moreover, the α-Au₂S monolayer appears to show strong oxidation resistibility. The novel electronic properties, coupled with structural and chemical stabilities, endow the new gold sulfide monolayers to be highly promising for future applications in nanoelectronics.

Laser ablation synthesis of carbon-phosphides from graphene/nanodiamond-phosphorus composite precursors: Laser desorption ionisation time-of-flight mass spectrometry

RATIONALE

Carbon-phosphides are new and promising strategic materials with applications e.g. in optoelectronics. However, their chemistry and methods of synthesis are not completely understood, and only a limited number of C-P clusters have been detected up to now. Laser ablation synthesis (LAS) or laser desorption ionisation (LDI) has great potential to generate C_m P_n clusters in the gas phase and to act as the basis for the development of new technology.

METHODS

The LAS of carbon phosphides using mixtures of nano-carbon sources (graphene, nanodiamonds) with phosphorus allotropes (red, black, and phosphorene) was examined. Since phosphorene is not commercially available, it was synthesised. A reflectron time-of-flight mass spectrometer was used to produce and identify the C-P clusters. A transmission electron microscope was used to characterise the prepared composites.

RESULTS

LDI of various carbon-phosphorus composites generated a range of carbon-phosphides. From graphene-red phosphorus, C_m P⁺ (m = 3-47), C_m P₂ ⁺ (m = 2-44), C_m P₃ ⁺ (m = 1-42), C_m P₄ ⁺ (m = 1-39), C_m P₅ ⁺ (m = 1-37), C_m P₆ ⁺ (m = 1-34), C_m P₇ ⁺ (m = 1-31), C_m P₈ ⁺ (m = 1-29), C_m P₉ ⁺ (m = 1-26), C_m P₁₀ ⁺ (m = 1-24), C_m P₁₁ ⁺ (m = 1-21), and C_m P₁₂ ⁺ (m = 1-19) clusters were detected, while nanodiamond composites with red/black phosphorus and with phosphorene yielded C₂₄ P_5 + 2n ⁺ (n = 0-28), C₂₄ P_5 + 2n ⁺ (n = 0-16), and C₂₄ P_5 + 2n ⁺ (n = 0-14) clusters, respectively. In total, over 300 new carbon-phosphide clusters were generated.

CONCLUSIONS

The novel series of carbon-phosphide clusters generated from graphene or nanodiamond composites with red/black phosphorus or with phosphorene demonstrated rich carbon-phosphide chemistry that might inspire the development of novel nano-materials with specific properties.